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

Abstract

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 electron 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. 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 ultra-small ratiometric pH nanoindicators, but also provides new insights on the origin of photoluminescent (PL) emission of metal NCs. the physicochemical properties of metal NCs [33,34] . Several strategies have been developed to the synthesis of bimetallic Ag-Au NCs, such as one-pot co-reduction method [35][36][37] (spontaneous reduction of as-mixed Ag and Au precursors through balancing the redox potentials of metal pairs by thiol ligand [38,39] ), classical galvanic replacement reaction approach [40] (involves the spontaneous reduction of a noble-metal cationic by a less noble metal in solution driven by the difference in redox potentials), abnormal anti-galvanic replacement reaction approach [41,42] (inverse process for galvanic replacement reaction recently observed for the synthesis of thiolate-protected Ag-Au NCs) and addition reaction [43][44][45] (a hydride-mediated controlled growth process), etc. And intriguingly, as-formed heteroatom doped metal NCs generally exhibited dramatically enhancement of luminescent and catalytic performance [30,[46][47][48][49][50][51][52] , which are expected to have synergistic effects in their physicochemical properties compared with their mono-metallic analogues [53] . Nevertheless, the origin of PL or the nature of emitter of heteroatom doped metal NCs remain unclear and even controversial, which limits the rational design of metal nanoclusters with improved and tailored optical and catalytic properties.Luminescent metal NCs with dual-wavelength emission have been exploited as ratiometric nanoprobes for sensing and imaging [54] , rather than absolute intensity-dependent signal readout of single-emissive metal NCs, ratio...