A wireless passive NFC tag type gas sensor for freshness monitoring of fresh food and a preparation method thereof

Abstract

The application discloses a wireless passive NFC tag type gas sensor for fresh food freshness monitoring and a preparation method thereof. The tag type gas sensor comprises a flexible substrate, a radio frequency antenna, an NFC chip, interdigital electrodes, a sensitive material layer and a tuning capacitor. The application determines the target gas emission concentration based on the resistance change of the sensitive material layer after contacting the target gas, and further determines the deterioration of fresh food. The application establishes a linear relationship between the sensing interface resistance and the target gas concentration, adjusts the radio frequency antenna structure and the patch resistor to control the threshold resistance at the on-off point of the NFC tag type gas sensor, judges the sensing interface resistance through the on-off state of the NFC tag type sensor and the communication terminal (mobile phone, reader, etc.), and realizes the judgment of the freshness of fresh food. The NFC tag type gas sensor is flexible and wearable, is convenient for attaching to fruits and fresh food surfaces, does not need power supply, has simple structure and low cost, and is convenient for mass production.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of sensors and NFC technology, in particular to a wireless passive NFC tag type gas sensor for freshness monitoring of fresh food and a preparation method thereof. BACKGROUND

[0002] Flexible gas sensors have the characteristics of simple structure, high sensitivity and wide applicability, and can be designed arbitrarily according to the requirements of measurement conditions. At present, the main methods widely used in gas detection are fluorescence probe method, electrochemical luminescence method and gas chromatography method. Unfortunately, although these methods have high sensitivity, the sampling process is complex, the analysis time is long, and the detection cost is high, which cannot meet the on-site rapid detection of gas in the actual environment. At present, chemical resistance type sensors are expected to convert chemical reactions into electrical signals and realize detection through resistance changes of sensing materials. Combining flexible wearable gas sensors with NFC technology not only meets the diversification requirements of the surface of the target to be measured, but also efficiently and conveniently detects the real-time concentration or content of the target to be measured.

[0003] NFC technology is developed on the basis of non-contact radio frequency identification (RFID) and interconnection technology, and can realize identification and data exchange with compatible devices in short distance. After improvement and optimization in various fields, NFC tags with various functions have many advantages: wireless passive, i.e. no external power supply is needed, and electromagnetic induction provided by reading devices (such as smart phones) is used for work; low cost, i.e. low production cost, suitable for mass production; high ductility, i.e. easy to combine with various flexible materials, suitable for various application scenarios; convenient reading, i.e. can be directly read by smart phones and other devices, facilitating users to obtain information. NFC tags not only retain their original advantages, but also realize real-time monitoring of environmental or biochemical parameters, providing innovative solutions for food safety, medical health, environmental monitoring and other fields.

[0004] Although the tag type sensor has many advantages and overcomes some shortcomings of rigid sensors, it still cannot get rid of digital source table, network analyzer and impedance analyzer in terms of data acceptance and processing, i.e. it cannot realize truly wireless passive detection. These instruments are bulky, expensive and complex to operate, which cannot meet the actual application requirements. The proposed NFC tag type sensor not only has the characteristics of flexibility, small size and simple preparation, but also can be directly attached to the surface of various fresh foods and fruits or integrated in the integrated environment required to be detected, and is free from the shackles of large-scale analysis equipment, and only needs a smart phone or a simple reader to realize high sensitivity and rapid detection of the target. SUMMARY

[0005] The application aims to provide a wireless passive NFC tag type gas sensor for fresh food freshness monitoring with high sensitivity, low detection cost and easy industrial production, and a preparation method of the wireless passive NFC tag type gas sensor for fresh food freshness monitoring based on target gas concentration change, and a new fresh food freshness judgment and detection method.

[0006] The wireless passive NFC tag type gas sensor for fresh food freshness monitoring comprises a flexible substrate, an NFC coil, a resistance reserved bit, an interdigital electrode, a chip capacitance reserved bit, a patch resistor and an NFC chip, the NFC coil is arranged on the flexible substrate, the resistance reserved bit and the chip capacitance reserved bit are arranged at the NFC coil, wherein the resistance reserved bit is used for placing the patch resistor, and the chip capacitance reserved bit is used for placing the NFC chip; the interdigital electrode is installed on the back of the NFC coil; the interdigital electrode is connected in series or parallel with the NFC chip; when connected in series, the interdigital electrode is connected with the through holes at both ends of the resistance reserved bit, that is, the interdigital electrode is connected in series at both ends of the NFC chip; when connected in parallel, the interdigital electrode is connected with the through holes at both ends of the chip capacitance reserved bit, that is, the interdigital electrode is connected in parallel at both ends of the NFC chip.

[0007] In the above technical solution, the area where the interdigital electrode is located is an interdigital sensing area, the interdigital sensing area is arranged on the back of the NFC coil, and the interdigital sensing area is integrated on the NFC tag in two modes, series connection mode and parallel connection mode, in the series connection mode, the interdigital sensing area on the back of the NFC tag is connected with the through holes at both ends of the resistance reserved bit and is connected in series at both ends of the NFC chip, and in the parallel connection mode, the interdigital sensing area on the back of the NFC tag is connected with the through holes at both ends of the chip capacitance reserved bit and is connected in parallel at both ends of the NFC chip.

[0008] In the above technical solution, when the interdigital sensing interface contacts hydrogen sulfide, ethylene and nitrogen dioxide, the resistance of the interdigital sensing interface changes according to the target gas concentration change, thereby causing the impedance of the NFC tag to change, resulting in the change of the resonant frequency or the reflection coefficient of the NFC antenna, and further affecting the on-off state of the NFC tag.

[0009] Further, the flexible substrate is polyimide (PI), polyethylene terephthalate (PET), polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE) or paper.

[0010] Further, the NFC coil is a multi-turn coil, and the shape is one of a circle, a rectangle, a round rectangle and a triangle, and the antenna material comprises at least one of gold, silver, copper, aluminum and carbon.

[0011] Further, the NFC chip is selected from NTAG213, NTAG215, S50, PN532, PN533, PN544, PN7150, and PN7462.

[0012] Further, the label gas sensor further comprises a tuning capacitor, which is installed on the resistance reserved bit (3) and has a capacitance range of 1-100 pF.

[0013] Further, the interdigital electrode is composed of a comb-shaped electrode with a micron to millimeter gap and a finger width, and the surface of the comb-shaped electrode is modified with a sensitive material, wherein the sensitive material is composed of any two of carbon nanotubes, molybdenum disulfide, metal nanoparticles, and metal organic frameworks.

[0014] Further, the carbon nanotubes are single-walled carbon nanotubes, multi-walled carbon nanotubes, or carboxylated carbon nanotubes; the metal nanoparticles are at least one of Au, Ag, Pd, and Pt; and the metal organic framework is one of MOF-74, MOF199, and ZIF-8.

[0015] Further, the preparation method of the sensitive material is as follows:

[0016] The carboxylated single-walled carbon dispersion liquid is added with an AgNO3 solution, stirred and heated, and a dispersed sodium citrate solution is slowly dropped, continuously stirred and reacted, and the product is collected, which is the sensitive material.

[0017] Preferably, the concentration of the AgNO3 solution is 0.1 M, the concentration of the carboxylated single-walled carbon dispersion liquid is 0.1-1.4 mg / mL, the volume ratio of SWCNT to Ag is 20-8:2-20, and the continuous stirring and reaction time is 10-80 min.

[0018] On the other hand, the present application provides a preparation method of the wireless passive NFC label gas sensor for freshness monitoring of fresh food, comprising the following steps:

[0019] (1) NFC coil appearance design is performed, the target threshold resistance of the NFC label is determined according to the type and concentration of the gas to be measured, and the equivalent inductance of the coil is adjusted to match the target threshold resistance;

[0020] (2) The design parameters of the NFC coil are imported into CAD, and at least one of laser printing, photoetching, silk screen printing, and electroplating is used to manufacture the NFC coil;

[0021] (3) The passive elements on the NFC label, including the NFC chip, the capacitor, and the resistance, are welded by conductive silver paste, tin wire, solder paste, and carbon paste;

[0022] (4) The nanocomposite material is modified in the interdigital electrode region, i.e., the interdigital sensing region. Based on the linear relationship established between the resistance change of the interdigital sensing region and the gas concentration, the relationship between the readability of the NFC tag when the sensing region is at different resistance values ​​under different concentrations of target gas is obtained;

[0023] When exposed to the target gas, if the resistance of the interdigital sensing area decreases, the NFC tag adopts the series mode, and the NFC tag changes from being unable to communicate to being able to communicate; if the resistance of the interdigital sensing area increases, the NFC tag adopts the parallel mode, and the NFC tag changes from being unable to communicate to being able to communicate; if the resistance changes in the corresponding series and parallel modes are opposite, the communication changes are also opposite.

[0024] Furthermore, an NFC terminal device is used to read the NFC tag, wherein the NFC terminal device is a smart phone with an NFC module or an NFC reader / writer.

[0025] On the other hand, the present invention provides a method for judging and detecting the freshness of fresh food, which uses a wireless passive NFC tag-type gas sensor to judge and detect the freshness of fresh food. When the wireless passive NFC tag-type gas sensor contacts the target gas, if the resistance of the interdigital sensing area decreases, the NFC tag adopts a series mode, and the NFC tag changes from being unable to communicate to being able to communicate; if the resistance of the interdigital sensing area increases, the NFC tag adopts a parallel mode, and the NFC tag changes from being unable to communicate to being able to communicate; if the resistance changes in the corresponding series and parallel modes are opposite, the communication changes are also opposite.

[0026] The principle of the present invention is as follows: the target gas undergoes a specific physical and chemical reaction on the surface of the interdigital electrode modified with a nanocomposite material, causing the resistance of the interdigital sensing interface to change, which in turn affects the impedance of the NFC tag, changes the resonant frequency and reflection coefficient of the NFC tag, and ultimately changes the readability of the NFC tag and the communication terminal. After determining the gas concentration and nanocomposite material to be detected, the series or parallel mode is locked, and according to the change in the resistance of the interdigital sensing interface, the NFC coil parameters or the fixed resistor size in the NFC tag are adjusted to match the appropriate threshold resistance. Finally, the gas concentration in the target environment is determined by the readability of the communication terminal and the NFC tag.

[0027] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0028] (1) NFC tag sensors are flexible, small, and easy to prepare. They can be directly attached to the surface of various fresh produce or fruits or integrated into the environment where they need to be detected.

[0029] (2) low cost, easy to mass production. The proposed NFC tag sensor manufacturing process is simple, the materials are cheap and easy to get, which is beneficial to mass production and industrialization;

[0030] (3) low energy consumption, good interaction, strong portability. It is free from the shackles of large-scale analysis equipment, and only a smart phone or a simple reader can realize high sensitivity and rapid detection of target objects;

[0031] (4) high sensitivity, low detection limit. The interdigital sensing area integrated in the proposed NFC tag sensor has high sensitivity and can detect ppb level target gas. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is the physical diagram of the wireless passive NFC tag gas sensor of the application;

[0033] Figure 2 It is the HFSS simulation design schematic diagram of the coil in the NFC tag of the application;

[0034] Figure 3 It is the equivalent circuit diagram of the wireless passive NFC tag gas sensor of the application, wherein, L a is the equivalent inductance, R IC is the chip resistance, C total is the equivalent capacitance, C IC is the chip capacitance, C ant NFC is the antenna parasitic capacitance, C t is the tuning capacitance;

[0035] Figure 4 It is the series and parallel equivalent circuit diagram of the NFC tag gas sensor of the application, wherein, L a is the equivalent inductance, R IC is the chip resistance, C total is the equivalent capacitance, R s is the series resistance, R senso is the gas sensor resistance;

[0036] Figure 5 It is the NFC tag manufacturing process diagram of the application;

[0037] Figure 6 It is the response optimization diagram of the synthesis parameters of the nano-sensitive material in the NFC tag of the application;

[0038] Figure 7 It is the dynamic sensitivity response and linear fitting diagram of hydrogen sulfide detection of the application;

[0039] Figure 8 It is the dynamic monitoring data diagram of the wireless passive NFC tag gas sensor of the application on salmon; DETAILED DESCRIPTION

[0040] The technical solutions of the present application are further described below with reference to the drawings.

[0041] Example 1

[0042] Figure 1 In the figure, 1 is a flexible substrate, 2 is an NFC coil, 3 is a resistance reserved bit, 4 is an interdigital electrode, 5 is a chip capacitor reserved bit, and 6 is a copper ring.

[0043] The wireless passive NFC tag type gas sensor of the present application comprises a flexible substrate 1, an NFC coil 2, a resistance reserved bit 3, a chip capacitor reserved bit 5, an interdigital electrode 4, a patch resistor, and an NFC chip, wherein the resistance reserved bit 3 and the chip capacitor reserved bit 5 are provided on the NFC coil 2, the resistance reserved bit 3 is used for placing the patch resistor and a tuning capacitor, the chip capacitor reserved bit 5 is used for placing the NFC chip, and the resistance reserved bit 3 and the chip capacitor reserved bit 5 are both located on the front face of the NFC tag (the face on which the NFC coil is installed is the front face of the NFC tag). The front and back faces of the NFC coil are processed by a copper ring to form a through hole, the back face of the NFC coil is integrated with the interdigital electrode, and the surface of the interdigital electrode is modified with a sensitive material layer.

[0044] The interdigital electrode and the surface sensitive material layer thereof together constitute a sensor, the sensor exhibits an increase in resistance in a hydrogen sulfide environment, and in order to realize the early warning effect of the NFC tag, the NFC tag changes from being incommunicable to being communicable.

[0045] As shown in Figure 3 , the equivalent circuit of the NFC tag is composed of an equivalent inductance L a , a chip resistor R IC , and an equivalent capacitor C total , wherein the equivalent capacitor is composed of a chip capacitor C IC , an NFC antenna parasitic capacitor C ant , and a tuning capacitor C t .

[0046] The NFC coil and the interdigital electrode are in series or parallel relationship. As shown in Figure 4 , when they are in series, the interdigital electrode (4) is connected to the through hole between the two ends of the resistance reserved bit, i.e., the interdigital electrode is connected in series to the two ends of the NFC chip; when they are in parallel, the interdigital electrode (4) is connected to the through hole between the two ends of the chip capacitor reserved bit, i.e., the interdigital electrode is connected in parallel to the two ends of the NFC chip.

[0047] Optionally, the flexible substrate is PI, PET, PTFE, paper or the like flexible substrate; the NFC coil is a multi-turn coil, the shape is one of a circle, a rectangle, a rounded rectangle, a triangle, the antenna material is at least one of gold, silver, copper, aluminum and carbon, and the NFC chip is NTAG213, NTAG215, S50, PN532, PN533, PN544, PN7150 and PN7462.

[0048] Optionally, the sensitive material of the interdigital electrode surface modification is a composite material of single-walled, multi-walled, carboxylated carbon nanotubes or molybdenum disulfide and metal nanoparticles and metal organic framework, or two of the above three types of materials are combined, the metal nanoparticles are at least one of Au, Ag, Pd and Pt, and the metal organic framework is one of MOF-74, MOF-199 and ZIF-8.

[0049] Embodiment 2

[0050] In the application, a single-sided rectangular NFC antenna is selected, in order to make the antenna and the chip have good impedance matching, the antenna is designed and optimized in the HFSS simulation software, and the main steps include:

[0051] (1) determining the resonant frequency and the required bandwidth according to the NFC communication protocol;

[0052] (2) querying the NFC chip NTAG213 data manual to determine the chip capacitance and calculating the required equivalent inductance value of the antenna through the resonant frequency calculation formula;

[0053] (3) determining that the antenna shape is a single-sided rectangle, reserving the required positions of the NFC chip, the tuning capacitor, the patch resistor and the interdigital electrode;

[0054] (4) initially modeling the antenna in the HFSS software, defining and setting the initial variables: the number of turns n of the coil is 6, the line width w is 0.6mm, the line spacing s is 0.2mm, and the substrate thickness g is 0.0125mm;

[0055] (5) drawing the PI flexible substrate through the cuboid modeling tool in HFSS, and setting the parameters to 40*40mm. (In HFSS, the related parameters of PI need to be defined, the relative dielectric constant is set to 3.5, and the mass density is set to 1450kg / m 3 ).

[0056] (6) modeling from the outermost coil to the inner side successively through the rectangular modeling tool, setting the antenna to an ideal conductor material after the antenna modeling is completed, and setting an excitation source at the reserved chip position instead of the chip.

[0057] (7) Set the air wall with length, width and height of 500mm and set the scanning frequency of 0-25MHz.

[0058] (8) Observe the equivalent inductance value change at 13.56MHz, and adjust the number of turns n, the line width w, the line spacing s,

[0059] Change the equivalent inductance value of the antenna to meet the results calculated by the formula before.

[0060] Example 3

[0061] This embodiment provides a preparation method for modifying the sensitive material on the surface of the interdigital electrode, comprising the following steps:

[0062] (1) Take 8mg of dispersed carboxylated single-walled carbon nanotubes (abbreviated as SWCNT, purchased from Pioneer Nanotechnology Co., Ltd.) and dissolve in 16mL of deionized water;

[0063] (2) Under the action of ultrasonic waves, ultrasonic waves at a power of 60% for 1h to obtain a well-dispersed carboxylated single-walled carbon dispersion;

[0064] (3) Take 10mL of 0.1M AgNO3 solution for standby;

[0065] (4) Take 189.2mg of sodium citrate and dissolve in 19mL of deionized water, ultrasonic dispersion is uniform, standby;

[0066] (5) Take the carboxylated single-walled carbon dispersion, and add the AgNO3 solution under the action of a magnetic stirrer;

[0067] (6) While stirring, the solution is heated to 95℃, and the well-dispersed sodium citrate solution is slowly added dropwise, and continuously stirred for 50min;

[0068] (7) Centrifuge the product at 10000rpm for 10min for 3 times to collect the product, and vacuum dry at 60℃ for 12h.

[0069] (8) Take 4mg of dried finished product and disperse in 2mL of deionized water, take 40μL of dispersion and drop coat on the interdigital electrode area, and dry at room temperature;

[0070] Optimization comparison experiment is conducted on the ratio of SWCNT and Ag in the above method to obtain the sensitive material with the best performance. The concentration of SWCNT, the volume ratio of SWCNT and Ag, the reaction time, the concentration of the modification solution and the volume of the modification solution are optimized by the control variable method. Except for the variable, other conditions are the same as the previous experimental conditions.

[0071] The main variables are as follows:

[0072] SWCNT concentration is 0.1, 0.2, 0.5, 0.8, 1.1, 1.4 mg / mL respectively;

[0073] The volume ratio of SWCNT and Ag is 20:2, 20:12, 20:16, 16:20, 8:20 respectively;

[0074] The continuous stirring reaction time is 10, 30, 50, 70, 80 min respectively;

[0075] The concentration of the modification solution is 1, 2, 3, 4, 5 mg / mL respectively;

[0076] The volume of the modification solution is 10, 20, 30, 40, 50 μL respectively.

[0077] The interdigital electrodes obtained by the above variables are tested for response, and the test method is: at room temperature (25℃), a 0.1V voltage is applied by a digital source table to collect the real-time resistance signal of the gas sensor, and after waiting for a stable resistance each time (about 300s), 200ppb H2S is introduced for 880s, and the flow rate of H2S and N2 is controlled to be 20sccm:480sccm to realize, wherein the concentration of the H2S gas source used is 5ppm.

[0078] The test results are shown in Figure 6 , and Figure 6 It can be seen that the optimal result obtained is that the SWCNT concentration is 0.5 mg / mL, the volume ratio of WCNT and Ag is 16:20, the continuous stirring reaction time is 50 min, the concentration of the modification solution is 2 mg / mL, and the volume of the modification solution is 40 μL, and the highest response can reach 254.7%.

[0079] Example 4

[0080] The embodiment provides a preparation method of a wireless passive NFC tag type gas sensor for fresh food freshness monitoring, which comprises the following steps:

[0081] (1) designing and preparing a wireless passive NFC tag type gas sensor, wherein the NFC coil and the interdigital electrode are in series in the embodiment:

[0082] (11) using HFSS software to design the appearance of the NFC coil, determining the required target threshold resistance according to the type and concentration of the to-be-detected gas, and adjusting the equivalent inductance of the coil to match the target threshold resistance;

[0083] (12) importing the design parameters of the NFC coil into CAD, and using at least one process of laser printing, photoetching, silk screen printing and electroplating to manufacture the NFC coil;

[0084] (13) welding the passive elements on the NFC tag, including the NFC chip, the capacitor and the resistance, by using conductive silver paste, tin wire, solder paste and carbon paste.

[0085] (2) Synthesis of nanocomposites for H2S detection:

[0086] (21) Take 8 mg of dispersed carboxylated single-walled carbon nanotubes and dissolve them in 16 mL of deionized water;

[0087] (22) Under the action of ultrasound, ultrasonic at a power of 60% for 1 h to obtain a well-dispersed carboxylated single-walled carbon dispersion;

[0088] (23) Take 10 mL of 0.1M AgNO3 solution for standby;

[0089] (24) Take 189.2 mg of sodium citrate and dissolve it in 19 mL of deionized water, and ultrasonically disperse it uniformly for standby;

[0090] (25) Take the carboxylated single-walled carbon dispersion and add the AgNO3 solution under the action of a magnetic stirrer;

[0091] (26) While stirring, warm the solution to 95°C, slowly add the well-dispersed sodium citrate solution, and continuously stir for 50 min;

[0092] (27) Centrifuge the product 3 times at 10,000 rpm for 10 min using deionized water, and vacuum dry at 60°C for 12 h.

[0093] (3) Take 4 mg of dried finished product and disperse it in 2 mL of deionized water, take 40 μL of the dispersion and drop coat it on the interdigital electrode area, and dry at room temperature;

[0094] (4) Establish a linear relationship between the resistance change of the interdigital sensing area and the gas concentration;

[0095] (5) Obtain the change relationship of the readability of the NFC tag at different resistance values of the sensing area under different concentrations of target gas.

[0096] (6) Attach the NFC tag to the surface of the object to be measured, open the NFC function of the mobile phone and perform a read scan operation, and if the NFC tag communicates, the object to be measured has deteriorated.

[0097] Example 5

[0098] The gas sensitive test is mainly realized by the gas platform built for optimizing the sensor mentioned in embodiment 4, and the H2S concentration used is 200 ppb, which is realized by controlling the flow rate of H2S and N2 to be 20 sccm: 480 sccm, wherein the concentration of the H2S gas source used is 5 ppm. In the step sensor part, the H2S concentrations used are 200 ppb, 400 ppb, 600 ppb, 800 ppb and 1000 ppb respectively, and the corresponding flow rates between H2S and N2 are 20 sccm: 480 sccm, 40 sccm: 460 sccm, 60 sccm: 440 sccm, 80 sccm: 420 sccm and 100 sccm: 400 sccm respectively.

[0099] Based on the salmon spoilage concentration threshold of about 200 ppb, we selected five concentrations of H2S to verify the sensing performance of the proposed gas sensor. The time of each H2S input is 880 s, and the sensor recovery time is 380 s. A voltage of 0.1 V is applied at room temperature (25℃) to collect the resistance signal of the gas sensor. During the concentration step change process, the sensor maintains a good linear response, successfully proving the excellent sensitivity of the invented sensor in detecting H2S. As shown in Figure 7 , in the concentration range of 200 ppb-1000 ppb, the sensing response is 66.6%, 101.5%, 127.1%, 137.6% and 154.3% respectively at intervals of 200 ppb, and accordingly a fitting equation of sensing response (y) and H2S concentration (x) is established, which can be expressed as y = 54 + 0.1x, and the regression coefficient (R 2 ) is 0.98, further proving that the proposed sensor has good linear response and stability.

[0100] As shown in Figure 8 , the proposed NFC tag sensor is used to test the change of resistance and readability of 150 g salmon from fresh to spoilage during the test. The time interval of each test is 12 h, and from the figure it can be seen that the sensor resistance rises obviously from 822.9 Ω to 2246.2 Ω during 72-84 h, realizing the transition of NFC tag from unreadable to readable, indicating that the invention has strong practical application value and meets the current actual sample detection needs.

[0101] Embodiment 6

[0102] The embodiment provides a method for judging the readability of an NFC tag, which comprises:

[0103] (1) using a smart phone or an NFC card reader with an NFC module to perform reading and scanning operation, and judging whether the tag is readable from the display interface of the phone and the card reader;

[0104] (2) Analyze the actual performance of the NFC tag by the network analyzer, and the specific steps are as follows:

[0105] (21) First, calibrate the network analyzer;

[0106] (22) After calibration, connect the network with the self-made antenna (the self-made antenna is made of copper wire winding, and the size is approximately the same as the antenna);

[0107] (23) Detect the performance of the NFC tag through the self-made antenna, measure the resistance value change caused by different gas concentrations, and mark the corresponding S 11 Change and mark the corresponding resonance frequency

[0108] (3) According to the change of S 11 and the shift of resonance frequency, verify the reason of the change of tag readability from the principle.

Claims

1. A wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food, characterized in that: The tag-type gas sensor comprises a flexible substrate (1), an NFC coil (2), a resistor reserved position (3), a forked electrode (4), a chip capacitor reserved position (5), a patch resistor and an NFC chip, wherein the NFC coil (2) is arranged on the flexible substrate (1), the resistor reserved position (3) and the chip capacitor reserved position (5) are both arranged on the NFC coil (2) and located on the front of the NFC tag, and the side on which the NFC coil is installed is the front of the NFC tag, wherein the resistor reserved position (3) is used to place the patch resistor, and the chip capacitor reserved position (5) is used to place the NFC chip; the forked electrode (4) is installed on the back of the NFC coil (2); the forked electrode (4) and the NFC chip are connected in series or in parallel; when connected in series, the forked electrode (4) is connected to the through holes at both ends of the resistor reserved position, that is, the forked electrode string The forked electrodes (4) are connected to both ends of the NFC chip; when connected in parallel, the forked electrodes (4) are connected to the through holes at both ends of the chip capacitor reserved position, that is, the forked electrodes are connected in parallel to both ends of the NFC chip; the surface of the forked electrodes is modified with a sensitive material, and the sensitive material is a material sensitive to H2S, wherein the sensitive material is a composite of any two materials selected from carbon nanotubes, molybdenum disulfide, metal nanoparticles and metal organic frameworks; when contacting the target gas, if the sensing area of ​​the forked electrodes shows a decrease in resistance, the NFC tag adopts a series mode, and the NFC tag changes from being unable to communicate to being able to communicate; if the sensing area of ​​the forked electrodes shows an increase in resistance, the NFC tag adopts a parallel mode, and the NFC tag changes from being unable to communicate to being able to communicate; if the resistance change is opposite in the corresponding series and parallel modes, the communication change is also opposite.

2. The wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food according to claim 1, characterized in that: The flexible substrate (1) is polyimide, polyethylene terephthalate, polydimethylsiloxane, polytetrafluoroethylene or paper; and / or the NFC coil (2) is a multi-turn coil, the shape of which is one of circular, rectangular, rounded rectangular and triangular, and the antenna material includes at least one of gold, silver, copper, aluminum and carbon.

3. The wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food according to claim 1, characterized in that: The NFC chip is selected from NTAG213, NTAG215, S50, PN532, PN533, PN544, PN7150, and PN7462.

4. The wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food according to claim 1, characterized in that: The interdigitated electrodes are composed of comb-shaped electrodes with gaps and finger widths ranging from micrometers to millimeters, and their surfaces are modified with sensitive materials.

5. The wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food according to claim 1, characterized in that: The carbon nanotubes are single-walled carbon nanotubes, multi-walled carbon nanotubes or carboxylated carbon nanotubes; the metal nanoparticles are at least one of Au, Ag, Pd and Pt; and the metal organic framework is any one of MOF-74, MOF199 and ZIF-8.

6. The wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food according to claim 1, characterized in that: The sensitive material is composed of a composite of carbon nanotubes and metal nanoparticles, wherein the carbon nanotubes are carboxylated single-walled carbon and the metal nanoparticles are Ag. The preparation method of the sensitive material is as follows: Add AgNO3 solution to the carboxylated single-walled carbon dispersion, stir and heat, slowly drop the dispersed sodium citrate solution, continuously stir to react, collect the product, and the product is the sensitive material.

7. The wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food according to claim 6, characterized in that: The concentration of AgNO3 solution is 0.1 M, the concentration of carboxylated single-walled carbon dispersion is 0.1-1.4 mg / mL, the volume ratio of SWCNT and Ag is 20-8:2-20, and the continuous stirring reaction time is 10-80 minutes.

8. The wireless passive NFC tag-type gas sensor for monitoring the freshness of fresh food according to claim 1, characterized in that: Use an NFC terminal device to read NFC tags, where the NFC terminal device is a smartphone with an NFC module or an NFC reader / writer.

Images