Fifty years ago, when the celebrated science-fiction writer Isaac Asimov visited the 1964 World’s Fair in New York, he was inspired to predict which technologies would feature in the 2014 exhibition (New York Times 16 August 1964). With uncanny accuracy he forecast that ready meals and Skype-like video communications would become the norm, along with a prevalence of electronic and cordless devices. But even Asimov did not foresee that next-generation "electronic" devices might not rely on electrons after all.

Indeed, researchers are now developing new technologies based on the movement of ions and molecules rather than electrons. In a recent issue of Nanotechnology Weihua Guan, Sylvia Xin Li and Mark Reed at Yale University explain how this approach "offers a unique opportunity to integrate wet ionics with dry electronics seamlessly". The Yale researchers provide an overview of controlled ion and molecule movement in devices that resemble a transistor, as well as describing the theory governing these systems, fabrication techniques, and potential applications in energy conversion and in biological and chemical analysis.

Nanopore transport in natural systems

Studying the movement of particles in nanochannels is not a new idea. In 1940 Harold Abramson and Manuel Gorin working in New York noted in a report that "when an electric current is applied across the living human skin, the skin may be considered to act like a system of pores through which transfer of substances like ragweed pollen extract may he achieved both by electrophoretic and by diffusion phenomena". And Klinkenberg, whose studies of rock permeability in the 1940s and 1950s led to the "Klinkenberg correction" familiar to many geophysicists, described an analogy between diffusion and electrical conductivity in porous rocks as far back as 1951.

More recently, researchers have studied transport in living systems through pore structures on a much smaller scale. The selective transport of ions and small organic molecules across the cell membrane is crucial for a number of functions, including communication between cells, nerve conduction and signal transmission. Understanding these processes may benefit a wide range of potential applications such as selective separation, biochemical sensing, and controlled release and drug-delivery processes.

In 2012 Saima Nasir, Mubarak Ali and Wolfgang Ensinger at the Technische Universität Darmstadt and the GSI Helmholtz Centre for Heavy Ion Research in Germany successfully demonstrated controlled ionic transport through nanopores functionalized with amine-terminated polymer brushes. The polymer nanobrushes swell and shrink in response to changes in temperature, thus opening and closing the nanopore passage to ionic molecules. "This process should permit the thermal gating and controlled release of ionic drug molecules through the nanopores modified with thermoresponsive polymer chains across the membrane," they explain.

Carbon nanochannel structures

With their intrinsic nanoscale features, carbon nanomaterials are obvious candidates for nanochannel systems. Jae Hyun Park and Narayana Aluru at the University of Illinois at Urbana-Champaign and Susan Sinnott at the University of Florida in the US have developed molecular dynamics simulations of a potassium chloride solution passing through carbon nanotubes joined together to form a Y-shaped junction. In particular they considered the case where the nanotubes of the different arms of the Y have different diameters, and their results suggest that these "Y junctions" could be used to separate the potassium and chloride ions in the solution based on their different sizes. (Nanotechnology 17 046).

A collaboration of researchers at Shanghai University in China and Northwestern University in the US also used molecular dynamics simulations to calculate the movement of ions in solution through a graphene nanopore under an applied electric field. Their results confirm that the electric conductance is proportional to the size of the nanopore, which helps to understand how these structures might be exploited in applications such as fast DNA sequencing. If the pore is large enough to allow DNA bases through, but small enough to allow only one to pass at a time, a current value can be assigned to each base, allowing the DNA to be sequenced by measuring the ionic currents.

Nanochannel transport in devices

In their review, Guan, Li and Reed also describe some of the interactions that can affect the behaviour of DNA molecules in nanoscale channels. Researchers have shown that transistor-like gated structures can be used to control the rate at which a DNA molecule passes through a nanopore over a range of three orders of magnitude, an impressive performance that has been attributed to the interplay of electrophoresis and electro-osmosis effects. However, Reed and colleagues note in their review that "a clear physical picture of the gated DNA translocation through the nanopore is still lacking", suggesting that further research is needed to understand the regulation mechanism in these structures.

Other applications of controlled ion and molecule transport described in the review include protein-based molecular switches, ionic charge-coupled devices (ionic CCDs), diodes and enhanced energy-conversion systems, with current work focused on understanding the mechanisms at play and how they can be manipulated. The review also describes the theory behind effects that originate from the nanoscale feature sizes, as well as bottom-up and top-down approaches to fabricating nanochannels in different dimensions.