SERRS of free-base porphyrins on immobilized metal gold and silver nanoparticles

SERRS of three differently charged free-base porphyrins, namely cationic 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin (TMPyP), anionic 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (TSPP), and neutral 5,10,15,20-tetrakis(tetraphenylporphyrin) (TPP), was measured on SERS-active substrates prepared by immobilization of Ag or Au nanoparticles to silanized glass plates. Au surfaces were suitable for TMPyP and TSPP detection while Ag surfaces for TMPyP and TPP. SERRS spectra exhibited excellent reproducibility, very good stability in time (relative standard deviation lower than 10%) and did not contain any sign of the porphyrin metalation or perturbation of its native structure. Even SERRS spectra of water-insoluble porphyrin (TPP) were obtained without any interference of solvent Raman signal. Estimated limits of detection (LOD) were between 3 × 10−9 and 8 × 10−8 M porphyrin concentrations in soaking solution. SERRS on immobilized Ag and Au nanoparticles was found a suitable analytical method for detection of free-base (including water-insoluble) porphyrins, advantagenous considerably with respect to routinely used metal sols.