It could be amazing to transport carbon nanotubes in membranes to advance human health

The artistic performance of rapid ion filtration within single-walled carbon nanotubes. Small ions such as potassium, chloride, and sodium are traversed through the internal volume of nanometer-wide carbon nanotubes at speeds that exceed the diffusion of loose water in the order of magnitude. Credit: Image by Francesco Fornasiero / LLNL.

Researchers at the Lawrence Livermore National Laboratory (LLNL) have found that the membrane pores of carbon nanotubes will allow rapid dialysis processes that would greatly reduce the treatment time of hemodialysis patients.

The ability to distinguish molecular components in complex solutions is essential for many biological and man-made processes. One way is to apply a concentration gradient to a porous membrane. This drives ions or molecules smaller than the diameter of the pores while blocking anything that is too large to enter the pores from one side of the membrane to the other.

In nature, biological membranes of the kidneys or liver can perform complex filtrations while still maintaining high performance. Synthetic membranes, however, often struggle with the familiar exchange between selectivity and permeability. The same material properties that promise what can and cannot pass through the membrane inevitably reduce the rate at which filtration can occur.

In an astonishing discovery published in the journal Advanced Science, LLNL researchers have found that pores in carbon nanotubes (graphite cylinders with diameters thousands of times smaller than human hair) can provide a solution between permeability and selectivity. When the concentration gradient was used as the driving force, small ions, such as potassium, chloride, and sodium, propagated through these small pores faster than in order of magnitude than when they moved in solution.

“This result was unexpected because there is a general consensus in the literature that pore diffusion rates of this diameter should be well below or below what we see,” said Steven Buchsbaum, lead author of the article.

“Our findings enrich the exciting and often misunderstood nanofluidic phenomena recently discovered at the closure of a few nanometers,” added project lead researcher Francesco Fornasiero.

The group believes that this work has several technological implications. Membranes used as channels for transporting carbon nanotubes would allow rapid hemodialysis processes, which would greatly reduce treatment time. Also, the cost and time of purifying proteins and other biomolecules, as well as recovering valuable products from electrolytic solutions, could be drastically reduced. Improved ion transport in small graphite pores can provide power to high-density supercapacitors even if they approach ions in pore sizes.

To carry out these studies, the team used pre-developed membranes, allowing them to be transported only from the inside of the aligned carbon nanotubes with a small diameter of nanometers. Using a custom diffusion cell, a concentration gradient was applied to these membranes and the rate of transport of various salts and water was measured. “We have developed rigorous control tests to ensure that there is no other possible explanation for the large recorded ion fluxes, such as transport that occurs through leaks or defects in our membranes,” Buchsbaum said.

To better understand why this behavior occurs, the team sought the help of some LLNL experts. Anh Pham and Ed Lau used computational simulations, and April Sawvel used nuclear magnetic resonance spectroscopy to study the motion of ions inside carbon nanotubes. Several possible explanations have been successfully discarded to make the picture clearer. However, a full and quantitative understanding of the observed transport rates is being developed.

Reference: “Rapid permeation of small ions in carbon nanotubes” by Steven F. Buchsbaum, Melinda L. Jue, April M. Sawvel, Chiatai Chen, Eric R. Meshot,
Sei Jin Park, Marissa Wood, Kuang Jen Wu, Camille L. Bilodeau, Fikret Aydin, Tuan Anh Pham, Edmond Y. Lau and Francesco Fornasiero, December 20, 2020, Advanced Science.
DOI: 10.1002 / advs.202001802

Other contributors to this work include Melinda Jue, Chiatai Chen, Eric Meshot, Sei Jin Park, Marissa Wood and Kuang Jen Wu of the LLNL and Camille Bilodeau of the Rensselaer Polytechnic Institute. This work was supported by the Department of Chemical and Biological Technologies of the Agency for the Reduction of Threats of Defense in “Dynamic Multifunctional Secondary Skin Materials”[MS]2“program.

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