Surface Engineering of Gold Nanoclusters Protected with 11-Mercaptoundecanoic Acid for Photoluminescence Sensing

11-Mercaptoundecanoic acid-protected gold nanoclusters (Au@MUA NCs) were synthesized and investigated as a model to understand the photoluminescence (PL) properties of water-soluble, structurally unknown Au NCs. Surface engineering, including ligand exchange where the length of the alkane chain was changed, ligand conjugation where the charged terminal carboxyl group of the ligands was blocked, and effects of external chemical reducing and oxidizing agents, was carried out. PL profiles were monitored to reveal the impact of surface engineering on the PL. We found that surface ligands, especially the ligand's length and ligand functionalities, play a significant role in the PL properties of Au@MUA NCs. The results also show interesting properties of these NCs as a tunable PL sensor.