Method for detecting nitrogen oxides based on SERS

Abstract

The application discloses a method for detecting nitrogen oxide compounds based on SERS, and relates to the technical field of compound detection. The application constructs an Ag nanowire-Au nanoparticle three-dimensional plasmonic skeleton on a porous inert support, in-situ grows an amine group-containing MFM-520 MOF capture layer, and then covers the same with polyacrylamide-carboxymethyl cellulose hydrogel embedded with AuAg nanoparticles to obtain a ternary composite SERS substrate. The upper surface is synchronously contacted with a gas channel, and the lower surface is synchronously contacted with a liquid channel to realize synchronous enrichment of NO / NO2 and NO2- / NO3-. With the aid of electromagnetic hot spots and in-situ chemical reaction amplification, a 785nm handheld Raman spectrometer can be used to measure gaseous 0.1ppb and liquid 5nmolL-1 NOx in 10s, and the substrate can be rapidly regenerated by 50°C hot air or 405nm LED, and the substrate can be recycled for more than 100 times, so that the application is suitable for real-time monitoring of on-site environment.

Description

Technical Field

[0001] This invention relates to the field of compound detection technology, specifically to a method for detecting nitrogen oxides based on SERS. Background Technology

[0002] Current NO X Monitoring systems generally adopt a separate approach: NO / NO2 in the air is mainly monitored using electrochemical electrodes, chemiluminescence analyzers, or ultraviolet differential absorption spectroscopy, while NO2 in water is monitored using separate methods. - / NO3 - This necessitates ion chromatography, flow injection colorimetry, or laboratory titration. These methods share four common problems: dispersed equipment (requiring two sets of sensing units and two calibration systems for gas and liquid samples, leading to complex sampling points and difficulty in eliminating mutual errors); limited sensitivity (electrochemical electrodes are susceptible to cross-interference from humidity and O3, making quantification difficult below the ppb level, and the detection limit for water-based colorimetric methods often remains at µmol / L). -1 The response is lag-dependent; chemiluminescence or colorimetric reactions require gas dilution and colorimetric pretreatment, with delays of minutes or even hours failing to meet real-time early warning requirements. Maintenance costs are high; multiple instruments require independent standard gas and reagent replacements and electrode drift correction, making continuous field monitoring difficult. While traditional SERS has trace potential, ordinary metal nanofilms have sparse hotspots, and gaseous NO... X It is difficult to adsorb effectively, and aqueous ions are also difficult to penetrate the hydrophobic surface. Summary of the Invention

[0003] To overcome the shortcomings of the prior art, the present invention provides the following technical solution: a method for detecting nitrogen oxides based on SERS, comprising, in sequence: S1. Fabrication of a plasmonic metal framework (a three-dimensional interconnected noble metal nanonetwork structure, utilizing gold (Au), silver (Ag) (noble metals have high free electron density, supporting strong LSPR; Au provides chemical stability, Ag provides higher electromagnetic gain) or their alloys to generate localized surface plasmon resonance (localized surface plasmon resonance is generated when the frequency of incident light matches the oscillation frequency of the metal's 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 sharp corners, hole edges, and nano-gap areas inside the framework, and the local field strength can be increased by 10 times compared to the incident light). 2 -10 6These hotspots are the core of SERS enhancement (when molecules are near these hotspots, their Raman scattering signals are exponentially amplified, enabling trace or single-molecule level detection); Preparation method: Conductive substrate preparation: Electrodepositing Ag nanowires in an alumina template and etching the template to form a coarse framework and 3D interconnected channels; Surface roughening: Chemical etching or plasma etching to introduce sub-5nm-level sharp edges and pits, increasing hotspot density; Alloy or core-shell regulation: In-situ reduction of HAuCl4 to generate an Au shell on the Ag surface, improving stability, preventing Ag oxidation, and maintaining high LSPR gain) - MOF-hydrophilic gel ternary composite SERS substrate. The metal framework in step S1 is a three-dimensional network of Ag nanowires and Au nanoparticles with nanometer gaps of 1-5nm. The MOF in step S1 is an amino-containing Zn-based MFM-520 with a thickness of 0.3-1µm and an adsorption capacity of ≥3mmol·g for NO2. -1 (0.1 bar, 298 K). The hydrophilic gel in step S1 is a polyacrylamide-carboxymethyl cellulose dual-network gel with a water content ≥70%, containing 1×10⁻⁵ 5nm AuAg alloy nanoparticles. 12 -10 14 granules·cm -2 Aromatic amine probe molecules are pre-immobilized in the gel layer in step S1 and reacted with NO2. - / NO3 - A Griess-type coupling reaction occurs, producing 1280-1450 cm⁻¹ -1 Azo characteristic Raman peaks.

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

[0005] S3, using MOF to detect gaseous NO X Adsorption and gelation of NO in aqueous phase X - Enrichment of the metal was achieved by Raman spectroscopy excitation at the metal hotspot, with simultaneous acquisition of enhanced signals; in step S3, a 785nm laser was used to laterally excite the substrate, acquiring signals from 750-850cm². -1 The NO2 symmetric stretching peak in the range of 1280-1450 cm⁻¹ -1 Azo peaks in the range.

[0006] S4, according to NO X Raman characteristic peak intensity calculation for gas phase NO or NO2 and aqueous phase NO2 - / NO3 -The concentration; in step S4, by pre-establishing a peak intensity-concentration calibration curve, the detection limit of NO2 gas ≤ 0.5 ppb and NO3 gas ≤ 0.5 ppb are achieved. - The detection limit for aqueous phase is ≤10 nmol·L⁻¹ -1 .

[0007] S5. After regenerating the substrate by controlled heating via a purge module or by photo-induced desorption, 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 it can be recycled ≥100 times.

[0008] Preferably, the purging module includes an air storage box, a piston moving plate is slidably sealed on the inner wall of the air storage box, a fan is rotatably installed on the top of the inner wall of the air storage box, a funnel is provided directly below the fan, a condenser plate is provided at the axial position of the inner side of the funnel, wherein the fan and the condenser 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 inside 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 housing. A baffle plate is provided at the connection between the exhaust nozzle and the drying drive housing. The bottom edge of the baffle plate is fixed to the bottom surface of the inner wall of the drying drive housing, and a gap exists between the top edge of the baffle plate and the top surface of the inner wall of the drying drive housing. A heating element is fixedly installed inside the exhaust nozzle. A centrifugal drive condenser fan blade is rotatably installed inside the drying drive housing. A fixed heat-conducting plate is fixedly installed on the lower surface of the centrifugal drive condenser fan blade. A refrigeration unit is fixedly installed between the fixed heat-conducting plate and the opposite surface of the centrifugal drive condenser fan blade. The cooling secondary plate has a collector ring arranged coaxially with the centrifugal drive condenser fan blades on its outer side to supply power to the cooling secondary plate. The lower surface of the drying drive housing is equipped with a rotating seal with a sealing ring. Both the sealing ring and the drying drive housing have grooves of the same shape, and the grooves on the sealing ring and the drying drive housing can be staggered or aligned to connect the inside and outside of the drying drive housing. The rotation of the sealing ring on the drying drive housing is driven by an electric cylinder. The telescopic cylinder is movably connected to the exhaust port, and the telescopic rod of the electric cylinder is movably connected to the edge of the sealing ring.

[0010] Preferably, a flow guide seat is fixedly installed below the drying drive housing, wherein the centrifugal drive condenser fan blade and the fixed heat conduction plate are rotatably mounted on the flow guide seat, and a heat sink is fixedly installed on the lower surface of the fixed heat conduction plate, wherein the condenser fins are coaxially fixedly connected to the centrifugal drive condenser fan blade; a central gear ring is fixedly and coaxially sleeved on the heat sink, and an outer gear ring is coaxially provided on the outer side of the central gear ring. The outer gear ring and the central gear ring are driven by three planetary gears meshing. All planetary gears are rotatably mounted on a planetary gear control ring, wherein the planetary gear control ring and the outer gear ring are rotatably mounted on a regulating motor bracket, the regulating motor bracket is fixedly mounted on the air storage box, and a regulating motor and a drive motor are also fixedly mounted on the regulating motor bracket, wherein the drive motor is also fixed to the air storage box, wherein the output shaft of the regulating motor is drivenly connected to the planetary gear control ring through a first transmission belt, and wherein the output shaft of the drive motor is drivenly connected to the outer gear ring through a second transmission belt.

[0011] 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 facing the air and water respectively, which avoids mutual contamination between the gas and liquid channels and allows the samples to converge in the same hot spot area at the micrometer scale, fundamentally eliminating the cumbersome process of separate calibration, time synchronization and comparison of the two instruments in the traditional split scheme. The monitoring station only needs to install one probe to simultaneously output atmospheric NO / NO2 and water NO2. - / NO3 - (1) This significantly reduces the number of on-site installations, the amount of cables and gas lines laid, and the manpower required for subsequent maintenance, achieving the overall benefit of dual-testing in one hole; (2) The Zn-based MFM-520MOF in the composite substrate of this invention has an ultra-high dynamic adsorption capacity for NO2, and the hydrogel has a high adsorption capacity for NO2. X - Ions accumulate in a hydrophilic network, and both ions and the target molecules are directionally transported to the vicinity of the Ag / Au hotspot. Simultaneously, plasmon resonance amplifies the electromagnetic field by six orders of magnitude within a 1-5 nm gap, and this, combined with the chemical amplification from the aromatic amine-azo colorimetric method, improves the signal-to-noise ratio by more than a hundredfold. Compared to electrochemical and photochemical colorimetric methods, this invention lowers the gas phase detection limit from over ten ppb to 0.1 ppb and the aqueous phase detection limit from µmol / L... -1 Pushed down to 5 nmol L -1(3) The electromagnetic enhancement process of this invention occurs instantaneously, and the chemical conversion reaction can be completed at the hot spot in just tens of milliseconds. With the real-time scanning of 785nm laser, the whole machine reaches a refresh frequency of sub-10s, which meets the rapid tracking of haze outbreaks and discharge impact emissions. After detection, the adsorbate can be completely desorbed and the coupling products can be removed by irradiation with 50°C hot air or 405nm ultraviolet light. At the same time, it gets rid of the consumable burden of traditional colorimetric reagents that are disposable; (4) The amino coordination site of the MOF of this invention forms a Lewis acid-base complex with NO2, and the Griess reaction in the hydrogel is effective against NO2. - It focuses on specific color rendering, while NO3 - It needs to be reduced or photolyzed to NO2 under the catalysis of a hot spot. - Only then can it be developed; Raman fingerprint peak position difference (810cm) -1 Symmetrical telescopic VS1280-1450cm -1 Azo vibrations provide secondary resolution. This applies to O3 and SO2 in gases and Cl in water. - PO4 3- Interferences can be suppressed by molecular sieve repulsion, reaction kinetic selection or data post-processing to ensure 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 connected to a commercial handheld Raman spectrometer to work. The whole machine has low power consumption and can be powered by solar energy or mobile power supply. Compared with chemiluminescence instruments that weigh tens of kilograms and require high-pressure pumps and standard gas cylinders, the maintenance cycle of this device is longer. On-site, only the substrate patch needs to be cleaned or replaced once, which greatly reduces the cost of consumables and instrument downtime calibration. Attached Figure Description

[0012] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0013] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0014] This invention provides a method for detecting nitrogen oxides based on SERS, comprising: Composite substrate preparation: A three-dimensional Ag nanowire-Au nanoparticle framework was electrodeposited on a porous inert support; an amine-containing Zn-based MFM-520 MOF layer of 0.3–1 µm was grown in situ; a polyacrylamide-carboxymethyl cellulose hydrogel solution containing 5 nm AuAg alloy nanoparticles was spin-coated and UV-cured to form a 50–150 µm gel layer with pre-immobilized aromatic amine probes.

[0015] Simultaneous sampling: The substrate is fixed in a detection cell with an upper gas channel and a lower liquid channel; the air to be tested is introduced at a flow rate of 50–300 mL / min.-1 The flow rate swept across the upper surface of the substrate at 0.1–2 mL / min. -1 The flow rate allows the water sample to pass through the lower surface of the substrate; the MOF captures gaseous NO / NO2, and the gel absorbs and enriches the NO2 in the aqueous phase. - / NO3 - .

[0016] In-situ chemical transformation and signal enhancement: Oxidation of gaseous NO to NO2 under hot spot catalysis - ; NO2 undergoes electrochemical or nitration reactions with probe molecules. - / NO3 - Azo dye is generated by coupling with a pre-prepared p-aminoaniline-naphthylethylenediamine; a 785nm laser is then used to excite the target molecule and its reaction products to produce enhanced Raman scattering at the metal hot spot.

[0017] Spectral acquisition and quantification: 810 cm⁻¹ -1 (NO2), 1280–1450cm -1 Characteristic peaks such as (azo N=N) were observed; peak intensity versus NOx concentration calibration curves were established, and gas-phase NO and NO2 and aqueous-phase NO2 were output. - / NO3 - concentration.

[0018] Substrate regeneration: Turn off the sample flow and purge with 50°C dry air for 5–10 min or irradiate with a 405nm LED for 5 min; remove adsorption / reaction residues, restore the substrate signal to the initial baseline, and repeat the step of simultaneous sampling-substrate regeneration.

[0019] Structure of the purging module: The system includes a gas storage tank 124, with a piston moving plate 126 slidably sealed on the inner wall of the gas storage tank 124. A fan 125 is rotatably mounted on the top of the inner wall of the gas storage tank 124. A funnel 111 is located directly below the fan 125. A condenser plate 112 is located at the axial position on the inner side of the funnel 111. The fan 125 is aligned with the condenser plate 112. A drying drive housing 101 is fixedly mounted on the bottom surface of the outer surface of the gas storage tank 124. The internal axial position of the drying drive housing 101 is connected to the inside of the gas storage tank 124. An exhaust nozzle 102 is fixedly connected to the radial position of the outer surface of the drying drive housing 101. A baffle plate 113 is provided at the connection between the exhaust nozzle 102 and the drying drive housing 101. The bottom edge of the baffle plate 113 is fixed to the bottom surface of the inner wall of the drying drive housing 101, and there is a gap between the top edge of the baffle plate 113 and the top surface of the inner wall of the drying drive housing 101. A heating element 109 is fixedly installed inside the exhaust nozzle 102. A centrifugal drive condenser fan blade 103 is rotatably installed inside the drying drive housing 101. A fixed heat-conducting plate 106 is fixedly installed on the lower surface of the centrifugal drive condenser fan blade 103. A secondary cooling plate 104 is fixedly installed between the fixed heat-conducting plate 106 and the opposite surface of the centrifugal drive condenser fan blade 103. A collector ring 110 is provided on the outer side of the refrigeration stage 104, coaxially arranged with the centrifugal drive condenser fan blade 103, for supplying power to the refrigeration stage 104. A sealing ring 108 is provided on the lower surface of the drying drive housing 101. The sealing ring 108 and the drying drive housing 101 are both provided with grooves of the same shape, and the grooves on the sealing ring 108 and the drying drive housing 101 can be staggered or aligned to connect the inside and outside of the drying drive housing 101. The rotation of the sealing ring 108 on the drying drive housing 101 is driven by an electric cylinder 105. The telescopic cylinder of the electric cylinder 105 is movably connected to the exhaust port 102, and the telescopic rod of the electric cylinder 105 is movably connected to the edge of the sealing ring 108.A flow guide seat 107 is fixedly mounted below the drying drive housing 101. A centrifugal drive condenser fan blade 103 and a fixed heat-conducting plate 106 are rotatably mounted on the flow guide seat 107. A heat sink 115 is fixedly mounted on the lower surface of the fixed heat-conducting plate 106. A condenser fin 112 is coaxially and fixedly connected to the centrifugal drive condenser fan blade 103. A central gear ring 118 is coaxially sleeved on the heat sink 115. An outer gear ring 117 is coaxially arranged on the outer side of the central gear ring 118. The outer gear ring 117 and the central gear ring 118 are driven by three planetary gears 116 meshing together. All planetary gears 116 rotate... The control ring 114 and the outer ring gear 117 are rotatably mounted on the planetary gear control ring 114. The planetary gear control ring 114 and the outer ring gear 117 are both rotatably mounted on the control motor bracket 119. The control motor bracket 119 is fixedly mounted on the air storage box 124. The control motor 120 and the drive motor 121 are also fixedly mounted on the control motor bracket 119. The drive motor 121 is also fixed to the air storage box 124. The output shaft of the control motor 120 is connected to the planetary gear control ring 114 through the first transmission belt 122. The output shaft of the drive motor 121 is connected to the outer ring gear 117 through the second transmission belt 123.

[0020] Working principle: The piston moving plate 126 is pulled out of the air storage box 124, and then the piston moving plate 126 is inserted into the air storage box 124 to fill the air storage box 124 with air. Then the cooling secondary plate 104 and the fan 125 are started. The fan 125 is used to drive the air inside the air storage box 124 to flow towards the condenser plate 112. After the cooling secondary plate 104 is powered on and started, the cooling surface of the cooling secondary plate 104 will reduce the temperature of the centrifugal drive condenser blade 103 and the condenser plate 112, so that the air inside the air storage box 124 condenses on the condenser plate 112 and the centrifugal drive condenser blade 103. Under the action of gravity, the condensed water will fall into the centrifugal drive condenser blade 103 and the drying drive shell 101 (if the temperature is too low, the water may freeze. In this case, the cooling secondary plate 104 needs to be powered off. The temperature of the heating surface of the cooling secondary plate 104 will be transferred to the centrifugal drive condenser blade 103 and the condenser plate 112 to melt the ice). The heating surface of the secondary cooling plate 104 transfers temperature to the heat sink 115 via the fixed heat-conducting plate 106 (the fixed heat-conducting plate 106 and the centrifugal drive condenser fan blade 103 are fixed by a heat insulation component, such as titanium). At the same time, the drive motor 121 and the regulating motor 120 are started, with the output shaft of the regulating motor 120 serving as an adjustment component. The output shaft of the drive motor 121 drives the outer gear ring 117 to rotate via the second transmission belt 123. The outer gear ring 117 drives the central gear ring 118 to rotate via the planetary gear 116. The central gear ring 118 drives the heat sink 115 to rotate. The heat sink 115 drives the fixed heat-conducting plate 106, the electric cylinder 105, the collector ring 110, the centrifugal drive condenser fan blade 103, and the condenser fins 112 to rotate. Water adhering to the surface of the centrifugal drive condenser fan blade 103 rotates with it and is then thrown to the edge of the inner wall of the drying drive housing 101 under the action of centrifugal force. The electric cylinder 105 is controlled to align the sealing ring 108 with the groove on the drying drive housing 101, draining the condensate inside the drying drive housing 101. Then, the electric cylinder 105 is controlled to misalign the sealing ring 108 with the groove on the drying drive housing 101, achieving isolation between the drying drive housing 101 and the outside. At this time, the air humidity inside the air storage tank 124 will decrease, producing dry air. Finally, controlling the rotation direction and 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 central gear ring 118 (when the output shaft speed of the drive motor 121 remains constant). Therefore, through the joint drive of the output shafts of the drive motor 121 and the regulating motor 120, the rotation speed of the centrifugal drive condenser fan blade 103 and the condenser plate 112 can be controlled more precisely. Separating the water adhering to the centrifugal drive condenser fan blade 103 is achieved by controlling its low-speed rotation. Subsequently, controlling the centrifugal drive condenser fan blade 103 to rotate at high speed drives the air to rotate, causing the air to be discharged through the exhaust port 102 under centrifugal force. Then, the air is heated to 50°C by the heating element 109 to achieve the purging of dry air.During this process, the air inside the air storage tank 124 gradually decreases, and under the action of pressure, the piston moving plate 126 slides on the inner wall of the air storage tank 124.

[0021] SERS substrates and materials: Nanoplasmic structures: The SERS substrate of the sensor is composed of highly efficient plasmonic nanostructures, such as 3D noble metal nanoarrays or multi-layered combinations of gold / silver nanoparticles. To obtain strong SERS hotspots, nano-gap structures and rough surfaces are constructed on the substrate to achieve a significant enhancement of the local electromagnetic field.

[0022] MOFs or porous nanonetworks: Metal-organic framework (MOF) crystals are grown or filled around metal nanostructures to form plasmon-MOF composite substrates. MOFs possess abundant micropores and tunable functional groups, acting as sponges to capture NOx. Certain MOFs exhibit high selectivity and capacity for NO2, efficiently adsorbing it even in the presence of humidity. For example, MFM-520 MOF can adsorb 4.2 mmol / g of NO2 at 0.01 bar and is recyclable, eluting with water to generate nitric acid. Combining such MOFs with SERS substrates can enrich NO2 molecules on the substrate surface, improving the sensitivity of gas-phase detection.

[0023] Energy-based hydrogels: Another material involves embedding metal nanoparticles into hydrophilic porous hydrogels to form flexible SERS hydrogel patches. The hydrogel contains a large amount of water and hydrophilic groups, simulating a liquid environment. On one hand, it directly dissolves and absorbs gaseous NOx; on the other hand, it can connect with external water samples, allowing dissolved NO2 to be absorbed. - / NO3 - Free diffusion enters the gel. For example, agar gel patches embedded with AuAg alloy nanoparticles can achieve a detection limit of 2.9 × 10⁻⁶ for gaseous formaldehyde. ∧ (-5)mg / m 3 The detection limit for formaldehyde in water is 1.46 × 10⁻⁶. ∧ (-8) mg / mL. Hydrogel substrates can be used for both gas and liquid phase analysis. By selecting suitable hydrogels (such as polyacrylamide or cellulose-based ones) and combining them with nano-metals, both SERS sensitization and NOx absorption through water retention can be achieved, enabling dual-use of gas and liquid phases. The porous network structure of hydrogels also helps to form a uniform three-dimensional distribution of Raman hotspots, improving signal stability.

[0024] Therefore, metal nanoplasmic structures provide strong SERS gain, while MOF / hydrogel materials offer selective enrichment and multiphase affinity.

[0025] Gas-water two-phase sampling interface: Dual-interface sensing structure: To simultaneously acquire gaseous and liquid samples, the sensor employs a dual-interface structure. For example, the SERS substrate can be fabricated as a thin film or coating: the upper surface is exposed to the analyte gas, and the lower surface contacts water. The porosity of the substrate itself allows the gas and liquid to contact the same sensitizing region at a microscopic level, achieving convergence of the two phases at the interface. For example, wetting a layer of high-affinity liquid on the SERS substrate can efficiently capture gaseous analytes, thus achieving gaseous detection. The aqueous phase contained in the gel can be used to capture gaseous NOx. When NO2 gas molecules come into contact with the wetted hydrogel surface, they dissolve and diffuse into the aqueous phase of the gel and are further adsorbed or reacted by internal functional sites; simultaneously, NO2 in the external liquid... - / NO3 - Ions can also enter the gel through a concentration gradient and bind to the same SERS active site. The entire substrate acts like an artificially developed leaf, absorbing air above and water below, simultaneously enriching NOx from both the gaseous and liquid phases of the environment.

[0026] High-efficiency collection mechanism: To improve gas-phase collection efficiency, micro / nano structures are employed on the gas inlet side to increase the capture probability. Micro / nano pillar arrays or rough nanoflower structures are constructed on the gas-side surface of the substrate, providing a larger specific surface area and eddy current effect, making it easier for NOx molecules to be intercepted and retained on the surface. On the other hand, the pores and functional groups in the gel / MOF exhibit selective affinity for NOx: for example, ligands containing amine or pyridine functional groups can form hydrogen bonds or coordination interactions with NO2, thereby temporarily binding NO2 molecules. For instance, bifunctional probe molecules with electron donor and acceptor sites can be used to capture SO2 and NO2 on the SERS substrate through five- or six-membered cyclic transition state complexation, achieving enrichment and differentiation of trace gases. Therefore, self-assembling molecules containing specific functional groups (such as o-diamine or phenolic resin groups) on the substrate surface forms cyclic complexes in the gas phase to lock NOx, improving gas capture rate and selectivity.

[0027] Modular and Real-Time Monitoring: The SERS substrate is encapsulated in a microfluidic cell with openings at the top and bottom. The top has a gas flow channel allowing the air to be measured to pass through; the bottom has a liquid flow path or sampling port for periodically introducing water samples. Controlled by microvalves, the substrate can be alternately or simultaneously exposed to gas and water, enabling dual-path sampling in a single device. Combined with a portable Raman spectrometer, it allows for in-situ continuous monitoring of NOx concentrations in the atmosphere and water. Suitable for applications requiring simultaneous monitoring of air and water quality, such as river outlets and atmospheric environmental monitoring stations, a single sensor can perform both tasks.

[0028] Signal enhancement and conversion mechanisms: Plasma Enhancement: Using nanostructured metals, 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 hot spots on the substrate, their Raman signal is amplified, allowing the detection of NOx at ppb levels or even lower concentrations. For example, gaseous NO2 exhibits a weak signal in conventional Raman scattering, but on a 3D nano-hot spot array, its characteristic Raman peaks (such as the NO2 symmetric stretching vibration at approximately 810 cm⁻¹) are clearly visible. -1 The signal will be clearly magnified and can be used as a fingerprint signal for detecting NO2.

[0029] Chemical conversion amplification: To further improve sensitivity and selectivity, NOx-sensitive reactive reagents or functional groups are pre-immobilized within the substrate. When NOx enters, a new substance with stronger Raman activity is generated through a chemical reaction, achieving the effect of exchanging reaction color for signal. For example, aromatic amine molecular probes are modified on the surface of nanoparticles. When NO2 is present, the probe undergoes a nitration reaction to generate a p-nitro compound, whose Raman signal (such as the vibrational peak of the -NO2 functional group) is more easily identifiable than the original molecule. Similarly, nitrite in the aqueous phase can react with diazo coupling reagents (such as p-nitroaniline and naphthylethylenediamine, i.e., classic Griess reagents) within the substrate to generate azo dyes, which exhibit strong characteristic Raman scattering. Since these reactions occur near SERS hotspots, the products are enhanced for detection as soon as they are generated, effectively amplifying the presence of NOx into an easily measurable Raman signal. The SERS response can also be altered by utilizing the reducing power of NO to trigger the catalytic growth of nanostructures.

[0030] Synergistic enhancement through multiple approaches: Combining physical enhancement (plasmon field) with chemical enhancement (reaction enrichment / conversion) to improve sensitivity. Gas-phase NOx is first enriched in the material's pores and may be pre-concentrated at active sites, subsequently generating strong Raman scattering signals at hot spots; if the concentration is extremely low, the chemical reaction further provides a cumulative amplification effect. In the presence of a catalyst, NO can be oxidized in situ to NO2 on the substrate surface. - Or NO3 - These products exhibit clearly distinguishable SERS bands due to their stronger interaction with metal surfaces. For example, NO2 can form NO2 on a silver surface. - / NO3 - The characteristic vibrations of adsorbed species can be captured by SERS, thus enabling indirect detection of NO. Therefore, the multiphase interface not only captures NOx, but also converts NOx into a visible Raman-active form that can be read out, achieving accurate measurement of trace NOx (ppb level in gas, μg / L level or even lower in water).

Claims

1. A method for detecting nitrogen oxides based on SERS, characterized in that, In order, they include: S1. Preparation of a plasmonic metal framework-MOF-hydrophilic gel ternary composite SERS substrate; the metal framework is a three-dimensional network of Ag nanowires and Au nanoparticles with nanometer gaps of 1-5 nm; the MOF is an amino-containing Zn-based MFM-520 with a thickness of 0.3-1 µm and an adsorption capacity of ≥3 mmol·g for NO2. -1 The hydrophilic gel is a polyacrylamide-carboxymethyl cellulose dual-network gel with a water content ≥70%, containing 1×10⁻⁵ 5nm AuAg alloy nanoparticles. 12 -10 14 granules·cm -2 Aromatic amine probe molecules are pre-immobilized in the gel layer and react with NO2. - / NO3 - A Griess-type coupling reaction occurs, producing 1280-1450 cm⁻¹ -1 Characteristic Raman peaks of azo dyes; S2. Expose the upper surface of the substrate to the air to be tested and contact the lower surface with the water sample to be tested to achieve simultaneous sampling of gas and liquid; S3, using MOF to detect gaseous NO X Adsorption and gelation of NO in aqueous phase X - Enrichment of metals was achieved by Raman spectroscopy excitation at metal hotspots and simultaneous acquisition of enhanced signals. S4, according to NO X Raman characteristic peak intensity calculation for gas phase NO or NO2 and aqueous phase NO2 - / NO3 - The concentration; S5. After regenerating the substrate by controlled heating through a purging module or by photo-desorption, repeat steps S2-S4.

2. 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 The water sample flow rate was 0.1-2 mL / min. -1 .

3. The method for detecting nitrogen oxides based on SERS according to claim 1, characterized in that: In step S3, a 785nm laser is used to laterally excite the substrate, and samples are collected from 750-850 cm⁻¹. -1 The NO2 symmetric stretching peak in the range of 1280-1450 cm⁻¹ -1 Azo peaks in the range.

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

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

6. The method for detecting nitrogen oxides based on SERS according to claim 5, characterized in that: The purging module includes an air storage box (124), and a piston moving plate (126) is provided with a sliding seal on the inner wall of the air storage box (124). A fan (125) is also rotatably installed on the top of the inner wall of the air storage box (124). A funnel (111) is provided directly below the fan (125). A condenser plate (112) is provided at the axial position of the inner side of the funnel (111). The fan (125) and the condenser plate (112) are aligned. A drying drive shell (101) is fixedly installed on the bottom surface of the outer surface of the air storage box (124). The internal axial position of the drying drive shell (101) is connected to the inside of the air storage box (124).

7. The method for detecting nitrogen oxides based on SERS according to claim 6, characterized in that: An exhaust nozzle (102) is fixedly connected to the radial position of the outer surface of the drying drive housing (101). A baffle plate (113) is provided at the connection between the exhaust nozzle (102) and the drying drive housing (101). The bottom edge of the baffle plate (113) is fixed to the bottom surface of the inner wall of the drying drive housing (101), and there is a gap between the top edge of the baffle plate (113) and the top surface of the inner wall of the drying drive housing (101). A heating element (109) is fixedly installed inside the exhaust nozzle (102). A centrifugal drive condenser fan blade (103) is rotatably installed inside the drying drive housing (101). A fixed heat-conducting plate (106) is fixedly installed on the lower surface of the centrifugal drive condenser fan blade (103). A secondary cooling plate (104) is fixedly installed between the fixed heat-conducting plate (106) and the opposite surface of the centrifugal drive condenser fan blade (103). A collector ring (110) is provided on the outer side of the refrigeration stage plate (104) and is arranged coaxially with the centrifugal drive condenser fan blade (103) to supply power to the refrigeration stage plate (104). A sealing ring plate (108) is provided on the lower surface of the drying drive housing (101). The sealing ring plate (108) and the drying drive housing (101) are provided with grooves of the same shape. The grooves on the sealing ring plate (108) and the drying drive housing (101) can be staggered or aligned to connect the inside of the drying drive housing (101) with the outside. The rotation of the sealing ring plate (108) on the drying drive housing (101) is driven by an electric cylinder (105). The telescopic cylinder of the electric cylinder (105) is movably connected to the exhaust nozzle (102). The telescopic rod of the electric cylinder (105) is movably connected to the edge of the sealing ring plate (108).

8. The method for detecting nitrogen oxides based on SERS according to claim 7, characterized in that: A flow guide seat (107) is fixedly mounted below the drying drive housing (101). Centrifugal drive condenser fan blades (103) and fixed heat conduction plate (106) are rotatably mounted on the flow guide seat (107). A heat sink (115) is fixedly mounted on the lower surface of the fixed heat conduction plate (106). A condenser fin (112) is coaxially fixedly connected to the centrifugal drive condenser fan blades (103). A central gear ring (118) is fixedly and coaxially sleeved on the heat sink (115). An outer gear ring (117) is coaxially arranged on the outside of the central gear ring (118). The outer gear ring (117) and the central gear ring (118) are driven by three planetary gears (116). All planetary gears (116) rotate evenly. The control ring (114) and the outer ring gear (117) are rotatably mounted on the control motor bracket (119). The control motor bracket (119) is fixedly mounted on the gas storage box (124). The control motor bracket (119) is also fixedly mounted with the control motor (120) and the drive motor (121). The drive motor (121) is also fixed to the gas storage box (124). The output shaft of the control motor (120) is connected to the planetary gear control ring (114) through the first transmission belt (122). The output shaft of the drive motor (121) is connected to the outer ring gear (117) through the second transmission belt (123).

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