Hybrid nanomaterial comprising octahedral crystalline clusters of formula [mo 6i i 8(OH) a 4(H2O) a 2]·2h 2o on graphene

Abstract

The present invention relates to a hybrid nanomaterial comprising an octahedral crystalline cluster of formula [M06l'8(OH)a4(H2O)a2]-2H2O, wherein the crystalline cluster is supported on graphene. Furthermore, the invention relates to a method for preparing the hybrid nanomaterial of the invention, and to a gas detection system comprising at least: (a) a gas sensor provided with a hybrid nanomaterial according to the present invention; and (b) optical interrogation means and / or electrical interrogation means configured to interrogate the gas sensor.

Description

[0001] HYBRID NANOMATERIAL COMPRISING OCTAHEDRAL CRYSTALLINE CLUSTERS OF FORMULA [MoersíOHJ HzO^J ^HzO ON GRAPHENE

[0002] DESCRIPTION

[0003] OBJECT OF THE INVENTION

[0004] The present invention belongs to the technical field of nanotechnology and relates to a hybrid nanomaterial comprising an octahedral crystalline cluster of molybdenum iodide supported on graphene, useful as a gas sensor through two different transduction schemes: optical and electrical.

[0005] BACKGROUND OF THE INVENTION

[0006] Air pollution is arguably the single greatest threat to environmental health worldwide, accounting for 7 million premature deaths each year. The detection of gases such as nitrogen dioxide (NO2) and ammonia (NH3) is useful in numerous different technical fields, including, but not limited to, environmental applications related to the detection of pollutants, the food industry, and safety applications within the petrochemical industry.

[0007] In light of this, various pollutant detection techniques have been developed, such as gas chromatography coupled with mass spectroscopy, which can selectively detect contaminants at trace levels. However, these instrumental techniques are expensive, bulky, use consumables, and require frequent attention from qualified personnel. For this reason, there are currently only a few automated air quality monitoring stations, which are poorly distributed in specific areas. This is clearly insufficient for effective pollution monitoring, since this can only be achieved through a denser network of analysis locations (or points) (greater granularity) distributed over larger areas.

[0008] In addition, various technologies have been used for the development of gas sensors, for example, chemoresistive devices, which have attracted significant research efforts due to their simple conduct and readout instrumentation, durability, high miniaturization potential, and industrial scalability. For decades, metal oxide (MOX)-based semiconductors have dominated the field of gas sensing due to their high sensitivity, non-toxicity, and availability. However, their poor selectivity and high power consumption still hinder their effective implementation in commercial applications.

[0009] In recent years, the possibility of using carbon nanomaterial-based sensors, especially those using graphene, has been suggested, and they have become a common approach to developing devices that operate at room temperature. Gas sensors operating at room temperature are energy-efficient and help reduce manufacturing costs, making them very attractive for integration into air quality monitoring networks. Despite graphene's notable properties, such as a high surface area-to-volume ratio, low noise levels, high carrier density and mobility, and affordable production costs, developing such a sensor using this material would require overcoming several associated drawbacks. Thus, graphene, especially in its pristine form, exhibits limited sensitivity and selectivity.

[0010] Therefore, significant research efforts have been directed toward modifying or functionalizing carbon-based films to improve their sensing properties. The most straightforward functionalization strategies are probably the modification of graphene using plasma treatments, to create defects and replace carbon atoms with others of a different nature, or the use of solvent chemistry processes for covalent or noncovalent functionalization of the graphene surface. These approaches generally improve the sensitivity of graphene layers, but the problem of selectivity generally persists, given the low specificity of the atoms and functional groups present on the graphene surface.

[0011] Another well-known strategy is the functionalization of graphene with organic compounds such as macrocycles, aptamers, or cavitands. This approach significantly improves selectivity due to the high specificity of organic compounds (i.e., they act as chemical receptors) toward specific gases. However, the organic nature of this functionalization, prone to degradation under operating conditions, can compromise long-term device stability. High specificity can result in limited reversibility of gas-solid interactions, compromising response repeatability. Sensitivity can be significantly increased and selectivity can be fine-tuned to a certain extent by a widely used strategy involving the decoration of graphene with nanoparticles.However, these MOX nanoparticles typically require moderate to high operating temperatures to activate their sensing properties. Therefore, the need to develop cost-effective, low-power devices may be jeopardized.

[0012] As an alternative, the use of molybdenum chalcogenides and their derivatives in gas sensing applications has been explored in recent years. However, these nanostructured semiconductors vary in composition and morphology, which adversely affects sensing performance.

[0013] In the field of technology, there is still a need to develop new gas sensors capable of monitoring low concentrations of flammable, combustible, or toxic gases in real time. Such devices should also offer other essential properties, such as reliability, sensitivity, selectivity, repeatability, accuracy, and long-term stability, as well as low cost and low power consumption. It is also desirable for such sensors to be capable of being integrated into portable gas detection systems, as well as integrated into personal detectors for workers at risk of exposure.

[0014] BRIEF DESCRIPTION OF THE FIGURES

[0015] Figure 1: shows the orthogonal structure of the crystalline cluster comprised in the hybrid nanomaterial of the invention, where M represents the orthogonal positions of molybdenum (Mo), X' represents the internal positions of iodide (I), and L arepresents the apical positions of water (H2O) and hydroxo (OH).

[0016] Figure 2: (a) shows the Raman spectrum of the Moe@Graphene hybrid. (b) shows the Raman region of the Moe sample measured before (black line) and after (gray line) heat treatment (60 °C). All spectra were obtained at 514 nm.

[0017] Figure 3: (a) shows the variation in the electrical resistivity of a hybrid nanomaterial according to the material invention, in the presence of NO2 (range 250-1000 ppb) and operated at room temperature, (b) compares the calibration curves obtained in the detection of NO2, using pure graphene and the hybrid nanomaterial of the invention.

[0018] Figure 4: (a) shows the dynamic response towards NO2 (range 50-250 ppb) under room temperature operation for the Moe@Graphene nanocomposite according to the present invention, (b) compares the calibration curves obtained in the detection of NO2, in a dry and humid atmosphere.

[0019] Figure 5: shows a repeatability experiment for a hybrid nanomaterial of the invention applying 15 pulses of 100 ppb of NO2.

[0020] Figure 6: (a) shows an example of the dynamic responses of the hybrid nanomaterial sensor of the invention when detecting NH3. (b) compares the calibration curves obtained for pure graphene and for the hybrid nanomaterial of the invention, for the detection of NH3.

[0021] Figure 7: (a) shows the room temperature photoluminescence (PL) spectra of a pure Moe thin film deposited on a quartz substrate and exposed under Ar, air, NO2 and NH3 gases. (b) shows the room temperature PL spectra measured directly on Moe@Graphene coated on an alumina substrate used in resistive transduction and exposed under Ar, air, NO2 and NH3 gases.

[0022] DESCRIPTION OF THE INVENTION

[0023] The present invention relates to a hybrid nanomaterial comprising an octahedral crystalline cluster of molybdenum iodide supported on graphene, useful as a gas sensor through two different transduction schemes: optical and electrical. Thus, the present invention also relates to a gas detection system provided with at least one gas sensor provided with the hybrid nanomaterial of the invention, comprising the crystalline clusters supported on graphene. The gas detection systems, according to the present invention, can operate under two different transduction schemes: optical and electrical, since the nanohybrids provided therein modify both their electrical resistivity and their optical properties (for example, their photoluminescence (PL)) in the presence of gases.Thanks to these extraordinary properties, which are characteristic of the nanohybrids of the invention, it is possible to develop highly selective and low-cost gas detection systems that operate at room temperature and also exhibit high stability and repeatability in dry and humid environments.

[0024] Thus, in a first aspect, the invention relates to a hybrid nanomaterial comprising a crystalline cluster of formula [M06l'8(OH) a 4(H2O) a 2]-2H2O, with an octahedral structure, where the crystalline cluster is supported on graphene. An illustration showing the cluster [M06l'8(OH) a 4(H2O) a 2]-2H2O is shown in Figure 1. Thus, the hybrid nanomaterial comprises a crystalline cluster of formula [M06l'8(OH) a 4(H2O) a2]-2H2O, according to the octahedral structure shown in Figure 1, where M represents the orthogonal positions of molybdenum (Mo), X' represents the internal positions of iodide (I), and L a represents the apical positions of water (H2O) and hydroxo (OH), where the crystal cluster is supported on graphene. The crystal cluster with formula [M06l'8(OH) a 4(H2O) a 2]-2H2O can be found as either a cis isomer or a trans isomer.

[0025] In the context of the present invention, the term “nanomaterial” refers to a material having a largest dimension less than 1 µm (i.e., 1000 nm). In particular embodiments of the present invention, the largest dimension of the nanomaterial is less than 1000 nm, less than 900 nm, less than 800 nm, less than 700 nm, less than 600 nm, less than 500 nm, less than 400 nm, less than 300 nm, less than 200 nm, less than 10 µm, less than 90 nm, less than 80 nm, less than 70 nm, less than 60 nm, less than 50 nm, less than 40 nm, less than 30 nm, less than 20 nm, less than 10 nm, or less than 5 nm.

[0026] In the context of the present invention, the term "graphene" refers to an allotrope of carbon formed by carbon atoms organized in a hexagonal pattern, where the covalent bonds between the bonded carbon atoms are generated from the overlapping of sp hybrid orbitals. 2. In one embodiment of the invention, the graphene is provided in the form of graphene nanoplatelets, graphene nanoribbons, graphene helixes, graphene superlattices, or combinations thereof. In a preferred embodiment of the present invention, the graphene is provided in the form of graphene nanoplatelets. In a preferred embodiment of the present invention, the graphene is provided in the form of graphene nanoribbons. In a preferred embodiment of the present invention, the graphene is provided in the form of a graphene helix. In a preferred embodiment of the present invention, the graphene is provided in the form of graphene superlattices. In the context of the invention, graphene nanoplatelets refer to a multilayered form of graphene that is available in a range of thicknesses and average particle diameters, where the thickness of a single graphene sheet is 0.345nm (in the case of several stacked sheets, the thickness can be a few nanometers), and where the diameters are in a range of a few hundred nanometers to several microns, and which have a 2D carbon structure on a nanometer scale in a planar / laminar form. Alternatively, in particular embodiments of the present invention, the solid support may be graphene and / or modified graphene, where the modified graphene is selected from the group consisting of graphene oxide, reduced graphene oxide, N- and S-doped graphene, and combinations thereof.

[0027] In one embodiment of the present invention, the crystalline cluster of formula [M06l'8(OH) a 4(H2O) a 2]-2H2O is supported on the graphene in the crystalline or polyhedral form of the cluster. That is, in the hybrid nanomaterial of the invention, the cluster does not lose its crystalline form during the hybridization reaction with the graphene support.

[0028] In a further aspect, the invention relates to a method for preparing a hybrid nanomatehal according to the present invention, wherein the method comprises: a) Dispersing a precursor of formula (Bu4N)2[Mo6r8(02CCH3) a 6] in a mixture of water, acetone and thetilamine (TEA) at room temperature under continuous stirring; b) Heat the suspension resulting from step (a) to 35 eC for 1 hour; c) Slowly evaporating the solution resulting from step (b) at room temperature such that microchstals are formed; d) Collecting the microchstals resulting from step (c) when the solution becomes clear. e) Suspending the microchstals resulting from step (d) in dichloromethane under sonication; f) Suspending graphene in dichloromethane under sonication; g) Adding the suspension resulting from step (e) to the suspension resulting from step (f); h) Homogenizing the mixture resulting from step (g) under sonication. In another aspect, the invention relates to a gas detection system comprising, at least:

[0029] - a gas sensor provided with a hybrid nanomaterial comprising an octahedral crystalline cluster of general expression [M6X'8L a 6] n+ / n+ on a solid support; and

[0030] - optical interrogation means or electrical interrogation means configured to interrogate the gas sensor.

[0031] In this aspect of the invention, the hybrid nanomaterial comprises an octahedral crystalline cluster of formula [M6X'8L a 6] n+ / n+ where M is a metal selected from molybdenum or tungsten, X' is an internal ligand of the octahedral crystal structure according to the structure of Figure 1, where the internal ligand is selected from chlorine, bromine or iodine, and where L a is an apical ligand of the octahedral crystal structure according to the structure of Figure 1 , where the apical ligand is generally selected from halogens, O-, S- and N-doper ligands. In one embodiment of this aspect of the invention, the solid support is selected from the group consisting of graphene, graphene oxide, reduced graphene oxide and N- and S-doped graphene.

[0032] In a final aspect, the present invention relates to a gas detection system comprising, at least:

[0033] - a gas sensor provided with a hybrid nanomaterial according to the first aspect of the present invention; and

[0034] - optical interrogation means or electrical interrogation means, configured to interrogate the gas sensor.

[0035] As described above, gas detection systems provided with gas sensors made with hybrid nanomaterials according to the present invention have, among others, the following advantages: they are easy to use, can be miniaturized, operate correctly at room temperature, have a low manufacturing cost, consume little energy and show excellent gas detection properties.

[0036] In the context of the present invention, the expression "interrogating the gas sensor" should be understood to mean detecting changes in a physically measurable variable associated with said gas sensor. Thus, for example, in a preferred embodiment of the invention, the optical interrogation means are configured to detect photoluminescence from the gas sensor. In another preferred embodiment of the invention, the electrical interrogation means are configured to detect variations in the electrical resistivity of the gas sensor. Both interrogation means capable of detecting photoluminescence from the gas sensor and interrogation means capable of detecting variations in the electrical resistivity of the gas sensor are already known in the prior art. These interrogation means are similar to those used in the context of the present invention; therefore, they will not be described in further detail herein.In a preferred embodiment of the invention, the gas detection system comprises both optical interrogation means and electrical interrogation means.

[0037] The gas detection system of the present invention can be used for the detection of NO2 and / or NH3. In a particular embodiment, the gas detection system of the present invention can be used for the simultaneous detection of NO2 and NH3.

[0038] This gas detection system is particularly advantageous because it simultaneously employs two different transduction schemes: electrical (e.g., resistive) and optical. Thus, in one embodiment of the present invention, the simultaneous use of these two different interrogation schemes (resistive and optical) is expressly contemplated. In the context of the present invention, the simultaneous use of the two different interrogation schemes (resistive and optical) is referred to as "orthogonal sensing." The combination of these electrical and optical transduction schemes in a single system increases the selectivity of gas detection and also enables multi-parametric detection, allowing for its implementation in air quality monitoring systems due to its ease of use, low cost, low power consumption, and excellent gas detection properties.Furthermore, the simultaneous use of two different detection mechanisms considerably increases the system's versatility and enables it to detect more different gaseous species. More specifically, and as already seen, electrical interrogation means would allow for very good detection of NO2: the quantification limit would be on the order of 10 ppb, and excellent response repeatability would be achieved (0.9% relative error). In contrast, optical interrogation means would not be effective in detecting this particular gas. Likewise, in the case of NH3, optical interrogation means would allow for very good detection of said gas, while electrical interrogation means would not adequately perform this task.

[0039] All terms and embodiments described herein are equally applicable to all aspects of the invention. It should be noted that, as used in the specification and the appended claims, the singular forms "a" and "the" include their plural referents, unless the context clearly indicates otherwise. Similarly, the term "comprises" or "comprising" as used herein also describes "consists of" or "consisting of" in accordance with generally accepted patent practice.

[0040] EXAMPLES

[0041] The present invention is in no way limited to the embodiments disclosed herein. Other possible embodiments of this invention will be apparent to those skilled in the art in light of this description. Consequently, the scope of protection of the present invention is defined exclusively by the claims.

[0042] Example 1: Materials and methods.

[0043] The synthesis of molybdenum clusters was achieved by dispersing the cluster precursor (Bu4N)2[Mo6r8(02CCH3) a 6] (30 mg) in a mixture (15 ml) of water, acetone and triethylamine (TEA) (50 / 45 / 5% v / v, respectively) in a round-bottom flask with continuous magnetic stirring (500 rpm) at room temperature. Subsequently, a bubbler was connected to the flask and the resulting orange-yellow suspension was heated at 35 °C for 1 hour. Over time, the mixture evolved into a solution, which was added to a 10 mL vial sealed with perforated parafilm for slow evaporation over 5-7 days. Finally, red microcrystals were collected from the remaining solution. The crystalline material was washed twice with acetone and once with Milli-Q water (10 ml), to provide the solid identified as [M06r8(OH) a 4(H2O) a2]-2H2O (hereafter Moe). It is noteworthy that the optimum crystal size is achieved when the solution becomes transparent.

[0044] To support molybdenum clusters on graphene, two solutions were prepared in parallel. First, a 10 mL graphene suspension (0.5 mg / mL) was made in dichloromethane. Then, the resulting suspension was placed in an ultrasonic tip to achieve proper graphene exfoliation. In particular, pulsed sonication (1 s on / 2 s off) was applied at 280 W for 90 minutes. A 1 mL suspension of Moe (0.25 mg / mL) in dichloromethane was prepared by sonication for 30 minutes in an ultrasonic bath. Then, the Moe mixture was added to the graphene suspension, resulting in 5% Moe-loaded graphene. The mixture was homogenized for 10 minutes with stirring. Subsequently, the solution was sonicated in an ultrasonic bath for 1 h, resulting in a proper distribution of graphene-supported molybdenum clusters (hereafter referred to as Moe@Graphene).The resulting nanomaterial, according to the present invention, was deposited by spray coating. Specifically, the compound was deposited on screen-printed platinum electrodes on alumina substrates. To prevent aggregate degradation, a moderate temperature (60. e C) during spray coating deposition and nitrogen was used as carrier gas.

[0045] UV-Vis spectra were acquired at 20 °C using a Varian Cary 50 Conc spectrophotometer equipped with 10 x 10 mm quartz cells. Steady-state PL measurements were recorded on an Edinburgh Instruments FLS1 100 spectrofluorometer using a 450 W xenon lamp equipped with a dual monochromator for excitation and emission coupled to a cooled photomultiplier (PMT-980). Raman spectra were obtained from solid samples previously deposited on quartz substrates using a Renishaw “Reflex” spectrometer equipped with an Olympus microscope. The excitation wavelength was 514 nm from an Ar ion laser. +while the laser power was -10-25 mW; 20 acquisitions were taken for each spectrum. Powder X-ray diffraction (XRD) patterns were obtained using a Philips X'Pert diffractometer and copper radiation (CuKa = 1.541 178 Á). FESEM images were recorded with a Zeiss Ultra 55 field FESEM apparatus (Atlanta, GA, USA) equipped with a backscattered electron (BSE) detector.

[0046] Once homogeneous layers were achieved, the gas sensors were placed in an airtight testing chamber (dead volume of 35 cm 3). The chamber was then connected to a gas mixing and supply system consisting of several calibrated gas cylinders. A pure, dry air atmosphere (Air Premier purity: 99.999%) was used. The total flow rate was adjusted to a low rate (100 ml / min) using a set of flow controllers and solenoid valves. The effect of ambient humidity on sensor performance was also evaluated by using a mixer evaporator controller to humidify the atmosphere.

[0047] The sensor resistances were monitored using a multimeter, recording the resistance changes induced by different gas concentrations. Specifically, the sensors were exposed to given concentrations of target gases for 5 minutes and subsequently stabilized for 15 minutes under dry air flow. Various concentrations of gas species were applied by successive dilutions with pure dry air, and the detection responses were defined as (AR / Ro) expressed as a percentage. Where Ro is defined as the sensor resistance level in air, while AR corresponds to the resistance changes obtained during 5 minutes of exposure to the gas.

[0048] Samples for UV-Vis and photoluminescence (PL) experiments were prepared by dispersing 5 mg of the sample (Moe or Moe@Graphene) in 1 ml of acetonitrile and sonicated for 10 min. The suspensions were then drop-cast onto an acid-treated quartz substrate (Moe sample) and an alumina substrate (Moe@Graphene sample) placed on a hotplate at 50 °C. The samples were then placed inside a screw-capped cuvette and secured with a small piece of commercial Blu-Tack. Samples were purged for 20 min with Ar, air, and NO2 (equilibrated with pure synthetic air) before each in situ measurement. NH3 measurements were performed by gas diffusion of 10 pL of ammonium hydroxide solution (28% NH3wt, Sigma Aldrich) placed at the bottom of the cuvette.The amount of NH3 gas was calculated from Henry's law (Hcp = 0.58 M / Pa at 298.15 K) and considering the equilibrium concentration of ammonia in solution (kb = 1.8-10-5).

[0049] Example 2: Characterization of nanomatehal

[0050] The crystalline material [M06Í8(OH) a 4(H2O) a 2]-2H2O (Moe) and its derivative nanohybrid (Moe@Graphene) were analyzed using various techniques such as Raman spectroscopy, powder X-ray diffraction and field emission scanning electron microscopy (FESEM). The immobilization of Moe on graphene was performed by mixing the respective precursors dispersed in dichloromethane and was confirmed by Raman spectroscopy (Figure 2a), resulting in Moe@Graphene. In this regard, Raman shifts below 360 cm' 1are associated with the presence of molybdenum clusters, while at higher frequencies, the characteristic D, G, 2D and D' bands of graphene appear at 1347, 1588, 2712 and 2938 cm-1 , respectively. It is worth noting that the D band is associated with the presence of structural defects, disordered carbon atoms in the sp configuration. 2 and carbonaceous impurities. While the G band is related to the in-plane vibrations of sp2 carbon bonds. Therefore, the D / G ratio revealed a poorly crystalline graphene, and the presence of defects and oxygenated functional groups grafted onto the graphene likely supports the immobilization of Mo clusters. The inorganic groups immobilized on the graphene surface are essential for gas sensing performance, allowing for greater interactions with gas compounds.

[0051] Furthermore, the crystallinity and integrity of the cluster were studied after the heat treatment (60 °C) required for the preparation of the resulting Moe@Graphene:

[0052] - Raman: Figure 2b shows intense bands with Raman shifts at 129, 153, 292 and 357 cm -1 , associated with the Mo-Mo, Mo-I, and Mo-O vibrations of the cluster. These Raman shifts are similar to those reported for similar hexamolybdenum clusters with iodinated materials, revealing that the heat treatment applied during the experimental protocol does not damage the cluster crystallinity due to the lack of differences between the two spectra.

[0053] - X-ray diffraction (XRD): The synthesis and integrity of the clusters after heat treatment was also confirmed by powder X-ray diffraction, where the Moe cluster diffractogram shows the characteristic monocrystal diffraction peaks. This family of planes is associated with a preferential orientation of the crystalline material when deposited on a flat surface. Moreover, the diffractograms of the resulting hybrid (Moe@Graphene) before and after the thermal process applied during the deposition step show no variation. It is noteworthy that the cluster-related peaks observed in the Moe cluster diffractogram can also be observed in the resulting nanohybrid (e.g. before exposure to the thermal process), revealing that the cluster orientation is maintained. Furthermore, there are no significant differences in the Moe@Graphene diffractograms before and after heat treatment.Thus, XRD also confirms that the developed synthesis and deposition processes prevented the degradation of the Mo clusters. The robustness of the prepared cluster materials is an advantage for preparing gas sensors and performing the resistive and optical measurements described below.

[0054] - Field emission scanning electron microscopy (FESEM): The surface of the Moe@Graphene nanomaterial was observed by FESEM, revealing a highly porous surface, which could be useful for gas sensing. These observations also confirm the homogeneous distribution of crystalline particles formed by Mo clusters supported on graphene.

[0055] Example 3: Gas detection through electrical measurements

[0056] The ability of the hybrid nanomaterial according to the invention to detect various gas species was evaluated under operating conditions at room temperature. This approach allows for the development of gas sensors with low power consumption and reduced manufacturing costs.

[0057] Different concentrations (250, 500, 750, and 1000 ppb) of NO2 were applied during several consecutive measurement cycles (Figure 3a), obtaining significant differences when graphene is loaded with molybdenum clusters compared to its pure counterpart. Specifically, the calibration curves (Figure 3b) revealed that the hybrid nanomaterial sample according to the invention (Moe@Graphene) exhibits electrical responses up to 5 times higher (i.e., the intensity of the resistance changes induced by exposure to NO2) than pure graphene. Regarding the sensitivity, which is given by the slope of the curves shown in Figure 3b, the hybrid nanomaterial layer of the invention shows a four times higher sensitivity towards NO2 than pure graphene.

[0058] Furthermore, lower NO2 concentrations were tested to evaluate the performance of the inventive hybrid nanomaterial in environmental monitoring applications. Considering the threshold limit values ​​(TLVs) defined by the Clean Air Act (United States Environmental Protection Agency) and the Ambient Air Quality (European Union), NO2 concentrations ranging from 50 to 250 ppb were tested. Figure 4a shows intelligible and repeatable detection of the target gas by the inventive hybrid nanomaterial (Moe@Graphene), enabling its potential use in commercial applications. Notably, even in this low range of NO2 concentrations, the developed sensor displays good reference stability under room temperature operating conditions.In contrast, pure graphene exhibits poor sensing performance in this concentration range, experiencing low repeatability due to slight resistance changes induced by exposure to the target gas.

[0059] However, ambient humidity is an element that interferes with gas detection performance, so NO2 was also detected in the 50–250 ppb range under humid conditions. Consequently, the previous experiment performed under dry conditions was reproduced at 60% relative humidity for the cluster-loaded graphene. Figure 4b shows a comparison of the Moe@graphene sample in both environments, revealing a significant increase in detection response (up to twofold) when the sensor is operated under humid conditions. However, the sensitivity (i.e., the slope of the calibration curve) remains almost unchanged under dry or humid conditions, which is an outstanding result for implementation in environmental monitoring devices.

[0060] Considering the intelligible detection responses and low noise levels recorded, the limit of detection and quantification (LOD and LOQ, respectively) were evaluated by measuring trace levels of NO2 ranging from 10 to 25 ppb. The calibration curve was used to estimate both parameters using the following equations: where Sy are the standard deviations of the residuals and , while b corresponds to the sensitivity (slope) of the calibration curve. In addition, a factor of 10 and 3 is applied to the LOQ and LOD, respectively. As a result, the sensitive layer of the hybrid nanomaterial reveals a LOQ of 10.3 ppb, while the estimated LOD is 3.1 ppb. It is worth noting that these values ​​are well below the TLV required for environmental monitoring applications. The device operating conditions (i.e., 5 min of gas exposure and room temperature working conditions) result in advantageous features such as low power consumption, high sensitivity, and rapid detection of gas species. Nevertheless, the repeatability of the gas sensor is a key influencing parameter that determines the potential of the developed clustered graphene layer for use in gas sensing.Consequently, successive cycles of 5 min of NO2 exposure followed by 15 min of recovery in dry air were recorded (Figure 5). As a result, the inventive hybrid nanomaterial revealed excellent sensor repeatability, showing an error of approximately 0.9%.

[0061] In addition to the detection of an electron-accepting gas such as NO2, the sensing performance of pure and cluster-loaded graphene was also evaluated against an electron-donating species such as ammonia (NH3). Figure 6a shows the resistance changes obtained for the inventive hybrid nanomaterial sample when detecting NH3 in the range of 25–100 ppm. A very significant baseline drift and poor response can be observed. Indeed, it is worth noting that the resistive response of pure graphene is higher than that of cluster-loaded graphene, as shown in Figure 6b. However, considering that the sensitivity is given by the slope of the calibration curve, it can be concluded that the inventive hybrid nanomaterial is practically insensitive to NH3, since the responses are less than 0.1%, and they hardly change when exposed to different ammonia concentrations.

[0062] In view of the results in Figures 6a and 6b, it was necessary to verify whether the absence of a resistive response to ammonia could be due to the decomposition of the Mo clusters due to the basicity of NH3. In this perspective, the robustness of Moe was evaluated by powder X-ray diffraction analysis. Specifically, a thin film of the molybdenum cluster was prepared and exposed to NH3. XRD diffractograms of a sample of the hybrid nanomaterial according to the invention Moe@Graphene were performed before and after exposure to ammonia, in which no changes appear. This reveals the excellent stability of the Moe clusters, which do not degrade when exposed to ammonia or nitrogen dioxide.

[0063] Example 4: Oases detection through optical measurements

[0064] The gas-sensing optical properties of Moe and Moe@Graphene materials were preliminarily investigated using films deposited on quartz and alumina transducer substrates, respectively, in the presence of NO2 (1 ppm) and NH3 (250 ppm) equilibrated in air. Photoluminescence (PL) spectroscopy provides insight into the interaction between molybdenum clusters and the target gases. The steady-state emission spectrum of pure Moe crystalline material exhibits a broad emission band centered at 725 nm when recorded under an argon atmosphere (Figure 7a). This wavelength is characteristic of the excited state of the electron triplet of the {Moel's} cluster core. 4*- A similar spectrum was recorded for the Moe@Graphene device under an inert atmosphere, but a red shift was detected at 770 nm (Figure 7b). This change can be attributed to two main factors: the polarity conferred by the graphene environment and the interactions between the defective graphene and the Moe material. Both films were exposed to NO2, NH3U O2 gases (air) and the PL changes were measured. In the in situ characterization, intensity variations in the emission spectra were observed with respect to the analyte gases. A decrease in PL was observed in the presence of air with respect to exposure under argon. This is due to an efficient PL quenching by oxygen leading to the formation of O2 ( 1 Ag), which is an intrinsic characteristic of the cluster core materials {Moel's} 4*- A decrease in PL was also observed after exposure to NO2, and this PL quenching is achieved to a lesser extent than in the presence of air. In contrast, a high PL response was detected in the presence of ammonia. The different responses of the molybdenum crystalline material to NH3 and NO2 gases follow the same trend as for the Moe@Graphene nanomaterial. In addition to the donor / acceptor properties of the gas molecules, these phenomena could be associated with a strong interaction between NH3 and the molybdenum clusters.

[0065] Example 5: Orthoonal sensing

[0066] As seen above, the inventive hybrid nanomaterial film coatings (Moe@Graphene) on standard alumina substrates can be interrogated using two different interrogation modes: resistive and optical, which can be used, for example, for the detection of nitrogen dioxide and ammonia, respectively. This behavior would correspond to orthogonal sensing, which is a term coined in the context of the detection of different toxic gases or even multi-component gas mixtures using sensor arrays, i.e., in the context of gas detection through optical and electrical measurements performed in parallel.

[0067] Ideally, orthogonal detection would be achieved by having at least one highly specific sensor present in the array for each target species. In a real-world application, orthogonality can be achieved either by modifying / optimizing the gas-sensitive film to make it more specific for its target species (in this approach, specificity or selectivity is generally improved to some extent) or by constructing orthogonal metasensors based on the combination of response characteristics of a sensor array with partially overlapping selectivity. The resistive and optical results described in this work pave the way for designing a non-conventional strategy to achieve orthogonal detection.

Claims

CLAIMS A hybrid nanomaterial comprising an octahedral crystalline cluster of formula [M06l'8(OH) a 4(H2O) a 2]-2H2O, where the crystalline cluster is supported on graten or. The hybrid nanomaterial of claim 1 wherein the graphene comprises graphene nanoplatelets. A method for preparing a hybrid nanomaterial according to any one of claims 1 or 2, wherein the method comprises: a) Dispersing a precursor of formula (Bu4N)2[Mo6r8(02CCH3) a 6] in a mixture of water, acetone and triethylamine (TEA) at room temperature under continuous stirring; b) Heat the suspension resulting from step (a) to 35 eC for 1 hour; c) Slowly evaporating the solution resulting from step (b) at room temperature, such that microcrystals are formed; d) Collecting the microcrystals resulting from step (c) when the solution becomes transparent; e) Suspending the microcrystals resulting from step (d) in dichloromethane under sonication; f) Suspending graphene in dichloromethane under sonication; g) Adding the suspension resulting from step (e) to the suspension resulting from step (f); h) Homogenizing the mixture resulting from step (g) under sonication. A gas detection system comprising at least: a gas sensor provided with a hybrid nanomaterial comprising an octahedral crystalline cluster of formula [M6X'8L a 6] n+ / n+on a solid support selected from a group consisting of graphene, graphene oxide, reduced graphene oxide and N- and S-doped graphene, where M is a metal consisting of molybdenum or tungsten, X' is selected from a group consisting of chlorine, bromine and iodine, and L a is selected from a group consisting of halogens and O-, S-, and N-donor ligands; and an electrical interrogation means or interrogation means optical and electrical interrogation means configured to interrogate the gas sensor. The gas detection system according to claim 4, wherein the gas sensor is provided with a hybrid nanomaterial according to any one of claims 1 or 2. The gas detection system according to any one of claims 4 or 5, wherein optical interrogation means are configured to detect photoluminescence from the gas sensor. The gas detection system according to any one of claims 4 or 5, wherein electrical interrogation means are configured to detect variations in the electrical resistivity of the gas sensor.

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