High-Yield Analysis of Individual Ions and Molecules Through the Interior of Carbon Nanotubes

<b>The nanopore technique is one of the promising tools for single-molecule analysis due to its comparable size to a molecule. Each molecule can be analyzed based on the Coulter counting method that observes the change of ion current during the translocation of the molecule through the pore. Recently, the interior of carbon nanotube (CNT) has been used as a nanopore material because of its nanoscale dimensions, atomically smooth and contaminant-free inner surface, and large slip length. The high aspect ratio with the entire structure uniformity of the CNT offers an ideal environment for observing molecular transport phenomena. Until now, there have been several studies on the CNT ion channels as Coulter counter, but further studies are challenging due to the poor reproducibility of platforms and low detection efficiency. To solve these problems, we have developed a new CNT nanopore platform with a high fabrication yield and detection efficiency. Horizontal arrays of CNTs are synthesized on a centimeter scale by chemical vapor deposition (CVD). After characterizing each CNT, one of the CNTs is isolated and embedded in a polymer matrix followed by slicing into numerous membranes containing an identical length and diameter of the CNT channel. Then, the membrane is attached to a glass capillary to fabricate a membrane – capillary assembly, which is a freely movable measurement platform. We observe translocation events of individual solvated cation K+, Na+, and Li+ in a single-walled CNT (SWNT) with a 1.28 nm diameter. Electrophoretic mobility of each cation is estimated 7.6 × 10−8, 5.2 × 10−8, and 3.9 × 10−8 m2 Vs−1 for K+, Na+, and Li+, respectively (K+ &gt; Na+ &gt; Li+). Beyond the detection of solvated ions, molecular transport phenomena in a CNT channel are reported. In this study, the polyethylene glycol (PEG), one of the widely used water-soluble molecules, is translocated through a multi-walled CNT (MWNT) having a 3.36 nm diameter. Since the molecule is neutral, electroosmotic flow, induced by the negative surface charge of CNT, drives the transport of the molecule. Here, in order to understand the relationship between the molecular properties and the molecular transport phenomena, various molecular weights (200, 600, 1000, and 1500 g/mol) of PEG are tested. At the constant electric field, electroosmotic velocities of PEG molecules are calculated as 2.77 × 10-4, 1.32 × 10-4, 5.00 × 10-5, and 1.74 × 10-5 m/s for PEG-200, 600, 1000, and 1500, respectively, which has an exponential relationship with molecular weights. Furthermore, the change of molecular conformation in the nano-confined space is studied by molecular dynamic (MD) simulation. According to the simulation, the molecule is attracted to the CNT wall and stretched due to the interaction between the molecule and the CNT wall.</b>