Quantitative surface plasmon spectroscopy: Determination of the infrared optical constants of living cells

Fourier transform infrared (FTIR) spectroscopy gives a great amount of information on the distribution of chemical components in biological objects, in particular cells, in a label-free manner. We report on the development of the spectroscopic technique that combines the surface plasmon resonance (SPR) and the FTIR. Our method is based on the original processing of SPR measurements at varying angles and wavelengths and yields the complex refractive index of the analyte in a broad wavelength region. Contrary to previous SPR studies which yielded information about refractive index variation only, our technique gives absolute optical constants of the examined medium. Using this approach, we studied living Madin Darby canine kidney (MDCK) epithelial cells cultured in their natural aqueous environment and measured their optical constants. We showed that our technique has the ability to distinguish absorption lines of certain chemical components of the cells, such as the absorption lines of a CHn bonds which are characteristic mostly for cell membrane lipids.