Transport channels with ultrahigh K+-selectivity against other monovalent ions (Na+) are crucial for living beings, but it is still a bottleneck to construct such biological ionic channels with promising selectivity and permeability. This research designs a kind of MXene-based lamellar membrane with an asymmetric structure consisting of a recognition layer (RL) and an enhancement layer (EL) that shows a fantastic Matthew amplification effect for K+ sieving.
The research team from Tsing University and South China University of Technology designed a kind of Matthew membrane with an asymmetric structure as an amplifier of K+ separation to address the K+ separation problem in the field of univalent ion separation. The term "Matthew effect" is commonly used by sociologists and economists to reflect a state of polarization in society, which can be simplified as "the rich get richer and the poor get poorer"; it was coined by the sociologist Robert K. Merton in 1968 and is derived from the biblical Gospel of Matthew. In the case of ionic transport through a membrane, the preferred transport of K+ is amplified by the Matthew effect. A Matthew membrane is a bilayer membrane composed of an ion-selective RL and an ion-permeable EL.
The research team researches that the amplifying effect of K+ separation through the Matthew membrane exhibits unipolarity. In other words, it is only when the ions pass through the Matthew membrane from the RL to the EL that the K+-separation performance is amplified. It can be concluded that the promising K+-separation performance of the membrane is related not only to the optimal asymmetric structure but also to the direction of K+ ions passing through. The targeted K+ ions must be recognized and enriched by the RL before they are removed through the EL in order to exhibit an amplified K+-separation performance.
The research team clearly observes that the diffusion rate of water molecules and ions in a confined channel of the EL is several times higher than that in the aqueous bulk solution, which indicated that the EL can be regarded as an amplifier that promotes the permeation of the K+ previously selected by the RL and thus increases the K+/Na+ selectivity. In comparison, the MXene-CE (CE: 1-aza-18-crown-6 ether) consists of only an RL but lacks an inducible transport layer EL that can quickly detach the K+ from the affinity site through a steep K+ concentration gradient; thus, the MXene-CE exhibits a poor permeation rate and average selectivity.
In summary, an MXene-based Matthew membrane is designed and exhibited an outstanding K+-sieving performance with a K+/Na+ selectivity of up to about 9. Experiments and simulations reveal that the excellent performance can be attributed to the following reasons: ① Crown ether incorporation into the RL provides a strong affinity for K+ in comparison with Na+, leading to K+ recognition and selection; and ② the reasonable design of the EL permits faster diffusion rate of the K+ enriched in the RL via the confinement effect of the EL, in comparison with the transport of Na+. This smart design of a suitably matching RL and EL endows the asymmetric MXene (Ti3C2Tx) membrane with a Matthew K+-transport amplification effect. The resulting membrane exhibits high K+/Na+ selectivity with fast K+ permeability, based on a combination of a specific K+-recognition effect in the RL and fast transport of the hydrated K+ through the slit-shaped nanochannels between the MXene (Ti3C2Tx) nanosheets of the EL. Considering the remarkable monovalent ion-sieving performance of Matthew membrane, this work provides new insight into the ion sieving of 2D materials-based membranes.
The paper "A Matthew MXene (Ti3C2Tx
