Testing anionic spacers by SERRS (surface-enhanced resonance Raman scattering) of a cationic free-base porphyrin in systems with laser-ablated Ag colloids

Abstract

Thiopheneacetic acid and several thiol-based anionic species were tested as prospective spacers between the surface of Ag colloidal particles (prepared by laser ablation) and a chromophoric cationic adsorbate. Time-evolution of the SERRS signal of a cationic free base porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridiniumyl)porphine (H 2TMPyP) attached to the surface of laser-ablated Ag colloidal particles modified by adsorption of either 3-thiopheneacetic acid or a thiol-based anionic species was measured on 2 h time-scale and analyzed. While H 2TMPyP adsorbed on bare surfaces of laser-ablated Ag colloidal particles is rapidly converted into AgTMPyP surface complex (spectrally identical with its synthetic analogue), denaturation of the native structure of the porphyrin by formation of the same AgTMPyP surface species was not observed upon adsorption on the modified colloidal surfaces. The native structure of H 2TMPyP was fully preserved upon adsorption on Ag colloid modified by 3-thiopheneacetic acid and good quality SERRS spectra of H 2TMPyP were obtained from Ag colloid/3-thiopheneacetic acid/H 2TMPyP system. By contrast, none of the thiol-based spacers enabled to obtain SERRS spectrum of unperturbed H 2TMPyP. Modification of Ag colloid by 3-mercaptopropionic acid, 2-mercaptoethanesulfonic acid, 2-mercaptoethanesulfonate and 3-mercaptopropanesulfonate did not prevent metalation of the porphyrin, however, it affected the structure of the resulting AgTMPyP surface complex, as witnessed by the difference of the characteristic marker bands (384, 1005, 1366 and 1566 cm −1) from those of synthetically prepared AgTMPyP. Modification of the colloid by mercaptoethanol prevents porphyrin metalation, however, it induces another kind of perturbation of the native structure of H 2TMPyP.