Semiconductor nanocrystals are expected to play an important role in the development of novel electronic and optoelectronic devices, and have already shown promise in the area of biological reporters for medical screening and sensor applications. We have been involved in developing luminescent CdSe/ZnS core-shell nanocrystals (or quantum dots, QDs) for use in self-assembled structures and as fluorescent reporters in immunoassay-based biodetectors. As part of our efforts to bind semiconductor CdSe/ZnS quantum dots to antibody proteins for our immunoassay work, we functionalized the nanocrystal surfaces with a variety of organic acid salts to impart water solubility to the nanocrystals.
During the course of working with these derivatized, water-soluble quantum dots, we observed significant differences in their chemical reactivities and physical characteristics compared to those of underivatized CdSe/ZnS nanocrystals. One of the most striking differences observed is the reactivity of the derivatized and underivatized nanocrystals with stainless steel surfaces. The fluorescence of aqueous mixtures of our water-soluble nanocrystals is immediately quenched upon exposure of the mixtures to stainless steel (SS) surfaces or to chromium oxide, whereas underivatized quantum dots exhibit little or no reactivity at all. As has been reported by several other laboratories, the water-soluble nanocrystals also exhibit significantly lower quantum yields compared to the underivatized nanocrystals. We discuss the unusual reactivity exhibited by these nanocrystals, and suggest possible explanations for their interesting chemical behavior. We also describe methods to prevent the quenching of water-soluble derivatized quantum dots by stainless steel and metal oxides.