Method for detecting oxynitride based on SERS

Abstract

The invention discloses a method for detecting oxynitride based on SERS (Surface Enhanced Raman Scattering), and relates to the technical field of compound detection. The preparation method comprises the following steps: constructing an Ag nanowire-Au nanoparticle three-dimensional plasmon skeleton on a porous inert support body, performing in-situ growth of an amino-containing MFM-520MOF capture layer, and covering the capture layer with an AuAg nanoparticle embedded polyacrylamide-carboxymethyl cellulose hydrogel to obtain the ternary composite SERS substrate. The upper surface is in synchronous contact with the gas channel, and the lower surface is in synchronous contact with the liquid channel, so that synchronous enrichment of NO / NO and NO / NO is realized. By means of double amplification of an electromagnetic hot spot and an in-situ chemical reaction, gas-phase 0.1 ppb NOx and liquid-phase 5nmol L NOx can be measured within 10s only by using a 785nm handheld Raman spectrometer, a substrate is quickly regenerated through 50-DEG C hot air or a 405nm LED, the substrate can be recycled for more than 100 times, and the device is suitable for real-time monitoring of a field environment.

Description

Technical Field

[0001] The present invention relates to the technical field of compound detection, and in particular to a method for detecting nitrogen oxides based on SERS. Background Art

[0002] Current NOx monitoring systems generally adopt a separate approach: NO / NO2 in air relies primarily on electrochemical electrodes, chemiluminescence, or UV differential absorption spectroscopy, while NO2⁻ / NO3⁻ in water requires ion chromatography, flow injection colorimetry, or laboratory titration. These approaches suffer from four common problems: Distributed equipment requires two sets of sensor units and two calibration systems for gas and liquid, resulting in complex deployment and difficulty resolving mutual errors; limited sensitivity: electrochemical electrodes are subject to cross-interference from humidity and O3, making sub-ppb quantification difficult, and aqueous colorimetric detection limits often remain in the µmol / L⁻¹ range; response lag: chemiluminescence or colorimetric reactions require gas dilution and colorimetric pretreatment, resulting in delays of minutes or even hours, making real-time warnings unfeasible; and high maintenance costs: multiple instruments require independent calibration gases, reagent replacement, and electrode drift correction, making continuous field monitoring difficult. While traditional SERS has potential for trace detection, the sparse hotspots of common metal nanofilms hinder effective adsorption of gaseous NOx, and aqueous ions also struggle to penetrate the hydrophobic surface. Summary of the Invention

[0003] To overcome the above-mentioned defects of the prior art, the present invention provides the following technical solution: a method for detecting nitrogen oxides based on SERS, comprising: S1. Fabrication of a plasmonic metal skeleton (a three-dimensional interconnected noble metal nano-network structure, using gold (Au), silver (Ag) (noble metals have high free electron density and can support strong LSPR; Au provides chemical stability, Ag provides higher electromagnetic gain) or their alloys to generate localized surface plasmon effects at the nanoscale (localized surface plasmons are generated when the frequency of the incident light matches the oscillation frequency of the metal free electrons), thereby forming a large number of high-intensity electromagnetic hotspots inside and on the surface of the structure (the electromagnetic field is extremely compressed at the sharp corners, hole edges and nanogaps inside the skeleton, and the local field strength can be enhanced by 10²–10 compared to the incident light). 6times). These hotspots are the core of SERS enhancement (when molecules are located near these hotspots, their Raman scattering signals are exponentially amplified, enabling trace or single-molecule detection). Preparation method: Conductive substrate preparation: Ag nanowires are electrodeposited on an alumina template and the template is etched to form a coarse framework with 3D interconnected channels. Surface roughening: Chemical or plasma etching is used to introduce sub-5nm edges and pits, increasing the density of hotspots. Alloy or core-shell manipulation: In situ reduction of HAuCl₄ forms an Au shell on the Ag surface, improving stability, preventing Ag oxidation, and maintaining high LSPR gain. The metal framework in step S1 is a three-dimensional network of Ag nanowires and Au nanoparticles with nanogaps of 1-5 nm. The MOF in step S1 is amine-containing Zr-based MFM-520, with a thickness of 0.3-1 µm and an NO₂ adsorption capacity of ≥3 mmol·g⁻¹ (0.1 bar, 298 K). The hydrophilic gel in step S1 is a polyacrylamide-carboxymethyl cellulose double network gel with a water content of ≥70%, in which 5 nm AuAg alloy nanoparticles 1×10¹²-10¹ are dispersed. 4 In step S1, the aromatic amine probe molecules are pre-immobilized in the gel layer and undergo a Griess-type coupling reaction with NO2⁻ / NO3⁻ to produce azo-characteristic Raman peaks at 1280-1450 cm⁻¹.

[0004] S2. Expose the upper surface of the substrate to the air to be tested and the lower surface to the water sample to be tested to achieve simultaneous sampling of gas and liquid; in step S2, the gas flow rate is 50-300 mL·min⁻¹, and the water sample flow rate is 0.1-2 mL·min⁻¹.

[0005] S3. With the help of MOF's adsorption of gaseous NOx and gel's enrichment of aqueous NOx⁻, Raman spectroscopy is excited at the metal hotspot and the enhanced signal is simultaneously collected. In step S3, a 785nm laser is used to laterally excite the substrate to collect the NO2 symmetric stretching peak in the range of 750-850cm⁻¹ and the azo peak in the range of 1280-1450cm⁻¹.

[0006] S4. Calculate the concentrations of gaseous NO or NO2 and aqueous NO2⁻ / NO3⁻ based on the NOx Raman characteristic peak intensity. In step S4, a peak intensity-concentration calibration curve is pre-established to achieve a NO2 gas detection limit of ≤0.5 ppb and a NO3⁻ aqueous phase detection limit of ≤10 nmol·L⁻¹.

[0007] S5: After regenerating the substrate by controlled heating or light desorption via the purge module, repeat steps S2-S4. In step S5, regeneration is performed using 45-55°C hot air or 405nm LED light for 5-10 minutes. The substrate signal recovery rate is ≥95%, and the substrate can be recycled ≥100 times.

[0008] Preferably, the purge module includes an air storage box, the inner wall of the air storage box is slidingly sealed with a piston moving plate, the top of the inner wall of the air storage box is also rotatably installed with a fan, a funnel is provided directly below the fan, and a condensing plate is provided at the axial position inside the funnel, wherein the fan and the condensing plate are aligned, and a drying drive shell is fixedly installed on the bottom surface of the outer surface of the air storage box, and the internal axial position of the drying drive shell is connected to the interior of the air storage box.

[0009] Preferably, an exhaust nozzle is fixedly connected to the radial position of the outer surface of the drying drive shell, a water baffle is provided at the connection between the exhaust nozzle and the drying drive shell, the bottom edge of the water baffle is fixed to the bottom surface of the inner wall of the drying drive shell, and there is a gap between the top edge of the water baffle and the top surface of the inner wall of the drying drive shell, a heating plate is fixedly installed inside the exhaust nozzle, a centrifugal driven condensing fan blade is rotatably installed inside the drying drive shell, a fixed heat conducting plate is fixedly installed on the lower surface of the centrifugal driven condensing fan blade, and a refrigeration fan is fixedly installed between the fixed heat conducting plate and the opposite surface of the centrifugal driven condensing fan blade. The outer side of the second-stage refrigeration plate is provided with a collector ring coaxially arranged with the centrifugal driven condensing fan blade, which is used to supply power to the second-stage refrigeration plate. The rotary seal on the lower surface of the drying drive shell is equipped with a sealing ring plate. The sealing ring plate and the drying drive shell are provided with grooves of the same shape, and the grooves on the sealing ring plate and the drying drive shell can be staggered or aligned, which is used to connect the inside of the drying drive shell with the outside. The rotation of the sealing ring plate on the drying drive shell is driven by the electric cylinder. The telescopic cylinder of the electric cylinder is movably connected to the exhaust nozzle, and the telescopic rod of the electric cylinder is movably connected to the edge of the sealing ring plate.

[0010] Preferably, a guide seat is fixedly installed overhead below the drying drive shell, wherein the centrifugal driven condensing fan blades and the fixed heat conducting plate are rotatably installed on the guide seat, and the lower surface of the fixed heat conducting plate is fixedly installed with a heat sink, wherein the condensing plate is coaxially fixedly connected with the centrifugal driven condensing fan blades; a fixed coaxial sleeve on the heat sink is provided with a central gear ring, and an outer ring gear ring is coaxially provided on the outer side of the central gear ring, and the outer ring gear ring and the central gear ring are meshed with three planetary gears, and all the planetary gears are rotatably installed on the planetary gear control ring, wherein the planetary gear control ring and the outer ring gear ring are rotatably installed on the control motor bracket, and the control motor bracket is fixedly installed on the air storage box, and the control motor and the drive motor are also fixedly installed on the control motor bracket, wherein the drive motor is also fixed to the air storage box, wherein the output shaft of the control motor is connected to the planetary gear control ring through a first transmission belt, and wherein the output shaft of the drive motor is connected to the outer ring gear ring through a second transmission belt. Compared with the prior art, the present invention has the following advantages: (1) The present invention utilizes the upper and lower surfaces of the same composite SERS substrate to face the air and water respectively, which not only avoids the mutual contamination of the gas and liquid channels, but also allows the samples to converge in the same hotspot area at the micrometer scale, fundamentally eliminating the tedious process of calibrating, timing and comparing the two instruments separately in the traditional split-type solution. The monitoring station only needs to install one probe to simultaneously output atmospheric NO / NO2 and water quality NO2⁻ / NO3⁻, greatly reducing the number of on-site deployment points, the amount of cables and gas line laying, and the manpower for subsequent maintenance, thus achieving the overall benefit of dual measurement in one hole; (2) The Zr-based MFM-520MOF in the composite substrate of the present invention has an ultra-high dynamic adsorption capacity for NO2, and the hydrogel enriches NOx⁻ ions in a hydrophilic network. The two transport the target molecules to the Ag / Au hotspot in a directional manner; at the same time, the plasmon resonance can amplify the electromagnetic field by six orders of magnitude in the 1-5nm gap, and the chemical amplification of the aromatic amine-azo color development is superimposed to improve the signal-to-noise ratio by more than 100 times. Compared with electrochemical and photochemical colorimetric methods, the present invention reduces the gas phase detection limit from more than 10 ppb to 0.1 ppb, and the water phase detection limit from µmolL⁻¹ to 5 nmolL⁻¹, providing sufficient margin for early warning and ecological tracing. (3) The electromagnetic enhancement process of the present invention occurs instantaneously, and the chemical conversion reaction can be completed at the hot spot in just tens of milliseconds. Combined with 785nm laser real-time scanning, the entire device achieves a refresh rate of sub-10 seconds, which meets the requirements for rapid tracking of haze outbreaks and outlet impact emissions. After detection, the adsorbate can be completely desorbed and the coupling product can be removed by irradiation with 50°C hot air or 405nm ultraviolet light. At the same time, it gets rid of the burden of disposable consumables of traditional colorimetric reagents; (4) The amino coordination sites of the MOF of the present invention form Lewis acid-base complexes with NO2, and the Griess reaction in the hydrogel specifically displays color for NO2⁻, while NO3⁻ needs to be reduced or photolyzed to NO2⁻ under hot spot catalysis before it can be displayed; the difference in Raman fingerprint peak positions (810cm⁻¹ symmetric stretching vs. 1280-1450cm⁻¹ azo vibration) provides secondary resolution. The interference of O3, SO2 in the gas and Cl⁻, PO4³⁻ in the water can be suppressed by molecular sieve exclusion, reaction kinetic selection or data post-processing, ensuring the reliability of the measurement; (5) The three-dimensional metal skeleton of the present invention has self-supporting strength and can be processed into chips or flexible films. After being assembled into a detachable microfluidic box, it can be docked with a commercial handheld Raman spectrometer to work. The whole machine has low power consumption and can be powered by solar energy or a mobile power supply. Compared with chemiluminescence analyzers that weigh dozens of kilograms and require high-pressure pumps and standard gas cylinders, this device has a longer maintenance cycle and only requires cleaning or replacing the base patch once on site, greatly reducing the cost of consumables and instrument downtime and adjustment. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Flowchart of the method of the present invention. DETAILED DESCRIPTION

[0012] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific implementation methods.

[0013] The present invention provides a method for detecting nitrogen oxides based on SERS, which comprises: Composite substrate preparation: Electrodeposition of a three-dimensional Ag nanowire-Au nanoparticle skeleton on a porous inert support; in situ growth of a 0.3–1 µm layer of amino-containing Zr-based MFM-520MOF; spin coating of a polyacrylamide-carboxymethyl cellulose hydrogel solution containing 5 nm AuAg alloy nanoparticles, followed by UV curing to form a 50–150 µm gel layer and pre-immobilization of aromatic amine probes.

[0014] Synchronous sampling: The substrate is fixed to a detection cell with an upper gas channel and a lower liquid channel; the air to be tested is passed over the upper surface of the substrate at a flow rate of 50–300 mL·min⁻¹, while the water sample to be tested is passed over the lower surface of the substrate at a flow rate of 0.1–2 mL·min⁻¹; the MOF captures gaseous NO / NO2, and the gel absorbs and enriches aqueous NO2⁻ / NO3⁻.

[0015] In situ chemical conversion and signal enhancement: Gaseous NO is oxidized to NO2⁻ under hotspot catalysis; electrochemical or nitration reactions occur with probe molecules, and NO2⁻ / NO3⁻ couples with pre-placed p-aminoaniline-naphthylethylenediamine to form azo dyes; 785nm laser excitation causes the target molecule and its reaction products to produce enhanced Raman scattering at the metal hotspot.

[0016] Spectral acquisition and quantification: Acquire characteristic peaks such as 810 cm⁻¹ (NO2) and 1280–1450 cm⁻¹ (azo N=N); establish a calibration curve between peak intensity and NOx concentration, and output gas phase NO and NO2 and aqueous phase NO2⁻ / NO3⁻ concentrations.

[0017] Substrate regeneration: Turn off the sample flow and allow 50°C dry air to flow for 5–10 minutes or irradiate with a 405nm LED for 5 minutes to remove adsorption / reaction residues and restore the substrate signal to the initial baseline. Repeat the steps of simultaneous sampling and substrate regeneration.

[0018] The structure of the purge module: It includes an air storage box 124, the inner wall of which is slidingly sealed with a piston moving plate 126, and the top of the inner wall of the air storage box 124 is also rotatably installed with a fan 125, a funnel 111 is provided directly below the fan 125, and a condensing sheet 112 is provided at the axial position inside the funnel 111, wherein the fan 125 is aligned with the condensing sheet 112, and a drying drive shell 101 is fixedly installed on the bottom surface of the outer surface of the air storage box 124, and the internal axial position of the drying drive shell 101 is connected to the interior of the air storage box 124. The radial position of the outer surface of the drying drive shell 101 is fixedly connected and equipped with an exhaust nozzle 102. A water baffle 113 is provided at the connection between the exhaust nozzle 102 and the drying drive shell 101. The bottom edge of the water baffle 113 is fixed to the bottom surface of the inner wall of the drying drive shell 101. There is a gap between the top edge of the water baffle 113 and the top surface of the inner wall of the drying drive shell 101. A heating plate 109 is fixedly installed inside the exhaust nozzle 102. A centrifugal driven condensing fan blade 103 is rotatably installed inside the drying drive shell 101. A fixed heat conducting plate 106 is fixedly installed on the lower surface of the centrifugal driven condensing fan blade 103. A refrigeration secondary plate 104 is fixedly installed between the opposite surface of the fixed heat conducting plate 106 and the centrifugal driven condensing fan blade 103. A collector ring 110 is provided on the outer side of the secondary refrigeration plate 104 and is coaxially arranged with the centrifugal driven condensing fan blade 103, which is used to supply power to the secondary refrigeration plate 104. A sealing ring piece 108 is provided for the rotary seal on the lower surface of the drying drive shell 101. The sealing ring piece 108 and the drying drive shell 101 are both provided with grooves of the same shape, and the grooves on the sealing ring piece 108 and the drying drive shell 101 can be staggered or aligned to connect the inside and outside of the drying drive shell 101. The rotation of the sealing ring piece 108 on the drying drive shell 101 is driven by the electric cylinder 105. The telescopic cylinder of the electric cylinder 105 is movably connected to the exhaust nozzle 102, and the telescopic rod of the electric cylinder 105 is movably connected to the edge of the sealing ring piece 108.A guide seat 107 is fixedly installed overhead below the drying drive housing 101, wherein the centrifugal driven condensing fan blades 103 and the fixed heat conducting plate 106 are rotatably installed on the guide seat 107, and a heat sink 115 is fixedly installed on the lower surface of the fixed heat conducting plate 106, wherein the condensing plate 112 is coaxially fixedly connected with the centrifugal driven condensing fan blades 103; a central gear ring 118 is fixedly provided on the coaxial sleeve of the heat sink 115, and an outer ring gear ring 117 is coaxially provided on the outer side of the central gear ring 118, and the outer ring gear ring 117 and the central gear ring 118 are meshed and driven by three planetary gears 116, and all the planetary gears 116 are rotated. The planetary gear control ring 114 is rotatably mounted on the planetary gear control ring 114, wherein the planetary gear control ring 114 and the outer ring gear 117 are both rotatably mounted on the regulating motor bracket 119, and the regulating motor bracket 119 is fixedly mounted on the air storage tank 124. The regulating motor 120 and the drive motor 121 are also fixedly mounted on the regulating motor bracket 119, wherein the drive motor 121 is also fixed to the air storage tank 124, wherein the output shaft of the regulating motor 120 is transmission-connected to the planetary gear control ring 114 via a first transmission belt 122, wherein the output shaft of the drive motor 121 is transmission-connected to the outer ring gear 117 via a second transmission belt 123.

[0019] Working principle: Pull the piston moving plate 126 out of the air storage box 124, and then insert the piston moving plate 126 into the air storage box 124 to allow air to accumulate in the air storage box 124, and then start the refrigeration secondary plate 104 and the fan 125, wherein the fan 125 is used to drive the air inside the air storage box 124 to flow toward the condensing plate 112. After the refrigeration secondary plate 104 is powered on and started, the cooling surface of the refrigeration secondary plate 104 will reduce the temperature of the centrifugal-driven condensing fan blades 103 and the condensing plate 112, thereby allowing the air inside the air storage box 124 to condense on the condensing plate 112 and the centrifugal-driven condensing fan blades 103. Under the action of gravity, the condensed water will fall into the centrifugal-driven condensing fan blades 103 and the inside of the drying drive shell 101 (if the temperature is too low, it may cause the water to freeze. At this time, the refrigeration secondary plate 104 needs to be powered off, and the temperature of the heating surface of the refrigeration secondary plate 104 will be transferred to the centrifugal-driven condensing fan blades 103 and the condensing plate 112 to melt the ice). The heating surface of the secondary refrigeration plate 104 transfers the heat to the heat sink 115 via the fixed heat conducting plate 106 (the fixed heat conducting plate 106 and the centrifugally driven condensing fan blades 103 are fixed by a thermal insulator, such as titanium). Simultaneously, the drive motor 121 and the control motor 120 are activated, with the output shaft of the control motor 120 serving as the regulating component. The output shaft of drive motor 121 rotates outer ring gear 117 via second drive belt 123. Outer ring gear 117 rotates center ring gear 118 via planetary gears 116. Center ring gear 118 rotates heat sink 115, which in turn rotates fixed heat conducting plate 106, electric cylinder 105, slip ring 110, centrifugally driven condenser blades 103, and condenser blades 112. Water adhering to the surface of centrifugally driven condenser blades 103 rotates with the rotation and, under the action of centrifugal force, is flung to the edge of the inner wall of drying drive housing 101. Electric cylinder 105 is controlled to align sealing ring 108 with the groove in drying drive housing 101, draining the condensed water from drying drive housing 101. Electric cylinder 105 is then controlled to offset sealing ring 108 from the groove in drying drive housing 101, isolating drying drive housing 101 from the outside. At this point, the humidity inside air storage tank 124 decreases, producing dry air. Finally, controlling the rotation direction and rotation speed of the output shaft of the regulating motor 120 can control the transmission ratio between the output shaft of the drive motor 121 and the center gear ring 118 (when the output shaft speed of the drive motor 121 remains unchanged). Therefore, by driving the output shafts of the drive motor 121 and the regulating motor 120 together, the rotation speeds of the centrifugal-driven condensing fan blades 103 and the condensing sheets 112 can be more accurately controlled, wherein the water attached to the centrifugal-driven condensing fan blades 103 is separated by controlling the centrifugal-driven condensing fan blades 103 to rotate at a low speed. Subsequently, the centrifugal-driven condensing fan blades 103 are controlled to rotate at a high speed, driving the air to rotate so that the air is discharged through the exhaust nozzle 102 under the action of centrifugal force. It is then heated to 50°C by the heating sheet 109 to achieve the purging of dry air.During this process, the air inside the air storage box 124 gradually decreases, and the piston moving plate 126 slides on the inner wall of the air storage box 124 under the action of pressure.

[0020] SERS substrates and materials: Nanoplasmonic structures: The sensor's SERS substrate is composed of highly efficient plasmonic nanostructures, such as 3D noble metal nanoarrays or multi-layered gold / silver nanoparticle combinations. To achieve strong SERS hotspots, nanogaps and roughened surfaces are constructed on the substrate to achieve a significant local enhancement of the electromagnetic field.

[0021] MOF or porous nanonetwork: Metal-organic framework (MOF) crystals are grown or filled around metal nanostructures to form a plasmon-MOF composite substrate. MOFs, with their abundant micropores and tunable functional groups, can act as sponges for NOx capture. Certain MOFs exhibit high selectivity and capacity for NO2, effectively adsorbing it even in the presence of humidity. For example, MFM-520MOF can adsorb 4.2 mmol / g of NO2 at 0.01 bar and can be recycled and eluted with water to produce nitric acid. Combining these MOFs with SERS substrates can enrich NO2 molecules on the substrate surface, improving gas-phase detection sensitivity.

[0022] Energy-enhanced hydrogels: Another material involves embedding metal nanoparticles within a hydrophilic porous hydrogel to form a flexible SERS hydrogel patch. The hydrogel contains a large amount of water and hydrophilic groups, simulating a liquid environment. This allows it to directly dissolve and absorb gaseous NOx while also connecting to external water samples, allowing dissolved NO⁻ / NO⁻ to diffuse freely into the gel. For example, an agarose gel patch embedded with AuAg alloy nanoparticles has a detection limit of 2.9×10^(-5) mg / m³ for gaseous formaldehyde and 1.46×10^(-8) mg / mL for formaldehyde in water. The hydrogel substrate enables both gas-phase and liquid-phase analysis. Selecting a suitable hydrogel (such as polyacrylamide or cellulose) combined with the nanometal provides both SERS sensitivity enhancement and water storage for NOx absorption, achieving dual gas-liquid applications. The hydrogel's porous network structure also helps to create a uniform three-dimensional distribution of Raman hotspots, improving signal stability.

[0023] Therefore, the metal nanoplasmonic structure provides strong SERS gain, while the MOF / hydrogel material provides selective enrichment and multiphase affinity.

[0024] Gas-water two-phase sampling interface: Dual-interface sensing structure: To simultaneously sample gas and liquid samples, sensors employ a dual-interface structure. For example, the SERS substrate is fabricated as a thin film or coating: the top surface is exposed to the gas to be detected, while the bottom surface contacts the water. The inherent porosity of the substrate allows the gas and liquid to microscopically contact the same sensitive region, enabling convergence of the two phases at the interface. For example, wetting a layer of high-affinity liquid on the SERS substrate efficiently captures gaseous analytes, enabling gas-phase detection. The aqueous phase contained within the gel is used to capture gaseous NOx. When NO2 gas molecules contact the wetted hydrogel surface, they dissolve and diffuse into the gel's aqueous phase, where they are further adsorbed or reacted by functional sites within. Simultaneously, NO2⁻ / NO3⁻ ions in the external liquid can also enter the gel through a concentration gradient and bind to the same SERS active sites. The entire substrate acts like a layer of artificial leaves, drawing air from above and water from below, simultaneously enriching NOx from both the gas and liquid phases.

[0025] Efficient Collection Mechanism: To improve gas-phase collection efficiency, micro-nanostructures are employed on the gas inlet side to increase capture probability. Micro-nanopillar arrays or rough nanoflower structures are constructed on the gas-side surface of the substrate, providing a larger surface area and eddy currents, making it easier for NOx molecules to be intercepted and retained on the surface. Furthermore, the pores and functional groups in the gel / MOF exhibit a selective affinity for NOx. For example, ligands containing functional groups such as amines and pyridyls can form hydrogen bonds or coordination interactions with NO2, temporarily binding NO2 molecules. For example, bifunctional probe molecules with electron donor and acceptor sites are used to capture SO2 and NO2 on a SERS substrate via five- or six-membered cyclic transition state complexation, achieving both enrichment and differentiation of trace gases. Therefore, molecules containing specific functional groups (such as ortho-diamines and phenolic resins) are self-assembled on the substrate surface to form cyclic complexes in the gas phase, locking NOx and improving gas capture efficiency and selectivity.

[0026] Modularity and real-time monitoring: Modular detachable probe form. The SERS substrate is encapsulated in a microfluidic cell with upper and lower openings: there is a gas flow channel at the top to allow the air to be tested to pass through; there is a liquid flow path or sampling port at the bottom to periodically introduce water samples. Through microvalve control, the substrate can be exposed to gas and water alternately or simultaneously, realizing dual-path sampling in one device. Combined with a portable Raman spectrometer, in-situ continuous monitoring of NOx concentrations in the atmosphere and water bodies can be performed. It is suitable for occasions such as river outlets and atmospheric environment monitoring stations where simultaneous monitoring of air and water quality is required. One sensor can perform two tasks.

[0027] Signal enhancement and conversion mechanism: Plasmon enhancement: Leveraging nanometallic structures, the sensing substrate generates a strong localized electromagnetic field in response to incident laser light, significantly enhancing the Raman scattering signal of NOx or its derivatives. When NOx molecules are trapped near hotspots on the substrate, their Raman signal is amplified, enabling detection of NOx concentrations at the ppb level or even lower. For example, gaseous NO2 has a weak signal in conventional Raman scattering, but on a 3D nanohotspot array, its characteristic Raman peaks (such as the NO2 symmetric stretching vibration at approximately 810 cm⁻¹) are clearly amplified, serving as a fingerprint for NO2 detection.

[0028] Chemical conversion amplification: To further enhance sensitivity and selectivity, NOx-sensitive reagents or functional groups are pre-immobilized within the substrate. Upon NOx ingress, a chemical reaction generates a new, more Raman-active species, effectively exchanging color for signal. For example, an aromatic amine molecular probe modified on the nanoparticle surface undergoes nitration in the presence of NO2 to form a p-nitro compound, whose Raman signal (such as the -NO2 functional group vibrational peak) is more easily detected than the original molecule. Similarly, nitrite in the aqueous phase reacts with diazo coupling reagents (such as p-nitroaniline and naphthylethylenediamine, the classic Griess reagents) within the substrate to form azo dyes, which exhibit strong, characteristic Raman scattering. Because these reactions occur near the SERS hotspot, the product is immediately detected, effectively amplifying the presence of NOx into a readily detectable Raman signal. The SERS response is altered by exploiting the reducibility of NO to trigger the catalytic growth of the nanostructure.

[0029] Multiple enhancement synergy: Combining physical enhancement (plasmon field) with chemical enhancement (reaction enrichment / conversion) improves sensitivity. Gas-phase NOx is first enriched by the material pores and potentially pre-concentrated at active sites, generating a strong Raman scattering signal at hotspots. At extremely low concentrations, chemical reactions further provide a cumulative amplification effect. In the presence of a catalytic promoter, NO can be in situ oxidized to NO₂⁻ or NO₃⁻ on the substrate surface. These products, due to their stronger interaction with the metal surface, produce distinct SERS bands. For example, NO₂ on a silver surface can form adsorbed species such as NO₂⁻ / NO₃⁻, whose characteristic vibrations can be captured by SERS, enabling indirect NO detection. Therefore, the multiphase interface not only captures NOx but also converts it into a visible Raman-active form for readout, enabling accurate measurement of trace NOx levels (ppb levels in gas, μg / L or even lower in water).

Claims

1. A method for detecting nitrogen oxides based on SERS, characterized in that: Including in order: S1. Preparation of plasmonic metal skeleton-MOF-hydrophilic gel ternary composite SERS substrate; S2, exposing the upper surface of the substrate to the air to be tested and the lower surface to the water sample to be tested, so as to achieve simultaneous sampling of gas and liquid; S3. By utilizing the MOF’s adsorption of gaseous NOx and the gel’s enrichment of aqueous NOx⁻, Raman spectroscopy is excited at the metal hotspot and the enhanced signal is collected simultaneously. S4. Calculate the concentrations of gaseous NO or NO2 and aqueous NO2⁻ / NO3⁻ based on the intensity of the NOx Raman characteristic peak; S5. After regenerating the substrate by controlled temperature increase of the purge module or light desorption, repeat steps S2-S4.

2. The method for detecting nitrogen oxides based on SERS according to claim 1, characterized in that: The metal skeleton in step S1 is a three-dimensional network of Ag nanowires and Au nanoparticles with a nanogap of 1-5 nm; The MOF in step S1 is Zr-based MFM-520 containing amine groups, with a thickness of 0.3-1µm and an adsorption capacity of NO2 ≥3mmol·g⁻¹; The hydrophilic gel in step S1 is a polyacrylamide-carboxymethyl cellulose double network gel with a water content of ≥70%, in which 5 nm AuAg alloy nanoparticles 1×10¹²-10¹ are dispersed. 4 particle·cm⁻²; In step S1, aromatic amine probe molecules are pre-immobilized in the gel layer and undergo a Griess-type coupling reaction with NO2⁻ / NO3⁻ to generate azo characteristic Raman peaks at 1280-1450 cm⁻¹.

3. The method for detecting nitrogen oxides based on SERS according to claim 1, characterized in that: In step S2, the gas flow rate is 50-300 mL·min⁻¹, and the water sample flow rate is 0.1-2 mL·min⁻¹.

4. The method for detecting nitrogen oxides based on SERS according to claim 1, wherein: In step S3, a 785 nm laser is used to laterally excite the substrate to collect the NO2 symmetric stretching peak in the range of 750-850 cm⁻¹ and the azo peak in the range of 1280-1450 cm⁻¹.

5. The method for detecting nitrogen oxides based on SERS according to claim 1, characterized in that: In step S4, the peak intensity-concentration calibration curve is pre-established to achieve a NO2 gas detection limit of ≤0.5ppb and a NO3⁻ aqueous phase detection limit of ≤10nmol·L⁻¹.

6. The method for detecting nitrogen oxides based on SERS according to claim 1, characterized in that: In step S5, the substrate is regenerated by using a purge module with 45-55°C hot air or 405nm LED light for 5-10 minutes. The substrate signal recovery rate is ≥95%, and the substrate can be recycled for ≥100 times.

7. The method for detecting nitrogen oxides based on SERS according to claim 6, characterized in that: The purge module includes an air storage box (124), the inner wall of the air storage box (124) is provided with a piston moving plate (126) in a sliding seal, the top of the inner wall of the air storage box (124) is also rotatably installed with a fan (125), a funnel (111) is provided directly below the fan (125), and a condensing plate (112) is provided at an axial position inside the funnel (111), wherein the fan (125) and the condensing plate (112) are aligned, and a drying drive shell (101) is fixedly installed on the bottom surface of the outer surface of the air storage box (124), and the internal axial position of the drying drive shell (101) is connected to the interior of the air storage box (124).

8. The method for detecting nitrogen oxides based on SERS according to claim 7, characterized in that: An exhaust nozzle (102) is fixedly connected to the radial position of the outer surface of the drying drive shell (101), and a water baffle (113) is provided at the connection between the exhaust nozzle (102) and the drying drive shell (101). The bottom edge of the water baffle (113) is fixed to the bottom surface of the inner wall of the drying drive shell (101), and a gap is present between the top edge of the water baffle (113) and the top surface of the inner wall of the drying drive shell (101). A heating plate (109) is fixedly installed inside the exhaust nozzle (102), and a centrifugal driven condensing fan blade (103) is rotatably installed inside the drying drive shell (101). A fixed heat conducting plate (106) is fixedly installed on the lower surface of the centrifugal driven condensing fan blade (103), and a refrigeration secondary plate (104) is fixedly installed between the opposite surfaces of the fixed heat conducting plate (106) and the centrifugal driven condensing fan blade (103). The outer side of the refrigeration secondary plate (104) is provided with a collector ring (110) coaxially arranged with the centrifugal drive condensing fan blade (103) for supplying power to the refrigeration secondary plate (104). The lower surface of the drying drive shell (101) is provided with a sealing ring plate (108) for rotary sealing. The sealing ring plate (108) and the drying drive shell (101) are both provided with grooves of the same shape, and the grooves on the sealing ring plate (108) and the drying drive shell (101) can be staggered or aligned, for connecting the inside of the drying drive shell (101) with the outside. The rotation of the sealing ring plate (108) on the drying drive shell (101) is driven by the electric cylinder (105). The telescopic cylinder of the electric cylinder (105) is movably connected to the exhaust nozzle (102), and the telescopic rod of the electric cylinder (105) is movably connected to the edge of the sealing ring plate (108).

9. The method for detecting nitrogen oxides based on SERS according to claim 8, characterized in that: A guide seat (107) is fixedly installed overhead below the drying drive housing (101), wherein the centrifugal driven condensing fan blade (103) and the fixed heat conducting plate (106) are rotatably installed on the guide seat (107), and a heat sink (115) is fixedly installed on the lower surface of the fixed heat conducting plate (106), wherein the condensing plate (112) is coaxially fixedly connected to the centrifugal driven condensing fan blade (103); a central gear ring (118) is fixedly provided on the coaxial sleeve of the heat sink (115), and an outer ring gear ring (117) is coaxially provided on the outer side of the central gear ring (118), and the outer ring gear ring (117) and the central ring gear (118) are meshed and driven by three planetary gears (116), and all the planetary gears (116) are rotated. The planetary gear control ring (114) is rotatably mounted on the planetary gear control ring (114), wherein the planetary gear control ring (114) and the outer ring gear (117) are both rotatably mounted on the regulating motor bracket (119), the regulating motor bracket (119) is fixedly mounted on the air storage box (124), and the regulating motor bracket (119) is also fixedly mounted with a regulating motor (120) and a driving motor (121), wherein the driving motor (121) is also fixed to the air storage box (124), wherein the output shaft of the regulating motor (120) is transmission-connected to the planetary gear control ring (114) through a first transmission belt (122), and wherein the output shaft of the driving motor (121) is transmission-connected to the outer ring gear (117) through a second transmission belt (123).

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