Effect of nanoporous structure and polymer brushes on the ionic conductivity of poly(methacrylic acid)/anode aluminum oxide hybrid membranes

Anode aluminum oxide (AAO) porous materials have been widely used in ionic translocation for many biological and chemical studies. However, the lack of stimuli-response of this material limits its applications for the precise control of ionic transportation by the external environment. In this study, we functionalized the internal nanopores of AAO membranes to generate polyelectrolyte-filled pH-responsive membranes whose ionic conductivity could be readily controlled by changing the pH value. AAO membranes with different pore sizes (25, 75 and 100 nm) were modified with poly(methacrylic acid) (PMAA) by a "grafting-to" approach. Thermogravimetric and SEM analysis revealed that the extent of PMAA infiltration strongly depends upon the relative sizes of the nanopores and the PMAA concentration. Increasing the size of the nanopores enables the infiltration of PAA solution with a higher concentration. Electrochemical impedance spectroscopy demonstrated that the membrane conductivity decreases from 7.87 × 10^-4 S cm^-1 at pH 1 to 5.72 × 10^-5 S cm^-1 at pH 7. The functionalized AAO nanopores showed significant sensitivity to pH value, whereas a valve effect was observed in the pH range between 4 and 5. Our fabricated PMAA-AAO membranes show promising potential to be used as pH sensors and smart valves in micro-/nano-total analysis chips for biomedical and chemical applications.