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In this study, we synthesized Co₃O₄ thin films using a sol–gel spin coating method and investigated the effect of heat treatment on their carbon dioxide (CO₂) gas sensing properties. To optimize their crystallinity and enhance their CO₂ sensitivity, the thin films were annealed at temperatures ranging from 400^∘C to 550^∘C. We characterized the morphological, structural, electrical, and optical properties of the Co₃O₄ thin films to gain insights into their behavior in the low operating temperature range (OTR). Our X-ray diffraction (XRD) analysis revealed that all the thin films exhibited a cubic structure, with improved crystallinity observed at higher annealing temperatures. We observed a decrease in band edges in the optical transmittance spectra of the thin films as the annealing temperature increased, indicating a redshift in the absorption edge. Notably, the highest electrical conductivity was observed for the sample annealed at 550^∘C, suggesting enhanced crystallinity and improved CO₂ gas sensitivity. These findings provide valuable insights into the optimization of Co₃O₄ thin films for CO₂ gas sensing applications.

Both selectivity and sensitivity of chemical sensors can be significantly improved by exploiting the information contained in microfluctuations present in the sensor system. We call our collection of methods to extract information from these microfluctuations Fluctuation-Enhanced Sensing (FES). In this review paper we discuss general FES principles and two types of applications; gas sensing with commercial solid state sensors and the Sensing of Phage-Triggered Ion Cascde (SEPTIC) technique to detect and identify bacteria.

Co-axial Zn_1−xMg_xO core, ZnO shell structures were grown using metal organic chemical vapor deposition (MOCVD), with Mg mole fractions of 2, 5 and 10%. The co-axial core shell structure, with the respective Mg concentration is verified using scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS). The response times (ṟise time and fall time) of the devices, after being exposed to methanol, varied with Mg mole fraction at the core, r-0.17s and, f-0.37s & f-0.48s for 2% Mg, r-0.81s and, f-5.98s & f-0.89s for 5% Mg and r-0.33s and f-0.13s for 10% Mg. The sensitivity of the devices at room temperature increased with the increment of Mg mole fraction at the core, 25%, 48% and 50% with Mg concentration of 0.02, 0.05 and 0.1, respectively, under high concentration of methanol. The estimated activation energy, corresponds to doubly charged oxygen vacancy (Vo²⁺).

ZnO polyhedral structures were fabricated via a rapid thermal evaporation process. The ZnO nanostructures grown had faceted trumpet-like morphologies. The intensity of visible emission in photoluminescence spectra strongly related to growth temperature, indicating different concentrations of oxygen vacancy. These structures exhibited typical sensing property to gaseous ethanol. The sensing property was related to the oxygen vacancies.

First-principle calculation was carried out to systematically investigate carbon monoxide (CO) adsorption on pristine and cobalt (Co)-doped phosphorenes (Co-bP). Whether or not CO is adsorped, pristine phosphorene is a direct-band-gap semiconductor. However, the bandgap of Co-bP experiences direct-to-indirect transition after CO molecule adsorption, which will affect optical absorption considerably, implying that Co doping can enhance the sensitivity of phosphorene as a CO gas sensor. Moreover, Co doping can improve an adsorption energy of CO to 1.31 eV, as compared with pristine phosphorene (0.12 eV), also indicating that Co-bP is energetically favorable for CO gas sensing.

The priority of research efforts over the past two decades has been focused on the effective detection of harmful gases and the development of small, efficient, and reliable nanodimensional sensors. The adsorption of carbon monoxide (CO) gas molecules on zigzag gallium phosphide nanoribbons (zGaPNRs) has been studied using first-principle calculations within the context of Density Functional Theory (DFT). Here, we have evaluated the potential of single-atom thick zGaPNRs in different configurations of nanoribbons for the detection of CO. Many possible configurations of studying the CO molecule adsorption on zGaPNRs have been explored. It is established that the interaction of CO molecules has an impact on the electrical and transport properties of zGaPNRs. The H-GaP-H is the most stable structure with the binding energy ($E_b$) of −5.70eV. The stability is compromised with CO adsorption with CO-GaP-CO being the least stable structure. Pristine structure is semiconducting with the energy band gap ($E_G$) of −3.95eV. The computed sensitivity (S) values are found to be highest for Co-GaPNRs with the S value of $1.77\times 10^{18}$ and the least sensitive structure is H-GaP-CO with the computed S as $3.84\times 10^{17}$. Additionally, it is noted that CO molecules always establish a stable chemical bond with the nanoribbon edges through the C-side. The unique behavior is revealed by the transport characteristics, which demonstrates that when CO adsorption occurs near the Ga edge, the current magnitude is noticeably greater. Our research demonstrates the potential of specific CO adsorption and detection for the development of nanosensors.

Present communication reports the LPG and NH₃ sensing properties of Co₃O₄ films prepared on throughly cleaned stainless steel (SS) and copper (CU) substrates by using DC electrochemical deposition method followed by air annealing at 350°C/2 h. The resultant films are characterized by using X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The LPG and NH₃ gas sensing properties of these films are measured at room temperature (RT) by using static gas sensing system at different concentrations of test gas ranging from ~ 25 ppm to 350 ppm. The XRD and Raman spectroscopy studies clearly indicated the formation of pure cubic spinel Co₃O₄ in all films. The LPG and NH₃ gas sensing properties of films showed (i) the increase in sensitivity factor (S.F.) with gas concentrations and (ii) more sensibility to LPG as compared to NH₃ gas. In case of NH₃ gas (conc. ~ 150 ppm) and LPG gas (conc. ~ 60 ppm) sensing, the maximum S.F. = 270 and 258 are found for the films deposited on CU substrates, respectively. For all films, the response time (3–5 min.) is found to be much higher than the recovery time (30–50 sec). For all films, the response and recovery time are found to be higher for LPG as compared to NH₃ gas. Further, repeatability–reproducibility in gas sensing properties is clearly noted by analysis of data for number of cycles recorded for all films from different set of depositions.

Polypyrrole (PPy)–Zn₂SnO₄ nanocomposites with different weight percentages (0–20%) of Zn₂SnO₄ were successfully prepared by chemical oxidative polymerization. The prepared nanocomposites were deposited on epoxy glass substrate using a spin coating technique and have been characterized using various techniques such as X-ray diffractometer, field emission scanning electron microscopy (FESEM) and Fourier transform infrared (FTIR) spectrometer. The physicochemical characterization confirmed well-formed dodecylbenzene (DBSA)-doped PPy–Zn₂SnO₄ nanocomposites with granular morphology and high porosity. Among various nanocompositions, DBSA-doped PPy–Zn₂SnO₄ (10 wt.%) nanocomposite was found to be highly sensitive towards NH₃ vapor at room temperature i.e. with a chemiresistive response of 5.44% at 27 ppm with a reasonably fast recovery time of 76 s. Additionally, it shows a linear response and appropriate recovery time at all concentrations of NH₃ vapor. The DBSA-doped PPy–Zn₂SnO₄ nanocomposite response is four times better than pure PPy toward NH₃ vapor at room temperature. Therefore, it is expected that such material with excellent gas sensing properties at room temperature can be used for high-performance NH₃ sensors.

Indium oxide (In₂O₃) films have been prepared by thermal oxidation of pre-deposited indium films onto glass substrate kept at room temperature (35°C). These films were dipped into an aqueous solution (0.1 M) of lithium chloride (LiCl) and aluminum chloride (AlCl₃) followed by being fired at 500°C. Based on X-ray diffraction results, it has been observed that pure and Li modified In₂O₃ films are polycrystalline in nature while Al modified In₂O₃ film has a prominent peak corresponding to 222 plane of In₂O₃. Field emission scanning electron microscopy of pure film shows smaller grains which get transformed to bigger ones for Li modified In₂O₃ film. In case of Al modified In₂O₃ film agglomerated small grains are observed. This film also reveals the response of 60% for 100 ppm of ammonia vapors at room temperature. The transparency increased from 23–36% to 53–67% in visible region with Li modification of pure In₂O₃ film.

A ZnO nanoparticles (NPs)/reduced graphene oxide (rGO) composite was fabricated via a simple one-step solvothermal method with graphene oxide (GO) and Zn(NO₃)₂ ⋅ 6H₂O as the precursors. The morphology, crystal structure and optical properties of the synthesized materials were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) spectroscopy. The synthesized composite exhibited rGO layers assorted with tiny ZnO NPs. rGO supposedly acted as a template in the solvothermal process, that may promote the preferential attachment of ZnO NPs and prevented the agglomeration of ZnO NPs in the synthesized composite. It was also found that the electrical properties of the composite improved markedly with bare ZnO NPs, without significantly changing the morphology and crystal structure of the ZnO NPs. The main aim of this research is to develop an efficient sensor and to understand the effect of graphene in sensing characteristics. The synthesized composite was exposed to H₂, CO and C₂H₂ gases to confirm its feasibility for gas sensing, and the results showed preferential detection of reducing gases at low temperature.

Density functional theory combined with Gibbs free energy calculations is used to study the sensing behavior of tin dioxide (SnO${}_2)$ clusters towards chlorine gas molecules. Studied SnO₂ clusters’ results show the known property of tin dioxide being an oxygen-deficient semiconductor with the preferred stoichiometry SnO${}_{1.8}$. The kind of reactions that result in sensing Cl₂ molecules is investigated. These include oxygen replacement, chlorine molecule dissociation and van der Waals attachment. Oxygen replacement shows an increase in energy gap which is the case experimentally. Optimum sensing operating temperature towards Cl₂ molecules that results from the intersection of the highest SnO₂ adsorption and desorption Gibbs free energy lines is at 275^∘C in agreement with the experimentally measured temperature of 260^∘C.

Spin-coated thin films of copper phthalocyanine (CuPc) were fabricated using different rotation speeds from 250 rpm to 1250 rpm. The structural characterization of these films was analyzed using UV-vis spectroscopy and atomic force microscopy (AFM). Gas sensing properties of these spun thin films were investigated against different volatile organic compounds such as chloroform, dichloromethane and toluene using surface plasmon resonance (SPR) technique. CuPc thin films were found to be highly sensitive to chloroform and dichloromethane vapor with fast response and recovery times. These measurements clearly indicated that the CuPc molecule is a promising material for the development of the room temperature vapor sensing applications with sensitivities between $4\times 10^{-6}$ and $15\times 10^{-6}$ percent response ppm${}^{-1}$. Three different functional groups of CuPc structures coded as CuPc I, II and III were investigated which differ from each other in their chemical structures in terms of their microcycle ring groups and peripheral groups, all attached to the same free base porphyrin skeleton. The number of microcycle ring groups and peripheral groups were found to be efficient on the gas sensing properties. The calculated refractive index and extinction coefficients using SPR curves were $1.54,0.84$ for CuPc I thin film, $1.64,0.13$ for CuPc II thin film and $1.71,0.22$ for CuPc III thin film, respectively. For different substrate rotation speeds, the thin film thicknesses vary between 2nm and 6nm for CuPc I and CuPc III thin films whereas it ranges between 4nm and 9nm for CuPc II thin film.

Resistance noise data from a single gas sensor can be utilized to identify gas mixtures. We calculated the power spectral density. higher order probability densities and the bispectrum function of the recorded noise samples; these functions are sensitive to different natural vapors and can be employed to select a proper detection criterion for gas composites and odors.

Room temperature gas sensing properties of CuO nanowires synthesized by thermal oxidation of copper foils was studied in different configurations: (i) isolated nanowires aligned between two electrodes, (ii) as grown CuO foil consisting of nanowires and (iii) CuO nanowire films. Sensors were studied for response to different gases. Different sensors showed qualitatively different response on exposure to H₂S. Isolated nanowires showed high sensitivity, (~200% for 10 ppm of gas) and fast response (30 s) and recovery times (60 s). In these samples, the resistance mainly decreased on exposure to H₂S (though a small initial increase was observed). In CuO foils, resistance increased for low concentrations (5–10 ppm) but decreased at high concentrations. In the case of CuO nanowire films, resistance only increased on exposure of H₂S (upto 400 ppm). Since CuO is a p-type semiconductor, on exposure to H₂S an increase in resistance is expected due to oxygen adsorption related process. Decrease in resistance in some of the sensors was understood in terms of reaction of CuO with H₂S resulting in the formation of CuS.

The synthesis of metallophthalocyanines (M $=$ Co, Cu, Mn) bearing four ethyl 7-oxy-4,8-dimethylcoumarin-3-propanoate moieties was performed. These novel compounds were characterized by elemental analysis, ${}^{\mathrm{\text{1}}}$H-NMR spectroscopy, FT-IR, UV-vis and mass spectral data. DC and AC electrical properties of the films of metallophthalocyanines were investigated in the temperature range of 295–523 K. AC measurements were performed in the frequency range of 40–10${}^{\mathrm{\text{5}}}$ Hz. Activation energy values of the films took place between 0.55 eV–0.93 eV. Impedance spectroscopy measurements revealed that bulk resistance decreases with increasing temperature, indicating semiconductor properties. DC conductivity results also supported this result. Their gas sensing properties were also investigated for the vapors of Volatile Organic Compounds (VOCs), $n$-butyl acetate (200–3200 ppm) and ammonia (7000–56000 ppm) between temperatures 25–100°C. Sensitivity and response times of the films for the tested vapors were reported. The results were found to be reversible and sensitive to the vapors of $n$-butyl acetate and ammonia. It was found that Mn(OAc)Pc showed better sensitivity than CoPc and CuPc for $n$-butyl acetate vapors at all measured vapor concentrations and temperatures. Mn(OAc)Pc also showed better sensitivity than CoPc and CuPc for ammonia vapors at 22°C.

A new hybrid material has been developed by mixing a sandwich-type double-decker, Eu[Pc(OC₄H₉)₈]₂ = 2,3,9,10,16,17,23,24-octabutoxyphthalocyaninate] with acidified multiwalled carbon nanotubes (aMWCNTs) through non-covalent interactions. The UV-vis spectrum, X-ray diffraction and scanning electron microscope have been employed to reveal the $J$-aggregate mode and optimized morphology of Eu[Pc(OC₄H₉)₈]₂ molecules in the Eu[Pc(OC₄H₉)₈]₂/aMWCNTs hybrid material. The gas-sensing devices based on this hybrid material are fabricated by a simple solvent-processing quasi-Langmuir–Shäfer (QLS) progress. The $n$-type and $p$-type response is shown by the Eu[Pc(OC₄H₉)₈]₂/aMWCNTs hybrid film at room temperature. The detection limit of the hybrid for ammonia and nitrogen dioxide gas is 0.5 ppm and 0.3 ppm, respectively.

Octa-substituted metallophthalocyanines [M = Ni(II), Zn(II), Co(II), and Cu(II)] carrying 3,4-dialkoxyphenyl tosylamino groups at the peripheral positions have been synthesized from 1,2-dicyano-4,5-bis[(3,4-dialkoxyphenyl-tosylamino)methyl]benzene in the presence of the corresponding anhydrous metal salt. Next to the metal ion center, the length of the alkyl chains in the dialkoxyphenyl moiety ($n=4$, 5, 6, and 12) was varied. In total, sixteen soluble phthalocyanines have been characterized by elemental analysis, FT-IR and ${}^{\mathrm{\text{1}}}$H-NMR spectroscopy as well as mass spectrometry. Furthermore, the gas sensing properties of these new compounds have been studied using quartz crystal microbalance transducers. The sensing properties are described on the basis of sensor responses to nine different test analytes comprising volatile organic compounds, toxic gases, and chemical warfare agent simulants. The influence of the metal ion center and substituents on sensor selectivity and sensitivity is discussed. The compounds show good performance in the gas-sensing experiments with diverse responses to the analytes. Phthalocyanine species with pronounced selectivity for polar analytes, hydrocarbons or amines have been identified among the set of sensors with the help of multivariate data exploration methods. The results reveal that quite a high diversity in terms of selectivity is introduced through the minute variations to the phthalocyanine structure.

In this paper, the opportunities and challenges which future applications based on nanoparticles offer to engineers are described. New high-added value products induce us to rethink ways to control nanoparticle design and handling. Several challenges are described and typical solutions are given: (1) finding relevant engineering tasks outside traditional process industry; (2) the importance of mixing reactants; (3) obtaining monodisperse particles; (4) gaining efficient control via electric forces and (5) developing multi-step processes which allow more control over particle properties.

In this paper, three-dimensional (3D) Co₃O₄ flower-like microspheres have been successfully synthesized via a facile ethylene glycol (EG)-mediated solvothermal method followed by calcination. The as-prepared flower-like precursors microspheres are formed from the assembly of 2D nanosheets in the presence of hexadecyltrimethylammonium bromide (CTAB). The flower-like architectures of the prepared precursors could be tailored by changing the amount of CTAB. Furthermore, when evaluated as a gas sensor, the obtained Co₃O₄ flower-like microspheres exhibit a good response and sensitivity toward ethanol gas, suggesting their promising potential for gas sensors application.

A novel composite of Au-functionalized porous silicon (PS)/V₂O₅ nanorods (PS/V₂O₅:Au) was prepared to detect NO₂ gas. PS/V₂O₅ nanorods were synthesized by a heating process of pure vanadium film on PS, and then the obtained PS/V₂O₅ nanorods were functionalized with dispersed Au nanoparticles. Various analytical techniques, such as field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), have been employed to investigate the properties of PS/V₂O₅:Au. Herein, the PS/V₂O₅:Au sample exhibited improved NO₂-sensing performances in response, stability and selectivity at room temperature (25${}^{\circ }$C), compared with the pure PS/V₂O₅ nanorods. These phenomena were closely related to not only the dispersed Au nanoparticles acting as a catalyst but also the p-n heterojunctions between PS and V₂O₅ nanorods. Whereas, more Au nanoparticles suppressed the improvement of response to NO₂ gas.