Living organisms such as bacteria and yeasts have been known to synthesize nanocrystalline CdS semiconductors [Dameron, 1989; Holmes, 1997]. Glutathione and related phytochelatin peptides were shown to be the biomolecules that capped CdS nanocrystallites formed by selected yeasts. Our recent studies have shown that cysteine and cysteine-containing peptides can be used in vitro to dictate the formation of discrete sizes of CdS and ZnS nanocrystals [Bae 97a; Bae98a-c; Nguyen, 1998]. Our procedure for the synthesis of relatively homogeneous ZnS or CdS nanocrystals involves three steps: (1) formation of metallo-complexes of cysteine or cysteine-containing peptides, (2) introduction of stoichiometric amounts of inorganic sulfide into the metallo-complexes and finally (3) solvent fractionation of relatively homogeneous nanocrystals. The uniformity of the sizes is determined by size-fractionation and optical spectroscopy. Photo-catalytic studies suggest that glutathione-capped ZnS nanocrystals prepared by our procedure are dramatically more efficient than TiO2 in a model system (Fig. 1, 2). For example ZnS completely degraded p-nitrophenol in 20 min at a catalyst to substrate ratio of 5:1. TiO2 was completely ineffective at the same ratio (Fig. 1). A considerably higher ratio (62.5:1) of TiO2: p-nitrophenol was needed to cause degradation of p-nitrophenol (Fig. 2).
[Holmes 97] Holmes, J. D., Richardson, D. J., Saed, S., Evans-Gowing, R., Russell, D. A., and Sodeau, J. R. "Cadmium-specific formation of metal sulfide 'Q-particles' by Klebsiella pneumoniae," Microbiol., 143, 2521-30 (1997).
[Dameron 89] Dameron, C. T., Reese, R. N., Mehra, R. K., Kortan, A. R., Carroll, P. J., Steigerwald, M. L., Brus, L. E., and Winge, D. R. "Biosynthesis of Cadmium Sulfide Quantum Semiconductor Crystallites." Nature, 338, 596-7 (1989).
[Bae 97a] Bae, W., Abdullah, R., Henderson, D., and Mehra, R. K. "Characteristics of glutathione-capped ZnS nanocrystallites." Biochem. Biophys. Res. Comm., 237, 16-23 (1997).
[Bae 97b] Bae, W. and Mehra, R. K. "Metal-binding characteristics of a phytochelatin analog (Glu-Cys)2Gly." J. Inorg. Biochem., 68, 201-210 (1997).
[Bae 98a] Bae, W. and Mehra, R. K. "Properties of glutathione- and phytochelatin-capped CdS bionanocrystallites." J. Inorg. Biochem., 69, 33-43 (1998).
[Bae 98b] Bae, W., Abdullah, R., and Mehra, R. K. "Cysteine-mediated synthesis of CdS bionanocrystallites." Chemosphere, 37, 363-85 (1998).
[Bae 98c] Bae, W. and Mehra, R. K. "Cysteine-capped ZnS nanocrystallites:preparation and characterization." J. Inorg. Biochem., 70, 125-35 (1998).
[Nguyen 98] Nguyen, L.,Kho, K., Bae, W. and Mehra, R. K. "Glutathione as a matrix for the synthesis of CdS bionanocrystallites." Chemosphere (in press).