I'm an undergrad researcher in material science and nanomaterials. For years I've wanted a simple science experiment that a child could do with parental or adult supervision for nanoscience outreach. I'm tired of simple baking soda volcanoes. You are better than that.... So lets make nanomaterials in the kitchen! Today, you're gonna make the tiniest Instructable ever.
Specifically, we are making Carbon Quantum Dots. Quantum dots are nanometer scale particles, typically of semiconductor. They are so small that when high energy light hits them, the photons will be absorbed then emitted at a lower energy state. QD's are usually made from toxic chemicals but today I'll show you how to make non-toxic quantum dots in your home. Also, they can be used as fluorescent dyes, inks, and possibly paints. I will probably be making things glow around my apartment in the coming days.
The original paper at the bottom used baking soda and fructose over the course of an hour. I modified it to use sucrose and take 10 minutes.
Disclaimer: This is as dangerous as making candy. This is simply cooking, be safe and use common sense, molten sugar is the worst thing to burn yourself with.
Tempored Glass bowl(I prefer Borosilicate but we aren't rapidly changing temperatures)
Heat resistant gloves(perhaps with silicone for grip)
A few containers for you samples
Glass vials to store
UV or Blacklight
Water(I used tap water)
Baking soda(Sodium Bicarbonate)
White Vinegar(Acetic Acid)
Step 1: Prep the Saccharide
Add to the glass bowl
1 cup of water
1/4th cup of vinegar
6 tablespoons of sugar(80 grams)
Microwave for 5 minutes
The sucrose must be broken down in a process called inversion, the sucrose is broken into fructose and glucose. Sucrose inversion is heavily used candy making because it makes the sugar less grainy in sweets. The acid decreases the time needed to do this from hours to minutes, especially with the application of heat.
Step 2: Neutralize the Acid
Allow the bowl to cool to be safely handled
Slowly add 20 grams of baking soda to the solution.(See video to see what happens when you do it all at once.)
The solution must be basic for the synthesis to occur so we add enough baking soda to neutralize the acid and make raise the pH.
Step 3: Synthesize the Quantum Dots
Microwave the basic solution for 5 minutes. The liquid will yellow then darken.
As the concentration of QD's increase, the liquid will get darker and darker. I was able to get formation within 3 minutes but I wanted to minimize any chance of failure for anyone reproducing this experiment.
Step 4: Dilute the Quantum Dots
Allow the bowl to cool then pipette small amounts to a half filled vial of water. The Quantum Dots/sugar mixture will disburse and glow Green when exposed to an ultraviolet light.
If the QD's are too concentrated then they wont glow as well, you have most likely made several cups or a liters worth of Quantum Dots in 10 minutes, Congratulations.
Step 5: Discussion
This project was originally much more difficult but made an additional color. It originally took significantly longer to make the QDs.
Photo 1:I Took a sample at 1 minute, 20 minutes, 30 minutes, 40 minutes, and an hour.
The sucrose(a disaccharide) is not as easily converted to Quantum Dots as simple sugars(monosaccharides). I spent an hour and a half to get QD's just using baking soda. The sucrose would only decompose to Glucose and Fructose at high temperatures for extended periods of time. When simple sugars are in a basic solution, they can form the quantum dots. So using the acid and heat, I made lots of free fructose and glucose, but then I had to make the solution basic again.
So we added baking soda to neutralize the acid, but that isn't enough, when acetic acid and baking soda mix, they form sodium acetate which acts as a weakly acidic buffer solution. So i added plenty of sodium bicarbonate to the instructions to overcome the sodium acetate's buffering properties.
All in all, it was a fascinating challenge and I hope I've satisfied everyone's curiosity today.
Additional Reading and references
Fascinating paper using Smartphones and carbon dots. https://www.nature.com/articles/s41598-017-11572-8
Nitrogen doping CQD's. https://www.nature.com/articles/srep05294