Membranes have ubiquitous application ranging from food, agricultural industry, potable water production, water treatment, biotechnology, and nanofiltration to desalination of sea water and saline aquifer. The challenge of current membrane technology is to combine two antagonistic properties: high ion permittivity and selectivity with the same efficiency as biological cell membranes having well-controlled ion transport and outstanding permeability and selectivity. This will help in the creation of tunable nanodevices such as microfluidic transistors, diodes, and sensors that can differentiate between ions and recognize single molecules. Two avenues have been explored in the past. One is confining artificial water channel aquaporin and Gramicidin A in a copoly-mer matrix, but this lacks the required mechanical stability. The second was by using a mechanically strong inorganic material like a carbon nanotube as a confining matrix; however, this approach was never tried in reality. Recently, researchers from Montpellier and Franche-Comte Universities, France have prepared a hybrid nanoporous membrane by confining Gramicidin A (GA) in cylindrical nanopores of track-etched polycarbonate thin film (see Figure) and obtained enhanced ionic permeability.

As reported in the February 9th issue of Nano Letters (DOI: 10.1021/nl103841m; p. 712), S. Balme and co-researchers used hydrophobic polyvinylpyrrolidone (PVP)-coated track-etched polycarbonate membranes which are 5 μm thick with a nanopore diameter of 15 nm and a density of 7 × 10⁸ per cm³. These were treated in ethanol to make the outer surface less hydrophobic compared to the inner surface since GA prefers a hydrophobic surface. When this ethanol-treated membrane was soaked in a GA-containing solution for 72 hours, the GA molecules become attached inside the nanopores.

The bioinspired hybrid membrane shows the orientation of Gramicidine A inside the cylindrical nanopores. Reproduced with permission from Nano Lett. 11 (2)(2011), DOI: 10.1021/nl103841m; p. 712. © 2011 American Chemical Society.

Fluorescent signal measurement using labeled protein confirmed that GA is uniformly inserted throughout the nanopores of the membrane and in a higher concentration for an ethanol-treated membrane. GA-impregnated membranes exhibited improved ion diffusion for 10⁻¹, 10⁻², and 5 × 10⁻³ mol L⁻¹ solution of Na⁺, K⁺, Ca⁺, and Mg⁺ chlorine solutions. Furthermore, molecular dynamics simulation revealed that double-stranded (ds) GA dimer conformation is more stable inside the nanopores than a single-stranded (ss) one. Since ds-dimers can accommodate either monovalent or divalent ions, these hybrid membranes lack ion selectivity as compared to the biological membranes where GA is mostly single stranded.

According to the researchers, this work opens a promising avenue for research in nanobiofiltration and tunable nanodevices with differential ion conduction.