Well-Defined Nanoclusters as Fluorescent Nanosensors: A Case Study on Au25(SG)18

The fluorescence of nanoparticles has attracted much attention in recent research, but in many cases the underlying mechanisms are difficult to evaluate due to the polydispersity of nanoparticles and their unknown structures, in particular the surface structures. Recent breakthroughs in the syntheses and structure determinations of well-defined gold nanoclusters provide opportunities to conduct in-depth investigations. Devising well-defined nanocluster sensors based on fluorescence change is of particular interest not only for scientific studies but also for practical applications. Herein, the potential of the glutathionate (SG)-capped Au25 nanocluster as a silver ion sensor is evaluated. The Ag+ detection limit of approximately 200 nM, based on the fluorescence enhancement and good linear fluorescence response in the silver ion concentration range from 20 nM to 11 mu M, in combination with the good selectivity among 20 types of metal cations, makes Au25(SG)18 a good candidate for fluorescent sensors for silver ions. Further experiments reveal three important factors responsible for the unique fluorescence enhancement caused by silver ions: 1) the oxidation state change of Au25(SG)18; 2) the interaction of neutral silver species (Ag0, reduced by Au25(SG)18-) with Au25(SG)18; and 3) the interaction of Ag+ with Au25(SG)18. Experiments demonstrate the very different chemistry of hydrophobic Au25(SC2H4Ph)18 and hydrophilic Au25(SG)18 in the reaction with silver ions. This work indicates another potential application of gold nanoclusters, offers new strategies for nanocluster-based chemical sensing, and reveals a new way to influence nanocluster chemistry for potential applications.