Preparation of magnetic metal organic framework composites for the extraction of neonicotinoid insecticides from environmental water samples

Abstract

MOF-199/Fe₃O₄ nanoparticles were explored for the first time to purify environmental water samples and adsorb neonicotinoid insecticides as hybrid adsorbents. The composites were successfully synthesized by an in situ method at room temperature with electrostatic interaction to chemically stabilize the nanoparticles and metal ions. The nanoparticles were uniformly encoded with metal organic frameworks (MOFs). With the lowest loading amount of Fe₃O₄ applied to magnetic solid-phase extraction (MSPE) of six neonicotinoid insecticides in environmental water samples followed by high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) analysis, the extraction efficiency was the highest. The main influencing factors including the solution ionic strength, solution pH, extraction time, desorbing organic solvent, and solvent volume, were also evaluated. Considering the adsorbing conditions and insecticide structures, the adsorbing mechanism was preliminarily discovered to be largely dependent upon the π–π interaction with the benzene ring in MOF-199 and delocalized large π bonds in the insecticide molecules. Under optimal conditions, the limits of detection (LODs) were 0.3–1.5 ng mL⁻¹ with a signal-to-noise ratio (S/N) of 3. All the analytes exhibited good linearity with correlation coefficients (r²) of higher than 0.9947. The relative standard deviations (RSDs) for six neonicotinoid insecticides in environmental samples in five replicates ranged from 1.5% to 11.6%, and good recoveries from 88.0% to 107.0% were obtained, indicating that the MOF-199/Fe₃O₄ composites are feasible for analysis of trace analytes in environmental water samples.

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1. Introduction

Neonicotinoid insecticides are a group of relatively new active ingredients that are derived from plant-sourced nicotine in pesticides.¹ The insecticides have been growing fastest in the world market² and have been largely used in agricultural fields, but research showed that they could pose a serious risk to human health and honeybees with colony collapse disorder (CCD), due to their special interactions with acetylcholinesterase receptors and proteins.³ Neonicotinoid insecticides in environmental samples were mainly determined by liquid–liquid extraction (LLE) and solid-phase extraction (SPE), coupled with liquid chromatography (LC),^4,5 gas chromatography mass spectrometry (GC-MS),⁶ and liquid chromatography tandem mass spectrometry (LC-MS/MS).⁷ Although the process of SPE has several disadvantages,⁸ it is considered to be superior to the LLE due to less consumption of organic solvent. With the development of SPE into magnetic solid-phase extraction (MSPE) based on magnetic nanoparticles, it has showed superiorities including easy operations within a short time, and little or no consumption of organic solvents. Therefore, in order to develop an easy MSPE method for neonicotinoid insecticides and effectively monitor trace residues of neonicotinoid insecticides in environment samples, a highly sensitive multi-residual determination method coupled with MSPE should be established.

Recently, metal organic frameworks (MOFs) have emerged as interesting mesoporous materials because of their remarkable and considerable properties including the tunable topology and porosity, extremely high surface area, adjustable pore size and simple functionality.⁹ The materials have been extensively used for chemical detection, constituent separation,¹⁰ catalysis,¹¹ sensing, biomedical therapy¹² and other fields. However, the use of intrinsic magnetic properties of MOFs seems to be infeasible for MSPE process due to their limited magnetization. Nowadays, subtle coupling of MOFs and magnetic nanoparticles have been preliminarily used for adsorbing and desorbing organic contaminants with rapid, convenient separation and high selectivity from complicated matrices.¹³ Since Yan and co-workers first magnetized Cr–MOF microcrystals in two steps by respectively synthesizing Fe₃O₄ nanoparticles and MOFs and then mixing them via electrostatic interactions,¹⁴ Several reports about multi-residual detection of pollutants have emerged. Chen et al. also synthesized magnetic Fe–MOF core–shell microspheres for adsorbing seven polychlorinated biphenyls with GC-MS detection, and the magnetic property could be well maintained.¹⁵ More recently, Hu et al.¹⁶ synthesized magnetic Zn–MOFs by chemically bonding functionalization of Fe₃O₄ nanoparticles with MOFs to adsorb polycyclic aromatic hydrocarbons and gibberellic acid, Zhang et al.¹⁷ synthesized magnetic Fe–MOFs using an solvothermal method in one step to adsorb organophosphorus pesticides. These methods require some prior modification for Fe₃O₄ nanoparticles, or involve complicated, time-consuming operations. Thus, our object was to fabricate magnetic MOFs applied into MSPE using an easy and fast synthetic procedure. In addition to the above mentioned applications, MOFs can be used to extract other pollutants such as heavy metal ions,^18,19 herbicides,²⁰ pharmaceuticals and personal care products (PPCPs),²¹ dyes,^22,23 volatile organic compounds (VOCs),^24,25 and nitrogen-containing compounds (NCCs).²⁶ Furthermore, plausible adsorption or interaction mechanisms between pollutants and MOFs have been proposed in several literatures, mainly including electrostatic interaction, acid–base interaction, hydrogen bonding, stacking/interaction and hydrophobic interaction.²⁷ However, to the best of our knowledge, there is no report on direct applications of magnetic MOFs to adsorptive removal of neonicotinoid insecticides and exploration of the interaction mechanism.

In this paper, the proposed synthetic procedure involved in situ fabrication of magnetic MOF-199 using a room-temperature method. The electrostatic interaction between negatively charged Fe₃O₄ and positively charged metal ions was expected to chemically stabilize the magnetic MOFs/nanoparticle composites to successfully generate a homogeneous magnetic product. The performance of the composites applied to the environmental water samples to adsorb neonicotinoid insecticides was evaluated and the adsorption mechanism was preliminarily studied. Also, different loading amounts of Fe₃O₄ nanoparticles were compared. The established method offered excellent sensitivity, a wide linear range, and simple operations, as well as satisfactory recoveries.

2. Experimental

2.1 Reagents and standards

FeCl₃·6H₂O (99%), FeCl₂·4H₂O (98%), 1,3,5-benzenetricarboxylic acid (H₃BTC, 95%) and sodium chloride (99%) were purchased from Sigma Corporation (St. Louis, MO, US). Copper acetate monohydrate was from Aladdin Reagent Corporation (Shanghai, China). Deionized water obtained from a Milli-Q system (France) was used. Methanol, acetone, acetonitrile and ethanol were obtained from Tedia (Fairfield, OH, US). Seven neonicotinoid insecticides including dinotefuran, thiamethoxam, clothianidin, acetamiprid, nitenpyram, imidacloprid and thiacloprid were purchased from Accustandard (Germany). These standards were stored under refrigeration at 4 °C and used to prepare working standard solutions. All other reagents were of analytical grade.

2.2 Instrumentation

The samples were analyzed with an Agilent HPLC system (1200, Agilent, USA) coupled with a triple quadrupole mass spectrometer (AB2000, AB Sciex, USA) equipped with an electrospray ionization (ESI) source. X-ray diffraction (XRD) measurements were performed on a D-Max 2200 VPC diffractometer (Japan). Fourier transform infrared (FT-IR) was carried out using PerkinElmer spectrum 100. Scanning electron microscopy (SEM) images were recorded on a JSM-6300 SEM instrument (Japan). The hysteresis loop of magnetic compounds was measured using a LakeShore7407 vibrating sample magnetometer (VSM, U.S). KQ-500DB CNC ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd.) was also used.

2.3 Chromatographic conditions

Chromatographic separation was performed on an Eclipse XDB-C₁₈ column (2.1 × 150 mm, id: 3.5 μm, Agilent, America). The mobile phase consisted of (A) 0.1% formic acid solution and (B) acetonitrile. The elution conditions were as follows: 0–3 min, a linear gradient was kept at 10% B; 3–9 min, a linear gradient was from 10% to 70% B; 9–12 min, a linear gradient was kept at 70% B; 12–12.1 min, a linear gradient was changed from 70% to 10% B; 12.1–15 min, a linear gradient was kept at 10% B. The flow rate was 0.3 mL min⁻¹. The column temperature was maintained at 35 °C. The injection volume was 5 μL.

2.4 Mass spectrometric conditions

Mass spectrometry was conducted on a tandem quadrupole mass spectrometer (AB-MDS Sciex, API-2000) equipped with an ESI source. The conditions of the ESI source were as follows: ion-spray voltage (IS): 5500 V; source temperature: 450 °C; curtain gas: 45 psi; collision gas: 5 psi; ion source gas 1: 45 psi; ion source gas 2: 45 psi; focusing potential: 400 V; entrance potential: 10 V; declustering potential (V), collision energy (eV), qualitative ion and quantitative ion were listed in Table 1. All the seven compounds were analyzed in positive ESI mode and multiple reaction monitoring (MRM) mode was selected for quantification.

Table 1 Optimized multiple reaction monitoring (MRM) parameters and structures of neonicotinoid insecticides

Compound	Structure	Declustering potential (V)	Collision energy (eV)	Qualitative ion pair (m/z)	Quantitative ion pair (m/z)
Dinotefuran		25, 25	23, 26	203.3 → 129.3, 203.3 → 87.1	203.3 → 129.3
Thiamethoxam		35, 35	18, 24	271.3 → 225.1, 271.3 → 99.0	271.3 → 225.1
Clothianidin		20, 20	23, 37	292.1 → 211.2, 292.1 → 132.3	292.1 → 211.2
Acetamiprid		25, 25	20, 27	250.2 → 132.1, 250.2 → 169.0	250.2 → 132.1
Nitenpyram		26, 26	25, 40	256.1 → 209.0, 256.1 → 175.1	256.1 → 209.0
Imidacloprid		28, 28	33, 43	223.1 → 126.1, 223.1 → 55.9	223.1 → 126.1
Thiacloprid		45, 45	32, 64	253.1 → 125.7, 253.1 → 90.2	253.1 → 125.7

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2.5 Synthesis of Fe₃O₄/MOF-199 composites

Fe₃O₄ nanoparticles were synthesized with a chemical co-precipitation method.²⁸ Briefly, FeCl₃·6H₂O (2.35 g) and FeCl₂·4H₂O (0.86 g) were dissolved in 100 mL of deionized water with stirring under nitrogen atmosphere at 60–70 °C. Then, 10 mL of 25% ammonia solution was added, and the mixture was stirred vigorously for 30 min at 80 °C. After cooled to room temperature, black precipitates were isolated from the solution by external magnetic fields and then alternatively washed with deionized water and ethanol several times until the pH level of the washing approached neutral.

MOF-199/Fe₃O₄ composites was prepared with a slightly modified room temperature method.^17,29 The procedure was as follows: the freshly prepared wet magnetic Fe₃O₄ nanoparticles (50 mg of dried Fe₃O₄) were dispersed in 40 mL of Cu(CH₃COO)·H₂O (860 mg) solution with ultrasonication for 5 min. Then, 500 mg H₃BTC solution (80 mL, DMF : ETOH = 1 : 1) was added with ultrasonication for another 5 min. Triethylamine (0.5 mL) was added to the reaction mixture and vigorously stirred for 5 h. Thereafter, the gray-blue solid was separated from the reaction medium with external fields, alternatively washed with deionized water and hot ethanol several times to remove impurities, and dried overnight at 70 °C. The resulting product was identified as MOF-199/Fe₃O₄ composites.

2.6 MSPE procedure

Firstly, 20 mg of Fe₃O₄/MOF-199 was added into 10 mL of water sample. In order to completely adsorb analytes, a 15 mL tube was ultrasonicated for 1 min and then shaken in an oscillator at 200 rpm for 20 min. Secondly, the adsorbent (MOF-199/Fe₃O₄) was isolated from the water sample with external magnetic fields and water was removed completely. Thirdly, 0.5 mL acetone was added as an eluate for ultrasonication lasting a few minutes (repeated 3 times). The desorption solutions were combined together and transferred to a 5 mL tube, and then evaporated to dryness under a mild nitrogen stream. The residues were dissolved in 2 mL of deionized water and filtered through a 0.22 μm membrane. Finally, 5 μL was injected into the HPLC-MS/MS system for analysis.

3. Results and discussion

3.1 Characterization of Fe₃O₄/MOF-199 magnetic composites

The aim of this work was to obtain hybrid composites of MOF-199/Fe₃O₄ with a high adsorbing capacity for insecticides by an in situ method under room temperature. In the synthesizing process, Cu²⁺ could be adsorbed on the nanoparticles with electrostatic interactions, and then the MOFs could grow around the nanoparticles and form a uniform structure. As we could see from Fig. 1, the composites were evaluated with different loading amounts of Fe₃O₄ nanoparticles (50 mg, 100 mg and 200 mg). With a decreasing amount of Fe₃O₄ nanoparticles, the saturation magnetization was gradually decreased. In fact, the composites with the lowest loading amount of Fe₃O₄ nanoparticles (50 mg) could be easily separated from the solution with external magnetic fields. However, the MOFs could not be completely separated from the samples with a loading amount below 50 mg.

Fig. 1 Magnetization curves of MOF-199/Fe₃O₄ composites with different loading amounts of Fe₃O₄ nanoparticles: (a) 50 mg; (b) 100 mg; (c) 200 mg.

From Fig. 2, the SEM graphs revealed the morphologies of the composites. The graph of pure MOF-199 (Fig. 2d) crystals synthesized in room temperature in this work was in accordance with the literature reported with a solvothermal method³⁰ indicating a successful synthesis. Compared with pure MOF-199, the loaded nanoparticles of 50 mg could be uniformly attached on the surface of the composites with a good magnetic property (Fig. 2a) and the loading of 100 mg (Fig. 2b) and 200 mg (Fig. 2c) could lead to heterogeneous surfaces of the composites, with free Fe₃O₄ nanoparticles near the composites, which caused a loss and unstable magnetic property. So, in comprehensive consideration, the composites were used in the following procedures with the loading amount of 50 mg nanoparticles.

Fig. 2 SEM images of MOF-199/Fe₃O₄ composites with the loading amounts of 50 mg (a), 100 mg (b) and 200 mg (c) nanoparticles and with pure MOF-199 (d).

As shown in Fig. 3, the composites were further characterized by XRD and FT-IR. We can obviously see that the six characteristic peaks for Fe₃O₄ nanoparticles and those for MOF-199 in the composites (Fig. 3a) were in agreement with the earlier literature.²⁹ All the characteristic peaks of Fe₃O₄ nanoparticles could be observed in the XRD graph of MOF-199/Fe₃O₄ composites, indicating that the hybrid composites were successfully synthesized, being mainly composed of Fe₃O₄ nanoparticles and MOF-199.

Fig. 3 XRD patterns (a) and FT-IR spectra (b) of Fe₃O₄, pure MOF-199 and MOF-199/Fe₃O₄.

The FT-IR spectra were shown in Fig. 3b. For Fe₃O₄ nanoparticles, the characteristic features at 3428 cm⁻¹ were the adsorption bands with the O–H stretching vibrations. And 584 cm⁻¹ was attributed to the Fe–O bond vibration of Fe₃O₄. For MOF-199, the five typical bands were almost identical with those for the Fe₃O₄/MOF-199 composites, indicating that the MOFs played a major role in the hybrid composites. Among the four bands, the peak at 730 cm⁻¹ was attributed to the out-of-plane bending vibration of C–H in the benzene ring of H₃BTC. In the region of 1800–800 cm⁻¹, the bands at 1642 cm⁻¹ and 1445 cm⁻¹ were assigned to the asymmetric stretching of carboxyl groups in H₃BTC, and 1374 cm⁻¹ was assigned to the symmetric stretching of carboxyl groups in H₃BTC.³¹

3.2 Effect of the Fe₃O₄ amounts on the adsorption capacity

Experiments were performed in triplicate to obtain the average values for the evaluation of the effect of Fe₃O₄ loading amounts on the adsorption capacity. The effect was expressed as the extraction efficiency (R%), which was calculated using the equation:
where C, C₀, V and V₀ presented the analyte concentration in the reconstituted solvent, analyte concentration in the initial water, the volumes of the reconstituted solvent and initial water volume (10 mL), respectively.

The extraction efficiency of the Fe₃O₄/MOF-199 composites with different amounts of Fe₃O₄ (200 mg, 100 mg and 50 mg) was evaluated by enriching seven neonicotinoid insecticides. Different loading amounts of the MOF-199/Fe₃O₄ composites were compared with pure Fe₃O₄ particles and MOF-199 crystals for the performance evaluation. The result showed that pure MOF-199 crystals had the highest extraction efficiency for the analytes among all adsorbent materials (Fig. 4). The Fe₃O₄ nanoparticles showed the lowest extraction recovery and the extraction recovery of the composites increased gradually with decreased loading amounts of Fe₃O₄. This indicated MOF crystals played a major role in adsorbing insecticides due to its large surface with a benzene ring and high porosity inside the MOFs. In addition, with the increasing amounts of Fe₃O₄, the pore could be occupied by the nanoparticles, leading to decreasing porosity and lower adsorption. Furthermore, among the seven insecticides, dinotefuran was hardly adsorbed by the MOF-199/Fe₃O₄ composites. Therefore, in order to guarantee the good magnetic property and high extraction recovery, the loading amount of 50 mg Fe₃O₄ was selected and employed in the following studies.

Fig. 4 Extraction efficiency of neonicotinoid insecticides of pure MOF-199, Fe₃O₄ and the MOF-199/Fe₃O₄ composites by loading different Fe₃O₄ nanoparticles, with 10 mL sample water volume and extraction time of 20 min (n = 3).

3.3 Optimization of the MSPE conditions

3.3.1 Effects of the NaCl concentrations on the adsorption capacity. In order to conveniently identify the optimal condition in the experiment process, elution peak areas were used to evaluate different parameters. The salt-out effect was evaluated by altering the NaCl concentrations from 0% to 20% (Fig. 5). With the NaCl concentrations varying from 0% to 10%, the elution peak areas of the analytes decreased; when the NaCl concentration was 15%, the peak area was the highest except for the blank sample whose NaCl concentration was 0%. The peak areas of dinotefuran witnessed no changes at all concentrations. Therefore, NaCl was not added in the following procedures.

Fig. 5 Effects of different NaCl concentrations on the eluting peak areas (n = 5).

3.3.2 Effects of the pH values on the adsorption capacity. The pH value of the sample solutions played a vital role in adsorbing analytes into the pores of MOFs, which affected not only the forms of MOF crystals and charge species, but also the existing forms of analytes. This study demonstrated the elution peak areas with the sample pH value ranging from 3 to 11 (Fig. 6). When the pH value ranged from 3 to 7, except for slightly decreased thiamethoxam, the peak areas of the other insecticides increased insignificantly or almost had no changes. When the pH value ranged from 7 to 11, except for dinotefuran, the peak areas of all six insecticides decreased significantly. The reason could be that the neonicotinoid insecticides were easily degraded or the frameworks would be collapsed in strong alkaline conditions.³² Because the pH value of the environmental water samples is almost in the range from 5 to 7, there is no need to adjust the pH value of the samples in the following procedures.

Fig. 6 Effects of different pH on the eluting peak areas (n = 5).

3.3.3 Effects of the extraction time. During the equilibrium-based MSPE process, the extraction time is one of primary factors that influence the extraction efficiency. R% was also used to evaluate the optimal extraction time. The extraction time ranging from 5 to 30 min (Fig. 7) were investigated while other parameters being held constant. The result revealed that the extraction recovery increased when the time ranged from 5 to 20 min except for dinotefuran, and then remained nearly stable from 20 to 30 min. Therefore, the extraction time of 20 min was selected.

Fig. 7 Effects of different extraction time on the extraction efficiency (n = 5).

3.3.4 Effects of the elute solvent. In this work, four most commonly used organic solvents including ethanol, methanol, acetone and acetonitrile were investigated for desorption of neonicotinoid insecticides. Each type of organic solvent of 0.5 mL was studied. The result showed that the elution ability of acetone was much stronger than methanol and acetonitrile. Thus, acetone was selected as the desorption solvent. Furthermore, the elution efficiency was affected by the elution volume of acetone. It was found from the test result that using 0.5 mL acetone in ultrasonication for several minutes with three repeats was the optimal volume.

3.4 Adsorption mechanism

According to the reports in literatures, the MOF adsorption mechanisms mainly include electrostatic interaction, acid–base interaction, hydrogen bonding, π–π stacking/interaction, and hydrophobic interaction. Understanding the mechanism will be very helpful for not only the adsorption process but also the application of adsorption in selective removal.²⁷ With the neonicotinoid structure considered (Table 1), all of the analytes have delocalized large π bonds, except for dinotefuran. They have nitrogen-containing groups or hydrophobic groups. From the above experiments and discussion, when the pH value was changed in the samples, dinotefuran had no changes, but the peak areas of other insecticides decreased because of degraded or collapsed frameworks, indicating that charge species had no effects on the adsorption efficiency. Moreover, when the NaCl concentration was changed, despite the larger peak areas appearing at 15%, they almost had no difference with 0% NaCl. However, dinotefuran changed less because it had no benzene ring or delocalized large π bond, indicating less interactions of the hydrophobic and hydrogen bonding. A conclusion may be drawn that the mechanism of adsorbing neonicotinoid insecticides largely depended on the π–π interaction with the benzene ring in the MOF-199 frameworks and delocalized large π bonds in the insecticides, also, the electrostatic interaction played small part. However, experiments should be conducted and validated further for interaction mechanisms.

3.5 Method validation

Fig. 8a presented the TIC graph of seven standard solutions. We knew from the above discussion that the composites had almost no adsorption for dinotefuran, and so the detection method was established for the other six insecticides. A series of aqueous solutions containing each of the neonicotinoids at seven concentration levels of 5, 10, 20, 40, 200, 400 and 800 ng mL⁻¹ for thiamethoxam, clothianidin and imidacloprid, and 2, 4, 8, 16, 80, 160, and 320 ng mL⁻¹ for nitenpyram, acetamiprid and thiacloprid, were prepared for calibration curve establishment. For each level, analysis was performed in five replicates. Quantitative parameters such as the linear range, correlation coefficient (r²), and limit of detection (LOD) were evaluated (Table 2). All the analytes exhibited good linearity with the r² value of higher than 0.9947. The LODs based on the signal-to-noise ratio (S/N) of 3 fell into the range of 0.3–1.5 ng mL⁻¹. In order to evaluate the precision of the method, a repeatability study was carried out by performing five parallel experiments for the spiked samples at the concentration of 5 ng mL⁻¹ for each of the neonicotinoids, and the result showed that the relative standard deviations (RSDs) varied from 2.1% to 8.1%. This indicated the method had high sensitivity and good repeatability with the calibration data.

Fig. 8 The TIC chromatography of water samples with seven neonicotinoids (a), lake water samples (b) and lake samples spiked with 8 ng mL⁻¹ of nitenpyram, acetamiprid and thiacloprid, and 20 ng mL⁻¹ of thiamethoxam, clothianidin and imidacloprid (c).

Table 2 Linearity, limit of detection and limit of quantification for the six neonicotinoids

Neonicotinoid	Linear range (ng mL^−1)	Regression equation	r^2	LOD (ng mL^−1)	LOQ (ng mL^−1)	RSD (%) (n = 5)
Nitenpyram	2–320	y = 81.5 + 237x	0.9947	0.3	1	8.1
Clothianidin	5–800	y = 385 + 125x	0.9960	1	3	7.5
Thiamethoxam	5–800	y = −3.14 + 51.6x	0.9974	1.5	4.5	3.5
Imidacloprid	5–800	y = −140 + 120x	0.9972	1.5	4.5	4.9
Acetamiprid	2–320	y = 226 + 495x	0.9989	0.4	1.2	2.1
Thiacloprid	2–320	y = 49.2 + 260x	0.9995	0.4	1.2	3.1

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3.6 Analysis of environmental water samples

To validate the applicability of the developed method, the method was employed to analyze neonicotinoids in different environmental water samples, including tap water, lake water and well water samples. The results shown in Table 3 revealed that no residues were detected in well water and tap water, and 2 ng mL⁻¹ imidacloprid was detected in lake water (Fig. 8b). The recoveries of six neonicotinoids were studied by respectively spiking the standard solution into water samples at the concentrations of 4, 8 and 16 ng mL⁻¹ for nitenpyram, acetamiprid and thiacloprid, and 10, 20 and 40 ng mL⁻¹ for thiamethoxam, clothianidin and imidacloprid. The recoveries for the neonicotinoids in tap water, lake water and well water samples were in the range from 88.0% to 107.0% and the RSDs ranged from 1.5% to 11.6% (Table 3).

Table 3 The recoveries of the six neonicotinoids from tap water, lake water and well water

Pesticide	Spiked (ng mL^−1)	Tap water (ng mL^−1)			Lake water (ng mL^−1)			Well water (ng mL^−1)
		Found (ng mL^−1)	R (%)	RSD (%)	Found (ng mL^−1)	R (%)	RSD (%)	Found (ng mL^−1)	R (%)	RSD (%)
a nd means not detected.
Nitenpyram	0	nd^a			nd			nd
	4	3.89	97.2	7.9	4.1	102.5	4.6	3.9	97.5	8.2
	8	7.17	89.6	7.2	8.1	101.2	2.4	7.9	98.7	1.8
	16	15.6	97.5	3.1	15.5	96.8	4.8	16.7	104.3	3.4
Clothianidin	0	nd			nd			nd
	10	10.7	107.0	7.7	9.8	98.0	7.4	9.6	96.0	8.5
	20	19.6	98.0	9.1	21.4	107.0	9.1	20.1	100.5	6.5
	40	39.2	98.0	6.2	42.2	105.5	11.6	39.8	99.5	2.3
Thiamethoxam	0	nd			nd			nd
	10	10.1	101.0	5.4	10.8	108.0	8.3	9.6	96.0	3.8
	20	19.5	97.5	5.5	19.6	98.0	4.1	18.6	93.0	8.1
	40	38.7	96.7	1.9	38.6	96.5	1.5	39.9	99.7	6.4
Imidacloprid	0	nd			2			nd
	10	9.96	99.6	2.1	9.4	94.0	11.0	9.51	95.1	5.4
	20	20.35	101.7	5.2	20.2	101.1	4	20.70	103.5	7.1
	40	39.21	98.0	5.3	37.4	93.5	6.4	39.26	98.1	4.6
Acetamiprid	0	nd			nd			nd
	4	3.55	88.7	5.6	3.52	88.0	8.0	3.52	88.0	7.9
	8	7.90	98.7	4.1	7.09	88.7	5.9	7.94	99.2	8.3
	16	14.72	92	3.6	15.75	98.4	7.3	16.35	102.1	8.1
Thiacloprid	0	nd			nd			nd
	4	3.99	99.7	10.5	3.59	89.7	4.7	3.97	99.2	9.3
	8	7.85	98.1	8.2	7.90	98.7	5.1	7.93	99.1	7.6
	16	14.6	91.2	4.1	14.43	90.1	3.3	15.85	99.1	2.4

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4. Conclusions

In the present work, magnetic MOF-199 composites were easily synthesized and successfully used as an effectively adsorbent for the first time to adsorb some neonicotinoid insecticides in environmental water samples prior to high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). The magnetic sorbent can be easily and quickly isolated from water samples with external magnetic fields, avoiding a time-consuming column passing procedure in SPE. By changing the experimental conditions, it was found that the adsorption mechanism mainly depended on the π–π interaction. The developed method offered excellent sensitivity, a wide linear range, and simple operations, as well as satisfactory recoveries and repeatability under the optimized conditions. The method was successfully employed to analyze real environmental water samples. Most importantly, the results demonstrated that the MOF-199/Fe₃O₄ composites were potential materials to purify samples and adsorb other trace analytes in MSPE.

Acknowledgements

This work was supported by the Special Fund for Agro-Scientific Research in the Public Interest (201209094) of China, and Key Laboratory of Agri-Food Safety and Quality of ministry of agriculture of China (2016-KF-11).

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