Quantitative Assessment of Molecular Transport through Sub-3 nm Silica Nanochannels by Scanning Electrochemical...

Scanning electrochemical microscopy (SECM) has been proved to be a powerful technique to study molecular transport across ionic channels in biomembranes and artificial nanoporous membranes. In this work SECM was used to study the dynamics of molecular transport across the ultrathin silica nanoporous membrane consisting of sub-3 nm diameter perpendicular channels. We focused on the quantitative assessment of permselectivity and permeability of the membrane and the effect of radial electrical double layer (EDL) on them. By SECM imaging, it was phenomenologically observed that the membrane with negatively charged surface exhibited permselectivity to anionic molecule, for instance hexacyanoruthenate(II) (Ru(CN)₆^4-). And the permselective transport of Ru(CN)₆^4- was obviously more favored at a higher concentration of KCl. Precise membrane permeability to Ru(CN)₆^4- was quantitatively determined by overlapping experimental SECM approach curves with the ones generated by finite element simulations. The high permeability up to 35 μm s^-1 was ascribed to the straight channel structure and ultrahigh channel density of 4 × 10¹² cm^-2. Moreover, the permeability was varied from 35 μm s^-1 to 2.5 μm s^-1 when decreasing the concentration of KCl from 1.0 to 0.01 M, corroborating the electrostatic origin of membrane permselectivity. On the other hand, the simulated concentration profiles at both sides of the membrane suggested that the molecular transport across the membrane was mainly driven by the large transmembrane concentration gradient while the tip-induced transport was relatively negligible. These results help to quantitatively understand the molecular transport selectivity and dynamics across nanoporous membranes and to rationally design artificial molecular sieving membranes.