Hollow‐Structured Mesoporous Materials: Chemical Synthesis,Functionalization and Applications

Hollow‐structured mesoporous materials (HMMs), as a kind of mesoporous material with unique morphology, have been of great interest in the past decade because of the subtle combination of the hollow architecture with the mesoporous nanostructure. Benefitting from the merits of low density, large void space, large specific surface area, and, especially, the good biocompatibility, HMMs present promising application prospects in various fields, such as adsorption and storage, confined catalysis when catalytically active species are incorporated in the core and/or shell, controlled drug release, targeted drug delivery, and simultaneous diagnosis and therapy of cancers when the surface and/or core of the HMMs are functionalized with functional ligands and/or nanoparticles, and so on. In this review, recent progress in the design, synthesis, functionalization, and applications of hollow mesoporous materials are discussed. Two main synthetic strategies, soft‐templating and hard‐templating routes, are broadly sorted and described in detail. Progress in the main application aspects of HMMs, such as adsorption and storage, catalysis, and biomedicine, are also discussed in detail in this article, in terms of the unique features of the combined large void space in the core and the mesoporous network in the shell. Functionalization of the core and pore/outer surfaces with functional organic groups and/or nanoparticles, and their performance, are summarized in this article. Finally, an outlook of their prospects and challenges in terms of their controlled synthesis and scaled application is presented.     

Materials and Techniques for Implantable Nutrient Sensing Using Flexible Sensors Integrated with Metal–Organic Frameworks

The combination of novel materials with flexible electronic technology may yield new concepts of flexible electronic devices that effectively detect various biological chemicals to facilitate understanding of biological processes and conduct health monitoring. This paper demonstrates single‐ or multichannel implantable flexible sensors that are surface modified with conductive metal–organic frameworks (MOFs) such as copper‐MOF and cobalt‐MOF with large surface area, high porosity, and tunable catalysis capability. The sensors can monitor important nutriments such as ascorbicacid, glycine, l ‐tryptophan (l ‐Trp), and glucose with detection resolutions of 14.97, 0.71, 4.14, and 54.60 × 10^?6 m , respectively. In addition, they offer sensing capability even under extreme deformation and complex surrounding environment with continuous monitoring capability for 20 d due to minimized use of biological active chemicals. Experiments using live cells and animals indicate that the MOF‐modified sensors are biologically safe to cells, and can detect l ‐Trp in blood and interstitial fluid. This work represents the first effort in integrating MOFs with flexible sensors to achieve highly specific and sensitive implantable electrochemical detection and may inspire appearance of more flexible electronic devices with enhanced capability in sensing, energy storage, and catalysis using various properties of MOFs.     

Laser‐Induced Freestanding Graphene Papers: A New Route of Scalable Fabrication with Tunable Morphologies and Properties for Multifunctional Devices and Structures

The recently emergent laser‐induced graphene (LIG) technology has endowed the fabrication of smart devices with one‐step processing and scalable/designable features. Graphene paper (GP), an important architecture of 2D layered carbon, however, is never produced through LIG. Herein, a novel strategy is reported for production of freestanding GP through LIG technology. It is first determined that the unique spatial configuration of polyimide (PI) paper is critical for the preparation of GP without the appearance of intense shape distortion. Benefiting from the mechanism, the as‐produced laser‐induced graphene paper (LIGP) is foldable, trimmable, and integratable to customized shapes and structures with the largest dimension of 40 × 35 cm². Based on the processing–structure–property relationship study, one is capable of controlling and tuning various physical and chemical properties of LIGPs, rendering them unique for assembling flexible electronics and smart structures, e.g., human/robotic motion detectors, liquid sensors, water–oil separators, antibacterial media, and flame retardant/deicing/self‐sensing composites. With the key findings, the escalation of LIGP for commercialization, roll‐to‐roll manufacturing, and multidisciplinary applications are highly expected.     

Hybrid Copper‐Nanowire–Reduced‐Graphene‐Oxide Coatings: A “Green Solution” Toward Highly Transparent,Highly Conductive,and Flexible Electrodes for (Opto)Electronics

This study reports a novel green chemistry approach to assemble copper‐nanowires/reduced‐graphene‐oxide hybrid coatings onto inorganic and organic supports. Such films are robust and combine sheet resistances (<30 Ω sq^?1) and transparencies in the visible region (transmittance > 70%) that are rivalling those of indium–tin oxide. These electrodes are suitable for flexible electronic applications as they show a sheet resistance change of <4% after 10 000 bending cycles at a bending radius of 1.0 cm, when supported on polyethylene terephthalate foils. Significantly, the wet‐chemistry method involves the preparation of dispersions in environmentally friendly solvents and avoids the use of harmful reagents. Such inks are processed at room temperature on a wide variety of surfaces by spray coating. As a proof‐of‐concept, this study demonstrates the successful use of such coatings as electrodes in high‐performance electrochromic devices. The robustness of the electrodes is demonstrated by performing several tens of thousands of cycles of device operation. These unique conducting coatings hold potential for being exploited as transparent electrodes in numerous optoelectronic applications such as solar cells, light‐emitting diodes, and displays.     

Flexible Metal–Organic Framework‐Based Mixed‐Matrix Membranes: A New Platform for H2S Sensors

Metal–organic framework (MOF)–polymer mixed‐matrix membranes (MMMs) have shown great potential and superior performance in gas separations. However, their sensing application has not been fully established yet. Herein, a rare example of using flexible MOF‐based MMMs as a fluorescent turn‐on sensor for the detection of hydrogen sulfide (H₂S) is reported. These MOF‐based MMMs are readily prepared by mixing a highly stable aluminum‐based nano‐MOF (Al‐MIL‐53‐NO₂) into poly(vinylidene fluoride) with high loadings up to 70%. Unlike the intrinsic fragility and poor processability of pure‐MOF membranes, these MMMs exhibit desirable flexibility and processability that are more suitable for practical sensing applications. The uniform distribution of Al‐MIL‐53‐NO₂ particles combined with the permanent pores of MOFs enable these MMMs to show good water permeation flux and consequently have a full contact between the analyte and MOFs. The developed MMM sensor (70% MOF loading) thus shows a highly remarkable detection selectivity and sensitivity for H₂S with an exceptionally low detection limit around 92.31 × 10^?9m , three orders of magnitude lower than the reported powder‐form MOFs. This work demonstrates that it is feasible to develop flexible luminescent MOF‐based MMMs as a novel platform for chemical sensing applications.     

Skin‐Inspired Multifunctional Autonomic‐Intrinsic Conductive Self‐Healing Hydrogels with Pressure Sensitivity,Stretchability, and 3D Printability

The advent of conductive self‐healing (CSH) hydrogels, a class of novel materials mimicking human skin, may change the trajectory of the industrial process because of their potential applications in soft robots, biomimetic prostheses, and health‐monitoring systems. Here, the development of a mechanically and electrically self‐healing hydrogel based on physically and chemically cross‐linked networks is reported. The autonomous intrinsic self‐healing of the hydrogel is attained through dynamic ionic interactions between carboxylic groups of poly(acrylic acid) and ferric ions. A covalent cross‐linking is used to support the mechanical structure of the hydrogel. Establishing a fair balance between the chemical and physical cross‐linking networks together with the conductive nanostructure of polypyrrole networks leads to a double network hydrogel with bulk conductivity, mechanical and electrical self‐healing properties (100% mechanical recovery in 2 min), ultrastretchability (1500%), and pressure sensitivity. The practical potential of CSH hydrogels is further revealed by their application in human motion detection and their 3D‐printing performance.