Semi-quantitative analysis of indigo carmine, using silver colloids, by surface enhanced resonance Raman spectroscopy (SERRS) I.T. Shadi, B.Z. Chowdhry, M.J. Snowden, R. Withnall * Vibrational Spectroscopy Centre, School of Chemical and Life Sciences, University of Greenwich, Chatham Maritime Campus, Kent ME4 4TB, UK Received 15 June 2002; accepted 15 August 2002 Abstract The application of surface enhanced resonance Raman spectroscopy (SERRS) to the semi-quantitative analysis of the dye, indigo carmine, has been examined using citrate-reduced silver colloids. Good linear correlations are observed for the dye band at 1580 cm 1 in the concentration range 10 7  /10 5 and 10 9 /10 5 mol dm 3 , using laser exciting wavelengths of 514.5 [(R /0.9983)] and 632.8 nm [(R /0.9978)], respectively. At concentrations of dye above 10 6 M the concentration dependence of the SERRS signals is non-linear due to the coverage of the surface of the colloidal particles by the dye being in excess of a full monolayer. At concentrations above 10 6 M resonance Raman spectroscopy (RRS) can be employed for the quantitative analysis of the dye. An internal standard was used and a good linear correlation (R /0.997) was observed for the dependence of dye signal intensities at 1580 cm 1 in the concentration range 10 5 /10 4 M using a laser exciting wavelength of 514.5 nm. The limits of detection of indigo carmine by SERRS (514.5 nm), SERRS (632.8 nm) and solution RRS (514.5 nm) are found to be 0.9, 1 and 38 ppm, respectively. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Indigo carmine; Silver sol; Surface enhanced resonance Raman spectroscopy (SERRS); Resonance Raman spectroscopy (RRS); Semi-quantitative analysis; Internal standard 1. Introduction Indigo carmine is the sodium salt of 5,5?- indigodisulfonic acid (Fig. 1). It is used as a clinical diagnostic stain [1,2], biological stain (of sectioned cells) [3] and as a food colouring agent [4]. In this study, surface-enhanced Raman and resonance Raman spectra of indigo carmine have been obtained after the addition of an aqueous solution of the dye to silver sol. To our knowledge SERRS spectra of indigo carmine using silver sols have not been reported. * Corresponding author. Tel.:  /44-208-331-8691; fax:  /44- 208-331-9983. E-mail address: r.withnall@gre.ac.uk (R. Withnall). Spectrochimica Acta Part A 59 (2003) 2201  /2206 www.elsevier.com/locate/saa 1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-1425(03)00063-5