A method for measuring the mechanical properties of microplastics based on atomic force microscopy

Abstract

The present invention discloses a method for measuring the mechanical properties of microplastics based on an atomic force microscope, comprising the following steps: S1. Adhere a polycarbonate filter membrane on a glass slide and fix the microplastic sample on the polycarbonate filter membrane; S2. Perform mechanical calibration on the probe of the atomic force microscope; S3. Use the atomic force microscope to perform high-resolution scanning of the surface morphology of the microplastic sample in the peak force quantitative nanomechanical scanning mode, select a relatively flat position on the surface for nanoindentation testing, and obtain indentation data; S4. Analyze the indentation data to calculate the hardness data of the measured microplastic sample. The microplastic sample preparation method proposed by the present invention has excellent fixing effect, avoids the shaking of the microplastic sample during the scanning process of the diamond probe, and successfully performs the nanoindentation measurement process on the surface of the measured sample of powdery microplastic particles with a particle size of only a few micrometers, thereby obtaining the mechanical property information of the microplastics.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental analysis, and more particularly, to a method for measuring the mechanical properties (hardness) of microplastics based on an atomic force microscope. Background Art

[0002] In modern society, plastics are a widely used material with advantages such as durability, light weight, and low cost, and are already indispensable in many fields such as industry, agriculture, and medicine. However, with the discovery of plastic debris in Antarctica by researchers, the pollution of plastic waste has escalated into a worldwide problem. Every year, approximately 300 million tons of plastics are produced worldwide. It is reported that there are more than 150 million tons of plastics in the ocean, and it is predicted that about 270,000 tons of them will gradually break up and fragment, eventually becoming 5.25 trillion plastic remnants floating on the ocean surface. The aging of plastics in the environment is a common phenomenon and is defined as the loss of physical integrity and polymer degradation caused by processes such as mechanical wear, ultraviolet radiation, chemical oxidation, stabilizer leaching, thermal effects, and biodegradation. Generally, after aging, the surface of plastics will show changes in morphology, roughness, hydrophilicity, and chemical composition, resulting in changes in mechanical properties, such as embrittlement, which reduces the hardness of plastics.

[0003] Plastic debris will break into smaller particles after the natural aging process. Microplastics with a particle size less than 5 mm are the most abundant plastic debris, and due to their unique properties, microplastics have attracted extensive interest from researchers. As an emerging pollutant, microplastics have been separated and extracted from different aquatic environments such as urban rivers, inland lakes, and the ocean. The natural aging process of microplastics is generally considered to be a very slow process, especially in aquatic environments. This is because the molecular structure of plastic polymers is relatively stable, and additives are often added to increase stability. After aging, microplastics show significant changes in surface roughness, morphology, chemical composition, and hydrophilicity. Oxygen-containing groups such as -OH and C=O are generated after microplastic aging, thereby increasing the hydrophilicity, polarity, and charge of the surface. During the aging process, plastic additives and microplastic-derived intermediates such as phthalates, bisphenol A, nonylphenol, and brominated flame retardants will be released, further increasing the ecological risk of microplastics. In order to reduce the uncertainty of environmental risk assessment, it is very necessary to understand the aging process of microplastics and its potential impacts. And an important characteristic reflecting the aging degree of microplastics is the change in mechanical properties. Generally, the hardness of the microplastic surface will decrease with the increase in the aging degree because aging will increase the embrittlement and corrosion degree of microplastics. However, due to the very small particle size of microplastics themselves, there is currently no relatively perfect and convenient quantitative measurement method for the hardness of microplastics.

[0004] Atomic Force Microscope (AFM for short) is an advanced surface imaging technology with atomic-level high resolution. Generally, the lateral resolution of an atomic force microscope can reach 0.2 nanometers, and the longitudinal resolution can reach 0.1 nanometers. The working principle of an atomic force microscope is to use the interaction between the atoms of the tip and the atoms of the sample surface to perform imaging of the pattern. The probe of an atomic force microscope can be divided into a tip and a cantilever. The tip is fixed at one end of the elastic micro-cantilever beam. A laser beam is reflected by the back of the elastic micro-cantilever beam into a photodetector. When the tip contacts the sample surface, an extremely weak repulsive force will be generated between the atoms at the tip of the tip and the atoms of the sample surface. Due to the unevenness of the sample surface, the tip will move up and down in the direction perpendicular to the sample surface, which will drive the micro-cantilever to undergo a small bending deformation, causing a change in the reflection optical path, and thus making the laser spot reflected on the photodetector move accordingly. The position changes of the micro-cantilever corresponding to each scanned point on the sample surface measured by this optical detection method can be used to obtain information about the surface topography of the sample.

[0005] Nanoindentation is used to characterize and measure the mechanical properties of materials at the sub-micron and nano scales. This mechanical measurement technique applies a certain pressure to the tip of a probe, causing it to come into contact with the material surface and produce a certain deformation. At the same time, the cantilever deformation-displacement curve during the process of the tip pressing into the material surface is recorded and converted into a force-displacement curve, so as to study the relationship between the structure and function of the material. Through this technique, the mechanical properties such as the hardness, elastic modulus, elasticity, plastic deformation of various material surfaces, and the adhesion between the thin film and the sample substrate can be measured and studied. At present, the application fields of this technique involve various semiconductor thin films in integrated circuits, surface metal coatings of magnetic storage media, polymer coatings with specific properties on the surface of polymer materials, thin films on the surface of microelectromechanical component materials, etc. Nanoindentation technology has been widely used in the field of measuring the mechanical properties of material surfaces. However, microplastic particles with a size of even a few microns or nanometers are difficult to fix the sample position due to their small particle size, which will affect the needle insertion process of the tip and the selection of the indentation position during nanoindentation operation, resulting in a research gap in this field. Chinese Patent CN102200543A discloses a measurement device and method for nanoindentation on the surface of micro-particles based on an atomic force microscope. Double-sided tape is used to fix the powdered micro-particle sample to be measured, and then nanoindentation on the surface of the micro-particles is measured based on the atomic force microscope. However, the atomic force microscope is a very delicate technology in mechanics, highly sensitive, and easily affected by various subtle forces. Therefore, the huge adhesion force on the surface of the double-sided tape will surely affect the scanning trajectory of the probe for micro-nano scale samples to a certain extent. Moreover, Chinese Patent CN102200543A measures the thin film coating on the surface of micro-particle materials with a particle size of 20-80 microns, and is not suitable for measuring the hardness of plastic particles with a particle size of only a few microns or even nanometers itself. Summary of the Invention

[0006] The object of the present invention is to overcome the above-mentioned defects and deficiencies existing in the prior art, and provide a method for measuring the mechanical properties of microplastics based on an atomic force microscope.

[0007] The above object of the present invention is achieved by the following technical solutions:

[0008] A method for measuring the mechanical properties of microplastics based on an atomic force microscope, comprising the following steps:

[0009] S1. Adhere a polycarbonate filter membrane on a glass slide, and fix the microplastic sample on the polycarbonate filter membrane;

[0010] S2. Perform mechanical calibration on the probe of the atomic force microscope;

[0011] S3. Use an atomic force microscope to perform high-resolution scanning of the surface topography of the microplastic sample in the peak force quantitative nanomechanics scanning mode. Select a relatively flat position on the surface for nanoindentation testing to obtain indentation data;

[0012] S4. Analyze the indentation data to calculate and obtain the hardness data of the measured microplastic sample.

[0013] The polycarbonate filter membrane is a sterile membrane made by using advanced dielectric technology under ultra-clean conditions. In the prior art, it is mainly used for molecular filtration. During filtration, particles larger than the pore size are retained on the surface of the filter membrane, and it is an ideal tool for sieving particles and precision filtration. The present invention creatively uses the polycarbonate filter membrane as the fixed substrate for the microplastic sample. Due to the closely arranged micropores on the polycarbonate filter membrane, the microplastic particle sample can be caught in them and thus firmly embedded on the substrate. Moreover, the process is simple, the cost is low, and the required time is short.

[0014] As described above, the mechanical properties of microplastics are very important. However, due to the complex characteristics of their force curves, it is difficult to quantitatively measure their hardness using the ordinary atomic force microscope scanning mode. A new emerging atomic force microscope imaging mode based on a constant peak force, the peak force quantitative nanomechanics scanning mode, can simultaneously perform high-resolution topography imaging and quantitative mechanical property mapping. In the peak force quantitative nanomechanics scanning mode, the probe intermittently contacts the sample surface in a short time, and controls the maximum force (peak force) at each pixel to obtain the force-distance curve, which is then used as the feedback signal. Since the maximum force on the sample is directly adjusted and remains constant throughout the scanning process, the deformation caused by the force applied by the probe can be well controlled, and at the same time, the contact area between the probe tip and the sample is minimized, which is very crucial for measuring the mechanical properties of soft and fragile samples. Therefore, the peak force quantitative nanomechanics scanning mode is a true contact measurement, which is very suitable for providing the real topography information of microplastics and directly determining their mechanical properties from the force curve. The present invention focuses on the mechanical properties of microplastics. First, obtain the morphology of microplastics at different peak forces through the peak force quantitative nanomechanics scanning mode (the hardness of different microplastic samples is different. If the peak force is too large, it will damage the sample surface; if the peak force is too small, the accuracy of the sample topography map will be reduced. Therefore, the peak force size needs to be adjusted to find the most suitable peak force) to select the appropriate force. Then, perform a nanoindentation experiment to obtain the hardness of the microplastic sample.

[0015] After fixing microplastic particles on a polycarbonate filter membrane substrate, the peak force quantitative nanomechanical scanning mode of an atomic force microscope is used to analyze the morphology and mechanical properties (hardness) of the plastic particles, avoiding the shaking of the microplastic sample during the probe scanning process. The nanoindentation measurement process is successfully carried out on the surface of the powder-like microplastic particle sample with a particle size of only a few micrometers, thereby obtaining the mechanical property information of the microplastics.

[0016] Preferably, in step S1, adhering the polycarbonate filter membrane on the glass slide is to first prepare a clean glass slide, wash it with deionized water, and dry the area around the glass slide with a sterile lens paper. The central area of the glass slide does not need to be wiped and remains moist; then gently place the polycarbonate filter membrane on the moist central area of the glass slide, avoiding wrinkles. Due to the presence of moisture, the whole filter membrane will quickly become wet and at the same time tightly stick to the surface of the glass slide; after about two to three hours, the glass slide dries naturally, and the polycarbonate filter membrane adheres firmly to the surface of the glass slide.

[0017] Further preferably, the glass slide is about 4 - 5 cm long, about 2 cm wide, and about 2 mm thick; the area size of the central area of the glass slide that does not need to be wiped is about 2 cm × 2 cm; the diameter of the polycarbonate filter membrane is 2 cm.

[0018] Preferably, in step S1, fixing the microplastic sample on the polycarbonate filter membrane is to evenly sprinkle an appropriate amount of microplastic sample powder on the polycarbonate filter membrane, and then blow the surface of the filter membrane with nitrogen for 3 - 5 seconds and repeat several times. Under the nitrogen purge, the microplastic sample will move on the surface of the filter membrane and a considerable part of it will get stuck in the micropores. Since the purge direction of nitrogen is perpendicular to the surface of the filter membrane, under the repeated purge of nitrogen, the particles with weak fixation will be blown off, while the particles with relatively strong fixation will get stuck tighter. This can blow off the microplastic particle samples that are not fixed and make the fixed particles more firm.

[0019] Preferably, the pore size of the polycarbonate filter membrane in step S1 is 2 - 10 micrometers. The pore size depends on the particle size of the microplastic sample. For micro- and nano-plastic particle samples, filter membranes with pore sizes of 2 or 8 micrometers are usually selected.

[0020] Preferably, the atomic force microscope models in steps S2 and S3 are Dimension Icon TM , and the manufacturer is Bruker Corporation, USA.

[0021] Preferably, the probe in step S2 is a probe DDESP-V2 with a diamond coating, and the manufacturer is Bruker Corporation, USA. The spring constant value provided by the manufacturer is 80 N / m. Before conducting a nanoindentation experiment to evaluate the mechanical resistance generated by the microplastics when subjected to an external force, the probe of the atomic force microscope needs to be calibrated mechanically.

[0022] Preferably, the mechanical calibration in step S2 is to calibrate the mechanical properties of the probe using a standard sapphire substrate. Specifically, first perform a thermal tuning on the probe, and then use the standard sapphire substrate provided by the instrument manufacturer to calibrate the mechanical properties of the probe, calibrating the deflection sensitivity and spring constant of the probe.

[0023] Preferably, the scanning rate of the probe is 0.8 Hz, the scanning resolution is set to 128×128 pixels, and the probe pressure is 10 - 100 μN. The magnitude of the probe pressure depends on the hardness of the microplastic sample. For a harder sample, a higher pressure should be set. Generally, it is set to press on the microplastic with a force of 10 - 100 μN, which is sufficient to produce obvious indentations or holes on the surface of the microplastic.

[0024] Preferably, step S4 is to use the data analysis software Nanoscope of the atomic force microscope to analyze the indentation data, and the hardness data of the measured sample can be obtained through simple calculations.

[0025] Further preferably, the hardness of the microplastic is calculated using the following formula:

[0026] H = F max / A

[0027] where H represents hardness, F max is the peak force when an indentation is produced on the microplastic, and A is the projected area of the indentation produced on the sample surface. After saving the relevant data file and opening it in the Nanoscope analysis software, these two values can be directly obtained. Substituting them into the above formula gives the hardness data.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The present invention provides a method for measuring the mechanical properties of microplastics based on an atomic force microscope. After fixing microplastic particles on a polycarbonate membrane substrate, the peak force quantitative nano-mechanical scanning mode of the atomic force microscope is used to analyze the morphology and mechanical properties of the plastic particles. The sample preparation method proposed by the present invention has excellent fixing effects, avoiding the shaking of the microplastic sample during the probe scanning process, successfully performing the nano-indentation measurement process on the surface of the measured sample of powdery microplastic particles with a particle size of only a few micrometers, thereby obtaining the mechanical property information of the microplastics; not only the probe landing point is accurate, the projected area of the nano-indentation is of appropriate size, the nano-indentation depth is appropriate, but also the data calculation process is simple, the method is scientific, and the results are accurate; in addition, the consumables used in the entire mechanical property measurement are only glass slides and polycarbonate membranes, with low costs. Description of the Drawings

[0030] Figure 1This is the three-dimensional morphology diagram of two plastic particle samples that have completed nanoindentation experiments in Example 1 of the present invention.

[0031] Figure 2 This is the surface morphology diagram of a microplastic particle sample that has undergone several nanoindentation experiments in Example 2 of the present invention. Detailed implementation manners

[0032] The present invention will be further described below in conjunction with the specification drawings and specific embodiments, but the embodiments do not limit the present invention in any form. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the technical field.

[0033] Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0034] Example 1 A method for measuring the mechanical properties of microplastics based on an atomic force microscope

[0035] 1. Prepare a glass slide (in this example, a sailing brand glass slide, about 4 - 5 cm long, about 2 cm wide, and about 2 mm thick). Wash it with deionized water and dry the surrounding area with lens paper. The middle area (2 cm × 2 cm) of the glass slide does not need to be wiped, but remains wet. Then, pick up a polycarbonate filter membrane (Isopore TM Merck Millipore, with a pore size of 2 microns) with a diameter of 2 cm with forceps and gently place it on the wet area in the center of the glass slide. After about two or three hours, the glass slide is completely dry, and the polycarbonate filter membrane is also firmly adhered to it.

[0036] 2. Uniformly coat the microplastic particles mainly composed of polypropylene on the polycarbonate filter membrane, blow nitrogen onto the surface of the filter membrane for 3 - 5 seconds, and repeat three times. Naturally, some of the samples will be blown off, and the remaining sample particles will be relatively firmly fixed on the polycarbonate filter membrane.

[0037] 3. Carefully load the probe of the atomic force microscope onto the probe holder, and place the probe holder under the probe head of the atomic force microscope. The model of the atomic force microscope used in this example is Dimension Icon TM , and the probe used is a probe DDESP-V2 with a diamond coating, and its spring constant value is 80 N / m. The manufacturer is Bruker Corporation of the United States.

[0038] 4. Calibrate the mechanical properties of the probe using a standard sapphire substrate. The actually measured spring constant of the probe cantilever is 94 N / m. Set the scanning rate of the probe to 0.8 Hz and the scanning resolution to 128×128 pixels. Then, adjust the probe tip and set it to press on the microplastics with forces of 10, 20, 30, and 40 μN respectively. Carefully observe and evaluate the generated surface topography images, and finally set the peak force to 20 μN.

[0039] 5. Characterize the morphology of the microplastic particle samples using an atomic force microscope, and select a relatively flat area on the sample surface for nanoindentation experiments, repeating several times. For each batch of plastic particle samples, a relatively flat position should be selected for multiple indentation tests, and each individual plastic particle can also be tested multiple times. Finally, take the average value.

[0040] 6. As Figure 1 shown, several nanoindentation experiments were carried out on the surface of the microplastic particle samples, and the indentations were very obvious. Then, save the analysis file data one by one.

[0041] 7. Open the relevant analysis file using the Nanoscope program to directly obtain the value of the peak force F max and the value of the projected area A of the indentation. Substitute them into the formula H = F max / A (where H represents hardness, F max is the peak force when generating an indentation on the microplastics, and A is the projected area of the indentation generated on the sample surface), and the hardness of the microplastic sample can be obtained. Finally, take the average value, and the result is 37.4 GPa.

[0042] Example 2 A method for measuring the mechanical properties of microplastics based on an atomic force microscope

[0043] 1. Prepare a glass slide (in this example, a sailing brand glass slide, about 4 - 5 cm long, about 2 cm wide, and about 2 mm thick). Wash it with deionized water and dry the surroundings with lens paper. The middle area (2 cm × 2 cm) of the glass slide does not need to be wiped, but remains wet. Then, pick up a polycarbonate filter membrane with a diameter of 2 cm (Isopore TM Merck Millipore, with a pore size of 2 μm) with forceps and gently place it on the wet area in the center of the glass slide. After about two or three hours, the glass slide is completely dry, and the polycarbonate filter membrane is firmly adhered to it.

[0044] 2. Uniformly coat the microplastic particles mainly composed of polystyrene on the polycarbonate filter membrane, blow the surface of the filter membrane with nitrogen for 3 - 5 seconds, and repeat three times. Naturally, some samples will be blown off, and the remaining sample particles will be relatively firmly fixed on the polycarbonate filter membrane.

[0045] 3. Carefully load the probe of the atomic force microscope onto the probe holder and place the probe holder under the probe tip of the atomic force microscope. The model of the atomic force microscope used in this embodiment is Dimension Icon TM . The probe used is a diamond-coated probe DDESP-V2 with a spring constant value of 80 N / m, and the manufacturer is Bruker Corporation of the United States for both.

[0046] 4. Calibrate the mechanical properties of the probe with a standard sapphire substrate. The actually measured spring constant of the probe cantilever is 97 N / m. Set the scanning rate of the probe to 0.8 Hz and the scanning resolution to 128×128 pixels. Then, adjust the probe tip and set it to press on the microplastic with forces of 30, 40, 50, and 60 micronewtons respectively. Carefully observe and evaluate the generated surface topography images, and finally set the peak force to 50 micronewtons.

[0047] 5. Use the atomic force microscope to perform topography characterization on the microplastic particle sample, and select a relatively flat area on the sample surface for nanoindentation experiments, repeating several times. For each batch of plastic particle samples, a relatively flat position should be selected for multiple indentation tests, and each individual plastic particle can also be tested multiple times, and finally the average value is taken.

[0048] 6. As Figure 2 shown, several nanoindentation experiments were carried out on the surface of the microplastic particle sample, and the indentations were very obvious. Then, save the analysis file data one by one.

[0049] 7. Use the Nanoscope program to open the relevant analysis file to directly obtain the value of the peak force F max and the value of the projected area A of the indentation. Substitute them into the formula H = F max / A (where H refers to hardness, F max is the peak force when generating an indentation on the microplastic, and A is the projected area of the indentation generated on the sample surface), and the hardness of the microplastic sample can be obtained. Finally, take the average value, and the result is 82.5 GPa.

[0050] Example 3 A method for measuring the mechanical properties of microplastics based on an atomic force microscope

[0051] 1. Prepare a glass slide (in this example, a sailing brand glass slide with a length of about 4 - 5 cm, a width of about 2 cm, and a thickness of about 2 mm). Clean it with deionized water and dry the surroundings with lens paper. The middle area (2 cm×2 cm) of the glass slide does not need to be wiped but remains wet. Then, pick up a polycarbonate filter membrane with a diameter of 2 cm (Isopore TMGently place a Merck Millipore polycarbonate filter membrane (with a pore size of 8 μm) in the moist area at the center of the glass slide. After about two or three hours, when the glass slide is completely dry, the polycarbonate filter membrane will adhere firmly to it.

[0052] 2. Uniformly coat the microplastic particles mainly composed of polystyrene on the polycarbonate filter membrane, blow nitrogen onto the surface of the filter membrane for 3 - 5 seconds, and repeat three times. Naturally, a part of the sample will be blown off, while the remaining sample particles will be relatively firmly fixed on the polycarbonate filter membrane.

[0053] 3. Carefully load the probe of the atomic force microscope onto the probe holder, and place the probe holder under the probe tip of the atomic force microscope. The model of the atomic force microscope used in this example is Dimension Icon TM , and the probe used is a diamond-coated probe DDESP-V2 with a spring constant value of 80 N / m, both produced by Bruker Corporation in the United States.

[0054] 4. Calibrate the mechanical properties of the probe with a standard sapphire substrate. The actually measured spring constant of the probe cantilever is 97 N / m. Set the scanning rate of the probe to 0.75 Hz and the scanning resolution to 128×128 pixels. Then, adjust the probe tip, and set it to press on the microplastic with forces of 30, 40, 50, and 60 μN respectively. Carefully observe and evaluate the generated surface topography images, and finally set the peak force to 50 μN.

[0055] 5. Use the atomic force microscope to perform topography characterization on the microplastic particle sample, and select a relatively flat area on the sample surface for nanoindentation experiments, repeating several times. For each batch of plastic particle samples, a relatively flat position should be selected for multiple indentation tests. Each individual plastic particle can also be tested multiple times, and finally take the average value.

[0056] 6. Perform several nanoindentation experiments on the surface of the microplastic particle sample. The indentations are clearly visible, and then save the analysis file data one by one.

[0057] 7. Open the relevant analysis file using the Nanoscope program to directly obtain the value of the peak force F max and the value of the projected area A of the indentation. Substitute them into the formula H = F max / A (where H represents hardness, F max is the peak force when generating an indentation on the microplastic, and A is the projected area of the indentation generated on the sample surface), and then obtain the hardness of the microplastic sample. Finally, take the average value, and the result is 86.2 GPa.

Claims

1. A method for measuring the mechanical properties of microplastics based on an atomic force microscope, characterized in that, It includes the following steps: S1. Attach a polycarbonate filter membrane to a glass slide and fix the microplastic sample on the polycarbonate filter membrane; S2. Perform mechanical calibration on the probe of the atomic force microscope; S3. Use the atomic force microscope to perform high-resolution scanning of the surface morphology of the microplastic sample in the peak force quantitative nanomechanical scanning mode, select a relatively flat position on the surface for nanoindentation testing, and obtain indentation data; S4. Analyze the indentation data and calculate to obtain the hardness data of the measured microplastic sample; In step S1, attaching the polycarbonate filter membrane to the glass slide means first preparing a clean glass slide, washing it with deionized water, and drying the area around the glass slide with a sterile lens paper. The central area of the glass slide does not need to be wiped and remains moist; then gently place the polycarbonate filter membrane on the moist central area of the glass slide, avoiding wrinkles, and after natural drying, the polycarbonate filter membrane can be firmly attached to the surface of the glass slide; In step S1, fixing the microplastic sample on the polycarbonate filter membrane means evenly spreading an appropriate amount of microplastic sample powder on the polycarbonate filter membrane, and then blowing nitrogen onto the surface of the filter membrane and repeating several times; Step S4 is to use the atomic force microscope data analysis software Nanoscope to analyze the indentation data and calculate to obtain the hardness data of the measured sample; the hardness of the microplastic is calculated using the following formula: H = F max / A Among them, H is the hardness, and F max is the peak force when an indentation is produced on the microplastic, and A is the projected area of the indentation produced on the sample surface. After saving the relevant data file, these two values can be directly obtained by opening it in the Nanoscope analysis software. Substituting these values into the above formula gives the hardness data.

2. The method according to claim 1, wherein The pore size of the polycarbonate filter membrane described in step S1 is 2-10 microns.

3. The method according to claim 1, characterized in that The atomic force microscope models described in steps S2 and S3 are Dimension Icon TM , and the manufacturer is Bruker Corporation of the United States.

4. The method according to claim 1, wherein The probe described in step S2 is a probe DDESP-V2 with a diamond coating, and the manufacturer is Bruker Corporation, USA.

5. The method according to claim 1, wherein The mechanical calibration described in step S2 is to perform mechanical property calibration on the probe using a standard sapphire substrate.

6. The method according to claim 1, wherein The scanning rate of the probe is 0.8 Hz, the scanning resolution is set to 128×128 pixels, and the probe pressure is 10-100 μN.

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