Silver cation-mediated dual-emissive Au–Ag bimetallic nanoclusters for pH ratiometric sensing

Metal nanoclusters (NCs) with intrinsic dual-emission are not only significant for fundamental research but also for accurate ratiometric sensing and imaging. Using 1-dodecanethiol (DT) as protected-cum-reduced ligand, the core-shell structured Au-Ag bimetallic NCs with well-resolved dual-emission bands centered at 440 nm and at 630 nm were successfully synthesized by a facile one-pot approach.A combined characterizations of X-ray photoelectron spectroscopy(XPS), transmission rlectron microscope (TEM) and optical spectrum revealed that the Au-Ag bimetallic NCs had a core-shell structure with a metallic Au(0) core coated by a shell of Au(I)- and Ag(I)-thiolate motifs, and very interestingly, the dual-emissive Au-Ag NCs exhibited unique ratiometric pH-dependent emissions, which could be used an ideal pH ratiometric nanosensors. Most importantly, our observed Ag+ mediated pH-dependent dual-emissive behavior suggests that the metal NCs core is not real emitter center, while structural water molecules (SWs) confined at the nanoscale interface of metal core are true luminous centers, which answers that the optical properties of metal NCs are very sensitive to the delicate change of surrounding microenvironment of metal NCs, including the dosing of Ag+, pH and packing mode of surface ligands. Our discovery not only opens up new possibilities for the design of ultrasmall ratiometric pH nanoindicators, but also provides new insights on the origin of photoluminscent (PL) emission of metal NCs.