Reusable Ag (at) g-C3N4 / TiO2 cotton fabric SERS (Surface Enhanced Raman Scattering) substrate as well as preparation method and application thereof

Abstract

The invention provides a reusable Ag (at) g-C3N4 / TiO2 cotton fabric SERS (Surface Enhanced Raman Scattering) substrate as well as a preparation method and application thereof, and belongs to the technical field of molecular detection. According to the substrate, silver nanoparticles, titanium dioxide and graphite-phase carbon nitride are orderly integrated on a flexible cotton cloth substrate to form a composite material, and the composite material has excellent SERS (Surface Enhanced Raman Scattering) performance and photocatalytic pollutant degradation capacity. Through the synergistic effect between TiO2 and g-C3N4, the photocatalytic performance is remarkably improved, meanwhile, the substrate is endowed with the self-cleaning characteristic, and the SERS effect is effectively enhanced. The local surface plasmon resonance effect of AgNPs not only promotes the enhancement of SERS signals, but also improves the effective separation of photo-induced electrons and holes, thereby enhancing the photocatalytic degradation efficiency. The substrate can rapidly degrade organic pollutants under visible light irradiation, keeps high sensitivity in multiple detection periods, and shows excellent reusability.

Description

Technical Field

[0001] The present invention relates to a reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate and a preparation method and application thereof, belonging to the technical field of molecular detection. Background Art

[0002] Surface-enhanced Raman scattering (SERS) is a highly sensitive molecular detection technology widely used in chemistry, physics, medicine and the environment. The core principle of SERS is the interaction between probe molecules and rough noble metal surfaces (such as gold and silver), which can enhance the Raman signal through the localized surface plasmon resonance (LSPR) effect, thereby improving the detection sensitivity. This technology can significantly enhance the signal, so that even very small amounts of substances can be detected. However, although traditional noble metal substrates have a high signal enhancement effect, they also face problems such as high cost, structural instability and poor reusability, which to some extent limit their promotion in large-scale applications. Therefore, the development of low-cost, stable and highly reusable SERS substrates has become an important direction of current research.

[0003] In recent years, flexible substrates have attracted increasing attention due to their excellent flexibility, high adaptability, and potential for large-scale production. However, relatively few studies have examined dual-functional flexible substrates, and most have not delved deeply into their applications in the field of SERS. To overcome the limitations of noble metal substrates, researchers have attempted to combine flexible substrates with functional nanomaterials to create reusable and highly sensitive SERS substrates.

[0004] Based on this, the present invention orderly integrates AgNPs, TiO2 and g-C3N4 nanomaterials onto a flexible cotton substrate, successfully preparing a reusable, highly sensitive SERS substrate. This substrate not only provides a strong SERS signal amplification effect, but also exhibits good self-cleaning and photocatalytic repair functions. The synergistic effect of this composite material effectively improves the adsorption capacity of the probe molecules, while enhancing the SERS signal through the local surface plasmon resonance effect, thereby significantly improving the detection performance. Compared with traditional silver substrates, this new composite substrate has a lower detection limit, higher sensitivity and stronger reusability, and exhibits excellent photocatalytic performance in the degradation of environmental pollutants.

[0005] In addition, the flexible Ag@g-C3N4 / TiO2 cotton fabric SERS substrate developed by the present invention has significant application potential in the field of food safety. Utilizing the flexibility of cotton fabric, pollutants or harmful substances can be collected efficiently and non-destructively on the surface of food, and instant quantitative analysis can be achieved through SERS technology. In addition, the self-cleaning and photocatalytic degradation functions of the substrate make it reusable in practical applications. Therefore, this new composite substrate not only provides a more convenient and efficient means for food safety testing, but also can significantly reduce testing costs and improve detection efficiency, and has broad application prospects in the food industry. Summary of the Invention

[0006] This paper designs a reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate. Through the synergistic effect of three components, this substrate enables highly sensitive SERS detection and rapid photocatalytic self-cleaning, exhibiting excellent performance. Its structure, SERS performance, and recyclability are characterized. Specifically:

[0007] (1) This paper successfully fabricated a highly sensitive SERS substrate by systematically integrating AgNPs, TiO2, and g-C3N4 onto a flexible cotton fabric substrate using a hydrothermal method combined with a chemical reduction method. SEM, XRD, and XPS experimental results effectively demonstrated the successful preparation of the substrate.

[0008] (2) This study explored the role of TiO2 and g-C3N4 in substrate SERS enhancement. The results showed that the synergistic effect between the three components significantly enhanced the SERS signal. Furthermore, malachite green (MG) was selected as a probe molecule to evaluate the sensitivity of the substrate in detecting MG molecules.

[0009] (3) The substrate prepared by the present invention can self-clean through photocatalysis after SERS detection, achieving rapid repair.

[0010] (4) The present invention investigated the recyclability of the substrate, and the results showed that the substrate could maintain its photocatalytic and SERS performance in multiple detection cycles, and had high sensitivity and reusability for the detection of multiple target molecules.

[0011] (5) The substrate prepared by the present invention has flexible application characteristics, can realize in-situ, non-destructive detection, and is suitable for various actual samples.

[0012] Technical solution of the present invention

[0013] The present invention uses sol-gel-assisted hydrothermal synthesis technology to ultrasonically treat g-C3N4 nanosheets and tetraisopropoxytitanium (TBT) with anhydrous ethanol to achieve their uniform and stable dispersion. Subsequently, the premixed solution, pretreated cotton fabric, and deionized water are transferred to a Teflon-lined autoclave. Under hydrothermal reaction conditions, TBT hydrolyzes and forms a Ti-O-Ti network, which combines g-C3N4 and TiO2 and solidifies them into the fiber structure of the cotton fabric. Finally, AgNPs are in situ reduced and grown on the surface of the g-C3N4 / TiO2 composite cotton fabric. This preparation method has high stability and photocatalytic degradation function, and can maintain excellent SERS performance after multiple uses, making it suitable for the detection and degradation of environmental pollutants.

[0014] The technical solution of the present invention is as follows: a reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate, the preparation method of which is as follows:

[0015] 1) Pretreatment of cotton fabric: soak the cotton fabric in KOH solution, place it in a Teflon-lined autoclave for treatment, wash it until neutral, and freeze-dry it;

[0016] 2) Preparation of g-C3N4 / TiO2 cotton fabric substrate: TBT and g-C3N4 nanosheets were added to anhydrous ethanol, and after ultrasonic treatment, the pretreated cotton fabric obtained in step 1) and deionized water were added, and the mixture was transferred to a Teflon-lined autoclave for hydrothermal treatment, followed by washing and vacuum freeze-drying to obtain a g-C3N4 / TiO2 cotton fabric substrate;

[0017] 3) Preparation of Ag@g-C3N4 / TiO2 cotton fabric substrate: The g-C3N4 / TiO2 cotton fabric substrate obtained in step 2) was placed in an AgNO3 solution for treatment, and then transferred to an ascorbic acid solution for immersion, rinsed, and vacuum-dried to obtain an Ag@g-C3N4 / TiO2 cotton fabric substrate.

[0018] The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate is characterized in that, in step 1), the treatment is carried out in an autoclave at 130°C with an 80% filling rate for 5 hours.

[0019] The above-mentioned reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate is characterized in that, in step 2), the hydrothermal treatment is performed at 120°C for 5 hours.

[0020] The above-mentioned reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate is characterized in that, in step 3), the concentration of the AgNO3 solution is 0.02M, and the treatment is soaking for 3 hours.

[0021] The above-mentioned reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate is characterized in that, in step 3), the soaking in the ascorbic acid solution is in a 0.01M ascorbic acid solution for 20 minutes.

[0022] Application of the above-mentioned Ag@g-C3N4 / TiO2 cotton fabric SERS substrate in photodegradation of malachite green.

[0023] Application of the above-mentioned Ag@g-C3N4 / TiO2 cotton fabric SERS substrate in the qualitative detection of malachite green.

[0024] Application of the above-mentioned Ag@g-C3N4 / TiO2 cotton fabric SERS substrate in the quantitative detection of malachite green.

[0025] Application of the above-mentioned Ag@g-C3N4 / TiO2 cotton fabric SERS substrate in detecting malachite green on the surface of fish.

[0026] The specific steps are as follows

[0027] 1. Pretreatment of cotton fabrics

[0028] Approximately 5 g of cotton fabric was soaked in 28 ml of 12 M KOH solution, placed in a 35 ml Teflon-lined autoclave, and treated at 130°C with an 80% fill rate for 5 h. Subsequently, the cotton fabric was thoroughly rinsed with anhydrous ethanol and deionized water until neutral and then freeze-dried in a vacuum.

[0029] 2. Preparation of g-C3N4 / TiO2 cotton fabric substrate

[0030] 0.35 ml of TBT and 0.035 g of g-C3N4 nanosheets were added to 12.6 ml of anhydrous ethanol and subjected to ultrasonic dispersion for 40 minutes. The mixed solution, 0.35 g of pretreated cotton fabric, and 14 ml of deionized water were then transferred to a 35 ml Teflon-lined autoclave and hydrothermally treated at 120°C for 5 hours. After treatment, the mixture was rinsed sequentially with anhydrous ethanol and deionized water to neutralize and remove any residual particles. Finally, the material was freeze-dried in a vacuum oven.

[0031] 3. Preparation of Ag@g-C3N4 / TiO2 cotton fabric substrate

[0032] The g-C3N4 / TiO2 cotton fabric was cut into 1 cm x 1 cm pieces, placed in a beaker, and immersed in a solution containing 5 ml of 0.02 M AgNO3 for 3 hours. The fabric was then transferred to a solution containing 5 ml of 0.01 M ascorbic acid and soaked for 20 minutes. After rinsing with deionized water, the fabric was freeze-dried in a vacuum oven.

[0033] 4. SERS spectroscopy measurement

[0034] The SERS performance of the composite flexible substrate was evaluated by the Raman characteristic peaks of the MG molecules. 0.0365 g of MG powder was accurately weighed and 100 ml of 10 -3 Then, MG solutions with different concentration gradients (10 -6 M to 10 -12 M). Subsequently, 20 μL of each of these solutions with different concentrations was added dropwise to the Ag@g-C3N 4 / TiO2 cotton fabric flexible substrate surface. After air drying at room temperature, SERS measurements were performed directly on the flexible SERS substrate.

[0035] 5. Photocatalytic measurements and reusable SERS detection

[0036] To investigate the catalytic activity of the composite flexible substrates, we performed photocatalytic degradation measurements under simulated sunlight (xenon lamp, 300W, 20A) by evaluating the changes in Raman signal intensity of probe molecules (e.g., MG and R6G) on different substrates. The distance between the simulated sunlight source and the sample was maintained at 5 cm. SERS analysis was performed using Raman microscopy before and after irradiation.

[0037] 6. Preparation and testing of actual samples

[0038] Take 20 μL 10 -6 The MG solution with a concentration of M was dropped onto the surface of different types of fish samples. After natural drying, the prepared Ag@g-C3N 4 / The sample surface was directly wiped with a TiO2 cotton fabric flexible substrate. To enhance contact between the substrate and the sample, the flexible substrate was lightly moistened before wiping to facilitate ethanol evaporation and facilitate the migration of MG to the surface. Subsequently, SERS signals were directly acquired using an excitation wavelength of 633 nm and an accumulation time of 10 seconds.

[0039] The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate described in this invention amplifies SERS signals by leveraging the localized surface plasmon resonance (LSPR) effect of AgNPs, the large surface area of g-C3N4, and its ability to adsorb more target molecules via π-π interactions, significantly improving the substrate's SERS detection performance. The g-C3N4 and TiO2 interact synergistically to effectively separate photogenerated electrons and holes, while the AgNPs act as electron traps, further suppressing the recombination of electron-hole pairs. This enhances photocatalytic reactions and improves the substrate's practicality in environmental pollution control.

[0040] The present invention has the following beneficial effects

[0041] 1) Highly sensitive SERS performance: By using a three-component composite flexible substrate of AgNPs, g-C3N4 and TiO2, the Raman signal can be effectively enhanced, the detection sensitivity of the target molecules can be improved, and it is suitable for the detection of trace molecules.

[0042] 2) Self-cleaning function: The photocatalytic degradation ability of TiO2 and g-C3N4 enables the substrate to rapidly degrade surface pollutants by photocatalysis after SERS detection.

[0043] 3) Good reusability: After photocatalytic self-cleaning, the SERS substrate can restore its original performance and has a long service life, which reduces the frequency of substrate replacement and reduces detection costs.

[0044] 4) Simple operation process: It can be directly detected in situ by wiping. It is easy to operate and does not require complicated instruments or special conditions, which is convenient for on-site analysis and detection.

[0045] 5) Environmental protection and economy: The base material and operation method are simple and environmentally friendly, with high economy and sustainability, and suitable for large-scale application.

[0046] 6) Taking advantage of the flexible properties of cotton fabric, it is suitable for rapid, in-situ detection of pollutants in the food field, and effectively removes surface pollutants through photocatalytic degradation function.

[0047] 7) The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate described in the present invention has good reusability and can still maintain a high SERS signal enhancement and photocatalytic degradation effect after multiple cycles of use, and is suitable for long-term pollutant detection and environmental governance.

[0048] 8) The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate described in the present invention has a significantly enhanced SERS effect and simultaneously has the dual function of photocatalytic degradation of pollutants. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 SEM images of original cotton fabric (a), pretreated cotton fabric (b), g-C3N4 / TiO2 cotton fabric (c), and Ag@g-C3N4 / TiO2.

[0050] Figure 2 XRD patterns of pretreated cotton fabric, g-C3N4 / TiO2 cotton fabric and Ag@g-C3N4 / TiO2 cotton fabric.

[0051] Figure 3 XPS patterns of g-C3N4 / TiO2 cotton fabric and Ag@g-C3N4 / TiO2 cotton fabric

[0052] Figure 4 MG powder samples and 10 -6 SERS spectra of MMG adsorbed on different substrates

[0053] Figure 5 SERS signals of MG with different concentrations on Ag@g-C3N4 / TiO2 cotton fabric substrate (a), the peak of Ag@g-C3N4 / TiO2 cotton fabric is at 1616cm -1 Linear fitting of the peak at and MG concentration (b).

[0054] Figure 6 Using simulated sunlight to degrade 10 -6 SERS spectra of MMG adsorbed on Ag@g-C3N4 / TiO2 cotton fabric substrate.

[0055] Figure 7 During the cyclic photocatalysis on Ag@g-C3N4 / TiO2 cotton fabric, 10 -6 SERS spectra of MMG at 1616 cm -1 Raman intensity changes of the peaks during each “detection-cleaning” cycle (b).

[0056] Figure 8 Schematic diagram of the reusable Ag@g-C3N4 / TiO2 cotton fabric (a). Alternate photocatalytic degradation on Ag@g-C3N4 / TiO2 cotton fabric 10 -6 SERS spectra recorded during MMG and R6G processes (b).

[0057] Figure 9 Schematic diagram of the reusability of the actual sample tested on Ag@g-C3N4 / TiO2 cotton fabric (a). MG of different fish species in the “test-clean” cycle at 1616 cm -1 Raman intensity changes of the band (b). MG Raman spectra of Ag@g-C3N4 / TiO2 cotton fabric before and after self-cleaning on herring (c). MG Raman spectra of Ag@g-C3N4 / TiO2 cotton fabric before and after self-cleaning on pomfret (d).

[0058] Figure 10 Schematic diagram of the preparation process of Ag@g-C3N4 / TiO2 cotton fabric and its reusable photocatalytic degradation. DETAILED DESCRIPTION

[0059] In order to better understand the technical solution of the present invention, a specific embodiment is given to further illustrate it in detail, but the solution is not limited thereto.

[0060] Example 1 (Preparation and Characterization of Ag@g-C3N4 / TiO2 Cotton Fabric Substrate)

[0061] a. Pretreatment of cotton fabrics

[0062] Approximately 5 g of cotton fabric was soaked in 28 ml of 12 M KOH solution, placed in a 35 ml Teflon-lined autoclave, and treated at 130°C with an 80% fill rate for 5 h. Subsequently, the cotton fabric was thoroughly rinsed with anhydrous ethanol and deionized water until neutral and then freeze-dried in a vacuum.

[0063] b, g Preparation of -C3N4 / TiO2 cotton fabric substrate

[0064] 0.35 ml of TBT and 0.035 g of g-C3N4 nanosheets were added to 12.6 ml of anhydrous ethanol and subjected to ultrasonic dispersion for 40 minutes. The mixed solution, 0.35 g of pretreated cotton fabric, and 14 ml of deionized water were then transferred to a 35 ml Teflon-lined autoclave and hydrothermally treated at 120°C for 5 hours. After treatment, the mixture was rinsed sequentially with anhydrous ethanol and deionized water to neutralize and remove any residual particles. Finally, the material was freeze-dried in a vacuum oven.

[0065] c. Preparation of Ag@g-C3N4 / TiO2 cotton fabric substrate

[0066] The g-C3N4 / TiO2 cotton fabric was cut into 1 cm x 1 cm pieces, placed in a beaker, and immersed in a solution containing 5 ml of 0.02 M AgNO3 for 3 hours. The fabric was then transferred to a solution containing 5 ml of 0.01 M ascorbic acid and soaked for 20 minutes. After rinsing with deionized water, the fabric was freeze-dried in a vacuum oven.

[0067] Figure 1 This is the preparation process of Ag@g-C3N4 / TiO2 cotton fabric, and the morphology of the samples was observed by SEM. Figure 1 a is the typical form of original cotton fabric, showing natural wrinkles and smooth surface, with one-dimensional tubular and tree-like fiber structure. Figure 1 b is cotton fabric that has undergone hydrophilic heat treatment, and its fiber morphology and structure remain intact, laying the foundation for constructing a flexible matrix. Figure 1 c is g-C3N4 / TiO2 cotton fabric. After hydrothermal treatment with TBT and g-C3N4 nanosheets, the fiber surface becomes rough, with obvious particle aggregates and exhibits a distinct pore structure. Figure 1 d is Ag@g-C3N4 / TiO2 cotton fabric. Due to the in situ growth of Ag NPs on the fiber surface, the surface of the cotton fabric presents a fish-scale structure.

[0068] Figure 2Figure 2 shows the XRD patterns of pretreated cotton fabric, g-C3N4 / TiO2 cotton fabric, and Ag@g-C3N4 / TiO2 cotton fabric. As shown, all substrates exhibit a broad diffraction peak at approximately 21°, which is primarily attributed to the amorphous morphology of the cotton fabric. After treatment with TBT and g-C3N4 nanosheets, diffraction peaks of TiO2 (JCPDS 21-1272) and g-C3N4 (JCPDS 87-1526) appeared on the flexible g-C3N4 / TiO2 cotton fabric substrate. Characteristic peaks of Ag (JCPDS No. 04-0783) also appeared on the Ag@g-C3N4 / TiO2 cotton fabric substrate.

[0069] Figure 3 The XPS spectra of g-C3N4 / TiO2 cotton fabric and Ag@g-C3N4 / TiO2 cotton fabric are shown. As shown, sharp photoelectron peaks at 285, 399, 532, and 458 eV for C1, N1, O1, and Ti 2p, respectively, further confirm the successful growth of TiO2 and g-C3N4 nanosheets on the cotton fabric. In addition to the characteristic peaks shared with g-C3N4 / TiO2 cotton fabric, the flexible Ag@g-C3N4 / TiO2 cotton fabric substrate exhibits a sharp peak in the Ag 3d spectrum at approximately 367 eV. These characteristics effectively demonstrate the effective loading of the ternary components of AgNPs, g-C3N4, and TiO2 on the cotton fabric.

[0070] Example 2 Evaluation of SERS performance of Ag@g-C3N4 / TiO2 cotton fabric substrate

[0071] a. Comparison of SERS signals of substrates with different components

[0072] MG organic pollutants were selected as probe molecules to evaluate the SERS performance of the composite flexible substrate. - 6 M), pretreated cotton fabric, g-C3N4 / TiO2 cotton fabric, Ag cotton fabric, and Ag@g-C3N4 / TiO2 cotton fabric were naturally dried, and the Raman signals of all substrates were detected and compared with those of the MG powder sample.

[0073] Accurately weigh 0.0365 g of MG powder and prepare 100 ml of 10 -3 Then, MG solutions with different concentration gradients (10 -6 M to 10 -12 M). Subsequently, 20 μL of each of these solutions with different concentrations was added dropwise to the Ag@g-C3N 4 / TiO2 cotton fabric flexible substrate surface. After air drying at room temperature, SERS measurements were performed directly on the flexible SERS substrate.

[0074] Figure 4 MG powder samples and 10 -6 SERS spectra of MMG adsorbed on different substrates. The MG powder sample has a peak at 916 cm -1 、1173cm -1 、1219cm -1 、1366cm -1 、1397cm -1 and 1616cm -1 There are 6 strong Raman characteristic peaks at 916cm -1 、1173cm -1 and 1219cm -1 The characteristic peaks at 1366 cm are related to out-of-plane CH bending, in-plane CH bending and CH rocking. -1 and 1397cm -1 The characteristic peak at 1616cm is due to the symmetric stretching vibration of the CN group. -1 The most prominent characteristic peak is caused by the stretching vibration of the C-C ring. Except for the apparent characteristic peaks of MG in the Ag cotton fabric flexible substrate and the Ag@g-C3N4 / TiO2 cotton fabric flexible substrate, no characteristic peaks of MG were detected in the Raman spectra of other flexible substrates. Compared with other flexible substrates, Ag NPs provide SERS signals due to their significant LSPR effect, thereby increasing the Raman vibration peaks of MG. In addition, due to the large specific surface area of g-C3N4, which adsorbs more probe molecules, Ag@g-C3N4 / TiO2 obtains the strongest SERS signal.

[0075] b. Sensitivity detection of Ag@g-C3N4 / TiO2 cotton fabric substrate

[0076] In order to evaluate the SERS sensitivity of Ag@g-C3N4 / TiO2 cotton fabric flexible substrate, the SERS detection results were analyzed under the MG concentration gradient (10 -6 M to 10 -12 M) range was recorded.

[0077] Figure 5 a is the SERS signal of MG with different concentrations on Ag@g-C3N4 / TiO2 cotton fabric substrate, 5b is the SERS signal of Ag@g-C3N4 / TiO2 cotton fabric with peak at 1616 cm -1 The linear fitting of the peak at 1616 cm and the MG concentration is shown in the figure. As shown in the figure, the SERS signal intensity of Ag@g-C3N4 / TiO2 cotton fabric substrate gradually decreases with the decrease of MG concentration. -1The SERS signal at 100 nm was used as a quantitative characteristic peak to establish a linear relationship between the MG peak intensity and the logarithm of the concentration. 2 ) has a square value of 0.997. The linear regression equation can be expressed as: 1616 cm -1 =58499.5+4805.2log C, where "I" represents 1616 cm -1 The SERS intensity at , C represents the concentration of MG. When the signal-to-noise ratio is 3, the theoretical detection limit (LOD) of MG is calculated using the linear regression equation, and its value is 6.74×10 -13 Compared with previous studies on MG detection, this method has higher sensitivity and lower LOD.

[0078] Example 3 Photocatalytic activity of Ag@g-C3N4 / TiO2 cotton fabric substrate

[0079] To investigate the reusability of the composite flexible substrate, we evaluated its photocatalytic degradation of probe molecules under simulated sunlight. In the experiment, a xenon lamp was used as the simulated sunlight source, with a power of 300W and a current of 20A. The distance between the source and the sample was maintained at 5cm. The photocatalytic reaction was carried out under continuous simulated sunlight, and the SERS signal of the sample was collected every 6 minutes to monitor the extent of molecular degradation. Figure 6 It uses simulated sunlight to degrade 10 -6 SERS spectra of MMG adsorbed on an Ag@g-C3N4 / TiO2 cotton fabric substrate. The Raman signal intensity on the composite flexible substrate surface decreases with increasing irradiation time. After 30 minutes of irradiation, the characteristic Raman peak signal almost completely disappears, indicating a 94.6% degradation rate of the probe molecule on the flexible substrate surface under simulated sunlight. g-C3N4 and TiO2 impart photocatalytic activity to the substrate, while Ag NPs significantly enhance the light absorption capacity of the Ag@g-C3N4 / TiO2 cotton fabric flexible substrate, particularly in the visible light range. Furthermore, Ag NPs effectively capture electrons from g-C3N4 and TiO2, promoting the separation of photogenerated electrons and holes. The synergistic effect of AgNPs, TiO2, and g-C3N4 enhances the photocatalytic activity by generating free radicals and electron holes, accelerating the decomposition of the MG dye and achieving substrate renewal.

[0080] Example 4 Reusability of Ag@g-C3N4 / TiO2 Cotton Fabric Substrate

[0081] The reusability of the composite flexible substrate was demonstrated through a series of repeated testing and self-cleaning treatment experiments conducted under simulated sunlight exposure for 30 minutes and a current intensity of 20A. -6Using MMG as a probe molecule, the first SERS measurement was performed on an Ag@g-C3N4 / TiO2 cotton fabric substrate. Subsequently, the substrate was exposed to simulated sunlight for 30 minutes to self-clean, restoring its SERS performance and enabling the next SERS measurement and self-cleaning cycle. This experiment was repeated 10 times on the same substrate. Figure 7 a is the Ag@g-C3N4 / TiO2 cotton fabric substrate on 10 -6 The SERS spectrum 7b of the MMG detection-photocatalytic self-cleaning cycle experiment is the SERS spectrum of MG at 1616 cm in each “detection-cleaning” cycle. -1 The Raman intensity of the band changes. The main characteristic peak of MG can be clearly observed during the detection process and disappears after self-cleaning. In 10 consecutive regeneration experiments, the intensity of the SERS detection spectrum obtained by adding MG solution in each cycle remained almost unchanged, with a relative standard deviation (RSD) of 4.8%. This shows that the substrate has good reusability and can maintain the stability of the SERS signal during multiple "detection-cleaning" cycles.

[0082] In order to test the universality of the substrate, an alternating detection-degradation cycle experiment of different dye molecules was carried out on the Ag@g-C3N4 / TiO2 cotton fabric substrate. -6 The MMG solution was dropped on the substrate to detect the SERS signal, and then the second SERS test was performed after 30 minutes of simulated sunlight irradiation. -6 MR6G solution was added dropwise to the same substrate, and SERS analysis was performed before and after degradation. This cycle was repeated four times. Figure 8 a is a schematic diagram of the alternate detection-degradation and reusable Ag@g-C3N4 / TiO2 cotton fabric. Figure 8 b Alternating photocatalytic degradation of Ag@g-C3N4 / TiO2 cotton fabric 10 -6 SERS spectra recorded during the MMG and R6G processes. As can be seen from the figure, a clear SERS signal is displayed before each self-cleaning step; however, after self-cleaning, the signal disappears without affecting the next measurement. This demonstrates the excellent repeatability and self-cleaning ability of the Ag@g-C3N4 / TiO2 cotton fabric.

[0083] Example 5 Actual sample detection of Ag@g-C3N4 / TiO2 cotton fabric substrate

[0084] The illegal use of aquaculture drugs, especially MG, poses a major challenge to the environment and public health. These chemicals may be transmitted through the food chain and affect human health. In order to verify the application potential of Ag@g-C3N4 / TiO2 cotton fabric flexible substrate in food safety, an experiment was conducted to directly detect MG residues in real samples. First, MG (20 μL, 10 -6 M) was applied to the surface of herring and wiped with Ag@g-C3N4 / TiO2 flexible substrate, followed by SERS analysis. After 30 minutes of simulated sunlight irradiation, we conducted a second SERS test on the sample. -6 M) was smeared on the surface of a pomfret, wiped with the same Ag@g-C3N4 / TiO2 flexible substrate, and subjected to SERS analysis. After 30 minutes of simulated sunlight irradiation, a fourth SERS test was performed.

[0085] Figure 9 a is the process of actual sample testing. Figure 9 b is the MG at 1616cm in the “detection-cleaning” cycle for different fish species. -1 The Raman intensity of the band changes. Figure 9 c is the MG Raman spectra of Ag@g-C3N4 / TiO2 cotton fabric before and after self-cleaning by wiping herring. Figure 9 d is the MG Raman spectrum of Ag@g-C3N4 / TiO2 cotton fabric before and after self-cleaning. The experimental results show that the SERS peak of the Ag@g-C3N4 / TiO2 cotton fabric is prominent before the self-cleaning process. However, after self-cleaning, the SERS signal almost completely disappears. This demonstrates that the Ag@g-C3N4 / TiO2 cotton fabric flexible substrate has excellent self-cleaning capabilities, effectively removing contaminants from the sample surface and providing a clean detection surface for the next round of testing, ensuring accurate and reproducible detection.

[0086] Figure 10Schematic diagram of the preparation process of a flexible Ag@g-C3N4 / TiO2 cotton fabric substrate and its subsequent reusable photocatalytic degradation. We successfully prepared g-C3N4 / TiO2 cotton fabric using a sol-gel-assisted hydrothermal synthesis method. First, TBT and g-C3N4 nanosheets were ultrasonically treated in anhydrous ethanol for 40 minutes to obtain a homogeneous premixed solution. Due to the strong polarity of g-C3N4 nanosheets, upon binding with TBT, the solution formed a stable, uniform dispersion. Next, this mixed solution, pretreated cotton fabric, and deionized water were transferred to a reactor for a hydrothermal reaction. During this process, TiO2 and g-C3N4 were firmly embedded in the cotton fabric fiber structure, significantly enhancing the substrate's stability and imparting photocatalytic properties. Finally, Ag NPs were deposited on the g-C3N4 / TiO2 cotton fabric surface by in situ reduction of AgNO3 with the dropwise addition of ascorbic acid. This open structure not only facilitates the uniform distribution of silver nanoparticles but also promotes their direct interaction with target molecules, thereby forming a composite substrate with efficient photocatalytic degradation performance and significant SERS signal amplification effect. In summary, the synergistic effect of TiO2, g-C3N4, and Ag NPs enables this composite cotton fabric substrate to exhibit remarkable performance in photocatalytic degradation and SERS signal amplification. By precisely controlling the synthesis process of the composite material, not only is the photocatalytic activity of the substrate effectively improved, but its reusability is also ensured, thus providing a new and efficient platform for the removal of environmental pollutants and molecular detection.

Claims

1. A reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate, characterized in that: The preparation method is as follows, 1) Pretreatment of cotton fabric: soak the cotton fabric in KOH solution, place it in a Teflon-lined autoclave for treatment, wash it until neutral, and freeze-dry it; 2) Preparation of g-C3N4 / TiO2 cotton fabric substrate: TBT and g-C3N4 nanosheets were added to anhydrous ethanol, and after ultrasonic treatment, the pretreated cotton fabric obtained in step 1) and deionized water were added, and the mixture was transferred to a Teflon-lined autoclave for hydrothermal treatment, followed by washing and vacuum freeze-drying to obtain a g-C3N4 / TiO2 cotton fabric substrate; 3) Preparation of Ag@g-C3N4 / TiO2 cotton fabric substrate: The g-C3N4 / TiO2 cotton fabric substrate obtained in step 2) was placed in an AgNO3 solution for treatment, and then transferred to an ascorbic acid solution for immersion, rinsed, and vacuum-dried to obtain an Ag@g-C3N4 / TiO2 cotton fabric substrate.

2. The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1, characterized in that: In step 1), the treatment is carried out in an autoclave at 130° C. and 80% filling rate for 5 hours.

3. The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1, characterized in that: In step 2), the hydrothermal treatment is performed at 120° C. for 5 hours.

4. The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1, characterized in that: In step 3), the concentration of the AgNO3 solution is 0.02M, and the treatment is soaking for 3 hours.

5. The reusable Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1, characterized in that: In step 3), the soaking in the ascorbic acid solution is a 0.01 M ascorbic acid solution for 20 minutes.

6. Use of the Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1 in the photodegradation of malachite green.

7. Use of the Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1 in the qualitative detection of malachite green.

8. Use of the Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1 in the quantitative detection of malachite green.

9. Use of the Ag@g-C3N4 / TiO2 cotton fabric SERS substrate according to claim 1 in detecting malachite green on the surface of fish.

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