ANIONIC ORGANIC SPECIES DETECTION USING SILVER NANOPARTICLES?

Silver nanoparticles for spectroscopic applications, namely for surface-enhanced (resonance) Raman scattering (SE(R) RS), are usually prepared by a chemical reduction in aqueous solutions. The as-prepared nanoparticles mostly reveal negative zeta potential values among other important characteristic features. Therefore, it is really questionable if anionic organic species and/or pollutants dissolved in aqueous solutions can be directly and immediately detected by SE(R) RS using the as-prepared silver nanoparticles. In this contribution, we are going to show our results concerning SERRS detection of two anionic (4,4', 4'', 4'''-(21H, 23H-porphine-5,10,15,20-tetrayl) tetrakis(benzenesulfonic acid, TSPP), 4,4', 4'', 4'''-(21H, 23H-porphine-5,10,15,20-tetrayl) tetrakis(benzoic acid), TCPP) and, for a direct comparison, of cationic (5,10,15,20-tetrakis(4-trimethyl-ammoniophenyl) porphyrin tetra(p-toluenesulfonate), TTMAPP) porphyrins serving as model compounds. Several different types of silver nanoparticles prepared by diverse chemical reduction processes are tested in this study. Based on our data, SERRS spectral detection of selected water-soluble porphyrins strongly depends on their charge in aqueous solutions and on the type of silver nanoparticles being exploited for SERRS measurements. Furthermore, it can be claimed that the cationic porphyrin enables its immediate detection at low concentrations, while the anionic ones need an additional time in order to be detected.