Carbon Dot Color Tuning with Smartphone Spectra

ISEF Category: Materials Science

Subcategory: Electronic, Optical, and Magnetic Materials  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

A tiny change in chemistry can make one material glow blue, green, or yellow. That makes carbon dots a great science fair topic. You can make them from common ingredients, then measure their light instead of just guessing by eye. A DIY smartphone spectrometer lets you turn that glow into real data.

What Is It?

Carbon dots are tiny carbon-based particles that can fluoresce, which means they absorb one kind of light and emit another. Think of them like microscopic glow beads. When you change how you make them, their structure can change too, and that can shift the color of light they give off.

This project asks you to connect synthesis conditions to optical behavior. In plain terms, you make the dots, shine light on them, and record the emitted spectrum. A spectrum is just a graph of brightness versus color wavelength. Your smartphone spectrometer turns that graph into something you can compare across samples.

The big idea is that chemistry controls structure, and structure controls light. That makes this project a nice bridge between materials science, chemistry, and measurement design.

Why This Is a Good Topic

This topic works well because you can test one variable at a time, like heating conditions, starting ratios, or purification choices, and then measure a clear output, the emission spectrum. It connects to real uses such as sensors, bioimaging, and optical materials. You can learn synthesis planning, calibration, spectral analysis, and how to defend your controls, all without needing a university lab.

Research Questions

• How does the citric acid to urea ratio affect the emission peak of carbon dots?
• What is the effect of microwave heating time on the brightness of carbon dot fluorescence?
• Does the purification method change the measured emission spectrum of the dots?
• To what extent does reaction temperature shift the color of emitted light?
• Which synthesis condition gives the narrowest and most repeatable emission peak?
• How does storage time affect the fluorescence intensity of prepared carbon dots?

Basic Materials

• Citric acid powder, food-grade or lab grade.
• Urea powder, food-grade or lab grade.
• Microwave-safe glass container.
• Digital kitchen scale with 0.1 g accuracy.
• Disposable pipettes or transfer droppers.
• Distilled water.
• Clear plastic cuvettes or small transparent sample vials.
• Blue or ultraviolet LED flashlight.
• DIY smartphone spectrometer with diffraction grating.
• Dark box or shoebox to reduce ambient light.
• White printer paper for background control.
• Gloves, safety goggles, and lab notebook.

Advanced Materials

• Analytical balance.
• Hot plate with temperature control.
• Centrifuge for removing larger particles.
• Dialysis tubing or filtration setup for purification comparison.
• UV-Vis spectrometer.
• Fluorescence spectrometer.
• Transmission electron microscope if particle size validation is available.
• pH meter.
• Zeta potential analyzer.
• FTIR spectrometer for surface chemistry comparison.
• Conductivity meter for solution property checks.

Software & Tools

• ImageJ: Measures spectral peak position and relative brightness from photographed spectra.
• Python: Helps you smooth spectra, plot calibration curves, and compare samples.
• Google Sheets: Organizes sample labels, measurements, and summary statistics.
• Tracker: Supports careful image timing and repeatability checks if you record glow changes over time.
• PubChem: Helps you look up the starting materials and basic chemical properties.

Experiment Steps

1. Define one variable you will change first, such as starting ratio, heating time, or purification method.
2. Plan a control sample so you can compare every carbon dot batch against the same baseline.
3. Design a way to capture emission spectra under fixed lighting and camera settings.
4. Build a calibration plan that turns pixel position in your smartphone spectrometer into wavelength.
5. Decide how you will summarize each spectrum, such as peak wavelength, intensity, and peak width.
6. Plan repeat trials and a comparison table before you make any samples.

Common Pitfalls

• Changing the phone camera settings between photos, which makes spectra look different even when the sample did not change.
• Using uneven lighting in the spectrometer box, which shifts the apparent peak position.
• Skipping purification and then blaming impurities on the synthesis variable.
• Comparing samples by eye instead of by measured peak wavelength and intensity.
• Making only one batch per condition, which leaves you with no way to tell real trends from random variation.

What Makes This Competitive

A stronger project goes beyond a simple color change demo. You can compare several synthesis variables, include real repeats, and use calibration to report peak shifts in numbers. You can also test whether the smartphone spectrum matches a lab instrument for a subset of samples. That kind of careful measurement and validation makes the work feel more like materials research and less like a classroom lab.

Project Variations

• Test how different citric acid to urea ratios change emission color and brightness.
• Compare microwave synthesis with oven or hot-plate heating to see whether the reaction path changes the spectrum.
• Study how purification or storage affects fluorescence stability over time.

Learn More

• NASA Nanoscale Science, Engineering, and Technology pages: Search NASA for background on nanoscale materials and fluorescence-related imaging concepts.
• PubChem: Look up citric acid and urea for structure, properties, and safety basics.
• NIH PubMed: Search for review articles on carbon dots, fluorescence, and optical properties.
• Carbon Dots: Synthesis, Properties and Applications in a peer-reviewed journal: Search university library access or PubMed-linked reviews for accessible summaries.
• MIT OpenCourseWare: Search for chemistry and spectroscopy lectures that explain emission, absorption, and calibration.
• NOAA Science resource pages: Use for plain-language explanations of light, spectra, and measurement concepts.