The application of metal-organic frameworks (MOFs) as a sensing layer has been attracting great interest over the last decade, due to their uniform properties in terms of high porosity and tunability, which provides a large surface area and/or centers for trapping/binding a targeted analyte. Here we report the fabrication of a highly sensitive humidity sensor that is based on composite thin films of HKUST-1 MOF and carbon nanotubes (CNT). The composite sensing films were fabricated by spin coating technique on a quartz-crystal microbalance (QCM) and a comparison of their shift in resonance frequencies to adsorbed water vapor (5 to 75% relative humidity) is presented. Through optimization of the CNT and HKUST-1 composition, we could demonstrate a 230% increase in sensitivity compared to plain HKUST-1 film. The optimized CNT-HKUST-1 composite thin films are stable, reliable, and have an average sensitivity of about 2.5×10−5 (Δf/f) per percent of relative humidity, which is up to ten times better than previously reported QCM-based humidity sensors. The approach presented here is facile and paves a promising path towards enhancing the sensitivity of MOF-based sensors.
In this work, we present a highly sensitive and selective capacitive humidity sensor. Trianglamine hydrochloride is used as the sensing material, which is synthesized by a [3+3] cyclocondensation reaction between terpthaldehyde and 1R,2R-cyclohexanediamine followed by addition of hydrochloric acid and vapor diffusion of acetone. The crystalline trianglamine hydrochloride salts are dispersed in acetonitrile and then coated on interdigitated electrode substrates by drop casting. The sensor response is characterized for relative humidity (RH) ranging from 5% to 95%. The sensor has a nonlinear response, where the sensitivity increases with an increase in RH. The sensor demonstrates, on average, normalized sensitivities of 0.015 and 6.9 per percent of RH below and above 65% RH, respectively. In addition, the sensor is characterized for hysteresis, long-term stability, effect of temperature, and selectivity. The hysteresis of the sensor is a maximum of about 20% RH and is stable for over 25 days. Temperature analysis of the sensors shows that the sensitivity decreases with increase in temperature. The material is shown to be highly selective with respect to volatile organic compounds (VOCs) and toxic/corrosive gasses. Overall, trianglamine hydrochloride is a promising material for developing a highly sensitive and selective capacitive transduction-based humidity sensor.
Biosensor development includes the deposition of (nano)materials onto a conductive electrode surface, which is a crucial step for obtaining improved performance from the constructed biosensors. Various methods have been used to create a successful matrix of (nano)materials that ensures proper contact between the material and electrode surface. The purpose of (nano)material deposition is to provide a high surface area to improve the electroanalytical performance of biosensors by supporting the stable immobilization of enzymes in a more significant quantity as well as enhancing the catalytic or bioaffinity features. For decades, researchers have been using increasingly advanced methods not only for improving sensing performance, but also for improving stability, reproducibility, and mass production. In this review, we summarized the methods used for (nano)material deposition onto an electrode surface for efficient biosensor fabrication. An enhanced and optimized (nano)material deposition method is crucial for the mechanical stability and fabrication reproducibility of electrodes when designing a suitable biosensing device. In addition, we discussed the problems faced during biosensor application as well as the present challenges and prospects for superior deposition methods.
Morphology, nanoscale features, and tunable properties are the fundamentals of nanomaterials to many applications in use nowadays. Exciting approaches are utilized for developing and exploring new nanomaterials that enable broader impact on a variety of application areas. In this study, we synthesized thin nickel oxide (NiO) nanosheets using wet chemistry process and modified their surface with gold (Au) nanoparticles (NPs) via reduction method to obtain new hybrid (Au NPs-NiO nanosheets) nanomaterial. The morphological analysis of NiO and Au NPs-NiO nanosheets revealed small, uniform, thin, and smooth surface of NiO nanosheets formation, which become rough and decorated with small Au NPs uniformly over the NiO nanosheets surface. The synthesized NiO nanosheets and hybrid (Au NPs-NiO nanosheets) nanomaterials were further utilized to modify the glassy carbon electrode for the fabrication of electrochemical-based hydrazine sensors. The fabricated sensors were applied to detect hydrazine using cyclic voltammetry (CV). And the obtained sensing properties of the hybrid nanomaterial-based sensor were comparatively better than the only NiO nanosheets based sensors. Further, hybrid nanomaterial-based sensors characterized in detail, which showed an excellent sensitivity of 31.75 μAnM cm. The lower detectable limit of hydrazine sensor was as low as ˜0.05 nM, which is considerably better than the other metal oxide-based hydrazine sensors. Better sensing performance of hybrid nanomaterial-based sensor likely stems from nanosheets small size, tiny thickness, and Au modification that significantly improve the surface area and lead a positive synergistic effect (an effect arises between two or more materials that produce an enhanced effect compared to their individual effects) for electrocatalytic reaction. We believe that this hybrid nanomaterial with excellent catalytic properties could be utilized as an efficient electrode material to design other chemical and biological sensors.
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