Optical Microrheology of Protein Solutions Using Tailored Nanoparticles

Abstract

This work represents a critical re-examination of the application of dynamic light scattering (DLS)-based tracer particle microrheology to measure the zero shear viscosity of aqueous solutions of different proteins up to very high concentrations. It is demonstrated that a combination of surface-functionalized tracer particles, the use of the so-called 3D-DLS technique, and carefully chosen parameters for the scattering experiments is essential for a reliable and artifact-free determination of the viscosity of highly diverse protein solutions, while keeping the amount of protein to a minimum. The major challenges that arise in such microrheology experiments with protein solutions are discussed and used as guiding principles for the synthesis of all-purpose tracer particles with optimal size and an efficient surface functionalization, and the choice of the appropriate amount of tracers in the sample. Potential problems arising from depletion attractions between the tracer particles induced by the proteins are addressed, and compelling evidences for the absence of such effects are presented. The validity of the approach is corroborated by the perfect agreement between the zero shear viscosity obtained from 3D-DLS-based microrheology and literature data from classical rheological measurements for two vastly different protein-solvent systems up to concentrations close to the arrest transition.

Spectroscopic Analysis of Protein‐Crowded Environments Using the Charge‐Transfer Fluorescence Probe 8‐Anilino‐1‐Naphthalenesulfonic Acid

“…The viscosity (black circles) in Figure 3(a) is the experimental value by the 3D-DLS technique, which can provide the precise measurement from lower to higher viscosity by suppressing multiple scattering, using 3D cross correlation technique. [35] The 3D-DLS-based viscosity shows the good agreement with the classical rheological measurement. The viscosity (black line) in Figure 3(a) is the theoretical value by the Ross-Minton equation, which predicts the viscosity (η) in highly concentrated solutions by including short-ranged intermolecular interaction,…”

“…The viscosity (black circles) in Figure (a) is the experimental value by the 3D‐DLS technique, which can provide the precise measurement from lower to higher viscosity by suppressing multiple scattering, using 3D cross correlation technique . The 3D‐DLS‐based viscosity shows the good agreement with the classical rheological measurement.…”

“…The red and blue dashed lines are the fitting results of the positions and intensities of peak maximum as a function of the concentration, respectively. The black circles and black line in Figure 3(a) show the concentration dependences of the experimental value of viscosity from the reference [35] and the theoretical value of viscosity estimated by the Ross-Minton equation, respectively. [36,37] The horizontal axis is set to a logarithmic scale to compare the results in higher concentrations clearly.…”

“…Since the viscosity is correlated to the excluded volume effect, the ratio of MCR score against viscosity is plotted in Figure 10(b). The viscosity is the experimental data from the reference, [35] which was also employed in Figure 3(a). According to the Förester-Hoffmann equation, [52,53] the ratio of MCR scores (I ratio ) and viscosity (η) are directly related through ð4Þ where A and x are constant depending on protein solution.…”

Microrheology with optical tweezers: peaks & troughs

“…Moreover, microrheology measurements can be performed in situ in an environment that cannot be reached by a conventional rheology experiment, for instance inside a living cell [29,30]. The 25 most popular microrheology techniques are: video particle tracking microrheology [31], diffusing wave spectroscopy [32,33], dynamic light scattering [34,35], atomic force microscopy [36,37], magnetic tweezers [38,22] and optical tweezers [39,40,41,42,43,44,45,46,47]. These are classified as either 'active' or 'passive' techniques, depending on whether the particle displacement is induced 30 by an external force field or generated by the thermal fluctuations of the fluid molecules surrounding the probe particle, respectively.…”