Absorption and resonance Raman characteristics of β-carotene in water-ethanol mixtures, emulsion and hydrogel

Absorption or resonance Raman scattering are often used to identify and even quantify carotenoids in situ. We studied the absorption spectra, the Raman spectra and their resonance behavior of β-carotene in different molecular environments set up as mixtures from lipid (emulsion) and non-polar (ethanol) solvents and a polar component (water) with regard to their application as references for in situ measurement. We show how both absorption profiles and resonance spectra of β-carotene strongly depend on the molecular environment. Most notably, our data suggests that the characteristic bathochromic absorption peak of J-aggregates does not contribute to carotenoid resonance conditions, and show how the Raman shift of the C=C stretching mode is dependent on both, the molecular environment and the excitation wavelength. Overall, the spectroscopic data collected here is highly relevant for the interpretation of in situ spectroscopic data in terms of carotenoid identification and quantification by resonance Raman spectroscopy as well as the preparation of reference samples. In particular, our data promotes careful consideration of appropriate molecular environment for reference samples.