The University of Massachusetts Amherst
University of Massachusetts Amherst

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How to Make a Quantum Dot

University of Massachusetts Amherst chemical engineering major Brendan Walker, a senior honors student from Yarmouth Port, Massachusetts, is doing some critical research on those strange and wonderful quantum dots. These luminescent nanoparticles promise to revolutionize medical diagnostic devices, photovoltaic cells, and many industries from healthcare to home entertainment. Even as an undergrad, Walker is working on the leading edge of quantum-dot research, helping to scale up a revolutionary new production process from the lab bench to large-scale chemical reactors required to create quantum dots at the industrial level.

Quantum dots are single crystalline nanoparticles of semiconducting material on the order of two to 10 nanometers in diameter that exhibit unusually high brightness, sensitivity, and photo stability, thus making them especially useful as luminescent tags for biomolecules.

Picture giving two people in a crowded room glow sticks and then turning off the lights. They would be extremely easy to locate. This is the how the medical field hopes to make use of quantum dots.

Walker works on the research team of Professor T.J. Lakis Mountziaris, head of the UMass Amherst Chemical Engineering Department and one of the world’s leading researchers on quantum dots. His team has come up with a promising new way to produce quantum dots. The traditional way of manufacturing quantum dots is through injection of reactants into a solvent solution. To form the crystalline structure of quantum dots in such a solution, you must heat a solvent to a very high temperature and inject reactants into it at very specific rates with very specific timing.

“Now the problem with that is it’s actually very hard to get everything right, because it’s manually injected,” explains Walker. “What that means for industrial production is that it would be very difficult to scale that process up to a large size. It’s even difficult to do on a lab bench. Imagine how difficult to do for a gigantic reactor.”

By contrast, the microemulsion method of the Mountziaris group is much easier to carry out and doesn’t require high temperatures. “Basically, all you do is start the microemulsion and mix the chemicals,” says Walker. “Then you put it in the reactor, add certain reactants, manage the flow of the gas into the reactor, and you get a product. It should be very easy to scale up.”

But one key question about the new microemulsion process is the quality of the crystalline structures in the quantum dots it produces. The crystal quality of a quantum dot is essential because the fluorescence depends upon it. If the quality is poor, then the quantum dot won’t be as bright. The high temperature of the hot-injection process forms pure crystals of high quality. With microemulsion, quantum dots are synthesized at room temperature, and they could possibly have a lower quality.

“The goal of my research is to optimize the crystal quality of the microemulsion quantum dots,” says Walker. “I will also attempt to examine how the crystaline quality differs from quantum dots created in hot injection.”

The results of Walker’s experiments could go a long way toward applying quantum dots, especially in the medical field. Quantum dots are 3,000 times brighter than conventional dyes now used in medical diagnostics.

“Quantum dots emit bright light in a color range that varies with their size,” explains Walker about the nanoparticles’ most significant characteristic. “The shape and size of quantum dots can be tailored to fluoresce specific colors.”

Because the color of a quantum dot depends on its size, quantum dots of different sizes and colors can be used to test for multiple indications simultaneously. One result would be a single all-purpose clinical diagnostics device much smaller than today’s machines.

“This could lead to diagnostic equipment being in nearly every medical office,” says Walker. “It would greatly reduce the time needed to diagnose conditions and also reduce the amount of blood that would need to be sampled.”

Such equipment would be especially beneficial in Third World countries, where many people have no access to diagnostic machines. Walker notes that he gets a lot of fulfillment out of that kind of benefit. He did an internship at Abbott Labs, doing data analysis on an HIV drug called Aluvia, the only co-formulated protease inhibitor tablet that doesn’t require refrigeration and can be taken without food, two important factors in delivering medicine to developing countries.

“I ended up finding out something that cut degradation in the drug during production by 90 percent,” Walker recalls. “It was great knowing that what I did led to us being able to manufacture the drug less expensively and thus sell it less expensively in Africa. Likewise, I really like that my quantum dot research will actually help people.”

March 2010