Enhanced extraction of organophosphorus pesticides from fruit juices using magnetic effervescent tablets composed of the NiFe₂O₄@SiO₂@PANI-IL nanocomposites

Abstract

The reported ionic liquid (IL)-based magnetic effervescent tablets are a result of direct addition of ILs and magnetic nanoparticles (MNPs). In effervescent reaction-enhanced microextraction procedures, the dissociation between ILs and MNPs easily leads to loss of ILs due to aqueous solubility, thereby decreasing the extraction efficiency. Herein, we attached a hydrophilic IL ([BMIM]Br) onto the surface of NiFe₂O₄@SiO₂@polyaniline (NiFe₂O₄@SiO₂@PANI-IL) to prepare novel core–shell-like multi-layer nanocomposites. Magnetic effervescent tablets were composed of Na₂CO₃ as an alkaline source, tartaric acid as an acidic source and as-synthesized nanocomposites as an extractant. The nanocomposites were used in an effervescent reaction-enhanced magnetic solid-phase extraction (ERMSE) for the extraction of four organophosphorus pesticides (OPPs) in fruit juices prior to HPLC-DAD detection. Under optimized conditions, this method provided low limits of detection (0.06–0.17 μg L⁻¹), high recoveries (80.6–97.3%) and excellent precision (1.1–5.2%) for OPP quantification in five fruit juices. Notably, the three-layer core–shell nanocomposites were efficiently recycled for at least eight extraction cycles with a recovery loss of <10%. The novelty of this study lies in: (1) for the first time, the ILs-based hybrid magnetic nanocomposites were prepared with appropriate pore size/volume and more active sites for OPPs; (2) the combination of the nanocomposites with effervescent tablets realizes rapid dispersion of CO₂ bubbles, and convenient magnetic separation/collection into one synchronous step; and (3) due to there being no requirement of electrical power, it is feasible for use in field conditions. Thus, the ERMSE method has excellent potential for conventional monitoring of trace-level OPPs in complex fruit juice matrices.

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1. Introduction

Organophosphorus pesticides (OPPs) play an important role in controlling agricultural insects; however, their high toxicity has raised great public concern, especially in various foods.¹ The residues of OPPs in fruit juices constitute a serious risk to human health.² The Chinese government set the maximum residue limits of 10–50 μg kg⁻¹ for OPPs in fruits and their related products (GB2763-2016, China Food and Drug Administration).³ To ensure safety of fruit juices for human consumption, sensitive, rapid and robust analytical methodologies are required for OPP detection in fruit juices.

Compared to traditional methods, magnetic solid-phase extraction (MSPE) overcomes several shortcomings, such as use of large volumes of organic solvent, requirement for specialized devices and time-consuming processing.^4,5 Among magnetic adsorbents utilized for MSPE, magnetic nanoparticles (MNPs) are widely used in the adsorption of OPPs due to their excellent characteristics. Among them, NiFe₂O₄ has received wide attention because of its advantageous properties, such as high surface area and strong magnetic response.^6–8 However, due to the high chemical reactivity of bare NiFe₂O₄, it readily oxidizes in air leading to a loss of dispersibility and adsorption capacity. Hence, a protective layer is often applied to ensure chemical stability and enhance dispersibility.^9–11

Silica is often selected as a coating layer for NiFe₂O₄ MNPs because of its chemical stability, biocompatibility and ease for surface modification.^9,10 For example, methyl blue was adsorbed from aqueous solution by magnetic 0.9NiFe₂O₄/0.1SiO₂ nanocomposites.¹¹ Some modification of nanocomposite surfaces by either functionalization or coating with other solid support or functional groups (e.g., natural, conductive, and synthetic polymers) is required to enhance resolution and extraction efficiency.¹² Among the conductive polymers, polyaniline (PANI) has attracted a great deal of attention due to its multifunctional properties (e.g., hydrophobicity, acid–base character, π–π interactions and electrical activity), easy synthesis and permeable porous structure.^13,14 In the synthesis of PANI, it is possible to add amphiphilic compounds, called surfactants, which interact with each other to form molecular aggregates, called micelles, which form a porous media.¹⁵ Consequently, PANI is applied in separation science, in areas such as packing material in MSPE and fiber material for SPME.¹⁶ Especially, PANI has been modified with MNPs to extract various types of pollutants from environmental matrices, such as prepared Fe₃O₄@SiO₂@polyaniline (PANI) MNPs to adsorb Se and Te,¹⁷ PANI nanofiber synthesized on graphene oxide (PANI-GO) surfaces for extraction of Pb²⁺ in food samples¹⁸ and restricted access macroporous magnetic polyaniline for coumarin detection in rat plasma.¹⁵

Ionic liquids (ILs) refer to a class of organic salts composed of various inorganic and organic anions and organic cations.^19,20 By changing the structure of ionic moieties or polymerization, the properties of ILs can be modified and optimized for different target analytes.²¹ In recent years, ILs have been employed to increase the extraction efficiency by loading them onto the surface of polymers or MNPs,²² which can interact with organic chemicals via electrostatic interactions, hydrogen bonding, π–π stacking and hydrophilic/hydrophobic interactions. Generally, the prominent benefits of using ILs as coating materials on MNPs are to improve the selectivity, sensitivity and extraction efficiency for target analytes.²³ Notably, the integration of ILs with PANI will supply more surface-active groups (e.g., carboxyl, hydroxyl and ammonium groups), yielding stronger molecular interactions between PANI-IL and targeted chemicals.

Based on the above considerations, magnetic nanocomposites based on the combination of NiFe₂O₄@SiO₂, PANI and ILs were prepared, hereafter referred to as NiFe₂O₄@SiO₂@PANI-IL. The MNPs were employed as an adsorbent in an effervescent reaction-enhanced microextraction for efficient extraction of OPPs in fruit juices. The NiFe₂O₄@SiO₂@PANI-IL nanocomposites demonstrated several advantages aimed at improving the dispersibility of nanocomposites and extraction efficiency, such as moderate specific surface area and pore size, an abundance of surface-active groups, rapid magnetic separation and reduced agglomeration of nanoparticles. Nowadays, effervescent reaction-enhanced microextraction is based on a simple reaction that generates CO₂ bubbles, which accelerates the dispersion of MNPs in solution.^24,25 The process of magnetic effervescent tablet-assisted dispersive extraction can eliminate the use of dispersive solvents in microextraction, realize rapid magnetic separation, avoid centrifugation/stirring steps and save processing time.

The aim of this work was to develop a novel magnetic nanocomposite based on IL-coated NiFe₂O₄@SiO₂@PANI hybrid MNPs. Subsequently, the hybrid nanocomposites were employed in an effervescent reaction-enhanced magnetic solid-phase extraction (ERMSE) for preconcentration/extraction of four common OPPs from fruit juice samples prior to detection by liquid chromatography. Under optimized conditions, the ERMSE methodology demonstrated efficient adsorption/extraction of trace-level OPPs, thereby showing excellent potential for routine monitoring of OPPs in fruit juices.

2. Experimental

2.1. Reagents and materials

Analytical standards for four OPPs (methamidophos, malathion, parathion, diazinon) with 99% purity, and analytical-grade FeCl₃·6H₂O and Ni(NO₃)₂·6H₂O were purchased from Zhongke Quality Inspection Biotechnology (Beijing, China). Chromatographic-grade methanol, acetonitrile, ethanol, acetone and ethyl acetate were obtained from Aladdin (Shanghai, China). A series of analytical-grade chemicals were acquired from Tansoole (Shanghai, China): sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), hydrochloride acid (HCl), polyethylene glycol (PEG), tartaric acid (TTA), tetraethoxysilane (TEOS), aniline, ammonium persulfate (APS) and the ionic liquid, 1-butyl-3-methylimidazolium bromide ([BMIM]Br). Each OPP standard was dissolved in methanol to prepare a stock solution (100 mg L⁻¹) and stored at 4 °C. Ultrapure water (>18.2 MΩ-cm, Millipore, Billerica, MA, USA) was used throughout this study. The 0.22 μm nylon (NL) and 0.45 μm polyether sulfone (PES) membrane filters were purchased from Anpel Scientific Instrument (Shanghai, China).

2.2. Instrumentation

Quantification of OPPs was performed on a Shimadzu HPLC-DAD (LC-20AT). A Zorbax Eclipse XDB-C₁₈ column (5 μm, 150 mm × 4.6 mm) was used for separating the analytes. HPLC-DAD operational conditions were as follows: flow rate, 0.9 mL min⁻¹; column temperature, 30 °C; mobile phase, methanol–water at 65%:35% v/v; detection wavelength, 270 nm and injection volume, 20 μL. The structure and morphology of nanomaterials were characterized by scanning electron microscopy (SEM, Sigma300, Germany) and transmission electron microscopy (TEM, JEM2010, USA). Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet IS50 spectrometer with a range of 400–4000 cm⁻¹. Powder X-ray diffraction (XRD) of nanomaterials was obtained by an EDX-720 X-ray power diffractometer (Agilent, USA) with a Cu Kα source (1.54056 Å) at room temperature between 5° and 90° 2θ. Brunauer–Emmett–Teller (BET) surface area was measured by N₂ adsorption–desorption at 77 K using an ASAP 2020 system (Quantachrome, USA). Magnetic properties were investigated using a vibrating sample magnetometer (VSM, San Diego, CA, USA) with an applied field between −20 000 Oe and 20 000 Oe at room temperature. Zeta potential was measured using a zeta potential analyzer (Malvern, UK). Thermogravimetric analysis (TGA) was determined at a heating rate of 10 °C min⁻¹ over a temperature range of 50 to 700 °C under nitrogen flow (SDT Q600 Thermal Analyzer, Perkin Elmer, Waltham, MA, USA).

2.3. Synthesis of NiFe₂O₄@SiO₂@PANI-IL nanocomposites

2.3.1. NiFe₂O₄ MNPs. Magnetite NiFe₂O₄ nanoparticles were prepared by using a hydrothermal method.²⁴ First, FeCl₃·6H₂O (5.4 g) and Ni(NO₃)₂·6H₂O (2.9 g) were dissolved in 100 mL ultrapure water. After 10 min ultrasound, 0.11 g of PEG was added and further sonicated for 5 min.²⁶ Then, the pH of the resulting solution was adjusted to 7.0 using 0.9 mol L⁻¹ NaOH. Next, the mixture was transferred to a 100 mL Teflon-lined stainless autoclave and heated at 160 °C for 12 h followed by cooling to room temperature. Finally, the precipitates were collected using a magnet and washed for three cycles with ultrapure water and ethanol, before drying in a vacuum oven at 60 °C for 6 h.

2.3.2. NiFe₂O₄@SiO₂ nanomaterials. NiFe₂O₄ MNPs (3.0 g) were dispersed in 25 mL of ethanol and ultrasonicated for 10 min. The solution was stirred at ambient conditions under a N₂ flow with the pH adjusted to 11 (15%, NH₃·H₂O). Next, 2 mL of TEOS was added to the solution and continuously stirred for 1 h. Subsequently, NiFe₂O₄@SiO₂ nanomaterials were magnetically separated and washed for 3 cycles with ultrapure water and ethanol, followed by drying in a vacuum oven for 3 h at 60 °C.

2.3.3. NiFe₂O₄@SiO₂@PANI-IL nanocomposites. First, 1.5 g of ammonium persulfate (APS) and 1.0 g of aniline were dissolved in 40 mL of 0.25 M HCl. The mixture was then stirred for 2 h and stored at 4 °C. Next, the aforementioned mixture was added dropwise to 60 mL ethanol containing 1.5 g of NiFe₂O₄@SiO₂ and 1.0 g of [BMIM]Br. After 12 h of stirring, the synthesized NiFe₂O₄@SiO₂@PANI-IL nanocomposites were rinsed for 3 cycles with ethanol and water, before drying in a vacuum oven at 60 °C. Finally, the dried nanomaterial was ground into a powder and sealed for further use (Fig. 1A).

Fig. 1 Schematic representation of the ERMSE procedures. Note: (A) synthesis of the NiFe₂O₄@SiO₂@PANI-IL nanocomposites; (B) preparation of effervescent tablets; (C), operational procedures for the ERMSE method.

2.4. Preparation of the magnetic effervescent tablets

Initially, Na₂CO₃ and TTA were dried in an oven for 1 h at 60 °C and stored in a desiccator before use. Then, a mixture of Na₂CO₃ (212 mg) and TTA (300 mg), and 15 mg of prepared NiFe₂O₄@SiO₂@PANI-IL nanocomposites were mixed and manually blended in an agate mortar. After grinding, the power was compressed into a magnetic effervescent tablet (8 mm diameter × 2.5 mm thickness) by a 5T/8MM 5T Punch Press (Shanghai, China). The as-pressed effervescent tablets were stored in a desiccator at room temperature prior to use (Fig. 1B).

2.5. Fruit juice sample preparation

Five types of fruit juice samples (apple, pear, orange, peach and mango) were provided by Jialefu Supermarket (Suzhou, China). Aliquots (30 mL) of the fresh juice samples were centrifuged at 5000 rpm for 10 min and the supernatants filtered through a 0.45 μm membrane filter. Finally, the filtrate was diluted 1 : 2 using Milli-Q ultrapure water for use in the following ERMSE procedures.

2.6. ERMSE procedures

A schematic illustration of the ERMSE method is depicted in Fig. 1C. First, an aliquot (8 mL) of the pretreated juice was introduced into a 15 mL conical centrifuge tube. Next, a magnetic effervescent tablet was placed into the tube where it produced a vigorous reaction of bubbles (CO₂) throughout the reaction vessel. The effervescent tablet remained active for ∼3–4 min, resulting in homogeneous dispersion of the nanocomposites throughout the aqueous solution. Then, the nanocomposite adsorbent enriched with the analytes was collected by placing an external magnet alongside the centrifuge tube. The supernatant was carefully discarded by pipette, and an appropriate volume of elution solvent added to elude the OPPs from the nanocomposites. The collected eluate was filtered using a 0.45 μm filter, and redissolved in 100 μL methanol after being dried with a gentle nitrogen gas flow. Finally, 20 μL of the resulting solution was subjected to HPLC/DAD detection.

3. Result and discussion

3.1. Characterization of NiFe₂O₄@SiO₂@PANI-IL MNPs

3.1.1. SEM and TEM analyses of coated nanocomposites. Morphologies of the NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL were examined by SEM and TEM (Fig. 2). NiFe₂O₄ nanoparticles exhibited an obvious agglomeration phenomenon (Fig. 2A and D). The SiO₂ attached on the NiFe₂O₄ surface protected the MNPs from being corroded and oxidized in the air, and further agglomerated in the aqueous phase. NiFe₂O₄@SiO₂ showed sphere-shaped nanoparticles with increased particle size as compared to NiFe₂O₄ nanoparticles (Fig. 2B and E). Compared to the pure NiFe₂O₄ MNPs, SEM images of NiFe₂O₄@SiO₂@PANI-IL displayed a multi-layer sphere, but their surface became rough (Fig. 2C). TEM observations showed smaller nanoparticles attached onto the surface of the NiFe₂O₄ core as PANI coated the surface of the SiO₂ layer, and PANI acted as a bridge to load ILs (Fig. 2F). Introduction of ILs onto the surface of the core–shell structure led to more surface active sites on the MNPs, contributing to potentially higher extraction efficiency for OPPs.

Fig. 2 SEM images of NiFe₂O₄ (A), NiFe₂O₄@SiO₂ (B) and NiFe₂O₄@SiO₂@PANI-IL (C); TEM images of NiFe₂O₄ (D), NiFe₂O₄@SiO₂ (E) and NiFe₂O₄@SiO₂@PANI-IL (F).

3.1.2. EDS elemental mapping. Elemental analysis by EDS mapping demonstrated the presence of O, Fe, Ni, Si, C and N in the synthesized nanocomposites (Fig. 3); atomic percentages are reported in ESI Table 1.† As for NiFe₂O₄@SiO₂@PANI-IL nanocomposites, the elemental percentages for O, Fe, Ni, Si, C and N were 16.6, 2.5, 1.2, 3.4, 65.6 and 6.7%, respectively. Compared to NiFe₂O₄ and NiFe₂O₄@SiO₂ nanoparticles, the NiFe₂O₄@SiO₂@PANI-IL surface had lower O, Fe, Ni and Si contents, but higher C and N concentrations indicating successful synthesis of magnetic core–shell nanocomposites. EDS mapping of NiFe₂O₄@SiO₂@PANI-IL identified the presence of Fe (from NiFe₂O₄), Ni (from NiFe₂O₄), O (from NiFe₂O₄@SiO₂), N (from PANI and [BMIM]Br), and C (from PANI and [BMIM]Br). These results confirm the uniform distribution of multi-layer coatings (SiO₂, PANI and [BMIM]Br) over the surface of the NiFe₂O₄ nanoparticles, and also the successful synthesis of the core–shell structure of NiFe₂O₄@SiO₂@PANI-IL nanocomposites.

Fig. 3 EDS mapping of the NiFe₂O₄@SiO₂@PANI-IL nanocomposites.

3.1.3. FT-IR characterization. FT-IR spectra for NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL MNPs are displayed in Fig. 4A. NiFe₂O₄ MNPs displayed absorption bands at 454, 600, 1342, 1628 and 3419 cm⁻¹ corresponding to the stretching vibration peaks for Ni–O, Fe–O, C=O, N–O and O–H, respectively. Compared to the NiFe₂O₄ spectrum, new peaks around 700–1000 and 1100 cm⁻¹ appeared in both NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL nanocomposites, which were attributed to the C–H stretching vibration and Si–O–Si lattice vibration of SiO₂, respectively. These results indicate that SiO₂ was successfully encapsulated on the surface of the NiFe₂O₄ MNPs by physical and chemical adsorption. The peaks at 1299 and 1487 cm⁻¹ were related to the C=O and C=C groups of PANI-IL. The typical stretching vibration peak of C=N at 1233 cm⁻¹, resulting from PANI and [BMIM]Br, indicated that the PANI-IL was successfully attached onto the surface of NiFe₂O₄@SiO₂. Hence, FT-IR analysis verified the successful preparation of the NiFe₂O₄@SiO₂@PANI-IL nanocomposites.

Fig. 4 FT-IR spectrum of NiFe₂O₄ (A-a), NiFe₂O₄@SiO₂ (A-b) and NiFe₂O₄@SiO₂@PANI-IL (A-c); XRD patterns of NiFe₂O₄ (B-a), NiFe₂O₄@SiO₂ (B-b) and NiFe₂O₄@SiO₂@PANI-IL (B-c); Magnetic properties of nanocomposites (C); zeta potentials of nanocomposites at different pH values (D).

3.1.4. XRD characterization. The crystalline structures of NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL were confirmed by XRD (Fig. 4B). The main peaks at 2θ values of 18.3, 30.4, 44.2, 54.7, 57.1 and 63.5° were assigned to the (111), (220), (400), (422), (511) and (440) reflections of NiFe₂O₄ (PDF#74-2081 standard pattern), respectively. Compared to NiFe₂O₄, the peak for NiFe₂O₄@SiO₂ around 24.6° was assigned to the (002) plane of SiO₂.²⁷ There were no prominent differences in XRD profiles between NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL, implying that the PANI-IL coating did not affect the crystal structure of NiFe₂O₄@SiO₂ MNPs. Results of XRD characterization provided further evidence for successful synthesis of the nanocomposites.

3.1.5. Magnetic properties. Saturated magnetization values for NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL MNPs were 36.2, 29.5 and 23.9 emu g⁻¹, respectively (Fig. 4C). Higher magnetic saturation values reflect stronger magnetic properties. Although the saturation magnetization values of the nanocomposites were reduced by surface coatings, rapid separation from aqueous solution was still achieved due to the relatively high saturation magnetization of NiFe₂O₄@SiO₂@PANI-IL (Inset of Fig. 3C).

3.1.6. Zeta potential characterization. The point of zero charge (PZC) is an important variable for assessing the surface charge characteristics of nanocomposites. The PZC of NiFe₂O₄@SiO₂@PANI-IL nanocomposites was pH = 5.8, indicating that the net surface charge of the nanocomposites was positive at pH < 5.8, but negative at pH > 5.8 (Fig. 4D). At a near-neutral charge state (pH = 5.8), the nanomaterials should possess their highest extraction efficiency for the molecular-state OPPs.

3.1.7. N₂ adsorption–desorption isotherms. The BET surface area along with pore volume and size were measured for NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL using N₂ adsorption–desorption dynamics. BET surface areas of NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL were 104.5, 173.1 and 81.1 m² g⁻¹, respectively (Fig. 5; ESI Table 2†). Compared to NiFe₂O₄ and NiFe₂O₄@SiO₂, the decreased surface area of NiFe₂O₄@SiO₂@PANI-IL nanocomposites appears to result from the introduction of multiple surface functional groups, accompanying with partial agglomeration of the MNPs.²⁸ Relative to the NiFe₂O₄ (0.302 cm³ g⁻¹), the decreased pore volume of NiFe₂O₄@SiO₂ (0.059 cm³ g⁻¹) was likely due to the encapsulation of SiO₂ nanoparticles into NiFe₂O₄. However, compared to the NiFe₂O₄@SiO₂, the higher pore volume of NiFe₂O₄@SiO₂@PANI-IL (0.136 cm³ g⁻¹, Fig. 5C) may result from generation of mesoporous structures between the interconnected nanofibers in PANI-IL, providing more active paths and sites for adsorption. Mean pore size of NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL was 13.7, 14.1 and 17.9 nm, which were all in the mesoporous size range (2–50 nm). Generally, the moderate surface area and mesoporous structure provide abundant sites for sorption of pollutants, rendering NiFe₂O₄@SiO₂@PANI-IL an effective adsorbent for OPPs.

Fig. 5 Nitrogen adsorption–desorption isotherms for NiFe₂O₄ (A), NiFe₂O₄@SiO₂ (B) and NiFe₂O₄@SiO₂@PANI-IL nanocomposites (C); TGA profiles of the different nanocomposite coatings (D).

3.1.8. Thermogravimetric properties. Thermal stability of the three MNPs was assessed by TGA in the temperature range of 50–700 °C under nitrogen flow. Mass loss below 200 °C was in the range of 5–9%, which was ascribed to evaporation of sorbed water (Fig. 5D). In the temperature range of 200–450 °C, mass loss of NiFe₂O₄@SiO₂@PANI-IL nanocomposites was about 20%, which may represent loss of unbound HCl.²⁹ A further mass loss of 30% at temperatures between 450–575 °C was attributed to pyrolysis of the PANI chains. In sharp contrast, the TGA curve of NiFe₂O₄ showed a mass loss of ∼8% over the 30–700 °C temperature range assigned to the loss of physically and chemically bound water molecules (Fig. 5D). The similar low-level mass loss for NiFe₂O₄@SiO₂ and NiFe₂O₄ verifies the good thermal stability of MNPs in the core–shell microspheres.

3.2. Optimization of ERMSE operational parameters

3.2.1. Composition of acidic and basic salts for effervescent tablet. Effervescent tablets (mixture of effervescent precursors) play an important role in the ERMSE procedures by acting as a vigorous dispersant. Effervescent tablets are composed of three main components: acidic salt, alkaline salt and extractant/adsorbent.²³ According to previous reports,^30,31 we tested two conventional acidic salts and two alkaline salts as potential tablet components, Na₂CO₃ and NaHCO₃ as alkaline salts and NaH₂PO₄ and tartaric acid (TTA) as acidic salts. We assessed four different tablet combinations: Na₂CO₃ + NaH₂PO₄, NaHCO₃ + NaH₂PO₄, Na₂CO₃ + TTA, and 2NaHCO₃ + TTA. The Na₂CO₃ + TTA combination provided the highest overall mean extraction recoveries (ERs) for the four OPPs. With respect to individual OPP ERs, Na₂CO₃ + TTA had the highest ERs for methamidophos, malathion and parathion, while Na₂CO₃ + NaH₂PO₄ had the highest ER for diazinon (Fig. 6A). The Na₂CO₃ + TTA tablet, composed of 212 mg of Na₂CO₃, 300 mg of C₄H₆O₆ and 15 mg nanocomposites, produced a vigorous effervescence intensity and reaction time (∼4 min), consistent with previous results obtained by Ding et al.³⁰ and Wu et al.³¹ Given its superior performance, the Na₂CO₃+ TTA tablet was selected as the acidic/alkaline source in preparation of magnetic effervescent tablets.

Fig. 6 Effects of experimental variables on the ERs for OPPs. Note: (1) (A), effects of different composition of effervescent tablets on ERs; (B), amount of the nanocomposites; (C), extraction time; (D), type of elution solvent; (E) volume of elution solvent; (F) comparison of ERs with effervescent reaction versus 4 min hand-shaking (no effervescence); (2) error bar indicates the standard deviation (n = 3); (3) spiked level of each OPP species at 20 μg L⁻¹.

3.2.2. Amount of NiFe₂O₄@SiO₂@PANI-IL nanocomposites as adsorbent. Magnetic nanocomposites enhance the adsorption/extraction of OPPs from complex matrices due to their high specific surface area, appropriate PZC yielding suitable surface charge characteristics, abundant surface adsorption groups and efficient collection from solution by a strong magnetic field.³ The effects of the amount of MNPs on the ERs were investigated over the range of 5–30 mg. The ERs for all four OPPs increased with increasing adsorbent doses from 5 to 15 mg, but decreased with further increases from 20 to 30 mg (Fig. 6B). As a consequence, 15 mg was chosen as the optimum quantity of adsorbent for the extraction process.

3.2.3. Extraction time. Extraction time indicates the interval from dispersion to separation of the nanocomposites and is an important parameter for optimizing the ERs for OPPs in the ERMSE procedures. An appropriate extraction time leads to improvements in adsorption by MNPs, purification effects and extraction efficiency.³² The effect of extraction time was examined in the range of 2–10 min (Fig. 6C). The ERs for the four OPPs increased gradually with increasing extraction time from 2 to 6 min, followed by a decrease from 6 to 10 min. Accordingly, 6 min was adopted as the optimum extraction time for subsequent experiments.

3.2.4. Type and volume of elution solvent. The elution solvent has a significant effect on the ERs for OPPs due to differential polarities. Five common eluent solvents (methanol, acetonitrile, acetone, ethanol and ethyl acetate) were compared for their elution efficiency. Acetonitrile gave the highest average ERs for the four OPPs (∼85%), which was much higher than for methanol (∼75%) (Fig. 6D). Acetone, ethanol and ethyl acetate produced relatively lower ERs (∼65%) for the four OPPs. Thus, acetonitrile was selected as the optimal solvent in further optimization studies.

Elution solvent volume was examined over the range of 0.4 to 2.0 mL for ERs of the four OPPs. The ERs increased from 0.4 to 0.8 mL of acetonitrile, but decreased with further increases from 0.8 to 2.0 mL (Fig. 6E). When the volume of elution solvent was <0.8 mL, OPPs were not completely desorbed, which consequently contributed to a lower ER. Theoretically, the larger the volume of eluent, the higher the extraction recovery. However, the volume of eluent is always chosen as a compromise between achieving complete immersion of the nanomaterials and minimizing analyte dilution to reduce the required volume of elution solvent.³² In this investigation, when the volume of acetonitrile was larger than 0.8 mL, the hybrid nanocomposites were over-immersed. Under such circumstance, due to saturated or excessive adsorption by nanocomposites, the volume of eluent collected after elution would be greatly lost, thus leading to the decrease of extraction recovery. Thus, an elution solvent volume of 0.8 mL was selected as the optimum volume.

Based on the optimization studies, the selected experimental conditions were: Na₂CO₃ + TTA as tablet acidic and alkaline sources; eluent, 0.8 mL of acetonitrile; composite mass, 15 mg; and extraction/adsorption time, 6 min.

3.3. Comparison of ERs with effervescence and no effervescence (hand-shaking)

To evaluate whether an effervescent reaction is conductive to enhanced dispersive efficiency for the NiFe₂O₄@SiO₂@PANI-IL nanocomposites, we compared the ERs for OPPs in the presence or absence of effervescent reaction. Direct addition of nanocomposites with a 4 min hand-shaking yielded ERs for the four OPPs in the range of 79.3 to 85.4% (Fig. 6F). In contrast, use of the effervescent tablets (CO₂ bubbles lasting for ∼4 min) increased the average ERs to the range of 88.4 to 93.7%, an absolute increase of ∼9.0% as compared to hand-shaking treatments. In particular, ERs for methamidophos and parathion reached as high as 93.7 and 92.8%, respectively, upon effervescent reaction (Fig. 6F). Therefore, we posit that integration of NiFe₂O₄@SiO₂@PANI-IL into the effervescent tablet contributed to greater dispersion of the nanocomposites, thereby increasing the ERs for OPPs in the ERMSE method. Moreover, utilization of effervescent tablets avoids the need for an external physical energy source, such as ultrasound or vortexing, making the methodology feasible for remote and outdoor use.

3.4. Comparison of ERs by NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL

The effects of SiO₂ and PANI-IL coatings on the ERs for OPPs were investigated among the three MNPs (NiFe₂O₄, NiFe₂O₄@SiO₂ and NiFe₂O₄@SiO₂@PANI-IL) using the aforementioned optimized conditions. When NiFe₂O₄ was used as the adsorbent/extractant, the average ER for OPPs was 56.7%, which compared to 75.4 % for the NiFe₂O₄@SiO₂ treatment (Fig. S1†). This ∼19% increase in ERs can be ascribed to the SiO₂-coated NiFe₂O₄ having increased contact area with analytes in the aqueous solution and reduced agglomeration of MNPs, thereby improving the adsorption efficiency. In contrast, SiO₂@PANI-IL-coated NiFe₂O₄ nanoparticles yielded average ERs as high as 88.3%, an increase of ca. 31% and 13% over those of NiFe₂O₄ and NiFe₂O₄@SiO₂, respectively. These results demonstrate that the introduction of SiO₂, PANI and IL onto the surface of NiFe₂O₄ nanoparticles prominently enhance adsorption/extraction efficiency for OPPs.

3.5. Validation of the ERMSE/HPLC-DAD method

Under optimized conditions, this proposed method was evaluated in the context of linear range (LR), coefficient of determination (R²), limits of detection (LODs, based on S/N = 3), limits of quantitation (LOQs, based on S/N = 10), as well as intra- and inter-day precisions. The LRs were 0.21–0.58–500 μg L⁻¹ for methamidophos, malathion, parathion and diazinon (Table 1). R² values ranged from 0.9945 to 0.9991, and the LODs and LOQs were 0.06–0.17 μg L⁻¹ and 0.21–0.58 μg L⁻¹ for the four OPPs, respectively. Intra- and inter-day precisions, expressed as relative standard deviations (RSDs), were 1.5–4.3% and 1.1–5.2%, respectively, at three fortification levels (5, 20 and 200 μg L⁻¹; n = 6).

Table 1 Analytical performance of the ERMSE/HPLC-DAD method in fruit juice samples^a

Analyte	Liner range (μg L^−1)	R^2	LODs (μg L^−1)	LOQs (μg L^−1)	Intra-day RSDs (%, n = 6)			Inter-day RSDs (%, n = 6)
					Low	Medium	High	Low	Medium	High
a (1) LRs, linear ranges; R^2, coefficients of determination; LODs, limits of detection at S/N = 3; LOQs, limits of quantitation at S/N = 10; and RSDs, relative standard deviations (n = 6). (2) Precision experiments were conducted under the following conditions: (a) OPPs at the three fortification levels in apple juice; (b) NiFe2O4@SiO2@PANI-IL nanocomposites as absorbent, 15 mg; acetonitrile as eluant, 0.8 mL; extraction time, 6 min; Na2CO3 + TTA, 8 mm diameter × 2.5 mm thickness. (3) “Low, Medium and High” indicates 5, 20 and 200 μg L^−1 fortification levels in apple fruit samples, respectively.
Methamidophos	0.45–500	0.9963	0.13	0.45	2.3	4.2	3.2	1.1	3.4	4.8
Malathion	0.37–500	0.9987	0.11	0.37	2.4	1.5	4.3	4.4	3.7	5.2
Parathion	0.17–500	0.9991	0.17	0.58	3.3	1.8	3.6	3.2	2.1	4.6
Diazinon	0.21–500	0.9945	0.06	0.21	1.9	3.8	3.5	2.8	4.5	3.6

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3.6. Real sample analysis

To evaluate real-world applicability, five fruit juice samples (apple, pear, orange, peach, mango) were extracted by the ERMSE method, and subsequently analyzed by HPLC/DAD. Good ERs were obtained for all four OPPs from the fruit juice samples: methamidophos = 81.3–96.5%, malathion = 80.6–95.4%, parathion (83.5–97.1%) and diazinon (81.4–97.3%) (Table 2). Fig. 7 shows typical chromatograms of the four OPPs detected in fruit juices by the ERMSE/HPLC-DAD method. In summary, the newly developed method satisfies the technical requirements for trace-level detection of OPPs in complex fruit juice matrices with high precision and accuracy.

Table 2 The fortified recoveries for OPPs in fruit juice samples^a

Sample	Methamidophos			Malathion			Parathion			Diazinon
	Added (μg L^−1)	Found (μg L^−1)	ER (%)	Added (μg L^−1)	Found (μg L^−1)	ER (%)	Added (μg L^−1)	Found (μg L^−1)	ER (%)	Added (μg L^−1)	Found (μg L^−1)	ER (%)
a (1) ER indicates extraction recovery; (2) each treatment includes three replicates (n = 3); (3) each detected value is mean ± SD (standard deviation, n = 3); (4) experimental conditions: NiFe2O4@SiO2@PANI-IL as absorbent, 15 mg; 0.8 mL of acetonitrile as eluent; extraction time, 6 min.
Apple juice	5	4.2 ± 0.04	84.3	5	4.8 ± 0.05	95.4	5	4.7 ± 0.02	94.6	5	4.6 ± 0.01	91.1
	20	18.3 ± 0.13	91.3	20	18.6 ± 0.04	93.2	20	19.4 ± 0.17	97.1	20	17.5 ± 0.14	87.4
	200	191.2 ± 0.05	95.6	200	172.2 ± 0.28	86.1	200	173.2 ± 0.21	86.6	200	192.4 ± 0.37	96.2
Pear juice	5	4.5 ± 0.06	90.2	5	4.4 ± 0.10	88.3	5	4.4 ± 0.03	87.1	5	4.3 ± 0.02	87.3
	20	16.3 ± 0.21	81.3	20	18.2 ± 0.21	91.7	20	16.7 ± 0.24	83.5	20	16.5 ± 0.33	82.6
	200	177.2 ± 0.34	88.6	200	161.8 ± 0.35	80.9	200	182.0 ± 0.22	91.0	200	162.8 ± 0.24	81.4
Orange juice	5	4.8 ± 0.01	96.5	5	4.7 ± 0.02	93.6	5	4.4 ± 0.04	88.6	5	4.7 ± 0.02	93.5
	20	18.9 ± 0.54	94.3	20	16.1 ± 0.25	80.6	20	18.7 ± 0.13	93.4	20	18.9 ± 0.21	94.8
	200	166.2 ± 0.24	83.1	200	175.0 ± 0.13	87.5	200	179.4 ± 0.28	89.7	200	191.0 ± 0.30	95.5
Peach juice	5	4.2 ± 0.08	84.3	5	4.6 ± 0.01	91.6	5	4.3 ± 0.02	85.3	5	4.9 ± 0.01	97.3
	20	16.6 ± 0.15	82.9	20	17.7 ± 0.22	88.4	20	19.4 ± 0.08	96.8	20	17.5 ± 0.17	87.3
	200	178.2 ± 0.22	89.1	200	163.8 ± 0.18	81.9	200	170.6 ± 0.15	85.3	200	169.8 ± 0.41	84.9
Mango juice	5	4.7 ± 0.11	93.7	5	4.2 ± 0.04	83.4	5	4.2 ± 0.03	84.5	5	4.5 ± 0.03	90.4
	20	17.1 ± 0.23	85.5	20	18.7 ± 0.36	93.7	20	18.7 ± 0.19	93.6	20	17.8 ± 0.26	89.1
	200	181.2 ± 0.37	90.6	200	171.6 ± 0.21	85.8	200	188.4 ± 0.28	94.2	200	167.2 ± 0.33	83.6

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Fig. 7 Typical chromatograms of four OPPs by the ERMSE/HPLC-DAD method. Note: Optimized ERMSE conditions: OPP fortification level of 5 μg L⁻¹; volume of eluent (acetonitrile), 0.8 mL; composite mass, 15 mg; no salt addition and extraction/adsorption time, 6 min.

3.7. Recycling of the NiFe₂O₄@SiO₂@PANI-IL nanocomposites

MNP reusability (i.e., recycling) is an important metric for improving the application potential of nanocomposites.³³ We analyzed reusability of the NiFe₂O₄@SiO₂@PANI-IL adsorbent by regenerating with three washing cycles of ethanol and water, and subsequent drying for use in another effervescent-tablet preparation and extraction cycle. As a result, the three-layer core–shell nanomaterial could be reused for at least eight cycles with ER losses of <10%, suggesting that the nanocomposites retains excellent recyclability and stability (Fig. S2†). Thus, the NiFe₂O₄@SiO₂@PANI-IL nanomaterial possesses excellent characteristics for long-term use in the monitoring of trace OPPs in fruit juice samples.

3.8. Comparison of the ERMSE/HPLC-DAD method with previously reported methods

A comparison of the present method with previously reported methods for the determination of OPPs was conducted in the context of type of adsorbent, RSD, LODs, extraction time and ERs (Table 3).^4,5,28,29,34 The ERMSE/HPLC-DAD method has the following advantages: (1) it is fast with the whole adsorption/extraction process completed within 6 min. Effervescent tablet-assisted diffusion provides rapid and effective dispersion of the nanocomposites. The adsorption/extraction time (6 min) for the new method is prominently lower than those of MOFs-based MSPE (20 min)⁴ and Fe₃O₄@SiO₂@KIT-6-based MSPE (10 min);⁵ (2) it possesses higher sensitivity for OPPs with LODs of 0.06–0.17 μg L⁻¹ and higher repeatability with intra- and inter-day precisions of 1.1–5.2% as compared to those of MIL-101-based MSPE-HPLC-DAD (LODs of 0.3–1.5 μg L⁻¹)⁴ and Fe₃O₄/C-based MSPE-HPLC-UV (LODs of 4.3–47.4 μg L⁻¹).²⁸ It provides comparable LODs with those of MHMS-MCNPs-based MSPE/HPLC-UV (0.07 μg L⁻¹)²⁹ and PIL-MNPs-based MSPE/HPLC-UV (0.01 μg L⁻¹);³⁴ (3) it avoids the utilization of traditional organic dispersive solvents (methanol, acetonitrile, acetone, etc.) in conventional microextraction procedures; (4) the utilization of a “green” solvent in the nanocomposites and recyclable MNCs make the method more environment friendly. Overall, the NiFe₂O₄@SiO₂-PANI-IL-based ERMSE/HPLC-DAD method is simple, rapid, easy to use and benign to the environment, and thus shows great prospective for routine trace monitoring of OPP residues in complex fruit juice matrices.

Table 3 Comparison of the ERMSE/HPLC-DAD method with previously reported methods for the determination of OPPs in fruit juices^a

Pretreatment method	Type of nanomaterial as adsorbent	RSD (%)	LOD (μg L^−1)	Extraction time (min)	ERs (%)	References
a (1) MOFs, metal organic frameworks; (2) Fe3O4/C, carbon coated Fe3O4; (3) MHMS-MCNPs, mixed hemimicelle SDS-coated magnetic chitosan nanoparticles; (4) Fe3O4@SiO2@KIT-6, mesoporous KIT-6-magnetite composite; (5) PIL-MNPs, poly (ionic liquid) immobilized magnetic nanoparticles; (6) MSPE-HPLC-UV, magnetic solid-phase extraction combined with high-performance liquid chromatography-ultraviolet detection; (7) MSPE-HPLC-DAD, magnetic solid-phase extraction combined with high-performance liquid chromatography-diode array detection.
MSPE/HPLC-DAD	MOFs, MIL-101	1.1–7.8	0.3–1.5	20	80.2–107.5	7
MSPE/HPLC-UV	Fe3O4/C	2.7–7.6	4.3–47.4	5	79.6–103.5	36
MSPE/HPLC-UV	MHMS-MCNPs	<4.6	0.07	3	74–104.8	29
MSPE/HPLC-UV	Fe3O4@SiO2@KIT-6	0.1–5.5	0.005–0.01	10	86.6–98.8	8
MSPE/HPLC-UV	PIL-MNPs	4.5–11.3	0.01	2	81.4–112.6	34
ETMSE/HPLC-DAD	NiFe2O4@SiO2@PANI-IL	1.1–5.2	0.06–0.17	6	80.6–97.3	This work

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3.9. Recycling of the NiFe₂O₄@SiO₂@PANI-IL nanocomposites

MNP reusability (i.e., recycling) is an important metric for improving the application potential of nanocomposites.³³ We analyzed reusability of the NiFe₂O₄@SiO₂@PANI-IL adsorbent by regenerating with three washing cycles of ethanol and water, and subsequent drying for use in another effervescent-tablet preparation and extraction cycle. As a result, the three-layer core–shell nanomaterial could be reused for at least eight cycles with ER losses of <10%, suggesting that the nanocomposites retains excellent recyclability and stability (Fig. S2†). Thus, the NiFe₂O₄@SiO₂@PANI-IL nanomaterial possesses excellent characteristics for long-term use in the monitoring of trace OPPs in fruit juice samples.

4. Conclusions

An ERMSE method, based on utilization of effervescent tablets and NiFe₂O₄@SiO₂@PANI-IL nancomposites, was developed for the enhanced extraction of OPPs in fruit juices prior to HPLC-DAD detection. Vigorous CO₂ effervescent bubbles contributed to homogeneous dispersion of nanocomposites, which enhanced interactions between OPPs and the nanosorbents. The superparamagnetism of nanocomposites was conductive to rapid separation/collection of the adsorbent from the aqueous phase. Moreover, the attachment of SiO₂, PANI and ILs onto the surface of NiFe₂O₄ enhanced active sites, pore size/volume and stability, thereby enhancing extraction capacity. Under optimized conditions, the ERMSE method gave high precision with RSDs of 1.1–5.2%, low LODs of 0.06–0.17 μg L⁻¹ and satisfactory recoveries of 80.6–97.3% in apple, pear, orange, peach and mango fruit juices. Notably, the NiFe₂O₄@SiO₂@PANI-IL nancomposites can be recycled at least eight times with ER losses <10%. Consequently, the newly developed method provides a simple, efficient, and green method requiring no dispersive solvents or auxiliary devices, thereby providing wide application value in conventional monitoring of OPPs in fruit juice matrices and potential other food/environmental matrices.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was jointly funded by the Natural Science Foundation of China (21876125 and 22076134), Project of Suzhou Sci&Tech Bureau (SYG201875, SNG2018047 and SNG2018051) and Jiangsu Postgraduate Scientific Research and Practice Innovation Program in 2020 (SJCX20_1103 and KYCX20_2781).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra09100f

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