Fabrication of thermo-responsive nano-valve by grafting-to in melt of poly(N-isopropylacrylamide) onto nanoporous silicon nitride membranes

• Successful grafting of PNIPAM onto ultrathin nanoporous silicon nitride membranes.
• Thermo responsiveness of grafted PNIPAM polymer brushes demonstrated.
• Nanovalve behavior with reversible opening and closing of the pores demonstrated.
• Membrane with temperature controlled cut-off down to 3 nm hydrodynamic radius.

This article reports the grafting-to in melt of amino and carboxy-terminated poly(N-isopropylacrylamide) (PNIPAM) onto ultrathin nanoporous silicon nitride membranes (NSiNMs) with a mean pore size of 85±15 nm functionalized respectively with a self-assembled monolayer (SAM) of isocyanatopropyltriethoxysilane (ICPES) and glycidylpropyloxytrimethoxysilane (GPS) to achieve permeation control and obtain thermo-responsive nano-valves. The presence of PNIPAM onto the surface was controlled via X-ray photoelectron spectroscopy (XPS) analysis and the grafting characteristics such as the grafting density and the distance between grafting sites were calculated from the ellipsometric measurements of the dry PNIPAM films. The modified NSiNMs were then integrated into a diffusion cell and used for vitamin B12 and fluorescein isothiocyanate-dextrans (FITC-dextrans) diffusion experiments to demonstrate the reversible and reproducible thermo-responsive behavior of the whole device. By controlling the temperature of the system below and above the lower critical solution temperature (LCST) of the PNIPAM (32 °C), the nanopores can switch from a closed state (swollen state of the chains below the LCST) to an opened state (collapsed state of the chains above the LCST). The clear ON-OFF behavior of the grafted NSiNMs has been observed for the diffusion of FITC-dextran molecules with a hydrodynamic radius superior to 3 nm.

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