Electrokinetic Transport Of Methanol And Lithium Ions Through A 2.25-Nm-Diameter Carbon Nanotube Nanopore

The flow enhancement and molecular selectivity of carbon nanotubes make them unique nanopore conduits. In this study, we examine separately the transport of Li+ and methanol through a 2.25 nm-diameter, 200-mu m long single-walled carbon nanotube (SWNT). Threshold voltages of 200 mV and 700 mV were found for Li+ and methanol, respectively, to exhibit pore blocking. As the applied electric field was increased, the pore-blocking currents for Li+ and for methanol were both found to generally increase in the range of 1 to 6 pA. A simple volumetric model for methanol and hydrated Li+ is consistent with these observations. For applied voltages between 200 and 1000 mV, the dwell times for Li+ transport varied from 200 to 1200 ms and scaled linearly with inverse electric field. These results indicate an electrophoretic mobility of 1.6 x 10(7) m2 V-1 s(1), in agreement with previous measurements of alkali metal ions in SWNTs. Conversely, for applied voltages between 700 and 1000 mV, the dwell times for methanol remained relatively constant at an average of 88 ms, consistent with the expected behavior of a neutral molecule. The average velocity of methanol was found to be 2.3 x 10(3) m/s, which is in agreement with an electro-osmotic flow model of neutral molecules through a small-diameter nanopore. By comparing charged and neutral pore blocking species in this way, this approach promises to deconvolve the effects of electrokinetics and electro-osmotic flow in molecularly sized nanopore conduits.