Cysteine-capped ZnS nanocrystallites: Preparation and characterization

Abstract

Cysteine-capped ZnS nanocrystalline semiconductors (NCs) were prepared by titrating sodium sulfide into preformed Zn–cysteine complexes. Only a maximum of ∼40% of Zn(II) in Zn–cysteine complex was converted into ZnS NCs when the reaction was carried out at room temperature for 30 min. However, incubation of the reaction mixture at 45°C for 60 min significantly enhanced the production of ZnS NCs as a maximum of ∼75% of Zn(II) was converted into NCs. Cysteine capping produced NCs that were smaller than those capped by glutathione. Furthermore, cysteine-capped ZnS exhibited a narrow range of size distribution as suggested by UV/VIS spectroscopy of the NCs separated on a size-fractionation column. Unreacted Zn–cysteine complex could be removed from ZnS NC preparations by selective precipitation with ethanol. The precipitation procedure also led to the isolation of ZnS NCs that appeared more uniform by gel-filtration analysis. Ethanol precipitation procedures allowed preparation of large quantities of powdered NCs which retained their colloidal nature upon resuspension in water or buffers. pH titration experiments indicate that the average size of the particles was smallest in the pH range 7–10, but the size increased as the samples were made acidic or alkaline. Cysteine-capped NCs were capable of causing photoreduction of methylviologen, basic fuchsin and naphthol blue black.