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Metal/covalent-organic framework (MOF/COF) membranes have attracted increasing research interest and have been considered as state-of-the-art platforms applied in various environment- and energy-related separation/transportation processes. To break the trade-off between permeability and selectivity to achieve ultimate separation, recent studies have been oriented towards how to design and exploit ultrathin MOF/COF membranes (i.e. sub-1 μm-thick). Given great advances made in the past five years, it is valuable to timely and systematically summarize the recent development and shed light on the future trend in this multidisciplinary field. In this review, we first present the advanced strategies in fabricating ultrathin defect-free MOF/COF membranes such as in situ growth, contra-diffusion method, layer-by-layer (LBL) assembly, metal-based precursor as the pre-functionalized layer, interface-assisted strategy, and laminated assembly of MOF/COF nanosheets. Then, the recent progress in some emerging applications of ultrathin MOF/COF membranes beyond gas separation is highlighted, including water treatment and seawater desalination, organic solvent nanofiltration, and energy-related separation/transportation (i.e. lithium ion separation and proton conductivity). Finally, some unsolved scientific and technical challenges associated with future perspectives in this field are discussed, inspiring the development of next-generation separation membranes.

Structural colors are in increasing demand in many fields such as color displays, sensor devices, and anti-counterfeiting labels. Herein, large-scale and angle-independent structural colors were constructed from polydopamine (PDA) coatings with a high refractive index and uniformity. This strategy can be utilized to facilely fabricate various color patterns. These structural colors also display high solvent resistance and excellent universality to a series of substrates. Moreover, stimuli-responsive structural colors can be easily synthesized via the co-deposition of PDA with poly(N-isopropylacryl-amide). To the best of our knowledge, this is the first time that structural colors from PDA-based coatings have been reported.

Two-dimensional (2D) nanosheets have attracted extensive attention and have been considered as compelling nanofillers of polyamide-based nanofiltration membranes (NFMs) with thin film nanocomposite (TFN) frame. However, there still remains a big challenge, the poor compatibility between these 2D nanofillers and the polyamide matrix, significantly restricting their practical applications. Herein, we present TFN NFMs by incorporating tannic acid (TA) functionalized molybdenum disulfide (MoS2) nanosheets into the polyamide skin layers via typical interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC). TA was employed to assist the aqueous exfoliation of bulk MoS2 into 2D nanosheets, and simultaneously served as the functional molecule to modify and stabilize the as-exfoliated MoS2 nanosheets in aqueous solutions. Thanks to the contribution of TA, the MoS2 nanosheets can be covalently linked into the skin layers through the reaction of TA and TMC, thus exhibiting excellent compatibility with polyamide matrix. Besides, the as-prepared TFN NFMs show promoted water permeation as high as 17.0 L/m2⋅h⋅bar without compromising the salt rejection of divalent ions (98.5%). Moreover, these high-performance TFN NFMs also possess outstanding long-term stability, with great implication for the practical applications.