Tailoring the interaction between a gold nanocluster and a fluorescent dye by cluster size : creating a toolbox of range-adjustable pH sensors

We present a novel strategy for tailoring the fluorescent azadioxatriangulenium (KU) dye-based pH sensor to the target pH range by regulating the pKa value of the gold nanoclusters. Based on the correlation between the pKa and surface curvature of ligand-protected nanoparticles, the pKa value of the gold nanoclusters was controlled by size. In particular, three different-sized para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters, Au25(p-MBA)18, Au102(p-MBA)44, and Au210–230(p-MBA)70–80 were used as the regulator for the pH range of the KU response. The negatively charged gold nanoclusters enabled the positively charged KU to bind to the surface, forming a complex and quenching the …