Graphene-based capacitive sensor for detecting moisture content in lubricating oil, preparation method of graphene-based capacitive sensor and method for detecting moisture content in lubricating oil

Abstract

The invention provides a graphene-based capacitive sensor for detecting the moisture content in lubricating oil, a preparation method of the graphene-based capacitive sensor and a method for detecting the moisture content in the lubricating oil, and belongs to the technical field of moisture content detection. The graphene-based capacitive sensor for detecting the moisture content in the lubricating oil comprises a graphene-based electrode and a gold nanoparticle layer which are arranged in a laminated manner, the graphene-based electrode comprises a laser-induced graphene film and doped nanoparticles loaded on the surface and in pores of the laser-induced graphene film, and the doped nanoparticles are silver nanoparticles and copper oxide particle clusters; array holes are formed in the gold nanoparticle layer, the diameter of holes in the array holes is larger than the diameter of water molecules and smaller than the diameter of lubricating oil molecules, and the depth of the holes in the array holes is consistent with the thickness of the gold nanoparticle layer. The graphene-based capacitive sensor can realize real-time online detection of the moisture content in the lubricating oil, and is high in detection sensitivity and simple to operate.

Description

Technical Field

[0001] The present invention relates to the technical field of moisture content detection, and in particular to a graphene-based capacitive sensor for detecting the moisture content in lubricating oil, a preparation method thereof, and a method for detecting the moisture content in lubricating oil. Background Art

[0002] Trace moisture in lubricating oil will reduce its lubricating performance. Moisture will damage the oil film formed by the lubricating oil, resulting in a decrease in the oil film strength, an increase in the friction coefficient, and further an increase in equipment wear; moisture may also cause changes in the viscosity of the lubricating oil, affecting its fluidity and lubricating ability; after moisture and lubricating oil are mixed, an emulsion may be formed. An emulsion is an oil-water mixture, usually milky white. Emulsification will cause poor fluidity of the lubricating oil, reduce the lubricating effect, and may block the oil circuit, affecting the normal operation of the system. Therefore, it is of great significance to accurately detect the moisture content in lubricating oil.

[0003] Traditional methods for detecting the moisture content in lubricating oil are mostly laboratory off-line detections, and the procedures are cumbersome. Summary of the Invention

[0004] The purpose of the present invention is to provide a graphene-based capacitive sensor for detecting the moisture content in lubricating oil, a preparation method thereof, and a method for detecting the moisture content in lubricating oil. By using the graphene-based capacitive sensor of the present invention, real-time on-line detection of the moisture content in lubricating oil can be achieved, and the detection sensitivity is high and the operation is simple.

[0005] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions: The present invention provides a graphene-based capacitive sensor for detecting the moisture content in lubricating oil, including a graphene-based electrode and a gold nanoparticle layer arranged in a stacked manner; the graphene-based electrode includes a laser-induced graphene film and doped nanoparticles loaded on the surface and pores of the laser-induced graphene film, and the doped nanoparticles are silver nanoparticles and copper oxide particle clusters; an array of holes is provided on the gold nanoparticle layer, and the diameter of the holes in the array of holes is larger than the diameter of water molecules and smaller than the diameter of lubricating oil molecules, and the depth of the holes in the array of holes is the same as the thickness of the gold nanoparticle layer.

[0006] Preferably, the thickness of the laser-induced graphene film is 0.017~0.021 mm; the particle size of the silver nanoparticles is 10~100 nm; the particle size of the copper oxide particle clusters is 100~125 nm.

[0007] Preferably, the thickness of the gold nanoparticle layer is 850~950 nm; the gold nanoparticle layer is formed by gold nanoparticles, and the particle size of the gold nanoparticles is 15~25 nm; The diameter of each hole in the array of holes is independently 0.6 to 0.9 nm; based on the center of the hole, the distance between two adjacent holes is independently 0.005 to 0.01 mm.

[0008] Preferably, the graphene-based electrode is an interdigital electrode.

[0009] The present invention provides a method for preparing the graphene-based capacitive sensor described in the above technical solution, including the following steps: Provide a laser-induced graphene film, which is obtained by subjecting an organic polymer film to a first laser treatment; Coat a silver salt solution on the surface of the laser-induced graphene film, and after drying, perform a second laser treatment to obtain a silver nanoparticle-doped film; Coat a copper salt solution on the surface of the silver nanoparticle-doped film, and after drying, perform a third laser treatment to obtain a film co-doped with silver nanoparticles and copper oxide particle clusters; Use magnetron sputtering to deposit a gold nanoparticle layer on the surface of the film co-doped with silver nanoparticles and copper oxide particle clusters, and then perform laser array drilling on the surface of the gold nanoparticle layer to obtain the graphene-based capacitive sensor.

[0010] Preferably, the organic polymer film includes a polyimide film or a polybenzoxazine resin film; the conditions of the first laser treatment include: laser power of 3.78 to 4.15 W, laser scanning speed of 200 to 300 mm / s, initial defocus amount of 0 to 8 mm, and laser scanning spacing of 0.01 to 0.05 mm; The silver salt solution is a silver nitrate solution, the concentration of the silver salt solution is 0.4 to 0.6 mol / L, and the coating amount of the silver salt solution is 0.05 to 0.20 μL / mm 2 ; the conditions of the second laser treatment include: laser power of 3.96 to 4.70 W, laser scanning speed of 200 to 300 mm / s, defocus amount of 5 mm, and laser scanning spacing of 0.03 mm; The copper salt solution is a copper nitrate solution, the concentration of the copper nitrate solution is 0.4 to 0.6 mol / L, and the coating amount of the copper nitrate solution is 0.05 to 0.20 μL / mm 2 ; the conditions of the third laser treatment include: laser power of 4.32 to 4.90 W, laser scanning speed of 200 to 250 mm / s, defocus amount of 4 mm, and laser scanning spacing of 0.03 mm.

[0011] Preferably, the method for preparing the gold nanoparticle layer comprises the following steps: coating a mixed solution containing chloroauric acid and a reducing agent on the surface of the co-doped film of silver nanoparticles and copper oxide particle clusters, and performing magnetron sputtering to form a gold nanoparticle layer on the surface of the co-doped film of silver nanoparticles and copper oxide particle clusters.

[0012] Preferably, the conditions for the laser array drilling include: a laser power of 10.3-10.9 W, a laser scanning speed of 150-200 mm / s, and a defocus amount of 2 mm.

[0013] The present invention provides an application of the graphene-based capacitive sensor described in the above technical solution or the graphene-based capacitive sensor prepared by the preparation method described in the above technical solution in detecting the water content in lubricating oil.

[0014] Preferably, the lubricating oil includes gear oil, hydraulic oil, transformer oil or engine oil.

[0015] The present invention provides a graphene-based capacitive sensor for detecting the water content in lubricating oil, comprising a graphene-based electrode and a gold nanoparticle layer arranged in a stacked manner; the graphene-based electrode includes a laser-induced graphene film and doped nanoparticles loaded on the surface and pores of the laser-induced graphene film, and the doped nanoparticles are silver nanoparticles and copper oxide particle clusters; an array of holes is provided on the gold nanoparticle layer, the diameter of the holes in the array is larger than the diameter of water molecules and smaller than the diameter of lubricating oil molecules, and the depth of the holes in the array is the same as the thickness of the gold nanoparticle layer. In the present invention, doping silver nanoparticles in the laser-induced graphene film can provide a larger specific surface area for interaction with water molecules, thereby enhancing the response of the graphene-based capacitive sensor to water; moreover, silver nanoparticles can provide a high carrier concentration, thereby increasing the conductivity of the laser-induced graphene film, reducing its resistance, and enhancing the Joule heat effect, which is beneficial to removing water in the sample through the Joule heating reaction, thereby ensuring the accuracy of the test results when using the graphene-based capacitive sensor for the next experiment; in the present invention, copper oxide particle clusters can improve the hydrophilicity of the laser-induced graphene film, which is beneficial to increasing the surface energy of graphene and further improving its surface wettability; by providing a gold nanoparticle layer and arranging an array of holes thereon, only water molecules can pass through, and lubricating oil molecules cannot pass through, which increases the channels for water molecules to contact graphene and is beneficial to removing water in the sample through the Joule heating reaction, thereby ensuring the accuracy of the test results when using the graphene-based capacitive sensor for the next experiment. Using the graphene-based capacitive sensor provided by the present invention can realize the real-time on-line detection of the water content in lubricating oil, and has high detection sensitivity and simple operation, and can timely discover and handle the problem of excessive water content in lubricating oil, meeting the requirements of the rapid and effective operation of modern equipment. Description of the Drawings

[0016] Figure 1 Flow chart of the preparation method of the graphene-based capacitive sensor in the embodiment; Figure 2 Schematic structural diagram of the graphene-based capacitive sensor in the embodiment; wherein, 1 is an organic polymer film, 2 is a laser-induced graphene film, 3 is a silver nanoparticle-doped film, 4 is a co-doped film of silver nanoparticles and copper oxide particle clusters, 5 is a gold nanoparticle layer, and 6 is an array of holes; Figure 3 SEM image of the LIG film in Example 1; Figure 4 SEM image of the Ag / LIG film in Example 1; Figure 5 SEM image of the Ag / LIG / CuO film in Example 1; Figure 6 TEM image of the Ag / LIG / CuO film in Example 1; Figure 7 SEM image of the graphene-based capacitive sensor in Example 1; Figure 8 Test result graph of the pore size on the surface of the gold nanoparticle layer of the graphene-based capacitive sensor in Example 1; Figure 9 Capacitance-moisture content standard curve obtained based on the graphene-based capacitive sensor in Example 1; Figure 10 Capacitance-moisture content standard curve obtained based on the graphene-based capacitive sensor in Example 2; Figure 11 Capacitance-moisture content standard curve obtained based on the graphene-based capacitive sensor in Comparative Example 1. Detailed implementation manners

[0017] The present invention provides a graphene-based capacitive sensor for detecting the moisture content in lubricating oil, including a graphene-based electrode and a gold nanoparticle layer arranged in a stacked manner; the graphene-based electrode includes a laser-induced graphene film and doped nanoparticles loaded on the surface and pores of the laser-induced graphene film, and the doped nanoparticles are silver nanoparticles and copper oxide particle clusters; an array of holes is provided on the gold nanoparticle layer, the diameter of the holes in the array of holes is larger than the diameter of water molecules and smaller than the diameter of lubricating oil molecules, and the depth of the holes in the array of holes is the same as the thickness of the gold nanoparticle layer.

[0018] The graphene-based capacitive sensor of the present invention includes a graphene-based electrode, and the graphene-based electrode includes a laser-induced graphene film and doped nanoparticles loaded on the surface and pores of the laser-induced graphene film. The doped nanoparticles are silver nanoparticles and clusters of copper oxide particles. As an embodiment of the present invention, the thickness of the laser-induced graphene film can be 0.017 - 0.021 mm, specifically 0.020 mm; the particle size of the silver nanoparticles can be 10 - 100 nm; the clusters of copper oxide particles are in the shape of cauliflower-like particles, and the particle size of the clusters of copper oxide particles can be 100 - 125 nm. As an embodiment of the present invention, the graphene-based electrode can be an interdigital electrode, and the size parameter can be 30×30 mm (the size parameter of the overall graphene interdigital electrode). The present invention uses a laser-induced graphene film as the matrix material, which has good hydrophilicity, and has micron pores and nano pores, a large specific surface area, and good electrical conductivity; at the same time, by loading silver nanoparticles and clusters of copper oxide particles on the surface and pores of the laser-induced graphene film, the present invention can further improve the electrical conductivity and hydrophilicity of the laser-induced graphene film.

[0019] The graphene-based capacitive sensor provided by the present invention includes a gold nanoparticle layer stacked with the graphene-based electrode. The gold nanoparticle layer is provided with array holes, and the diameter of the holes in the array holes is larger than the diameter of water molecules and smaller than the diameter of lubricating oil molecules. The depth of the holes in the array holes is the same as the thickness of the gold nanoparticle layer. The fact that the gold nanoparticle layer of the present invention is provided with array holes specifically means that the gold nanoparticle layer is provided with holes distributed in an array. As an embodiment of the present invention, the thickness of the gold nanoparticle layer can be 850 - 950 nm, specifically 880 - 900 nm; the gold nanoparticle layer is formed by gold nanoparticles, and the particle size of the gold nanoparticles can be 15 - 25 nm. As an embodiment of the present invention, the diameter of each hole in the array holes can be independently 0.6 - 0.9 nm, specifically 0.7 - 0.8 nm; based on the center of the hole, the spacing between two adjacent holes can be independently 0.005 - 0.01 mm, specifically 0.008 - 0.01 mm. The present invention limits the size of the array holes within the above range, which is beneficial to ensuring the accurate detection of the water content in the lubricating oil by the graphene-based capacitive sensor; specifically, the diameter of water molecules is between 0.3 - 0.9 nm, and the diameter of lubricating oil molecules is between 1 nm and more than a dozen nanometers. Due to the excellent hydrophilic property of the laser-induced graphene, when the oil-water mixture passes through the laser-induced graphene, the water molecules and lubricating oil molecules can be initially separated by passing through the array holes of the gold nanoparticle layer first, and then based on the good hydrophilic surface of the graphene-based electrode, the water molecules in the lubricating oil can be further absorbed, facilitating the accurate determination of the water molecule content.

[0020] The present invention provides a preparation method of the graphene-based capacitive sensor described in the above technical solution, including the following steps: Providing a laser-induced graphene film, which is obtained by subjecting an organic polymer film to a first laser treatment; Coating a silver salt solution on the surface of the laser-induced graphene film, drying it, and then subjecting it to a second laser treatment to obtain a silver nanoparticle-doped film; Coating a copper salt solution on the surface of the silver nanoparticle-doped film, drying it, and then subjecting it to a third laser treatment to obtain a film co-doped with silver nanoparticles and copper oxide particle clusters; Forming a gold nanoparticle layer on the surface of the film co-doped with silver nanoparticles and copper oxide particle clusters by magnetron sputtering, and then performing laser array drilling on the surface of the gold nanoparticle layer to obtain the graphene-based capacitive sensor.

[0021] In the present invention, unless otherwise specified, the raw materials used are commercially available products well-known to those skilled in the art or are prepared by methods well-known to those skilled in the art.

[0022] The present invention first provides a laser-induced graphene film, which is obtained by subjecting an organic polymer film to a first laser treatment. As an embodiment of the present invention, the organic polymer film may include a polyimide (PI) film or a polybenzoxazine resin (Poly(PH-ddm)) film; the thickness of the organic polymer film may be 115-135 nm, specifically 125 nm. The present invention preferably pre-treats the organic polymer film and then performs the first laser treatment; the pre-treatment preferably includes ultrasonic washing and drying in sequence, and the reagents used for ultrasonic washing may be acetone, isopropanol, and water in sequence, and the water may specifically be deionized water; the conditions for ultrasonic washing with each reagent include: the ultrasonic frequency may independently be 800-1000 Hz, and the ultrasonic time may independently be 10-15 min; the drying temperature may be 60-85 °C, and the time may be 3-5 min.

[0023] As an embodiment of the present invention, the conditions for the first laser treatment may include: the laser power is 3.78-4.15 W, the laser scanning speed is 200-300 mm / s, and the laser scanning spacing is 0.01-0.05 mm. In the embodiments of the present invention, when using a PI film, the conditions for the first laser treatment specifically include: the laser power is 4.15 W, the laser scanning speed is 300 mm / s, and the laser scanning spacing is 0.03 mm; when using a Poly(PH-ddm) film, the conditions for the first laser treatment specifically include: the laser power is 3.78 W, the laser scanning speed is 200 mm / s, and the laser scanning spacing is 0.03 mm.

[0024] In the embodiment of the present invention, the laser used for the first laser treatment is specifically an ultraviolet picosecond laser. The ultraviolet picosecond laser is equipped with a CCD camera and a three-dimensional moving processing platform, which can adjust the sample processing position to make the imaging on the PC side clear. In the embodiment of the present invention, specifically, the organic polymer thin film is fixed on the three-dimensional moving processing platform, and the organic polymer thin film is placed at the center position of the light spot, so that the laser beam is aligned with the working surface, and then focusing and defocusing are carried out. The initial defocus amount can be set to 0-8 mm, specifically 6 mm, and other processing parameters are set to perform the first laser treatment to obtain a laser-induced graphene thin film.

[0025] Through the first laser treatment, the present invention can roughen the surface of the organic polymer thin film. At the same time, through laser-induced carbonization of the organic polymer, a grid-like graphene structure is formed. During this process, gases (such as carbon dioxide, etc.) are also released, thereby promoting the formation of micron pores and nano pores in the laser-induced graphene. Finally, a laser-induced graphene thin film with a rough surface and a pore structure is obtained (the laser-induced graphene thin film shows a grid state under an optical microscope). This laser-induced graphene thin film has good hydrophilicity, which is convenient for subsequent full absorption of silver salt solution and copper salt solution. The silver salt solution and copper salt solution can fully wet the graphene, so that silver ions and copper ions are evenly distributed on the surface and inside the pores of the entire laser-induced graphene thin film. Finally, a laser-induced graphene thin film with silver nanoparticles and copper oxide particle clusters loaded on the surface and in the pores is obtained, which is beneficial to increasing the loading amount of silver nanoparticles and copper oxide particle clusters, and the silver nanoparticles and copper oxide particle clusters are evenly dispersed. The surface of the organic polymer thin film without the first laser treatment is too smooth, which is not conducive to the subsequent absorption of silver salt solution and copper salt solution and the loading of silver nanoparticles and copper oxide particle clusters.

[0026] After obtaining the laser-induced graphene thin film, the present invention coats the silver salt solution on the surface of the laser-induced graphene thin film, and after drying, performs the second laser treatment to obtain a silver nanoparticle-doped thin film. As an embodiment of the present invention, the silver salt solution can be a silver nitrate solution, and the concentration of the silver salt solution can be 0.4-0.6 mol / L, specifically 0.5 mol / L; the coating amount of the silver salt solution can be 0.05-0.20 μL / mm 2 , specifically 0.15-0.20 μL / mm 2As an embodiment of the present invention, the drying can be carried out in air; the drying time can be 5 - 8 min, specifically 6 min; the drying can be carried out under the condition of 20 - 30 °C, specifically at room temperature (25 °C). As an embodiment of the present invention, the conditions of the second laser treatment can include: laser power of 3.96 - 4.70 W, laser scanning speed of 200 - 300 mm / s, defocus amount of 5 mm, and laser scanning pitch of 0.03 mm. In the embodiments of the present invention, when using PI film, the conditions of the second laser treatment include: laser power of 4.70 W, laser scanning speed of 300 mm / s, defocus amount of 5 mm, and laser scanning pitch of 0.03 mm; when using Poly(PH-ddm) film, the conditions of the second laser treatment include: laser power of 3.96 W, laser scanning speed of 200 mm / s, defocus amount of 5 mm, and laser scanning pitch of 0.03 mm. In the embodiments of the present invention, the laser used for the second laser treatment is specifically an ultraviolet picosecond laser. The laser-induced graphene film of the present invention has good hydrophilicity. When a silver salt solution is coated on the surface of the laser-induced graphene film, after drying, the laser-induced graphene film still absorbs some moisture, which can provide more hydrophilic functional groups during the second laser treatment process. Through the second laser treatment of the present invention, the silver salt forms silver nanoparticles and is loaded on the surface and pores of the laser-induced graphene film, obtaining a silver nanoparticle-doped film; doping silver nanoparticles in the laser-induced graphene film can provide a larger specific surface area to interact with water molecules, thereby enhancing the response of the graphene-based capacitive sensor to moisture; moreover, silver nanoparticles can provide a high carrier concentration, thereby increasing the conductivity of the laser-induced graphene film, reducing its resistance, enhancing the Joule heat effect, and being beneficial to removing moisture in the sample through the Joule heating reaction, so as to ensure the accuracy of the test results when using the graphene-based capacitive sensor for the next experiment.

[0027] After obtaining the silver nanoparticle-doped film, the present invention coats a copper salt solution on the surface of the silver nanoparticle-doped film, dries it, and then performs a third laser treatment to obtain a film co-doped with silver nanoparticles and copper oxide particle clusters. As an embodiment of the present invention, the copper salt solution can be a copper nitrate solution, and the concentration of the copper nitrate solution can be 0.4 - 0.6 mol / L, specifically 0.5 mol / L; the coating amount of the copper salt solution can be 0.05 - 0.20 μL / mm 2 , specifically 0.15 - 0.20 μL / mm 2As an embodiment of the present invention, the drying can be carried out in air; the drying time can be 5 to 8 minutes, specifically 6 minutes; the drying can be carried out under the condition of 20 to 30 °C, specifically at room temperature (25 °C). As an embodiment of the present invention, the conditions of the third laser treatment may include: the laser power is 4.32 to 4.90 W, the laser scanning speed is 200 to 250 mm / s, the defocus amount is 4 mm, and the laser scanning pitch is 0.03 mm. In the embodiment of the present invention, when using a PI film, the conditions of the second laser treatment include: the laser power is 4.90 W, the laser scanning speed is 250 mm / s, the defocus amount is 4 mm, and the laser scanning pitch is 0.03 mm; when using a Poly(PH-ddm) film, the conditions of the second laser treatment include: the laser power is 4.32 W, the laser scanning speed is 200 mm / s, the defocus amount is 4 mm, and the laser scanning pitch is 0.03 mm. In the embodiment of the present invention, the laser used for the third laser treatment is specifically an ultraviolet picosecond laser. The silver nanoparticle-doped film of the present invention has good hydrophilicity. When a copper salt solution is coated on the surface of the silver nanoparticle-doped film, after drying, the silver nanoparticle-doped film still absorbs some moisture, and more hydrophilic functional groups can be provided during the third laser treatment. Through the third laser treatment of the present invention, copper oxide particle clusters are formed from the copper salt and loaded on the surface and pores of the silver nanoparticle-doped film, and a silver nanoparticle and copper oxide particle cluster co-doped film is obtained; among them, under the high-temperature condition of laser irradiation, some copper ions in the copper salt react with oxygen to form copper oxide, and some copper ions react with carbon atoms in graphene to generate copper, and then copper reacts with oxygen under the condition of residual heat to form copper oxide; at this time, the copper oxide particles gradually accumulate to form cauliflower-shaped nanoparticle clusters, that is, copper oxide particle clusters. The copper oxide particle clusters are evenly dispersed on the surface and in the pores of the silver nanoparticle-doped film, and a silver nanoparticle and copper oxide particle cluster co-doped film is obtained, whose hydrophilicity is further increased, and the moisture in the sample can be removed through the Joule heating reaction, thereby ensuring the accuracy of the test results when using the graphene-based capacitance sensor for the next experiment.

[0028] After obtaining the co-doped film of silver nanoparticles and copper oxide particle clusters, the present invention uses magnetron sputtering to deposit a gold nanoparticle layer on the surface of the co-doped film of silver nanoparticles and copper oxide particle clusters, and then performs laser array drilling on the surface of the gold nanoparticle layer to obtain the graphene-based capacitive sensor. The present invention can laser cut the co-doped film of silver nanoparticles and copper oxide particle clusters according to the required shape of the graphene-based electrode (such as the shape of an interdigital electrode) to obtain a graphene-based electrode, and then deposit a gold nanoparticle layer on the surface of the graphene-based electrode; or the present invention can also directly cut the organic polymer film into the required shape and then perform operations such as the first laser treatment; that is, the present invention has no special limitation on the timing of forming the required shape of the graphene-based electrode. In the embodiment of the present invention, specifically, after obtaining the co-doped film of silver nanoparticles and copper oxide particle clusters, the co-doped film of silver nanoparticles and copper oxide particle clusters is laser cut to obtain a graphene-based electrode, and then a gold nanoparticle layer is deposited on the surface of the graphene-based electrode. As an implementation manner of the present invention, the conditions for the laser cutting may include: the power may be 40-45%, specifically 40-42%; the Burst number may be 3-4; the repetition frequency may be 100-110 kHz, specifically 100-105 kHz; the pulse width may be 9500-10500 fs, specifically 9800-10000 fs. The laser used for laser cutting in the embodiment of the present invention is specifically an infrared femtosecond laser.

[0029] As an embodiment of the present invention, the preparation method of the gold nanoparticle layer includes the following steps: coating a mixed solution containing chloroauric acid and a reducing agent on the surface of the co-doped film of silver nanoparticles and copper oxide particle clusters (i.e., the graphene-based electrode), and performing magnetron sputtering to form a gold nanoparticle layer on the surface of the co-doped film of silver nanoparticles and copper oxide particle clusters (i.e., the graphene-based electrode). As an embodiment of the present invention, the reducing agent may include glycine, sodium borohydride, or trisodium citrate. In the embodiment, glycine is specifically used. As a reducing agent, glycine can reduce chloroauric acid to form gold, and at the same time, it can also act as a stabilizer, which is beneficial to the formation of the gold nanoparticle layer. As an embodiment of the present invention, the molar ratio of chloroauric acid to the reducing agent in the mixed solution containing chloroauric acid and the reducing agent may be 1:1 to 1.5, specifically 1:1 to 1.2; the mixed solution containing chloroauric acid and the reducing agent may be obtained by mixing an aqueous solution of chloroauric acid and an aqueous solution of the reducing agent; the concentration of the aqueous solution of chloroauric acid may be 0.3 to 0.6 mol / L, specifically 0.4 mol / L; the concentration of the aqueous solution of the reducing agent may be 0.01 to 1.0 mol / L, further 0.1 to 0.4 mol / L. Specifically, when using an aqueous solution of sodium borohydride, the concentration of the aqueous solution of sodium borohydride may be 0.01 to 0.1 mol / L, and when using an aqueous solution of trisodium citrate, the concentration of the aqueous solution of trisodium citrate may be 0.1 to 1.0 mol / L. In the embodiment of the present invention, an aqueous solution of glycine with a concentration of 0.4 mol / L is specifically used.

[0030] As an embodiment of the present invention, the coating amount of the mixed solution containing chloroauric acid and the reducing agent may be 0.04 to 0.06 μL / mm 2 , specifically 0.05 μL / mm 2 . As an embodiment of the present invention, the conditions of the magnetron sputtering include: the sputtering power may be 125 to 140 W, specifically 130 W; the sputtering time may be 30 to 60 min, specifically 45 min. In the embodiment of the present invention, specifically, the film obtained after coating is placed in the vacuum chamber of the magnetron sputtering equipment, evacuated to below 10 -4 Pa, and then filled with high-purity argon gas to keep the working gas pressure at 0.6 Pa, and the magnetron sputtering is performed to form the gold nanoparticle layer. As an embodiment of the present invention, preferably, drying is also included after the magnetron sputtering. The present invention has no special limitation on the drying, as long as it can ensure sufficient drying.

[0031] After obtaining the gold nanoparticle layer, the present invention performs laser array drilling on the surface of the gold nanoparticle layer to obtain the graphene-based capacitive sensor. Specifically, the present invention performs laser array drilling according to the array hole parameters (including the diameter of the holes, the depth of the holes, and the spacing between two adjacent holes) on the gold nanoparticle layer of the graphene-based capacitive sensor. As an embodiment of the present invention, the conditions for the laser array drilling include: the laser power can be 10.1 - 11.4 W, specifically 10.6 W; the laser scanning speed can be 100 - 200 mm / s, specifically 150 mm / s; the defocus amount can be 2 mm. In the embodiments of the present invention, the laser used for the laser array drilling is specifically an ultraviolet picosecond laser, and the movement path of the laser beam is controlled by a high-precision galvanometer system to obtain the required array holes.

[0032] The present invention provides a method for detecting the water content in lubricating oil, comprising the following steps: Drop the lubricating oil to be measured onto the graphene-based capacitive sensor described in the above technical solution or the graphene-based capacitive sensor prepared by the preparation method described in the above technical solution, measure the capacitance of the graphene-based capacitive sensor, and obtain the water content in the lubricating oil to be measured according to the predetermined capacitance-water content standard curve.

[0033] As an embodiment of the present invention, the lubricating oil to be measured may include gear oil, hydraulic oil, transformer oil or engine oil; the gear oil may include 320# lubricating oil or 220# lubricating oil; the hydraulic oil may be 46# hydraulic oil; the transformer oil may be 25# transformer oil; the engine oil may be 10W-40.

[0034] In an embodiment of the present invention, taking the lubricating oil to be measured as 320# lubricating oil and the graphene-based electrode in the graphene-based capacitance sensor as an interdigital electrode (with size parameters of 30×30 mm) as an example, the amount of the lubricating oil to be measured can be 200-300 μL, specifically 250 μL. As an implementation manner of the present invention, the method for obtaining the capacitance-moisture content standard curve includes the following steps: Measure the initial capacitance value of the graphene-based electrode in the graphene-based capacitance sensor (specifically, an interdigital electrode with size parameters of 30×30 mm) and record it as C0; Drop the standard sample (a lubricating oil sample with a moisture content of 0-2 wt%, specifically 0, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, 2.0 wt%; the volume can be 250 μL) onto the surface of the gold nanoparticle layer on the graphene-based electrode in the graphene-based capacitance sensor, measure the capacitance of the graphene-based electrode every 30 s for a total of 7 min, that is, obtain 14 groups of measurement data, and take the average value and record it as Cx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, corresponding to the moisture content in the standard sample respectively); Calculate the capacitance change rate of the standard sample with different water contents ε = ΔCx / C0×100%, and perform linear fitting with the moisture content as the vertical coordinate and the capacitance change rate as the vertical coordinate to obtain the curve of the capacitance change rate varying with the moisture content, which is the capacitance-moisture content standard curve. As an implementation manner of the present invention, calculate ε / x (x represents the moisture content, with the unit of wt%; ε represents the capacitance change rate, with the unit of %) and record it as the slope, which is the sensitivity of the graphene-based capacitance sensor.

[0035] As an implementation manner of the present invention, drop the lubricating oil to be measured onto the graphene-based capacitance sensor, measure the capacitance of the graphene-based capacitance sensor, and then the moisture content in the lubricating oil to be measured can be obtained according to the capacitance-moisture content standard curve; Subsequently, the present invention preferably applies a current to the graphene-based electrode in the graphene-based capacitance sensor. At this time, the graphene-based electrode acts as a conductor and has a resistance. When the current passes through the graphene-based electrode, the electrons inside the graphene-based electrode collide with atoms to generate heat (the applied voltage and current can be 40 V and 100 A respectively). Since graphene has good hydrophilicity, the heat generated by Joule heating evaporates the water molecules adsorbed by graphene, thereby removing the moisture in the sample and ensuring the accuracy of the test results when using the graphene-based capacitance sensor for the next experiment.

[0036] Figure 1 It is a flowchart of the preparation method of the graphene-based capacitance sensor in the embodiment. Figure 2Schematic diagram of the structure of the graphene-based capacitive sensor in the embodiment. Herein, 1 is an organic polymer film, 2 is a laser-induced graphene film, 3 is a silver nanoparticle-doped film, 4 is a co-doped film of silver nanoparticles and copper oxide particle clusters, 5 is a gold nanoparticle layer, and 6 is an array of holes.

[0037] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than 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 efforts shall fall within the protection scope of the present invention.

[0038] Example 1 Step S1: The polyimide (PI) film (with a thickness of 125 nm) is ultrasonically washed successively with acetone, isopropanol, and deionized water. The ultrasonic frequency is set to 1000 Hz, and the ultrasonic washing time for each reagent is 15 min. After the ultrasonic washing is completed, the PI film is placed on a hot plate at a temperature of 60 °C and dried for 5 min. Step S2: The ultrasonically washed and dried PI film is fixed to the three-dimensional moving processing platform of the ultraviolet picosecond laser. The laser beam is aligned with the area to be processed on the PI film, and the PI film is adjusted to be located at the center of the light spot to make the imaging clear at the PC end. The processing parameters of the ultraviolet picosecond laser are set for the first laser treatment to obtain a laser-induced graphene (LIG) film (with a thickness of 0.020 mm). The conditions of the first laser treatment include: laser power of 4.15 W, laser scanning speed of 300 mm / s, initial defocus amount of 6 mm, and laser scanning pitch of 0.03 mm. Step S3: 245 μL of a silver nitrate solution with a concentration of 0.5 mol / L is coated on the surface of the LIG film (with a size of 35 mm × 35 mm). After drying in air at room temperature (25 °C) for 6 min, the second laser treatment is carried out using an ultraviolet picosecond laser to obtain an LIG film with silver nanoparticles loaded on the surface and in the pores, that is, a silver nanoparticle-doped film (denoted as Ag / LIG film). The conditions of the second laser treatment include: laser power of 4.70 W, laser scanning speed of 300 mm / s, defocus amount of 5 mm, and laser scanning pitch of 0.03 mm. The particle size of the silver nanoparticles is 10 - 100 nm. Step S4: Coat 245 μL of copper nitrate solution with a concentration of 0.5 mol / L on the surface of the Ag / LIG film. After drying in air at room temperature for 6 min, perform the third laser treatment using an ultraviolet picosecond laser to obtain an Ag / LIG film with copper oxide particle clusters loaded on the surface and in the pores, that is, a silver nanoparticle and copper oxide particle cluster co-doped film (denoted as Ag / LIG / CuO film); the conditions of the third laser treatment include: laser power of 4.90 W, laser scanning speed of 250 mm / s, defocus amount of 4 mm, and laser scanning pitch of 0.03 mm; the copper oxide particle clusters are in the shape of cauliflower-like particles, and the particle size of the copper oxide particle clusters is 100 - 125 nm; Step S5: Place the Ag / LIG / CuO film in an infrared femtosecond laser, and obtain an Ag / LIG / CuO interdigital electrode by laser cutting according to the preset interdigital electrode size parameters; the conditions of the laser cutting include: power of 40%, Burst number of 3, repetition frequency of 100 kHz, and pulse width of 10000 fs; the overall size parameters of the interdigital electrode are 30 mm × 30 mm; Step S6: Stir and mix evenly an aqueous solution of chloroauric acid with a concentration of 0.4 mol / L and an aqueous solution of glycine with a concentration of 0.4 mol / L in a volume ratio of 1:1 to obtain a mixed solution; coat the mixed solution on the surface of the Ag / LIG / CuO interdigital electrode (the coating amount of the mixed solution is 0.05 μL / mm 2 ) and then place it in the vacuum chamber of a magnetron sputtering device. Evacuate to below 10 -4 Pa, then fill with high-purity argon gas, keep the working gas pressure at 0.6 Pa, perform magnetron sputtering for 45 min under a sputtering power of 130 W, and then dry to obtain an interdigital electrode with a gold nanoparticle layer deposited on the surface; the gold nanoparticle layer is formed by gold nanoparticles, the thickness of the gold nanoparticle layer is 900 nm, and the particle size of the gold nanoparticles is 15 - 25 nm; Step S7: According to the preset array hole parameters, use an ultraviolet picosecond laser to perform the fourth laser treatment on the interdigital electrode with the gold nanoparticle layer deposited on the surface to form array holes on the surface of the gold nanoparticle layer, and obtain a graphene-based capacitive sensor; the conditions of the fourth laser treatment include: laser power of 10.6 W, laser scanning speed of 150 mm / s, and defocus amount of 2 mm; during the fourth laser treatment, the movement path of the laser beam is controlled by a high-precision galvanometer system; the diameter of each hole in the array holes is distributed between 0.6 - 0.9 nm, the depth of the holes is 900 nm, and based on the center of the holes, the distance between adjacent two holes is about 0.01 mm.

[0039] Figure 3SEM image of the LIG thin film in Example 1; the results show that the LIG presents a flaky network structure, and the graphene sheets have a smooth surface.

[0040] Figure 4 SEM image of the Ag / LIG thin film in Example 1; the results show that, compared with the pure LIG thin film, it is obvious that abundant Ag nanoparticles are attached to the surface of the Ag / LIG thin film. The successful attachment of these Ag nanoparticles can provide a larger specific surface area to interact with water molecules, thereby enhancing the response of the graphene-based capacitive sensor to moisture.

[0041] Figure 5 SEM image of the Ag / LIG / CuO thin film in Example 1; the results show that abundant nanoparticles are attached to the surface of the Ag / LIG / CuO thin film.

[0042] Figure 6 TEM image of the Ag / LIG / CuO thin film in Example 1, where the right side is an enlarged view of the left side; the results show that abundant nanoparticles are attached to the surface of the Ag / LIG / CuO thin film, and the clusters of CuO are within the solid and dashed circles.

[0043] Figure 7 SEM image of the graphene-based capacitive sensor in Example 1, where the right side is an enlarged view of the left side; the results show that array holes are formed on the surface of the gold nanoparticle layer of the graphene-based capacitive sensor.

[0044] Figure 8 Test result graph of the pore size on the surface of the gold nanoparticle layer of the graphene-based capacitive sensor in Example 1. Specifically, the pore size is obtained by performing N2 adsorption-desorption tests on the thin film structure. The results show that the diameter of the pores is distributed between 0.6 and 0.9 nm.

[0045] Example 2 Operate according to the method of Example 1, with the difference that: Replace the PI thin film in step S1 with a polybenzoxazine resin (Poly(PH-ddm)) thin film, and the drying temperature is 85 °C; The conditions for the first laser treatment in step S2 include: laser power is 3.78 W, laser scanning speed is 200 mm / s, initial defocus amount is 6 mm, and laser scanning pitch is 0.03 mm; The conditions for the second laser treatment in step S3 include: laser power is 3.96 W, laser scanning speed is 200 mm / s, defocus amount is 5 mm, and laser scanning pitch is 0.03 mm; The conditions for the second laser treatment in step S4 include: laser power of 4.32 W, laser scanning speed of 200 mm / s, defocus amount of 4 mm, and laser scanning pitch of 0.03 mm.

[0046] Comparative Example 1 Operate according to the method of Example 1, except that step S3 and step S4 are omitted, that is, a graphene-based capacitive sensor is prepared using a LIG interdigital electrode.

[0047] Test Example 1 Prepare a LIG film and an Ag / LIG film according to the method of Example 1, and measure the resistance of the LIG film and the Ag / LIG film (specifically, measure 10 times, measure once every 1 min, that is, measure for a total of 10 min, and take the average value of the 10 capacitance measurements as the detection result). The results show that the average resistance of the LIG film is 315 KΩ, and the average resistance of the Ag / LIG film is 227 KΩ, indicating that loading silver nanoparticles on the LIG film can effectively reduce its resistance.

[0048] Test Example 2 Prepare a copper oxide particle cluster-doped film (denoted as LIG / CuO film) according to the method of Example 1, that is, omit step S3 after step S2 is completed and directly perform step S4; at the same time, prepare a LIG film according to the method of Example 1; then measure the contact angles of the LIG film and the LIG / CuO film (specifically, prepare 10 groups of samples and measure the contact angles respectively). The results show that the contact angle range of the LIG film is 76.5° - 87.0°, and the average value is 79.8°; the contact angle range of the LIG / CuO film is 1.1° - 13.2°, and the average value is 5.0°; the smaller the contact angle, the better the wettability and the higher the hydrophilicity. Therefore, it can be proved that loading copper oxide particle clusters on the LIG film is beneficial to improving its hydrophilic ability.

[0049] Test Example 3 Prepare an Ag / LIG / CuO interdigital electrode as a graphene-based capacitive sensor according to the method of Example 1 (that is, without a gold nanoparticle layer and without array holes, denoted as a comparative sample); at the same time, prepare a graphene-based capacitive sensor according to the method of Example 1 (that is, with a gold nanoparticle layer and provided with array holes, denoted as an experimental sample); then measure the capacitances of the two graphene-based capacitive sensors (specifically, prepare 10 groups of samples and measure the capacitance respectively, and take the average value as the detection result). The results show that the average capacitance of the experimental sample is 88.1 pF, while the average capacitance of the comparative sample is 25.3 pF, indicating that setting a gold nanoparticle layer can effectively increase the capacitance of the graphene-based capacitive sensor.

[0050] Test Example 4 The graphene-based capacitive sensors prepared by the examples and comparative examples were used to measure the capacitance of oil-water mixtures with different water contents, as follows: (1) Obtaining the capacitance-water content standard curve, including the following steps: Measuring the initial capacitance value of the interdigital electrodes in the graphene-based capacitive sensor and recording it as C0; dropping a standard 320# lubricating oil sample (a 320# lubricating oil sample with a water content of 0-2 wt%, specifically 0, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, 2.0 wt%; with a volume of 250 μL) onto the surface of the gold nanoparticle layer of the interdigital electrodes in the graphene-based capacitive sensor, measuring the capacitance of the interdigital electrodes every 30 s for a total of 7 min, obtaining 14 groups of measurement data, and taking the average value and recording it as Cx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, corresponding to the water content in the standard sample respectively); calculating the capacitance change rate ε = ΔCx / C0 × 100% of the standard 320# lubricating oil sample with different water contents, and performing linear fitting with the water content as the ordinate and the capacitance change rate as the ordinate, obtaining the curve of the capacitance change rate varying with the water content, which is the capacitance-water content standard curve, and the slope of the capacitance-water content standard curve is the sensitivity of the graphene-based capacitive sensor; (2) Determination of the water content in the 320# lubricating oil sample to be measured: dropping the 320# lubricating oil sample to be measured onto the surface of the gold nanoparticle layer of the interdigital electrodes in the graphene-based capacitive sensor, measuring the capacitance of the graphene-based capacitive sensor, and then obtaining the water content in the 320# lubricating oil sample to be measured according to the capacitance-water content standard curve.

[0051] The detection results were characterized by sensitivity. Each graphene-based capacitive sensor was respectively subjected to 5 parallel experiments, and the measurement results were linearly fitted and averaged. Figure 9 is the capacitance-water content standard curve obtained based on the graphene-based capacitive sensor in Example 1, Figure 10 is the capacitance-water content standard curve obtained based on the graphene-based capacitive sensor in Example 2, Figure 11 is the capacitance-water content standard curve obtained based on the graphene-based capacitive sensor in Comparative Example 1; the results show that the sensitivity of the graphene-based capacitive sensor prepared in Example 1 is 66.04079 (as Figure 9 shown), the sensitivity of the graphene-based capacitive sensor prepared in Example 2 is 63.7323 (as Figure 10 shown), while the sensitivity of the graphene-based capacitive sensor prepared in Comparative Example 1 is 39.73151 (as Figure 11As shown, it shows that after doping silver nanoparticles and clusters of copper oxide particles in Embodiment 1 of the present invention, the sensitivity of the obtained graphene-based capacitive sensor is significantly improved.

[0052] In addition, in order to verify the accuracy of the graphene-based capacitive sensor of the present invention for measuring the water content in lubricating oil, a 320# lubricating oil sample to be tested with a water content of 0.6 wt% was prepared, and the water content in the 320# lubricating oil sample to be tested was measured according to the above steps. The results show that the water content in the 320# lubricating oil sample to be tested is 0.56 wt%, and the difference from its actual water content is only 0.04 wt%. This shows that the graphene-based capacitive sensor of the present invention can accurately measure the water content in lubricating oil.

[0053] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A graphene-based capacitive sensor for detecting the water content in lubricating oil, characterized in that, It includes a graphene-based electrode and a gold nanoparticle layer arranged in a laminated manner; the graphene-based electrode includes a laser-induced graphene film and doped nanoparticles loaded on the surface and pores of the laser-induced graphene film, and the doped nanoparticles are silver nanoparticles and clusters of copper oxide particles; an array of holes is provided on the gold nanoparticle layer, and the diameter of the holes in the array is larger than the diameter of water molecules and smaller than the diameter of lubricating oil molecules, and the depth of the holes in the array is consistent with the thickness of the gold nanoparticle layer.

2. The graphene-based capacitive sensor according to claim 1, wherein The thickness of the laser-induced graphene film is 0.017 - 0.021 mm; the particle size of the silver nanoparticles is 10 - 100 nm; the particle size of the clusters of copper oxide particles is 100 - 125 nm.

3. The graphene-based capacitive sensor according to claim 1 or 2, characterized in that, The thickness of the gold nanoparticle layer is 850 - 950 nm; the gold nanoparticle layer is formed by gold nanoparticles, and the particle size of the gold nanoparticles is 15 - 25 nm; The diameter of each hole in the array of holes is independently 0.6 - 0.9 nm; based on the center of the hole, the distance between two adjacent holes is independently 0.005 - 0.01 mm.

4. The graphene-based capacitive sensor according to claim 1, wherein The graphene-based electrode is a interdigitated electrode.

5. The preparation method of the graphene-based capacitance sensor according to any one of claims 1 to 4, characterized in that, It includes the following steps: Provide a laser-induced graphene film, which is obtained by subjecting an organic polymer film to a first laser treatment; Coat a silver salt solution on the surface of the laser-induced graphene film, dry it and then perform a second laser treatment to obtain a silver nanoparticle-doped film; Coat a copper salt solution on the surface of the silver nanoparticle-doped film, dry it and then perform a third laser treatment to obtain a film co-doped with silver nanoparticles and clusters of copper oxide particles; Use magnetron sputtering to deposit a gold nanoparticle layer on the surface of the film co-doped with silver nanoparticles and clusters of copper oxide particles, and then perform laser array drilling on the surface of the gold nanoparticle layer to obtain the graphene-based capacitive sensor.

6. The preparation method according to claim 5, characterized in that, The organic polymer film includes a polyimide film or a polybenzoxazine resin film; the conditions of the first laser treatment include: laser power is 3.78 - 4.15 W, laser scanning speed is 200 - 300 mm / s, initial defocus amount is 0 - 8 mm, and laser scanning pitch is 0.01 - 0.05 mm; The silver salt solution is silver nitrate solution, the concentration of the silver salt solution is 0.4 to 0.6 mol / L, and the coating amount of the silver salt solution is 0.05 to 0.20 μL / mm 2 ; The conditions of the second laser treatment include: laser power is 3.96 to 4.70 W, laser scanning speed is 200 to 300 mm / s, defocus amount is 5 mm, and laser scanning pitch is 0.03 mm; The copper salt solution is a copper nitrate solution, the concentration of the copper nitrate solution is 0.4 - 0.6 mol / L, and the coating amount of the copper nitrate solution is 0.05 - 0.20 μL / mm 2 ; The conditions of the third laser treatment include: the laser power is 4.32 - 4.90 W, the laser scanning speed is 200 - 250 mm / s, the defocus amount is 4 mm, and the laser scanning pitch is 0.03 mm.

7. The preparation method according to claim 5, characterized in that, The preparation method of the gold nanoparticle layer includes the following steps: coat a mixed solution containing chloroauric acid and a reducing agent on the surface of the film co-doped with silver nanoparticles and clusters of copper oxide particles, perform magnetron sputtering to form a gold nanoparticle layer on the surface of the film co-doped with silver nanoparticles and clusters of copper oxide particles.

8. The preparation method according to claim 5, wherein The conditions of the laser array drilling include: laser power is 10.3 - 10.9 W, laser scanning speed is 150 - 200 mm / s, and defocus amount is 2 mm.

9. A method for detecting the moisture content in lubricating oil, characterized in that, It includes the following steps: Drop the lubricating oil to be measured onto the graphene-based capacitive sensor according to any one of claims 1 - 4 or the graphene-based capacitive sensor prepared by the preparation method according to any one of claims 5 - 8, measure the capacitance of the graphene-based capacitive sensor, and obtain the water content in the lubricating oil to be measured according to a predetermined capacitance-water content standard curve.

10. The method according to claim 9, wherein The lubricating oil to be measured includes gear oil, hydraulic oil, transformer oil or engine oil.

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