Cysteine-directed fluorescent gold nanoclusters for the sensing of pyrophosphate and alkaline phosphatase

Good biocompatible fluorescent gold nanoclusters (Au NCs) stabilized by cysteine with blue-green emission were first synthesized through a core etching process. The as-prepared clusters were found to be useful as label-free fluorescent sensors for the detection of the biologically important molecules pyrophosphate (PPi) and alkaline phosphatase (ALP). The approach was based on the competition between cysteine and PPi for coordination of Cu2+. The fluorescence of Au NCs could be quenched by Cu2+. PPi could recover the Cu2+ -quenched Au NCs fluorescence selectively, and in this way PPi could be detected (with a detection limit of 2 mu M). Upon addition of ALP, PPi was hydrolyzed and Cu2+ was released, which re-quenched the recovered fluorescence. As a result, an assay for ALP activity was established (allowing detection of 0.1 mU mL(-1) of ALP). The strategy employed here offers a new way to construct novel nanomaterial-based biosensors.