JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2007; 38: 1259–1266 Published online 26 June 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jrs.1761 Surface-enhanced Raman scattering study of the red dye laccaic acid M. V. Ca ˜ namares * and M. Leona The Metropolitan Museum of Art, 1000 Fifth Avenue, New York, NY 10028-0198, USA Received 13 February 2007; Accepted 12 April 2007 FT-Raman and surface-enhanced Raman scattering (SERS) spectroscopy were applied to the study of lac dye, a highly fluorescent anthraquinone red dye. The SERS spectra were obtained at different pH values, on Ag nanoparticles prepared by chemical reduction with citrate and hydroxylamine, and at several excitation wavelengths, in order to find the best experimental conditions for the detection of the lac dye. The lower detection limit was achieved using nanoparticles prepared by reduction with hydroxylamine, excitation at 514.5 nm, and slightly acidic pH conditions, thus exploiting a combination of factors including lower electrostatic repulsion between dye and nanoparticles and resonance Raman enhancement. A comparison between the adsorption of laccaic acid (LA) and carminic acid (CA), another anthraquinone red dye, was also done, based on the SERS spectra of both dyes. Copyright  2007 John Wiley & Sons, Ltd. KEYWORDS: laccaic acid; carminic acid; SERS; art and archaeology; natural dyes INTRODUCTION The red dyes of scale insect origin are of great importance in the history of early textiles and lake pigments. Their identification on ancient and historical textiles can be of importance to answer questions of age and place of origin. The main colouring matters of these dyes are similar in chemical composition: all of them are based on a hydroxyanthraquinonic structure. One of the most important insect dyes is lac, the scarlet dye present in the eggs of the insect Laccifer lacca, which develop in a resinous cocoon, knows as sticklac, on the twigs of over 160 host trees in an arc from northern India through to Indochina. 1 The main colouring components of lac dye are laccaic acid A (LA) (71–96%) (Fig. 1(a)) and B (0–20%) 2 ; laccaic acids C, D and E are just minor components. Lac was used in India and Japan since antiquity. It seems to have been extensively used in European dyeing practice only in the late 18th century. The high fluorescence of LA seriously limits the applica- tion of normal Raman spectroscopy to its study. FT-Raman and surface-enhanced Raman spectroscopy (SERS) can, how- ever, be successfully used to characterize this dye in aqueous media. The applicability of FT-Raman spectroscopy to the study of LA is limited to the pure dye in the solid phase, as the Raman effect is very weak at near-infrared excitation. L Correspondence to: M. V. Ca ˜ namares, The Metropolitan Musem of Art, 1000 Fifth Avenue, New York, NY 10028 USA. E-mail: mvca@iqfr.csic.es SERS on the other hand, can be successfully used to study very dilute aqueous solutions, with the additional advan- tage of the fluorescence quenching occurring on the metal surface. 3,4 Recent articles have discussed the adsorption and acidic behaviour of anthraquinone derivatives, 5–9 including the red dyes alizarin 8 and carminic acid (CA). 9 The identification of lac dye by SERS spectroscopy has also been reported. 10 – 12 In this paper we present a study of the adsorption of LA on silver nanoparticles prepared by chemical reduction with citrate and hydroxylamine. The SERS study was carried out at different pH values and excitation wavelengths in order to determine the best experimental conditions to detect low concentrations of LA in samples from artifacts such as textiles or paintings. A comparison of the behaviour of LA and CA on the silver nanoparticles was also carried out. The assignment of vibrational modes for LA was assisted by a theoretical calculation. EXPERIMENTAL Materials LA was purchased from Tokyo Kaisei Co. (98%) and CA from Sigma (96%). Stock solutions of the dyes at a concentration of 10 3 M were prepared in water. All the reagents employed were of analytical grade and purchased from Sigma and Merck. The aqueous solutions were prepared by using 18 M ultrapure water (Millipore MilliQ). Silver colloids were prepared by reduction of silver nitrate with trisodium Copyright  2007 John Wiley & Sons, Ltd.