Preparation and application of a sweat ascorbic acid detection sensor based on screen-printed electrode and wood substrate

Abstract

The present application belongs to the field of wearable sensor, and discloses a preparation and application of a sweat ascorbic acid sensor with simple structure and low cost, which adopts silk screen printing and wood substrate. The sensor of the present application combines silk screen printed electrodes and hydrophilic wood substrate. In order to overcome the shortcomings of past sensors, such as hydrophobicity and high cost, the silk screen printing technology is adopted for manufacturing. The silk screen printed carbon electrode has high flexibility and good conductivity, and the hydrophilic wood paper is used as the substrate, which has high ability of collecting, storing and transporting sweat. The sensor has the characteristics of high sensitivity (0.032 μA L μmol ‑1 ), high selectivity, low detection limit (10 μA L μmol ‑1 ), wide linear range (10-1000 μmol / L), good repeatability and good stability. In general, the wearable sensor developed in this study shows great potential in the prognosis, diagnosis and treatment of ascorbic acid disease, and is a promising and cost-effective strategy.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of wearable sensors, and particularly relates to a simple and low-cost preparation and application of a sweat ascorbic acid detection sensor using screen printing and wood substrates. BACKGROUND

[0002] Sweat-based ascorbic acid detection devices have gradually gained attention due to their real-time, convenience, and other advantages. As a body fluid rich in biomarkers, sweat is easy to collect and store, making it an ideal medium for determining ascorbic acid levels. In sweat ascorbic acid detection sensors, sensors based on electrochemical technology are the most prominent due to their high sensitivity, fast response, and miniaturization. Sensors typically include a detection electrode and a substrate.(1) For the detection electrode, there are enzyme catalysis and non-enzyme catalyst catalysis methods. Although enzyme electrodes are favored for their excellent selectivity, the easy deactivation of enzymes can affect the stability of the detection, and the development and production of such electrodes are costly and complex. Non-enzyme catalytic electrodes use graphene, noble metals, and other catalysts to enhance the reaction, but the preparation process of graphene is complex and costly, and the deposition process of metal catalysts is tedious and expensive, increasing the overall production cost and making it difficult to promote large-scale use.(2) In terms of substrates, although common materials such as polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) are easy to obtain and have a certain degree of flexibility, their low hydrophilicity can affect the uniform distribution of sweat on the sensor surface, negatively impacting performance. In addition, the hydrophobic nature of these substrates can disrupt the natural moist environment of the skin surface, reducing comfort and biological compatibility during long-term wear. Another problem is that these substrate materials cannot achieve directional transport of analytes in sweat, relying only on diffusion, which slows the process of sweat and its contained ascorbic acid molecules reaching the detection electrode, potentially leading to a decrease in analyte concentration, affecting the response speed and detection sensitivity of the sensor. In summary, although the above electrochemical ascorbic acid detection technology in sweat shows certain reliability in measuring ascorbic acid concentration in sweat, the preparation of electrodes is not simple, the cost is high, the low hydrophilicity of substrate materials, and the inability to directionally transport sweat are the main challenges faced by current technology, which will limit the overall performance and application range of the sensor. Therefore, it is necessary to prepare an electrochemical sensor that is simple to prepare, low in cost, has high hydrophilicity, and can directionally transport sweat to optimize the detection process of ascorbic acid. SUMMARY

[0003] In order to overcome the shortcomings and deficiencies of the prior art, the primary purpose of the present application is to provide a simple and low-cost preparation method for a sweat ascorbic acid detection sensor based on screen-printed electrodes and wood substrates.

[0004] Another object of the present application is to provide a simple, low-cost and highly hydrophilic sensor for ascorbic acid detection in sweat prepared by the above method. The sensor comprises a three-electrode and a flexible wooden substrate. Carbon paste and Ag / AgCl dropping are screen-printed onto the flexible wooden substrate to form a three-electrode. The flexible wooden substrate is treated to remove some lignin for collecting, storing and transporting sweat to the electrode.

[0005] Another object of the present application is to provide the use of the simple, low-cost and highly hydrophilic sensor in ascorbic acid detection in sweat. In the detection process, ascorbic acid is oxidized on the electrode, and the concentration of ascorbic acid is analyzed by measuring the current of the oxidation reaction.

[0006] The objects of the present application are achieved by the following solutions:

[0007] A method for preparing a sensor for ascorbic acid detection in sweat based on screen-printed electrodes and wooden substrates, comprising the following steps:

[0008] Step (1): Preparation of a flexible wooden substrate

[0009] The wood piece is immersed in a mixed aqueous solution containing NaOH and Na2SO3, then vacuum impregnated, and after the end of the process, immersed in boiling water to remove the chemicals, and then dried to obtain a flexible wooden substrate with part of the lignin removed;

[0010] Step (2): Screen-printing treatment

[0011] The flexible wooden substrate is fixed on a screen printing table, and solid wax is applied to the flexible wooden substrate through the screen, then the flexible wooden substrate is heated to melt the solid wax particles passing through the mesh into liquid and absorbed by the substrate to form a hydrophobic barrier, the uncoated solid wax is a hydrophilic area, separating the hydrophobic and hydrophilic areas, and then cooled to room temperature to obtain a wax-printed patterned flexible wooden substrate;

[0012] Step (3): Three-electrode printing

[0013] The three-electrode, i.e. working electrode, counter electrode and reference electrode, is printed on the hydrophilic area of the wax-printed patterned flexible wooden substrate, and then dried to obtain a sensor for ascorbic acid detection in sweat based on screen-printed electrodes and wooden substrates.

[0014] Screen printing technology, as a simple, efficient and relatively mature preparation process, shows significant cost-effectiveness in the field of sensor preparation. This method is suitable for various substrate materials, including wood, plastic and metal, providing a low-cost solution for quickly customizing electrode patterns on a variety of substrates. Through the template, screen printing can accurately transfer different types of ink to the material surface, achieving rapid and accurate fabrication of electrode patterns. In addition, this technology can also distinguish between hydrophilic and hydrophobic regions of the substrate through simple screen wax printing, providing more flexibility and functionality for sensor design. In terms of preparation cost, the advantage of screen printing technology is the economy of raw materials and preparation tools. Whether it is ink or printing equipment, low-cost commercial materials and devices can be selected, greatly reducing the production cost of sensors. The application of screen printing technology in sensor preparation provides the possibility for simple and low-cost production of sensors. On the other hand, the use of flexible wood as a substrate material for sensors is beneficial for the collection and transmission of sweat. After removing part of the lignin, the wood substrate not only has the flexibility and water absorption similar to paper, but also has higher mechanical strength than paper. In addition, the natural pores and porous structure of wood are extremely suitable for transporting and storing sweat. These fine channels simulate the plant's water transport system, which can directly and quickly transport sweat to the electrode. This transport mechanism does not rely on diffusion, reducing time delay and sample loss, thereby improving the accuracy and stability of the sensor, making it an ideal sensor substrate material. In summary, by combining screen printing technology and flexible wood substrate, a sensor with simple preparation, low cost, high hydrophilicity and sweat directional conduction can be manufactured.

[0015] The wood chips in step (1) are preferably basswood wood chips.

[0016] The concentrations of NaOH and Na2SO3 in the mixed aqueous solution containing NaOH and Na2SO3 in step (1) are ≥1 mol / L and ≥0.4 mol / L, respectively.

[0017] The amount of the mixed aqueous solution containing NaOH and Na2SO3 used in step (1) is sufficient to completely immerse the wood chips in the mixed aqueous solution containing NaOH and Na2SO3.

[0018] The time for vacuum impregnation in step (1) is 2-12 h. To ensure sufficient impregnation, vacuum impregnation under stirring is preferred.

[0019] The removal of chemicals by immersing in boiling water in step (1) refers to the removal of NaOH and Na2SO3.

[0020] The drying in step (1) is preferably freeze-drying to prevent shrinkage of the wood chips during drying.

[0021] The flexible wood substrate obtained in step (1) can be further cut and shaped as needed, wherein the cutting is preferably cutting into a rectangular flexible wood substrate with a length of 4 cm, a width of 3 cm, and a thickness of 1 mm.

[0022] The silk screen in step (2) is preferably a 200-mesh wooden frame screen made of nylon;

[0023] The solid wax in step (2) is at least one of beeswax and paraffin wax;

[0024] The heating in step (2) is heating at 60-100°C for 0.5-3 min, and is preferably heating at 80°C for 1 min;

[0025] The cooling to room temperature in step (2) refers to cooling to room temperature in less than 10 s.

[0026] The working electrode in step (3) is a conductive carbon electrode; the counter electrode is a conductive carbon electrode; and the reference electrode is an Ag / AgCl electrode;

[0027] Preferably, the reference electrode in step (3) is an Ag / AgCl electrode, and a conductive carbon layer is further included between the Ag / AgCl electrode and the hydrophilic region of the wax-printed patterned flexible wood substrate to increase the electrical conductivity.

[0028] In step (3), during printing, the silk screen frame is fixed on the printing table, the three-electrode pattern of the silk screen is aligned with the hydrophilic region of the wax-printed patterned flexible wood substrate, the flexible wood substrate is fixed, the conductive carbon paste ink is poured at one end of the silk screen template, the ink is evenly scraped across the silk screen using a squeegee, and the ink is transferred to the substrate through the apertures to form a pattern; after printing, the printed wood piece is placed in air until the ink solvent is completely volatilized, and after drying, the Ag / AgCl paste is squeegeed on the reference electrode, and then dried to obtain a sweat ascorbic acid detection sensor based on a silk screen printed electrode and a wood substrate.

[0029] In step (3), to improve the electrical conductivity, the ink can be squeegeed repeatedly, and is preferably squeegeed 5 times.

[0030] In the above preparation method, if the temperature is not specified, it is performed at room temperature.

[0031] A sweat ascorbic acid detection sensor based on a silk screen printed electrode and a wood substrate prepared by the above method.

[0032] The sweat ascorbic acid detection sensor based on a silk screen printed electrode and a wood substrate described above is used for detecting ascorbic acid in sweat.

[0033] A method for detecting ascorbic acid in sweat based on a screen-printed electrode and a wood substrate, comprising the following steps:

[0034] (1) Prepare artificial sweat with different ascorbic acid concentrations, and contact or immerse the flexible wood substrate of the sensor without wax dyeing and the side printed with three electrodes in the artificial sweat, then obtain the oxidation current peak of the artificial sweat with different ascorbic acid concentrations at 0.1 V voltage by the LSV test method of the electrochemical workstation, and obtain the standard curve of the concentration of ascorbic acid and the oxidation current by taking the concentration of ascorbic acid as the abscissa and the oxidation current peak as the ordinate.

[0035] (2) Test the oxidation current peak of the actual sweat at 0.1 V voltage by the LSV test method of the electrochemical workstation, and then substitute the oxidation current peak into the standard curve in step (1) to obtain the concentration of ascorbic acid in the actual sweat.

[0036] The mechanism of the present application is as follows:

[0037] The developed new sensor can provide a solution for the past low hydrophilicity and high cost sweat ascorbic acid sensor. The present application aims to solve this problem by constructing a carbon electrode on a flexible wood substrate with part of the lignin removed through screen printing technology for detecting the concentration of ascorbic acid in sweat. This sensor not only optimizes the production process and production cost, but also has excellent electrochemical performance. Moreover, thanks to the excellent water absorption of the flexible wood substrate and the use of the wood conduit water transport system, it can directly and quickly transport sweat to the electrode, showing its high efficiency in practical application. In terms of ascorbic acid detection, the sensor shows excellent performance indicators, including high sensitivity (0.032 mu A mu M -1 ), excellent selectivity, low detection limit (10 mu mol / L) and wide linear response range, can quickly and accurately monitor the small changes in ascorbic acid concentration, and shows good repeatability and stability, providing a new strategy with practical application value for monitoring and managing health problems related to human ascorbic acid concentration.

[0038] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0039] (1) The wood substrate of the sensor of the present application has excellent hydrophilicity. Its water contact angle is close to 0°, and the sweat sample can be transported through the original conduit of the wood.

[0040] (2) The sensor prepared by the present application has good selectivity, and under the interference of various active factors, the oxidation peak of ascorbic acid can be observed at a specific oxidation potential of 0.1 V.

[0041] (3) The prepared sensor has good durability. Under the influence of different bending cycles, different bending radii and different temperatures, the oxidation peak of ascorbic acid can be observed at a specific oxidation potential of 0.1 V. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 A process diagram for preparing a hydrophilic ascorbic acid sensor of a screen printing electrode and a wood substrate from raw materials.

[0043] Figure 2 A schematic diagram of the working principle of a flexible wood sensor.

[0044] Figure 3 A flexible wood substrate sensor prepared in Example 1 Micro-morphology and delignification analysis diagram, wherein a is the cross-sectional SEM diagram of the wood substrate sensor, b is the actual diagram of the wood substrate sensor, c is the cross-sectional SEM diagram of the working electrode; d is the actual diagram of the baxi wood before and after delignification; e is the analysis of wood cellulose, hemicellulose and lignin composition before and after delignification; f is the change of wood mass decrease before and after delignification.

[0045] Figure 4 The change of water absorption of baxi wood before and after delignification prepared in Example 1. a is the digital photo of liquid absorption at different times before and after delignification of baxi wood; b is the liquid diffusion diagram of baxi wood before and after delignification; c is the liquid absorption diagram of baxi wood before and after delignification.

[0046] Figure 5 The performance diagram of screen wax printing and screen carbon printing, wherein a is the change diagram of wood hydrophilicity and hydrophobicity before and after screen wax printing; b is the conductive performance diagram of wood after screen carbon printing; c is the EIS test diagram.

[0047] Figure 6 The sensitivity test of the sensor prepared in Example 1; the sensitivity comparison of the baxi wood substrate sensor after delignification with (a) baxi wood substrate sensor without delignification and (b) filter paper substrate sensor; CV test of baxi wood substrate sensor after delignification with different scanning rates (c) and fitting (d).

[0048] Figure 7 The performance test of the sensor prepared in Example 1, wherein a and b are DPV tests of different concentrations of ascorbic acid in ABS solution and fitting, c and d are continuous concentration i-t test of the sensor and the enlarged diagram.

[0049] Figure 8 The influence of different bending times (a) and bending diameters (b) on the sensitivity of the sensor prepared in Example 1.

[0050] Figure 9 The current change of ascorbic acid production was measured for the sensor prepared in Example 1 under the influence of different temperatures (a) and the influence of storage time (b).

[0051] Figure 10 The simulated application of the sensor prepared in Example 1, wherein (a) the anti-interference test of the sensor current-time (i-t) characteristics, (b) the DPV detection of ascorbic acid in synthetic sweat, (c) the DPV simulation test of human skin synthetic sweat; (d) the sensor attached to the body surface. DETAILED DESCRIPTION

[0052] The application will be further described in conjunction with the examples and drawings, but the embodiments of the application are not limited thereto.

[0053] The reagents used in the examples were commercially available unless otherwise specified. Ascorbic acid, l-lactic acid, uric acid, tyrosine, acetic acid, sodium acetate, sodium chloride, potassium chloride, potassium ferricyanide (III), sodium hydroxide, sodium sulfite were purchased from Macklin Company. White wax was purchased from Henan Century Industrial Co., Ltd. Conductive carbon paste and printing materials were purchased from Dongguan Jiecheng Silk Screen Co., Ltd. Artificial sweat was purchased from Jiaying Scientific Research Company. Ag / AgCl ink was purchased from ALS Company, Japan. Wood was from Guangzhou Paper Mill.

[0054] In the examples, flexible wood pieces were obtained after delignification of wood pieces, which could be cut and shaped according to needs. First, the cut wood pieces were screen-printed to separate the hydrophobic and hydrophilic regions. Then, carbon electrode printing was performed. After printing, the printed wood pieces were placed in air until the ink solvent was completely volatilized. After drying, Ag / AgCl ink was printed on the reference electrode to obtain a flexible wood sensor.

[0055] First, the water absorption speed of the wood paper substrate was tested using a water absorption instrument. Then, the hydrophilicity of the wood paper substrate was measured using a contact angle tester (DCAT 21). Field emission scanning electron microscopy (HITACH) was used to perform cross-sectional and surface scanning electron microscopy (SEM) imaging of the electrode and wood substrate. Finally, electrochemical tests were performed using an electrochemical workstation (CHI 660e).

[0056] The performance of the wood substrate in the examples was characterized by the following two methods:

[0057] Wood composition analysis: The content of cellulose, hemicellulose, and lignin in wood was determined using the NREL method, which involves grinding the wood to 40-60 mesh. Subsequently, a series of solvent extractions using water and alcohol were performed to remove non-cellulosic materials such as fats and proteins from the sample. Then, the sample was hydrolyzed in two steps with 72% concentrated sulfuric acid and 4% dilute sulfuric acid. After the hydrolysis was completed, the solid residue (mainly lignin) was separated from the monosaccharides in the solution. The monosaccharides in the solution were quantified using high-performance liquid chromatography (HPLC), while the solid residue was weighed and further chemically analyzed to determine the lignin content.

[0058] Water absorption properties: The water absorption rate of the wood substrate was tested using a water absorption tester. A rectangular wood strip with a width of 3 centimeters, a length of 10 centimeters, and a thickness of 1 millimeter was placed on the water absorption tester for the water absorption test. The amount of liquid absorbed within a specified time was recorded to measure the water absorption rate, and the rate of liquid interface movement was recorded to evaluate its diffusion rate.

[0059] The performance of the sensors in the examples was characterized by the following methods:

[0060] Electrochemical impedance spectroscopy: The test was performed in a solution containing 5 mmol / L K3[Fe(CN)6] and 0.2 mol / L KCl.

[0061] Conductivity test: A conductive carbon paste was printed on the wood substrate, and a battery and an LED lamp were connected to form a series circuit, and the conductivity was judged by the brightness of the LED lamp.

[0062] Sensitivity characterization: i-t tests of different substrate sensors were performed in an ABS solution with an ascorbic acid concentration of 25 μmol / L; CV tests of the sensors were performed at different scan rates of 0.05-0.175 V / s in a solution containing 5 mmol / L K3[Fe(CN)6] and 0.2 M KCl; DPV tests of ascorbic acid at concentrations of 10-1000 μmol / L were performed in an ABS solution; and i-t tests were performed by adding 1 drop of 25 mmol / L ascorbic acid aqueous solution every 30 seconds to an ascorbic acid aqueous solution with a concentration of 25 μmol / L.

[0063] Bending resistance performance characterization: DPV tests were performed after 0-500 bends (bend diameter 40 mm) and 1 bend with a diameter of 20 mm in an ABS solution with an ascorbic acid concentration of 200 μmol / L, and the peak current was recorded.

[0064] Temperature change resistance and durability characterization: DPV tests were performed in an ABS solution with an ascorbic acid concentration of 25 μmol / L, and the peak current was recorded.

[0065] Selective characterization: The test was carried out in ABS solution, and the i-t test process was carried out in ABS solution containing 25 μmol / L ascorbic acid, and glucose, dopamine, lactic acid, tyrosine and uric acid were added dropwise during the test.

[0066] Sweat simulation characterization: Ascorbic acid was added to the artificial sweat to a concentration of 25 μmol / L, and DPV test was carried out.

[0067] ABS solution: The concentration was 0.01 mol / L, the pH value was 4.7, and it contained 50 mmol / L NaCl.

[0068] i-t (current-time) test: Carried out by an electrochemical workstation (CHI 660e) at a voltage of 0.1 V, sampling interval: 0.1 s; running time: 400 s; sensitivity: 1 x 10 -4 A / V.

[0069] Cyclic voltammetry (CV) test: Carried out by an electrochemical workstation (CHI 660e) in the range of -0.8 to 0.8 V; sensitivity: 1 x 10 -4 A / V.

[0070] Differential pulse voltammetry (DPV) test: Carried out by an electrochemical workstation (CHI 660e) in the range of -0.4 to 0.4 V; incremental potential: 0.004 V; pulse amplitude: 0.05 V; pulse width: 0.05 s; pulse period: 0.5 s; sensitivity: 1 x 10 -4 A / V.

[0071] The process of preparing the substrate from raw materials and preparing the sensor by silk screen printing in the examples is shown in Figure 1 After delignification, flexible wood pieces are obtained, which can be cut and shaped as needed. First, the cut wood pieces are silk screen printed to separate the hydrophobic and hydrophilic regions. Then carbon electrode printing is carried out, and after printing is completed, the printed wood pieces are placed in air until the ink solvent is completely volatilized. After drying, Ag / AgCl ink is printed on the reference electrode to obtain a flexible wood sensor.

[0072] Example 1: Preparation and application of the sensor

[0073] (1) Preparation of flexible wood substrate

[0074] A piece of wood with a width of 3 cm, a length of 10 cm, and a thickness of 1 mm was completely immersed in 500 mL of a mixed aqueous solution containing 1 mol / L of NaOH and 0.4 mol / L of Na2SO3, and then quickly transferred to a vacuum chamber. The liquid was made to flow into the xylem by vacuum, stirring, and soaking treatment for 6 h. The treated wood piece needed to be immersed in boiling deionized water several times to remove the chemicals. Then, in order to prevent the wood piece from shrinking during the drying process, the wood piece needed to be dehydrated by a freeze-drying method to form the final product of the flexible wood substrate.

[0075] (2) Silk screen wax printing

[0076] The wood substrate was fixed on the silk screen printing table, and industrial petroleum white wax was applied to the wood through silk screen scraping. Then the wood was placed in an oven at 80°C for 1 min to melt the solid wax particles through the mesh, become liquid and be absorbed by the substrate to form a hydrophobic barrier. After baking, it was taken out of the oven and allowed to cool to room temperature (<10 s) to form a 4 cm 2 hydrophilic reaction space.

[0077] (3) Silk screen carbon printing

[0078] The silk screen frame was fixed on the printing table so that the three-electrode pattern of the silk screen was in the central hydrophilic area of the wood. After the wood piece was fixed, conductive carbon paste was poured onto one end of the silk screen template, and a scraper was used to evenly scrape the ink across the silk screen. The ink was transferred to the substrate through the pores to form a pattern. The scraping was repeated 5 times to improve the conductivity. After printing was completed, the printed wood piece was placed in the air until the ink solvent was completely volatilized. After drying, Ag / AgCl ink was printed on the reference electrode, and a flexible wood electrode was obtained after drying.

[0079] For comparison, a sensor with a non-delignified substrate was set up, which was basically the same as Example 1, except that the wood substrate was not delignified. The other preparation procedures were the same, i.e., without step (1), directly performing step (2) and step (3) on a wood piece with a width of 3 cm, a length of 10 cm, and a thickness of 1 mm, to obtain a wood substrate sensor without delignification.

[0080] For comparison, a sensor with a filter paper substrate was set up, which was basically the same as Example 1, except that a filter paper with a width of 3 cm, a length of 10 cm, and a thickness of 1 mm was directly subjected to step (2) and step (3) to obtain a paper-based sensor.

[0081] (4) Application of the sensor

[0082] A sensor with a size of 4 cm*2.5 cm was made.

[0083] The process diagram of fabricating the screen-printed electrode and wood substrate hydrophilic ascorbic acid sensor from raw materials and the schematic diagram of sensor working principle in this example are shown in Figure 1 and Figure 2 respectively. This sensor adopts a novel method to combine flexible wood substrates with screen-printed electrodes to monitor ascorbic acid in sweat. The monitoring electrode consists of three electrodes, a working electrode (WE), a counter electrode (CE), and a reference electrode (RE). These sensors are directly applied to the skin surface, interact with sweat, and analyze biochemical markers. The electrode is designed to specifically interact with ascorbic acid, facilitating its conversion to dehydroascorbic acid through the oxidation process at the electrode interface. The presence of a wax layer can control the evaporation of sweat, ensuring that only ascorbic acid reaches the electrode. In addition, the flexible wood substrate below the electrode has a hydrophilic channel that can effectively transport sweat to the electrode surface. This design takes advantage of the natural microstructure of wood, which is modified to enhance its hydrophilicity, ensuring effective sweat absorption and transport. The practical application of this sensor has proven its ability to continuously monitor ascorbic acid levels, which is crucial for assessing health conditions such as sepsis. The output of the sensor can be converted into a measurable electrical signal indicating the concentration of ascorbic acid, which can be transmitted to a portable device for real-time monitoring.

[0084] Performance tests:

[0085] (1) Water absorption performance of wood substrate

[0086] Appropriate wood delignification can improve its flexibility and water absorption capacity. To further detail the process of lignin removal, we quantified the delignification process and introduced the changes in the microstructure, physical properties, and potential of wood as a flexible electrode substrate. The properties of the wood substrate are shown in Figure 3 and Figure 4 , Figure 3Figure 1 shows the results of the above experiments. a is the cross-sectional SEM image of the wood substrate sensor, it can be seen that the flexible wood electrode surface still retains the original channel structure of the wood, which can utilize the wood pores to transport liquid upward by capillary action, which is conducive to the response of the electrode; b is a physical photograph, and c is a cross-sectional SEM image of the working electrode, which further shows how these channels extend through the wood sheet to the top of the carbon electrode. These pores form a sponge-like structure, which helps to absorb and transport liquid. After the substrate absorbs sweat, it is directly transported to the working electrode through these channels, making it an ideal choice for flexible electrode substrates that require rapid absorption and transport of sweat; d is the physical image of the wood before and after removing lignin from Dalbergia sissoo; it illustrates the transformation from untreated wood to delignified wood, which involves soaking the wood in an alkaline solution, which dissolves most of the lignin and fat, resulting in a significant change in color and transparency, indicating the effect of chemical treatment. e is the analysis of the cellulose, hemicellulose and lignin components of the wood before and after lignin removal, it can be clearly seen that during the delignification process, the lignin content decreased from 23.8% to 19.3%, resulting in the removal of nearly 20% of the lignin, while retaining enough lignin to maintain the stability of the substrate structure. The alkaline solution also causes some hydrolysis of cellulose and hemicellulose. f is the change in the mass ratio of the wood before and after lignin removal; the absolute dry mass of the wood after delignification decreased by 13.1%, confirming the partial removal of lignin. Figure 4 Figure 2 shows the results of the above experiments. a is a digital photograph of the liquid absorption of Dalbergia sissoo over time before and after lignin removal (where the liquid is water + green pigment); b is a liquid diffusion graph before and after lignin removal from Dalbergia sissoo, after delignification treatment, the liquid rises to almost the same height in the same time, which shows that the delignification treatment does not destroy the original structure of the wood, maintaining its good liquid diffusion ability; c is a liquid absorption graph before and after lignin removal from Dalbergia sissoo, showing that the water absorption rate of the wood substrate after delignification has improved, which is due to the increase in hydrophilicity, meaning that the wood can more effectively attract and retain moisture.

[0087] (2) Conductivity of screen-printed electrodes

[0088] A battery and an LED lamp were connected in series on the flexible wood substrate prepared in step (1) of Example 1 to form a series circuit, and the conductivity was judged by the brightness of the LED lamp. The results obtained are shown in Figure 1 b, which shows that it has good conductivity. The sensor prepared in Example 1 was also tested by electrochemical impedance spectroscopy (EIS), and the results are shown in Figure 1 c, which shows that the resistance of the sensor is about 480 ohms, indicating good conductivity. Figure 5 Figure 5

[0089] (3) Sensitivity characterization

[0090] ​​To compare the electrochemical performance of the flexible wood-based sensor after absorbing sweat and the performance of direct immersion in solution, we took its performance in ascorbic acid aqueous solution (25 pmol / L) as the benchmark (blue curve) and compared it with sensors based on delignified basswood, non-delignified basswood, and paper-based sensors, as shown in Figure 6 . Figure 6 The a in Figure 2 shows the current response of the delignified wood sensor and the non-delignified wood sensor after absorbing ascorbic acid solution to saturation. Compared with the non-delignified wood sensor (brown curve), the current response of the delignified wood sensor (orange curve) shows a higher initial current value and remains stable throughout the test, closely matching the blue benchmark line. This indicates that delignified wood as a base material has better current transmission ability after absorbing the solution, because part of the lignin is removed, enhancing the water retention of the wood and thus improving the sensitivity of the sensor. Figure 6 The b in Figure 2 compares the current response of the delignified wood-based sensor and the standard paper-based sensor. In this data set, the current response of the delignified wood-based sensor (orange curve) is significantly better than that of the paper-based sensor (green curve). The paper-based sensor shows much lower current after absorbing the solution than the benchmark, which may be due to the insufficient water absorption capacity of the paper, resulting in reduced availability of reactants. In addition, the mechanical properties of the paper deteriorate after absorbing water, making it prone to tearing and unsuitable for wearable sensing during human movement. In contrast, the delignified wood-based sensor maintains good electrical signals and mechanical integrity after water absorption, which is crucial for ensuring the accuracy and reliability of electrochemical sensors. Figure 6 The set of CV graphs in Figure 2c shows the current response of the sensor at different scan rates, where the curves represent the recorded current values at different scan rates (0.05 V / s ~ 0.175 V / s). As can be seen from the figure, at different scan rates, the shapes of all CV curves are almost the same and show clear oxidation peaks, and the oxidation peak height increases regularly with the increase of scan rate, and there is a linear relationship between the peak current and the square root of voltage and scan rate. If a similar linear relationship is fitted, the linear relationship between the size of the oxidation peak current I and the scan rate (SR 1 / 2 ) and voltage U 1 / 2 can be obtained. Figure 6 The d in Figure 2 shows that the fitting degree (R 2 ) reaches 0.9876, indicating that the sensor has very good electrochemical performance. This reflects that the sensor has good reaction kinetics and can withstand faster scan rates, which is very important for electrochemical applications that require fast detection and fast response, allowing the sensor to effectively perform electrochemical analysis in a short time.

[0091] (4) Sensitivity and stability of the sensor

[0092] To explore the sensitivity and stability of the sensor, we tested it in different concentrations of ascorbic acid aqueous solution. Figure 7 Figure 2a shows the current response curves of the sensor at different concentrations. It can be seen that as the concentration increases, the current also increases accordingly, showing clear step changes, indicating that the sensor has high sensitivity to different concentrations of analytes. Figure 7 Figure 2b is a linear fit of the data in Figure 2a, showing the relationship between analyte concentration and current. The blue markers represent experimental data points, while the brown line represents the linear fit curve. It can be observed that the data points are closely distributed around the fitting line, with a fitting degree (R 2 ) of 0.9573, indicating that there is a strong positive correlation between the current response of the sensor and the concentration of the analyte, and the linear relationship is maintained throughout the measurement range. Figure 7 Figure 2c depicts the stable growth of current with increasing concentration. When the picture is enlarged ( Figure 7 Figure 2d), it can be seen that the current stabilizes very quickly after the concentration increases, stabilizing within 3s and showing the stability and repeatability of the current signal over a long period of time. The linear growth of the current indicates that the sensor performance is stable during continuous monitoring, which is an important feature for long-term concentration change monitoring applications. In summary, Figure 7 it is shown that the sensor we developed not only has high sensitivity and good linear response range, but also maintains stable performance during continuous use.

[0093] (5) Durability of the sensor

[0094] To study the mechanical durability of the sensor, we tested its performance after different degrees of folding. Figure 8 Figure 3a shows the current response of the sensor after different numbers of folds. It can be seen that even after as many as 500 folds, the current response only decreases slightly. This indicates that the sensor has good mechanical durability, and its performance remains relatively stable even after repeated folding. Figure 8 Figure 3b shows the current response of the sensor at different bending radii. The results show that even with very small bending radii, the current response remains stable and does not change significantly, further confirming that the sensor can withstand large mechanical deformation without affecting its electrochemical performance. Overall, these results show that the sensor can maintain its detection ability even after physical changes such as folding and bending, which is crucial for developing stable wearable biosensors. This high level of mechanical stability ensures that the sensor can provide continuous and accurate data in practical applications such as sports activity monitoring or health status tracking.

[0095] (6) Environmental tolerance of the sensor

[0096] To investigate the environmental tolerance of the sensor, we compared its performance at different temperatures and different storage times. Figure 9 Figure 2a demonstrates the current response of the sensor to temperature changes. As observed, the current increases slightly with increasing temperature (22°C, 32°C, 42°C, 52°C), but the increase is not significant. This indicates that normal human body temperature variations will not have a substantial impact on the sensor performance, and the device can provide reliable data in daily activities even in the case of external temperature or body temperature fluctuations. Figure 9 Figure 2b shows that the sensor maintains good performance even after 180 days of storage, with little change in electrode performance within the first 60 days of preparation. This demonstrates the excellent durability of the sensor, providing a significant advantage for wearable applications and market distribution.

[0097] (7) Interference resistance of the sensor

[0098] To investigate the interference resistance of the sensor, we conducted interference resistance tests by adding various interfering substances during the ascorbic acid detection process (i-t). Figure 10 Figure 3a shows a current curve over time, which represents the sensor's current response after adding different interfering substances one by one to the detection system during the detection of ascorbic acid concentration. As can be seen from the figure, after adding 5 mmol / L glucose, 50 μmol / L tyrosine, 0.5 mmol / L uric acid, 5 mmol / L lactic acid, and 50 μmol / L dopamine as interfering substances, the current signal did not show significant changes, indicating that the sensor is not affected by these interfering substances. The curve remains basically flat, meaning that the sensor can still stably detect the concentration of ascorbic acid in the presence of interfering substances. This interference-free performance is a very important feature in the design of biosensors, especially when detecting in complex biological samples such as sweat or blood, as these samples may contain a variety of chemical substances.

[0099] To test the performance of the sensor when worn on the human body, we conducted simulation tests on the sensor and tested it on the human body surface. Figure 10 Figure 3b demonstrates the electrochemical response of pure artificial sweat (black curve) and artificial sweat with 10 μmol / L ascorbic acid added (brown curve). It can be observed that after adding ascorbic acid, the curve shows a clear peak, indicating that the sensor can still effectively distinguish between samples containing ascorbic acid and samples not containing ascorbic acid in a strong interference environment, thus proving the sensor's good interference resistance and selectivity. Figure 10Figure 4c shows the current response of the sensor when tested on the surface of human skin with artificial sweat containing 10 μmol / L ascorbic acid. The current curve shows a clear peak at a certain voltage, which indicates that the sensor can measure ascorbic acid stably at a specific voltage even in human activities. Figure 10 Figure 4d shows that the sensor is designed to be flexible, which can be attached to the surface of human skin and used in sports without affecting its performance. This flexibility and wearability make the sensor very suitable for continuous monitoring of human health conditions, such as in sports or other daily activities.

[0100] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods, and shall be included in the protection scope of the present application.

Claims

1. A method for detecting ascorbic acid in sweat using a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate, characterized in that The following steps are involved: (1) Artificial sweat with different ascorbic acid concentrations was prepared, and the side of the flexible wood substrate of the sensor that was not wax-dyed and printed with three electrodes was contacted with the artificial sweat or immersed in the artificial sweat. Then, the LSV test method of the electrochemical workstation was used to obtain the peak oxidation current of the artificial sweat with different ascorbic acid concentrations at a voltage of 0.1 V. The standard curve of ascorbic acid concentration and oxidation current was obtained with the ascorbic acid concentration as the horizontal axis and the oxidation current peak as the vertical axis. (2) The peak value of the oxidation current of the actual sweat at a voltage of 0.1 V is measured by the LSV test method of the electrochemical workstation, and then the peak value of the oxidation current is substituted into the standard curve of step (1) to obtain the concentration of ascorbic acid in the actual sweat; The method for preparing the sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate comprises the following steps: Step (1): Preparation of flexible wood substrate The wood chips are immersed in a mixed aqueous solution containing NaOH and Na2SO3, followed by vacuum impregnation. After completion, they are immersed in boiling water to remove chemicals, and then dried to obtain a flexible wood substrate with some lignin removed. Step (2): Silkscreen wax printing A flexible wood substrate is fixed on a screen printing table, and solid wax is applied to the flexible wood substrate through a screen. The flexible wood substrate is then heated to melt the solid wax particles that pass through the mesh, turning them into liquid and being absorbed by the substrate to form a hydrophobic barrier. The areas not coated with solid wax are hydrophilic, separating the hydrophobic and hydrophilic regions. The substrate is then cooled to room temperature to obtain a wax-printed patterned flexible wood substrate. Step (3): Three-electrode printing Three electrodes, namely a working electrode, a counter electrode and a reference electrode, were printed on the hydrophilic area of ​​a wax-patterned flexible wood substrate, and then dried to obtain a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate.

2. The method for detecting ascorbic acid in sweat using a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate according to claim 1, wherein: The concentrations of NaOH and Na2SO3 in the mixed aqueous solution containing NaOH and Na2SO3 in step (1) preparing the flexible wood substrate are ≥1 mol / L and ≥0.4 mol / L, respectively; The amount of the mixed aqueous solution containing NaOH and Na2SO3 used in step (1) of preparing the flexible wood substrate is sufficient to allow the wood chips to be completely immersed in the mixed aqueous solution containing NaOH and Na2SO3; The time of vacuum impregnation in step (1) of preparing the flexible wood substrate is 2-12 hours.

3. The method for detecting ascorbic acid in sweat using a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate according to claim 1, wherein: The solid wax described in step (2) silk screen wax printing is at least one of beeswax and paraffin wax.

4. The method for detecting ascorbic acid in sweat using a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate according to claim 1, wherein: The heating in step (2) of the screen wax printing process is heating at 60-100° C. for 0.5-3 min; The cooling to room temperature mentioned in step (2) of the screen wax printing process refers to cooling to room temperature in less than 10 seconds.

5. The method for detecting ascorbic acid in sweat using a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate according to claim 1, wherein: In step (3) three-electrode printing, the working electrode is a conductive carbon electrode; the counter electrode is a conductive carbon electrode; and the reference electrode is an Ag / AgCl electrode.

6. The method for detecting ascorbic acid in sweat using a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate according to claim 5, characterized in that: A conductive carbon layer was also included between the Ag / AgCl electrode and the hydrophilic region of the wax-printed patterned flexible wood substrate.

7. The method for detecting ascorbic acid in sweat using a sweat ascorbic acid detection sensor based on screen-printed electrodes and a wood substrate according to claim 5, wherein: During printing in step (3), the screen frame is fixed on the printing table so that the three-electrode pattern of the screen is aligned with the hydrophilic area of ​​the wax-printed flexible wood substrate. The flexible wood substrate is fixed, and the conductive carbon paste ink is poured on one end of the screen template. The ink is evenly scraped across the screen using a scraper, and the ink is transferred to the substrate through the pores to form a pattern. After printing, the printed wood chips are placed in the air until the ink solvent is completely evaporated. After drying, Ag / AgCl slurry is scraped on the reference electrode and then dried to obtain a sweat ascorbic acid detection sensor based on screen-printed electrodes and wood substrate.

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