Luminescent Gold pH Sensor Based On Nanoparticle-Supported Molecular Brush

A gold nanoparticle-based pH sensor utilizing the mechanism of molecular monolayer brush collapse has been described. In this material, a bifunctional molecular ligand containing thiol and carboxylic acid functional groups forms a collapsible molecular brush on the surface of ~2 nm diameter gold nanoparticles. The thiol-based linkage creates a polarity sensitive photoluminescent charge transfer complex at the particle periphery. When a pH change eliminates the charge on the carboxylic acid, van der waals interactions lead to collapse of the ligands and the photoluminescent species responds to the change in local polarity. Electron microscopy, electrophoresis, dynamic light scattering and mass spectrometry were used to characterize the materials. Absorption, fluorescence emission and excitation, and zeta potential measurements were used to understand the reversible pH response, and a model for the interactions between ligand complexes and the gold nanoparticles is discussed. This material can be used as a pH sensor with an operational range between pH 5 and 8.