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Page 2 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

outlook in the further exploration of separation mechanism, large-scale manufacturing, and the eventual
commercialization of the membranes.

Keywords: 2D nanosheet, nanochannel, membrane, separation, regulation, construction

INTRODUCTION
Membrane separation technologies have become revolutionized various industries by offering numerous
benefits, such as low cost, high efficiency, and environmental sustainability. These technologies utilize thin
membrane barriers that separate specific substances based on variations in size, charge, or other properties.
Membranes can be roughly categorized into macrochannel membranes, microchannel membranes, and
nanochannel membranes, depending on the size of the channels and their characteristic scale.
Macrochannel membranes possess larger channel dimensions, typically in the millimeter to centimeter
range. They are designed for applications involving larger particles or higher flow rates and are commonly
used in pre-treatment processes in various industries, including chemical, environmental, pharmaceutical,
mining, and more. Microchannel membranes have smaller channel dimensions than macrochannels but are
still relatively large when compared to nanochannels. Typically, their channel sizes range from tens to
hundreds of microns in diameter. They find a variety of applications in areas such as microfluidics,
chemical synthesis, and biomedical devices. Nanochannel membranes feature channels with dimensions
typically in the nanometer range. These membranes are engineered with extremely small channel sizes to
precisely control molecular or ion transport. They are extensively used in diverse applications such as water
treatment, ion selectivity, gas separation, DNA sequencing, protein analysis, drug delivery systems, etc. Each
scale of the membrane has its own strengths and weaknesses (as shown in Table 1 below), and the choice of
the membrane depends on the specific separation requirements, target molecules or ions, and operating
[1-7]
conditions .
Compared with other membranes, nanochannel membranes have unique advantages due to their extremely
small channel size, offering the potential for highly selective separations, reduced sample consumption, and
single-molecule analysis. These properties make nanochannel membranes particularly suitable for
applications that require precise control of molecular transport and analysis and have received increasing
attention from the scientific and engineering communities [8-14] . To meet specific requirements, ultrathin
nanochannel membranes can be directly assembled from a variety of inorganic or organic materials. Two-
dimensional (2D) materials are of particular interest in the field of nanochannel engineering due to their
unique physical and chemical properties, including atomic-level thickness, hydrophilicity, and ductility. A
[15]
diverse range of 2D materials are currently under investigation, such as graphene , graphene oxide
(GO) , transition metal carbides/nitrides (MXenes) , hexagonal boron nitride (h-BN) , graphitic carbon
[16]
[18]
[17]
[22]
[19]
nitride (g-C N ) , transition metal dichalcogenides (TMDs) [20,21] , layered double hydroxides (LDHs) , 2D
4
3
zeolites , 2D metal-organic frameworks (2D MOFs) , 2D covalent organic frameworks (2D COFs) ,
[23]
[24]
[25]
and beyond. These materials offer a wealth of possibilities for creating high-performance 2D-material-based
nanochannel membranes. The superb separation performance achieved by nanochannel membranes based
on 2D materials is mainly attributed to the unique properties and structural characteristics of the materials.
The atomic-level thickness of 2D materials allows the formation of nanochannels with precise dimensions,
which can effectively restrict the passage of certain molecules or ions depending on their size. In addition,
the layer spacing and surface chemistry of the nanochannels can be modulated by introducing functional
groups or modifying the surface, further affecting the selectivity of the membrane. In addition, the high
aspect ratio and large surface area of 2D materials provide abundant active sites for interaction with the
target material, thus enabling molecular sieving and preferential ion transport. In addition, the ordered
arrangement of nanosheets in the membrane structure facilitates the formation of continuous and uniform