SURFACE-ENHANCED RAMAN-SPECTROSCOPY INVESTIGATION OF FLUOROQUINOLONE DNA DNA GYRASE MG2+ INTERACTIONS .1. ADSORPTION OF PEFLOXACIN ON COLLOIDAL SILVER - EFFECT OF DRUG CONCENTRATION, ELECTROLYTES, AND PH/

Surface-enhanced Raman spectra (SERS at Creighton colloidal silver) an d UV/visible spectra have been recorded for an antimicrobial agent (pe floxacin) at a biologically active concentration (ca. 10(-6) mol/L(-1) ). The adsorption of pefloxacin on the silver surface occurs both via the carboxylate group and the carbonyl of the pyridinone ring. The con jugated part of the molecule is tilted and gives rise to a charge tran sfer between the drug and the plasmon surface. However the orientation of the drug on the colloid varies with the concentration of pefloxaci n, salt addition, and pH. Adsorption via only the carboxylate is privi leged in presence of strongly competitive anions such as Cl-. Thus the carbonyl of the pyridinone ring is desorbed, and the charge transfer is not detected. In basic medium the competitive OH- ion leads to simi lar orientation changes. For acidic pHs few residual molecules having a carboxylate function, or few carboxylic species bonded via the pyrid inone C=O group, remain adsorbed on the aggregated and unstable silver surface. The spectroscopic analyses and the measurements of the parti cle size of the colloid show that added salt increases the aggregation and enhances the pefloxacin SERS signals if the anion is not competit ive. in the presence of NaNO3 an increase of the plasmon oscillations of the metal and a larger number of adsorption sites could explain the SERS amplitude. The competitive anions Cl- and OH- to a lesser extent limit the colloid aggregation, pefloxacin adsorption, and SERS enhanc ement. The influence of the charge transfer on the Raman intensity app ears to be weak. (C) 1996 John Wiley & Sons, Inc.