SERS procedure using photoreduced substrates and reflection FTIR spectroscopy for the study of natural organic colourants

Abstract

AbstractIn this work, we showed the potential of photoreduced surface‐enhanced Raman spectroscopy or surface‐enhanced Raman scattering (SERS) substrate for the detection of organic colourants (in mixtures) in lipid and proteinaceous paint layers. Different organic colourants such as madder lake pigment, cochineal lake pigment and lac dye were included in the study. SERS procedure with different approaches was tested, namely, direct application, soaking (incubation) of the sample in the substrate and hydrolysis with the hydrofluoric (HF) acid vapours. For the analysis of colourants bound in linseed oil, a pretreatment step was required (soaking/incubating or hydrolysing with HF). Moreover, exposing a cross‐section of a sample taken from a polychrome work of art to the vapours of HF enabled SERS detection of the cochineal lake pigment in the paint layer of this cross‐section. Therefore, the SERS substrate could be used also for the study of stratigraphy of real cultural heritage samples. As the analyses by means of SERS are (minimally) invasive, the potential of noninvasive reflection FTIR‐spectroscopy analyses for the identification of organic colourants was tested as well. Reflection infrared spectra of organic colourants are presented and discussed. Furthermore, within this study, it was shown that madder lake pigment could be identified in paint layers based on the characteristic bands of the hydrated alumina.