Permselective Ion Transfer at the Nanoscopic Ities Array

Abstract

Free-standing silica nanochannel membrane (SNM) with perforated channels was utilized to create arrays of nanoscale interfaces between two immiscible electrolyte solutions (nano-ITIES), at which permselective ion transfer and detection were achieved. The SNM consisted of a high density of straight nanochannels with a diameter of 2.3 nm and a length of 70 nm. The silicon wafer coated by 150-nm-thick porous silicon nitride film (p-SiNF) with pores of 5 mm-in-diameter was used to support the SNM in a form of nanochannel-on-micropore. Considering the material surface lipophilicity, the nano-ITIES array was formed at the boundary between SNM and p-SiNF, with a diffusion geometry equivalent to two back-to-back inlaid microdisc interfaces. Thus, the transfer of tetraethylammonium (TEA+) across the nano-ITIES array yielded symmetric sigmoidal current responses in cyclic voltammogram. In addition, due to the ultrasmall size and negatively charged surface of silica nanochannels, the nano-ITIES displayed obvious size and charge permselectivities. Transfer of ions with a size comparable with or larger than the nanochannel, such as meso-tetra(N-methyl-4-pyridyl) porphine and iron(III) meso-tetra(4-sulfonatophenyl)porphine, was sterically blocked. And that of anions with a size smaller than the nanochannels, such as methyl orange, encountered the strong electrostatic repulsion from channel walls, showing obvious dependence on the ionic strength of aqueous solution in accordance with the prediction of classical Gouy-Chapman theory. The present approach is facile and inexpensive for building nano-ITIES array with potential applications in ion detection and separation. Figure 1