Ag Nanoparticles Sensitized In2O3 Nanograin for the Ultrasensitive HCHO Detection at Room Temperature

Structure-Function Relationship of Highly Reactive CuOx Clusters on Co3 O4 for Selective Formaldehyde Sensing at Low Temperatures

Designing reactive surface clusters at the nanoscale on metal-oxide supports enables selective molecular interactions in low-temperature catalysis and chemical sensing. Yet, finding effective material combinations and identifying the reactive site remains challenging and an obstacle for rational catalyst/sensor design. Here, the low-temperature oxidation of formaldehyde with CuOx clusters on Co3 O4 nanoparticles is demonstrated yielding an excellent sensor for this critical air pollutant. When fabricated by flame-aerosol technology, such CuOx clusters are finely dispersed, while some Cu ions are incorporated into the Co3 O4 lattice enhancing thermal stability. Importantly, infrared spectroscopy of adsorbed CO, near edge X-ray absorption fine structure spectroscopy and temperature-programmed reduction in H2 identified Cu+ and Cu2+ species in these clusters as active sites. Remarkably, the Cu+ surface concentration correlated with the apparent activation energy of formaldehyde oxidation (Spearman's coefficient ρ = 0.89) and sensor response (0.96), rendering it a performance descriptor. At optimal composition, such sensors detected even the lowest formaldehyde levels of 3 parts-per-billion (ppb) at 75°C, superior to state-of-the-art sensors. Also, selectivity to other aldehydes, ketones, alcohols, and inorganic compounds, robustness to humidity and stable performance over 4 weeks are achieved, rendering such sensors promising as gas detectors in health monitoring, air and food quality control.

Environmental formaldehyde sensing at room temperature by smartphone-assisted and wearable plasmonic nanohybrids

Formaldehyde is a toxic and carcinogenic indoor air pollutant. Promising for its routine detection are gas sensors based on localized surface plasmon resonance (LSPR). Such sensors trace analytes by converting tiny changes in the local dielectric environment into easily readable, optical signals. Yet, this mechanism is inherently non-selective to volatile organic compounds (like formaldehyde) and yields rarely detection limits below parts-per-million concentrations. Here, we reveal that chemical reaction-mediated LSPR with nanohybrids of Ag/AgO_x core-shell clusters on TiO₂ enables highly selective formaldehyde sensing down to 5 parts-per-billion (ppb). Therein, AgO_x is reduced by the formaldehyde to metallic Ag resulting in strong plasmonic signal changes, as measured by UV/Vis spectroscopy and confirmed by X-ray diffraction. This interaction is highly selective to formaldehyde over other aldehydes, alcohols, ketones, aromatic compounds (as confirmed by high-resolution mass spectrometry), inorganics, and quite robust to relative humidity changes. Since this sensor works at room temperature, such LSPR nanohybrids are directly deposited onto flexible wristbands to quantify formaldehyde between 40-500 ppb at 50% RH, even with a widely available smartphone camera (Pearson correlation coefficient r = 0.998). Such chemoresponsive coatings open new avenues for wearable devices in environmental, food, health and occupational safety applications, as demonstrated by an early field test in the pathology of a local hospital.

Humidity Effect on Low-Temperature NH3 Sensing Behavior of In2O3/rGO Composites under UV Activation

The nature of the constituent components of composite materials can significantly affect the character of their interaction with the gas phase. In this work, nanocrystalline In₂O₃ was synthesized by the chemical precipitation method and was modified using reduced graphene oxide (rGO). The obtained composites were characterized by several analysis techniques-XRD, TEM, SEM, FTIR and Raman spectroscopy, XPS, TGA, and DRIFTS. The XPS and FTIR and Raman spectroscopy results suggested the formation of interfacial contact between In₂O₃ and rGO. The results of the gas sensor's properties showed that additional UV illumination led to a decrease in resistance and an increase in sensor response at room temperature. However, the presence of humidity at room temperature led to the disappearance of the response for pure In₂O₃, while for the composites, an inversion of the sensor response toward ammonia was observed. The main reason may have been the formation of NH₄NO₃ intermediates with further hydrolysis and decomposition under light illumination with the formation of nitrite and nitrate species. The presence of these species was verified by in situ DRIFT spectroscopy. Their strong electron-accepting properties lead to an increase in resistance, which possibly affected the sensor signal's inversion.

Practical and Efficient: A Pocket-Sized Device Enabling Detection of Formaldehyde Adulteration in Vegetables

Formaldehyde, as a carcinogenic substance, is often intentionally used to adulterate vegetables to increase their shelf life, and the adhesive tape used to attach labels can also leave formaldehyde on the surface of vegetables. However, as the "gold" standard, gas chromatography (GC) and high-performance liquid chromatography (HPLC) are expensive for individual tests and confined to the laboratory owing to their size and a suitable detector (low-cost, portable, fast detection speed) to check formaldehyde contamination in vegetables not being available. Here, we tested formaldehyde contamination in vegetables using a low-cost and hand-held detector combined with a screen-printed electrode (SPE) amperometric sensor and an open-sourced potentiostat. The analyzer can detect a concentration of 100 μmol/L formaldehyde and achieve a good linear range between 100 and 1000 μmol/L. Furthermore, the detector successfully identified formaldehyde contamination in 53 samples of six different kinds of vegetables even after residual formaldehyde on the surface was evaporated. Most importantly, under the practicability-oriented idea, a cost-effective strategy was implemented for this detector design rather than using other pricey methods (e.g., photolithography, electron-beam evaporation, chemical deposition), which enormously reduces the cost (under ∼USD 0.5 per test) and meets all of the requirements of ASSURED device. We believe this cheap, portable detector could help law-enforcing authorities, healthcare workers, and customers to screen formaldehyde contamination easily. Also, the cost-saving strategy is appropriate for low-income areas, where there is a lack of laboratories, funds, and trained experts.

Ultrasensitive ppb-level trimethylamine gas sensor based on p-n heterojunction of Co3O4/WO3

Trace poisonous and harmful gases in the air have been harming and affecting people's health for a long time. At present, effective and accurate detection of ppb-level harmful gas is still a bottleneck to be overcome. Herein, we report a ppb-level triethylamine (TEA) gas sensor based on p-n heterojunction of Co₃O₄/WO₃, which is prepared with ZIF-67 as the precursor and provides Co₃O₄deposited tungsten oxide flower-like structure. Due to the introduction of Co₃O₄and the 3D flower-like structure of WO₃, the Co₃O₄/WO₃-2 gas sensor shows excellent gas sensing performance (1101 for 10 ppm at 240 °C), superb selectivity, good long-term stability and linear response for TEA concentration. Moreover, the experimental results indicate that the Co₃O₄/WO₃-2 gas sensor also possesses a good response to 50 ppb TEA, in fact, the theoretical limit of detection is 0.6 ppb. Co₃O₄not only improves the efficiency of electron separation/transport, but also accelerates the oxidation rate of TEA. This method of synthesizing p-n heterojunction with ZIF as the precursor provides a new idea and method for the preparation of low detection limit gas sensors.

Effect of Ag Addition on the Gas-Sensing Properties of Nanostructured Resistive-Based Gas Sensors: An Overview

Nanostructured semiconducting metal oxides (SMOs) are among the most popular sensing materials for integration into resistive-type gas sensors owing to their low costs and high sensing performances. SMOs can be decorated or doped with noble metals to further enhance their gas sensing properties. Ag is one of the cheapest noble metals, and it is extensively used in the decoration or doping of SMOs to boost the overall gas-sensing performances of SMOs. In this review, we discussed the impact of Ag addition on the gas-sensing properties of nanostructured resistive-based gas sensors. Ag-decorated or -doped SMOs often exhibit better responsivities/selectivities at low sensing temperatures and shorter response times than those of their pristine counterparts. Herein, the focus was on the detection mechanism of SMO-based gas sensors in the presence of Ag. This review can provide insights for research on SMO-based gas sensors.

Low Temperature HCHO Detection by SnO2/TiO2@Au and SnO2/TiO2@Pt: Understanding by In-Situ DRIFT Spectroscopy

In this work we analyze the effectiveness of decoration of nanocrystalline SnO2/TiO2 composites with gold nanoparticles (Au NPs) and platinum nanoparticles (Pt NPs) in enhancing gas sensor properties in low-temperature HCHO detection. Nanocrystalline SnO2/TiO2 composites were synthesized by a chemical precipitation method with following modification with Pt and Au NPs by the impregnation method. The nanocomposites were characterized by TEM, XRD, Raman and FTIR spectroscopy, DRIFTS, XPS, TPR-H2 methods. In HCHO detection, the modification of SnO2 with TiO2 leads to a shift in the optimal temperature from 150 to 100 °C. Further modification of SnO2/TiO2 nanocomposites with Au NPs increases the sensor signal at T = 100 °C, while modification with Pt NPs gives rise to the appearance of sensor responses at T = 25 °C and 50 °C. At 200 °C nanocomposites exhibited high selectivity toward formaldehyde within the sub-ppm concentration range among different VOCs. The influence of Pt and Au NPs on surface reactivity of SnO2/TiO2 composite and enhancement of the sensor response toward HCHO was studied by DRIFT spectroscopy and explained by the chemical and electronic sensitization mechanisms.

Strategies for Improving the Sensing Performance of Semiconductor Gas Sensors for High-Performance Formaldehyde Detection: A Review

Formaldehyde is a poisonous and harmful gas, which is ubiquitous in our daily life. Long-term exposure to formaldehyde harms human body functions; therefore, it is urgent to fabricate sensors for the real-time monitoring of formaldehyde concentrations. Metal oxide semiconductor (MOS) gas sensors is favored by researchers as a result of their low cost, simple operation and portability. In this paper, the mechanism of formaldehyde detection by gas sensors is introduced, and then the ways of ameliorating the response of gas sensors for formaldehyde detection in recent years are summarized. These methods include the control of the microstructure and morphology of sensing materials, the doping modification of matrix materials, the development of new semiconductor sensing materials, the outfield control strategy and the construction of the filter membrane. These five methods will provide a good prerequisite for the preparation of better performing formaldehyde gas sensors.

A high-response formaldehyde sensor based on fibrous Ag-ZnO/In2O3 with multi-level heterojunctions

Timely detection of formaldehyde is pivotal because formaldehyde is slowly released from the indoor decorative materials, jeopardizing our healthy. Herein, a high-response formaldehyde gas sensor based on Ag-ZnO/In₂O₃ nanofibers was successfully fabricated. Compared with all the control samples, the hybrid exhibits superior sensitivity (0.65 ppm^-1), excellent selectivity (≥ 12.5) and durable stability (the deviation value ≤ 3%). Particularly, an ultra-high response value of about 186 towards 100 ppm of formaldehyde at 260 °C was achieved, heading the list of outstanding candidates. Additionally, the limit of detection is as low as 9 ppb. The enhanced gas sensing properties can be mainly attributed to multi-level heterojunctions (n-n heterojunction and Ohmic junction) and the "spill-over" effect of Ag, ultimately increasing the adsorption of gas molecules on the surface of sensing material. This work verifies that proper design of multi-level heterojunctions significantly upgrade the sensing performance.

Recent advances in energy-saving chemiresistive gas sensors: A review

With the tremendous advances in technology, gas-sensing devices are being popularly used in many distinct areas, including indoor environments, industries, aviation, and detectors for various toxic domestic gases and vapors. Even though the most popular type of gas sensor, namely, resistive-based gas sensors, have many advantages over other types of gas sensors, their high working temperatures lead to high energy consumption, thereby limiting their practical applications, especially in mobile and portable devices. As possible ways to deal with the high-power consumption of resistance-based sensors, different strategies such as self-heating, MEMS technology, and room-temperature operation using especial morphologies, have been introduced in recent years. In this review, we discuss different types of energy-saving chemisresitive gas sensors including self-heated gas sensors, MEMS based gas sensors, room temperature operated flexible/wearable sensor and their application in the fields of environmental monitoring. At the end, the review will be concluded by providing a summary, challenges, recent trends, and future perspectives.

Enhanced performance of an acetone gas sensor based on Ag-LaFeO3 molecular imprinted polymers and carbon nanotubes composite

High performance acetone gas sensors were fabricated with molecular imprinted polymers of Ag-LaFeO₃ (ALFOMMIPs) and multi walled carbon nanotubes (CNTs) composite using the microwave assisted sol-gel method. The crystalline structure, functional groups, grain size and surface appearance of the synthesized materials were analyzed via different characterization techniques and the gas responses of the samples were examined. The detailed acetone gas sensing tests and analysis revealed that the CNTs and ALFOMIPs nanocomposite (CNT/ALFOMIP) sample possessed a higher response than that of the ALFOMIPs sample. Where 0.75 wt% CNTs were added into the ALFOMIPs (0.75% CNT/ALFOMIP nanocomposite) sensor, an excellent gas sensing performance was exhibited. The response of this sensor was up to 59 for 5 ppm acetone vapors and the response and recovery times were 58 and 33 s at low working temperature of 86 °C, respectively. In addition, it had the best selectivity only to acetone vapors due to the use of the molecular imprinting technique.

Platinum-Supported Cerium-Doped Indium Oxide for Highly Sensitive Triethylamine Gas Sensing with Good Antihumidity

Triethylamine is extremely harmful to human health, and chronic inhalation can lead to respiratory and hematological diseases and eye lesions. Hence, it is essential to develop a triethylamine gas-sensing technology with high response, selectivity, and stability for use in healthcare and environmental monitoring. In this work, a simple and low-cost sensor based on the Pt- and Ce-modified In₂O₃ hollow structure to selectively detect triethylamine is developed. The experimental results reveal that the sensor based on 1% Pt/Ce₁₂In exhibits excellent triethylamine-sensing performance, including its insusceptibility to water, reduced operating temperature, enhanced response, and superior long-term stability. This work suggests that the enhancement of sensing performance toward triethylamine can be attributed to the high relative contents of O_V and O_C, large specific surface area, catalytic effect, the electronic sensitization of Pt, and the reversible redox cycle properties of Ce. This sensor represents a unique and highly sensitive means to detect triethylamine, which shows great promise for potential applications in food safety inspection and environmental monitoring.