Table of Contents

    Topic review

    Nanostructured Materials for VOC Sensing

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    Contributors: Kien Wen Sun , Muthaiah Shellaiah
    Submitted by: Kien Wen Sun

    Definition

    Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources.

    1. Introduction

    Volatile organic compounds (VOCs) are the well-known cause of indoor pollution, which can harm the human health and leads to various disorders [1][2][3]. Long-time exposure to VOCs, such as phosgene, can lead to death and severe chronic diseases [4,5]. Another example is Benzene (a well-known carcinogen), which have the great potential to damage human tissues such as spleen, stomach, liver, kidneys, etc., and also affect the nervous, circulatory, immune, cardiovascular, and reproductive and respiratory systems [4][5]. Thus, to overcome such potential hazards of VOCs, numerous environmental safety agencies like Environmental Protection Agency (EPA), National Institute of Occupational Safety and Health (NIOSH), and European Agency for Safety and Health at Work (EU-OSHA), have fixated the accepted limit of certain VOCs down to sub-ppb concentrations [4]. To quantify such harmful VOCs, researchers focused to develop various sensory materials and to achieve the signals by means of fluorescent, electrochemical, or current–voltage (I-V) fluctuations, etc [4][5][6][7][8]. Wherein, with the uniqueness and high sensitivity, chemiresistor device-based VOCs detection seems to be an interesting research topic [9][10]. In this track, nanomaterial-based semiconducting chemiresistor devices for VOCs monitoring are impressive in terms of sensitivity and selectivity [11][12].

    Materials with internal or external dimensions at nanoscale (<100 nm) are defined as nanomaterials [13][14]. They can be available in one to three dimensions (1D to 3D) with diverse structures like nanoparticles, nanocrystals, nanowires, nanocubes, nanosheets, nanotubes, nanosheets, nanocages, etc. [15]. Those nanostructured semiconducting materials play a vital role in sustainable applications such as conductivity studies, transistors, solar cells, healthcare diagnostics, sensors, and so forth [16][17][18][19][20]. Among them, utilization of semiconducting nanomaterials-based sensing of VOCs/gases seems to be more attractive [21][22]. Therefore, many reviews are available on 1D to 3D inorganic-nanomaterials-based semiconducting gas/VOCs sensors [23][24][25][26][27]. Similarly, the utilization of semiconducting polymeric/hybrid/organic-based semiconducting nanomaterials were also proposed towards selective sensing of VOCs [28][29][30][31][32][33][34][35]. However, contrast to those polymeric/hybrid/organic nanostructures, inorganic semiconducting nanomaterials are highly admirable due to the low cost processing, durability, environment affordability, and high reproducibility, etc. [36][37][38]. Therefore, the effects of diverse nanostructured properties of inorganic semiconducting materials towards the determination of VOCs require more discussions. For example, Tin dioxide (SnO2) seems to exist in diverse nanostructured forms, like nanoparticles, nanowires, hollow micro/nano-spheres, nanofibers, and cubahedra, with specific selectivity to different VOCs, such as acetone, ethanol, butanol, and toluene [36][37][38][39][40][41]. Thus, it is essential to gather information on consumption of diverse nanostructured materials in the device based detection of VOCs.

    To this path, reviews on analytes quantification by means of electrochemical devices have already been reported by many researchers [42][43]. However, the information regarding the utilization of diverse inorganic-nanostructures in chemiresistor device-based VOCs determination, which is necessary for upcoming researchers, is currently deficient. Therefore, we deliver a comprehensive review on diverse nanostructured semiconducting nanomaterials that has been employed in device-based VOCs quantifications based on recent reports (more than 340 references published since 2016).

    In this review, valuable information on diverse nanostructures employed in chemiresistor device-based VOCs detection and quantification (Figure 1) is revealed and discussed. Moreover, the mechanisms and effects of semiconducting nanostructures are clarified with charge/electron transport properties of those sensory nanomaterials. Lastly, synthesis of diverse nanostructured materials, advantages and disadvantages of those nanostructures, and their future scope towards VOCs quantification are summarized with justification.

    Figure 1. Schematic illustration of device based sensing of diverse nanostructures to volatile organic compounds (VOCs).

    2. Materials and Diverse Nanostructure Selections

    In general, design and development of semiconducting nanomaterial-based gas sensors need to follow some important rules as follows.

    1. For device-based detection, the materials must have the unique p- or n- type semiconducting properties, which can be further boosted by combining with other materials to form a p-n or p-p or n-n heterojunction towards specific analyte quantification [44].

    2. The selected nanomaterials must have large surface to enhance the adsorption of gaseous VOCs, which can induce signal responses, such as I-V fluctuations, electrochemical responses, fluorescent deviations, etc. However, for device-based sensing, signals are obtained as either I-V or electrochemical responses.

    3. To attain the discrimination of diverse VOCs, different 1D to 3D nanostructures (nanoparticles/quantum dots, nanocrystals, nanorods/nanowires/nanoneedles, nanofibers/nanobelts, nanotubes, nanocubes, nanocages, nanowalls, nanosheets, nanoflakes/nanoplates, nanospheres, nanoflowers, porous-nanostructures, hierarchical nanostructures, and so on) can be adapted via optimizing their selectivity and sensitivity to specific analyte [45][46][47][48][49][50][51][52][53][54][55][56].

    4. Diverse nanostructures can be developed by chemical synthesis, hydrothermal, chemical vapor deposition (CVD), combustion synthesis, sputtering, electrospinning, impregnation, sol–gel, solid-state reaction, hybrid composite synthesis, etc., to achieve specific sensitivity to the target VOC [45][46][47][48][49][50][51][52][53][54][55][56]. However, this should be done without affecting semiconducting properties of the proposed nanostructure, otherwise the sensitivity and reproducibility may be affected significantly.

    5. Fabricated nanostructures on device must withstand different VOC exposures and at diverse humid/temperature conditions. Many devices may be affected by humidity of the environment and lead to malfunction, thereby special attention should be paid to the effect of humidity on the sensory devices. Similarly, certain semiconducting/hybrid materials can function at either higher or lower temperatures. Therefore, justification of operating temperature is a must for the sensory devices.

    6. Diverse Nanostructures in Acetone Detection

    Detection of acetone vapor by diverse nanostructures and materials follows the general mechanism of adsorption of volatile vapor over the surface of the substrate, resulting to the release of electrons and induce resistance changes (Ra/Rg; Ra and Rg are resistance without and with the target gas). During the injection of acetone vapor in the gaseous chamber consist of diverse nanostructured materials, the vapor reacts with the oxygen species that is already adsorbed, as descripted in the Equations (1) and (2), which releases electrons to be detected as sensor signal (Ra/Rg).

    (CH3COCH3)g → (CH3COCH3)ads

    (1)

    (CH3COCH3)ads+ 8O → 3CO2 + 3H2O + 8e

    (2)

    The sensitivity can be varied depending on the adsorption surface area, which might be the concern with diverse nanostructures towards VOCs detection. Such quantifications can also be significantly affected by the semiconducting properties, such as -type, -type, p-n, p-p, and n-n heterojunctions. Thus, information on miscellaneous nanostructured materials that has been reported in acetone detection and sensitivities are outlined below.

    Among various nanostructures, the semiconducting nanoparticles (NPs) are capable of providing uniform surface for acetone adsorption to deliver unique resistance changes at certain temperatures. To this path, TiO2 NPs, α-Fe2O3 NPs, Mn@ZnO NPs (MZO NPs; p + n interlock, field effect transistor), Pt-decorated Al-doped ZnO (Pt-AZO NPs), AZO NPs, B-TiO2@Ag NPs, La1-xYxMnO3-⸹ NPs, and Bi1-xLaxFeO3 NPs were consumed in selective device-based quantification of acetone with part per billion/parts per million (ppb/ppm) detection limits (LODs) [57][58][59][60][61][62][63][64]. These nanoparticles are generally synthesized via hydrothermal (TiO2 NPs, MZO NPs, Pt-AZO NPs, AZO NPs, and B-TiO2@Ag NPs), reverse micro-emulsion (α-Fe2O3 NPs), and sol–gel (La1-xYxMnO3-⸹ NPs and Bi1-xLaxFeO3 NPs) methods that operate between 250 and 500 °C (as summarized in ). Among these nanoparticles, Pt-AZO NPs are quite unique with a Ra/Rg response of 421, due to the doping and decoration of Al and Pt, correspondingly. Wherein, decoration of Pt enhances the sensor responses via increased O2 adsorption (which reacts with acetone to release more electrons) on the surface, as shown in Figure 2. However, the main drawback of this work is its operating temperature (450 °C).

    Figure 2. Schematic illustrating the sensing reaction mechanism of (a) Al-doped ZnO (AZO) and (b) Pt-decorated AZO (Pt-AZO) sensors in air and acetone (Reproduced with the permission from Ref. [60]).

    Similar to NPs, nanocrystalline materials were also involved in volatile acetone discrimination. For example, Zhang et al. developed the Pd-doped SmFe0.9Mg0.1O3 nanocrystals by a sol–gel tactic and utilized in acetone detection under light illumination at 220 °C [65]. Wherein, the Pd:SmFe0.9Mg0.1O3 displayed an impressive response of 7.16 (for 0.5 ppm) with a LOD of 0.01 ppm. To this track, hydrothermally synthesized WO3 nanocrystals were also displayed a good response (Ra/Rg = 3.8; 0.25 ppm) at 320 °C with a satisfactory LOD of 0.075 ppm [66]. Moreover, Rhodium (Rh) additive in the solvothermally synthesized TiO2 nanocrystals (TiO2-5Rh) slightly improved the sensor response (Ra/Rg = 9.6; 50 ppm) at 300 °C, as shown in Figure 3 [67]. However, this work still needs further optimization on the operating temperature and LOD. Considering the acetone quantitation, nanowires (NWs) were fabricated to afford device-based sensors. Functionalization or attachment of certain materials on the surface of NWs may enhance the sensitivity.

    Figure 3. (a) HRTEM micrograph of the 400 °C TiO2-5Rh sample, detail of the orange squared region and its corresponding power spectrum; (b) Comparison between the TiO2-5Rh acetone responses at various operating temperatures. The typical volcano behavior can be observed (Reproduced with the permission from Ref [67]).

    To this light, Kim et al. and Singh et al. demonstrated an exceptional acetone sensitivity (refer to Table 1) of Co3O4 NPs modified SnO2 NWs and self-assembled monolayer (SAM) functionalized ZnO NWs [68][69]. These sensory materials can be synthesized via vapor–liquid–solid (VLS), sol–gel, and thermal annealing processes to detect acetone with an LOD of 0.5 ppm. Additionally, p-n heterojunction NWs were proposed with ZnO branched p-CuxO@n-ZnO NWs (fabricated by hydrothermal method and atomic layer deposition) for acetone sensing [70]. Wherein, the sensor operated at 250 °C and displayed low sensor responses of 3.39–6.38 (5–50 ppm), as depicted in Figure 4. Therefore, the device requires further optimization to attain a higher response to achieve excellent performance.

    Figure 4. TEM image of ZnO branched p-CuxO @n-ZnO heterojunction nanowires and its sensor responses to acetone (Reproduced with the permission from Ref. [70]).

    Discrimination of acetone was also demonstrated by hydrothermally/chemically synthesized nanorods (NRds) and nanoneedles at certain operating temperatures with good response/recovery time, as seen in Table 1 [71][72][73][74][75]. Among these nanorods (Cr doped ZnO NRds, SnS2 NRds, Au/Pd-doped ZnO NRds, and α-Fe2O3/NiO NRds), the α-Fe2O3 NPs-doped NiO NRds [74] displayed a high response of 290 for 100 ppm acetone at 280 °C with a LOD of approximately 5 ppm. This might be due to its p-n heterojunction property, but the operation temperature of the sensor operation requires further reduction. Because the Ag-doped ZnO nanoneedles [75] did not display any exceptional response (Ra/Rg = 30.233; for 200 ppm at 370 °C); therefore, they did not play an important role in volatile acetone detection. Devices with hydrothermally synthesized nanoarrays, such as La-doped SnO2, hybrid 1D/2D α-Fe2O3-SnO2, and ZnTiO3 were engaged in acetone gas estimation [76][77][78]. Wherein, in contrast to other nanoarrays, ZnTiO3 nanoarrays [78] showed exceptional sensor responses of 78/94 for 12.5 ppm at 45 °C/350 °C with the LODs of 0.09 and 0.01 ppm, correspondingly. This work is impressive because its operation at dark and light conditions can be completed with less than 3 min response/recovery time.

    Volatile acetone quantitation was also authenticated by many semiconducting nanofibers fabricated by hydrothermal, electrospinning, and calcination tactics. Nanofibers like Ag-decorated SnO2, PrFeO3, Pt-ZnO-In2O3, Au@WO3-SnO2, In-doped ZnSnO3, ZnO, and Ru-doped SnO2 showed selective sensitivity to acetone at various operating temperatures (150–300 °C) [79][80][81][82][83][84][85]. Among them, Au@WO3-SnO2 [84] developed by Shao et al. displayed a high response of 196.1 for 10 ppm at 150 °C, with an estimated LOD of <0.5 ppm and response/recovery time of <2 min. In this track, functionalized/sensitized/assembled/decorated nanotubes (NTs) and multi-walled carbon nanotubes (MWCNTs) were reported by many research groups towards acetone sensing. Pd/Pt functionalized SnO2 NTs, PdO@ZnO−SnO2 NTs, α-Fe2O3 NRds-MWCNTs, Co3O4—MWCNTs, Pt-CuFe2O4 NTs, Pd@WO3–SnO2 NTs, and ZnO-Decorated In/Ga Oxide NTs (synthesized by hydrothermal, encapsulation, electrospinning, and soaking) were reported for exceptional acetone detection at various operating temperatures (225–400 °C) [86][87][88][89][90][91][92]. These decoration of certain catalytic substrates may enhance acetone selectivity by varying the operating temperature. For example, Pt or Pd loaded multidimensional SnO2 NTs [88] displayed an exceptional sensitivity to acetone (Ra/Rg = 93.55 for 5 ppm at 350 °C) with a LOD of <1 ppm. Similarly, PdO−ZnO composite on hollow SnO2 NTs [89] showed a good sensor response to acetone even at low concentration (Rair/Rgas = 5.06 for 1 ppm at 400 °C) with a LOD of 0.01 ppm. This sensor operates under higher humidity (95% RH) than that of others, thereby is quite noticeable. The sensitivity can be enhanced by forming multiple heterojunctions and by chemical sensitization effect of nanocatalyst.

    For instance, the sensitivity of Pd@WO3–SnO2 NTs [91] to acetone was enhanced by multiple heterojunctions formation (WO3–SnO2 n–n junctions, PdO–SnO2, and PdO–WO3 p-n junctions) and by the sensitization effect of Pd nanocatalyst. Doped SnO2 nanobelts were employed as sensor materials for the discrimination of acetone with LODs down to sub-ppm level [95,96]. Li et al., and Chen et al. reported the development of Y- or Eu-doped SnO2 nanobelts by thermal evaporation tactic above 1350 °C and utilized them in acetone sensing with verified enhanced performance (refer to Table 1) than those of pure SnO2 nanobelts. Volatile acetone gas detection has been well demonstrated by nanocubes derived from p-type Co3O4, hybrid In2O3@RGO, Ag functionalized ZnSnO3, ZnO−CuO p-n heterojunction, p-type NiFe2O4, NiO/ZnO composite, and MOF derived-ZnO/ZnFe2O4 [93][94][95][96][97][98][99]. Among them, ZnO−CuO p-n heterojunction displayed a good sensor response of 11 to 1 ppm acetone at 200 °C with an impressive LOD of 0.009 ppm. Moreover, the material has a stable response up to 40 days, thereby becomes a notable candidate. Similarly, hybrid In2O3@RGO nanocubes [94] showed discriminative sensing to acetone and formaldehyde at 175 °C and 225 °C, respectively, but the interference studies were still lacking in this report.

    The acetone sensing capability of nanocages has been established by PdO functionalized Co3O4 hollow nanocages, ZnO/ZnFe2O4 hollow nanocages, PdO functionalized NiO/NiCo2O4 truncated nanocages, and Ag@CuO-TiO2 hollow nanocages [100][101][102][103]. Wherein, PdO acted as a catalyst (in PdO functionalized Co3O4 hollow nanocages and PdO functionalized NiO/NiCo2O4 truncated nanocages) to enhance the sensitivity and PdO@Co3O4 hollow nanocages [100] displayed a response of 2.51 to 5 ppm acetone at 350 °C with a LOD of 0.1 ppm. Moreover, PdO@Co3O4 hollow nanocages can be operable at 90% humid condition. Next, ZnO/ZnFe2O4 hollow nanocages [101] also detect the acetone gas to certain extend (Ra/Rg = 25.8 for 100 ppm at 290 °C), but require optimization on operating temperature. On the contrary, multicomponent Ag@CuO-TiO2 hollow nanocages [103] enhance the acetone sensing at low operating temperature (Ra/Rg = 6.2 for 100 ppm at 200 °C) with a calculated LOD of ~1 ppm, as illustrated in Figure 5. However, more focus is required to improve the response.

    Figure 5. (a,b) The TEM images of Ag@CuO-TiO2 hollow nanocages; (c) Gas responses to 100 ppm various target gases at 200 °C (B, benzene; D, dimethylbenzene; A, acetone; M, methanol; F, formaldehyde; E, ethanol; T, toluene); (d) Gas responses of different compositions to 100 ppm acetone at 200 °C (A–D, A: Ag@CuO; B: TiO2; C: CuO-TiO2; D: Ag@CuO-TiO2) and 375 °C (E: TiO2) (e) Responses vs. acetone concentrations at 200 °C; (The inset figure is the linear relationship between response and concentration) (f) Response and recovery curves to 100 ppm acetone at 200 °C (Reproduced with the permission from Ref [103]).

    Researchers synthesized various nanosheets (NShs) by means of post-thermal, hydrothermal, impregnation, liquid exfoliation, precipitation, and multistep approaches and applied them in gaseous acetone quantification. Co3O4 NShs, ZnO NShs, SnO2/Fe2O3 multilayer NShs, NiO NShs, and F-doped TiO2 NShs were engaged in acetone sensing at different operating temperatures as summarized in Table 1 [104][105][106][107][108]. In particular, F-doped TiO2 NShs grown on Ti foam [108] function linearly in the detection range of 25–800 ppm at 25 °C and are stable at divers humid conditions (20–90% RH), thereby become exceptional material for acetone detection. Subsequently, materials with nanowalls were utilized in acetone sensory, which showed the unique advantages of wide surface area for volatile gas adsorption. Nb-doped ZnO nanowalls (synthesized by radio-frequency (RF) magnetron sputtering), CuO nanowalls (from Oxidation of Cu foil in aqueous NH4OH), NiO nanowalls (from chemical bath deposition (CBD), and ZnO deposited carbon nanowalls (from microwave plasma-enhanced chemical vapor deposition (MPECVD) were reported as acetone sensors with extensive selectivity [109][110][111][112]. Herein, Nb-doped ZnO nanowalls [109] operate at 200 °C and deliver a high response of 89.13 (for 100 ppm) with good linear response between 20 and 100 ppm. Moreover, NiO nanowalls [111] exhibit exceptional response to acetone (Ra/Rg 30; for 10 ppm at 250 °C) with a LOD of 0.2 ppm and exhibit the dynamic response as shown in Figure 6. Similarly, Choi et al. established wide surface area interaction of acetone to ZnO deposited carbon nanowalls [112]; therefore, development of such nanowalls-based VOCs sensors are highly anticipated.

    Figure 6. (i) (a) Plan-view SEM image of the NiO nanowalls (the inset shows the NiO-based sensor), (b) grey value profile along the line perpendicular to the nanosheet marked with the red circle, and (c) cross-view SEM image of the NiO nanowalls; (ii) (a) Dynamic responses of the NiO-based sensor to acetone pulses in the concentration range 1–40 ppm at 250 °C and (b) calibration curve at 250 °C (Reproduced with the permission from Ref. [111]).

    Towards acetone selective sensing, nanoflakes were incorporated in devices, which showed good sensitivity as other nanostructures. To this approach, α-MoO3 nanoflakes, SnS nanoflakes, and Au NPs incorporated MoS2 nanoflakes were developed by researchers by means of RF sputtering, solid-state reaction, and chemical exfoliation, respectively [113][114][115]. Wherein, SnS nanoflakes seems to be an impressive candidate with a sensor response of >1000 at low operating temperature (100 °C). Moreover, the sensor response is stable even after six weeks with a LOD of <5 ppm and response/recovery time of <15 s. Similar to the microspheres [116][117], materials with nanosphere (NSP) structures are also effectively applied in the acetone estimation as noted below. Liu et al. and Zhu et al. developed the NiO/ZnO and WO3-SnO2 hollow composite NSPs to engage in effective quantitation of acetone via solvothermal and hydrothermal methods, correspondingly [118][119]. As shown in Figure 7, solvothermally synthesized NiO/ZnO NSPs show great responses to acetone (Ra/Rg = 29.8 for 100 ppm at 275 °C) with an LOD down to sub-ppm level [118].

    Figure 7. (ad) Schematic representation of solvothermal synthesize, SEM images and sensory response of of NiO/ZnO hollow spheres towards acetone (Reproduced with the permission from Ref. [118]).

    In fact, the greater sensor response of NiO/ZnO NSPs was attributed to the decoration of NiO nanoparticles over the ZnO NSPs, thus become a notable candidate in acetone detection. On the other hand, the WO3-SnO2 forms two kind of NSPs, namely, the WO3-SnO2 160 NSPs and the WO3-SnO2 190 NSPs (prepared by heating at 160 °C and 190 °C, respectively) which display good response to acetone (Ra/Rg = ~8 & 16 for 50 ppm at 275 °C). However, it still requirse further optimization to attain high linear responses and low LOD.

    To this track, researchers described the acetone quantitation by flower-like structures of Na-doped p-type ZnO, cubic-rhombohedral-In2O3, Au NPs functionalized ZnO, and RuO2 modified ZnO [120][121][122][123]. In particular, the Na-doped ZnO nanoflowers revealed the acetone sensing under ultraviolet (UV) illumination with a LOD of 0.2 ppm [120]. Contrast to the cubic-rhombohedral-In2O3 microflowers (Ra/Rg = 13.6 for 50 ppm at 250 °C; response/recovery time = 2 s/46 s; LOD = 0.01 ppm) and RuO2 modified ZnO nanoflowers (Ra/Rg = 125.9 for 100 ppm at 172 °C; response/recovery time = 1 s/52 s; LOD 25 ppm) [121][123], Wang et al. described an exceptional sensor response of Au NPs functionalized ZnO nanoflowers [122] as shown in Table 1. Due to the loading of Au NPs over ZnO surface, the response is 2900 for 100 ppm acetone with a LOD of <20 ppm at higher operating temperature (~365 °C), thereby become a notable candidate in acetone quantification. However, the working temperature requires further optimization.

    Majority of nanostructures showed the importance of the pore effect on acetone detection as described in the following. Porous nanoparticles (Au sensitized Fe2O3 NPs and Au/ZnO NPs), porous nanorods (ZnFe2O4 NRds and α-Fe2O3/SnO2 NRds), porous nanospheres (Pt sensitized W18O49), nanoporous fibers (ZnO/C), porous hierarchical nanostructures (Pt doped 3D SnO2 and Ni doped ZnO), and porous nanocomposites (CuFe2O4/α-Fe2O3 and (WO3/Au) were utilized by researchers towards acetone quantification [123][124][125][126][127][128][129][130][131][132][133]. These nanostructures were synthesized through various hydrothermal/atomic layer deposition/template mediated tactics. As shown in Table 1, many of these porous structures revealed exceptional acetone sensing due to the factor of pore effect in the gaseous species adsorption. In particular, Pt-sensitized W18O49 nanospheres and Pt-doped 3D porous SnO2 hierarchical structures [128][130] reported exceptional sensing of acetone (Ra/Rg = 85 (for 20 ppm) and 505.7 (for 100 ppm) at 180 °C and 153 °C, respectively) with the LODs down to sub-ppm level (~0.05 ppm), thereby become the unique candidates. Owing to the porous effect on VOCs detection, ZnO/C nanoporous fibers [129] evidenced the sensitivity to both acetone and ethanol (Ra/Rg = 53.222 and 59.273 (for 100 ppm) at 370 °C) with LODs down to sub-ppm level. But further optimizations are required on the operating temperature and selectivity to particular VOC.

    Other than porous nanostructures, hierarchical nanostructures synthesized by either hydrothermal or thermal oxidation tactics were effectively applied in acetone detection. Hierarchical nanostructures of ZnO NWs-loaded Sb-doped SnO2-ZnO, 3D flower-like ZnO, and Au NPs-SnO2 NTs seems to be impressive in the selective detection of acetone at operating temperatures ≥ 200 °C as presented in Table 1 [134][135][136]. Wherein, Wang et al. described the direct transformation of SnS2 NShs to hierarchical porous SnO2 NTs via thermal oxidation, which evinced a better selectivity upon Au NPs decoration at 200 °C with a LOD of 0.445 ppm, thereby stated as an exceptional candidate in acetone sensing [136]. To this light, differently shaped nanostructures were synthesized by the tactics like hydrothermal, impregnation, templated synthesize, etc. 3D inverse opal (3DIO) In2O3–CuO nano-architecture, 3D grass-like carbon-doped ZrO2 nano-architecture, Sm2O3 loaded mulberry shaped SnO2 hierarchical nanostructure, cactus-like WO3-SnO2 nanocomposite, walnut like architecture of Fe and C co-doped WO3, Urchin like Cr-doped WO3 hollow nanospheres, and core-shell heterostructure of ZnO/MoS2 NShs were engaged in effective detection of acetone as shown in Table 1 [137][138][139][140][141][142][143]. Wherein, 3D grass-like carbon-doped ZrO2 architecture film displays the selectivity to both alcohols and acetone [138], thereby cannot be stated as a better candidate for acetone detection. Similarly, as seen in Figure 8, walnut like architecture of Fe and C co-doped WO3, lying underneath microstructure rather than nanostructure, shows high selectivity to acetone (Ra/Rg = ~18 for 10 ppm) at 300 °C even after 12 weeks [141]. Moreover, it can detect the acetone even at sub-ppm concentration (~0.2 ppm) and the sensor can operates at 90% humid condition, thus become an exceptional candidate in acetone quantitation. Apart from the aforementioned diverse nanostructures, nanocomposites that are not described under any unique nano-architectures also show their high selective sensitivity to acetone [144][145][146][147][148][149] as shown in Table 1. Wherein, a few of them operate at diverse humid conditions (10–90% RH) and some others require optimization of operating temperature to attain high sensitivity.

    Figure 8. (a,b) Top-view and cross-sectional SEM image of Fe and C codoped WO3 (red arrow signifying the walnut like architecture); (c) Dynamic response-recovery curves of the three sensors based on the prepared pure WO3, C codoped WO3 () and Fe and C codoped WO3 (FW3) sensors to different concentrations of acetone at 300 °C (Reproduced with the permission from Ref [141]).

    Table 1. Detection concentration, response/recovery time, operating temperature (Temp.) and limits of dection (LODs) to acetone gas by diverse nanostructured material.

    Materials/Nanostructure

    Analyte/Concentration

    Gas Response (S = Rair/Rgas)

    Response/Recovery

    Temp.

    LOD

    Ref

    TiO2/nanoparticles

    Acetone/500 ppm

    9.19

    10 s/9 s

    270 °C

    0.5 ppm

    [57]

    α-Fe2O3/nanoparticles

    Acetone/100 ppm

    8.8

    NA

    340 °C

    5 ppm

    [58]

    Mn doped ZnO/nanoparticles

    Acetone/2 ppm

    3.7

    17 s/NA

    340 °C

    1.8 ppm

    [59]

    Pt-decorated Al-doped ZnO/nanoparticles

    Acetone/10 ppm

    421

    2.9 s/440 s

    450 °C

    ~0.1 ppm

    [60]

    Al doped ZnO/nanoparticles

    Acetone/10 ppm

    11.8

    11 s/793 s

    500 °C

    0.01 ppm

    [61]

    B-TiO2@Ag/nanoparticles

    Acetone/50 ppm

    68.19

    12 s/41 s

    250 °C

    0.887 ppm

    [62]

    La1-xYxMnO3-/nanoparticles

    Acetone/500 ppm

    27.2

    NA

    300 °C

    NA

    [63]

    Bi1-xLaxFeO3/nanoparticles

    Acetone/0.05 ppm

    8

    15 s/13 s

    260 °C

    0.05 ppm

    [64]

    SmFe1−xMgxO3/nanocrystals

    Acetone/0.5 ppm

    7.16

    32 s/8 s

    220 °C

    0.01 ppm

    [65]

    WO3/nanocrystals

    Acetone/0.25 ppm

    3.8

    4 s/5 s

    320 °C

    0.0075 ppm

    [66]

    TiO2-5Rh/nanocrystals

    Acetone/50 ppm

    9.6

    NA

    300 °C

    10 ppm

    [67]

    Co3O4 NPs attached SnO2/nanowires

    Acetone/50 ppm

    70

    NA/122 s

    300 °C

    0.5 ppm

    [68]

    self-assembled monolayer (SAM) functionalized ZnO/nanowires

    Acetone/50 ppm

    170 & 90

    2 min/24 min &

    3 min/29 min

    300 °C

    0.5 ppm

    [69]

    Branched p-CuxO @ n-ZnO/nanowires

    Acetone/5–50 ppm

    3.39–6.38

    62 s/90 s

    250 °C

    ~5 ppm

    [70]

    Cr doped ZnO single-crystal/nanorods

    Acetone/70 ppm

    70

    NA

    300 °C

    ~10 ppm

    [71]

    SnS2/nanorods

    Acetone/ 10 ppm

    25

    NA

    300 °C

    Down to sub-ppm

    [72]

    Au@ZnO & Pd@ZnO/nanorods

    Acetone/100 ppm

    44.5 & 31.8

    8 s/5 s & 17 s/11 s

    150 °C

    0.005 ppm

    [73]

    α-Fe2O3-NiO/nanorods

    Acetone/100 ppm

    290

    28 s/40 s

    280 °C

    ~5 ppm

    [74]

    Ag-doped ZnO/nanoneedles

    Acetone/200 ppm

    30.233

    10 s/21 s

    370 °C

    ~10 ppm

    [75]

    La-doped SnO2/nanoarrays

    Acetone/200 ppm

    69

    6–12 s/20 s

    290 °C

    5 ppm

    [76]

    α-Fe2O3-SnO2/nanoarrays

    Acetone/1 ppm

    5.37

    14 s/70 s

    340 °C

    1 ppm

    [77]

    ZnTiO3/nanoarrays

    Acetone/12.5 ppm

    78 & 94

    117 and 141 s/99 and 131 s & 75 and 81 s/50 and 69 s (dark & light)

    45 °C & 350 °C

    0.01 & 0.09 ppm

    [78]

    Ag-decorated SnO2/nanofibers

    Acetone/50 ppm

    40

    6 s/10 s

    160 °C

    5 ppm

    [79]

    PrFeO3/nanofibers

    Acetone/200 ppm

    141.3

    7 s/6 s

    180 °C

    10 ppm

    [80]

    Pt-ZnO-In2O3/nanofibers

    Acetone/100 ppm

    57.1

    1 s/44 s

    300 °C

    0.5 ppm

    [81]

    Au@WO3-SnO2/nanofibers

    Acetone/10 ppm

    196.1

    ~2 min (for both)

    150 °C

    <0.5 ppm

    [82]

    Au functionalized In-doped ZnSnO3/nanofibers

    Acetone/50 ppm

    19.3

    10 s/13 s

    200 °C

    ~10 ppm

    [83]

    ZnO/nanofibers

    Acetone/100 ppm

    50–124

    65–130 s/75–135 s

    250 °C

    NA

    [84]

    Ru-doped SnO2/nanofibers

    Acetone/100 ppm

    118.8

    1 s/86 s

    200 °C

    ~0.5 ppm

    [85]

    Pd-SnO2/nanotubes

    Acetone/5 ppm

    93.55

    NA

    350 °C

    <1 ppm

    [86]

    PdO@ZnO-SnO2/nanotubes

    Acetone/1 ppm

    5.06

    20 s/64 s

    400 °C

    0.01 ppm

    [87]

    α-Fe2O3 nanorods-MWCNTs/nanotubes

    Acetone/100 ppm

    38.7

    2 s/45 s

    225 °C

    0.5 ppm

    [88]

    Pt-decorated CuFe2O4/nanotubes

    Acetone/100 ppm

    16.5

    16 s/299 s

    300 °C

    ~5 ppm

    [90]

    WO3–SnO2/nanotubes

    Acetone/50 ppm

    63.8

    NA

    275 °C

    0.05 ppm

    [91]

    ZnO-Decorated In/Ga Oxide/nanotubes

    Acetone/100 ppm

    12.7 & 27.1

    6.8 s/6.1 s & 11.8 s/11.6 s

    300 °C

    0.2 ppm

    [92]

    Y doped SnO2/nanobelts

    Acetone/100 ppm

    11.4

    9–25 s/10–30 s

    210 °C

    0.9024 ppm

    [150]

    Eu doped SnO2/nanobelts

    Acetone/100 ppm

    8.56

    15 s/19 s

    210 °C

    0.131 ppm

    [151]

    Co3 O4/nanocubes

    Acetone/500 ppm

    4.88

    2 s/5 s

    240 °C

    ~10 ppm

    [93]

    Ag-ZnSnO3/nanocubes

    Acetone/100 ppm

    30

    2 s/3 s

    280 °C

    1 ppm

    [95]

    ZnO−CuO/nanocubes

    Acetone/1 ppm

    11.14

    NA

    200 °C

    0.009 ppm

    [96]

    NiFe2O4/nanocubes

    Acetone/1 ppm

    1.9

    8 s/40 s

    160 °C

    0.52 ppm

    [97]

    NiO/ZnO/nanocubes

    Acetone/200 ppm

    58

    24 s/133 s

    340 °C

    ~10 ppm

    [98]

    MOF derived-ZnO/ZnFe2O4/nanocubes

    Acetone/5 ppm

    9.4

    5.6 min/6 min

    250 °C

    <0.5 ppm

    [99]

    PdO@Co3O4/nanocages

    Acetone/5 ppm

    2.51

    NA

    350 °C

    0.1 ppm

    [100]

    ZnO/ZnFe2O4/nanocages

    Acetone/100 ppm

    25.8

    8 s/32 s

    290 °C

    1 ppm

    [101]

    PdO-NiO/NiCo2O4/nanocages

    Acetone/100 ppm

    6.7

    <20 s/<30 s

    210 °C

    NA

    [102]

    Ag@CuO- TiO2/nanocages

    Acetone/100 ppm

    6.2

    56 s/9 s

    200 °C

    ~1 ppm

    [103]

    Co3O4/ nanosheets

    Acetone/100 ppm

    16.5

    NA

    111 °C

    ~5 ppm

    [104]

    ZnO/nanosheets

    Acetone/5 ppm

    6.7

    <60 s (for both)

    300 °C

    <5 ppm

    [105]

    SnO2/Fe2O3/nanosheets

    Acetone/10 ppm

    9.8

    0.8 s/3.4 s

    260 °C

    NA

    [106]

    NiO/nanosheets

    Acetone/150 ppm

    >90

    80 s/82 s

    200 °C

    0.83 ppm

    [107]

    Fluorine doped TiO2/nanosheets

    Acetone/800 ppm

    17.42

    162 s/220.5 s

    25 °C

    ~25 ppm

    [108]

    Nb-doped ZnO & ZnO/nanowalls

    Acetone/100 ppm

    84.62 & 89.13

    NA

    200 °C & 200 °C

    <20 ppm

    [109]

    CuO/nanowalls

    Acetone/500 ppm

    4

    82 s (50 ppm)/NA

    320 °C

    2 ppm

    [110]

    NiO/nanowalls

    Acetone/10 ppm

    >30

    NA

    250 °C

     0.2 ppm

    [111]

    α-MoO3/nanoflakes

    Acetone/10-100 ppm

    NA

    NA

    150 °C

    NA

    [113]

    SnS/nanoflakes

    Acetone/100 ppm

    >1000

    3 s/14 s

    100 °C

    <5 ppm

    [114]

    NiO/ZnO/nanospheres

    Acetone/100 ppm

    29.8

    1 s/20 s

    275 °C

    Down to sub-ppm

    [118]

    WO3-SnO2/nanospheres

    Acetone/50 ppm

    ~8 & 16

    16 s/12 s & 15 s/11 s

    240 °C

    ~50 ppm

    [119]

    Na:ZnO/nanoflowers

    Acetone/100 ppm

    3.35

    18.2 s/63 s

    NA

    0.2 ppm

    [120]

    ZnO/nanoflowers

    Acetone/100 ppm

    2900 & 300

    5 s/NA

    365 °C & 248 °C

    <20 ppm

     

    [122]

    RuO2-modified ZnO/nanoflowers

    Acetone/100 ppm

    125.9

    1 s/52 s

    172 °C

    <25 ppm

    [123]

    Au NPs-Fe2O3/porous nanoparticles

    Acetone/10 ppm

    6.1

    5 s/20 s

    200 °C

    0.132 ppm

    [124]

    Au/ZnO/porous nanoparticles

    Acetone/1 ppm

    17.1

    231 s/215 s

    275 °C

    <0.1 ppm

    [125]

    ZnFe2O4/porous nanorods

    Acetone/100 ppm

    52.8

    1 s/11 s

    260 °C

    <10 ppm

    [126]

    α-Fe2O3/SnO2/porous nanorods

    Acetone/100 ppm

    53.1

    9 s/2.5 s

    260 °C

    <10 ppm

    [127]

    W18O49/Pt/porous nanospheres

    Acetone/20 ppm

    85

    13 s/11 s (50 ppm)

    180 °C

    0.052 ppm

    [128]

    Pt-doped-3D-SnO2/porous hierarchical structure

    Acetone/100 ppm

    505.7

    130 s/140 s

    153 °C

    <0.05 ppm

    [130]

    Ni doped ZnO/porous hierarchical structure

    Acetone/100 ppm

    68

    6 s/2 s

    340 °C

    0.116 ppm

    [131]

    CuFe2O4/ α-Fe2O3/porous composite shell

    Acetone/100 ppm

    14

    NA

    275 °C

    0.1 ppm

    [132]

    3D- WO3/Au/porous nano- composite

    Acetone/1.5 ppm

    7.6

    7 s/8 s

    410 °C

    0.1 ppm

    [133]

    ZnO nanowires-loaded Sb-doped SnO2-ZnO/hierarchical structure

    Acetone/5 ppm

    12.1 & 27.8

    <16 s & 32 s (res)/NA (rec)

    400 °C

    4.3 & 8.1 ppm

    [134]

    ZnO/3D-flower-like hierarchical structure

    Acetone/100 ppm

    18.6

    7 s/NA

    300 °C

    NA

    [135]

    Au-SnO2/ hierarchical structure

    Acetone/100 ppm

    40.42

    22 s/90 s

    200 °C

    0.445 ppm

    [136]

    In2O3–CuO/3D-inverse opals structure

    Acetone/0.5 ppm

    4.8

    13 s/20 s

    370 °C

    0.05 ppm

    [137]

    SnO2/Sm2O3/mulberry- shaped structure

    Acetone/100 ppm

    41.14

    NA

    250 °C

    0.1 ppm

    [139]

    WO3-SnO2/cactus like nano-composite

    Acetone/600 ppm

    26

    NA

    360 °C

    NA

    [140]

    Cr doped WO3/urchin-like hollowspheres

    Acetone/10 ppm

    13.3

    NA

    250 °C

    0.467 ppm

    [142]

    ZnO/MoS2 nanosheets/core-shell nanostructure

    Acetone/5 ppm & 20 ppm (No UV and UV)

    14.4 & 4.67

    71 s/35 s & 56 s/69 s

    300 °C & 100 °C

    0.1 ppm

    [143]

    RGO-h-WO3/nano-composite

    Acetone/ 200 ppm

    1.5

    14 s/NA

    Room Temp.

    <1 ppm

    [144]

    2D-C3N4-SnO2/nano-composite

    Acetone/100 ppm

    29

    10 s/11 s

    380 °C

    0.067 ppm

    [145]

    In loaded WO3/SnO2/nano-composite

    Acetone/50 ppm

    66.5

    11 s/5.5 s

    200 °C

    <1 ppm

    [146]

    Co3O4 nanowires–hollow carbon spheres/nano-composite

    Acetone/200 ppm

    23

    NA

    200 °C

    <1 ppm

    [147]

    Fe2O3/In2O3/nano-composite

    Acetone/100 ppm

    >15

    8 s/6 s

    200 °C

    NA

    [148]

    CuO-Ga2O3/nano-composite

    Acetone/1.25 ppm

    ~1.3

    NA

    300 °C

    0.1 ppm

    [149]

    NA = Not available; Temp. = Temperature; s = seconds; in = minutes.

    4. Alcoholic Vapor Detection by Miscellaneous Nanostructures

    In regard to the development of alcoholic vapor detection, various nanostructured materials were placed in the gaseous chamber to interact with the adsorbed oxygen species follow the reactions described in Equations (3) and (4) to release electrons.

    (CnH2n+1 OH)gas → (CnH2n+1 OH)ads

    (3)

    (CnH2n+1 OH)ads + 3nO → nCO2 + (n+1)H2O + 3ne

    (4)

    Similar to the above detection process, metal oxide can oxidize the alcoholic vapor to aldehyde and convert them to water and carbon-dioxide to produce electrons as described in Equations (5)–(7).

    (CnH2n+1 OH)ads → (CnH2n+1 O)ads + H+

    (5)

    (CnH2n+1 O-)ads + H+ → (CnH2n+1 O)ads + H2

    (6)

    (CnH2n+1 O)ads + (3n − 1)O-ads → nCO2 + (n + 1)H2O + 3(n−1)e

    (7)

    In the above two sensory processes, the released electrons lead to changes in resistance, which is then used as a sensor signal (Ra/Rg). To this track, numerous reports with diverse nanostructures have been reported towards the sensing of alcoholic gases as detailed as following.

    Through the synthetic tactics like soft-chemical approach, hydrothermal, calcination, solvothermal, and sol–gel methods, researchers proposed several nanoparticles syntheses and applications in the alcoholic vapors sensing [152][153][154][155][156]. Sn3N4 NPs, Ni-doped SnO2 NPs, C-doped TiO2 NPs, Pr-doped In2O3 NPs, and Au/Cl co-modified LaFeO3 NPs were engaged in the detection of alcoholic vapors. In particular, Sn3N4 NPs and Au and Cl co-modified LaFeO3 NPs [154,158] displayed good sensor responses to ethanol vapor with decent response/recovery time at an optimum operating temperature of 120 °C as shown in . Whereas, C-doped TiO2 NPs were demonstrated in the detection of n-pentanol at 170 °C with a LOD down to sub-ppm level [154]. In this light, Ni-doped SnO2 NPs were noted as an exceptional candidate, which showed sensitivity to both n-butanol and formaldehyde via tuning the doping concentrations of Ni ions [153]. Two percent Ni-doped SnO2 NPs displayed a remarkable sensor response (Ra/Rg = 1690.7) to n-butanol at 160 °C with a response/recovery time of 10 s/>10 min. Moreover, 4% Ni-doped SnO2 NPs demonstrated a sensor response (Ra/Rg = 1298) at 100 °C with the response/recovery time of 6 s/>10 min. Their detection of n-butanol displayed a linear response from 1 to 100 ppm with a LOD of ~ 1 ppm, thereby can be considered for future development.

    Similar to the NPs, nanocrystalline materials were also used in the sensing applications of alcohols. Cao and co-workers explored the ethanol sensing utilities of unmodified/Cl-modified LaFexO3-δ nanocrystals [157][158]. Both LaFexO3-δ and Cl-modified LaFexO3-δ nanocrystals displayed sensor responses of 132 and 79.2 (for 1000 and 200 ppm, correspondingly) with the response/recovery time of <10 s at 140 °C and 136 °C, respectively. However, the proposed LODs of both nanocrystals require further optimization. Ethanol sensing was also delivered by α-MoO3 and copper oxide (CuO/Cu2O) nanocrystals [159][160], but their operating temperatures seems to be >250 °C (see Table 2), thus requires more interrogations. Xiaofeng et al. described the methanol detection by Gd1–xCaxFeO3 (x = 0–0.4) nanocrystalline powder [161]. This material showed a sensor response of Ra/Rg = 117.7 (for 600 ppm) at 260 °C with a response/recovery time of <2 min and a LOD of <50 ppm. However, it still requires investigations on more interfering gaseous.

    Towards the detection of ethanol, doped/functionalized nanowires, such as Au modified ZnO NWs, Fe2O3 NPs coated SnO2 NWs, In2O3 NPs decorated ZnS NWs, and Sr-doped cubic In2O3/rhombohedral In2O3 homojunction, NWs displayed extensive sensitivity [162][163][164][165]. These NWs were synthesized by vapor–liquid–solid (VLS) method, hydrothermal or electrospun tactics. All NWs can operate at 300 °C except the Au modified ZnO NWs, which operate at 350 °C. In particular, Sr-doped cubic In2O3/rhombohedral In2O3 homojunction NWs [165] are impressive in terms of the response/recovery time (<1 min) with sub-ppm LOD (0.025 ppm). Similar to the NWs, nanorods are also utilized in the quantitation of ethanol as follows. Cr2O3 NPs functionalized WO3 NRds, ZnO NRds, Pd NPs decorated ZnO NRds, and SnO2-ZnO heterostructure NRds were synthesized by thermal evaporation, hydrothermal, and chemical vapor deposition (CVD) methods and employed in ethanol sensing [166][167][168][169][170]. Shankar et al. reported the fabrication of three kinds of polyvinyl alcohol (PVA)-ZnO NRds calcined composites (NR1, NR2, and NR3) and demonstrated their exceptional sensitivity to ethanol at room temperature as shown in Figure 9 and Table 2 [167]. In this work, NR3 displays a good sensing performance (Ra/Rg = 23; response/recovery time =26 s/43 s; LOD 5 ppm). In view of this, SnO2-ZnO heterostructure NRds (operating temperature is 275 °C) seems to be another good candidate with the LOD of 1 ppm, but it still requires optimization to reduce the operation temperature.

    Figure 9. (i) (a) FESEM images of PVA–ZnO nanocomposites calcined at 873 K for different time durations—1 h (NR1), 3 h (NR2), and 6 h (NR3) and (b) schematic of the ZnO nanorods (NR1, NR2, and NR3); (ii) Sensing responses of the calcined samples towards 100 ppm of acetone, acetaldehyde, methanol, and ethanol at room temperature (299 K) (Reproduced with the permission from Ref. [167]).

    On the other end of the spectrum, Perfecto et al. reported the discrimination of Iso-propyl alcohol (IPA) by rGO-WO3.0.33H2O nanoneedles (rGO: reduced graphene oxide) [171]. This material was synthesized by the combination of ultrasonic spray nozzle (USN) and microwave-assisted hydrothermal (MAH) methods and displayed a sensor response of 4.96 to 100 ppm IPA at room temperature and at 55% RH with a LOD of 1 ppm. Therefore, it becomes an impressive candidate in IPA sensing. However, more investigations are required to improve the sensitivity. Sm-doped SnO2 nanoarrays were developed through a hydrothermal tactic and applied in IPA sensing by Zhao and co-workers [172]. In which, it showed a high sensing response to IPA (Ra/Rg = 117.7; response/recovery time = 12 s/20 s; at 252 °C) with a LOD of ~ 1 ppm, therby become a notable material. However, this work requires more efforts in the reduction of operating temperature.

    With regard to different alcoholic vapors detections, materials with nanofibers structures were proposed by many research groups. For example, Han et al. developed the rough SmFeO3 nanofibers via electrospinning and calcination processes and used in the determination of ethylene glycol [173]. The sensor response reached 18.19 to 100 ppm of ethylene glycol at 240 °C with a LOD of ~5 ppm. Moreover, both response/recovery time were less than a min, thus become a prominent material in ethylene glycol sensor. Similar to the above report, Feng and co-workers synthesized the In-doped NiO nanofibers through electrospinning method and employed in the detection of methanol [174]. At 300 °C, the sensor response of In-doped NiO nanofibers reached 10.9 for 200 ppm methanol (response/recovery time = 273 s/26 s), which was five times higher than that of pure NiO nanofibers. However, this work needs further optimization to minimize the working temperature and LOD (25 ppm). In view of this, nanofibrous materials (SiO2@SnO2 core-shell nanofibers and Yb doped In2O3 nanofibers) were also engaged in the sensing of ethanol [175][176]. SiO2@SnO2 core-shell nanofibers [177] were synthesized by electrospinning and calcination process, but details on the sensor studies seems to be deficient (see Table 2) and need more interrogations. In contrast, Yb-doped In2O3 nanofibers (developed by electrospinning) is an impressive candidate with the capability of operation at room temperature and a sensor response of 40 for 10 ppm ethanol with a LOD of 1 ppm [176].

    Next, nanotubes were employed in the alcoholic vapor detection by using changes in resistance as the sensor signal. For example, Alali and co-workers developed the p-p heterojunction CuO/CuCo2O4 NTs through electrospinning and applied in the sensing of n-propanol at room temperature as shown in Figure 10 [178]. The importance of this work lies on its sensor signal to 10 ppm of n-propanol (Ra/Rg = 14; response/recovery time = 6.3 s/4.1 s). Similar to the above work, p-p type CuO–NiO heterojunction NTs were synthesized via calcination treatment and utilized in the detection of glycol at 110 °C [179]. Herein, the sensor responses reached 10.35 for 100 ppm glycol with a response/recovery time of 15 s/45 s and a LOD down to sub-ppm level (0.078 ppm), thus become a remarkable material. Ethanol sensing was demonstrated by coated and doped materials with nanotubes structures (NiO decorated SnO2 NTs, Ca-doped In2O3 NTs, Ni-doped In2O3 NTs, and W-doped NiO NTs) at diverse operating temperatures [180][181][182][183]. These nanotubes were synthesized by hydrothermal or electrospinning tactics and successfully used in the discrimination of ethanol. The NiO decorated SnO2 NTs [180] showed ethanol sensing ability of vertical standing NTs at 250 °C, but was low in sensitivity and LOD (Ra/Rg = 123.7 for 1000 ppm; response/recovery time = 10 s/58 s). On the other hand, enhanced sensor responses were observed in the rest of the doped NTs at ≥ 160 °C with a LODs of ̴ 5 ppm. For example, Ca-doped In2O3 NTs [181] reached a highest sensor response of 183.3 for 100 ppm ethanol (response/recovery time = 2 s/56 s) with a LOD of <5 ppm at 240 °C, thus, is noted as a good material for ethanol sensors.

    Discrimination of ethanol vapor was demonstrated by materials with nanobelt structures as discussed in the following. In2O3 NPs deposited TiO2 nanobelts, α-MoO3 nanobelts, and Zn-doped MoO3 nanobelts were hydrothermally prepared and engaged in ethanol detection at 100 °C, 300 °C, and 240 °C, respectively [184][185][186]. Among them, In2O3 NPs deposited TiO2 nanobelts [184] seems to be an impressive candidate with a sensor response of >9 for 100 ppm ethanol, working temperature at 100 °C, response/recovery time of 6 s/3 s), and a LOD of 1 ppm as noted in Table 2. To this track, Wang et al. described the n-butanol sensing utility of nanocube structured Fe2O3 that was derived from MOF via calcination [187]. Upon the exposure to 100 ppm n-buatnol, the MOF derived Fe2O3 nanocubes displayed a sensor response of ~6 (for 100 ppm) at 160–230 °C, response/recovery time is <2 min with a LOD of <1 ppm. Similarly, Nguyen et al. delivered the ethanol sensor property of hydrothermally synthesized In2O3 nanocubes [188]. In which, the sensor response reached 85 at 300 °C for 100 ppm ethanol with lower response/recovery time (15 s/60 s) and a LOD of <5 ppm, thus is noted as a noteworthy material in ethanol detection. However, further optimization is required in reducing the working temperature.

    Figure 10. (a) A diagram of the electrospinning process with a photograph of as-spun fiber mat and SEM image of composite fibers before calcination treatment. (b,e) Photographs of the sensor without and with covering by sensing materials, respectively. (c) Schematic diagram of the sensor and its components. (d) A schematic of the sensor circuit and its elements (Reproduced with the permission from Ref. [178]).

    Other than the microcages [189], nanocages were also reported in ethanol quantitation by the researchers. ZIF-8 derived ZnO hollow nanocages, ZIF-8 derived-Ag-functionalized ZnO hollow nanocages, and Cu2O hollow dodecahedral nanocages were employed in the exceptional detection of ethanol [190][191][192]. Among them, both ZIF-8 derived ZnO hollow nanocages and ZIF-8 derived-Ag-functionalized ZnO hollow nanocages were reported with high sensor responses to 100 ppm ethanol (Ra/Rg = 139.41; response/recovery time = 2.8 s/56.4 s at 325 °C and Ra/Rg = 84.6; response/recovery time = 5 s/10 s at 275 °C, correspondingly) with their LODs down to sub-ppm levels (0.025 and 0.0231 ppm, respectively). In the light of this, such materials developments are appreciated with further interrogations to reduce the working temperature. Nanosheet structured materials were also developed for the quantification of gaseous ethanol. Wherein, Al-doped ultrathin ZnO NShs, NiO NPs decorated SnO2 NShs, and CuO NPs decorated ultrathin ZnO NShs displayed their sensor responses to ethanol vapor [193][194][195]. However, apart from sensor responses and LODs, these materials require optimization for working temperatures, which are >250 °C, as noted in Table 2.

    In addition to diverse nanostructure-based alcoholic vapor detection, SnS2 and CdS nanoflakes were proposed for the sensing of methanol and IPA [196][197]. Bharatula and co-workers identified the methanol sensing performance of SnS2 nanoflakes [196]. As illustrated in Figure 11, the SnS2 nanoflakes exhibit an exceptional sensor response of 1580 for 150 ppm gas exposure at room temperature with the response/recovery time of 67 s/5 s, thereby can be commercialized towards the detection of methanol. However, more investigations on interference studies are required. In view of this, Liu et al. demonstrated the IPA sensing properties of CdS nanoflakes [197] with a LOD down to sub-ppm level (0.05 ppm). However, the sensor studies of CdS nanoflakes require high temperature (275 °C) and also lack interference studies.

    Figure 11. (i) TEM analysis of SnS2 nanoflakes (a and b) low magnification TEM images, (c) high resolution TEM image and (d) selected area diffraction (SAED) pattern; (ii) Alcohol sensing performance of SnS2 nanoflakes (all alcohols at 150 ppm) at 25 °C (a) response vs. alcohols and inset shows the response vs. concentration, (b) resistance vs. alcohols, (c) typical I–t plot and (d) bar diagram of response and recovery time of the test alcohols (Reproduced with the permission from Ref. [196]).

    Materials such as Co-doped ZnO hexagonal nanoplates, ZIF-8 derived α-Fe2O3/ZnO/Au hexagonal nanoplates, and ZnO nanoplates were reported in the ethanol detection [198][199][200]. The sensor responses of those nanoplates were found as 570, 170, and 8.5 for 300, 100, and 1000 ppm of ethanol at 300 °C, 280 °C, and 164°C, correspondingly. Wherein, hydrothermally synthesized Co-doped ZnO hexagonal nanoplates displayed a high sensor response to both ethanol and acetone [198]. However, further works on the interference studies as well as optimization for working temperature are required. On the other hand, compared to ZnO nanoplates [200], samples of ZIF-8 derived α-Fe2O3/ZnO/Au hexagonal nanoplates (synthesized by multi-step reaction process) [199] seems to be impressive in terms of response/recovery time (5 s/4 s) and LOD (~10 ppm) as shown in Figure 12. Materials with NSPs structures towards alcoholic gas detection were proposed by the researchers.

    Figure 12. (i) SEM images of Sample 1–4: (ad) before calcination; (eh) after calcination; TEM images of Sample 1–4 after calcination (il); (ii) (a) The response of samples to 100 ppm ethanol at different operating temperature; (b) the response and recovery curves of samples upon exposure to 10–1000 ppm ethanol; (c) the response curves of samples to ethanol concentrations; (d) the linear relationship of log(S-1)-log(C) plot to ethanol at the optimum operating temperatures (Reproduced with the permission from Ref. [199]).

    Nanosphere-shaped materials such as Zn2SnO4, monodispersed indium tungsten oxide, Ag@In2O3, ZnSnO3, ZnO, and α-Fe2O3 were effectively applied in the discrimination of alcoholic vapors as presented in Table 2 [201][202][203][204][205][206]. Wherein, Zn2SnO4 NSPs, and Ag@In2O3 core-shell NSPs [201][203] were found to be effective in detecting the ethanol with sensor response of 23.4 and 72.56 (for 50 ppm ethanol at 180 °C and 220 °C, individually) with response/recovery time of <1 min and LODs of ~ 5 ppm and ~ 2ppm, correspondingly. Similarly, monodispersed indium tungsten oxide ellipsoidal NSPs and α-Fe2O3 hollow NSPs [202][206] were engaged in the quantitation of methanol at higher working temperature (>250 °C). However, α-Fe2O3 hollow NSPs were found to be more impressive with a sensor response of 25 for 10 ppm methanol at 280 °C (response/recovery time = 8 s/9 s) and with a LOD of 1 ppm. Subsequently, perovskite type ZnSnO3 NSPs and ZnO hollow NSPs [204][205] were applied in the detection of n-propanol and n-butanol (for 500 ppm at 200 °C and 385 °C, respectively). Between them, ZnSnO3 NSPs seems to be a better candidate with respect to their working temperature and LOD (0.5 ppm).

    In light of this, materials with modified nanoflower structures (PdO NPs modified ZnO, rGO nanosheets modified NiCo2S4 and Pd and rGO modified TiO2) and grained nanoflowers (NiO) were utilized in alcoholic gases assays [207][208][209][210]. PdO NPs modified ZnO nanoflowers displayed their enhanced sensing capability of methanol via decoration of PdO NPs over the surface of ZnO as shown in Figure 13. In a similar fashion, reduced graphene oxide (rGO) nanosheets modified NiCo2S4 nanoflowers and Pd and rGO modified TiO2 nanoflowers were demonstrated in the detection of ethanol as noted in Table 2.

    Figure 13. (a) X-ray diffraction patterns of the synthesized pristine ZnO NFs, NiO modified ZnO NFs, and PdO modified ZnO NFs. (b) FESEM image of pristine ZnO NF; inset shows magnified view of (i) pristine, (ii) NiO modified and (iii) PdO modified ZnO NF. (c) Cross-sectional view of grown ZnO NF on Si/SiO2 substrate. (d) Transient response characteristics (response magnitude (%) as a function of time) of the PdO–ZnO NF hybrid structure towards methanol, ethanol, and 2-propanol in the concentration range of 0.5–700 ppm at 150 °C (Reproduced with the permission from Ref. [207]).

    In particular, rGO modified nanoflowers were highly impressive in terms of the operating temperature (≤100 °C) and can be commercialized in future. Nanomaterials with porosity plays a vital role in the sensing studies of volatile alcoholic compounds. Porous structures of Ag-functionalized ZnO, Al-doped ZnO, Au loaded WO3, 3D-ordered In-doped ZnO, Si@ZnO NPs, Ag loaded graphitic C3N4, hierarchical mixed Pd/SnO2, SnO2 fibers, hierarchical branched TiO2-SnO2, and hierarchical Co-doped ZnO were reported for their alcohol sensing utilities [211][212][213][214][215][216][217][218][219][220]. These porous nanostructures were synthesized by combustion method, nanocasting method, template mediated synthesis, microemulsion method, microdispensing method, solvothermal method, and calcination tactics. As noted in Table 2, these meso-/macro-porous nanostructures displays their good responses to alcoholic vapors at different working temperatures (lie between 150 and 350 °C). For example, hierarchical Co-doped ZnO mesoporous structure [220] revealed its exceptional sensitivity to ethanol (Ra/Rg = 54 for 50 ppm at 180 °C; response/recovery time = 22 s/53 s) with a LOD of 0.0454 ppm, thereby can be attested as a good candidate for ethanol sensing studies.

    Next, hierarchical nanostructures/nanocomposites were reported towards the detection of alcoholic gases. For instance, hierarchical Fe2O3 NRds on SnO2 NSPs nanocomposites and MoO3-mixed SnO2 hierarchical aerogel nanostructures were employed in the quantification of ethanol at 320 °C and 260 °C, respectively [221][222]. The sensors responses of these materials are 23.512 and 714, correspondingly, with response/recovery time <1 min/7 min, as shown in Table 2. In a similar trend, hierarchical In2O3 NPs decorated ZnO nanostructure [223] displayed discrimination of n-butanol (Ra/Rg = 218.3 for 100 ppm at 260 °C; response/recovery time = 22 s/53 s) with a LOD down to sub-ppm level, thereby become a notable material. Apart from hierarchical nanostructures, diverse shaped nanostructures were also used in volatile alcohols identification. Honeycomb-like SnO2-Si-NPA nanostructure, rambutan-like SnO2 hierarchical nanostructure, ZnO nano-tetrapods, raspberry-like SnO2 hollow nanostructure, snowflake-like SnO2 hierarchical architecture, sea cucumber-like indium tungsten oxide, hollow Pentagonal-Cone-Structured SnO2 architecture, and neck-connected nanostructure film of ZIF-8 derived ZnO were proposed in the detection of alcohols as noted in Table 2 [224][225][226][227][228][229][230][231]. These materials were synthesized by solvothermal, hydrothermal, thermal-annealing, calcination, or CVD tactics. However, ZnO nano-tetrapods [226] can be ruled out due to their combined sensing applicability in hydrocarbon detection.

    As described in Table 2, nanocomposite architectures were also demonstrated to be effective towards the quantification of alcohols. Wherein, nanocomposites of flower like LaMnO3@ZnO, SnO2-Pd-Pt-In2O3, RGO-SnO2 NPs, SnO2-V2O5, ZnO:Fe, g-C3N4-SnO2, and Co3O4 nanosheet array-3D carbon foam are more impressive towards alcohols sensing studies [232][233][234][235][236][237][238]. Among them, SnO2-V2O5 nanocomposite [226] was demonstrated with its sensitivity to ethanol (~66% for 160 ppm) through local grain-to-grain conductivity, but details on other sensor properties, such as response/recovery time and LOD were not available. Moreover, RGO-SnO2 NPs composite [234] seems to be significant in terms of its capable operation between 24 and 98% humid conditions. This may be due to the presence of reduced graphene oxide along with the SnO2 NPs. ZnO:Fe nanostructured film [236] morphology was improved by UV treatment, which further enhanced its sensor response. Similarly, upon modification of SnO2 by g-C3N4 NShs [237], the ethanol sensitivity was improved.

    Table 2. Detection concentration, response/recovery time, operating temperature (Temp.) and LODs to volatile alcohols by diverse nanostructured materials.

    Materials/Nanostructure

    Analyte/Concentration

    Gas Response (Rair/Rgas)

    Response/Recovery

    Temp.

    LOD

    Ref

    Sn3N4/ nanoparticles

    Ethanol /100 ppm

    51.3

    NA

    120 °C

    0.07 ppm

    [152]

    C doped TiO2/nanoparticles

    n-Pentanol/100 ppm

    11.12

    100 s/675 s

    170 °C

    0.5 ppm

    [154]

    Pr doped In2O3/nanoparticles

    Ethanol/50 ppm

    106

    16.2 s/10 s

    240 °C

    <1 ppm

    [155]

    Au and Cl Comodified LaFeO3/nanoparticles

    Ethanol/100 ppm

    102.1 & 220.7

    <40 s/NA

    120 °C

    <10 ppm

    [156]

    LaFexO3 −/nanocrystals

    Ethanol/1000 ppm

    132

    1 s/1.5 s

    140 °C

    <50 ppm

    [157]

    Cl doped LaFexO3 −/nanocrystals

    Ethanol/200 ppm

    79.2

    9 s/5 s

    136 °C

    <100 ppm

    [158]

    α-MoO3/nanocrystals

    Ethanol/100 ppm

    >55

    34 s/70 s

    350 °C

    NA

    [159]

    CuO/Cu2O/nanocrystals

    Ethanol/100 ppm

    10 & 9.5

    5 s/10 s & 4.1 s/10.5 s

    300 °C & 275 °C

    <10 ppm

    [160]

    Gd1–xCaxFeO3/nanocrystals

    Methanol/600 ppm

    117.7

    1 min/1.1 min

    260 °C

    <50 ppm

    [161]

    Au modified ZnO/nanowires

    Ethanol/500 ppm

    12.35

    215 s/180 s

    350 °C

    <10 ppm

    [162]

    Fe2O3 nanoparticles coated SnO2/nanowires

    Ethanol/200 ppm

    57.56

    300 s/100 s

    300 °C

    <5 ppm

    [163]

    In2O3 nanoparticles decorated ZnS/nanowires

    Ethanol/500 ppm

    >25

    400 s/100 s

    300 °C

    <10 ppm

    [164]

    Sr-doped cubic In2O3/rhombohedral In2O3/nanowires

    Ethanol/1 ppm

    21

    <1 m (both)

    300 °C

    0.025 ppm

    [165]

    Cr2O3 nanoparticles functionalized WO3/nanorods

    Ethanol/200 ppm

    5.58

    51.35 s/48.65 s

    300 °C

    <5 ppm

    [166]

    ZnO/nanorods

    Ethanol/100 ppm

    23

    26 s/43 s

    Room Temp.

    <5 ppm

    [167]

    1D-ZnO/nanorods

    Ethanol/100 ppm

    44.9

    6 s/31 s

    300 °C

    <10 ppm

    [168]

    Pd nanoparticles decorated ZnO/nanorods

    Ethanol/500 ppm

    81

    6 s/95 s

    260 °C

    <100 ppm

    [169]

    SnO2/ZnO/nanorods

    Ethanol/100 ppm

    18.1

    2 s/38 s

    275 °C

    <1 ppm

    [170]

    rGO-WO3.0.33H2O/nanoneedles

    Isopropanol/100 ppm

    4.96

    <90 s/NA

    Room Temp.

    1 ppm

    [171]

    Sm-doped SnO2/nanoarrays

    Isopropanol/100 ppm

    117.7

    12 s/20 s

    252 °C

    ~1 ppm

    [172]

    SmFeO3/nanofibers

    Ethylene glycol/100 ppm

    18.19

    41 s/47 s

    240 °C

    ~5 ppm

    [173]

    In doped NiO/nanofibers

    methanol/200 ppm

    10.9

    273 s/26 s

    300 °C

    25 ppm

    [174]

    SiO2@SnO2/core-shell nanofibers

    Ethanol/200 ppm

    37

    13 s/16 s

    NA

    NA

    [175]

    Yb doped In2O3/nanofibers

    Ethanol/10 ppm

    40 & 5

    NA

    Room Temp.

    <1 & 5 ppm

    [176]

    CuO/CuCo2O4/nanotubes

    n-Propanol/10 ppm

    14

    6.3 s/4.1 s

    Room Temp.

    <10 ppm

    [178]

    CuO-NiO/nanotubes

    Glycol/100 ppm

    10.35

    15 s/45 s

    110 °C

    0.078 ppm

    [179]

    NiO decorated SnO2/vertical standing nanotubes

    Ethanol/1000 ppm

    123.7

    10 s/58 s

    250 °C

    NA

    [180]

    Ca doped In2O3/nanotubes

    Ethanol/100 ppm

    183.3

    2 s/56 s

    240 °C

    <5 ppm

    [181]

    Au and Ni doped In2O3/nanotubes

    Ethanol/100 ppm

    16.16 & 49.74

    5 s/64 s & 3 s/49 s

    160 °C & 220 °C

    <5 ppm (for both)

    [182]

    W doped NiO/nanotubes

    Ethanol/100 ppm

    40.56

    54 s/22 s

    200 °C

    <5 ppm

    [183]

    In2O3 NPs deposited TiO2/nanobelts

    Ethanol/100 ppm

    >9

    6 s/3 s

    100 °C

    1 ppm

    [184]

    α-MoO3/nanobelts

    Ethanol/800 ppm

    173

    <65 s/>15 s

    300 °C

    <50 ppm

    [185]

    Zn doped MoO3/nanobelts

    Ethanol/1000 ppm

    321

    <121 s (for both)

    240 °C

    5 ppm

    [186]

    MOF derived Fe2O3/nanocubes

    Ethanol/100 ppm

    ~6

    <120 s/<60 s

    160 - 230 °C

    <1 ppm

    [187]

    In2O3/nanocubes

    Ethanol/100 ppm

    85

    15 s/60 s

    300 °C

    <5 ppm

    [188]

    ZIF-8 derived ZnO/hollow nanocages

    Ethanol/100 ppm

    139.41

    2.8 s/56.4 s

    325 °C

    0.025 ppm

    [190]

    ZIF-8 derived Ag functionalized ZnO/hollow nanocages

    Ethanol/100 ppm

    84.6

    5 s/10 s

    275 °C

    0.0231 ppm

    [191]

    Cu2O/hollow dodecahedral nanocages

    Ethanol/100 ppm

    4.6

    112.4 s/157.5 s

    250 °C

    NA

    [192]

    Al-doped ZnO/nanosheets

    Ethanol/100 ppm

    90.2

    1.6 s/1.8 s

    370 °C

    <1 ppm

    [193]

    NiO NPs decorated SnO2/nanosheets

    Ethanol/100 ppm

    153

    NA

    260 °C

    <5 ppm

    [194]

    CuO NPs decorated ZnO/nanosheets

    Ethanol/200 ppm

    97

    <7 s/<40 s

    320 °C

    <1 ppm

    [195]

    SnS2/nanoflakes

    Methanol/150 ppm

    1580

    67 s/5 s

    Room Temp.

    NA

    [196]

    Co doped ZnO/hexagonal nanoplates

    Ethanol/300 ppm

    570

    50 s/5 s

    300 °C

    ~50 ppm

    [198]

    ZIF-8 derived α-Fe2O3/ZnO/Au/nanoplates

    Ethanol/100 ppm

    170

    4 s/5 s

    280 °C

    ~10 ppm

    [199]

    ZnO/nanoplates

    Ethanol/1000 ppm

    8.5

    154.4 s (125 ppm)/114.2 s (1500 ppm)

    164 °C

    NA

    [200]

    Zn2SnO4/nanospheres

    Ethanol/50 ppm

    23.4

    18 s/45 s

    180 °C

    ~5 ppm

    [201]

    Indium Tungsten Oxide/ellipsoidal nanospheres

    Methanol/400 ppm

    >5

    2 s/9 s

    312 °C

    ~20 ppm

    [202]

    Ag@In2O3/core-shell nanospheres

    Ethanol/50 ppm

    72.56

    13 s/8 s

    220 °C

    ~2 ppm

    [203]

    ZnSnO3/hollow nanospheres

    n-Propanol/500 ppm

    64

    NA

    200 °C

    0.5 ppm

    [204]

    ZnO/hollow nanospheres

    n-Butanol/500 ppm

    292

    36 s/9 s

    385 °C

    ~10 ppm

    [205]

    α-Fe2O3/hollow nanospheres

    Methanol/10 ppm

    25

    8 s/9 s

    280 °C

     1 ppm

    [206]

    PdO NPs modified ZnO/nanoflowers

    Methanol/150 ppm

    >80

    18 s/52.2 s

    150 °C

    <0.5 ppm

    [207]

    NiO/grained nanoflowers

    Ethanol/150 ppm

    35

    3 s/6 s

    200 °C

    2.6 ppm

    [208]

    rGO nanosheets modified NiCo2S4/nanoflowers

    Ethanol/100 ppm

    >2.5

    4.56 s/10.38 s

    100 °C

    <10 ppm

    [209]

    Pd and rGO modified TiO2/nanoflowers

    Ethanol/700 ppm

    >64% (for both)

    6.55 s/186.97 s & 75.64 s/147.16 s

    90 °C

    <10 ppm

    [210]

    Ag-functionalizedZnO/macro-/mesoporous- nanostructure

    n-Butanol/100 ppm

    994.8

    66 s/25 s

    240 °C

    <1 ppm

    [211]

    Al-doped ZnO/macro-/mesoporous- nanostructure

    n-Butanol/100 ppm

    751.96

    25 s/23 s

    300 °C

    ~1 ppm

    [212]

    Au loaded WO3/mesoporous- nanostructure

    n-Butanol/100 ppm

    14.35

    10 s/35 s

    250 °C

    <10 ppm

    [213]

    In doped ZnO/three dimensionally ordered mesoporous- nanostructure

    Ethanol/100 ppm

    88

    25 s/10 s

    250 °C

    <5 ppm

    [214]

    Si@ZnO NPs/ mesoporous- nanostructure

    Ethanol/300 ppm

    62.5

    0.27 min/3.5 min

    400 °C

    <50 ppm

    [215]

    Ag loaded g-C3N4/mesoporous- nanostructure

    Ethanol/50 ppm

    49.2

    11.5 s/7 s

    250 °C

    <1 ppm

    [216]

    Pd/SnO2/porous- nanostructure

    Ethanol/5 -200 ppm

    ~90%

    1.5 s/18 s

    300 °C

    <5 ppm

    [217]

    SnO2/mesoporous- nanofibers

    n-Butanol /300 ppm (for both)

    556.5 & 415.3

    195 s/100 s & 64 s/36 s

    150 °C & 200 °C

    <10 ppm

    [218]

    TiO2–SnO2/hierarchical branched mesoporous nano- composite

    Ethanol/50 ppm

    40

    7 s/5 s

    350 °C

    0.2 ppm

    [219]

    Co-Doped ZnO/hierarchical mesoporous- nanostructure

    Ethanol/50 ppm

    54

    22 s/53 s

    180 °C

    0.0454 ppm

    [220]

    Fe2O3 nanorods on SnO2 nanospheres/hierarchical nano- composite

    Ethanol/100 ppm

    23.512

    5 s/12 s

    320 °C

    <50 ppm

    [221]

    MoO3-mixed SnO2/ hierarchical nanostructure

    Ethanol/100 ppm

    714

    1 s (for all)/357 s, 8 s and 85 s

    260 °C

    <10 ppm

    [222]

    In2O3 Nanoparticles Decorated ZnO/hierarchical nanostructure

    n-Butanol /100 ppm

    218.3

    5 s/12 s

    260 °C

    Down to sub-ppm

    [223]

    SnO2 – Si-NPA/honeycomb like nanostructure

    Ethanol/50 ppm

    7.7

    10 s/9 s

    320 °C

    <10 ppm

    [224]

    SnO2/ rambutan-like hierarchical nanostructure

    n-Butanol /100 ppm

    44.3

    8 s/5 s

    140 °C

    <20 ppm

    [225]

    SnO2/ raspberry-like hollow nanostructure

    n-Butanol /100 ppm

    303.49

    163 s/808 s

    160 °C

    1 ppm

    [227]

    SnO2/ snowflake-like hierarchical nanostructure

    Ethanol/250 ppm

    ~55

    6 s/7 s

    400 °C

    NA

    [228]

    SnO2/ pentagonal-cone assembled with nanorods

    Ethanol/200 ppm

    98

    11 s/18 s

    220 °C

    1 ppm

    [230]

    ZIF-8 derived ZnO/neck- connected nanostructure films

    Ethanol/50 ppm

    124

    120 s/70 s

    375 °C

    0.5 ppm

    [231]

    LaMnO3@ZnO/nano- composite

    n-Butanol /100 ppm

    6

    8 s/17 s

    300 °C

    NA

    [232]

    SnO2-Pd-Pt- In2O3/nano- composite

    Methanol/100 ppm

    320.73

    32 s/47 s

    160 °C

    0.1 ppm

    [233]

    RGO-SnO2/ nano- composite

    Ethanol /100 ppm

    43

    8 s/NA

    300 °C

    ~5 ppm

    [234]

    ZnO:Fe/ nano- composite films

    Ethanol /100 ppm

    61 & 36

    1.1 s/1.45 s & 0.23 s/0.34 s

    250 °C & 350 °C

    ~10 ppm

    [236]

    g-C3N4-SnO2/nano- composite

    Ethanol/500 ppm

    240

    15 s/38 s

    300 °C

    ~50 ppm

    [237]

    Co3O4 nanosheet array-3D carbon foam/ nano- composite

    Ethanol/100 ppm

    10.4

    45 s/140 s

    100 °C

    0.2 ppm

    [238]

    NA = Not available; Temp. = Temperature; s = seconds; in = minutes.

    5. Various Nanostructures in Volatile Aldehyde Detection

    In addition to acetone or alcoholic vapors detection, volatile organic aldehydes quantitation is also become essential and diverse nanostructured materials were reported in the aldehyde gases quantification. This section describes published reports in detail. Majority of the research papers generally focused on the assay of formaldehyde (HCHO) via the following reaction mechanism [239], which are also applicable for other aldehydes detection. As described in Equations (8) and (9), the adsorbed oxygen in the sensor chamber first reacts to form the acid followed by interaction with oxygen anion to release electrons, which can be adopted as the sensor signal.

    HCHO + O(ads) → HCOOH + e

    (8)

    HCHO + 2O → CO2 + H2O + 2e

    (9)

    Nanoparticles, such as In2O3 NPs, molecularly imprinted polymers (MIPs) NPs, amorphous Eu0.9Ni0.1B6 NPs, Ni-doped SnO2 NPs, and NiO granular NPs films were utilized to detect the formaldehyde at diverse operating temperatures with sub-ppm LODs, as noted in [240][241][242][243][244]. Among them, molecularly imprinted polymers (MIPs) NPs discriminated the formaldehyde by means of quartz crystal microbalance (QCM) tactic with a LOD of 0.5 ppm, thus can be noted as an additional approach for the formaldehyde detection. Moreover, amorphous Eu0.9Ni0.1B6 NPs [242] were noted as an exceptional candidate in formaldehyde quantitation due do their sensor response (Ra/Rg 7 for 20 ppm; response/recovery time 20 s (for both)) at room temperature. Therefore, commercialization of amorphous Eu0.9Ni0.1B6 NPs towards HCHO detection is desirable.

    Regarding the HCHO quantification, researchers developed nanowires (SnO2 NWs, p-CuO/n-SnO2 core-shell NWs, ZnO meso-structured NWs and RGO coated Si NWs) and nanorods (Co doped In2O3 NRds and Ag-functionalized and Ni-doped In2O3 NRds) via VLS, atomic layer deposition, low-temperature chemical synthesis, metal-assisted chemical etching method (MACE), thermal annealing methods and applied them in effective sensing of HCHO gas [245][246][247][248][249][250]. As noted in Table 3, many of these materials display exceptional sensitivity to HCHO at dif

    ferent working temperatures. Note that ZnO meso-structured NWs [248] are capable of operating at room temperature and show a high sensor response of 1223% to 50 ppm HCHO with a LOD of 0.005 ppm under UV, thereby attest as a motivational research. In view of this, Zhang et al. reported the utility of Ag-LaFeO3 with spheres, fibers, and cages architectures with sensor responses of 16, 14, and 23 for 1 ppm HCHO at 82 °C, 110 °C, and 70 °C, correspondingly [251]. This work has driven the utilization of diverse nanostructures in VOCs determination as described below.

    Materials with nanofibers structures were also attested their sensing ability to HCHO vapor. Ag-doped LaFeO3 nanofibers, Co3O4-ZnO core-shell nanofibers, WO3/ZnWO4—1D nanofibers, and Pr-doped BiFeO3/hollow nanofibers were synthesized by electrospinning method with the combination of calcination tactic and utilized in HCHO detection at 190–230 °C [252][253][254][255]. As shown in Table 3, WO3/ZnWO4—1D hetrostructured nanofibers are found to be a good candidate in terms of the sensor response (Ra/Rg = 44.5 for 5 ppm at 220 °C; response/recovery time = 12 s/14 s) with a LOD of 1 ppm [254]. However, further optimization to reduce the working temperature is still needed. Liang and co-workers presented the alkaline earth metals-doped In2O3 NTs for the sensing of HCHO [256]. In which, Ca-doped In2O3 NTs showed a sensor response of 116 for 100 ppm HCHO at 130 °C with the response/recovery time of 1 s/328 s and a LOD of 0.06 ppm. Therefore, it can be authorized as an exceptional material for the HCHO sensor studies. Doped nanobelts were employed in the discriminative assay of HCHO as detailed below. Er-doped SnO2 and Pt-decorated MoO3 nanobelts were reported for the quantitation of volatile HCHO [257][258]. As shown in Figure 14, Er-doped SnO2 nanobelts [257] display a distinct response to HCHO vapor (Ra/Rg = 9 for 100 ppm at 230 °C; response/recovery time = 17 s/25 s) with a LOD of 0.141 ppm. On the other hand, Pt-decorated MoO3 nanobelts [258] are more impressive with a sensor response (Ra/Rg = ~25% for 100 ppm; response/recovery time = 17.8 s/10.5 s; LOD = 1 ppm) at room temperature, thereby become a noteworthy material in HCHO sensory research.

    Figure 14. (i) SEM images of Er–SnO2 nanobelts (a) at a low magnification, (b) at a high magnification, (c) SEM image of pure SnO2 nanobelts, and (d) EDS spectra of Er–SnO2 nanobelts; (ii) (a) The responses (Rs) versus temperature (T) of Er–SnO2 nanobelt to 100 ppm gas from 150 °C to 260 °C, (b) The responses (Rs) versus temperature (T) of SnO2 nanobelt to 100 ppm gas from 150 °C to 260 °C, (c) Histogram of the sensitive responses at 230 °C (Reproduced with the permission from Ref. [257]).

    Nanocube-shaped materials were consumed in the quantification of HCHO vapor at diverse operating temperatures. Multi-shelled hollow nanocubes (ZnSnO3 and ZnSn(OH)6; synthesized by co-precipitation method) were engaged in HCHO sensing at 220 °C and 60 °C, respectively [259][260]. The responses reached 37.2 and 56.6 (for 100 ppm; response/recovery time = 1 s/59 s and 1 s/89 s, respectively) with the LODs of <10 ppm and 1ppm, correspondingly. Both materials performed remarkablely in HCHO sensory studies. Similarly, MOF (zeolite imidazolate framework-67; ZIF-67)-derived Co3O4/CoFeO4 double-shelled nanocubes were utilized in the detection of HCHO [261]. As illustrated in Figure 15, the ZIF-67-derived Co3O4/CoFeO4 double-shelled nanocubes were synthesized by MOF route and found to be more effective in the sensing of HCHO even at 1 ppm concentration.

    Figure 15. (i) Schematic of the process to fabricate porous Co3O4 HNCs, CCFO DSNCs, and CFO SSNCs; (ii) (a) Responses of the sensors to 10 ppm of formaldehyde at 80–230 °C. (b) Responses of the sensors to 10 ppm of various gases at the optimal operating temperatures. Dynamic response–recovery curves of the CCFO DSNCs and Co3O4 HNCs sensors to xylene in the ranges (c,e) 1–20 ppm and (d,f) 50–500 ppm under their optimal operating temperatures. (g) Response of the sensors in the ranges from 1 to 500 ppm of xylene. (h) Responses of the CCFO DSNCs sensor as a function of low formaldehyde concentration (1–10 ppm) (Reproduced with the permission from Ref. [261]).

    For 10 ppm HCHO, the sensor response reaches 12.7 at low operating temperature (139 °C) with short response/recovery time (4 s/9 s) and sub-ppm LOD (0.3 ppm). Therefore, development of such nanocube materials are highly anticipated in HCHO quantitation. Nanosheets with and without decoration were effectively applied in the sensing of HCHO. High resolution SEM and TEM images of nanosheets are dispayed in Figure 16. WO3 clusters decorated In2O3 NShs (synthesized by impregnating method), SnO2 NShs, ZnO NShs, Au atom dispersed In2O3 NShs, and PdAu bimetal decorated SnO2 NShs were exploited in the discrimination of HCHO at various operating temperatures [262][263][264][265][266]. However, the ZnO NShs (adopted from hydrothermal method; Figure 16) were engaged in aqueous phase detection of HCHO with linear regression of 10 nM to 1 mM and a LOD of 210 nM; thus, it cannot be listed as device-based assays [267]. Compared to other HCHO sensory reports, Au atom dispersed In2O3 NShs (developed by light assisted reduction method) were highly fascinated with respect to its sensor reposes (Ra/Rg = 85.67 for 50 ppm at 100 °C; response/recovery time = 25 s/198 s) with an exceptional LOD of 0.00142 ppm [265]. PdAu bimetal decorated SnO2 NShs (synthesized by hydro-solvothermal treatment) were reported for the detection of both acetone and HCHO at 250 °C and 110 °C, respectively, with a LODs down to sub-ppm level [266]. The device also works at high humid condition (94% RH), but interference studies still need more clarification., Hayashi et al. proposed the utilization of SnS2 nanoflake device to detect the HCHO gas [268], which had a LOD down to sub-ppm (0.001/0.02 ppm) and operated at 210 °C. However, details on other sensory properties are currently missing.

    Figure 16. Field Emission Scanning Electron Microscopy (FESEM) image (a) cross section image (b), energy dispersive spectroscopy (EDS) mapping image (c) and transmission electron microscope (TEM) image (d) (inner HR TEM and selected area diffraction (SAED) pattern) of ZnO NShs (Reproduced with the permission from Ref. [267]).

    Nanospheres were also fabricated to quantify the HCHO as described in the following. Hussain and co-workers demonstrated the utilization of 0D ZnO NSPs and NPs (developed by low temperature hydrothermal route) to discriminate the formaldehyde [269]. Wherein, 0D ZnO NSPs displayed a higher sensor response (Ra/Rg = 95.4 for 100 ppm at 295 °C; response/recovery time = 11 s/8 s; LOD as ~5 ppm) than that of 0D ZnO NPs (Ra/Rg = 68.2 for 100 ppm at 295 °C; response/recovery time = 11 s/8 s; LOD = ~10 ppm). However, the working temperature needs to be reduced before commercialization. In light of this, hydrothermally synthesized Ag-doped Zn2SnO4/SnO2 hollow NSPs [270] were reported in the detection of HCHO at low working temperature (Ra/Rg = 60 for 50 ppm at 140 °C; response/recovery time = 9 s/5 s; LOD = 5 ppm), thereby is attested as a better candidate. Similar to NSPs, microspheres were also still applied in the detection of HCHO. For example, MOF-derived ZnO/ZnCo2O4 microspheres [271] were used in HCHO sensing with a response of 26.9 for 100 ppm gas with response/recovery time of 9 s/14 s and a LOD of 0.2 ppm. Therefore, research on NSPs/microspheres-based VOCs detection is highly anticipated in future.

    As an alternative to HCHO sensors, materials with nanoflowers like structures were developed. Hierarchical SnO2 and Sn3O4/rGO hetrostructured nanoflowers (synthesized by hydrothermal method) were reported for formaldehyde assay [272][273]. As noted in Table 3, Sn3O4/rGO hetrostructured nanoflowers are more effective in sensing HCHO (Ra/Rg = 44 for 100 ppm at 150 °C; response/recovery time = 4 s/125 s) than that of Hierarchical SnO2 nanoflowers with a LOD of 1 ppm. Moreover, the presence of rGO enhances the sensor response and long-term stability, thereby is noted as a good addition to HCHO sensors.

    Exploitation of different nanostructures with porosity in the quantitation of HCHO detection has been demonstrated. Porous nanostructures such as Au-loaded In2O3 hierarchical porous nanocubes, Ag-loaded ZnO porous hierarchical nanocomposite, Pd–WO3/m-CN mesoporous nanocubes, GO/SnO2-2D mesoporous nanosheets, ZnSnO3-2D mesoporous nanostructure, LaFeO3 porous hierarchical nanostructure, Bi doped Zn2SnO4/SnO2 porous nanospheres, ZnO porous nanoplates, and Au@ZnO mesoporous nanoflowers were reported as HCHO sensors at diverse operating temperatures as summarized in Table 3 [274][275][276][277][278][279][280][281][282]. The higher sensor responses achieved from those materials are attributed to the presence of porosity, which increases the adsorption of the HCHO gas significantly to enhance the signal (changes in resistance). Among the aforementioned porous materials, 2D mesoporous GO/SnO2 nanosheets [277] revealed an exceptional sensor response of 2275.7 at low operating temperature (60 °C) with the response/recovery time of 81.3 s/33.7 s and a LOD of 0.25 ppm. Therefore, this extraordinary work has demonstrated possible commercialization in future. Similarly, mesoporous Pd–WO3/m-CN nanocubes [276] performed quite impressively with operation at 95% dry humid condition. In the light of this, TiO2/ZnCo2O4 porous nanorods were engaged in the detection of HCHO and trimethylamine (TEA) at 220 °C and 130 °C [283]. However, response in TEA detection was higher than that of HCHO and it was lack of interference studies.

    Hierarchical nanostructures of Zn2SnO4/SnO2, Pt/MnO2-Ni(OH)2 hybrid nanoflakes, SnO2 nanofiber/nanosheets, In2O3@SnO2 composite, and cedar-like SnO2 were demonstrated as HCHO sensors [284][285][286][287][288]. These materials were synthesized by means of chemical route, hydrothermal, or electrospinning tactics. Wherein, hierarchical Pt/MnO2-Ni(OH)2 hybrid nanoflakes [285] demonstrated only the formaldehyde oxidation activity at room temperature, thereby more work was needed to study the exact sensor response. Among these hierarchical nanostructures, In2O3@SnO2 hierarchical composite displayed a good sensor response (Ra/Rg = 180.1 for 100 ppm at 120 °C; response/recovery time = 3 s/3.6 s) with a LOD of 0.1 ppm. Different shaped nanostructures were also proposed by researchers as an alternative to aldehyde sensors. Following the similar synthetic approaches, urchin-like In2O3 hollow nanostructure, butterfly-like SnO2 hierarchical nanostructure, SnO2 hollow hexagonal prisms, and NiO/NiFe2O4 composite nanotetrahedrons were synthesized and used in aldehydes detection as illustrated in Table 3 [289][290][291][292]. However, butterfly-like SnO2 hierarchical nanostructure [290] performs better in the detection of acetaldehyde than that of HCHO as noted in Figure 17. It displays a high sensor response to acetaldehyde (Ra/Rg = 178.3 for 100 ppm at 243 °C; response/recovery time = 28 s/58 s) with a LOD of <0.5 ppm, thereby cab be noted as an exceptional candidate. In the detection of HCHO, nanocomposite materials were also effectively employed. Vertical graphene (VG) decorated SnO2, multiwalled carbon nanotubes-polyethyleneimine, and n-n TiO2@SnO2 nanocomposites (synthesized by ALD method) were utilized in the discrimination of HCHO with LODs down to sub-ppm level [293][294][295]. In particular, multiwalled carbon nanotubes-polyethyleneimine composites [286] operated at room temperature with good response/recovery time (<1 min for both). On the other hand, n-n TiO2@SnO2 nanocomposites [295] detected the HCHO under UV and dark conditions at 50 °C. Therefore, development of such nanocomposite-based VOC sensor devices are much anticipated.

    Figure 17. (i) Schematic illustration of the formation of butterfly-like SnO2 architectures. (ii) The response comparison of the butterfly-like SnO2 architectures to 200 ppm of various VOCs at the optimal operating temperature. (Reproduced with the permission from Ref. [290]).

    Table 3. Detection concentration, response/recovery time, operating temperature (Temp.) and LODs to volatile organic aldehyde gas by diverse nanostructured materials.

    Materials/Nanostructure

    Analyte/Concentration

    Gas Response (Rair/Rgas)

    Response/Recovery

    Temp.

    LOD

    Ref

    p-CuO/n-SnO2/core-shell nanowires

    Formaldehyde/50 ppm

    2.42

    52 s/80 s

    250 °C

    <1.5 ppm

    [246]

    ZnO/meso-structured nanowires (under UV)

    Formaldehyde/50 ppm

    1223%

    NA

    Room Temp.

    0.005 ppm

    [247]

    RGO coated Si/nanowires

    Formaldehyde/10 ppm

    6.4

    30 s/10 s

    300 °C

    0.035 ppm

    [248]

    Co doped In2O3/nanorods

    Formaldehyde/10 ppm

    23.2

    60 s/120 s

    130 °C

    1 ppm

    [249]

    Ag-functionalized Ni-doped In2O3/nanorods

    Formaldehyde/100 ppm

    123.97

    1.45 s/58.2 s

    160 °C

    <2.5 ppm

    [250]

    Ag doped LaFeO3/nanofibers

    Formaldehyde/5 ppm

    4.8

    2 s/4 s

    230 °C

    ~5 ppm

    [252]

    Co3O4-ZnO/core-shell nanofibers

    Formaldehyde/100 ppm

    >5

    6 s/9 s

    220 °C

    <10 ppm

    [253]

    WO3/ZnWO4/nanofibers

    Formaldehyde/5 ppm

    44.5

    12 s/14 s

    220 °C

    1 ppm

    [254]

    Pr-doped BiFeO3/hollow nanofibers

    Formaldehyde/50 ppm

    17.6

    17 s/19 s

    190 °C

    5 ppm

    [255]

    Ca doped In2O3/nanotubes

    Formaldehyde/100 ppm

    116

    1 s/328 s

    130 °C

    0.06 ppm

    [256]

    Er-doped SnO2/nanobelt

    Formaldehyde/100 ppm

    9

    17 s/25 s

    230 °C

    0.141 ppm

    [257]

    Pt-decorated MoO3/nanobelt

    Formaldehyde/100 ppm

    ~25%

    17.8 s/10.5 s

    Room Temp.

    1 ppm

    [258]

    ZnSnO3/multi-shelled nanocubes

    Formaldehyde/100 ppm

    37.2

    1 s/59 s

    220 °C

    <10 ppm

    [259]

    ZnSn(OH)6/multi-shelled nanocubes

    Formaldehyde/100 ppm

    56.6

    1 s/89 s

    60 °C

    1 ppm

    [260]

    MOF-derived Co3O4/CoFeO4/double-

    shelled nanocubes

    Formaldehyde/10 ppm

    12.7

    4 s/9 s

    139 °C

    0.3 ppm

    [261]

    WOx clusters decorated In2O3/nanosheets

    Formaldehyde/100 ppm

    ~25

    1 s/67 s

    170 °C

    0.1 ppm

    [262]

    SnO2/nanosheets

    Formaldehyde/200 ppm

    207.7

    30 s/57 s

    200 °C

    0.1 ppm

    [263]

    Au atom dispersed In2O3/nanosheets

    Formaldehyde/50 ppm

    85.67

    25 s/198 s

    100 °C

    0.00142 ppm

    [265]

    SnS2/nanoflakes film

    Formaldehyde/NA

    NA

    NA

    210 °C

    0.001/0.02 ppm

    [268]

    0D ZnS/nanospheres and nanoparticles

    Formaldehyde/100 ppm

    95.4 & 68.2

    11 s/8 s

    295 °C

    ~5 & 10 ppm

    [269]

    Ag doped Zn2SnO4/SnO2/hollow nanospheres

    Formaldehyde/50 ppm

    60

    9 s/5 s

    140 °C

    5 ppm

    [270]

    SnO2/nanoflowers (hierarchical)

    Formaldehyde/100 ppm

    34.6

    64 s/10 s

    300 °C

    5 ppm

    [272]

    Sn3O4/rGO/nanoflower (hetero- structure)

    Formaldehyde/100 ppm

    44

    4 s/125 s

    150 °C

    1 ppm

    [273]

    Au-loaded In2O3/porous hierarchical nanocubes

    Formaldehyde/100 ppm

    37

    3 s/8 s

    240 °C

    10 ppm

    [274]

    Ag-loaded ZnO/porous hierarchical nano- composite

    Formaldehyde/100 ppm

    170.42

    12 s/90 s

    240 °C

    1 ppm

    [274]

    Pd–WO3/m-CN/mesoporous nanocubes

    Formaldehyde/25 ppm

    24.2

    6.8 s/4.5 s

    120 °C

    1 ppm

    [276]

    GO/SnO2/2D mesoporous nanosheets

    Formaldehyde/100 ppm

    2275.7

    81.3 s/33.7 s

    60 °C

    0.25 ppm

    [277]

    ZnSnO3/2D mesoporous nanostructure

    Formaldehyde/100 ppm

    45.8

    3 s/6 s

    210 °C

    0.2 ppm

    [278]

    LaFeO3/porous hierarchical nanostructure

    Formaldehyde/50 ppm

    116

    7 s/24 s

    125 °C

    0.05 ppm

    [279]

    Bi doped Zn2SnO4/SnO2/porous nanospheres

    Formaldehyde/50 ppm

    23.2

    16 s/9 s

    180 °C

    10 ppm

    [280]

    ZnO/porous nanoplates

    Formaldehyde/100 ppm

    12

    80 s/60 s

    240 °C

    10 ppm

    [281]

    Au@ZnO/mesoporous nanoflowers

    Formaldehyde/100 ppm

    45.28

    NA

    220 °C

    NA

    [282]

    Zn2SnO4/SnO2/hierarchical octahedral nanostructure

    Formaldehyde/100 ppm

    >60

    76 s/139 s (for 20 ppm)

    200 °C

    2 ppm

    [284]

    SnO2 nanofiber/nanosheet/hierarchical nanostructure

    Formaldehyde/100 ppm

    57

    4.7 s/11.6 s

    120 °C

    ~0.5 ppm

    [286]

    In2O3@SnO2/hierarchical nano- composite

    Formaldehyde/100 ppm

    180.1

    3 s/3.6 s

    120 °C

    0.01 ppm

    [287]

    SnO2/cedar like hierarchical nano-micro structure

    Formaldehyde/100 ppm

    13.3

    <1 s/13 s

    200 °C

    ~5 ppm

    [288]

    In2O3/urchin like hollow nanostructure

    Formaldehyde/1 ppm

    20.9

    46 s/90 s

    140 °C

    0.05 ppm

    [289]

    SnO2/Butterfly like hierarchical nanostructure

    Acetaldehyde/100 ppm

    178.3

    28 s/58 s

    243 °C

    <0.5 ppm

    [290]

    SnO2/hollow hexagonal prisms

    Formaldehyde/2 ppm

    882

    19 s/NA

    120 °C

    <2 ppm

    [291]

    NiO/NiFe2O4/nano- tetrahedrons composite

    Formaldehyde/100 ppm

    22.5

    9 s/3 s

    240 °C

    0.2 ppm

    [292]

    VG/SnO2/nano- composite

    Formaldehyde/5 ppm

    >5

    46 s/95 s

    Room Temp.

    0.02 ppm

    [293]

    NA = Not available; Temp. = Temperature; s = seconds; in = minutes.

    6. Various Nanostructures in Volatile Organic Amines Detection

    The well-known toxic volatile organic amines detection by nanomaterials are highly anticipated. The mechanism follows the similar trend as proposed in other VOCs sensors. Upon interaction with anionic oxide species in the chamber, the volatile amines reacts and releases electrons resulting changes in resistance, which is adopted as sensor signals. A simple scheme represents the reaction mechanisms in triethylamine (TEA) detection is given in Equations (10)–(12). Similar to this mechanism, other volatile organic amine detections also follow the same sequence.

    In chamber O2 (ads) + e → O2 (ads)

    (10)

    In chamber O2 (ads) + e → 2O (ads)

    (11)

    Under TEA (C2H5)3N + O (ads) → N2 + CO2 + H2O + e

    (12)

    TEA detection was demonstrated by nanoparticle-based devices as explained below. Co3O4/ZnO hybrid NPs, Ho-doped SnO2 NPs, and CuCrO2 NPs (synthesized by hydrothermal or gas–liquid phase chemical method and annealing) were utilized in the sensing of TEA at 285 °C, 175 °C, and 140 °C, respectively, as shown in [296][297][298]. Wherein, Co3O4/ZnO hybrid NPs [296] shows a high response (Ra/Rg = 282.3 for 200 ppm at 285 °C; response/recovery time = 25 s/36 s) with a LOD of ~ 10 ppm. However, more investigations are needed to lower the working temperature. Subsequently, multi-metal functionalized tungsten oxide NWs (Ag/Pt/W18O49 NWs) and 1D SnO2 coated ZnO hybrid NWs were reported in the discrimination of TEA and n-Butylamine, correspondingly, at 240 °C. The Ag/Pt/W18O49 NWs [299] displayed a high sensor response to TEA (Ra/Rg = 813 for 50 ppm at 240 °C; response/recovery time = 15 s/35 s (for 2 ppm) with a LOD of 0.071 ppm, thereby is noted as a remarkable candidate. 1D SnO2 coated ZnO hybrid NWs (synthesized by solvothermal and calcination tactics) revealed sensitivity to n-Butylamine (Ra/Rg = 7.4 for 10 ppm at 240 °C; response/recovery time is 40 s/80 s) with an estimated LOD of 1 ppm, thereby can be included in n-Butylamine detection [300].

    Apart from NWs, nanorods were also engaged in the quantitation of volatile organic amines as detailed in the following. The sensing of Diethylamine (DEA), Trimethylamine (TMA), and TEA were demonstrated by V2O5-decorated α-Fe2O3 NRds, Au NPs decorated WO3 NRds, Ag NPs decorated α-MoO3 NRds, Cr doped α-MoO3 NRds, acidic α-MoO3 NRds, and NiCo2O4 microspheres assembled by hierarchical NRds as illustrated in Table 4 [301][302][303][304][305][306]. All these NRds were synthesized through electrospinning, calcination, wet-chemical reduction, thermal annealing, or by hydrothermal methods. Wherein, V2O5-decorated α-Fe2O3 NRds and Au NPs decorated WO3 NRds [293][294] demonstrated sensor utility towards DEA and TMA with the responses of 8.9 and 76.7 (response/recovery time = 2 s/40 s and 6 s/7 s, respectively) at 350 °C and 280 °C, respectively. The LODs of DEA and TMA are estimated as ~ 5 ppm, apart from the working temperature, it is worthy of continuing research. All other NRds [303][304][305][306] can detect the TEA at various operating temperatures (between 180 and 300 °C) as revealed in Table 4. Among them, Ag NPs decorated α-MoO3 NRds [295] show an exceptional sensor response to TEA (Ra/Rg = 408.6 for 100 ppm at 200 °C; response/recovery time = 3 s/107 s) with a LOD of 0.035 ppm. On the other hand, NiCo2O4 microspheres assembled by hierarchical NRds [306] has a low sensor response to TEA (Ra/Rg 1.5 for 50 ppm at 180 °C; response/recovery time = 49 s/54 s) with a LOD of 0.145 ppm, hence further investigations are mandatory on this material to improve the response.

    Towards enhanced sensing of TEA, Xu and co-workers fabricated the Au@SnO2/α-Fe2O3 core-shell nanoneedles directly on alumina tubes via pulsed laser deposition (PLD) and DC-sputtering methods [307]. As shown in Figure 18, Au@SnO2/α-Fe2O3 core-shell nanoneedles displayed a higher response to TEA (Ra/Rg = 39 for 100 ppm at 300 °C; response/recovery time = 4 s/203 s) than that of α-Fe2O3 nanoneedles and SnO2/α-Fe2O3 core-shell nanoneedles with a LOD of ~ 2 ppm. However, more interrogations are required to increase the response and to reduce the operating temperature.

    Figure 18. (i) (a) Gas sensor of α-Fe2O3 nanosheets fixed on an electronic bracket (b) Al2O3 cube covered with a film of sensing materials; (c) SEM image of SnO2 nanosheet in cross-sectional view; (d) SEM image of α-Fe2O3 nanoneedles (FNs) directly grown on Al2O3 tubes; (e) SEM images of SFNs; (f,g) SEM images of Au@SnO2/α-Fe2O3 nanoneedles (ASFNS) after implantation of SnO2 shell and Au nanoparticles; (h) EDS spectrum of ASFNs; (ii) The selectivity of different gases with same concentration at 300 °C (Reproduced with the permission from Ref. [307]).

    By means of calcination and electrospinning, nanofibers (Al2O3/α-Fe2O3 nanofibers and In2O3 hierarchical nanofibers with in situ growth of octahedron particles) were proposed by the researchers for TEA sensing studies [308][309]. As depicted in Figure 19, the Al2O3/α-Fe2O3 nanofibers [308] evidence the sensor signal even at 0.5 ppm. The sensor response to TEA is 15.19 (for 100 ppm) at 250 °C (response/recovery time = 1 s/17 s), but it is important to further reduce the working temperature in this report. In contrast, In2O3 hierarchical nanofibers with in situ growth of octahedron particles [309] revealed the highest response to TEA at 40 °C (Ra/Rg = 87.8 for 50 ppm; response/recovery time = 148 s/40 min) with a LOD of ~5 ppm, thereby is noted as an excellent work. Subsequently, TiO2 membrane NTs and sidewall modified single-walled carbon nanotubes (SWCNTs) were employed in the detection of TMA vapor [310][311]. The flexible TiO2 membrane NTs showed a response of 40 for 400 ppm of TMA, but details regarding the working temperature and response/recovery time were not clear. Similarly, sidewall modified SWCNTs were demonstrated with selectivity to both ammonia and TMA at room temperature, thereby cannot be stated as a successful work. In view of this, Galstyan et al. described the DMA sensing utility of Nb doped TiO2 nanotubes at 300 °C as illustrated in Table 4 [312]. This work requires additional interrogations to attain a high response at low temperature.

    Figure 19. (i) FESEM images of S1 (a), S2 (b), S3 (c), S4 (d), high-magnification SEM images of S1 (e) and S3 (f); (Here S1–S4 represents Al2O3/α-Fe2O3 nanofibers); (ii) (a) Gas responses of the S3 sensor as a function of trimethylamine (TEA) concentrations at 250 °C. (b) Dynamic response-recovery curves of the sensor S3 to different concentrations of TEA at the operating temperature (Reproduced with the permission from Ref. [308]).

    Nanobelts such as Au NPs decorated MoO3 nanobelts, W doped MoO3 nanobelts, RuO2 NPs decorated MoO3 nanobelts, and ZnO-SnO2 nanobelts were developed by hydrothermal, soaking, and two step synthesis, etc., and applied in the discrimination of TMA and TEA [313][314][315][316]. Doping and decoration are the two important steps to enhance the senor signals. As shown in Table 4, these nanobelts, except the RuO2 NPs decorated MoO3 nanobelts, used in the TMA and TEA sensing interrogations require more efforts to enhance the response and to minimize the operating temperature. The RuO2 NPs decorated MoO3 nanobelts evidenced a good response (Ra/Rg = 75 for 10 ppm at 260 °C; response/recovery time = 2 s/10 s) with a LOD of ~1 ppm, but the working temperature still needs further optimization. Towards TEA detection, Zhang and co-workers described the utilization of hydrothermally synthesized In2O3 nanocubes [317], which showed a high response (Ra/Rg = 175 for 100 ppm at 260 °C; response/recovery time = 11 s/14 s) with a LOD of ~ 10 ppm, thereby is noted as an encouraging research.

    As an important candidate in the TEA sensory studies, nanosheets were intensively studied by many research groups. WO3 NShs, Au@ZnO-SnO2 NShs, TiO2 NPs decorated CuO NShs, and Rh-SnO2 NShs were synthesized by precipitation, PLD, water bath treatment and solution etching methods, and surface impregnation precipitation and heat treatment method, respectively, and applied in TEA detection [318][319][320][321]. Among them, WO3 NShs operate at room temperature and show a sensor response of ~ 14 for 1000 ppm as shown in Table 4. Though this work is an impressive one, but the sensor response is not sufficiently high. On the other hand, Rh-SnO2 NShs displayed a great response to TEA (Ra/Rg = 607.2 for 100 ppm at 325 °C; response/recovery time = 49 s/24 s) with a LOD of ~ 1 ppm, but further investigation is necessary to lower the working temperature. In light of this, Yan et al. reported the hydrothermally synthesized Ag modified Zn2SnO4 hexagonal nanoflakes-hollow octahedron for the enhanced sensing of TEA [322]. This material performed remarkably in terms of the sensor response (Ra/Rg = 83.6 for 50 ppm at 220 °C; response/recovery time 1s/24 s) with a LOD of ~ 1 ppm. Subsequently, Zn2SnO4-doped SnO2 hollow NSPs and CeO2-SnO2 nanoflowers (by Hydrothermal synthesize) were engaged in the sensing investigations of volatile organic amines [323][324]. Wherein, Zn2SnO4-doped SnO2 hollow NSPs [323] is an exceptional material, which detected the phenylamine (Ra/Rg = 4.53 for 50 ppm at 300 °C; response/recovery time = 10 s/4 s) with a LOD of ~ 1 ppm. On the other hand, CeO2-SnO2 nanoflowers [324] can be noted as an alternative in the TEA sensing at 310 °C.

    Similar to diverse nanostructures, materials with porosity were vastly used in the detection of volatile organic amines as described below. WO3-SnO2 mesoporous nanostructures, CuO porous particles with diverse morphologies, In2O3 mesoporous nanocubes, CeO2 porous nanospheres, Au decahedrons-decorated α-Fe2O3 porous nanorods, ZnCo2O4 porous nanostructures, NiCo2O4 porous nanoplates, SnO2 porous thin films, Fe2O3/ZnFe2O4 porous nanocomposite, and Au-Modified ZnO porous hierarchical nanosheets were reported for the quantification of TMA or TEA as summarized in Table 4 [325][326][327][328][329][330][331][332][333][334]. Hydrothermal, solvothermal, calcination, impregnation, template methods, wet-chemical methods, etc, were used to synthesize these porous nanomaterials. Porosity scaled from nano to micro can enhance the capture of volatile amines. A schematic of WO3-SnO2 mesoporous nanostructure formation and its utilization in sensors is shown in Figure 20 [335].

    Among them, In2O3 mesoporous nanocubes and Au-Modified ZnO porous hierarchical nanosheets were engaged in the sensing of TMA with decent LODs [327][334]. Similarly, CeO2 porous nanospheres and SnO2 porous thin films were reported with remarkable performance in sensing of TEA at room temperature [328][332]. In particular, SnO2 porous thin films [332] revealed a high sensitivity to TEA (Ra/Rg = 150.5 for 10 ppm at RT; response/recovery time = 53 s/120 s) with a LOD of 0.11 ppm, thereby can be stated as excellent material towards commercialization.

    Figure 20. Schematic representation for nanocasting synthesis of mesoporous WO3–SnO2 by hard templating of SBA-15 (Reproduced with the permission from Ref. [335]).

    Like other nanostructures, various hierarchical nanostructures were discussed in volatile amines detection. In light of this, α-Fe2O3 snowflake-like hierarchical nanostructure, Zn2SnO4–ZnO hierarchical nanocomposite, MoS2/GO 3D hierarchical nanocomposite, Au NPs decorated Co3O4 hierarchical nanochains, and WO3 hierarchical flower like spheres were developed and employed in TEA detection [335][336][337][338][339]. These materials were synthesized via the combination of solvothermal, annealing, hydrothermal, calcination, template route, and precipitation methods. These hierarchical nanostructures operate between 205 and 260 °C to enhance sensor responses to TEA as noted in Table 4. For example, Au NPs decorated Co3O4 hierarchical nanochains displayed a sensor response to TEA even at 10 ppm as shown in Figure 21 [338]. However, among the aforementioned hierarchical nanomaterials, Zn2SnO4–ZnO hierarchical nanocomposite [336] revealed a high response to TEA (Ra/Rg = 175.5 for 100 ppm at 200 °C; response/recovery time = 12 s/25 s) with a LOD of 0.4 ppm, thereby can be noted as an inspiring research in TEA sensors.

    Apart from the utilization of distinct nano-architectures, numerous diversified nano-shaped materials were employed in the quantitation of volatile organic amines. In view of this, ZnO/Au hemishperical nanostructure, SnO2/Au/Fe2O3 nanoboxes, Au decorated ZnO nest-like nanostructure, Pd doped ZnO agaric like nanostructure, Co3O4@MnO2 shish-kebab like nanostructure, and Au@ZnO core-shell nanostructure were demonstrated in TEA and aniline detection as noted in Table 4 [340][341][342][343][344][345]. In particular, Pd-doped ZnO agaric like nanostructure [343] was used in aniline sensing with a high response (Ra/Rg = 182 for 100 ppm at 280 °C; response/recovery time = 29 s/23 s) with a LOD of 0.5 ppm, thereby is noted as a distinct research. Among the other materials in TEA discrimination, Au@ZnO core-shell nanostructure [345] is an exceptional material with excellent sensor performance at 50 °C (Ra/Rg = 12.2% for 5 ppm; response/recovery time is 27 s/46 s) and a LOD of ~ 1 ppm. In parallel to diverse nanostructures, composite materials without any structural modifications were also utilized in TEA and TMA sensors. Materials such as ZnO/ZnFe2O4 composites, Au/Co3O4/W18O49 hollow composite nanospheres, α-Fe2O3@α-MoO3 composite, CuO/ZnO 3D diamond shaped MOF, rGO decorated W-doped BiVO4 hierarchical nanocomposite, and Au@MoS2 nanocomposite were effectively consumed in TEA or TMA detection [346][347][348][349][350][351]. Herein, ZnO/ZnFe2O4 composite displayed a good sensitivity to TEA under irradiation (Ra/Rg 9 for 500 ppm at 80 °C) and requires additional interrogations to enhance the response [346]. Similarly, p-n CuO/ZnO 3D diamond shaped MOF composite authenticated the selectivity to TEA and methanol at 220 °C and 260 °C, respectively [349]. However, this composite showed a better response to TEA (Ra/Rg ≥ 400 for 500 ppm; response/recovery time = 11 s/~ 60 min) than that of methanol with a LOD of 0.175 ppm. However, this research still needs further work. Compared to the other composite materials, Au@MoS2 nanocomposite is a unique material with its sensory response to TEA at 30 °C (Ra/Rg = 44 for 50 ppm; response/recovery time = 9 s/91 s) and a LOD of ~ 2 ppm as noted in Table 4 [351].

    Figure 21. (i) TEM and HRTEM images of the prepared (a,b) pure and (c,d) Au/Co3O4 nanochains (Au/Co3O4-2) (ii) (a) Dynamic response-recover curves of the nanochain sensors towards various concentrations of TEA and their concentration-dependent responses within the TEA concentration range of (b) 10e500 ppm and (c)10–200 ppm; (d) the response and recovery curves of the sensors towards 300 ppm TEA (Reproduced with the permission from Ref. [338]).

    Table 4. Detection concentration, response/recovery time, operating temperature (Temp.) and LODs to volatile organic amine gas by diverse nanostructured materials.

    Materials/Nanostructure

    Analyte/Concentration

    Gas Response (Rair/Rgas)

    Response/Recovery

    Temp.

    LOD

    Ref

    Co3O4/ZnO/hybrid nanoparticles

    Triethylamine/200 ppm

    282.3

    25 s/36 s

    280 °C

    ~10 ppm

    [296]

    Ho-doped SnO2/nanoparticles

    Triethylamine/50 ppm

    12

    2 s/2 min

    175 °C

    ~5 ppm

    [297]

    CuCrO2/nanoparticles

    Triethylamine/100 ppm

    ~5

    90 s/120 s

    140 °C

    ~10 ppm

    [298]

    Ag/Pt/W18O49/nanowires

    Triethylamine/50 ppm

    813

    15 s/35 s (for 2 ppm)

    240 °C

    0.071 ppm

    [299]

    1D SnO2 coated ZnO/hybrid nanowires

    n-Butylamine/10 ppm

    7.4

    40 s/80 s

    240 °C

    ~1 ppm

    [300]

    V2O5 -decorated α-Fe2O3/nanorods

    Diethylamine/100 ppm

    8.9

    2 s/40 s

    350 °C

    ~5 ppm

    [301]

    Au NPs decorated WO3/nanorods

    Trimethyl-amine/100 ppm

    76.7

    6 s/7 s

    280 °C

    ~5 ppm

    [302]

    Ag NPs decorated α-MoO3/nanorods

    Triethylamine/100 ppm

    408.6

    3 s/107 s

    200 °C

    0.035 ppm

    [303]

    Cr dopedα-MoO3/nanorods

    Triethylamine/100 ppm

    150.25

    7 s/80 s

    200 °C

    ~1 ppm

    [304]

    Acidic α-MoO3/nanorods

    Triethylamine/100 ppm

    101.74

    4 s/88 s

    300 °C

    0.1 ppm

    [305]

    Au@SnO2/α-Fe2O3/core-shell nanoneedles on alumina tubes

    Triethylamine/100 ppm

    39

    4 s/203 s

    300 °C

    ~2 ppm

    [307]

    Al2O3/α-Fe2O3/nanofibers

    Triethylamine/100 ppm

    15.19

    1 s/17 s

    250 °C

    ~0.5 ppm

    [308]

    In2O3/hierarchical nanofibers (with nanoparticles)

    Triethylamine/50 ppm

    87.8

    148 s/40 min

    40 °C

    ~5 ppm

    [309]

    Nb doped TiO2/nanotubes

    Dimethyl-amine/50 ppm

    9.1

    ≥300 s (for both)

    300 °C

    ~5 ppm

    [312]

    Au NPs decorated MoO3/nanobelts

    Trimethyl-amine/50 ppm

    70

    6 s/9 s

    280 °C

    ~5 ppm

    [313]

    W doped MoO3/nanobelts

    Trimethyl-amine/50 ppm

    13.8

    6 s/11 s

    280 °C

    ~5 ppm

    [314]

    RuO2 NPs decorated MoO3/nanobelts

    Triethylamine/10 ppm

    75

    2 s/10 s

    260 °C

    ~1 ppm

    [315]

    ZnO-SnO2/nanobelts

    Triethylamine/100 ppm

    9.9

    1.8 s/18 s

    220 °C

    ~1 ppm

    [316]

    In2O3/nanocubes

    Triethylamine/100 ppm

    175

    11 s/14 s

    180 °C

    ~10 ppm

    [317]

    WO3/nanosheets

    Triethylamine/1000 ppm

    ̴14

    NA

    Room Temp.

    ~5 ppm

    [318]

    Au@ZnO- SnO2/nanosheets

    Triethylamine/100 ppm

    115

    7 s/30 s

    300 °C

    ~2 ppm

    [319]

    TiO2 NPs decorated CuO/nanosheets

    Triethylamine/5 ppm

    12.7

    45 s/202 s

    160 °C

    0.5 ppm

    [320]

    Rh-SnO2/nanosheets

    Triethylamine/100 ppm

    607.2

    49 s/24 s

    325 °C

    ~1 ppm

    [321]

    Ag modified Zn2SnO4/hexagonal nanoflakes- hollow octahedron

    Triethylamine/50 ppm

    83.6

    <1 s/20 s

    220 °C

    ~1 ppm

    [322]

    Zn2SnO4- doped SnO2/hollow nanospheres

    Phenylamine/50 ppm

    4.53

    10 s/12 s

    300 °C

    ~1 ppm

    [323]

    CeO2-SnO2/nanoflowers

    Triethylamine/200 ppm

    252.2

    NA

    310 °C

    ~20 ppm

    [324]

    WO3-SnO2/mesoporous nanostructure

    Triethylamine/50 ppm

    87

    6 s/7 s

    220 °C

    ~1 ppm

    [325]

    CuO/porous particles with diverse morphologies

    Triethylamine/100 ppm

    5.6–102

    <40 s/<260 s

    230 °C

    ~5 ppm

    [326]

    In2O3/mesoporous nanocubes

    Trimethyl-amine/10 ppm

    57

    4 s/11 s

    160 °C

    ~5 ppm

    [327]

    CeO2/porous nanospheres

    Triethylamine/100 ppm

    4.67

    13 s/<230 s

    Room Temp.

    ~5 ppm

    [328]

    Au decahedrons-decorated α-Fe2O3/porous nanorods

    Triethylamine/50 ppm

    17

    12 s/18 s

    40 °C

    1 ppm

    [329]

    ZnCo2O4/porous nano- structures

    Triethylamine/100 ppm

    14

    7 s/57 s

    200 °C

    ~5 ppm

    [330]

    NiCo2O4/porous nanoplates

    Triethylamine/10 ppm

    2.58

    <33 s/42 s

    220 °C

    0.5 ppm

    [331]

    SnO2/porous thin films

    Triethylamine/10 ppm

    150.5

    53 s/120 s

    Room Temp.

    0.11 ppm

    [332]

    Fe2O3/ZnFe2O4/porous nano- composite

    Triethylamine/20 ppm

    60.24

    2 s/7 s

    300 °C

    0.2 ppm

    [333]

    Au-Modified ZnO/porous hierarchical nanosheets

    Trimethyl-amine/30 ppm

    65.8

    3.3 s/64 s

    260 °C

    0.01 ppm

    [334]

    α-Fe2O3/snowflake-like hierarchical nanostructure

    Trimethyl-amine/100 ppm

    10.9

    0.9 s/1.5 s

    260 °C

    ~5 ppm

    [335]

    Zn2SnO4–ZnO/hierarchical nano- composite

    Triethylamine/100 ppm

    175.5

    12 s/25 s

    200 °C

    0.4 ppm

    [336]

    MoS2/GO/3D hierarchical nano- composite

    Triethylamine/100 ppm

    192

    20 s/18 s

    260 °C

    1 ppm

    [337]

    Au NPs decorated Co3O4/hierarchical nanochains

    Triethylamine/300 ppm

    >40

    94 s/100 s

    210 °C

    ~10 ppm

    [338]

    WO3/hierarchical flower like spheres

    Triethylamine/10 ppm

    11.6

    3 s/55 s

    205 °C

    0.083 ppm

    [339]

    ZnO/Au/hemishperical nanostructure

    Triethylamine/100 ppm

    104.8

    5 s/2 s

    260 °C

    ~10 ppm

    [340]

    SnO2/Au/Fe2O3/nanoboxes

    Triethylamine/100 ppm

    126.84

    7 s/10 s

    240 °C

    0.05 ppm

    [341]

    Au decorated ZnO/nest-like nanostructure

    Triethylamine/200 ppm

    625

    4 s/26 s

    320 °C

    1 ppm

    [342]

    Pd doped ZnO/agaric like nanostructure

    Aniline/100 ppm

    182

    29 s/23 s

    280 °C

    0.5 ppm

    [343]

    Co3O4@MnO2/shish-kebab like nanostructure

    Triethylamine/100 ppm

    9.13

    93 s/92 s

    250 °C

    ~10 ppm

    [344]

    Au@ZnO/core-shell nanostructure

    Triethylamine/5 ppm

    12.2%

    27 s/46 s

    50 °C

    ~1 ppm

    [345]

    Au/Co3O4/W18O49/hollow composite nanospheres

    Triethylamine/2 ppm

    16.7

    9 s/14 s

    270 °C

    0.081 ppm

    [347]

    α-Fe2O3@α- MoO3/nano- composite

    Triethylamine/50 ppm

    76

    4 s/370 s

    280 °C

    ~2 ppm

    [348]

    rGO decorated W doped BiVO4/hierarchical nano- composite

    Trimethyl-amine/20 ppm

    12.8

    16 s/NA

    135 °C

    0.63 ppm

    [350]

    Au@MoS2/nano- composite

    Triethylamine/50 ppm

    44

    9 s/91 s

    30 °C

    ~2 ppm

    [351]

    NA = Not available; Temp. = Temperature; s = seconds; in = minutes.

    7. Volatile Hydrocarbons Detection by Distinct Nanostructures

    Following the similar mechanism mentioned earlier in acetone and alcohols discrimination, hydrocarbons sensors were reported using various nanostructured materials. Nanoparticles were oftenly investigated by many researchers. Chen et al. reported Ag-LaFeO3 NPs developed via two different tactics like MIP and lotus-leaf templated synthesis and engaged in xylene detection at 99 °C and 125 °C, correspondingly [352][353]. As noted in Table 5, Ag-LaFeO3 NPs synthesized via MIP tactic seems to be good in terms of sensor response (Ra/Rg = 36.2 for 5 ppm at 99 °C; response/recovery time = 114 s/55 s) with a LOD of <1 ppm, thereby Ag-LaFeO3 NPs can be attested as an inspiring candidate in xylene sensors. Similar to above studies, Au loaded ZnO NPs and cobalt porphyrin (CoPP)-functionalized TiO2 NPs were described for the quantitation of xylene and BTX (Benzene, Toluene and Xylene) vapors at 377 °C and 240 °C, respectively [354][355]. Wherein, cobalt porphyrin (CoPP)-functionalized TiO2 NPs showed a high response (Ra/Rg 5 for 10 ppm at 240 °C; response/recovery time = 40 s/80 s) with a LOD of 0.005 ppm [355]. However, further studies are needed to minimize the operating temperatures in both cases. Other than NPs, Park and co-workers described the utility of In-doped ZnO Quantum dots (QDs) towards the sensing of acetylene gas at 400 °C with the LOD down to sub-ppm level (0.1 ppm) [356]. However, the working temperature in this report was high, which requires more work. In view of this, Xu et al. exploited the BTEX (Benzene, Toluene, Ethyl benzene, and Xylene) detection by MOF derived nanocrystals at room temperature [357]. However, anti-interference studies with this material is still in question, thereby cannot be attested as excellent work.

    To discriminate the volatile toluene, α-Fe2O3/SnO2 NW arrays and Pt NPs sensitized Si NW-TeO2 NWs (opted from ultrasonic spray pyrolysis, hydrothermal, and sputtering tactics) were proposed with sensor responses >40 at 90 °C and 200 °C, respectively, as noted in Table 4 [358][359]. Such NWs-based sensory research is noted as an innovative one. Pd NPs decorated TiO2 NRds were employed in the sensing of liquefied petroleum gas (LPG), but further interrogations are required for authentication [360]. Following above work, Lee et al. fabricated the cobalt porphyrin (CoPP)-ZNO NRds towards the detection of Toluene (Ra/Rg = 3.3 for 10 ppm) and estimated a LOD of 0.002 ppm [361]. Moreover, this material can operates in the 0 to 85% humid conditions, thereby attest as a nice work. Qin and co-workers described the Y-doped or undoped α-MoO3 nanoarrays towards the enhanced sensing of Xylene at 370 °C as noted in Table 5 [362][363]. As shown in Figure 22, authors conformed the 1% Y doping over α-MoO3 nanoarrays displayed an improved sensor response than that of other doping concentrations or non-doping (0, 3%, and 5%). However, both works do not merit the commercialization due to the higher operating temperature.

    Nanofibers, such as MOF-driven metal-embedded metal oxide (Pd@ZnO-WO3) nanofibers, V2O5 nanofibers, and Pd functionalized SnO2 nanofibers were engaged in the determination of toluene, xylene, and butane, correspondingly [364][365][366]. Pd@ZnO-WO3 nanofibers showed a high response to toluene (Ra/Rg = 22.22 for 1 ppm at 350 °C; response/recovery time 20 s/not available) with a LOD of 0.1 ppm, thereby become an impressive candidate apart from the working temperature [356]. Subsequently, V2O5 nanofibers were noted as highly remarkable material to be employed in xylene detection at room temperature (Ra/Rg = 191 for 500 ppm at RT; response/recovery time = 80 s/50 s (for 100 ppm)) with an estimated LOD of ̴ 5 ppm [367]. Similar to above research, Pd functionalized SnO2 nanofibers [366] used in the discrimination of butane at 260 °C (see Table 5), but its sensor response needs to be improved with further investigations. As an inclusion to the VOC sensory research, nanotubes (Pt-decorated CNTs, 3D TiO2/G-CNTs and hierarchical NiCo2O4 NTs) were demonstrated towards toluene or xylene discrimination [367–369]. As seen in Table 5, TiO2 NPs decorated G-CNTs seems to be an excellent candidate towards toluene detection at room temperature with a calculated LOD of 0.4 ppm [368].

    Figure 22. (a,b) The SEM and TEM images of nanoarrays; (c) Response curve of samples to 100 ppm different Gases at the working temperature of 370 °C; (d) Response curve of samples to 100 ppm xylene at the different working temperature (Reproduced with the permission from Ref. [363]).

    Nanobelts (Fe doped MoO3 nanobelts and Au decorated ZnO/In2O3 belt-tooth nanostructure), nanocages (ZnO/ZnCo2O4 hollow nanocages), nanosheets (Au functionalizedWO3·H2O NShs, porous h-BN 3D NShs, and Nb-doped NiO NShs), nanoflakes (CdO hexagonal nanoflakes and ZnO-CeO2 triangular nanoflakes), and nanospheres (ZnFe2O4 NSPs and Pt doped CoCr2O4 hollow NSPs) were demonstrated to volatile hydrocarbons quantitation by the researchers as noted in Table 5 [369][370][371][372][373][374][375][376][377][378]. Among them, Nb-doped NiO NShs and CoCr2O4 hollow NSPs [374][378] are highly notable with their sensor responses to xylene vapor (Ra/Rg = 335.1 for 100 ppm at 370 °C and Ra/Rg = 559 for 5 ppm at 275 °C, respectively) with LODs down to sub-ppm level (0.002 ppm and 0.0187 ppm, correspondingly). In light of this, involvement of porous nanostructures is anticipated due to the high adsorption nature through the pores [373]. Yao et al. discussed the pore-effect of Pd-SnO2 nanoporous composite [379] for the capture and sensing of methane gas as illustrated in Figure 23. The material shows reasonable sensor response (see Table 5) in methane sensory research. In view of this, p-n Co3O4–TiO2 mesoporous hierarchical nanostructures were elaborated in the sensing of xylene at an operating temperature of 115 °C [380]. In which, the response is well enough (Ra/Rg = 113 for 50 ppm; response/recovery time = 130 s/150 s) due to the pore-effect of the material. In parallel to utilization of distinct nanostructures in volatile hydrocarbons discriminations, many hierarchical nanomaterials were also employed as described below. Hierarchical nanostructures like Au loaded MoO3 hollow NSPs, Pt-SnO2 hollow NSPs, NiO/NiMoO4 NSPs, Co3O4, and WO3 were demonstrated in the sensing of toluene, xylene, methane, or acetylene at various temperature (180–340 °C) as noted in Table 5 [381][382][383][384][385]. Among them, hierarchical nanostructure of Co3O4 displayed a sensor response to toluene at 180 °C even after one month, thereby is noted as a reliable system [384]. In this work, authors described the sensory performance of cube shaped Co3O4 (C-Co3O4), rod shaped Co3O4 (R- Co3O4), and sheet shaped (S- Co3O4) nanostructures synthesized via hydrothermal tactic. Wherein, sheet shaped (S- Co3O4) hierarchical nanostructure showed a higher response than that of others as shown in Figure 24.

    Figure 23. (a,b) Schematic diagram of the sensing reaction mechanism of the Pd-SnO2 composite nanoporous structure (Reproduced with the permission from Ref. [379]).

    Figure 24. Resistance curves of the sensors based on (a) Co3O4-C, (b) Co3O4-R, (c) Co3O4-S towards 200 ppm toluene at 180 °C, respectively; (d) responses of three sensors towards 200 ppm toluene at different working temperatures; (e) responses of three sensors to 200 ppm different target gases at 180 °C; (f) dynamic resistance curve of the sensor based on Co3O4-S to different concentration of toluene; (g) relationship between response and toluene concentration; (h) response and morphology stability of Co3O4-S-based sensor to 200 ppm toluene during 30 days (measurement number = 3); (i) schematic of sensor exposed to air and target gas (Reproduced with the permission from Ref. [384]).

    In addition, PbS NPs decorated CdO necklace like nanobeads were discussed in the quantitation of LPG gas at RT by Sonawane and co-workers [386]. Nevertheless, further investigation is necessary to authenticate the material’s reliability. In light of this, Au loaded TiO2 hedgehog-like nanostructure was proposed to detect xylene at 375 °C, thereby can be included as an additional candidate [387]. These materials were synthesized by hydrothermal method followed by simple isometric impregnation route. They are required to be optimized to minimize the working temperature during the sensor studies.

    Similar to microstructure-based volatile hydrocarbon vapors quantification (example: Sn2+ doped NiO microspheres in Xylene detection with sub-ppm LOD) [388], nanocomposites, such as Pd/PdO/S-SnO2 nanocomposite film, rGO/Co3O4 nanocomposite, WO3 decorated TiO2 NPs nanocomposite, BGQD/Ag–LaFeO3 nanocomposite, Ag/Bi2O3 nanocomposite, AgO loaded LaFeO3 nanocomposite, CuO NPs-Ti3C2Tx MXene nanocomposite, and Graphene/SnO2 NPs nanocomposite were effectively applied in the detection of hydrocarbons as detailed in Table 5 [389][390][391][392][393][394][395][396]. Wherein, Ag/Bi2O3 nanocomposite and Graphene/SnO2 NPs nanocomposite were found to perform remarkably in terms of their operation temperature (at room temperature) [393][396]. Recently, Luo et al. described the sensitivity of a polymer-SWCNTs composite toward BTX with an exceptional LOD of 5 ppm [398]. However, this work also depends on the polymer properties and requires more detailed investigations.

    Detection concentration, response/recovery time, operating temperature (Temp.), and LODs to volatile hydrocarbons gases by diverse nanostructured materials.

    Materials/Nanostructure

    Analyte/Concentration

    Gas Response (Rair/Rgas)

    Response/Recovery

    Temp.

    LOD

    Ref

    Ag-LaFeO3/nanoparticles

    Xylene/5 ppm

    36.2

    114 s/55 s

    99 °C

    <1 ppm

    [352]

    Ag-LaFeO3/nanoparticles

    Xylene/10 ppm

    16.76

    68 s/36 s

    125 °C

    0.2 ppm

    [353]

    Au-ZnO/nanoparticles

    Xylene/100 ppm

    92

    4 s/6 min

    377 °C

    NA

    [354]

    cobalt porphyrin (CoPP)-functionalized TiO2/nanoparticles

    Benzene, Toluene and Xylene (BTX)/10 ppm

    >5

    40 s/80 s

    240 °C

    0.005 ppm

    [355]

    In-doped ZnO/Quantum dots

    Acetylene/10 ppm

    19.3

    ~100 s/NA

    400 °C

    0.1 ppm

    [356]

    Metal organic Frameworks/nanocrystals

    Benzene, Toluene, Ethyl benzene and Xylene (BTEX)/50 ppm

    >20

    NA

    Room Temp.

    0.4 ppm

    [357]

    α-Fe2O3/SnO2/nanowire arrays

    Toluene/100 ppm

    49.7

    20 s/15 s

    90 °C

    ~50 ppm

    [358]

    Pt NPs sensitizedSi NW-TeO2/nanowires

    Toluene/50 ppm

    45

    20 s/500 s

    200 °C

    ~10 ppm

    [359]

    CoPP-ZnO/nanorods

    Toluene/10 ppm

    >2.5

    NA

    NA

    0.002 ppm

    [361]

    α-MoO3/nanoarrays

    Xylene/100 ppm

    19.2

    1 s/≤20 s

    370 °C

    ~10 ppm

    [362]

    Y doped α-MoO3/nanoarrays

    Xylene/100 ppm

    28.3

    1 s/~15 s

    370 °C

    ~5 ppm

    [363]

    MOF-driven metal- embedded metal oxide (Pd@ZnO- WO3)/nanofibers

    Toluene/1 ppm

    22.22

    <20 s/NA

    350 °C

    0.1 ppm

    [364]

    V2O5/nanofibers

    Xylene/500 ppm

    191

    80 s/50 s (100 ppm)

    Room Temp.

    ~5 ppm

    [365]

    Pd functionalized SnO2/nanofibers

    Butane/3000 ppm

    47.58

    3.20 s/6.28 s

    260 °C

    ~10 ppm

    [366]

    Pt-decorated CNTs/nanotubes

    Toluene/5 ppm

    5.06

    90 s/520 s

    150 °C

    ~1 ppm

    [367]

    3D TiO2/G-CNT/nanotubes

    Toluene/500 ppm

    42.9%

    9–11 s (for both)

    Room Temp.

    0.4 ppm

    [368]

    NiCo2O4/nanotubes (hierarchical)

    Xylene/100 ppm

    9.25

    20 s/9 s

    220 °C

    ~1 ppm

    [397]

    Fe doped MoO3/nanobelts

    Xylene/100 ppm

    6.1

    20 s/75 s

    206 °C

    ~5 ppm

    [369]

    Au decorated ZnO/In2O3/belt-tooth nanostructure

    Acetylene/100 ppm

    5

    8.5 s/NA

    90 °C

    ~25 ppm

    [370]

    ZnO/ZnCo2O4/hollow nanocages

    Xylene/100 ppm

    34.26

    NA

    320 °C

    0.126 ppm

    [371]

    Au functionalizedWO3·H2O/nanosheets

    Toluene/100 ppm

    50

    2 s/9 s

    300 °C

    ~10 ppm

    [372]

    Nb-doped NiO/nanosheets

    Xylene/100 ppm

    335.1

    63 s/66 s

    370 °C

    0.002 ppm

    [374]

    CdO/hexagonal nanoflakes

    liquefied petroleum gas (LPG)/500 ppm

    ~27.5

    8.6 s/10 s

    270 °C

    ~10 ppm

    [375]

    ZnO-CeO2/triangular nanoflakes

    BTEX/50 ppm

    10–21

    8 s/10 s

    200 °C

    0.01 ppm

    [376]

    ZnFe2O4/nanospheres

    Toluene/100 ppm

    9.98

    18.14 s/29.2 s

    300 °C

    ~1 ppm

    [377]

    Pt doped CoCr2O4/hollow nanospheres

    Xylene/5 ppm

    559

    300 s/600 s

    275 °C

    0.0187 ppm

    [378]

    Pd-SnO2/nanoporous composite

    Methane/3000 ppm

    17.6

    3 s/5 s

    340 °C

    ~100 ppm

    [379]

    Co3O4–TiO2/mesoporous hierarchical nanostructure

    Xylene/50 ppm

    113

    130 s/150 s

    115 °C

    ~5 ppm

    [380]

    Au loaded MoO3/hollow nanospheres (hierarchical)

    Toluene and Xylene/100 ppm

    17.5 & 22.1

    19 s/6 s & 1.6 s/2 s

    250 °C

    0.1 & 0.5 ppm

    [381]

    Pt-SnO2/hollow nanospheres (hierarchical)

    Methane/250 ppm

    4.88 & 4.33

    NA

    300 °C & 340 °C

    ~25 ppm

    [382]

    NiO/NiMoO4/hierarchical nanospheres

    p-Xylene/5 ppm

    101.5

    10-50 s/20-200 s

    375 °C

    0.02 ppm

    [398]

    Co3O4/hierarchical nanostructure

    Toluene/200 ppm

    8.5

    10 s/30 s

    180 °C

    ~5 ppm

    [384]

    WO3/hierarchical nanostructure

    Acetylene/200 ppm

    32.31

    12 s/17 s

    275 °C

    <5 ppm

    [385]

    PbS NPs decorated CdO/necklace like nanobeads

    LPG/1176 ppm

    ~50

    148 s/142 s

    Room Temp.

    ~300 ppm

    [386]

    Au loaded TiO2/hedgehog-like nanostructure

    Xylene/100 ppm

    6.49

    5 s/2 s

    375 °C

    ~2 ppm

    [387]

    Pd/PdO/S-SnO2/nano- composite film

    methane/300 ppm

    12.3

    8 s/12 s

    240 °C

    <50 ppm

    [389]

    rGO/Co3O4/nano- composite

    Toluene/5 ppm

    11.3

    NA

    110 °C

    ≥0.5ppm

    [390]

    WO3 decorated TiO2 NPs/nano- composite

    Xylene/10 ppm

    92.53

    410 s/2563 s

    160 °C

    1 ppm

    [391]

    BGQD/Ag–LaFeO3/nano- composite

    Benzene/1 ppm

    17.5

    NA

    65 °C

    <1 ppm

    [392]

    Ag/Bi2O3/nano- composite

    Toluene/50 ppm

    89.2%

    NA

    Room Temp.

    ~10 ppm

    [393]

    AgO loaded LaFeO3/nano- composite

    Acetylene/100 ppm

    60

    6.1 min/4.7 min

    200 °C

    ~5 ppm

    [394]

    CuO NPs-Ti3C2TxMXene/nano- composite

    Toluene/50 ppm

    11.4

    270 s/10 s

    250 °C

    0.32 ppm

    [395]

    Graphene/SnO2 NPs nano-composite

    BTX/0.2–11 ppm

    1–28

    NA

    RT and 250 °C

    ~0.2 ppm

    [396]

    NA = Not available; Temp. = Temperature; s = Seconds; in = minutes.

    8. Nanostructures in other VOCs Determinations

    Apart from the quantitation of acetone, alcohols, aldehydes, amines, and hydrocarbons, nanomaterials were also consumed in the discrimination of other VOCs as detailed in this section. For instance, Ma et al. reported the acetic acid sensing properties of Co-doped LaFeO3 nanofibers [399]. The fabricated nanofibers detected the acetic acid at 130 °C than that of other interferences (ethanol, methanol, acetone, N,N’-dimethylformamide (DMF), ammonia, and benzene). This work requires further investigations to identify various sensory details. Mesoporous tungsten oxides with crystalline framework were utilized in the highly selective detection of foodborne pathogen “3-hydroxy-2-butanone” and proposed in food safety by Zhu and co-workers [400]. Compared to all other competing species at high concentration (50 ppm), the material displayed a high response to 3-hydroxy-2-butanone at low concentration (Ra/Rg 25 for 5 ppm) at 290 °C with response/recovery time of <1 min and a LOD of 0.1 ppm. The porosity of this material plays a vital role in the sensory studies with similar mechanism proposed in acetone. This is an excellent and representative work in food safety. Aqueous electrochemical-based detection of chloroform was proposed by Hamid and co-workers [401]. Wherein, Fe2O3 NPs decorated ZnO NRds were used in this work, which showed linear response from 10 µM to 10 mM with a calculated LOD of 0.6 µM. However, this material needs to be investigated to optimize the resistance-based sensor responses. For example, MOF derived core-shell PrFeO3 functionalized α-Fe2O3 nano-octahedrons were employed in the quantitation of ethyl acetate at 206 °C as seen in Figure 25 [402]. For 100 ppm ethyl acetate, the sensor response reached 22.85 with response/recovery time of <15 s with LOD down to 1 ppm. Therefore, this work is noted as an excellent one towards ethyl acetate discrimination in the presence of competing species (ethanol, acetone, xylene, formaldehyde, and benzene).

    Figure 25. (a) Typical FESEM images of MIL-53 nano-octahedrons; (b) SEM images of MIL-53/Pr–Fe hydroxide precursor; (c) typical FESEM image of pure core-shell α-Fe2O3 nano-octahedrons; (d) Selectivity of four sensors upon exposure to 100 ppm various interfering gases at 255 °C (S1) and 206 °C (S2–S4), respectively, (reproduced with the permission from Ref. [402]).

    Gao and his research team discussed the detection ability of ZnO NRds bundles towards three kinds of abused drugs (C10H15NO, C9H13NO and C8H9NO2; 100 ppm) at 252 °C via the similar mechanism in VOCs detection [403]. These ZnO NRds bundles were synthesized by hydrothermal method, and upon exposure to those abused drugs vapors, the sensor responses were higher (>40) than that of other interferences. Firstly, the n-type ZnO NRds bundles adsorb O2 molecules in air to capture free electrons in conduction band (at the device surface) and convert them to adsorbed oxygen species (O2−, O2, O). Secondly, decrease in the number of electrons in the conduction band leads to the formation of a depletion layer, which increases resistance in ZnO. Consequently, the electrons are released back to conduction band upon interaction with target drug vapors, which affects the depletion layer and the resistance of ZnO. The changes in resistance can provide sensor responses of interactions with target drug vapor. Moreover, this work performed quite well at high humid condition (>90% RH). In a similar fashion, Ag–ZnO–SWCNT field effect transistor (FET) device demonstrated its sensing performance to organophosphorus pesticide “methyl parathion” with linear response from 1 × 10−16 to 1 × 10−4 M and a LOD of 0.27 × 10−16 M [404]. The response and recovery time of this sensor is 1 s and 3 s, respectively. Moreover, this work also attested by real time analysis in rice and soil, thereby is noted as good innovation.

    A FET device based on electrospun InYbO nanofibers was proposed towards the sensing of DMF at room temperature by Chen and co-workers [405]. The sensor showed remarkable response (Ra/Rg = 89) to 2 ppm DMF, with an estimated LOD of 0.00118 ppm. Moreover, it delivered fast response/recovery time of 36 s/67 s, thereby is noted as encouraging research with a wide scope. In view of this, MgGa2O4/graphene composites were synthesized by simple hydrothermal route and utilized in the sensing of acetic acid at room temperature [406]. At 0.1 wt% graphene, the material displayed a high sensor response (Ra/Rg = 363.3 for 100 ppm; response/recovery time = 50 s/35 s) with a calculated LOD of 0.001 ppm. Therefore, it is noted as an exceptional material and can be consumed in acetic acid sensory studies in future.

    9. Advantages and Limitations

    Consumption of distinct nanostructured materials in VOCs detection also has a few advantages and limitations as noted below.

    1. By tuning the nanostructural features, materials with similar compositions are able to detect diverse VOC analytes.

    2. Modulation of nanostructures might be able to tune the availability of surface area to enhance adsorption of VOC analytes, thereby the sensor response can be improved significantly.

    3. By adopting different nanostructures, the operating temperature for the sensing of specified VOC can be reduced or lowered to room temperature.

    4. Divergent nanostructures formation via the combination of diverse materials and their decoration or doping or functionalization may enhance/tune the conducting and charge/electron transport properties, which can help the device to detect specific VOC from other competing species.

    5. Utilization of known and easily fabricated semiconducting materials with diverse nanostructures may led to the generation of cost-effective and reliable devices with social impact.

    6. The less toxic nature of some semiconducting materials is noted as an advantage and can be implemented in upcoming health care devices.

    7. Synthesis of majority of diverse nanostructures seems to be more complicated with involvement of multiple synthetic tactics, such as hydrothermal, CVD, impregnation, electrospinning, etc. The requirement of processing at high temperature further increases the production cost.

    8. Reliability of numerous diverse semiconducting nanostructures to specific VOC is still in question due to the higher operating temperature.

    9. Reports on temperature dependent multiple analyte sensors by a few nanostructured materials are still not convincing with the interference studies; therefore, applicability of those materials is limited and requires more interrogations.

    10. Fabrication of diverse nanostructured materials is also limited by the sophisticated clean room atmosphere and characterizations using costly equipment, such as SEM, TEM, electrochemical instruments, etc.

    11. Majority of reported nanostructures are still not authenticated by real time applications, which limits the operation of those devices in VOC detection.

    12. In general, porous nanostructures display the high/low responses to VOCs due to their pore-effect but are limited in operation by their uneven results.

    13. Stability of sensory reports is considerably affected by high humid conditions and is restricted in real time applications.

    10. Conclusions and Perspectives

    This review gives a concise summary of the diverse nanostructured material-based detection of VOCs (more than 340 references that have been published since 2016), such as acetone, alcohols, aldehydes, amines, hydrocarbons, and other volatile organic compounds. The underlying mechanism of those temperature tuned semiconducting device-based assays of VOCs is also discussed in brief with given advantages and limitations. Moreover, the VOC sensory utilities of diverse nanostructures and nanocomposites with p-p, p-n, n-n heterojunction structures are also presented. Wherein, numerous inorganic-nanostructured materials such as metal oxides, perovskites and composited structures are currently noted as effective candidates towards VOCs detection. Further, majority of reports mainly follows the similar trend to detect the aforementioned VOCs. In addition to those sensory details, following points also require much attention.

    1. Synthetic complications on the development of semiconducting materials towards VOCs sensors must be reduced with respect to cost-effect and reliability.

    2. Justification regarding the role of semiconducting property in the sensing studies seems to be deficient in some reports, which requires more investigations.

    3. Majority of the reports did not provide any theoretical or in-depth explanation regarding, “Why the attested nanostructure becomes more specific to the certain target?” Therefore, extensive research must be done in future.

    4. Utilization of diverse nanostructures with similar material compositions towards different VOCs still needs more interpretations.

    5. Numerous VOCs sensors operate at higher temperature, thereby optimization research should be conducted to make the devices operable at low temperature or even room temperature.

    6. Studies on how to resolve the factors that affecting the sensor response (such as humidity, interferences, temperature, etc.) are necessary.

    7. So far, majority of VOC sensors make use of metal-oxide nanostructures, thereby implementation of devices with emerging halide perovskite nanomaterials can be much anticipated in VOCs detection.

    8. Many reports describe the dopant and composited materials tuned sensory responses towards specific VOCs, but there is no-valid information regarding the efficacy and the role of such dopants or composited materials on the VOCs sensing, which requires clarification.

    9. Enormous amount of reports are available for acetone and alcohols chemiresistor sensors, thus upcoming researchers must focus towards commercialization rather than simply developing the new materials.

    10. Due to the high toxic effect of aldehydes and amines over the eco-systems, chemiresistor sensors coupled with eco-friendly instrumental set up is requires much focus.

    11. Mainstream of BTEX assays by distinct nanostructures are still need to be tuned towards specific target due to their ineffectiveness towards mixed analytes.

    12. Detection of other toxic VOCs (such as carbon tetrachloride (CCl4), chloroform (CHCl3), phosgene, etc.) needs much attention in future.

    13. Standardized procedure is become mandatory to attain the specific nanostructures and its VOC sensing performance.

    14. Development of stable and commercial devices in the determination of VOCs is still in demand, therefore, much attention is required for commercialization.

    15. More research is necessary to justify the exact production cost of reliable devices in VOCs detection with social importance.

    Though mechanistic aspects of VOCs detection have been clarified by numerous reports; however, theoretical and in-depth discussions regarding charge/electron transport in semiconducting properties are yet to be improved. As an essential research in health care innovations, many scientists are currently trying to develop and commercialize cost-effective devices towards specific VOC targets, which may improve the safety of the healthcare and food products in future.

    The entry is from 10.3390/s21020633

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