Quantitative Photophysics of Photoluminescent and Scintillating Nanoparticles

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Quantum dots (QDs) are fluorescent crystals of semiconductor materials (such as cadmium selenide, CdSe) that range from ~ 2 to 5 nm in diameter. This is less than 1/1000th the diameter of a typical living cell. QDs are capped with active molecules that enable them to remain in a stable suspension in water or other solutions. The energy and reactivity of QDs--determined by the material, capping agent, and size--determine the ability of a QD to transfer electrons to, or accept electrons from, other molecules in the solution. This electron transfer is very important for a large variety of applications and problems. Unwanted electron transfer can damage or kill biological tissue. Since it happens on the nanoscale, damage can occur deep inside cells, including the nucleus. On the other hand, electron transfer can be used to kill unwanted cells, such as cancer cells or bacteria. Transferring of electrons from QDs to other materials also allows QDs to be used as materials for solar cells, although currently the process is not well-enough understood to create high-efficiency materials. In our recent work we used a mathematical model to describe the fluorescence properties of QDs in the presence of electron-donating molecules. This model helped describe the mechanisms of QD fluorescence and electron transfer, and also provided insight into the off-on "blinking" seen in individual QDs. Our goals for this proposal are to extend and develop this model to provide a general quantitative photophysical description of QD fluorescence and blinking, with a particular emphasis on QDs in water exposed to electron-donating molecules. We will also expand to study other types of nanoparticles, such as those that emit luminescence under X-ray instead of visible light illumination (radioluminescent nanoparticles). This research will provide a framework useful to all scientists working with nanoparticle photophysics in biomedical or alternative energy applications. Our laboratory's long-term goals are to use these results to create anti-bacterial and anti-cancer agents based upon nanoparticles. These agents will be specifically targeted to the cancer or pathogen, and so show lower toxicity to healthy cells than many chemotherapy drugs.