Experimental measurements in single-nanotube fluidic channels
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Introduction Carbon nanotubes (CNTs) are well suited for nanofluidic channels1–3 owing to their remarkable structural features, including nanoscale diameter, high-aspect ratio, and structural uniformity. The graphitic inner wall of a CNT provides atomic smoothness and hydrophobicity that contribute to a nonzero velocity along the wall. Molecules passing through a CNT can attain average velocities that are orders of magnitude higher than the values estimated by the Hagen–Poiseuille Law, making the experimental measurement of flux possible even for a single CNT. A unique feature of CNT-based nanofluidic channels is that unlike protein-ion channels, which close and open via conformational changes, CNT channels maintain a structurally open state, so the role of a blocking species is crucial when characterizing molecular transport. The blockers tested thus far are molecular species with sizes comparable to the diameter of CNTs, including dyes,4 DNA,5,6 nanoparticles,7 and hydrated ions.8,9 Recent advances in CNT research allow us to control or measure the diameter, length, and pore properties of CNTs, hence CNT fluidic channels have great potential for singlemolecule studies, which have been challenging to conduct reliably using other nanofluidic systems. Several studies have addressed molecular transport through the small diameter of a CNT channel, proposing applications
in desalination of seawater,10 molecular filtering, drug delivery,11 and nanoscale reactions.12 Most of the systems studied were composed of large numbers of CNTs that leave ambiguities in understanding the molecular transport. Single-CNT channels enable the study of single-molecule transport through CNTs with known diameters, lengths, and electrical properties, and thus have become promising platforms for further nanofluidics studies. The design and fabrication of a reliable platform for experimental measurements of a single-CNT channel, however, remains challenging, owing to the small dimensions of these systems. In this article, we first introduce single-CNT platforms for experimentally measuring transport of various species along the interior of a CNT, including hydrated ions, DNA, and nanoparticles. We then discuss stochastic pore-blocking events from single-CNT channels and the information that can be extracted from the events. We also describe optical detection of molecular translocation through the nanochannel. Finally, we discuss future research directions of nanopore studies and the challenges to overcome.
Experimental platforms for studying singlenanotube fluidic channels Isolating a single carbon nanotube and assembling it into an experimental platform may sound like a daunting task.
Hyegi Min, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, South Korea; [email protected] Yun-Tae Kim, School of Life Sciences, Ulsan National Institute of Science and Technology, South Korea; [email protected] Chang Young Lee, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology
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