A Humidity-Induced Large Electronic Conductivity Change of 107 on a Metal-Organic Framework for Highly Sensitive Water Detection.

The electronic conductivity (EC) of metal-organic frameworks (MOFs) is sensitive to strongly oxidizing guest molecules. Water is a relatively mild species, however, the effect of H2 O on the EC of MOFs is rarely reported. We explored the effect of H2 O on the EC in the MOFs (NH2 )2 -MIL-125 and its derivatives with experimental and theoretical investigations. Unexpectedly, a large EC increase of 107 on H2 SO4 @(NH2 )2 -MIL-125 by H2 O was observed. Brønsted acid-base pairs formed with the -NH2 groups, and H2 SO4 played an important role in promoting the charge transfer from H2 O to the MOF. Based on H2 SO4 @(NH2 )2 -MIL-125, a high-performance chemiresistive humidity sensor was developed with the highest sensitivity, broadest detection range, and lowest limit of detection amongst all reported sensing materials to date. This work not only demonstrated that H2 O can remarkably influence the EC of MOFs, but it also revealed that post-modification of the structure of MOFs could enhance the influence of the guest molecule on their EC to design high-performance sensing materials.

Dangling bond formation on COF nanosheets for enhancing sensing performances.

Dangling bond formation for COF materials in a rational manner is an enormous challenge, especially through post-treatment which is a facile strategy while has not been reported yet. In this work, a "chemical scissor" strategy is proposed for the first time to rationally design dangling bonds in COF materials. It is found that Zn2+ coordination in post-metallization of TDCOF can act as an "inducer" which elongates the target bond and facilitates its fracture in hydrolyzation reactions to create dangling bonds. The number of dangling bonds is well-modulated by controlling the post-metallization time. Zn-TDCOF-12 shows one of the highest sensitivities to NO2 in all reported chemiresistive gas sensing materials operating under visible light and room temperature. This work opens an avenue to rationally design a dangling bond in COF materials, which could increase the active sites and improve the mass transport in COFs to remarkably promote their various chemical applications.

Pore Size Modulation in Flexible Metal-Organic Framework Enabling High Performance Gas Sensing.

Pore size plays a critical role in determining the performance of metal-organic frameworks (MOFs) in catalysis, sensing, and gas storage or separation. However, revealing the pore-size/property relationship remains extremely challenging because ideal structure models possessing different pore sizes but having the same components are lacking. In this work, a solvent-coordination directed structure swelling method was developed for modulating the ratio between the large and narrow pore phases of a flexible MOF, MIL-88B. Pore-size-dependent gas sensitivity and selectivity were studied for the first time in the MIL-88B samples. The optimized MIL-88B-20 % sample showed one of the best sensing performances among all the reported MOF-based H2 S-sensing materials. This work not only provides a method to synthesize ideal structure models for revealing the relationship between pore-size and properties, but also may inspire the development of high-performance gas sensing materials.

The Growth Mechanism of a Conductive MOF Thin Film in Spray-based Layer-by-layer Liquid Phase Epitaxy.

The layer-by-layer liquid-phase epitaxy (LBL-LPE) method is widely used in preparing metal-organic framework (MOF) thin films with the merits of controlling thickness and out-of-plane orientation for superior performances in applications. The LBL-LPE growth mechanism related to the grain boundary, structure defect, and orientation is critical but very challenging to study. In this work, a novel "in-plane self-limiting and self-repairing" thin-film growth mechanism is demonstrated by the combination study of the grain boundary, structure defect, and orientation of Cu3 (HHTP)2 -xC thin film via microscopic analysis techniques and electrical measurements. This mechanism results a desired high-quality MOF thin film with preferred in-plane orientations at its bottom for the first time and is very helpful for optimizing the LBL-LPE method, understanding the growth cycle-dependent properties of MOF thin film, and inspiring the investigations of the biomimetic self-repairing materials.

Layer-by-Layer Growth of Preferred-Oriented MOF Thin Film on Nanowire Array for High-Performance Chemiresistive Sensing.

High-quality MOF thin films with high orientation and controlled thickness are extremely desired for applications. However, they have been only successfully fabricated on flat substrates. Those MOF 2D thin films are limited by low exposed area and slow mass transport. To overcome these issues, MOF 3D thin films with good crystallinity, preferred orientation, and precisely controllable thickness in nanoscale were successfully prepared in a controllable layer-by-layer manner on nanowire array substrate for the first time. The as-prepared Cu-HHTP 3D thin film is superior to corresponding 2D thin films and showed one of the highest sensitivity, lowest LOD, and fastest response among all reported chemiresistive NH3 sensing materials at RT. This work provides a feasible approach to grow preferred-oriented 3D MOF thin film, offering new perspectives for constructing MOF-based heterostructures for advanced applications.

A Covalent Organic-Inorganic Hybrid Superlattice Covered with Organic Functional Groups for Highly Sensitive and Selective Gas Sensing.

Organic-inorganic hybrid superlattices (OIHSLs) hold attractive physical and chemical properties, while the construction of single-crystal covalent OIHSLs has not been achieved. Herein a coordination assembly strategy was proposed to create a single-crystal covalent OIHSL PbBDT (BDT=1,4-benzenedithiolate), where layered [PbS2 ] sublattice covalently connects with benzene sublattice. The covalent bonding offers better thermo-/chemi-stability, inter-sublattice electron transport, and unique organic-group-functionalized surface, which may enable better performances in chemical applications than non-covalent OIHSL. These features endow PbBDT with the highest sensitivity, the lowest detection limit and excellent selectivity towards NO2 at room temperature among all chemiresistive gas-sensing materials with reported response time less than 2 min without the need of light assistance.

Boosting Room Temperature Sensing Performances by Atomically Dispersed Pd Stabilized via Surface Coordination.

The urgent requirement of monitoring air pollution worldwide evokes intensive research interest in developing chemiresistive gas sensing techniques. To overcome the limits in sensitivity and selectivity of room temperature (RT) chemiresistive sensing materials, a new strategy using single-atom catalysts (SACs) via surface coordination is proposed. As a proof-of-concept, single Pd atoms on TiO2 (Pd1-TiO2) possess high efficiency in generating adsorbed O2- as well as high activity and selectivity in catalyzing CO oxidation at RT. As a result, Pd1-TiO2 shows record high sensitivity among the reported RT sensing materials, which is even comparable to those of the best materials working at high temperature. It also provides an approximately 1 order of magnitude lower limit of detection than the best CO sensing materials. Moreover, Pd1-TiO2 presents high selectivity toward 12 kinds of interference gases. This work not only paves a way to design high-performance RT gas sensing materials but also extends the application of SACs.

Organic "receptor" fully covered few-layer organic-metal chalcogenides for high-performance chemiresistive gas sensing at room temperature.

Organic-metal chalcogenides (OMCs) are proposed as a new family of two-dimensional (2D) chemiresistive sensing materials. Few-layer Ag(SPh-NH2), one of the OMCs, fully and orderly covered with predesigned -NH2 groups as "receptors", shows the highest sensitivity, excellent selectivity and reversibility in response to NO2 among all the reported 2D chemiresistive sensing materials at room temperature.

Fabrication of ß-cyclodextrin-coated poly (diallyldimethylammonium chloride)-functionalized graphene composite film modified glassy carbon-rotating disk electrode and its application for simultaneous electrochemical determination colorants of sunset yellow and tartrazine.

We proposed a green and facile approach for the synthesis of ß-cyclodextrin-coated poly(diallyldimethylammonium chloride)-functionalized graphene composite film (ß-CD-PDDA-Gr) by using L-ascorbic acid (L-AA) as the reducing agent at room temperature. The ß-CD-PDDA-Gr composite film modified glassy carbon-rotating disk electrode (GC-RDE) was then developed for the sensitive simultaneous determination of two synthetic food colorants: sunset yellow (SY) and tartrazine (TT). By cyclic voltammetry (CV), the peak currents of SY and TT increased obviously on the developed electrochemical sensor. The kinetic parameters, such as diffusion coefficient D and standard heterogeneous rate constant kb, were estimated by linear sweep voltammetry (LSV). Under the optimal conditions, the differential pulse voltammetry (DPV) signals of SY and TT on the ß-CD-PDDA-Gr modified GC-RDE were significantly enhanced. The enhanced anodic peak currents represented the excellent analytical performance of simultaneous detection of SY and TT in the range of 5.0×10(-8) to 2.0×10(-5) mol L(-1), with a low limit of detection (LOD) of 1.25×10(-8) mol L(-1) for SY and 1.43×10(-8) mol L(-1) for TT (SN(-1)=3). This proposed method displayed outstanding selectivity, good stability and acceptable repeatability and reproducibility, and also has been used to simultaneously determine SY and TT in some commercial soft drinks with satisfactory results. The obtained results were compared to HPLC of analysis for those two colorants and no significant differences were found. By the treatment of the experimental data, the electrochemical reaction mechanisms of SY and TT both involved a one-electron-one-proton-transfer process.