Application of PS@Ag@ZIF-8 composite film in preparation of optical sensor

Abstract

The application belongs to the technical field of environmental protection, and particularly relates to application of a PS@Ag@ZIF-8 composite film in preparation of an optical sensor. The application takes a single-layer colloidal crystal sample as a framework, and prepares PS@Ag@ZIF-8 which is loaded with silver AgNPs on the surface and coated with ultrathin MOFs, and the PS@Ag@ZIF-8 can be used as a high-efficiency optical sensor device. The application increases the refractive index difference of the ordered array, increases the p-v ratio of the film, and improves the color saturation by means of the ingenious design of the AgNPs. The ultrathin two-dimensional structure is combined with the AgNPs, and high-efficiency optical signal conversion and color output are realized for VOCs.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection technology, specifically relating to the application of PS@Ag@ZIF-8 composite films in the preparation of optical sensors. Background Technology

[0002] In recent years, with the continuous advancement of industrialization and the rapid development of the social economy, the number of motor vehicles has continued to grow rapidly, and the consumption of gasoline and diesel fuel has also increased significantly, resulting in a year-on-year increase in the amount of volatile organic compounds (VOCs). VOCs are characterized by low concentrations but high toxicity. They have a strong toxic effect on the central nervous system, can reduce human immunity, and may even cause cancer. Therefore, effective monitoring of VOC pollution has attracted great attention from researchers. Traditional chemical sensors mainly rely on substances such as semiconductors or polymers, which undergo changes in optical, electrical, or mechanical properties when absorbing or reacting with target VOCs. In recent years, metal-organic frameworks (MOFs), which are formed by the self-assembly of organic ligands and metal ions or metal clusters through coordination bonds, have been widely used in gas storage and separation, catalysis, and sensing due to their advantages such as rich and diverse pore structures, simple synthetic routes, and large specific surface areas. Furthermore, due to their unique pore structure, high porosity, and tunable framework structure, MOFs can more effectively adsorb or enrich analytes in the environment, making them highly selective and sensitive chemical sensing materials. MOFs also possess a certain degree of stability and can undergo reversible adsorption; therefore, many researchers are dedicated to developing MOF-based sensors. Effectively converting the type and concentration of the analyte into a readable signal is a crucial step in realizing MOF sensors. Currently, relevant research has utilized different mechanisms to achieve signal conversion, such as electrochemical methods, mass change methods, and optical methods. Most MOF sensors are based on fluorescent MOFs, but this method suffers from signal instability due to specific quenching. Sensors based on other non-fluorescent MOFs mainly achieve signal conversion through electrical, mechanical, and optical principles. These methods generally require concentrating the MOFs in thin films within a device capable of providing signal conversion. Although electrical methods are widely used in sensors, this approach is limited to specific conductive MOFs, while most MOFs are insulating. Mechanical methods, such as those using quartz crystal microbalances (QCM), surface acoustic waves, and microcantilever structures, achieve good sensitivity by detecting changes in the mass of MOFs before and after analysis. However, these devices are complex and unsuitable for portable and miniaturized applications. In contrast, MOF sensors utilizing external optical elements and employing optical transmission modes that do not involve light absorption or emission by MOFs, enabling in-situ and visual monitoring through color changes, represent an ideal approach.

[0003] To date, some teams have developed optically responsive devices by utilizing the effective adsorption of analytes, leading to changes in refractive index. One approach is based on localized surface plasmon resonance spectroscopy, loading MOFs onto refractive index-sensitive plasmon nanoparticles. Another approach involves Hupp's team's micrometer-scale ZIF-8 thin film (G. Lu, JTHupp, Metal-Organic Frameworks as Sensors: A ZIF-8 Based Fabry-Perot Device as a Selective Sensor for Chemical Vapors and Gases, J. Am. Chem. Soc. 2010, 132, 7832-7833.), which, due to the Fabry-Pérot principle, exhibits weak differentiation of different VOCs in the visible light region. Furthermore, there are reports of modulating the photonic bandgap in photonic crystals by changing the refractive index and converting it into optical signals. Nevertheless, developing optical sensors with both high performance and compatibility with practical applications of MOFs remains a significant challenge.

[0004] For optical sensors, sensitivity is a crucial factor in evaluating performance. For a given concentration of analyte, a thicker MOF film requires the adsorption of more material to cause a change in refractive index, which introduces a new problem: such structures slow down the sensor's response time. Therefore, constructing thinner MOF structures allows for efficient transmission of optical sensing signals without reducing device sensitivity. Furthermore, shifts in the reflectance spectrum can be effectively reflected in colorimetric sensing. This is particularly important for real-time monitoring of analyte color changes when there is only one reflection peak in the visible light region. Generally, maximizing the pv ratio (the ratio of reflection peak to reflection valley) of the film yields colorimetric detection signals with higher color saturation, which is difficult to achieve when using MOF sensors alone. Summary of the Invention

[0005] This invention fabricates PS@Ag@ZIF-8 based on Ag NPs loaded on an MCC surface and coated with ultrathin MOFs using an in-situ growth method, which is then used as an optical sensor. The invention utilizes the ingenious design of the Ag NPs to increase the refractive index difference of the ordered array, thereby increasing the pv ratio of the film and improving color saturation. The combination of the ultrathin two-dimensional structure and Ag NPs achieves high-efficiency optical signal conversion and color output for VOCs.

[0006] The present invention specifically adopts the following solution:

[0007] The application of PS@Ag@ZIF-8 composite films in the fabrication of optical sensors, specifically, the sensors are used to detect organic compounds, such as VOCs.

[0008] The method for fabricating the aforementioned optical sensor includes:

[0009] Step 1: Submicron-sized polystyrene microspheres (PS) are prepared by emulsion-free polymerization; a PS solution uniformly dispersed in water and ethanol is prepared, and then a brightly colored monolayer colloidal crystal (MCC) sample is obtained by gas-liquid interface self-assembly. The MCC is dried in a drying oven at 80°C for 24 hours to increase the adhesion between the PS microspheres and the substrate.

[0010] The PS particle size distribution in the PS solution is less than 7%. The silicon wafer is a single-sided polished silicon wafer.

[0011] Step 2: Process the dried MCC sample to obtain non-close-packed MCC.

[0012] A silver nitrate (AgNO3) solution was prepared using an equal volume ratio of methanol and water. The silver nitrate solution was stirred at room temperature, and then triethylamine solution was added and stirred again to form a silver growth solution. Two-thirds of a non-close-packed MCC substrate was vertically immersed in the silver growth solution and grown in situ at room temperature in the dark for 1-6 hours. Afterward, the sample was removed, rinsed with methanol, and dried with N2 to obtain the PS@Ag substrate.

[0013] The ratio of silver nitrate, water, and methanol used was 0.0848 g: 25 mL: 25 mL. The preferred in-situ growth time in the dark was 3 hours. The preparation method of the non-close-packed MCC was as follows: the dried MCC was treated with O2 plasma for 1 minute in a 200 L / min O2 stream using a 150 W plasma cleaner to obtain the non-close-packed MCC.

[0014] Step 3: Vertically immerse the PS@Ag substrate in a fresh mixed methanol solution of Zn(NO3)2 and 2-methylimidazole, and grow vertically (two-thirds of the substrate is immersed in the solution). Maintain at room temperature for 10 minutes, then remove the substrate from the growth solution, rinse with plenty of methanol, and dry with N2 gas. Repeat the growth cycle multiple times to obtain the PS@Ag@ZIF-8 composite film.

[0015] The molar ratio of Zn(NO3)2 to 2-methylimidazole in the Zn(NO3)2 and 2-methylimidazole methanol solution is 1:2; the growth conditions are: growth once every 10 minutes at room temperature. Preferably, the number of growth cycles is 4.

[0016] Compared with the prior art, the beneficial effects of the present invention are:

[0017] This invention utilizes MCC as a framework to fabricate PS@Ag@ZIF-8, a substrate containing silver-loaded Ag NPs and coated with ultrathin MOFs. The PS@Ag@ZIF-8 can serve as a high-efficiency optical sensor. This sensor converts changes in effective refractive index under different vapor environments into a highly efficient and readable signal for output. The sensor exhibits superior selectivity for acetonitrile and alcohols, and due to the cleverly designed Ag NPs, it can also obtain colorimetric detection reports with more pronounced color differences. The device has an ultrafast response speed (<4s), good linear response over a wide dynamic vapor concentration range, and excellent cycling performance. The device's sensitivity and color saturation are closely related to the MOF film thickness and the Ag NP loading. This device allows analytes to rapidly diffuse and adsorb into the pores, achieving efficient signal conversion.

[0018] This invention enriches the research on the application of sensors based on the combination of MCC, Ag NPs and MOFs in VOCs detection. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the fabrication process of the PS@Ag@ZIF-8 optical sensor.

[0020] Figure 2 SEM images of an MCC assembled from PS with a diameter of 520 nm before and after oxygen plasma etching: before oxygen plasma etching (a, b); top view (c) and side view (d) after 1 min of oxygen plasma etching.

[0021] Figure 3 TEM images (300 nm and 100 nm) of PS@Ag at different growth times of 1 h (a, d), 3 h (b, e) and 6 h (c, f).

[0022] Figure 4 SEM images of PS@Ag at different growth times: 1h (a, d), 3h (b, e), and 6h (c, f).

[0023] Figure 5 TEM images (200 nm) (a, e), elemental distribution maps (b, c, d), and HRTEM images (f, g) of PS@Ag grown with 3hAg.

[0024] Figure 6 SEM images of PS@Ag@ZIF-8 at different growth times of Ag: 3h (a, b) and 6h (c, d).

[0025] Figure 7 Theoretical simulation XRD results for PS@Ag-3, PS@Ag-3@ZIF-8, PS@Ag-6@ZIF-8 ordered arrays and ZIF-8.

[0026] Figure 8 The reflectance spectra and optical photographs of ZIF-8 grown in PS@Ag-3@ZIF-8 at different growth times (0, 1, 2, 3, 4 times) are shown (the inserted optical photographs correspond to the ZIF-8 growth times of 0, 1, 2, 3, 4 times from left to right).

[0027] Figure 9 Reflectance spectra and optical photographs of PS@ZIF-8 and PS@Ag-3@ZIF-8 grown for different numbers of ZIF-8 cycles (a: 3 cycles; b: 4 cycles).

[0028] Figure 10 TEM image (a) and N2 adsorption-desorption isotherm (77K) (b) of ZIF-8 nanoparticles.

[0029] Figure 11 Optical responses of PS@Ag-3@ZIF-8 to various vapors: reflectance spectra (a); shift of the reflectance peak (b); optical photographs of exposure to different vapor conditions (c: from left to right: water, tert-amyl alcohol, ethanol, and acetonitrile).

[0030] Figure 12 Optical response of PS@Ag-3@ZIF-8 thin film under acetonitrile atmosphere of different concentrations: reflectance spectrum (a); spectral shift of the reflectance peak (b) (original position at 561 nm).

[0031] Figure 13 The dynamic response of the PS@Ag-3@ZIF-8 film in a saturated acetonitrile environment is shown in (a), the reflection peak position is shown in (b) when alternately exposed to N2 and saturated acetonitrile vapor, and the stability of the PS@Ag-3@ZIF-8 film over time is shown in (c). Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments, but this does not limit the scope of protection of the present invention.

[0033] Example 1

[0034] 1. Test Methods

[0035] 1.1 Reagents and Raw Materials

[0036] Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) was purchased from Sigma-Aldric H.

[0037] 2-Methylimidazole (C4H6N) is from Aladdin.

[0038] Styrene (≥90%) was purchased from Alfaesa (China) Chemical Co., Ltd.

[0039] Methanol (CH3OH), ethanol (CH3CH2OH), sodium hydroxide (NaOH), sodium dodecyl sulfate (C 12 H 25 OSO3Na, potassium persulfate (K2S2O8), and silver nitrate (AgNO3) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

[0040] Acetonitrile (CH3CN) and n-propanol (CH3(CH2)2OH) were purchased from Macklin Inc. (China) Biochemical Technology Co., Ltd.

[0041] Triethylamine (C6H) 15 N) is provided by Tianjin Kemio Chemical Reagent Co., Ltd.

[0042] All chemicals in this embodiment are analytical grade and were used without any purification process.

[0043] The water used in the experiment was ultrapure water (≥18.2MΩ).

[0044] The silicon wafers were purchased from Beijing Ruihua Zhiyuan Co., Ltd.

[0045] 1.2 Characterization

[0046] The morphology of the samples was observed using transmission electron microscopy (TEM, JEM-1400) and scanning electron microscopy (SEM, JSM-7001F). The X-ray diffraction patterns of the samples were obtained using an X-ray diffractometer (XRD, Bruker D8, CuKα radiation). The N2 adsorption-desorption isotherms of the samples were obtained using a gas adsorption analyzer (ASAP2020HD88, USA) at 77 K, and the specific surface area was estimated using Barrett-Emmett-Teller (BET) theory. Optical spectra were acquired using a Marine Optics USB2000 fiber optic spectrophotometer and a Leica DM2700M optical microscope. Optical images were captured under white LED illumination using a Leica DFC450 digital color camera connected to the microscope and equipped with a 10x objective lens.

[0047] 1.3 Preparation process

[0048] (1) Synthesis of polystyrene microspheres (PS) and preparation of monolayer colloidal crystals (MCC)

[0049] First, PS with a particle size of 520 nm (particle size dispersion less than 7%) was prepared by emulsion-free polymerization according to the literature (B. T. Holland, C. C. Blanford, T. Do, A. Andreas, Synthesis of Highly Ordered, Three-Dimensional, Macroporous Structures of Amorphous or Crystalline Inorganic Oxides, Phosphates, and Hybrid Composites. Chem. Mater. 1999, 11(3), 795-805.).

[0050] Then, cut the silicon wafer (one side polished) into 1cm x 1cm square pieces, and use piranha solution (V 浓H2SO4 ∶V H2O2 Soak in a mixture of 7:3 for 3 hours, then rinse thoroughly with plenty of water and dry under N2 airflow for later use.

[0051] Next, the PS microemulsion was washed once each with anhydrous ethanol and ultrapure water by centrifugation at speeds of 2000 rpm / min and 3000 rpm / min, respectively, for 30 min each time. After each centrifugation, the supernatant was removed, and finally, the PS solution was dispersed in water and ethanol to obtain a uniformly dispersed PS solution (the ratio of PS, anhydrous ethanol, and ultrapure water was 0.1 g : 0.6 mL : 0.6 mL).

[0052] Then, an MCC sample on the silicon wafer is obtained through a gas-liquid interface self-assembly method.

[0053] (2) Preparation of PS@Ag thin films

[0054] The MCCs assembled on the silicon wafer were dried at 80°C for 24 hours to increase the adhesion between the PS microspheres and the substrate. Then, the dried MCCs were treated with O2 plasma for 1 minute in a 150W plasma cleaner at a flow rate of 200 L / min to obtain non-close-packed MCCs.

[0055] Add AgNO3 (10 mM, 2.5 mL) to methanol and aqueous solution (V) 甲醇 ∶V 水Add 5 ml of water and 5 ml of methanol sequentially to a 1:1 mixture, stir in the dark for 20 min, then add 0.5 mL of triethylamine solution (Caution! Triethylamine is toxic and highly irritating; take precautions when using it), stir in the dark for 5 min to form a silver growth solution. Then, vertically immerse two-thirds of the non-close-packed MCC substrate in the silver growth solution and allow it to grow in the dark for 1 h, 3 h, and 6 h. Afterward, remove the sample, rinse with methanol, and dry with nitrogen. Films with different Ag growth times are named PS@Ag-X (X represents a growth time of X h).

[0056] (3) Preparation of PS@Ag@ZIF-8 thin films

[0057] The PS@Ag-X substrate was vertically immersed in a fresh mixed methanol solution (4 mL) of Zn(NO3)2 (12.5 mM) and 2-methylimidazole (25 mM) and kept at room temperature for 10 min. It was then rinsed with a large amount of methanol and dried with an N2 stream. This series of steps was marked as one cycle. The process was repeated a different number of times to obtain ZIF-8 coatings of varying thicknesses, labeled as PS@Ag@ZIF-8 films.

[0058] (4) Synthesis of ZIF-8 nanoparticles

[0059] A methanol solution of Zn(NO3)2 (12.5 mM, 200 mL) was added to a methanol solution of 2-methylimidazole (25 mM, 200 mL). The mixture was kept at room temperature. After 30 min, the growth solution was collected, washed with plenty of methanol, centrifuged three times, and dried overnight in a vacuum oven at 60 °C for later use.

[0060] 1.4 Sensor Performance Test

[0061] Before testing, the PS@Ag@ZIF-8 sample was evacuated at room temperature to remove the solvent from ZIF-8. First, the sample was fixed in a self-made transparent quartz flow cell (1.0 cm × 4.0 cm × 1.0 cm), with a small cotton ball placed on its side. The flow cell was placed on the stage of an optical microscope. Then, 10 μL of VOCs analyte was dropped onto the cotton ball using a pipette, and the reflectance spectra in the visible light region before and after VOCs exposure were collected at 293 K. A series of acetonitrile vapors of different concentrations were obtained by passing them through a container containing the corresponding solvent at a flow rate of 100 mL / min at 293 K to obtain saturated vapor. The ratio of acetonitrile-saturated nitrogen and pure nitrogen gas flow was varied using two flow controllers, and the mixture was stirred in a stainless steel gas mixer. Finally, the mixed gas flow was introduced into the quartz flow cell, and the changes in reflectance spectra were recorded. The time resolution of the reflectance spectra was 100 ms.

[0062] 2. Results

[0063] 2.1 Characteristics of the PS@Ag@ZIF-8 ordered array structure constructed using two-dimensional colloidal crystals as templates

[0064] 2.1.1 Structural characteristics of monolayer colloidal crystals (MCC)

[0065] from Figure 2 As can be seen from ab, the PS microspheres with a diameter of 520nm have a uniform particle size distribution, and the self-assembled MCCs are arranged in a hexagonal close-packed manner on the silicon substrate, presenting a large area of ​​ordered single crystal domain regions.

[0066] To increase the hydrophilicity of the PS microsphere surface and the intersphere spacing, a non-close-packed ordered array was obtained through oxygen plasma etching. To ensure that the PS microspheres did not shift during etching, the assembled MCC was heat-treated at 80°C for 24 hours before etching to increase the adhesion between the PS microspheres and the substrate. Figure 2 c found that after 1 minute of oxygen plasma etching, the PS microsphere particle size decreased to 516 nm, and the intersphere spacing increased significantly. However, the PS microspheres still maintained good order, providing a good growth substrate for the next step of growing Ag nanoparticles and ZIF-8 particles.

[0067] Through SEM side view ( Figure 2 d) It can be seen that the prepared ordered structure is a typical two-dimensional colloidal crystal ordered array, and the PS microspheres are arranged in an orderly manner on the silicon wafer substrate.

[0068] 2.1.2 Structural characteristics of PS@Ag

[0069] Figure 3 TEM images of Ag grown on the surface of PS microspheres at different times at 300 nm and 100 nm are presented. Figure 3 As shown in this embodiment, Ag NPs can be grown in situ on the surface of PS microspheres after oxygen plasma etching. When the Ag growth time is 1 hour, Ag NPs with a size of approximately 2-13 nm are dispersed on the PS surface. Figure 3 a), d), these particles are firmly fixed to the PS surface. From Figure 4 a and Figure 4 The SEM images of d show that Ag NPs are evenly distributed on the PS surface. If the Ag growth time is extended to 3 hours... Figure 3 b, e) and 6h ( Figure 3 c, f), the Ag NPs size increased to 33 nm and 52 nm, respectively, which is consistent with the phenomenon observed by SEM. Figure 4 b, e and Figure 4 c, f). However, when the Ag growth time is 6 hours... Figure 3The image clearly shows that Ag NPs have a significant coverage on the PS surface, exhibiting obvious aggregation. From... Figure 4 It can also be seen that the process of growing Ag NPs on the surface of PS microspheres does not destroy the ordered array of two-dimensional colloidal crystals, which provides an ideal substrate for the next step of growing ZIF-8 to obtain an ordered PS@Ag@ZIF-8 array.

[0070] The elemental distribution diagram of the PS@Ag structure of Ag grown for 3 hours proves that Ag is uniformly distributed on the surface of C. Figure 5 bd). From the high-resolution transmission microscope image ( Figure 5 f, g), the lattice fringes of Ag NPs grown on the PS surface are clearly visible. The lattice spacing is 0.234 nm, which corresponds exactly to the (111) crystal plane of Ag (JCPDS, No. 87-0718), thus proving that Ag ions are reduced to Ag elemental on the PS surface after the addition of a reducing agent.

[0071] Because Ag NPs(n Ag The introduction of (=0.2) can increase the refractive index (n) of PS@Ag@ZIF-8. PS =1.58, n ZIF-8 =1.54-1.59) difference, thus making the color of the final constructed PS@Ag@ZIF-8 film brighter, so ZIF-8 was coated on the surface of PS@Ag samples grown for 3h and 6h. By growing ZIF-8 nanostructures in multiple cycles, ultrathin ZIF-8 films can be obtained on PS@Ag two-dimensional arrays. After four ZIF-8 cycles, ZIF-8 particles are uniformly coated on the surface of PS@Ag-3, with a particle size of about 90nm. There are very few scattered ZIF-8 particles on the ordered array surface of PS@Ag-3@ZIF-8, no cracks appear on the structure surface, the order of MCC is well maintained, and large-area order is observed. Figure 6 a, b), which provides the foundation for subsequent efficient optical sensing signals. However, when ZIF-8 is grown four times again on a PS surface that has already grown Ag NPS for 6 hours ( Figure 6(c) and (d) Obvious ZIF-8 particles are scattered on the film surface, with more pronounced ZIF-8 particle edges and larger particles reaching 240 nm in size. Furthermore, due to the relatively limited surface space of PS@Ag-6, the surface stress of the film increases significantly with the increase of ZIF-8 thickness, causing obvious cracks in the PS@Ag-6@ZIF-8 film array. These cracks further affect the output of the optical signal. Compared to the PS@Ag-3 structure, the PS@Ag-6 surface has less space remaining for ZIF-8 growth, resulting in a reduced proportion of ZIF-8 structure. This significant reduction in ZIF-8 composition will significantly affect the adsorption capacity of this film structure for VOCs during optical detection. Therefore, an ordered PS@Ag-3@ZIF-8 array with Ag NPs grown on the PS surface for 3 hours is selected as the optimal sequence for optical sensing.

[0072] like Figure 7 As shown, Ag elemental (JCPDS, No. 87-0718) is generated in the ordered arrays of PS@Ag-3, PS@Ag-3@ZIF-8, and PS@Ag-6@ZIF-8. 2θ = 38.50° and 44.80° correspond to the (111) and (200) crystal planes of silver elemental, respectively, which is consistent with the HRTEM results. Comparison of the ZIF-8 crystal structure simulation data with the results of PS@Ag-3@ZIF-8 and PS@Ag-6@ZIF-8 also confirms the formation of a ZIF-8 structure. The strong diffraction peak at 2θ = 33.06° corresponds to a diffraction peak on the single-crystal silicon substrate.

[0073] 2.2 Study on the tunable optical properties of PS@Ag@ZIF-8 composite structure

[0074] To investigate the optical properties of the PS@Ag@ZIF-8 composite structure and demonstrate its tunability, this embodiment detects and observes the reflectance spectra and optical photographs of PS@Ag-3@ZIF-8 with different ZIF-8 growth cycles. Then, PS@Ag-3@ZIF-8 with the optimal ZIF-8 growth cycle is selected, and its response to VOCs gases such as water, ethanol, n-propanol, and acetonitrile is examined. An ordered array is assembled on a high-refractive-index single-crystal silicon wafer (n...). si ≈3.5), its optical response is essentially a dielectric thin film, and the interference between the thin film and the substrate and between the thin film and air produces Fabry-Pérot interference peaks. When the incident light is perpendicular, the position of the peak of its reflection spectrum conforms to formula (1):

[0075] mλ max =2n eff d (1)

[0076] d is the film thickness, n effd is the refractive index of the dielectric film, and m is a positive integer (this formula is referenced from G. Lu, JTHupp, Metal-Organic Frameworks as Sensors: A ZIF-8 Based Fabry-Perot Device as a Selective Sensor for Chemical Vapors and Gases, J. Am. Chem. Soc. 2010, 132, 7832-7833.). For PS@Ag-3@ZIF-8 with different ZIF-8 growth cycles, the corresponding d and n are... eff They are not the same. Subsequently, depending on the type and concentration of the analyte adsorbed in the ZIF-8 pores, n eff It also changes accordingly, thus the reflection peak λ max It will also change accordingly, accompanied by a visible change in the film's color, which is more intuitive than observing its reflectance spectrum and enables visual monitoring.

[0077] The thickness of the ZIF-8 coating on the PS@Ag-3 surface affects its optical properties. This is because as the number of ZIF-8 growth cycles increases, the ZIF-8 (n = 1.54-1.59) replaces the air between PS@Ag cells (n = 1), leading to a decrease in the thickness of the ZIF-8 layer. eff As d increases, so does d, and according to Formula 1, the corresponding reflection peak λmax increases accordingly. Figure 8 It can be seen that with the increase of ZIF-8 growth times, the peaks at wavelengths of 502.40, 510.42, 530.42, 538.60, and 560.67 nm show a gradual redshift. Growing ZIF-8 four times on a PS@Ag substrate results in a peak redshift of approximately 59 nm. The ratio of peak to trough increases with the number of ZIF-8 growth times, thereby improving the color saturation of the structure. Figure 8 The inserted optical photographs show that the composite structure's color indeed becomes increasingly bright, eventually approaching a bright yellow. Furthermore, with the increase in the number of ZIF-8 growth cycles, the number of ZIF-8 particles coated on the PS@Ag surface increases, leading to a certain degree of film thickness. The specific surface area of ​​the PS@Ag-3@ZIF-8 nanostructure increases, resulting in a greater ability to adsorb VOCs gases, thus enhancing its optical response. Considering these factors, an ordered PS@Ag-3@ZIF-8 array with four ZIF-8 growth cycles was selected for subsequent optical performance studies.

[0078] To further demonstrate the role of AgNPs, this embodiment investigated the reflectance spectra and optical photographs of PS@ZIF-8 and PS@Ag-3@ZIF-8 nanostructures. Figure 9As shown, when the number of ZIF-8 growth cycles is the same, the pv ratio (ratio of reflection peak to reflection valley) of PS@Ag-3@ZIF-8 is larger and the half-width peak is narrower compared to PS@ZIF-8. Observing its optical photographs, under the same conditions, the ordered array of PS@Ag@ZIF-8 has a brighter color than the PS@ZIF-8 structure. These phenomena indicate that growing AgNPS on the surface of PS microspheres and then encapsulating it with ZIF-8 can significantly enhance its color saturation, thus improving the sensitivity of PS@Ag@ZIF-8 nanomaterials for detecting different VOCs.

[0079] 2.3 Study on the steam sensing performance of PS@Ag@ZIF-8 composite structure

[0080] Figure 10 Image a is a TEM image of ZIF-8 nanoparticles grown under the same conditions. The particles have a typical rhombic dodecahedral shape, are well dispersed and uniform in size, about 130 nm, and are free of impurities. Figure 10 b is the corresponding N2 adsorption-desorption isotherm curve, with a micropore volume of 0.57 cm³. 3 g -1 The specific surface area of ​​BET is 1460.57 m². 2 g -1 The specific surface area of ​​the micropores is 1255.42 m². 2 g -1 Other specific surface areas are 205.15 m². 2 g -1 Therefore, it can be deduced that when the ordered structure of the thin film adsorbs analytes, the ZIF-8 micropores dominate, but the packing pores between ZIF-8 particles can also effectively promote the diffusion of analytes. Due to the optical properties and flexibility of MCC and the advantages of MOF materials such as high adsorption capacity, chemical specificity, and size screening, the PS@Ag ordered array and ZIF-8 microporous structure are effectively combined. When ZIF-8 adsorbs analytes, the nadsorption of the composite structure varies depending on the adsorbed substance and concentration. eff The varying degrees of increase are converted into readable signals, resulting in a spectral redshift, accompanied by visualized colorimetric sensing. Therefore, this composite structure should have superior sensing performance.

[0081] The adsorption capacity of ZIF-8 for different analytes depends on their chemical affinity, steric hindrance, and kinetic diameter. For example... Figure 11 ab shows the optical response of the PS@Ag-3@ZIF-8 film under various vapor atmospheres. Although the cage diameter of ZIF-8 ( While slightly smaller, the ZIF-8 framework possesses a degree of flexibility due to the elasticity of the imidazole ring, allowing larger alcohols to enter the channels. For example, methanol (… ), ethanol ) and n-propanol ( As the diameter (d) of the tested substance molecules increases, the reflectance spectrum shows a significant red shift. However, for n-butanol, due to its diameter (…),… The adsorption capacity is relatively large, and adsorption is hindered at the ZIF-8 pore openings, resulting in a slightly smaller red shift in the spectrum compared to n-propanol. Furthermore, because tert-amyl alcohol ( The steric hindrance and size are too large, so the adsorption capacity of the film is significantly weakened, the increase in film refractive index is small, and the red shift of the spectrum is slightly smaller. In addition to size sieving selectivity, PS@Ag@ZIF-8 films also exhibit selectivity for different functional groups in sensing. Acetonitrile ( As a polar molecule, its structure is in a singlet state, although methanol (n=1.32) and acetonitrile ( (n=1.33) have similar refractive indices and molecular sizes, but because the -OH group has a weaker affinity for ZIF-8 than the -CN group, the composite film structure more readily adsorbs large amounts of acetonitrile, thus causing n eff The increase is significant, causing acetonitrile to exhibit a greater redshift than methanol or even other alcohols, with a redshift of approximately 37 nm. Figure 11 ab). Although water molecules ( The diameter of the pores is smaller than that of ZIF-8, but due to the hydrophobicity of ZIF-8, water molecules still have difficulty entering the channels, so the film only has a slight optical response to water molecules. Colorimetric detection enables visual monitoring, which is of great significance in practical applications. Because the ZIF-8 structure is regrown on PS microspheres encapsulated in Ag NPS, its color saturation is significantly enhanced, thus improving the film's sensitivity to the detection of different VOCs. Figure 11 c is an optical photograph of the corresponding thin film structure exposed to air, tert-amyl alcohol, ethanol, and acetonitrile in sequence. Its colors are highly distinguishable, especially for acetonitrile, which changes from the original bright yellow to brownish-red, creating a strong visual impact.

[0082] Linear response over a wide dynamic range is an important indicator of superior sensor performance. To demonstrate this performance, the optical response of the thin-film composite structure exposed to different concentrations of acetonitrile (0, 500, 2000, 5000, 10000, 20000, 30000, 40000, 50000, 60000, 70000 ppm) was investigated. The results are as follows: Figure 12 As shown, due to the spectral resolution of 1 nm, the reflectance peak position red-shifted by 2.3 nm when the structure is in an acetonitrile atmosphere of 500 ppm. Within the acetonitrile concentration range of 500 ppm to 70000 ppm, the sensor in this embodiment exhibits good linearity (R0). 2=0.98). Such a superior linear response over a wide concentration range is mainly attributed to the advantages of this thin film design, such as ultrathin thickness, MOF microporous structure, introduction of AgNPs and grain boundaries. These advantages enable the efficient conversion of small concentration changes in the analyte atmosphere into readable signals.

[0083] In practical VOCs detection, real-time monitoring is required, making a rapid sensor response to the analyte crucial. Therefore, the kinetic response of the PS@Ag-3@ZIF-8 thin film to saturated acetonitrile vapor was investigated. Figure 13 As can be seen, the reflection peak position redshifts very quickly, reaching equilibrium in just 3 seconds. This result is faster than the previous result, which took 5 seconds to reach equilibrium when an ultrathin MOF coating was applied to MCC (L. Li, X. Jiao, D. Chen, BV Lotsch, C. Li, Facile Fabrication of Ultrathin Metal-Organic Framework-Coated Monolayer Colloidal Crystals for Highly Efficient Vapor Sensing. Chem. Mater. 2015, 27, 7601-7609.). The ultrafast response speed is mainly due to the fact that the ultrathin MOF film structure facilitates rapid molecular transport. Repeatability and reversibility are important factors affecting sensor performance. After alternatingly exposing the sample to N2 and saturated acetonitrile vapor for 6 cycles, the initial peak position and the reflection peak position after adsorbing acetonitrile molecules remained almost unchanged. Figure 13 b). Since this embodiment utilizes the rapid and reversible physical adsorption capability of ZIF-8, it avoids the drawbacks of requiring heating or vacuuming to cycle the sensor during the cycling process. This reduces production costs and losses to a certain extent and is more conducive to the practical application of the sensor. Figure 13 c studied the optical performance of the sensor after being placed for different periods of time and found that the device has good stability (sealed bag), and the optical performance does not change significantly even after 180 days.

[0084] The above description is merely a preferred embodiment of this application and is not intended to limit this application in any way. Any modifications or alterations made by those skilled in the art without departing from the scope of the technical solution disclosed above are equivalent to equivalent implementations and fall within the protection scope of this invention.

Claims

1. The application of PS@Ag@ZIF-8 composite thin films in the fabrication of optical sensors, characterized in that, The sensor is used to detect organic compounds, specifically VOCs. The method for preparing the PS@Ag@ZIF-8 composite film includes: Step 1: Submicron-sized polystyrene microspheres were prepared by emulsion-free polymerization; then a PS solution uniformly dispersed in water and ethanol solutions was prepared, and a monolayer colloidal crystal sample attached to a silicon wafer substrate was obtained by gas-liquid interface self-assembly. Step 2: Prepare a silver nitrate solution using methanol and water in equal volume ratios as solvents; at room temperature, stir the silver nitrate solution, then add triethylamine solution and stir again to form a silver growth solution; vertically immerse two-thirds of the non-close-packed monolayer colloidal crystal substrate in the silver growth solution and grow in situ at room temperature in the dark for 1-6 hours; remove the substrate, rinse with methanol, and dry with N2 to obtain a PS@Ag substrate; Step 3: The PS@Ag substrate was vertically immersed in a fresh mixed methanol solution of Zn(NO3)2 and 2-methylimidazole and grown vertically. The growth was carried out at room temperature. The substrate was then removed from the growth solution, rinsed with methanol, and dried with N2 gas. The growth cycle was repeated multiple times to obtain the PS@Ag@ZIF-8 composite film. The effective combination of PS@Ag ordered array and ZIF-8 microporous structure allows for varying refractive indices of the composite dielectric film as ZIF-8 adsorbs analytes, depending on the adsorbed substance and concentration. neff The varying degrees of increase are converted into readable signals, resulting in a spectral redshift, accompanied by visual colorimetric sensing.

2. The application according to claim 1, characterized in that, In step 1, the polystyrene microspheres in the polystyrene microsphere solution have a particle size dispersion of less than 7%; the silicon wafer is a single-sided polished silicon wafer.

3. The application according to claim 1, characterized in that, In step 2, the ratio of silver nitrate, water, and methanol used is 0.0848g:25mL:25mL.

4. The application according to claim 1, characterized in that, The preparation method of non-densely packed MCC is as follows: the monolayer colloidal crystal substrate treated at 80℃ is treated with O2 plasma in an O2 flow of 200L / min for 1min to obtain the non-densely packed monolayer colloidal crystal substrate.

5. The application according to claim 1, characterized in that, In step 2, the in-situ growth time in the dark is 3 hours.

6. The application according to claim 1, characterized in that, In step 3, the molar ratio of Zn(NO3)2 to 2-methylimidazole in the Zn(NO3)2 and 2-methylimidazole methanol solution is 1:

2.

7. The application according to claim 1, characterized in that, In step 3, the growth conditions are as follows: growth is carried out once every 10 minutes at room temperature; the number of growth cycles is 4.

Images