Flexible breathable plant leaf moisture monitoring sensor as well as preparation method and application thereof

Abstract

The invention discloses a flexible breathable plant leaf moisture monitoring sensor as well as a preparation method and application thereof, and relates to the technical field of plant moisture monitoring. The porous electrode which is easy to prepare and has flexibility, air permeability and light transmission is obtained through the preparation method, the preparation process of the electrode is different from that of electrostatic spinning, and the porous structure of the electrode can be obtained through a sacrifice template method; the sacrificial particles with different particle sizes, such as sodium chloride or lactose, can be used for preparing micron-scale and nano-scale PDMS substrates with different pore sizes, so that the controllability and the stability of a microstructure are realized; meanwhile, the conductive silver layer is uniformly evaporated on the surface of the PDMS thin film with the microcosmic porous structure in a vacuum evaporation mode, and the thickness of the silver layer can be accurately controlled through a film thickness gauge. And the nano-scale silver particles almost have no influence on the porous structure, so that the air permeability and the light transmission of the electrode are ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of plant moisture monitoring, and in particular to a flexible breathable plant leaf moisture monitoring sensor and a preparation method and application thereof. Background Art

[0002] In the field of plant moisture monitoring, the water status of plants can be directly reflected by monitoring the capacitance signal of plants. This is because the relative content of water in plants is very high, and the dielectric constant is much higher than other components in plant tissues. There are currently two main monitoring modes for plant capacitance signals, invasive and non-invasive. In invasive monitoring, researchers place needle electrodes in the plant body and monitor them with an LCR monitor to obtain the capacitance signal of the plant tissue. Then, the physiological indicators of the plant are analyzed based on the monitoring results. The non-invasive monitoring method uses metal parallel plates as electrodes and plant tissues as dielectric layers to form parallel plate capacitors. The working principles of the two are similar, and both judge the moisture content of the sample to be tested by analyzing the capacitance signal. However, the invasive method causes damage to plant growth, and the non-invasive rigid electrodes are difficult to ensure that they fit the plant surface, and the measurement results have large errors.

[0003] With the development of flexible electronic technology, researchers use flexible substrates and use methods such as magnetron sputtering, vacuum evaporation, and electrospinning to manufacture conductive layers, obtain electrodes with good flexibility, and then attach them to leaves to achieve in-situ sensing of plant moisture. Some researchers also directly attach liquid metal electrodes, conductive polymer electrodes, etc. to the surface of leaves, and use impedance spectroscopy to analyze the electrical signals of plants to obtain information on the physiological status of plants.

[0004] The water content in the leaves reaches 80% to 90%, and the dielectric constant of water is about 80, which is much higher than other sufficient dielectric constants in plant tissues. Moreover, the leaves, as poor conductors, can form parallel plate capacitors together with parallel plate electrodes. The capacitance of a capacitor is affected by the plate area, the plate spacing, and the dielectric constant of the dielectric layer. The water content of the leaves of the leaf capacitor is the main factor causing the change in the dielectric constant. The thickness of the leaves remains basically unchanged. Therefore, the change in the parallel plate capacitance value of the leaves as a dielectric can be directly used to reflect the change in the water content of the plant leaves. When the water content of the leaves increases, the dielectric constant changes, and the leaf capacitance will also increase accordingly. Conversely, the leaf capacitance will decrease.

[0005] However, traditional rigid electrodes cannot effectively fit the blades, which not only reduces the accuracy of the results, but also causes mechanical damage to the blades, and the monitoring results also have a lag.

[0006] Although the use of flexible electrodes to monitor leaf capacitance achieves in-situ sensing of plant moisture and reduces mechanical damage, the electrodes placed on the leaves also need to consider the impact of the electrodes on leaf photosynthesis, transpiration and other material exchange activities, which significantly hinders the gas exchange of plant leaves. Moreover, the moisture produced by leaf transpiration will partially accumulate between the leaves and the electrodes, causing inaccurate monitoring results. In addition, the complex electrode preparation method increases the cost and is difficult to use on a large scale.

[0007] Therefore, a new solution to the above problems needs to be proposed. Summary of the invention

[0008] The purpose of the present invention is to provide a flexible breathable plant leaf moisture monitoring sensor and a preparation method and application. By optimizing the preparation process and adjusting various preparation parameters, a low-cost, flexible breathable electrode is developed to solve the technical problems raised in the background technology.

[0009] To achieve the above object, the present invention provides the following technical solution: a method for preparing a flexible breathable plant leaf moisture monitoring sensor, comprising at least the following steps:

[0010] S1: Use a commercial engraving machine to cut the film carrier;

[0011] S2: Prepare PDMS solution and stir it thoroughly with a magnetic stirrer;

[0012] S3: Coat the PDMS solution on the PET carrier, then place the PET on a coating machine and rotate it at 250 rpm for 60 seconds to obtain a PDMS film with uniform thickness;

[0013] S4: Use sodium chloride as sacrificial particles and disperse an appropriate amount of it on the PDMS film, then let it stand for 20 minutes;

[0014] S5: placing the PDMS and sodium chloride mixed film after standing in a vacuum drying oven to accelerate the curing of PDMS;

[0015] S6: placing the solidified PDMS and sodium chloride mixed film in pure water to fully dissolve it, and after fully dissolving, placing it in a vacuum drying oven to dry it to obtain a film;

[0016] S7: attach the dried film to the substrate of the vacuum evaporation coating equipment, put it into the cavity, and place an appropriate amount of pure silver particles and an appropriate amount of chromium particles on two different tungsten boats respectively;

[0017] S8: Close the vacuum chamber door and start automatic vacuum until the vacuum degree in the chamber reaches below 1×10-3MPa;

[0018] S9: Set the evaporation temperature and start coating. First, coat 5nm of chromium, then coat silver. When the film thickness meter shows that the thickness of the silver layer reaches 80nm, stop coating.

[0019] S10: Use a laser cutter to cut the silver-coated PDMS into 15 mm circular electrodes.

[0020] Furthermore, the thin film carrier cut in S1 is circular with a diameter of 8 cm.

[0021] Furthermore, the ratio of the PDMS solution in S2 is 10:1.

[0022] Furthermore, the vacuum drying oven parameters in S5 are set to 90° C. for 1 hour.

[0023] Furthermore, there is one pure silver particle in the tungsten boat in S7, and the weight is 0.3 g to 0.5 g, and there are one to two chromium particles in the tungsten boat in S7, and the total weight is 0.05 g to 0.15 g.

[0024] A new technical solution is further proposed:

[0025] The flexible breathable plant leaf moisture monitoring sensor is obtained by processing the above-mentioned flexible breathable plant leaf moisture monitoring sensor preparation method.

[0026] Further, the flexible breathable plant leaf moisture monitoring sensor includes a silver layer and porous PDMS;

[0027] The porous PDMS is used as a flexible substrate. Due to the presence of the micron-nano stepped pore structure, the pressure difference generated by the stepped pore size on the diffusion channel is more conducive to the gas-liquid transfer rate through the Bernoulli effect and capillary action, providing good air permeability, which is more conducive to the absorption and desorption of water, so that the sensor has good air permeability and will not affect the physiological behaviors such as transpiration and photosynthesis of leaves.

[0028] The silver layer serves as a conductive layer and as a capacitor electrode.

[0029] A new technical solution is further proposed:

[0030] The application of the flexible breathable plant leaf moisture monitoring sensor is to use the flexible breathable plant leaf moisture monitoring sensor for monitoring the capacitance of plant leaves. The flexible breathable plant leaf moisture monitoring sensor is used as a flexible breathable electrode when used on plant leaves.

[0031] Furthermore, the process and verification of the flexible breathable plant leaf moisture monitoring sensor for plant leaf detection includes at least the following steps:

[0032] The plant leaves are used as dielectric layers and the flexible breathable electrodes are used as plates to form parallel plate capacitors.

[0033] The flexible gas-permeable electrodes are attached in pairs to the upper and lower surfaces of mature leaves while avoiding the main veins of the leaves;

[0034] Connect the porous electrode and the test instrument, record the leaf capacitance signal of the sensor, and draw a curve;

[0035] The leaves with electrodes attached were removed from the plants and placed on an electronic balance, and the capacitance of the leaves was monitored using an LCR tester.

[0036] Record the current capacitance value and leaf mass every hour, and calculate the difference between the lost leaf water mass and leaf capacitance;

[0037] A linear fitting curve of leaf capacitance change value and leaf water mass change value was drawn;

[0038] It can be seen from the linear fitting curve that the leaf capacitance is linearly correlated with the change of leaf moisture, so the leaf capacitance can be used to directly reflect the leaf moisture.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. The present invention obtains a porous electrode that is easy to prepare and has flexibility, air permeability and light transmittance through a preparation method. The preparation process of the electrode is different from electrospinning. Its porous structure can be obtained by using a sacrificial template method. The sacrificial particles of different particle sizes such as sodium chloride or lactose can be used to prepare PDMS substrates with different pore sizes at the micrometer and nanometer levels, achieving the controllability and stability of the microstructure; at the same time, a conductive silver layer is uniformly evaporated on the surface of the PDMS film with a microscopic porous structure by vacuum evaporation, and the thickness of the silver layer can be accurately controlled by a film thickness meter. In addition, the nanometer-level silver particles have almost no effect on the porous structure, ensuring the air permeability and light transmittance of the electrode.

[0041] 2. The present invention uses a commercial engraving machine to cut PET film as a carrier of PDMS solution, which not only saves costs, but also the commercial engraving machine can cut multiple PET carriers at the same time, which has a large-scale preparation basis. The use of a glue machine can make a PDMS film with uniform thickness, and the speed and time of the glue machine can be adjusted to control the thickness of the PDMS film. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0043] Figure 1 Schematic diagram of the electrode structure of the present invention;

[0044] Figure 2 This is the electrode permeability test diagram;

[0045] Figure 3 is the linear fitting curve of the change of leaf capacitance and water mass loss when the leaves gradually lose water;

[0046] Figure 4 The electrodes and blades form a blade capacitance sensor, which monitors the results 24 hours a day;

[0047] Figure 5 This is the electrode stability test diagram;

[0048] Figure 6 This is a photo of the leaf after the electrode was removed after 30 days of attachment.

[0049] Figure 7 This is the change curve of leaf capacitance of Pothos sinensis during one day of waterless culture.

[0050] In the figure: 1. Silver layer; 2. Porous PDMS. DETAILED DESCRIPTION

[0051] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0052] Embodiment 1:

[0053] The preparation method of the flexible breathable plant leaf moisture monitoring sensor comprises at least the following steps:

[0054] S1: Use a commercial engraving machine to cut the film carrier;

[0055] S2: Prepare PDMS solution and stir it thoroughly with a magnetic stirrer;

[0056] S3: Coat the PDMS solution on the PET carrier, then place the PET on a coating machine and rotate it at 250 rpm for 60 seconds to obtain a PDMS film with uniform thickness;

[0057] S4: Use sodium chloride as sacrificial particles and disperse an appropriate amount of it on the PDMS film, then let it stand for 20 minutes;

[0058] S5: placing the PDMS and sodium chloride mixed film after standing in a vacuum drying oven to accelerate the curing of PDMS;

[0059] S6: placing the solidified PDMS and sodium chloride mixed film in pure water to fully dissolve it, and after fully dissolving, placing it in a vacuum drying oven to dry it to obtain a film;

[0060] S7: attaching the dried film to the substrate of the vacuum evaporation coating device and placing it in the cavity. In this embodiment, one (about 0.4 grams) of pure silver particles and one to two (about 0.1 grams) of chromium particles are placed in two different tungsten boats respectively. Different amounts may be used in different embodiments.

[0061] S8: Close the vacuum chamber door and start automatic vacuum until the vacuum degree in the chamber reaches below 1×10-3MPa;

[0062] S9: Set the evaporation temperature and start coating. First, coat 5nm of chromium, then coat silver. When the film thickness meter shows that the thickness of the silver layer reaches 80nm, stop coating.

[0063] S10: Use a laser cutter to cut the silver-coated PDMS into 15 mm circular electrodes.

[0064] The film carrier cut in S1 is a circular shape with a diameter of 8 cm.

[0065] The ratio of the PDMS solution in S2 is 10:1.

[0066] The vacuum drying oven parameters in S5 were set to 90°C for 1 h.

[0067] Embodiment 2:

[0068] See also Figure 1 , this embodiment specifically obtains a flexible breathable plant leaf moisture monitoring sensor based on the method proposed in the above embodiment 1;

[0069] The flexible breathable plant leaf moisture monitoring sensor includes a silver layer 1 and a porous PDMS 2;

[0070] Porous PDMS2 is used as a flexible substrate. Due to the presence of micron-nano stepped pore structure, the pressure difference generated by the stepped pore size on the diffusion channel is more conducive to the gas-liquid transfer rate through the Bernoulli effect and capillary action, providing good air permeability. For specific data and results, please refer to Figure 2The air permeability of the porous electrode reaches 51.98g h-1m-2, which is about 10 times that of the non-porous electrode, and is much higher than the gas exchange capacity of natural transpiration of leaves (20g h-1m-2), and will not limit the transpiration of leaves.

[0071] The silver layer 1 serves as a conductive layer and as a capacitor electrode;

[0072] At the same time, the flexible electrodes can fit the blade surface well, ensuring the detection accuracy of the physical signals during the test;

[0073] Therefore, this wearable sensor based on micro-nano stepped pore structure can achieve accurate detection of physiological signals over a long period of time.

[0074] Embodiment three:

[0075] This embodiment proposes an application of a flexible breathable plant leaf moisture monitoring sensor based on the above-mentioned embodiment 2;

[0076] The flexible breathable plant leaf moisture monitoring sensor is used to monitor the capacitance of plant leaves. The flexible breathable plant leaf moisture monitoring sensor is used as a flexible breathable electrode when used on plant leaves.

[0077] In this embodiment, the plant leaves used are green radish leaves, which may be different plant leaves in different embodiments.

[0078] The specific operation of using the leaves of green radish as the dielectric layer is further proposed:

[0079] In the present invention, the green radish leaves are used as the dielectric layer, and the flexible breathable electrodes are used as the plates to form a parallel plate capacitor. The electrodes are attached in pairs to the upper and lower surfaces of the mature leaves, and are careful to avoid the main veins of the leaves. The porous electrodes and the testing instrument are connected, the leaf capacitance signal of the sensor is recorded, and a curve is drawn. The green radish leaves with electrodes attached are removed from the plant and placed on an electronic balance, and the leaf capacitance value is monitored using an LCR tester. The current capacitance value and leaf mass are recorded every hour, and the difference between the lost leaf water mass and the leaf capacitance is calculated. The following curves are drawn: Figure 3 The linear fitting curve of the leaf capacitance change value and the leaf water mass change value is shown. It can be seen from the figure that the leaf capacitance is linearly related to the leaf water mass change, so the leaf capacitance can be used to directly reflect the leaf water mass.

[0080] Specific data and results of the experiments based on the above description:

[0081] Through the above three embodiments, it is further proposed to use the leaves of green radish as the dielectric layer. Figure 3 - Figure 5 :

[0082] Part of the process in the experiment is the same as in Example 3. The leaves of the green radish are used as the dielectric layer and the flexible breathable electrodes are used as the plates to form a parallel plate capacitor. The electrodes are attached in pairs to the upper and lower surfaces of mature leaves, and are careful to avoid the main veins of the leaves. The porous electrodes and the test instrument are connected to record the leaf capacitance signal of the sensor and draw a curve. In the 24-hour continuous monitoring, the capacitance of the leaf capacitance sensor shows a trend of being low during the day and high at night. Figure 4 shown.

[0083] This is consistent with the results of leaf moisture testing by previous researchers.

[0084] Compared with traditional commercial electrodes, the flexible electrode monitors moisture changes of up to 20%, while the moisture change rate monitored by commercial electrodes measured in the same period is less than 10%, which fully demonstrates the high sensitivity of this flexible electrode.

[0085] Furthermore, the stability of the sensor in external environments was explored, where PET was used as a dielectric layer, e.g. Figure 5 As shown. The capacitance value of the sensor maintains good stability under different humidity conditions, and the relative capacitance change (RCC) does not exceed 2%; the capacitance value of the sensor maintains good stability under different temperatures, and the relative capacitance change (RCC) does not exceed 1%; under different light, the relative capacitance changes in the range of -1% to 1%. The long-term stability of the sensor was tested at room temperature, and the relative capacitance changed by less than 0.5%, indicating that the sensor is reliable under continuous monitoring and exhibits good stability. The porous electrode can adapt to different environmental conditions and has certain application potential in real-time monitoring of leaf moisture.

[0086] The health of the tested plants and leaves was followed up and observed. During the entire test process, the electrodes had no significant effect on the growth and development of the plants. After the electrodes were removed after 30 days, the leaf morphology did not change significantly. This shows that the electrodes have good biocompatibility. Figure 6 shown.

[0087] A flexible gas-permeable electrode with micron-nano stepped pores was prepared by a simple sacrificial template method, with a water vapor permeability of up to 51.98gh-1m-2;

[0088] The flexible electrodes fit perfectly to the leaves, enabling real-time monitoring of leaf moisture.

[0089] After the electrode was attached to the leaf for 30 days, there was no obvious impact on the leaf morphology, which proved the excellent biocompatibility of the electrode.

[0090] Embodiment 4:

[0091] The steps of the fourth embodiment are the same as the early steps of the third embodiment, except that:

[0092] The normally cultivated green radish was placed in a dry glass cup and the leaf capacitance value was monitored for a whole day. The relative change rate of its leaf capacitance was calculated and plotted. Figure 7 The curve of the relative change rate of leaf capacitance over time is shown. As can be seen from the figure, the capacitance value is on a downward trend as a whole. And there are some fluctuations, which indicates that the green radish plant is still carrying out corresponding physiological activities (transpiration, respiration, etc.), the water in the plant is still being transported in the body, and the plant is still very active. However, from the overall negative change rate, it can be seen that the overall water content of the plant has been continuously lost, indicating that the plant is facing severe water stress.

[0093] In summary:

[0094] The present invention uses the water vapor passage rate to characterize the air permeability of the electrode. The test result is 51.98gh-1m-2, which is higher than the average transpiration rate of the leaves, and the air permeability is much higher than that of ordinary PDMS electrodes that have not been porous treated, indicating that the porous electrode has excellent air permeability. The stability of the electrode was also tested. The electrode was placed under different humidity, temperature, illumination and 12h of standing, and its capacitance change value was tested respectively. From the capacitance change rate in the figure, it can be seen that the electrode has good stability and can be used in actual tests to obtain reliable monitoring results. After wearing for up to 30 days, there is no obvious restriction on the normal physiological activities of the leaves, showing good biocompatibility.

[0095] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

Claims

1. A method for preparing a flexible breathable plant leaf moisture monitoring sensor, characterized in that: At least the following steps are included: S1: Use a commercial engraving machine to cut the film carrier; S2: Prepare PDMS solution and stir it thoroughly with a magnetic stirrer; S3: Coat the PDMS solution on the PET carrier, then place the PET on a coating machine and rotate it at 250 rpm for 60 seconds to obtain a PDMS film with uniform thickness; S4: Use sodium chloride as sacrificial particles and disperse an appropriate amount of it on the PDMS film, then let it stand for 20 minutes; S5: placing the PDMS and sodium chloride mixed film after standing in a vacuum drying oven to accelerate the curing of PDMS; S6: placing the solidified PDMS and sodium chloride mixed film in pure water to fully dissolve it, and after fully dissolving, placing it in a vacuum drying oven to dry it to obtain a film; S7: attach the dried film to the substrate of the vacuum evaporation coating equipment, put it into the cavity, and place an appropriate amount of pure silver particles and an appropriate amount of chromium particles on two different tungsten boats respectively; S8: Close the vacuum chamber door and start automatic vacuum until the vacuum degree in the chamber reaches below 1×10-3MPa; S9: Set the evaporation temperature and start coating. First, coat 5nm of chromium, then coat silver. When the film thickness meter shows that the thickness of the silver layer reaches 80nm, stop coating. S10: Use a laser cutter to cut the silver-coated PDMS into 15 mm circular electrodes.

2. The method for preparing the flexible breathable plant leaf moisture monitoring sensor according to claim 1, characterized in that: The film carrier cut in S1 is circular with a diameter of 8 cm.

3. The method for preparing the flexible breathable plant leaf moisture monitoring sensor according to claim 1, characterized in that: The ratio of the PDMS solution in S2 is 10:

1.

4. The method for preparing the flexible breathable plant leaf moisture monitoring sensor according to claim 1, characterized in that: The vacuum drying oven parameters in S5 are set to 90° C. for 1 h.

5. The method for preparing the flexible breathable plant leaf moisture monitoring sensor according to claim 1, characterized in that: There is one pure silver particle in the tungsten boat in S7, and the weight is 0.3g to 0.5g. There are one to two chromium particles in the tungsten boat in S7, and the total weight is 0.05g to 0.15g.

6. Flexible breathable plant leaf moisture monitoring sensor, characterized by: The flexible breathable plant leaf moisture monitoring sensor is obtained by the preparation method of claims 1-5.

7. The flexible breathable plant leaf moisture monitoring sensor according to claim 6, characterized in that: It comprises a silver layer (1) and porous PDMS (2); The porous PDMS (2) is used as a flexible substrate. Due to the presence of the micron-nano stepped pore structure, the pressure difference generated by the stepped pore size on the diffusion channel is more conducive to the gas-liquid transfer rate through the Bernoulli effect and capillary action, providing good air permeability, which is more conducive to the absorption and desorption of water, so that the sensor has good air permeability and will not affect the physiological behaviors of the leaves such as transpiration and photosynthesis; The silver layer (1) serves as a conductive layer and as a capacitor electrode.

8. Application of flexible breathable plant leaf moisture monitoring sensor, characterized by: The flexible breathable plant leaf moisture monitoring sensor described in claim 6 is used for monitoring the capacitance of plant leaves. The flexible breathable plant leaf moisture monitoring sensor is used as a flexible breathable electrode when used on plant leaves.

9. The application of the flexible breathable plant leaf moisture monitoring sensor according to claim 8, characterized in that: The process and verification of the flexible breathable plant leaf moisture monitoring sensor for plant leaf detection includes at least the following steps: The plant leaves are used as dielectric layers and the flexible breathable electrodes are used as plates to form parallel plate capacitors. The flexible gas-permeable electrodes are attached in pairs to the upper and lower surfaces of mature leaves while avoiding the main veins of the leaves; Connect the porous electrode and the test instrument, record the leaf capacitance signal of the sensor, and draw a curve; The leaves with electrodes attached were removed from the plants and placed on an electronic balance, and the capacitance of the leaves was monitored using an LCR tester. Record the current capacitance value and leaf mass every hour, and calculate the difference between the lost leaf water mass and leaf capacitance; A linear fitting curve of leaf capacitance change value and leaf water mass change value was drawn; It can be seen from the linear fitting curve that the leaf capacitance is linearly correlated with the change of leaf moisture, so the leaf capacitance can be used to directly reflect the leaf moisture.

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