A nanopore is a very small hole. 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).
It may be considered a Coulter counter for much smaller particles.
Nanopores may be formed by pore-forming proteins, 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 and electron beams.
Nanopores may also be used to identify analytes other than DNA. Professor Hagan Bayley’s Research team at the University of Oxford has published research that uses protein nanopores to differentiate between enantiomers of small molecules such as ibuprofen and thalidomide, identify specific biomarkers and screen ion channels. These might have broader applications in clinical medicine and drug development.
Nanopore measurement in track etched membrane
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 transports such as Bubble Point11 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;19 Molecular Transport.
More recently, the use of light transmission technique  as a method for nanopore size measurement has been proposed.
Ion current rectification for analytical chemistry
Ion current rectification is a very important phenomenon for nanopore. Siwy research group has contribute significantly to understand this experimentally and theoretially. Jiang lei group has been pioneering in constructing smart switching nanopore via incorporating intelligent polymer or molecules into the nanopore.
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.
Such sequencing, if successful, could revolutionize the field of genomics, as sequencing would be simplified and have the potential for dramatic improvements in power and cost over current versions that use fluorescence/luminescence and optical instrumentation to detect this photon signal. 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. Using this principle, the world's only commercial tunable nanopore-based particle detection system has been developed by Izon Science Ltd.
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. This way 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.
Inspired by the biological ion channel, a synthetic film with a single nanopore structure was prepared as described by Apel et al. Unlike the fragile lipid-bilayer membrane in which most natural ion channels are embedded, this synthetic film is mechanically and chemically robust. Even though ion channels in living organisms have been studied by a mimic method using synthetic nanopores during the past several decades, how to endow these synthetic nanopores with intelligence is still a challenging task. Prof. Lei Jiang and his colleagues extend the function of molecule — nanopore systems by using G-quadruplex DNA. In their biomimetic nanochannel system, there is an ion concentration effect, which is a very important phenomenon in a living body and other systems do not have. Their novel biomimetic nanochannel system was responsive to potassium ion within a certain concentration range and simulated these processes in a pH-neutral environment as in a natural organism. In their work, the situation of the grafting G-quadruplex DNA on a single nanopore can closely imitate the in vivo condition because the G-rich telomere overhang is attached to the chromosome. Therefore, their artificial system could promote a potential to conveniently study biomolecule conformational change in confined space by the current measurement, which is significantly different from the nanopore sequencing. Moreover, such a system may also potentially spark further experimental and theoretical efforts to simulate the process of ion transport in living organisms and can be further generalized to other more complicated functional molecules for the exploitation of novel bioinspired intelligent nanopore machines.
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