Salt-assisted mechanistic analysis of chitosan/tripolyphosphate micro- and nanogel formation

Self-assembled micro- and nanogels are frequently prepared by mixing tripolyphosphate (TPP) with dilute chitosan solutions. Upon its addition, the TPP ionically cross-links the chitosan molecules into gel-like colloids that range from tens of nanometers to micrometers in diameter. These particles are biocompatible, mucoadhesive and, because they are easy to prepare under very mild conditions, attract widespread interest in the encapsulation of drugs, neutraceuticals, and other bioactive payloads. Despite their broad use, however, their formation mechanism has remained largely obscured by the very fast kinetics of their self-assembly. To this end, we have tuned the TPP and monovalent salt (NaCl) concentrations to dramatically slow down this process (to occur on the time scale of days instead of milliseconds), and then probed the evolution in the size and morphology of micro- and nanogels during their formation. This revealed that the micro- and nanogel formation rates are extremely sensitive to NaCl and TPP concentrations, and that the formation process occurs in two stages: (1) formation of small primary nanoparticles and (2) aggregation of primary particles into larger, higher-order colloids that are obtained at the end of the ionotropic gelation process.