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Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11 Page 17 of 35

situ fabricated nanoparticles@GO membrane uniformly introduced nanoparticles within GO laminates
without damaging their initial ordered stacking structures and hence exhibited ultrahigh water permeance
and excellent rejections for various solutes in water, as well as good stability under high pressure and cross-
flow operation. This general concept of intercalation can also be demonstrated by several kinds of
nanoparticles such as TiO 2 [123] , silica , and hydroxy sodalite nanocrystals , endowing nanoparticles
[124]
[125]
intercalated GO membranes with additional multifunctions. Compared with nonporous metal oxide
nanoparticles, intrinsically porous nanocrystals are more beneficial for fast nanofluidic transport when they
are intercalated into 2D nanochannels. By incorporating MOFs [126,127] , COFs [128,129] , and biomimetic water
channels with extra internal pathways through sub-nano-sized apertures as the microporous fillers into
[130]
2D laminates, both the size and the number of nanochannels are increased, leading to greatly enhanced
nanofluidic transport performance. Li et al. described a route for fabricating MOFs channeled graphene
composite membranes with molecular sieving properties using in-situ crystallization. A series of MOFs,
including ZIF-8, ZIF-7, CuBTC, and MIL-100, were showcased to be impregnated into interlayers of GO
laminates, which firmly anchored and bolstered up GO laminates by coordination bonds to form a highly
porous architecture with uniform nanochannels .
[127]
Khan et al. created a hybrid laminar membrane by assembling COFs and GO nanosheets . The
[128]
incorporation of COFs nanosheets provides a large number of pores that shorten the transport pathway
while retaining the interlayer distance. Similarly, Sui et al. also intercalated rigid 2D COFs into GO
[129]
laminates to realize a robust GO/COF laminar membrane . The atomically thin 2D COFs with pores serve
as a nano spacer to increase the interlayer spacing between GO nanosheets and provide direct transfer
channels, thereby reducing water transfer resistance. On the other hand, the COFs enhance the self-
supporting capacity of GO networks on a substrate with large pores. As a result, this strategy led to a
significant increase in the water permeance of the optimized GO/COF laminate membrane compared to the
pristine GO membrane without compromising its rejection rates to organic dyes. Mao et al. embedded
imidazole-ureido bola-amphiphile-imidazole compound as biomimetic imidazole-quartet water channels
into assembled GO laminates to enhance water transport selectivity over butanol benefiting from the
hydrophilic water-preferential nanochannels along the imidazole-ureido molecular scaffolds .
[130]
As the GO membrane is a showpiece that provides flexible platforms for developing versatile novel 2D-
material membranes, the methods mentioned above for regulating channel size are adaptable for other 2D-
material membranes. For example, Wang et al. related the performance of MoS membranes to the size of
2
[131]
their nanochannels in different hydration states . They found that the water-impermeable behavior of the
dry MoS nanochannel, which has a 0.62 nm interlayer spacing, is caused by the irreversible nanosheet
2
restacking during a drying process. In comparison, the fully hydrated MoS membrane possesses a stable
2
1.2 nm interlayer spacing, leading to high water permeability and moderate-to-high ionic and molecular
rejection. Meanwhile, the MoS nanochannel has a much stronger van der Waals attraction force than the
2
GO nanochannel, which prevents the interlayer spacing from increasing, thereby ensuring the aqueous
stability of MoS membranes. Inspired by the structural characteristics of GO membranes, where oxidized
2
zones act as spacers to provide a relatively large interlayer distance to accommodate water molecules, Ran et
al. incorporated appropriate acid spacers between g-C N interlayers to enlarge the width of the 2D
3
4
channels . The intercalation molecules successfully break up the tightly stacked structure of g-C N
[132]
4
3
laminates. Accordingly, the modified g-C N membranes give rise to two orders of magnitude higher water
4
3
permeance without sacrificing the separation efficiency. In a similar route, Wang et al. devised a method to
partially exfoliate dg-C N nanosheets with artificial nanopores and unstripped fragments as self-supporting
3
4
spacers and assemble them into 2D laminar membranes for water purification . The artificial nanopores
[19]
and the spacers between the partially exfoliated g-C N nanosheets provide numerous nanochannels for
3
4