An innovative surface-enhanced Raman spectroscopy (SERS) method for the identification of six historical red lakes and dyestuffs

Surface-enhanced Raman spectroscopy (SERS) was used in this work to obtain highly detailed spectra of artists' red lake pigments and colorants. In the past, Raman spectroscopy has been successfully employed to identify many pigments and modern synthetic dyes. Unfortunately, red lake pigments and dyes commonly employed in artistic production from antiquity to the mid-nineteenth century are often extremely fluorescent, making identification with Raman spectroscopy difficult or impossible. This work presents an innovative SERS technique that quenches fluorescence, significantly enhances the weak Raman scattering effect, and requires very little sample material and minimal sample handling. A silver island film (AgIF), approximately 6-8 nm thick, is deposited on the substrate by electron beam (e-beam) deposition. The SERS-active surface is then analyzed with a confocal dispersive Raman microscope, at an excitation wavelength of 632.8 nm. Reference materials including the synthetic dyestuffs alizarin, purpurin, and eosin, high-purity carminic acid, and historic red lake pigments such as madder lake, cochineal, brazilwood, lac lake, and kermes were studied. The proposed method has great potential for the unambiguous identification of red dyes applied in different media on a variety of substrates, as demonstrated by the highly detailed Raman spectra presented here. Copyright (C) 2006 John Wiley & Sons, Ltd.