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Communication

Fabrication of Magnetic Molecularly Imprinted Polymers for Selective Extraction of Dibutyl Phthalates in Food Matrices

by
Lina Li
,
Yunzhu Lu
,
Chengtao Wang
and
Lei Cheng
*
Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Engineering and Technology Research Center of Food Additives, Beijing Technology & Business University (BTBU), Beijing 100048, China
*
Author to whom correspondence should be addressed.
Foods 2024, 13(9), 1397; https://doi.org/10.3390/foods13091397
Submission received: 30 March 2024 / Revised: 28 April 2024 / Accepted: 29 April 2024 / Published: 1 May 2024
(This article belongs to the Section Food Analytical Methods)
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Abstract

:
In this study, a novel magnetic molecularly imprinted polymeric material (Fe3O4@MOF@MIP-160) with a metal-organic backbone (Fe3O4@MOF) carrier was prepared using dibutyl phthalate (DBP) as a template. The material can be used for the efficient, rapid, and selective extraction of trace amounts of phthalic acid esters (PAEs) in food and can detect them via gas chromatography-mass spectrometry (GC-MS). The synthesis conditions of the materials were optimized to prepare the Fe3O4@MOF@MIP160 with the highest adsorption performance. Transmission electron microscopy (TEM), Fourier Transform Infrared Spectra (FT-IR), Vibration Sample Magnetic (VSM), and the Brunauer–Emmett–Teller (BET) method were used to characterize the materials. Compared with Fe3O4@MOF and the magnetic non-imprinted polymeric material (Fe3O4@MOF@NIP), Fe3O4@MOF@MIP-160 possesses the advantages of easy and rapid manipulation of magnetic materials, the advantages of high specific surface area and the stability of metal–organic frameworks, and the advantages of high selectivity of molecularly imprinted polymers. Fe3O4@MOF@MIP-160 has good recognition and adsorption capacity for di-butyl phthalate (DBP) and diethylhexyl phthalate (DEHP): the adsorption capacity for DBP and DEHP is 260 mg·g−1 and 240.2 mg·g−1, and the adsorption rate is fast (reaching equilibrium in about 20 min). Additionally, Fe3O4@MOF@MIP160 could be recycled six times, making it cost-effective, easy to operate, and time-saving as compared to traditional solid-phase extraction materials. The phthalate ester content in drinking water, fruit juice, and white wine was analyzed, with recoveries ranging from 70.3% to 100.7%. This proved that Fe3O4@MOF@MIP160 was suitable for detecting and removing PAEs from food matrices.

1. Introduction

Phthalate esters (PAEs) are endocrine disruptors commonly found in the environment [1]. Humans are exposed to plasticizers through diet, air inhalation, and dermal contact penetration. Thus, food products with high levels of plasticizers are hazardous to human health [2]. When added to food, plasticizers such as diethylhexyl phthalate (DEHP), di-butyl phthalate (DBP), and diisononyl phthalate (DINP) can damage human health [3]. Phthalates are added to food as they can increase flexibility, extensibility, and swelling. Consequently, exposure to these chemicals during food processing and in food packaging materials might cause plasticizer leakage and contamination [4,5]. The molecular structure of PAEs is similar to that of hormones, and they are classified as a suspected environmental hormone. Once PAEs enter the human body, due to their solubility in fats and organic solvents and insolubility in water, they will accumulate in the human body and not be easily discharged; prolonged intake will lead to high residual concentrations of PAEs in the human body. If ingested over a long period, PAEs can interfere with the secretion of human hormones, leading to abnormalities in the sex of children born to pregnant women, jeopardizing the reproductive ability of males, promoting precocious puberty in females, menstrual disorders, and infertility, and leading to other damage to the reproductive system. Moreover, some phthalates or their metabolites are suspected to be human carcinogens and endocrine disruptors [6,7,8]. Some phthalates have been included in the list of pollutants in different countries because they pose significant risks to human health and the environment [9]. PAEs are fat-soluble substances. After entering the human body, they accumulate in fatty tissues and are not easily excreted. If ingested over a long time, high levels of toxic phthalates can accumulate in the human body [10,11]. With the ever-increasing attention to food safety, the concentration of phthalates has been decreasing in food. However, food testing and analysis have serious matrix interference [12]. Thus, it is the need of the hour to develop simple, highly selective, and sensitive sample pretreatment techniques to detect PAEs in complex food matrices. The Notice on the Notification of Maximum Residue Levels of Phthalates in Foods and Food Additives issued by the General Office of the Ministry of Health of China in 2011 (Weiwei Office Supervision Letter (2011) No. 551) has already listed the permissible maximum residue in foods: the maximum residue for DBP is set at 0.3 mg·kg−1 and for DEHP at 1.5 mg·kg−1. In white spirits, the maximum acceptable concentrations for DEHP and DBP are 5 mg·kg−1 and 1 mg·kg−1, respectively.
Due to the advantages of simple operation, low cost, and ease of use, solid-phase extraction (SPE) is an important separation and enrichment technique [13]. The core of the solid-phase extraction (SPE) technique is the extraction material; this is the key factor in determining the extraction and separation efficiency. Therefore, it is essential to develop efficient and selective adsorbents. In recent times, the preparation and application of novel high-efficiency extraction materials have generated significant attention. Commonly used solid-phase extraction materials include C8, C18, cotton fiber, silica, graphitized carbon black composite, and neutral alumina. These materials adsorb targeted small molecules via molecular forces such as hydrophobicity, hydrogen bonding, π-πstacking, and ion exchange [14]. Once adsorbed, the impurities can be easily removed by washing and elution. This leads to the selective purification and enrichment of the target components [15,16,17,18,19]. Magnetic solid-phase extraction (MSPE) is convenient, adjustable, and widely used in biotechnology, medicine, and analytical chemistry [20,21,22]. In this method, magnetic nanomaterials are used as extraction materials. Typically, the magnetic cores of these materials are iron oxides. The outermost layer of the material is often modified chemically to increase its adsorption and selectivity [23].
Metal–organic frameworks (MOFs) have attracted much attention due to their large specific surface area, ultra-high porosity, and tunable pore size. However, the selectivity of the ferrous MOFs is poor, and they are unable to remove low concentrations of pollutants with specificity [24,25,26,27,28,29,30,31]. Molecularly imprinted polymers (MIPs) have been widely used to separate and enrich PAEs, as they possess specific recognition sites that are complementary to the size and shape of the template molecules [32,33,34,35,36,37]. These studies demonstrate that MOF-MIP materials have a high potential for separation and enrichment of molecules, as they combine the high selectivity of MIPs with the high adsorption capacity of MOFs.
There have been many studies on nanocomposites, such as iron oxide nanostructures (IONs) combined with graphene or its derivatives (e.g., graphene oxide and reduced graphene oxide), which hold great promise for the engineering of efficient nanocomposites [38]. It is conceivable that novel magnetic molecularly imprinted polymers with multifunctional properties can be prepared if the three concepts of metal–organic backbone materials, molecularly imprinted polymers, and magnetic separation are combined in a single system. Modification of magnetic nanoparticles (Fe3O4) with the superparamagnetic effect and modification with MOFs can increase the specific surface area of the carrier and at the same time increase the adsorption of the material to the target compounds; further modification with MIPs can improve the selectivity of the material to the targets. The resultant magnetic materials will not only exhibit high selectivity and high adsorption capacity for the target molecules but also have high magnetic responsiveness [39,40,41,42,43,44,45,46,47,48,49].
Using dibutyl phthalate (DBP) as a template, this study prepared a novel magnetic molecularly imprinted polymeric material (Fe3O4@MOF@MIP160) to detect phthalate esters. The conditions for the synthesis of Fe3O4@MOF@MIP160 were optimized. The prepared Fe3O4@MOF@MIP160 was characterized by FTIR, SEM, and nitrogen adsorption–desorption. The reusability of Fe3O4@MOF@MIP160 and the PAEs’ adsorption capacity were also investigated. Finally, the prepared Fe3O4@MOF@MIP160 was coupled with GCMS to detect PAEs in real samples, such as drinking water, fruit juice, and liquor.
This study presents the development of a novel magnetic solid-phase extraction agent through the ingenious integration of magnetic solid-phase extraction technology with advanced functional new materials. It is anticipated that this innovation will demonstrate greater potential in the composite construction and surface modification of nanomaterials, potentially revolutionizing current extraction techniques. Addressing the drawbacks of the existing national standard methods, which are often cumbersome and lack universality, this research aims to establish a more efficient, straightforward, universally applicable, and environmentally friendly detection method. The main focus is on the most critical safety concerns facing the food industry, providing a robust complement and new perspective to the current food testing systems.

2. Materials and Methods

2.1. Materials

Analytical-grade chemicals were used in this study. Di-butyl phthalate (DBP; C16H22O4, 99.5%), Iron (III) chloride hexahydrate, iron (II) chloride tetrahydrate, N, N-Dimethylformamide (DMF, 99.5%), and benzoic acid (99.5%) were obtained from Aladdin Reagent (Shanghai, China). PVP (Mw = 40,000) and tetrakis(4-carboxyphenyl) porphyrin (TCPP, 97%) were procured from TCI (Tokyo, Japan). Zirconyl chloride octahydrate (99.9%) was obtained from InnoChem Technology (Beijing, China). Methacrylic acid (MAA, C4H6O2, 99.0%), azobisisobutyronitrile (AIBN, C8H12N4,99.0%), and ethylene glycol dimethacrylate (EGDMA, C10H14O4, 99.0%) were supplied by Aladdin Chemistry (Shanghai, China). Ammonia (NH3·H2O), trisodium citrate dihydrate, methyl alcohol (MeOH, CH4O, 99.5%), ethanol (99.5%), and acetonitrile were obtained from Macklin (Shanghai, China). Ultrapure water (Millipore Mill-Q system, Merck Millipore, Darmstadt, Germany) was used for the experiments.

2.2. Synthesis

2.2.1. Preparation of Fe3O4@MOF

The chemical coprecipitation method was used to synthesize the magnetic nanoFe3O4. Briefly, FeCl3·6H2O (1.329 g) and FeCl2·4H2O (0.489 g) were dissolved in deionized water (30 mL) and ethanol (30 mL) under N2 atmosphere and stirred mechanically at 800 rpm, followed by the addition of ammonia. After stirring for 10 min, external permanent magnets were used to collect black nanoFe3O4. Next, the Fe3O4 product was washed thrice using deionized water. The PVP powder (5×) was added to the prepared Fe3O4 and shaken at 1200 rpm in a vortex shaker for 15 h at room temperature. Then, it was washed twice with water and transferred to the DMF solution to obtain Fe3O4@PVP. Finally, Fe3O4@PVP, TCPP (30 mg·mL−1), ZrOCl2·2H2O (15 mg·mL−1), and benzoic acid (280 mg·mL−1) were slowly stirred at 90 °C for 6 h. Finally, the Fe3O4@PVP@MOF product was washed thrice using DMF. Finally, the Fe3O4@PVP@MOF product was stored in chromatography-grade methanol solvent [50].

2.2.2. Preparation of Fe3O4@MOF@MIP

The following procedure was used to synthesize Fe3O4@PVP@MOF-based DBP-imprinted polymer. First, MAA (0.2 mmol) and DBP (0.05 mmol) were dissolved in acetonitrile (20 mL), followed by ultrasonication for 30 min. After adding Fe3O4@PVP@MOF (30 mg), EGDMA (3 mmol), and AIBN (10 mg), the obtained solution was inflated with N2 gas for 10 min to remove O2, followed by stirring at 60 °C for the next 24 h. The product was washed with a mixture of methanol and acetic acid (6:1, v/v) and redistilled water to sufficiently denature the template DBP until no template molecules were detectable, resulting in Fe3O4@MOF@MIP160. Finally, these polymers were dried at 60 °C for 12 h in an oven. NIP was prepared following the same procedures without the use of DBP.

2.3. Characterization of Fe3O4@MOF@MIP160

The morphology of Fe3O4@MOF@MIP160 was observed using transmission electron microscopy (TEM) using a Nippon Electron-JEOL JEM-f200 (JEOL Ltd., Tokyo, Japan) instrument. Fourier transform infrared (FTIR) spectra were recorded between 400 and 2500 cm−1 at 25 °C using a Thermo Fisher-Nicolet IS5 (Thermo Fisher Scientific Inc., Waltham, MA, USA). A Bruker D8 Advance X-ray diffraction (XRD) spectrometer (Scientific Technology Co. Ltd., Billerica, MA, USA), was used to obtain the XRD maps. Surface charge and particle size were determined using zeta potential determination and dynamic light scattering (DLS). The Autosorb-IQ-MP (Quantachrome Instruments, Boynton Beach, FL, USA) was used to measure the N2 adsorption–desorption isotherms at 77 K to assess the pore structure of the samples. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. A Lake Shore 8600 VSM (Lake Shore Cryotronics, Ltd., Westerville, OH, USA) vibrating sample magnetometer was used to characterize the hysteresis loops with a maximum field of 1.5 T at 25 °C [51,52].

2.4. Sample Preparation

Drinking water, fruit juices, and white spirits were procured from local supermarkets. Ultrasonic degassing was used for degassing all samples for 30 min, followed by filtration of the insoluble impurities using a 0.22 μm filter membrane and storage in glassware at 4 °C for further use.
The separation procedure for the analysis of food samples was as follows: 5 mL of the pre-treated sample solution was measured and mixed with 10 mg of Fe3O4@MOF@MIP160 for 20 min. It was left to stand for one minute, and then Fe3O4@MOF@MIP160 was separated from the solution by an external magnet and added to 2 mL of methanol to desorb DBP and DEHP; the supernatant enriched with the target phthalate esters was successfully separated. Finally, it was analyzed by GCMS.

2.5. GC-MS Analysis Parameters for the Samples

Chromatographic separation was done using a Thermo Fisher TSQ 8000 EVO (Thermo Fisher Scientific Inc., Waltham, MA, USA) gas chromatography-mass spectrometer (GC-MS) with an HP-5 ms capillary column (30 m × 250 mm × 0.25 mm) and a triple quadrupole mass spectrometer. After setting the split/unsplit injector to 260 °C, 1 μL of the sample was injected without split. The carrier gas was high-purity helium (99.999%) at a flow rate of 1.0 mL·min−1. The column temperature procedure was as follows: the initial column temperature was set at 60 °C and held for 1 min, followed by increasing to 220 °C at the rate of 20 °C·min−1 and held for 1 min. Then, the temperature was increased to 250 °C at 5 °C·min−1 and held for 1 min and then increased to 290 °C at 20 °C·min−1 and held for 7.5 min.
Using the standard isotope internal standard method, initially, a standard stock solution at a concentration of 10 μg·mL−1 and a deuterated internal standard stock solution at the same concentration were prepared. Subsequently, a series of standard working solutions were formulated by accurately pipetting the two-phthalate ester standard stock solutions (10 μg·mL−1) and sequentially diluting them with chromatography-grade ethyl acetate to achieve concentrations of 10, 25, 50, 100, 250, 500, 1000, 2000, 3000, 4000, and 5000 ppb. Concurrently, a deuterated internal standard stock solution (10 μg·mL−1) was added to ensure that the internal standard concentration was 100 ppb for the standard series solutions ranging from 10 to 1000 ppb, and 1000 ppb for the standard series solutions ranging from 1000 to 5000 ppb. These solutions were prepared immediately before use. The proposed method was validated in terms of linearity, limit of detection (LOD), and limit of quantitation (LOQ). Every sample was tested three times by GC-MS to obtain the standard deviation. These results were analyzed by Anova software (SPSS version 22.0).

2.6. Dynamic Binding Experiments

By altering the synthetic conditions of the material (the ratio of template molecules to functional monomers, the ratio of template molecules to EGDMA, and the reaction time), a mixture of 4 mL PAEs (DBP and DEHP) at a concentration of 100 mg·L−1 was adsorbed onto 10 mg of Fe3O4@MOF@MIP160 material to ascertain the impact of these regulatory factors on the adsorption capacity. Additionally, multiple washing cycles were conducted to confirm the number of washes required to completely remove the template molecules.
The adsorption capacity for two phthalate acid esters (PAEs) was determined through adsorption kinetics experiments using the prepared Fe3O4@MOF@MIP160. Initially, 10 mg of Fe3O4@MOF@MIP160, Fe3O4@MOF@NIP, and Fe3O4@MOF adsorbents was immersed in 4 mL of a phthalate solution at a concentration of 100 mg·L−1. After allowing for adsorption for 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, and 30 min, the supernatant was taken for subsequent analysis. The concentrations of dibutyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) in the post-adsorption solution were measured using gas chromatography-mass spectrometry (GC-MS). The following equations were employed to calculate the adsorption capacity (Qe, milligrams per gram) and the recovery rate (R, %), as shown in (1) and (2):
Q e = ( C 0 −   C t ) × V m
R ( % ) = C 0 −   C t C 0 × 100
where V represents the volume of PAE solution (mL); C0 and Ct indicate the initial and real-time concentrations of the PAE solution (mg·L−1), respectively; Qe (mg·g−1) represents the amount of PAEs adsorbed by the adsorbent; and m is the mass of Fe3O4@MOF@MIP160 and Fe3O4@MOF@NIP.

2.7. Stability Experiment

To examine the stability of the materials, 10 mg of Fe3O4@MOF@MIP160 was removed from the magnetic material stock solution as an extractant for the adsorption of the two phthalate esters at 15-day intervals for 45 days of continuous experiments.

2.8. Selective Research

Selective research was conducted by adding known levels of common interfering components in foods (vitamin C, whey protein, calcium ions) as well as benzylamine under the simulation system. Benzylamine is an aromatic amine compound that may be found in foods and has a structure similar to PAEs. The above four substances were utilized to explore the adsorption selectivity properties of the composites for the targets.

2.9. Regeneration of Adsorption of Fe3O4@MOF@MIP160

Fe3O4@MOF@MIP160 (10 mg) was suspended in 4 mL of a methanol solution of phthalates (50 mg·L−1). The adsorption process was conducted as follows: initially, 10 mg of Fe3O4@MOF@MIP160 adsorbent was immersed in 4 milliliters of a phthalate ester solution with a concentration of 100 mg·L−1. After 20 min of adsorption, which allowed the system to reach adsorption equilibrium, the supernatant was collected for further analysis. After adsorption, the material was washed thrice with methanol and dried at 60 °C until constant weight was obtained. This adsorption–desorption cycle was repeated six times.

3. Results

3.1. Optimization of the Synthesis Conditions

For magnetic composites, the core material must maintain its superparamagnetism. Thus, the conditions for the synthesis of Fe3O4@MOF@MIP160 were optimized to achieve the best adsorption performance of the composites.

3.1.1. Effect of the Ratio of Template Molecules to Functional Monomer

The imprinting efficiency and affinity of MIP depend on the molar ratio between the template (DBP) and the functional monomer (MAA). This study examined the effect of three different ratios of DBP to MAA—2:1, 1:1, 1:2, and 1:4—on the PAE adsorption performance of Fe3O4@MOF@MIP160 (Figure 1). When the DBP to MAA ratio was 2:1, there was not enough monomer to form a complete recognized cavity, resulting in low recovery. With the increase of the monomer ratio, the adsorption capacity of Fe3O4@MOF@MIP160 gradually enhanced. The highest recoveries (DBP: 80.2%, DEHP: 70.3%) were obtained for Fe3O4@MOF@MIP160 when the ratio reached 1:1. This might be because the ester group on DBP formed a complete complex with the hydrogen on the amino group in MAA, resulting in a complete recognition cavity. When the DBP-to-MAA ratios were 1:2 and 1:4, the recovery was lower due to the decrease in the number of templates, which, in turn, decreased the amount of effective MAA–DBP complexes. Since there were not enough MAA–DBP complexes, the overall selectivity of the MIP was reduced. Thus, 1:1 was chosen as the optimal ratio of template molecule to functional monomer.

3.1.2. Effect of the Ratio of Template Molecules to EGDMA

The content of EGDMA, a cross-linking agent, impacts the polymer network structure as well as the polarity of the adsorbent. This affects the recognition and adsorption of the test substance on Fe3O4@MOF@MIP160. Therefore, the adsorption properties of Fe3O4@MOF@MIP160 were examined at the following DBP-to-EGDMA ratios: 2:5, 1:5, 1:10, 1:20, and 1:30 (Figure 2). The highest recovery rate (DBP: 73.7%, DEHP: 85.4%) was observed at the DBP–EGDMA of 1:20. At the other ratios, i.e., 2:5, 1:5, and 1:10, the lack of EGDMA and the inability of MIP to form a stable pore structure resulted in a high rate of MIP loss. In contrast, when the ratio was 1:30, the recovery rate decreased because of the presence of excessive EGDMA. This led to a denser polymer network structure in Fe3O4@MOF@MIP160, which increased the spatial site resistance of the molecularly imprinted sites. Therefore, 1:20 was chosen as the optimal template molecule-to-EGDMA ratio.

3.1.3. Effect of Reaction Time

The effectiveness of magnetic material preparation, as well as the adsorption properties of the composites, are affected by reaction time. The adsorption properties of Fe3O4@MOF@MIP160 were examined at the following reaction times: 1, 3, 6, 12, 24, and 36 h. As shown in Figure 3, the highest recovery (DBP: 99.2%, DEHP: 89.33%) was obtained when the reaction time was 24 h. At reaction times < 24 h, the molecular imprinting polymerization was incomplete, while at reaction times > 24 h, the product color was darker brown due to oxidation that may have occurred, leading to poor binding.

3.1.4. Number of Washes Required to Completely Remove Stencil Molecules

The last step requires the removal of the stencil molecules from the product to finally obtain the molecularly imprinted polymerization with a precise 3D imprinted cavity structure, which is complementarily matched with the template molecules in terms of geometric morphology, dimensions, and layout of the functional groups to ensure a high degree of selectivity for the target molecules, as well as to expand the adsorption capacity and improve the adsorption ability of the material.
To ensure that no template molecules remained in the fabricated Fe3O4@MOF@MIP160, the template molecule DBP was denatured by washing with methanol and acetic acid (6:1, v/v) and distilled water several times until no template molecules were detected. As shown in Figure 4, the results showed that most of the template molecules had been removed by the second wash, and there were no template molecules left at all by the fourth wash, so the final choice was to wash four times.

3.2. Characterization of Adsorbent Materials

3.2.1. TEM, DLS, and Zeta Analysis

TEM was used to characterize the morphology and structure of the material (Figure 5).
The results showed that the nanospheres are monodisperse irregular spheres with uniform particle size. Figure 5e,f shows that the surface of Fe3O4@MOF@MIP160 is rough, with many irregular deposits around it, which are considered imprinted polymers, suggesting that the MIP has been covered on the surface of Fe3O4@MOF in the form of a thin layer. The TEM results showed that the particle size of Fe3O4@MOF is in the range of 200–210 nm and Fe3O4@MOF@MIP160 is in the range of 350–400 nm. As shown in Figure 6b, the particle size of the nanoparticles was then characterized by DLS. The uncoated nano-Fe3O4@MOF has a uniform size distribution with an average diameter of (221 ± 0.32) nm, and the coated nano-Fe3O4@MOF@MIP160 shows a uniform size distribution with an average diameter of (410 ± 0.92) nm. The MIP layer was uniformly dispersed on the surface of Fe3O4@MOF, and the thickness of the MIP layer deposited on the surface of Fe3O4@MOF was about 95 nm. This structure accelerates the rate of adsorption and facilitates the adsorption of target molecules. The particle size results for TEM were slightly smaller than the DLS test; this was because the TEM test samples of nanoparticles were dried, while the DLS test was carried out in solution. Since the surfactant swells in liquids, the DLS test particle size data were generally slightly larger. The polymer dispersibility index (PDI) was 0.162, which shows good dispersibility. The results of the Zeta analysis are shown in Figure 6a, demonstrating that the prepared material has a large positive and negative potential span (43.2 mV), indicating good stability.

3.2.2. XRD Analysis

The synthesized crystal structures and phase purities were further investigated by characterizing the X-ray single crystal diffraction (XRD) of Fe3O4, Fe3O4@MOF, and Fe3O4@MOF@MIP160 (Figure 7). The peaks at 2 θ = 30.3, 35.6, 43.3, 53.7, 57.5, and 62.9 can be assigned to the (220), (311), (400), (422), (511), and (440) characteristic diffraction peaks of the Fe3O4 nanoparticles, respectively (JCPDS no. 19-629). Comparison of XRD diffraction analysis of Fe3O4@MOF revealed that no impurity peaks were found, except for the crystalline Fe3O4 and MOF peaks (at 4.54°, 6.44°, 7.88°, and 9.14°), which indicated that the direct crystallization degree of the two was good and the incorporation of the MOF material did not destroy the original crystal structure of Fe3O4. No significant difference was found between Fe3O4@MOF@MIP160 and Fe3O4@MOF.

3.2.3. VSM Analysis

Magnetic field properties are crucial for separating magnetic materials. The saturation magnetization strength of Fe3O4 was around 60.7 emu·g−1. The VSM method was used to determine the magnetic properties of Fe3O4@MOF and Fe3O4@MOF@MIP160. The application and the magnetic field magnetization intensity of the materials were plotted. The magnetization saturation values of Fe3O4@MOF and Fe3O4@MOF@MIP160 were 30.12 emu·g−1 and 27.06 emu·g−1, respectively (see Figure 8a). Moreover, there was no observable magnetic hysteresis in the hysteresis curves, which was indicative of the excellent superparamagnetic properties of the materials. The degree of magnetization of both materials was lower than Fe3O4 because of the modification of MOF. Due to the assembly of MIPs, Fe3O4@MOF@MIP160 was found to be slightly less magnetized than Fe3O4@MOF. However, it was of sufficient size to be quickly and effectively separated by an external magnet. The results revealed that the nanocomposites have a higher separation efficiency and can be used for magnetic solid-phase extraction.

3.2.4. FT-IR Analysis

The characteristic functional groups on the surface of the prepared nanocomposites were identified by FT-IR (Figure 8b). The spectrum of Fe3O4@MOF@MIP160 retained several characteristic absorption peaks of Fe3O4@MOF, such as at 1410 cm−1 (-C-O stretching vibration). Additionally, the absorption band at 525 cm−1 was attributed to the Fe-O stretching vibration, suggesting that the metal center ion was coordinated with the organic ligand. The spectrum of Fe3O4@MOF@MIP160 showed a new characteristic absorption peak at 725 cm−1, which could be attributed to a = C-H bending vibration arising from the polymerization between EGDMA and the benzene ring. This is because the MIP layer was grafted on the Fe3O4@MOF surface.

3.2.5. Nitrogen Adsorption–Desorption

In addition to superparamagnetism, the extraction capability is also of great importance for magnetic solid-phase extractants. Notably, extraction performance is closely related to the specific surface area and pore size of the material. Both these properties can be quantitatively estimated by N2 adsorption, and desorption experiments can be used. Figure 8c,d shows that the Fe3O4@MOF@MIP160 adsorption isotherm belongs to the I-type isotherm. At low pressure, the composite rapidly adsorbed nitrogen to reach equilibrium, suggesting that the initial adsorption is dominated by small-sized pores. Once the system reached equilibrium, capillary condensation or multimolecular layer adsorption occurred, indicating that the material had a microporous or mesoporous structure. The specific surface area and total pore volume of Fe3O4@MOF@MIP160 were calculated to be 95.604 m2·g−1 and 0.292 cm3·g−1, respectively. Using density-functional-transfer-theoretic (DFT) calculations, the average pore size of the material was estimated to be 1.40 nm in the N2 isotherm. Thus, the nanocomposite’s high specific surface area and high pore size provided more active sites for adsorbing target substances. On the other hand, the small pore size structure favored the high selectivity of small molecules.

3.3. Analysis of Adsorption Performance of Fe3O4@MOF@MIP160

Figure 9 shows the dynamic adsorption curves of PAEs adsorbed on Fe3O4@MOF@MIP160, Fe3O4@MOF@NIP, and Fe3O4@MOF. The adsorption of PAEs on Fe3O4@MOF@MIP160 occurred rapidly in the first 10 min. After the first 10 min, the rate of increase slowed down, and adsorption equilibrium was reached after 20 min. The adsorption behaviors of Fe3O4@MOF@NIP and Fe3O4@MOF were similar. However, the adsorption capacity of Fe3O4@MOF@MIP160 (260.2 mg·g−1) for DBP was significantly higher than Fe3O4@MOF@NIP (111.4 mg·g−1) and Fe3O4@MOF (98.5 mg·g−1). Fe3O4@MOF@MIP160 (240.2 mg·g−1) also displayed a significantly higher adsorption capacity for DEHP than Fe3O4@MOF@NIP (90.4 mg·g−1) and Fe3O4@MOF (89.0 mg·g−1). The adsorption ability of the magnetic materials was improved by the presence of molecularly imprinted sites, especially in the selective recognition of the template molecule DBP. This is because the binding of molecularly imprinted sites best matches the size and shape of DBP. Moreover, the structure of DEHP is similar to that of DBP, resulting in better selective recognition. This suggests that specific imprinted recognition sites on the surface of Fe3O4@MOF@MIP160 match the DBP well in terms of size and spatial arrangement. The high specific surface area of the magnetic Fe3O4@MOF material also contributes to its large adsorption capacity.

3.4. Stability of Fe3O4@MOF@MIP160

To verify the stability of the fabricated composite material Fe3O4@MOF@MIP160, we impregnated the synthesized samples in methanol solvent and kept them in storage for up to 45 days. An appropriate amount of adsorbent material was extracted every two weeks to examine whether its extraction efficacy for the target analytes was stable at different time intervals. As shown in Figure 10, the adsorption rate was unchanged and remained above 93% (DBP) and 84% (DEHP) after 45 days, which indicates that the material has strong stability.

3.5. Selectivity of Fe3O4@MOF@MIP160

Because of the complexity of food matrices, it is important to consider the possible interfering effects of other co-existing substances when selecting magnetic solid-phase extraction agents and to ensure that the molecularly imprinted polymers (MIPs) used demonstrate excellent selectivity. Therefore, the adsorption properties of the resulting materials for the target compounds were systematically examined by introducing known concentrations of interfering components into the simulated system. As shown in Figure 11, the results indicate that the adsorption efficiency of the synthesized Fe3O4@MOF@MIP160 for the target compound phthalate was not significantly suppressed under the complex matrix conditions, and the highly selective adsorption of the target was still maintained. Moreover, it can be seen that the novel magnetic molecularly imprinted polymeric material (Fe3O4@MOF@MIP160) prepared with dibutyl phthalate (DBP) as a template has a higher adsorption rate for phthalate than the non-imprinted polymeric material (Fe3O4@MOF@NIP) prepared without a template and achieves a better exclusion of the common interfering components of foodstuffs, including vitamin C, whey protein, calcium ions, benzoyl methane, and benzylamine. This suggests that Fe3O4@MOF@MIP160 is selective for phthalates and can be used to extract target phthalates without interference in complex food matrices.

3.6. Regeneration of Fe3O4@MOF@MIP160

The regeneration of Fe3O4@MOF@MIP160 is important to make the material cost-effective and increase its applications. As can be seen in Figure 12, the adsorption capacity of Fe3O4@MOF@MIP160 for PAEs did not change after six adsorption–desorption cycles; adsorption equilibrium time was also basically maintained at about 20 min. Thus, compared with the traditional SPE materials, Fe3O4@MOF@MIP160 can result in significant cost savings in sample pretreatment. Moreover, these materials possess a high magnetization strength and can expedite the sample pretreatment process.

3.7. GC-MS Methodology Evaluation

We optimized the chromatographic and mass spectrometric parameters (Figure 13).
The respective working curves were constructed for drinking water, fruit juice, and white spirit samples. Considering the wide concentration distribution range of the target in the samples and to achieve more accurate quantitative determination, we adopted the strategy of segmented standard curves for quantitative analysis in the ranges of 10–1000 ppb and 1000–5000 ppb, respectively. The results of the standard curves as well as the correlation coefficients are shown in Table 1; the results indicate that all samples have a good linear relationship with the correlation coefficient (R2) ≥ 0.9991.
The limits of detection (LOD) and quantification (LOQ) of DBP in drinking water, carbonated beverages, fruit juices, and white wine samples were calculated based on the signal-to-noise ratios of analyte concentrations of 3:1 and 10:1, as well as the limits of detection (LOD) and quantification (LOQ) of DEHP. The results are shown below: The limits of detection (LOD) and quantification (LOQ) for DBP in drinking water were 0.091 µg·L−1 and 0.08 µg·L−1, respectively; for DEHP in drinking water, the LOD and LOQ were 0.13 µg·L−1 and 0.18 µg·L−1, respectively. The limits of detection (LOD) and quantification (LOQ) for DBP in fruit juice were 0.01 µg·L−1 and 0.06 µg·L−1, respectively; for DEHP in fruit juice, the LOD and LOQ were 0.04 µg·L−1 and 0.08 µg·L−1, respectively. The limits of detection (LOD) and quantification (LOQ) for DBP in white spirits were 0.06 µg·L−1 and 0.07 µg·L−1, respectively; for DEHP in white spirit, the LOD and LOQ were 0.01 µg·L−1 and 0.05 µg·L−1, respectively.
In this study, the accuracy of the proposed method was verified by performing spiked recovery experiments in three blank food sample matrices—drinking water, fruit juice, and white wine—by adding three gradients (high, medium, and low) of standard solutions to explore the recovery of target substances in each matrix. In addition, the precision was determined by investigating the relative standard deviation (RSD) of the measurements. The results are shown in Table 2. The results showed that the extraction rate was mostly in the range of 80–100% for both phthalates tested at several different spiked levels with good precision and accuracy.
In conclusion, the proposed method is suitable for the quantitative analysis and detection of phthalates in a wide range of food samples.

3.8. Application to Real Sample Analysis

In this study, three common commercially available beverages, i.e., drinking water, fruit juice, and white wine, were analyzed and tested. Different concentration levels of DBP and DEHP were successfully identified in the above samples, and the results are shown in Table 3. Comparison of the experimental results with the national maximum residue limits (MRLs) showed that all the detected levels of DBP and DEHP were significantly lower than the national limits, thus confirming that the levels of these two phthalate esters in the commercially available beverages were within the national safety limits.

4. Conclusions

By combining the high specific surface area, ultra-high porosity, and excellent stability of metal–organic frameworks (MOFs) with the specific recognition of the molecularly imprinted polymer MIP, this study prepared a novel surface molecularly imprinted polymer, Fe3O4@MOF@MIP160. First, the synthesis of Fe3O4@MOF@MIP160 was standardized by optimizing the ratio of template molecule to functional monomer, the ratio of template molecule to EGDMA, and the reaction time. The prepared Fe3O4@MOF@MIP160 displayed an excellent adsorption performance, with 99.2% recovery of DBP and 89.33% recovery of DEHP. The prepared nanocomposites were characterized by TEM, FT-IR, VSM, and BET. The results confirmed that the molecularly imprinted polymers were successfully immobilized on the surface of Fe3O4@MOF. Fe3O4@MOF@MIP160 demonstrated good specific recognition, large adsorption capacity, and sufficient magnetization strength. Further, the material showed good performance even after six cycles, suggesting that the cost of sample pretreatment during detection would be significantly lesser than traditional national standard SPE columns. The magnetic properties of Fe3O4@MOF@MIP160 would also simplify the operation steps and greatly save the pretreatment time. The nanocomposites were also used to determine phthalate esters in drinking water, fruit juices, and white wine with high accuracy and precision. The proposed method established in this study greatly simplifies the sample processing steps, saves time and costs in experiments, and is more environmentally friendly than the traditional method, while ensuring the accuracy and precision of the experiment. The findings of this study indicate that the combination of surface-imprinted polymers and magnetic composites has great potential for enriching and separating trace pollutants.
In recent years, the field of food safety testing has persistently explored methods for determining the levels of phthalate esters in the food supply, with innovative developments in novel magnetic solid-phase extraction (MSPE) techniques providing new solutions to these challenges. Within this suite of detection processes, the steps of extraction play a pivotal role. However, traditional techniques often come with significant drawbacks, including high costs, complex operational procedures, extended processing times, and environmental pollution due to the extensive use of organic solvents.
As a result, research into the composite construction and surface modification of nanomaterials has become increasingly valued. It is anticipated that these new materials will demonstrate enhanced adsorption capabilities, potentially transforming current extraction methodologies and thus improving the efficiency and environmental sustainability of food safety testing. It is foreseeable that by integrating materials with individual properties in order to modifying them, composite materials with comprehensive and superior performance can be obtained. Combining the enrichment techniques of these novel materials with sophisticated detection equipment is likely to establish a more effective detection system for trace substances in the food supply. This would render the detection methods more accurate, environmentally benign, time-saving, and cost-effective, presenting a promising prospect for the future.

Author Contributions

Conceptualization, L.C.; data curation, L.L.; formal analysis, L.L.; funding acquisition, L.C.; investigation, L.L. and Y.L.; methodology, L.L. and Y.L.; project administration, L.C. and C.W.; resources, L.C.; software, L.L.; supervision, L.C.; validation, L.L. and Y.L.; visualization, L.L. and L.C.; writing—original draft, L.L. and L.C.; writing—review and editing, L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC, Grant No. 32372299).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Effect of the ratio of template molecule to functional monomer on the adsorption performance of Fe3O4@MOF@MIP160.
Figure 1. Effect of the ratio of template molecule to functional monomer on the adsorption performance of Fe3O4@MOF@MIP160.
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Figure 2. Effect of the ratio of template molecules to EGDMA on the adsorption performance of Fe3O4@MOF@MIP160.
Figure 2. Effect of the ratio of template molecules to EGDMA on the adsorption performance of Fe3O4@MOF@MIP160.
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Figure 3. Effect of reaction time on the preparation of Fe3O4@MOF@MIP160.
Figure 3. Effect of reaction time on the preparation of Fe3O4@MOF@MIP160.
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Figure 4. The number of washes required to completely remove stencil molecules.
Figure 4. The number of washes required to completely remove stencil molecules.
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Figure 5. TEM images of (a–c) Fe3O4@MOF; (d–f) Fe3O4@MOF@MIP160.
Figure 5. TEM images of (a–c) Fe3O4@MOF; (d–f) Fe3O4@MOF@MIP160.
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Figure 6. (a) Zeta potential; (b) DLS testing of Fe3O4@MOF and Fe3O4@MOF@MIP160.
Figure 6. (a) Zeta potential; (b) DLS testing of Fe3O4@MOF and Fe3O4@MOF@MIP160.
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Figure 7. Wide-angle XRD patterns of Fe3O4, Fe3O4@MOF, and Fe3O4@MOF@MIP160.
Figure 7. Wide-angle XRD patterns of Fe3O4, Fe3O4@MOF, and Fe3O4@MOF@MIP160.
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Figure 8. (a) Hysteresis curves of Fe3O4@MOF and Fe3O4@MOF@MIP160; (b) FTIR spectra of Fe3O4@MOF and Fe3O4@MOF@MIP160; (c,d) N2 adsorption isotherm and pore size distribution curve of Fe3O4@MOF@MIP160.
Figure 8. (a) Hysteresis curves of Fe3O4@MOF and Fe3O4@MOF@MIP160; (b) FTIR spectra of Fe3O4@MOF and Fe3O4@MOF@MIP160; (c,d) N2 adsorption isotherm and pore size distribution curve of Fe3O4@MOF@MIP160.
Foods 13 01397 g008
Foods 13 01397 g009
Figure 9. Adsorption curves of Fe3O4@MOF@MIP160, Fe3O4@MOF@NIP, and Fe3O4@MOF on DBP (a) and DEHP (b).
Figure 9. Adsorption curves of Fe3O4@MOF@MIP160, Fe3O4@MOF@NIP, and Fe3O4@MOF on DBP (a) and DEHP (b).
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Foods 13 01397 g010
Figure 10. Effect of storage time on the adsorption of phthalate esters by Fe3O4@MOF@MIP160.
Figure 10. Effect of storage time on the adsorption of phthalate esters by Fe3O4@MOF@MIP160.
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Foods 13 01397 g011
Figure 11. Adsorption effects of magnetic molecularly imprinted polymeric material Fe3O4@MOF@MIP160 and magnetic non-molecularly imprinted polymeric material Fe3O4@MOF@NIP on coexisting components in foods.
Figure 11. Adsorption effects of magnetic molecularly imprinted polymeric material Fe3O4@MOF@MIP160 and magnetic non-molecularly imprinted polymeric material Fe3O4@MOF@NIP on coexisting components in foods.
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Foods 13 01397 g012
Figure 12. Reuse results of Fe3O4@MOF@MIP160 over six times.
Figure 12. Reuse results of Fe3O4@MOF@MIP160 over six times.
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Foods 13 01397 g013
Figure 13. (a) Chromatogram of DBP in standard solution; (b) chromatogram of DEHP in standard solution; (c) chromatogram of DBP in food samples; (d) chromatogram of DEHP in food samples (The blue circle represents the selection range of the chromatographic peak area region); (e) mass spectra of DBP in standard solution; (f) mass spectra of DEHP in standard solution; (g) mass spectra of DBP in food samples; (h) mass spectra of DEHP in food samples.
Figure 13. (a) Chromatogram of DBP in standard solution; (b) chromatogram of DEHP in standard solution; (c) chromatogram of DBP in food samples; (d) chromatogram of DEHP in food samples (The blue circle represents the selection range of the chromatographic peak area region); (e) mass spectra of DBP in standard solution; (f) mass spectra of DEHP in standard solution; (g) mass spectra of DBP in food samples; (h) mass spectra of DEHP in food samples.
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Table 1. Standard curve equations and correlation coefficients for different ranges in different foods.
Table 1. Standard curve equations and correlation coefficients for different ranges in different foods.
RangeDBPDEHP
10–1000 ppb1000–5000 ppb10–1000 ppb1000–5000 ppb
Drinking waterstandard curvey = 0.432x + 0.562y = 1.22x + 0.322y = 0.426x + 0.228y = 1.82x + 0.237
R20.99980.99990.99910.9993
Fruit juicestandard curvey = 0.335x + 0.340y = 0.763x + 0.036y = 0.662x + 0.716y = 0.613x + 0.028
R20.99980.99970.99920.9991
White spiritsstandard curvey = 0.55216x + 4.046y = 0.287x + 0.887y = 0.629x + 1.22y = 3.27x + 0.008
R20.99930.99960.99910.9991
Table 2. Recoveries of real samples spiked with PAEs (n = 3).
Table 2. Recoveries of real samples spiked with PAEs (n = 3).
SampleAnalyteAdded (0.1 μg/L)Added (1 μg/L)Added (10 μg/L)
Extraction Rate
(%)		RSD (%) Extraction Rate
(%)		RSD (%)Extraction Rate
(%)		RSD (%)
Drinking
water		DBP93.5 ± 2.62.490.9 ± 1.92.4100.7 ± 3.43.37
DEHP85.3 ± 2.41.787.3 ± 1.42.290.3 ± 2.52.34
Fruit juiceDBP90.8 ± 1.33.290.1 ± 1.21.997.7 ± 2.12.18
DEHP80.3 ± 2.11.481.3 ± 1.63.787.3 ± 1.53.16
White spiritsDBP80.8 ± 2.31.586.1 ± 1.23.2897.3 ± 3.11.29
DEHP70.3 ± 2.12.778.3 ± 1.92.4587.3 ± 1.01.96
Table 3. DBP and DEHP content in actual beverage samples.
Table 3. DBP and DEHP content in actual beverage samples.
Sample 1Sample 2Sample 3Sample 4 (White Spirits)
DBP (mg/kg)0.010.190.090.71
DEHP (mg/kg)0.820.661.023.22
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Li, L.; Lu, Y.; Wang, C.; Cheng, L. Fabrication of Magnetic Molecularly Imprinted Polymers for Selective Extraction of Dibutyl Phthalates in Food Matrices. Foods 2024, 13, 1397. https://doi.org/10.3390/foods13091397

AMA Style

Li L, Lu Y, Wang C, Cheng L. Fabrication of Magnetic Molecularly Imprinted Polymers for Selective Extraction of Dibutyl Phthalates in Food Matrices. Foods. 2024; 13(9):1397. https://doi.org/10.3390/foods13091397

Chicago/Turabian Style

Li, Lina, Yunzhu Lu, Chengtao Wang, and Lei Cheng. 2024. "Fabrication of Magnetic Molecularly Imprinted Polymers for Selective Extraction of Dibutyl Phthalates in Food Matrices" Foods 13, no. 9: 1397. https://doi.org/10.3390/foods13091397

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