Superlubricity of carbon nanotubes as a prerequisite for mechanically activated ionic transport

The field of nanofluidics is the study of the behavior, control and manipulation of fluids that are confined in extremely small structures. The dimensions of these minute structures typically range from 1 nanometer to 100 nanometers (1 nm = 10^-9 m). The fluids constrained within these nanostructures demonstrate different physical behaviors compared to those demonstrated by fluids in larger structures due to the extremely small characteristic scaling length. Consequently, researchers working in the nano fluidics field are not able to rely on the knowledge gained in previous research into the physical behaviors of fluids in larger structures but rather must start almost from the scratch. The situation is complicated by the fact that the equipment required to study the flow in the nanotubes has not been available until recently.

While the field of nano fluidics has grown, manifold systems such as nanopores and nanotubes, along with the development of new instruments, have opened the door for scientists to study the fluid transport processes in these systems. These investigations have made it clear that synthetic nanofluidic devices continue to be far overshadowed by those biological devices existing in the natural world of plants and animals. Nano fluidic structures contained within biologic systems continue to be far more effective and efficient in such areas as information storage and ionic pumping, as well as in processes such as mechanically and electrically activated transport.

Biological ionic structures demonstrate a vast range of sophisticated transport functions and processes. Recently, the discovery of mechano transduction channels has brought to light an intricate response, which involves the reaction of mechanically activated ionic excitable cells when they experience mechanical pressure or stretching.

Recreating these responses in artificial devices has proven to be a challenge. Alice Marcotte, Timothée Mouterde, Antoine Nigués, Alessandro Siria and Lydéric Bocquet from the Laboratoire de Physique de l’Ecole normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université in Paris, France took up the challenge to recreate the advanced biological response in a synthetic system.

Alice Marcotte and her team used minuscule (single digit) carbon nanotubes with radii of only 2 nanometers or less to investigate how ionic transport in these mechano transduction channels are affected by mechanical pressure. To date, such investigations in carbon nanotubes have been impossible so the team relied on transmembrane nanotube methodology to make what was previously impossible, possible.

One interesting finding of the scientific team regards the similarities between mechano sensitive biological channels and ‘single-digit’ carbon nanotubes. Mechano transduction refers to any mechanism used by cells to convert a mechanical stimulus into an electrochemical activity (converts physical forces in biochemical signals). Mechano transduction is an essential component of many biological senses and physiological processes. The senses relying on mechano transduction include that are considered to be conscious senses, such as hearing and touch, or unconscious senses, such as regulating blood pressure.

Mechano transduction incorporates a mechanically gated ion channel, which allows movement, pressure, or sound to work in excitable cells (which have the ability to generate electrical signals) and change the excitability level of said sensory cells. Stimulating a mechano receptor opens mechanically sensitive ion channels in order to produce a current which changes the cell membrane potential. These mechano sensitive biological channels are proven to demonstrate a non-linear dependence of ionic currents while the biological mechanisms respond to changing tension in the membrane along with changes to the spatial arrangements of molecules with the protein structure.

While ‘single-digit’ carbon nanotubes do not exhibit any response to changes in spatial arrangements, they do exhibit a response to pressure changes when ionic species, which are pressure dependent, accumulate under voltage. The quadratic pressure dependence of the ionic response in the carbon nanotubes relies on the extremely low friction of water on the carbon surface (superlubricity state). As a result, single-digit carbon nanotubes have the ability to respond to local pressure signals at the nanoscale.

Experiments confirm that the conductance of carbon nanotubes is quadratically related to the applied pressure. This finding opens the doors to a wide variety of applications to create new types of local pressure sensors. One possible use would be in providing us with the ability to produce artificial sensing devices for senses such as touch.

Further details: Alice Marcotte et al. Mechanically activated ionic transport across single-digit carbon nanotubes, Nature Materials (2020). DOI: 10.1038/s41563-020-0726-4