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Semiconductor nanocrystals are finding use in a wide variety of scientific applications, although biologists are probably most familiar with their 'quantum dot' incarnation. Currently, these are produced by chemical syntheses in specialized laboratories. However, in recent years, University of Texas investigators Brent Iverson and Angela Belcher (now at MIT) have become interested in the possibility of using biological systems to assemble these specialized reagents.

Biosynthesis of such crystals is not an entirely new concept, as earlier studies have demonstrated nanocrystal biosynthesis in yeast (Dameron et al., 1989). Additionally, an earlier collaboration between Belcher and Iverson identified peptides that, when expressed on the surface of M13 bacteriophage, were capable of nucleating the formation of semiconductor nanowires from zinc or cadmium sulfide (Mao et al., 2003). Iverson, Belcher and colleagues now sought to expand this strategy to bacteria and designed a system for expressing bacterial surface peptides that they hoped would mirror the effects observed in viruses. What they got instead, according to Iverson, was a surprise: “The negative controls worked too..., and what we realized was that Escherichia coli have this ability—and it had nothing to do with the peptide that we were putting on the surface of the bacteria.”

Indeed, hours after incubating bacteria of the ABLE C strain with cadmium chloride and sodium sulfide, the investigators observed densely packed deposits of nanocrystals by electron microscopy (Sweeney et al., 2004). Further analysis revealed that this process takes place primarily in stationary-phase cells (Fig. 1) and that only certain bacterial strains are capable of efficient nanocrystal formation.

Figure 1: Electron microscope images show that ABLE C cells in stationary phase (a) contain substantial deposits of cadmium sulfide nanocrystals, whereas cells in late log phase (b) do not.
figure 1

Bar indicates 200 nm; inset depicts enlarged image of cell. Reprinted from Chemistry & Biology with permission from Elsevier.

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Beyond these determinations, however, this biosynthetic process remains poorly understood, and initial experiments investigating links between cellular thiol content and crystal formation proved inconclusive. “All possibilities are open for discussion,” says Iverson, although early indications suggest that some endogenous molecule is initiating a cytosolic nucleation process. “We're trying to isolate that critical piece that's missing—what's the molecule that's causing it—because we assume that it's nucleation.”

Emory University professor Shuming Nie, who has worked extensively with quantum dots, lauds the potential of this system: “It's a really innovative, excellent approach to use biological systems to synthesize these semiconductor nanocrystals, [and] it shows that these nanocrystals are intrinsically compatible with biological systems.” Nonetheless, he warns that the quality of such biosynthetic preparations at present “can not really compete with the quality—either in size, distribution or optical efficiency—of materials synthesized in the laboratory.” The authors agree that this is only the beginning, and Belcher says that optimizing the quality and size distribution of these biosynthetic nanocrystals are top priorities for future investigation.

Nevertheless, these findings provide encouraging evidence of the potential benefits that could be reaped using bacteria for the synthesis of a variety of products and materials. “I think that it's an exciting new process, just in terms of harnessing the potential of organisms to make materials that they normally don't make,” says Belcher. Iverson concurs and sees broad possibilities for optimizing such biological pathways: “This [process] is clearly happening without intervention, and I think there is absolutely no reason to believe that you can't start intervening by putting known nucleators in and controlling their expression and abundance, and try to do things in a very rational way... because these are really good little reactors.”