A wearable plant humidity sensor and a method of manufacturing the same

Abstract

This invention discloses a wearable plant humidity sensor and its preparation method, belonging to the field of plant moisture detection technology. The preparation method includes: fabricating Ag interdigitated electrodes on a flexible PET substrate; reacting MA2SnCl6 precursor solution by dissolving MACl and SnCl4·5H2O in a solvent, and optionally optimizing the solution with isopropanol or ammonia; spin-coating the precursor solution onto the electrodes and annealing to form an MA2SnCl6 thin film. This invention is the first to apply lead-free double perovskite MA2SnCl6 to plant humidity sensing. The resulting sensor is flexible, wearable, and deformation-resistant, allowing it to fit closely to plant leaves, enabling real-time, in-situ, and non-destructive detection of surface moisture in living plants (such as pumpkin seedlings). It also boasts advantages such as high sensitivity, good repeatability, and strong stability, solving the problems of interference susceptibility and poor applicability of traditional detection methods.

Description

Technical Field

[0001] This invention relates to the field of plant moisture detection technology, and in particular to a wearable plant humidity sensor and its preparation method. Background Technology

[0002] Real-time and accurate monitoring of plant moisture status is crucial for preventing drought stress, guiding precision irrigation, and ensuring agricultural production and forestry management. Currently, plant moisture detection technologies are mainly divided into two categories: non-contact detection and contact detection. Although non-contact methods (such as those based on visible / near-infrared spectroscopy and thermal imaging) do not require direct contact with plants, they are generally susceptible to changes in ambient light, atmospheric conditions, and background interference, resulting in low detection sensitivity and poor specificity. Furthermore, the raw data often requires complex algorithms for processing, making it difficult to achieve stable and convenient in-situ real-time monitoring.

[0003] Contact-based electrical sensors, especially humidity sensors based on resistance or capacitance principles, can offer high detection resolution. However, their core drawback lies in their typically rigid substrates (such as silicon wafers, glass, or rigid printed circuit boards). These sensors are inflexible and rigid, making them severely incompatible with the physical characteristics of soft, varied, and evolving plant organs (such as leaves). This incompatibility prevents the sensor from forming a tight, conformal fit with the plant surface, limiting the types of plants it can be used with. Furthermore, the additional weight and pressure can hinder the normal expansion, movement, and growth of plant leaves, effectively causing physical interference with the observed object and failing to meet the basic requirements of non-destructive testing.

[0004] For continuous monitoring of moisture in pumpkin seedling leaves, existing technologies face a significant challenge: reliable signal acquisition relies on stable, close physical contact. However, pumpkin seedling leaves, as dynamically growing, soft, and fragile living organs, naturally repel any rigid, non-deformable external attachments. Therefore, developing a sensor that adapts to the morphology of pumpkin seedling leaves, deforms synchronously with their natural growth and movement, and does not interfere with their normal transpiration and photosynthesis is crucial for achieving truly in-situ, long-term, and non-destructive moisture monitoring. Although flexible electronics technology has made progress, combining it with novel sensitive materials specifically to address the challenge of wearable moisture detection in pumpkin seedlings remains to be explored. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a wearable plant humidity sensor and its preparation method. This sensor is flexible, thin, and wearable, allowing it to closely adhere to the plant surface and achieve real-time, in-situ, and non-destructive monitoring of the moisture content (especially the surface humidity) of living pumpkin seedling leaves. It also boasts advantages such as high sensitivity, strong anti-interference ability, and good stability.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a wearable plant humidity sensor, comprising the following steps: A method for preparing a wearable plant humidity sensor, characterized by comprising the following steps: S1. The flexible substrate is ultrasonically cleaned with ultrapure water and ethanol, dried, and vapor-deposited to obtain Ag interdigitated electrodes. S2. Dissolve MACl and SnCl4·5H2O in a solvent and react to obtain a precursor solution; S3. The precursor solution obtained in step S2 is dropped onto the Ag interdigitated electrode prepared in step S1. After spin coating and annealing, an MA2SnCl6 thin film is formed on the electrode, thus obtaining the wearable plant humidity sensor.

[0007] Based on the above technical solutions, preferably, tin-based perovskites can effectively avoid the toxicity problems of lead-based perovskites, while stable Sn... 4+ The oxidation state endows A2Sn(IV)X6 perovskite with advantages such as high symmetry, good air stability, and excellent photoelectric properties. The tin-based perovskite with the molecular formula MA2SnCl6, compared to other metal oxide semiconductors and lead-based perovskites, exhibits a significant characteristic of being sensitive to defects, thus readily reacting with water molecules to induce changes in electrical signals, thereby achieving humidity sensing.

[0008] More preferably, in step S2, the MACl is methylammonium chloride, and the molar ratio of MACl to SnCl4·5H2O is 1:1-5:1.

[0009] More preferably, the molar ratio of MACl to SnCl4·5H2O is 2:1.

[0010] More preferably, in step S2, the solvent is selected from at least one of ultrapure water, N,N-dimethylformamide, and dimethyl sulfoxide.

[0011] More preferably, in step S2, the reaction conditions are: a temperature of 90-110℃ and a stirring speed of 700-800 r / min for 2-3 hours.

[0012] More preferably, in step S2, after obtaining the precursor solution, the precursor solution is mixed with isopropanol or ammonia and centrifuged, and the precipitate is redispersed to obtain an optimized precursor solution for use in step S3.

[0013] More preferably, the centrifugation process is carried out at a speed of 7000-9000 r / min for 5-6 minutes.

[0014] More preferably, in step S1, the drying is performed by blowing with a nitrogen gas stream; the size of the Ag interdigitated electrode is 1cm × 2cm, and it is subjected to ultraviolet ozone treatment for 5-6 minutes after vapor deposition.

[0015] More preferably, in step S3, the spin coating speed is 1000-3000 r / min and the time is 1-2 minutes; the annealing is performed at 70-90℃ for 1-2 hours.

[0016] Secondly, the present invention provides a wearable plant humidity sensor prepared by the above-described preparation method.

[0017] Based on the above technical solutions, preferably, the wearable plant humidity sensor includes a flexible substrate, an Ag interdigitated electrode disposed on the flexible substrate, and an MA2SnCl6 humidity-sensitive film covering the Ag interdigitated electrode.

[0018] More preferably, the flexible substrate comprises a PET film.

[0019] Thirdly, the present invention provides the application of the above-mentioned wearable plant humidity sensor in detecting the surface moisture of living pumpkin seedling leaves.

[0020] The present invention has the following advantages over the prior art: (1) The core breakthrough of the sensor in this invention lies in its flexible and wearable characteristics. Using an ultra-thin (125μm), lightweight PET flexible substrate, the device can be bent and deformed, and can achieve perfect conformal fit with the soft, non-planar surface of plant organs such as the leaves of live pumpkin seedlings. This fitting method does not require binding or applying pressure, fundamentally avoiding the problem of traditional rigid sensors hindering the normal expansion, growth and transpiration of leaves due to weight and pressure, and realizing long-term, in-situ water monitoring without interfering with the natural physiological processes of plants.

[0021] (2) Lead-free double perovskite MA2SnCl6 was used as a plant humidity sensing material for the first time. This material is sensitive to water adsorption, and its impedance changes significantly with ambient humidity. More importantly, by optimizing the precursor solution with isopropanol or ammonia, a sensitive material with a rougher surface and a larger specific surface area was obtained, which provides more adsorption sites for water molecules, thereby significantly improving the sensitivity and response speed of the sensor.

[0022] (3) This invention not only works well under stable laboratory conditions, but also maintains the reliability of the detection signal in complex field environments with varying light and natural bending of the sensor due to plant growth and wind, thus solving the problem that traditional optical detection methods are easily affected by ambient light interference.

[0023] (4) The preparation process of this invention is simple and low in cost, easy to scale up production, and can be effectively applied to real-time water monitoring and drought differentiation of living pumpkin seedlings and other plants, providing precise services for agricultural irrigation decision-making. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is the XPS spectrum of the humidity-sensitive material MA2SnCl6 in the H2O group in Example 1 of the present invention.

[0026] Figure 2 The images show the specific surface area nitrogen adsorption-desorption isotherm and pore size distribution of the H2O group humidity-sensitive material MA2SnCl6 in Example 1 of this invention.

[0027] Figure 3 (a) is a SEM morphology characterization image of the humidity-sensitive material MA2SnCl6 in the H2O group in Example 1 of the present invention; Figure 3 (b) is a SEM morphology characterization image of the humidity-sensitive material MA2SnCl6 in the H2O-IPA group in Example 2 of the present invention; Figure 3 (c) is a SEM morphology characterization image of the humidity-sensitive material MA2SnCl6 in the H2O-NH3·H2O group in Example 2 of the present invention.

[0028] Figure 4 This is a diagram showing the dimensions of the interdigitated electrodes in Embodiment 3 of the present invention.

[0029] Figure 5 This paper examines the effect of different test voltages on the impedance fluctuation range of the device prepared in Example 3 of the present invention at relative humidity of 2%, 40%, 77%, and 93%.

[0030] Figure 6 This study examines the effect of different test frequencies on the impedance variation range of the device fabricated in Example 3 of this invention at relative humidity levels of 2%, 40%, 59%, 77%, and 93%.

[0031] Figure 7 The graph shows the response / recovery repeatability test results of the device prepared in Example 3 of the present invention at relative humidity of 2% - 93%.

[0032] Figure 8 The graph shows the stability test results of the device prepared in Example 3 of the present invention under multiple consecutive tests over 35 days.

[0033] Figure 9 These are actual photos of four groups of pumpkin seedlings experiencing different periods of water shortage in Application Example 6 of this invention.

[0034] Figure 10 This is a physical image of the device used in Application Example 6 of the present invention to detect the surface humidity of pumpkin seedling leaves.

[0035] Figure 11 The image shows the humidity detection results of the device in Application Example 6 of this invention on the surface of pumpkin seedling leaves after different periods of water shortage. Detailed Implementation

[0036] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0037] Table 1: Material Source Description Table

[0038] Example 1: This embodiment provides the preparation of MA2SnCl6 humidity-sensitive material, with the following steps: Weigh 0.1350 g of MACl and 0.3506 g of SnCl4·5H2O into a 5 mL transparent reagent bottle, and add 1 mL of ultrapure water (labeled as H2O group), N,N-dimethylformamide (labeled as DMF group), and dimethyl sulfoxide (labeled as DMSO group), respectively. Then, place them on a constant temperature shaker (rotation speed set to 800 r / min, temperature set to 100℃) and react for 2 h. After cooling to room temperature, the H2O group yields a slightly turbid solution, while the DMF and DMSO groups yield clear and transparent solutions, i.e., precursor solutions.

[0039] To obtain the elemental composition information of the prepared humidity-sensitive material, XPS tests were performed on the H2O group humidity-sensitive material MA2SnCl6, such as... Figure 1 As shown.

[0040] To measure the surface properties of the prepared material and further investigate its adsorption performance, this invention conducted nitrogen adsorption-desorption tests on the solid powder synthesized using ultrapure water as a solvent. The sample pretreatment temperature was 100℃. The test results are as follows: Figure 2 As shown.

[0041] To investigate the film formation of humidity-sensitive materials, thin films of the H2O-based humidity-sensitive materials were fabricated using spin coating. Figure 3(a) shows the SEM image information of the humidity-sensitive material film of the H2O group.

[0042] Example 2: This embodiment further optimizes the MA2SnCl6 humidity-sensitive material synthesized in Example 1 using ultrapure water as the solvent using different solvents, aiming to improve the humidity sensing performance of the material. The steps are as follows: 160 μL of the H2O precursor solution prepared in Example 1 was added to 15 mL centrifuge tubes containing 1200 μL of the following different solutions: ① Isopropanol (labeled as H2O-IPA group); ② Ammonia water (labeled as H2O-NH3·H2O group). The mixture was then centrifuged at 8000 r / min for 5 min. Both groups yielded milky white precipitates. The supernatant was discarded, and the precipitates were dispersed in 400 μL of IPA and 400 μL of NH3·H2O, respectively, to obtain the optimized precursor solutions.

[0043] To investigate the film formation of humidity-sensitive materials, thin films of H2O-IPA and H2O-NH3·H2O humidity-sensitive materials were prepared by spin coating. Figure 3 (b) shows the SEM image information of the humidity-sensitive material thin film of the H2O-IPA group. Figure 3 (c) shows the SEM image information of the humidity-sensitive material thin film of the H2O-NH3·H2O group.

[0044] Example 3: This embodiment provides the fabrication of a wearable plant humidity sensor based on MA2SnCl6. The steps are as follows: A 125 μm thick transparent PET film (1.5 × 2.5 cm) was cleaned for 30 min each in an ultrasonic bath of ultrapure water and ethanol, then dried in a nitrogen stream and placed in a custom mask. Finally, an Ag interdigitated electrode with dimensions of 1 cm × 2 cm was deposited using vapor deposition. After UV ozone treatment for 5 min, the film was spin-coated. 60 μL of the precursor solution obtained in Example 1 using ultrapure water as a solvent was dropped onto the 1 μm thick Ag interdigitated electrode, and then spin-coated at 2000 r / min for 60 s. Finally, the device was annealed on an 80℃ hot plate for 1 h to obtain the MA2SnCl6 wearable plant humidity sensor. The actual wearable plant humidity sensor is shown in the figure. Figure 4 As shown.

[0045] To investigate the effect of voltage on the device's response performance, the device was tested at four different relative humidities (2%, 40%, 77%, and 93%), and the data changes over a 1-minute test period were observed when different voltages were applied. Figure 5 As shown.

[0046] The device fabricated in this invention is an impedance-type humidity sensor. Applying an AC voltage and the operating frequency during testing will affect the impedance response characteristics of the device. Figure 6 The graphs show the humidity-impedance variation of the device at different frequencies under five different relative humidities: 2%, 40%, 59%, 77%, and 93%.

[0047] Figure 7 The impedance response / recovery repeatability is shown after six cycles of continuous exposure of the device to the lowest (2%) and highest (93%) relative humidity conditions.

[0048] To investigate the stability of the device under long-term, continuous, and repeated testing, the impedance value of the same device was tested at regular intervals for 35 days. The test results are as follows: Figure 8 As shown.

[0049] Example 4: S1. A transparent PET film with a thickness of 125 μm and a size of 1.5 cm × 2.5 cm is ultrasonically cleaned sequentially in ultrapure water and ethanol for 30 minutes each time. After cleaning, it is dried with a nitrogen stream. The dried film is placed in a custom mask, and Ag interdigitated electrodes with a size of 1 cm × 2 cm are prepared on its surface using a vapor deposition method. After vapor deposition, the electrodes are subjected to ultraviolet ozone treatment for 5 minutes.

[0050] S2. In a reaction vessel, weigh approximately 0.0675 g of MACl and 0.3506 g of SnCl₄·5H₂O in a 1:1 molar ratio and dissolve them in ultrapure water. Place the mixed solution on a constant-temperature shaker and react for 2 hours at 90°C and 700 rpm. After the reaction is complete, cool to room temperature to obtain the precursor solution. Take a portion of the above precursor solution, mix it with isopropanol, and place it in a centrifuge tube. Centrifuge at 7000 rpm for 5 minutes, discard the supernatant, and redisperse the resulting solid precipitate in fresh isopropanol to obtain the optimized precursor solution.

[0051] S3. The optimized precursor solution obtained in step S2 is dropped onto the Ag interdigitated electrode prepared in step S1. Then, it is spin-coated at 1000 r / min for 1 minute. After spin-coating, the device is placed on a hot stage and annealed at 70°C for 1 hour. After annealing, an MA2SnCl6 humidity-sensitive film is formed on the electrode surface, obtaining the wearable plant humidity sensor.

[0052] Example 5: S1. A transparent PET film with a thickness of 125 μm and a size of 1.5 cm × 2.5 cm was ultrasonically cleaned sequentially in ultrapure water and ethanol for 30 minutes each time. After cleaning, it was dried with a nitrogen stream. The dried film was placed in a custom mask, and Ag interdigitated electrodes with a size of 1 cm × 2 cm were fabricated on its surface using a vapor deposition method. After vapor deposition, the electrodes were subjected to ultraviolet ozone treatment for 6 minutes.

[0053] S2. In a reaction vessel, weigh 0.3375 g of MACl and 0.3506 g of SnCl₄·5H₂O in a molar ratio of 5:1 and dissolve them in dimethyl sulfoxide. Place the mixed solution on a constant-temperature shaker and react for 3 hours at 110℃ and a stirring speed of 800 r / min. After the reaction is complete, cool to room temperature to obtain a precursor solution. Take a portion of the above precursor solution, mix it with ammonia water, and place it in a centrifuge tube. Centrifuge at 9000 r / min for 6 minutes, discard the supernatant, and redisperse the resulting solid precipitate in fresh ammonia water to obtain the optimized precursor solution.

[0054] S3. The optimized precursor solution obtained in step S2 is dropped onto the Ag interdigitated electrode prepared in step S1. Then, it is spin-coated at 3000 r / min for 2 minutes. After spin-coating, the device is placed on a hot plate and annealed at 90°C for 2 hours. After annealing, an MA2SnCl6 humidity-sensitive film is formed on the electrode surface, obtaining the wearable plant humidity sensor.

[0055] Application Example 6: The MA2SnCl6 wearable plant humidity sensor prepared in Example 3 was used to detect the surface moisture content of pumpkin seedlings. The principle is based on the transpiration of plant leaf surfaces. Two wires were attached to the two electrodes of the prepared device with conductive silver paste. Double-sided tape was applied to the blank ends along the length of the device. Finally, the device was attached to the surface of the third leaf of four groups of pumpkin seedlings under different drought stress levels. The device was then connected to a CHI 760 electrochemical workstation for impedance value testing.

[0056] The four groups of pumpkin seedlings used in the practice, which were subjected to different periods of water shortage, are as follows: Figure 9 As shown.

[0057] The MA2SnCl6 wearable plant humidity sensor, used to detect plant humidity, is attached to the surface of pumpkin seedling leaves, such as... Figure 10 As shown.

[0058] The device detected the humidity on the surface of pumpkin seedling leaves after different periods of water shortage, as follows: Figure 11As shown in the figure, under the same test period, the fourth group of pumpkin seedlings, which were watered normally every day, had the lowest impedance value and the highest leaf surface humidity. Meanwhile, the impedance values ​​of the third, second, and first groups, which had gradually increased with the number of dry days, gradually increased, indicating that their leaf surface humidity was gradually decreasing.

[0059] Comparative Examples 7-8: Unlike Example 1, this example uses 0.3375 g of MACl and 0.338 g of SnCl4·5H2O (molar ratio of MACl to SnCl4·5H2O is 0.5:1) and 6.75 g of MACl and 13.5 g of SnCl4·5H2O (molar ratio of MACl to SnCl4·5H2O is 10:1), respectively. The remaining steps are the same as in Example 1 and will not be repeated here.

[0060] Comparative Example 9: Unlike Example 3, SnO2 is used instead of MA2SnCl6 sensitive layer. The remaining steps are the same as in Example 3, and will not be repeated here.

[0061] Comparative Example 10: Unlike Example 3, Cs2SnI6 is used instead of MA2SnCl6. The remaining steps are the same as in Example 3, and will not be repeated here.

[0062] Comparative Example 11: Unlike Example 3, in step S2, the optimization process of "mixing with isopropanol or ammonia, centrifuging, and redispersing" is completely omitted, and the precursor solution obtained from the preliminary reaction is directly used for spin coating to form a film.

[0063] A comprehensive comparison of the above examples and comparative examples reveals that Examples 1-5 and Comparative Examples 7-8 demonstrate that the molar ratio of MACl to SnCl4·5H2O is crucial for synthesizing high-performance MA2SnCl6 sensitive materials. Comparative Examples 7 (molar ratio 0.5:1) and 8 (molar ratio 10:1), due to significant deviations from the stoichiometric ratio required for the formation of the MA2SnCl6 double perovskite, are expected to result in the formation of a large amount of impurity phases, poor crystallinity, or compositional deviations, thereby severely impacting the final sensor's humidity response sensitivity, stability, and repeatability.

[0064] Application Example 6 demonstrates that the MA2SnCl6-based sensor can effectively distinguish the moisture status of live pumpkin seedlings. Comparative Examples 9 (using SnO2 material) and 10 (using Cs2SnI6) serve as counterexamples, illustrating that simply replacing the MA2SnCl6 of this invention with other sensitive materials (even the lead-free tin-based perovskite Cs2SnI6) does not yield equivalent overall performance. The expected comparative results show that the sensors prepared in Comparative Examples 9-10 will be significantly inferior to the device of Example 3 of this invention in one or more aspects, such as sensitivity, response / recovery characteristics, long-term stability, or resistance to environmental interference (e.g., light exposure). This highlights the criticality and non-obviousness of the specific material choice of MA2SnCl6 for achieving the inventive objective of "highly accurate and interference-resistant plant moisture detection."

[0065] Comparative Example 11 (omitting this optimization step) directly confirms the necessity of this process. After omitting the optimization step, the morphology of the sensitive material (comparative) Figure 3 (a) and Figure 3 (b)(c) and the specific surface area will deteriorate, directly leading to a decrease in the humidity response sensitivity and a slower response speed of the prepared sensor. This indicates that the optimization process is not a routine step that can be disregarded, but rather a key technological innovation that can substantially improve the core performance of the sensor of this invention.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a wearable plant humidity sensor, characterized in that, Includes the following steps: S1. The flexible substrate is ultrasonically cleaned with ultrapure water and ethanol, dried, and vapor-deposited to obtain Ag interdigitated electrodes. S2. Dissolve MACl and SnCl4·5H2O in a solvent and react to obtain a precursor solution; S3. The precursor solution obtained in step S2 is dropped onto the Ag interdigitated electrode prepared in step S1. After spin coating and annealing, an MA2SnCl6 thin film is formed on the electrode, thus obtaining the wearable plant humidity sensor.

2. The preparation method according to claim 1, characterized in that, In step S2, the molar ratio of MACl to SnCl4·5H2O is 1:1-5:

1.

3. The preparation method according to claim 1, characterized in that, In step S2, the solvent is selected from at least one of ultrapure water, N,N-dimethylformamide, and dimethyl sulfoxide.

4. The preparation method according to claim 1, characterized in that, In step S2, the reaction conditions are: a temperature of 90-110℃ and a stirring speed of 700-800 r / min for 2-3 hours.

5. The preparation method according to claim 1, characterized in that, In step S2, after obtaining the precursor solution, the precursor solution is mixed with isopropanol or ammonia and centrifuged. The precipitate is then redispersed to obtain an optimized precursor solution for use in step S3.

6. The preparation method according to claim 1, characterized in that, In step S1, the drying is performed by blowing with a nitrogen gas stream; the Ag interdigitated electrode has a size of 1 cm × 2 cm and is subjected to ultraviolet ozone treatment for 5-6 minutes after vapor deposition.

7. The preparation method according to claim 1, characterized in that, In step S3, the spin coating speed is 1000-3000 r / min and the time is 1-2 minutes; the annealing is performed at 70-90℃ for 1-2 hours.

8. A wearable plant humidity sensor prepared by the method according to any one of claims 1-7, characterized in that, It includes a flexible substrate, an Ag interdigitated electrode disposed on the flexible substrate, and an MA2SnCl6 humidity-sensitive film covering the Ag interdigitated electrode.

9. The wearable plant humidity sensor as described in claim 8, characterized in that, The flexible substrate is a PET film.

10. The application of the wearable plant humidity sensor as described in claim 8 in detecting surface moisture of live pumpkin seedling leaves.

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