ELSEVIER zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Efficient production of microbially synthesized cadmium sulfide quantum semiconductor crystallites P. Williams,* E. Keshavarz-Moore, and P. Dunnill zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC *Westlakes Research Institute, The Princess Royal Building, Westlakes Science and Technology Park, Moor Row, Cumbria, United Kingdom; Advanced Centre for Biochemical Engineering, Department of Chemical and Biochemical Engineering, University College London, London, United Kingdom In 1989, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Dameron et al. reported that two yeasts, Schizosaccharomyces pombe and Candida glabrata, produced cadmium @ de in the form of peptide-capped crystallites as a result of being cultured in the presence of cadmium salts. These can function as quantum semiconductor crystallites. W e have now found that the intra- cellular cadmium sulfide (CdS) quantum semiconductor crystallites approximately 1.8 nm in diameter can be selectively released from S. pombe cells by the effects offreeze-thaw. This method of production does not require cells to be broken as do previously reported methods for CdS crystallite preparation and so the starting material, for subsequent downstream processing, is significantly cleaner and much simpler to purify. W e also have exploited the effects of “aging” partially purified CdS quantum semiconductor crystallites at 4°C to minimize the extent of protein cross-contamination and select for the most stable CdS quantum semiconductors. Breakdown of imperfectly coated CdS crystallites evidently removed these. S. pombe was used in preference to C. glabrata for the production of CdS quantum semiconductor crystallites for reasons of microbiological safety. S. pombe also produces a size range of CdS quantum semiconductor crystallites which can be separated by gel filtration to provide quantum semiconductor crystallites with different, highly specific electrooptical properties. zyxwvutsrqponmlkjihgfedcbaZY Keywords: Cadmium sulfide; quantum semiconductor; purification Introduction Research into nanotechnology is advancing because com- mercial applications of materials in molecular electronics are developing. Quantum well semiconductors first developed by AT&T Bell Laboratories and IBM in the 1970s have now become commonplace. They form the basis of today’s laser diodes found in compact disc players and microwave receivers of satellite information. It is now possible, not only to confine electrons in two and one dimensional planes (quantum wires), but also as zero dimensional dots.’ Nanometer scale cadmium sulfide (CdS) quantum semiconductor crystallites can act as quantum dots. They possess specific electronic Address reprint requests to Dr. P. Williams, Westlakes Research Institute, The Princess Royal Building, Westlakes Science and Technology Park, Moor Row, Cumbria CA24 3LN, United Kingdom Received 23 February 1995; revised 11 October 1995; accepted 25 October 1995 properties depending upon their size which is different to those of the bulk material. As the size of the quantum semi- conductor crystallite decreases, its band gap energy in- creases to cause a blue shift of the absorption threshold (to higher energies). *y3 At the same time, discrete absorption bands can appear in the spectrum which correlate with quantum semiconductor crystallite size. It has been sug- gested that arrays of densely packed quantum dots could form a substrate for computers of unprecedented power.4 Also, the capabilities of size-tuned quantum dots to absorb and emit light at predetermined wavelengths are character- istics which would make semiconductor lasers more effi- cient and precisely tuned than those now in existence.’ CdS quantum semiconductor crystallites have been pre- pared chemically in a number of media. The technique for synthesizing inorganic quantum semiconductor crystallites involves controlling crystallite size by restraining the reac- tion environment. This has been achieved by various meth- ods in aqueous and/or organic solutions using reverse mi- celles,’ porous glasses,6 zeolites,7 and polymers;8 however, zyxwvut Enzyme and Microbial Technology 19:206-213, 1996 0 1996 by Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0141-0229/96/$15.00 SSDI 0141-0229(95)00233-2