Ultrastable Hydrophilic Gold Nanoclusters Protected by Sulfonic Thiolate Ligands

Increasing surface charge is a commonly used and efficient strategy to improve the stability of colloidal nanoparticles in solution, but the use of this strategy in metal nanoclusters (NCs) has not been studied yet. Here, we demonstrate that increasing the surface charge can effectively solve the long-term poor stability of hydrophilic gold (Au) NCs in solution. We choose the well-studied Au25 NCs as the model system and introduce a new thiolate ligand p-mercaptobenzenesulfonic acid (p-MBSA) as the protecting ligand. Compared with the conventional hydrophilic Au NCs protected by thiolate ligands with carboxylic group, the sulfonic group of p-MBSA can increase the surface charge of Au NCs unprecedentedly, thereby improving the stability of Au NCs in solution. In order to confirm this hypothesis, p-mercaptobenzoic acid (p-MBA) is selected as the representative of carboxylic thiolate ligand and used to synthesize Au25 NCs as a control. The results show that the ζ potential of the as-synthesized [Au25(p-MBSA)18]− can reach −100 mV, which is in sharp contrast with [Au25(p-MBA)18]− (about −50 mV). The high surface charge density of [Au25(p-MBSA)18]− improves its stability in a wide pH range (pH = 3–10), high salt concentration (2 M NaCl) and high temperature (80 °C). Au25(p-MBSA)18 with enhanced stability in solution can be used as ultrastable nanozymes under different working environments, and it can mimic the properties of peroxidases under acidic conditions (pH = 4–6). This study suggests that thiolate ligands with sulfonic groups will be promising surface ligands to protect and functionalize Au NCs for biomedical and catalytic applications.