Instant Pot Quantum Dots (Fluorescent Carbon Dots)

You've probably heard some buzz about quantum dots (we'll call them QDs) in the context of things like QD TVs. The TVs use inorganic QDs containing toxic elements like cadmium, which are not very kitchen-chemistry friendly. There is an Instructable on making those called "Make Quantum Dots (Cadmium Selenide Type)" by NurdRage that you could check out, if you have access to the needed chemicals, expertise, and equipment.

Arguably, what we will make here are not technically QDs, but they do the same cool thing, which is absorb ultraviolet light and emit visible light. These are like the carbon dots in "DIY-Quantum-Dots Nanotech-in-Your-Kitchen" by StrangelyAmusing, but (in my opinion) bigger, better, and brighter.

There are many fun things you can do with these QDs. I wanted them for fluorescent droplet photography as shown in the images above. For that, having a large supply that I can discard after use is convenient. For those who are interested, I'll explain the fluorescent droplet photography at the end.

The process for making these QDs is based on a paper I co-authored a few years ago ( see https://pubs.acs.org/doi/abs/10.1021/acs.langmuir.....) It uses what we call hydrothermal (i.e. hot water) synthesis, which usually requires high-pressure lab equipment. However, in this case, we were going only to a temperature of 110 °C (230 °F), which is reachable in an ordinary pressure cooker. The ingredients are safe and relatively inexpensive, which means we can make big (liters/quarts at a time) batches.

So, let's make a big batch of QDs!

Pressure cooker (I used an 8 quart Instant Pot, because that's what I had)

Citric Acid (less than $10 for a pound on Amazon), 125 grams for a 2 liter batch

Polyethyleneimine (PEI), 25 grams for a 2 liter batch. This may be $25 to $75 worth total (sorry!)

Distilled water (2 liters, for a 2 liter batch)

Kitchen scale

UV light to see the results (e.g., this UV LED flashlight, $10.99 on Amazon)

The only ingredient that is expensive and hard to find is the PEI. Use whatever you are able to buy. I don't think the molecular weight (MW) or whether it is a branched or linear form of the polymer matters for this. For the paper, we used PEI with a MW of 1800, but I have also used branched PEI with a MW of 25,000 from Sigma-Aldrich, and PEI with a MW of 75,000 provided as solution of 30 wt. % PEI in water, which I found on e-bay. If what you can buy is a solution of PEI in water, then you will have to do some math to get the right total amount. For example, if you have 30 wt. % PEI in water, use 83.3 grams of it (0.3 * 83.3 = 25 g PEI).

Of course you can double, halve, or otherwise scale these amounts to fit your pressure cooker and your needs.

Measure 2 liters of distilled water, and add it to the pressure cooker.

Use your kitchen scale to weigh out 125 grams of citric acid, and add it to the pressure cooker.

If you have pure PEI, weigh out 25 grams of it and add it to the pressure cooker. Depending on exactly what kind of PEI you got, it is likely to be extremely viscous and sticky. Do the best you can - it doesn't have to be exact. One approach is to weigh it in a divot on the citric acid powder to keep it off the walls of the bowl then dump them both in together.

If you have a PEI solution in water, then do the math and weigh out the appropriate amount. At least it will be less sticky.

Stir things together in the pressure cooker until you have a clear solution. Try not to leave anything stuck on the bottom. Start the cooker and set it to cook on the high pressure setting for 2 hours. You may notice a little bit of an ammonia-like smell, which I believe is harmless.

After two hours, safely vent the steam from the cooker and remove the lid when it is safe to do so. Shine your UV flashlight in the pot and be impressed by the blue glow!

Allow the liquid to cool, and then have fun with it!

Once they have cooled, you can pour these into your empty distilled water bottle and save them for future fun activities.

Some water evaporates during the whole process, so you will get a little less than two liters of the final QD dispersion. There really are tiny carbon particles in there that are responsible for the light emission. Unfortunately, unless you have access to a transmission electron microscope, you are not going to be able to see them. I added a transmission electron microscope image above, and just trust me, that is more-or-less what they would look. They are roughly 5 nanometers (5 billionths of a meter) in diameter, which means that you could line up about 10,000 of them across the width of a human hair.

But, macroscopically, it will look like a jug of urine (likely also fluorescent, but not nearly as bright). Over time, the dispersion will get darker, but because the particles are so small and because they are positively charged and therefore repel each other, they will never settle out of the solution. The darker jug in the above images is a batch that is several months old. The QDs still seem to glow just as bright.

I think the darkening is related to the byproducts of the reaction that are left in the jug with the QDs. In the lab, we would separate out the QDs from the other material, using an ultracentrifuge, which we don't have in the kitchen. There are actually a bunch of instructables on making centrifuges, but I am not sure any of them will be able to separate out the QDs.

One fun thing to do is to put an ultrasonic mist generator ($10-15 on Amazon) in a bowl of water, then add a little bit of the QD solution and shine your UV flashlight on it. Of course, you could get a lot fancier with this, but it illustrates the basic idea.

Basically, anything that you would do with the "invisible ink" extracted from a highlighter as described in Instructables like this one or this one, you can do with your QDs. And you have plenty to share!

Because the QD dispersion is water-based, it generally wipes off of things, and is pretty much only visible under UV light anyway.

The simplest approach is to put a little bit in a bowl and use a cotton swab for a brush, but you could get as fancy as you want.

Note that while I think the QD dispersion is pretty harmless, you shouldn't drink it and I can't rule out the possibility that it will irritate your skin. It did not irritate the back of my hand. The banana seemed to turn brown faster than the other ones in the bowl (only the peel). I ate it anyway. While the PEI that we used is certainly toxic in sufficient doses, it is also approved for use in food packaging material, so you have probably also eaten food that has been in contact with it.

So, this is what originally motivated me to make a big batch of QDs. For the photos shown here and at the top, I was dropping the QD dispersion into a bowl that initially had pure water in it. Over time, the amount of QDs in the bowl increased, so we get varying levels of contrast between the stuff dropping into the bowl and what is already in the bowl. If you want to do this, you need:

(1) A droplet photography setup to synchronize your camera flash with a device that generates droplets. There are great Instructables on creating these setups from scratch, like this one or this one or this one. Alternatively, you can get a commercial device. I used the one from MIOPS. It is $149.99, but if you are patient, it may go on sale for as little as $99.99. Of course, you also need an appropriate DSLR or mirrorless camera that can work with your droplet generator and flash triggering setup. I am a total novice at droplet photography, so I won't bother showing you my setup.

(2) A UV flash. This is a very fun thing to have independently of the droplet photography. This Instructable by inkybreadcrumbs explains how to convert a regular flash to a UV flash by removing the plastic parts covering the flash (which block UV) and taping on a filter that blocks everything except UV. I basically did what he said, except that I only added one filter to the flash, and I used this square filter from Edmund Optics. As of this writing, it is on clearance for $49.99. It will let some infrared light through, along with the UV, but that was OK for my purposes here.

One nice feature of doing this fluorescent droplet photography is that the resulting images are basically monochromatic. So, if you don't like the blue color, it is very simple to replace it with something else in your favorite photo-editing software (see red example above).