Tailored ad-hoc methods must be developed for successful identification of minute amounts of natural dyes on works of art using Surface-Enhanced Raman Spectroscopy (SERS). This article details two of these successful approaches using silver film over nanosphere (AgFON) substrates and silica gel coupled with citrate-reduced Ag colloids. The latter substrate functions as the test system for the coupling of thin-layer chromatography and SERS (TLC-SERS), which has been used in the current research to separate and characterize a mixture of several artists' dyes. The poor limit of detection of TLC is overcome by coupling with SERS, and dyes which co-elute to nearly the same spot can be distinguished from each other. In addition, in situ extractionless non-hydrolysis SERS was used to analyze dyed reference fibers, as well as historical textile fibers. Colorants such as alizarin, purpurin, carminic acid, lac dye, crocin, and Cape jasmine were thus successfully identified.

This study aims to provide a better understanding of the Raman spectra of cobalt-based pigments in works of art. Systematic analyses of 21 industrial blue cobalt-based pigments were carried out by using elemental energy dispersive spectroscopy and phase analyses by X-ray diffraction (XRD) and Raman microspectroscopy, which led to the identification of 17 spinel Co-based pigments. All the minerals contained in each pigment were characterised by all three techniques and subsequently attributed to diluents, extenders, mineralisers or unreacted reagents or to the main mineral phase, whether it was pure or contained doping or lattice modifier elements. In the particular case where doping elements were in low concentration in the host mineral lattice, their distinction by XRD from non-doped spinel was impossible. On the contrary, Raman microspectroscopy turned out to be a perfect tool for detecting the presence of the doping agents in the spinel lattice. The determination of the elemental and mineralogical composition of the industrial blue cobalt-based pigments commercially available for artists and studied in this survey represents a significant increase in the amount of analytical data available for this type of pigment as well as a valuable addition in the characterisation by Raman microspectroscopy of these compounds in situ, on works of art.

The pH-dependent solution voltammetry (pH 1–13) of phosphate-stabilized, small (2 nm diameter) iridium oxide nanoparticles (IrIVO x NPs) is described and compared with that of (flocculated) films of the same size nanoparticles on electrodes. The IrIVO x NPs show waves with one electron/one proton formal potential dependency for the IrV/IV redox transformation and (below pH 6) for the IrIV/III reaction. Above pH 6, the IrIV/III reaction becomes a one-electron/two-proton process, unlike the one-electron/one-proton reactivity of the nanoparticles in films. The change is associated with surface oxide acid–base sites having pK A = 6 for solution phase nanoparticles that apparently are inactivated by the flocculation chemistry. Spectral isosbestic points are observed over the pH range of 5–8 for the dissolved nanoparticles. Controlled potential coulometry demonstrates that all of the Ir sites, throughout the nanoparticle, undergo the IrIV/III redox transformation. Whereas the IrIVO x NPs are stabilized at different pH values by phosphate ligation, the associated equilibria are somewhat sluggish, as indicated by small spectral differences for equi-pH nanoparticle solutions mixed with phosphate by different procedures. The IrIVO x NPs can also be capped with hydrophobic carboxylic acids, which allows extraction into nonpolar solvents such as CH2Cl2.