Graphene as a subnanometre trans-electrode membrane

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Abstract
Atomically thin layers of graphite — known as graphene — are highly electronically conducting across the plane of the material. Now researchers from Harvard University and the Massachusetts Institute of Technology show that, when used as a membrane separating two liquid reservoirs, graphene is strongly ionically insulating, while its in-plane electronic properties are strongly dependent on the inter-facial environment. The membrane prevents ions and water from flowing through it, but can attract various ions and other molecules to its two atomically close surfaces. A variety of analytical applications may result. For instance, the authors show that by drilling pores a few nanometres in diameter into these 'trans-electrode' membranes, it is possible to thread a long DNA molecule through the graphene nanopore. The DNA blocks the flow of ions, resulting in a characteristic electrical signal reflecting the size and conformation of the molecule. Such a system has potential as the basis of devices that could significantly reduce the cost of DNA sequencing. Graphene is highly electronically conducting across the plane of the material. These authors show that a graphene membrane separating two ionic solutions in electrical contact is strongly ionically insulating despite being atomically thin and has in-plane electronic properties dependent on the interfacial environment. Numerical modelling reveals that very high spatial resolution is possible using this system, and the researchers propose that drilled membranes could form the basis of DNA sequencing devices. Isolated, atomically thin conducting membranes of graphite, called graphene, have recently been the subject of intense research with the hope that practical applications in fields ranging from electronics to energy science will emerge1. The atomic thinness, stability and electrical sensitivity of graphene motivated us to investigate the potential use of graphene membranes and graphene nanopores to characterize single molecules of DNA in ionic solution. Here we show that when immersed in an ionic solution, a layer of graphene becomes a new electrochemical structure that we call a trans-electrode. The trans-electrode’s unique properties are the consequence of the atomic-scale proximity of its two opposing liquid–solid interfaces together with graphene’s well known in-plane conductivity. We show that several trans-electrode properties are revealed by ionic conductance measurements on a graphene membrane that separates two aqueous ionic solutions. Although our membranes are only one to two atomic layers2,3 thick, we find they are remarkable ionic insulators with a very small stable conductance that depends on the ion species in solution. Electrical measurements on graphene membranes in which a single nanopore has been drilled show that the membrane’s effective insulating thickness is less than one nanometre. This small effective thickness makes graphene an ideal substrate for very high resolution, high throughput nanopore-based single-molecule detectors. The sensitivity of graphene’s in-plane electronic conductivity to its immediate surface environment and trans-membrane solution potentials will offer new insights into atomic surface processes and sensor development opportunities.
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