Onion Skin Carbon Dots for pH Sensing

ISEF Category: Materials Science

Subcategory: Nanomaterials  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

Onion skins can glow under the right conditions. That glow can act like a tiny sensor for pH, which is the measure of how acidic or basic something is. You can test whether adding boric acid changes how wide that sensing range becomes. That gives you a real materials science project from a grocery store waste stream.

What Is It?

This project studies carbon dots, which are tiny carbon particles that can fluoresce, meaning they emit light after being excited by a lamp or UV source. Scientists sometimes make them from plant scraps, like onion skins, because those scraps contain carbon-rich compounds that can be heated and processed into glowing particles.

Think of each carbon dot like a tiny light bulb with a chemical mood ring. Its brightness or color can change when the surrounding pH changes. That happens because acid or base can change the particle surface, which changes how it absorbs and emits light. If you add boric acid during synthesis, you may alter the surface chemistry and shift the pH range where the dots respond best.

Your job is to compare untreated carbon dots with boric-acid-doped ones and see which version gives a wider, clearer, or more predictable pH signal. That is a real materials question, because you are changing the structure of the material to change its function.

Why This Is a Good Topic

This is a strong science fair topic because you can test a clear variable, boric acid doping, and measure a clear outcome, fluorescence response across pH. It connects to real problems in sensing, water testing, food quality checks, and low-cost diagnostics. You can also collect enough data to compare signal strength, range, and repeatability, which makes the project feel more like real research than a one-off demo.

Research Questions

• How does boric acid doping change the fluorescence intensity of onion-skin carbon dots across a pH range? ?
• What is the effect of boric acid concentration on the pH range where the dots show the largest signal change? ?
• Does boric acid doping improve the repeatability of fluorescence measurements between batches? ?
• To what extent does excitation wavelength change the pH sensitivity of untreated and doped carbon dots? ?
• Which synthesis condition produces the largest fluorescence difference between acidic and basic samples? ?
• How does storage time affect the fluorescence stability of boric-acid-doped carbon dots? ?

Basic Materials

• Dried onion skins.
• Boric acid.
• Distilled water.
• pH buffer solutions or household solutions with measured pH.
• Small glass jars or beakers.
• Coffee filters or filter paper.
• Disposable pipettes or transfer pipettes.
• UV flashlight or blue LED excitation source.
• Clear cuvettes or small transparent sample vials.
• Smartphone camera with manual exposure control.
• White background and dark box or drawer for imaging.
• Digital scale with 0.01 g or 0.1 g accuracy.
• Safety goggles and nitrile gloves.

Advanced Materials

• Teflon-lined hydrothermal reactor or sealed heating vessel.
• Centrifuge tubes.
• Benchtop centrifuge.
• Laboratory hot plate or oven.
• pH meter with calibration buffers.
• Fluorescence spectrometer or plate reader.
• Quartz cuvettes.
• Dialysis tubing or membrane filtration setup.
• Analytical balance.
• Magnetic stirrer and stir bars.
• UV-Vis spectrophotometer.
• Transmission electron microscopy access for particle size confirmation.
• FTIR or XPS access for surface chemistry checks.

Software & Tools

• ImageJ: Measures fluorescence intensity from photos under fixed lighting and compares samples across pH values.
• Python: Organizes data, fits calibration curves, and makes plots with error bars.
• Google Sheets: Tracks sample labels, pH values, and replicate measurements in one place.
• GeoGebra: Helps you fit trend lines or curves when you want a quick check on your calibration.
• NIH ImageJ macro tools: Automates repeated image analysis for batches of fluorescence images.

Experiment Steps

1. Define the exact sensing question, then choose one response metric such as brightness, color shift, or intensity ratio.
2. Plan two carbon-dot batches, one untreated and one boric-acid-doped, so you can compare material changes directly.
3. Set up a pH series with clear controls, then decide how you will keep lighting, camera settings, and sample thickness constant.
4. Build a calibration plan that turns fluorescence readings into a numeric pH response curve.
5. Choose a repeatability strategy, such as multiple batches or multiple measurements per sample, so you can test reliability, not just one-time signal.
6. Decide how you will compare dynamic range, sensitivity, and stability, then pick the statistics that match those goals.

Common Pitfalls

• Using changing room light or auto camera settings, which makes the fluorescence signal look stronger or weaker for the wrong reason.
• Comparing samples with different particle concentration, which can hide whether pH or sample amount caused the brightness change.
• Skipping pH verification, which means your labels may not match the real acidity of the test solutions.
• Letting residue from one solution contaminate the next, which creates false shifts in the fluorescence response.
• Treating one batch as proof, which makes it impossible to know whether the boric-acid effect is repeatable.

What Makes This Competitive

A competitive version of this project goes beyond making a glowing sample. You compare multiple doping levels, track repeatability across batches, and quantify dynamic range with real calibration curves. Strong projects also test whether the signal stays stable over time, in different lighting setups, or in different sample matrices. That kind of careful measurement makes your work look like materials research, not just a classroom demo.

Project Variations

• Test onion-skin carbon dots made with different plant waste sources, such as tea leaves, citrus peel, or onion skins.
• Compare boric acid doping with another surface modifier, such as citric acid, to see which one improves pH response more.
• Measure fluorescence stability in real samples, such as sports drinks, vinegar dilutions, or soap solutions, instead of only buffer solutions.

Learn More

• PubChem: Look up boric acid properties and safety information, then search by compound name on the NIH PubChem database.
• PubMed: Search review articles on carbon dots, fluorescence sensing, and pH-responsive nanomaterials.
• Carbon Dots and Their Applications: Search for review chapters through university library access or Google Scholar to get background on synthesis and sensing.
• NASA Earth Observatory: Review simple explanations of spectroscopy and light interaction with materials, then connect them to fluorescence concepts.
• MIT OpenCourseWare: Search materials science and chemistry courses for lectures on nanomaterials, spectroscopy, and surface chemistry.
• NIH NIGMS: Read free educational pages on pH, buffers, and acid-base chemistry for quick concept review.