Covalent organic frameworks (COFs) are a type of crystalline organic porous material with specific features and interesting structures, including porosity, large surface area, and biocompatibility. These features enable COFs to be considered as excellent candidates for applications in various fields. Recently, COFs have been widely demonstrated as promising materials for biomedical applications because of their excellent physicochemical properties and ultrathin structures. In this review, we cover the recent progress of COF materials for applications in photodynamic therapy, gene delivery, photothermal therapy, drug delivery, bioimaging, biosensing, and combined therapies. Moreover, the critical challenges and further perspectives with regards to COFs for future biology-facing applications are also discussed.

Reliable and nondestructive monitoring of food quality is of great importance in sustaining life and promoting good health. Herein, we developed sensitive, fast, reversible, and nanometer-thick fluorescent films for the nondestructive evaluation of fish freshness. The nanofilms were prepared via the dynamic condensation of tetraphenylethylene derivative (TPEBA) with Calix[4]pyrrole derivative (CPTH) at the humid air/DMSO interface. The amorphous nanofilm is uniform with the thickness in the range of 12∼58 nm. Owing to the aggregation-induced emission (AIE) property of TPEBA, the nanofilm is highly emissive with a Stokes shift of ∼175 nm. The typically designed chemical composition and nanostructure endow the film-preferable affinity to amine vapors, and the networked structure allows fast mass transfer, which lays foundation for high-performance sensing. With an optimized nanofilm-based sensor, biogenetic amines were sensitively, selectively, and reversibly detected. The detection limit (DL) for trimethylamine (TMA) is 0.89 ppm. Typically, interference from water can be neglected; thus, the nondestructive evaluation of fish freshness was realized. Moreover, a portable seafood freshness detector was conceptually built.

The modular construction of defect-free nanofilms with a large area remains a challenge. Herein, we present a scalable strategy for the preparation of calix[4]­pyrrole (C[4]­P)-based nanofilms through acryl hydrazone reaction conducted in a tetrahydrazide calix[4]­pyrrole (CPTH)-based self-assembled layer at the air/DMSO interface. With this strategy, robust, regenerable, and defect-free nanofilms with an exceptionally large area (∼750 cm2) were constructed. The thickness and permeability of the film systems can be fine-tuned by varying the precursor concentration or by changing another building block. A typical nanofilm (C[4]­P-TFB, ∼67 nm) depicted high water flux (39.9 L m–2 h–1 under 1 M Na2SO4), narrow molecular weight cut-off value (∼200 Da), and promising antifouling properties in the forward osmosis (FO) process. In addition, the nanofilms are stable over a wide pH range and tolerable to different organic solvents. Interestingly, the introduction of C[4]P endowed the nanofilms with both outstanding mechanical properties and unique group-selective separation capability, laying the foundation for wastewater treatment and pharmaceutical concentration.

Lead-free (1-x)(K 0.37 Na 0.63 )NbO 3 -xCa(Sc 0.5 Nb 0.5 )O 3 (x=0.050, 0.070, 0.090, 0.095 and 0.100) transparent ferroelectric ceramics have been fabricated by pressureless sintering procedure. Transmittance of 0.91(K 0.37 Na 0.63 )NbO 3 -0.09Ca(Sc 0.5 Nb 0.5 )O 3 ceramics sintered in sealed alumina crucible was 15% higher than those sintered unsealed in air. By increasing the content of Ca(Sc 0.5 Nb 0.5 )O 3 , the phase structure of (K 0.37 Na 0.63 )NbO 3 ceramics transformed from orthorhombic to tetragonal symmetry first and then to pseudo cubic symmetry. The 0.91(K 0.37 Na 0.63 )NbO 3 -0.09Ca(Sc 0.5 Nb 0.5 )O 3 ceramics exhibited high density (98%), high transmittance (60%) in the near-IR region and relatively good electrical properties ( r = 1914, tan = 0.037, T c = 147 °C, P r = 6.88 μC/cm 2 , E c = 8.49 kV/cm). Meanwhile, the introduction of Ca(Sc 0.5 Nb 0.5 )O 3 induced a composition fluctuation in the (K 0.37 Na 0.63 )NbO 3 lattice and made the ceramics more relaxor-like, which would lead to a further reduction of light scattering. These results demonstrated that 0.91(K 0.37 Na 0.63 )NbO 3 -0.09Ca(Sc 0.5 Nb 0.5 )O 3 could be promising lead-free transparent ferroelectric ceramics.

Although it is widely used in industry and food products, formic acid can be dangerous owing to its corrosive properties. Accurate determination of formic acid would not only benefit its qualified uses but also be an effective way to avoid corrosion or injury from inhalation, swallowing, or touching. Herein, we present a nanofilm-based fluorescent sensor for formic acid vapor detection with a wide response range, fast response speed, and high sensitivity and selectivity. The nanofilm was synthesized at a humid air/dimethyl sulfoxide (DMSO) interface through dynamic covalent condensation between two typically designed building blocks, de-tert-butyl calix[4]arene-tetrahydrazide (CATH) and 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)tetra-benzaldehyde (ETBA). The as-prepared nanofilm is uniform, flexible, fluorescent, and photochemically stable. The thickness and fluorescence intensity of the nanofilm can be facilely adjusted by varying the concentration of the building blocks and the sensing performance of the nanofilm can be optimized accordingly. Based on the nanofilm, a fluorescent sensor with a wide response range (4.4 ppt−4400 ppm) for real-time and online detection of formic acid vapor was built. With the sensor, a trace amount (0.01%) of formic acid in petroleum ether (60−90 °C) can be detected within 3 s. Besides, fluorescence quenching of the nanofilm by formic acid vapor can be visualized. It is believed that the sensor based on the nanofilm would find real-life applications in corrosion and injury prevention from formic acid.