In Vitro Enzymatic Synthesis of Phytochelatin-Capped CdS Nanocrystals

Semiconductor nanocrystals, also known as quantum dots, have been widely used as a fluorescent dye in biological applications. To synthesize biocompatible quantum dots, E. coli was designed to produce phytochelatin synthase (PCS), an enzyme which catalyzes the synthesis of a naturally occurring metal-chelating peptide phytochelatin (PC) upon the addition of glutathione (GSH). Presence of a hexa-histidine tail enabled the direct immobilization of PCS onto Ni-resins. Addition of inorganic salt precursors and GSH resulted in the room-temperature assembly of CdS nanocrystals using PC as the capping agents. High-resolution transmission electron microscopy confirmed the presence of CdS nanocrystals of the wurtzite (hexagonal) structure. These nanocrystals exhibited an absorption peak at 313nm and a fluorescent peak at 345 nm. Moreover, by varying the concentration of metal ion precursor, different compositions of PC (PC2, PC3 and PC4) can be induced, forming CdS nanocrystals of different sizes. Since PC is known to bind a wide range of heavy metals such as Zn, Pb, and As, this approach may be generalized to guide the self assembly of a wide range of semiconductor nanocrystals with different compositions.