Porphyrins as SERRS spectral probes of chemically functionalized Ag nanoparticles

The results of surface-enhanced resonance Raman scattering (SERRS) spectral probing of citrate- and/or citric acid-modified Ag nanoparticles by selected free-base porphyrins, namely a tetracationic 5,10,15,20-tetrakis(1-methyl-4-pyridiniumyl)porphine and neutral 5,10,15,20-tetra(pyridyl)porphine (H2TPyP) and 5,10,15,20-tetrakis(4-aminophenyl)porphine (H2TAPP) are reported, along with a novel procedure of the functionalized Ag nanoparticle hydrosols preparation by laser ablation of a Ag target in aqueous sodium citrate and/or citric acid solutions of various concentrations. SERRS spectra obtained from the Ag nanoparticle hydrosol/porphyrin system were analyzed using the spectral marker bands of free-base, Ag metallated and diacid forms. In freshly prepared SERRS-active systems, adsorbed citrate was found to function as an efficient molecular spacer for positively charged porphyrin species both in the pH-neutral and in the acidic media, allowing for SERRS spectral detection of not only cationic, but also additionally protonized neutral porphyrins in the native free-base (and/or, at low pH, in the diacid) form without denaturation by Ag incorporation. Furthermore, a substantial increase of the SERRS signal observed for H2TPyP and H2TAPP in systems with Ag nanoparticles prepared by laser ablation in 1 × 10−2 M citric acid solutions is attributed to both the electromagnetic enhancement increase stemming from the presence of hot spots in compact aggregates of touching and intergrown Ag nanoparticles (visualized by /HR/-TEM), and from the molecular resonance enhancement increase originating from a close match between the Soret band of the diacid form (440 nm) and the 457.9 nm excitation. For H2TAPP, the large SERRS enhancement manifests itself in the 1 × 10−10 M SERRS spectral detection limit.