Micro-scale kinetics and heterogeneity of a pH triggered hydrogel

Particle tracking microrheology is used to study the gelation process of the amino acid derivative fluorenylmethoxycarbonyl-tyrosine (Fmoc-Y). Gelation is triggered by the addition of glucono-delta-lactone, which gradually lowers the pH over several hours and has been observed to produce particularly clear gels compared to other gelation agents. Microrheology is shown to be effective in tracking particles in this system for incubation times up to 6 h. After this time, the particle mean-squared displacements (MSD) remain constant with incubation time. The gel point, defined as the incubation time at which particle trajectories switch from a diffusive to a sub-diffusive regime is estimated to be 3600s. MSD plots also show that gelation occurs via a percolation transition. Examining the individual particle trajectories allows the heterogeneity of the sample to be determined through various analytical techniques, specifically the Heterogeneity Ratio, HR and the non-Gaussian parameter, N. The results demonstrate that glucono-delta-lactone allows the triggering of gradual gelation on the micro-scale with an exceptionally low degree of local heterogeneity which remains approximately constant after the gel point.