Enhanced Separation Performance of Polyamide Thin-Film Nanocomposite Membranes with Interlayer by Constructed Two-Dimensional Nanomaterials: A Critical Review

Thin-film composite (TFC) polyamide (PA) membrane has been widely applied in nanofiltration, reverse osmosis, and forward osmosis, including a PA rejection layer by interfacial polymerization on a porous support layer. However, the separation performance of TFC membrane is constrained by the trade-off relationship between permeability and selectivity. Although thin-film nanocomposite (TFN) membrane can enhance the permeability, due to the existence of functionalized nanoparticles in the PA rejection layer, the introduction of nanoparticles leads to the problems of the poor interface compatibility and the nanoparticles agglomeration. These issues often lead to the defect of PA rejection layers and reduction in selectivity. In this review, we summarize a new class of structures of TFN membranes with functionalized interlayers (TFNi), which promises to overcome the problems associated with TFN membranes. Recently, functionalized two-dimensional (2D) nanomaterials have received more attention in the assembly materials of membranes. The reported TFNi membranes with 2D interlayers exhibit the remarkable enhancement on the permeability, due to the shorter transport path by the “gutter mechanism” of 2D interlayers. Meanwhile, the functionalized 2D interlayers can affect the diffusion of two-phase monomers during the interfacial polymerization, resulting in the defect-free and highly crosslinked PA rejection layer. Thus, the 2D interlayers enabled TFNi membranes to potentially overcome the longstanding trade-off between membrane permeability and selectivity. This paper provides a critical review on the emerging 2D nanomaterials as the functionalized interlayers of TFNi membranes. The characteristics, function, modification, and advantages of these 2D interlayers are summarized. Several perspectives are provided in terms of the critical challenges for 2D interlayers, managing the trade-off between permeability, selectivity, and cost. The future research directions of TFNi membranes with 2D interlayers are proposed.


Introduction
Membrane technology has been rapidly developing in the past 60 years, and it is widely used to remove unwanted contaminants in drinking water, wastewater treatment, and seawater desalination [1]. Polyamide (PA) thin-film composite (TFC) membrane, as the most successful commercialization, has experienced tremendous development since the concept of interfacial polymerization (IP) was first introduced by Mogan [2]. The typical PA-TFC membrane consists of the porous support layers and the PA rejection layer formed by the IP reaction from 1,3-phenylenediamine (MPD) or piperazine (PIP) with the 1,3,5benzenetricarbonyl trichloride (TMC) (Figure 1a). The state-of-the-art PA-TFC membranes are mainly employed for reverse osmosis (RO), nanofiltration (NF), and forward osmosis (FO) processes with the prominent water permeability and water/salts selectivity. However, different 2D nanomaterials' interlayers were summarized, which focused on their unique physicochemical and molecular sieving behaviors for high-efficiency separation membranes. Applications of prepared TFNi membranes in water and solvent separation systems were emphasized. This review also provided useful guidance on the rational selection of 2D nanomaterials, as well as preparation methods of the 2D interlayer for the development of high-performance TFNi membranes. (a) Interfacial polymerization reactions between the 1,3-phenylenediamine or piperazine with the 1,3,5-benzenetricarbonyl trichloride, as well as the molecular structure of the corresponding PA layers from molecular dynamics simulations, reproduced with permission from Ref. [48] with copyright permission from © 2020 American Chemical Society; (b) schematic illustration of the structures of TFC, TFN, TFCn, and TFNi membranes; (c) Recent publications on the TFNi membranes with 2D interlayers. All data were obtained by searching the keyword "TFNi" in the database of Scopus, under May 2022, with further manual screening. Figure 1. (a) Interfacial polymerization reactions between the 1,3-phenylenediamine or piperazine with the 1,3,5-benzenetricarbonyl trichloride, as well as the molecular structure of the corresponding PA layers from molecular dynamics simulations, reproduced with permission from Ref. [48] with copyright permission from © 2020 American Chemical Society; (b) schematic illustration of the structures of TFC, TFN, TFCn, and TFNi membranes; (c) Recent publications on the TFNi membranes with 2D interlayers. All data were obtained by searching the keyword "TFNi" in the database of Scopus, under May 2022, with further manual screening.

Two-Dimensional Nanomaterial's Interlayers
Although the reported TFNi membranes with 2D interlayer have been rapidly increased during the past five years, as shown in Figure 1c, the 2D nanomaterial's interlayer for TFNi membranes has not been well summarized yet. Based on the above advantages and scalability of the 2D interlayer, an overview of the current development was proposed on the 2D nanomaterial's intercalated TFNi membranes. The TFNi membranes with the different 2D nanomaterials' interlayers were summarized, which focused on their unique physicochemical and molecular sieving behaviors for high-efficiency separation membranes. Applications of prepared TFNi membranes in water and solvent separation systems were emphasized. This review also provided useful guidance on the rational selection of 2D nanomaterials, as well as preparation methods of the 2D interlayer for the development of high-performance TFNi membranes.

Two-Dimensional Nanomaterial's Interlayers
The 2D nanomaterials, as interlayers, provide unique prospects on the performance enhancement of TFNi membranes [49][50][51]. The 2D nanomaterials with one or few atoms thick- ness are appropriate materials for producing the thin and homogenous nanoscale interlayers [52]. The interlayers of TFNi membranes are constructed by stacking 2D nanomaterials, which may generate the nano-scale channels in the nanosheets slit and gallery [42]. These nano-scale channels can increase the transfer and achieved high-efficiency ion/molecular sieving. Furthermore, the certain 2D nanomaterials have promising mechanical properties and chemical stability, allowing for the utilization of organic solvent separation, even in the hostile chemical conditions [53,54]. In addition, the 2D nanomaterial's interlayers could improve IP reaction and, thus, the formation of better-quality PA layers. In the following sections, we emphasized the representative 2D nanomaterials as the interlayers for the prepared TFNi membranes, as well as the applications of TFNi membrane in water and solvent separation systems.

Graphene Oxide
Graphene oxide (GO), a chemical derivative of 2D graphene, attracted extensive research for developing high performance membranes [39,55]. With a high area-to-thickness ratio and abundant functional groups on the surface, the GO nanosheets can be stacked on top of each other to form large-area thin films or membranes [56,57]. The well-arranged GO nanosheets showed the promising gas and liquid separation performance. Furthermore, the control of microstructures and modifications of GO nanosheets using various methods have further enhanced the GO membrane performances [58]. However, controlling the pores and scaling up the GO membranes remain great challenges.
The GO nanosheets have received significant attention as promising interlayers. For instance, the GO nanosheets being directly deposited on the surface of support layers [59] and formation of the PA layer on the GO modified support layers were prepared by the IP reaction with MPD and TMC (Figure 2a). The GO nanosheets with oxygen-containing groups could adsorb more amino-contained MPD molecules, which facilitated the controlled formation of PA layers. The results demonstrated that confined IP reaction could fabricate defect-free, highly permeable, and excellently selective RO membranes ( Figure 2b). Subsequently, an ultrathin hydrophilic interlayer was proposed for the fabrication of TFNi membranes, which comprised the polydopamine (PDA) modified GO nanosheets by selfpolymerization [60]. This hydrophilic interlayer enhanced the pure water permeability of the support layer membranes. The prepared TFNi membrane, with a PDA-modified GO interlayer, showed great improvement of water flux (57.6%) without severe loss of the reverse solute flux under the FO process. In addition, a new technique of plasma-enhanced chemical vapor deposition (PECVD) was proposed for the modification of GO nanosheets. Seah et al. [61] employ the PECVD method to synthesize different types of surface-coated GO nanosheets as interlayers for the fabrication of TFNi membranes using emerging mistbased IP reaction. The effects of hydrophobic/hydrophilic coating on the GO properties and how the changes on its surface chemistry could influence the properties of TFNi membranes for salt/dye mixture treatment were investigated. The prepared TFNi membrane could recover 79-86% of NaCl from the feed, producing saline permeate containing <0.3% pigment.

Mxenes
MXenes are emerging 2D nanomaterials derived from the layered ternary Mn+1AXn, or MAX phases, where 'M' is a transition metal, 'A' is a group IV-V element, and 'X' is carbon and/or nitrogen [62,63]. Currently, more than 30 MXenes nanomaterials have been experimentally synthesized, containing transition metal carbides, nitrides, or carbonitrides [64]. The MXenes with the high surface charge, excellent hydrophilicity, large specific surface area, the changeable molecular structures, and chemical compositions [65], have great potentials in designing and fabricating functional composite membranes.
For instance, Ti3C2Tx, as a representative of MXenes, is hydrophilic with a negatively charged surface similar to GO nanosheets [66]. The functional groups of grafted Ti3C2Tx nanosheets were employed to fabricate TFN membranes for organic solvent nanofiltration [67]. The 2D MXenes, as the interlayer between the substrates and PA layer, had attracted to improve the performance of FO membranes. Wu et al. [68]  Compared with the TFC membrane, the prepared TFNi membrane featured an interlayer of the CNT-intercalated MXene, which attained a water flux that was four times higher and a lower specific salt flux under FO tests. Thus, it showed that a highly permeable CNT-intercalated MXene interlayer could provide an enhanced "gutter" effect, and it improved the FO performance of the corresponding TFNi membranes.

Mxenes
MXenes are emerging 2D nanomaterials derived from the layered ternary M n+1 AX n , or MAX phases, where 'M' is a transition metal, 'A' is a group IV-V element, and 'X' is carbon and/or nitrogen [62,63]. Currently, more than 30 MXenes nanomaterials have been experimentally synthesized, containing transition metal carbides, nitrides, or carbonitrides [64]. The MXenes with the high surface charge, excellent hydrophilicity, large specific surface area, the changeable molecular structures, and chemical compositions [65], have great potentials in designing and fabricating functional composite membranes.
For instance, Ti 3 C 2 T x , as a representative of MXenes, is hydrophilic with a negatively charged surface similar to GO nanosheets [66]. The functional groups of grafted Ti 3 C 2 T x nanosheets were employed to fabricate TFN membranes for organic solvent nanofiltration [67]. The 2D MXenes, as the interlayer between the substrates and PA layer, had attracted to improve the performance of FO membranes. Wu et al.  Figure 3d,e), as well as the facilitation of water and ethanol transportation by the interlayer gallery, between Ti 3 C 2 T x nanosheets. In addition, Sun et al. [40] fabricated a carbon nanotubes (CNT)-intercalated MXene interlayer for a novel TFNi membrane. Compared with the TFC membrane, the prepared TFNi membrane featured an interlayer of the CNT-intercalated MXene, which attained a water flux that was four times higher and a lower specific salt flux under FO tests. Thus, it showed that a highly permeable CNT-intercalated MXene interlayer could provide an enhanced "gutter" effect, and it improved the FO performance of the corresponding TFNi membranes.

Molybdenum Disulfide
Molybdenum disulfide (MoS2) is another 2D nanomaterials that has recently attracted greater attention than GO nanosheets [41,69,70]. The MoS2 has the three atomic layers, containing the molybdenum atomic layer, sandwiched between two sulfur atomic layers. Then, the MoS2 nanosheets show a unique layered morphology that resembled a "sandwich" structure. Simultaneously, the stacked MoS2 nanosheets may generate smooth and stiff sub-nanoscale channels, contributing to the high water flux and ion/molecular selectivity [71]. A hierarchical flower-like structure of molybdenum sulfide (HF-MoS2) was deposited on the polyethersulfone (PES) ultrafiltration substrate by vacuum filtration [72] (Figure 4a). The excellent hydrophilicity and large specific surface area of HF-MoS2 was beneficial to the dispersion of PIP solution by IP reaction; then, a loose and  [68], with copyright permission from © 2020 American Chemical Society.

Molybdenum Disulfide
Molybdenum disulfide (MoS 2 ) is another 2D nanomaterials that has recently attracted greater attention than GO nanosheets [41,69,70]. The MoS 2 has the three atomic layers, containing the molybdenum atomic layer, sandwiched between two sulfur atomic layers. Then, the MoS 2 nanosheets show a unique layered morphology that resembled a "sandwich" structure. Simultaneously, the stacked MoS 2 nanosheets may generate smooth and stiff sub-nanoscale channels, contributing to the high water flux and ion/molecular selectivity [71]. A hierarchical flower-like structure of molybdenum sulfide (HF-MoS 2 ) was deposited on the polyethersulfone (PES) ultrafiltration substrate by vacuum filtration [72] ( Figure 4a). The excellent hydrophilicity and large specific surface area of HF-MoS 2 was beneficial to the dispersion of PIP solution by IP reaction; then, a loose and defect-free PA layer was obtained. The results showed that the TFNi membranes with specific density of HF-MoS 2 had excellent separation performance (e.g., M3 membrane showed the pure water permeance of 18.3 L m −2 h −1 bar −1 , and the rejections of Na 2 SO 4 and MgSO 4 were above 98%) (Figure 4b). In addition, compared with the TFC membrane (M0), the TFNi membranes (M3) exhibited excellent structural stability in the long-term NF performance evaluation (Figure 4c).
Membranes 2022, 12, x FOR PEER REVIEW 7 of 16 defect-free PA layer was obtained. The results showed that the TFNi membranes with specific density of HF-MoS2 had excellent separation performance (e.g., M3 membrane showed the pure water permeance of 18.3 L m −2 h −1 bar −1 , and the rejections of Na2SO4 and MgSO4 were above 98%) (Figure 4b). In addition, compared with the TFC membrane (M0), the TFNi membranes (M3) exhibited excellent structural stability in the long-term NF performance evaluation (Figure 4c). A performance summary of TFNi membranes with the above 2D nanomaterial's interlayers was shown in Table 1. It was noting that the 2D interlayer intercalated TFNi membranes exhibited the simultaneous enhancement on water/solvent's fluxes and rejections for RO, NF, and FO processes. In particular, the 2D interlayers were obtained by advanced preparation techniques, e.g., chemical modification and CVD, which showed an enhanced "gutter" effect and improved the separation performance. Then, it was believed that the 2D interlayer may pave a brand-new avenue for designing and fabricating high-performance TFNi membranes.  A performance summary of TFNi membranes with the above 2D nanomaterial's interlayers was shown in Table 1. It was noting that the 2D interlayer intercalated TFNi membranes exhibited the simultaneous enhancement on water/solvent's fluxes and rejections for RO, NF, and FO processes. In particular, the 2D interlayers were obtained by advanced preparation techniques, e.g., chemical modification and CVD, which showed an enhanced "gutter" effect and improved the separation performance. Then, it was believed that the 2D interlayer may pave a brand-new avenue for designing and fabricating high-performance TFNi membranes.

Interlayers from Emerging Nanosheets
Metal-organic framework (MOF) and covalent organic framework (COF) are burgeoning porous nanomaterials, which had wide interests in recent years because of their interesting chemistry and potential applications. In membrane science, researchers have extensively explored the use of MOF and COF for gas and liquid separations [75]. For instance, zeolitic-imidazolate framework (ZIF), a subclass of MOF, has been gaining attention in membrane materials for H 2 /CO 2 separation [76] and ethanol recovery [29]. The ability to exfoliate MOF and COF into ultrathin nanosheets enables a range of new opportunities for the interlayer materials of TFNi membranes.

MOF Nanosheets
MOF is a kind of nano-porous crystalline materials containing the transition metal ions as nodes and organic ligands as connecting chains [77,78]. Exfoliated MOF nanosheets with ultrathin thickness, large surface area, and high surface-to-volume atom ratios are attracting increasing attention in the last few years [48,69,79]. The MOF nanosheets can exploit the voids on the surface of nanosheets to provide a unique diffusion pathway for water or solvent molecules. Meanwhile, the subatomic surface function of MOF nanosheets can effectively enhance the compatibility between PA and MOF, which was beneficial to improve the water/solvent flux and selectivity of TFNi membranes [80]. In this section, we summarize the recent advances in the utilization of MOF nanosheets as interlayer for the prepared TFNi membranes and how they improved the separation performance.
To increase water permeability and dye rejections, Carlos et al. [81] created a unique TFNi membrane for the NF process by depositing a ZIF-93 interlayer on the surface of the polyimide (P84) hollow fiber membrane. The ZIF-93 interlayer includes aldehyde functional groups and could be reacted with MPD monomer. This process will neutralize the HCl during interfacial polymerization and form a compact PA layer. Compared with TFC membrane, the TFNi membrane with ZIF-93 interlayer not only improved a fourfold increase in water permeability but also increased the rejection of dye (Sunset yellow, 450 Da) from 88% to 98%. Wen et al. [69] used ultrathin 2D MOF nanosheets (ZnTCPP) as interlayer to fabricate the TFNi membrane (Figure 5a). Compared with the conventional TFC membrane (TFC-MOF-0) (Figure 5b), the larger size and wider leaf-like features of PA layer were observed, and the thinner and defect-free PA layer on the ZnTCPP modified substrates was produced (Figure 5c). Then, the optimal TFNi membrane (TFC-MOF-2) simultaneously increased water permeance (4.82 L m −2 h −1 bar −1 ) and NaCl rejection (94.7%) under the RO mode test (Figure 5d). Zhao et al. [42] focused on the development of high-performance RO membranes, which used UiO-66-NH 2 MOF as interlayer to improve the separation efficiency of TFNi membrane. The obtained PA layer on the UiO-66-NH 2 modified PSf substrate showed a greater surface area, which allowed the formation of In addition, the TFNi membranes with MOF interlayer have promising applications in an organic solvent system. For instance, Lu's group [29] demonstrated the facile fabrication of ZIF-8 interlayers for TFNi membranes by simultaneous phase inversion and the crosslinking process. The prepared TFNi membranes with ZIF-8 interlayer had remarkable water and solvent permeances under the RO mode, as well as the higher ethanol flux under organic solvent forward osmosis (OSFO). Zhang et al. [82] successfully synthesized a stable modified ZIF-8 (MZIF-8), as an interlayer on a ceramic support, before interfacial polymerization. The prepared TFNi membranes with MZIF-8 interlayer exhibited higher dehydrating performance for ethanol-water mixtures.

COF Nanosheets
COF is a new class of crystalline organic porous materials, which constructed the periodic networks connected by covalent bonds and are usually referred to as "organic zeolites" [83]. The variety of organic ligand structures may be used to build multifarious COF crystals with 2D structures [84]. Even in strong acid, strong alkali, and hostile solvent conditions, COF offers superior thermal and chemical stability to MOF [43]. The 2D COF nanosheets were prepared by two categories: top-down and bottom-up, which possessed a wide variety of tunable properties [85]. Thus, the 2D COF nanosheets have shown great promising applications in separation [86], energy storage [87], catalysis [88], etc.
Compared with GO nanosheets, the 2D COF nanosheets may serve as ideal nanointerlayers for constructed TFNi membranes, offering additional solvent channels and solute selectivity due to their variable pore size and 2D flat-plate structures [89]. Some competitive fabrication methods, such as in-situ growth and vacuum filtration, have been explored to produce continuous and defect-free 2D COF interlayers [90,91]. However, how In addition, the TFNi membranes with MOF interlayer have promising applications in an organic solvent system. For instance, Lu's group [29] demonstrated the facile fabrication of ZIF-8 interlayers for TFNi membranes by simultaneous phase inversion and the crosslinking process. The prepared TFNi membranes with ZIF-8 interlayer had remarkable water and solvent permeances under the RO mode, as well as the higher ethanol flux under organic solvent forward osmosis (OSFO). Zhang et al. [82] successfully synthesized a stable modified ZIF-8 (MZIF-8), as an interlayer on a ceramic support, before interfacial polymerization. The prepared TFNi membranes with MZIF-8 interlayer exhibited higher dehydrating performance for ethanol-water mixtures.

COF Nanosheets
COF is a new class of crystalline organic porous materials, which constructed the periodic networks connected by covalent bonds and are usually referred to as "organic zeolites" [83]. The variety of organic ligand structures may be used to build multifarious COF crystals with 2D structures [84]. Even in strong acid, strong alkali, and hostile solvent conditions, COF offers superior thermal and chemical stability to MOF [43]. The 2D COF nanosheets were prepared by two categories: top-down and bottom-up, which possessed a wide variety of tunable properties [85]. Thus, the 2D COF nanosheets have shown great promising applications in separation [86], energy storage [87], catalysis [88], etc.
Compared with GO nanosheets, the 2D COF nanosheets may serve as ideal nanointerlayers for constructed TFNi membranes, offering additional solvent channels and solute selectivity due to their variable pore size and 2D flat-plate structures [89]. Some competitive fabrication methods, such as in-situ growth and vacuum filtration, have been explored to produce continuous and defect-free 2D COF interlayers [90,91]. However, how to fabricate 2D COF interlayers with structural integrity and stability is still facing challenges.
Recent studies showed that the uniform deposition of COF nanosheets on the PES support layer, by self-assembly method, can effectively control the interfacial polymerization and reduce the thickness of the PA selective layer (~10 nm). This method reduced the internal concentration polarization (ICP) and improved water transport [90]. The approximately eight-fold enhancement in the water flux of TFNi membrane with COF interlayer was found, without affecting the rejections. Jiang et al. [91] proposed that the COF interlayer enhanced the anti-fouling performance and long-term stability of the final TFNi membrane. They used the direct in-situ growth of COF nanosheets on the PSf hollow fibers (HF) (Figure 6a). The excellent anti-fouling performance (99.1% higher than TFC membrane) was obtained by the BSA fouling experiments (Figure 6c). Meanwhile, the TFNi membranes with COF interlayer exhibited excellent long-term stability (Figure 6d).
He et al. [89] successfully prepared sulfonated COF interlayer on the TMC modified surface of the nylon membrane support by vacuum filtration. The introduction of sulfonated COF interlayer rendered the TFNi membrane with lower water contact angle, more negative charge, thinner PA thickness, and reduced structure parameter. Then, the permeability and selectivity of TFNi membranes were significant enhancements. The prepared membrane overcame the trade-off between the flux and rejection in FO mode, and a superior selectivity (J w /J s = 24.7 L/g) was proposed. In addition, the COF interlayer with structural integrity and stability was beneficial for the preparation of ultra-smooth and ultra-thin PA rejection layers. Yang et al. [43], in-situ, constructed a nano-porous interlayer of 2D COF (TpPa-1) on the PI support for organic solvent nanofiltration (OSN). The introduced COF interlayer interacted with the MPD, leading to an accelerated self-sealing and self-termination of the IP process. The synergetic effect resulted in an ultra-thin PA rejection layer and a greatly increased ethanol permeance. Then, the prepared TFNi membrane showed an ethanol permeance of 60 L m −2 h −1 MPa −1 , and the rejection rate of Rhodamine B was higher than 99% (Table 2). proximately eight-fold enhancement in the water flux of TFNi membrane with COF inter layer was found, without affecting the rejections. Jiang et al. [91] proposed that the CO interlayer enhanced the anti-fouling performance and long-term stability of the final TFN membrane. They used the direct in-situ growth of COF nanosheets on the PSf hollow f bers (HF) (Figure 6a). The excellent anti-fouling performance (99.1% higher than TFC membrane) was obtained by the BSA fouling experiments (Figure 6c). Meanwhile, th TFNi membranes with COF interlayer exhibited excellent long-term stability (Figure 6d

Conclusions and Perspectives
The TFNi membranes with interlayer have been receiving significant attention in recent years, and most of the current research mainly focuses on water and/or solvent separation processes. The interlayer constructed from 2D nanomaterials has injected new vitality into the TFNi membrane. For instance, the 2D interlayer was facilitated to regulate IP reaction (e.g., distribution of the amine monomer), leading to uniform and dense PA selective layers. The 2D COF nanosheets interlayer reduces the thickness of the PA selective layer by IP reaction. Furthermore, there are significant enhancements on the long-term stability, mechanical strength, chlorine resistance, and anti-fouling capabilities of the final TFNi membranes. In this review, a new generation composite membrane-TFNi with 2D interlayer-was reviewed for water and solvent separation processes. The 2D nanomaterials usually have high aspect ratios and large lateral size; thus, they can easily cover the macropores on the substrate through several layers of stacking, providing a smooth and uniform interlayer needed for IP process. Moreover, the oriented assembly of 2D nanomaterials provides a low friction flow [92], which ensures a low permeation resistance and improves the "gutter" effects.
Nevertheless, the 2D nanomaterials' interlayers facing major challenges are facile preparation, low-cost, and stability. Based on these considerations, the dip-coating method may be the most practical way for preparation of a 2D interlayer in view of its simple operation, relatively low-cost, and its existing use in commercial membrane production lines. Moreover, the stability of emerging 2D MOF or COF nanosheets' interlayers is still needed to develop for industrial implementation. Meanwhile, the performance of TFNi membranes strongly depends on microstructures and functionalization of 2D interlayers. Future studies need to perform a more systematic development on the scalable methods for deliberate control of 2D interlayers, especially microstructures and functionalization. More novel and accurate transport models and experiments should be proposed for the unique 2D interlayer in the TFNi membranes. It is significantly important to understand the interaction between PA and 2D interlayer (either physically or chemically), as well as the actual transport mechanism of solutes in the TFNi membranes, particularly those incorporated with nanosheets/nanoporous nanomaterials. Ultimately, the selection and utilization of 2D nanomaterials as interlayers for the fabrication of TFNi membranes should depend on the constituent of feed solutions, e.g., salt ions, dyes, active pharmaceutical ingredients, etc.

Conflicts of Interest:
The authors declare no conflict of interest.