Impedance-Based Detection of NO2 Using Ni-MOF-74: Influence of Competitive Gas Adsorption

Exceptional Electrical Detection of Trace NO2 via Mixed Metal MOF-on-MOF Film-Based Sensors

The tunability of metal-organic frameworks (MOFs) makes them exceptional materials for the development of highly selective, low-power sensors for toxic gas detection. Herein, we demonstrate enhanced detection of NO2 gas by a MOF-based electrical impedance sensor made using a unique mixed metal MOF-on-MOF synthesis. A combined experimental and computational study was performed using the exemplar NixMg1-x-MOF-74 to understand the fundamental structure-property relationships behind metal mixing and MOF film synthesis methods on sensor performance. Density functional theory results indicated that the presence of Ni in Mg-MOF-74 increased framework stability and increased the electron density of states at lower energies near the HOMO, as well as enhanced the NO2-Mg adsorption interaction. Impedance data of the NixMg1-x-MOF-74 films with larger Ni contents showed greater impedance change after exposure to 1 ppm of NO2 gas. Furthermore, when synthesized through either a drop-cast or direct solvothermal film growth approach, the monometallic Ni-based sensors had the best performance. However, the mixed metal NixMg1-x-MOF-74 sensors synthesized through a MOF-on-MOF approach resulted in the highest impedance change, outperforming all monometallic Ni-based sensors. In particular, the mixed metal Ni-on-Mg-MOF-74 film was the best-performing sensor with an impedance change of 309 upon trace NO2 exposure. Change in impedance response after NO2 exposure was improved by 52% compared to the best monometallic Ni-on-Ni-MOF-74 sensor. Structural analysis of the Ni-on-Mg film showed that the first Mg-MOF-74 layer acts as a structural template controlling the structural features of the final film after metal exchange with Ni. This led to improved film quality, evidenced by the greater crystallinity and larger MOF grain sizes, and resulted in enhanced sensor performance which was not achievable through other metal mixing methods. Altogether, this study identifies structure-property relationships and synthetic templating methods that inform MOF-based sensor design, allowing for improved detection of toxic compounds.

Bimetallic MOF-Based Sensor for Highly Sensitive Detection of Ammonia Gases

The demand for the detection of ultralow concentrations of ammonia gas is growing. A bimetallic metal-organic framework (MOF) comprising Prussian blue analogs (PBAs) was used to achieve highly sensitive and stable detection of ammonia gas at room temperature in this study. First, PB was enriched by using ammonia for improved gas sensing properties. Second, a sensitive membrane with more vacancies was formed by partially replacing Fe3+ with Cu2+ through a cation-exchange strategy. Finally, a capacitive sensor was developed for ultralow-concentration ammonia detection using a Cu-Fe PBA sensitive membrane and interdigitated electrodes (IDEs). To investigate the adsorption efficiency of the designed composite sensitive film for ammonia, PBAs nanoparticles were deposited on a quartz microcrystal balance (QCM) via cyclic voltammetry and a hydrothermal method. Approximately 10 ppm of ammonia was adsorbed under 1 atm by the Cu-Fe PBA film prepared using a reaction time of 36 h, and the adsorption efficiency was measured to be 2.2 mmol g-1 using the QCM frequency response. The Cu-Fe PBAs were also tested using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Brunauer-Emmett-Teller theory. The introduction of Cu2+ ions significantly increased the specific surface area of Cu-Fe PBAs MOF, and the number of adsorption sites for ammonia also increased; however, its skeleton structure remained similar to that of PB. Then, the capacitive sensor based on Cu-Fe PBA sensitive membrane and IDE was fabricated and the gas sensing detection device was established for ammonia detection. Overall, the developed capacitive sensor exhibits a linear response of 75-1000 ppb and a detection limit of 3.8 ppb for ultralow ammonia concentrations, which makes it superior to traditional detection methods and thus allows excellent application prospects.