Charge-Related Spectral Shift of Localized Surface Plasmon Resonance in Silver Nanoplates Induced by Deprotonation and Complexation of Surface Carboxyl Groups with Metal Cations

We investigated the spectral shift of localized surface plasmon resonance (LSPR) in a monolayer of mercaptopropionic-acid-stabilized silver nanoplates induced by deprotonation or complexation of surface carboxyl groups with metal cations in an aqueous solution. The deprotonation-induced LSPR red shift of about 10 nm in magnitude linearly depends on the degree of protonation of the carboxyl groups, while complexation with divalent or trivalent cations induces an additional red shift. We apply a simple model to establish a correlation between LSPR spectral shifts and pH, the stability constants of metal–carboxylate complexes, and the type and concentration of metal cations in the solution. Unlike complexation with metal cations, electrostatic adsorption of cationic macromolecules (polyethylenimine and trypsin) induces only a minor spectral shift due to incomplete compensation of the surface negative charges. Our results pave the way for the development of new optical chemical sensors based on the charge-related spectral shift of the LSPR in colloidal plasmonic nanoparticles.