Porous hollow sphere photonic crystal sensor and method for detecting xylene gas

Abstract

A porous hollow spherical photonic crystal sensor for detecting xylene gas and a method thereof, comprising synthesizing PS@SiO2 core-shell colloids, assembling into spherical photonic crystals by microfluidics, calcining to remove the polystyrene inner core to form a hollow SiO2 structure, and introducing a porous structure by controllable chemical etching. Subsequently, functionalization is performed by using P(VBC-co-MMA) to obtain a porous hollow spherical photonic crystal sensor, which is endowed with strong molecular affinity for xylene, and selective gas adsorption of the porous photonic crystal is realized. Benefiting from the super-symmetrical spherical structure, the obtained photonic crystal sensor exhibits an inherent angle-independent optical response, overcoming the limitation of angle dependence of thin film photonic crystal sensors. Exposure to xylene gas shows a significant reversible red shift of the optical stop band and a clear color change, which is due to the increase of the effective refractive index caused by vapor diffusion and adsorption. By monitoring the movement of the optical stop band in air and saturated xylene, visual detection of xylene gas can be realized.

Description

Technical Field

[0001] This invention relates to xylene gas detection technology, and in particular to a porous hollow spherical photonic crystal sensor and method for detecting xylene gas. Background Technology

[0002] Volatile organic compounds (VOCs) are widely used as solvents and intermediates in industries such as chemical manufacturing, coatings, printing, and petrochemicals. Xylene, in particular, is extremely dangerous due to its high volatility, flammability, and significant neurotoxicity. Long-term exposure to xylene gas can lead to serious health problems, including neurological disorders, respiratory irritation, and blood damage, posing a serious threat to occupational safety and environmental health. Therefore, developing sensitive, selective, and reliable xylene detection strategies is crucial for environmental monitoring and industrial safety.

[0003] Photonic crystals are periodic submicron structures composed of materials with different refractive indices arranged in an ordered manner, enabling effective control over the propagation characteristics of light. When the concentration or type of gas molecules in the external environment changes, the structural color of the photonic crystal changes accordingly. Therefore, due to their excellent optical properties and photonic bandgap performance, photonic crystals are increasingly being applied in the field of gas detection.

[0004] Conventional analytical techniques for detecting xylene, such as gas chromatography, mass spectrometry, and related spectroscopic methods, offer high precision but typically require expensive instrumentation, complex sample preparation, and skilled operators, severely limiting their applicability in real-time and field monitoring. In contrast, photonic crystal (PC)-based optical chemical sensors offer an attractive alternative due to their label-free operation, rapid response, and intuitive colorimetric readout. The periodic dielectric structure in photonic crystals creates optical band stops whose spectral positions are highly sensitive to variations in lattice spacing and effective refractive index, enabling the direct conversion of chemical stimuli into optical signals. Over the past two decades, photonic crystal-based sensing platforms have been widely used for detecting gases, volatile organic compounds (VOCs), ions, and biomolecules. Despite these advances, two-dimensional reflective photonic crystals (like most thin-film and low-symmetry PC sensors) exhibit inherent angle dependence. This angle dependence complicates signal acquisition, reduces measurement repeatability, and hinders practical deployment outside of laboratory settings, highlighting the limitations of current photonic crystal detection.

[0005] To overcome these challenges, photonic crystal structures with inherent angle-independent optical responses are highly desirable. Spherical photonic crystals, also known as photonic crystal beads, inherently eliminate angle dependence due to their supersymmetric geometry, ensuring a consistent photonic response regardless of the observation orientation. Furthermore, spherical structures possess excellent mechanical strength, structural uniformity, and compatibility with microfluidic fabrication processes, making them ideal for scalable and reproducible sensing platforms. However, despite these advantages, integrating molecular selectivity and high sensitivity into spherical photonic crystal sensors for the detection of hazardous volatile organic compounds remains largely unexplored. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a porous hollow spherical photonic crystal sensor and method for detecting xylene gas. The sensor is constructed through multiple steps, including the synthesis of a polystyrene@silica (PS@SiO2) core-shell colloid, microfluidic assembly into a spherical photonic crystal, calcination to remove the polystyrene core to form a hollow SiO2 structure, and controlled chemical etching to introduce a porous structure. Subsequently, it is functionalized using poly(vinylbenzyl chloride-co-methyl methacrylate) [P(VBC-co-MMA)] to obtain the porous hollow spherical photonic crystal sensor [P(VBC-co-MMA) Functionalized Porous SiO2Hollow Opal Photonic Crystal, P(VBC-co-MMA)-P-SiO2 SHOPC], endowing it with a strong molecular affinity for xylene and enabling selective gas adsorption by the porous photonic crystal. Thanks to its supersymmetric spherical structure, the resulting photonic crystal sensor exhibits an inherent angle-independent optical response, overcoming the angle-dependent limitation of thin-film photonic crystal sensors. Exposure to xylene gas manifests as a significant reversible redshift of the optical bandgap and a marked color change, stemming from an increase in effective refractive index due to gas diffusion and adsorption. Visual detection of xylene gas can be achieved by monitoring the shift of the optical bandgap in air and saturated xylene.

[0007] The technical solution of the present invention is as follows:

[0008] A porous hollow spherical photonic crystal sensor for detecting xylene gas includes porous spherical hollow photonic crystal beads and a surface-functionalized polymer recognition layer. The sensor is characterized by: polystyrene@silica (PS@SiO2) core-shell colloidal particles; a spherical photonic crystal assembled via microfluidic control; hollow silica structural units formed by calcination to remove the polystyrene core; hierarchical pores introduced by controllable chemical etching; and poly(vinylbenzyl chloride-co-methyl methacrylate) [P(VBC-co-MMA)] coating the spherical photonic crystal beads. The sensor is functionalized to achieve selective adsorption within a porous photonic crystal framework. Thanks to its supersymmetric spherical structure, the sensor exhibits an inherent angle-independent optical response. When exposed to xylene gas, the gas diffuses and adsorbs within the pores, causing an increase in the effective refractive index, which in turn leads to a significant and reversible redshift in the optical bandgap accompanied by a noticeable color change. This sensor combines a porous hollow structure with polymer-assisted molecular recognition, resulting in rapid response, good selectivity, and excellent reusability. Sensitive and visual detection of xylene is achieved based on the shift in the optical bandgap.

[0009] The spherical photonic crystal is formed from PS@SiO2 monodisperse droplets prepared by microfluidic method, and the microparticles in the spherical photonic crystal are hollow SiO2.

[0010] The porous structure was obtained by etching a hollow spherical photonic crystal in an aqueous solution of sodium carbonate and hexadecyltrimethylammonium bromide (CTAB) for 12 h, 14 h, 16 h and 18 h.

[0011] The polymer P (VBC-co-MMA) that can functionalize porous hollow spherical photonic crystals is polymerized from vinyl benzyl chloride (VBC) and polymethyl methacrylate (MMA).

[0012] The porous hollow spherical photonic crystal sensor [P(VBC-co-MMA)-P-SiO2 SHOPC] is made by immersing a porous hollow spherical photonic crystal in a tetrahydrofuran (THF) solution containing P(VBC-co-MMA), resulting in P(VBC-co-MMA)-P-SiO2 SHOPCs with mass fractions of 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt%, respectively.

[0013] A method for fabricating a porous hollow spherical photonic crystal sensor for detecting xylene gas, characterized by comprising the following steps:

[0014] Step A: Preparation of P(VBC-co-MMA) probe molecules. These probe molecules were synthesized by free radical polymerization of vinyl benzyl chloride (VBC) and polymethyl methacrylate (MMA). Using 4.5 mL of VBC and 2.7 mL of MMA as monomers and 30.3 mg of azobisisobutyronitrile (AIBN) as an initiator, the reaction was carried out in 60 mL of THF solution for 48 h. After the reaction, the mixture was stirred in 400 mL of chilled petroleum ether for 2 h, resulting in an insoluble white precipitate. This precipitate was then vacuum-dried overnight to obtain the P(VBC-co-MMA) probe molecules.

[0015] Step B: Using P(VBC-co-MMA) as solute and THF as solvent, P(VBC-co-MMA)-THF solutions with mass concentrations of 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt% were prepared respectively. The porous hollow spherical photonic crystal was immersed in the solutions of different mass concentrations for 1 h and then removed to obtain four probe concentrations of P(VBC-co-MMA)-P-SiO2SHOPC sensors for detecting xylene.

[0016] The preparation of the porous hollow spherical photonic crystal in step B includes the following steps:

[0017] Step B1, Synthesis of PS colloidal particles: Under a nitrogen atmosphere, polystyrene, polyvinylpyrrolidone (PVP), 2,2'-azobisisobutylamidine dihydrochloride (AIBA) and deionized water are mixed and reacted in a water bath under stirring. After the reaction, the polystyrene colloidal solution containing PVP is prepared by centrifugation and washing.

[0018] Step B2, Synthesis of PS@SiO2 Colloidal Particles: A polystyrene ethanol solution, ammonia water and tetraethyl orthosilicate (TEOS) are mixed and heated in a water bath under magnetic stirring. After the reaction, the PS@SiO2 colloidal solution is prepared by centrifugation and washing.

[0019] Step B3, Preparation of SiO2 Hollow Opal Photonic Crystal: Dimethyl silicone oil and methyl fluorosilicone oil were used as the continuous phase, and PS@SiO2 dispersion was used as the dispersed phase. Under stable flow conditions, monodisperse emulsion droplets generated in a microfluidic device were collected. The droplets were then placed in an oven to evaporate moisture, washed with n-hexane, and calcined in a muffle furnace to obtain SiO2 hollow optical photonic crystal (Silica Hollow Opal Photonic Crystal, SiO2 SHOPC).

[0020] Step B4, Preparation of SiO2 Porous Hollow Opal Photonic Crystal: Mix and stir hexadecyltrimethylammonium bromide aqueous solution (CTAB) and anhydrous sodium carbonate, add SiO2 SHOPC, let stand and etch to obtain SiO2 porous hollow spherical photonic crystal (P-SiO2 SHOPC).

[0021] A method for selectively detecting xylene gas using a porous hollow spherical photonic crystal sensor, characterized by comprising the following steps:

[0022] Step 1: Construct an optical microscope setup for measuring the reflection spectrum of a spherical photonic crystal. Insert a spectrometer above the eyepiece of the optical microscope and connect the spectrometer to an optical fiber. Transmit the reflection spectrum data through the optical fiber. The portion of the reflected light that passes through the spectrometer and then enters the spectrometer through the optical fiber generates real-time reflection spectrum data. Install an industrial camera above the spectrometer and connect a camera sensor to achieve simultaneous detection of the OM diagram and the reflection spectrum.

[0023] Step 2: Place the sensor in a sealed gas chamber equipped with micropores. The gas chamber is placed on the stage of an optical microscope, and a certain volume of volatile solvent is injected through the micropores using a microsyringe. When the sample is exposed to the solvent gas environment, the changes in the OM diagram and the optical band of the reflectance spectrum are obtained and recorded.

[0024] The technical effects of this invention are as follows: This invention provides a porous hollow spherical photonic crystal sensor and method for detecting xylene gas. Based on the high selective adsorption capacity of a polymer-functionalized porous hollow silica structure for xylene gas, the porous hollow spherical photonic crystal, obtained by controllable chemical etching of SiO2 hollow spherical photonic crystals, is functionalized by introducing P(VBC-co-MMA) probe molecules to specifically adsorb SiO2. When the sensor is exposed to an environment containing xylene gas, xylene molecules are selectively adsorbed and diffused into the porous structure of the photonic crystal bead with polymer assistance. Since the refractive index of xylene is higher than that of air, the effective refractive index of the porous photonic crystal framework increases. This refractive index change induces a significant and reversible redshift in the photonic bandgap, accompanied by a noticeable color change. By monitoring the shift in the peak wavelength of the reflection spectrum before and after the presence of xylene gas, sensitive, rapid, and visual detection of xylene gas can be achieved.

[0025] This invention provides a porous spherical hollow photonic crystal sensor. The sensor employs a multi-step strategy to synthesize PS@SiO2 core-shell colloids, remove the polymer core to form hollow SiO2 units, microfluidically assemble them into spherical photonic crystal beads, and introduce a porous structure through controlled chemical etching, resulting in porous hollow silica photonic crystal beads. These beads are then functionalized with P(VBC-co-MMA) to obtain a sensor capable of selectively identifying xylene gas. Based on the high affinity and selective adsorption of xylene molecules by P(VBC-co-MMA), xylene gas diffuses and adsorbs within the porous photonic crystal framework, leading to an increase in its effective refractive index. This, in turn, induces a reversible shift in the optical bandgap accompanied by a significant color change, thereby achieving label-free, sensitive, and visual detection of xylene gas. This sensor possesses a supersymmetric spherical structure, achieving an angle-independent optical response and overcoming the angle-dependent limitations of traditional planar sensors. The sensor manufacturing process is relatively simple, the raw materials are readily available, the cost is low, and it has advantages such as fast response, good selectivity, and reusability. It has broad application prospects in the field of on-site detection of harmful volatile organic compounds.

[0026] The present invention has the following characteristics: (1) The raw materials of the present invention are abundant, the sensor cost is low, and the preparation process is simple. (2) The sensor structure of the detection instrument of the present invention is simple, the response time is short, and it has good linear response. (3) The present invention is the first to use the volume phase change of smart materials caused by the specific binding reaction of functionalized spherical photonic crystal sensor with xylene to detect xylene gas. (4) The sensor of the present invention has excellent selectivity, good sensitivity and stability. (5) The sensor of the present invention has reversibility. (6) The present invention optimizes the performance of the sensor. By continuously adjusting the etching time of the spherical photonic crystal beads and the mass concentration of P(VBC-co-MMA)-THF solution, it was found that the sensor prepared under the following composition ratio can achieve the best performance: the etching time of the spherical photonic crystal beads is 18 h, and the mass concentration of P(VBC-co-MMA)-THF solution is 2.0 wt%. (7) The sensor has good sensitivity and the detection limit is 0.45 μg mL. -1 (105 ppm). Attached Figure Description

[0027] Figure 1 This is a diagram illustrating the fabrication process and sensing mechanism of a porous hollow spherical photonic crystal sensor for detecting xylene gas, which is an embodiment of the present invention. Figure 1The sensor comprises porous spherical hollow photonic crystal beads and a polymer recognition layer functionalized on their surface. The polystyrene@silica (PS@SiO2) is a core-shell colloidal particle. The spherical photonic crystal is obtained through microfluidic assembly. The hollow silica structural unit is formed by calcination to remove the polystyrene core. Controlled chemical etching introduces hierarchical pores. Poly(vinylbenzyl chloride-co-methyl methacrylate) [P(VBC-co-MMA)] functionalizes the spherical photonic crystal beads, achieving selective adsorption within the porous photonic crystal framework. Thanks to its supersymmetric spherical structure, the sensor exhibits an inherent angle-independent optical response. When exposed to xylene gas, the gas diffuses and adsorbs within the pores, causing an increase in the effective refractive index, which in turn leads to a significant and reversible redshift in the optical bandgap accompanied by a noticeable color change. This sensor, combining a porous hollow structure with polymer-assisted molecular recognition, exhibits rapid response, good selectivity, and excellent reusability. Sensitive and visual detection of xylene is achieved based on the shift in the optical bandgap. In this context, "Silicone oil" refers to silicone oil, "Evaporation" means evaporation, "Calcination" means calcination, "Cross-sectional diagram" is a cross-sectional view of photonic crystal particles, "Etching" means etching, "Na2CO3" is anhydrous sodium carbonate, "Introduce probe" means to introduce P(VBC-co-MMA) probe molecules, "Xylene gas" is xylene gas, and "Air" is air.

[0028] Figure 2 This is a diagram of the detection device and internal optical path of a porous hollow spherical photonic crystal sensor for detecting xylene gas according to the present invention. Figure 2 The right side of the diagram shows the detection device, which can simultaneously and in real-time acquire the reflectance spectrum and optical microscopic image (OM image) of the P(VBC-co-MMA)-P-SiO2SHOPC sensor, and quantitatively analyze the concentration of volatile solvents based on the characteristics of the reflectance spectrum and changes in the OM image. The sensor is placed in a sealed gas chamber with a micro-injection port, which is fixed on the stage of an optical microscope. A fixed volume of volatile solvent is injected through the injection port using a micro-syringe at room temperature to create a controllable gas-phase detection environment. Figure 2The left-hand side of the diagram shows the internal optical path. The light beam emitted from the light source passes through a mirror to the objective lens and is focused onto the sensor surface. The reflected light signal is then transmitted to a spectrometer connected to the microscope, enabling real-time acquisition of the reflection spectrum. Simultaneously, a continuous zoom lens is placed between the microscope and the camera to improve the imaging resolution and spatial sampling accuracy of the optical microscopy image, significantly enhancing these aspects. The diagram includes the following components: Optical Micrograph (optical micrograph), Beam Splitter (beam splitter), Objectives (objectives), Gas Chamber (gas chamber), Specimen (sample), Light Source (light source), Microscopy Spectrum (microscope spectrum), Lightpath diagram (optical path diagram), Optical Microscope (optical microscope), Spectrometer (spectrometer), Vacuum Pump (vacuum pump), and Reflectance Spectrum (reflectance spectrum).

[0029] Figure 3 In saturated xylene, the reflectance spectra of the response of the P(VBC-co-MMA)-P-SiO2 SHOPC sensor were measured under illumination using four different concentrations of probe molecules. Figure 3 Four concentrations of P(VBC-co-MMA)-tetrahydrofuran (THF) solutions were selected (etching time 18 hours): 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt%. Figure 3 The horizontal axis represents wavelength (unit: nanometers, nm), with scale values ​​including 450, 500, 550, 600, 650, 700, and 750. Figure 3 The vertical axis represents reflectance (unit: arbitrary unit, au). Figure 3 The figure includes sensor performance curves for four probe molecule concentrations, from top to bottom: 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt%. As shown in the figure, the optical bandgap shift of the sensor gradually increases with increasing probe molecule concentration from 1.0 wt% to 2.0 wt%. Under the influence of saturated xylene gas, the reflectance spectra of these sensors show a significant wavelength shift. When the concentration of the immersed polymer solution is further increased to 2.5 wt%, the optical bandgap shift decreases. A larger bandgap shift indicates better sensor response performance. The P(VBC-co-MMA)-P-SiO2 SHOPC sensor prepared using a 2.0 wt% P(VBC-co-MMA)-THF solution exhibits the largest optical bandgap redshift of 37 nm, demonstrating the best performance.

[0030] Figure 4 The reflectance spectrum of the response of a P(VBC-co-MMA)-P-SiO2 SHOPC sensor containing 2.0 wt% probe was measured under illumination in saturated xylene at four etching times. Figure 4 Four etching times for P-SiO2 SHOPC were selected: 12 h, 14 h, 16 h and 18 h. Figure 4 The horizontal axis represents wavelength (unit: nanometers, nm), with scale values ​​including 450, 500, 550, 600, 650, 700, and 750. Figure 3 The vertical axis represents reflectance (unit: arbitrary unit, au). Figure 3 The figure includes sensor performance curves for four etching times, from top to bottom: 12 h, 14 h, 16 h, and 18 h. As shown in the figure, the corresponding cutoff band redshift gradually increases with the etching time from 12 h to 18 h. When the etching time is further extended to 20 h, the P-SiO2 SHOPC becomes very fragile and unsuitable for sensing measurements. A larger stopband shift results in better sensor response performance. The P(VBC-co-MMA)-P-SiO2 SHOPC sensor prepared using P-SiO2 SHOPC with an etching time of 18 h exhibits the largest cutoff band redshift of 37 nm, demonstrating the best performance.

[0031] Figure 5 This is a comparison of the stopband displacement of a porous hollow spherical photonic crystal sensor exposed to seven different saturated solvent gases. Figure 5 The vertical axis represents the stopband shift (nm stands for nanometer), with scales including 0, 5, 10, 15, 20, 25, 30, 35, 40, and 45. Figure 5The response performance of the P(VBC-co-MMA)-P-SiO2 SHOPC sensor to seven saturated solvent gases was studied. The reagents, from left to right, are ultrapure water, acrylic acid, methanol, ethanol, N-propanol, acetonitrile, and xylene. To evaluate the selectivity of the P(VBC-co-MMA)-P-SiO2 SHOPC sensor to xylene, its optical band shift in different solvent saturated gases was recorded. As a control, the spectral response of the unmodified probe molecule P-SiO2 SHOPC was also tested. Both P(VBC-co-MMA)-P-SiO2 SHOPC and P-SiO2 SHOPC exhibited an optical band shift (redshift) when exposed to all test gases. A significant optical band shift of approximately 37 nm was observed when P(VBC-co-MMA)-P-SiO2 SHOPC was exposed to xylene gas. In contrast, the optical stopband redshifts induced by other solvents were all less than 25 nm and were independent of the presence or absence of the P(VBC-co-MMA) modification layer. These results indicate that the sensor exhibits a selective response to xylene. Detailed Implementation

[0032] The following is in conjunction with the attached diagram ( Figures 1-5 The present invention will be described in conjunction with the embodiments.

[0033] Figure 1 This is a diagram illustrating the fabrication process and sensing mechanism of a porous hollow spherical photonic crystal sensor for detecting xylene gas, which is an embodiment of the present invention. Figure 2 This is a diagram of the detection device and internal optical path of a porous hollow spherical photonic crystal sensor for detecting xylene gas according to the present invention. Figure 3 In saturated xylene, the reflectance spectra of the response of the P(VBC-co-MMA)-P-SiO2 SHOPC sensor were measured under illumination using four different concentrations of probe molecules. Figure 4 The reflectance spectrum of the response of a P(VBC-co-MMA)-P-SiO2 SHOPC sensor containing 2.0 wt% probe was measured under illumination in saturated xylene at four etching times. Figure 5 This is a comparison of the stopband shifts of a porous hollow spherical photonic crystal sensor exposed to seven different saturated solvent gases. (Reference) Figures 1 to 5As shown, a porous hollow spherical photonic crystal sensor for detecting xylene gas includes porous spherical hollow photonic crystal beads and a polymer recognition layer functionalized on their surface. The polystyrene@silica (PS@SiO2) core-shell structured colloidal particles, the spherical photonic crystal obtained by microfluidic assembly, the hollow silica structural unit formed by calcination to remove the polystyrene core, the hierarchical porosity introduced by controllable chemical etching, and the poly(vinylbenzyl chloride-co-methyl methacrylate) [P(VBC-co-MMA)] functionalizing the spherical photonic crystal beads... The sensor utilizes a porous photonic crystal framework to achieve selective adsorption. Thanks to its supersymmetric spherical structure, it exhibits an inherent angle-independent optical response. When exposed to xylene gas, the gas diffuses and adsorbs within the pores, causing an increase in the effective refractive index. This results in a significant and reversible redshift of the optical bandgap, accompanied by a noticeable color change. Combining a porous hollow structure with polymer-assisted molecular recognition, the sensor offers rapid response, good selectivity, and excellent reusability. Sensitive and visual detection of xylene is achieved based on the shift in the optical bandgap.

[0034] The spherical photonic crystal is formed from PS@SiO2 monodisperse droplets prepared by microfluidic method, and the microparticles in the spherical photonic crystal are hollow SiO2.

[0035] The porous structure was obtained by etching a hollow spherical photonic crystal in an aqueous solution of sodium carbonate and hexadecyltrimethylammonium bromide (CTAB) for 12 h, 14 h, 16 h and 18 h.

[0036] The polymer P (VBC-co-MMA) that can functionalize porous hollow spherical photonic crystals is polymerized from vinyl benzyl chloride (VBC) and polymethyl methacrylate (MMA).

[0037] The porous hollow spherical photonic crystal sensor [P(VBC-co-MMA)-P-SiO2 SHOPC] is made by immersing a porous hollow spherical photonic crystal in a tetrahydrofuran (THF) solution containing P(VBC-co-MMA), resulting in P(VBC-co-MMA)-P-SiO2 SHOPCs with mass fractions of 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt%, respectively.

[0038] A method for fabricating a porous hollow spherical photonic crystal sensor for detecting xylene gas, characterized by comprising the following steps:

[0039] Step A: Preparation of P(VBC-co-MMA) probe molecules. These probe molecules were synthesized by free radical polymerization of vinyl benzyl chloride (VBC) and polymethyl methacrylate (MMA). Using 4.5 mL of VBC and 2.7 mL of MMA as monomers and 30.3 mg of azobisisobutyronitrile (AIBN) as an initiator, the reaction was carried out in 60 mL of THF solution for 48 h. After the reaction, the mixture was stirred in 400 mL of chilled petroleum ether for 2 h, resulting in an insoluble white precipitate. This precipitate was then vacuum-dried overnight to obtain the P(VBC-co-MMA) probe molecules.

[0040] Step B: Using P(VBC-co-MMA) as solute and THF as solvent, P(VBC-co-MMA)-THF solutions with mass concentrations of 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt% were prepared respectively. The porous hollow spherical photonic crystal was immersed in the solutions of different mass concentrations for 1 h and then removed to obtain four probe concentrations of P(VBC-co-MMA)-P-SiO2SHOPC sensors for detecting xylene.

[0041] The preparation of the porous hollow spherical photonic crystal in step B includes the following steps: Step B1, synthesis of PS colloidal particles: Under a nitrogen atmosphere, polystyrene, polyvinylpyrrolidone (PVP), 2,2'-azobisisobutylamidine dihydrochloride (AIBA), and deionized water are mixed and reacted in a water bath under stirring. After the reaction, the mixture is centrifuged and washed to prepare a polystyrene colloidal solution containing PVP; Step B2, synthesis of PS@SiO2 colloidal particles: An ethanol solution of polystyrene, ammonia, and tetraethyl orthosilicate (TEOS) are mixed and reacted in a water bath under magnetic stirring. After the reaction, the mixture is centrifuged and washed to prepare a PS@SiO2 colloidal solution; Step B3, preparation of SiO2 hollow photonic crystals: Dimethyl silicone oil and methyl fluorosilicone oil are used as the continuous phase, and PS@SiO2 dispersion is used as the dispersed phase. Monodisperse emulsion droplets generated in a microfluidic device are collected under stable flow conditions. Then, the water is evaporated in an oven, washed with n-hexane, and calcined in a muffle furnace to obtain a SiO2 hollow photonic crystal (SiO2 SHOPC); Step B4, preparation of SiO2 porous hollow spherical photonic crystal: hexadecyltrimethylammonium bromide aqueous solution (CTAB) and anhydrous sodium carbonate are mixed and stirred, SiO2 SHOPC is added to it, and the mixture is allowed to stand and etched to obtain a SiO2 porous hollow spherical photonic crystal (P-SiO2 SHOPC).

[0042] A method for selectively detecting xylene gas using a porous hollow spherical photonic crystal sensor is characterized by the following steps: Step 1, constructing an optical microscope setup for measuring the reflectance spectrum of the spherical photonic crystal. A spectrometer is inserted above the eyepiece of the optical microscope and connected to an optical fiber. Reflectance spectrum data is transmitted via the optical fiber. Real-time reflectance spectrum data is generated from the portion of the reflected light that passes through the spectrometer and enters the spectrometer via the optical fiber. An industrial camera is installed above the spectrometer and connected to a camera sensor to simultaneously detect the OM (Optical Oscillation) image and the reflectance spectrum. Step 2, placing the sensor in a sealed gas chamber equipped with micropores. This gas chamber is placed on the stage of the optical microscope, and a certain volume of volatile solvent is injected through the micropores using a microsyringe. When the sample is exposed to the solvent gas environment, the OM image and the changes in the optical bandgap of the reflectance spectrum are obtained and recorded.

[0043] A functionalized porous hollow spherical photonic crystal sensor for selectively detecting xylene gas includes porous spherical hollow photonic crystal beads and a polymer recognition layer on their surface. The beads consist of polystyrene@silica (PS@SiO2) core-shell colloidal particles, a microfluidically assembled spherical photonic crystal, and hollow silica structural units formed by calcination to remove the polystyrene core. Controlled chemical etching introduces hierarchical pores. The P(VBC-co-MMA) can specifically adsorb SiO2. The photonic crystal has a supersymmetric spherical structure, and the sensor exhibits an inherent angle-independent optical response. When exposed to xylene gas, because xylene has a higher refractive index than air, the gas diffuses and adsorbs within the pores, causing an increase in the effective refractive index. This results in a significant and reversible redshift of the optical bandgap, accompanied by a noticeable color change. Sensitive and visual detection of xylene is achieved by detecting this optical bandgap shift.

[0044] The spherical photonic crystal is formed from PS@SiO2 monodisperse droplets prepared by microfluidic method, and the microparticles in the spherical photonic crystal are hollow SiO2.

[0045] The porous structure was obtained by etching a hollow spherical photonic crystal in an aqueous solution of sodium carbonate and hexadecyltrimethylammonium bromide (CTAB) for 12 h, 14 h, 16 h and 18 h.

[0046] The polymer P (VBC-co-MMA) that can functionalize porous hollow spherical photonic crystals is polymerized from vinyl benzyl chloride (VBC) and polymethyl methacrylate (MMA).

[0047] The porous hollow spherical photonic crystal sensor [P(VBC-co-MMA)-P-SiO2 SHOPC] is made by immersing a porous hollow spherical photonic crystal in a tetrahydrofuran (THF) solution containing P(VBC-co-MMA), resulting in P(VBC-co-MMA)-P-SiO2 SHOPCs with mass fractions of 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt%, respectively.

[0048] A method for fabricating a porous hollow spherical photonic crystal sensor for detecting xylene gas includes the following steps:

[0049] Step A: Preparation of P(VBC-co-MMA) probe molecules, which are synthesized by free radical polymerization of VBC and MMA. 30.3 mg of AIBN was added to a 250 mL three-necked round-bottom flask containing 4.5 mL of VBC, 2.7 mL of MMA, and 60 mL of THF solution. Cooling water was circulated through a condenser. A magnetic stirrer was added to the mixture, and nitrogen gas was introduced for 30 min. The apparatus was sealed, and the reaction was carried out at 62 °C under a nitrogen atmosphere for 48 h. After the reaction was complete, the resulting mixture was added in portions to 400 mL of frozen petroleum ether under vigorous stirring, forming an insoluble white precipitate. After stirring for 2 h, the white solid was collected by vacuum filtration, washed five times with petroleum ether, and vacuum dried overnight at 50 °C in a vacuum oven to obtain the P(VBC-co-MMA) probe molecules.

[0050] Step B: Using P(VBC-co-MMA) as solute and THF as solvent, P(VBC-co-MMA)-THF solutions with mass concentrations of 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt% were prepared. The solutions were shaken for 30 min to ensure uniform mixing. The porous hollow spherical photonic crystals were then immersed in the solutions of different mass concentrations for 1 h and removed. After drying, four P(VBC-co-MMA)-P-SiO2 SHOPC sensors for detecting xylene with different probe concentrations were obtained.

[0051] The preparation of the porous hollow spherical photonic crystal in step B includes the following steps:

[0052] Step B1, Synthesis of PS Colloidal Particles: Under a nitrogen atmosphere, 8 mL of styrene, 1.5 g of polyvinylpyrrolidone (PVP), and 80 mL of deionized water were added to a three-necked round-bottom flask. The mixture was stirred at 300 r / min and heated to 75 °C. 0.1450 g of AIBA was dissolved in 20 mL of deionized water and magnetically stirred for 30 min until completely dissolved. When the reaction temperature reached 75 °C, the AIBA solution was slowly injected into the reaction system using a syringe to initiate the polymerization reaction. The reaction was carried out at 75 °C for 5 h. After centrifugation and washing, a solution of PS colloidal particles containing PVP was obtained.

[0053] Step B2, synthesis of PS@SiO2 colloidal particles: 50 mL of a 0.5 wt% PS dispersion in ethanol was transferred to an Erlenmeyer flask, 4.3 mL of ammonia and 1.5 mL of TEOS were added, and the mixture was reacted at 50 °C for 3 h. After centrifugation and washing, a PS@SiO2 solution was obtained.

[0054] Step B3, Preparation of SiO2 hollow photonic crystals: Using a mixed oil phase composed of dimethyl silicone oil and methyl fluorosilicone oil (15:2, volume ratio) as the continuous phase, and 15 wt% PS@SiO2 dispersion as the dispersed phase, under stable flow conditions, an injection pump was used to inject the mixture at 200 µL / h. -1 and 3000 µL h -1 The dispersed and continuous phases were injected at a flow rate of [missing information - likely a flow rate], and the resulting emulsion droplets in the microfluidic device were collected in polytetrafluoroethylene (PTFE) petri dishes to obtain a monodisperse emulsion with uniform droplet size. The petri dishes were then placed in an oven at 75 °C to evaporate moisture, yielding silicone oil-coated PS@SiO2 superspheres. The obtained superspheres were washed with n-hexane at least three times to remove residual silicone oil, and then calcined in a muffle furnace at 650 °C for 3 h to obtain SiO2 SHOPCs with enhanced mechanical strength.

[0055] Step B4, Preparation of SiO2 porous hollow spherical photonic crystal: Dissolve 0.3125 g CTAB and 1.06 g anhydrous sodium carbonate in 50 mL deionized water and stir magnetically for 30 min. After removing the stirring rod, add SiO2 SHOPC and etch at 35℃ for 12 h, 14 h, 16 h and 18 h respectively to obtain P-SiO2 SHOPC.

[0056] A method for selectively detecting xylene gas using a porous hollow spherical photonic crystal sensor includes the following steps:

[0057] Step 1: Construct an optical microscope setup for measuring the reflection spectrum of a spherical photonic crystal. A spectrometer is inserted above the eyepiece of the optical microscope and connected to an optical fiber. Light reflected from the sample under the optical microscope is transmitted to the spectrometer via the fiber, thus transmitting the reflection spectrum data. To prevent overexposure of the OM image from the microscope, an industrial camera is mounted above the spectrometer. By adjusting the working distance of the lens, the additional optical path introduced by the spectrometer is compensated, and the focal plane of the eyepiece is again matched with the camera sensor. After connecting the camera sensor, the OM image is clearly recovered, meeting the requirement of real-time acquisition of the sample's OM image. The portion of light that passes through the spectrometer and enters the spectrometer via the optical fiber generates real-time reflection spectrum data, thereby achieving real-time detection.

[0058] Step 2: Place the sensor in a sealed gas chamber equipped with micropores. The gas chamber is placed on the stage of an optical microscope. A certain volume of volatile solvent is injected through the micropores using micro-injectors of 10 µL, 20 µL, 50 µL, and 100 µL. When the sample is exposed to the solvent gas environment, the changes in the OM diagram and the optical band of the reflectance spectrum are obtained and recorded.

[0059] This invention discloses a method for preparing a novel sensor for detecting xylene gas. First, a polystyrene@silica core-shell colloid is synthesized. Then, the polymer core is removed to form hollow silica structural units. These structural units are then assembled into spherical photonic crystal beads using microfluidic technology. Hierarchical pores are introduced into the spheres through controlled chemical etching to prepare porous spherical hollow photonic crystal beads. Finally, the photonic crystal beads are functionalized with poly(vinylbenzyl chloride-co-methyl methacrylate) [P(VBC-co-MMA)] to obtain a polymer-functionalized spherical hollow photonic crystal sensor. The sensor prepared by this invention exhibits excellent sensitivity and selectivity, rapid response, and good angle-independent optical performance. The spherical hollow photonic crystal sensor prepared by this invention is recyclable.

[0060] The inventive concept of this invention is to utilize the high affinity and selective adsorption capacity of poly(vinylbenzyl chloride-co-methyl methacrylate) [P(VBC-co-MMA)] for xylene molecules, functionalizing it onto porous spherical hollow photonic crystal beads to obtain a novel angle-independent sensor, thereby achieving selective detection of xylene gas. When the sensor is exposed to xylene gas, xylene molecules are selectively adsorbed and diffused into the porous framework of the photonic crystal bead with polymer assistance. Since the refractive index of xylene is higher than that of air, the effective refractive index of the porous photonic crystal structure increases. This change in effective refractive index induces a significant and reversible redshift in the g-optical bandgap, accompanied by a noticeable color change. By constructing a microspectroscopy-coupled system, the shift in the peak wavelength of the reflectance spectrum of the sensor before and after the presence of xylene gas can be monitored in real time, enabling sensitive, rapid, and visual detection of xylene gas concentration. Thanks to its supersymmetric spherical structure, this sensor fundamentally overcomes the angle-dependent limitations of traditional planar photonic crystal sensors, providing a universal and reusable sensing design for the detection of harmful volatile organic compounds.

[0061] A porous hollow spherical photonic crystal sensor for detecting xylene gas includes a spherical photonic crystal composed of hollow silica particles. The spherical photonic crystal has a porous structure formed by etching. The outer spherical surface, the interstitial surface, and the pore wall surface of the spherical photonic crystal are all covered with a polymer recognition layer sensitive to xylene gas. The polymer recognition layer is poly(vinylbenzyl chloride-co-methyl methacrylate) P(VBC-co-MMA). When the porous hollow spherical photonic crystal is exposed to xylene gas, the gas diffuses and is adsorbed within the pores, causing an increase in the effective refractive index. This leads to a significant and reversible redshift of the optical bandgap, accompanied by a noticeable color change. Sensitive and visual detection of xylene is achieved based on the shift of the optical bandgap.

[0062] The hollow silica particles are made by calcining PS@SiO2 to remove the polystyrene core. The spherical photonic crystal is formed by calcining PS@SiO2 monodisperse droplets prepared by microfluidic method to remove water from the droplets and the polystyrene core in PS@SiO2. The etching is performed by controlled chemical etching to introduce hierarchical pores.

[0063] A porous hollow spherical photonic crystal sensor and method for detecting xylene gas are disclosed, comprising the synthesis of PS@SiO2 core-shell colloids, microfluidic assembly into spherical photonic crystals, calcination to remove the polystyrene core to form a hollow SiO2 structure, and controllable chemical etching to introduce a porous structure. Subsequently, functionalization with P(VBC-co-MMA) yields a porous hollow spherical photonic crystal sensor, endowing it with a strong molecular affinity for xylene, enabling selective gas adsorption by the porous photonic crystal. Thanks to its supersymmetric spherical structure, the resulting photonic crystal sensor exhibits an inherent angle-independent optical response, overcoming the angle-dependent limitation of thin-film photonic crystal sensors. Exposure to xylene gas manifests as a significant reversible redshift of the optical bandgap and a noticeable color change, stemming from the increase in effective refractive index due to vapor diffusion and adsorption. Visual detection of xylene gas can be achieved by monitoring the shift of the optical bandgap in air and saturated xylene.

[0064] Contents not described in detail in this specification are prior art known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand this invention, but does not limit the scope of protection of this invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essential content of this invention fall within the scope of protection of this invention.

Claims

1. A porous hollow spherical photonic crystal sensor for detecting xylene gas, characterized in that, The invention comprises a spherical photonic crystal composed of hollow silica particles. The spherical photonic crystal has a porous structure formed by etching. The outer spherical surface, the interstitial surface, and the pore wall surface of the spherical photonic crystal are all covered with a polymer recognition layer sensitive to xylene gas. The polymer recognition layer is poly(vinylbenzyl chloride-co-methyl methacrylate) P(VBC-co-MMA). When the porous hollow spherical photonic crystal is exposed to xylene gas, the gas diffuses and is adsorbed within the pores, causing an increase in the effective refractive index. This leads to a significant and reversible redshift of the optical bandgap, accompanied by a noticeable color change. Sensitive visual detection of xylene is achieved based on the shift of the optical bandgap.

2. The porous hollow spherical photonic crystal sensor for detecting xylene gas according to claim 1, characterized in that, The hollow silica particles are made by calcining PS@SiO2 to remove the polystyrene core. The spherical photonic crystal is formed by calcining PS@SiO2 monodisperse droplets prepared by microfluidic method to remove water from the droplets and the polystyrene core in PS@SiO2. The etching is performed by controlled chemical etching to introduce hierarchical pores.

3. The porous hollow spherical photonic crystal sensor for detecting xylene gas according to claim 1, characterized in that, The porous structure was obtained by etching a hollow spherical photonic crystal in an aqueous solution of sodium carbonate and hexadecyltrimethylammonium bromide (CTAB) for 12 h, 14 h, 16 h and 18 h.

4. The porous hollow spherical photonic crystal sensor for detecting xylene gas according to claim 1, characterized in that, The P(VBC-co-MMA) is polymerized from vinyl benzyl chloride (VBC) and polymethyl methacrylate (MMA).

5. The porous hollow spherical photonic crystal sensor for detecting xylene gas according to claim 1, characterized in that, The porous hollow spherical photonic crystal sensor is named P(VBC-co-MMA)-P-SiO2 SHOPC, which contains the following chemical formula. The polymer recognition layer is formed by immersing the sensor in a tetrahydrofuran (THF) solution containing P(VBC-co-MMA). The mass fractions of P(VBC-co-MMA) in the P(VBC-co-MMA)-P-SiO2 SHOPC are 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt%.

6. A method for fabricating a porous hollow spherical photonic crystal sensor for detecting xylene gas as described in any one of claims 1-5, characterized in that, Includes the following steps: Step A: Prepare P(VBC-co-MMA) probe molecules. The probe molecules are formed by free radical polymerization of vinyl benzyl chloride (VBC) and polymethyl methacrylate (MMA). Using 4.5 mL of VBC and 2.7 mL of MMA as monomers and 30.3 mg of azobisisobutyronitrile (AIBN) as initiator, the reaction is carried out in 60 mL of tetrahydrofuran (THF) solution for 48 h. Then, the resulting mixture is stirred in 400 mL of frozen petroleum ether for 2 h to obtain an insoluble white precipitate, which is dried under vacuum overnight to obtain P(VBC-co-MMA) probe molecules. Step B: Using P(VBC-co-MMA) as the solute and tetrahydrofuran (THF) as the solvent, P(VBC-co-MMA)-THF solutions with mass concentrations of 1.0 wt%, 1.5 wt%, 2.0 wt%, and 2.5 wt% were prepared respectively. The porous hollow spherical photonic crystal was immersed in the solutions of different mass concentrations for 1 h and then removed to obtain four probe concentrations of P(VBC-co-MMA)-P-SiO2 SHOPC sensors for detecting xylene.

7. The method for preparing a porous hollow spherical photonic crystal sensor for detecting xylene gas according to claim 6, characterized in that, Step B includes the following steps: Step B1, Synthesis of polystyrene (PS) colloidal particles: Under a nitrogen atmosphere, polystyrene, polyvinylpyrrolidone (PVP), 2,2'-azobisisobutylamidine dihydrochloride (AIBA), and deionized water are mixed and reacted in a water bath under stirring. After the reaction, the polystyrene colloidal solution containing PVP is prepared by centrifugation and washing. Step B2, Synthesis of PS@SiO2 Colloidal Particles: An ethanol solution of polystyrene, ammonia water and tetraethyl orthosilicate (TEOS) are mixed and reacted in a water bath under magnetic stirring. After the reaction, the PS@SiO2 colloidal solution is prepared by centrifugation and washing. Step B3, Preparation of SiO2 hollow photonic crystals: Using dimethyl silicone oil and methyl fluorosilicone oil as the continuous phase and PS@SiO2 dispersion as the dispersed phase, monodisperse emulsion droplets generated in a microfluidic device were collected under stable flow conditions. The droplets were then placed in an oven to evaporate moisture, washed with n-hexane, and calcined in a muffle furnace to obtain SiO2 hollow photonic crystals (SiO2SHOPCs). Step B4, Preparation of SiO2 porous hollow spherical photonic crystal: Mix hexadecyltrimethylammonium bromide aqueous solution CTAB with anhydrous sodium carbonate, add SiO2 SHOPCs into the mixture, allow to stand and etch to obtain SiO2 porous hollow spherical photonic crystal P-SiO2SHOPC.

8. A method for selectively detecting xylene gas using a porous hollow spherical photonic crystal sensor as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Construct an optical microscope setup for measuring the reflection spectrum of a spherical photonic crystal. Insert a spectrometer above the eyepiece of the optical microscope and connect the spectrometer to an optical fiber. Transmit the reflection spectrum data through the optical fiber. The portion of the reflected light that passes through the spectrometer and then enters the spectrometer through the optical fiber can generate real-time reflection spectrum data. Install an industrial camera above the spectrometer and connect a camera sensor to achieve simultaneous detection of the optical microscopic image (OM image) and the reflection spectrum. Step 2: Place the sensor in a sealed gas chamber equipped with micropores. The gas chamber is placed on the stage of an optical microscope, and a certain volume of volatile solvent is injected through the micropores using a microsyringe. When the sample is exposed to the solvent gas environment, the changes in the OM diagram and the optical band of the reflectance spectrum are obtained and recorded.

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