Properties of glutathione- and phytochelatin-capped CdS bionanocrystallites

The incorporation of inorganic sulfide into cadmium-glutathione (GSH) led to the formation of a variety of GSH-capped CdS (GSH-CdS) complexes that differed in sulfide/Cd(II) ratios, optical spectroscopic properties and Cd(II)-binding capacity of GSH. The size-fractionation of GSH-CdS complexes indicated that the Cd(II)/GSH molar ratio increased from a minimum of 0.3 to a maximum of 25 as sulfide/Cd(II) molar ratio increased from 0 to similar to 1.0 equivalent. The absorption shoulders in the 290-400 nm range, photoluminescence in the 400-550 nm range and the ability to reduce methylviologen indicated that these GSH-CdS complexes behaved like semiconductor nanocrystallites (NCs). The predicted radii of (GSH-CdS) NCs varied from 10.8 to 17.3 Angstrom. Unlike GSH, phytochelatins (PCs) formed CdS crystallizes that appeared uniform in size as was indicated by their similar optical properties. Although the properties of PC-capped CdS (PC-CdS) complexes were controllable by altering the amounts of sulfide titrated, sulfide-induced transitions in the electronic absorption spectrum (indicative of the crystallite size) were limited to the blue of 318 nm. Thus, the maximum predicted radius of PC-capped crystallites was 11.8 Angstrom. The titration of PCs into GSH-CdS led to the replacement of GSH with PCs, Interestingly, the displacement of GSH by PCs did not alter the size of CdS particles as indicated by lack of changes in emission lambda(max) or in the characteristic absorption shoulder at 358 nm. However, emission yields were quantitatively decreased upon displacement of GSH with PCs. (C) 1998 Elsevier Science Inc. All rights reserved.