Surface-enhanced resonance raman scattering from copper(II) 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin adsorbed on aggregated and nonaggregated silver colloids

Surface-enhanced resonance Raman scattering (SERRS) from copper(II) 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (CuTMPyP(4)) adsorbed on a borohydride-reduced silver colloid was studied as a function of CuTMPyP(4) concentration ranging from 1 x 10(-9) to 1 x 10(-6) M, using the 441.6 nm excitation line of a He-Cd laser. The OH stretching band of bulk water was employed as an internal intensity standard for normalization of the porphyrin SERRS spectra, The surface coverage of Ag colloidal particles by porphyrin molecules as well as the surface plasmon absorption (SPA) and morphology of the Ag colloid/CuTMPyP(4) systems in dependence on the total porphyrin concentration in the systems were investigated and related to the concentration dependence of the SERRS signal. The plots of the intensities of 12 SERRS spectral bands of the Al, porphyrin modes against the porphyrin concentration are found to be fairly linear for systems with the total porphyrin concentrations 1 x 10(-8) to 2 x 10(-7) M, in which all porphyrin molecules introduced into the system are adsorbed on isolated Ag colloidal particles and/or small aggregates of ca. 30-70 particles. By contrast, these plots become highly nonlinear and mode-frequency dependent for systems with total porphyrin concentrations greater than or equal to 3 x 10(-7) M containing large aggregates of > 100 porphyrin-covered Ag colloidal particles as well as fractions of residual nonadsorbed porphyrin molecules. While the former systems are stable for at least one month and provide a highly reproducible SERRS signal, the stability of the latter ones is limited to several hours. Employment of the 441.6 nm excitation coincident with both the Soret band of the porphyrin and the onset of the SPA band of isolated Ag colloidal particles is thus shown to provide a rather unique possibility of obtaining reproducible SERRS spectra with a linear concentration dependence from pico- to nanomolar quantities of a cationic metalloporphyrin adsorbed on nonaggregated and/or weakly aggregated Ag colloidal particles.