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This study aims at synthesizing a novel Surface-Enhanced Raman Spectroscopy (SERS) sensor based on a nanostructured substrate, calcium silicate hydrate (C-S-H), the main hydration product of Portland cement. The procedure involves first the incorporation of silver within the nanostructure of the gel (C-S-Ag-H) and second the modification of the surface of pellets of the newly synthesized material by laser irradiation at 532 nm or 355 nm. The assessment of the SERS activity revealed no amplification of the organic molecule rhodamine B (RdB) signal, used as a probe, on the C-S-Ag-H before laser irradiation. However, after laser irradiation, the SERS effect was comparable to that of reference colloidal Ag nanoparticles (NPs). This SERS response is attributed to an increase in the concentration of metallic silver on the surface of the C-S-Ag-H pellets upon laser irradiation and material ablation as revealed by X-ray Photoelectron Spectroscopy (XPS). Further enhancement of the SERS signals was induced by depositing colloidal Ag NPs on the irradiated pellets, giving rise to interparticle hot spots where the electromagnetic field is further intensified.

Varnishes are employed in painted artworks for protection from atmospheric pollution and oxidation and for improving the esthetic appearance by providing an even and brilliant surface finish. Also, in some cases, colored varnishes are applied intentionally to modify the chromatic appearance of paintings. Varnishes undergo complex and differentiated structural and chemical changes over time depending on their composition and conservation conditions. The present work investigates the extent of degradation of the outermost layers of varnish coatings due to aging as a function of depth by using nonlinear optical microscopy (NLOM) in the modality of multiphoton excitation fluorescence (MPEF). This non-invasive technique has been employed for the determination of the thickness, with a microscopic axial and lateral resolution, of the affected regions of pictorial terpenoid varnish layers, such as dammar, mastic, shellac and sandarac, subjected to various types and degrees of aging, artificial, natural and a combination of both. A homemade nonlinear optical microscope, based on a tightly focused pulsed femtosecond laser emitting at 800 nm, was used for the investigation. Single-photon laser-induced fluorescence (LIF) measurements served to determine the degree of surface aging and the optimum NLOM-MPEF operating conditions and helped to interpret the results obtained by applying the latter. These results signpost the correlations between the extent of degradation, non-invasively determined by NLOM-MPEF, the type and length of aging, the nature of the varnish and the initial thickness of the varnish layer.