Microbial Synthesis of CdS Nanocrystals in Genetically Engineered E.[emsp14]coliWe thank the NSF (BES0422791 and BES0329482) for financial support of this research. We also thank Dr. Georgiou for kindly providing E.[emsp14]coli strain R189

Semiconductor nanocrystals have been shown to possess unique optical, electrical, and optoelectronic properties for a wide range of applications. In particular, the exploitation of semiconductor nanocrystals, often referred to as quantum dots, in biological applications has increased dramatically because of their unique spectral properties, which enable simultaneous multiplex labeling and detection. More importantly, the spectral properties of these semiconductor nanocrystals can be controlled effectively by tuning the size, composition, surface properties, and crystal structure of the nanocrystals. Conventional chemical synthesis based on high-temperature organometallic processes is extremely toxic and expensive, and involves unstable species. For practical purposes, an alternative “green chemistry” scheme that is safe, simple, inexpensive, and suitable for industrial upscaling is extremely attractive. One promising alternative to chemical synthesis is the use of biological templates for the synthesis of nanocrystals. Many different biological templates, including peptides, nucleotides, and fusion proteins, are known to act as capping agents to regulate the synthesis of CdS, CdSe, and CdTe. Biological templates not only guide the nucleation of inorganic materials, but also control the crystal structure and size, under aqueous and ambient conditions. Biological approaches to nanocrystal synthesis can also be extended to living biological systems. Peptides capable of nucleating nanocrystal growth were displayed on the surface of M13 bacteriophage; the genetically engineered phage promoted the synthesis of crystalline nanowires, while preserving the exquisite regulation of material composition, size, and shape. Furthermore, the fission yeast Schizosaccharomyces pombe (S. pombe) has been used to promote the synthesis of CdS nanocrystals. In response to cadmium toxicity, S. pombe synthesizes phytochelatins (PCs) with repeating gGlu-Cys units to trap cadmium as nontoxic complexes. This low-molecular-weight complex composed of only cadmium and PCs is then transported actively into the vacuole and converted into highmolecular-weight PC–Cd–S complexes by the incorporation of sulfide. This process results in the formation of nanocrystals. During the nucleation process, PCs serve as a binding template/nucleation site for the metal ions and stabilize the nanocrystal core against continued aggregation. Although S. pombe has the intrinsic ability to form semiconductor nanocrystals by storing the peptide–metal complex in the vacuole as a defense mechanism, the multicompartment requirement makes it difficult to control and fine-tune the properties of the nanocrystals produced. Prokaryotes, such as bacteria, are ideal for engineering the synthesis of nanocrystals with precisely tailored size and crystallinity because of their single-compartment property. Escherichia coli (E. coli) is of particular interest, as the genetic tools and cellular metabolisms associated with this bacterium are well understood. Therefore, the guided assembly of genetic traits necessary for nanocrystal synthesis is possible. A recent study demonstrated the biosynthesis of CdS nanocrystals in E. coli without any genetic modification. However, only samples cultured for 24 h were shown to produce CdS nanocrystals, for which a large distribution in size from 2–5 nm was observed. More importantly, the mechanism of nanocrystal synthesis was not elucidated, and only a small subset of strains was able to synthesize nanocrystals. Inspired by the PC-based detoxification mechanism of S. pombe and the ability of this microorganism to create CdS nanocrystals, we genetically modified E. coli to establish a generalized approach to CdS-nanocrystal synthesis on the basis of the PC-directed method. To explore the feasibility of using E. coli as a biofactory for the controlled synthesis of CdS nanocrystals, the E. coli strain JM109 was endowed with the ability to produce PCs by expressing SpPCS, the PC synthase of S. pombe. A feedbackdesensitized g-glutamylcysteine synthetase (GSHI*), which catalyzes the synthesis of the PC precursor glutathione (GSH), was cotransformed to enhance the level of PC synthesis 10-fold, as reported elsewhere. Cells designed to synthesize PCs were grown in an LB medium (lysogeny broth) containing the appropriate antibiotics; isopropyl-b-d1-thiogalactopyranoside (IPTG) and cadmium chloride were added during the early exponential growth to promote PC synthesis and cadmium binding. Sodium sulfide was added 3 h after the addition of cadmium chloride to induce the formation of CdS nanocrystals for an additional 1 h. The formation of PC-templated CdS was first suggested by SDSPAGE analysis. When the CdS luminescence was visualized [*] S. H. Kang, Prof. N. V. Myung, Prof. A. Mulchandani, Prof. W. Chen Department of Chemical and Environmental Engineering University of California, Riverside Bourns Hall A242, Riverside, CA 92521 (USA) Fax: (+1)951-827-5696 E-mail: [email protected]