Nanocrystals of semiconducting materials, otherwise included in the term quantum dots (QDs), have fascinated physicists, chemists and electronic engineers since the 1970s. The most striking feature of these materials is that their chemical and physical properties differ markedly from those of the bulk solid. [1] Since their quantum size effects are understood, fundamental and applied research on these systems has become increasingly popular. One of the most interesting applications is the use of nanocrystals as luminescent labels for biological systems. [2][3][4][5] Quantum dots have several advantages over conventional fluorescent dyes: they emit light at a variety of precise wavelengths depending on their size and have long fluorescent lifetimes.
Numerous methods exist for the syntheses of semiconductor nanocrystals, but most processes are costly, require sophisticated equipment or extreme reaction conditions and result in low product yields. [2,3,4,6] These synthetic methods are impractical for applications requiring larger quantities or higher concentrations of nanocrystals. However, Candida glabrata yeasts can produce CdS nanocrystallites by chelating Cd with phytochelatins (PCs) or glutathione (GSH) to form a peptide-Cd complex. [7] Then, labile sulfide is introduced to produce the peptide-capped CdS nanocrystallites.
Based on the yeast's use of peptides as naturally powerful chelators of foreign metal species and our previous experiments on sulfide-protected glyconanoparticles, [8] we have developed a simple production method that yielding gram quantities of stable, water-soluble CdS nanocrystals by using the non-natural amino acid tiopronin (N-2-mercaptopropionylglycine) as a capping agent. Tiopronin is a pharmaceutically important drug used for the treatment of cystinuria and rheumatoid arthritis. [9] Importantly, tiopronin has a free terminal -CO 2 H group that provides a handle for further reactivity. The chemical functionality of this capping agent gives the nanocrystals a very high stability and has allowed us to functionalize the QDs with a HIV-1 Tat protein-derived peptide sequence. This pep-[a] Dr.