Improving the properties of a gold nanoparticle barium sensor through mixed-ligand shells

This work proposes an improvement of gold nanoparticle sensors using mixed-ligand shells composed of ligands with a shared affinity. As a proof of concept, two ligands, 2-Sulfanylethanesulfonate (MSA) and 11-mercaptoundecanoic acid (MUA), sharing an affinity towards Ba²⁺ were chosen. The two ligands were then combined in a mixed ligand shell, and the Ba²⁺ sensor properties of both the mixed-ligand and mono-ligand shell nanoparticles were examined. The sensor properties of the mixed-ligand nanoparticles were then shown to be superior to both mono-ligand particles. The mixed-ligand nanoparticles retained a high sensitivity for Ba²⁺, while greatly improving the sensor linear range and selectivity. Next, nanoparticles stabilized by a mixed-ligand monolayer consisting of either MSA or MUA with a ligand unreactive towards Ba²⁺ were shown to have no reactivity towards Ba²⁺. This result indicates a cooperative effect between the MSA and MUA ligands and highlights the importance of the reactivity of both ligands for sensor optimization.