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Original Article
ARTICLE IN PRESS
doi:
10.25259/AJC_1236_2025

Fabrication of magnetic surface molecularly imprinted polymers via semi-covalent imprinting strategy for selective recognition of bisphenol A

, , ,
aCollege of Materials Science and Engineering, Hebei University of Engineering, Handan, PR China
bTechnology Innovation Center of Modified Plastics of Hebei Province, Handan, PR China

* Corresponding author: E-mail address: wangzehu12@126.com (Z. Wang)

Received: , Accepted: ,
© 2026 Arabian Journal of Chemistry - Published by Scientific Scholar
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Abstract

The magnetic responsiveness could endow material with excellent separation performance. In this work, magnetic surface molecularly imprinted polymers (MIPs) were prepared by a semi-covalent imprinting strategy. Firstly, a covalent template-monomer complex was synthesized through the reaction of bisphenol A (BPA) and 3-isocyanatopropyltriethoxysilane (IPTS). Secondly, Fe3O4 nanoparticles were prepared through the co-precipitation of Fe2+ and Fe3+. Thirdly, the gelation of template-monomer complex (BPA-IPTS) and tetraethoxysilane was performed on the surface of Fe3O4 magnetic cores. After the removal of BPA by the thermal cleavage, Fe3O4@MIPs with core-shell structure were obtained. BPA-IPTS was characterized by Fourier transform infrared spectroscopy (FTIR) and 1H NMR. Fe3O4@MIPs were detected by FTIR, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The spectra of FTIR and 1H NMR indicate that BPA-IPTS has been synthesized successfully. The TEM images show that Fe3O4@MIPs possess a typical core-shell structure with the diameter range from 25 to 40 nm. The XRD patterns indicate that the presence of MIPs shell layer does not affect the crystal structure of Fe3O4 nanoparticles. The FTIR spectrum of Fe3O4@MIPs indicates that BPA-IPTS has been coated on the surface of Fe3O4 nanoparticles. The successful building of imprinted sites on the shell layer was validated by a series of binding experiments, including binding kinetics, binding isotherm and binding selectivity. Due to the presence of Fe3O4 cores, the imprinted sites are distributed in the external shell layer, which decreases the mass transfer resistance significantly. The results show that Fe3O4@MIPs have fast binding rate with the equilibrium binding time of 60 min, which is consistent with the expected result. Besides, Fe3O4@MIPs appear considerable binding affinity and high selectivity towards BPA, the binding capacity is apparently higher than that of Fe3O4@NIPs with the imprinting factor of 1.74. The experiment results indicate that Fe3O4@MIPs are promising material for the separation of BPA. This study provides an alternative strategy for the building of magnetic surface MIPs.

Keywords

Bisphenol A
Core-shell structure
Fe3O4 nanoparticles
Molecularly imprinted polymers
Semi-covalent imprinting

1. Introduction

As an endocrine disruptor, bisphenol A (BPA) could interfere with the reproductive and endocrine systems of humans and wildlife, reduce their immune function and increase their risk of getting cancer [1,2]. At present, BPA is widely used in the synthesis of polycarbonates and epoxy resins, which are further processed into various daily necessities, such as food containers, outer packaging materials, water pipes, beverage bottles, tableware, milk bottles and children’s toys [3,4]. BPA could easily migrate from these products and enter into the environment as time goes on. Up to now, BPA has been detected from food, beverages and surface water, which is bound to threaten the reproductive health and immune function of humans and wildlife [5,6]. Therefore, the precise detection and efficient isolation of BPA have become especially crucial. In recent years, lots of efforts have been devoted to detect and remove BPA present in the environment. Among various methods, adsorption is adopted widely [7]. Unfortunately, BPA often exists at extremely low concentration in the environment and coexists with other substances, which are high in content, low in toxicity and biodegradable. Conventional adsorbents are subject to certain limitations in practical application on account of the absence of selectivity towards BPA. Consequently, there is an urgent need to develop new adsorbents featuring outstanding selectivity.

Molecular imprinting technology (MIT) is a powerful approach for manufacturing the polymer materials with specific recognition property by copolymerizing functional monomers and cross-linking agents in the presence of template molecules, the resulting polymer materials are called as molecularly imprinted polymers (MIPs) [8]. MIPs belong to a class of artificially synthesized receptor materials with high affinity and excellent selectivity towards target molecule. In addition, MIPs have numerous advantages, including low preparation cost, simple synthesis method, high stability and good reusability [9]. At present, MIPs have been widely applied in solid-phase extraction [10], chromatographic separation [11], chemical sensors [12], drug delivery [13] and artificial antibodies [14].

At present, the main methods for preparing MIPs include the reversible covalent bonds [15] and the non-covalent interactions [16]. Due to the higher stability of covalent bonds, the covalent imprinting strategy could yield much more uniform imprinted sites. Even so, the covalent imprinting is still defined as an inflexible approach since the preparation process requires the template molecules and functional monomers to form reversible covalent bonds in advance. Therefore, only template molecules with specific functional groups are suitable for covalent imprinting, which limits its application scope. Compared with the covalent imprinting, the non-covalent imprinting has no such requirement, which only relies on various non-covalent interactions between template molecules and functional monomers, such as hydrogen bonding, ionic interactions, van der Waals forces, π-π interactions. The obtained MIPs could rebind the template molecules via the same non-covalent interactions. Compared with the covalent imprinting, the variety of compounds that could be imprinted is significantly broadened [17].

Besides the above two imprinting strategies, the semi-covalent imprinting is also an alternative method for the fabrication of MIPs. During the preparation process, the reversible covalent bonds between templates molecules and functional monomers were formed, which is same as the covalent imprinting. After the removal of covalently bonded templates molecules, the obtained MIPs could rebind target molecules through the non-covalent interactions, which is different from covalent imprinting [18]. The semi-covalent imprinting combines organically the high affinity of covalent imprinting with gentle operating conditions of non-covalent imprinting organically. Up to now, the researches involving semi-covalent imprinting have been reported from time to time. Chang and coworkers prepared molecularly imprinted silica particles for the recognition of estrone by semi-covalent imprinting [19]. Liu and coworkers prepared molecularly imprinted silica with immiscible ionic liquid as solvent for selective recognition of testosterone [20]. Ozin et al prepared molecularly imprinted mesoporous organosilica using semi-covalent imprinting technique [21]. He and coworkers prepared dummy molecularly imprinted mesoporous silica by semi-covalent imprinting for solid-phase extraction of BPA [22]. Even so, the research is very limited. Therefore, the further research and exploration are still needed.

Bulk polymerization is the most traditional method for the fabrication of MIPs. The functional monomers, template molecules, cross-linking agents, initiators and porogen were mixed in a certain proportion. After polymerization and removal of the template through physical or chemical means, the bulk MIPs were obtained [23]. Bulk polymerization has various advantages, such as simple synthesis equipment and strong universality. However, the bulk MIPs usually requires preprocessing procedures before use, including crushing, grinding and sieving. These procedures are time-consuming, labor-intensive and results in inevitably low yield. Besides, the grinding process will produce irregular particles, destroy the affinity of recognition sites with the reduction of the binding efficiency [24]. These problems restrict the application of bulk polymerization. Subsequently, suspension polymerization [25], precipitation polymerization [26], swelling polymerization [27], miniemulsion polymerization [28] and sol-gel method [29] were developed successively for the fabrication of spherical MIPs, which could omit the preprocessing procedures of crushing, grinding and sieving.

Even so, the above-mentioned methods belong to the embedding method. The prepared spherical MIPs still face the following disadvantages. Firstly, the embedding of template molecules within spherical MIPs makes their elution difficult and incomplete, which will affect inevitably the accuracy of quantitative analysis. Secondly, the spherical MIPs usually have a slow binding rate and a low binding capacity for target molecules. To address these shortcomings, surface MIT emerges, which involves grafting or coating a thin MIPs layer on the surface of matrix materials [30]. The polymers prepared by surface MIT are called as surface MIPs (S-MIPs). Compared with conventional MIPs, S-MIPs possess the following advantages. On the one hand, the thin imprinted layer could facilitate the removal and re-binding of template molecules, thus possesses a fast mass transfer rate. On the other hand, the morphology of S-MIPs depends on the used matrix materials, which could realize the designability of shape [31].

Recently, magnetic MIPs (M-MIPs) have gained increasing interest [32,33]. The incorporation of magnetic component grants MIPs magnetic separation performance and thus avoids the centrifugation or filtration procedure, which are laborious, time-consuming but essential in the process of synthesis and application of traditional MIPs [34,35]. The rebinding tests of M-MIPs are achieved by directly dispersing samples into solutions, and the samples could be easily isolated from the suspension solutions using an external magnetic field, offering a high efficiency, convenient and economical approach [36]. Typically, M-MIPs are prepared by the coating of imprinting layer on the surface of magnetic materials, in which magnetic materials are used as solid support for MIPs layer [37].

It is predictable that the separation property and binding rate would be improved significantly if magnetic separation technique and surface MIT could be combined effectively. In this contribution, Fe3O4 nanoparticles were chosen as magnetic support, and Fe3O4@MIPs were prepared by a semi-covalent imprinting strategy combined with surface MIT for specific recognition of BPA. The specific preparation procedure is illustrated in Scheme 1. Firstly, the covalent template-monomer complex (BPA-IPTS) was synthesized by the direct coupling of phenol moieties and isocynanate groups. Meanwhile, Fe3O4 nanoparticles were also prepared through the co-precipitation of Fe2+ and Fe3+. Secondly, the gelation of BPA-IPTS and tetraethoxysilane was carried out on the surface of Fe3O4 nanoparticles. After the removal of BPA by the thermal cleavage, Fe3O4@MIPs with core-shell structure were obtained, which could rebind selectively BPA. Additionally, the presence of Fe3O4 cores endows MIPs with excellent magnetic separation performance, which could achieve rapid separation during use. The core-shell structure of obtained Fe3O4@MIPs was observed by transmission electron microscope. The successful fabrication of the imprinted sites was verified by a series of binding experiments. The results show that Fe3O4@MIPs could rebind precisely BPA and present fast binding rate as expected. Meanwhile, the presence of Fe3O4 core makes the separation of Fe3O4@MIPs much easier, which could achieve rapid separation with the aid of an external magnetic field and omit the cumbersome procedures of centrifugation or filtration. The excellent binding property and magnetic separation performance provide promising application prospect for Fe3O4@MIPs.

Schematic diagram for the preparation of Fe3O4@MIPs via a semi-covalent imprinting strategy.
Scheme 1.
Schematic diagram for the preparation of Fe3O4@MIPs via a semi-covalent imprinting strategy.

2. Materials and Methods

2.1. Materials

3-isocyanatopropyltriethoxysilane (IPTS), tetrahydrofuran (THF), ammonium hydroxide, isopropanol, tetraethoxysilane (TEOS), absolute ethanol, toluene and dimethyl sulfoxide (DMSO) were all purchased from Beijing Chemical Reagent Co. Ltd. Bisphenol A (BPA), 4, 4’-biphenol (BIP) and p-tert-butylphenol (BP) were all obtained from Aladdin Chemistry Co. Ltd. Hydrazine hydrate, ferric chloride hexahydrate (FeCl3·6H2O) and ferrous sulfate (FeSO4·7H2O) were all supplied by Shanghai Chemical Reagent Co. Ltd. The water used in the experiments was deionized water from ULUPURE purification system.

2.2. Instruments

Transmission electron microscope (TEM, H-800, Hitachi) was used to observe the size and structure of Fe3O4, Fe3O4@BPA-IPTS and Fe3O4@MIPs. X-ray powder diffraction (XRD) spectra of Fe3O4, Fe3O4@NIPs and Fe3O4@MIPs were recorded on a X-ray powder diffractometer (D/max-UltimallI, Rigaku). Fourier transform infrared (FTIR) spectra were recorded on an infrared spectrometer (TENSOR37, BRUKER) using KBr as background spectrum. The 1H NMR was recorded with the help of superconducting ultra-shielded fourier transform nuclear magnetic resonance spectrometer (AVANCE 600, BRUKER). Deuterated chloroform (CDCl3) was used as the solvent of sample in the process of testing. The content of BPA and its structural analogues was measured using a UV-vis spectrophotometer (UV-3150, SHIMADZU).

2.3. Synthesis of BPA-IPTS complex

BPA-IPTS complex was synthesized by coupling reaction based on the literature with minor modifications [21]. Briefly, BPA (2.751 g, 12 mmol) was added to a 100 mL three-necked flask, followed by the addition of 25 mL anhydrous THF. The mixture was subjected to magnetically stirring under nitrogen gas. After complete dissolution, IPTS (5.97 mL, 24 mmol) was added dropwise with the help of a constant pressure dropping funnel under the mechanical agitation. After the addition was completed, the solution was heated to 70°C using an oil bath, and the reaction proceeded continuously for 24 h. After the removal of THF by rotary evaporator, the yellow oily BPA-IPTS complex was obtained.

2.4. Preparation of Fe3O4 nanoparticles

Fe3O4 nanoparticles were prepared through co-precipitation method using ammonium hydroxide as the precipitating agent [38]. Briefly, 5 g of FeCl3·6H2O was added into 50 mL of deionized water, the mixture was stirred magnetically until a uniform brownish-yellow solution was formed. After that, 1 mL of hydrazine hydrate and 2 g of FeSO4·7H2O was added into the above solution successively, followed by magnetically stirring until a uniform yellow green solution was formed. Then, 10 mL of ammonium hydroxide (27%) was added quickly into the homogenous solution. The reaction proceeded for 30 min at 25°C and the generated black product was incubated at 80°C for 1 h. The black product was separated from the solution using the magnet and washed thoroughly with deionized water and absolute ethanol. The bulk black product was obtained after vacuum-dry at 70°C for 24 h. After thorough grinding, Fe3O4 nanoparticles were obtained.

2.5. Preparation of Fe3O4@MIPs

0.4 g of Fe3O4 nanoparticles was added into the mixed solvent composed of 80 mL of isopropanol and 6 mL of deionized water. The obtained mixture was ultrasonicated for 15 min to obtain a uniform dispersion solution. Then, 2 mL of ammonium hydroxide (27%) was added into the dispersion solution, followed by magnetically stirring for 15 min. 0.8 g of BPA-IPTS and 4 mL of TEOS was added into 25 mL of isopropanol, the obtained mixture was magnetically stirred to obtain the homogenous solution. The homogenous solution was added into dropwise the above-mentioned Fe3O4 dispersion solution. After the drip was finished, the reaction proceeded for 8 h at 25°C under magnetic stirring. The obtained product was separated using a magnet and washed thoroughly with deionized water and absolute ethanol, then vacuum-dried for 24 h at 50°C. 0.8 g of above-mentioned product was added into the mixed solvent of 6 mL of deionized water and 30 mL of DMSO, followed by ultrasonic dispersion for 10 min. The obtained mixture was heated up to 160°C and maintained for 5 h under the protection of nitrogen gas. After cooling to room temperature, the obtained product was separated using a magnet and washed with deionized water and absolute ethanol thoroughly. The Fe3O4@MIPs were obtained finally after vacuum drying for 24 h at 25°C. In addition, non-imprinted polymers (Fe3O4@NIPs) were also synthesized, the sole distinction is that BPA-IPTS was excluded from the gelation process.

2.6. Batch binding experiments

In this work, toluene was chosen as the solvent for dissolving BPA and its structure analogues. Firstly, the binding kinetics test was carried out as follows. 20 mg of Fe3O4@MIPs or Fe3O4@NIPs was added into 10 mL of BPA toluene solution with the concentration of 100 mg L-1. The obtained suspensions were incubated through constant temperature shaker for a predetermined time at 25°C. After reaching the scheduled time, Fe3O4@MIPs or Fe3O4@NIPs were separated with the aid of a magnet, respectively. The remaining concentration of BPA in the supernatant was measured by means of UV-vis. The binding capacity of Fe3O4@MIPs or Fe3O4@NIPs at different times could be calculated by subtracting the residual concentration of BPA from its initial concentration using the following formula (1).

(1)
B t = C 0 − C t v / m

where C0 (mg L-1) denotes the initial concentration, Ct (mg L-1) indicates the remaining concentration at the predetermined time, v (mL) represents the solution volume, m (mg) refers to the mass of Fe3O4@MIPs or Fe3O4@NIPs, and Bt is the binding capacity of Fe3O4@MIPs or Fe3O4@NIPs towards BPA at the predetermined time.

The binding isothermal experiment was performed according to the following steps. 20 mg of Fe3O4@MIPs or Fe3O4@NIPs was added into 10 mL of toluene solution containing different BPA concentrations ranging from 20 to 300 mg L-1. The obtained suspensions were incubated through constant temperature shaker for 12 h at 25°C. After the binding reached equilibrium, Fe3O4@MIPs or Fe3O4@NIPs were separated with the aid of a magnet, respectively. The residual BPA content in the supernatant was measured by means of UV-vis. The equilibrium binding capacities of Fe3O4@MIPs or Fe3O4@NIPs were calculated by subtracting the equilibrium concentration of BPA from its initial concentration according to the formula (2).

(2)
B = C 0 − F v / m

where C0 (mg L-1) denotes the initial concentration, F (mg L-1) indicates equilibrium concentration. v (mL) represents the solution volume, m (mg) refers to the mass of Fe3O4@MIPs or Fe3O4@NIPs, and B stands for the equilibrium binding capacity of Fe3O4@MIPs or Fe3O4@NIPs towards BPA.

2.7. Selective experiments

To assess the binding selectivity of Fe3O4@MIPs, BIP and BP were selected as the structure analogues of BPA in the selectivity test. Briefly, 20 mg of Fe3O4@MIPs or Fe3O4@NIPs was added into 10 mL of toluene solutions with 100 mg L-1 of BPA, BIP and BP, respectively. The obtained suspensions were incubated through constant temperature shaker for 12 h at 25°C. After the binding reached equilibrium, Fe3O4@MIPs or Fe3O4@NIPs were separated with the aid of a magnet, respectively. The residual contents of BPA, BIP and BP in the supernatant were determined by means of UV-vis, respectively. The equilibrium binding capacities of Fe3O4@MIPs or Fe3O4@NIPs towards BPA, BIP and BP were calculated by the above formula (2).

3. Results and Discussion

3.1. The preparation of Fe3O4@MIPs

In order to create the well-defined imprinted sites, the semi-covalent imprinting strategy was adopted in this work. The template molecule of BPA could be covalently bound to the functional monomer of IPTS through a thermally reversible carbamate bond, which was achieved by the direct coupling of phenol moieties and isocynanate groups. The precursor, BPA-IPTS, was produced from the coupling reaction of a 1:2 stoichiometric mixture of BPA and IPTS, and the synthesis route is shown in Figure 1. The direct coupling reaction occurs easily, in which no catalyst is needed. After the removal of solvent with the help of rotary evaporator, the target product of BPA-IPTS was obtained. To verify the formation of carbamate bond, BPA, IPTS and BPA-IPTS were all characterized by FTIR, and the spectra are shown in Figure 2. For BPA, the characteristic peaks at 2966 cm-1, 1600 cm-1 and 1510 cm-1 are attributed to the C-H stretching vibration of methyl, C=C skeletal vibration of benzene ring. The C=O stretch of carbamate at 1720 cm-1 is clearly visible in the spectrum of BPA-IPTS. Meanwhile, the isocyanate (N=C=O) stretch of IPTS at 2270 cm-1 disappears in the spectrum of BPA-IPTS, implying that the original N=C=O group of IPTS has been converted to the carbamate. To obtain a more comprehensive characterization of the composition of BPA-IPTS, 1H-NMR was employed and the detailed results are as follows. 1H NMR (600 MHz, CDCl3): δ (ppm) 7.18 (d, 4H, J=8.6 Hz), 7.00 (d, 4H, J=8.4 Hz), 5.41 (t, 2H, J=5.6 Hz), 3.84 (q, 12H, J=7.0 Hz), 3.25 (q, 4H, J=6.5 Hz), 1.87-1.83 (m, 6H), 1.64 (s, 4H), 1.24 (t, 18H, J=7.0 Hz), 0.73-0.64 (m, 4H). 1H NMR results further confirm that BPA-IPTS has been synthesized successfully.

Synthesis route of BPA-IPTS by a direct coupling reaction of BPA and IPTS.
Figure 1.
Synthesis route of BPA-IPTS by a direct coupling reaction of BPA and IPTS.
FTIR spectra of BPA, IPTS and BPA-IPTS.
Figure 2.
FTIR spectra of BPA, IPTS and BPA-IPTS.

Additionally, Fe3O4, Fe3O4@BPA-IPTS and Fe3O4@MIPs were also characterized by FTIR, and the results are presented in Figure 3. For Fe3O4 nanoparticles, the absorption peak at 588 cm-1 is attributed to the stretching vibration of Fe-O. The absorption peak around 3424 cm-1 is assigned to the stretching vibration of -OH, which is generated by the active -OH groups located at the surface of Fe3O4 nanoparticles, resulting from the strong surface energy and hygroscopicity of Fe3O4 nanoparticles. The peak at 1633 cm-1 belongs to the bending vibration of -OH. Compared with Fe3O4 nanoparticles, Fe3O4@BPA-IPTS exhibits a stretching vibration absorption peak of -C=O at 1712 cm-1, indicating that the BPA-IPTS complex has been coated on the surface of Fe3O4 nanoparticles. For Fe3O4@MIPs, the disappearance of the -C=O stretching vibration absorption peak at 1712 cm-1 suggests that the template molecule of BPA has been removed completely.

FTIR spectra of Fe3O4, Fe3O4@BPA-IPTS and Fe3O4@MIPs.
Figure 3.
FTIR spectra of Fe3O4, Fe3O4@BPA-IPTS and Fe3O4@MIPs.

To determine the influence of MIPs shell layer on the crystal structure, Fe3O4, Fe3O4@MIPs and Fe3O4@NIPs were investigated by XRD, and the XRD patterns are shown in Figure 4. It could be found that Fe3O4 exhibits 6 distinguishable characteristic peaks, which appear at 30.0o, 35.5o, 43.2o, 53.6o, 57.1o, 62.7o, corresponding to the diffraction planes of (220), (311), (400), (422), (511) and (440) located the inverse spinel structure of Fe3O4, indicating that the Fe3O4 prepared by the co-precipitation method has a standard inverse spinel crystal structure. The peaks of Fe3O4@MIPs and Fe3O4@NIPs are basically consistent with the characteristic peaks of Fe3O4, the only difference is that the intensity of the peaks weakens, indicating that MIPs and NIPs shell layer does not affect their crystal structure.

XRD patterns of (a) Fe3O4, (b) Fe3O4@NIPs and (c) Fe3O4@MIPs.
Figure 4.
XRD patterns of (a) Fe3O4, (b) Fe3O4@NIPs and (c) Fe3O4@MIPs.

The morphology of Fe3O4 nanoparticles, Fe3O4@BPA-IPTS and Fe3O4@MIPs was all observed by TEM, and the images are shown in Figure 5. It is clearly shown that the Fe3O4 nanoparticles appear spherical in shape, and the particle size is between 25 and 40 nm. In addition, it could be found that there is a relatively serious agglomeration phenomenon, which results from the small size and large surface energy of Fe3O4 nanoparticles. The Fe3O4@BPA-IPTS exhibits a typical core-shell structure with a shell thickness of about 10 nm. The morphology and structure of Fe3O4@MIPs does not show an obvious difference with that of Fe3O4@BPA-IPTS, Fe3O4 core and MIPs shell layer could be clearly distinguished from the image, indicating that the high temperature process during template removal has no impact on the imprinted polymer shell layer. These results indicate that Fe3O4@MIPs have been prepared successfully.

TEM images of (a) Fe3O4, (b) Fe3O4@BPA-IPTS and (c) Fe3O4@MIPs.
Figure 5.
TEM images of (a) Fe3O4, (b) Fe3O4@BPA-IPTS and (c) Fe3O4@MIPs.

Figure 6 shows the magnetic separation phenomenon of Fe3O4@MIPs under the action of an external magnetic field. When an external magnetic field is not present, Fe3O4@MIPs particles are uniformly dispersed in the toluene solution and form a brown suspension. When an external magnetic field is imposed, Fe3O4@MIPs particles are quickly attracted to the side of the bottle wall where the external magnetic field is present, and the toluene solution instantly becomes transparent. These results indicate that Fe3O4@MIPs have good magnetic responsiveness, which facilitates the separation of Fe3O4@MIPs particles from solution and therefore simplifies the post-processing procedures.

(a) The dispersion and separation of Fe3O4@MIPs in the absence and (b) presence of an external magnet.
Figure 6.
(a) The dispersion and separation of Fe3O4@MIPs in the absence and (b) presence of an external magnet.

3.2. Rebinding kinetics and isotherm

The core-shell structure make the imprinted sites of Fe3O4@MIPs distribute in the outer layer, which exhibits low mass transfer resistance and could promote effectively the removal and rebinding of template molecules. Hence, Fe3O4@MIPs are expected to exhibit rapid rebinding kinetics. Figure 7 shows the rebinding kinetics curves of Fe3O4@MIPs and Fe3O4@NIPs. It could be observed clearly that the amounts of BPA adsorbed by Fe3O4@MIPs and Fe3O4@NIPs at different time show a similar change trend, but the binding capacity of Fe3O4@MIPs towards BPA is significantly higher than that of Fe3O4@NIPs at every detection time point, implying the successful construction of high affinity BPA imprinted sites. In addition, it could be also found that the binding process of BPA on Fe3O4@MIPs is obviously divided into two stages: a rapid growth period (the initial 60 min) and an equilibrium period. The equilibrium binding capacity could be achieved in the rapid growth period, which is attributed to the presence of a large number of accessible imprinted sites. After 60 min, the binding process enters the equilibrium period and the binding capacity no longer shows a visible change, which results from the reduction of residual BPA concentration and the decrease in the number of unoccupied imprinted sites.

Binding kinetics curves of Fe3O4@MIPs and Fe3O4@NIPs.
Figure 7.
Binding kinetics curves of Fe3O4@MIPs and Fe3O4@NIPs.

To gain a deeper understanding of the binding property of Fe3O4@MIPs, the equilibrium binding tests under varying initial BPA concentrations were conducted, and the binding capacities of Fe3O4@MIPs and Fe3O4@NIPs towards BPA are presented in Figure 8. The two curves are both drawed by plotting the equilibrium binding capacity against initial BPA concentration. It could be found clearly that the saturated binding capacities of Fe3O4@MIPs and Fe3O4@NIPs both increase gradually with the increase of the initial BPA concentration. Although the binding curve of Fe3O4@MIPs displays a similar variation tendency to that of Fe3O4@NIPs, the binding capacities of Fe3O4@MIPs are significantly higher than that of Fe3O4@NIPs over the whole tested concentration range. Furthermore, it also could be found that the difference value of binding capacity between Fe3O4@MIPs and Fe3O4@NIPs becomes much more prominent as the initial BPA concentration increases. These results indicate that the imprinted sites have been built successfully on the surface of Fe3O4 nanoparticles.

The saturated binding capacities of Fe3O4@MIPs and Fe3O4@NIPs under different initial BPA concentration.
Figure 8.
The saturated binding capacities of Fe3O4@MIPs and Fe3O4@NIPs under different initial BPA concentration.

3.3. Selectivity analysis

The selective binding property of MIPs towards target molecule is an important indicator for evaluating the molecular recognition performance of materials. To investigate the binding selectivity of Fe3O4@MIPs, the selective binding tests were conducted using BIP and BP as the structure analogs of BPA. Figure 9 presents the selective binding capacities of Fe3O4@MIPs and Fe3O4@NIPs towards BPA, BIP and BP. It could be found that Fe3O4@MIPs exhibit certain binding capacities for the three compounds, but their binding capacities towards BIP and BP are significantly lower than that towards BPA, which follow an order of BPA > BIP > BP. The result is attributed to the presence of abundant BPA imprinted sites on the surface of Fe3O4@MIPs. These imprinted sites are completely complementary to BPA in terms of shape, size and structure, thus showing highly selective binding performance towards BPA. The difference values in binding capacities between Fe3O4@MIPs and Fe3O4@NIPs for BPA, BIP and BP are 6.01, 1.10 and 0.14 mg g-1 with corresponding imprinting factors of 1.74, 1.17 and 1.02. It is indisputable that Fe3O4@MIPs present the largest binding capacity and imprinting factor towards BPA. These results indicate that Fe3O4@MIPs have good binding selectivity towards BPA.

The binding capacities of Fe3O4@MIPs and Fe3O4@NIPs towards BPA, BIP and BP, respectively.
Figure 9.
The binding capacities of Fe3O4@MIPs and Fe3O4@NIPs towards BPA, BIP and BP, respectively.

4. Conclusions

In this work, the semi-covalent imprinting strategy and surface MIT were combined to prepare Fe3O4@MIPs with excellent binding performance and rapid separation property. Fe3O4 nanoparticles were prepared via co-precipitation of Fe2+ and Fe3+, which endowed Fe3O4@MIPs with excellent magnetic separation property. BPA-IPTS was synthesized through the direct coupling reaction of BPA and IPTS. The gelation of BPA-IPTS and TEOS was performed on the surface of Fe3O4 nanoparticles, followed by the removal of BPA by the thermal cleavage, which built specific recognition sties. The fabrication of Fe3O4@MIPs was verified by FTIR spectrum and TEM image. The rebinding experiment results indicate that Fe3O4@MIPs display rapid binding rate, which could reach binding equilibrium within 60 min because of the presence of core-shell structure. In addition, Fe3O4@MIPs also display outstanding binding specificity towards BPA with the imprinting factor of 1.74. This study offers an alternative approach for the building of Fe3O4@MIPs towards BPA by semi-covalent imprinting strategy, and Fe3O4@MIPs also exhibit the application potential in the domains of environmental protection and detection.

Acknowledgment

This work was funded by the Central Government Guides Local Science and Technology Development Fund (Grant No. 206Z1201G), the 333 Talent Project of Hebei Province (Grant No. C20231113) and the Natural Science Foundation of Hebei Province (Grant No. E2023402024) and Natural Science Foundation of Hebei Province (Grant No. E2023402024).

CRediT authorship contribution statement

Zehu Wang: Writing - original draft, Investigation, Conceptualization; Xiaoliang Zhang: Formal analysis. Software; Yanming Wang:Writing - review & editing, Supervision; Xuan Yue: Project administration, Funding acquistion.

Declaration of competing interest

There are no conflicts of interest.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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