pH Dependent changes in the optical properties of carboxylic acid derivatized silver colloidal particles

The optical properties of silver colloidal particles derivatized using an aromatic bifunctional molecule, 4-carboxythiophenol, are presented in this communication. The capping molecule forms a thiolate bond with the silver colloidal particle yielding a carboxylic acid terminal functionality which may then be charged to varying degrees by controlling the silver hydrosol. pH. A progressive red shift together with a damping and broadening of the surface plasmon feature (lambda(max)) of the silver particles is observed as the pH is reduced from 10 to 3. Ag colloidal particles with high surface coverage of the bifunctional molecule showed negligible flocculation with time at high solution pH indicating good stabilization due to coulombic repulsive interactions. At low pH, considerable flocculation was observed even for high surface coverage due to lack of coulombic stabilization. A tentative explanation is put forward to explain changes in the optical properties of the colloidal particles due to variation in the pH dependent surface charge of the particles. (C) 1997 Elsevier Science B.V.