Protein-Directed Au(0)-Rich Gold Nanoclusters as Ratiometric Luminescence Sensors for Auric Ions via Comproportionation-Induced Emission Enhancement.

Gold nanoclusters (Au NCs) are widely used as fluorescent probes in biomedical sensing and imaging due to their versatile optical properties and low cytotoxicity. Surface engineering of gold nanoclusters (Au NCs) aims to design a surface with versatile physicochemical performances, but previous investigations have primarily focused on the acquisition of the "brightest" species. This has resulted in other types of Au NC being neglected. In the present study, our group prepared a series of Au NCs that were rich in surface Au(0), using the "aged" form of bovine serum albumin (BSA) via controlling the pH during synthesis. We found that slight increases of alkalinity during synthesis over that which produced Au NCs with the most intensive photoluminescence generated the "darkest" Au NCs, which exhibited the strongest absorption. These Au NCs included more Au atoms and had a higher Au(0) content. Furthermore, the addition of Au quenched the emission of the "brightest" Au NCs, but increased that of the "darkest" Au NCs. The increased Au(I) proportion observed in the Au-treated "darkest" Au NCs resulted in a novel comproportionation-induced emission enhancement effect, which we utilized to construct a "turn-on" ratiometric sensor for toxic Au. The addition of Au generated simultaneous, opposite effects on blue-emissive diTyr BSA residues and red-emissive Au NCs. After optimization, we successfully constructed ratiometric sensors for Au with high sensitivity, selectivity, and accuracy. This study will inspire a new pathway to redesign the protein-framed Au NCs and analytical methodology via comproportionation chemistry.