Fengwei Huo

Imparting functionality to a metal–organic framework material by controlled nanoparticle encapsulation

Microporous metal-organic frameworks (MOFs) that display permanent porosity show great promise for a myriad of purposes. The potential applications of MOFs can be developed further and extended by encapsulating various functional species (for example, nanoparticles) within the frameworks. However, despite increasing numbers of reports of nanoparticle/MOF composites, simultaneously to control the size, composition, dispersed nature, spatial distribution and confinement of the incorporated nanoparticles within MOF matrices remains a significant challenge. Here, we report a controlled encapsulation strategy that enables surfactant-capped nanostructured objects of various sizes, shapes and compositions to be enshrouded by a zeolitic imidazolate framework (ZIF-8). The incorporated nanoparticles are well dispersed and fully confined within the ZIF-8 crystals. This strategy also allows the controlled incorporation of multiple nanoparticles within each ZIF-8 crystallite. The as-prepared nanoparticle/ZIF-8 composites exhibit active (catalytic, magnetic and optical) properties that derive from the nanoparticles as well as molecular sieving and orientation effects that originate from the framework material.

Polymer pen lithography

We report a low-cost, high-throughput scanning probe lithography method that uses a soft elastomeric tip array, rather than tips mounted on individual cantilevers, to deliver inks to a surface in a “direct write” manner. Polymer pen lithography merges the feature size control of dip-pen nanolithography with the large-area capability of contact printing. Because ink delivery is time and force dependent, features on the nanometer, micrometer, and macroscopic length scales can be formed with the same tip array. Arrays with as many as about 11 million pyramid-shaped pens can be brought into contact with substrates and readily leveled optically to ensure uniform pattern development.

Smart responsive phosphorescent materials for data recording and security protection

Smart luminescent materials that are responsive to external stimuli have received considerable interest. Here we report ionic iridium (III) complexes simultaneously exhibiting mechanochromic, vapochromic and electrochromic phosphorescence. These complexes share the same phosphorescent iridium (III) cation with a N-H moiety in the N^N ligand and contain different anions, including hexafluorophosphate, tetrafluoroborate, iodide, bromide and chloride. The anionic counterions cause a variation in the emission colours of the complexes from yellow to green by forming hydrogen bonds with the N-H proton. The electronic effect of the N-H moiety is sensitive towards mechanical grinding, solvent vapour and electric field, resulting in mechanochromic, vapochromic and electrochromic phosphorescence. On the basis of these findings, we construct a data-recording device and demonstrate data encryption and decryption via fluorescence lifetime imaging and time-gated luminescence imaging techniques. Our results suggest that rationally designed phosphorescent complexes may be promising candidates for advanced data recording and security protection.

Microstructured Graphene Arrays for Highly Sensitive Flexible Tactile Sensors

A highly sensitive tactile sensor is devised by applying microstructured graphene arrays as sensitive layers. The combination of graphene and anisotropic microstructures endows this sensor with an ultra-high sensitivity of -5.53 kPa(-1) , an ultra-fast response time of only 0.2 ms, as well as good reliability, rendering it promising for the application of tactile sensing in artificial skin and human-machine interface.

A Family of Metal‐Organic Frameworks Exhibiting Size‐Selective Catalysis with Encapsulated Noble‐Metal Nanoparticles

The encapsulation of noble-metal nanoparticles (NPs) in metal-organic frameworks (MOFs) with carboxylic acid ligands, the most extensive branch of the MOF family, gives NP/MOF composites that exhibit excellent shape-selective catalytic performance in olefin hydrogenation, aqueous reaction in the reduction of 4-nitrophenol, and faster molecular diffusion in CO oxidation. The strategy of using functionalized cavities of MOFs as hosts for different metal NPs looks promising for the development of high-performance heterogeneous catalysts.

Engineering ZIF‐8 Thin Films for Hybrid MOF‐Based Devices

Patterned metal-organic framework, ZIF-8 thin films can be generated by using standard photolithography or via selective growth with the aid of microcontact printing. The alternate chemical deposition (of ZIF-8) and physical deposition (of metallic materials) allow the insertion of metal layers in the ZIF-8 film that could serve as multifunctional chemical sensors for vapors and gases.

Scanning probe block copolymer lithography

Integration of individual nanoparticles into desired spatial arrangements over large areas is a prerequisite for exploiting their unique electrical, optical, and chemical properties. However, positioning single sub-10-nm nanoparticles in a specific location individually on a substrate remains challenging. Herein we have developed a unique approach, termed scanning probe block copolymer lithography, which enables one to control the growth and position of individual nanoparticles in situ. This technique relies on either dip-pen nanolithography (DPN) or polymer pen lithography (PPL) to transfer phase-separating block copolymer inks in the form of 100 or more nanometer features on an underlying substrate. Reduction of the metal ions via plasma results in the high-yield formation of single crystal nanoparticles per block copolymer feature. Because the size of each feature controls the number of metal atoms within it, the DPN or PPL step can be used to control precisely the size of each nanocrystal down to 4.8 ± 0.2 nm.

Coating Two-Dimensional Nanomaterials with Metal–Organic Frameworks

We demonstrate the coating of various 2D nanomaterials including MoS2 nanosheets, graphene oxide (GO), and reduced graphene oxide (rGO) with zeolitic imidazolate frameworks (i.e., ZIF-8) via a facile procedure. Additionally, ternary core-shell structures like Pt-MoS2@ZIF-8, Pt-GO@ZIF-8, and Pt-rGO@ZIF-8 have also been prepared. As a proof-of-concept application, a memory device based on MoS2@ZIF-8 hybrid was fabricated and it exhibited write-once-read-many-times (WORM) memory effect with high ON/OFF ratio and long operating lifetime. It is expected that MOF coated 2D nanomaterials may find wide applications in energy storage and conversion, catalysis, sensing, and information storage devices.

Mesoporous Metal–Organic Frameworks with Size‐, Shape‐, and Space‐Distribution‐Controlled Pore Structure

W. Zhang, Z. Xu, Prof. W. Huang, Prof. F. Huo Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM) Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM) Nanjing Tech University (NanjingTech) 30 South Puzhu Road Nanjing 211816 , PR China W. Zhang, Y. Liu, Prof. G. Lu, Prof. S. Li, C. Cui, J. Wu, Prof. F. Huo School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798 , Singapore E-mail: fwhuo@ntu.edu.sg Prof. Y. Yang School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive Singapore 637459 , Singapore Dr. Y. Wang, Prof. H. Chen Division of Chemistry and Biological Chemistry Nanyang Technological University 21 Nanyang Link Singapore 637371 , Singapore Z. Xu, Prof. D. Tian College of Science Nanjing Tech University (NanjingTech) Puzhu Road Nanjing 211816 , PR China J. S. DuChene, Prof. W. D. Wei Department of Chemistry and Center for Nanostructured Electronic Materials University of Florida Gainesville , FL 32611 , USA

Freestanding graphene paper decorated with 2D-assembly of Au@Pt nanoparticles as flexible biosensors to monitor live cell secretion of nitric oxide.

We report the development of a new type of flexible electrochemical biosensors based on graphene paper loaded with closely-packed Au@Pt core-shell nanoparticles as a freestanding cell culture substrate for real-time monitoring cell secretion of nitric oxide. The hybrid electrode was fabricated through a modular approach in which 2D-assembly of nanoparticles formed at the oil-water interface was transferred onto graphene paper by dip-coating. We have shown that the independently optimized metal nanostructures and graphene paper were integrated into functional electrodes with high electrocatalytic activity. When used for the detection of nitric oxide, the flexible electrodes have demonstrated high sensitivity, a wide linear range, and a low detection limit, which, in combination with its biocompatibility, offer unique opportunities for the real-time monitoring of nitric oxide secretion by human endothelial vein cells grown on the electrode. These interesting findings collectively demonstrate the potential of our modular approach for designing high-performance flexible electrodes with tailored surface properties.