The influence of pH and anions on the adsorption mechanism of rifampicin on silver colloids

The influence of pH and anions on the adsorption mechanism of rifampicin on colloidal silver nanoparticles has been analysed by electronic absorption, resonance Raman (RR) and surface-enhanced resonance Raman spectroscopy (SERRS). Rifampicin is a widely used antibiotic with a zwitterionic nature. SERRS spectra of rifampicin adsorbed on silver sols, prepared using hydroxylamine hydrochloride as reducing agent, undergo dramatic changes upon lowering the pH. The spectral form changes progressively from that characteristic of chemisorbed rifampicin (at pH > 7) to one very similar to the rifampicin RR spectrum (at lower pH), indicative of a modification of the adsorption mechanism on the surface of the Ag nanoparticles. The RR-type SERRS spectrum is proposed to result from formation of an ion pair between rifampicin and Cl- anions, which, deriving from the colloid preparation, are adsorbed on the Ag surface. The addition of anions to the hydroxylamine hydrochloride sol facilitates conversion from the chemisorbed to ion pair form and leads to an order of magnitude increase in the SERRS signal. Copyright (C) 2007 John Wiley & Sons, Ltd.