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

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