Detection and capture of toxic nitrogen oxides (NO x) is important for emissions control of exhaust gases and general public health. The ability to directly electrically detect trace (0.5-5 ppm) NO 2 by a metal-organic framework (MOF)-74-based sensor at relatively low temperatures (50 °C) is demonstrated via changes in electrical properties of M-MOF-74, M = Co, Mg, Ni. The magnitude of the change is ordered Ni > Co > Mg and explained by each variant's NO 2 adsorption capacity and specific chemical interaction. Ni-MOF-74 provides the highest sensitivity to NO 2 ; a 725× decrease in resistance at 5 ppm NO 2 and detection limit <0.5 ppm, levels relevant for industry and public health. Furthermore, the Ni-MOF-74-based sensor is selective to NO 2 over N 2 , SO 2 , and air. Linking this fundamental research with future technologies, the high impedance of MOF-74 enables applications requiring a near-zero power sensor or dosimeter, with the active material drawing <15 pW for a macroscale device 35 mm 2 with 0.8 mg MOF-74. This represents a 10 4-10 6 × decrease in power consumption compared to other MOF sensors and demonstrates the potential for MOFs as active components for long-lived, near-zero power chemical sensors in smart industrial systems and the internet of things.

Iodine detection is crucial for nuclear waste clean-up and first responder activities. For ease of use and durability of response, robust active materials that enable the direct electrical detection of I2 are needed. Herein, a large reversible electrical response is demonstrated as I2 is controllably and repeatedly adsorbed and desorbed from a series of metal–organic frameworks (MOFs) MFM-300(X), each possessing a different metal center (X = Al, Fe, In, or Sc) bridged by biphenyl-3,3′,5,5′-tetracarboxylate linkers. Impedance spectroscopy is used to evaluate how the different metal centers influence the electrical response upon cycling of I2 gas, ranging from 10× to 106× decrease in resistance upon I2 adsorption in air. This large variation in electrical response is attributed not only to the differing structural characteristics of the MOFs but also to the differing MOF morphologies and how this influences the degree of reversibility of I2 adsorption. Interestingly, MFM-300(Al) and MFM-300(In) displayed the largest changes in resistance (up to 106×) yet lost much of their adsorption capacity after five I2 adsorption cycles in air. On the other hand, MFM-300(Fe) and MFM-300(Sc) revealed more moderate changes in resistance (10–100×), maintaining most of their original adsorption capacity after five cycles. This work demonstrates how changes in MOFs can profoundly affect the magnitude and reversibility of the electrical response of sensor materials. Tuning both the intrinsic (resistivity and adsorption capacity) and extrinsic (surface area and particle morphology) properties is necessary to develop highly reversible, large signal-generating MOF materials for direct electrical readout for I2 sensing.

Metal–organic frameworks (MOFs) are poised to improve modern-day gas sensors. Their unparalleled tunability and chemical selectivity across a wide span of chemical species have made them attractive for use in highly selective gas sensors. In this work, we delve into one of the biggest strengths of many MOFs, their irreversible chemisorption of gas molecules, and how this can be leveraged to create unique, low cost irreversible gas sensors (dosimeters) enabled by changes in MOF electrical properties. On the other hand, robust chemical bonding to the sensor substrate is often one of the biggest practical challenges facing MOF sensors for long-life applications. “Hold on tight”, be it to adsorbed molecules or the substrate itself, is a key factor in development of future MOF gas sensors. With continued advances, MOF materials are positioned to make both highly innovative and significantly impactful improvements to highly selective gas sensors used throughout society.

A novel sensor composed of a nickel-loaded SSZ-13 zeolite membrane on interdigitated electrodes (IDEs) is demonstrated as a direct electrical NO2 readout sensor. This NO2 adsorbent is made by homogeneously loading SSZ-13 zeolite with nickel­(II) through a liquid-phase ion-exchange procedure. Exposure of the zeolite-based sensor to trace NO2 gas elicits an electrical impedance response measured at a single frequency. The sensor shows the same final change in impedance magnitude upon equilibration to different concentrations of trace NO2 in N2, suggesting that the occupation and eventual saturation of adsorption sites lead to the impedance change. However, the NO2 concentration can be determined through analysis of the rate of impedance change, where lower concentrations of NO2 lead to larger time constants with a logarithmic relationship to the NO2 concentration. Two time constants were observed from the linearized impedance plots, a fast one (τ1) and a slow one (τ2), where τ2 showed a larger dependence on the NO2 concentration, increasing faster than τ1 as the NO2 concentration decreased. Furthermore, the Ni-SSZ-13 sensor response is partially reversible in an inert gas environment, indicating the reversible adsorption of NO2 at nickel surface sites. Under exposure to humid air, differentiation between humid air and dry 5 ppm NO2 is accomplished by examination of the real component of the impedance signal. The resulting NO2 atmosphere shows an increase in the real component (more resistive), whereas the humid air shows a decrease (more capacitive). These results indicate that control of metal-ion loading into SSZ-13 may allow these NO2 selective catalytic reduction catalysts to be further leveraged as low-temperature NO2 sensors.

Nanoporous materials, including metal–organic frameworks (MOFs) and inorganic zeolites, are gaining attention as gas sensor materials due to their chemical selectivity and robustness. To advance industrial viability of these materials as sensors, long-term, variable environment testing is needed to evaluate their stability and continued chemical exposure response. Nanoporous materials-based direct electrical readout sensors were evaluated for 3 months under dry or humid conditions at 74 °C. The sensors were comprised of either Ni-MOF-74, Mg-MOF-74, or Ni-SSZ-13 zeolite. Additionally, we describe the development of multichambered sensor testing platforms that allows uninterrupted direct impedance monitoring of each sensor over long test periods. Results indicate relative stability in dry conditions for the sensors over time. In contrast, degradation of the active sensing material is evident in the humid environment. Collectively, these results demonstrate need for long-term testing of emerging nanoporous sensor materials under specific environmental conditions.