SERS excitation profiles of phthalazine adsorbed on single colloidal silver aggregates as a function of cluster size

The wavelength-dependent SERS enhancement and photoreactivity of phthalazine (pht) adsorbed on single Ag colloid fractal clusters deposited out of solution onto a glass surface were determined as a function of cluster size from the excitation profiles of the SERS spectra of three individual colloid clusters with dimensions 1.9 x 1.9 mu m, 3.7 x 2.8 mu m and 7.5 x 4.7 mu m. The spectra were correlated with a given cluster by using imaging Raman microspectroscopy. The SERS excitation profiles obtained from various bands observed in the SERS spectra could be understood in terms of two wavelength-dependent contributions: (i) the photochemical activity which grows monotonically toward the blue in the spectral range (470-650 nm) used and (ii) the SERS enhancement function which increases toward the red. The variety of shapes observed for the excitation profiles can be explained in terms of the relative contribution of the pht reagent and the photoproduct to the sometimes overlapping SERS bands. The relative contributions (or lack of overlap) suggested by the SERS excitation profiles are in good agreement with the independent assignments of those bands to pht or its photoproduct. The wavelength-dependent SERS enhancement and the photoreactivity were found to be approximately independent of cluster size for the three clusters studied in keeping with predictions made in the literature relating to the optical properties of fractal clusters.