CYSTEINE-CAPPED ZNS NANOCRYSTALLITES - PREPARATION AND CHARACTERIZATION

Cysteine-capped ZnS nanocrystalline semiconductors (NCs) were prepared by titrating sodium sulfide into preformed Zn-cysteine complexes. Onl y a maximum of similar to 40% of Zn(LI) in Zn-cysteine complex was con verted into ZnS NCs when the reaction was carried out at room temperat ure for 30 min. However, incubation of the reaction mixture at 45 degr ees C for 60 min significantly enhanced the production of ZnS NCs as a maximum of similar to 75% of Zn(II) was converted into NCs. Cysteine capping produced NCs that were smaller than those capped by glutathion e. Furthermore, cysteine-capped ZnS exhibited a narrow range of size d istribution as suggested by UV/VIS spectroscopy of the NCs separated o n a size-fractionation column. Unreacted Zn-cysteine complex could be removed from ZnS NC preparations by selective precipitation with ethan ol. The precipitation procedure also led to the isolation of ZnS NCs t hat appeared more uniform by gel-filtration analysis. Ethanol precipit ation procedures allowed preparation of large quantities of powdered N Cs which retained their colloidal nature upon resuspension in water or buffers. pH titration experiments indicate that the average size of t he 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 an d naphthol blue black. (C) 1998 Elsevier Science Inc. All rights reser ved.