Conductive Stimuli-Responsive Coordination Network Linked with Bismuth for Chemiresistive Gas Sensing

A comparison of microcrystal electron diffraction and X-ray powder diffraction for the structural analysis of metal-organic frameworks

This study successfully implemented microcrystal electron diffraction (microED) and X-ray powder diffraction (XRPD) for the crystal structure determination of a new phase, TAF-CNU-1, Ni(C8H4O4)·3H2O, solved by microED from single microcrystals in the powder and refined at the kinematic and dynamic electron diffraction theory levels. This nickel metal-organic framework (MOF), together with its cobalt and manganese analogues with formula M(C8H4O4)·2H2O with M = MnII or CoII, were synthesized in aqueous media as one-pot preparations from the corresponding hydrated metal chlorides and sodium terephthalate, as a promising 'green' synthetic route to moisture-stable MOFs. The crystal structures of the two latter materials have been previously determined ab initio from X-ray powder diffraction. The advantages and disadvantages of both structural characterization techniques are briefly summarized. Additional solid-state property characterization was carried out using thermogravimetric analysis, scanning electron microscopy and Fourier transform infrared spectroscopy.

Electronic Chemical Sensors Based on Conductive Framework Materials

The development of portable electronic chemical sensors is key to solving a number of challenges, including monitoring environmental and industrial hazards, as well as understanding and improving human health. Framework materials possess several desirable characteristics that make them well-suited for electroanalytical applications, including high surface area, atomically precise distribution of active sites, and tunable properties that can be leveraged through modular reticular chemistry. This review highlights the emergence of conductive framework materials as active components in electrically transduced chemical sensors, including the development of new materials for the detection of a wide variety of analytes in both gas and liquid phase. The efforts to gain fundamental understanding of the molecular interactions and sensing mechanisms between framework materials and analytes are described, along with applications of these materials on portable and flexible substrates. The review suggests areas for further study, including the study of material-analyte interactions at the molecular level and the continued development of scalable methods for the integration of framework materials into low-power, portable sensing devices.

Smart Textile: Electrohydrodynamic Jet Printing of Ionic Liquid-Functionalized Cu3(HHTP)2 Metal-Organic Frameworks for Gas-Sensing Applications

This study presents the development and characterization of a smart textile gas sensor based on the integration of ionic liquid (IL)-functionalized Cu3(HHTP)2 metal-organic frameworks (MOFs), using electrohydrodynamic jet (e-jet) printing. The sensor was designed for the detection of nitric oxide (NO) gas, a critical target in various environmental and safety applications. Cu3(HHTP)2 MOFs were synthesized and subsequently functionalized with 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM+ Otf-) ionic liquid to enhance their chemiresistive performance toward NO gas. The functionalized MOF was then e-jet printed onto electrospun polylactic acid (PLA) substrates to fabricate smart textile sensors. The IL-functionalized Cu3(HHTP)2 sensors demonstrated a 582× increase in conductivity compared to previously reported MOF-based sensors. Additionally, IL functionalization enhanced sensor sensitivity, with a response increasing from less than 5% in pristine MOF@PLA sensors to approximately 570% at 100 ppm of NO gas. Performance was systematically evaluated across NO concentrations ranging from 5 to 300 ppm, achieving a theoretical limit of detection of 3.7 ppm. The sensors exhibited partial reversibility and retained functionality over extended periods and under humid conditions. Comprehensive analyses using SEM, EDX, FTIR, and XRD were performed to assess the crystallinity of MOF deposits and elucidate the sensing mechanism. These findings highlight the potential of e-jet printing of IL-functionalized MOFs for the development of advanced, flexible gas sensors with applications in both civilian and military settings and implications for personal protective wearable technologies.

Mechanistic Insight into the Formation and Deposition of Conductive, Layered Metal-Organic Framework Nanocrystals

This paper describes the use of the layered conductive metal-organic framework (MOF) (nickel)3-(hexahydroxytriphenylene)2 [Ni3(HHTP)2] as a model system for understanding the process of self-assembly within this class of materials. We confirm and quantify experimentally the role of the oxidant in the synthetic process. Monitoring the deposition of Ni3(HHTP)2 with in situ infrared spectroscopy revealed that MOF formation is characterized by an initial induction period, followed by linear growth with respect to time. The presence and identity of oxidizing agents is critical for the coordination-driven self-assembly of these materials and impacts both the length of the induction period and the observed rate of MOF growth. A large excess of hydrogen peroxide results in a 2× increase in the observed deposition rate (9.6 ± 6.8 × 10-4 vs 5.0 ± 2.8 × 10-4 min-1) over standard reaction conditions, but leads to the formation of large, irregularly shaped particles. Slower deposition rates in the presence of oxygen favor the formation of uniformly sized nanorods (98 ± 38 × 25 ± 6 nm). These quantitative insights into the mechanism of HHTP-based MOF formation provide valuable information about the fundamental aspects of coordination and polymerization that are critical for nanoscale crystal engineering of structure-property relationships in this class of materials.

Dynamic Interchain Motion in 1D Tetrathiafulvalene-Based Coordination Polymers for Highly Sensitive Molecular Recognition

The application of electrically conductive 1D coordination polymers (1D CPs) in nanoelectronic molecular recognition is theoretically promising yet rarely explored due to the challenges in their synthesis and optimization of electrical properties. In this regard, two tetrathiafulvalene-based 1D CPs, namely [Co(m-H2TTFTB)(DMF)2(H2O)]n (Co-m-TTFTB), and {[Ni(m-H2TTFTB)(CH3CH2OH)1.5(H2O)1.5]·(H2O)0.5}n (Ni-m-TTFTB) are successfully constructed. The shorter S···S contacts between the [M(solvent)3(m-H2TTFTB)]n chains contribute to a significant improvement in their electrical conductivities. The powder X-ray diffraction (PXRD) under different organic solvents reveals the flexible and dynamic structural characteristic of M-m-TTFTB, which, combined with the 1D morphology, lead to their excellent performance for sensitive detection of volatile organic compounds. Co-m-TTFTB achieves a limit of detection for ethanol vapor down to 0.5 ppm, which is superior to the state-of-the-art chemiresistive sensors based on metal-organic frameworks or organic polymers at room temperature. In situ diffuse reflectance infrared Fourier transform spectroscopy, PXRD measurements and density functional theory calculations reveal the molecular insertion sensing mechanism and the corresponding structure-function relationship. This work expands the applicable scenario of 1D CPs and opens a new realm of 1D CP-based nanoelectronic sensors for highly sensitive room temperature gas detection.

Conductive Framework Materials for Chemiresistive Detection and Differentiation of Toxic Gases

ConspectusSensing complex gaseous mixtures and identifying their composition and concentration have the potential to achieve unprecedented improvements in environmental monitoring, medical diagnostics, industrial safety, and the food/agriculture industry. Electronically transduced chemical sensors capable of recognizing and differentiating specific target gases and transducing these chemical stimuli in a portable electronic device offer an opportunity for impact by bridging the utility of chemical information with global wireless connectivity. Among electronically transduced chemical sensors, chemiresistors stand out as particularly promising due to combined features of low-power requirements, room temperature operation, non-line-of-sight detection, high portability, and exceptional modularity. Relying on changes in resistance of a functional material triggered by variations in the surrounding chemical environment, these devices have achieved part-per-billion sensitivities of analytes by employing conductive polymers, graphene, carbon nanotubes (CNTs), metal oxides, metal nanoparticles, metal dichalcogenides, or MXenes as sensing materials. Despite these tremendous developments, the need for stable, selective, and sensitive chemiresistors demands continued innovation in material design in order to operate in complex mixtures with interferents as well as variations in humidity and temperature.To fill existing gaps in sensing capabilities, conductive metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have recently emerged as a promising class of materials for chemiresistive sensing. In contrast to previously reported chemiresistors, these materials offer at least three unique features for gas sensing applications: (i) bottom-up synthesis from molecularly precise precursors that allows for strategic control of material-analyte interactions, (ii) intrinsic conductivity that simultaneously facilitates charge transport and signal transduction under low power requirements, and (iii) high surface area that enables the accessibility of abundant active sites and decontamination of gas streams by coordinating to and, sometimes, detoxifying harmful analytes. Through an emphasis on molecular engineering of structure-property relationships in conductive MOFs and COFs, combined with strategic innovations in device integration strategies and device form factor (i.e., the physical dimensions and design of device components), our group has paved the way to demonstrating the multifunctional utility of these materials in the chemiresistive detection of gases and vapors. Backed by spectroscopic assessment of material-analyte interactions, we illustrated how molecular-level features lead to device performance in detection, filtration, and detoxification of gaseous analytes. By merging the bottom-up synthesis of these materials with device integration, we show the versatility and scalability of using these materials in low-power electronic sensing devices. Taken together, our achievements, combined with the progress spearheaded on this class of materials by other researchers, establish conductive MOFs and COFs as promising multifunctional materials for applications in electronically transduced, portable, low-power sensing devices.

Efficient Capture and Electroreduction of Dilute CO2 into Highly Pure and Concentrated Formic Acid Aqueous Solution

High-purity CO2 rather than dilute CO2 (15 vol %, CO2/N2/O2 = 15:80:5, v/v/v) similar to the flue gas is currently used as the feedstock for the electroreduction of CO2, and the liquid products are usually mixed up with the cathode electrolyte, resulting in high product separation costs. In this work, we showed that a microporous conductive Bi-based metal-organic framework (Bi-HHTP, HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) can not only efficiently capture CO2 from the dilute CO2 under high humidity but also catalyze the electroreduction of the adsorbed CO2 into formic acid with a high current density of 80 mA cm-2 and a Faradaic efficiency of 90% at a very low cell voltage of 2.6 V. Importantly, the performance in a dilute CO2 atmosphere was close to that under a high-purity CO2 atmosphere. This is the first catalyst that can maintain exceptional eCO2RR performance in the presence of both O2 and N2. Moreover, by using dilute CO2 as the feedstock, a 1 cm-2 working electrode coating with Bi-HHTP can continuously produce a 200 mM formic acid aqueous solution with a relative purity of 100% for at least 30 h in a membrane electrode assembly (MEA) electrolyzer. The product does not contain electrolytes, and such a highly concentrated and pure formic acid aqueous solution can be directly used as an electrolyte for formic acid fuel cells. Comprehensive studies revealed that such a high performance might be ascribed to the CO2 capture ability of the micropores on Bi-HHTP and the lower Gibbs free energy of formation of the key intermediate *OCHO on the open Bi sites.

Enhanced cycloaddition between CO2 and epoxide over a bismuth-based metal organic framework due to a synergistic photocatalytic and photothermal effect

The cycloaddition reaction between CO2 and epoxide is an efficient way to convert CO2 into high value-added chemicals. Therefore, it is particularly important to develop efficient catalysts that can catalyze the reaction under mild conditions. In this work, a metal-organic framework (Bi-HHTP, consisting of bismuth (Bi) as metal dots and 2,3,6,7,10,11-hexahydroxy-triphenylene (HHTP) as organic linkers) with zigzagging corrugated topology was successfully synthesized, which shows excellent catalytic activity under visible light irradiation. Various characterizations suggest that the excellent activity is derived from the following reasons: (1) the abundant exposed Bi sites provide Lewis sites for adsorption of epoxides and CO2; (2) the free holes produced over Bi-HHTP under light irradiation which could oxidize epoxide, which consequently facilitateing the subsequent ring-opening reaction; and (3) the existence of synergistic photocatalytic and photothermal effect in Bi-HHTP. This study provides a new avenue of developing bismuth-based metal organic frameworks to promote the efficiency of cycloaddition of CO2 under mild conditions.

Negative electrodes for supercapacitors with good performance using conductive bismuth-catecholate metal-organic frameworks

Metal-organic frameworks (MOFs) have attracted increasing research interest in various fields. Unfortunately, the poor conductivity of most traditional MOFs considerably hinders their application in energy storage. Benefiting from the full charge delocalization in the atomic plane, two-dimensional conductive coordination frameworks achieve good electrochemical performance. In this work, π-π coupling conductive bismuth-catecholate nanobelts with tunable lengths, Bi(HHTP) (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), are synthesized by a simple hydrothermal reaction and their length-dependent electrochemical properties are also investigated. The Bi(HHTP) nanobelts (about 10 μm in length) possess appropriate porosity, numerous redox active sites and good electrical conductivity. Being a negative electrode for supercapacitors, Bi(HHTP) nanobelts display a high specific capacitance of 234.0 F g^-1 and good cycling stability of 72% after 1000 cycles. Furthermore, the mechanism of charge storage is interpreted for both battery-type and surface-capacitive behavior. It is believed that the results of this work will help to develop battery-type negative electrode materials with promising electrochemical performance using some newly designed π-π coupling conductive coordination frameworks.

Redox-Active Mixed-Linker Metal-Organic Frameworks with Switchable Semiconductive Characteristics for Tailorable Chemiresistive Sensing

Metal-organic frameworks (MOFs), with diverse metal nodes and designable organic linkers, offer unique opportunities for the rational engineering of semiconducting properties. In this work, we report a mixed-linker conductive MOF system with both tetrathiafulvalene and Ni-bis(dithiolene) moieties, which allows the fine-tuning of electronic structures and semiconductive characteristics. By continuously increasing the molar ratio between tetrathiafulvalene and Ni-bis(dithiolene), the switching of the semiconducting behaviors from n-type to p-type was observed along with an increase in electrical conductivity by 3 orders of magnitude (from 2.88×10^-7  S m^-1 to 9.26×10^-5  S m^-1 ). Furthermore, mixed-linker MOFs were applied for the chemiresistive detection of volatile organic compounds (VOCs), where the sensing performance was modulated by the corresponding linker ratios, showing synergistic and nonlinear modulation effects.