Elastic films of single-crystal two-dimensional covalent organic frameworks.

The properties of polycrystalline materials are often dominated by defects; two-dimensional (2D) crystals can even be divided and disrupted by a line defect1-3. However, 2D crystals are often required to be processed into films, which are inevitably polycrystalline and contain numerous grain boundaries, and therefore are brittle and fragile, hindering application in flexible electronics, optoelectronics and separation1-4. Moreover, similar to glass, wood and plastics, they suffer from trade-off effects between mechanical strength and toughness5,6. Here we report a method to produce highly strong, tough and elastic films of an emerging class of 2D crystals: 2D covalent organic frameworks (COFs) composed of single-crystal domains connected by an interwoven grain boundary on water surface using an aliphatic bi-amine as a sacrificial go-between. Films of two 2D COFs have been demonstrated, which show Young's moduli and breaking strengths of 56.7 ± 7.4 GPa and 73.4 ± 11.6 GPa, and 82.2 ± 9.1 N m-1 and 29.5 ± 7.2 N m-1, respectively. We predict that the sacrificial go-between guided synthesis method and the interwoven grain boundary will inspire grain boundary engineering of various polycrystalline materials, endowing them with new properties, enhancing their current applications and paving the way for new applications.

Linker-Guided Growth of Single-Crystal Covalent Organic Frameworks.

The core features of covalent organic frameworks (COFs) are crystallinity and porosity. However, the synthesis of single-crystal COFs with monomers of diverse reactivity and adjustment of their pore structures remain challenging. Here, we show that linkers that can react with a node to form single-crystal COFs can guide other linkers that form either COFs or amorphous polymers with the node to gain single-crystal COFs with mixed components, which are homogeneous on the unit cell scale with controlled ratios. With the linker-guided crystal growth method, we created nine types of single-crystal COFs with up to nine different components, which are more complex than any known crystal. The structure of the crystal adapted approximately to that of the main component, and its pore volume could be expanded up to 8.8%. Different components lead to complex and diverse pore structures and offer the possibilities to gain positive synergies, as exemplified by a bicomponent COF with 2200 and 733% SO2 uptake capacity of that of the two pure-component counterparts at 298 K and 0.002 bar. The selectivity for separation of SO2/CO2 ranges from 1230 to 4247 for flue gas based on ideal adsorbed solution theory, recording porous crystals. The bicomponent COF also exhibits a 1300% retention time of its pure-component counterparts for SO2 in a dynamic column breakthrough experiment for deep desulfurization.

Interfacially Locked Metal Aerogel Inside Porous Polymer Composite for Sensitive and Durable Flexible Piezoresistive Sensors.

Flexible pressure sensors play significant roles in wearable devices, electronic skins, and human-machine interface (HMI). However, it remains challenging to develop flexible piezoresistive sensors with outstanding comprehensive performances, especially with excellent long-term durability. Herein, a facile "interfacial locking strategy" has been developed to fabricate metal aerogel-based pressure sensors with excellent sensitivity and prominent stability. The strategy broke the bottleneck of the intrinsically poor mechanical properties of metal aerogels by grafting them on highly elastic melamine sponge with the help of a thin polydimethylsiloxane (PDMS) layer as the interface-reinforcing media. The hierarchically porous conductive structure of the ensemble offered the as-prepared flexible piezoresistive sensor with a sensitivity as high as 12 kPa-1 , a response time as fast as 85 ms, and a prominent durability over 23 000 compression cycles. The excellent comprehensive performance enables the successful application of the flexible piezoresistive sensor as two-dimensional (2D) array device as well as three-dimensional (3D) force-detecting device for real-time monitoring of HMI activities.

Double Lock Label Based on Thermosensitive Polymer Hydrogels for Information Camouflage and Multilevel Encryption.

Developing extra safety encryption technologies to prevent information leakage and combat fakes is in high demand but is challenging. Herein, we propose a "double lock" strategy based on both lower critical solution temperature (LCST) and upper critical solution temperature (UCST) polymer hydrogels for information camouflage and multilevel encryption. Two types of hydrogels were synthesized by the method of random copolymerization. The number of -CO-NH2 groups in the network structure of the hydrogels changed the enthalpic or entropic thermo-responsive hydrogels, and ultimately precisely controlled their phase transition temperature. The crosslink density of the polymer hydrogels governs the diffusion kinetics, resulting in a difference in the time for their color change. The combination of multiple LCST and UCST hydrogels in one label realized information encryption and dynamic information identification in the dimensions of both time and temperature. This work is highly interesting for the fields of information encryption, anti-counterfeiting, and smart responsive materials.

Reinforced Topological Nanoassemblies: 2D Hexagon-Fused Wheel to 3D Prismatic Metallo-Lamellar Structure with Molecular Weight of 119 K Daltons.

By a precise metallo-ligand design, the advanced coordination-driven self-assembly could succeed in the preparation of giant molecular weight of the metallo-architectures. However, the synthesis of a single discrete high-molecular-weight (>100 K Da) structure has not been demonstrated due to the insurmountable synthetic challenge. Herein, we present a two-dimensional wheel structure (W1) and a gigantic three-dimensional dodecagonal prism-like architecture (P1), which were generated by multicomponent self-assembly of two similar metallo-organic ligands and a core ligand with metal ions, respectively. The giant 2D-suprastructure W1 with six hexagonal metallacycles that fused to the central spoke wheel was first achieved in nearly quantitative yield, and then, directed by introducing a meta-substituted coordination site into the key ligand, the supercharged (36 Ru2+ and 48 Cd2+ ions) double-decker prismatic structure P1 with two wheel structure W1s serve as the surfaces and 12 connectivities serve as the edges, where a molecular weight up to 119 498.18 Da was accomplished. The expected molecular composition and size morphology was unequivocally characterized by nuclear magnetic resonance, mass spectrometry, and transmission electron microscopy investigations. The introduction of a wheel structure is able to add considerable stability and complexity to the final architecture. These well-defined scaffolds are expected to play an important role in the functional materials field, such as molecular encapsulation and medicine sustained release.

A robust hybrid nanozyme@hydrogel platform as a biomimetic cascade bioreactor for combination antitumor therapy.

The development of highly effective and minimally invasive approaches for cancer treatment is the ultimate goal. Herein, an injectable hybrid hydrogel as a biomimetic cascade bioreactor is designed for combination antitumor therapy by providing spatiotemporally-controlled and long-term delivery of therapeutic agents. This hybrid nanozyme@hydrogel (hPB@gellan) is doped with Prussian blue (PB) nanoparticles via the in situ nanoprecipitation method in the polysaccharide gellan matrix. The obtained PB nanoparticles have a small size of 10 nm and play dual roles as a photothermal agent with a photothermal conversion efficiency of 59.6% and as a nanozyme to decompose hydrogen peroxide into oxygen. By incorporating glucose oxidase (GOD) into the hybrid hydrogel, a cascade bioreactor is formed for PB-promoted glucose consumption. Owing to its shear-thinning and self-recovery properties, the hybrid hydrogel is locally administered into tumors, and shows long-term resistance against body clearance and metabolism. The in vivo antitumor results demonstrate that the tumors in the group of combined photothermal and starvation therapy (GOD/hPB@gellan + NIR) are greatly eliminated with a tumor suppression rate of 99.7% 22 days after the treatment. The outstanding antitumor performance is attributed to the main attack by NIR-triggered hyperthermia and the holding attack by GOD-mediated starvation from the catalytic bioreactor of the hybrid hydrogel. Taking into consideration the advantages of biosafety, simple synthetic approaches and facile manipulation in treatment, the hybrid hydrogel has great potential for clinical translation.

Robust organohydrogel with flexibility and conductivity across the freezing and boiling temperatures of water.

A tough double-network (DN) organohydrogel, obtained by simply soaking a poly(2-acrylamido-2-methylpropane sulfonic acid)/polyacrylamide (PAMPS/PAAm) hydrogel in an ethylene glycol solution of lithium chloride, retains high mechanical performance, flexibility (-80 to 120 °C) and conductivity (-20 to 120 °C), paving the way towards broad applications.

Near-infrared light-responsive hydrogels via peroxide-decorated MXene-initiated polymerization.

Two-dimensional MXene Ti3C2T x nanosheets with peroxide decoration (p-Ti3C2T x ) are synthesized by a sonication-assisted MILD etching method. The obtained MXenes can generate hydroxyl radical species and act as an initiator for free-radical polymerization of a series of acrylic monomers without the use of light illumination or co-initiators. The monomers analyzed include acrylamide, N-isopropylacrylamide (NIPAM), N,N-dimethylacrylamide, methyl methacrylate, and hydroxyethyl methacrylate. By simply mixing N-isopropylacrylamide monomers and p-Ti3C2T x nanosheets under deoxygenated conditions, PNIPAM-based nanocomposite hydrogels are synthesized using a high concentration of the monomer. The nanocomposite hydrogels have a photothermal conversion efficiency of 34.7% and photothermal stability superior to that of pristine Ti3C2T x . Taking advantage of the thermal responsive behavior of PNIPAM, the nanocomposite hydrogels are successfully exploited as remotely near-infrared light controlled "smart" windows, fluidic valves and photodetectors.