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.

Silver nanoparticle hydrosol formation by laser ablation (LA) of a Ag target immersed in pure water and in aqueous electrolyte solutions (HCl, NaCl, NaOH, AgNO 3 , and/or Na 2 S 2 O 3 ) of various concentrations has been followed by surface plasmon extinction (SPE) spectral measurements and, for selected samples, by transmission electron microscopy (TEM), scanning electron microscopy (SEM) imaging, and energy dispersive X-ray (EDX) analysis. The laser ablation process accompanied by the Ag nanoparticle fragmentation (NF) has been performed with nanosecond laser pulses, by employing either a continuous or an intermittent irradiation regime. SPE spectra have been recorded either after each irradiation step of the stepwise procedure or at the end of the continuous one and, additionally, during a subsequent aging of Ag hydrosols. The presence of HCl, NaCl, and/or NaOH during LA/NF has led to the stabilization of the resulting Ag nanoparticles, while the presence of AgNO 3 and Na 2 S 2 O 3 has shown a destabilizing effect. The alternations of light and dark periods in LA/NF process, along with the presence of electrolytes (having different affinities toward Ag nanoparticle surfaces and toward Ag + ions as well) can be considered as a new physicochemical parameter affecting the outcome of LA/NF. Moreover, the light/dark alternations are useful for separating the effects of ions on Ag nanoparticle hydrosols directly under irradiation from those occurring during further aging. Ag hydrosols prepared by LA/NF in 1 × 10 -3 M NaCl and aged for 1-2 days have been shown to be excellent substrates for surface-enhanced Raman scattering (SERS) measurements owing to the presence of compact aggregates with numerous interstices between nanoparticles and of reduced Ag(0) adsorption sites on chloridemodified Ag nanoparticle surfaces.