The primary goal of this project is to determine which physical and chemical characteristics of semiconductor quantum dots (Qdots) are associated with toxicity using cells and tissues culture in vitro. Recent advances in nanotechnology have produced a new class of fluorescent nanoparticles, semi-conductor Qdots. These nanometer-sized crystals have unique photochemical and photophysical properties that are not available from either isolated molecules or bulk solids, and consequently have enabled new opportunities in many areas including optoelectronics, anti-counterfeiting inks, and photovoltaics. More recently, the research interest in Qdots has shifted toward the life sciences, where material scientists, chemists and biologists are working together to develop these quantum-confined nanocrystals as fluorescent probes for biomedical imaging. Compared with organic dyes and fluorescent proteins, semiconductor Qdots offer several unique advantages, such as size- and composition-tunable emission from visible to infrared wavelengths, large absorption coefficients across a wide spectral range, and very high levels of brightness and photostability. Compared to other imaging modalities, optical imaging with Qdots is highly sensitive, quantitative, and is capable of multiplexing. Recent research by us and others [1-13] has shown that nanometer-sized Qdots can be covalently linked with biorecognition molecules such as peptides, antibodies, nucleic acids, and small molecule ligands for use as fluorescent probes. For example, we have conjugated antibodies targeting PSMA (prostate specific membrane antigen) to CdSe/ZnS core/shell Qdots (emission 640 nm) for simultaneous imaging and targeting prostate tumors in mouse models [14]. The results demonstrate that polymerencapsulated Qdot probes are stable under in vivo conditions and are at least 100 times more sensitive than organic fluorophores in detection of subcutaneously implanted tumors. In spite of these achievements, the toxicity of many Qdots is largely unknown. High-quality Qdots are generally made from group ll-VI and lll-V elements in the chemical periodic table including Cd, Se and Hg, which are toxic heavy metals. Indeed, many studies have shown that core constituents can leach out of Qdot crystals causing toxicity, unless they are capped (usually with ZnS) and further modified with amphiphilic coatings to stabilize them. Our preliminary results indicate that polymer-protected Qdots remain intact in live cells and animals for up to 2-4 months and have relatively low toxicity. However, their long-term degradation, metabolism, and clearance are still unknown. Furthermore, many Qdots produced for industrial applications are not necessarily designed with biocompatibility in mind, and therefore occupational and environmental exposures to these materials have become major concerns. In order to address the potential for Qdots to elicit toxicity, in this project we will employ in vitro toxicity screens using multiple mouse and human target cell types exposed cjirectly to Qdots in the medium. In addition to these more simple systems, in collaboration with Drs. Faustman and Yu (Projects 2 and 3) we will also use embryonic CNS micromass cells and Sertoli/gonocyte cultures. In coordination with Dr. Yost (Core 1) we will expose mouse respiratory epithelium from multiple strains of inbred mice and grown in air-liquid interface cultures to aerosolized Qdots. We will examine the effects of various surface modifications on the Qdots, use nanogold particle controls that have been similariy modified, as well as CdClg in solution. The selection of Qdots and their modifications will be based on differences in their size, stability, charge, and bioavailability, and related to their practical applications in photonics, optics, solar cells, sensors and biomedical imaging (see Core 1 for description of the particles). We hypothesize that in addition to their core metal composition, the colloidal stability and surface properties (such as surface ligands and functional groups) will be primary determinants of the toxicity of Qdots.
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