An ionic liquid and lithium chloride doped metal organic framework material and a humidity sensor using the same as a material and a preparation method thereof

Abstract

This invention discloses a metal-organic framework material doped with ionic liquid and lithium chloride, a humidity sensor using the same material as a sensing element, and its preparation method, belonging to the field of humidity sensor technology. This invention uses a solvothermal method to prepare the metal-organic framework material, and employs an in-situ encapsulation method to introduce the ionic liquid followed by lithium chloride doping for composite modification, resulting in a metal-organic framework material co-modified by ionic liquid and lithium chloride. The low-humidity sensor of this invention consists of a substrate with silver interdigitated electrodes on its surface and a sensing layer drop-coated onto the substrate and the silver interdigitated electrodes. The humidity sensor prepared using the ionic liquid and lithium chloride doped metal-organic framework material exhibits good humidity-sensing characteristics in the low humidity range, including high sensitivity, good linearity, long-term stability, low hysteresis, and fast response recovery. This invention utilizes the high ionic conductivity of the ionic liquid and the strong hygroscopicity of lithium chloride to enhance humidity response through an ion migration mechanism, avoiding the use of precious metals, reducing costs, and simultaneously improving the sensor's environmental adaptability and controllability, providing a more economical and efficient solution for low-humidity detection.

Description

Technical Field

[0001] This invention relates to a humidity sensor using an ionic liquid and lithium chloride-doped metal-organic framework material as a sensing element, and a method for preparing the same, belonging to the field of humidity sensing technology. Background Technology

[0002] Sensors, as the core foundation for realizing the Internet of Things in the information age, are key devices that convert measured physical or chemical signals into readable data. Humidity, as a crucial physical quantity characterizing the water molecule content in the air, is indispensable for accurate monitoring in numerous fields vital to national welfare and people's livelihoods. For example, in the healthcare field, ventilators, infant incubators, and drug storage environments require precise humidity control to ensure patient safety and drug effectiveness; in industrial production, processes such as semiconductor manufacturing, precision instrument assembly, and food processing are extremely sensitive to environmental humidity, with even minor fluctuations affecting product yield; in agricultural science, humidity control in greenhouses directly impacts crop growth and pest and disease control; and in extreme environments such as national defense and aerospace, ammunition storage and cabin environment control place stringent demands on the reliability and stability of humidity monitoring.

[0003] Traditional humidity sensors, such as those based on polymers or metal oxides, still face numerous bottlenecks in practical applications. They generally suffer from low sensitivity, slow response recovery, narrow operating temperature range, insufficient long-term stability, and susceptibility to cross-gas interference. Especially under high temperature, high humidity, or corrosive conditions, sensor performance is prone to degradation or even failure, making it difficult to meet the demands for accurate, real-time, and reliable monitoring in complex application scenarios. Therefore, developing novel humidity sensors that combine high sensitivity, fast response, low power consumption, excellent stability, and strong anti-interference capabilities has become a research hotspot and urgent need in the field of sensor technology.

[0004] Metal-organic frameworks (MOFs) offer a novel solution to overcome the performance bottlenecks of traditional humidity sensors due to their unique structural properties. These crystalline porous materials, formed by the self-assembly of metal ions or clusters with organic ligands through coordination bonds, possess extremely high specific surface areas, precisely tunable pore sizes and channel structures, a rich variety of chemical functional sites, and excellent structural designability. These characteristics enable MOFs to exhibit superior adsorption capacity for water molecules and specific host-guest interactions, holding promise for rapid and highly sensitive detection across a range from low to high humidity. Summary of the Invention

[0005] The purpose of this invention is to provide a humidity sensor and its preparation method using an ionic liquid and lithium chloride-doped metal-organic framework material as the sensing material, aiming to solve the shortcomings of existing low humidity sensors in terms of sensitivity, cost and environmental adaptability.

[0006] This invention employs a solvothermal method to prepare metal-organic framework materials (MOFs). An in-situ encapsulation method is used to introduce an ionic liquid, followed by lithium chloride doping for composite modification, resulting in a MOF co-modified by the ionic liquid and lithium chloride. The ionic liquid and lithium chloride-doped MOF described in this invention is obtained through the following steps: (1) Dissolve 0.25~2.5 mmol of zirconium source and 0.27~2.7 mmol of 2-aminoterephthalic acid in 10~100 mL of N,N-dimethylformamide; (2) Add 0.2~2 mmol of 1,3-dimethylimidazolium chloride to the solution obtained in step (1); (3) Add 1~7 mL of glacial acetic acid and 1~5 mL of deionized water to the mixed solution obtained in step (2), disperse it evenly by ultrasonication, and then transfer it to a hydrothermal reactor and treat it at 120℃ for 24~36 hours. (4) Cool the solution in the hydrothermal reactor in step (3) to room temperature, centrifuge to obtain reddish-brown powder, wash the reddish-brown powder with methanol and ethanol, centrifuge and dry to obtain reddish-brown powder. (5) Weigh 30-120 mg of the reddish-brown powder obtained in step (4) and disperse it in 5-20 mL of deionized water. Disperse it evenly by ultrasonication. Then add 3-12 mg of lithium chloride and stir for 15-60 minutes to obtain a mixed solution. (6) After the mixed solution obtained in step (5) is left to stand or centrifuged at room temperature, it is dried at 40~80℃ to obtain the metal-organic framework material doped with ionic liquid and lithium chloride.

[0007] In the preparation of an ionic liquid and lithium chloride-doped metal-organic framework material according to the present invention, the mass percentage of lithium chloride is 5-15 wt%.

[0008] In the preparation of an ionic liquid and lithium chloride-doped metal-organic framework material according to the present invention, the zirconium source is one or more of zirconium chloride, zirconium oxychloride, or zirconium nitrate.

[0009] The present invention discloses a humidity sensor with low humidity sensing characteristics. The sensor structure includes a substrate, interdigitated electrodes on the substrate, and a composite sensing material layer prepared by the above method coated on the electrodes. When the sensing layer comes into contact with water molecules in the environment, it adsorbs them, resulting in a significant change in impedance. By measuring this change, low humidity can be detected.

[0010] The humidity sensor described in this invention has low humidity sensing characteristics. The change in impedance between the interdigitated electrodes before and after moisture absorption is measured by an impedance analyzer, which can obtain the sensor's sensitivity, response time, repeatability and other performance indicators (test voltage AC 1V, frequency 500Hz).

[0011] The humidity sensor with low humidity sensing characteristics described in this invention is prepared as follows: (1) The ionic liquid and lithium chloride-doped metal-organic framework material were mixed with deionized water. The concentration of the metal-organic framework material was 5~20 mg / mL. Then, the mixture was ultrasonically dispersed to obtain a uniformly dispersed solution. (2) Take 1~10μL of the dispersion from step (1) and drop it onto a substrate with silver interdigitated electrodes on its surface; (3) Dry the device obtained in step (2) at 40~80℃ for 6~12 hours; (4) The device obtained in step (3) is aged at 60-80% relative humidity for 1-6 hours to obtain a humidity sensor based on ionic liquid and lithium chloride doped metal-organic framework material.

[0012] Advantages of this invention: (1) This invention utilizes the inherent high ionic conductivity of ionic liquids and the strong hygroscopicity and ionization properties of lithium chloride to construct a humidity-sensitive mechanism dominated by ion migration, which significantly enhances the change in the conductivity of the material. (2) This invention avoids excessive use of precious metal gold and toxic chemical reducing agents. The materials used, ionic liquid and lithium chloride, are inexpensive and readily available, greatly reducing raw material costs. At the same time, the preparation process is greener, safer, and more environmentally friendly, meeting the economic and environmental requirements of large-scale industrial production. (3) The ionic liquid and lithium chloride involved in this invention can stably fill or modify the pores of the metal-organic framework material, and the composite sensing material has a stable structure. (4) The silver cross-finger electrode in the humidity sensor prepared by the present invention has excellent chemical inertness and conductivity stability, which improves the signal-to-noise ratio and measurement accuracy while maintaining good long-term stability. Attached Figure Description

[0013] Figure 1 These are scanning electron microscope images of the ionic liquid (dimethylimidazolium bromide) and the lithium chloride-doped metal-organic framework material prepared in Example 2.

[0014] Figure 2 These are scanning electron microscope images of the ionic liquid (1,3-dimethylimidazolium chloride) and the lithium chloride-doped metal-organic framework material prepared in Example 1.

[0015] Figure 3 This document shows the substrate and corresponding parameters of the interdigitated electrodes of the humidity sensor made from the ionic liquid and lithium chloride-doped metal-organic framework material prepared in Example 1, along with a physical image.

[0016] Figure 4 The humidity sensor prepared in Example 1, using ionic liquid and lithium chloride-doped metal-organic framework materials, exhibits humidity sensing characteristics under low to medium humidity conditions.

[0017] Figure 5 The data is a repeatability curve of the humidity sensor prepared in Example 1 using ionic liquid and lithium chloride-doped metal-organic framework material under medium and low humidity conditions.

[0018] Figure 6 The humidity hysteresis curves of the humidity sensor prepared in Example 1 using ionic liquid and lithium chloride-doped metal-organic framework materials are shown at low to medium humidity. Specific implementation methods

[0019] Example 1: (1) Dissolve 279.7 mg of zirconium chloride and 216.6 mg of 2-aminoterephthalic acid in 48 mL of N,N-dimethylformamide to obtain a mixed solution of zirconium chloride and 2-aminoterephthalic acid. (2) Add 127.2 mg of 1,3-dimethylimidazolium chloride, labeled as 20 wt% ionic liquid, to the solution obtained in step (1); (3) Add 3.5 mL of glacial acetic acid and 2 mL of deionized water to the mixed solution obtained in step (2), disperse it evenly by ultrasonication, and then transfer it to a hydrothermal reactor and treat it at 120°C for 24 hours. (4) Cool the solution in the hydrothermal reactor in step (3) to room temperature, centrifuge to obtain reddish-brown powder, wash the reddish-brown powder with methanol and ethanol respectively, centrifuge and repeat 3 times to obtain reddish-brown powder. (5) Weigh 30mg of the reddish-brown powder obtained in step (4) and disperse it in 5mL of deionized water. Disperse it evenly by ultrasonication. Then add 3mg of lithium chloride and stir magnetically for 60 minutes to obtain a mixed solution labeled as a metal-organic framework material with 20wt% ionic liquid and 9wt% lithium chloride. (6) Before each use, ultrasonically disperse the metal-organic framework material of 20wt% ionic liquid and 9wt% lithium chloride obtained in step (5) for 15 minutes, transfer 2.5μL of the dispersion in step (5), drop the dispersion onto the substrate with silver interdigitated electrodes on the surface, and dry it at 60°C for 6 hours. (7) The device obtained in step (6) is aged at 60% relative humidity for 6 hours to obtain a humidity sensor based on ionic liquid and lithium chloride doped metal-organic framework material.

[0020] Example 2: (1) Dissolve 279.7 mg of zirconium chloride and 216.6 mg of 2-aminoterephthalic acid in 48 mL of N,N-dimethylformamide to obtain a mixed solution of zirconium chloride and 2-aminoterephthalic acid. (2) Add 127.2 mg of dimethylimidazolium bromide to the solution obtained in step (1); metal-organic framework material labeled as 20 wt% ionic liquid; (3) Add 3.5 mL of glacial acetic acid and 2 mL of deionized water to the mixed solution obtained in step (2), disperse it evenly by ultrasonication, and then transfer it to a hydrothermal reactor and treat it at 120°C for 24 hours. (4) Cool the solution in the hydrothermal reactor in step (3) to room temperature, centrifuge to obtain reddish-brown powder, wash the reddish-brown powder with methanol and ethanol respectively, centrifuge and repeat 3 times to obtain reddish-brown powder. (5) Weigh 34mg of the reddish-brown powder obtained in step (4) and disperse it in 7mL of deionized water. Disperse it evenly by ultrasonication. Then add 6mg of lithium chloride and stir magnetically for 60 minutes to obtain a mixed solution, labeled as a metal-organic framework material with 20wt% ionic liquid and 15wt% lithium chloride. (6) Before each use, ultrasonically disperse the metal-organic framework material of 20wt% ionic liquid and 15wt% lithium chloride obtained in step (5) for 15 minutes, transfer 2.5μL of the dispersion in step (5), drop the dispersion onto the substrate with silver interdigitated electrodes on the surface, and dry it at 60°C for 6 hours. (7) The device obtained in step (6) is aged at 60% relative humidity for 6 hours to obtain a humidity sensor based on ionic liquid and lithium chloride doped metal-organic framework material.

[0021] Example 3: (1) Dissolve 279.7 mg of zirconium chloride and 216.6 mg of 2-aminoterephthalic acid in 48 mL of N,N-dimethylformamide to obtain a mixed solution of zirconium chloride and 2-aminoterephthalic acid. (2) Add 127.2 mg of 1,3-dimethylimidazolium chloride, labeled as 20 wt% ionic liquid, to the solution obtained in step (1); (3) Add 3.5 mL of glacial acetic acid and 2 mL of deionized water to the mixed solution obtained in step (2), disperse it evenly by ultrasonication, and then transfer it to a hydrothermal reactor and treat it at 120°C for 24 hours. (4) Cool the solution in the hydrothermal reactor in step (3) to room temperature, centrifuge to obtain reddish-brown powder, wash the reddish-brown powder with methanol and ethanol respectively, centrifuge and repeat 3 times to obtain reddish-brown powder. (5) Weigh 19mg of the reddish-brown powder obtained in step (4) and disperse it in 4mL of deionized water. Disperse it evenly by ultrasonication. Then add 1mg of lithium chloride and stir magnetically for 60 minutes to obtain a mixed solution, labeled as a metal-organic framework material with 20wt% ionic liquid and 5wt% lithium chloride. (6) Before each use, ultrasonically disperse the metal-organic framework material of 20wt% ionic liquid and 5wt% lithium chloride obtained in step (5) for 15 minutes, transfer 2.5μL of the dispersion in step (5), drop the dispersion onto the substrate with silver interdigitated electrodes on the surface, and dry it at 60°C for 6 hours. (7) The device obtained in step (6) is aged at 60% relative humidity for 6 hours to obtain a humidity sensor based on ionic liquid and lithium chloride doped metal-organic framework material.

[0022] Scanning electron microscope images of the ionic liquid (dimethylimidazolium bromide) and lithium chloride-doped metal-organic framework material prepared in Example 2 are shown below. Figure 1 As shown. By Figure 1 It can be seen that the ionic liquid (dimethylimidazolium bromide) and lithium chloride-doped metal-organic framework materials are mainly composed of a large number of irregularly shaped particles, which are densely packed and distributed without obvious regular arrangement; the particle size is submicron and the specific surface area is relatively small; the ionic liquid may be more loosely distributed in the channels, the mass transfer path is slightly longer, and the response speed is slightly slower.

[0023] Scanning electron microscope images of the ionic liquid (1,3-dimethylimidazolium chloride) and the lithium chloride-doped metal-organic framework material prepared in Example 1 are shown below. Figure 2 As shown. By Figure 2 It can be seen that the main body of the ionic liquid (1,3-dimethylimidazolium chloride) and lithium chloride-doped metal-organic framework material consists of a large number of small particles that are tightly aggregated and randomly arranged. The particle size is at the nanoscale, and the total specific surface area is larger, which can expose more MOF channels and hydrophilic sites of ionic liquid, which is conducive to the adsorption / desorption of water molecules and improves the humidity response sensitivity.

[0024] Details of the substrate and interdigitated electrode parameters and physical images of the humidity sensor made of ionic liquid and lithium chloride-doped metal-organic framework material prepared in Example 1 are shown below. Figure 3 As shown, the humidity sensor has a reasonable substrate structure, uniformly distributed interdigitated electrodes, is easy to process, and has a low cost. In addition, the silver interdigitated electrodes have excellent chemical inertness and conductivity stability, which improves the signal-to-noise ratio and measurement accuracy while maintaining good long-term stability.

[0025] The humidity sensor prepared in Example 1, using an ionic liquid and lithium chloride-doped metal-organic framework, exhibits the following humidity sensing characteristics under low to medium humidity conditions: Figure 4 As shown, the impedance change rate of the humidity sensor reaches 99% in the relative humidity range of 0-50%, indicating that the humidity sensor with ionic liquid and lithium chloride doped metal-organic framework materials has a good response to low and medium humidity.

[0026] The response recovery curves of the humidity sensor prepared in Example 1, using an ionic liquid and lithium chloride-doped metal-organic framework material, over 420 seconds between 0% and 20% relative humidity are shown below. Figure 5 As shown, the humidity sensor underwent three cycles within 420 seconds, and its sensitivity did not change significantly during the cycles. The response and recovery times were 31 seconds and 34 seconds, respectively, further demonstrating that the humidity sensor based on ionic liquid and lithium chloride-doped metal-organic framework materials has a fast response and good stability to low and medium humidity.

[0027] The humidity hysteresis curves of the humidity sensor prepared in Example 1 using ionic liquid and lithium chloride-doped metal-organic framework materials under medium to low humidity conditions are shown below. Figure 6 As shown in the figure, the humidity sensor exhibits very low hysteresis and high stability within the relative humidity range of 0-50%.

Claims

1. A metal-organic framework material doped with ionic liquid and lithium chloride, characterized in that: By utilizing the high ionic conductivity of ionic liquids and the strong hygroscopicity of lithium chloride, the humidity response is enhanced through an ion migration mechanism.

2. The metal-organic framework material as described in claim 1, characterized in that: It is obtained through the following steps: (1) Dissolve 0.25–2.5 mmol of zirconium source and 0.27–2.7 mmol of 2-aminoterephthalic acid in 10–100 mL of N,N-dimethylformamide; (2) Add 0.2~2 mmol of 1,3-dimethylimidazolium chloride to the solution obtained in step (1); (3) Add 1-7 mL of glacial acetic acid and 1-5 mL of deionized water to the mixed solution obtained in step (2), disperse it evenly by ultrasonication, and then transfer it to a hydrothermal reactor for treatment at 120-140℃ for 24-36 hours; (4) Cool the solution in the hydrothermal reactor in step (3) to room temperature, centrifuge to obtain a reddish-brown powder, wash the reddish-brown powder with methanol and ethanol, centrifuge and dry to obtain a reddish-brown powder; (5) Weigh 30-120 mg of the reddish-brown powder obtained in step (4) and disperse it in 5-20 mL of deionized water. Disperse it evenly by ultrasonication. Then add 3-12 mg of lithium chloride and stir for 15-60 minutes to obtain a mixed solution. (6) After the mixed solution obtained in step (5) is left to stand or centrifuged at room temperature, it is dried at 40~80℃ to obtain the metal-organic framework material doped with ionic liquid and lithium chloride.

3. The metal-organic framework material doped with ionic liquid and lithium chloride as described in claim 1, characterized in that: The zirconium source is one or more of zirconium chloride, zirconium oxychloride, or zirconium nitrate.

4. The metal-organic framework material doped with ionic liquid and lithium chloride as described in claim 1, characterized in that: The mass percentage of lithium chloride is 5-15 wt%.

5. A humidity sensor with low humidity sensing characteristics, comprising a substrate with silver interdigitated electrodes on its surface, and a sensing layer drop-coated on the substrate and the silver interdigitated electrodes, characterized in that: The sensing layer is the metal-organic framework material doped with ionic liquid and lithium chloride as described in claim 1.

6. The humidity sensor as described in claim 5, characterized in that: It is obtained through the following steps: (1) The metal-organic framework material according to claim 1 is mixed with deionized water, the concentration of the metal-organic framework material is 5~20 mg / mL, and then ultrasonically dispersed to obtain a uniformly dispersed dispersion; (2) Take 1~10μL of the dispersion from step (1) and drop it onto a substrate with silver interdigitated electrodes on its surface; (3) Dry the device obtained in step (2) at 40~80℃ for 6~12 hours; (4) The device obtained in step (3) is aged at 60-80% relative humidity for 1-6 hours to obtain a humidity sensor based on ionic liquid and lithium chloride doped metal-organic framework material.

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