When water and ions move together through channels only a nanometer wide, they behave in unusual ways. In these tight spaces, water molecules line up in single file. This forces ions to shed some of the water molecules that normally surround them, leading to the unique physics of ion transport. Biological channels are especially adept at this behavior, often choreographing channel openings and closings to achieve complex behaviors such as signals in the nervous system.
In a recent study published in Nano Letters, researchers at Lawrence Livermore National Laboratory (LLNL) and the University of Maryland engineered carbon nanotubes with openings that can reversibly open and close depending on pH. The result is a synthetic "molecular gate" that mimics the behavior of barrel-shaped proteins called porins, which form holes in cell membranes to let specific molecules pass through.
The researchers used a chemical process to create incredibly short, fluorescent nanotubes with specific lid-like structures at their ends. They then inserted these tiny tubes into fatty membranes that mimic cell walls, forming sub-nanometer channels that force water and ions into a single-file line.
The team discovered that attaching a particular "lid" to the nanotube rim allowed them to control the flow of molecules.
"We saw that at acidic pH, the molecular lid closed, physically blocking the pore. At neutral pH, the lid rotated open, allowing ions and water to pass almost unhindered," said Jobaer Abdullah, author and a graduate student at University of California, Merced and LLNL.
To confirm the effectiveness of the lid, the team combined their measurements with machine learning-accelerated first-principles molecular dynamics simulations. The simulations showed how the lid's conformational changes altered the barriers for ion entry.
"Our simulations revealed that the probability of the channels staying open is significantly lowered under acidic pH conditions, directly linking molecular motion to macroscopic flow," said Margaret Berrens, author and LLNL scientist.
The ability to engineer responsive nanofluidic channels like those proposed here has significant implications.
"Synthetic membranes that can dynamically adjust their permeability could benefit desalination, biosensing and drug-delivery technologies, while providing new tools for studying how biological channels achieve selective ion transport," said Aleksandr Noy, lead author and LLNL scientist.
Author and LLNL scientist Anh Pham added, "This work expands the design space for nanofluidic systems by showing that even a single functional group, or lid, at the pore entrance can transform a static nanotube into an active, environmentally responsive gate."
This work was supported by the Center of Nanofluidic Transport, an Energy Frontier Research Center established by the Department of Energy Office of Science's Basic Energy Sciences. Other LLNL co-authors on the paper include Zhongwu Li and Marcos Calegari Andrade, who is now an assistant professor at the University of California, Santa Cruz. The research is also supported by the LLNL Grand Challenge Program.
