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.

Figure 1

Brillouin spectra of aqueous glycerol (a) and lysozyme (b) solutions for representative solute concentrations. Elastic line (at zero frequency) comes from the reference beam [21].

Figure 2

Brillouin linewidth (a) and line frequency (b) of the biorelevant aqueous solutions versus solute concentration of nonaqueous components: glycine (down triangles), triglycine (up triangles), glycerol (squares), sucrose (circles), lysozyme (diamonds), BSA (right triangles), gelatin (left triangles). The solid lines are predictions from Eq. (5) for (a) and Eq. (4) for (b). The dashed line in (a) is a fit from Eq. (6); in (b) it is an average between Eqs. (4) and (8) predictions. The dotted line in (b) corresponds to the aqueous solution of ethanol according to the sound velocities reported in [22].

Figure 3

Brillouin linewidth (a) and line frequency (b) of biorelevant aqueous solutions versus viscosity of nonaqueous components: glycine (down triangles), triglycine (up triangles), glycerol (squares), sucrose (circles), lysozyme (diamonds), BSA (right triangles), gelatin (left triangles). Crossed circles and star correspond to egg white and cell cytosol, respectively.

Figure 4

Brillouin line frequency of the biorelevant aqueous solutions versus concentration of nonaqueous components. Symbols are the same as in Fig. 2. The solid line is the prediction from Eq. (4) and the dashed line is the prediction from Eq. (A8). The inset represents the geometry of the biphasic model.