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

Abstract

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 in the nanopore based on Coulter counting method that observes change of ion current during 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 entire structure uniformity of the CNT offers an ideal environment for observing molecular transport phenomena. Until now, there have been several promising studies on detection of individual molecule and ion in CNT ion channels, but further studies are challenging due to the poor reproducibility of platforms and low detection efficiency. As a solution of these problems, we have developed a new CNT nanopore platform which has the high fabrication yield and detection efficiency. Horizontal arrays of CNTs are synthesized in 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 identical length and diameter of the CNT channel. Then, the membrane is attached onto a glass capillary to fabricate a membrane – capillary assembly which is a freely movable measurement platform. Through the platform, we observe translocation events of individual solvated cation K+, Na+, and Li+ in a single-walled CNT (SWNT) having 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+ > Na+ > Li+). At the second part, molecular transport phenomena in a CNT channel is reported. In this study, the polyethylene glycol (PEG), one of the widely used water soluble molecule, is translocated through a multi-walled CNT (MWNT) having 3.36 nm diameter. Since the molecule is neutral, electroosmotic flow induced by negative surface charge of CNT drives transport of the molecule. Here, in order to understand effects of the molecular properties on 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 exponential relationship with molecular weights. Furthermore, 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. At the last part, we demonstrate single-cell analysis by detecting neuropeptides released from a neuron. For experiment, we extract each bag cell neuron from an animal model Apysia kurodai, and culture the cell in a PDMS micro-well containing artificial sea water (ASW) and antibiotics. Chemical stimulation with high concentration of K+ contents induces depolarization of the cell membrane releasing neuropeptides. Under constant electric field, the released neuropeptides are transported toward a counter electrode through a CNT ion channel with 2.8 nm diameter depending on their intramolecular net charge. In order to identify each neuropeptide from a cell releasate, we collect and analyze signals from artificially synthesized standard peptides as references. Combination of channel blockage properties offers clue for identification of each molecule.