Enhanced detection of low concentration volatile organic compounds using advanced doped zinc oxide sensors

Pure zinc oxide nanoparticles, as well as those doped with 3% calcium, aluminum, and gallium, were synthesized using a sol-gel method and then deposited onto an alumina substrate for sensing tests. The resulting nanoparticles were characterized using a variety of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), UV-VIS-NIR absorption spectroscopy, and photoluminescence (PL) measurements, to examine their structural, morphological, and optical properties. The prepared nanoparticles were found to have the hexagonal wurtzite structure of ZnO with a P63mC space group. The UV-Vis-IR spectra showed that the samples are highly absorbent in the UV range, while the PL spectra confirmed the presence of many defects in the ZnO structure, such as oxygen vacancies and zinc interstitials. The doped samples exhibited more defects than the pure sample. SEM images of the deposited film surface showed agglomerates with a spherical shape and confirmed the nanometer scale size of our prepared samples, as corroborated by the TEM images. The EDX spectra indicated the high purity of the ZnO deposited films, with a high presence of Zn and O and the presence of the doped elements (Ca, Al, and Ga) in each doped sample. Sensing tests were performed on ZnO, Ca3%-doped ZnO (C3ZO), Al3%-doped ZnO (A3ZO), and Ga3%-doped ZnO (G3ZO) sensors in the presence of volatile organic compounds (VOCs) gases such as ethanol, formaldehyde, methanol, and acetone at low concentrations. The sensors exhibited high responses to low ppm level concentrations of the VOCs gases. At a low operational temperature of 250 °C, the C3ZO sensor had the highest response to 5 ppm of ethanol, methanol, and formaldehyde gases compared to the pure and other doped sensors. Additionally, the A3ZO sensor exhibited the highest response to acetone gas. In conclusion, our findings suggest that the doping of zinc oxide can enhance the low concentration detection of VOCs gases, with the C3ZO and A3ZO sensors showing the highest response to specific gases.

Species-Selective Detection of Volatile Organic Compounds by Ionic Liquid-Based Electrolyte Using Electrochemical Methods

The detection of volatile organic compounds (VOCs) is an important topic for environmental safety and public health. However, the current commercial VOC detectors suffer from cross-sensitivity and low reproducibility. In this work, we present species-selective detection for VOCs using an electrochemical cell based on ionic liquid (IL) electrolytes with features of high selectivity and reliability. The voltammograms measured with the IL-based electrolyte absorbing different VOCs exhibited species-selective features that were extracted and classified by linear discriminant analysis (LDA). The detection system could identify as many as four types of VOCs, including methanol, ethanol, acetone, formaldehyde, and additional water. A mixture of methanol and formaldehyde was detected as well. The sample required for the VOCs classification system was 50 μL, or 1.164 mmol, on average. The response time for each VOC measurement is as fast as 24 s. The volume of VOCs such as formaldehyde in solution could also be quantified by LDA and electrochemical impedance spectroscopy techniques, respectively. The system showed a tunable detection range for 1.6 and 16% (w/v) CH2O solution by adjusting the composition of the electrolyte. The limit of detection was as low as 1 μL. For the 1.6% CH2O solution, the linearity calibration range was determined to be from 5.30 to 53.00 μmol with a limit of detection at 0.53 μmol. The mechanisms for VOCs determination and quantification are also thoroughly discussed. It is expected that this work could provide a new insight into the concept of electrochemical detection of VOCs with machine learning analysis and be applied to both VOCs gas monitoring and fluid detection.

Facet-energy inspired metal oxide extended hexapods decorated with graphene quantum dots: sensitive detection of bisphenol A in live cells

The development of crystal-facet metal oxide heterostructures has been of great interest owing to their rational design and multifunctional properties at the nanoscale level. Herein, we report a facile solution-based method for the synthesis of single-crystal Cu₂O nanostructures (i.e. Cu₂O-CuO) as a core. Graphene quantum dots (GQDs) with varying concentrations are fabricated on the surface of Cu₂O extended hexapods (EHPs) in ethanol solution at room temperature via self-assembly, where copper acts as a sacrificial model and a stabilizer as well. The Cu₂O crystals displayed a good sensing activity toward BPA oxidation owing to their high energy facets, dangling bonds and great proportion of surface copper atoms. Structural, morphological, chemical and vibrational investigations were performed in detail, presenting high crystallinity of hybrid nanocomposites and Cu₂O-CuO heterojunction positions along with the growth of GQDs on the core of Cu₂O-CuO crystals. The electrochemical sensing performance of the as-fabricated Cu₂O-CuO@GQD EHPs was monitored for the determination of bisphenol A (BPA) as an early diagnostic marker and environmental contaminant. The synergistic effects of the boosted surface area, exposed Cu {111} crystallographic planes and mixed copper valences enhance redox reaction kinetics by increasing the electron shuttling rate at the electrode-analyte junction. Benefitting from the improved electrocatalytic activity for BPA oxidation, the electrochemical sensor displayed the lowest limit of detection (≤1 nM), good chemical stability, a broad linear range (2 nM-11 mM), and high sensitivity (636 μA mM^-1 cm^-2). The Cu₂O-CuO@GQD EHP-based sensing platform was used for BPA detection in water and human serum samples. We have also constructed a pioneering electrochemical sensing platform for BPA detection in live cells, which might be used as a marker for early disease diagnosis.

Underpinning the Interaction between NO2 and CuO Nanoplatelets at Room Temperature by Tailoring Synthesis Reaction Base and Time

An approach to tailor the morphology and sensing characteristics of CuO nanoplatelets for selective detection of NO₂ gas is of great significance and an important step toward achieving the challenge of improving air quality and in assuring the safety of mining operations. As a result, in this study, we report on the NO₂ room temperature gas-sensing characteristics of CuO nanoplatelets and the underlying mechanism toward the gas-sensing performance by altering the synthesis reaction base and time. High sensitivity of ∼40 ppm^-1 to NO₂ gas at room temperature has been realized for gas sensors fabricated from CuO nanoplatelets, using NaOH as base for reaction times of 45 and 60 min, respectively at 75 °C. In both cases, the crystallite size, surface area, and hole concentration of the respective materials influenced the selectivity and sensitivity of the NO₂ gas sensors. The mechanism underpinning the superior NO₂ gas sensing are thoroughly discussed in terms of the crystallite size, hole concentration, and surface area as active sites for gas adsorption.

3D Hollow Quasi-Graphite Capsules/Polyaniline Hybrid with a High Performance for Room-Temperature Ammonia Gas Sensors

Designing sensing materials with novel morphologies and compositions is eminently challenging to achieve high-performance gas sensor devices. Herein, an in situ oxidative polymerization approach is developed to construct three-dimensional (3D) hollow quasi-graphite capsules/polyaniline (GCs/PANI) hierarchical hybrids by decorating protonated PANI on the surface of GCs; as a result, an immensely active and sensitive material was developed for sensing ammonia gas at room temperature. Moreover, the GCs possessed a capsule-like hollow/open structure with partially graphitized walls, and PANI nanospheres were uniformly decorated on the GC surfaces. Furthermore, the inflexible and rigid 3D ordered chemistry of these materials provides the resulting hybrids with a large interfacial surface area, which not only allows for rapid adsorption and charge transfer but also provides the necessary structural stability. The 3D hollow GCs/PANI hybrids exhibit excellent performance; the GCs/PANI-3 hybrid is highly sensitive (with a response value of 1.30) toward 10 ppm NH₃ gas and has short response and recovery times of 34 and 42 s, respectively. The GCs/PANI-3 hybrid also demonstrates a good selectivity, repeatability, and long-term stability, which are attributed to the substantial synergistic effect of the GCs and PANI. The design of such a unique 3D ordered framework provides a promising pathway to achieve room-temperature gas sensors for commercial applications.

Synthesis of CuO–CdS composite nanowires and their ultrasensitive ethanol sensing properties

One dimension CuO/CdS composites with an average diameter of 30 nm were synthesized by a solvothermal method. The operating temperature of the sensors is 182 °C, and their responses were improved by 6 times. The ultrafast response–recovery time was obtained.

Konjac glucomannan-templated synthesis of three-dimensional NiO nanostructures assembled from porous NiO nanoplates for gas sensors

Biopolymer template synthesis has attracted extensive interest for fabricating highly porous metal oxide nanostructures. In this report, a green template-based approach for the synthesis of three-dimensional (3D) NiO nanostructures assembled from porous NiO nanoplates is introduced using a konjac glucomannan (KGM) template. The Ni–KGM composites, which were formed by the immersion of KGM nanofibrils in nickel nitrate solution, were annealed in air at 600 °C to obtain the highly porous NiO nanoplates. The KGM nanofibrils were used as a sacrificial template, which was combusted at a high temperature for the formation of the porous nanostructures. The gas sensor properties of the porous NiO architecture were systematically investigated with four reduced gases including hydrogen sulfide, ammonia, carbon monoxide and hydrogen. The results indicate that the porous NiO nanoplates show a good detection of hydrogen sulfide with a rapid response and recovery speed at low concentrations.

Biopolymer template synthesis has attracted extensive interest for fabricating highly porous metal oxide nanostructures.

P–p heterojunction CuO/CuCo2O4 nanotubes synthesized via electrospinning technology for detecting n-propanol gas at room temperature

Composite CuO/CuCo₂O₄ nanotubes were synthesized by electrospinning technology. The large specific surface area, complex tubular structure, and p–p heterojunction are the potential reasons for the excellent room temperature gas sensing performance toward n-propanol vapor.

B. RK, Jones LA, Mayes ELH, Ippolito SJ, Sunkara MV. Modulating interleaved ZnO assembly with CuO nanoleaves for multifunctional performance: perdurable CO2 gas sensor and visible light catalyst

Efficient CO₂ gas detection and visible light photocatalysis performance shown by interleaved CuO/ZnO heterostructures ascribed primarily to the high surface area, p/n nano-interfaces and catalytic role of Ag.

Room-Temperature High-Performance H2S Sensor Based on Porous CuO Nanosheets Prepared by Hydrothermal Method

Porous CuO nanosheets were prepared on alumina tubes using a facile hydrothermal method, and their morphology, microstructure, and gas-sensing properties were investigated. The monoclinic CuO nanosheets had an average thickness of 62.5 nm and were embedded with numerous holes with diameters ranging from 5 to 17 nm. The porous CuO nanosheets were used to fabricate gas sensors to detect hydrogen sulfide (H2S) operating at room temperature. The sensor showed a good response sensitivity of 1.25 with respond/recovery times of 234 and 76 s, respectively, when tested with the H2S concentrations as low as 10 ppb. It also showed a remarkably high selectivity to the H2S, but only minor responses to other gases such as SO2, NO, NO2, H2, CO, and C2H5OH. The working principle of the porous CuO nanosheet based sensor to detect the H2S was identified to be the phase transition from semiconducting CuO to a metallic conducting CuS.

A highly sensitive gas sensor based on CuO nanoparticles synthetized via a sol–gel method

In this paper, CuO nanoparticles were synthetized via a sol–gel method and their corresponding gas sensor was achieved simultaneously.