Brillouin spectroscopy of biorelevant fluids in relation to viscosity and solute concentration.

The measurement of intracellular viscoelastic properties by Brillouin scattering is a rapidly developing field in biophysics and medicine. Here, the Brillouin spectroscopy is applied for a number of aqueous solutions of biorelevant molecules to reveal relations between the Brillouin line parameters (frequency and width) and viscosity or solute concentration. It is found that for the majority of the studied biorelevant molecules the solute concentration governs the Brillouin frequency in a universal manner. On the other hand, the relations between the macroscopic viscosity and Brillouin peak parameters are different for different solutes. We conclude that for biological fluids the viscosity evaluation from Brillouin data needs prior knowledge about the chemical composition. This result challenges the fidelity of the indirect experimental determinations of the cellular viscosity, when small molecule solutions are used for the calibration.