Nanopore

A nanopore is a pore of nanometer size. It may, for example, be created by a pore-forming protein or as a hole in synthetic materials such as silicon or graphene.

When a nanopore is present in an electrically insulating membrane, it can be used as a single-molecule detector. It can be a biological protein channel in a high electrical resistance lipid bilayer, a pore in a solid-state membrane or a hybrid of these - a protein channel set in a synthetic membrane. The detection principle is based on monitoring the ionic current passing through the nanopore as a voltage is applied across the membrane. When the nanopore is of molecular dimensions, passage of molecules (e.g., DNA) cause interruptions of the "open" current level, leading to a "translocation event" signal. The passage of RNA or single-stranded DNA molecules through the membrane-embedded alpha-hemolysin channel (1.5 nm diameter), for example, causes a ~90% blockage of the current (measured at 1 M KCl solution).[1]

It may be considered a Coulter counter for much smaller particles.

Biological and protein nanopores

Nanopores may be formed by pore-forming proteins,[2] typically a hollow core passing through a mushroom-shaped protein molecule. Examples of pore-forming proteins are alpha hemolysin and MspA porin. In typical laboratory nanopore experiments, a single protein nanopore is inserted into a lipid bilayer membrane and single-channel electrophysiology measurements are taken.

Solid state nanopores

Solid-state nanopores are generally made in silicon compound membranes, one of the most common being silicon nitride. Solid-state nanopores can be manufactured with several techniques including ion-beam sculpting[3] and electron beams.[4]

More recently, the use of graphene[5] as a material for solid-state nanopore sensing has been explored. Another example of solid-state nanopores is a box-shaped graphene (BSG) nanostructure.[6] The BSG nanostructure is a multilayer system of parallel hollow nanochannels located along the surface and having quadrangular cross-section. The thickness of the channel walls is approximately equal to 1 nm. The typical width of channel facets makes about 25 nm.

Nanopore measurement in track etched membranes

Since the discovery of track-etched technology in the late 1960s, filter membranes with needed diameter have found application potential in various fields including food safety, environmental pollution, biology, medicine, fuel cell, and chemistry. These track-etched membranes are typically made in polymer membrane through track-etching procedure, during which the polymer membrane is first irradiated by heavy ion beam to form tracks and then cylindrical pores or asymmetric pores are created along the track after wet etching.

As important as fabrication of the filter membranes with proper diameters, characterizations and measurements of these materials are of the same paramount. Until now, a few of methods have been developed, which can be classified into the following categories according to the physical mechanisms they exploited: imaging methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM); fluid transport such as bubble point and gas transport; fluid adsorptions such as nitrogen adsorption/desorption (BEH), mercury porosimetry, liquid-vapor equilibrium (BJH), gas-liquid equilibrium (permoporometry) and liquid-solid equilibrium (thermoporometry); electronic conductance; ultrasonic spectroscopy; and molecular transport.

More recently, the use of light transmission technique [7] as a method for nanopore size measurement has been proposed.

Ion current rectification

Ion current rectification (ICR) is an important phenomenon for nanopore. Ion current rectification can also be used as a drug sensor [8][9] and be employed to investigate charge status in the polymer membrane.[10]

Nanopore based sequencing

The observation that a passing strand of DNA containing different bases results in different blocking levels has led to the nanopore sequencing hypothesis. Oxford Nanopore Technologies and Professor Hagan Bayley's laboratories have shown identification of individual nucleotides including methylated cytosine as they pass through a modified hemolysin nanopore.[11]

Apart from rapid DNA sequencing, other applications include separation of single stranded and double stranded DNA in solution, and the determination of length of polymers. At this stage, nanopores are making contributions to the understanding of polymer biophysics, as well as to single-molecule analysis of DNA-protein interactions.

Size tunable nanopores

Size-tunable elastomeric nanopores have been fabricated, allowing accurate measurement of nanoparticles as they occlude the flow of ionic current.This measurement methodology can be used to measure a wide range of particle types. In contrast to the limitations of solid-state pores, they allow for the optimisation of the resistance pulse magnitude relative to the background current by matching the pore-size closely to the particle-size. As detection occurs on a particle by particle basis, the true average and polydispersity distribution can be determined.[12][13] Using this principle, the world's only commercial tunable nanopore-based particle detection system has been developed by Izon Science Ltd. The box-shaped graphene (BSG) nanostructure can be used as a basis for building devices with changeable pore sizes.[6]

Alternative definition

These can be about 20 nm in a diameter. They are integrated into artificially constructed encapsulated cells of silicon wafers. These pores allow small molecules like oxygen, glucose and insulin to pass however they prevent large immune system molecules like immunoglobins from passing. As an example, rat pancreatic cells are microencapsulated, they receive nutrients and release insulin through nanopores being totally isolated from their neighboring environment i.e. foreign cells. This knowledge can help to replace nonfunctional islets of Langerhans cells in the pancreas (responsible for producing insulin), by harvested piglet cells. They can be implanted underneath the human skin without the need of immunosuppressants which put diabetic patients at a risk of infection.

See also

References

  1. ↑ Akeson M, Branton D, Kasianowicz JJ, Brandin E, Deamer DW (December 1999). "Microsecond time-scale discrimination among polycytidylic acid, polyadenylic acid, and polyuridylic acid as homopolymers or as segments within single RNA molecules". Biophys. J. 77 (6): 3227–33. doi:10.1016/S0006-3495(99)77153-5. PMC 1300593Freely accessible. PMID 10585944.
  2. ↑ Bayley H (June 2009). "Membrane-protein structure: Piercing insights". Nature. 459 (7247): 651–652. doi:10.1038/459651a. PMID 19494904.
  3. ↑ Li J, Stein D, McMullan C, Branton D, Aziz MJ, Golovchenko JA (July 2001). "Ion-beam sculpting at nanometre length scales". Nature. 412 (6843): 166–9. doi:10.1038/35084037. PMID 11449268.
  4. ↑ Storm AJ, Chen JH, Ling XS, Zandbergen HW, Dekker C (August 2003). "Fabrication of solid-state nanopores with single-nanometre precision". Nat Mater. 2 (8): 537–40. doi:10.1038/nmat941. PMID 12858166.
  5. ↑ Garaj S, Hubbard W, Reina A, Kong J, Branton D, Golovchenko J (September 2010). "Graphene as a sub-nanometer trans-electrode membrane". Nature. 467 (7312): 190–3. doi:10.1038/nature09379. PMC 2956266Freely accessible. PMID 20720538.
  6. 1 2 R. V. Lapshin (2016). "STM observation of a box-shaped graphene nanostructure appeared after mechanical cleavage of pyrolytic graphite" (PDF). Applied Surface Science. Netherlands: Elsevier B. V. 360: 451–460. doi:10.1016/j.apsusc.2015.09.222. ISSN 0169-4332.
  7. ↑ Li Yang; Qingfeng Zhai; Guijuan Li; Hong Jiang; Lei Han; Jiahai Wang; Erkang Wang (October 2013). "Light Transmission Technique for Pore Size Measurement in Track-Etched Membranes". Chemical Communication. 49 (97): 11415–7. doi:10.1039/c3cc45841e. PMID 24169442.
  8. ↑ JiaHai Wang; Charles R Martin (2008). "A new drug-sensing paradigm based on ion-current rectification in a conically shaped nanopore". Nanomedicine. 3 (1): 13–20. doi:10.2217/17435889.3.1.13. PMID 18393663.
  9. ↑ Zhijun Guo; Jiahai Wang; Erkang Wang (October 2013). "Selective discrimination of small hydrophobic biomolecules based on ion-current rectification in conically shaped nanochannel.". Talanta. 89: 253–7. doi:10.1016/j.talanta.2011.12.022. PMID 22284488.
  10. ↑ Zhijun Guo; Jiangtao Ren; Jiahai Wang; Erkang Wang (October 2013). "pH-reversed ionic current rectification displayed by conically shaped nanochannel without any modification". Nanoscale. 3 (9): 3767–73. doi:10.1039/c1nr10434a. PMID 21826328.
  11. ↑ Clarke J, Wu HC, Jayasinghe L, Patel A, Reid A, Bayley H (2009). "Continuous base identification for single-molecule nanopore DNA sequencing". Nature Nanotechnology. 4 (4): 265–270. doi:10.1038/nnano.2009.12. PMID 19350039.
  12. ↑ G. Seth Roberts, Darby Kozak, Will Anderson, Murray F. Broom, Robert Vogel and Matt Trau. Tunable Nano/Micropores for Particle Detection and Discrimination: Scanning Ion Occlusion Spectroscopy". Small (2010) - Volume 6, Issue 23, pages 2653–2658.
  13. ↑ Stephen J. Sowerby, Murray F. Broom, George B. Petersen. "Dynamically resizable nanometre-scale apertures for molecular sensing" Sensors and Actuators B: Chemical Volume 123, Issue 1 (2007), pages 325-330

Further reading

External links and references

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