Fluorescent iridium nanoclusters for selective determination of chromium(VI)

Fluorescent iridium nanoclusters (IrNCs) consisting of up to 7 Ir atoms were prepared by heating IrCl3 in N, N-dimethylformamide. No other reagents are required. High resolution transmission electron microscopy (HRTEM) shows the IrNCs to be monodispersed with an average size of 0.9 +/- 0.2 nm. They are well soluble in polar solvents and stable in these solvents for at least 6 months. Under photoexcitation with 365 nm light, they emit strong bluish green fluorescence with peaks that depend on the excitation wavelength and range from 530 to 650 nm. The fluorescence lifetime typically is 2.2 ns and the quantum yield is 8.3%. Fluorescence is quenched by Cr(VI) ion (chromate), and the emission peak is gradually red-shifted. According to the absorbance spectra of IrNCs in the presence and absence of Cr(VI) and Stern-Volmer quenching behavior study, static quenching is involved. Based on these findings, a selective assay was developed for the determination of Cr(VI). It has a linear response in the 0.1 to 100 mu M chromate concentration range and a 25 nM detection limit.