Temperature–pH sensitivity of bovine serum albumin protein-microgels based on cross-linked poly(N-isopropylacrylamide-co-acrylic acid)

Monodispersed poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM/AAc) microgels with various contents of acrylic acid (AAc) and cross-linker N,N′-methylenebisacrylamide (MBAAm) were synthesized by surfactant-free emulsion polymerization. The microgels exhibited affinity of binding bovine serum albumin (BSA) with increase of AAc contents. The adsorption amount of BSA was strongly dependent on suspension pH. The maximum adsorption was observed at around pH 4.0, while both low pH and basic conditions induced the rather low adsorption. In addition, AFM images showed that the microgel particles underwent bridging aggregation by loading BSA. The resultant BSA-microgel particles became more hydrophobic after loading BSA. Importantly, the adsorption of BSA onto PNIPAM/AAc microgel was found to alter the swelling/deswelling behavior of the resultant microgel suspensions. With the increase of BSA concentration, the volume phase transition temperature of BSA-microgel particles shifted to the lower temperature.