New SERS feature of -carotene: consequences for quantitative SERS analysis

While recording SERS spectra of pure -carotene at sub-micromole concentrations for reference purpose, we discovered an unusual spectral response never reported before. In pre-resonance conditions with the 532-nm line, SERS of -carotene with AgNPs exhibits among the strong upsilon(CC) mode at 1512cm(-1) unshifted from normal Raman spectrum, additional strong bands at 1649, 1575 and 1387cm(-1) as well as other medium bands not observed in the Raman spectrum of the crystalline powder. Such behavior is explained in terms of selection rules relaxation upon cyclohexene terminal rings of the -carotene interaction with the NP surface. AFM images of the SERS system suggested dimers and trimers clustering of the nanoparticles with adsorbed -carotene. In light of the new SERS feature the consequences in correct interpretation of the SERS imaging from complex biosystems containing carotenoids are discussed. Relative intensity ratio of the -carotene band at 1512cm(-1) and water against concentration allowed a reliable SERS calibration curve for 50 to 500nmoll(-1) concentration range and provided quantitative SERS assessment of the carotenoid content in the sea urchin (Paracentrotus lividus) gonads extracts. Copyright (c) 2015 John Wiley & Sons, Ltd.