Specific Binding of Ligand-Functionalized Thermoresponsive Microgels: Effect of Architecture, Ligand Density, and Hydrophobicity.

The biomolecular interaction of ligand-presenting switchable microgels is studied with respect to the polymer type, composition, and structure of the microgels. Monodisperse microgels are prepared through precipitation polymerization of N -isopropylacrylamide (PNIPAM microgels) or oligo(ethylene glycol methacrylamide)s (POEGMA microgels) in the presence of crosslinkers or in their absence (self-crosslinked). Functionalization with mannose or biotin as model ligands and affinity measurements upon heating/cooling are conducted to obtain mechanistic insights into how the microgel phase transition affects the specific interactions. In particular, we are interested in adjusting the crosslinking, swelling degree, and ligand density of mannose-functionalized microgels to reversibly catch and release mannose binding Escherichia coli by setting the temperature below or above the microgels' volume phase transition temperature (VPTT). The increased mannose density for collapsed microgels above the VPTT results in stronger E. coli binding. Detachment of E. coli by reswelling the microgels below the VPTT is achieved only for self-crosslinked microgels showing a stronger decrease in ligand density compared to microgels with dedicated crosslinkers. Owing to a reduced mannose density in the shell of POEGMA microgels, their E. coli binding was lower compared to PNIPAM microgels, as supported by ultraresolution microscopy. Importantly, an inverse temperature-controlled binding of microgels decorated with hydrophilic mannose and hydrophobic biotin ligands is observed. This indicates that hydrophobic ligands are inaccessible in the collapsed hydrophobic network above the VPTT, whereas hydrophilic mannose units are then enriched at the microgel-water interface and thus are more accessible.