pc5271AY2526wiki - User contributions [en]

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-12T01:56:30Z

Yiteng: /* repeatability check */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepared fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

We describe the process of CQD synthesis:

# 1.0 g of citric acid and 1.0 g of urea were weighed and placed in a test tube.
# The mixture was moistened with approximately 3 drops of deionized water.
# Then we heated the mixture in a household microwave oven at full power. During heating, the reaction mixture frothed and gradually darkened.
# The heating process was stopped when the mixture became dark brown and the frothing had mostly subsided, which typically occurred after 2–3 min.

We excited the photoluminescence of the CQD with a commercial UV LED laser pointed at an angle about 37° as it provides effective excitation of the CQD solution while positioning the brightest fluorescent region toward the spectrometer and reducing direct scattered excitation light entering the detector.

We measured the fluorescence with an Ocean Optics [ST03418] spectrometer device. Since carbon quantum dots typically produce relatively weak photoluminescence, only a small fraction of the emitted light would reach the spectrometer without optical collection, resulting in a lower signal level and poorer signal-to-background ratio. Therefore, we conducted the experiment in a dark room to minimize interference from ambient light. Moreover, a collection lens was used to increase the collection efficiency by gathering a larger solid angle of emitted light and directing it toward the detector.

In our setup, the lens (focal length=18mm) was placed approximately 3.3 cm from the emitting region. Based on the thin-lens equation,

<math display="block">
\frac{1}{f}=\frac{1}{u}+\frac{1}{v},
</math>

where we substitute <math>f=18</math>mm and <math>u=33</math>mm, we get <math>v\approx3.95</math>cm.

This positive value of <math>v</math> indicates that the lens forms a real image of the emitting region. In practice, this means the lens can collect fluorescence from the sample and focus it more effectively into the detection path. A short focal length such as 18 mm is appropriate because it allows strong light collection from a nearby source and is well suited to weak fluorescence measurements.
The collection can also be estimated from the solid angle. For a lens at distance <math>u</math>, the collection half-angle is approximately
<math display="block">
\theta\approx \tan^{-1}(\frac{D/2}{u}),
</math>
where <math>D</math> is the lens diameter.

[[File:Experiment.jpeg|500px|center]]
[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Additionally, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer integral time had been increased to its maximum(6s). A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

For each setup, we measure two sets of spectra:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution.

[[File:Dataset1.pdf|400px|center]]
[[File:Dataset2.pdf|400px|center]]
he first dataset corresponds to the initially prepared solution, while the second dataset corresponds to the solution after one week of storage followed by re-dilution.

Both datasets show a CQD-related emission feature , indicating CQD photoluminescence is present before and after dilution. The undiluted peak position is stable around <math>\lambda_{\text{peak}} \approx 547.5\,\text{nm}</math> (std <math>\sim 3.0\,\text{nm}</math>), while the diluted mean remains near <math>\lambda_{\text{peak}} \approx 544.6\,\text{nm}</math> but with much larger scatter (std <math>\sim 31\,\text{nm}</math>).The undiluted sample exhibits a strong, repeatable peak <math>I_{\text{peak}} \approx 689\,\text{a.u.}</math> (RSD <math>\sim 11.6\%</math>). After dilution, the peak collapses to <math>I_{\text{peak}} \approx 32.5\,\text{a.u.}</math> (<math>\sim 21\times</math> lower) and repeatability worsens (RSD <math>\sim 45\%</math>). CQD emission remains, but the usable fluorescence signal approaches the background level, amplifying sensitivity to environment light noise.

In the undiluted case, <math>I_{\mathrm{int}} \approx 6.91\times10^{4}\,\mathrm{a.u.}\cdot\mathrm{nm}</math> with RSD <math>\sim 11.2\%</math>, indicating a robust total-emission metric. After dilution, <math>I_{\mathrm{int}} \approx 1.46\times10^{3}\,\mathrm{a.u.}\cdot\mathrm{nm}</math>(<math>\sim 47\times</math> lower) and RSD rises to <math>\sim 86\%</math>,at the same time,the diluted dataset reports an unrealistically narrow and highly unstable width: FWHM <math>\approx 7.0\,\text{nm}</math> (RSD <math>\sim 57\%</math>). Such behavior is characteristic of low-SNR artifacts, where noise spikes or local fluctuations are misidentified as the main emission peak.Although both datasets show some degree of repeatability, the re-mixed and re-diluted solution exhibits a clear degradation in CQD quality.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high quality measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result after dilution. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, although our conclusions are not sufficiently rigorous due to various limitations, the project successfully demonstrated fluorescent CQD synthesis and established that our experimental and measurement workflow can be used to assess the quality of the CQD solution.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T17:10:17Z

Yiteng: /* repeatability check */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepared fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

We describe the process of CQD synthesis:

# 1.0 g of citric acid and 1.0 g of urea were weighed and placed in a test tube.
# The mixture was moistened with approximately 3 drops of deionized water.
# Then we heated the mixture in a household microwave oven at full power. During heating, the reaction mixture frothed and gradually darkened.
# The heating process was stopped when the mixture became dark brown and the frothing had mostly subsided, which typically occurred after 2–3 min.

We excited the photoluminescence of the CQD with a commercial UV LED laser pointed at an angle about 37° as it provides effective excitation of the CQD solution while positioning the brightest fluorescent region toward the spectrometer and reducing direct scattered excitation light entering the detector.

We measured the fluorescence with an Ocean Optics [ST03418] spectrometer device. Since carbon quantum dots typically produce relatively weak photoluminescence, only a small fraction of the emitted light would reach the spectrometer without optical collection, resulting in a lower signal level and poorer signal-to-background ratio. Therefore, we conducted the experiment in a dark room to minimize interference from ambient light. Moreover, a collection lens was used to increase the collection efficiency by gathering a larger solid angle of emitted light and directing it toward the detector.

In our setup, the lens (focal length=18mm) was placed approximately 3.3 cm from the emitting region. Based on the thin-lens equation,

<math display="block">
\frac{1}{f}=\frac{1}{u}+\frac{1}{v},
</math>

where we substitute <math>f=18</math>mm and <math>u=33</math>mm, we get <math>v\approx3.95</math>cm.

This positive value of <math>v</math> indicates that the lens forms a real image of the emitting region. In practice, this means the lens can collect fluorescence from the sample and focus it more effectively into the detection path. A short focal length such as 18 mm is appropriate because it allows strong light collection from a nearby source and is well suited to weak fluorescence measurements.
The collection can also be estimated from the solid angle. For a lens at distance <math>u</math>, the collection half-angle is approximately
<math display="block">
\theta\approx \tan^{-1}(\frac{D/2}{u}),
</math>
where <math>D</math> is the lens diameter.

[[File:Experiment.jpeg|500px|center]]
[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Additionally, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer integral time had been increased to its maximum(6s). A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

For each setup, we measure two sets of spectra:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution.

[[File:Dataset1.pdf|550px|center]]
[[File:Dataset2.pdf|550px|center]]

Both datasets show a CQD-related emission feature , indicating CQD photoluminescence is present before and after dilution. The undiluted peak position is stable around <math>\lambda_{\text{peak}} \approx 547.5\,\text{nm}</math> (std <math>\sim 3.0\,\text{nm}</math>), while the diluted mean remains near <math>\lambda_{\text{peak}} \approx 544.6\,\text{nm}</math> but with much larger scatter (std <math>\sim 31\,\text{nm}</math>).The undiluted sample exhibits a strong, repeatable peak <math>I_{\text{peak}} \approx 689\,\text{a.u.}</math> (RSD <math>\sim 11.6\%</math>). After dilution, the peak collapses to <math>I_{\text{peak}} \approx 32.5\,\text{a.u.}</math> (<math>\sim 21\times</math> lower) and repeatability worsens (RSD <math>\sim 45\%</math>). CQD emission remains, but the usable fluorescence signal approaches the background level, amplifying sensitivity to environment light noise.

In the undiluted case, <math>I_{\mathrm{int}} \approx 6.91\times10^{4}\,\mathrm{a.u.}\cdot\mathrm{nm}</math> with RSD <math>\sim 11.2\%</math>, indicating a robust total-emission metric. After dilution, <math>I_{\mathrm{int}} \approx 1.46\times10^{3}\,\mathrm{a.u.}\cdot\mathrm{nm}</math>(<math>\sim 47\times</math> lower) and RSD rises to <math>\sim 86\%</math>,at the same time,the diluted dataset reports an unrealistically narrow and highly unstable width: FWHM <math>\approx 7.0\,\text{nm}</math> (RSD <math>\sim 57\%</math>). Such behavior is characteristic of low-SNR artifacts, where noise spikes or local fluctuations are misidentified as the main emission peak.Although both datasets show some degree of repeatability, the re-mixed and re-diluted solution exhibits a clear degradation in CQD quality.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high quality measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result after dilution. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, although our conclusions are not sufficiently rigorous due to various limitations, the project successfully demonstrated fluorescent CQD synthesis and established that our experimental and measurement workflow can be used to assess the quality of the CQD solution.

File:Dataset2.pdf

2026-04-11T16:55:11Z

Yiteng:

File:Dataset1.pdf

2026-04-11T16:54:51Z

Yiteng:

File:Dataset.pdf

2026-04-11T16:53:29Z

Yiteng:

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T16:18:28Z

Yiteng: /* Conclusion */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepared fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

We describe the process of CQD synthesis:

# 1.0 g of citric acid and 1.0 g of urea were weighed and placed in a test tube.
# The mixture was moistened with approximately 3 drops of deionized water.
# Then we heated the mixture in a household microwave oven at full power. During heating, the reaction mixture frothed and gradually darkened.
# The heating process was stopped when the mixture became dark brown and the frothing had mostly subsided, which typically occurred after 2–3 min.

We excited the photoluminescence of the CQD with a commercial UV LED laser pointed at an angle about 37° as it provides effective excitation of the CQD solution while positioning the brightest fluorescent region toward the spectrometer and reducing direct scattered excitation light entering the detector.

We measured the fluorescence with an Ocean Optics [ST03418] spectrometer device. Since carbon quantum dots typically produce relatively weak photoluminescence, only a small fraction of the emitted light would reach the spectrometer without optical collection, resulting in a lower signal level and poorer signal-to-background ratio. Therefore, we conducted the experiment in a dark room to minimize interference from ambient light. Moreover, a collection lens was used to increase the collection efficiency by gathering a larger solid angle of emitted light and directing it toward the detector.

In our setup, the lens (focal length=18mm) was placed approximately 3.3 cm from the emitting region. Based on the thin-lens equation,

<math display="block">
\frac{1}{f}=\frac{1}{u}+\frac{1}{v},
</math>

where we substitute <math>f=18</math>mm and <math>u=33</math>mm, we get <math>v\approx3.95</math>cm.

This positive value of <math>v</math> indicates that the lens forms a real image of the emitting region. In practice, this means the lens can collect fluorescence from the sample and focus it more effectively into the detection path. A short focal length such as 18 mm is appropriate because it allows strong light collection from a nearby source and is well suited to weak fluorescence measurements.
The collection can also be estimated from the solid angle. For a lens at distance <math>u</math>, the collection half-angle is approximately
<math display="block">
\theta\approx \tan^{-1}(\frac{D/2}{u}),
</math>
where <math>D</math> is the lens diameter.

[[File:Experiment.jpeg|500px|center]]
[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Additionally, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer integral time had been increased to its maximum(6s). A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

For each setup, we measure two sets of spectra:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution; however, the diluted sample exhibited very poor repeatability.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high quality measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result after dilution. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, although our conclusions are not sufficiently rigorous due to various limitations, the project successfully demonstrated fluorescent CQD synthesis and established that our experimental and measurement workflow can be used to assess the quality of the CQD solution.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T15:53:09Z

Yiteng: /* Conclusion */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For CQD synthesis, 1.0 g of citric acid and 1.0 g of urea were weighed and placed in a test tube. The mixture was moistened with approximately 3 drops of deionized water and then heated in a household microwave oven at full power. During heating, the reaction mixture frothed and gradually darkened. The heating process was stopped when the mixture became dark brown and the frothing had mostly subsided, which typically occurred after 2–3 min.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[ST03418]''' spectrometer, as the detector,
* a focusing lens（focal length=18 mm) for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.
* a mount used to fix the UV LED light source in a specific position and orientation.
[[File:Experiment.jpeg|500px|center]]
[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Due to the extremely low brightness of the fluorescence, the experiment was conducted in a dark room. The UV LED was positioned at an angle relative to the beaker so that the most strongly excited fluorescent region was located on the side wall directly facing the spectrometer.

Similarly, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer integral time had been increased to its maximum(6s). A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

To collect more light, we incorporated focusing lens in the setup to increase the collection angle. On the opposite side of the lens, we extended the optical path by 4 cm according to the image distance, and coupled the collected light into an optical fiber connected to the spectrometer.

<math display="block">
f=\frac{d_o d_i}{d_o+d_i}
</math>

For each setup, we measure two sets of spectra:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution; however, the diluted sample exhibited very poor repeatability.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high quality measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result after dilution. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, even some further quantitative analysis was not all achieved, the project successfully demonstrated fluorescent CQD synthesis and established that our experimental and measurement workflow can, to some extent, be used to assess the quality of the CQD solution.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T15:51:59Z

Yiteng: /* Measurement pipelines */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For CQD synthesis, 1.0 g of citric acid and 1.0 g of urea were weighed and placed in a test tube. The mixture was moistened with approximately 3 drops of deionized water and then heated in a household microwave oven at full power. During heating, the reaction mixture frothed and gradually darkened. The heating process was stopped when the mixture became dark brown and the frothing had mostly subsided, which typically occurred after 2–3 min.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[ST03418]''' spectrometer, as the detector,
* a focusing lens（focal length=18 mm) for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.
* a mount used to fix the UV LED light source in a specific position and orientation.
[[File:Experiment.jpeg|500px|center]]
[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Due to the extremely low brightness of the fluorescence, the experiment was conducted in a dark room. The UV LED was positioned at an angle relative to the beaker so that the most strongly excited fluorescent region was located on the side wall directly facing the spectrometer.

Similarly, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer integral time had been increased to its maximum(6s). A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

To collect more light, we incorporated focusing lens in the setup to increase the collection angle. On the opposite side of the lens, we extended the optical path by 4 cm according to the image distance, and coupled the collected light into an optical fiber connected to the spectrometer.

<math display="block">
f=\frac{d_o d_i}{d_o+d_i}
</math>

For each setup, we measure two sets of spectra:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution; however, the diluted sample exhibited very poor repeatability.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high-precision measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result after dilution. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, even some further quantitative analysis was not all achieved, the project successfully demonstrated fluorescent CQD synthesis and established that our experimental and measurement workflow can, to some extent, be used to assess the quality of the CQD solution.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T15:33:28Z

Yiteng: /* Apparatus set up/experiment process */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For CQD synthesis, 1.0 g of citric acid and 1.0 g of urea were weighed and placed in a test tube. The mixture was moistened with approximately 3 drops of deionized water and then heated in a household microwave oven at full power. During heating, the reaction mixture frothed and gradually darkened. The heating process was stopped when the mixture became dark brown and the frothing had mostly subsided, which typically occurred after 2–3 min.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[ST03418]''' spectrometer, as the detector,
* '''[insert lens specs]''' for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.

Two spectra are measured under identical conditions:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

The cleaned CQD emission spectrum is obtained by subtracting the water reference from the solution signal:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

The excitation-leakage / Rayleigh-scattering region near the LED wavelength is excluded from analysis, and isolated detector spikes are removed before final metric extraction.

'''[INSERT IMAGES]'''

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

[[File:Experiment.jpeg|500px|center]]

=== Apparatus set up/experiment process ===

[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Due to the extremely low brightness of the fluorescence, the experiment was conducted in a dark room. The UV LED was positioned at an angle relative to the beaker so that the most strongly excited fluorescent region was located on the side wall directly facing the spectrometer.

Similarly, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer integral time had been increased to its maximum(6s). A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

To collect more light, we incorporated a focusing lens（focal length=18 mm)in the setup to increase the collection angle. On the opposite side of the lens, we extended the optical path by 4 cm according to the image distance, and coupled the collected light into an optical fiber connected to the spectrometer.

<math display="block">
f=\frac{d_o d_i}{d_o+d_i}
</math>

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution; however, the diluted sample exhibited very poor repeatability.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high-precision measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result after dilution. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, even some further quantitative analysis was not all achieved, the project successfully demonstrated fluorescent CQD synthesis and established that our experimental and measurement workflow can, to some extent, be used to assess the quality of the CQD solution.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T15:05:42Z

Yiteng: /* Conclusion */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For CQD synthesis, 1.0 g of citric acid and 1.0 g of urea were weighed and placed in a test tube. The mixture was moistened with approximately 3 drops of deionized water and then heated in a household microwave oven at full power. During heating, the reaction mixture frothed and gradually darkened. The heating process was stopped when the mixture became dark brown and the frothing had mostly subsided, which typically occurred after 2–3 min.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[ST03418]''' spectrometer, as the detector,
* '''[insert lens specs]''' for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.

Two spectra are measured under identical conditions:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

The cleaned CQD emission spectrum is obtained by subtracting the water reference from the solution signal:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

The excitation-leakage / Rayleigh-scattering region near the LED wavelength is excluded from analysis, and isolated detector spikes are removed before final metric extraction.

'''[INSERT IMAGES]'''

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

[[File:Experiment.jpeg|500px|center]]

=== Apparatus set up/experiment process ===

[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Due to the extremely low brightness of the fluorescence, the experiment was conducted in a dark room. The UV LED was positioned at an angle relative to the beaker so that the most strongly excited fluorescent region was located on the side wall directly facing the spectrometer.

Similarly, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer exposure time had been increased to its maximum. A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution; however, the diluted sample exhibited very poor repeatability.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high-precision measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result after dilution. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, even some further quantitative analysis was not all achieved, the project successfully demonstrated fluorescent CQD synthesis and established that our experimental and measurement workflow can, to some extent, be used to assess the quality of the CQD solution.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T15:02:05Z

Yiteng: /* Results and discussions */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For synthesis, we dissolve '''1 g citric acid''' and '''1 g urea''' in '''deionized water''' using standard laboratory glassware (beakers, volumetric flasks, and pipettes), then heat the solution in a '''microwave oven''' under controlled conditions until CQDs form. The specific steps are as follows:

1. Prepare the Reagents: Weigh approximately 1 gram of urea and 1 gram of citric acid, and add them both into a test tube.

2. Moisten the Mixture: Add about 3 drops of deionized water to the reaction mixture in the test tube to moisten it.

3, Microwave Heating: Place the test tube in a household microwave oven and heat it on full power.

4. Observe and Stop: During the heating process, the mixture will froth and gradually turn darker. Stop the heating once the mixture becomes dark brown and the frothing has mostly subsided. This usually takes about 2 to 3 minutes.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[ST03418]''' spectrometer, as the detector,
* '''[insert lens specs]''' for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.

Two spectra are measured under identical conditions:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

The cleaned CQD emission spectrum is obtained by subtracting the water reference from the solution signal:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

The excitation-leakage / Rayleigh-scattering region near the LED wavelength is excluded from analysis, and isolated detector spikes are removed before final metric extraction.

'''[INSERT IMAGES]'''

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

[[File:Experiment.jpeg|500px|center]]

=== Apparatus set up/experiment process ===

[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Due to the extremely low brightness of the fluorescence, the experiment was conducted in a dark room. The UV LED was positioned at an angle relative to the beaker so that the most strongly excited fluorescent region was located on the side wall directly facing the spectrometer.

Similarly, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer exposure time had been increased to its maximum. A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

=== repeatability check ===
[[File:Raw_rep.pdf|750px|center]]
[[File:Raw_dil.pdf|750px|center]]

There was a 7-day interval between the measurements for the first and second graphs. Due to solvent evaporation, we re-diluted the solution; however, the diluted sample exhibited very poor repeatability.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high-precision measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, even some further quantitative analysis was not all achieved, the project successfully demonstrated fluorescent CQD synthesis and established the key experimental bottlenecks for future improvement.

File:Raw dil.pdf

2026-04-11T14:57:27Z

Yiteng:

File:Raw rep.pdf

2026-04-11T14:56:31Z

Yiteng:

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-11T14:35:08Z

Yiteng:

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For synthesis, we dissolve '''1 g citric acid''' and '''1 g urea''' in '''deionized water''' using standard laboratory glassware (beakers, volumetric flasks, and pipettes), then heat the solution in a '''microwave oven''' under controlled conditions until CQDs form. The specific steps are as follows:

1. Prepare the Reagents: Weigh approximately 1 gram of urea and 1 gram of citric acid, and add them both into a test tube.

2. Moisten the Mixture: Add about 3 drops of deionized water to the reaction mixture in the test tube to moisten it.

3, Microwave Heating: Place the test tube in a household microwave oven and heat it on full power.

4. Observe and Stop: During the heating process, the mixture will froth and gradually turn darker. Stop the heating once the mixture becomes dark brown and the frothing has mostly subsided. This usually takes about 2 to 3 minutes.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[insert specs]''' spectrometer, as the detector,
* '''[insert lens specs]''' for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.

Two spectra are measured under identical conditions:

1. the '''water spectrum''', representing solvent and setup background,

2. the '''solution spectrum''', containing both CQD emission and background contributions.

The cleaned CQD emission spectrum is obtained by subtracting the water reference from the solution signal:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

The excitation-leakage / Rayleigh-scattering region near the LED wavelength is excluded from analysis, and isolated detector spikes are removed before final metric extraction.

'''[INSERT IMAGES]'''

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artefacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

[[File:Experiment.jpeg|500px|center]]

=== Apparatus set up/experiment process ===

[[File:Setup_R.jpg|500px|center]]
[[File:Setup.jpg|450px|center]]

Due to the extremely low brightness of the fluorescence, the experiment was conducted in a dark room. The UV LED was positioned at an angle relative to the beaker so that the most strongly excited fluorescent region was located on the side wall directly facing the spectrometer.

Similarly, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer exposure time had been increased to its maximum. A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

== Conclusion ==

On the synthesis side, we achieved the first objective: the prepared solution exhibited clear fluorescence under UV excitation, and we observed a distinct diffusion of fluorescence upon adding droplets into the solution. These macroscopic signatures, together with the peak position visible in our recorded spectra, strongly suggest that fluorescent carbon quantum dot species were successfully produced.

However, quantitative characterization proved to be the main limitation. Using the Ocean Optics fiber spectrometer, the emission intensity remained too weak for high-precision measurements. Even after adding a focusing lens and increasing the integration time to its maximum, the signal-to-noise ratio was insufficient, and repeated measurements under identical conditions cannot give us a fully reliable result. The lack of a true dark environment further increased background fluctuations and reduced repeatability.

In addition, practical constraints in sample handling affected data quality over the multi-week timeline. We did not have dedicated storage conditions for high-stability sample preservation, and evaporation and precipitation gradually changed the effective concentration and composition, compromising control over experimental variables. Combined with limited instrumental sensitivity, these factors prevented several planned follow-up analyses.

Overall, even some further quantitative analysis was not all achieved, the project successfully demonstrated fluorescent CQD synthesis and established the key experimental bottlenecks for future improvement.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-08T08:36:59Z

Yiteng: /* Apparatus set up/experiment process */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For synthesis, we dissolve '''1 g citric acid''' and '''1 g urea''' in '''deionized water''' using standard laboratory glassware (beakers, volumetric flasks, and pipettes), then heat the solution in a '''microwave oven''' under controlled conditions until CQDs form. The specific steps are as follows:
1. Prepare the Reagents: Weigh approximately 1 gram of urea and 1 gram of citric acid, and add them both into a test tube.
2. Moisten the Mixture: Add about 3 drops of deionized water to the reaction mixture in the test tube to moisten it.
3, Microwave Heating: Place the test tube in a household microwave oven and heat it on full power.
4. Observe and Stop: During the heating process, the mixture will froth and gradually turn darker. Stop the heating once the mixture becomes dark brown and the frothing has mostly subsided. This usually takes about 2 to 3 minutes.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[insert specs]''' spectrometer, as the detector,
* '''[insert lens specs]''' for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.

Two spectra are measured under identical conditions:
# the '''water spectrum''', representing solvent and setup background,

# the '''solution spectrum''', containing both CQD emission and background contributions.

The cleaned CQD emission spectrum is obtained by subtracting the water reference from the solution signal:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

The excitation-leakage / Rayleigh-scattering region near the LED wavelength is excluded from analysis, and isolated detector spikes are removed before final metric extraction.

'''[INSERT IMAGES]'''

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artifacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

[[File:Experiment.jpeg|750px|center]]

=== Apparatus set up/experiment process ===

[[File:Setup_R.jpg|750px|center]]
[[File:Setup.jpg|750px|center]]

Due to the extremely low brightness of the fluorescence, the experiment was conducted in a dark room. The UV LED was positioned at an angle relative to the beaker so that the most strongly excited fluorescent region was located on the side wall directly facing the spectrometer.

Similarly, the solution concentration had a significant impact on the measurement results. Usable data were obtained only after the solution had been diluted until it became completely colorless and the spectrometer exposure time had been increased to its maximum. A possible explanation is that an excessive concentration of fluorescent species in the solution may partially block the emitted light.

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-04-08T07:34:42Z

Yiteng: /* Apparatus set up */

== Introduction ==
Carbon quantum dots (CQDs) are fluorescent nanomaterials that can be formulated into water-based coatings and inks, enabling UV-activated visuals for applications such as exhibition displays, interactive graphics, and covert markings.
In this project, we focus on CQDs synthesized from citric acid and urea because the system is low-cost, relatively safe, and easy to prepare with typical teaching-lab equipment, and it commonly yields bright visible fluorescence (often in the green region) that can be excited using a simple UV source.

Citric acid + urea CQDs are typically made by heating an aqueous precursor solution (e.g., hydrothermal or microwave). During heating, the molecules dehydrate and partially carbonize, forming tiny carbon-rich particles while leaving oxygen- and nitrogen-containing groups on the surface. These CQDs fluoresce because UV/blue light excites electrons to higher energy, and when the electrons relax, they emit visible light; the brightness and color depend strongly on the size/structure of the carbon core and, especially, on surface “states” created by surface groups and defects.
In this system, the heating conditions can also produce small fluorescent byproducts in addition to the dots, so two batches made with slightly different time/power/temperature can have noticeably different intensity and spectral shape.
Therefore, our goal is to build and use a repeatable fluorescence sensing setup (UV excitation + spectrometer readout) to quantitatively evaluate CQD “quality” using objective metrics such as fluorescence intensity and spectral consistency, and to relate these metrics back to controllable synthesis parameters.

In the sections that follow, we outline the sensing rationale, define our experimental goals and measurables, and then detail the synthesis and fluorescence-measurement methodology used to quantify CQD quality.

== Report objectives and scope==

The objective of this work is to quantify the '''optical fluorescence quality''' of citric-acid/urea carbon quantum dots (CQDs) using a repeatable optical setup based on '''UV LED excitation''' and '''spectrometer detection'''. The goal is to generate metrics that allow comparison of CQD batches produced under different synthesis conditions.

The analysis is designed to:

# isolate true photoluminescence (PL) from optical artifacts,
# extract robust spectral quality metrics,
# assess whether the emission is relatively uniform or heterogeneous,
# evaluate whether observed differences are real or caused by measurement noise or drift.

In this project, “quality” refers to optical fluorescence quality, not full chemical or structural quality.

A higher-quality CQD solution is defined as one that shows:
* stronger fluorescence under fixed measurement conditions,
* a cleaner and more stable emission spectrum,
* a narrower and more uniform emission band,
* better repeatability across repeated measurements.

Thus, CQD quality is evaluated through four dimensions:

<math display="block">
\text{Quality}\sim\text{brightness} + \text{spectral purity} + \text{spectral uniformity} + \text{measurement repeatability}
</math>

== Measurement pipelines ==

We prepare fluorescent carbon quantum dots (CQDs) by heating an aqueous mixture of citric acid and urea, which promotes dehydration and condensation of the precursors, followed by partial carbonization to form nanoscale carbonaceous particles. Citric acid serves as the carbon source, while urea introduces nitrogen-containing surface functional groups that improve water dispersibility and typically increase fluorescence intensity.

For synthesis, we dissolve '''1 g citric acid''' and '''1 g urea''' in '''deionized water''' using standard laboratory glassware (beakers, volumetric flasks, and pipettes), then heat the solution in a '''microwave oven''' under controlled conditions until CQDs form. The specific steps are as follows:
1. Prepare the Reagents: Weigh approximately 1 gram of urea and 1 gram of citric acid, and add them both into a test tube.
2. Moisten the Mixture: Add about 3 drops of deionized water to the reaction mixture in the test tube to moisten it.
3, Microwave Heating: Place the test tube in a household microwave oven and heat it on full power.
4. Observe and Stop: During the heating process, the mixture will froth and gradually turn darker. Stop the heating once the mixture becomes dark brown and the frothing has mostly subsided. This usually takes about 2 to 3 minutes.

The optical setup uses:
* a commercial UV LED light pen as the excitation source,
* an Ocean Optics '''[insert specs]''' spectrometer, as the detector,
* '''[insert lens specs]''' for better light capturing
* deionized water (same as used in the sample) as the reference/background measurement.

Two spectra are measured under identical conditions:
# the '''water spectrum''', representing solvent and setup background,

# the '''solution spectrum''', containing both CQD emission and background contributions.

The cleaned CQD emission spectrum is obtained by subtracting the water reference from the solution signal:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

The excitation-leakage / Rayleigh-scattering region near the LED wavelength is excluded from analysis, and isolated detector spikes are removed before final metric extraction.

'''[INSERT IMAGES]'''

== Data-processing workflow ==
To ensure the reported spectra accurately reflect the intrinsic photoluminescence (PL) of the Carbon Quantum Dots (CQDs), the following post-acquisition processing protocol was implemented:

# Acquire raw spectra of water and CQD solution under the identical optoelectronic configurations conditions using the '''OceanView''' software.
# Align the spectra in wavelength.
# Subtract the solvent blank from the solution spectrum.
# Exclude the wavelength region proximal to the excitation wavelength to remove excitation leakage and Rayleigh scattering.
# Remove isolated detector spikes and nonphysical subtraction artifacts.
# Use the processed spectrum for metric extraction.

Subsequently, we report the following quantities for each batch.

[[File:Experiment.jpeg|750px|center]]

=== Apparatus set up/experiment process ===

[[File:Setup_R.jpg|750px|center]]
[[File:Setup.jpg|750px|center]]

=== 1. Cleaned emission spectrum ===

The primary processed output is the cleaned CQD photoluminescence spectrum:

<math display="block">
I_{\text{clean}}(\lambda) = I_{\text{solution}}(\lambda) - \alpha I_{\text{water}}(\lambda)
</math>

where:
* <math>I_{\text{solution}}(\lambda)</math> is the measured CQD solution spectrum,
* <math>I_{\text{water}}(\lambda)</math> is the water reference spectrum,
* <math>\alpha</math> is the background scaling factor.

This cleaned spectrum is used for all subsequent metric extraction.

=== 2. Peak wavelength ===

The peak wavelength is defined as:

<math display="block">
\lambda_{\text{peak}} = \text{argmax}_{\lambda} I_{\text{clean}}(\lambda)
</math>

This indicates the dominant emission position and serves as a spectral-color / emissive-state indicator. Stable values of <math>\lambda_{\text{peak}}</math> across repeated measurements or across batches suggest more consistent synthesis and more reproducible emissive behavior.

=== 3. Peak intensity ===

The peak intensity is defined as:

<math display="block">
I_{\text{peak}} = \max_{\lambda} I_{\text{clean}}(\lambda)
</math>

This gives the maximum signal level in the cleaned spectrum. It is useful as a simple brightness indicator, but it is more sensitive to noise and alignment than integrated intensity.

=== 4. Integrated intensity ===

The integrated emission intensity is defined as:

<math display="block">
I_{\text{int}} = \int_{\lambda_1}^{\lambda_2} I_{\text{clean}}(\lambda)\, d\lambda
</math>

where <math>\lambda_1</math> and <math>\lambda_2</math> define the valid emission range used for analysis.

This is the main brightness metric used in this work because it captures the total emitted signal and is more robust than peak intensity for comparing different batches under fixed measurement conditions.

=== 5. Full width at half maximum (FWHM) ===

The spectral width is quantified using the full width at half maximum:

<math display="block">
\mathrm{FWHM} = \lambda_{\text{right}} - \lambda_{\text{left}}
</math>

where <math>\lambda_{\text{left}}</math> and <math>\lambda_{\text{right}}</math> are the wavelengths at which the cleaned spectrum reaches half of the peak intensity.

FWHM is used as the main spectral-uniformity metric. A smaller FWHM suggests a narrower and more uniform emission band, while a larger FWHM suggests broader emission and greater heterogeneity of emissive states.

=== 6. Signal-to-background ratio (SBR) ===

To evaluate measurement quality and confidence in the extracted signal, the signal-to-background ratio is defined as:

<math display="block">
\mathrm{SBR} = \frac{I_{\text{peak}}}{\sigma_{\text{baseline}}}
</math>

where <math>\sigma_{\text{baseline}}</math> is the standard deviation of the cleaned signal in a wavelength region where no real CQD photoluminescence is expected.

A larger SBR indicates that the detected fluorescence is strong relative to the residual background noise, and therefore more reliable.

=== 7. Repeatability / precision ===

To assess whether differences between samples are real rather than caused by setup drift or noise, repeated measurements are used. For any metric <math>x</math>, the mean and standard deviation are computed as:

<math display="block">
\bar{x} = \frac{1}{n}\sum_{k=1}^{n} x_k
</math>

<math display="block">
s_x = \sqrt{\frac{1}{n-1}\sum_{k=1}^{n}(x_k - \bar{x})^2}
</math>

The relative standard deviation (RSD) is then defined as:

<math display="block">
\mathrm{RSD}(x)\% = \frac{s_x}{\bar{x}} \times 100\%
</math>

In this work, the most useful repeatability metrics are:
* <math>\mathrm{RSD}(\lambda_{\text{peak}})</math>
* <math>\mathrm{RSD}(I_{\text{int}})</math>
* <math>\mathrm{RSD}(\mathrm{FWHM})</math>

A lower RSD indicates better measurement precision and higher confidence in batch-to-batch comparisons.

=== 8. Summary of key quality metrics ===

The primary quality metrics used in this work are:
* <math>I_{\text{int}}</math> as the main brightness metric,
* <math>\lambda_{\text{peak}}</math> as the dominant emission-position metric,
* <math>\mathrm{FWHM}</math> as the main spectral-uniformity / heterogeneity metric,
* <math>\mathrm{SBR}</math> as a measurement-confidence metric,
* repeatability metrics based on <math>\mathrm{RSD}</math> to quantify precision.

Together, these quantities provide a practical framework for comparing the optical fluorescence quality of CQD batches produced under different synthesis conditions.

== Results and discussions ==
[[File:cqd_water_solution_compare.pdf|750px|center]]

The background-subtracted spectrum shows a clear broad positive emission band, confirming that the CQD solution produces genuine photoluminescence rather than only solvent or instrumental background. The dominant emission peak is located at <math>\lambda_{\mathrm{peak}} = 548.28\ \mathrm{nm}</math>, placing the fluorescence in the green region of the visible spectrum. The extracted peak intensity is <math>I_{\mathrm{peak}} = 717.72\ \mathrm{a.u.}</math>, while the integrated intensity is <math>I_{\mathrm{int}} = 73457.29\ \mathrm{a.u.\ nm}</math>, indicating that the sample is clearly fluorescent under the present measurement conditions.

The emission width is relatively broad, with <math>\mathrm{FWHM} = 91.73\ \mathrm{nm}</math>. This suggests that the fluorescence is not produced by a highly uniform single emissive state, but more likely by a distribution of emissive environments or overlapping emissive centers. In other words, the sample shows strong fluorescence but only moderate spectral uniformity.

Several narrow spikes and sharp dips remain in the cleaned raw trace. These are much narrower than the main emission band and are therefore attributed to detector or subtraction artifacts rather than intrinsic CQD features. Overall, the result indicates a fluorescent green-emitting CQD sample with noticeable spectral heterogeneity.

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-03-22T05:42:21Z

Yiteng: /* Data-processing workflow */

File:Setup.jpg

2026-03-22T05:40:17Z

Yiteng:

File:Setup R.jpg

2026-03-22T05:39:28Z

Yiteng:

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-03-17T05:05:19Z

Yiteng: /* Data-processing workflow */

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-03-17T05:04:54Z

Yiteng: /* Measurement pipelines */

Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control

2026-03-17T05:02:43Z

Yiteng: /* Introduction */

Main Page

2026-02-10T06:18:16Z

Yiteng: /* Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control */

'''Welcome to the wiki page for the course PC5271: Physics of Sensors "(in AY25/26 Sem 2)!'''

This is the repository where projects are documented. You will need to create an account for editing/creating pages. If you need an account, please contact Christian.

'''Logistics''':
Our '''location is S11-02-04''', time slots for "classes" are '''Tue and Fri 10:00am-12:00noon'''.

==Projects==

===[[Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control]]===

Group menber: Zhang yiteng, Li Xiaoyue, Peng Jianxi

This project aims to develop a low-cost, repeatable optical sensing system to quantify the quality of Carbon Quantum Dots (CQDs). We synthesize CQDs using a microwave-assisted method with citric acid and urea, and characterize their fluorescence properties using a custom-built setup comprising a UV LED excitation source and a fiber-optic spectrometer. By analyzing spectral metrics such as peak wavelength, intensity, and FWHM, we establish a robust quality control protocol for nanomaterial production.

===[[Inductive Sensors of Ultra-high Sensitivity Based on Nonlinear Exceptional Point]]===
Team members: Yuan Siyu; Zhu Ziyang; Wang Peikun; Li Xunyu

We are building two coupled oscillating circuits: one that naturally loses energy (lossy) and one that gains energy (active) using a specific amplifier that saturates at high amplitudes. When tuning these two circuits to a nonlinear Exceptional Point (NEP), the system becomes extremely sensitive to small perturbations in inductance, following a steep cubic-root response curve, while remaining resistant to noise.

'''CK:''' We likely have all the parts for this, but let us know the frequency so we can find the proper amplifier and circuit board.

'''SY:''' Thanks for your confirmation. The operating frequency is around 70-80 kHz.

===[[EA Spectroscopy as a series of sensors: Investigating the Impact of Film-Processing Temperature on Mobility in Organic Diodes]]===
Team members: Li Jinhan; Liu Chenyang

We will use EA spectroscopy, which will include optical sensors, electrical sensors, and lock-in amplifiers, among other components as a highly sensitive, non-destructive optical sensing platform to measure the internal electric field modulation response of organic diodes under operating conditions, and to quantitatively extract carrier mobility based on this measurement. By systematically controlling the thin film preparation temperature and comparing the EA response characteristics of different samples, the project aims to reveal the influence of film preparation temperature on device mobility and its potential physical origins.

===[[Optical Sensor of Magnetic Dynamics: A Balanced-Detection MOKE Magnetometer]]===
Team members: LI Junxiang; Patricia Breanne Tan Sy

We will use a laser-based magneto-optical Kerr effect setup featuring a high-sensitivity differential photodiode array to measure the Kerr rotation angle induced by surface magnetism. This system serves as a versatile optical platform to investigate how external perturbations such as magnetic fields or radiation source alter the magnetic ordering of materials, allowing for the quantitative extraction of the magneto-optical coupling coefficients of various thin films.

===[[Precision Measurement of Material and Optical Properties Using Interferometry]]===
Team members: Yang SangUk; Zhang ShunYang; Xu Zifang

We will be constructing an interferometer and use it as a tool for precision measurement. One primary objective is determination of the refractive index of various gases by analyzing the resulting shift interference fringes.
===[[Precision Thermocouple Based Temperature Measurement System]]===
Team members: Sree Ranjani Krishnan; Nisha Ganesh ; Burra Srikari

We will design, build, and validate a precision thermocouple-based temperature measurement system using the Seebeck effect. The system will convert the extremely small thermoelectric voltage generated by a thermocouple into accurate, real-time temperature data. Since the output voltage is really small we will be using an instrumentation amplifier to amplify the output voltage and use an Arduino to digitalize the results.

Materials needed: K-type thermocouple/Thermophile;Arduino

==Resources==
===Books and links===
* A good textbook on the Physics of Sensors is Jacob Fraden: Handbook of Mondern Sensors, Springer, ISBN 978-3-319-19302-1 or [https://link.springer.com/book/10.1007/978-3-319-19303-8 doi:10.1007/978-3-319-19303-8]. There shoud be an e-book available through the NUS library at https://linc.nus.edu.sg/record=b3554643
* Another good textbook: John B.Bentley: Principles of Measurement Systems, 4th Edition, Pearson, ISBN: 0-13-043028-5 or https://linc.nus.edu.sg/record=b2458243 in our library.

===Software===
* Various Python extensions. [https://www.python.org Python] is a very powerful free programming language that runs on just about any computer platform. It is open source and completely free.
* [https://www.gnuplot.info Gnuplot]: A free and very mature data display tool that works on just about any platform used that produces excellent publication-grade eps and pdf figures. Can be also used in scripts. Open source and completely free.
* Matlab: Very common, good toolset also for formal mathematics, good graphics. Expensive. We may have a site license, but I am not sure how painful it is for us to get a license for this course. Ask if interested.
* Mathematica: More common among theroetical physicists, very good in formal maths, now with better numerics. Graphs are ok but can be a pain to make looking good. As with Matlab, we do have a campus license. Ask if interested.

===Apps===
Common mobile phones these days are equipped with an amazing toolchest of sensors. There are a few apps that allow you to access them directly, and turn your phone into a powerful sensor. Here some suggestions:

* Physics Toolbox sensor suite on [https://play.google.com/store/apps/details?id=com.chrystianvieyra.physicstoolboxsuite&hl=en_SG Google play store] or [https://apps.apple.com/us/app/physics-toolbox-sensor-suite/id1128914250 Apple App store].

===Data sheets===
A number of components might be useful for several groups. Some common data sheets are here:
* Photodiodes:
** Generic Silicon pin Photodiode type [[Media:Bpw34.pdf|BPW34]]
** Fast photodiodes (Silicon PIN, small area): [[Media:S5971_etc_kpin1025e.pdf|S5971/S5972/S5973]]
* PT 100 Temperature sensors based on platinum wire: [[Media:PT100_TABLA_R_T.pdf|Calibration table]]
* Thermistor type [[Media:Thermistor B57861S.pdf|B57861S]] (R0=10kΩ, B=3988Kelvin). Search for [https://en.wikipedia.org/wiki/Steinhart-Hart_equation Steinhart-Hart equation]. See [[Thermistor]] page here as well.
* Humidity sensor
** Sensirion device the reference unit: [[media:Sensirion SHT30-DIS.pdf|SHT30/31]]
* Thermopile detectors:
** [[Media:Thermopile_G-TPCO-035 TS418-1N426.pdf|G-TPCO-035 / TS418-1N426]]: Thermopile detector with a built-in optical bandpass filter for light around 4μm wavelength for CO2 absorption
* Resistor color codes are explained [https://en.wikipedia.org/wiki/Electronic_color_code here]

** Fluxgate magnetometer [[media:Data-sheet FLC-100.pdf|FCL100]]
* Lasers
** Red laser diode [[media:HL6501MG.pdf|HL6501MG]]
* Generic amplifiers
** Instrumentation amplifiers: [[media:Ad8221.pdf|AD8221]] or [[media:AD8226.pdf|AD8226]]
** Conventional operational amplifiers: Precision: [[media:OP27.pdf|OP27]], General purpose: [[media:OP07.pdf|OP07]]
** Transimpedance amplifiers for photodetectors: [[media:AD8015.pdf|AD8015]]

==Some wiki reference materials==
* [https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:FAQ MediaWiki FAQ]
* [[Writing mathematical expressions]]
* [[Uploading images and files]]

==Old wikis==
You can find entries to the wiki from [https://pc5271.org/PC5271_AY2425S2 AY2024/25 Sem 2] and [https://pc5271.org/PC5271_AY2324S2 AY2023/24 Sem 2].

Main Page

2026-02-10T06:17:30Z

Yiteng: /* Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control */

'''Welcome to the wiki page for the course PC5271: Physics of Sensors "(in AY25/26 Sem 2)!'''

This is the repository where projects are documented. You will need to create an account for editing/creating pages. If you need an account, please contact Christian.

'''Logistics''':
Our '''location is S11-02-04''', time slots for "classes" are '''Tue and Fri 10:00am-12:00noon'''.

==Projects==

===[[Fluorescence Sensor for Carbon Quantum Dots: Synthesis, Characterization, and Quality Control]]===
Group menber: Zhang yiteng, Li Xiaoyue, Peng Jianxi
This project aims to develop a low-cost, repeatable optical sensing system to quantify the quality of Carbon Quantum Dots (CQDs). We synthesize CQDs using a microwave-assisted method with citric acid and urea, and characterize their fluorescence properties using a custom-built setup comprising a UV LED excitation source and a fiber-optic spectrometer. By analyzing spectral metrics such as peak wavelength, intensity, and FWHM, we establish a robust quality control protocol for nanomaterial production.

===[[Inductive Sensors of Ultra-high Sensitivity Based on Nonlinear Exceptional Point]]===
Team members: Yuan Siyu; Zhu Ziyang; Wang Peikun; Li Xunyu

We are building two coupled oscillating circuits: one that naturally loses energy (lossy) and one that gains energy (active) using a specific amplifier that saturates at high amplitudes. When tuning these two circuits to a nonlinear Exceptional Point (NEP), the system becomes extremely sensitive to small perturbations in inductance, following a steep cubic-root response curve, while remaining resistant to noise.

'''CK:''' We likely have all the parts for this, but let us know the frequency so we can find the proper amplifier and circuit board.

'''SY:''' Thanks for your confirmation. The operating frequency is around 70-80 kHz.

===[[EA Spectroscopy as a series of sensors: Investigating the Impact of Film-Processing Temperature on Mobility in Organic Diodes]]===
Team members: Li Jinhan; Liu Chenyang

We will use EA spectroscopy, which will include optical sensors, electrical sensors, and lock-in amplifiers, among other components as a highly sensitive, non-destructive optical sensing platform to measure the internal electric field modulation response of organic diodes under operating conditions, and to quantitatively extract carrier mobility based on this measurement. By systematically controlling the thin film preparation temperature and comparing the EA response characteristics of different samples, the project aims to reveal the influence of film preparation temperature on device mobility and its potential physical origins.

===[[Optical Sensor of Magnetic Dynamics: A Balanced-Detection MOKE Magnetometer]]===
Team members: LI Junxiang; Patricia Breanne Tan Sy

We will use a laser-based magneto-optical Kerr effect setup featuring a high-sensitivity differential photodiode array to measure the Kerr rotation angle induced by surface magnetism. This system serves as a versatile optical platform to investigate how external perturbations such as magnetic fields or radiation source alter the magnetic ordering of materials, allowing for the quantitative extraction of the magneto-optical coupling coefficients of various thin films.

===[[Precision Measurement of Material and Optical Properties Using Interferometry]]===
Team members: Yang SangUk; Zhang ShunYang; Xu Zifang

We will be constructing an interferometer and use it as a tool for precision measurement. One primary objective is determination of the refractive index of various gases by analyzing the resulting shift interference fringes.
===[[Precision Thermocouple Based Temperature Measurement System]]===
Team members: Sree Ranjani Krishnan; Nisha Ganesh ; Burra Srikari

We will design, build, and validate a precision thermocouple-based temperature measurement system using the Seebeck effect. The system will convert the extremely small thermoelectric voltage generated by a thermocouple into accurate, real-time temperature data. Since the output voltage is really small we will be using an instrumentation amplifier to amplify the output voltage and use an Arduino to digitalize the results.

Materials needed: K-type thermocouple/Thermophile;Arduino

==Resources==
===Books and links===
* A good textbook on the Physics of Sensors is Jacob Fraden: Handbook of Mondern Sensors, Springer, ISBN 978-3-319-19302-1 or [https://link.springer.com/book/10.1007/978-3-319-19303-8 doi:10.1007/978-3-319-19303-8]. There shoud be an e-book available through the NUS library at https://linc.nus.edu.sg/record=b3554643
* Another good textbook: John B.Bentley: Principles of Measurement Systems, 4th Edition, Pearson, ISBN: 0-13-043028-5 or https://linc.nus.edu.sg/record=b2458243 in our library.

===Software===
* Various Python extensions. [https://www.python.org Python] is a very powerful free programming language that runs on just about any computer platform. It is open source and completely free.
* [https://www.gnuplot.info Gnuplot]: A free and very mature data display tool that works on just about any platform used that produces excellent publication-grade eps and pdf figures. Can be also used in scripts. Open source and completely free.
* Matlab: Very common, good toolset also for formal mathematics, good graphics. Expensive. We may have a site license, but I am not sure how painful it is for us to get a license for this course. Ask if interested.
* Mathematica: More common among theroetical physicists, very good in formal maths, now with better numerics. Graphs are ok but can be a pain to make looking good. As with Matlab, we do have a campus license. Ask if interested.

===Apps===
Common mobile phones these days are equipped with an amazing toolchest of sensors. There are a few apps that allow you to access them directly, and turn your phone into a powerful sensor. Here some suggestions:

* Physics Toolbox sensor suite on [https://play.google.com/store/apps/details?id=com.chrystianvieyra.physicstoolboxsuite&hl=en_SG Google play store] or [https://apps.apple.com/us/app/physics-toolbox-sensor-suite/id1128914250 Apple App store].

===Data sheets===
A number of components might be useful for several groups. Some common data sheets are here:
* Photodiodes:
** Generic Silicon pin Photodiode type [[Media:Bpw34.pdf|BPW34]]
** Fast photodiodes (Silicon PIN, small area): [[Media:S5971_etc_kpin1025e.pdf|S5971/S5972/S5973]]
* PT 100 Temperature sensors based on platinum wire: [[Media:PT100_TABLA_R_T.pdf|Calibration table]]
* Thermistor type [[Media:Thermistor B57861S.pdf|B57861S]] (R0=10kΩ, B=3988Kelvin). Search for [https://en.wikipedia.org/wiki/Steinhart-Hart_equation Steinhart-Hart equation]. See [[Thermistor]] page here as well.
* Humidity sensor
** Sensirion device the reference unit: [[media:Sensirion SHT30-DIS.pdf|SHT30/31]]
* Thermopile detectors:
** [[Media:Thermopile_G-TPCO-035 TS418-1N426.pdf|G-TPCO-035 / TS418-1N426]]: Thermopile detector with a built-in optical bandpass filter for light around 4μm wavelength for CO2 absorption
* Resistor color codes are explained [https://en.wikipedia.org/wiki/Electronic_color_code here]

** Fluxgate magnetometer [[media:Data-sheet FLC-100.pdf|FCL100]]
* Lasers
** Red laser diode [[media:HL6501MG.pdf|HL6501MG]]
* Generic amplifiers
** Instrumentation amplifiers: [[media:Ad8221.pdf|AD8221]] or [[media:AD8226.pdf|AD8226]]
** Conventional operational amplifiers: Precision: [[media:OP27.pdf|OP27]], General purpose: [[media:OP07.pdf|OP07]]
** Transimpedance amplifiers for photodetectors: [[media:AD8015.pdf|AD8015]]

==Some wiki reference materials==
* [https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:FAQ MediaWiki FAQ]
* [[Writing mathematical expressions]]
* [[Uploading images and files]]

==Old wikis==
You can find entries to the wiki from [https://pc5271.org/PC5271_AY2425S2 AY2024/25 Sem 2] and [https://pc5271.org/PC5271_AY2324S2 AY2023/24 Sem 2].