The template molecules (0.2867 g of Trichlorfon and 9.2244 g of Cypermethrin) were dissolved in 12 mL chloroform in a three-neck flask, followed by the addition of 2.756 g of methacrylic acid (8 mmol) as the functional monomer. The mixture was stirred at room temperature for 30 minutes to ensure thorough mixing. Ethylene glycol dimethacrylate (3.964 g, 20 mmol) was added as the crosslinking agent, followed by 400 mg of azobisisobutyronitrile as the initiator. The reactions were configured with 0.5 mmol of template monomer, a template:functional monomer:crosslinker ratio of 1:4:20 (n:m:n), 0.44 mmol of initiator, and a pore-forming agent consisting of a mixed solution of acetonitrile/methanol (volume ratio = 3/1). After thorough mixing, the flask was sealed and sonicated for 15 mins. After nitrogen gas was introduced for 15 minutes to remove oxygen, the flask was sealed and placed in a 60 °C water bath for 24 h [11, 12].

Soxhlet Extraction for Removal of Pesticide Template Molecules from the Polymer: After the polymerization process was completed, an amber-colored solid was obtained. The solid was thoroughly ground in a mortar and sieved through a 200-mesh screen. The resulting powder was tightly wrapped in a filter paper thimble and placed into the extraction chamber of a Soxhlet apparatus. A mixture of methanol and glacial acetic acid (120 mL:40 mL, 3:1 v/v) was used as the extraction solvent. The extraction system was assembled and heated in a water bath at 93 °C. Siphoning occurred approximately every 20 min. After 12 h of continuous extraction, the thimble was removed. The extracted polymer was then washed with methanol until neutral pH was achieved and dried in an oven at 50 °C for 12 h to obtain the final molecularly imprinted polymer (MIP) [15, 16, 22].

The non-imprinted polymer (NIP) was prepared following the same procedure, except that no pesticide template molecule was added during the polymerization.

The surface morphologies of trichlorfon, the molecularly imprinted polymer (MIP), and the non-imprinted polymer (NIP) were characterized using scanning electron microscopy (SEM) to evaluate structural and topological features. A drop of nanometer compound was taken from the copper-covered network with membrane, subjected to natural air and 2% phosphotungstic acid dye for 30 min in the copper network carrying the nanometer compound. A filter paper was used to absorb the excess dye. Then, the sample was scanned with a transmission electron microscope using an acceleration volt-age of 80 kV [11, 12].

FTIR spectroscopy was employed to characterize trichlorfon, MIP, and NIP. By comparing the stretching vibration peaks of specific functional groups in the infrared spectra, the elution efficiency of the pesticide molecules was evaluated. The samples’ infrared spectrum analysis was performed using the KBr method, with a wave number in the range of 400 to 4000 cm^-1. Scanning was conducted for 32 times at a resolution of 4 cm^-1. This analysis also helped determine whether chemical bonding occurred between the trichlorfon molecules and the functional monomer, methacrylic acid [28, 29].

A standard aqueous solution of trichlorfon or cypermethrin at a concentration of 300 mg/L was accurately prepared. All experiments were performed in triplicate: 20 mg of molecularly imprinted polymer (MIP) was weighed into seven 50-mL volumetric flasks, followed by the addition of 10 mL of the 300 mg/L trichlorfon or cypermethrin standard solution. The flasks were shaken in an incubator at room temperature for periods ranging from 5 to 240 mins. After shaking, the mixtures were centrifuged at 4000 rpm for 30 mins. The supernatant was then carefully collected, and its absorbance was measured at 190 nm using a UV-Vis spectrophotometer. The same procedure was applied in parallel to evaluate the non-imprinted polymer (NIP) [10, 12].

To evaluate the adsorption performance of the two types of molecularly imprinted polymers, equilibrium binding studies were conducted with trichlorfon and cypermethrin. Exactly 20 mg of MIP was weighed into a 50-mL volumetric flask, and 10 mL of trichlorfon or cypermethrin solution at concentrations ranging from 0 to 300 mg/L was added. The mixtures were oscillated at room temperature for 4 hours and then centrifuged at 4000 rpm for 15 minutes. The supernatant was filtered through a 0.22 µm aqueous-phase membrane, and a 50 µL aliquot was injected for HPLC analysis at a detection wavelength of 200 nm [15, 23].

Measure 15 mL of 0.01 mol/L silver nitrate standard solution into a 150 mL volumetric flask. Dilute to volume with ultrapure water. Add a clean, dry magnetic stir bar and heat while stirring at 100 °C. Once boiling commences with uniform bubble formation, slowly add 3 mL of 1% trisodium citrate solution dropwise while maintaining stirring and heating for approximately 30 minutes. Cease heating and allow the mixture to cool naturally to room temperature. Take 40 g of the solution and centrifuge at 3500 rpm for 2 min to remove larger silver nanoparticle particles. Ultrasonically disperse the supernatant for 20 min to ensure thorough mixing, yielding the AgNPs. Store away from light [19, 30].

Five silver nitrate solutions were prepared at different concentrations: 0.2, 0.6, 1.0, 1.4, and 1.8 $\times$ 10^-3 mol/L. From each concentration of silver nitrate, 150 mL was taken to synthesise AgNPs as SERS-enhanced substrates. Five sodium chloride solutions were also prepared at concentrations of 0.2, 0.6, 1.0, 1.4, and 1.8 $\times$ 10^-2 mol/L, (5 mL each) for subsequent use. The 10^-2 mol/L stock standard solutions of trichlorfon and cypermethrin were serially diluted with methanol to obtain concentrations ranging from 10^-2 to 10^-8 mol/L. The diluted solutions were stored in sealed volumetric flasks to provide standard solutions of the two pesticide molecules at various concentrations [31].

Raman and SERS spectra of the trichlorfon and cypermethrin standard solutions were acquired using a HORIBA HR Evolution microconfocal Raman spectrometer. Prior to the experiment, the instrument was calibrated using a silicon wafer. A 5 µL aliquot of AgNPs was mixed with 5 µL of the test solution on a microscope slide, thoroughly mixed, and the slide was placed on the stage after 2 min. Exposure to light was avoided throughout. Measurements were performed using the Raman spectrometer to collect the spectra. This method was applied to each concentration of the pesticide solutions. Unless otherwise specified, the spectral acquisition parameters were as follows: laser wavelength, 532 nm; laser intensity, 50%; laser spot diameter, 200 µm; grating, 600 grooves/mm (centre wavelength 500 nm); and a 50$\times$ objective lens on the confocal microscope. Ten SERS spectra were collected for each concentration [19, 30, 32].

Common leeks, which may retain residues of trichlorfon and cypermethrin, were selected as the actual samples for testing. The leek samples were spiked with known amounts of both pesticides. By combining the MIPs to adsorb the pesticide molecules with the prepared AgNPs as the SERS enhancement substrate, SERS spectra were acquired from the pesticide-contaminated leek samples. The leeks were rinsed with ultrapure water, air-dried in a fume hood for 30 min, immersed in the pesticide solutions for 5 min, removed and then left in a cool place for 24 h. This procedure ensured that the spiked pesticide concentrations closely approximated real-world contamination levels [17, 33].

Three grams of each of the two prepared pesticide-specific molecularly imprinted polymers (MIPs) were weighed and dissolved in 3 mL of methanol. Chopped leeks were placed on a glass slide, 5 µL of the molecularly imprinted polymer solution was added, and the MIPs were allowed to adsorb fully onto the sample and dry. Subsequently, 5 µL of AgNPs was added. After 2 min, surface-enhanced Raman spectroscopy signals were collected from ten uniformly distributed points on the sample surface. Measurements were taken multiple times at each concentration, and the average value was selected once the signal intensity has stabilised; this average was used as the final measurement result. The instrument parameters were as follows: laser wavelength, 532 nm; laser intensity, 50%; laser spot diameter, 200 µm; grating centred at 500 nm; and a 50$\times$ objective lens on the confocal microscope [30, 34].

The molecularly imprinted polymers (MIPs) synthesized using trichlorfon and cypermethrin as template molecules, along with the non-imprinted polymer (NIP), were characterized by SEM. As shown in Fig. 1, molecularly imprinted polymer shown in the left figure has a microspherical surface with a large specific surface area. It is porous and has a rough, uneven texture. This morphological difference can be attributed to the interactions between the pesticide template molecules and the functional monomer during polymerization, leading to the formation of numerous microspheres complementary in size and shape to the templates [23, 25]. In contrast, the NIP, synthesized in the absence of template molecules, lacks such specific cavities, resulting in a comparatively smoother surface with fewer pores.

Fig. 1.

Scanning electron micrographs of molecularly imprinted polymers. Scanning electron micrographs of molecularly imprinted polymers (A) ($\times$10,000) and non-imprinted polymers (B) ($\times$10,000).

The FTIR spectra of the MIP, NIP, and trichlorfon are shown in Fig. 2. In the spectrum of MIP (Fig. 2A), the absorption peak at 1240.84 cm^-1 corresponds to the stretching vibration of P=O [17]. Both the MIP (Fig. 2A) and NIP (Fig. 2B) show absorption near 1730 cm^-1, attributed to the C=O stretching vibration. 2962.02 corresponds to C-H stretching vibrations of methyl (-CH₃) or methylene (-CH₂) groups. A broad absorption peak observed at 3503.93 cm^-1 in the MIP spectrum (Fig. 2C) indicates the presence of O–H stretching vibrations due to intermolecular hydrogen bonding. This suggests that the P=O group of trichlorfon interacted with the –OH group of methacrylic acid, confirming successful binding between the mixed templates and the functional monomer during MIP synthesis [11, 15]. The similarity between the FTIR spectra of the MIP and NIP (Fig. 2A,B) indicates analogous molecular structures, confirming the effective removal of template molecules from the MIP.

Fig. 2.

Infrared characterization of polymers and pesticides. Infrared characterization of polymers and pesticides. Note: (A) Molecularly imprinted polymer; (B) non-imprinted polymer; (C) trichlorfon.

The adsorption capacity was measured at different time intervals (Fig. 3). The results demonstrate that the adsorption capacity of the MIP increased over time, reaching a maximum value of 32.24 mg/g, indicating high adsorption affinity for trichlorfon [23, 25].

Fig. 3.

Kinetic uptake plots of the imprinted polymer towards trichlorfon and cypermethrin at 300 mg/L. Kinetic uptake plots of the imprinted polymer towards trichlorfon and cypermethrin at 300 mg/L. Note: Lowercase letters indicate a 0.05 significance level.

As shown in Fig. 3, after 30 minutes of shaking, the adsorption capacities of the common MIP toward trichlorfon and cypermethrin were 17.4 mg/g and 18.9 mg/g, representing 61.3% and 62.4% of the saturated adsorption capacity, respectively. Adsorption equilibrium was essentially reached within 180 mins. These results suggest that the common MIP exhibits rapid adsorption kinetics for organophosphorus pesticides and can be effectively used as a sorbent in matrix solid-phase dispersion extraction for rapid extraction of such pesticides from samples [24, 26].