(Nanowerk Spotlight) Ion-transport-based sensory systems can be categorized into three groups – ionic sensors, ionic processors, and ionic interfaces – all of which center on ion transport.
The concept of ionic sensory systems is inspired by ion transport in biological systems, which is the movement of ions across a membrane, passively through ion channels or actively through ion pumps.
Many biological systems transmit signals via ions or molecules, whereas man-made sensors are mainly based on electron transport systems, i.e. electronics. This is one of the reasons why electron-transport-based sensors still suffer from limitations when interacting with biological systems. For instance, it is difficult to realize a direct human-computer interface, i.e. a bioelectronic interface, without a transducer, since both sides speak 'different languages'.
Biological systems contain numerous nanoscale ionic elements, which exist in the form of ion channels and ion pumps in cell membranes ('ion pumping' is a process of consuming energy to decrease entropy, in which ions are actively transported from low concentration to high concentration). They work together to control the ion concentration gradients across cell membranes, enabling information encoding/decoding through action potentials.
This is the biological mode of communication within an organism and also the way of detecting and interpreting information from the environment. For example, mammalians have high sensing capability to external mechanical stimuli, represented by the senses of touch, balance, hearing, and homeostasis, which all are derived from the phenomenon of ionic mechanotransduction that constitute various physiological processes.
A recent paper in Advanced Materials ("Bio-inspired Ionic Sensory Systems: The Successor of Electronics") introduces the essential principles of (accurate) ion transport, and reviews the recent progress in the development of the individual elements for ionic sensory system.
In biological systems, the success of initiation, processing, and transmission of information is closely related to the accurate ion transport across cell membranes. The precise ion selectivity, directionality, and ion transport against concentration gradients in cells are the molecular basis for all electrical activities.
Although sensory systems based on accurate ion transport are still in their early development, recent progress in nano-ionic sensory systems underlines their significant potential.
Figure 1. Basic ion-related effects and ion-transport-based sensory systems. A) Schematic diagram of the electrical double layer (EDL) formation and corresponding cyclic voltammogram. B) Schematic diagram of the electrochemical gating mechanism and corresponding cyclic voltammogram. C–E) Ion transport including ion selectivity, ion rectification and ion pump. C) Ion selectivity: the electrical double layers (Stern layer and Diffusion layer) near the surface are overlapping when the diameter of the nanochannel is comparable with the Debye length. This leads to the co-ions being excluded from the channel while the counter-ions can pass through. D) Ion rectification in an asymmetric charged nanochannel: the ions pass through the channel with a preferential direction which is suppressed in another direction when the nanochannel or membrane is asymmetric in structure and/or surface charge. E) The ion pump is an active ion transport from low concentration to high concentration along with energy consumption. F) A typical example of an ionic sensory system, which is based on accurate ion transport and can realize the signal transformation cascade: external stimuli → ionic signal → biological signal. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The three different component of ion-transport-based sensory systems are illustrated in Figure 1 above: ionic sensors, ionic processors, and ionic interfaces. In order to mimic these biological ionic processes, artificial ionic sensors will have to realize similar functions with ultrahigh sensitivity and operational stability.
And although various ion transport functions have been realized based on different (nano)materials (for instance graphene), the concepts and layouts of artificial ionic sensory systems are still in the toddler stage compared with today's electronic sensory systems.
Mechanoreceptor sensors in mammalians are vital to protection from external injury. Piezoelectric materials can be easily used to build electron-transport-based artificial mechanoreceptor sensors. Subsequently, ionic artificial mechanoreceptor sensors can be achieved by combining piezoelectric films with artificial ion-channel systems (see Figure 2A).
Another function of mammalian skin is to alert to external thermal stimuli, which is one of the self-protection functions in mammals. The detection of external thermal stimuli originates from the temperature-sensitive transient receptor potential channels in the thermoreceptor cells of the skin, which can transduce thermal signals to ionic signals, then to action potentials for information transfer. Inspired by this biological thermosensory process, researchers developed an ionic thermoelectric conversion behavior by an ion selective membrane, by which the external temperature stimuli could be transduced into ionic signals (Figure 2B).
Artificial visual systems work similarly and already, researchers developed an ion-transport-based photodetector by polymeric carbon nitride nanotube membrane, which is self-powered and also has the advantages of high selectivity, high sensitivity, and high stability (Figure 2C).
Ion-transport-based processes can be applied for signal processing and information storage. Transistors form the backbone of microelectronics and modern industry and are mainly fabricated from inorganic semiconductor materials, in most cases silicon. Some electrolyte-gated field-effect transistors and organic electrochemical transistors already work via ion accumulation induced double layer capacitance or ions penetration, but are still primarily based on electron transport.
Development of an all-ion-transport-based transistor will provide a unique opportunity for real-time regulation/control of signals from living organisms. Already, scientists have shown that both protons and ions can be used as charge carriers to fabricate 'ionic transistors' (see: "Fast, flexible ionic transistors for bioelectronic devices").
However, ionic transistors are difficult to modulate in most cases because both proton and ion transport in nanoconfinement is subject to either a local electric field arising from surface charge or an externally applied potential, which on the other hand also provides a unique opportunity.
For instance, a recently developed electrically modulated ionic transistor is based on a tunable nanoconfinement in layered graphene-based nanoporous membranes (Figure 2D). Due to the excellent conductivity of graphene, the gate potential can be applied directly to the membrane, which can tune the electric double layer thickness enclosed between the layers of graphene materials, and then control the ion flux, i.e., ionic current.
In biology, cells and tissues use finely regulated ion fluxes for their intra- and intercellular communication. These ionic interfaces are ion-transport-based connection elements that can act as an interfacing device between electronic and biological systems.
Research to date has shown that ionic interfaces are a powerful bridging module between external signals – including light signal, electrical signal, and thermal signal – and biological signals.
However, future development of ionic interfaces should not only focus on 'speaking' to biological tissue, but also on 'reading' of the biological language. Once such bidirectional communication is reached, the real communication between artificial and biological intelligence should not be far away.
The ultimate goal, as the authors conclude, is to construct an integrated ionic sensory system, able to realize seamless signal transduction between external stimuli, responsive artificial machines, and biological systems.
To mimic the real-time processing and manipulation of biological signals and information in living organisms, future ionic sensory systems should contain all essential elements: ionic sensors, ionic central processors, and ionic interfaces (Figure 3). By integrating these components, a 'smarter' artificial device with signal transduction and information processing functions will be achieved, operating in ways indistinguishable from living systems.
By Michael Berger – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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