Study of laccaic acid and other natural anthraquinone dyes by Surface-Enhanced Raman Scattering spectroscopy

The red dyes of natural origin, such as lac dye, cochineal and madder, are of great importance in the history of early textiles and lake pigments. The main colouring substances of these dyes (laccaic acid, carminic acid and alizarin, respectively) are similar in chemical composition: all of them are based on a hydroxyanthraquinonic structure. In this work, we present the Surface Enhanced Raman Scattering spectroscopy (SERS) study of laccaic acid at 514.5 nm. The SERS spectra were obtained at different pH values and on differently prepared Ag nanoparticles, by means of chemical reduction of silver nitrate with tri-sodium citrate and hydroxylamine hydrochloride. The aim of this work is to find the best experimental conditions for the vibrational study and detection of lac dye. A comparison between the SERS spectra of laccaic acid and other anthraquinone dyes (carminic acid and alizarin) was also carried out.