Probing applications of laser ablated Ag colloids in SERS spectroscopy: Improvement of ablation procedure and SERS spectral testing

The possibilities to employ laser-ablated Ag colloids as stable active surfaces for reproducible SERS and SERRS spectral measurements were explored. Ag colloids prepared by laser ablation of Ag foil in water under varied conditions were characterized by surface plasmon absorption and transmission electron microscopy. The presence of large Ag particles in these colloids and their sedimentation induce undesired time dependence of the optical characteristics of the colloids. Fragmentation of these large particles by laser radiation in the course of the ablation process or by an additional irradiation of the resulting colloids was found to be the key to optimization of the particle size distribution of the ablated colloids, The presence of chlorides in the aqueous medium during ablation provides reduction of the average particle size, prevents formation of large particles, and increases the efficiency of the ablation process. However, it also reduces stability of the colloid against spontaneous aggregation and modifies the colloidal surface by Cl-. A chemical test based on SERRS spectral monitoring of the kinetics of a free base porphyrin metalation provided very good reproducibility when applied to the optimized Ag colloids ablated in pure water. By contrast, poor reproducibility of the same test was obtained for the chemically prepared colloids.