SERS study of fluorescent and non-fluorescent flavonoids: what is the role of excitation wavelength on SERS optical response?

Study of colorful compounds using Raman spectrometry can reveal crucial issues especially if fluorescent background is present. Fluorescence effect in flavonoids is shifted to higher wavelengths by the excited state intramolecular proton transfer. Therefore, new peaks arise in the visible spectral range. In this work, four flavonoids (flavone, 3-hydroxyflavone, chrysin and quercetin) are studied in the form of pure compounds in solid state and after adsorption on gold surface. Four excitation wavelengths 532, 633, 785, and 1064 nm are tested for normal Raman and surface-enhanced Raman scattering spectra of used flavonoids. Some of these flavonoids namely chrysin and quercetin exhibited high fluorescence background in normal Raman spectra and, therefore, it was not possible to obtain their good quality spectra using visible excitations. SERS spectrometry allowed detecting and identifying small amount of the colorful compounds and significantly reducing fluorescence background. Characteristic spectral bands were observed using SERS spectrometry at spectra of both fluorescent and non-fluorescent flavonoids.