A thermo-responsive affinity membrane with nano-structured pores and grafted poly(N-isopropylacrylamide) surface layer for hydrophobic adsorption

A novel thermo-responsive affinity membrane with nano-structured pores and grafted poly(N-isopropylacrylamide) (PNIPAM) surface layer is successfully fabricated for hydrophobic adsorption through the membrane surface wettability change tuned by environmental temperature. Shirasu porous glass (SPG) membranes with mean pore size of 1.8 μm are used as substrate membranes. Nano-structured pore surfaces are formed by depositing 125 nm SiO2 nano-particles onto the SPG membrane pore surfaces. PNIPAM brushes are then grafted on the nano-structured pore surfaces of membranes by plasma-induced grafting polymerization method. The formation and microstructures of the prepared membranes are investigated systematically by employing XPS, SEM, contact angle instrument, and mercury intrusion method. The results show that SiO2 nano-particles and PNIPAM-grafted layer are formed homogeneously on the SPG membrane pore surfaces. When the environmental temperature is 20 °C (below the lower critical solution temperature, LCST), the PNIPAM-grafted nano-structured membranes present very hydrophilic surfaces with water contact angle of 0°; on the other hand, when the environmental temperature is 40 °C (above the LCST), the PNIPAM-grafted nano-structured membranes present very hydrophobic surfaces with water contact angle of 130°. The thermo-responsive hydrophilic/hydrophobic surface wettability change of the prepared membranes is reversible and reproducible. Temperature-controlled hydrophobic-adsorption performance of the prepared membranes is investigated by studying the adsorption/desorption behavior of Bovine serum albumin (BSA) on the membrane surfaces with changing the environmental temperature across the LCST. The PNIPAM-grafted nano-structured membranes show satisfactory “adsorbing at temperature above the LCST - desorbing at temperature below the LCST” performance, and the desorption efficiency is as high as about 90%. The nano-structured architectures of the membrane pore surfaces are verified to be beneficial for the thermo-responsive hydrophobic adsorption of BSA molecules.