A method for detecting pesticide residues in plums

Abstract

The application discloses a kind of detection methods of pesticide residue of plum, it is related to the field of agricultural product safety detection, the detection method includes the following steps: S1, preparation has the core-shell structure nanoparticle dispersion liquid with photothermal effect and magnetocaloric effect;S2, the sealed detection cavity is formed on the surface of plum to be measured;S3, inject core-shell structure nanoparticle dispersion liquid into detection cavity;S4, at least one temperature cycle is applied to the core-shell structure nanoparticle in detection cavity;S5, after processing, core-shell structure nanoparticle dispersion liquid is collected, the scattering light intensity is measured using resonance light scattering method, and the pesticide residue concentration on the surface of plum is calculated according to the preset standard curve.The application integrates extraction, enrichment and detection in the same system, can realize trace detection of multiple organophosphorus and pyrethroid pesticides on the surface of plum, meet the food safety limit requirement, and operation is fast, in situ nondestructive, quantitative reproducibility is good.

Description

Technical Field

[0001] This invention relates to the field of agricultural product safety testing technology, and in particular to a method for detecting trace amounts of pesticide residues in plums using optical means. Background Technology

[0002] As a typical stone fruit, plums have a high water content in their flesh, and the pH value of the flesh tissue is between 3.0 and 4.0, exhibiting a highly acidic characteristic. After pesticide application, lipophilic pesticide molecules such as organophosphates and pyrethroids easily form a concentration gradient distribution at the interface between the peel and the flesh. Due to the barrier effect of the cuticle and waxy layer, the steady-state concentration of pesticide molecules at the interface can be 5-10 times higher than that inside the flesh. This interface enrichment effect provides a potential basis for signal amplification in surface detection. However, the high curvature of the plum surface makes in-situ optical detection difficult. At the same time, food safety standards have strict maximum residue limits for organophosphate pesticides, thus requiring high sensitivity of the detection methods.

[0003] Currently, pesticide residue detection mainly relies on gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography-mass spectrometry (HPLC-MS). These methods often require pretreatment operations such as homogenization, extraction, purification, and concentration of samples, with processing time for a single sample typically exceeding one hour, and they cannot achieve in-situ detection. Although surface-enhanced Raman spectroscopy and near-infrared spectroscopy can achieve in-situ measurement, the high curvature of the plum's spherical surface results in insufficient effective contact area between the optical probe and the peel; the actual area is too small, and uneven light spot focusing leads to poor quantitative reproducibility.

[0004] Traditional solid-phase microextraction (SPE) techniques require external ultrasonic or microwave energy to promote mass transfer. However, ultrasound can easily damage the integrity of the fruit peel, leading to the leakage of tissue fluid that interferes with detection. Microwaves, on the other hand, have poor selectivity and pose a risk of thermal damage. Visible light detection methods, such as colorimetry and resonant light scattering, are simple to operate and have low equipment costs, but their sensitivity is often limited by the mass transfer rate of low-concentration analytes, failing to meet food safety limits.

[0005] Therefore, how to achieve highly sensitive in-situ pesticide residue detection without damaging the integrity of plum fruit is an urgent problem to be solved. Summary of the Invention

[0006] This invention overcomes the shortcomings of existing technologies and provides a method for detecting pesticide residues in plums. Addressing the problems of low sensitivity and poor reproducibility in in-situ pesticide residue detection caused by the high water content, high acidity, and high curvature surface of plums, this invention utilizes the synergistic photothermal and magnetocaloric effects of core-shell structured nanoparticles to construct a cyclic microextraction process on the surface of intact plum peel, involving heating, vaporization, negative pressure, diffusion, and capture. Periodic temperature oscillations induce local pressure changes at the interface, driving pesticide molecules from deep within the pulp to migrate directionally to the surface and achieve in-situ enrichment on the nanoparticle surface. Finally, resonant light scattering technology is used for quantitative detection. This method enables trace detection of various organophosphorus and pyrethroid pesticides on the plum peel, thus meeting food safety limits. Furthermore, the method is rapid, non-destructive in situ, and exhibits good quantitative reproducibility.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The present invention provides a method for detecting pesticide residues in plums, comprising the following steps:

[0008] S1. Prepare a dispersion of core-shell structured nanoparticles with photothermal and magnetothermal effects, wherein the core of the core-shell structured nanoparticles is a magnetic nanomaterial and the shell is a noble metal nanomaterial.

[0009] S2. Install an element that can form a closed microcavity on the surface area of ​​the plum to be tested, so that the microcavity forms a sealed detection cavity with the surface of the plum;

[0010] S3. Inject a core-shell structured nanoparticle dispersion into the detection chamber to cover the surface area to be tested;

[0011] S4. Apply at least one heating and cooling cycle to the core-shell structured nanoparticles in the detection chamber. Each heating and cooling cycle includes a heating stage and a cooling stage. The heating stage involves irradiating the nanoparticles with an external light source to raise their temperature, and the cooling stage involves stopping the irradiation and allowing them to cool naturally.

[0012] S5. Collect the treated core-shell structured nanoparticle dispersion, measure its scattered light intensity using the resonant light scattering method, and calculate the pesticide residue concentration on the plum surface according to the preset standard curve.

[0013] In a preferred embodiment of the present invention, in step S1, the core is Fe3O4 nanoparticles and the shell is a gold nanolayer; the particle size of the Fe3O4 nanoparticles is 5-25 nm and the thickness of the gold nanolayer is 2-15 nm; the total particle size of the core-shell structured nanoparticles is 15-50 nm, and they have a surface plasmon resonance absorption peak at a wavelength of 540-550 nm.

[0014] In a preferred embodiment of the present invention, in step S1, the surface of the core-shell structured nanoparticles is subjected to non-specific adsorption and sealing treatment with a sealing agent; the sealing agent is selected from at least one of bovine serum albumin, casein, polyvinylpyrrolidone, and Tween series surfactants.

[0015] In a preferred embodiment of the present invention, in step S1, the method for preparing the core-shell structured nanoparticles includes the following steps:

[0016] a. Iron salt, anhydrous sodium acetate, and polyethylene glycol are dissolved in ethylene glycol and reacted solvothermally at 180-220 °C for 10-14 h to obtain Fe3O4 nanoparticles; wherein the molar ratio of iron salt to anhydrous sodium acetate is 1:4-8.

[0017] b. Disperse Fe3O4 nanoparticles in water, then add trisodium citrate solution and chloroauric acid solution sequentially, so that Au... 3+ Gold seeds are formed by ion adsorption on the surface of Fe3O4; a growth solution containing reducing agent and stabilizer is added, and the reaction is carried out at 20-40 °C for 30-90 min to obtain Au-Fe3O4 core-shell structured nanoparticles; wherein the mass ratio of chloroauric acid to Fe3O4 is 1-3:1.

[0018] In a preferred embodiment of the present invention, the reducing agent is selected from hydroxylamine, ascorbic acid, and sodium citrate; the stabilizer is selected from polyvinylpyrrolidone, polyethylene glycol, and hexadecyltrimethylammonium bromide.

[0019] In a preferred embodiment of the present invention, in step S2, the element that can form a closed microcavity is an annular washer made of elastic material, which is used to fit tightly against the spherical surface of the plum, with a fitting area ≥90%.

[0020] In a preferred embodiment of the present invention, in step S4, the heating and cooling cycles are repeated 2-8 times; wherein, the heating time of the heating stage is 10-30 s, raising the surface temperature of the plum to 55-70 ℃; the cooling time of the cooling stage is 15-45 s, lowering the surface temperature of the plum to 25-35 ℃; and the total cycle time of the heating and cooling stages is 45-75 s.

[0021] In a preferred embodiment of the present invention, in step S4, the external light source is a near-infrared laser with a wavelength of 780-850 nm, the diameter of the irradiated spot is smaller than the inner diameter of the enclosed microcavity, and the laser power density is 1.0-2.5 W / cm². 2 .

[0022] In a preferred embodiment of the present invention, in step S4, the heating stage causes the interfacial moisture to partially vaporize and form microbubbles, and the cooling stage causes the water vapor to condense, resulting in the collapse of the bubbles and the formation of a local negative pressure of -5 to -30 kPa in the closed microcavity. This negative pressure drives the deep tissue fluid of the plum to flow back to the surface, carrying pesticide molecules within a depth range of 100-800 μm from the surface of the peel to the surface of the nanoparticles for enrichment.

[0023] In a preferred embodiment of the present invention, in step S5, the excitation wavelength of the resonant light scattering method is 545-555 nm, the scanning wavelength is 450-650 nm, and the maximum scattered light intensity at a wavelength of 550-570 nm is recorded; the excitation and emission slit widths are both 3-10 nm.

[0024] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0025] (1) This invention provides a method for detecting pesticide residues in plums. Core-shell structured nanoparticles are used as photothermal probes and signal tags. A microcavity is formed on the plum surface and a nanoparticle dispersion is injected. Periodic irradiation with near-infrared laser is used. During the heating stage, the photothermal and magnetothermal processes of the nanoparticles synergistically raise the temperature, causing interfacial moisture to vaporize. During the cooling stage, condensation generates negative pressure, driving deep pesticide molecules to migrate directionally to the surface and accumulate in situ on the nanoparticle surface. Resonance light scattering technology is used to detect the intensity of the scattered light, and quantification is performed based on a standard curve. This invention integrates extraction, enrichment, and detection into a single system, enabling trace detection of various organophosphorus and pyrethroid pesticides on the plum peel, thus meeting food safety limits. Furthermore, the method is rapid, non-destructive in situ, and has good quantitative reproducibility.

[0026] (2) This invention utilizes Au-Fe3O4 core-shell nanoparticles under near-infrared laser irradiation to simultaneously excite the plasma photothermal effect of the gold shell and the photoabsorption thermal conversion of the Fe3O4 core. The two thermal mechanisms work synergistically at the molecular scale, enabling the nanoparticles to rapidly heat to the predetermined temperature within seconds. The heating rate is significantly improved compared to single materials, achieving instantaneous high-temperature heating of localized areas on the fruit peel surface, thereby avoiding thermal damage to the entire plum fruit. Compared to traditional destructive mass transfer promotion methods that require external ultrasound or microwaves, this invention achieves efficient heat input through in-situ heat generation of nanoparticles while maintaining the integrity of the fruit peel, providing an energy basis for the subsequent directional migration of pesticide molecules.

[0027] (3) This invention utilizes a periodic heating and cooling cycle. During the heating stage, interfacial moisture vaporizes to form microbubbles, and during the cooling stage, water vapor condenses, causing the bubbles to collapse. This generates alternating positive and negative pressure changes within the closed microcavity. Each negative pressure process acts as a suction effect on the deep tissue fluid, carrying pesticide molecules from the peel-pulp interface to the nanoparticle enrichment area. This significantly shortens the mass transfer distance of pesticide molecules from the high-concentration fruit pulp to the detection interface, improving extraction efficiency. Compared to the several hours required for passive diffusion at room temperature to reach equilibrium, this invention only requires a few cycles to achieve efficient enrichment, thus solving the problem of slow mass transfer of low-concentration pesticides in whole plums.

[0028] (4) This invention utilizes Au-Fe3O4 core-shell nanoparticles as a photothermal conversion medium, a pesticide capture carrier, and a resonant light scattering signal probe. It leverages the coordination bonds or hydrophobic interactions between the thiophosphate groups or aromatic rings in pesticide molecules and the gold surface to bridge adjacent nanoparticles into aggregates. Based on the principle of light scattering, the scattering intensity of the aggregates is proportional to the sixth power of the particle size. Therefore, a moderate increase in the nanoparticle size can trigger an exponential amplification of the scattering signal, enabling measurable resonant light scattering responses from trace amounts of pesticides. This allows for the trace detection of various organophosphates and pyrethroid pesticides on the surface of plums. Compared to the traditional visible light colorimetric method, which cannot meet the sensitivity requirements for food safety limits, this invention effectively improves its detection capability while maintaining the advantages of simple operation and in-situ detection. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a flowchart illustrating a method for detecting pesticide residues in plums according to the present invention.

[0031] Figure 2 This is a scanning electron microscope image of the Au-Fe3O4 core-shell nanoparticles of this invention;

[0032] Figure 3 This is a material standard curve diagram of chlorpyrifos in this invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0035] It should be noted that the raw materials, equipment and reagents used in this invention can all be purchased from the market or obtained through existing preparation methods.

[0036] The raw materials used in the examples and comparative examples are described below: ferric chloride hexahydrate (FeCl3·6H2O, purity ≥99%), anhydrous sodium acetate (NaOAc, purity ≥99%), ethylene glycol (purity ≥99.5%), polyethylene glycol (PEG-4000, average molecular weight 4000), chloroauric acid (HAuCl4·3H2O, purity ≥99.9%), trisodium citrate (purity ≥99%), and hydroxylamine (NH2OH·H2O). Cl (purity ≥98%), polyvinylpyrrolidone (PVP, K30, molecular weight 40000), bovine serum albumin (BSA, purity ≥98%), Tween-20 (polysorbate-20), chlorpyrifos standard (purity ≥99.5%), difenoconazole standard (purity ≥99%), diazinon standard (purity ≥99%), and malathion standard (purity ≥99%) were all purchased from Aladdin Reagent (Shanghai) Co., Ltd. Phosphate buffered saline (PBS, 10 mmol / L, pH 7.4) was purchased from Beijing Solarbio Science & Technology Co., Ltd. Polydimethylsiloxane (PDMS) prepolymer and curing agent (Sylgard 184) were purchased from Dow Corning Incorporated, Inc. All aqueous solutions were prepared using ultrapure water with a resistivity of 18.2 MΩ·cm.

[0037] The preparation method of Au-Fe3O4 core-shell nanoparticle dispersion includes the following steps:

[0038] a) Dissolve 1.35 g FeCl3·6H2O and 3.60 g anhydrous sodium acetate in 40 mL of ethylene glycol, add 1.00 g PEG-4000, stir to dissolve, and transfer to a 50 mL polytetrafluoroethylene-lined high-pressure reactor. React at 200 ℃ for 12 h. After natural cooling, magnetically separate the black precipitate, wash twice with anhydrous ethanol and three times with ultrapure water, and finally disperse in 10 mL of ultrapure water to obtain a Fe3O4 nanoparticle dispersion. The particle size was determined to be 12 nm by transmission electron microscopy.

[0039] b. Take 2 mL of Fe3O4 dispersion (containing approximately 10 mg of Fe3O4), add 20 mL of ultrapure water and 0.5 mL of 10 mmol / L trisodium citrate solution, stir for 10 min, then add 0.5 mL of 25 mmol / L HAuCl4 solution dropwise, and continue stirring for 15 min. Prepare a separate growth medium: add 10 mg of hydroxylamine, 10 mg of trisodium citrate, and 5 mL of 1% PVP solution sequentially to 50 mL of ultrapure water, adjust the pH to 8.0, slowly add the Fe3O4 dispersion containing gold seeds to the growth medium, and simultaneously add 2 mL of 1% HAuCl4 solution dropwise at a rate of 0.2 mL / min. React at 30 ℃ for 60 min. After magnetic separation, wash three times with ultrapure water, resuspend in 10 mL of PBS buffer to obtain Au-Fe3O4 core-shell nanoparticle dispersion. Transmission electron microscopy characterization showed that the Fe3O4 core particle size was 12 nm, the Au shell thickness was 8 nm, and its morphology was as follows. Figure 2 As shown, the total particle size is 28 nm, and the molar ratio of Au to Fe is 1.2:1.

[0040] c. Take 5 mL of Au-Fe3O4 dispersion, magnetically separate it, and resuspend it in 5 mL of PBS buffer containing 0.2% BSA. Mix by rotation at room temperature for 60 min, add 0.5 mL of 1% Tween-20 and continue mixing for 15 min. After magnetic separation and washing twice, resuspend it in 5 mL of PBS buffer, adjust the nanoparticle concentration to 200 μg / mL, and ultrasonically disperse for 5 min for later use.

[0041] The preparation of the ring gasket includes: mixing Sylgard 184 silicone rubber base liquid and curing agent at a mass ratio of 10:1, pouring the mixture into a ring mold with an inner diameter of 8 mm, an outer diameter of 12 mm, and a height of 2 mm, curing at 60 ℃ for 2 h, and obtaining a PDMS soft gasket ring with an inner diameter of 8 mm and a height of 2 mm after demolding.

[0042] Example 1:

[0043] like Figure 1 As shown in the figure, this embodiment provides a method for detecting pesticide residues in plums, including the following steps:

[0044] S1. Take the Au-Fe3O4 core-shell nanoparticle dispersion prepared above (total particle size 28 nm, Au shell thickness 8 nm, concentration 200 μg / mL) as a nanoprobe.

[0045] S2. Select fresh "Black Gem" plums, rinse the surface with ultrapure water for 5 seconds to remove surface dust, and allow to air dry. Mark 5 detection areas on the surface of the plums, each area approximately 0.5 cm². 2 Avoid the fruit stalk and calyx. Wipe the marked area with a cotton ball soaked in 75% ethanol and allow it to evaporate at room temperature for 30 seconds. Place the PDMS soft ring tightly against the marked area, ensuring that the edge of the ring is sealed to the fruit peel, with a contact area of ​​≥95%.

[0046] S3. Use a micropipette to draw 50 μL of Au-Fe3O4 nanoprobe dispersion and inject it into the closed cavity formed by the PDMS soft ring, ensuring that the liquid completely covers the detection area and there are no air bubbles.

[0047] S4. Use an 808 nm near-infrared laser as the external light source, adjust the spot diameter to 5 mm, and set the laser power density to 1.5 W / cm². 2 The system operates on a temperature cycling program: during the heating phase, the laser irradiates for 15 seconds, and during the cooling phase, the laser is turned off and the fruit cools naturally for 30 seconds. This cycle is repeated four times. Infrared thermal imaging monitoring shows that the surface temperature of the fruit peel rises to 62 ℃ during the heating phase and drops to 28 ℃ during the cooling phase.

[0048] S5. After the cycle, use a micropipette to aspirate all the nanoprobe suspension from the chamber, rinse the chamber twice with 20 μL PBS buffer, combine the solutions, and bring the volume to 100 μL. Resonance light scattering was detected using a fluorescence spectrophotometer (Hitachi F-7000). The excitation wavelength was set to 550 nm, the emission wavelength scanning range was 450-650 nm, the excitation and emission slit widths were both 5 nm, the scan rate was 300 nm / min, and the scattered light intensity I at 570 nm was recorded. max The intensity of scattered light I after the pesticide-free Au-Fe3O4 dispersion was treated with the same photothermal cycle. blank As a blank control, calculate ΔI = I max -I blank .like Figure 3 As shown, the pre-established standard curve equation is ΔI = 229.7 log C + 621.9, where C is the concentration of chlorpyrifos (ng / g), and the linear correlation coefficient R is... 2 =0.999, and by substituting ΔI into the pre-established standard curve equation, the residual concentration of chlorpyrifos was calculated.

[0049] To verify the detection performance of this embodiment, a standard spiking method was used. Fresh Black Gem plums were tested, and gas chromatography-mass spectrometry confirmed the absence of pesticide residues. 10 μL of chlorpyrifos standard solution (0.1% Tween 20 aqueous solution) was added to the surface of each detection area, resulting in final spiked concentrations of 0.5, 1, 5, and 10 ng / g. Each concentration was applied in triplicate to three areas. The areas were then air-dried for 30 min to allow for full pesticide penetration. The detection results are shown in Table 1.

[0050] Table 1:

[0051] Spiked concentration (ng / g) Measure the average concentration ± standard deviation (ng / g) Recovery rate (%) Relative standard deviation (%, n=3) 0.5 0.48±0.04 96.3 8.3 1 1.02±0.07 102.2 6.9 5 4.85±0.35 97.6 7.2 10 10.2±0.9 102.4 8.8

[0052] As shown in Table 1, within the spiked concentration range of 0.5 to 10 ng / g, the recovery rate was between 96% and 102%, and the relative standard deviation was between 6.9% and 8.8%, indicating that the detection accuracy and precision of this embodiment are good.

[0053] Example 2:

[0054] This embodiment is used to verify the consistency between the method of the present invention and the national standard method.

[0055] Three plum samples from different origins, designated Y1, Y2, and Y3, were purchased from three different stalls in the local farmers' market. Following the detection procedure in Example 1, five test areas were taken from each sample for analysis, and the average chlorpyrifos residue concentration was calculated. Simultaneously, approximately 5 g of the peel from the same sample was removed and analyzed using the national standard GB 23200.8-2016 method, namely QuEChERS pretreatment combined with gas chromatography-tandem mass spectrometry. The results are shown in Table 2.

[0056] Table 2:

[0057] Sample number Example 1: Measured value ± standard deviation (ng / g) National standard measured value ± standard deviation (ng / g) Relative standard deviation (%, n=5) Y1 1.2±0.1 1.35±0.12 11.8 Y2 0.3±0.05 0.28±0.04 6.9 Y3 Not detected Not detected 0

[0058] As shown in Table 2, the t-test showed no statistically significant difference between the two methods, with relative deviations within 15%, indicating that the detection results of the method of this invention are equivalent to those of the national standard method.

[0059] Example 3:

[0060] This embodiment is used to verify the detection capability of the method of the present invention for a variety of pesticides.

[0061] Four common pesticides—chlorpyrifos, difenoconazole, diazinon, and malathion—were spiked at 10 ng / g onto the surface of Black Gem plums. The assays were performed according to the procedure in Example 1, with each pesticide measured in triplicate. The limit of detection (LOD), recovery rate, and relative standard deviation (RSD) were calculated. The LOD was determined based on the concentration corresponding to a signal-to-noise ratio (SNR) of 3. The results are shown in Table 3.

[0062] Table 3:

[0063] pesticide name Limit of detection (ng / g) Recovery rate (%) at 10 ng / g spiked Relative standard deviation (%, n=5) Chlorpyrifos 0.10 96.5 6.8 difenoconazole 0.20 91.2 8.1 diazine 0.15 94.8 7.4 Malathion 0.25 89.5 9.2

[0064] As shown in Table 3, the method of the present invention has the ability to detect organophosphorus and pyrethroid pesticides at the ppb level, with a recovery rate between 89.5% and 96.5%, and a relative standard deviation of less than 10%, demonstrating good versatility.

[0065] To further simplify and make the present invention achieve its objectives and effects, the present invention will be further illustrated in conjunction with the following specific embodiments and comparative examples, but the present invention is not limited to the scope of the embodiments described herein.

[0066] Example 4:

[0067] This embodiment is basically the same as embodiment 1, except that the number of heating and cooling cycles is different. Specifically, the number of heating and cooling cycles in step S4 is 2. The remaining steps and process parameters are completely consistent with embodiment 1.

[0068] Example 5:

[0069] This embodiment is basically the same as Embodiment 1, except that the number of heating and cooling cycles is different. Specifically, the number of heating and cooling cycles in step S4 is 8. The remaining steps and process parameters are completely consistent with Embodiment 1.

[0070] Example 6:

[0071] This embodiment is basically the same as Embodiment 1, except that the heating stage time is different. Specifically, the laser irradiation time in step S4 is 10 seconds, which raises the surface temperature of the plum to 55°C. The remaining steps and process parameters are completely consistent with Embodiment 1.

[0072] Example 7:

[0073] This embodiment is basically the same as Embodiment 1, except that the heating stage time is different. Specifically, the laser irradiation time in step S4 is 30 seconds, which raises the surface temperature of the plum to 70°C. The remaining steps and process parameters are completely consistent with Embodiment 1.

[0074] Comparative Example 1:

[0075] This comparative example is basically the same as Example 1, except that the Au-Fe3O4 core-shell structured nanoparticles are replaced with Fe3O4 nanoparticles. Specifically, in step S1, only Fe3O4 nanoparticles are prepared without coating with an Au shell, and no BSA and Tween-20 modifications are performed. The Fe3O4 nanoparticles are dispersed in PBS buffer at a concentration of 200 μg / mL. The remaining steps and process parameters are completely consistent with those in Example 1.

[0076] Comparative Example 2:

[0077] This comparative example is basically the same as Example 1, except that the Au-Fe3O4 core-shell structured nanoparticles are replaced with Au nanoparticles. Specifically, in step S1, only Au nanoparticles are prepared without coating the Fe3O4 core. The Au nanoparticles have a particle size of 28 nm and are modified with BSA and Tween-20 at a concentration of 200 μg / mL. The remaining steps and process parameters are completely consistent with those in Example 1.

[0078] Comparative Example 3:

[0079] This comparative example is basically the same as Example 1, except that the surface of the Au-Fe3O4 core-shell structured nanoparticles is not sealed or modified. Specifically, in step S1, the Au-Fe3O4 core-shell structured nanoparticles are not treated with BSA and Tween-20 after preparation and are used directly. The remaining steps and process parameters are completely consistent with those of Example 1.

[0080] Comparative Example 4:

[0081] This comparative example is basically the same as Example 1, except that there is no heating and cooling cycle. Specifically, no heating and cooling cycle is applied in step S4. That is, after injecting the nanoparticle dispersion, it is left to stand for 3 minutes and no laser irradiation is performed. The remaining steps and process parameters are completely consistent with Example 1.

[0082] Comparative Example 5:

[0083] This comparative example is basically the same as Example 1, except that the number of heating and cooling cycles is different. Specifically, the heating and cooling cycle in step S4 is 1 time, while the other steps and process parameters are completely consistent with Example 1.

[0084] Comparative Example 6:

[0085] This comparative example is basically the same as Example 1, except that the number of heating and cooling cycles is different. Specifically, the number of heating and cooling cycles in step S4 is 10. The other steps and process parameters are completely consistent with Example 1.

[0086] Comparative Example 7:

[0087] This comparative example is basically the same as Example 1, except that the heating stage time is different. Specifically, the laser irradiation time in step S4 is 5 seconds, which raises the surface temperature of the plum to 45°C. The remaining steps and process parameters are completely consistent with Example 1.

[0088] Comparative Example 8:

[0089] This comparative example is basically the same as Example 1, except that the heating stage time is different. Specifically, the laser irradiation time in step S4 is 40 s, which raises the surface temperature of the plum to 80 ℃. The remaining steps and process parameters are completely consistent with Example 1.

[0090] To verify the technical effects of the embodiments and comparative examples of this invention, tests were conducted according to the following methods. All samples used chlorpyrifos as the target pesticide at a spiked concentration of 10 ng / g, and each sample was measured in parallel five times. Test items included: detection value, recovery rate, relative standard deviation, extraction efficiency, and pericarp integrity. The results are shown in Table 4.

[0091] Extraction efficiency determination method: The content of chlorpyrifos residue in plum peel and pulp after detection was determined by liquid chromatography-tandem mass spectrometry, and the extraction efficiency was calculated as the percentage of pesticide enriched on the surface of nanoparticles to the total amount of pesticide.

[0092] Peel integrity assessment: Observe the peel surface under a microscope for damage such as cracks and tissue fluid seepage. According to the degree of damage, it is divided into three levels: no damage, slight damage, and obvious damage.

[0093] Table 4:

[0094] Performance testing Average detected value (ng / g) Recovery rate (%) Relative standard deviation (%, n=5) Extraction efficiency (%) Fruit peel integrity Example 1 9.85 96.5 6.8 85.6 No damage Example 4 7.21 72.1 8.9 62.2 No damage Example 5 9.82 97.2 7.5 86.1 minor injury Example 6 7.85 78.5 7.8 68.6 No damage Example 7 9.88 96.8 7.1 84.7 minor injury Comparative Example 1 1.52 15.2 28.6 14.2 No damage Comparative Example 2 2.48 24.8 24.3 21.5 No damage Comparative Example 3 8.52 85.2 18.5 74.9 No damage Comparative Example 4 0.85 8.5 25.3 12.3 No damage Comparative Example 5 3.21 32.1 18.7 28.4 No damage Comparative Example 6 9.81 97.1 8.9 86.7 obvious damage Comparative Example 7 2.85 28.5 20.1 25.1 No damage Comparative Example 8 9.85 97.5 12.4 83.8 obvious damage

[0095] As shown in Table 4:

[0096] A comparison of Example 1 and Comparative Examples 1-2 reveals that: Comparative Example 1, using only Fe3O4 nanoparticles, lacks the surface plasmon photothermal effect of the Au shell. Under 808 nm laser irradiation, although Fe3O4 can absorb some light energy and convert it into heat through charge transfer transitions, its absorption cross-section is much smaller than that of the gold shell, resulting in a low heating rate. This leads to insufficient vaporization of interfacial moisture and insufficient local negative pressure, preventing deep pesticide molecules from effectively migrating to the surface. Furthermore, the Fe3O4 surface lacks active sites for forming coordination bonds and hydrophobic interactions between gold and thiophosphate groups or aromatic rings in pesticide molecules, resulting in weak pesticide adsorption capacity and a low enrichment factor. Consequently, the recovery rate is only 15.2%, and the extraction efficiency is only 14.2%. Comparative Example 2 uses only Au nanoparticles, which have good photothermal effects and pesticide adsorption capacity, but lack the magnetocaloric effect of Fe3O4 core and the convenience of magnetic separation. Under laser irradiation, the heating rate of Au nanoparticles is lower than that of the core-shell synergistic system, and without magnetic core-assisted thermal conversion, the overall thermal efficiency decreases. In addition, the lack of magnetic core makes it difficult for nanoparticles to be quickly magnetically separated after detection, resulting in poor dispersion uniformity, a relative standard deviation as high as 24.3%, and a recovery rate of only 24.8%.

[0097] A comparison between Example 1 and Comparative Example 3 reveals that the Au-Fe3O4 core-shell nanoparticles in Comparative Example 3, lacking BSA and Tween-20 blocking modification, exhibited significant non-specific adsorption of interfering substances such as proteins, polysaccharides, pigments, and microorganisms from plum epidermal tissue fluid during detection, in addition to specifically adsorbing target pesticide molecules. These interfering substances occupied the active sites on the gold surface, thereby blocking the coordination bonds and hydrophobic interactions between pesticide molecules and gold, resulting in a decrease in pesticide enrichment efficiency to 74.9% and a recovery rate to 85.2%. Furthermore, due to the random and uneven distribution of non-specific adsorbed objects, the degree of nanoparticle aggregation varied greatly in different detection areas or batches, leading to drastic fluctuations in the resonant light scattering signal and a relative standard deviation as high as 18.5%.

[0098] A comparison between Example 1 and Comparative Example 4 reveals that: Comparative Example 4 did not apply any heating or cooling cycles, i.e., it was left to stand for 3 minutes after the nanoprobe was injected. Under these conditions, pesticide molecules migrated from the deep pulp to the surface of the peel only through passive diffusion at room temperature. Due to the high water content and dense tissue of plum pulp, the diffusion coefficient of pesticide molecules in water-containing tissues is low. The mass transfer distance of passive diffusion for 3 minutes is only about 50-100 μm. Pesticides 500 μm deep from the peel surface cannot reach the interface, resulting in an extraction efficiency of only 12.3%. At the same time, without photothermal cycling, no vaporization or negative pressure occurs at the interface, and pesticide molecules cannot be actively pumped out and enriched on the surface of nanoparticles. Therefore, the recovery rate is only 8.5%.

[0099] A comparison of Examples 1, 4-5, and Comparative Examples 5-6 reveals the following: Example 4, with two heating and cooling cycles, achieved a recovery rate of 72.1% and an extraction efficiency of 62.2%; Example 1, with four cycles, achieved a recovery rate of 96.5% and an extraction efficiency of 85.6%; Example 5, with eight cycles, achieved a recovery rate of 97.2% and an extraction efficiency of 86.1%, but the fruit peel showed slight damage. Comparative Example 5, with only one cycle, achieved a recovery rate of only 32.1% and an extraction efficiency of 28.4%, because the equivalent mass transfer distance of a single cycle is approximately 150 μm, which only allows pesticide molecules within the surface 150 μm to migrate to the interface, leaving deeper pesticides unextracted. With two cycles, the total mass transfer distance reaches approximately 300 μm, increasing the extraction efficiency to 62%; with four cycles, the total mass transfer distance reaches approximately 600 μm, essentially covering the entire pesticide distribution area, and the extraction efficiency plateaus. Comparative Example 6 underwent 10 cycles. Although the extraction efficiency increased slightly to 86.7%, the cumulative heat effect from excessive heating and cooling caused the surface temperature of the peel to rise and fall repeatedly. This damaged the waxy structure of the cuticle, resulting in obvious cracking and exudation of tissue fluid, which interfered with the detection signal. The relative standard deviation increased to 8.9%, and the integrity of the peel was also compromised.

[0100] A comparison of Examples 1, 6-7, and Comparative Examples 7-8 reveals the following: In Example 6, laser irradiation for 10 seconds during the heating stage resulted in a surface temperature of 55°C, a recovery rate of 78.5%, and an extraction efficiency of 68.6%; in Example 1, heating for 15 seconds resulted in a temperature of 62°C, with a recovery rate of 96.5%; and in Example 7, heating for 30 seconds resulted in a temperature of 70°C, with a recovery rate of 96.8%, but slight damage to the pericarp. In Comparative Example 7, heating for 5 seconds resulted in a temperature of only 45°C, with a recovery rate of 28.5% and an extraction efficiency of 25.1%. This is because when the heating time is too short and the temperature is too low, the saturated vapor pressure of the interfacial moisture only increases from 3.17 kPa at 25°C to 9.59 kPa at 45°C, approximately three times, indicating weak vaporization, few and small microbubbles, insufficient negative pressure during cooling, and weak mass transfer driving force. When the temperature rises to 62 ℃, the saturated vapor pressure reaches 19.95 kPa, approximately 6.3 times that at 25 ℃. Water molecules gain sufficient kinetic energy to overcome surface tension and form dense microbubbles. Upon cooling, a strong negative pressure is generated, driving the efficient migration of deep-layer pesticides. In Comparative Example 8, heating for 40 s raises the temperature to 80 ℃. Although vaporization is vigorous, overheating causes denaturation of the pericarp proteins and rupture of cell membranes, resulting in a large amount of exudate from the tissue fluid. The interfering substances contained in this fluid cause the relative standard deviation to rise to 12.4%, and the pericarp is significantly damaged.

[0101] The above description is based on the preferred embodiments of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description, and all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0102] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for detecting pesticide residues in plums, characterized in that, Includes the following steps: S1. Prepare a dispersion of core-shell structured nanoparticles with photothermal and magnetothermal effects, wherein the core of the core-shell structured nanoparticles is a magnetic nanomaterial and the shell is a noble metal nanomaterial. S2. Install an element that can form a closed microcavity on the surface area of ​​the plum to be tested, so that the microcavity forms a sealed detection cavity with the surface of the plum; S3. Inject a core-shell structured nanoparticle dispersion into the detection chamber to cover the surface area to be tested; S4. Apply at least one heating and cooling cycle to the core-shell structured nanoparticles in the detection chamber. Each heating and cooling cycle includes a heating stage and a cooling stage. The heating stage involves irradiating the nanoparticles with an external light source to raise their temperature, and the cooling stage involves stopping the irradiation and allowing them to cool naturally. S5. Collect the treated core-shell structured nanoparticle dispersion, measure its scattered light intensity using the resonant light scattering method, and calculate the pesticide residue concentration on the plum surface according to the preset standard curve.

2. The method for detecting pesticide residues in plums according to claim 1, characterized in that, In step S1, the core is Fe3O4 nanoparticles and the shell is a gold nanolayer; the particle size of the Fe3O4 nanoparticles is 5-25 nm and the thickness of the gold nanolayer is 2-15 nm; the total particle size of the core-shell structured nanoparticles is 15-50 nm, and they have a surface plasmon resonance absorption peak at a wavelength of 540-550 nm.

3. The method for detecting pesticide residues in plums according to claim 1, characterized in that, In step S1, the surface of the core-shell structured nanoparticles is subjected to non-specific adsorption and sealing treatment with a sealing agent; the sealing agent is selected from at least one of bovine serum albumin, casein, polyvinylpyrrolidone, and Tween series surfactants.

4. The method for detecting pesticide residues in plums according to claim 2, characterized in that, In step S1, the method for preparing the core-shell structured nanoparticles includes the following steps: a. Iron salt, anhydrous sodium acetate, and polyethylene glycol are dissolved in ethylene glycol and reacted solvothermally at 180-220 °C for 10-14 h to obtain Fe3O4 nanoparticles; wherein the molar ratio of iron salt to anhydrous sodium acetate is 1:4-8. b. Disperse Fe3O4 nanoparticles in water, then add trisodium citrate solution and chloroauric acid solution sequentially, so that Au... 3+ Gold seeds are formed by ion adsorption on the surface of Fe3O4; a growth solution containing reducing agent and stabilizer is added, and the reaction is carried out at 20-40 °C for 30-90 min to obtain Au-Fe3O4 core-shell structured nanoparticles; wherein the mass ratio of chloroauric acid to Fe3O4 is 1-3:

1.

5. The method for detecting pesticide residues in plums according to claim 4, characterized in that, The reducing agent is selected from hydroxylamine, ascorbic acid, and sodium citrate; the stabilizer is selected from polyvinylpyrrolidone, polyethylene glycol, and hexadecyltrimethylammonium bromide.

6. The method for detecting pesticide residues in plums according to claim 1, characterized in that, In step S2, the element that can form a closed microcavity is an annular washer made of elastic material, used to fit tightly against the spherical surface of the plum, with a fitting area ≥90%.

7. The method for detecting pesticide residues in plums according to claim 1, characterized in that, In step S4, the heating and cooling cycles are repeated 2-8 times; the heating phase lasts for 10-30 seconds, raising the surface temperature of the plum to 55-70°C; the cooling phase lasts for 15-45 seconds, lowering the surface temperature of the plum to 25-35°C; and the total cycle time for the heating and cooling phases is 45-75 seconds.

8. The method for detecting pesticide residues in plums according to claim 1, characterized in that, In step S4, the external light source is a near-infrared laser with a wavelength of 780-850 nm, the diameter of the irradiation spot is smaller than the inner diameter of the enclosed microcavity, and the laser power density is 1.0-2.5 W / cm². 2 .

9. The method for detecting pesticide residues in plums according to claim 1, characterized in that, In step S4, the heating stage causes partial vaporization of interfacial moisture to form microbubbles, and the cooling stage causes water vapor to condense, resulting in bubble collapse and the formation of a local negative pressure of -5 to -30 kPa within the closed microcavity. This negative pressure drives the deep tissue fluid of the plum to flow back to the surface, carrying pesticide molecules within a depth range of 100-800 μm from the surface of the peel to the surface of the nanoparticles for enrichment.

10. The method for detecting pesticide residues in plums according to claim 1, characterized in that, In step S5, the excitation wavelength of the resonant light scattering method is 545-555 nm, the scanning wavelength is 450-650 nm, and the maximum scattered light intensity at a wavelength of 550-570 nm is recorded; the excitation and emission slit widths are both 3-10 nm.

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