Silver Island films as substrate for surface-enhanced Raman spectroscopy (SERS): A methodological study on their application to artists' red dyestuffs

Surface enhanced Raman spectroscopy (SERS) has enormous capabilities for the unambiguous identification of artists' red lake pigments and dyestuff. Until now, red lake pigments have been very poorly characterized with conventional Raman spectroscopy due to their high fluorescence and weak Raman scattering cross section. Furthermore, very small amount of dyestuff is necessary to achieve intense deep colors; therefore a characterization method with enhanced sensitivity is needed to accurately identify the dyestuff. This study focuses on two red dyestuffs, carminic acid and laccaic acid, which are extensively utilized in old masters' paintings. We propose an innovative sensing method using 6 nm silver island films (AglFs) fabricated with electron beam (e-beam) deposition on the colorant particles under investigation. The results of an in-depth methodological investigation into the signal-enhancement achievable with various experimental conditions applied to these substrates are presented. We explore the effects of varying laser excitation frequencies (532.15 nm 632.8nm and 785.7 nm), laser power at the sample and we investigate the dependence of S/N in the spectra from different spot sizes. Finally, the sensitivity of the method is determined. The results of the semi-quantitative analysis have allowed us to determine the best experimental conditions to achieve the highest sensitivity in the investigation of this important class of artists' colorants.