Luminescent Au Nanoparticles with a pH-Responsive Nanoparticle-Supported Molecular Brush

This is the authors' abstract. We don't add key points for this paper.

The pH-dependent properties of Au nanoparticles ∼2.2 nm in diameter decorated with bifunctional ligands possessing thiol and carboxylic acid functional groups are described. The Au−S linkage creates a polarity-sensitive photoluminescent charge transfer (CT) complex at the particle periphery, which acts as a built-in sensor for ligand configuration at the nanoparticle interface. When a pH change eliminates the charge on the exposed carboxylic acid group, van der Waals interactions between the carbon chain of the ligand and the Au nanoparticle lead to collapse of the ligands, and the photoluminescent Au−S CT species responds to the change in local polarity accordingly. Electron microscopy, electrophoresis, dynamic light scattering, and mass spectrometry were used to characterize the physical nature of the nanoparticles. Absorption, fluorescence emission and excitation, and zeta potential measurements were used to understand the reversible pH-dependent spectral response. The number of charged ligands on each particle and p K a for the brush collapse were estimated. The Lippert−Mataga equation was used to provide evidence of the change in local polarity of the CT complex coincident with observed spectral changes.