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Water-soluble, biologically compatible CdSe quantum dots (QDs) with l-cysteine as capping agent were synthesized in aqueous medium. Fluorescence (FL) spectra, absorption spectra, and transmission electron microscopy studies showed that both the molar ratio of Se/Cd and the reaction time are the determining factors for the size distribution of CdSe/l-cysteine QDs. The interaction of QDs bioconjugated to bovine serum albumin (BSA) was also studied by absorption and FL titration experiments. With addition of QDs, the FL intensity of BSA was largely quenched, which can be explained by static mechanism. However, when BSA was added to the solution of QDs, FL intensity of QDs was faintly enhanced at first and then quenched. Lysozyme could change the Escherchia coli (E. coli) membrane fluidity and permeability by FL polarimetry. Fluorescent imaging suggested that QDs can be designed as a probe to label the E. coli cell. These results suggested CdSe/l-cysteine QDs can be used as a probe for labeling biological molecule and bacteria cell.

The synthesis of water-soluble and low-cytotoxicity quantum dots (QDs) in aqueous solution has received much attention recently. A one-step and convenient method has been developed for synthesis of water-soluble glutathione (GSH)-capped and Zn(2+)-doped CdTe QDs via a refluxing route. Because of the addition of Zn ions and the epitaxial growth of a CdS layer, the prepared QDs exhibit superior properties, including strong fluorescence, minimal cytotoxicity, and enhanced biocompatibility. The optical properties of QDs are characterized by UV-vis and fluorescence (FL) spectra. The structure of QDs was verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), energy dispersive spectroscopy (EDS), atomic absorption spectrometry (AAS), and Fourier transform infrared spectroscopy (FTIR). Furthermore, the low cytotoxicity of the prepared QDs was proved by the microcalorimetric technique and inductively coupled plasma-atomic emission spectrometry (ICP-AES).