Layer-by-layer assembled stimuli-responsive nanoporous membranes

Stimuli-responsive materials that could change their properties and structures in response to external stimuli have recently received much attention owing to their many potential applications in biosensors, drug delivery systems, actuation devices, and many others. Among various preparation methods to realize these goals, layer-by-layer (LbL) assembly, based on polyelectrolytes leading to swellable multilayers, is a powerful processing method because it can be applied to substrates of any shape and size and, at the same time, the multilayer films assembled under specific conditions show a strong swelling behavior that can be modulated in response to post-treatment conditions. For molecular-level filtration, the pore size of membranes containing cylindrical pores should be less than tens of nanometers. However, it has been regarded as a challenging task to uniformly deposit polyelectrolytes on the sidewalls of nanoporous membranes with sub-100 nm pore diameters based on the LbL deposition method due to the well-known entropic entrance barrier problem. In the present study, by introducing pH-sensitive poly(allylamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) multilayers into nanoscaled pores with less than 100 nm diameter, we could additionally reduce and control the pore diameter of anodic aluminum oxide (AAO) membranes by adjusting the post-pH treatment, yielding smart ultrafiltration (UF) or nanofiltration (NF) membranes with various cutoffs realized in one membrane. Based on pH-sensitive nanopores in membranes, the translocation behavior of small spherical molecules as well as linear polymers through the stimuli-responsive membranes has been studied in detail.