Drastically different porphyrin adsorption and metalation processes in chemically prepared and laser-ablated SERS-active silver colloidal substrates

Processes of adsorption and metalation of a cationic water-soluble free base porphyrin, i.e., 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (TMPyP) on Ag colloids have been monitored by surface enhanced resonance Raman spectroscopy (SERRS) as a function of time and porphyrin concentrations. Ag colloids employed were prepared either by laser ablation or by chemical reduction (by sodium borohydride and citrate) of Ag salts. SERRS intensities of TMPyP depend on the morphology of the colloidal aggregates in each system; aggregation, in turn, is governed by a balance between the so-called "diffusion limited aggregation" and "contact limited aggregation" processes, strongly influenced by the TMPyP concentration and the initial state of the colloidal particles (i.e. the residuals ions stuck at the substrate surface). The time evolution of the overall SERRS intensities is thus a function of the colloid preparation procedure. On the other hand, the kinetics of metalation reflects primarily the accessibility of the substrate surface for porphyrin adsorption. Consequently, it is highly sensitive to porphyrin concentration, the covering of the colloid surface by porphyrin molecules being the main qualitative limiting factor. Moreover, the process of metalation is controlled by residual ions at the substrate surface and therefore completely different for laser-ablated and chemically prepared Ag colloids. In the former, the amount of the metalated species is only limited by the adsorbate concentration while in the latter it is also mediated by the removal of residual ions from the colloidal surface by porphyrin molecules.