image: For ion transport at nanoscale confinements, a cost of energy is discovered that originates from the rearrangement of the counter-ion shell, with the magnitude of which depending on the type of transport ion as well as the charging state of the ionic channel (top panel). Therefore, via tuning the electrical charges at the graphene-based heterogeneous membrane surface, a record mono-/bi-valent ion selectivity is achieved, together with a high permeation rate (bottom panel).
Credit: ©Science China Press
A research team led by scientists from Xiamen University and Nanjing University of Aeronautics and Astronautics has developed a novel artificial membrane that mimics the way biological membranes selectively control ion transport. The membrane exhibits an ultra-high selectivity between monovalent and bivalent metal ions, achieving a record selectivity ratio of over 10,000. The findings were recently published in Science Bulletin.
How does the ultra-selective membrane work?
The membrane combines functionalized graphene oxide (GO) nanosheets with polyacrylic acid (PAA) polymers, creating nanoscale channels with "smart gates" at their entrances. When signaling ions like aluminum (Al3+) are presented, they bind to the carboxyl groups on the PAA (as “gatekeepers”), and selectively blocks bivalent cations (e.g., Mg2+, Ca2+) while allowing monovalent ions (e.g., Li+, Na+, K+) to pass.
This chemical gating mechanism enables the membrane to “sense” changes in the surrounding chemical environment and adjust its permeability accordingly. It functions much like ion channel proteins in living cells, which can switch between open and closed states based on specific biological signals.
What is the molecular machinery of selective ion permeation?
Analysis using techniques like atomic force microscopy, zeta potential measurements, and TOF-SIMS confirmed that the PAA layer stays localized on the membrane’s surface, where it binds with high-valent ions to trigger the gating effect. Molecular dynamics simulations helped uncover the underlying mechanism: the gating effect originates from the high energy cost required to rearrange the counter-ion shells surrounding bivalent ions—a cost that is not shared by monovalent ions, which permeate fast.
Applications in critical separations
This membrane may find its potential applications in the following aspects:
- Lithium extraction: Li⁺/Mg²⁺ selectivity of 104 from salt-lake brines
- Water purification: Complete rejection of heavy metal ions
About the study
The study showcases the potential of integrating functioning macromolecules with two-dimensional materials to fabricate artificial heterogeneous membranes towards their biological counterparts. Further exploration using such adaptive systems could focus on Li+ ion extraction for lithium-ion batteries, as well as iontronic devices where stimuli-responsive ion transport is required. The work is mainly supported by the National Natural Science Foundation of China and the National Key Research and Development Program of China.