Gold nanoclusters (Au NCs) have received extensive attention because of their interesting optical properties[1–3], fluorescence[4, 5], magnetism, and redox properties, as well as their potential applications in many fields such as optics[8–10] and catalysis[11, 12]. To synthesize these highly fluorescent Au NCs, several methods have been developed. A method of synthesizing Au NCs is based on the reducing Au3+ ions in the presence of thiol ligands. For example, Dyer et al. exemplified the application of this approach by synthesizing Au NCs encapsulated by polyamidoamine dendrimers. Ying and coworkers developed a new method of Au NC synthesis using protein bovine serum albumin (BSA) as sole reduction agent at high pH. The other method of synthesizing Au NCs is based on the etching process using polymers. For example, Duan and Nie report a ligand-induced etching process for preparing highly fluorescent and water-soluble Au NCs. Jin et al. found that gold nanoparticles could be etched to produce ultrasmall clusters under reflux at high temperature. Inspired by this discovery, we wonder whether the etching method can be applied to using thiol ligands other than polymers.
Glutathione (GSH), the most abundant low-molecular weight thiol with naturally defined chemical components and structures, has been extensively used as ligands to induce the nucleation and growth of nanocrystals based on biomineralization or as templates to synthesize nanocrystals with well-defined morphologies. Au NCs, protected with GSH, have been well studied[17, 18]. Recently, Shichibu et al. discovered that some GSH-protected clusters could be etched by GSH, which generated GSH-protected Au25 as the main product[3, 19]. However, the quantum yield of these reported gold nanoclusters is relatively low. Hence, a method of creating Au NCs with monodispersed, ultra-small size, high quantum yield and near-infrared (NIR) emission would be extremely valuable for extending their fundamental properties and investigation of a wider range of applications.
In this work, we reported a novel synthetic method, based on the capability of a common commercially available thiol ligands, GSH, for the preparation of ultra-small size highly luminescent Au NCs at high temperature following with a further natural etching at RT with NIR emission (λem max = 650 nm, quantum yield (QY) ≈ 30 %). Our process is similar to the biomineralization behavior of organisms in nature: sequestering and interacting with organisms, followed by providing scaffolds for the minerals formed. The optical and structural properties of the etched Au NCs were performed by ultraviolet–visible (UV–vis) and photoluminescence (PL) spectroscopies, high-resolution transmission electron microscopy (HRTEM), and electrospray ionization mass spectrometry. The PL of the etched Au NCs was decreased obviously by the addition of Hg2+ and increased by the addition Pb2+ at certain concentration. Such etched Au NCs have great potential application in heavy metal ion detection in water and environmental monitoring.