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The rational design of composites based on graphene/metal oxides is one of the pillars for advancing their application in various practical fields, particularly gas sensing. In this study, a uniform distribution of ZnO nanoparticles (NPs) through the graphene layer was achieved, taking advantage of amine functionalization. The beneficial effect of amine groups on the arrangement of ZnO NPs and the efficiency of their immobilization was revealed by core-level spectroscopy, pointing out strong ionic bonding between the aminated graphene (AmG) and ZnO. The stability of the resulting Am-ZnO nanocomposite was confirmed by demonstrating that its morphology remains unchanged even after prolonged heating up to 350 °C, as observed by electron microscopy. On-chip multisensor arrays composed of both AmG and Am-ZnO were fabricated and thoroughly tested, showing almost tenfold enhancement of the chemiresistive response upon decorating the AmG layer with ZnO nanoparticles, due to the formation of p-n heterojunctions. Operating at room temperature, the fabricated multisensor chips exhibited high robustness and a detection limit of 3.6 ppm and 5.1 ppm for ammonia and ethanol, respectively. Precise identification of the studied analytes was achieved by employing the pattern recognition technique based on linear discriminant analysis to process the acquired multisensor response. [ABSTRACT FROM AUTHOR]

The 2D structure of MXenes attracts wide research attention toward an application of these materials in gas sensors. These structures are extremely sensitive to minor variations in their composition, which are employed for tuning their functional properties. Here, we consider the partially substituted MXenes of the composition of TixV2-xC, where x = 0.2, via quantum chemical calculations, and test their chemiresistive characteristics as a receptor component of the planar-type sensor and on-chip multisensor array. We thoroughly discuss the synthesis process of Ti0.2V1.8AlC MAX-phase and the corresponding MXenes, to prepare functional inks and, furthermore, deposit the films by microextrusion printing over an array of planar multi-electrode structures at the surface of a pen-sized chip. The crystal structure of the obtained materials is evaluated via X-ray diffraction analysis. The developed chip has been exposed upon few gaseous analytes, of alcohol VOCs, NH3, and H2O, of a 500–16,000 ppm concentration, at room temperature to ensure that we could observe the positive chemiresistive effect matured from resistance enhancing, with up to 10% vs. water vapors. The calculations carried in the framework of the density-functional theory for V2C, Ti2C, and Ti0.2V1.8C crystals ensured that the variations in their electronic structure were almost consistent with the experiment fundings: the most prominent effect is observed in relation to the H2O vapors. Therefore, these Ti0.2V1.8C structures could be considered for applying them in room temperature-operated hygrometers. [ABSTRACT FROM AUTHOR]

The growing demand of society for gas sensors for energy-efficient environmental sensing stimulates studies of new electronic materials. Here, we investigated quasi-one-dimensional titanium trisulfide (TiS3) crystals for possible applications in chemiresistors and on-chip multisensor arrays. TiS3 nanoribbons were placed as a mat over a multielectrode chip to form an array of chemiresistive gas sensors. These sensors were exposed to isopropanol as a model analyte, which was mixed with air at low concentrations of 1–100 ppm that are below the Occupational Safety and Health Administration (OSHA) permissible exposure limit. The tests were performed at room temperature (RT), as well as with heating up to 110 °C, and under an ultraviolet (UV) radiation at λ = 345 nm. We found that the RT/UV conditions result in a n-type chemiresistive response to isopropanol, which seems to be governed by its redox reactions with chemisorbed oxygen species. In contrast, the RT conditions without a UV exposure produced a p-type response that is possibly caused by the enhancement of the electron transport scattering due to the analyte adsorption. By analyzing the vector signal from the entire on-chip multisensor array, we could distinguish isopropanol from benzene, both of which produced similar responses on individual sensors. We found that the heating up to 110 °C reduces both the sensitivity and selectivity of the sensor array. [ABSTRACT FROM AUTHOR]

Engineering the physics and chemistry of 2D materials is a key to unlock the potential of the advanced e‐nose technologies limited by the current semiconductor technologies. Herein, the adjustment of the graphene's morphology, physics, and gas sensing properties upon its carboxylation via the developed photochemical method is demonstrated. Formation of matrices of nanoscale holes yet with the retention of the lamellar structure of the graphene layer is signified upon the introduction of up to 9.5 at% of carboxyl groups. The impact of the applied carboxylation on the conduction mechanism and electronic structure is demonstrated. The appearance of a set of the localized states in the valence band is revealed, originating from the molecular orbitals of carboxyls as is signified by the proposed approach for the identification of electronic states in graphene chemical derivatives. Given holey structure, predominance of highly affine carboxyls, and lateral inhomogeneity, the enhanced detection and discrimination of various alcohols, acetone, and ammonia vapors at room temperature is demonstrated. The opposite chemiresistive response toward ammonia in the humid air is also experimentally revealed and justified by the performed density functional theory modeling on the effect of ammonia, water, and their mix on electronic structure, and resistivity of the carboxylated graphene. [ABSTRACT FROM AUTHOR]

Herein, we review printing technologies which are commonly approbated at recent time in the course of fabricating gas sensors and multisensor arrays, mainly of chemiresistive type. The most important characteristics of the receptor materials, which need to be addressed in order to achieve a high efficiency of chemisensor devices, are considered. The printing technologies are comparatively analyzed with regard to, (i) the rheological properties of the employed inks representing both reagent solutions or organometallic precursors and disperse systems, (ii) the printing speed and resolution, and (iii) the thickness of the formed coatings to highlight benefits and drawbacks of the methods. Particular attention is given to protocols suitable for manufacturing single miniature devices with unique characteristics under a large-scale production of gas sensors where the receptor materials could be rather quickly tuned to modify their geometry and morphology. We address the most convenient approaches to the rapid printing single-crystal multisensor arrays at lab-on-chip paradigm with sufficiently high resolution, employing receptor layers with various chemical composition which could replace in nearest future the single-sensor units for advancing a selectivity. [ABSTRACT FROM AUTHOR]

MXenes, two-dimensional transition metal carbides or nitrides, have recently shown great promise for gas sensing applications. We demonstrate that the sensitivity of intrinsically metallic Ti3C2Tx MXene can be considerably improved via its partial oxidation in air at 350 °C. The annealed films of MXene sheets remain electrically conductive, while their decoration with semiconducting TiO2 considerably improves their chemiresistive response to organic analytes at low-ppm concentrations in dry air, which was used to emulate practical sensing environments. We demonstrate that partially oxidized MXene has a faster and a qualitatively different sensor response to volatile analytes compared to pristine Ti3C2Tx. We fabricated multisensor arrays of partially oxidized Ti3C2Tx MXene devices and demonstrate that in addition to their high sensitivity they enable a selective recognition of analytes of nearly the same chemical nature, such as low molecular weight alcohols. We investigated the oxidation behavior of Ti3C2Tx in air in a wide temperature range and discuss the mechanism of sensor response of partially oxidized MXene films, which is qualitatively different from that of pristine Ti3C2Tx. [ABSTRACT FROM AUTHOR]

The development of portable gas-sensing units implies a special care of their power efficiency, which is often approached by operation at room temperature. This issue primarily appeals to a choice of suitable materials whose functional properties are sensitive toward gas vapors at these conditions. While the gas sensitivity is nowadays advanced by employing the materials at nano-dimensional domain, the room temperature operation might be targeted via the application of layered solid-state electrolytes, like titanates. Here, we report gas-sensitive properties of potassium titanate whiskers, which are placed over a multielectrode chip by drop casting from suspension to yield a matrix mono-layer of varied density. The material synthesis conditions are straightforward both to get stable single-crystalline quasi-one-dimensional whiskers with a great extent of potassium replacement and to favor the increase of specific surface area of the structures. The whisker layer is found to be sensitive towards volatile organic compounds (ethanol, isopropanol, acetone) in the mixture with air at room temperature. The vapor identification is obtained via processing the vector signal generated by sensor array of the multielectrode chip with the help of pattern recognition algorithms. [ABSTRACT FROM AUTHOR]

Detection of heavy meals in aqueous media challenges worldwide research in developing particularly fast and affordable methods. Fluorescent sensors look to be an appropriate instrument for such a task, as recently they have been found to have made large progress in the detection of chemical analytes, primarily in the environment, along with biological fluids, which still suffer from not enough selectivity. In this work, we propose a new fluorescent method to selectively recognize heavy metals in an aqueous solution via employing an array of several fluorescent probes: acridine yellow, eosin, and methylene blue, which were taken as examples, being sensitive to a microsurrounding of the probe molecules. The exemplary sensor array generated six channels of spectral information through the use of various combinations of excitation and detection wavelengths. Following the known multisensor approach, we applied a linear discriminant analysis to selectively distinguish the vector signals from the sensor array from salts of heavy metals—Cu, Pb, Zn, Cd, and Cz—at the concentration ranges of 2.41 × 10−6–1.07 × 10−5 M, 2.8 × 10−5–5.87 × 10−4 M, 1.46 × 10−6–6.46 × 10−6 M, 1.17 × 10−8–5.2 × 10−8 M, and 2.11 × 10−6–9.33 × 10−6 M, respectively. The suggested approach was found to be promising due to it employing only one cuvette containing the test solution, simplifying a sample preparation when compared to preparing a variety of solutions in tests with single fluorescence probes. [ABSTRACT FROM AUTHOR]

While preparing oxide layers as gas sensors by a sol-gel approach, a high-temperature annealing makes a challenge to apply in numerous applications like flexible electronics with a heavy influence on the oxide microstructure. Therefore, its replacing by UV irradiation combined with a mild heating as "photo-annealing" paves the way to develop soft protocols when designing oxide-based gas sensors bearing a fine nanocrystallinity. Herein, we consider hierarchical sol-gel derived ZnO films which were a subject of conventional annealing and photoannealing to compare their gas-sensor performance when exposed to alcohol vapors. It is found that films obtained by photoannealing have an X-ray amorphous character, in contrast to ones being thermally annealed; although, the hierarchical organization of both samples revealed by SEM is almost identical. The DFTB modeling performed for ZnO crystal exposed to alcohol molecules and water has indicated the chemiresistive effect to be enhanced with a molecular weight of analytes. These observations were validated in experiment with sol-gel ZnO layers which exhibited an alcohol response in sub-ppm concentration range down to 10 ppb. To selectively compare the impact of various alcohols, we successfully applied a linear-discriminant analysis to the vector signal of the on-chip multisensor array. [Display omitted] • ZnO nanocrystalline layers were fabricated by annealing and UV irradiation/mild heating. • Photoannealed ZnO exhibited finer crystallinity than thermal-treated samples under same morphology. • The both-type ZnO layers exhibited a sensitivity to alcohol vapors down to 10 ppb concentration. • The alcohol response of ZnO layers matches results of DFT calculations. [ABSTRACT FROM AUTHOR]

• On-chip sensor array is developed with SnO 2 film gradually modified by IR laser. • On-chip sensor array operates at two modes, heating to 600 K and UV-LED irradiation. • On-chip sensor array can selectively detect ketones and alcohols down to 0.1 ppm. We consider an on-chip sensor array based on a mesoporous layer of SnO 2 nanoparticles to be screen printed on the multielectrode-supplied Si/SiO 2 substrate as a chemiresistive building platform for portable and personalized in situ instruments. To differentiate the local oxide layer properties we apply Nd:YAG laser whose scanning etched various layer areas at varied power driven by working current in 24.8 A–26.7 A range. As a result, the SnO 2 layer has dual-grad modified properties as, (i) a spatial modification of thickness down to nm-range, and (ii) the change of oxidation state with appearance of traces of SnO, which both result in a great varying of gas-sensing properties of local sensor elements over the array. To test the functionality of the chip, we could detect vapors of four ketones (acetone, cyclopentanone, cyclohexanone, 2-octanone) and four alcohols (methanol, ethanol, isopropanol, butanol), at sub-, down to ca. 100 ppb, and low, up to 10, ppm concentrations with their selective recognition via processing the array's vector signal by linear discriminant algorithm. Primarily, we show differences in the interaction of ketones and alcohols with SnO 2 surface by first-principle calculations in frames of density functional theory to serve as fundamental receptor pre-requisites for the analyte's selective discrimination by the oxide layer under the multisensor concept to employ here. We consistently show that two modes of the sensor operation could be rather equally applied to the array as, (i) UV LED, 366 nm wavelength, irradiation at room temperature, and (ii) heating up to approx. 583 K. While the heating provides faster and higher chemiresistive responses, the UV-excited mode provides more selective vector signals, lower energy consumption, and a higher signal-to-noise ratio. [ABSTRACT FROM AUTHOR]