Controlled ion permeability of ultrathin nanoporous SiO₂ films from silsesquioxane-containing polymer nanosheets

Bottom-up construction of nanoporous thin films with nanometer-scale controllability of film thickness and pore size persists as a key challenge for molecular separation and sensing materials with effective and selective molecular transportation ability. This report describes the controlled ion permeability of ultrathin nanoporous SiO₂ thin films with nanometer-scale controllability of the film thickness and pore size prepared from blend polymer Langmuir−Blodgett films (nanosheets) consisting of poly(N-dodecyl acrylamide) (pDDA) and silsesquioxane (SQ)containing random copolymer (p(DDA/SQ26)). Nanoporous SiO₂ thin films were prepared using photo-oxidation methods of p(DDA/SQ26) blend nanosheets under ambient conditions. The SiO₂ nanofilms exhibited tunable nanoporous structures with a wide range of porosity from 0.06 to 0.79. The film thickness was controlled to ca. 0.4 nm layer⁻¹ using varying numbers of deposition cycles. Furthermore, the average pore size was controlled with nanometer-scale accuracy by varying the p(DDA/SQ26) random copolymer ratio. Consequently, the size-selective ion permeability of nanoporous SiO₂ thin films was demonstrated by using two redox probes [Fc(CN)₆³⁻ and Ru(NH₃)₆²⁺], which have a similar molecular size. Cyclic voltammetry measurements revealed that the two probes exhibited the same permeation behavior in terms of the relative current density at high ionic strength conditions. These results demonstrate that ultrathin nanoporous SiO₂ films converted from p(DDA/SQ26) blend nanosheets are promising for the effective and selective molecular transportation for molecular separation and sensing applications.