Transport Of Amino Acid Cations Through A 2.25-Nm-Diameter Carbon Nanotube Nanopore: Electrokinetic Motion And Trapping/Desorption

In this study, we examine the individual ionic transport of four different amino acid cations through a 2.25-nm-diameter, 200-mu m-long single-walled carbon nanotube (SWNT) and compare them to previous work on Li+ and methanol in the same SWNT. For the amino acids, the pore-blocking current was found to increase with applied field, from 1 to 100 pA. The observed differences in pore-blocking currents of the different amino acids are mostly explained by a simple model of the volumes of the blockers. A threshold voltage was found for the amino acids to exhibit pore blocking, in agreement with previous results for monatomic ions and molecules which also encounter an energy barrier for entry into an SWNT. The average dwell time for each of the amino acids was found to be relatively constant for applied voltages between 400 and 1000 mV. Additionally, the distribution of dwell times for each amino acid exhibited a tail at long dwell times. These results are well-modeled by an adsorption/desorption model in which the amino acid molecules interact with the nanotube wall as they pass through the tube. Our results suggest that comparisons of conductance changes and mobilities could be used in some cases to identify the type of amino acid, although the identification might not be unambiguous.