Future generations of nanopores: solid-state
Protein nanopores are robust, easily reproducible at low cost, and easy to modify. However, future generations of nanopore sensing devices are likely to use nanopores fabricated from synthetic materials: solid-state nanopores. These have the potential to improve the cost and scale of nanopore analyses even further.
Oxford Nanopore has internal R&D projects and collaborations with research groups, developing innovative solid-state nanopore technologies into a next-generation nanopore technology. These collaborations include a broad intellectual property estate for solid-state nanopore sensing platforms in a variety of forms including silicon nitride, graphene, and modifications to these solid state materials for the sensing process.
In 2008, Oxford Nanopore established a collaboration with the laboratories of Professor Daniel Branton and Professor Jene Golovchenko at Harvard University. Early pioneers of nanopore sensing, and in particular in the development of methods of solid-state sequencing, Oxford Nanopore supports research in these laboratories and licenses the right to develop nanopore discoveries into a single molecule analysis technology. In 2011, a further collaboration was announced between Harvard and Oxford Nanopore, for the development of graphene as a solid-state nanopore sequencing device.
Solid-state nanopores
A solid-state nanopore is typically a nanometer sized hole formed in a synthetic membrane (usually SiNx or SiO2). The pore is usually fabricated by focused ion beam or focused electron beam, so the size of the pore can be tuned freely, although further development is necessary to reach the atomic precision naturally achieved by protein pores. Because of the pore geometry tunability and the superior mechanical and chemical stability of solid-state membranes, considerable R&D efforts have been attracted into this field, including alternative sequencing/diagnostic strategies, new membrane material, hybrid pore and integrated sensors.
Hybrid pores
Solid-state nanopores currently lack the chemical specificity of protein nanopores. A method under exploration is the integration of a protein pore into a solid-state membrane.
Nanopores with integrated sensors
Integrated sensors have been explored as technologies to supersede methods involving ionic current measurement. Proposed techniques include tunneling electrodes based detectors, capacitive detectors and graphene based nano-gap or edge state detectors. Recently, the local voltage signal generated by DNA translocation – which is proportional to ionic current signal – has been detected by transistor in experiment. This detection scheme may be an attractive alternative to ionic current because it preserves the information of ionic current signal with the potential to achieve much higher integration density and higher speed.
Graphene
Graphene is a robust, single atom thick ‘honeycomb’ lattice of carbon with high electrical conductivity. These properties make it an ideal material for high resolution, nanopore-based sequencing of single DNA molecules. The fine depth of the graphene membrane provides optimal spatial resolution along the DNA, and at the same time, graphene is extremely strong and chemically inert. Graphene itself is also a good electronic sensor material, which is sensitive to nearby molecules and the chemical/electrical environment.
In a landmark 2010 Nature publication (S. Garaj et al, Nature Vol 67,doi:10.1038/nature09379) the Harvard team and collaborators used graphene to separate two chambers containing ionic solutions, and created a hole - a nanopore – in the graphene. The group demonstrated that the graphene nanopore could be used as a trans-electrode, measuring a current flowing through the nanopore between two chambers. The trans-electrode was used to measure variations in the current as a single molecule of DNA was passed through the nanopore. This resulted in a characteristic electrical signal that reflected the size and conformation of the DNA molecule.
At one atom thick, graphene is believed to be the thinnest membrane able to separate two liquid compartments from each other. This is an important characteristic for DNA sequencing; a trans-electrode of this thickness would be suitable for the accurate analysis of individual bases on a DNA polymer as it passes through the graphene. Further developments are required to make high quality graphene pores with precise structure and edge chemistry that would be required for direct label-free sequencing.
Oxford Nanopore's in-house solid-state sequencing project is based in Cambridge, MA, USA.