Majumder et al fabricate membranes constructed from vertically aligned CNTs ~7 nm in diameter embedded in a polystyrene matrix and investigate the effect of anionically charged functional groups at the NT open ends. They show that these charged groups increase the flux of cationic species approximately fourfold compared to their unmodified CNT membrane. The effects of an applied voltage across the NTembedded membrane are investigated in subsequent work.60 Their membranes exhibit voltage-gated control of ionic transport through the cores of the NTs. For example, at a positive bias the tethered tips are drawn into the NT, occluding the pore. They demonstrate the effectiveness of such a device using a relatively large cation, ruthenium bi-pyridine (Ru(bipy)2+3 ). For example, increasing the voltage from 0 mV to 50 mV reduces the flux of Ru(bipy)2+3 to ~25%.