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Page 22 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

nucleophilic substitution condensation reaction between the oxygen-containing groups of amine and GO to
[105]
form a CeN covalent bond, which effectively suppressed the swelling effect . In-situ polymerization on the
surface of 2D materials to generate twins is also a commonly used strategy [72-75] . Kim et al. using N,N’-
methylenebisacrylamide (MBA) as the cross-linking agent, N-isopropylacrylamide (NIPAM) as the
monomer, and then ammonium persulfate (APS) as the initiator, successfully synthesized the GO@polymer
[158]
twins . The prepared membranes have excellent anti-swelling properties and desalination ability [158,159]
[Figure 9B].

The potential interaction of functional groups on the surfaces of different 2D materials makes the
superimposition of different 2D materials a possible strategy for modifying the surface chemistry of
materials [Figure 9C]. Xie et al. reported a Ti C T MXene-based membrane whose microstructure was
3
2 x
[160]
optimized by inserting rGO between the layers . The surface of the membrane was progressively
hydroxylated to increase the accessibility of Ti C T , thus improving the wettability of the film and
2
x
3
enhancing the adsorption and reduction of heavy metal ions.
Oxidation and reduction processes are commonly used to modify and prepare 2D materials, so surface
properties are often modified during the preparation process. For example, using a modified Hummers
method (i.e., oxidation procedures), Alkhouzaam et al. prepared a GO membrane with more active
[161]
surface properties than a graphene membrane, which facilitates its functionalization to meet the request
[Figure 9D].

The properties of nanochannel membranes based on 2D materials, including swelling resistance and water
permeability and retention, are closely related to the transport control effects of nanochannels and the
interleaving of surface properties. These factors play a key role in determining the overall performance and
functionality of the membranes. Therefore, it becomes imperative to thoroughly evaluate and consider the
effects of various modification methods on the nanochannels. This careful evaluation enables researchers to
select and optimize the most suitable 2D-material-based membranes for specific applications. By
understanding and exploiting the complex relationship between nanochannels and surface properties,
researchers can unlock the full potential of these membranes in areas as diverse as water purification to
energy storage.

APPLICATIONS OF 2D-MATERIAL-BASED NANOCHANNEL MEMBRANES
By rationalizing the structural and functional characteristics, 2D-material-based nanochannel membranes
can achieve many features, such as ultrafast transmission, selective sieving, sensitive sensing, controlled
gating, and rectification, among others. This section summarizes typical developments in liquid molecular
separation, gas separation, and ion sieving using state-of-the-art 2D-material-based nanochannel
membranes [Figure 10].

Liquid molecular separation
Membranes are highly susceptible to swelling effects when applied in liquid environments (i.e., liquid
molecular separations, including liquid solvent-molecular solute separations and liquid-liquid separations),
leading to the collapse of nanochannels, reduced selectivity, and shortened membrane lifespan. Therefore, it
is imperative to improve the swelling resistance of membranes in liquid environments compared to
membranes in other conditions.

A series of anti-swelling MXene nanochannel membranes were synthesized by Xing et al. using the
interaction forces generated by introducing negatively charged polymers into positively charged MXene