A removable SERS substrate for cultural heritage analysis: synthesis and evaluation

Abstract Raman spectroscopy is often applied in cultural heritage analysis, due to its high specificity and portability, but its results are usually affected by the occurrence of a highly fluorescent background or a weak signal. The use of metal nanoparticles as SERS substrates for in situ analyses is a common alternative to overcome Raman drawbacks, which in turn leads to new complications, since metal colloids cannot be easily removed from the analyzed artwork. The development and application of removable SERS substrates is one of the ways to overcome such difficulties when the measurements are carried out directly on objects such as paintings. In this work, we explored the preparation of silver and gold colloids and its inclusion in methylcellulose gels. Gels’ absorption behavior in the UV–Vis range was first evaluated in order to determine the occurrence of a Localized Surface Plasmon Resonance. The only suitable candidate for SERS substrate—the gel containing citrate-reduced gold nanoparticles—was further characterized via dynamic light scattering and scanning electron microscopy, and finally tested on a reference painted mock-up prepared following sixteenth century European recipes. The mock-up includes a modern synthetic dye, which was applied under the premise that it was a historical colorant, without knowing its true composition. This modern dye was also tested. Useful spectra of orpiment, indigo and the modern organic dye were acquired. Finally, the gel substrate was easily removed and the absence of gold contamination was verified with X-ray fluorescence spectroscopy. Graphical abstract