Preparation and application of a hydrogel humidity sensor based on optical detection method

Abstract

The invention relates to the preparation and application of a hydrogel humidity sensor based on an optical detection method, which belongs to the field of new sensors. Acrylamide monomers are doped into an acrylic ester hydrogel in a certain proportion, and copolymerization is achieved by ultraviolet light-induced polymerization, which can increase the swelling rate of the hydrogel. Based on nanoimprint technology, a high-quality hydrogel grating was prepared by a "two-step transfer method". The hydrogel grating can sense humidity using an optical detection method. The present invention develops a fast, simple and controllable hydrogel grating preparation technology with low cost; the prepared hydrogel grating humidity sensor has high response sensitivity, good linearity, adjustable response range and good repeatability, and has good practicality and development prospects in biosensing, environmental monitoring and other aspects.

Description

Technical Field

[0001] The invention discloses a preparation method and measurement application of an acrylate hydrogel grating based on an optical detection method, and belongs to the field of novel sensors. Background Art

[0002] From industrial and agricultural production to daily life, humidity is closely related to people's lives. In many fields such as precision electronic component manufacturing, meteorological monitoring, aerospace, and environmental protection, accurate measurement and control of environmental humidity are often required. Unlike the measurement of conventional environmental factors such as temperature, humidity measurement is affected by other factors (such as air pressure and temperature), making the measurement technology more complex. With the development of science and technology, the demand for humidity sensors is also increasing. Currently, commonly used humidity sensors usually work by measuring the capacitive reactance or impedance changes of polymer or metal oxide film layers. However, due to constraints such as process conditions, the linearity and performance parameters of such sensor devices are highly discrete, and the structure is complex and the manufacturing cost is high. On the other hand, in recent years, with the research and development of organic high-molecular functional polymers, their application as sensing devices has become increasingly widespread.

[0003] Organic humidity-sensitive materials contain numerous hydrophilic groups (such as hydroxyl and amine groups), which can interact with water molecules in the air to form hydrogen bonds, thereby responding to ambient humidity. By adjusting the specific surface area of ​​the humidity-sensitive material, more binding sites can be provided, resulting in faster detection, higher sensitivity, and a wider relative humidity range. Organic polymer humidity-sensitive materials are widely available and diverse, with hydrogels being the most widely used. Hydrogels are composed of a network of natural or synthetic hydrophilic polymer chains that has been physically or chemically cross-linked. Due to the internal cross-linked network, their structural integrity is not compromised by high water content. Due to their excellent biocompatibility and chemical and mechanical properties similar to those of human tissue, these soft materials are widely used in fields such as tissue engineering, medical dressings, and chemical sensors. Common hydrogel materials include cellulose, polyethylene, and acrylic acid and its derivatives (polyacrylic acid, polyacrylates, and polyacrylamide). Acrylic acid and its derivatives offer stable performance, high reactivity, and low cost, making them the most practical materials for the fabrication of optical devices.

[0004] A diffraction grating, often referred to as a "grating," is an optical element composed of periodic parallel lines. Based on the Fraunhofer principle of multi-slit diffraction and interference, a grating can effectively modulate the amplitude or phase (or both) of incident light. The relationship between the grating structure and the incident and diffracted light can be described by the grating equation mλ = d sin θ, where d is the grating period, m is the diffraction order, and θ is the diffraction angle. When the incident angle is fixed, each value of m that satisfies the grating equation corresponds to a different peak wavelength.

[0005] According to the manufacturing method, traditional gratings can be divided into ruled gratings, replicated gratings and holographic gratings. Ruled gratings are made by mechanically etching the surface of a material periodically, but this method requires very high precision of the equipment, and the quality of the gratings produced is poor, and ghost lines are easily generated. Holographic gratings are made by laser through double-beam interference lithography, followed by development and fixing. Although it overcomes the disadvantages of ghost lines, it has low diffraction efficiency and is selective for angles and spectra. Replica gratings are made by replicating the master grating, including primary and secondary replication, and are often called "nanoimprinting." This method is simple to operate, low in cost, and can be used with a wide range of materials.

[0006] The hydrogel's polymer chain network can lock into the periodic, ordered structure of the grating. When the grating's period, line height, duty cycle, and other parameters change due to the effects of ambient humidity, the amplitude or phase of the incident light can be manipulated. By measuring the intensity or peak position of the diffracted light, the device responds to changes in external stimuli within the readily identifiable visible or infrared bands, achieving humidity sensing. Summary of the Invention

[0007] The present invention aims to overcome the problems of the prior art by providing a rapid and simple method for preparing and detecting a hydrogel grating humidity sensor. Using hydroxyethyl methacrylate (HEMA) as the humidity-sensitive material, the sensor, based on optical detection, is fabricated via nanoimprinting technology. The sensor has promising application prospects for air humidity detection.

[0008] The technical solution of the present invention is a hydrogel humidity sensor based on optical detection methods, comprising a hygroscopic expansion unit, a test optical path, and a detection unit. The hygroscopic expansion unit is a hydrogel grating fabricated using nanoimprinting technology and fixed to a quartz substrate. The test optical path is a focusing-collimating optical path connected by infrared optical fibers, directing light perpendicularly onto the hydrogel grating. The detection unit is primarily a spectrometer, which detects and receives the characteristic diffraction spectrum of the hydrogel grating on the quartz substrate side and records the peak intensity changes of the first-order diffracted light. The detection angle is preferably fixed at 42°.

[0009] The grating period is 1-20 μm, preferably 1-10 μm, the modulation depth is half of the period, and the duty cycle is 1:1.

[0010] The raw material composition of the hydrogel grating based on controllable photopolymerization is shown in Table 1:

[0011] Table 1: Material composition and its function

[0012]

[0013]

[0014] The acrylate monomer is selected from hydroxyethyl methacrylate (HEMA). The acrylamide monomer is selected from methacrylamide (MAA). The crosslinker is selected from ethylene glycol dimethacrylate (EGDMA). The photoinitiator is selected from 2-hydroxy-2-methylpropiophenone (DMPA). The solvent is deionized water.

[0015] The preparation process of the above-mentioned hydrogel grating is as follows: put the monomer, cross-linker, photoinitiator and solvent into a small brown bottle according to the mass ratio, and stir for 2 hours under light-proof conditions until the mixed solution is uniform and transparent. Then, according to the two-step transfer method, first, select a grating master with suitable parameters, inject the PDMS solution into the "sandwich" structure, and after heating and curing, transfer to obtain a flexible PDMS grating template. In the second step, based on the controllable photopolymerization reaction, the hydrogel precursor solution is poured on the PDMS template. After UV curing, the PDMS grating master is peeled off to obtain a hydrogel grating.

[0016] During the research on the desired polymer material, in order to overcome the incomplete transfer of the hydrogel grating structure and improve the material's sensitivity to low relative humidity, the present invention selected a highly hydrophilic methacrylamide monomer for copolymerization with the acrylate hydrogel. The presence of acrylamide increases the swelling rate of the hydrogel material, which helps improve the transfer integrity of the hydrogel grating and effectively enhances the humidity sensitivity and response range of the hydrogel grating prepared using this material.

[0017] The method for preparing the hydrogel grating humidity sensor involves a two-step transfer process. First, a grating master with appropriate parameters is selected. A PDMS solution is injected into the sandwich structure. After heat curing, a flexible PDMS grating template is obtained by transfer printing. Second, a hydrogel precursor solution is cast onto the PDMS template using a controlled photopolymerization reaction. After UV curing, the PDMS grating master is peeled off to obtain the hydrogel grating.

[0018] The hydrogel grating sensor adopts an optical detection method, using a halogen lamp as a light source and a spectrometer to measure the spectrum of the diffracted light. By reading the peak position and peak change of the spectrum and comparing them with the values ​​of a reference humidity sensor, the relationship between the peak intensity of the first-order diffracted light and the humidity is obtained; then the humidity to be detected is detected, and the peak intensity of the first-order diffracted light is read to obtain the corresponding humidity.

[0019] Advantages of the present invention:

[0020] 1. Using acrylic hydrogel as the humidity-sensitive material, the swelling ratio approximately satisfies a linear relationship, and the material properties are stable, which can improve the reusability of the sensor;

[0021] 2. By doping the material with acrylamide monomer, the swelling rate of the material is increased, thereby improving the detection sensitivity of the sensor in the low humidity range;

[0022] 3. By changing the mass fraction of the cross-linker, the swelling rate of the hydrogel material can be changed, thereby regulating the response range of the hydrogel sensor;

[0023] 4. The optical detection readout method has a faster response time than traditional impedance measurement-based sensing systems;

[0024] 5. Based on nanoimprinting grating preparation technology, large-volume hydrogel gratings can be prepared, and the performance of the sensor can be further improved by adjusting the specific surface area. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the preparation of hydrogel gratings based on nanoimprinting method;

[0026] Figure 2 Structural characterization of the hydrogel grating (a) Optical microscopy; the upper right corner shows the structural dispersion at different observation angles (b) Scanning electron microscopy;

[0027] Figure 3 Experimental setup for measuring humidity correlation using hydrogel grating humidity sensor;

[0028] Figure 4 First-order diffraction spectrum of the hydrogel grating with 2% cross-linker mass fraction (a) and linear fitting of peak intensity and relative humidity (b);

[0029] Figure 5 First-order diffraction spectrum of the hydrogel grating with 3% cross-linker mass fraction (a) and linear fitting of the peak intensity and relative humidity (b);

[0030] Figure 6Repeatability test of the hydrogel grating humidity sensor with 4% cross-linker mass fraction (a) Peak intensity change of the first-order diffraction spectrum at 45% RH and 60% RH after 6 cycles; (b) Optical microscopy characterization of the hydrogel grating after two weeks at room temperature. DETAILED DESCRIPTION

[0031] The present invention will be further described below with reference to the examples, but the present invention is not limited to the following examples.

[0032] Example 1

[0033] In this example, a hydrogel grating was prepared using hydroxyethyl methacrylate and methacrylamide hydrogel precursor solution as materials, a 900-1700nm infrared optical fiber was used as the connecting optical path, and a USB-4000 spectrometer was used as the detection unit to build a hydrogel grating humidity sensor.

[0034] (1) Preparation of hydrogel grating solution

[0035] Add a clean magnetic rod to the cleaned Arnold Breast Augmentin (ABR) bottle and calculate the required weights of each component based on the mass ratio. Place 100 parts by mass of HEMA into the ABR, add 50 parts by mass of MAA and 25% by weight of deionized water (compared to the total weight of the two monomers). Stir these components on a magnetic stirrer for 5 minutes. Then, add 2% by weight of EGDMA and 1% by weight of DMPA (compared to the total weight of the two monomers). After all materials have been added, stir the ABR on a magnetic stirrer for 90 minutes until the mixture is transparent and homogeneous. Protect from light during this operation.

[0036] (2) Preparation of PDMS grating

[0037] A clean quartz plate was prepared as a substrate. Thin, 1cm-thick strips of gaskets were laid flat on both sides of the plate. The grating master was then placed on the gaskets and secured at both ends with binder clips to create a "sandwich" structure. The grating master had a period of 2μm, a modulation depth of 1μm, and a duty cycle of 1:1. The PDMS prepolymer was prepared by mixing an elastomer base (Sylgard 184, Dow Corning) with a curing agent in a 10:1 (w / w) ratio. After stirring for approximately 1 hour to achieve a uniform mixture, the PDMS material was vacuumed for 5 minutes to remove air bubbles. The prepolymer was then pipetted into the gaps in the sandwich structure and cured at 80°C for 2 hours. Finally, the master was peeled off to obtain a flexible PDMS negative template.

[0038] (III) Preparation of hydrogel gratings

[0039] A 2.5cm x 2.5cm quartz substrate was ultrasonically cleaned in acetone and deionized water for 5 minutes respectively. The PDMS template and quartz base were then cleaned in an oxygen plasma cleaner for 1 minute to improve the surface wettability. A 0.5mm thick gasket was added between the quartz sheet and the PDMS template, and the prepared hydrogel precursor solution was dripped into it. The two sides were fixed with binder clips to form a sandwich structure. Then it was cured under 365nm UV light for 1 minute. After the hydrogel material was fully polymerized, the PDMS template was peeled off to obtain a hydrogel grating. The prepared hydrogel grating was fixed on the quartz substrate.

[0040] (4) Construction of detection optical path

[0041] Based on the focusing-collimating optical path, a halogen lamp was used as the light source. This light was connected to the optical path via an infrared optical fiber. After focusing and collimation, it was vertically irradiated onto the surface of the hydrogel grating sample. Another infrared optical fiber was used as a detection probe to receive the characteristic diffraction spectrum of the hydrogel grating. It was connected to a USB-4000 spectrometer to record the peak intensity changes of the first-order diffracted light. The fixed detection angle was 42°. A humidifier was used to control the ambient humidity, and a commercial humidity sensor was used to compare the test results. The entire experimental setup was located in a sealed acrylic box measuring 50cm x 40cm x 25cm.

[0042] The experimental data of the hydrogel grating humidity sensor are as follows:

[0043] 1. Structural Characterization of Hydrogel Gratings

[0044] The effects of nanoimprinting technology on the preparation of large-area hydrogel gratings were investigated, and the prepared hydrogel gratings were characterized. The experimental results are as follows: Figure 2 As shown in Figure 1, the prepared hydrogel grating measures 1 cm x 1 cm. As can be seen in the optical image in the upper right corner of Figure (a), the hydrogel grating exhibits bright, uniform dispersion and angle-selective properties at the macroscopic level. The optical microscope image in Figure (a) and the scanning electron microscope image in Figure (b) demonstrate that the hydrogel grating is structurally intact at the microscopic level, with structural parameters very similar to those of the grating master. This demonstrates that our method can successfully fabricate large-scale, high-quality hydrogel gratings.

[0045] 2. Humidity-dependent properties of hydrogel gratings

[0046] To investigate the humidity dependence of hydrogel gratings, we constructed Figure 3 The focusing-collimating optical path shown in FIG. 3 is used to measure the first-order diffraction spectrum of the hydrogel grating with a cross-linker mass fraction of 4% at different humidity. The results are shown in FIG. Figure 4 As shown. Figure 4(a) It can be seen that as the humidity increases, the peak intensity of the first-order diffraction spectrum of the hydrogel grating at 1300nm gradually decreases, while the peak position remains unchanged. This is because when the humidity increases, the hydrogel absorbs water and swells, which increases the modulation depth of the grating in the vertical direction, resulting in a decrease in diffraction efficiency. However, the displacement of the hydrogel structure in the two horizontal directions is equal, so the grating period will not change significantly, so the position of the diffraction peak does not move. Fitting the humidity and peak intensity to obtain Figure (b), it can be seen that the peak intensity and humidity show an obvious linear correlation characteristic. It proves that humidity sensing detection of hydrogel gratings based on optical detection methods is feasible. We can also change the humidity response range of the grating by adjusting the cross-linking degree of the hydrogel grating material to make it more suitable for practical applications. Figure 5 The humidity dependence of the hydrogel grating with a mass fraction of 3%. Reducing the cross-linking degree can increase the swelling rate of the hydrogel material and improve its response to lower humidity. Figure 5 It can be seen that the hydrogel has a good linear humidity response from 40% RH to 65% RH.

[0047] Since this invention is applied in the field of sensors, we also investigated the repeatability and stability of the hydrogel grating in measuring humidity. We selected a hydrogel grating with a cross-linking degree of 4% and repeatedly measured its peak intensity under 45% RH and 60% RH conditions. Figure 6 As shown in Figure (a), after six cycles (multiple measurements at both 45% and 60% RH), the peak intensities of the first-order diffraction peaks of the hydrogel gratings at 45% RH and 60% RH are essentially the same. Figure (b) shows an optical microscopy image of the hydrogel gratings after two weeks under normal storage conditions, showing that the grating structure remains essentially unchanged. This demonstrates that our hydrogel humidity sensor exhibits excellent repeatability and stability, allowing it to be stored for several weeks.

Claims

1. A hydrogel grating humidity sensor based on an optical detection method, characterized in that: include: Hygroscopic expansion unit, test optical path and detection unit; The hygroscopic expansion unit is a hydrogel grating prepared by nanoimprinting technology and fixed on a quartz substrate; The test optical path is based on a focusing-collimating optical path, connected by an infrared optical fiber, and irradiates light vertically onto the hydrogel grating. The detection unit is a spectrometer, which is used to detect and receive the characteristic diffraction spectrum of the hydrogel grating on the quartz substrate side and record the peak intensity change of the first-order diffracted light. The grating period is 1-20μm, the modulation depth is half of the period, and the duty cycle is 1:

1. The raw materials of the hydrogel grating are as follows: The acrylate monomer is selected from hydroxyethyl methacrylate (HEMA), the acrylamide monomer is selected from methacrylamide (MAA), the crosslinker is selected from ethylene glycol dimethacrylate (EGDMA), the photoinitiator is selected from 2-hydroxy-2-methylpropiophenone (DMPA), and the solvent is deionized water.

2. The hydrogel grating humidity sensor based on the optical detection method according to claim 1, characterized in that: The grating period is 1-10 μm.

3. The hydrogel grating humidity sensor based on the optical detection method according to claim 1, characterized in that: 50 parts by mass of acrylamide monomers 4. The hydrogel grating humidity sensor based on the optical detection method according to claim 1, characterized in that: The preparation steps for hydrogel gratings are as follows: monomer, crosslinker, photoinitiator, and solvent are placed in a small brown bottle according to the mass ratio and stirred in the dark for 2 hours until the mixed solution becomes homogeneous and transparent. Then, according to the two-step transfer method, first, a grating master plate with appropriate parameters is selected, and the PDMS solution is injected into the "sandwich" structure. After heating and curing, the flexible PDMS grating template is transferred. In the second step, based on the controllable photopolymerization reaction, the hydrogel precursor solution is cast on the PDMS template. After UV curing, the PDMS grating master is peeled off to obtain the hydrogel grating.

5. Use of the hydrogel grating humidity sensor based on an optical detection method according to any one of claims 1 to 4 for detecting air humidity.

6. A method for detecting gas humidity using a hydrogel grating humidity sensor based on an optical detection method according to any one of claims 1 to 4, characterized in that: A halogen lamp is used as the light source, and a spectrometer is used to measure the spectrum of the diffracted light. By reading the peak position and peak change of the spectrum and comparing them with the values ​​of a reference humidity sensor, the relationship between the peak intensity of the first-order diffracted light and the humidity is obtained. Then, the humidity to be tested is detected, and the peak intensity of the first-order diffracted light is read to obtain the corresponding humidity.

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