Wireless passive flexible sensor and preparation method and application thereof

Abstract

The present application belongs to the technical field of humidity sensor, and particularly relates to a wireless passive flexible sensor and a preparation method and application thereof. The flexible sensor provided by the present application 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 single-substituted carboxyethyl viologen compound. In the present application, the single-substituted carboxyethyl viologen compound has good hydrophilicity and has the characteristic that the dielectric constant changes with the content of adsorbed water molecules, and the present application uses the same in the humidity-sensitive film to improve the sensitivity of the sensor in detecting humidity. Meanwhile, the present application realizes wireless transmission of signals through electromagnetic coupling. The flexible wireless passive sensor provided by the present application can be easily placed in a small and closed space and can precisely monitor the environmental humidity.

Description

Technical Field

[0001] The present 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 experienced rapid development. Due to their excellent flexibility, bendability, and stretchability, they have shown great application potential in fields such as human health monitoring, smart textiles, and environmental humidity monitoring. Environmental humidity monitoring is crucial in applications such as meteorology, agricultural production, measurement and control, and instrument and equipment protection. Commercial active humidity sensors offer the advantages of fast response, good stability, and intelligence. However, in some special circumstances, such as harsh sealed environments, it is difficult to place traditional humidity sensors in a confined, sealed environment for long-term monitoring. Furthermore, with the development and widespread application of precision instruments, higher requirements for environmental humidity are placed on them. Traditional humidity sensors, with their low sensitivity, cannot meet the more stringent humidity monitoring requirements. Therefore, the development of highly sensitive flexible humidity sensors is of great significance. Summary of the Invention

[0003] In view of this, the present invention provides a wireless passive flexible sensor and its preparation method and application. The flexible sensor provided by the present invention does not require external power supply, can transmit data wirelessly, has high sensitivity, and can accurately monitor environmental humidity in a small space, thereby expanding 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 distance 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 N of pairs of interdigital electrodes c 2 to 5 pairs, length L C The thickness h is 2 to 8 mm, the line width w is 0.2 to 1 mm, and the interval g is 1 to 2 mm.

[0010] Preferably, the preparation method of the monosubstituted carboxyethyl viologen compound comprises the following steps:

[0011] 2-bromoacetic acid and 4,4'-bipyridine are dissolved in dichloromethane to carry out Menshutkin reaction to obtain bipyridine monocarboxylate;

[0012] The bipyridine monocarboxylate, triethylamine and a second dichloromethane are mixed to undergo a 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 adding 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 use 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 disposed 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. The monosubstituted carboxyethyl viologen compound exhibits excellent hydrophilicity and a dielectric constant that varies with the content of adsorbed water molecules. Its use in the humidity-sensitive film improves the sensor's sensitivity in detecting humidity. Furthermore, the present invention utilizes electromagnetic coupling to achieve wireless signal transmission. The flexible wireless passive sensor provided by the present invention can be easily placed in small, confined spaces and accurately monitors ambient humidity. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0027] Figure 3 Schematic diagram of the test system for the flexible humidity sensor;

[0028] Figure 4 S is the reading coil of the flexible sensor with different diameters and different coupling distances. 11 - Frequency graph;

[0029] Figure 5 This is an SEM image of the 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 This is a curve showing the relationship between the resonant frequency change and the relative humidity of the flexible humidity sensor prepared in Example 1 at 0%, 20%, 50% and 80%;

[0032] Figure 8 The relationship curve of the response R of the flexible humidity sensor prepared in Example 1 as a function of relative humidity;

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

[0034] 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.

[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 It can be 10~30mm, the line distance W g It can be 1~2mm, line width W c It can be 0.2~1mm, the number of circles is 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 distance 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 embodiment of the present invention, the number N of pairs of interdigital electrodes 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] In one embodiment of the present invention, the flexible sensor is further coupled to an external reading coil. The diameter of the external reading coil can be 2 to 4 cm, or even 3 cm. The coupling distance can be 4 to 10 mm, or even 6 to 8 mm. The present invention utilizes HFSS software to optimize the reading coil size and coupling distance, achieving an optimal 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 the flexible substrate, 2 is the planar spiral inductor, 3 is the planar interdigital capacitor, 4 is the monosubstituted carboxyethyl viologen compound film, and 5 is the 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] 2-bromoacetic acid and 4,4'-bipyridine are dissolved in dichloromethane to carry out Menshutkin reaction to obtain bipyridine monocarboxylate;

[0044] The bipyridine monocarboxylate, triethylamine and a second dichloromethane are mixed to undergo a 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 perform a Menshutkin reaction to obtain a 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 0.7-2.8 g:7.5-30 mL, or 1.4-2.0 g:15-30 mL. 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 the reaction can be sufficient; the stirring time can be 1 to 2 days; the temperature of the Menshutkin reaction is room temperature, and 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 process, and conventional methods in the art can be used. As a specific embodiment of the present invention, the washing process can be performed 2 to 4 times; the drying process can be vacuum drying, which has no special requirements as long as it can remove the surface solvent.

[0048] After obtaining the bipyridine monocarboxylate, the present invention mixes the zwitterionic bipyridine monocarboxylate, triethylamine, and a second dichloromethane to perform a deprotonation reaction and then recrystallize 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 a second dichloromethane and triethylamine is added.

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

[0051] As a specific embodiment of the present invention, the volume ratio of the mass of the 2-bromoacetic acid to 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 stirring, as long as sufficient reaction can be achieved. The deprotonation reaction time can be 7.5 to 8.5 hours, specifically 8 hours. The deprotonation reaction temperature is room temperature, which 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 post-deprotonation system, washing the filtered solid with dichloromethane, and then drying. The present invention has no special requirements for the filtration process; conventional methods in the art may be employed. As a specific embodiment of the present invention, the washing process may be performed 2 to 4 times; the drying process may be vacuum drying. The present invention has no special requirements for the vacuum drying process, as long as the surface solvent can be removed.

[0054] As a specific embodiment of the present invention, the recrystallization solvent can be water, and the recrystallization temperature can be room temperature, which is 20-35° C. The structure obtained by recrystallization from water in the present invention comprises multiple water molecules, which are encapsulated in unit cells. The stacking of the unit cells organizes the water into extended hydrogen-bonded polymers.

[0055] As a specific embodiment of the present invention, the structural formula of the monosubstituted carboxyethyl viologen compound is NC6H4-C6H4NCH2CO2·3H2O.

[0056] As a specific embodiment 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 screen printing technology; 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 holding time of the first curing can be 25-35 minutes, and can be specifically 25 minutes, 30 minutes or 35 minutes.

[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 holding time of the second curing and the third curing can be consistent with the temperature and holding time of the first curing, which will not be repeated here. In the present invention, after the second printing, the second curing and the third curing, the sensitive capacitor and the planar spiral inductor are connected in series to form a complete LC resonant circuit, the resonant frequency of which changes with the capacitance value, and 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 adding the monosubstituted carboxyethyl viologen compound dispersion, 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 enables the monosubstituted carboxyethyl viologen compound dispersion to be evenly dispersed on the surface of the sensitive capacitor, improves the bonding performance of the monosubstituted carboxyethyl viologen compound film with the sensitive capacitor, and prevents the film from falling off during sensor flexing.

[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 for 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 environmental humidity, can accurately monitor environmental humidity, and can achieve wireless data transmission without the need for external power supply.

[0066] The working principle of the present invention using a flexible wireless passive sensor to monitor 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 through an external reading coil connected to a vector network analyzer, thereby achieving the purpose of humidity detection.

[0067]

[0068] In formula 1, Nc is the number of pairs of interdigitated electrodes, ε0 is the vacuum dielectric constant, and ε r is the dielectric constant of the mixture after the sensitive layer adsorbs 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 chamber, where the relative humidity inside can be freely and stably regulated as needed. 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 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. The humidity sensor's response is defined as R = |f - f0|, where f represents the sensor's resonant frequency at a specific humidity and f0 represents the sensor's resonant frequency at the beginning of a humidity range. This creates a "humidity-response curve." The resonant frequency at an unknown humidity is then measured using a vector network analyzer. The sensor's response at that humidity is calculated using R = |f - f0|, and the humidity can be calculated from the "humidity-response 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 the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0072] Example 1

[0073] Set the dimensions of the planar spiral inductor, interdigital 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 the best coupling distance for actual testing, the diameter of the reading coil and the coupling distance were optimized and simulated using HFSS software. 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 from 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 at 4 mm as the coupling distance between the two.

[0077] 1.4 g of 2-bromoacetic acid and 1.64 g of 4,4'-bipyridine were dissolved in 15 mL of CH2Cl2 and subjected to a Menshutkin reaction under stirring for 2 days. The reaction was then filtered, and the filtered solid was washed twice with CH2Cl2 and dried in vacuo to obtain bipyridine monocarboxylate.

[0078] The bipyridyl monocarboxylate was dispersed in 20 mL of CH2Cl2, and 5 mL of triethylamine was added to deprotonate the mixture under stirring for 8 h, followed by filtration. The filtered solid was washed twice with CH2Cl2 and then vacuum-dried. The vacuum-dried white powder was redissolved in water and recrystallized by evaporation at 30°C to obtain a monosubstituted carboxyethyl viologen compound.

[0079] A nested planar spiral inductor and planar interdigital electrode pattern was screen-printed on the top surface of a 125-μm-thick PET substrate and then dried at 110°C for 30 minutes for curing. Leads were then printed on the back of the PET substrate and dried at 110°C for 30 minutes for curing. Through holes were made at both ends of the leads and filled with conductive silver paste to connect the upper and lower leads, forming a complete circuit in series with the planar spiral inductor and the planar interdigital electrode. The circuit was then dried at 110°C for 30 minutes for curing.

[0080] After plasma hydrophilic treatment of the planar interdigitated electrode surface, 50 μL of ethanol dispersion of a monosubstituted carboxyethyl viologen compound with a concentration of 2 mg / mL 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 thin film of the monosubstituted carboxyethyl viologen compound in the flexible humidity sensor prepared in Example 1 was examined 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 Figure a 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 was tested according to the following method: the flexible humidity sensor prepared in the embodiment was placed in a sealed test chamber with a volume of 500 mL, and air of different humidity was quickly filled into it using a gas cylinder connected to a flow controller, so that the relative humidity inside the chamber could be stably and freely adjusted as needed, and the relative humidity was 0, 20%, 50%, and 80%, respectively. The resonant frequency of the sensor was extracted by sweeping a vector network analyzer, and the sweep range was from 120 MHz to 220 MHz. The reading coil used was a circular reading coil with a diameter of 3 to 6 cm, which was coaxially aligned with the sensor, and the reading distance was maintained at 4 to 10 mm. When the vector network analyzer swept to the resonant frequency of the sensor, its S 11 The parameter will drop to the minimum value. Thus, the S of the LC humidity sensor at different relative humidity equilibrium states is obtained. 11 -Frequency curve, when testing the sensor response time, the resonant frequency of the test sensor 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 swept between 120MHz and 220MHz to read the S of the flexible LC humidity sensor at different relative humidity equilibrium states.11 -Frequency curve graph, 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 as follows Figure 7 shown.

[0084] The LC wireless humidity sensor was tested in the relative humidity range of 0~80%RH. Figure 6 Given S 11 The relationship between the resonant frequency and relative humidity with the minimum 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 significantly, that is, as the relative humidity increases, the response increases.

[0086] The response of the flexible humidity sensor is calculated according to R = |f-f0|. The result shows 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. A dot-line graph of the response value and relative humidity is drawn, as shown in the figure. 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 of 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 shown in FIG. Figure 9 As shown. Figure 9 It can be seen from the resonant frequency change curve of the humidity sensor based on the sensitive material of the monosubstituted carboxyethyl viologen compound that was tested 5 times in a cycle at 50% relative humidity that the baseline frequency of the sensor and the frequency at 50% relative humidity fluctuated little in multiple tests and were relatively stable.

[0088] Although the above embodiment provides a detailed description of the present invention, 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 scope of protection 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; the humidity sensitive film is composed of a monosubstituted carboxyethyl viologen compound; The preparation method of the monosubstituted carboxyethyl viologen compound comprises the following steps: 2-bromoacetic acid and 4,4'-bipyridine are dissolved in a first dichloromethane to perform a Menshutkin reaction to obtain bipyridine monocarboxylate; the mass ratio of the 2-bromoacetic acid to the 4,4'-bipyridine is 0.7-2.8:0.82-3.28; the Menshutkin reaction is performed under stirring for 1-2 days; The bipyridyl monocarboxylate, triethylamine, and a second dichloromethane are mixed to undergo a deprotonation reaction and then recrystallized to obtain the monosubstituted carboxyethyl viologen compound; 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 for 7.5-8.5 h; the recrystallization solvent is water, and the recrystallization temperature is 20-35° C.

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 10-30 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 turns N i is 1-5, and the film thickness h of the planar spiral inductor is 10-50 μm; Number of interdigital electrodes N c 2~5 pairs, length L C The line width is 2~8mm, the thickness h is 10~50μm, the line width w is 0.2~1mm, and the interval g is 1~2mm.

3. 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.

4. The method for preparing the wireless passive flexible sensor according to any one of claims 1 to 3, 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.

5. The preparation method according to claim 4, characterized in that: Before adding the monosubstituted carboxyethyl viologen compound dispersion, the method further comprises: performing a hydrophilic treatment on the sensitive capacitor; The mass concentration of the monosubstituted carboxyethyl viologen compound dispersion is 1.8-2.2 mg / mL.

6. Use of the wireless passive flexible sensor according to any one of claims 1 to 3 or the flexible sensor prepared according to the preparation method according to claim 4 or 5 in monitoring environmental humidity.

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