The surface-enhanced Raman scattering (SERS) spectra of a group of flavonoids with related structures (quercetin, kaempferol, luteolin and apigenin) are reported here for the first time. It is demonstrated that the studied flavonoids undergo a chemical change on the Ag nanoparticles prepared by reduction with citrate used as a SERS substrate. Two labile chemical groups have been identified, one of them corresponding to the catechol-like moiety and another one corresponding to the C-ring, when a hydroxyl group is in position a with respect to the keto group.

Alizarin and carminic acid have been detected in reference wool and linen fibers dyed with madder and cochineal, respectively, through surface-enhanced Raman scattering (SERS) measurements carried out directly on the fiber, without previous hydrolysis of the mordant-dye complex. For such purpose, Ag nanoparticles were produced and immobilized in situ via the laser photoreduction of a silver nitrate aqueous solution in contact with the fiber. Control SERS spectra of pure dyes (alizarin, purpurin and carminic acid, as well as of mixtures of the first two) on similar Ag nanoparticles were also obtained. The method has been applied to one archeological Coptic textile (6th-8th A.D.) of Egyptian origin, where alizarin has been clearly identified. Other mordant dyes of the flavonoids family (luteolin, apigenin and flavonols) have also been identified by the same SERS method in wool fibers dyed with natural plants, mainly used in Central and South America (Dyer's greenweed, old fustic, onion and chilca) following pre-Columbian dying recipes.

Luteolin and apigenin flavonoid have been detected in silk and wool fibres dyed with weld (Reseda luteola L.) through surface-enhanced Raman scattering (SERS) measurements carried out 'on the fibre'. For such purpose, Ag nanoparticles were produced and immobilised in situ via the laser photoreduction of a silver nitrate water solution in contact with the fibre. Control SERS spectra of pure luteolin and apigenin, as well as of mixtures of them, on analogous Ag nanoparticles were also obtained. In this work flavonoids with a similar molecular structure were identified on dyed fibres for the first time without previously hydrolysing the mordant-dye complex.

The chemical modifications undergone by flavonoids under alkaline conditions in aqueous solution and on Ag nanoparticles studied by optical spectroscopy are presented and discussed.