In an article recently published in the Journal of the American Chemical Society, Bioengineering Assistant Professor Andrew Smith and his co-authors demonstrate a new process for using quantum dots to literally shed new light on molecular biology.
For many years, the use of fluorescent dyes had been the standard way to provide illumination and color to biological tissue. However, these dyes have problems. For one, they suffer from photo bleaching, meaning that the signal degrades relatively quickly. Also, the number of colors that can be used with fluorescent dyes is limited.
Quantum dots can solve some aspects of these problems. Quantum dots — tiny nanoparticles that can have special properties — have been used in research for nearly 20 years to illuminate tissue and cells. They do not suffer from photo bleaching, and they can provide a larger number of colors to distinguish a larger number of molecules in a biological sample, which is called spectral tunability — in fact, they could provide an order of magnitude more colors than what is available using fluorescent dyes. The spectral tunability of quantum dots would enable physicians and biologists to see multiple molecules and cellular events happening at the same time.
But prior quantum dots also presented some difficulties. The quantum dots most readily available from suppliers have been in the range of 15-35 nanometers — much larger than the size of the typical protein molecules they would be used to analyze, which are approximately five nanometers in size.
Developing a much smaller quantum dot is the major breakthrough that Smith and his team have accomplished. They succeeded in creating the smallest stable quantum dots to date. They accomplished this by greatly reducing the size of the shell needed to surround the nanoparticle.
Another breakthrough was the development of a method for incorporating “click” chemistry into the process. In click chemistry, a chemical reaction occurs almost instantaneously without the need for any purification process at the end — the materials just “click” together.
“This type of click chemistry had been around for more than 10 years, but it has not been very successful for quantum dots,” said Smith. “The big advantage is the ability to attach to something like an antibody with extremely high rate and efficiency.”
These new quantum dots are also quite stable. They can be stored for long periods of time without any degradation in their utility. The small quantum dots created using click chemistry that maintain stability have great promise for helping advance tissue studies for basic research and medical applications.
The quantum dots have some immediate use in tissue analysis. Using the data that is revealed by quantum dots, Smith envisions a time when doctors and researchers can learn about biological samples essentially at the microscope.
“We basically want to give a readout of all the important molecules that are there,” explained Smith. “If they can get a list of all the molecules that are there, they can really accurately diagnose the disease. The goal is improved accuracy with molecular assessment so that individuals with a disease can receive an individualized diagnosis and personalized therapy.”
Though they have made many accomplishments in this research project, Smith is looking ahead to further innovation. At about 7.4 nanometers, the team’s quantum dots are much smaller than any developed prior to this time, but the magic number for size would be about five nanometers.
“Five nanometers is a fundamental size in biology,” said Smith. “If you put something five nanometers in size in the human body, it can access almost any tissue and cell.”
Smith has been working on his research in collaboration with Paul Selvin, professor of Physics at Illinois. They eventually want to apply these quantum dots to study molecules in the brain to learn how brain cells interact.
“The goal is to understand brain diseases, and to do that we have to understand the molecules involved,” said Smith. “We want to see how these molecules behave in a healthy brain and then see how they misbehave in an unhealthy brain, such as one with Alzheimer’s disease.”
The work of Smith, Selvin, and their students has been supported by the National Institutes of Health through previous grants R00CA153914 and R21NS087413, and currently through grant R01NS097610.