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Curated by RSF Research Staff

New insights in how electrons travel in water

As quantum mechanics endows particles with surprising properties, it enables to explain physical processes like electrolysis. While thermodynamics gives a consistent account of them, independent of any mechanism, quantum mechanics provides a consistent explanation of electron fluxes crossing the interface between a metallic conductor and an aqueous environment.  Understanding how electron travel is important in many fields: hydrogen production, environmental scanning tunneling microscopy, scanning electrochemical microscopy and biosensing applications.

Biosensing applications are a very promising field of research. Biosensors can be defined as an analytical device that converts a biological response into an electrical signal. Such sensors must be highly specific, independent of physical parameters such as pH and temperature and should be reusable. Fabrication of biosensors, its material, transducing devices are very complex and requires multidisciplinary research in chemistry, biology, and engineering. New progress in understanding how electrons travel in water will be a big step forward.

The charge transfer processes that occur when water is placed between metallic electrodes to which voltage is applied, determine the voltage–current relation, which is central for biosensing of molecular interactions at interfaces. In a recent study, researchers from Sydney looked at these quantum mechanisms with electron tunneling into solution and its conditions.

Schematic of gold electrodes immersed in pure water showing the ion confguration. The positive charges in solution represent solvated hydronium ions and the negative charges represent solvated hydroxide ions. The blue shading indicates neutral water molecules. The interpretation of the contributions to the switch-on and switch-off transients with insets indicating ion movement near the cathode surface and charge classification. The pink area of switch-on represents Q, the sum of q1 and q2 and the pink area of switch-off represents only q2, while the red lines are the corresponding steady state on and off currents shown in the inset for 100 s after switch-off.

The team led by Professor McKenzie measured the low-voltage current flows between two gold electrodes placed in pure water. They evaluated the relative contribution to the steady current arising from tunneling of electrons. Their results showed that quantum tunneling of electrons to and from ions in solution near the electrodes is the larger current contribution to the current measured. They supported this claim with a new quantum model in agreement with observations.

This lays the basis for new and faster methods to detect biomedical impurities in water, with potentially important implications for biosensing techniques. […] A better understanding of electrolysis is becoming more important for applications in alternative energies in what is sometimes called the 'hydrogen economy'.

Professor McKenzie, Australian Institute for Nanoscale Science and Technology, University of Sydney

Their model assumes there are multiple one-dimensional parallel tunneling pathways linked to the macroscopic structure of ionized water. Their findings will help further development in biosensing at interfaces in water.

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