SE(R)RS excitation profile of free-base 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin on immobilized gold nanoparticles

Surface-enhanced (resonance) Raman scattering – SE(R)RS – excitation profile (dependence of SE(R)RS intensity on excitation wavelength) is a key point for optimizing of electromagnetic enhancement of particular SE(R)RS system. In this paper, we measured SE(R)RS excitation profile of free-base 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphyrin (TMPyP) on immobilized Au nanoparticles using confocal Raman microspectrometer with six excitation wavelengths (457.9, 488.0, 514.5, 530.9, 568.2 and 647.1 nm). Au colloidal nanoparticles were immobilized to a glass slide via a self-assembled monolayer of 3-aminopropyltrimethoxysilane. SE(R)RS excitation profile is correlated with the corresponding surface plasmon extinction (SPE) spectrum of immobilized Au nanoparticles although the maximum of SE(R)RS intensity is slightly shifted from maximum of SPE. In our case of resonant molecule, SE(R)RS enhancement is coupled with molecular resonance enhancement and SE(R)RS excitation profile for particular vibrational mode depends on its TMPyP molecular resonance contribution. SE(R)RS excitation profile shows that maximal SE(R)RS intensity is obtained for 568.2 nm excitation that provides limit of detection (LOD) of TMPyP 2 × 10−8 M in soaking solution.