Surface Adsorption of Antibiotics Ampicillin and Penicillin by SERS

Abstract

The surface adsorption of antibiotics with various functional groups have been investigated on the homogenous silver colloidal nanosurfaces by surface enhanced Raman spectroscopy (SERS). When analyte is very close to the metal surface weak Raman signals are highly enhanced. Because of enormous enhancement of signals by metal substrate the SERS has been established as an ultrasensitive detection as well as spectroscopic technique for the investigation of surface adsorption and spectral changes even at the single molecules level. Variations in the adsorption geometry due to changes of local environment (pH) have been observed to disclose the detailed information of the surface adsorption of antibiotics on the nanosurfaces. 6-Aminopenicillanic acid (6APA) is the common moiety of ampicillin and penicillin. In order to understand the surface adsorption of ampicillin and penicillin better, it would be helpful to understand the adsorption process of 6APA on the metal nanosurface. The surface adsorption process of 6APA on the colloidal nanosurface has been found to be quite different at lower pH as compared to higher pH. At lower pH, 6APA can be attached to the metal surface through NH3+ and carboxylate group. Only carboxylate group is observed to be attached with metal surface during deprotonation (pH >7.0). The attachment of NH3+ to the metal surface in the adsorption mechanism at lower pH is supported by the zwitterionic DFT simulations of 6APA. The B3LYP hybrid functional and 6-311G(d,p) basis set were used for the zwitterionic DFT simulations. It has been observed that the surface adsorption of ampicillin on the silver colloidal surfaces is similar to 6APA. This observation of adsorption of ampicillin has been confirmed by comparing pH dependent SERS spectra between 6APA and ampicillin. The adsorption process of penicillin has also been investigated such as ampicillin on the silver colloidal nanosurface at both lower and higher pH. The only difference in the chemical structure between ampicillin and penicillin is the existence of –NH2 group. The -NH2 functional group is absent in the chemical structure of penicillin. Therefore, there is no chance that penicillin exits in zwitterionic form. The band at 882 cm-1 in the SERS of 6APA and ampicillin has been observed as the result of protonation which is absent in penicillin. It is also observed that the pH SERS spectra of penicillin are almost same. Hence, the surface adsorption of penicillin in both acidic and basic environments would occur through carboxylate group only. In this thesis, it has been presented that SERS is a promising technique for the ultrasensitive detection of antibiotics. It is concluded that the surface adsorption process of ampicillin and penicillin are different at acidic and basic conditions.