CN114894891B - QCM humidity-sensitive sensor based on sodium alginate/polyacrylonitrile composite film, construction method and application - Google Patents

Abstract

The invention belongs to the technical field of humidity-sensitive sensors, and particularly relates to a Quartz Crystal Microbalance (QCM) humidity-sensitive sensor based on a sodium alginate/polyacrylonitrile (SA/PAN) composite film, and a preparation method and application thereof. A layer of Polyacrylonitrile (PAN) fiber network is directly constructed on the QCM sensor through an electrostatic spinning technology and is used as a supporting layer, and a layer of hydrophilic Sodium Alginate (SA) film is spin-coated on the PAN fiber network through a spin-coating technology, so that the QCM humidity sensor containing the SA/PAN humidity-sensitive film with a double-layer network structure is constructed. The preparation method comprises the following steps: (1) preparation of PAN nano-spun fibers; (2) preparation of sodium alginate solution; (3) coating the QCM sensor surface. The QCM humidity sensor is applied to humidity detection or a Morse code generator. The SA/PAN humidity sensor can be used for measuring skin humidity, has good sensing performance on skin humidity, has high sensitivity and quick recovery characteristic, utilizes fingertip movement to generate Moss code, and widens the variety of Moss code generators.

Description

QCM humidity sensor based on sodium alginate/polyacrylonitrile composite film and its construction method and application

Technical Field

The present invention belongs to the technical field of humidity sensors, and in particular relates to a QCM humidity sensor based on a sodium alginate/polyacrylonitrile (SA/PAN) composite film, a preparation method and an application thereof.

Background Art

Humidity detection plays an important role in industrial production, food industry, military aerospace, agricultural planting, drug storage, biomedicine and other fields. Too high humidity will reduce the safety of electrical appliances and accelerate the mildew of food; too low humidity will reduce the yield of crops and bring static electricity hazards. On the other hand, environmental humidity also has an important impact on human health. Too high humidity can easily cause asthma, rheumatism and rheumatoid arthritis, and can easily affect the body's sweating function; too low humidity can cause dry skin, nosebleeds, coughs and respiratory infections.

Quartz crystal microbalance (QCM) sensors have attracted much attention due to their advantages such as high precision, low power consumption, high sensitivity and low cost. QCM sensors are very sensitive to mass changes at the nanogram level. When the humidity-sensitive film adsorbs water molecules, the vibration frequency of the QCM slows down. Excessive coating materials and excessive humidity can cause the QCM to become unstable or even stop vibrating. Therefore, QCM sensors have higher requirements for the adsorption, film-forming properties, sensitivity and high-humidity stability of humidity-sensitive materials.

Sodium alginate (SA) is a natural polymer polyelectrolyte extracted from seaweed. Its molecular chain contains a large number of carboxylates and hydroxyls. The carboxyl group exists in the form of sodium carboxylate. In the hydrophilicity ranking of the groups, the carboxylates have a higher hydrophilicity, followed by the carboxyl group, and the hydrophilicity of the hydroxyl group is slightly weaker than that of the carboxyl group. Sodium alginate has attracted extensive attention from researchers due to its ideal fiber-forming, film-forming, hydrophilic, low price, wide source, non-toxic and environmentally friendly advantages. Polyacrylonitrile (PAN) has good mechanical stability and water insolubility, and is an excellent support membrane layer.

Therefore, there is an urgent need to improve the structure and preparation process of QCM humidity sensor film using sodium alginate and polyacrylonitrile to enhance the humidity-sensitive properties of the sensor, which can be used for skin humidity monitoring or contactless Morse code communication.

Summary of the invention

In view of the deficiencies in the prior art, the present invention provides a QCM humidity sensor based on sodium alginate/polyacrylonitrile (SA/PAN) composite film and a preparation method and application thereof to solve the problems involved in the background technology.

In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a QCM humidity sensor based on SA/PAN composite film, a layer of PAN fiber network is directly constructed on the QCM sensor as a support layer through electrospinning technology, and a layer of hydrophilic SA film is spin-coated on the PAN fiber network through spin coating technology to construct a QCM humidity sensor containing a double-layer network structure SA/PAN humidity-sensitive film.

A further technical solution of the present invention is:

The two materials on the surface of the QCM humidity sensor are tightly combined, the upper SA is uniform and dense, providing hydrophilic active sites for the film, while the lower electrospun PAN fibers support the upper SA film, providing diffusion channels for water molecules;

Or, the hydrophilic angle of the SA/PAN composite film is 39.42°.

The present invention also includes a method for preparing a QCM humidity sensor based on a SA/PAN composite film, the preparation steps comprising:

(1) Preparation of PAN nanospun fibers

The polyacrylonitrile was dissolved in dimethylformamide, heated in a water bath at 90°C and stirred vigorously to obtain a polyacrylonitrile spinning solution, and an electrospinning precursor solution was obtained by standing and degassing;

The electrospinning precursor solution was prepared into PAN nanofibers by electrospinning technology and attached to the pretreated QCM sensor surface;

(2) Preparation of sodium alginate solution

Dissolve sodium alginate powder in deionized water and stir magnetically to obtain a transparent sodium alginate solution;

(3) Coating onto the QCM sensor surface

The sodium alginate solution in step (2) is spin-coated onto the surface of the QCM sensor with PAN nanofibers attached in step (1), so that the sodium alginate solution diffuses into the surface layer of the PAN fiber structure. After drying, a QCM humidity sensor of SA/PAN film is obtained.

A further technical solution of the present invention is:

The parameters of the electrospinning technology in step (1) are set as follows: the distance between the flat receiver and the needle tip of the electrospinning instrument is set to 15 cm, a voltage of 16 kV is applied to both ends, the temperature in the electrospinning chamber is 25° C., the humidity inside the electrospinning instrument is 40% RH, the speed of the injector is 0.18 mL/h, and the spinning time is 1 minute.

A further technical solution of the present invention is:

In the step (1), the mass ratio of polyacrylonitrile to dimethylformamide is 5:83;

Or, the concentration of the sodium alginate solution in step (2) is 4 mg/mL;

Alternatively, in step (3), the volume ratio of the mass of PAN nanofibers on the surface of the QCM sensor to the sodium alginate solution is 1523:2 (ng: μL).

A further technical solution of the present invention is:

The spin-coated sensor in step (3) is dried at 60° C. for 2 hours.

A further technical solution of the present invention is:

The mass fraction of the polyacrylonitrile spinning solution in the step (1) is 6%.

Alternatively, the pretreatment of the QCM sensor is: the QCM without the coating material is repeatedly cleaned with ethanol and deionized water, and then dried with nitrogen.

The present invention also includes the application of a QCM humidity sensor based on the SA/PAN composite film, wherein the QCM humidity sensor is applied to humidity detection or a Morse code generator.

A further technical solution of the present invention is:

The humidity sensing mechanism is:

A double-layer network membrane structure with hydrophilic active sites and water diffusion channels is constructed on the surface of the QCM electrode, which is not only conducive to the adsorption of water molecules, but also conducive to the diffusion and desorption of water molecules to the lower membrane. The hydrophilic SA as the upper layer of the film contains many hydroxyl and carboxyl ions, which provides a large number of water molecule adsorption sites for the SA/PAN film. The electrospun polyacrylonitrile nanofibers greatly increase the specific surface area of the composite film and increase the pores. The water-insoluble electrospun fiber polyacrylonitrile as the diffusion channel of water molecules not only plays a supporting role, but also provides a diffusion channel for water molecules. When the humidity is low, the hydroxyl and carboxyl ions of the composite film combine with water molecules through hydrogen bonds for chemical adsorption. The film adsorbs water molecules and the frequency shift response of the sensor increases. When the humidity increases, water molecules are physically adsorbed through hydrogen bonds. The frequency shift response is affected by the mass effect and viscoelastic effect at the same time. Water molecules enter SA and PAN, causing the film to swell.

A further technical solution of the present invention is:

The QCM humidity sensor is used for skin humidity monitoring or contactless Morse code communication.

The QCM humidity sensor based on sodium alginate/polyacrylonitrile (SA/PAN) composite film of the present invention and its preparation method and application are beneficial as follows: The present invention uses an electrospinning process to prepare a SA/PAN composite film QCM sensor with excellent humidity sensing performance. The SA/PAN humidity sensor can measure skin humidity in the air and has good sensing performance for skin humidity. The sensor can respond instantly to fast-moving skin and recover quickly. The SA/PAN film QCM sensor has high sensitivity and fast recovery characteristics, generates Morse code by using fingertip movement, constructs a decoding and original code display interface, and broadens the types of Morse code generators.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the preparation process of an embodiment of the present invention:

Figure 2a-c are SEM characterization results of SA/PAN composite films according to an embodiment of the present invention;

d is the FT-IR spectra of SA, PAN and SA/PAN composite films of the embodiments of the present invention;

e is the water contact angle of SA, PAN and SA/PAN films in the embodiment of the present invention;

FIG3 is a graph showing the XPS characterization results of the SA/PAN composite film according to an embodiment of the present invention;

FIG4 a is a dynamic response curve of SA, PAN and SA/PAN sensors according to an embodiment of the present invention;

b is an exponential fitting curve diagram of frequency shift and humidity of three sensors in the embodiment of the present invention at a relative humidity of 0 to 97%;

c is a graph showing the dynamic adsorption and desorption of water molecules in the SA/PAN humidity sensor according to an embodiment of the present invention;

d is a humidity hysteresis curve diagram of the SA/PAN humidity sensor according to an embodiment of the present invention;

FIG5 a is a dynamic response/recovery measurement curve diagram of three sensors when switching from 0% relative humidity to 97% relative humidity according to an embodiment of the present invention;

b is the selectivity test result of the SA/PAN sensor of the embodiment of the present invention under four different gases of 43% relative humidity, 85% relative humidity and 400 ppm;

c is the long-term stability test result of the SA/PAN sensor of the embodiment of the present invention at 23%, 43%, 67% and 85% relative humidity within 36 days;

FIG6 a is a diagram showing the repeatability test results of the SA/PAN sensor according to an embodiment of the present invention under 23%, 52% and 85% relative humidity environments;

b is a graph showing the relationship between frequency shift and mass change under different humidity conditions in an embodiment of the present invention;

FIG7a is a conductivity spectrum of SA according to an embodiment of the present invention at different relative humidities;

b is the conductivity spectrum of PAN in the embodiment of the present invention at different relative humidity

c is the conductivity spectrum of PAN in the embodiment of the present invention at different relative humidity

d is the quality factor variation curve of the three sensors in the embodiment of the present invention

FIG8 is a schematic diagram of the process of water molecule adsorption by the SA/PAN composite film according to an embodiment of the present invention;

FIG9 is a schematic diagram of a fingertip movement test according to an embodiment of the present invention;

FIG10 a is a diagram of a fingertip motion test according to an embodiment of the present invention;

b is a graph showing changes in wrist skin humidity before and after exercise according to an embodiment of the present invention;

c is the skin humidity value at different positions of the human body in a calm state according to the embodiment of the present invention;

FIG11 is a diagram of Morse code information of “a qcm sensor” and “I love upc” according to an embodiment of the present invention;

FIG. 12 is a Morse code decoding and display interface according to an embodiment of the present invention.

DETAILED DESCRIPTION

The specific implementation manner will be further described below in conjunction with the accompanying drawings.

The reagents and instruments in the following examples are all conventional experimental reagents and instruments.

Embodiment 1:

QCM humidity sensor based on SA/PAN composite film and preparation method, Figure 1 is a schematic diagram of the preparation process:

(1) PAN nanospun fibers were prepared as the support layer.

1g of polyacrylonitrile was dissolved in 16.6g of dimethylformamide (DMF), heated in a 90℃ water bath and stirred vigorously for 8h to obtain a polyacrylonitrile spinning solution with a mass fraction of 6%. The electrospinning precursor solution was obtained by static degassing. The bare QCM wafer without film coating was repeatedly washed with ethanol and deionized water, and then blown dry with nitrogen. The above solution was transferred to the spinning syringe, and the No. 23 metal needle at the front of the syringe was connected to a high-voltage power supply, and the QCM was fixed on a flat receiver for electrospinning. The distance between the flat receiver and the needle tip was set to 15cm, and a voltage of 16kV was applied to both ends. The temperature in the electrospinning chamber was 25℃, and the humidity inside the electrospinning instrument was 40%RH. The speed of the injector was 0.18mL/h, and the spinning time was 1 minute. After the spinning was completed, the QCM covered with PAN nanofiber film was removed for use.

(2) Preparation of sodium alginate solution

0.2 g of sodium alginate powder was dissolved in 50 mL of deionized water and magnetically stirred for 3 hours to obtain a transparent sodium alginate solution (0.004 g/mL).

(3) Coating onto the QCM sensor surface

The PAN thin film QCM sensor with a spinning time of 1 minute was fixed on a spin coating device, and 2 μL of SA solution was spin coated to allow the SA solution to diffuse into the surface of the PAN fiber structure. The spin-coated sensor was dried at 60°C for 2 hours to allow SA and PAN to be tightly combined to obtain a SA/PAN thin film QCM sensor. The PAN fiber on the electrode of the QCM sensor was about 1523 ng.

Electrospinning technology can spin polymers without special morphology into fiber network films, thereby greatly increasing the porosity and adsorption capacity of the film and obtaining nanofibers with a larger specific surface area. Therefore, a layer of PAN fiber network was directly constructed on the QCM using electrospinning technology as a support layer, and a layer of hydrophilic SA film was spin-coated on the PAN film using spin coating technology to construct a SA/PAN humidity-sensitive film with a double-layer network structure, and a SA/PAN composite film QCM humidity sensor was prepared.

Comparative Example 1:

PAN humidity sensor with single CA film

1g of polyacrylonitrile was dissolved in 16.6g of dimethylformamide (DMF), heated in a 90℃ water bath and stirred vigorously for 8h to obtain a polyacrylonitrile spinning solution with a mass fraction of 6%. The electrospinning precursor solution was obtained by static degassing. The bare QCM wafer without film coating was repeatedly washed with ethanol and deionized water, and then blown dry with nitrogen. The above solution was transferred to the spinning syringe, and the No. 23 metal needle at the front of the syringe was connected to a high-voltage power supply, and the QCM was fixed on a flat receiver for electrospinning. The distance between the flat receiver and the needle tip was set to 15cm, and a voltage of 16kV was applied to both ends. The temperature in the electrospinning chamber was 25℃, and the humidity inside the electrospinning instrument was 40%RH. The speed of the injector was 0.18mL/h, and the spinning time was 7 minutes. A quartz crystal microbalance sensor of pure polyacrylonitrile film was prepared.

Comparative Example 2:

QCM humidity sensor with single SA film

(2) Preparation of sodium alginate solution

0.2 g of sodium alginate powder was dissolved in 50 mL of deionized water and magnetically stirred for 3 hours to obtain a transparent sodium alginate solution (0.004 g/mL).

(3) Coating onto the QCM sensor surface

The bare QCM chip without film coating was fixed on the chip holder of the spin coater, and 2 μL of SA solution was spin coated. The spin coated sensor was dried at 60 °C for 2 h to obtain a pure SA thin film QCM sensor.

1. Characterization characteristics analysis:

The frequency shift and quality information of the three sensor films of Example 1 and Comparative Examples 1 and 2 are shown in Table 1.

Table 1 Material film information of different QCM sensors

Figure 2 (a-c) shows the SEM characterization results of the SA/PAN composite film. As can be seen from the figure, the SA on the upper layer is uniform and dense, providing hydrophilic active sites for the film, while the electrospun PAN fibers on the lower layer support the upper SA film and provide diffusion channels for water molecules. The two materials are tightly combined. This double-layer film structure greatly improves the specific surface area, porosity and adsorption capacity of the material.

Figure 2(d) shows the FT-IR spectra of SA, PAN and SA/PAN nanocomposite films.

The chemical structure of the SA/PAN composite film was further analyzed by FT-IR, confirming the presence of SA and PAN in the composite film. In the FT-IR spectrum of sodium alginate SA, there is a broad and strong peak at 3430 cm ^-1 , which is the absorption peak of the hydroxyl (—OH) group. The absorption peak at 1613 cm ^-1 is the asymmetric stretching vibration peak of —COO—. The absorption peak at 1414 cm ^-1 is generated by the in-plane bending vibration of C—H. The peak at 1093 cm ^-1 is attributed to the C—O—C bond and C—C bond in the pyran ring. The peak at 1036 cm ^-1 is the stretching vibration peak of C—OH. In the spectrum of polyacrylonitrile PAN, the peak at 2939 cm ^-1 is the stretching vibration peak of the methylene (—CH2—) group. The peak at 2244 cm ^-1 is attributed to the stretching vibration of the nitrile (—CN) group. The peak at 1732 cm ^-1 is caused by C＝O. The weak peak detected at 1624 cm ^-1 is caused by the amino group. The peak at 1453 cm ^-1 is caused by the stretching vibration of the methylene (—CH2—) group. The peaks at 1367 cm ^-1 and 1236 cm ^-1 are attributed to the bending vibration of the C—H bond. The infrared spectrum of the SA/PAN composite film contains all the characteristic peaks of SA and PAN, indicating that the SA/PAN composite film has been successfully prepared. 2 μL of deionized water was dropped on the surface of the three films for hydrophilicity testing.

Figure 2(e) shows the water contact angles of SA, PAN and SA/PAN films.

The water contact angle of SA film is 32.13°, the water contact angle of PAN film is 55.34°, and the water contact angle of SA/PAN composite film is 39.42°. The hydrophilic angle of SA film is the smallest. The addition of SA improves the hydrophilicity of SA/PAN film, which is more conducive to humidity detection.

FIG3 is a graph showing the XPS characterization results of the SA/PAN composite film, where a is the measured spectrum; b is the C1s spectrum; c is the O1s spectrum; and d is the N1s spectrum.

The SA/PAN composite film is composed of C, N, O, H and Na elements. The results show that the intensity of the O1s peak is very high, which is because SA provides a large number of hydroxyl groups to the composite film. In Figure 3(b), two peaks can be found at 284.8eV and 285.1eV in the C1s spectrum, corresponding to the C—C bond and C—N bond in PAN, respectively. The peak at 286.7eV corresponds to C—O and C＝O in SA. In the O1s spectrum of Figure 3(c), the peak at 530.2eV corresponds to the —OH group in SA. The large area of the peak at 531.8eV corresponds to the C—O bond. The peak at 534.3eV corresponds to the C＝O bond in SA. In the N1s spectrum (as shown in Figure 3(d)), the two peaks at 399.6 and 399.7eV correspond to the C—N bond and —NH ₂ group of PAN, respectively. The results show that the composite film was successfully prepared and contains hydrophilic functional groups, which is conducive to the adsorption of water molecules.

2. Analysis of Humidity Sensitive Characteristics

The saturated salt solution method was used to provide a humidity environment with a relative humidity of 11-97%, and phosphorus pentoxide ( _{P2O5 )} was used as a desiccant to create a dry environment. The humidity sensitivity experiment was carried out in bottles with different humidity levels, with a switching interval of 100s and an experimental temperature of 25°C. In the test, the data sampling interval of the QCM tester was set to 0.5s.

Figure 4(a) shows the dynamic response curves of SA, PAN and SA/PAN sensors.

As humidity increases, the mass of water molecules adsorbed on the film gradually increases, and the frequency shift of the sensor gradually increases. The frequency shifts of single SA, single PAN and composite SA/PAN film sensors are -4190.41, -1958.09 and -6960.55 Hz, respectively. The humidity response of the SA/PAN sensor is significantly higher than that of a single film.

FIG4( b ) is an exponential fitting curve diagram of the frequency shift and humidity of the three sensors at a relative humidity of 0 to 97%.

The fitting equations of single SA, single PAN and composite SA/PAN film sensors are as follows: Δf = -15.56"e"^"x/17.34"-5.87, Δf = -13.06"e"^"x/19.63"-81.09 and Δf = -63.331"e"^"x/20.94"-298.96. The correlation coefficients (R2) of the three fitting equations are 0.9996, 0.9815 and 0.9900, respectively. The actual test curve has a high correlation with the exponential fitting curve.

Figure 4(c) is a graph showing the dynamic adsorption and desorption curves of water molecules in the SA/PAN humidity sensor.

The curve in the figure has good symmetry, and the frequency shift response values under positive and negative travel are very close.

Figure 4(d) is the humidity hysteresis curve of the SA/PAN humidity sensor.

The maximum humidity hysteresis value of the sensor is 2.57%RH, which occurs near 52%RH. The experimental results show that the humidity hysteresis characteristic of the sensor is excellent.

FIG5( a ) is a graph showing the dynamic response/recovery measurements of the three sensors when switching from 0% relative humidity to 97% relative humidity.

As can be seen from Figure 5(a), the response/recovery times of the single SA, single PAN, and composite SA/PAN film sensors are 19s/2.5s, 23s/3s, and 14.75s/2.5s, respectively.

Figure 5(b) shows the selectivity test results of the SA/PAN sensor at 43% relative humidity, 85% relative humidity and 400 ppm for four different gases (ethanol, ammonia, formaldehyde, and carbon monoxide).

The results showed that the sensor had good selectivity.

Figure 5(c) shows the long-term stability test results of the SA/PAN sensor at 23%, 43%, 67%, and 85% relative humidity over 36 days.

The results show that the sensor has good repeatability and long-term stability.

Figure 6(a) is a graph showing the repeatability test results of the SA/PAN sensor under 23%, 52% and 85% relative humidity environments.

It shows that the response curve of SA/PAN humidity sensor under three humidity conditions has good repeatability.

FIG6( b ) is a graph showing the relationship between frequency shift and mass change at different humidity levels.

As the humidity increases, the mass of water molecules adsorbed by the SA/PAN sensor and the frequency shift response both increase.

Figure 7(a) shows the conductivity spectra of SA at different relative humidities;

(b) is the conductivity spectrum of PAN at different relative humidity;

(c) is the conductivity spectrum of PAN at different relative humidity;

(d) is the quality factor variation curve of the three sensors;

Schematic diagram of the process of water molecules adsorbed by SA/PAN composite film

The above results are better than the humidity sensing performance of the existing sensors in Table 2.

Table 2 Comparison of humidity sensing performance of SA/PAN sensor and other sensors

Embodiment 2:

The present invention also includes the application of a QCM humidity sensor based on the SA/PAN composite film, wherein the QCM humidity sensor is applied to humidity detection.

The humidity sensing mechanism is:

FIG8 is a schematic diagram of the process of water molecule adsorption by SA/PAN composite film.

A double-layer network membrane structure with hydrophilic active sites and water diffusion channels is constructed on the surface of the QCM electrode, which is not only conducive to the adsorption of water molecules, but also conducive to the diffusion and desorption of water molecules to the lower membrane. As the upper layer of the film, the hydrophilic SA contains many hydroxyl and carboxyl ions, which provide a large number of water molecule adsorption sites for the SA/PAN membrane. The electrospun polyacrylonitrile nanofibers greatly increase the specific surface area of the composite film and increase the pores. As the diffusion channel of water molecules, the water-insoluble electrospun fiber polyacrylonitrile not only plays a supporting role, but also provides a diffusion channel for water molecules. When the humidity is low, the hydroxyl and carboxyl ions of the composite film combine with water molecules through hydrogen bonds for chemical adsorption, the film adsorbs water molecules, and the frequency shift response of the sensor increases. When the humidity increases, water molecules are physically adsorbed through hydrogen bonds. The frequency shift response is affected by both mass effect and viscoelastic effect. Water molecules enter SA and PAN, causing the film to swell. The fiber network polyacrylonitrile supports the upper SA membrane, provides a diffusion channel for water molecules, alleviates the swelling of SA, and shortens the time of water molecule adsorption and desorption.

The sodium alginate/polyacrylonitrile (SA/PAN) sensor was tested for the application of fingertip movement.

FIG9 is a schematic diagram of a fingertip movement test, in which the fingertip moves downward toward the sensor film, with the closest distance being 3 mm, and then moves upward, with the farthest distance being 5 cm.

In this work, the SA/PAN thin film QCM humidity sensor was used to monitor the humidity of human skin. The results show that the SA/PAN sensor has a good humidity-sensitive response to skin humidity. During the measurement, the sensor is non-contact with the skin and the sensor is 3mm away from the skin.

In this work, fingertip movement tests were conducted. Figure 10(a) shows the fingertip movement test diagram. The sensor showed excellent repeatability and ultra-fast recovery ability to fingertip movement. As shown in the figure, the frequency shift response value of the SA/PAN film sensor when the fingertip approaches is about -700Hz, and the frequency shift value fluctuates slightly each time the fingertip approaches. The SA/PAN film sensor has high sensitivity and fast recovery, and the detection results of medium and low humidity are satisfactory. The SA/PAN film sensor is used for application detection of skin humidity and non-contact Morse code communication.

The changes in wrist skin humidity before and after exercise were detected. Figure 10(b) shows the changes in wrist skin humidity before and after exercise. It can be seen that after exercise, the body surface is moist and the skin humidity increases.

Figure 10(c) shows the skin humidity values at different positions of the human body in a calm state.

The speed of fingertips was changed and the non-contact time was controlled to generate humidity signals with different half-peak widths. According to the rules of Morse code, a small half-peak width (<1.5) represents the "drip" in Morse code, and a large half-peak width (≥1.5) represents the "tap" in Morse code. There is no humidity response when the fingertips are removed. The fingertip removal time of less than 1 second represents the interval between letters, and the fingertip removal time of more than 1 second represents the interval between words. MATLAB was used to establish a mapping table of letters and numbers in Morse code, "drip" was replaced by 1, and "tap" was replaced by 0. Long intervals separate words, and short intervals separate letters.

According to the above Morse code rules, the "a qcmsensor" and "I love UPC" signals were sent to the sensor using non-contact fingertip movement. Figure 11 shows the Morse code information of "a qcm sensor" and "I love upc". The sensor can quickly capture, sense and display humidity signals.

In order to achieve communication by sending Morse code through fingertip movement, a Morse code converter was designed using MATLAB, and the display interface was built through App Designer. Figure 12 shows the Morse code decoding and display interface, which can successfully translate letters and numbers. The sensor can not only meet the needs of skin moisture detection, but also send Morse code through fingertip movement, broadening the types of Morse code generators and proving its potential for skin moisture monitoring and non-contact Morse code communication.

The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made based on the essence of the content of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. A QCM humidity sensor based on sodium alginate/polyacrylonitrile composite film is characterized in that: a layer of polyacrylonitrile fiber network is directly constructed on the QCM sensor as a supporting layer by electrospinning technology, and a layer of hydrophilic sodium alginate film is spin-coated on the polyacrylonitrile fiber network by spin coating technology to construct a QCM humidity sensor containing a double-layer network structure sodium alginate/polyacrylonitrile humidity-sensitive film. 2. The QCM humidity sensor based on sodium alginate/polyacrylonitrile composite film according to claim 1, characterized in that: The two materials on the surface of the QCM humidity sensor are tightly combined, the upper layer of sodium alginate is uniform and dense, providing hydrophilic active sites for the film, while the lower layer of electrospun polyacrylonitrile fibers supports the upper layer of sodium alginate film, providing diffusion channels for water molecules; Alternatively, the hydrophilic angle of the sodium alginate/polyacrylonitrile composite film is 39.42°. 3. The method for preparing a QCM humidity sensor based on a sodium alginate/polyacrylonitrile composite film according to any one of claims 1 to 2, characterized in that the preparation steps include: (1) Preparation of polyacrylonitrile nanospun fibers Dissolve polyacrylonitrile in dimethylformamide, heat in a 90°C water bath and stir vigorously to obtain a polyacrylonitrile spinning solution, and obtain an electrospinning precursor solution by standing and degassing; The electrospinning precursor solution was prepared into polyacrylonitrile nanofibers by electrospinning technology and attached to the pretreated QCM sensor surface; (2) Preparation of sodium alginate solution Dissolve sodium alginate powder in deionized water and stir magnetically to obtain a transparent sodium alginate solution; (3) Coating onto the QCM sensor surface The sodium alginate solution in step (2) is spin-coated onto the surface of the QCM sensor to which the polyacrylonitrile nanofibers are attached in step (1), so that the sodium alginate solution diffuses into the surface layer of the polyacrylonitrile fiber structure. After drying, a QCM humidity sensor of the sodium alginate/polyacrylonitrile film is obtained. 4. The method for preparing a QCM humidity sensor based on a sodium alginate/polyacrylonitrile composite film according to claim 3, characterized in that: The parameters of the electrospinning technology in step (1) are set as follows: the distance between the flat receiver and the needle tip of the electrospinning instrument is set to 15 cm, a voltage of 16 kV is applied to both ends, the temperature in the electrospinning chamber is 25° C., the humidity inside the electrospinning instrument is 40% RH, the speed of the injector is 0.18 mL/h, and the spinning time is 1 minute. 5. The method for preparing a QCM humidity sensor based on a sodium alginate/polyacrylonitrile composite film according to claim 3, characterized in that: In the step (1), the mass ratio of polyacrylonitrile to dimethylformamide is 5:83; Or, the concentration of the sodium alginate solution in step (2) is 4 mg/mL; Alternatively, in step (3), for every 1523 ng of polyacrylonitrile nanofibers on the surface of the QCM sensor, 2 μL of the sodium alginate solution is spin-coated onto the surface of the QCM sensor. 6. The method for preparing a QCM humidity sensor based on a sodium alginate/polyacrylonitrile composite film according to claim 3, characterized in that: The spin-coated sensor in step (3) is dried at 60° C. for 2 hours. 7. The method for preparing a QCM humidity sensor based on a sodium alginate/polyacrylonitrile composite film according to claim 3, characterized in that: The mass fraction of the polyacrylonitrile spinning solution in step (1) is 6%, Alternatively, the pretreatment of the QCM sensor is: the QCM without the coating material is repeatedly cleaned with ethanol and deionized water, and then dried with nitrogen. 8. Application of the QCM humidity sensor based on sodium alginate/polyacrylonitrile composite film according to any one of claims 1-2 or the QCM humidity sensor prepared by the method described in any one of claims 3-7, characterized in that: the QCM humidity sensor is applied to humidity detection or Morse code generator. 9. The use of the QCM humidity sensor based on sodium alginate/polyacrylonitrile composite film according to claim 8, characterized in that: The humidity sensing mechanism is: A double-layer network membrane structure with hydrophilic active sites and water diffusion channels is constructed on the surface of the QCM electrode, which is not only conducive to the adsorption of water molecules, but also conducive to the diffusion and desorption of water molecules to the lower membrane. The hydrophilic sodium alginate as the upper layer of the film contains many hydroxyl and carboxyl ions, which provides a large number of water molecule adsorption sites for the sodium alginate/polyacrylonitrile membrane. The electrospun polyacrylonitrile nanofibers greatly increase the specific surface area of the composite film and increase the pores. The water-insoluble electrospun fiber polyacrylonitrile as the diffusion channel of water molecules not only plays a supporting role, but also provides a diffusion channel for water molecules. When the humidity is low, the hydroxyl and carboxyl ions of the composite film combine with water molecules through hydrogen bonds for chemical adsorption. The film adsorbs water molecules and the frequency shift response of the sensor increases. When the humidity increases, water molecules are physically adsorbed through hydrogen bonds. The frequency shift response is affected by the mass effect and viscoelastic effect at the same time. Water molecules enter the sodium alginate and polyacrylonitrile, causing the film to swell. 10. The use of the QCM humidity sensor based on sodium alginate/polyacrylonitrile composite film according to claim 8, characterized in that: The QCM humidity sensor is used for skin humidity monitoring or contactless Morse code communication.