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Fluorescent inorganic nanoparticles are immerging novel materials that can be adopted for a large number of optical bioassays and chemical sensing probes.

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We have synthesised water soluble CdS/ZnS coreshell quantum dots (QDs) capped with mercaptoacetic acid (MAA). They were characterised by UV-vis absorption spectroscopy, fluorescence spectroscopy, FT-IR and transmission electron microscopy. Such QDs can be used as fluorescent probes for the determination of metal ions because they quench the fluorescence of the QDs. The QDs exhibit absorption and emission bands at 345 nm and 475 nm respectively, which is more longer wavelength compared to MAA-capped CdS QDs and obviously is the result of the larger particle size. The fluorescence intensity of CdS-based QDs is strongly enhanced by coating them with a shell of ZnS. In addition, such functionalised QDs are more sensitive to Hg(II) ions. Parameters such as pH, temperature and concentration of the QDs have been optimised. A high selectivity and sensitivity toward Hg(II) ions is obtained at pH 7.4 and a concentration of 12.0 mg of QDs per L. Under optimum conditions, the fluorescence intensity of CdS/ZnS QDs is linearly proportional to the concentration of Hg(II) in the range from 2.5 to 280 nM, with a detection limit of 2.2 nM. The effect of potentially interfering cations was examined and confirmed the high selectivity of this material.