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We report on the synthesis and characterization of cadmiumsulfide quantum dots (CdS-QDs) with three different types of capping ligands, mercaptoacetic acid (CdS-MAA), cysteamine, and 2-mercaptoethanesulfonic acid (CdS-CM), or tiopronin (CdS-TPN). All three types of QDs generate reactive oxygen species (ROS), in particular, superoxide-and hydroxyl radicals, upon irradiation with UV light. The ROS were analyzed using electron paramagnetic resonance (EPR) spectroscopy. Moreover, the hydroxyl radical generation from the three different QDs was monitored using a terephthalate-based fluorimetric assay. The three QDs were then used as photocatalyst to trigger the peroxidase activity of horseradish peroxidase.

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Colloidal CdS nanoparticles are conjugated with a variety of proteins, including enhanced yellow fluorescent protein, tobacco etch virus protease (TEV), lysozyme, and bacterial cytochrome P450 CYP152A1, and the photochemical properties of the resulting conjugates are analyzed by EPR spectroscopy and hydroxyl radical-specific fluorimetric assay. While irradiation of bare CdS colloids leads to photogeneration of hydroxyl and superoxide radicals, it is surprisingly observed that coating of the CdS particles with proteins effectively suppresses the production of these radical species and instead leads to increased formation of a long-lived reactive oxygen species, most likely H(2)O(2). A mechanism for the observed results is suggested. The empirical results are capitalized on for the assembly of a CdS-TEV nanohybrid, which shows significantly higher performance as a photocatalytic mediator for fatty acid hydroxylation by CYP152A1 than bare CdS nanoparticles.

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