Current understanding of the environmental effects and potential risks associated with purposely engineered nanoparticles is very limited. Due to the small size and high surface reactivity the nanoparticles have a potential to interact with microorganisms in a way different from that of the bulk substances, once discharged in the environment. Nonetheless, there is paucity of data and a limited conceptual understanding how the nanoparticles will interact with the biotic and abiotic components in the environment. Therefore the main goal of the present work is to improve our understanding of the effect of inorganic nanoparticles on microorganisms under environmentally relevant conditions. More specifically, bioavailability is explored as a key concept to link nanoparticles geochemistry to their potential ecotoxicity. Carboxylic- and amino-group functionalized CdSe/ZnS core/shell quantum dots (QDs) were chosen because of their water solubility and special tailoring for bioimaging application related to the cell uptake. Green alga Chlorella kesslerii and gram negative bacterium Ralstonia metalidurans were chosen as microorganisms of environmental importance.

The effect of pH, ionic strength, presence of dissolved organic matter on the potential agglomeration of QDs and their interaction with alga or bacterium were followed by the Fluorescence correlation spectroscopy (FCS). Diffusion time, number of particles and the fluorescence intensity were determined over the short term exposure for QDs concentrations in the range 2x10-10 to 5x10-7 mol L-1. Diffusion coefficients and hydrodynamic radius were obtained from the experimentally determined diffusion times, and used to evaluate the potential aggregation of QDs. The obtained results revealed that no carboxylic- nor amino-group functionalized QDs aggregate under most of the studied conditions. In addition, exponential decrease of the count per particle rate was obtained when QDs were in contact with alga or bacterium. It was hypothesized that the quenching of the QD fluorescence was induced by the reactive oxygen species formed during QD interaction with alga or bacterium. This hypothesis was further confirmed by flow cytometry measurements of the cell preloaded by 2',7'-dichlorodihydrofluorescein diacetate and exposed to QDs over 2h.

Bioavailability of the QDs to microalga or bacterium was further explored by using FCM and growth inhibition tests. No growth inhibition effect on alga or bacterium was observed for both carboxylic- and amino-group functionalized QDs. In contrast, the FCM measurements indicated association of the QDs to both algal and bacterial cells, but further studies should be performed to distinguish between QDs internalization and adsorption to the cell surface.