Fluorescent ZnO quantum dots synthesized with urea for the selective detection of Cr 6+ ion in water with a wide range of concentrations.

Nearly monodisperse Zinc oxide (ZnO) quantum dots (QDs) displayed yellow fluorescence were synthesized using urea as dispersant. In this paper, urea-ZnO QDs were used as fluorescent probe to detect Cr ⁶⁺ in solution. The emission from the as-synthesized urea-ZnO QDs is selectively quenched when Cr ⁶⁺ ions were added. Moreover, there are two linear relationships between the quenching of fluorescence intensity and the Cr ⁶⁺ concentrations ranging from 4 μM to 1000 μM, with the detection limit for Cr ⁶⁺ at 19.53 nM (on basis of 3σ/slope criterion). The quenching of fluorescence is attributed to aggregation of the QDs and charge transfer between the QDs and Cr ⁶⁺ by measurements of transmission electron microscopy (TEM) images, UV-visible absorption spectra and fluorescence lifetime.