The development of a simple sensor able to detect the presence of general classes of bacteria or parasites could mitigate serious worldwide health concerns, including food poisoning and drinking water contamination. One possible sensor is based upon colloidal semiconductor nanocrystals, which are inorganic semiconductor particles a few nanometers in diameter. Semiconductor nanocrystals are highly fluorescent, and their spectral properties are easily tunable with the size of the nanocrystal. Individual nanocrystals also have the potential ability to have their fluorescence intensity or wavelength controllably altered, thus providing the possibility for a simple colorimetric sensor for a given pathogen.
Multi-sized CdSe nanocrystals have been prepared via a one-pot synthesis, which yields fairly monodisperse particles. Capping the nanocrystal with a higher bandgap material (e.g. ZnS) can greatly improve its fluorescence quantum yield. To develop a nanocrystal-based colorimetric sensor for Gram(-) bacteria, water-soluble nanocrystals must be derivatized with a specific receptor molecule for lipid A, a primary component of the cell wall of Gram(-) bacteria. The figure shows colloids of different sized nanocrystals which have different colors due to the quantum size effect.
