Rapid preparation method of visual sensor and its application

Abstract

The present invention discloses a method for rapidly preparing a visual sensor and its application, namely, a method for rapidly preparing a close-packed microsphere template and an inverse opal photonic crystal hydrogel using an auxiliary horizontal deposition method and a sacrificial template method, for visual detection of different signals (such as stress, strain, and different temperatures). The preparation method of the visual sensor has the advantage of being faster than the traditional vertical deposition method. At the same time, the quality of the obtained microsphere template is better than that of the traditional horizontal deposition method. The double network hydrogel used in this method has good flexibility, elongation, electrical signal output and reusability. Under external stimulation, the applied stress and temperature changes can be visually detected based on the change in its color.

Description

Technical Field

[0001] The present invention relates to the technical field of sensors, and in particular to a rapid preparation method of a visualization sensor and application thereof. Background Art

[0002] Photonic crystals are optical materials composed of several dielectric materials with different refractive indices and dielectric constants, periodically distributed and arranged in space. When light (or electromagnetic waves) propagate through the material, they form bands of energy, known as photonic band gaps. Between these bands lies a "photonic band gap," which partially or completely blocks photons of a specific energy from propagating in a specific direction. The wavelengths that can be modulated by a photonic crystal are determined by factors such as its structural period and the material used.

[0003] Inverse opal is a typical photonic crystal material with orderly, uniform pores, typically at the micro-nanoscale. High-refractive-index materials are infused into the crevices of the opal through chemical or physical filling methods. The raw material is then removed through dissolution or chemical etching, resulting in an inverse opal material with densely packed air spheres within.

[0004] Inverse opal structures hold a crucial position in photonic crystal materials. Traditionally, inverse opal structures are prepared by vertical or horizontal deposition methods, typically using microsphere templates. This invention, based on an improved horizontal deposition method, develops a rapid method for fabricating visual sensors for strain and temperature sensing. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for quickly preparing a visual sensor and its application, that is, a method for quickly preparing a visual strain sensor or a temperature sensor, and provides the application of the visual sensor.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention provides a method for rapidly preparing a visual sensor, characterized by utilizing an assisted horizontal deposition method and a sacrificial template method to rapidly prepare a close-packed microsphere template and an inverse opal photonic crystal hydrogel for visual detection of different signals such as stress, strain, and temperature, comprising the following steps:

[0008] S1: Preparation of close-packed microsphere templates: With the assistance of hydrophilic fences, the microsphere dispersion produces a concave liquid surface under capillary action and gradually self-assembles into a close-packed structure on the substrate within 1 to 4 hours. The self-assembly speed of the photonic crystal is fast;

[0009] S2: Preparation of double network photonic crystal hydrogel: using hydrogel prepolymer solution to penetrate into the gaps between the nanospheres, crosslinking and curing, removing the microsphere template by chemical etching, and leaving nanopores in situ to obtain inverse opal photonic crystal hydrogel;

[0010] S3: Under the influence of external stimuli such as strain and temperature changes, the photonic crystal hydrogel expands or contracts, thereby generating a visual signal output, namely color change.

[0011] As a preferred embodiment, the auxiliary hydrophilic fence material is polydimethylsiloxane (PDMS); the PDMS is thermally initiated by monomers and cross-linking agents; the microspheres are silica microspheres; silica is formed by hydrolysis of tetraethyl orthosilicate under the catalysis of ammonia water.

[0012] Furthermore, in step S1: first, PDMS fences of different heights and diameters are prepared, the monomer and the cross-linking agent are mixed in a ratio of 10:1 and degassed, then poured into the fence template, and cross-linked at 90°C for 1 hour to form; then, oxygen plasma treatment is used to obtain a hydrophilic PDMS fence; thereafter, the fence is bonded to a hydrophilic glass sheet, and a silica microsphere dispersion is added to the fence, and the fence is placed at 35°C for 1 to 4 hours. As the water evaporates, a densely packed silica microsphere template can be obtained.

[0013] Furthermore, in step S2: the double network hydrogel and chitosan double network hydrogel prepolymer are slowly infiltrated into the gaps of the silica microsphere template by capillary force, and after solidification and molding, the silica microspheres are removed by hydrofluoric acid to obtain the inverse opal photonic crystal hydrogel.

[0014] Furthermore, the double network hydrogel is an interpenetrating network hydrogel formed by acrylic acid, acrylamide and chitosan; the hydrogel prepolymer is obtained by dissolving monomers of acrylic acid, acrylamide, chitosan, a crosslinker, an initiator and additives in water, and can be coagulated into a gel after thermal crosslinking at 60°C for 1 hour.

[0015] Furthermore, the cross-linking agent is N,N'-methylenebisacrylamide; the initiator is potassium persulfate; and the additive is multi-walled carbon nanotubes.

[0016] In a second aspect, the present invention provides an application of a method for rapid preparation of a visualization sensor based on any of the above-described methods, characterized in that: based on the output of optical signals and electrical signals of the photonic crystal hydrogel, it is prepared into a wearable sensor device; the wearable sensor device includes joint patches on the fingers, wrists and knees to serve as stretch or compression sensors.

[0017] The technical principles of the present invention are as follows:

[0018] The auxiliary fence material used in the present invention is polydimethylsiloxane (PDMS), which can also be other materials. PDMS is thermally initiated from monomers and a cross-linking agent. This material has excellent elasticity and formability, while also possessing a certain structural strength.

[0019] The microspheres used in the present invention are silica microspheres, which may also be microspheres made of other materials. The microspheres are formed by hydrolyzing tetraethyl orthosilicate under the catalysis of ammonia water and have good water dispersibility, roundness, and uniformity of particle size.

[0020] The double-network hydrogel used in this invention is an interpenetrating network hydrogel formed by poly(acrylic acid-co-acrylamide) hydrogel and chitosan. This hydrogel prepolymer is prepared by dissolving the monomers acrylic acid (AA), acrylamide (Am), and chitosan in water along with a crosslinker (N,N'-methylenebisacrylamide, MBAA), an initiator (potassium persulfate), and an additive (multi-walled carbon nanotubes, CNTs). The prepolymer is then thermally crosslinked at 60°C for one hour to form a gel. This hydrogel exhibits excellent elongation and reproducible stretchability.

[0021] In the present invention, PDMS fences of varying heights and diameters were first prepared. A monomer and cross-linker were mixed in a ratio of 10:1, degassed, and then poured into a fence template. Cross-linking was performed at 90°C for one hour to form the fence. The PDMS fence was then treated with oxygen plasma to render it hydrophilic. The fence was then bonded to a hydrophilic glass sheet, and a silica microsphere dispersion was added to the fence. The fence was then placed at 35°C for 1 to 4 hours. As the water evaporated, a densely packed silica microsphere template was obtained.

[0022] The present invention slowly infiltrates a poly(acrylic acid co-acrylamide) and chitosan dual network hydrogel prepolymer into the gaps of a silica microsphere template through capillary force. After curing and forming, the silica microspheres are removed using hydrofluoric acid to obtain an inverse opal photonic crystal hydrogel. When the hydrogel is subjected to tensile stress, the internal microstructure pores are squeezed, the structural period becomes smaller, and the macroscopic color undergoes a spectral blue shift. When the inverse opal photonic crystal hydrogel is placed in a high temperature environment, its internal network undergoes thermal expansion, which enlarges the structural period and causes a spectral red shift in the macroscopic color. Therefore, the present invention can achieve visualized stress sensing or temperature sensing.

[0023] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0024] By using the preparation method of the visual sensor of the present invention, a fast and excellent method for preparing silica microsphere templates can be developed. By improving the traditional horizontal deposition method, we have achieved high-speed self-assembly (1 to 4 hours) and obtained high-quality photonic crystals. This process can be carried out in a very mild environment (35°C). We introduced a PDMS fence into the traditional horizontal deposition method to form a concave liquid surface on the substrate. Therefore, the contact line appears in the center of the substrate, avoiding the coffee ring effect, thereby improving the quality of the photonic crystal. In addition, an inverse opal structure photonic crystal hydrogel was prepared using a flexible hydrogel to achieve visual sensing of stress / strain or temperature.

[0025] Conventional, conventional horizontal deposition methods typically operate at high temperatures (e.g., 60°C, 80°C). In comparison, the present invention's method for preparing photonic crystals using horizontal deposition can be performed under mild conditions (35°C), requiring minimal environmental requirements. Furthermore, compared to conventional horizontal deposition methods, the quality of the photonic crystals obtained using this method is significantly improved.

[0026] While literature also suggests methods for horizontally depositing photonic crystals onto patterned surfaces, these methods require large, high-precision, and expensive equipment. In contrast, this method utilizes conventional, hydrophilic PDMS enclosures and glass substrates, offering greater access to materials, ease of fabrication, and cost-effectiveness, without the need for large, high-precision, and expensive equipment.

[0027] In addition, vertical deposition, microfluidic self-assembly, block copolymer self-assembly, and cellulose nanocrystal self-assembly are used to assemble photonic crystals. These methods usually take a long time (days to weeks) to complete. In contrast, this method is faster and can be completed in 1 to 4 hours.

[0028] The double-network hydrogel used in this method has good flexibility, elongation, electrical signal output and reusability. Under external stimulation, the present invention can visually detect external stress and temperature changes based on its color change. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the rapid assembly of photonic crystals using the horizontal precipitation method of the present invention;

[0030] Figure 1 Middle: a is a schematic diagram of the preparation process; b is a schematic diagram of the self-assembly mechanism of nanospheres; c is the different parameters (radius and height) of the fence and the required assembly time; d is a SEM image of tightly packed silica nanospheres, scale bar is 500 nm.

[0031] Figure 2Color images and parameter influences of photonic crystals under different fence parameters of the present invention;

[0032] Figure 2 Middle: a is the optical image of the photonic crystal color obtained under different fence parameters; b is the effect of fence depth on the relative area of ​​color; c is the relationship between color brightness and fence depth.

[0033] Figure 3 Schematic diagram of the assembly effects of different photonic crystals of the present invention;

[0034] Figure 3 Middle: a is a structural color optical image assembled from 320, 300, 290, 260, 240, and 225 nm silica nanospheres, with a circle diameter of 1 cm; b is the reflectance spectrum of a photonic crystal obtained using silica nanospheres of different sizes; c is a special shape with different structural colors obtained from different PDMS molds, including WHU, NISE, and butterfly cartoon shapes, with a scale bar of 1 cm.

[0035] Figure 4 Schematic diagram of the preparation and characterization of the photonic crystal hydrogel of the present invention;

[0036] Figure 4 Middle: a is a schematic diagram of the preparation process of photonic crystal hydrogels; b and c are SEM images of hydrogels containing silica nanospheres (b) and inverse opal (c), respectively, with a scale of 500 nm; d is the tensile properties of hydrogels with different compositions; e is the cyclic tensile properties of the hydrogels; f is the stress response of the hydrogels in the last 10 stretching cycles; g is the conductivity of hydrogels with different compositions.

[0037] Figure 5 Schematic diagram of the strain response and optical and electrical signal output of the photonic crystal hydrogel of the present invention;

[0038] Figure 5 Middle: a) Images of the photonic crystal hydrogel under tensile strains of 0%-30%-90%-30%-0%. Scale bar: 1 cm. b) Relationship between the dominant wavelength of light reflected from the photonic crystal hydrogel and tensile strain. c) The dominant wavelength of light reflected from the hydrogel during a cyclic tensile test. d) The relative electrical resistance of the hydrogel under different tensile strains. e) The relative electrical resistance of the hydrogel during a cyclic tensile test.

[0039] Figure 6 This is a schematic diagram of the photonic crystal hydrogel sensor of the present invention used for human motion monitoring and real-time output of optical and electrical dual signals;

[0040] Figure 6Middle: a: A hue circle with hue values ​​marked at corresponding locations; b: A circuit diagram for measuring the resistance R of a photonic crystal hydrogel sensor; ce: The corresponding hue value change (c) and relative resistance change (e) of the hydrogel sensor during cyclic finger bending (d). fh: The corresponding hue value change (f) and relative resistance (h) of the hydrogel sensor during a cyclic knee-jerk reaction (g). ik: The hue value (i) and relative resistance (k) of the sensor under cyclic pressure (j).

[0041] Figure 7 Schematic diagram of the wearable photonic crystal hydrogel sensor of the present invention used for temperature sensing and achieving optical and electrical dual signal output;

[0042] Figure 7 Middle: a is an image of the structural color of the photonic crystal hydrogel at different water temperatures, with a scale of 1 cm; b is the hue value of the photonic crystal hydrogel at different water temperatures; c is an image of a wearable sensor, showing different color changes when the blue photonic crystal hydrogel is attached to a finger and exposed to water at different temperatures; d is the hue value of the sensor under cyclic temperature changes between 20°C (circles), 40°C (triangles), and 60°C (squares); e is the relative resistance of the sensor under cyclic temperature changes between 20°C (circles), 40°C (triangles), and 60°C (squares). DETAILED DESCRIPTION

[0043] The technical solution of the present invention will be further elaborated in detail below with reference to specific embodiments.

[0044] Example 1

[0045] 1. Rapid self-assembly of silica nanospheres

[0046] The photonic crystal periodic structure is prepared by an improved horizontal deposition method ( Figure 1 In order to obtain densely packed silica nanospheres, the silica nanosphere suspension was added to a container consisting of a piranha solution treated glass slide and a plasma treated PDMS fence ( Figure 1 (a) Due to the contact line between the concave liquid surface and the bottom substrate, as the solvent evaporates, the liquid will flow to the central area, driving the silica nanospheres to gather in the center ( Figure 1 (b) Therefore, the crystallization of the photonic crystal driven by capillary force starts from the center of the suspension. As the liquid in the crescent-shaped page evaporates, the three-phase boundary contact line extends outward, allowing the silica nanospheres to self-assemble into a densely packed structure behind the three-phase interface ( Figure 1The results show that the depth of the pores in the PDMS fence significantly affects the self-assembly time, and the greater the depth, the longer the self-assembly time. In this system, the self-assembly of nanospheres can be completed within a few hours, which is better than the commonly reported time (which often takes several days to weeks). The radius of the pores in the PDMS fence has no significant effect on the self-assembly time ( Figure 1 (c)

[0047] Figure 2 Figure a shows optical photographs of photonic crystals obtained with different enclosure parameters. Image J and Photoshop were used to extract the relative color area and color brightness of these photographs. The results show that the relative color area percentage and color brightness increase with increasing enclosure depth, but have no significant correlation with the radius of the enclosure hole. Figure 2 Middle b, Figure 2 (c)

[0048] Figure 3 Figure a shows an optical photograph of a photonic crystal self-assembled from silica nanoparticles of different particle sizes. As the particle size of the nanoparticles decreases, the macroscopic color gradually shifts to a blue spectrum ( Figure 3 (a). The reflection spectrum also shows the change of the main wavelength of the reflected light of the photonic crystal obtained by microspheres of different particle sizes ( Figure 3 In addition, the PDMS fence used in this method can adopt various special shapes to obtain photonic crystals of different shapes ( Figure 3 (c)

[0049] 2. Preparation of dual-network photonic crystal hydrogel

[0050] First, 0.2 mL of acrylic acid (AA), 0.1 g of acrylamide (Am), 0.0015 g of N,N'-methylenebisacrylamide (MBAA), 0.0050 g of potassium persulfate, 0.05 g of chitosan quaternary ammonium salt, and 0.001 g of multi-walled carbon nanotubes (CNTs) were added to deionized water. After oscillation and ultrasound, the hydrogel prepolymer was fully dissolved.

[0051] Through capillary force, the hydrogel prepolymer slowly penetrates into the gaps between the silica microspheres in the photonic crystal, solidifies at 60 ° C in a closed environment for 1 hour to form a gel, and then soaks in 10% hydrofluoric acid for 4 hours to etch away the silica microspheres, thus obtaining a double-network photonic crystal hydrogel ( Figure 4 (a) Figure 4 Zhongb and Figure 4 Middle c shows the scanning electron microscope (SEM) images of the photonic crystal hydrogel before and after etching.

[0052] The tensile mechanical properties of this double-network hydrogel were tested. The results showed that compared with pure poly (acrylic acid co-acrylamide) hydrogel, the addition of chitosan can significantly enhance the tensile strength and elongation of the hydrogel, while the further addition of carbon nanotubes will cause the elongation to decrease (but still higher than that of pure hydrogel), while the strength will increase slightly ( Figure 4 (d)

[0053] The cyclic stretching test showed that this double network hydrogel has excellent cyclic stretching properties ( Figure 4 Middle e, Figure 4 Similarly, the conductivity of the hydrogel was tested. With the addition of CNTs, the conductivity of the hydrogel increased several times ( Figure 4 (g).

[0054] 3. Strain sensing properties of photonic crystal hydrogels

[0055] The macroscopic color changes of the photonic crystal hydrogel were recorded by a camera under different tensile strains ( Figure 5 In (a), a visible light spectrometer is used to test the corresponding reflected light spectrum intensity, and a source meter is used to record the real-time changes in the resistance output signal.

[0056] The results show that the photonic crystal hydrogel prepared by this method can completely achieve color changes from red to green and then to blue under 90% strain; as the tensile strain increases, the main wavelength of its reflected light also changes from 625nm to 450nm ( Figure 5 (b) Cyclic stretching experiments show that the hydrogel can change color under 0-90% strain without being damaged ( Figure 5 At the same time, the hydrogel also showed changes in resistance signals, and its relative resistance (i.e. (real-time resistance - original resistance) / (original resistance)) increased with the increase of strain ( Figure 5 Similarly, the cyclic stretching experiment also proved the stability of the resistance signal output ( Figure 5 (e)

[0057] Application Example 1

[0058] 1. Strain sensing for wearable sensors

[0059] Based on the output of optical and electrical signals of the photonic crystal hydrogel, it was prepared into a wearable sensor device, and various applications of the strain sensor were demonstrated, including joint patches on fingers, wrists and knees, as stretch or compression sensors ( Figure 6 ).

[0060] In the case of bent finger joints ( Figure 6(d) When the bending angle increases from 0° to 30°, 60°, and 90°, the photonic crystal hydrogel changes from red to yellow, green, and then to blue, and the hue value changes from 0° to 70°, 120°, and 230°, respectively. Figure 6 Middle a, Figure 6 Meanwhile, the real-time relative resistance of the wearable sensor was recorded to verify the dual-signal response of the wearable device. The results showed that the device can achieve stable and repeatable response even during cyclic bending motion ( Figure 6 Similarly, when the wearable device with the photonic crystal hydrogel was applied to the wrist, the device also exhibited good sensing capabilities during cyclic bending of the wrist.

[0061] The wearable sensor can be used to detect rapid signal changes during processes such as knee-jerk reactions ( Figure 6 Middle f, Figure 6 Middle g, Figure 6 Middle h), also exhibited rapid detection behavior.

[0062] In addition to monitoring tensile strain behavior, wearable devices can also monitor compressive strain behavior ( Figure 6 Similarly, different degrees of pressure and strain will also cause different color changes of the sensor ( Figure 6 i) and relative resistance change ( Figure 6 (middle k).

[0063] 2. Temperature response of wearable sensors

[0064] Wearable sensors made from photonic crystal hydrogels can also respond to thermal stimuli ( Figure 7 The results showed that when the blue photonic crystal hydrogel was transferred from an aqueous solution at 20°C to 40°C and 60°C, the photonic crystal hydrogel showed obvious color changes, from blue to green and then to red, respectively ( Figure 7 Conversely, when transferred from 60℃ to 40℃ and 20℃ aqueous solutions, the color of the photonic crystal hydrogel changed from red to green and then back to blue ( Figure 7 In addition, the color changes very quickly during the heating process, and slowly during the cooling process ( Figure 7 b), which is related to the speed of heat exchange.

[0065] Similarly, photonic crystal hydrogels were integrated into wearable sensor devices as finger patches and demonstrated their application in temperature sensing ( Figure 7 (c). Cycling temperature testing of the wearable sensor device between 20°C, 40°C, and 60°C also showed stable and repeatable changes in the sensor's optical properties ( Figure 7Similarly, the electrical signal output (relative resistance) of the sensor device also exhibited good cycling characteristics during the monitoring of temperature changes ( Figure 7 (e)

Claims

1. A method for rapidly preparing a visual sensor, characterized in that: A method for rapidly preparing close-packed microsphere templates and inverse opal photonic crystal hydrogels using assisted horizontal deposition and sacrificial template methods for visual detection of various signals, including stress, strain, and temperature, includes the following steps: S1: Preparation of close-packed microsphere templates: With the assistance of hydrophilic fences, the microsphere dispersion produces a concave liquid surface under capillary action and gradually self-assembles into a close-packed structure on the substrate within 1 to 4 hours. The self-assembly speed of the photonic crystal is fast; S2: Preparation of dual-network photonic crystal hydrogel: The hydrogel prepolymer solution is infiltrated into the gaps between the nanospheres, cross-linked and cured, and the microsphere template is removed by chemical etching, leaving nanopores in situ to obtain the inverse opal photonic crystal hydrogel; S3: Under the action of external stimuli such as strain and temperature change, the photonic crystal hydrogel expands or contracts, thereby generating a visual signal output, i.e., color change; The auxiliary hydrophilic fence material is polydimethylsiloxane (PDMS); the PDMS is thermally initiated by a monomer and a cross-linking agent; the microspheres are silica microspheres; silica is formed by hydrolysis of tetraethyl orthosilicate under the catalysis of ammonia water; In step S1: first, PDMS fences of different heights and diameters are prepared, monomers and cross-linking agents are mixed at a ratio of 10:1, degassed, poured into a fence template, and cross-linked at 90°C for 1 hour to form a fence; Then, oxygen plasma treatment is used to obtain a hydrophilic PDMS fence; the fence is then bonded to a hydrophilic glass sheet, and a silica microsphere dispersion is added to the fence. The fence is placed at 35°C for 1 to 4 hours, and as the water evaporates, a densely packed silica microsphere template is obtained.

2. The method for rapidly preparing a visual sensor according to claim 1, wherein: In step S2, the hydrogel prepolymer is slowly infiltrated into the gaps of the silica microsphere template by capillary force, and after solidification and forming, the silica microspheres are removed by hydrofluoric acid to obtain the inverse opal photonic crystal hydrogel.

3. The method for rapidly preparing a visual sensor according to claim 2, wherein: The double network photonic crystal hydrogel is obtained by dissolving monomers of acrylic acid, acrylamide, chitosan, a crosslinking agent, an initiator, and additives in water, and can be coagulated into a gel through thermal crosslinking at 60° C. for 1 hour.

4. The method for rapidly preparing a visual sensor according to claim 3, wherein: The cross-linking agent is N,N'-methylenebisacrylamide; the initiator is potassium persulfate; and the additive is multi-walled carbon nanotubes.

5. An application of the rapid preparation method of a visualization sensor according to any one of claims 1 to 4, characterized in that: Based on the output of optical and electrical signals of the photonic crystal hydrogel, it is prepared into a wearable sensor device; the wearable sensor device includes joint patches on the fingers, wrists and knees to serve as stretch or compression sensors.

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