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.
Contents
- Biological and protein nanopores
- Solid state nanopores
- Nanopore measurement in track etched membranes
- Ion current rectification
- Nanopore based sequencing
- Size tunable nanopores
- Alternative definition
- References
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.
Biological and protein nanopores
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.
More recently, the use of graphene as a material for solid-state nanopore sensing has been explored. Another example of solid-state nanopores is a box-shaped graphene (BSG) nanostructure. 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 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 and be employed to investigate charge status in the polymer membrane.
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.
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. The box-shaped graphene (BSG) nanostructure can be used as a basis for building devices with changeable pore sizes.
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.