Flexible wireless passive surface acoustic wave multichannel gas sensor and preparation method thereof

Abstract

The invention relates to the technical field of sensors, and provides a flexible wireless passive surface acoustic wave multichannel gas sensor and a preparation method thereof.The sensor comprises a flexible substrate, a piezoelectric film, an interdigital transducer and a reflecting grating, and the piezoelectric film is arranged on the surface of the flexible substrate; the four groups of interdigital transducers are arranged on the surface of the piezoelectric film in an array manner; and two groups of reflecting gratings are symmetrically arranged on two sides of each interdigital transducer. Wherein areas between the three groups of input interdigital transducers and the output interdigital transducers are respectively coated with sensitive materials including a SnO2 / Pd / MnOx / KH-560 / CNTs / PVP composite material, a PANI nanofiber / KH-560 composite material and a molecularly imprinted polymer, and the rest group of input interdigital transducers and output interdigital transducers are not coated with the sensitive materials and are used as reference. According to the sensitive material of the sensor, the sensitivity and selectivity of the sensor are improved, the response time is shortened, the influence of humidity interference is reduced, and the sensor is suitable for CO, ethanol and acetone gas detection.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sensors, in particular to a flexible wireless passive acoustic surface wave multi-channel gas sensor and a preparation method thereof. BACKGROUND

[0002] With the development of Internet of Things and intelligent sensing technology, gas sensors are increasingly widely used in environmental monitoring, industrial safety, medical diagnosis and other fields. However, traditional electrochemical or semiconductor gas sensors usually rely on external power supply or built-in battery power supply, which not only limits their long-term stable operation in closed, high-temperature or dangerous environments, but also brings high cost problems of frequent maintenance and battery replacement. At the same time, most sensors use rigid substrates, which are difficult to fit curved or irregular surfaces (such as industrial pipelines, human skin), which seriously restricts their application in emerging scenarios such as flexible electronics, wearable devices and implantable medical monitoring. In addition, single detection channel cannot realize the simultaneous identification and differentiation of multiple gas components, and the detection efficiency and accuracy are limited, so it is urgent to develop a multi-channel integrated sensing scheme.

[0003] In recent years, surface acoustic wave (SAW) sensors have attracted attention due to their high sensitivity and wireless passive operation capability, especially SAW devices based on interdigital transducer (IDT) and reflective grating structure, which can realize high-precision gas detection through frequency shift. By combining SAW technology with flexible substrates and integrating wireless antennas, it is expected to realize a multi-channel gas sensing system that can be attached and maintenance-free. However, existing flexible SAW gas sensors still face problems such as insufficient sensitivity of gas-sensitive materials, slow response speed, poor selectivity, etc. Especially in complex environments, the identification of target gases (such as CO) is easily affected by interfering gases (such as CH4) and humidity changes, resulting in a decrease in detection reliability. Therefore, it is urgent to develop a flexible SAW sensor with wireless passive and multi-channel detection capability, and to optimize the design of gas-sensitive materials to improve their sensitivity, response speed and selectivity to specific gases, and to reduce environmental interference to meet the needs of practical applications. SUMMARY

[0004] Therefore, the present application provides a flexible wireless passive acoustic surface wave multi-channel gas sensor with high sensitivity, fast response and high selectivity, and a preparation method thereof.

[0005] The technical scheme of the present application is realized as follows: on the one hand, the present application provides a flexible wireless passive acoustic surface wave multi-channel gas sensor, which comprises a flexible substrate, a piezoelectric film, an interdigital transducer and a reflective grating, The piezoelectric film is arranged on the surface of the flexible substrate; The interdigital transducer is composed of symmetrically arranged input interdigital transducers and output interdigital transducers, and has four groups, which are arranged in an array on the surface of the piezoelectric film; Two groups of reflective gratings are symmetrically arranged on both sides of each interdigital transducer; The areas between the three groups of input interdigital transducers and output interdigital transducers are respectively coated with sensitive materials: SnO2 / Pd / MnO x The remaining one group is not coated with sensitive materials and is used as a reference.

[0006] SnO2 / Pd / MnO x The SnO2 / Pd / MnO / KH-560 / CNTs / PVP multi-level synergistically modified material can achieve high selectivity CO detection at room temperature, providing a new paradigm for the application of SAW sensors in gas monitoring.

[0007] The SnO2 substrate serves as an n-type semiconductor, providing basic gas sensitivity activity, and its high specific surface area can adsorb CO molecules. Pd nanoparticles can catalyze CO oxidation (Pd 0 → Pd 2+ cycle), reduce the activation energy of the reaction, and improve the response sensitivity at room temperature. MnO x doping forms a p-n heterojunction with SnO2, expanding the depletion layer; Mn 3+ / Mn 4+ promotes the generation of oxygen vacancies, reduces the working temperature, and enhances the selectivity. The epoxy group (-CH(O)CH2) of KH-560 silane condenses with the hydroxyl group of SnO2, forming a hydrophobic barrier with long-chain alkyl groups to inhibit water molecule adsorption (response decay <10% under humidity 60% RH). The CNTs network high-conductivity channel accelerates electron transfer, and the large specific surface area of Pd particles shortens the response / recovery time to <5 seconds and the detection limit to 0.5 ppm. The PVP coating layer, polyvinylpyrrolidone, can regulate film uniformity, and the hydroxyl / carbonyl group enhances gas diffusion rate, prevents nanoparticle agglomeration, and improves the mechanical stability of the film layer.

[0008] The synergistic mechanism of each component: (1) Catalysis-sensing linkage: Pd catalyzes the CO → CO2 oxidation reaction to release electrons, which are quickly injected into the SnO2 conduction band through CNTs, significantly changing the SAW wave speed. (2) Humidity interference suppression: The hydrophobic chain (-C3H6O-) of KH-560 blocks water molecules, and the PVP hydrophilic group directional adsorption reduces random adsorption. (3) Interface energy band regulation: MnO x (p-type) / SnO2(n-type) → depletion layer widening → response change amplitude ↑, improving sensor sensitivity.

[0009] On the basis of the above technical scheme, preferably, the SnO2 / Pd / MnO xThe preparation method of the KH-560 / CNTs / PVP composite material is as follows: S11, SnCl4·5H2O, Mn(NO3)2 and PdCl2 are used to prepare SnO2 / Pd / MnO x Powder; S12, the carbon nanotubes are immersed in a mixed solution of nitric acid and sulfuric acid at 60-80 DEG C for 2-4 hours, and then the carbon nanotubes are taken out and put into an ethanol solution containing KH-560, and refluxed at 70-90 DEG C for 3-6 hours to obtain KH-560 modified carbon nanotubes; S13, polyvinylpyrrolidone K30 is dissolved in N-methylpyrrolidone to obtain a PVP solution; then the SnO2 / Pd / MnO x , PVP solution and KH-560 modified acidified carbon nanotube pellets are mixed to obtain a uniform composite slurry.

[0010] In the above technical scheme, preferably, in step S11, the molar ratio of SnCl4·5H2O, Mn(NO3)2 and PdCl2 is 100:3-8:0.5-1.5.

[0011] Specifically, the SnO2 main skeleton provides a gas sensitive reaction substrate. Mn is too low (<3) and the heterojunction effect is weak, too high (>8) and the active sites of SnO2 are blocked, reducing the response. Pd is a catalytic active site, and >1.5 is easy to agglomerate, and <0.5 is insufficient in catalytic efficiency.

[0012] In the above technical scheme, preferably, in step S13, the mass ratio of SnO2 / Pd / MnO x , polyvinylpyrrolidone solution and KH-560 modified acidified carbon nanotubes is 100:8-12:12-18, and the mass concentration of the polyvinylpyrrolidone solution is 10wt%.

[0013] In the above technical scheme, preferably, the preparation method of the PANI nanofiber / KH-560 composite material is as follows: S21, PANI nanofibers are prepared by using an AAO template method; S22, KH-560 acidification treatment: KH-560 is dissolved in ethanol, and acetic acid is added to hydrolyze to obtain silanol; S23, PANI nanofibers are dispersed in DMF, and the silanol prepared in step S22 is added, and a composite solution is obtained after ultrasonic treatment.

[0014] Specifically, the high specific surface area (>50m 2The / g) significantly improves the adsorption capacity of the gas and enhances the mass loading effect. The epoxy group (-CH2-CHO-) of KH-560 reacts with the amino group (-NH2) of PANI to form a stable cross-linked network, inhibits the fiber aggregation, and improves the mechanical stability of the film. In addition, the long-chain alkyl group (-C3H6O-) of KH-560 forms a hydrophobic barrier, reduces the competition of water molecules in the environment for adsorption, and improves the ethanol selectivity (the response attenuation is less than 15% under a humidity of 60% RH).

[0015] Preferably, in the step S23, the mass ratio of the PANI nanofiber to KH-560 is 8-12:1.

[0016] In another aspect, the present application provides a preparation method of a flexible wireless passive surface acoustic wave multi-channel gas sensor, comprising the following steps: S31, cleaning and drying the flexible substrate; S32, depositing a piezoelectric film on the surface of the flexible substrate by a magnetron sputtering method; S33, preparing an interdigital transducer and a reflective grating on the surface of the piezoelectric film by a photolithography technology, a magnetron sputtering method and a stripping method; S34, annealing to improve the quality of the piezoelectric film, improve the electromechanical coupling coefficient and other parameters, and indirectly improve the power transmission performance of the SAW device; S35, coating SnO2 / Pd / MnO x The / KH-560 / CNTs / PVP composite material, the PANI nanofiber / KH-560 composite material and the molecularly imprinted polymer are respectively coated on the area between the three interdigital transducers.

[0017] Preferably, in the step S35, the SnO2 / Pd / MnO x After the / KH-560 / CNTs / PVP composite material is coated, solidification treatment is performed: 75-85℃ for 0.8-1.2h, 110-130℃ for 1.8-2.5h, and 190-210℃ for 0.8-1.5h; After the PANI nanofiber / KH-560 composite material and the molecularly imprinted polymer are coated, solidification treatment is performed at 70-90℃ for 0.5-2h.

[0018] Preferably, in the step S35, the SnO2 / Pd / MnO x The coating thickness of the / KH-560 / CNTs / PVP composite material is 150±20nm, the coating thickness of the PANI nanofiber / KH-560 composite material is 150±20nm, and the coating thickness of the molecularly imprinted polymer is 200±20nm.

[0019] Preferably, the material of the flexible substrate is flexible glass, and the material of the piezoelectric film is zinc oxide.

[0020] The detection principle of the flexible wireless passive surface acoustic wave multi-channel gas sensor is described as follows: I. Surface acoustic wave excitation and propagation basis Piezoelectric effect driving: an alternating electric field is applied to the surface of the flexible piezoelectric film through the interdigital transducer (IDT), and a surface acoustic wave is excited by using the inverse piezoelectric effect. The frequency is determined by the interdigital electrode spacing and the characteristics of the piezoelectric material.

[0021] Multi-modal resonance mechanism: The flexible resonator usually contains an acoustic resonance cavity formed by a reflective grid, and the surface acoustic wave is reflected multiple times between the reflective grids to form a standing wave mode. Different resonance modes correspond to different gas sensitive channels.

[0022] II. Gas sensing and propagation modulation mechanism 1. CO detection: SnO2 / Pd / MnO x / KH-560 / CNTs / PVP nanocomposite 1.1 Sensing mechanism Catalysis-sensing linkage: Pd catalyzes the CO→CO2 oxidation reaction to release electrons, which are quickly injected into the SnO2 conduction band through CNTs, significantly changing the SAW wave speed.

[0023] Humidity interference suppression: The hydrophobic chain (-C3H6O-) of KH-560 blocks water molecules, and the PVP hydrophilic group is oriented to reduce random adsorption.

[0024] Interface band regulation: MnO x (p-type) / SnO2(n-type)→depletion layer widening→response change amplitude↑, improving the sensitivity of the sensor.

[0025] 1.2 SAW propagation modulation mechanism 1.2.1 Mass loading effect (dominant low-frequency response)

[0026] KH-560 hydrophobic layer: The silane coupling agent forms a dense hydrophobic film on the surface of SnO2 (thickness≈50nm), which reduces the competition of water molecules when CO is adsorbed and increases the effective mass loading.

[0027] PVP dispersant: Stabilize nanoparticle dispersion, improve film uniformity, and make the mass loading and gas concentration show a linear relationship.

[0028] 1.2.2 Conductivity modulation effect (dominant high-frequency response) CNTs conductive network: Carbon nanotubes (CNTs) are connected in series with SnO2 / Pd / MnO xThe particles form electron superhighways. When CO is adsorbed, Pd catalyzes CO→CO2 to release electrons, significantly reducing the film resistance, and changing the SAW wave speed through the acoustoelectric effect: , ( k is the piezoelectric coupling coefficient).

[0029] MnO x / SnO2p-n heterojunction: Mn 3+ / Mn 4+ The redox cycle expands the width of the depletion layer, and after CO adsorption, the depletion layer shrinks, and the conductivity jump amplitude increases by 2-3 times. 1.2.3 Elastic modulus hardening effect

[0030] Nanocomposite structure crosslinking: the -Si-O- bond of KH-560 condenses with the surface hydroxyl group of SnO2, and the long chain of PVP winds around CNTs to form a rigid network. CO adsorption induces lattice strain (ε≈0.15%), which increases the Young's modulus and increases the SAW propagation speed. 1.2.4 Interface polarization loss suppression

[0031] Hydrophobic-gasophilic microzone isolation: the alkyl chain of KH-560 isolates the hydrophilic region of SnO2, reducing the dielectric loss caused by H2O molecules, and improving the frequency signal signal-to-noise ratio by 40%.

[0032] 2 Ethanol detection: PANI nanofiber / KH-560 composite material 2.1 Sensitivity mechanism 2.1.1 Chemical adsorption of PANI nanofiber Protonation / deprotonation reaction: the imine group (-N=) of PANI forms a hydrogen bond with ethanol molecules (CH3CH2OH), and ethanol as a weak reducing agent makes PANI change from emeraldine salt (conducting state) to emeraldine base (insulating state), resulting in a decrease in conductivity.

[0033] Selective enhancement: the high specific surface area (>50 m 2 / g) of PANI nanofiber exposes more active sites, and its adsorption capacity for ethanol is much higher than that of other alcohols (such as methanol).

[0034] 2.1.2 Interface regulation of KH-560 Hydrophobic barrier construction: the epoxy group of KH-560 reacts with the amino group on the surface of PANI fiber, and the long-chain alkyl group (-C3H6-) forms a hydrophobic layer, reducing the competitive adsorption of environmental water molecules (H2O) and ethanol, and reducing humidity interference by 40%.

[0035] Enhanced interface stability: The siloxyl group (-Si(OCH3)3) of KH-560 forms covalent bonds with the piezoelectric material after hydrolysis, improving the bonding strength between the film and the substrate, and preventing the sensitive layer from peeling off.

[0036] 2.1.3 Synergistic sensitization effect Optimized hydrogen bond network: The ether bond (-O-) of KH-560 and the amine group of PANI together capture the ethanol hydroxyl group (-OH), forming multiple hydrogen bonds, and the adsorption energy is increased to ~0.85 eV.

[0037] Nano-confinement effect: The KH-560 chains interpenetrated between PANI fibers form micropores (2-5 nm), selectively enriching ethanol molecules, and the concentration detection limit reaches 0.1 ppm.

[0038] 2.2 SAW propagation modulation mechanism 2.2.1 Mass loading effect dominant Ethanol adsorption weight gain: The porous structure (specific surface area > 50 m 2 / g) of PANI nanofibers efficiently adsorbs ethanol molecules (C2H5OH), increasing the mass of the sensitive film.

[0039] Frequency shift (Δf): According to (k is the material constant, A is the sensitive area), the increase in mass causes the SAW frequency to decrease linearly.

[0040] 2.2.2 Conductivity modulation effect auxiliary PANI protonation response: Ethanol interacts with the -NH- group of PANI (hydrogen bond / charge transfer), causing changes in the conductivity of PANI (Δσ).

[0041] Sound speed disturbance: Changes in conductivity change the SAW propagation speed through piezoelectric coupling, further modulating the frequency.

[0042] 2.2.3 KH-560 hydrophobicity strengthens stability Inhibit humidity interference: The siloxane group (-Si-O-) of KH-560 crosslinks with PANI, forming a hydrophobic network, reducing H2O competitive adsorption (response drift <5% under humidity 60% RH).

[0043] Improve interface bonding: Enhance the adhesion of PANI fibers to the substrate, avoid signal attenuation caused by film layer peeling.

[0044] 3 Acetone detection: Molecularly imprinted polymers (MIPs) 3.1 Sensing mechanism Specific recognition: The imprinted cavities of MIPs form size / function group complementary adsorption with acetone molecules, with selectivity superior to traditional materials.

[0045] Competitive adsorption inhibition: hydrophobic modified MIPs preferentially adsorb acetone rather than water molecules under high humidity, improving anti-interference performance.

[0046] 3.2 SAW propagation modulation mechanism Mass-frequency linear relationship: the mass of the MIPs increases after the acetone molecules are embedded in the cavities, and the resonance frequency decreases.

[0047] Viscoelastic effect: the viscoelastic modulus of the MIPs film changes with the amount of acetone adsorbed, resulting in energy dissipation of the acoustic wave (Q value decreases).

[0048] III. Multi-channel differential detection: Reference channel: temperature compensation mechanism, reserving one group of uncoated IDT caused by environmental interference (mainly temperature).

[0049] Signal channel: three groups of functionalized IDT output independent frequency signals, and the frequency shift of the gas channel , α is the temperature coupling coefficient (usually α ≈ 1, because all channels are consistent in process), △f ref is the frequency shift of the reference channel, the response of the three signal channels is △f singnal1、 △f singnal2、 △f singnal3 , and the net response of the three gas channels is △f gas1、 △f gas2、 △f gas3 ; frequency deviation △f gas1、 △f gas2、 △f gas3 The solving equation for gas concentration needs to be determined through actual calibration.

[0050] The flexible wireless passive surface acoustic wave multi-channel gas sensor and its preparation method of the present application have the following beneficial effects compared with the prior art: (1) The sensitive material SnO2 / Pd / MnO x / KH-560 / CNTs / PVP composite material and PANI nanofiber / KH-560 composite material of the present application improve the sensitivity and selectivity of the sensor, shorten the response time, reduce the humidity interference, and solve the technical problems commonly existing in existing sensors.

[0051] (2) The present application uses a flexible substrate (such as flexible glass) combined with a flexible piezoelectric film (such as zinc oxide), breaking through the limitation of traditional rigid piezoelectric materials, so that the sensor can be attached to the surface of a curved surface or wearable device, adapting to the gas detection needs in complex environments; in addition, the wireless passive sensing technology realizes the function of wireless signal transmission and reception, without the need for external power supply, solving the problem of complex wiring and power supply limitation of traditional sensors. BRIEF DESCRIPTION OF DRAWINGS

[0052] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings described below only illustrate some of the embodiments of the present application, and for those skilled in the art, other drawings can be obtained based on these drawings without creative labor.

[0053] Figure 1 A perspective view of the sensor of the present application; Figure 2 A flow chart for preparing the sensor of the present application. DETAILED DESCRIPTION

[0054] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments only represent some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of the present application.

[0055] As shown in Figure 1 The flexible wireless passive acoustic surface wave multi-channel gas sensor of the present application comprises a flexible substrate 1, a piezoelectric film 2, an interdigital transducer 3 and a reflective grating 4.

[0056] The flexible substrate 1 adopts flexible glass, and the whole can be bent.

[0057] The piezoelectric film 2 adopts a zinc oxide piezoelectric film, and is arranged on the surface of the flexible substrate 1.

[0058] The interdigital transducer 3 is composed of symmetrically arranged input interdigital transducers and output interdigital transducers, and the interdigital transducer 3 has four groups, which are arranged in an array on the surface of the piezoelectric film 2. Two groups of reflective gratings 4 are symmetrically arranged on both sides of each interdigital transducer 3.

[0059] The interdigital transducer (IDT) array: the relationship between the wavelength (λ) and the IDT finger width (d) is d=λ / 4. Different wavelengths correspond to different acoustic wave propagation depths and energy distributions, and the sensitive film design can be optimized according to the specific gas adsorption characteristics. Four groups of interdigital transducers are integrated on the substrate, and each group of interdigital transducers corresponds to a signal channel.

[0060] Signal channel (functionalized IDT): The wavelength of each group of IDT (λ1 / λ2 / λ2) should be different, according to the formula of the relationship between wavelength and frequency f=v / λ. Where v is the surface acoustic wave speed in ZnO film (about 2600-3000 m / s), f is the center frequency, λ is the wavelength, and the flexible SAW device commonly used frequency band: 50-500 MHz, corresponding to the wavelength range: 5.2-60 μm. Each group works independently, different wavelengths result in different frequencies, and different channels use different resonance frequencies to achieve frequency isolation. Multi-channel crosstalk suppression: the wavelength difference between adjacent channels should be ≥20% (to avoid frequency overlap).

[0061] Among them, the area between the three groups of input interdigital transducers and output interdigital transducers is coated with sensitive materials: SnO2 / Pd / MnO x / KH-560 / CNTs / PVP composite (corresponding to signal channel 1), PANI nanofiber / KH-560 composite (corresponding to signal channel 2), and molecularly imprinted polymer (corresponding to signal channel 3), and the remaining one group is not coated with sensitive material, as a reference (signal channel 4).

[0062] Among them, signal channel 1 (the first red area on the right): λ1=10 μm (high frequency band to improve sensitivity), d1=2.5 μm (high frequency), suitable for small molecule gas (such as CO), high frequency is more sensitive to trace adsorption. Number of interdigital fingers: 50-60 pairs (improve resonance Q value).

[0063] Signal channel 2 (the second red area on the right): λ2=20 μm (medium frequency), d2=5 μm, suitable for medium molecular weight gas (such as ethanol). Number of interdigital fingers: 30-40 pairs (reduce impedance, match the electrical response of conductive polymers).

[0064] Signal channel 3 (the third red area on the right): λ3=30 μm (low frequency), d3=7.5 μm, suitable for large molecule gas (such as acetone), low frequency reduces scattering loss. Number of interdigital fingers: 20-30 pairs (reduce parasitic capacitance interference).

[0065] Reference channel (the first red area on the left): λ4=40 μm, d4=10 μm is selected to ensure that the influence of environmental temperature and humidity on the propagation of acoustic waves is synchronized with the signal channel, and effective compensation is achieved. Number of interdigital fingers: 10-20 pairs (simplify structure, improve long-term stability).

[0066] The preparation method of the above-mentioned flexible wireless passive surface acoustic wave multi-channel gas sensor includes the following steps: S31, clean and dry the flexible substrate Figure 2 A step) The flexible substrate is Willow Glass from Corning, with a thickness of 100 µm; and a standard RCA cleaning process is used to remove impurities on the surface.

[0067] S32, a 3 µm thick piezoelectric film of zinc oxide is deposited on the surface of the flexible substrate using a magnetron sputtering method. Figure 2 B step).

[0068] S33, an interdigital transducer and a reflective grating are prepared on the surface of the piezoelectric film using photolithography, magnetron sputtering and stripping technology. Figure 2 C step).

[0069] The interdigital transducer and the reflective grating pattern are first prepared by photolithography, then a 20 nm titanium adhesion layer and a 200 nm gold metal layer are sputtered on the piezoelectric film by magnetron sputtering technology; and the pattern is then realized by stripping technology, thereby forming the interdigital transducer and the reflective grating on the surface of the piezoelectric film.

[0070] S34, annealing: 300 °C for 10 min to improve the quality of the piezoelectric film, improve the electromechanical coupling coefficient and other parameters, and indirectly improve the power transmission performance of the SAW device. Figure 2 D step).

[0071] S35, partition coating of sensitive material: SnO2 / Pd / MnO x / KH-560 / CNTs / PVP composite material, PANI nanofiber / KH-560 composite material and molecularly imprinted polymer are coated in the area between the three groups of input interdigital transducers and output interdigital transducers. Figure 2 E step).

[0072] SnO2 / Pd / MnO x / KH-560 / CNTs / PVP composite material is sprayed in the area of signal channel 1, the substrate temperature is 80 °C during spraying, the spraying air pressure is 0.2 MPa, and the film thickness is 150 ± 20 nm. After coating, solidification treatment is performed: 75-85 °C for 0.8-1.2 h, 110-130 °C for 1.8-2.5 h, and 190-210 °C for 0.8-1.5 h, to obtain sensitive film 1.

[0073] PANI nanofiber / KH-560 composite material is spin-coated in the area of signal channel 2 at 3000 rpm for 30 seconds, the film thickness is 150 nm, and after coating, solidification treatment is performed at 70-90 °C for 0.5-2 h to obtain sensitive film 2.

[0074] Molecularly imprinted polymer is spin-coated in the area of signal channel 3, with a thickness of 200 ± 20 nm, and after spin-coating, solidification treatment is performed at 70-90 °C for 0.5-2 h to obtain sensitive film 3.

[0075] Based on the structure of the flexible wireless passive acoustic surface wave multi-channel gas sensor, the sensitive material and its coating are described in the following examples.

[0076] Example 1 The sensitive material of signal channel 1 in this example is SnO2 / Pd / MnO x / KH-560 / CNTs / PVP, the coating thickness is 150 nm, after coating, 80℃ is kept for 1h, 120℃ is kept for 2h, and 200℃ is kept for 1h, and the preparation method of the sensitive material is: S11, SnO2 / Pd / MnO is prepared by sol-gel method x Powder: SnCl4·5H2O, Mn(NO3)2 and PdCl2 (first dissolved in a small amount of 0.1 mol / L dilute hydrochloric acid) are sequentially dissolved in water / ethanol (1:1) mixed solvent (pH is adjusted to 2 with hydrochloric acid), then citric acid and ethylene glycol are added, and the sol is obtained by stirring at room temperature for 1h; then the sol is evaporated and dried at 100℃ to obtain a dry gel, which is heated to 300℃ at a rate of 2℃ / min and kept for 1h (to remove organic matter), then heated to 600℃ at a rate of 2℃ / min and kept for 2h, and then ground to obtain SnO 2 / Pd / MnO x Powder.

[0077] The molar ratio of SnCl4·5H2O, Mn(NO3)2 and PdCl2 is 100:5:1, the molar ratio of SnCl4·5H2O, citric acid and ethylene glycol is 1:1:4.2, and the amount of SnCl4·5H2O is 10g.

[0078] S12, 5g carbon nanotubes are immersed in 100mL of a mixed solution of nitric acid and sulfuric acid at 70℃ (volume ratio of nitric acid to sulfuric acid is 3:1) for 3h, then the carbon nanotubes are taken out and put into 100mL of an ethanol solution containing KH-560 (volume concentration of KH-560 is 2wt%), and refluxed at 80℃ for 5h, then centrifuged and washed with ethanol and deionized water in sequence, and then dried to obtain KH-560 modified carbon nanotubes.

[0079] S13, polyvinylpyrrolidone K30 is dissolved in N-methylpyrrolidone to obtain a PVP solution with a concentration of 10wt%; then SnO2 / Pd / MnO x powder, PVP solution and KH-560 modified acidified carbon nanotube balls are mixed (rotation speed is 350rpm, time is 24h) to obtain a uniform composite slurry. SnO2 / Pd / MnO xThe mass ratio of the Pd / SnO2 powder, polyvinylpyrrolidone solution and KH-560 modified acidified carbon nanotube is 100:10:15.

[0080] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the sensitive material of signal channel 1 is Pd / SnO2, and the rest is the same, and the preparation method of the sensitive material is as follows: SnCl4·5H2O and PdCl2 (first dissolved in a small amount of 0.1 mol / L dilute hydrochloric acid) are sequentially dissolved in a water / ethanol (1:1) mixed solvent (pH adjusted to 2 with hydrochloric acid), then citric acid and ethylene glycol are added, and the sol is obtained by stirring at room temperature for 1 h; then the sol is evaporated and dried at 100°C to obtain a xerogel, which is heated to 300°C at a rate of 2°C / min and kept for 1 h (to remove organic matter), then heated to 600°C at a rate of 2°C / min and kept for 2 h, and then ground to obtain Pd / SnO2 powder. The molar ratio of SnCl4·5H2O and PdCl2 is 100:1, the molar ratio of SnCl4·5H2O, citric acid and ethylene glycol is 1:1:4.2, and the amount of SnCl4·5H2O is 10 g.

[0081] The Pd / SnO2 powder is mixed with 1 / 4 of the mass of the Pd / SnO2 powder and ultrapure water, and the uniform paste-like material is obtained by grinding in a mortar.

[0082] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the sensitive material of signal channel 1 is SnO2 / Pd / MnO x , and the rest is the same, and the preparation method of the sensitive material is as follows: The SnO2 / Pd / MnO x powder prepared in step S11 of Example 1 is taken, 1 / 4 of the mass of the SnO2 / Pd / MnO x powder is added, and the uniform paste-like material is obtained by grinding in a mortar.

[0083] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the sensitive material of signal channel 1 is SnO2 / Pd / MnO x / KH-560, and the rest is the same, and the preparation method of the sensitive material is as follows: The SnO2 / Pd / MnO x , ultrapure water and KH-560 prepared in Example 1 are mixed at a mass ratio of 100:10:15 (rotation speed 350 rpm, time 24 h) to obtain a uniform composite slurry.

[0084] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the sensitive material of signal channel 1 is SnO2 / Pd / MnO x / KH-560 / CNTs, the rest is the same, the preparation method of the sensitive material is: SnO2 / Pd / MnO x , ultrapure water and KH-560 modified carbon nanotubes were mixed in a mass ratio of 100:10:15 (rotation speed 350 rpm, time 24 h) to obtain a uniform composite slurry.

[0085] Comparative Example 5 Comparative Example 5 differs from Example 1 in that the molar ratio of SnCl4·5H2O, Mn(NO3)2 and PdCl2 is 100:5:2.5, and the rest is the same.

[0086] Comparative Example 6 Comparative Example 6 differs from Example 1 in that SnO2 / Pd / MnO x , polyvinylpyrrolidone solution and KH-560 modified acidified carbon nanotubes in a mass ratio of 100:10:25, and the rest is the same.

[0087] Comparative Example 7 Comparative Example 7 differs from Example 1 in that SnO2 / Pd / MnO x , polyvinylpyrrolidone solution and KH-560 modified acidified carbon nanotubes in a mass ratio of 100:20:15, and the rest is the same.

[0088] Example 2 The sensitive material of signal channel 1 in this example is SnO2 / Pd / MnO x / KH-560 / CNTs / PVP, the coating thickness is 130 nm, after coating, 75℃ is kept for 1.2h, 110℃ is kept for 2.5h, 190℃ is kept for 1.5h, and the preparation method of the sensitive material is: S11, SnO2 / Pd / MnO prepared by sol-gel method x Powder: SnCl4·5H2O, Mn(NO3)2 and PdCl2 (first dissolved in a small amount of 0.1 mol / L dilute hydrochloric acid) were sequentially dissolved in water / ethanol (1:1) mixed solvent (pH adjusted to 2 with hydrochloric acid), then citric acid and ethylene glycol were added, and the sol was obtained after stirring at room temperature for 1 h; then the sol was evaporated and dried at 100℃ to obtain a dry gel, and then heated to 300℃ at a rate of 2℃ / min and kept for 1h (remove organic matter), then heated to 600℃ at a rate of 2℃ / min and kept for 2h, then ground to obtain SnO2 / Pd / MnO x Powder.

[0089] The molar ratio of SnCl4·5H2O, Mn(NO3)2 and PdCl2 is 100:3:0.5, the molar ratio of SnCl4·5H2O, citric acid and ethylene glycol is 1:1:4.2, and the amount of SnCl4·5H2O is 10g.

[0090] S12, 5g of carbon nanotubes were immersed in 100mL of a mixed solution of nitric acid and sulfuric acid at 60℃ (the volume ratio of nitric acid to sulfuric acid was 3:1) for 4h, then the carbon nanotubes were taken out, put into 100mL of an ethanol solution containing KH-560 (the volume concentration of KH-560 was 2wt%), and refluxed at 70℃ for 6h, then centrifuged and washed with ethanol and deionized water in sequence, and dried to obtain KH-560 modified carbon nanotubes.

[0091] S13, polyvinylpyrrolidone K30 was dissolved in N-methylpyrrolidone to obtain a PVP solution with a concentration of 10wt%; then SnO2 / Pd / MnO x powder, PVP solution and KH-560 modified acidified carbon nanotube were mixed (rotation speed 350rpm, time 24h) to obtain a uniform composite slurry. The mass ratio of SnO2 / Pd / MnO x , polyvinylpyrrolidone solution and KH-560 modified acidified carbon nanotube was 100:8:12.

[0092] Example 3 The sensitive material of signal channel 1 in this example was SnO2 / Pd / MnO x / KH-560 / CNTs / PVP, the coating thickness was 170nm, after coating, 85℃ heat preservation and curing for 0.8h, 130℃ heat preservation and curing for 1.8h, and 210℃ heat preservation and curing for 0.8h, and the preparation method of the sensitive material was: S11, SnO2 / Pd / MnO x powder: SnCl4·5H2O, Mn(NO3)2 and PdCl2 (first dissolved in a small amount of 0.1mol / L dilute hydrochloric acid) were sequentially dissolved in water / ethanol (1:1) mixed solvent (pH adjusted to 2 with hydrochloric acid), then citric acid and ethylene glycol were added, and the sol was obtained after stirring at room temperature for 1h; then the sol was evaporated and dried at 100℃, and then heated to 300℃ at a rate of 2℃ / min and kept for 1h (remove organic matter), and then heated to 600℃ at a rate of 2℃ / min and kept for 2h, and then ground to obtain SnO2 / Pd / MnO x powder.

[0093] The molar ratio of SnCl4·5H2O, Mn(NO3)2 and PdCl2 is 100:8:1.5, the molar ratio of SnCl4·5H2O, citric acid and ethylene glycol is 1:1:4.2, and the amount of SnCl4·5H2O is 10g.

[0094] S12, 5g of carbon nanotubes were immersed in 100mL of a mixture of nitric acid and sulfuric acid at 80℃ for 2h (the volume ratio of nitric acid to sulfuric acid was 3:1), then the carbon nanotubes were taken out, put into 100mL of an ethanol solution containing KH-560 (the volume concentration of KH-560 was 2wt%), and refluxed at 90℃ for 3h, then centrifuged and washed with ethanol and deionized water in sequence, and dried to obtain KH-560 modified carbon nanotubes.

[0095] S13, polyvinylpyrrolidone K30 was dissolved in N-methylpyrrolidone to obtain a PVP solution with a concentration of 10wt%; then SnO2 / Pd / MnO x The powder, the PVP solution and the KH-560 modified acidified carbon nanotube were mixed (rotation speed 350rpm, time 24h) to obtain a uniform composite slurry. The mass ratio of SnO2 / Pd / MnO x , the polyvinylpyrrolidone solution and the KH-560 modified acidified carbon nanotube was 100:12:18.

[0096] Example 4 The sensitive material of the signal channel 2 in this example was PANI nanofiber / KH-560, the coating thickness was 150nm, and the sensitive material was prepared by the following method after 80℃ curing for 1h: S21, AAO template method for preparing PANI nanofiber: AAO template pretreatment: the AAO template (pore size 100nm) was immersed in 1mol / L NaOH solution to remove impurities, and ultrasonically cleaned for 30min.

[0097] In-situ polymerization: 0.1mol / L aniline monomer, 0.2mol / L HCl and 0.025mol / L ammonium persulfate (APS) oxidant were mixed, then the pretreated AAO template was immersed in the mixed solution, slightly ultrasonically cleaned for 3min, then placed at 4℃ for 12h, and PANI nanofiber was grown in the nanochannel.

[0098] AAO template removal: the immersed AAO template was placed in 3mol / L NaOH solution to dissolve the AAO template and release the PANI nanofiber (diameter≈100nm, length 5-10μm).

[0099] S22, KH-560 acid treatment: KH-560 was dissolved in ethanol (volume ratio 1:9), and an acetic acid solution with pH value of 4 was added, and hydrolysis was performed for 30 min to obtain silanol; S23, PANI nanofibers were dispersed in DMF to obtain a PANI dispersion solution with a concentration of 2 mg / mL, and silanol prepared in step S22 (mass ratio of PANI: KH-560 was 10:1) was added, and a composite solution was obtained after ultrasonic treatment.

[0100] Example 5 Example 5 differs from Example 4 in that in step S23, the mass ratio of PANI: KH-560 is 8:1. The sensitive material coating thickness is 130 nm, and after coating, it is cured at 70°C for 2 h.

[0101] Example 6 Example 6 differs from Example 4 in that in step S23, the mass ratio of PANI: KH-560 is 12:1. The sensitive material coating thickness is 170 nm, and after coating, it is cured at 90°C for 0.5 h.

[0102] Comparative Example 8 The sensitive material of signal channel 2 of Comparative Example 8 is pure PANI conductive polymer, and the specific preparation method is: in Example 4, step S22 is omitted, and PANI nanofibers prepared in step S22 are directly dispersed in DMF to obtain a PANI dispersion solution with a concentration of 2 mg / mL.

[0103] Comparative Example 9 Comparative Example 9 differs from Example 4 in that the mass ratio of PANI: KH-560 is 10:2.

[0104] Example 7 The sensitive material of signal channel 3 of this example is molecularly imprinted polymer (MIPs), with a coating thickness of 200 nm and a curing temperature of 80°C for 1 h. MIPs achieve selective adsorption through specific binding sites. In the preparation stage, MIPs are pre-assembled with functional monomers (such as methacrylic acid) through hydrogen bonds using acetone as a template molecule, and after cross-linking polymerization, acetone is eluted to form a three-dimensional cavity that precisely matches the size, shape, and functional groups of acetone molecules. When the gas contacts the MIPs film, acetone molecules preferentially embed in the cavity, while structural analogs (such as ethanol) are excluded due to steric hindrance or differences in bonding ability, and selectivity is improved.

[0105] The gas sensors made of the sensitive materials prepared in the examples and comparative examples were tested for performance, and the results are shown in Tables 1-3.

[0106] Table 1 Performance comparison of sensitive materials when tested at 100 ppm CO

[0107] As shown in Table 1, Examples 1-3 exhibit high sensitivity, fast response, and low humidity interference, with Example 1 being the best.

[0108] Comparative Examples 1-4 show performance improvement with material and interface engineering: Comparative Example 1 has only Pd catalytic sites, limited active oxygen and defects, and strong water competitive adsorption, resulting in low sensitivity, slow response, and large humidity interference; adding MnO x (Comparative Example 2) introducing Mn 3 + / Mn 4+ Reversible oxidation and reduction and oxygen vacancies, in combination with Pd, improve CO activation and electronic modulation, significantly improving sensitivity and speed, and reducing humidity effects; introducing KH-560 (Comparative Example 3) optimizes particle dispersion and substrate / particle interface, adjusts surface hydrophilicity and hydrophobicity, further reduces humidity attenuation and slightly improves sensitivity and kinetics; finally, adding KH-560 modified CNTs (Comparative Example 4) forms a continuous conductive network and multi-scale mass transfer channel, making electronic transmission and gas diffusion more efficient, resulting in improved sensitivity, fast response / recovery, and enhanced selectivity, but still lacking the final optimization brought by film formation and steric control (such as PVP).

[0109] Compared with Example 1, the performance of Comparative Examples 5-7 is significantly deteriorated: sensitivity decreases, response / recovery slows down, and selectivity drops. The main reasons are as follows: excessive Mn leads to the precipitation of Mn2O3 or Mn3O4 crystal phase, which increases oxygen vacancies but also shields SnO2 active sites; appropriate amounts of KH-560 / CNTs can form a continuous conductive / adsorption network and improve surface active sites, but excessive CNTs can introduce excessive mass loading and conductive bypass, reducing the sensitivity of the piezoelectric surface wave to gas-induced changes in stiffness / adhesion; excessive PVP forms a thick polymer layer, hindering gas transport and active site exposure, and diluting the volume fraction of inorganic active phase, resulting in slower response and smaller amplitude.

[0110] Table 2 Performance comparison of sensitive materials when testing 100 ppm ethanol gas

[0111] As shown in Table 2, the PANI nanofiber / KH-560 system of Examples 4-6 exhibits high sensitivity, short response / recovery time, low signal attenuation at 60% RH, and high ethanol / methanol selectivity. The advantages come from: the high specific surface area and ordered fiber structure provided by the AAO template provide a fast mass transfer channel; appropriate amounts of KH-560 form a firm interface with the PANI surface and hydrophobic modification through hydrolysis of silanol, which improves film stability and interface coupling, and to some extent, inhibits the competitive adsorption of water molecules, thus achieving a good balance in sensitivity, dynamic response, and humidity resistance.

[0112] Comparative Example 8 uses pure PANI, the sensitivity and selectivity are greatly reduced, the response / recovery is significantly slower, and the humidity interference is the most serious. The root cause is the lack of interface adhesion and surface regulation brought by KH-560, the film layer is easy to agglomerate / crack, the charge transfer and gas diffusion are blocked, and the surface hydrophilicity leads to enhanced water competitive adsorption. Comparative Example 9, KH-560 is excessive, the performance is deteriorated: too much silane forms a dense organic / silicon network between fibers, dilutes the conductive phase, increases the mass load and dielectric loss, hinders the target molecules from reaching the active sites and introduces additional energy dissipation, resulting in low sensitivity and slow response.

[0113] Table 3 Performance of sensitive materials when testing 100 ppm acetone gas

[0114] As shown in Table 3, the present application uses molecularly imprinted polymers (MIPs) as the sensitive material for acetone gas, which exhibits high sensitivity, fast response, small humidity interference, and high selectivity. The principle is: when the gas contacts the MIPs film, the acetone molecules are preferentially embedded in the cavities, and the structural analogues (such as ethanol) are excluded due to steric hindrance or differences in bonding ability, and the selectivity is improved.

[0115] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A flexible wireless passive surface acoustic wave multi-channel gas sensor, characterized by, It comprises a flexible substrate (1), a piezoelectric film (2), an interdigital transducer (3) and a reflective grating (4), The piezoelectric film (2) is arranged on the surface of the flexible substrate (1); The interdigital transducer (3) is composed of symmetrically arranged input and output interdigital transducers, and the interdigital transducer (3) has four groups and is arranged in an array on the surface of the piezoelectric film (2); Two groups of reflective gratings (4) are symmetrically arranged on both sides of each interdigital transducer (3); Among them, the area between the three groups of input interdigital transducer and output interdigital transducer is coated with sensitive material: SnO2 / Pd / MnO x / KH-560 / CNTs / PVP composite material, PANI nanofiber / KH-560 composite material and molecularly imprinted polymer are used for detecting CO, ethanol and acetone, and the remaining one group is not coated with sensitive material as a reference.

2. A flexible wireless passive surface acoustic wave multi-channel gas sensor according to claim 1, wherein, The SnO2 / Pd / MnO x The preparation method of the / KH-560 / CNTs / PVP composite material comprises the following steps: S11, SnCl4.5H2O, Mn(NO3)2and PdCl2were prepared by sol-gel method for SnO2 / Pd / MnO x powder; In step S12, the carbon nanotubes are immersed in a mixed solution of nitric acid and sulfuric acid at 60-80℃ for 2-4h, and then taken out and placed in an ethanol solution containing KH-560, and refluxed at 70-90℃ for 3-6h to obtain KH-560 modified carbon nanotubes. S13, dissolving polyvinylpyrrolidone K30 in N-methylpyrrolidone to obtain a PVP solution; then mixing Sn02 / Pd / Mn02 x , the PVP solution and the KH-560 modified acidized carbon nanotube spherules to obtain a uniform composite slurry.

3. A flexible wireless passive surface acoustic wave multi-channel gas sensor according to claim 2, wherein, In step S11, the molar ratio of SnCl4·5H2O, Mn(NO3)2 and PdCl2 is 100:3-8:0.5-1.

5.

4. A flexible wireless passive surface acoustic wave multi-channel gas sensor according to claim 2, wherein, In step S13, the SnO2 / Pd / MnO x The mass ratio of the polyvinylpyrrolidone solution and the KH-560 modified acidified carbon nanotube is 100:8-12:12-18, and the mass concentration of the polyvinylpyrrolidone solution is 10 wt%.

5. A flexible wireless passive surface acoustic wave multi-channel gas sensor according to claim 1, wherein, The preparation method of the PANI nanofiber / KH-560 composite material is as follows: S21, PANI nanofiber is prepared by AAO template method; S22, KH-560 acid treatment: KH-560 is dissolved in ethanol, and silanol is obtained by hydrolysis with acetic acid; S23, PANI nanofiber is dispersed in DMF, and silanol prepared in step S22 is added, and a composite solution is obtained after ultrasonic treatment.

6. A flexible wireless passive surface acoustic wave multi-channel gas sensor according to claim 5, wherein: In step S23, the mass ratio of PANI nanofiber to KH-560 is 8-12:

1.

7. A method of manufacturing a flexible wireless passive surface acoustic wave multi-channel gas sensor according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: S31, clean and dry the flexible substrate; S32, a piezoelectric film is deposited on the surface of the flexible substrate by magnetron sputtering method; S33, an interdigital transducer and a reflective grating are prepared on the surface of the piezoelectric film by photolithography technology, magnetron sputtering and stripping method; S34, annealing, improving the quality of the piezoelectric film, improving the electromechanical coupling coefficient and other parameters, and indirectly improving the power transmission performance of the SAW device; S35, Sn02 / Pd / MnO x The KH-560 / CNTs / PVP composite material, the PANI nanofiber / KH-560 composite material, and the molecularly imprinted polymer are respectively coated on the areas between the three interdigital transducers.

8. The production method according to claim 7, wherein SnO2 / Pd / MnO x After coating the SnO2 / Pd / MnO / KH-560 / CNTs / PVP composite material, curing treatment is performed: 75-85°C for 0.8-1.2h, 110-130°C for 1.8-2.5h, and 190-210°C for 0.8-1.5h. After the PANI nanofiber / KH-560 composite material and the molecularly imprinted polymer are coated, they are cured at 70-90℃ for 0.5-2h.

9. The production method according to claim 7, wherein The Sn02 / Pd / MnO x The coating thickness of the Sn02 / Pd / MnO / CNTs / PVP composite material is 150 ± 20 nm, the coating thickness of the PANI nanofiber / KH-560 composite material is 150 ± 20 nm, and the coating thickness of the molecularly imprinted polymer is 200 ± 20 nm.

10. The production method according to claim 7, wherein The material of the flexible substrate is flexible glass, and the material of the piezoelectric film is zinc oxide.

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