| We show that commercially available TiO2, SiO2, and ZnO nanoparticles are all internalized by C2BBe1 intestinal epithelial cells, but do not appear to be toxic, even after long term repeat-exposures. When particles were exposed to a simulated digestion protocol mimicking the stomach and intestinal environment, TiO2 particles did show mild toxicity by MTT assay, indicating a decrease in metabolic activity. IR spectra of these particles indicate presence of material from the digestion media, and these absorbed species may be responsible for the effects noted. Though the three particles were not significantly toxic, we note internalization by the intestinal epithelial cells, opening a possibility for absorption into circulation where they may localize in organs throughout the body. This will be observed by functionalizing the particles with fluorophores, after which they can be measured via fluorescence. To optimize the quantum yield efficiency, and thus the brightness, of one such fluorophore, we seek to improve a microwave synthesis of CdSe/CdS/ZnS quantum dots our lab has previously reported. By coupling the microwave reactor to a fluorescence spectrometer via fiber optic cables, we were able to monitor the development of the particles throughout the microwave heating. Time-dependent fluorescence shows the development of an early fluorescence peak at 502 nm attributed to CdSe cores. We then note two isosbestic points which we attribute to the development of CdS layer around CdSe cores, and eventually the formation of outer ZnS shell. We utilize this in situ monitoring along with a study of various nucleation temperatures ranging from 0 to 100°C, and pre-and-post microwave heating UV exposure treatments to obtain optimized CdSe/CdS/ZnS particles with a QY of 40%. This is an improvement over our previous particles' 13% QY, and the highest yet reported for an aqueous synthesis of CdSe/ZnS type particles. Finally, we incorporate these QDs as well as two organic fluorophores, rhodamine 6G and rhodamine 800, into silica shells for direct monitoring in intestinal epithelial cells and tissues of exposed mice. We show that, for small nanoparticles, a typical Stober-type ammonia driven synthesis does not yield stable fluorescence. This has been observed in literature and is attributed to incompletely hydrolyzed silica precursor causing partial dissolution of the silica shell. We remedy this by applying an arginine driven silica shell synthesis, which is known to produce a denser and more stable product at smaller particle sizes. We show that all three fluorophores can be coated in a simple generalized procedure, and the resulting particles all show stable fluorescence with no evidence of dye leakage. Using these particles, we demonstrate that silica nanoparticles can be observed internalizing into C2BBe1 intestinal epithelial cells, and in the tissues of mice that were fed the particles by gavage. We find direct evidence that the particles are absorbed into circulation and subsequently localize in organs throughout the body. Future efforts will attempt to better quantify this accumulation, as well as generalize the procedure to other food relevant nanoparticles such as TiO2. |
