Wireless passive flexible sensor and preparation method and application thereof

Abstract

The invention belongs to the technical field of humidity sensors, and particularly relates to a wireless passive flexible sensor and a preparation method and application thereof. The flexible sensor comprises a flexible substrate, a planar spiral inductor, a sensitive capacitor and a humidity-sensitive film, wherein the planar spiral inductor and the sensitive capacitor are arranged on the surface of the flexible substrate; the humidity-sensitive film covers the surface of the sensitive capacitor; the planar spiral inductor and the sensitive capacitor are connected in series; the humidity-sensitive film is composed of a mono-substituted carboxyethyl viologen compound. According to the invention, the monosubstituted carboxyethyl viologen compound has good hydrophilicity and has the characteristic that the dielectric constant changes along with the content of adsorbed water molecules, and the monosubstituted carboxyethyl viologen compound is applied to the humidity-sensitive film, so that the sensitivity of a sensor for detecting humidity is improved; meanwhile, wireless transmission of signals is achieved in an electromagnetic coupling mode; the flexible wireless passive sensor provided by the invention can be easily placed in a narrow closed space and can precisely monitor the environment humidity.

Description

Technical Field

[0001] The invention belongs to the technical field of humidity sensors, and in particular relates to a wireless passive flexible sensor and a preparation method and application thereof. Background Art

[0002] In recent years, flexible sensors have received widespread attention and rapid development. Due to their good flexibility, bendability and stretchability, they show great application potential in the fields of human health monitoring, smart textiles, environmental humidity monitoring, etc. Among them, environmental humidity monitoring is crucial in applications such as meteorology, agricultural production, measurement and control, and instrument and equipment protection. Commercial active humidity sensors have the advantages of fast response, good stability, and intelligence, but in some special cases, such as in harsh sealed environments, it is difficult to place traditional humidity sensors in a small sealed environment for long-term monitoring. At the same time, with the development and widespread application of precision instruments, they have higher requirements for environmental humidity. Traditional humidity sensors have low sensitivity and cannot meet more stringent humidity monitoring requirements. Therefore, it is of great significance to develop highly sensitive flexible humidity sensors. Summary of the invention

[0003] In view of this, the present invention provides a wireless passive flexible sensor and a preparation method and application thereof. The flexible sensor provided by the present invention does not require an external power supply, can transmit data wirelessly, has high sensitivity, can accurately monitor environmental humidity in a small space, and expands the application range of the sensor.

[0004] In order to solve the above technical problems, the present invention provides a flexible wireless passive sensor, comprising a flexible substrate, a planar spiral inductor and a sensitive capacitor arranged on the surface of the flexible substrate, and a humidity sensitive film covering the surface of the sensitive capacitor;

[0005] The planar spiral inductor and the sensitive capacitor are connected in series; and the humidity sensitive film is composed of a monosubstituted carboxyethyl viologen compound.

[0006] Preferably, the sensitive capacitor comprises a planar interdigital capacitor or a parallel plate capacitor;

[0007] The thickness of the humidity sensitive film is 10 to 30 μm;

[0008] The outer diameter W of the planar spiral inductor out 20~40mm, inner diameter W in 10~30mm, line spacing W g 1~2mm, line width W c 0.2~1mm, number of circles N i is 1 to 5, and the film thickness h of the planar spiral inductor is 10 to 50 μm;

[0009] The number of pairs of interdigital electrodes N c 2 to 5 pairs, length L C It is 2 to 8 mm, the thickness h is 10 to 50 μm, the line width w is 0.2 to 1 mm, and the interval g is 1 to 2 mm.

[0010] Preferably, the method for preparing the monosubstituted carboxyethyl viologen compound comprises the following steps:

[0011] Dissolving 2-bromoacetic acid and 4,4'-bipyridine in the first dichloromethane to carry out a Menshutkin reaction to obtain bipyridine monocarboxylate;

[0012] The bipyridine monocarboxylate, triethylamine and the second dichloromethane are mixed for deprotonation reaction and then recrystallized to obtain the monosubstituted carboxyethyl viologen compound.

[0013] Preferably, the mass ratio of 2-bromoacetic acid to 4,4'-bipyridine is 0.7-2.8:0.82-3.28;

[0014] The Menshutkin reaction is carried out under stirring conditions, and the stirring time is 1 to 2 days.

[0015] Preferably, the mass ratio of the 2-bromoacetic acid to the volume ratio of triethylamine is 0.7-2.8 g:5 mL;

[0016] The deprotonation reaction is carried out under stirring, and the deprotonation reaction time is 7.5 to 8.5 hours.

[0017] Preferably, the recrystallization solvent is water, and the recrystallization temperature is 20-35°C.

[0018] Preferably, the flexible substrate includes a PET substrate, a PI substrate, a PDMS substrate or a PTFE substrate.

[0019] The present invention also provides a method for preparing the wireless passive flexible sensor described in the above technical solution, comprising the following steps:

[0020] After a planar spiral inductor and a sensitive capacitor connected in series are arranged on the surface of a flexible substrate, a dispersion of a monosubstituted carboxyethyl viologen compound is dripped onto the surface of the sensitive capacitor to form a film, thereby obtaining the flexible wireless passive sensor.

[0021] Preferably, before the dropwise addition of the monosubstituted carboxyethyl viologen compound dispersion, the method further comprises: subjecting the sensitive capacitor to a hydrophilic treatment;

[0022] The mass concentration of the monosubstituted carboxyethyl viologen compound dispersion is 1.8-2.2 mg / mL.

[0023] The present invention also provides the application of the flexible sensor described in the above technical solution or the flexible wireless passive sensor prepared according to the preparation method described in the above technical solution in monitoring environmental humidity.

[0024] The present invention provides a flexible wireless passive sensor, comprising a flexible substrate, a planar spiral inductor and a sensitive capacitor arranged on the surface of the flexible substrate, and a humidity-sensitive film covering the surface of the sensitive capacitor; the planar spiral inductor and the sensitive capacitor are connected in series; the humidity-sensitive film is composed of a monosubstituted carboxyethyl viologen compound. In the present invention, the monosubstituted carboxyethyl viologen compound has good hydrophilicity and has the characteristic that the dielectric constant changes with the content of adsorbed water molecules. The present invention uses it in the humidity-sensitive film to improve the sensitivity of the sensor in detecting humidity; at the same time, the present invention realizes wireless transmission of signals by electromagnetic coupling; the flexible wireless passive sensor provided by the present invention can be easily placed in a small and confined space and can accurately monitor the environmental humidity. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the structure of the flexible sensor, where 1 is a flexible substrate, 2 is a planar spiral inductor, 3 is a planar interdigital capacitor, 4 is a monosubstituted carboxyethyl viologen compound film, and 5 is a reading coil;

[0026] Figure 2 Schematic diagram of the steps for preparing a flexible sensor;

[0027] Figure 3 This is a schematic diagram of a test system for a flexible humidity sensor;

[0028] Figure 4 S of the flexible sensor at different diameters of the reading coil and different coupling distances 11 - frequency graph;

[0029] Figure 5 This is a SEM image of a monosubstituted carboxyethyl viologen compound film in the flexible humidity sensor prepared in Example 1;

[0030] Figure 6 The S of the flexible humidity sensor prepared in Example 1 under 0%, 20%, 50% and 80% relative humidity environments 11 - frequency graph;

[0031] Figure 7 The relationship curve between the flexible humidity sensor prepared in Example 1 and the change of the resonance frequency at 0%, 20%, 50% and 80% relative humidity;

[0032] Figure 8 The relationship curve between the response R of the flexible humidity sensor prepared in Example 1 and the change of relative humidity;

[0033] Fig. 9 This is a resonant frequency variation curve of the flexible humidity sensor prepared in Example 1 after 5 cycles of testing at 50% relative humidity. DETAILED DESCRIPTION

[0034] The invention provides a flexible wireless passive sensor, comprising a flexible substrate, a planar spiral inductor and a sensitive capacitor arranged on the surface of the flexible substrate, and a moisture-sensitive film covering the surface of the sensitive capacitor.

[0035] As a specific embodiment of the present invention, the flexible substrate may include a PET substrate, a PI substrate, a PDMS substrate or a PTFE substrate; the thickness of the flexible substrate may be 120 to 130 μm, specifically 120 μm, 125 μm or 130 μm; the flexible substrate has good insulation properties, and the dielectric constant of the flexible substrate may be 2 to 5.

[0036] In the present invention, the planar spiral inductor and the sensitive capacitor are connected in series. As a specific embodiment of the present invention, the sensitive capacitor may include a planar interdigital capacitor or a parallel plate capacitor; the planar spiral inductor and the sensitive capacitor are nested and distributed on the surface of the flexible substrate, and the planar spiral inductor and the sensitive capacitor are connected in series through two back-through hole leads to form an LC resonant circuit. As a specific embodiment of the present invention, the outer diameter W of the planar spiral inductor is out It can be 20~40mm, inner diameter W in Can be 10~30mm, line spacing W g Can be 1~2mm, line width W c It can be 0.2~1mm, number of circles N i can be 1 to 5, the film thickness h of the planar spiral inductor can be 10 to 50 μm; the outer diameter W of the planar spiral inductor out It can also be 30-35mm, inner diameter W in It can also be 20-25mm, line spacing W g It can also be 1.3~1.5mm, line width W c It can also be 0.5~1mm, the number of circles N i It can also be 1 to 3, and the film thickness h of the planar spiral inductor can also be 20 to 40 μm.

[0037] As a specific implementation of the present invention, the number of pairs of interdigital electrodes N c It can be 2 to 5 pairs, length L C The number of pairs of interdigital electrodes N is 2 to 8 mm, the thickness h is 10 to 50 μm, the line width w is 0.2 to 1 mm, and the interval g is 1 to 2 mm. c It can also be 4 to 8 pairs, length LC It can also be 4 to 6 mm, the thickness h can also be 20 to 40 μm, the line width w can also be 0.4 to 0.6 mm, and the interval g can also be 1.4 to 1.8 mm.

[0038] The present invention utilizes HFSS electromagnetic simulation software to perform modeling and simulation analysis to determine the size parameters of the planar spiral inductor and the interdigital electrodes.

[0039] As a specific implementation of the present invention, the flexible sensor is also coupled with an external reading coil, the diameter of the external reading coil can be 2-4 cm, or 3 cm; the coupling distance can be 4-10 mm, or 6-8 mm. The present invention uses HFSS software to optimize and simulate the reading coil size and coupling distance, and obtains a better coupling distance.

[0040] Taking the sensitive capacitor as a planar interdigital capacitor as an example, Figure 1 Schematic diagram of the structure of the flexible sensor, where 1 is a flexible substrate, 2 is a planar spiral inductor, 3 is a planar interdigital capacitor, 4 is a monosubstituted carboxyethyl viologen compound film, and 5 is a reading coil.

[0041] The flexible humidity sensor provided by the present invention exhibits excellent performance when detecting different humidity levels, and has high sensitivity and good repeatability.

[0042] In the present invention, the humidity sensitive film is composed of a monosubstituted carboxyethyl viologen compound. As a specific embodiment of the present invention, the preparation method of the monosubstituted carboxyethyl viologen compound may include the following steps:

[0043] Dissolving 2-bromoacetic acid and 4,4'-bipyridine in the first dichloromethane to carry out a Menshutkin reaction to obtain bipyridine monocarboxylate;

[0044] The bipyridine monocarboxylate, triethylamine and the second dichloromethane are mixed for deprotonation reaction and then recrystallized to obtain the monosubstituted carboxyethyl viologen compound.

[0045] The present invention dissolves 2-bromoacetic acid and 4,4'-bipyridine in a first dichloromethane to carry out a Menshutkin reaction to obtain bipyridine monocarboxylate. As a specific embodiment of the present invention, the mass ratio of the 2-bromoacetic acid and 4,4'-bipyridine can be 0.7-2.8:0.82-3.28, or 1.4-2.0:1.64-3.0; the mass ratio of the 2-bromoacetic acid to the first dichloromethane can be 0.7-2.8g:7.5-30mL, or 1.4-2.0g:15-30mL. The present invention has no special requirements for the dissolution, as long as it can be completely dissolved.

[0046] As a specific embodiment of the present invention, the Menshutkin reaction can be carried out under stirring conditions. The present invention has no special requirements for the stirring, as long as it can fully react; the stirring time can be 1 to 2 days; the temperature of the Menshutkin reaction is room temperature, and the temperature of the room temperature is 20 to 35°C.

[0047] As a specific embodiment of the present invention, the Menshutkin reaction may further include: filtering the Menshutkin reaction system, washing the filtered solid with dichloromethane and then drying to obtain the bipyridyl monocarboxylate. The present invention has no special requirements for the filtration, and the conventional method in the art can be used. As a specific embodiment of the present invention, the washing times can be 2 to 4 times; the drying can be vacuum drying, and the present invention has no special requirements for the vacuum drying, as long as the surface solvent can be removed.

[0048] After obtaining the bipyridine monocarboxylate, the present invention mixes the zwitterionic bipyridine monocarboxylate, triethylamine and a second dichloromethane for deprotonation reaction and then recrystallization to obtain the monosubstituted carboxyethyl viologen compound. As a specific embodiment of the present invention, the mixing includes the following steps:

[0049] The bipyridyl monocarboxylate is dispersed in the second dichloromethane and then triethylamine is added.

[0050] The present invention has no particular limitation on the amount of the second dichloromethane, as long as it can be dispersed evenly.

[0051] As a specific embodiment of the present invention, the mass ratio of the 2-bromoacetic acid to the volume ratio of triethylamine can be 0.7-2.8 g:5 mL, and can be specifically 1.4 g:5 mL.

[0052] As a specific embodiment of the present invention, the deprotonation reaction can be carried out under stirring conditions. The present invention has no special requirements for the stirring, as long as the reaction can be fully reacted; the deprotonation reaction time can be 7.5 to 8.5 hours, specifically 8 hours; the deprotonation reaction temperature is room temperature, and the room temperature is 20 to 35° C. In the present invention, the product of the deprotonation reaction is a white powder.

[0053] As a specific embodiment of the present invention, the deprotonation reaction may further include: filtering the system after the deprotonation reaction, washing the filtered solid with dichloromethane and then drying. The present invention has no special requirements for the filtration, and the conventional method in the art can be used. As a specific embodiment of the present invention, the washing times can be 2 to 4 times; the drying can be vacuum drying, and the present invention has no special requirements for the vacuum drying, as long as the surface solvent can be removed.

[0054] As a specific embodiment of the present invention, the recrystallization solvent may be water, and the recrystallization temperature may be room temperature, which is 20 to 35° C. The structure obtained by recrystallization from water in the present invention comprises a plurality of water molecules, which are encapsulated in unit cells, and the stacking of the unit cells organizes the water into an extended hydrogen bond polymer.

[0055] As a specific embodiment of the present invention, the structural formula of the monosubstituted carboxyethyl viologen compound is NC 6 H 4 -C 6 H 4 NCH 2 CO 2 ·3H 2 O.

[0056] As a specific implementation of the present invention, the thickness of the humidity sensitive film may be 10 to 30 μm.

[0057] The present invention also provides a method for preparing the flexible sensor according to the above technical solution, comprising the following steps:

[0058] After a planar spiral inductor and a sensitive capacitor connected in series are arranged on the surface of a flexible substrate, a dispersion of a monosubstituted carboxyethyl viologen compound is dripped onto the surface of the sensitive capacitor to form a film, thereby obtaining the flexible sensor.

[0059] As a specific embodiment of the present invention, a method of setting a planar spiral inductor and a sensitive capacitor in series can be to perform a first printing on the surface of the flexible substrate using a screen printing technique; after the first printing is completed, a first curing is performed, and the temperature of the first curing can be 100-120°C, and can be specifically 100°C, 110°C or 120°C; the insulation time of the first curing can be 25-35min, and can be specifically 25min, 30min or 35min.

[0060] As a specific embodiment of the present invention, after the first curing, the following further comprises: performing a second printing on the back of the flexible substrate using screen printing technology and then performing a second curing to form a lead; making through holes at both ends of the lead, and injecting conductive silver paste into the through holes to connect the lead and the planar spiral inductor and the sensitive capacitor, and performing a third curing. As a specific embodiment of the present invention, the temperature and insulation time of the second curing and the third curing can be consistent with the temperature and insulation time of the first curing, which will not be repeated here. After the second printing, the second curing and the third curing, the present invention connects the sensitive capacitor and the planar spiral inductor in series to form a complete LC resonant circuit, and its resonant frequency changes with the capacitance value. The resonant frequency can be wirelessly read by an external reading coil to achieve the function of measuring humidity.

[0061] As a specific embodiment of the present invention, before the monosubstituted carboxyethyl viologen compound dispersion is added, the method may further include: subjecting the sensitive capacitor to a hydrophilic treatment; the hydrophilic treatment may be a plasma hydrophilic treatment. The hydrophilic treatment of the present invention can make the monosubstituted carboxyethyl viologen compound dispersion uniformly dispersed on the surface of the sensitive capacitor and improve the binding performance of the monosubstituted carboxyethyl viologen compound film with the sensitive capacitor, thereby preventing the film from falling off during the bending process of the sensor.

[0062] As a specific embodiment of the present invention, the solvent of the monosubstituted carboxyethyl viologen compound dispersion can be water or ethanol; the mass concentration of the monosubstituted carboxyethyl viologen compound dispersion can be 1.8-2.2 mg / mL, specifically 2 mg / mL.

[0063] As a specific embodiment of the present invention, the film-forming method can be drop coating, specifically drying the product after dropwise adding the monosubstituted carboxyethyl viologen compound dispersion; the drying temperature can be 45 to 55°C, specifically 45°C, 50°C or 55°C; the drying time can be 10 to 14h, specifically 10h, 12h or 14h.

[0064] The preparation method of the flexible sensor provided by the present invention has simple steps, is easy to operate, has high reliability, and is suitable for mass production. Figure 2 Schematic diagram of the steps for preparing flexible sensors.

[0065] The present invention also provides the use of the flexible sensor described in the above technical solution or the flexible wireless passive sensor prepared by the preparation method described in the above technical solution in monitoring environmental humidity. As a specific embodiment of the present invention, the environment can be a closed environment. The flexible wireless passive sensor provided by the present invention has high sensitivity to the humidity in the environment, can accurately monitor the environmental humidity, and can realize wireless data transmission without the need for external power supply.

[0066] The working principle of the flexible wireless passive sensor for monitoring ambient humidity is as follows: when the ambient humidity changes, the monosubstituted carboxyethyl viologen compound film on the surface of the sensitive capacitor interdigitated electrode adsorbs water molecules in the air, and its relative dielectric constant ε r The resonant frequency f of the entire resonant circuit changes with the change of the capacitance C of the sensor (see Formula 1 and Formula 2 for details). The resonant frequency can be wirelessly read by connecting the external reading coil of the vector network analyzer to achieve the purpose of detecting humidity.

[0067]

[0068] In formula 1, Nc is the number of pairs of interdigital electrodes, ε 0 is the dielectric constant of vacuum, ε r is the dielectric constant of the mixture after the sensitive layer absorbs water, K is the first kind of complete elliptic integral, L C is the length of the interdigital electrodes, w c is the total distance between the two interdigits, g c is the spacing between the interdigitated electrodes;

[0069] In formula 2, f is the resonant frequency of the sensor, L is the inductance of the sensor, and C is the capacitance of the sensor.

[0070] The sensor is placed in a humidity test box, and the relative humidity inside can be freely and stably regulated as needed. The test system diagram of the flexible humidity sensor is shown in the figure below. Figure 3 As shown in the figure, the sensor is attached to the upper surface of a cylindrical closed cavity, and its resonant frequency is extracted by scanning through a vector network analyzer connected to an external reading coil. When the vector network analyzer sweeps to the resonant frequency of the sensor, its S 11 (S 11 The reflection coefficient between the port impedance and the input impedance (from the source to the load) will be reduced to the minimum value, thus obtaining S 11 -Frequency curve diagram. Define the response of the humidity sensor R = |ff 0 |, where f represents the resonant frequency of the sensor when a certain humidity is reached, f 0 is the resonant frequency of the sensor at the beginning of a certain humidity range. Thus, a "humidity-response relationship curve" is established; then the resonant frequency under unknown humidity is measured by a vector network analyzer. 0 |After calculating the response value of the sensor under the humidity, the humidity can be calculated through the "humidity and response relationship curve".

[0071] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0072] Example 1

[0073] Set the dimensions of the planar spiral inductor, interdigitated electrodes, and external reading coil according to Table 1:

[0074] Table 1 Geometric parameter data of wireless passive flexible LC sensor design

[0075]

[0076] The coupling between the external reading coil and the sensor coil is related to the size, distance and relative position of the two coils. In order to obtain a better coupling distance for actual testing, the diameter of the reading coil and the coupling distance were optimized and simulated using HFSS software, and the S 11 The relationship between parameters and resonant frequency, such as Figure 4 As shown. Figure 4 As can be seen in a, when the diameter of the reading coil is 2cm, 3cm, and 4cm respectively, S 11 The parameters are -19.77dB, -30.38dB and -17.79dB respectively. Figure 4 As can be seen in Figure b, as the coupling distance increases, the resonant frequency basically remains around 158MHz, while S 11 The parameters change with the coupling distance. When the coupling distance is 4mm, 6mm, 8mm and 10mm, S 11 The parameter values ​​are -30.55dB, -27.81dB, -21.8dB and -16.54dB respectively. It can be seen that when the coupling distance is 10mm, S 11 The parameter has a minimum value, and when the coupling distance is 10mm, S 11 The parameter has a maximum value. Therefore, in the actual test platform, a reading coil with a diameter of 3 cm is selected, and the distance between the external readout coil and the sensor is fixed to 4 mm as the coupling distance between the two.

[0077] Dissolve 1.4 g of 2-bromoacetic acid and 1.64 g of 4,4'-bipyridine in 15 mL of CH 2 Cl 2 The Menshutkin reaction was carried out under stirring for 2 days, then filtered and washed with CH 2 Cl 2 The solid obtained by filtration was washed twice and then dried under vacuum to obtain bipyridine monocarboxylate;

[0078] Disperse bipyridyl monocarboxylate in 20 mL CH2 Cl 2 5 mL of triethylamine was added to the mixture and the mixture was deprotonated for 8 h under stirring. Afterwards, the mixture was filtered and washed with CH 2 Cl 2 The filtered solid was washed twice and then vacuum dried, and the vacuum dried white powder was redissolved in water and volatilized at 30° C. for recrystallization to obtain a monosubstituted carboxyethyl viologen compound;

[0079] A planar spiral inductor and a planar interdigital electrode pattern with nested distribution are printed on the upper surface of a PET substrate with a thickness of 125 μm by using screen printing technology, and then dried at a temperature of 110° C. for 30 minutes for curing; then, a lead is printed on the back of the PET substrate and then dried at a temperature of 110° C. for 30 minutes for curing, through holes are made at both ends of the lead, and conductive silver paste is used to fill the through holes to connect the upper and lower wires, so that the planar spiral inductor and the planar interdigital electrode are connected in series to form a complete loop, and then dried at a temperature of 110° C. for 30 minutes for curing;

[0080] After plasma hydrophilic treatment on the surface of the planar interdigitated electrode, 50 μL of 2 mg / mL ethanol dispersion of a monosubstituted carboxyethyl viologen compound was added dropwise using a pipette and then placed in an oven and dried at 50°C for 12 h to form a monosubstituted carboxyethyl viologen compound film with a thickness of 20 μm to obtain a flexible humidity sensor.

[0081] The monosubstituted carboxyethyl viologen compound film in the flexible humidity sensor prepared in Example 1 was detected by scanning electron microscopy to obtain a SEM image, as shown in FIG. Figure 5 As shown, b~d are element distribution diagrams Figure 5 Image a in the figure shows that the monosubstituted carboxyethyl viologen compound has an overall rectangular crystal structure (about 1 μm in length and about 300 nm in width) with a smooth surface. Figure 5 b~d show that the C, O, and N elements in the monosubstituted carboxyethyl viologen compound are evenly distributed.

[0082] The performance of the flexible humidity sensor is tested according to the following method: the flexible humidity sensor prepared in the embodiment is placed in a sealed test chamber with a volume of 500mL, and air with different humidity is quickly filled into it using a gas cylinder connected to a flow controller, so that the relative humidity inside can be stably and freely adjusted as needed, and the relative humidity is 0, 20%, 50%, and 80% respectively. The resonant frequency of the sensor is extracted by sweeping through a vector network analyzer, and the sweep range is from 120MHz to 220MHz. The reading coil used is a circular reading coil with a diameter of 3 to 6cm, which is coaxially aligned with the sensor, and the reading distance is maintained at 4 to 10mm. When the vector network analyzer sweeps to the resonant frequency of the sensor, its S 11The parameter will drop to the minimum value. Thus, the S of the LC humidity sensor under different relative humidity equilibrium conditions is obtained. 11 - Frequency graph, when testing the sensor response time, testing the sensor's resonant frequency when it changes between 0 and different relative humidity.

[0083] A vector network analyzer connected to the reading coil was used for frequency sweep extraction. The resonant frequency of the sensor was between 120MHz and 220MHz. The S of the flexible LC humidity sensor under different relative humidity equilibrium states was read. 11 -Frequency curve diagram, such as Figure 6 When testing the sensor response time, the resonant frequency of the sensor is tested when it changes between 0 and different relative humidity. The results are shown in Figure 7 shown.

[0084] The LC wireless humidity sensor was tested in the relative humidity range of 0 to 80% RH. Figure 6 Given S 11 The relationship between the resonant frequency and relative humidity with the smallest parameter. It can be seen that the resonant frequency and S 11 It gradually increases with the increase of relative humidity.

[0085] Depend on Figure 7 It can be seen that when the relative humidity of the environment in which the flexible LC humidity sensor is located increases from 0 to 20%, the resonant frequency of the sensor changes from 157.79 MHz to 167.13 MHz, and when the relative humidity of the environment drops to 0, the resonant frequency of the sensor returns to the initial frequency of 157.79 MHz (the shaded part in the figure is the relative humidity change stage); and as the detected relative humidity increases, the resonant frequency of the sensor changes more obviously, that is, as the relative humidity increases, the response increases.

[0086] According to R=|ff 0 | Calculate the response of the flexible humidity sensor. The result is that the response value of 20% relative humidity is 9.22MHz, the response value of 50% relative humidity is 15.04MHz, and the response value of 80% relative humidity is 23.48MHz. Draw a dot-line graph of its response value and relative humidity, as shown in Figure 8 As shown. Figure 8 It can be seen that as the relative humidity increases, the response of the flexible humidity sensor increases, and the response is approximately linear with the change in relative humidity. The sensitivity of the humidity sensor is 312.5kHz / %RH.

[0087] The resonant frequency change curve obtained by cyclically testing the flexible humidity sensor prepared in Example 1 at 50% relative humidity for 5 times is as follows: Fig. 9 As shown. Fig. 9It can be seen that from the resonant frequency change curve of the humidity sensor based on the sensitive material of the monosubstituted carboxyethyl viologen compound after 5 cycles of testing at 50% relative humidity, it can be found that the baseline frequency of the sensor and the frequency at 50% relative humidity fluctuate less in multiple tests and are relatively stable.

[0088] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A wireless passive flexible sensor, characterized in that: It comprises a flexible substrate, a planar spiral inductor and a sensitive capacitor arranged on the surface of the flexible substrate, and a humidity-sensitive film covering the surface of the sensitive capacitor; The planar spiral inductor and the sensitive capacitor are connected in series; and the humidity sensitive film is composed of a monosubstituted carboxyethyl viologen compound.

2. The wireless passive flexible sensor according to claim 1, characterized in that: The sensitive capacitor includes a planar interdigital capacitor or a parallel plate capacitor; The thickness of the humidity sensitive film is 10 to 30 μm; The outer diameter W of the planar spiral inductor out 20~40mm, inner diameter W in 10~30mm, line spacing W g 1~2mm, line width W c 0.2~1mm, number of circles N i is 1 to 5, and the film thickness h of the planar spiral inductor is 10 to 50 μm; The number of pairs of interdigital electrodes N c 2 to 5 pairs, length L C It is 2 to 8 mm, the thickness h is 10 to 50 μm, the line width w is 0.2 to 1 mm, and the interval g is 1 to 2 mm.

3. The wireless passive flexible sensor according to claim 1 or 2, characterized in that: The preparation method of the monosubstituted carboxyethyl viologen compound comprises the following steps: Dissolving 2-bromoacetic acid and 4,4'-bipyridine in the first dichloromethane to carry out a Menshutkin reaction to obtain bipyridine monocarboxylate; The bipyridine monocarboxylate, triethylamine and the second dichloromethane are mixed for deprotonation reaction and then recrystallized to obtain the monosubstituted carboxyethyl viologen compound.

4. The wireless passive flexible sensor according to claim 3, characterized in that: The mass ratio of 2-bromoacetic acid to 4,4'-bipyridine is 0.7-2.8:0.82-3.28; The Menshutkin reaction is carried out under stirring conditions, and the stirring time is 1 to 2 days.

5. The flexible sensor according to claim 3, characterized in that: The mass ratio of the 2-bromoacetic acid to the volume ratio of triethylamine is 0.7-2.8 g:5 mL; The deprotonation reaction is carried out under stirring, and the deprotonation reaction time is 7.5 to 8.5 hours.

6. The wireless passive flexible sensor according to claim 3, characterized in that: The solvent for recrystallization is water, and the temperature for recrystallization is 20-35°C.

7. The wireless passive flexible sensor according to claim 1, characterized in that: The flexible substrate includes a PET substrate, a PI substrate, a PDMS substrate or a PTFE substrate.

8. The method for preparing the wireless passive flexible sensor according to any one of claims 1 to 7, comprising the following steps: After a planar spiral inductor and a sensitive capacitor connected in series are arranged on the surface of a flexible substrate, a dispersion of a monosubstituted carboxyethyl viologen compound is dripped onto the surface of the sensitive capacitor to form a film, thereby obtaining the wireless passive flexible sensor.

9. The preparation method according to claim 8, characterized in that: Before adding the monosubstituted carboxyethyl viologen compound dispersion, the method further comprises: subjecting the sensitive capacitor to a hydrophilic treatment; The mass concentration of the monosubstituted carboxyethyl viologen compound dispersion is 1.8-2.2 mg / mL.

10. Use of the wireless passive flexible sensor according to any one of claims 1 to 7 or the flexible sensor prepared by the preparation method according to claim 8 or 9 in monitoring environmental humidity.

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