The human brain is a vast network of billions of biological cells called neurons that send out electrical signals that process information, resulting in our senses and thoughts. Atomic scale ion channels in each neuronal cell membrane play a key role in such ignitions which open and close ion flow in an individual cell by electrical voltage applied across the cell membrane, acting as a similar “biological transistor” to electronic transistors in computers. For decades, scientists have learned that biological ion channels are life transistors capable of passing extremely rapid and selective permeation of ions through atomic-scale selectivity filters to maintain vital functions. However, it remains a great challenge to this day to produce artificial structures to mimic these biological systems for fundamental understanding and practical applications.
Researchers led by Professor Xiang Zhang, President of the University of Hong Kong (HKU), developed an atomic-scale ion transistor based on electrically triggered graphene channels about 3 angstroms in width which demonstrated a highly selective ion transport. They also found that the ions moved a hundred times faster in such a tiny channel as in bulk water.
This breakthrough, recently reported in Science, not only provides a fundamental understanding of fast ion sifting at the atomic scale, but also leads to highly switchable ultra-fast ion transport that can find important applications in electrochemical and biomedical applications.
“This innovative ion transistor demonstrates the electrical switching of ultra-fast and selective ion transport simultaneously through atomic-scale channels like the biological ion channels running in our brains,” said Professor Xiang Zhang, Principal Investigator. “It deepens our fundamental understanding of ultra-total limit ion transport and will have a significant impact on important applications such as seawater desalination and medical dialysis.”
The development of artificial ion channels using traditional pore structures has been hampered by the tradeoff between permeability and selectivity for ion transport. The pore sizes exceeding the diameters of the hydrated ions make the ionic selectivity largely disappeared. High selectivity of monovalent metal ions can be achieved with precisely controlled channel size at the angstrom scale. However, these angstrom-scale channels significantly prevent rapid diffusion due to the steric resistance of hydrated ions to enter a narrower channel space.
“We have observed ultra-fast selective ion transport through the graphene channel at the atomic scale with an effective diffusion coefficient as high as Deff? 2.0´10-7 m2 / s.” said Yahui Xue, lead author of the study, a former postdoctoral researcher in Prof. Zhang’s group. “To our knowledge, this is the fastest diffusion observed in concentration-driven ion permeation across artificial membranes and even exceeds the intrinsic diffusion coefficient observed in biological channels.”
Scientists from Hong Kong and UC Berkeley first used the gate voltage to control the surface potential of graphene channels and achieved ultra-high charge filling density inside these channels. Neighboring charges exhibit strong electrostatic interaction with each other. This results in a state of dynamic charge equilibrium such that the insertion of a charge from one end of the channel would lead to the ejection of another at the other end. The resulting concerted load movement dramatically improves the overall speed and efficiency of transport.
“Our in situ optical measurements revealed a charge density as high as 1.8″ 1014 / cm2 at the largest applied gate voltage. said Yang Xia, a former doctoral student in Prof. Zhang’s group. “It is surprisingly high, and our theoretical mean-field modeling suggests that the ultra-fast transport of ions is attributed to very dense stacking of ions and their concerted movement within the graphene channels. “
The atomic scale ion transistor has also demonstrated superior switching capacity, similar to that of biological channels, arising from threshold behavior induced by the critical energy barrier for the insertion of hydrated ions. The channel size smaller than the hydration diameters of the alkali metal ions creates an intrinsic energy barrier which prohibits the entry of ions under the open circuit condition. By applying a triggering electrical potential, the hydration shell could be deformed or partially stripped to overcome the input energy barrier of ions, allowing the intercalation of ions and possibly permeable ion transport beyond a percolation threshold.
The atomic scale graphene channel consisted of a single flake of reduced graphene oxide flakes. This configuration has the advantage of intact layer structures for the study of fundamental properties and also preserves great flexibility for larger scale manufacturing in the future.
The sequence of selection of alkali metal ions through the ionic transistor at the atomic scale has been found to resemble that of biological potassium channels. It also involves a control mechanism similar to biological systems, which combines ionic dehydration and electrostatic interaction.
This work is a fundamental advance in the study of ion transport through solid pores at the atomic scale. Integrating atomic-scale ion transistors into large-scale networks can even make it possible to produce exciting artificial neural systems and even brain-like computers.
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