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 wit h the silver colloidal particle yielding a carboxylic acid terminal fu nctionality which may then be charged to varying degrees by controllin g the silver hydrosol. pH. A progressive red shift together with a dam ping and broadening of the surface plasmon feature (lambda(max)) of th e silver particles is observed as the pH is reduced from 10 to 3. Ag c olloidal particles with high surface coverage of the bifunctional mole cule showed negligible flocculation with time at high solution pH indi cating good stabilization due to coulombic repulsive interactions. At low pH, considerable flocculation was observed even for high surface c overage due to lack of coulombic stabilization. A tentative explanatio n is put forward to explain changes in the optical properties of the c olloidal particles due to variation in the pH dependent surface charge of the particles. (C) 1997 Elsevier Science B.V.