Pore Conductivity Control at the Hundred‐Nanometer Scale: An Experimental and Theoretical Study

Abstract We report on the observation of an unexpected mechanism that controls conductivity at the 100‐nm scale on track‐etched polycarbonate membranes. Transport measurements of positively charged methyl viologen performed by absorption spectroscopy under various pH conditions demonstrate that for 100‐nm‐diameter pores at pH 2 conductivity is blocked, while at pH 5 the ions move through the membrane according to diffusion laws. An oppositely charged molecular ion, naphthalene disulfonate, in the same membrane, shows the opposite trend: diffusion of the negative ion at pH 2 and very low conductivity at pH 5. The influence of parameters such as ionic strength and membrane surface coating are also investigated. A theoretical study of the system shows that at the 100‐nm scale the magnitude of the electric field in the vicinity of the pores is too small to account for the experimental observations; rather, it is the surface trapping of the mobile ion (Cl⁻ or Na⁺) that gives rise to the observed control of the conductivity. This surprising effect has potential applications for high‐throughput separation of large molecules and bio‐organisms.