Innovative label-free microspectroscopy, which can simultaneously collect Brillouin and Raman signals, is used to characterize the viscoelastic properties and chemical composition of living cells with sub-micrometric resolution. The unprecedented statistical accuracy of the data combined with the high-frequency resolution and the high contrast of the recently built experimental setup permits the study of single living cells immersed in their buffer solution by contactless measurements. The Brillouin signal is deconvoluted in the buffer and the cell components, thereby revealing the mechanical heterogeneity inside the cell. In particular, a 20% increase is observed in the elastic modulus passing from the plasmatic membrane to the nucleus as distinguished by comparison with the Raman spectroscopic marker. Brillouin line shape analysis is even more relevant for the comparison of cells under physiological and pathological conditions. Following oncogene expression, cells show an overall reduction in the elastic modulus (15%) and apparent viscosity (50%). In a proof-of-principle experiment, the ability of this spectroscopic technique to characterize subcellular compartments and distinguish cell status was successfully tested. The results strongly support the future application of this technique for fundamental issues in the biomedical field.

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Brillouin light scattering (BLS) can record particle resonances, which allow access to thermomechanical properties on a submicrometer scale. The full theoretical representation of BLS eigenmode spectra of polymer and silica colloids resolves a longstanding discussion showing that both even and odd l spheroidal modes are active. The BLS spectra of transparent silica suspensions fully support the predicted dependence of the strength of the individual resonance modes on the probing wavevector. It is this dependence that selects which spheroidal modes will be revealed experimentally.

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