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.