Reentrant phase transition behavior and sensitivity enhancement of a molecular recognition ion gating membrane in an aqueous ethanol solution

The response behavior of a molecular recognition ion gating membrane to an ion signal was investigated in an aqueous ethanol solution. The linear poly-N-isopropylacrylamide (NIPAM)-co-benzo[18]-crown-6-acrylamide (BCAm) copolymer showed phase separation in response to variation in ethanol mole fraction in the mixed solvent of water and ethanol, which is called a reentrant coil-to-globule-to-coil transition. The phase diagram of poly-NIPAM-co-BCAm changed in response to the concentration of a specific ion signal. Based on this reentrant phase transition behavior of the copolymer, the gating membrane, which was prepared by grafting the copolymer of NIPAM and BCAm onto the surface of a porous polyethylene film, opened and closed its pores by the swelling and shrinking of the grafted copolymer in an aqueous ethanol solution. In addition, the complex formation constant of BCAm increased with increasing ethanol mole fraction, which enhanced the response sensitivity and selectivity to ion species; thus, the sensitivity to BaCl2 was 10 times higher in 19.6 mol% aqueous ethanol solution than it was in pure water.