A method for preparing a SERS chip, the resulting product, and its application in ultra-trace mercury ion detection.

Abstract

This invention discloses a method for preparing a SERS chip, the resulting product, and its application in ultra-trace mercury ion detection. Using silver nitrate and melamine as raw materials, this invention employs a simple in-situ self-assembly and low-temperature solid-phase thermochemical reaction strategy to obtain a SERS chip composed of porous Ag@g-C3N4 composite nanowires. The chip obtained by this invention exhibits excellent selectivity for mercury ions, an ultra-wide linear detection range, and an ultra-low detection limit. Furthermore, the preparation method is simple and easily scaled up, convenient to apply, and offers fast detection speed, demonstrating promising application prospects.

Description

Technical Field

[0001] This invention relates to a method for preparing a SERS chip and the resulting product, specifically to a method for preparing a SERS chip that is simple to operate, has a concise process, is low in cost, and is easy to mass-produce. This SERS chip has the advantages of a wide detectable range of mercury ion concentration and low response sensitivity, and can be used for ultra-trace mercury ion detection, belonging to the field of spectroscopic analysis and detection technology. Background Technology

[0002] Mercury ions are widely present in industrial wastewater. In aquatic environments, inorganic mercury ions can be converted by microorganisms into the more toxic methylmercury, which accumulates rapidly through the aquatic food chain, posing a serious threat to the ecological environment and human health. Therefore, monitoring mercury ions has always been an important aspect of environmental protection, as well as a significant hot topic and challenge in analytical chemistry research.

[0003] Traditionally, quantitative detection of mercury ions has relied primarily on chromatography, spectrophotometry, atomic fluorescence spectrometry, and inductively coupled plasma mass spectrometry. However, these methods often involve expensive equipment, complex sample pretreatment, and high detection limits, and in particular, they fail to meet the requirements of rapid, sensitive, and accurate on-site detection. For rapid on-site detection of mercury ions, the detection limit is crucial because the long-term accumulation effect of mercury ions means that even low concentrations can eventually pose a serious threat to human health through the food chain.

[0004] Raman spectroscopy is a spectroscopic analysis method based on laser-excited characteristic Raman signals of analytes. It has many advantages, such as being sample-free, rapid, non-destructive, and in-situ. However, conventional Raman spectroscopy has a high detection limit, making it difficult to meet the requirements for ultra-trace detection. In recent years, surface-enhanced Raman scattering (SERS) technology has been gradually introduced into ultra-trace analysis and detection. SERS relies on the enhancement of Raman scattering signals generated by localized surface plasmon resonance in noble metal plasmas. This Raman scattering spectroscopy technique can significantly improve the detection signal intensity of the analyte, enabling trace-level detection, and even single-molecule-level analysis and research.

[0005] Unfortunately, SERS technology is more effective for detecting organic molecules with large scattering cross-sections, but it cannot directly detect mercury ions with extremely small scattering cross-sections. Therefore, an indirect SERS strategy is usually employed for mercury ion detection. In this indirect strategy, an organic molecule with a large scattering cross-section is typically selected as the Raman signal reporter, and then the mercury ion is detected by utilizing the positive enhancement (turn on) or negative attenuation (turn off) mechanism of the reporter signal. This indirect strategy makes ultra-sensitive detection of mercury ions using SERS technology possible.

[0006] Currently reported ultra-trace mercury ion detection techniques based on indirect SERS strategies typically involve multi-step pure liquid-phase synthesis processes. These methods suffer from numerous drawbacks, including numerous steps, expensive reagents, stringent conditions, and low preparation efficiency, failing to meet the practical demands for low-cost, large-scale applications. Therefore, researching and developing low-cost, easily scalable, high-performance indirect SERS active substrates for ultra-trace mercury ion detection is of significant scientific and practical importance. Summary of the Invention

[0007] In view of this, the present invention provides a method for preparing a SERS chip and the resulting product. The method is simple, low-cost, and easy to operate. The resulting SERS chip has excellent selectivity for mercury ions, an ultra-wide linear detection range, and an ultra-low detection limit, and has good prospects for market application.

[0008] The specific technical solution of this invention is as follows:

[0009] A method for fabricating a SERS chip, the method comprising the following steps:

[0010] (1) Add silver nitrate aqueous solution to melamine hot aqueous solution and stir to obtain hot colloidal solution;

[0011] (2) Drop the hot colloidal solution onto the substrate and let it stand until the water has completely evaporated;

[0012] (3) The substrate is subjected to rapid low-temperature heat treatment under gas protection to form Ag@g-C3N4 nanowires on the substrate surface, thus obtaining the SERS chip.

[0013] Furthermore, in step (1), melamine is dissolved in hot water to form a colorless and transparent solution, and the temperature of the solution is controlled between 60 and 70°C.

[0014] Furthermore, in step (1), in the system formed by melamine, water and silver nitrate, the concentration of melamine is 0.9~2.6 mg / ml and the concentration of silver nitrate is 0.8~1.8 mg / ml.

[0015] Furthermore, in step (1), the molar ratio of melamine to silver nitrate is controlled between 1.5 and 2.0.

[0016] Furthermore, in step (1), the present invention first dissolves a certain amount of white melamine powder (denoted as M) in hot water under stirring to obtain a colorless melamine hot solution, and then slowly adds a certain amount of silver nitrate (denoted as Ag) aqueous solution under vigorous stirring, and continues to stir for a period of time to obtain a colorless and transparent AgM coordination polymer hot melt solution.

[0017] Furthermore, in step (2), after the colloidal solution is dropped onto the substrate, a uniform liquid film is formed, and the substrate is left to stand at room temperature. During the standing process, the colloidal solution undergoes solvent evaporation and self-assembly on the substrate surface to obtain a white thin film of radial AgM coordination polymer nanowires.

[0018] Furthermore, in step (2), the substrate can be an inert substrate material that does not interfere with Raman signals, such as a single-crystal silicon wafer or a metal sheet (thin film).

[0019] Furthermore, in step (3), the substrate is rapidly heated to the target temperature of 280-340℃ at a relatively fast heating rate of 20-40℃ / min to carry out an isothermal solid-phase thermal transformation reaction. The isothermal reaction time is 120-240 min, realizing the thermal reduction from silver ions to elemental silver nanoparticles and the thermal transformation from melamine to graphitic carbon nitride (g-C3N4) polymer, and obtaining Ag@g-C3N4 porous composite nanowires composed of silver nanoparticles wrapped in graphitic carbon nitride as structural units.

[0020] Furthermore, in step (3), the substrate undergoes a isothermal solid-phase thermal transformation reaction under the protection of inert gases such as nitrogen and argon.

[0021] In this invention, readily available silver nitrate is used as the metal source, and melamine is used as both the carbon and nitrogen source. The method employs a simple in-situ self-assembly and solid-state thermochemical reaction strategy (see appendix). Figure 1 A porous SERS chip with Ag@g-C3N4 composite nanowires as the main component was obtained. This SERS chip can be used for ultra-trace mercury ion detection. g-C3N4 is a macromolecular polymer with a graphite-like structure composed of polytriazine structural units. This polymer has advantages such as stable physicochemical properties, large Raman scattering cross section, and obvious characteristic Raman signal. In the detection of mercury ions, the characteristic Raman signal of g-C3N4 serves as the reporter signal for mercury ion characteristics. I The SERS chip utilizes the principle that mercury ions can alter the surface-enhanced Raman scattering (SERS) capability of silver nanoparticles in a chip, thereby changing the intensity of the Raman signal characteristic of the Reporter. When the SERS chip is placed in a solution containing mercury ions, the mercury ions react with the silver nanoparticles in the Ag@g-C3N4 composite nanowires, forming a silver amalgam on the surface of the silver nanoparticles. This reduces the surface-enhanced Raman scattering capability of the silver nanoparticles, thus leading to a decrease in the intensity of the g-C3N4 Reporter characteristic Raman signal characteristic in the Ag@g-C3N4 composite nanowires. I R The intensity of the reduced Reporter signal ( I 0- I R ) and mercury ion concentration ( C It is directly proportional to .

[0022] Furthermore, when the SERS chip is used for qualitative detection of mercury ions, the initial characteristic signal of the Reporter in the SERS chip is first tested using a Raman spectrometer (either benchtop or handheld). I 0), then place the chip in the test solution for several minutes, remove it, and measure the characteristic signal of the Reporter in the chip again under the same conditions ( I R The intensity of the characteristic Raman signal in the two measurements is compared. If the characteristic Raman signal is significantly weakened, it indicates that the detected object contains mercury ions, and the signal weakening is proportional to the mercury ion concentration. If the characteristic Raman signal shows no significant change, it indicates that the detected object does not contain mercury ions or contains mercury ions at a concentration lower than the detection limit of the chip. When the SERS chip is applied to the quantitative detection of mercury ion solutions, a standard working curve (Δ) of the characteristic Raman signal attenuation amplitude versus the mercury ion concentration is first established using standard solutions with known mercury ion concentrations, following the above testing method. I = I 0 - I R ∝C Hg Because the resulting chip has an ultra-wide linear range, the relative strength of signal attenuation (Δ) can also be used. I / I 0) Plot the logarithm of concentration (lgC), then measure the unknown solution. Substitute the measurement results into the standard curve to obtain the concentration of the unknown mercury ion solution.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The raw materials and equipment required for the fabrication of the SERS chip of this invention are common and readily available, the fabrication cost is low, and the fabrication method is simple, easy to control, easy to scale up, and easy to apply on a large scale. The resulting chip exhibits excellent selectivity for mercury ions, an ultra-wide linear detection range, and an ultra-low detection limit.

[0025] 2. The SERS chip of this invention can be used only once, avoiding cross-contamination and performance degradation.

[0026] 3. The SERS chip of this invention exhibits excellent selectivity for mercury ions, an ultra-wide linear detection range, and an ultra-low detection limit. This chip is convenient and quick to use, with fast detection speed, requiring no sample pretreatment and can be used directly for testing. It is suitable for both laboratory testing and rapid on-site detection. Therefore, this invention has good prospects for market application and socio-economic benefits. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the fabrication route of the SERS chip of this invention.

[0028] Figure 2 This is a scanning electron microscope image of the porous Ag@g-C3N4 nanowires prepared in Example 1.

[0029] Figure 3 This is a transmission electron microscope image of the porous Ag@g-C3N4 nanowires prepared in Example 1.

[0030] Figure 4 This is the nitrogen adsorption-desorption isotherm diagram of the porous Ag@g-C3N4 nanowires prepared in Example 1.

[0031] Figure 5 This is the characteristic Raman spectrum of the porous Ag@g-C3N4 nanowires prepared in Example 1.

[0032] Figure 6 The porous Ag@g-C3N4 chip prepared in Example 1 exhibits selectivity for mercury ions.

[0033] Figure 7 The images show the Raman spectra of the porous Ag@g-C3N4 chip prepared in Example 1 after interaction with different mercury ion concentrations.

[0034] Figure 8 This is the linear range curve of mercury ion detection for the porous Ag@g-C3N4 chip prepared in Example 1.

[0035] Figure 9 This is an electron scanning image of the product obtained under air atmosphere conditions in Comparative Example 1.

[0036] Figure 10 This is an electron scanning image of the product after calcination at a higher thermal transformation temperature in Comparative Example 2.

[0037] Figure 11 This is an electron scanning image of the product after calcination at a lower thermal transition temperature in Comparative Example 3.

[0038] Figure 12 This is a scanned electron image of the product after calcination at a low heating rate, as shown in Comparative Example 4. Detailed Implementation

[0039] The technical solutions and advantages of the present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. It should be understood that the following embodiments are only for enabling those skilled in the art to better understand the present invention and are not intended to limit the scope of protection of the present invention.

[0040] Example 1

[0041] Accurately weigh 780 mg of melamine and add it to 300 mL of hot ultrapure water at 70°C with stirring. Continue stirring for 15 minutes to obtain a colorless and transparent hot melamine aqueous solution. Then, under stirring, add 10 mL of a 55 mg / mL silver nitrate aqueous solution to the above hot melamine aqueous solution and continue stirring for 30 minutes to obtain a colorless and transparent colloidal solution. Then, use a pipette to transfer the hot colorless colloidal solution to the surface of a clean monocrystalline silicon wafer, obtaining nearly hemispherical hot colloidal solution droplets. The size of the droplets can be flexibly adjusted by the pipette to ensure that a layer of colloidal solution is evenly spread on the surface of the monocrystalline silicon wafer. Then, it is allowed to cool naturally. As the droplet temperature decreases and the solvent evaporates, radial white coordination polymer nanowires gradually precipitate at the droplet's in-situ. After the solvent has completely evaporated, the single-crystal silicon wafer is transferred to a tube furnace and heated to 340 °C at a rate of 40 °C / min under an inert atmosphere. This temperature is then held for 240 min to allow the silver ions in the coordination polymer to be completely thermally reduced to elemental silver nanoparticles. Simultaneously, melamine undergoes a thermal condensation reaction and transforms into macromolecular graphitic carbon nitride (g-C3N4) on the surface of the silver nanoparticles. After natural cooling to room temperature, a SERS active chip composed of porous Ag@g-C3N4 nanowires is obtained.

[0042] Scanning electron microscope image of the SERS active chip as shown below Figure 2 As shown in the figure, a large number of nanowires exist on the surface of the single-crystal silicon wafer, and these nanowires are composed of nanoparticles. The transmission electron microscope image and nitrogen adsorption isotherm diagram of the SERS active chip are shown below. Figure 3 and Figure 4 As shown in the figure, the Ag@g-C3N4 nanowires exhibit obvious porous structure characteristics. Raman spectroscopy characterization of this SERS active chip is shown in the figure below. Figure 5 As shown in the figure, the obtained chip is at ~1360 cm⁻¹. -1 and ~1580 cm -1 It exhibits a very strong characteristic Raman signal.

[0043] Performance Evaluation

[0044] 1. The selectivity experimental results of the SERS active chip are as follows: Figure 6 As shown, the specific steps are as follows: Prepare mercury ion solutions and other common inorganic metal ion solutions with a concentration of 1.0 mg / L respectively, and then detect the initial 1580 cm⁻¹. -1 Characteristic Raman signal ( I The SERS active chips of 0 were placed in the above solutions respectively, allowed to stand for 10 minutes, and then removed. The 1580 cm⁻¹ of each chip was then measured again under the same conditions. -1 Characteristic Raman signal ( IR ), calculate the ( ) of various metal ion solutions I 0- I R ) / I R Numerical values ​​were calculated, and the results were plotted, as shown below. Figure 6 As shown in the figure, the chip of the present invention has excellent selectivity for mercury ions.

[0045] 2. Detection results showing the dependence of characteristic signals on mercury ion concentration. Figure 7 As shown, the specific steps are as follows: First, prepare a series of mercury ion aqueous solutions of different concentrations (1×10⁻⁶) using a stepwise dilution method. -1 ng / L ~ 1×10 7 Similarly, the initial characteristic Raman spectrum of the chip was first tested (ng / L). I 0), and then the chips were sequentially immersed in mercury ion solutions of different concentrations, from low to high, for 10 minutes each. After each concentration was completed, a characteristic Raman spectrum was obtained under the same conditions. I R Raman spectra of the chip after interaction with different mercury ion concentrations were obtained (attached). Figure 7 ).from Figure 7 As can be seen, the characteristic Raman signal of the chip gradually weakens as the concentration of mercury ions increases.

[0046] 3. Quantitative detection of linear curve results, as shown below. Figure 8 As shown, the specific steps are as follows: First, according to the method shown in performance characterization 2 above, the characteristic Raman signal spectra of the chip after being reacted with a series of standard mercury ion solutions of different concentrations are obtained. Then, under the same conditions, baseline correction is performed on each spectrum, and the 1580 cm⁻¹ value of each spectrum is read. -1 Characteristic Raman signal intensity ( I R ), and calculate the concentration of each mercury ion ( C ) corresponding to ( I 0- I R ) / I R Numerical values. Then, respectively using ( I 0- I R ) / I R The logarithm lg is commonly used for the vertical axis and concentration. C Plot the graph on the x-axis, then perform linear fitting using the least squares method to obtain the fitted working curve, as shown below. Figure 8As shown in the figure, the obtained chip exhibits an ultra-wide linear range for mercury ions. The standard equation, calculated using standard curve fitting, is as follows: Y =0.08 X +0.37, R 2 =0.99. Based on this linear fitting equation, it can be seen that the theoretical minimum detection concentration of this chip (i.e., when...) is... Y Concentration corresponding to =0 X ) is lg C =X=-4.625, considering actual testing requirements, the conservative calculation shows that the minimum detection limit is at least lg. C =-4.0~-3.0, which corresponds to the concentration C The concentration was 0.1~1.0 pg / L, indicating that the obtained chip has an ultra-low detection limit of picograms per liter.

[0047] Example 2

[0048] Accurately weigh 520 mg of melamine and add it to 300 mL of hot ultrapure water at 65°C with stirring. Continue stirring for 15 minutes to obtain a colorless and transparent hot melamine aqueous solution. Then, under stirring, add 10 mL of a 35 mg / mL silver nitrate aqueous solution to the above hot melamine aqueous solution and continue stirring for 20 minutes to obtain a colorless and transparent colloidal solution. Then, use a pipette to transfer the hot colorless colloidal solution to the surface of a clean monocrystalline silicon wafer, obtaining nearly hemispherical hot colloidal solution droplets. The size of the droplets can be flexibly adjusted by the pipette to ensure that a layer of colloidal solution is evenly spread on the surface of the monocrystalline silicon wafer. Then, the droplet was allowed to cool naturally. As the temperature of the droplet decreased and the solvent evaporated, radial white coordination polymer nanowires gradually self-assembled and precipitated at the droplet's in-situ. After the solvent had completely evaporated, the single-crystal silicon wafer was transferred to a tube furnace and heated to 300 °C at a rate of 30 °C / min under an inert atmosphere. This temperature was then held for 180 min to allow the silver ions in the coordination polymer to be completely thermally reduced to elemental silver nanoparticles. Simultaneously, melamine underwent a thermal condensation reaction and transformed into macromolecular graphitic carbon nitride (g-C3N4) on the surface of the silver nanoparticles. After natural cooling to room temperature, a SERS active chip composed of porous Ag@g-C3N4 nanowires was obtained. The microstructure of the obtained chip was similar to that of Example 1. The chip's detection performance for mercury ions was tested according to the performance evaluation method of Example 1, and it also exhibited excellent selectivity, ultra-low detection limit, and ultra-wide linear range.

[0049] Example 3

[0050] Accurately weigh 280 mg of melamine and add it to 300 mL of hot ultrapure water at 60°C with stirring. Continue stirring for 15 minutes to obtain a colorless and transparent hot melamine aqueous solution. Then, under stirring, add 10 mL of a 25 mg / mL silver nitrate aqueous solution to the above hot melamine aqueous solution and continue stirring for 10 minutes to obtain a colorless and transparent colloidal solution. Then, use a pipette to transfer the hot colorless colloidal solution to the surface of a clean monocrystalline silicon wafer, obtaining nearly hemispherical hot colloidal solution droplets. The size of the droplets can be flexibly adjusted by the pipette to ensure that a layer of colloidal solution is evenly spread on the surface of the monocrystalline silicon wafer. Then, the wafer was allowed to cool naturally. As the droplet temperature decreased and the solvent evaporated, radial white coordination polymer nanowires gradually self-assembled and precipitated at the droplet's in-situ. After the solvent had completely evaporated, the single-crystal silicon wafer was transferred to a tube furnace and heated to 280 °C at a rate of 20 °C / min under an inert atmosphere. This temperature was then held for 120 min to allow the silver ions in the coordination polymer to be completely thermally reduced to elemental silver nanoparticles. Simultaneously, melamine underwent a thermal condensation reaction, forming macromolecular graphitic carbon nitride (g-C3N4) on the surface of the silver nanoparticles. After natural cooling to room temperature, porous Ag@g-C3N4 nanowires composed of silver nanoparticles encapsulated in graphene carbon nitride were obtained on the surface of the single-crystal silicon wafer, thus yielding the SERS active chip. The resulting chip had a similar microstructure to that of Example 1. The chip's detection performance for mercury ions was tested according to the performance evaluation method of Example 1, and it also exhibited excellent selectivity, ultra-low detection limit, and ultra-wide linear range.

[0051] Comparative Example 1

[0052] SERS active chips were prepared according to the method in Example 1, except that the thermal transition was performed in an air atmosphere. The results are shown (see attached). Figure 9 The resulting product has an irregular morphology and cannot yield the target product prepared by this method. Due to the significant thermal loss of the characteristic organic reporter in the air atmosphere, the Raman signal of the obtained product is extremely poor, resulting in extremely poor mercury ion detection performance.

[0053] Comparative Example 2

[0054] SERS active chips were prepared according to the method in Example 1, except that the target isothermal thermal transition temperature was increased to a higher temperature (400°C). The results are shown (see attached). Figure 10 The product had an irregular morphology and could not yield the target product prepared by this method. The product was tested according to the performance evaluation method of Example 1; the characteristic organic reporter signal was weak, indicating poor detection performance for mercury ions.

[0055] Comparative Example 3

[0056] SERS active chips were prepared according to the method in Example 1, except that the target isothermal thermal transition temperature was lowered to a lower temperature (200°C). The results are shown (see attached). Figure 11 The product exhibited an intermediate transition morphology, indicating that the required temperature for the thermal transformation had not been reached, and the target product prepared by this method could not be obtained. Testing the product according to the performance evaluation method of Example 1 revealed that the characteristic signal of the target Reporter was not obtained, making ultra-trace detection of mercury ions impossible.

[0057] Comparative Example 4

[0058] SERS active chips were prepared according to the method in Example 1, except that the heating rate was 5 °C / min. The results showed that the product morphology consisted of relatively dispersed nanoparticles (see attached image). Figure 12 The target product prepared by this method could not be obtained. The product was tested according to the performance evaluation method of Example 1; the characteristic organic Reporter signal was weak, indicating poor detection performance for mercury ions.

[0059] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have similar substitutions and variations. Any modifications, equivalent substitutions, improvements, imitations, etc., made within the spirit and principles of the present invention are included within the protection scope of the present invention.

Claims

1. An application of a SERS chip in mercury ion detection, characterized in that, The method for fabricating the SERS chip includes the following steps: (1) Add silver nitrate aqueous solution to melamine hot aqueous solution and stir to obtain hot colloidal solution; (2) Drop the hot colloidal solution onto the substrate and let it stand until the water has completely evaporated; (3) The substrate is subjected to rapid low-temperature heat treatment under gas protection to form Ag@g-C3N4 nanowires on the substrate surface, thus obtaining the SERS chip.

2. The application according to claim 1, characterized in that: In step (1), in the system formed by melamine, water and silver nitrate, the concentration of melamine is 0.9~2.6 mg / ml and the concentration of silver nitrate is 0.8~1.8 mg / ml.

3. The application according to claim 1 or 2, characterized in that: In step (1), the molar ratio of melamine to silver nitrate is 1.5~2.0:

1.

4. The application according to claim 1 or 2, characterized in that: In step (1), the temperature of the melamine hot aqueous solution is 60~70℃.

5. The application according to claim 1, characterized in that: In step (2), after the colloidal solution is dropped onto the substrate, the substrate is left to stand at room temperature.

6. The application according to claim 1, characterized in that: In step (3), the substrate is heated to 280-340℃ at a heating rate of 20-40℃ / min for rapid low-temperature heat treatment, and the treatment time is 120-240 min.

7. The application according to claim 1 or 6, characterized in that: In step (3), the substrate is heat-treated under inert gas protection.

8. The application according to claim 1, characterized in that: Ag @g-C3N4 nanowires are composed of nanoparticles and have a porous structure.

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