Electrochemical surface-enhanced Raman spectroscopy (EC-SERS): a tool for the identification of polyphenolic components in natural lake pigments

The identification of natural organic pigments is important for the conservation, preservation, and historical interpretation of artwork. Due to the fugitive nature of the natural dye components in pigments, their analysis can be complicated by issues such as low concentration and sample complexity. In addition, these pigments are exceedingly diverse, and often represent complex mixtures which are difficult to analyse without a separation step. A particularly challenging class of dyes is the natural yellow polyphenols (i.e. quercetin, rhamnetin, emodin, etc.). Several techniques have been used successfully for the identification of phenolic compounds in a complex mixture, but the majority of these methods require advanced instrumentation and one or more separation steps. In addition, these methods may lack the sensitivity needed to detect minute amounts of pigment remaining in faded artwork. As a result, there is a need for innovative methods of analysis which can be applied to the interpretation of artworks containing natural dyes. In this work, cost-effective screen printed electrodes (SPEs) modified with silver nanoparticles (AgNP) were used to amplify the electrochemical SERS response of phenolic compounds. In particular, application of a voltage to the SERS substrate allows for a fine-tuning of the SERS signal, and was successfully used to separately characterize dye components in two natural yellow lake pigments, Reseda Lake and Stil de Grain. To our knowledge, this work represents the first electrochemical surface-enhanced Raman spectroscopy (EC-SERS) study of polyphenolic dye mixtures, and is the first application of EC-SERS for natural pigment analysis. This work establishes EC-SERS as a useful technique for the identification of complex natural dyes which may find potential use in the cultural heritage realm.