Identification of berberine in ancient and historical textiles by surface-enhanced Raman scattering

The highly fluorescent natural dye berberine can be easily identified in microscopic textile samples by surface-enhanced Raman spectroscopy employing citrate-reduced Ag colloid. The ordinary Raman (OR) and SERS spectra of berberine are presented and discussed in the light of a DFT calculation. Using FT-Raman and FT-SERS we could reliably compare relative intensity shifts and investigate the adsorption geometry of berberine on Ag nanoparticles. The significant enhancement in the FT-SERS spectrum of the out-of-plane ring system bending deformation mode at 729 cm(-1) relative to a group of in-plane vibrations at around 1500 cm(-1) was interpreted as evidence of a 'flat-on' adsorption geometry. SERS was successfully used to identify berberine in silk fiber samples coated with colloidal Ag following a pretreatment with HCl vapor. The SERS method allowed us to detect berberine in a microscopic sample of a single silk fiber from a severely degraded and soiled 17th Century Chinese textile fragment. Copyright (C) 2007 John Wiley & Sons, Ltd.