Surface‐enhanced raman spectroscopy investigation of fluoroquinolone/DNA/DNA gyrase/Mg2+ interactions: Part I. Adsorption of pefloxacin on colloidal silver—effect of drug concentration, electrolytes, and pH

Abstract Surface‐enhanced Raman spectra (SERS at Creighton colloidal silver) and UV/visible spectra have been recorded for an antimicrobial agent (pefloxacin) 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 conjugated part of the molecule is tilted and gives rise to a charge transfer between the drug and the plasmon surface. However the orientation of the drug on the colloid varies with the concentration of pefloxacin, salt addition, and pH. Adsorption via only the carboxylate is privileged in presence of strongly competitive anions such as C1−. 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 similar orientation changes. For acidic pHs few residual molecules having a carboxylate function, or few carboxylic species bonded via the pyridinone CO group, remain adsorbed on the aggregated and unstable silver surface. The spectroscopic analyses and the measurements of the particle size of the colloid show that added salt increases the aggregation and enhances the pefloxacin SERS signals if the anion is not competitive. 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 C1− and OH− to a lesser extent limit the colloid aggregation, pefloxacin adsorption, and SERS enhancement. The influence of the charge transfer on the Raman intensity appears to be weak. © 1995 John Wiley & Sons, Inc.