The development of membranes with high ion permeability and selectivity is crucial for sustainable water treatment, resource extraction, and energy storage. Biological ion channels, such as the K+ channel, achieve ultra-high selectivity through a synergistic effect of sub-nanoscale pore size, specific binding sites, and appropriate charge density. Mimicking these features in artificial membranes is a significant challenge. Previous attempts have focused on replicating structural characteristics using materials like graphene oxide, metal-organic frameworks, and perforated polyethylene terephthalate, but these often lack sufficient selectivity. Subsequent research incorporated functional groups to enhance selectivity based on chemical affinity, and charge density modulation has also been explored. However, precisely replicating all three features of biological channels remains a challenge. The transport and separation mechanisms within sub-nanoscale channels modulated by local charge density also need further investigation. This research aims to address these challenges by creating artificial ion channel membranes using MXene nanosheets and EDTA molecules as building blocks to precisely control channel size, binding sites, and tunable charge density, mimicking the KcsA K+ channel. MXene's unique layered structure and surface functionalities, combined with EDTA's ability to complex with divalent cations, offer a promising route towards high-performance ion separation membranes. This biomimetic approach promises to yield a more effective strategy for mono-/divalent ion selective membranes, which are greatly in demand for various applications.

The literature extensively covers the pursuit of high-performance ion-selective membranes. Initial research focused on mimicking the structural features of biological ion channels, employing materials like graphene oxide (GO) and metal-organic frameworks (MOFs) to create sub-nanometer channels. However, size exclusion alone proved insufficient for achieving high selectivity. Subsequent studies incorporated functional groups, such as sulfonate groups (-SO3-), to leverage chemical affinity for cation discrimination, especially between same-valent ions of similar size. Controlling channel charge density also emerged as a method to fine-tune ion selectivity. While these efforts captured aspects of biological ion channel behavior, achieving the precision and combined effects of size, binding sites, and charge density observed in nature has remained elusive. The lack of a comprehensive understanding of ion transport mechanisms within sub-nanoscale channels further hindered progress. This paper draws inspiration from successful examples like the use of polystyrene sulfonate within MOF membranes for lithium-ion separation, and the creation of artificial sodium-selective sub-nanochannels, providing context for the current approach based on MXene and EDTA.

This study employed a facile method to construct MXene laminar membranes functionalized with EDTA (MLM-EDTA). Firstly, Ti3C2Tx nanosheets were prepared using a minimally intensive layer delamination (MILD) method, etching away the Al layer from Ti3AlC2 using HCl and LiF. The resulting nanosheets, characterized by TEM and SAED, exhibited a single-layer nature with an average flake size of 1.0 µm and thickness of ~1.5 nm. XPS analysis confirmed the removal of Al and the presence of surface -OH, -O groups, and Lewis acid Ti sites. EDTA molecules were then grafted onto the Ti3C2Tx nanosheets, creating versatile EDTA-Ti3C2Tx connections via covalent and non-covalent bonding. This was confirmed by HAADF images, elemental mapping, zeta potential measurements, and XPS and FTIR spectroscopy. Membranes were assembled via vacuum-assisted filtration, named MLM-EDTA-X (X representing EDTA concentration). SEM and TEM imaging revealed highly aligned cross-sections and homogeneous EDTA distribution. Increasing EDTA loading enhanced membrane compactness, reducing channel spacing and improving anti-swelling properties, observed via XRD. The ion separation performance was evaluated using a custom U-shaped device measuring the permeation rates of a mixed alkali and alkaline earth ion solution. The K+/Mg2+ selectivity of MLM-EDTA membranes was significantly higher than that of pristine MLM membranes, reaching 121.2 for MLM-EDTA-1.5. DFT calculations explored EDTA-cation interactions, revealing stronger binding energies for divalent cations (Mg2+, Ca2+) than monovalent cations (Li+, Na+, K+), supporting the observed selectivity. The influence of charge density on separation was studied by varying pH, showing a linear correlation between charge density and K+/Mn+ selectivity. The impact of charge density was further investigated via molecular dynamics (MD) simulations, which analyzed ion entry and passing times through the channels. The simulations confirmed the importance of both size exclusion and electrostatic interactions, with charge density significantly affecting the transport energy barriers for different ions. The study employed various characterization techniques including XRD, SEM, TEM, XPS, FTIR, zeta potential measurements, ICP-OES, and DFT and MD simulations. Detailed calculations of ion permeation rates and selectivity were performed using established equations.

The MLM-EDTA membranes demonstrated exceptional cation sieving performance, particularly exhibiting an ultrahigh K+/Mg2+ selectivity of 121.2 in mixed salt solution, significantly surpassing previously reported values. This outstanding selectivity is attributed to a combination of factors: 1. **Biomimetic Channel Size:** The EDTA modification resulted in a channel size (~6.0 Å) comparable to the KcsA K+ channel, facilitating size-based ion exclusion. 2. **Cation Recognition by EDTA:** EDTA molecules exhibited a strong affinity for divalent cations (Mg2+, Ca2+), forming stable complexes that hinder their transport through the channels. This is supported by DFT calculations showing significantly higher binding energies for divalent ions compared to monovalent ions. 3. **Partial Dehydration Effects:** The sub-nanochannel size causes partial dehydration of hydrated ions upon entry. The energy penalty for dehydration is significantly higher for divalent cations, further impeding their transport. 4. **Charge Density Enhancement:** Increasing the local charge density by adjusting the pH value amplified the K+/Mg2+ selectivity. This is due to the intensified electrostatic attraction of monovalent cations to the negatively charged channels and the further enhancement of Mg2+-EDTA complexation at higher charge densities. Molecular dynamics simulations revealed how the interplay of these factors affects ion transport, showing that at low charge density, ion entry is the rate-limiting step, while at high charge density, ion passing is more crucial. The simulations showed that the entry time for Mg2+ increased due to size exclusion and dehydration, while the entry time for K+ decreased due to electrostatic attraction, illustrating the competitive effects at play. The transport energy barrier for Mg2+ was significantly higher than that for K+, especially at high charge densities, due to stronger Mg2+-EDTA complexation and increased hydrogen bonding with the channel walls. The MD simulations revealed that a second dehydration occurs in the channel as charge density increases, further enhancing Mg2+ resistance. The study demonstrated a clear linear relationship between charge density and K+/Mn+ selectivity (R2=0.98) indicating the strong influence of charge density on the membrane's ion sieving capability.

The findings address the research question by demonstrating a successful biomimetic approach for creating high-performance cation sieving membranes. The achieved K+/Mg2+ selectivity of 121.2 significantly surpasses previous reports, establishing a new benchmark in the field. The synergistic effects of size exclusion, EDTA-mediated cation recognition, partial dehydration, and charge density tuning offer a comprehensive explanation for the observed ultrahigh selectivity. These results validate the importance of integrating multiple features of biological ion channels for optimal performance in artificial membranes. The precise control over channel size and surface functionality provides a platform for tailoring selectivity towards specific ions, enabling applications in diverse fields.

This study successfully fabricated MLM-EDTA membranes with biomimetic sub-nanochannels, achieving ultrahigh and pH-tunable mono-/divalent cation selectivity. The combined effects of size exclusion, EDTA's cation recognition, partial dehydration, and charge density enhancement were shown to be crucial for selective ion sieving. This work offers a new strategy for designing high-performance ion separation membranes for applications including resource recovery, clean water production, and energy generation. Future research could focus on exploring other functional groups and MXene compositions to further optimize selectivity and expand applications.

While the MLM-EDTA membranes demonstrated exceptional performance, potential limitations include the scalability of the fabrication process and long-term stability under diverse operating conditions. Further investigation is warranted to optimize membrane stability and durability, assessing performance over extended periods and under varying environmental factors such as temperature, pressure, and the presence of other ions or contaminants. The study focused on a specific set of ions, and the generalizability to other ion pairs needs further investigation. The complexity of the MD simulations may require advanced computational techniques for more detailed and comprehensive modeling of transport mechanisms.