Cysteine-mediated synthesis of CdS bionanocrystallites

The synthesis of CdS bionanocrystallites has been studied by reacting Cd(II)-cysteine complexes with inorganic sulfide. Cysteine-mediated synthesis of CdS bionanocrystallites proceeded slightly faster at 45°C than at room temperature. The sizes of these CdS bionanocrystallites, as determined by optical spectroscopy, increased with increasing initial sulfide/Cd(II) ratios. Analyses of size-distribution showed significant heterogeneity in sizes only at sulfide/Cd(II) ratios of 0.25. An ethanol precipitation procedure was developed to remove unreacted Cd(II)-cysteine complexes. This procedure also resulted in the isolation of CdS bionanocrystallites (bioNCs) that appeared uniform in size and chemical composition. Cysteine behaved like glutathione insofar as the size-range of CdS particles was concerned. However, cysteine-mediated synthesis of CdS bioNCs resulted in uniformly sized-particles as has been observed previously with phytochelatins. Cysteine-capped CdS particles exhibited pH-dependent changes in their properties. pH-induced changes were more pronounced in emission than in absorption spectra. Photocatalytic activities of these bioNCs were indicated by reduction of three dyes. Irradiation of methylviologen, basic fuchsin and naphthol blue black at 366 nm in the presence of cysteine-capped CdS bioNCs caused reduction of these dyes. Samples exhibiting higher luminescence were also more active in the photocatalysis experiments.