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 pK(a) value of the gold nanoclusters. Based on the correlation between the pK(a) and surface curvature of ligand-protected nanoparticles, the pK(a) value of the gold nanoclusters was controlled by size. In particular, three different-sized para-mercaptobenzoic acid (p-MBA) protected gold nanoclusters, Au-25(p-MBA)(18), Au-102(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 fluorescence of the KU by the energy transfer process. The fluorescence was restored after adjusting the surface charge of the gold nanocluster by controlling the solution pH. In addition, the KU exhibited a significantly different pH response behaviour for each gold nanocluster. Au210-230(p-MBA)(70-80) showed a higher pH response range than Au-102(p-MBA)(44,) which was intuitive. However, Au-25(p-MBA)(18) showed an unexpectedly high pH response behaviour. pK(a) titration measurement, molecular dynamics simulations, and essential dynamics analysis showed that small nanoclusters do not follow the scaling between the curvature and the pK(a) value. Instead, the behaviour is governed by the distribution and interaction of p-MBA ligands on the nanocluster surface. This work presents an effective design strategy for fabricating a range adjustable pH sensor by understanding the protonation behaviour of the ultrasmall gold nanoclusters in an atomic range.