Dual-Stimuli-Responsive L-Serine-Based Zwitterionic UCST-Type Polymer with Tunable Thermosensitivity

The synthesis of L-serine-based zwitterionic polymers, poly(L-serinyl acrylate)s (PSAs), of controllable molecular weights and low polydispersities via reversible addition-fragmentation chain transfer (RAFT) polymerization in water at 70 degrees C is described. The obtained homopolymer PSA exhibits dual responsiveness toward pH and temperature in aqueous solution. The PSA exhibits an isoelectric point near pH 2.85 where the PSA molecules exist in its zwitterionic form. In the pH range of 2.3-3.5, the aqueous PSA solution appears as a two-phase system due to the formation of insoluble aggregates through the intra- and intermolecular electrostatic interaction between the pendent ammonium and carboxylate groups of the neighboring zwitterionic PSA molecules. Furthermore, in this pH range, the two-phase PSA solution becomes one-phase upon heating, exhibiting distinct reversible upper critical solution temperature (UCST)-type phase transition. The cloud point (T-p) is found to increase with increasing molecular weights of PSAs. It is also observed that the T-p changes with changing the solution pH, exhibiting highest T-p near the isoelectric point of PSA. Addition of an electrolyte such as brine solution also affects the T-p, of PSA solution following the antipolyelectrolyte effect. Finally, fluorescein isothiocyanate (FITC) tagged PSA with dual-responsiveness is prepared by the postmodification of pendent amino groups of PSA for futuristic applications in biosensors and bioimaging.