Molecular imaging systems (e.g., PET, gamma cameras) require electron-dense materials to effectively detect high-energy gamma rays. The traditional solution to increasing electron density in detectors has been to fabricate scintillators out of high-atomic-number materials (e.g., lanthanum bromide). High quality electron-dense scintillators tend to be expensive, due to difficulties in delivering crystal boules of high transparency and uniformity. Scintillators also have the disadvantage of requiring photodetectors (e.g., photomultipliers) to convert visible light into electrical signals, further adding to bulk and expense. Direct-detecting solid-state devices have better energy resolution than scintillators because of the reduced number of steps in the conversion process from gamma-rays to electrical signal. Several solid-state materials are available with excellent energy resolutions (e.g., CZT - cadmium zinc telluride), but have low stopping power for gamma-rays used in PET systems, and are relatively expensive. Although other semiconducting electron-dense compounds are available with high stopping power (i.e. PbS, PbSe), it has been challenging to design direct-detecting devices using them, because of unfavorable electronic properties of these compounds (e.g., low carrier mobility- lifetime product and/or narrow band-gap). Low carrier mobility-lifetime product results in poor energy resolution for detector of any practical thickness. Narrow band-gaps result in low resistivity, and consequently in high leakage current. Nanotechnology can provide confined materials (e.g., quantum dots) with electronic properties that significantly differ from those of the bulk formulations of the same compounds. This ability to fine-tune electronic properties has generated strong interest within the photovoltaic community. This proposal builds on the substantial work generated by one group that has embedded quantum structures in semiconducting plastics, forming conducting polymer donor- acceptor bulk heterojunctions with favorable mobility-lifetime and band-gap characteristics. Fortuitously from our point of view, quantum dots can be constructed of inexpensive compounds with high atomic number (e.g., lead sulfide). We propose to explore the use of inexpensive electron-dense quantum dot/polymer composites as x-ray and gamma-ray detectors. PUBLIC HEALTH RELEVANCE: Molecular imaging systems (PET and nuclear medicine) have become integrated into cancer diagnosis and treatment. These molecular imaging systems employ expensive detector materials in order to efficiently collect radiation. We propose the use of nanotechnology methods to fabricate novel radiation detector materials that will reduce cost and size of molecular imaging systems, and lower patients'radiation exposures.