A preparation method of a formaldehyde SERS sensor based on an Au@ZIF-8 core-shell structure

Abstract

This invention discloses a method for fabricating and applying a formaldehyde SERS sensor based on an Au@ZIF-8 core-shell nanostructure, belonging to the field of gas detection technology. Formaldehyde has an extremely small intrinsic Raman cross section, and ordinary SERS substrates lack the ability to capture it, resulting in weak detection signals at low concentrations. This sensor uses Au@ZIF-8 core-shell nanoparticles to assemble a two-layer thin film as the SERS substrate. The Au nanoparticles (AuNPs) serve as the electromagnetic enhancement core, while the ZIF-8 shell layer enriches and sieves formaldehyde molecules. A layer-to-layer deposition method is used to construct the double-layer thin film structure to obtain optimal SERS performance. This invention solves the problems of low sensitivity, poor specificity, insufficient stability, and difficulty in rapid on-site detection in existing formaldehyde detection technologies. The detection limit is as low as 0.16 ppt, and within a wide range from 0.01 ppb to 100 ppm, the characteristic peak intensity exhibits an excellent linear relationship with the logarithm of formaldehyde concentration (R² ≥ 0.995). It can specifically identify formaldehyde and exclude interference from other VOCs, making it suitable for rapid quantitative detection of formaldehyde in practical scenarios such as indoor environments and cigarette smoke.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, specifically to a formaldehyde SERS sensor based on an Au@ZIF-8 core-shell structure, its fabrication, and its application. Background Technology

[0002] In recent years, with the acceleration of industrialization and the widespread adoption of the interior decoration industry, volatile organic compound (VOC) pollution has become a major environmental issue of global concern. Formaldehyde, a typical highly toxic indoor pollutant, can lead to immune and nervous system dysfunction with long-term exposure, primarily originating from building materials, furniture, and adhesives. Currently, although traditional methods such as high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) offer high accuracy, their reliance on expensive instruments and cumbersome sample pretreatment make them unsuitable for the urgent need for rapid on-site detection. Surface-enhanced Raman scattering (SERS) technology, with its single-molecule-level sensitivity and fingerprint recognition capabilities, has become an ideal choice for on-site detection. However, the intrinsic Raman scattering cross-section of formaldehyde molecules is extremely small, and they are difficult to stably adsorb on metal surfaces, posing a significant challenge to direct detection. Furthermore, the high fluidity of gaseous formaldehyde makes signal acquisition difficult, and traditional precious metal substrates, lacking specific adsorption sites, exhibit insufficient sensitivity and poor signal-to-noise ratio under complex matrix interference. Metal-organic frameworks (MOFs) possess advantages such as high specific surface area and tunable pore size. Among them, ZIF-8 exhibits outstanding performance in gas enrichment due to its excellent thermal and chemical stability. Although previous studies have utilized Au@ZIF-8 core-shell structures to construct sensors, common challenges remain, including poor MOF structural stability, signal inhomogeneity, and the tendency for nanoparticles to aggregate. These defects directly lead to decreased detection sensitivity and repeatability, severely limiting the rapid quantitative application of SERS sensors in real-world complex environments (such as indoor air and cigarette smoke). Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a SERS formaldehyde gas sensor based on an Au@ZIF-8 core-shell structure and its preparation method. By optimizing the core-shell structure design and assembly process, and optimizing the AuNPs size, ZIF-8 shell thickness and the number of thin film layers, ultrasensitive and highly specific quantitative detection of formaldehyde gas can be achieved.

[0004] The present invention aims to solve the above-mentioned technical problems by providing a method for preparing the above-mentioned formaldehyde SERS sensor based on the Au@ZIF-8 core-shell structure.

[0005] This invention includes the following steps:

[0006] 1) Synthesis of AuNPs (gold nanospheres)

[0007] 2) Synthesis of Au@ZIF-8

[0008] 3) Preparation of multilayer nanoparticle thin film structures

[0009] The specific method for synthesizing AuNPs (gold nanospheres) in step 1) is as follows: HAuCl4 and CTAC are mixed, and fresh NaBH4 is added with vigorous stirring. The mixture is then diluted 10-fold with CTAC. Gold seeds and ascorbic acid are added to the CTAC solution, followed by the addition of HAuCl4 and stirring. The mixture is then allowed to stand at room temperature. Diluted sodium hypochlorite is added, followed by the addition of more HAuCl4, and the mixture is allowed to stand at room temperature. Excess reactants are removed by centrifugation, and the concentrate is stored at 4°C for later use.

[0010] The specific method for synthesizing Au@ZIF-8 in step 2) is as follows: Add CTAB to the AuNPs solution, stir until homogeneous, add Zn(NO3)2 aqueous solution and stir, centrifuge and wash, then bring to a final volume, add 2-methylimidazole aqueous solution and CTAB sequentially, stir and let stand, centrifuge and wash with methanol to complete the preparation of Au@ZIF-8.

[0011] The specific method for preparing the multilayer nanoparticle thin film structure in step 3) is as follows: The glassware and silicon wafer are immersed in a piranha solution, rinsed with ultrapure water, and then dried with nitrogen. Cyclohexane is added to the Au@ZIF-8 solution to form an organic / aqueous biphase. Anhydrous ethanol is injected to promote the aggregation of nanoparticles at the interface between the two phases to form a gold film. After the cyclohexane evaporates, the silicon wafer is inserted and removed to transfer the monolayer film. This process is repeated to form a bilayer film.

[0012] Compared with traditional methods, the present invention has the following outstanding advantages and technical effects:

[0013] 1) The sensor has good detection sensitivity and quantitative ability. The detection limit of the sensor for formaldehyde is as low as 0.16 ppt. In the concentration range of 0.01 ppb-100 ppm, the characteristic peaks are still clearly distinguishable even at low concentrations.

[0014] 2) Stable adsorption performance: It combines the dual functions of "SERS electromagnetic enhancement" of ZIF-8 and Au nanoparticles, which not only improves molecular capture efficiency but also amplifies signal response.

[0015] 3) It has outstanding specificity and anti-interference ability, allowing only small molecules such as formaldehyde to pass through, effectively blocking large molecular VOCs such as ethanol, toluene, and acetaldehyde, as well as interfering substances in complex environments, thus ensuring the specificity of detection from a structural perspective. In real-world scenarios such as mixed VOCs systems, complex environments in newly renovated rooms, and cigarette smoke, it can still accurately identify formaldehyde characteristic signals without interference peaks, and the detection results are reliable. Attached Figure Description

[0016] Figure 1 (Steps 1, 2, and 3) Figure 1The image shows scanning electron microscope (SEM) images of Au@ZIF-8 nanoparticle monolayer films prepared by interface self-assembly process; the inset in the upper right corner shows high-resolution transmission electron microscope (TEM) images of a single Au@ZIF-8 core-shell particle and its corresponding energy scattering spectroscopy (EDS) elemental distribution map.

[0017] Figure 2 (for step 4) Figure 2 This is a comparison of the SERS spectral responses of the Au@ZIF-8 substrate under various volatile organic compound (VOC) environments. The figure shows the differences in Raman signals of the sensor for formaldehyde (C2H4O) and interfering gases such as acetaldehyde, dichloromethane, toluene, and ethanol.

[0018] Figure 3 (for step 4) Figure 3 The Raman spectral evolution of formaldehyde gas at different concentration gradients using the Au@ZIF-8 sensor under optimal shell thickness (5 nm) is presented. The tested concentrations cover a wide range from 0.01 ppb to 100 ppm.

[0019] Figure 4 (for step 4) Figure 4 The actual detection spectra of the sensor in cigarette smoke under blank background, newly renovated indoor air, and different sampling methods were compared.

[0020] Figure 5 (Steps 1, 2, and 3) Figure 5 High-resolution transmission electron microscopy (TEM) image of a single Au@ZIF-8 core-shell particle. Detailed Implementation

[0021] Example 1: Preparation of Au@ZIF-8 core-shell nanoparticles

[0022] (1) Material preparation: chloroauric acid tetrahydrate (HAuCl4·4H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%), 2-methylimidazole (C4H6N2, 99%), ascorbic acid (C6H8O6), hexadecyltrimethylammonium bromide (C 19 H 42 BrN was purchased from Sigma-Aldrich. Methanol (CH4O, 99.5%), sodium borohydride (NaBH4), formaldehyde solution (CH2O, 37wt%), and hexadecyltrimethylammonium chloride (C) were also used. 19 H 42 ClN) and cyclohexane. The experimental water was Milli-Q ultrapure water with a resistivity greater than 18.2 MΩ·cm at 25℃. The VOCs gas containers were specially customized, and all glassware was cleaned with aqua regia before use.

[0023] (2) Preparation of AuNPs:

[0024] Using a seed synthesis method, 50 µL (0.05 M) HAuCl4 was added to 5 mL (0.1 M) CTAC solution, and 200 µL of freshly prepared NaBH4 (0.02 M) was injected under vigorous stirring. After 3 min, the solution was diluted 10-fold with CTAC (0.1 M) to synthesize gold seeds. Then, 900 µL of the seed solution and 40 µL (0.1 M) ascorbic acid were added to 10 mL (25 mM) CTAC solution, and 50 µL of HAuCl4 (0.05 M) solution was injected under vigorous stirring. The mixture was stirred for 3 min and allowed to stand at room temperature for at least 10 min. The resulting gold nanospheres exhibited an LSPR bandwidth centered at 520 nm. 10 nm gold nanospheres were synthesized. 20 µL of the 10 nm gold nanosphere suspension and 40 µL (0.1 M) ascorbic acid were added to 10 mL of CTAC (25 mM) solution. Add 50 µL (0.05 M) HAuCl4 solution under vigorous stirring and react for 60 min. Then, under rapid stirring, inject 10 µL of diluted sodium hypochlorite solution into 10 mL of the grown nanoparticle solution. After 5 min, add 3.75 µL (0.05 M) HAuCl4 solution under stirring and react undisturbed at room temperature for 90 min. Finally, centrifuge for 10 min (5000 rpm) to wash away excess reactants, concentrate the solution four times by volume, and store at 4 °C.

[0025] (3) Synthesis of Au@ZIF-8: The process of coating AuNPs with a ZIF-8 shell is as follows: Take 2 mL of the above AuNPs solution, add 200 µL of CTAB (1 mM) solution, then add 2 mL of Zn(NO3)2 aqueous solution (14.4 mM) to the reaction solution, stir for 3 hours, wash once with 4500 rpm, and make up to 2 mL. Add 1 mL of 2-methylimidazole aqueous solution (0.792 M) and CTAB (200 µL, 2 mM) to the solution in sequence, stir for 10 min, and let stand for 10 min. The obtained Au@ZIF-8 nanoparticles are washed twice with methanol by centrifugation for 5 min (3500 rpm). All stirring is carried out at room temperature.

[0026] Example 2: Preparation of a formaldehyde SERS sensor

[0027] (1) Silicon wafer pretreatment: Cut the silicon wafer into 1 cm × 1 cm size, soak it in piranha solution (H2SO4:H2O2=7:3, v / v) for 2 hours, take it out and rinse it 3 times with ultrapure water, blow it dry with nitrogen, and set it aside.

[0028] (2) Preparation of multilayer Au@ZIF-8 thin films: 2 mL of the methanol dispersion of Au@ZIF-8 core-shell nanoparticles prepared in Example 1 was added to a 5 mL glass beaker, and 800 µL of cyclohexane was slowly added to the solution surface to form an organic / water biphase interface; 2 mL of anhydrous ethanol was quickly injected until a mirror reflection appeared on the solution surface. At this time, Au@ZIF-8 nanoparticles aggregated at the interface to form a monolayer film; after the cyclohexane evaporated spontaneously, the pretreated silicon wafer was slowly immersed in the solution at a 30° angle, and then slowly removed at a speed of 1 cm / min, and the monolayer film was transferred to the surface of the silicon wafer; the above deposition process was repeated once to form 2 layers of Au@ZIF-8 thin films; after natural drying, a formaldehyde SERS sensor was obtained.

[0029] Example 3: Performance Testing of Formaldehyde SERS Sensor

[0030] (1) Sensitivity test: Formaldehyde gas standard samples with concentrations of 1 ppt, 1 ppb, 10 ppb, 100 ppb, 1 ppm, 10 ppm, and 100 ppm were prepared; the sensor prepared in Example 2 was placed in a sealed flow cell and purged with nitrogen for 5 min; formaldehyde standard gas of different concentrations was introduced respectively, and after equilibration for 10 min, it was detected by a portable Raman spectrometer with an excitation wavelength of 785 nm, and the value was recorded at 757 cm⁻¹. -1 The characteristic peak intensity was measured. Results showed that the sensor's detection limit for formaldehyde was as low as 0.16 ppt, within the concentration range of 0.01 ppb-100 ppm.

[0031] (2) Specificity test: Acetaldehyde, dichloromethane, toluene, and ethanol were selected as interfering VOCs, and gas samples with a concentration of 10 ppm were prepared. The samples were then tested using the same method described above, and the Raman spectra were recorded. The results showed that only formaldehyde gas showed a specificity at 757 cm⁻¹. -1 A distinct characteristic peak appeared at the substrate, while the Raman signals of other VOCs were consistent with those of the Au@ZIF-8 substrate, with no new characteristic peaks, indicating that the sensor has excellent specificity.

[0032] (3) Stability and Uniformity Test: Forty points were randomly selected on the sensor surface for Raman signal detection. The sensor was sealed and stored at 4℃, and Raman signal detection was performed on days 1, 3, 5, 10, and 15. This indicates that the sensor signal uniformity and stability are good. Figure 3As shown, the intensity of the characteristic peak at 757 cm⁻¹ exhibits a significant concentration-dependent change. Even at low concentrations, these fingerprint features remain clearly discernible, indicating that this sensor possesses a superior limit of quantitation (LOQ) compared to many reported metal-oxide-semiconductor sensors, {LOQ ≈ 10 times the blank noise (signal-to-noise ratio ≥ 10). Signal-to-noise ratio S / N = H} 信号 / H 噪声 34 Select 770 cm -1 up to 820 cm -1 As a blank noise signal, the target peak intensity is 1041.15 and the signal-to-noise ratio is 20.823.

[0033] Example 4: Actual Sample Detection

[0034] (1) Indoor air quality testing in newly renovated rooms: The sensors prepared in Example 2 were placed inside the newly renovated room and at the doorway, respectively. After exposure for 30 min, they were detected using a portable Raman spectrometer, and the formaldehyde concentration was calculated based on the linear fitting curve. The results showed that the formaldehyde concentration inside the room was 70-90 ppb, and at the doorway it was 55-70 ppb, which is consistent with the formaldehyde distribution pattern in newly renovated rooms.

[0035] (2) Cigarette smoke detection: Smoke exhaled after a deep inhalation of a cigarette and smoke exhaled during normal smoking were collected and passed into a sealed flow cell. The sensor was placed in the cell and allowed to equilibrate for 10 minutes before detection. The results showed that both smoke samples were at 757 cm⁻¹. -1 Characteristic peaks appeared at the locations, and the calculated formaldehyde concentrations were 10-15 ppm and 4-7 ppm, respectively, verifying the sensor's detection capability in complex matrices.

[0036] The above description is merely a specific embodiment of the present invention. Those skilled in the art can make various modifications or substitutions to the sensor structure and fabrication method of the present invention, all of which should be covered within the protection scope of the present invention. The protection scope of the present invention should be determined by the scope of the claims.

Claims

1. An Au@ZIF-8 core-shell structure based formaldehyde SERS sensor, characterized in that: The sensor comprises a substrate and a double-layer film of Au@ZIF-8 core-shell nanoparticles loaded on the substrate through layer-by-layer self-assembly; the Au@ZIF-8 particle is composed of a gold nanosphere core with a particle size of 60-70 nm and a ZIF-8 shell layer with a thickness of 4-6 nm; and the double-layer film structure is induced to form through an organic / water two-phase system.

2. The SERS sensor of claim 1, wherein, The Au nanoparticles are prepared through a seed-mediated growth method, and cetyltrimethylammonium chloride (CTAC) is used as a stabilizer and ascorbic acid is used as a reducing agent in the preparation process.

3. The SERS sensor of claim 1, wherein, The thickness of the ZIF-8 shell layer is regulated by adjusting the addition amount of cetyltrimethylammonium bromide (CTAB), and when the addition amount of CTAB is 300-600 μL (2 mM), the thickness of the ZIF-8 shell layer reaches the target range.

4. A method of preparing the SERS sensor of claim 1, comprising The method comprises the following steps: Step 1. Preparation of Au nanoparticles (AuNPs): Au seeds are prepared first, and then AuNPs with a particle size of about 60-70 nm are obtained through a growth solution reaction by using a seed synthesis method, and the AuNPs are stored after centrifugal purification; Step 2. Synthesis of Au@ZIF-8 core-shell nanoparticles: AuNPs solution and CTAB solution are mixed, and zinc nitrate aqueous solution and 2-methylimidazole aqueous solution are sequentially added, and then centrifugal washing is performed after stirring reaction to obtain Au@ZIF-8 core-shell nanoparticles; Step 3. Preparation of a multi-layer film: through layer-by-layer deposition, Au@ZIF-8 core-shell nanoparticles are transferred to a silicon substrate through self-assembly of an organic / water two-phase system, and a double-layer film is formed by repeating the assembly process, thereby obtaining a SERS sensor; Step 4. Detection and analysis: after nitrogen purging, smoke after different smoking is collected by using a gas sampling bag, and further SERS analysis is performed on the substrate after a period of time.

5. The preparation method according to claim 2, characterized in that, In step (1), the preparation process of the gold seed is as follows: 50 μL (0.05 M) of chloroauric acid tetrahydrate solution is added to 5 mL (0.1 M) of CTAC solution, 200 μL (0.02 M) of sodium borohydride solution is injected under vigorous stirring, and the solution is diluted by 10 times with (0.1 M) CTAC solution after 3 min of reaction; the growth process of the AuNPs is as follows: 900 μL of seed solution, 40 μL (0.1 M) of ascorbic acid is added to 10 mL (25 mM) of CTAC solution, 50 μL (0.05 M) of chloroauric acid tetrahydrate solution is injected under vigorous stirring, and the solution is reacted for 3 min, and then it is left to stand for 10 min, and sodium hypochlorite solution and chloroauric acid tetrahydrate solution are added for further reaction for 90 min, and then the solution is centrifugally purified.

6. The preparation method according to claim 2, characterized in that, In step (2), the volume ratio of the AuNPs solution to the zinc nitrate aqueous solution is 1:1, the concentration of the 2-methylimidazole aqueous solution is 0.792 M, the reaction temperature is room temperature 25-30℃, the stirring time is 10-15 min, and the centrifugal speed is 3500-4500 rpm.

7. The preparation method according to claim 2, characterized in that, In step (3), the organic / water two-phase system is composed of an Au@ZIF-8 core-shell nanoparticle aqueous solution and cyclohexane, anhydrous ethanol is used as an inducer to promote self-assembly of the nanoparticles, and the silicon substrate is pretreated by cleaning with a piranha solution.

8. Use of the SERS sensor according to claim 1, characterized in that, In step (4), the qualitative and quantitative detection of formaldehyde gas is carried out, and the detection process comprises the following steps: placing the sensor in a sealed flow cell, introducing the to-be-detected gas, detecting by using a Raman spectrometer with an excitation wavelength of 785 nm, and realizing formaldehyde identification through a characteristic Raman peak at 757 cm⁻¹.

9. Use according to claim 6, characterized in that, The detection limit of the formaldehyde is 0.16 ppt, the quantitative detection range is 0.01 ppb-100 ppm, and the determination coefficient R² of the characteristic peak intensity and the logarithm of the formaldehyde concentration is greater than or equal to 0.

995.

10. The use according to claim 6, characterized in that, In step (4), the to-be-detected gas includes indoor air, cigarette smoke and other actual environmental gases, the sensor can exclude the interference of acetaldehyde, dichloromethane, toluene, ethanol and other VOCs, and specific detection is realized.

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