1. Fabrication of MXene-Based Membranes

The ideal filtration membranes should be defect free, ultrathin, a dense film and mechanically robust with a high selectivity for small molecules along with good antifouling and antibacterial properties. Generally, methods such as spin coating, spray coating, vacuum filtration, Langmuir–Blodgett, drop casting, or direction evaporation and dip coating are used for the fabrication of 2D MXene lamellar membranes. Gogotsi et al. ^[1][63] used a vacuum filtration method for first time to prepare freestanding and PVDF-supported 2D Ti₃C₂T_x-based membranes. Such membranes demonstrated good hydrophilic properties because of the presence of the useful group in conjunction with excellent elasticity as well as a good mechanical strength, which is an ideal potential in separation membranes. Among these methods, the VF method is widely used for the fabrication of 2D MXene-based membranes due to its simplicity and ease of operation (Figure 1b). Ding et al. ^[2][64] also reported 2D MXene (Ti₃C₂T_x) with enhanced properties using a vacuum filtration method on a porous support whereas Kang et al. ^[3][65] fabricated MXene (Ti₃C₂T_x) and GO-based composite membranes by the same method. Sun and coworkers also fabricated GO/MXene lamellar membranes by the filtration method ^[4][66]. Wang and co-researchers worked out an improvement of the microstructure and physiochemical properties of an MXene membrane by mixing it with a polymer matrix. Recently, Wang et al. ^[5][67] reported Ti₃C₂T_x lamellar membranes produced by employing a multivalent ion as a hydrogel pillar in the interlayer spacing. Researchers for obtaining the uniform composition mixed a solution of sodium alginate (SA) and MXene Ti₃C₂T_x; this composite, SA-Ti₃C₂T_x, was then used for the lamellar SA-Ti₃C₂T_x membranes. Fascinatingly, a molecule of SA attached onto the MXene sheets by hydrogen bonding and Van der Waals forces. Finally, pillared SA-Ti₃C₂T_x laminates were arranged by submerging an SA-Ti₃C₂Tx membrane into a solution of different types of multivalent cations such as Ca²⁺, Ba²⁺, Mn²⁺, and Al³⁺. The pillar membrane showed a homogeneous structure similar to a nacre-like composite and it considerably decreased the swelling effect. Liu et al. ^[6][68] fabricated Ti₃C₂Tx-CNT hybrid membranes using vacuum filtration (Figure 1c,d). Liu and coworkers also fabricated pristine Ti₃C₂Tx and CNT membranes for comparative studies using the VF method (Figure 1g,h). They improved the mechanical stability and permeance of MXene by incorporating CNTs into Ti₃C₂Tx nanosheets. Huang et al. ^[7][48] used a phase inversion process to fabricate a PES-Ni@MXene membrane by using an external field and incorporated magnetic Ni@MXene nanoparticles with the upper layer of the PES membrane during a wet phase inversion process. MXene-based lamellar membranes were also prepared by a layer-by-layer (LbL) method ^[8][69]. Tian et al. assembled a tris(2-aminoethyl) amine (TAEA) molecule and Ti₃C₂Tx MXene using an LbL assembly and obtained highly ordered multilayer of MXene/TAEA with an interlayer distance ~1 Å. This strategy was a good addition to fabricate MXene-based multilayered membranes for large-scale applications.

From the above studies, it was concluded that the vacuum filtration method was mostly used to fabricate MXene membranes. However, there are several disadvantages associated with the vacuum filtration method. It needs a large volume of solvent, takes long time, and is definitely difficult to scale up. Therefore, alternative methods such as the shear alignment method, printing method, and spin coating method should be utilized to fabricate state-of-the-art MXene-based laminar membranes with advanced physicochemical properties to fully utilize the power of this wonder material.

2. Antibacterial Activity of MXene-Based Membranes

Pathogenic contamination is considered to be the most harmful issue worldwide and is responsible for various kinds of waterborne diseases ^[10][90]. It is directly responsible for the biofouling of any water filtration membrane; therefore, it is important that a membrane should be tested against antibacterial properties. Up to date, several bactericidal nanomaterials including graphene, TMDCs, and MXenes have been explored to meet these challenges. The antibacterial activity of graphite, graphite oxide, GO, rGO, MoS₂, and WS₂ against Gram-negative and Gram-positive bacteria have already been tested. Recently, MXenes with unique hydrophilic properties, a good adsorption, an ideal surface functionality, and excellent biocompatibility and photothermal properties have been widely tested for wastewater treatment and desalination, water purification, ion separation and other applications, as shown in Table 1. MXenes are expected to be resistant to biofouling and offer bactericidal properties ^[11][91]. However, very few studies ^{[11][12][13][14][15][16][17][18][19]}[91,92,93,94,95,96,97,98,99] have been carried out in this direction. An initial work by Rasool et al. ^[17][97] reported that Ti₃C₂Tx membranes could be an ideal platform for antibacterial studies (Figure 5a–d). Rasool et al. ^[17][97] further used Ti₃C₂T_x-based membranes to measure the antibacterial properties against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) by using bacterial growth curves based on optical densities (OD) and colony growth on agar nutritive plates (Figure 5b,c). The membranes showed a high antibacterial efficiency against both Gram-negative E. coli and Gram-positive B. subtilis compared with the GO membranes. Concentration-dependent antibacterial activity was observed and more than 98% of bacterial cell viability loss was found at 200 μg/mL in Ti₃C₂T_x for both bacterial cells within 4 h of exposure, as confirmed by a colony-forming unit (CFU) and regrowth curve (Figure 5d,e). In another study, Ti₃C₂T_x/PVDF composite membranes were tested to measure the antibacterial rate of E. coli and B. subtilis ^[11][91]. The composite membranes showed a ~73% and ~63% antibacterial rate for B. subtilis and E. coli, respectively, compared with the control PVDF membranes ^[11][91]. Additionally, the Ti₃C₂T_x membrane showed over a 99% growth inhibition of both bacteria under the same conditions. Mayerberger et al. ^[12][92] demonstrated Ti₃C₂Tz/chitosan composite nanofiber membranes for a passive antibacterial wound dressing application. The as-prepared composite membrane showed a 95% and 62% reduction in the colony-forming units of Gram-negative E. coli and Gram-positive Staphylococcus aureus (S. aureus), respectively. Jastrzebsa and coworkers also reported the antimicrobial properties of a Ti₃C₂ MXene-based nanocomposite, i.e., Ti₃C₂/SiO₂/Ag, Ti₃C₂/Al₂O₃/Ag, and Ti₃C₂/SiO₂/Pd ^[13][93]. They also demonstrated the outstanding bioactive properties of Ti₂C and Ti₃C₂ MXenes against a Gram-negative bacterial strain ^[19][99]. Recently, Zhu et al. ^[15][95] evaluated the effect of near-infrared (NIR) light on the antibacterial activities of silver (Ag), Ti₃C₂T_x, and an Ag/Ti₃C₂T_x composite. The as-prepared Ag/Ti₃C₂T_x composite showed a high efficacy against Gram-positive S. aureus and Gram-negative E. coli bacteria in an in vitro antibacterial test. Upon NIR irradiation, the antimicrobial effect of Ag/Ti₃C₂T_x significantly strengthened compared with the pristine Ag and Ti₃C₂T_x. The growth of E. coli was completely inhibited during the initial 0–6 h by 200 µg/mL of Ti₃C₂T_x due to the photothermal heat produced killing the bacteria in the surrounding area. The Ag/Ti₃C₂T_x composite exhibited the best antibacterial activities with the same dose of pristine Ag and Ti₃C₂T_x. After NIR irradiation, the Ti₃C₂T_x composite could completely restrain the E. coli growth when used at 100–200 µg/mL.