Influence of Electrostatic Interactions on Particle Tracking for Microrheology Analysis

Nur Hamideh^a, Yi-Syuan Guo^a, Leslie M. Shor^a,b

^a Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT USA

^b Center for Environmental Sciences and Engineering, University of Connecticut, Storrs, CT USA

Particle-tracking microrheology can be used to measure solution viscosity in-situ with high spatial resolution. In this technique, thermally-induced Brownian motions of fluorescent nano- or micro-sized spherical particles are tracked using an optical microscope. Viscoelastic drag on the spherical particles reduces the mean squared displacement of particles based on the Stokes-Einstein relation. This technique is increasingly used in biological applications using a range of different viscoelastic solutions, tracer particles, and chamber materials and dimensions. Here we evaluate accuracy of particle-tracking microrheology measurements as a function of charge interactions between tracer particles and the sample chamber. Two kinds of fluorescent tracers were used: 0.5 μm diameter negatively-charged carboxylated polystyrene (PS) beads and 0.2 μm diameter positively-charged aminated PS beads. Experiments were performed in an experimental assembly comprised of two glass microscope slides sealed with silicone grease. Both glass surfaces were pre-treated by applying 25 μL aliquots of four different types of biological solutions (tryptic soy broth, lysogeny broth, yeast extract, and bovine serum albumin), spread with a sterile swab, and allowed to dry. Particle trajectories of 50-80 particles were quantified in 40% glycerol-water solution and 58 μg/mL exopolysaccharide (EPS) solution using a Zeiss Axio Observer.Z1 inverted microscope and 40x objective, then analyzed using Fiji ImageJ and MatLab. Deviations between viscosity measured using particle tracking microrheometry and viscosity measured using traditional bulk-scale viscometry will be presented. These results will give insights into modulation of charge interactions by biological solution surface treatment in particle tracking microrheology.