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Article

Preparation of Molecularly Imprinted Magnetic Stir Bar for Bisphenol A and Its Analysis on Trace Bisphenol A in Actual Water Samples

by
Jing Wang
1,
Zhehao Wang
2,
Zhonglin Chen
2,
Pengwei Yan
2,*,
Jimin Shen
2 and
Jing Kang
2,*
1
AECC Shenyang Engine Research Institute, Shenyang 110015, China
2
State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China
*
Authors to whom correspondence should be addressed.
Water 2023, 15(19), 3361; https://doi.org/10.3390/w15193361
Submission received: 14 August 2023 / Revised: 17 September 2023 / Accepted: 21 September 2023 / Published: 25 September 2023
(This article belongs to the Special Issue Future Directions and Opportunities of Advanced Oxidation Technologies for Water Treatment)
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Abstract

:
In this paper, a new method for the preparation of a molecularly imprinted polymer (MIPs)-coated magnetic stir bar for bisphenol A (BPA) is proposed. The MIPs were prepared using BPA as the template molecule, and the sol-gel technique was employed to coat the MIPs onto the surface of a glass tube, which contained an internal magnetic core. The morphology and structure of the MIPs and the coating on the glass stir bar were analyzed using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Parameters affecting the extraction efficiency, such as extraction time, stirring speed, desorption solvent, and desorption time, were optimized. To evaluate the selective adsorption performance of the BPA-MIPs coating, molecularly imprinted stir bar sorptive extraction (MIPs-SBSE) was used in combination with high-performance liquid chromatography (HPLC) for the detection of BPA in deionized water, tap water, bottled water, and reservoir water. The results showed that under the optimized conditions, the MIPs coating exhibited better adsorption capacity and selectivity for BPA compared to the non-imprinted coating. The BPA-MIPs-SBSE could be reused for at least five cycles without a significant decrease in its selective adsorption ability. The recoveries of BPA in the four actual water samples ranged from 70.53% to 93.10%, with relative standard deviations of 4.49% to 8.69%. The practical application demonstrated that this method is simple, convenient, selective, and sensitive, making it suitable for the analysis and detection of trace amounts of BPA in complex samples.

1. Introduction

Endocrine disruptors are a class of chemicals that can potentially interfere with the normal function of the endocrine system in humans and animals. They exist in two forms in the natural environment: naturally occurring and synthetic [1]. Bisphenol A (BPA), as a typical endocrine disruptor, can have adverse effects on animal reproductive development [2]. In addition, human exposure to BPA can lead to sexual dysfunction, changes in immune function, type 2 diabetes, cardiovascular diseases, obesity, and liver function disorders [3,4]. On the other hand, BPA is an important raw material in the chemical industry, mainly used in the production of polycarbonates and epoxy resins. These polymers are widely used in the production of reusable everyday items such as food and beverage containers, baby bottles, electrical and electronic devices, sports safety equipment, automobiles, digital media, tableware, and medical devices, among others [5]. Due to the fact that these plastic products can release BPA when they are landfilled or degrade, BPA can be detected in water and environmental samples. Therefore, the detection of trace amounts of BPA in samples is a highly important task.
Currently, the main analytical methods for detecting BPA include high-performance liquid chromatography (HPLC), gas chromatography (GC), chemiluminescence (CL), and enzyme-linked immunosorbent assay (ELISA). However, due to the complexity of real samples and the presence of trace amounts of BPA in various forms, appropriate sample pretreatment is necessary before applying these analytical methods [4,5,6]. Existing sample pretreatment techniques include solid-phase extraction (SPE), liquid-liquid extraction, stir bar sorptive extraction (SBSE), solid-phase microextraction (SPME), liquid-phase microextraction, dispersive liquid-liquid extraction, matrix solid-phase dispersion, and ultrasound-assisted extraction [4,5,6]. Among these techniques, SBSE has gained wide popularity as a novel and effective sample pretreatment method. Its advantages over SPME lie in its larger adsorption capacity and higher extraction efficiency. SBSE can achieve the enrichment of target analytes through self-stirring, thus avoiding the competitive adsorption brought by external stirring. Despite being proposed as early as 1999, SBSE has made significant progress only in the past decade. In its early applications, PDMS was used as the coating material. PDMS, being nonpolar, exhibited good affinity for nonpolar compounds but weaker affinity for polar compounds. Therefore, researchers have proposed several new types of stir bar coatings, such as composite coatings [7,8,9], biphasic coatings [10,11], polymer phase coatings [12,13,14,15,16], integral coatings [17,18,19], restricted access material coatings [20], cation exchange membrane coatings [21], zirconia coatings [22], C18 coatings [23], graphene [24], and polydimethylsiloxane/metal-organic framework coatings [25]. However, the application of these new coatings has been limited by their lack of selectivity.
Molecularly imprinted polymers (MIPs) have the advantage of selective recognition of specific target analytes. This type of polymer was first proposed by Wulff et al. and has rapidly developed in areas such as pharmaceutical analysis [26], biosensors [27], and enzyme catalysis [28]. Nowadays, MIPs are widely applied in sample pretreatment processes due to their high selectivity in complex samples. There are also reports of using MIPs as stir bar coatings for trace target analyte adsorption and extraction. However, with most extraction techniques using MIPs as extraction coatings, such as stir bar sorptive extraction and solid-phase microextraction, it is challenging to form uniform and mechanically robust coatings during the preparation process.
In this study, a sol-gel method was successfully used to obtain a homogeneous and stable MIPs coating on the surface of a stir bar. The adsorption and selectivity of the MIPs-coated magnetic material for BPA were investigated. Furthermore, the MIPs-coated stir bar was utilized for the adsorption and extraction of trace BPA from spiked real water samples, followed by successful determination using HPLC. This approach allows for the efficient and selective analysis of trace BPA in complex environmental water samples.

2. Materials and Methods

2.1. Materials

BPA (bisphenol A, 98%) was purchased from TCI company in Japan. MAA (methacrylic acid, 99%), hydroxyl-terminated poly (dimethylsiloxane), and methanol (chromatographic grade) were purchased from Sigma company. EGDMA (ethylene glycol dimethacrylate), AIBN (azobisisobutyronitrile), DVB (divinylbenzene, 80%), MTMS (methyltrimethoxysilane, 98%), and poly (methylhydrosiloxane) were purchased from Aladdin company. TFA (trifluoroacetic acid, 95%) was purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd., Shanghai, China. Dichloromethane and sodium hydroxide were provided by Tianjin Fuyu Fine Chemical Co., Ltd., Tianjin, China. Other reagents used were of analytical grade, and all the experimental water used was Milli-Q water.

2.2. Experimental Equipment

Sample analysis was conducted using a high-performance liquid chromatography system equipped with a UV detector (HPLC/UV, 1200, Agilent Technologies, Santa Clara, CA, USA). All samples were separated using a C18 column (150 × 4.6 mm). The mobile phase consisted of methanol/water (70/30, v/v), and the flow rate was set at 1 mL/min. The detection wavelength was set at 280 nm. Infrared spectra were obtained using a Fourier-transform infrared spectrometer (FT-IR, Spectrum One, PerkinElmer, Waltham, MA, USA). The morphology of the MIPs and coating was observed using a scanning electron microscope (SEM, S-4800/350, Hitachi Ltd., Tokyo, Japan).

2.3. Analysis

2.3.1. The Preparation Method for Molecularly Imprinted Materials (MIPs)

The template molecule (BPA) and functional monomer (MAA) are mixed in a certain amount of solvent in a ratio of 1:15 and sonicated for 5 min. Then, the cross-linker, in a ratio of 1:30 to the template molecule, and a suitable amount of initiator are added, followed by another 5 min of sonication. After purging with nitrogen gas for 5 min to remove oxygen, the mixture is sealed. It is then heated at 60 °C in a water bath for 24 h to obtain a white powder. To ensure uniformity of the powder, the obtained product is ground in a mortar. To wash the bisphenol A molecularly imprinted polymers (BPA-MIPs), a mixture of methanol and 1 mol/L sodium hydroxide (in a volume ratio of 3:1) is used as the elution solution until no BPA is detected. Finally, the pH of the elution solution is adjusted to neutral using a mixture of methanol and water.
For the preparation of non-imprinted polymers (NIPs), the same method as for BPA-MIPs is followed, except that the template molecule BPA is not added.

2.3.2. The Pre-Treatment Method for a Glass Stirring Rod

After cleaning the glass rod with distilled water and dichloromethane, it should be soaked in a 1 mol/L sodium hydroxide solution for 24 h. After removal, rinse the rod with water, then soak it in a 0.1 mol/L hydrochloric acid solution for 1 h. Remove it, rinse it with water until the pH is 7, then dry it at 60 °C for 3 h.

2.3.3. The Preparation Method of a Magnetic Molecularly Imprinted Stirring Rod

Add 10 mg of bisphenol A molecularly imprinted polymer (BPA-MIPs), 100 mL of hydroxyl-terminated polydimethylsiloxane (OH-PDMS), 50 μL of divinylbenzene (DVB), 200 μL of dichloromethane (CH2Cl2), 100 μL of methyltrimethoxysilane (MTMS), 20 μL of hydrogen-containing silicone oil (PMHS), and 100 μL of trifluoroacetic acid (TFA) into a reaction vial. Sonicate the mixture for 5 min to completely dissolve the BPA-MIPs, forming a milky suspension. Immerse a pre-treated stirring rod into the milky suspension for the sol-gel process. After removal, immediately blow-dry the stirring rod using nitrogen gas for the gelation process. Repeat the impregnation coating steps 4–5 times until the surface of the stirring rod is uniformly coated with a certain thickness. Then, place the coated stirring rod in a 60 °C oven for 3 h to fix the surface coating. Before using the stirring rod, sonicate it in methanol for 10 min.
Immerse the stirring rod in a methanol solution and sonicate it for 20 min to remove surface impurities. In order to investigate the selectivity performance of the molecularly imprinted coating on the stirring rod, place the stirring rod with the molecularly imprinted (or non-imprinted) coating into a 50 mL conical flask. Add 20 mL of BPA solution with a concentration of 500 μg/L to each conical flask. After 100 min of adsorption experiment at room temperature, remove the stirring rod and place it into a small bottle containing 2 mL of desorption solvent. Sonicate for a certain period of time to completely release the adsorbed BPA from the stirring rod. Filter the desorption solution through a 0.22 μm membrane filter and analyze the BPA content using HPLC.
To prepare the standard solutions, add BPA to deionized water, tap water, bottled water, and reservoir water, respectively, to form two concentration levels of spiked solutions: 500 μg/L and 2000 μg/L. Immerse the BPA-MIPs-SBSE (bisphenol A molecularly imprinted polymer solid-phase microextraction) into the spiked solutions and perform adsorption under optimized conditions with stirring. After adsorption, remove the stirring rod and immerse it in a suitable amount of desorption solvent. Apply ultrasound to extract BPA from the stirring rod. Filter the desorption solution through a 0.22 μm filter membrane and analyze the extracted BPA concentration using HPLC (high-performance liquid chromatography).

3. Results and Discussion

3.1. Characterization of Molecularly Imprinted Polymers (MIPs) and Coatings

3.1.1. Scanning Electron Microscopy

Figure 1 shows SEM images of BPA-MIPs and BPA-NIPs at a magnification of 2000 times. It can be observed that both BPA-MIPs and BPA-NIPs prepared in the experiment have irregular elliptical shapes with an average particle size of 20.0 μm. Moreover, the surfaces of the elliptical particles are cross-linked and contain numerous cavities. Among them, Figure 1a represents BPA-MIPs, while Figure 1b represents BPA-NIPs. A comparison between the two images reveals that BPA-MIPs have significantly more cavities than BPA-NIPs. It is the presence of these cavities that contributes to the higher selective binding of BPA-MIPs towards BPA compared to BPA-NIPs. Please note that SEM images provide information about the morphology and structure of the materials but do not provide precise details about their chemical composition. To obtain a more comprehensive understanding of the materials’ characteristics, additional characterization techniques such as FT-IR analysis, thermal analysis, pore size, and surface area analysis should be employed. The combined application of these techniques can provide a more comprehensive assessment of the performance and properties of BPA-MIPs and BPA-NIPs.
Figure 2 shows SEM images of BPA-MIPs-SBSE coating (a) and BPA-NIPs-SBSE coating (b) at a magnification of 40,000 times. It can be observed that the molecularly imprinted stir bar sorptive extraction (MIPs-SBSE) coatings prepared using the sol-gel method exhibit noticeable cavity distribution on their surfaces. These cavities are capable of effectively binding the target analyte, BPA, enabling efficient adsorption and extraction of BPA. Among them, Figure 2a represents the BPA-MIPs-SBSE coating, while Figure 2b represents the BPA-NIPs-SBSE coating. A comparison between the two images clearly shows that the cavity distribution on the surface of BPA-MIPs-SBSE is more uniform than that of BPA-NIPs-SBSE, ensuring better adsorption and extraction of the target analyte.
SEM images provide a visual representation of the uniform distribution of cavities with specific selectivity for the target analyte, bisphenol A (BPA), on the surface of the molecularly imprinted stir bar sorptive extraction (MIPs-SBSE) coating. These cavities enable the effective binding of the target analyte, facilitating the adsorption and extraction of BPA.

3.1.2. Fourier Transform Infrared Spectroscopy

Figure 3 presents the Fourier transform infrared (FTIR) spectra of BPA and its molecularly imprinted polymers (MIPs). The spectra are as follows: (a) represents the FTIR spectrum of BPA; (b) represents the FTIR spectrum of the unwashed BPA-MIPs (molecularly imprinted polymers); (c) represents the FTIR spectrum of the washed BPA-MIPs; (d) represents the FTIR spectrum of BPA-NIPs (non-imprinted polymers).
In Figure 3a, several absorption peaks can be observed in the range of 3300–3000 cm−1, which corresponds to the stretching vibration of unsaturated carbon-hydrogen (C-H) bonds. Additionally, characteristic absorption peaks of the benzene ring in aromatic compounds can be seen at wavelengths such as 1598 cm−1, 1509 cm−1, and 1446 cm−1. In the range of 3500–3200 cm−1, there is a broad absorption peak attributed to the stretching vibration of hydrogen bonds (H-O) between BPA molecules. This peak overlaps with the absorption peak from the stretching vibration of unsaturated C-H bonds in the range of 3300–3200 cm−1, resulting in a larger absorption peak. The stretching absorption peak of the -CH3 group in BPA molecules can be observed at 2870 cm−1.
Comparing Figure 3b–d, it can be observed that in Figure 3b,c, there is a broad absorption peak in the range of 3500–3200 cm−1, which corresponds to the stretching vibration of hydrogen bonds (H-O) between BPA molecules. However, this characteristic peak is clearly not present in Figure 3d. This indicates that the template molecule BPA and the functional monomer MAA are polymerized together due to a large number of hydrogen bond interactions. Comparing Figure 3b,c, it can be noticed that the hydrogen bond interactions between the BPA-MIPs decrease after template washing. Absorption peaks of saturated C-H stretching vibrations are observed in the range of 3000–2950 cm−1 in Figure 3b–d. This indicates that the double bond (C=C) on the functional monomer MAA is opened and interacts with the crosslinking agent EGDMA during the polymerization process, forming saturated C-H bonds.
In conclusion, during the synthesis of molecularly imprinted polymers, the template molecule and the functional monomer are bound together through intermolecular hydrogen bond interactions. It is the presence of these interactions that allows the resulting molecularly imprinted polymers to possess a high degree of selectivity and recognition towards the target molecule BPA. Additionally, the functional monomer MAA plays a crucial role by opening the other end of the molecule’s C=C bond and forming a stronger attachment with the crosslinking agent, thereby improving the fixation of the template molecule.

3.1.3. Brunauer–Emmett–Teller Analysis

Figure 4 shows the Brunauer–Emmett–Teller (BET) analysis of the coating of the bisphenol A molecular imprinting stirring rod. According to the analysis results of specific surface area and pore volume, the BET specific surface area of the prepared material is 4.0 m2/g, and the total pore volume is 0.005 cm3/g.

3.2. Scatchard Model Analysis of Molecularly Imprinted Stirring Bar

The Scatchard model is a graphical method proposed by Scatchard in 1949 for analyzing the binding relationship between ions, drugs, and other molecules with proteins (including receptors). In this study, the model is used to analyze the binding relationship between the template molecule (BPA) and molecularly imprinted sites, in order to determine the maximum apparent binding capacity (Qm) of the molecularly imprinted sites for BPA.
In the Scatchard equation, the relationship between qe/Ce and qe is linear. By performing a linear fit, one can determine the equilibrium dissociation constant (Kd) of the binding sites and the maximum apparent binding capacity (Qm) of the binding sites. With Kd and Qm, the theoretical cavity content generated by the imprinting adsorbent can be determined. The Scatchard equation fitting of the magnetic molecularly imprinted stir bar for bisphenol A (BPA-MIPs-SBSE) is shown in Figure 5. As determined from Scatchard Equation, Qm = 33.79 μmol/g, and Kd = 1.828 μmol/L.
This indicates that in the preparation of BPA-MIPs-SBSE, the appropriate amounts of template molecule BPA, functional monomer MAA, and cross-linker EGDMA were used. MAA effectively binds BPA and forms functional imprinting sites. At the same time, BPA was completely removed during the elution process, allowing the imprinting cavities to form, thus successfully binding BPA in water samples.

3.3. Synthesis Mechanism of Bisphenol A Molecularly Imprinted Polymers

BPA (as the template) molecule contains two phenolic hydroxyl groups. Both of these groups can form hydrogen bonds with suitable residues in a non-protonic solvent. As a result, they can bind to the functional monomer methacrylic acid (which possesses a carboxyl group capable of hydrogen bonding) and achieve a high affinity and selectivity for BPA molecules. The other end of the functional monomer contains a C=C double bond structure, which, after the addition of a cross-linker, will open up and bind with the cross-linker. After the template molecule is removed, the entire polymer network should be uniformly distributed with effective imprinting sites. In this case, the chosen cross-linker is ethylene glycol dimethacrylate, whose chemical structure is similar to that of the methacrylic acid functional monomer. Therefore, their reactivity is similar, ensuring that the functional monomer or cross-linker does not preferentially polymerize over the other, resulting in the desired product with uniformly distributed active groups.
The hydrogen bonding between the functional monomer and the template molecule is disrupted by using a 1 mol/L NaOH solution in methanol. This allows the BPA bound to the imprinting sites to be eluted, creating cavities at the positions where BPA was bound. These cavities can then rebind with BPA from complex samples through intermolecular hydrogen bonding once again.

3.4. Molecular Imprinting Stir Bar Sorptive Extraction Conditions Optimization

3.4.1. Influence of Adsorption Time

The adsorption principle of SBSE is similar to that of SPME, both of which are based on equilibrium adsorption. The adsorption performance depends on the distribution coefficient of the target analyte between the adsorption phase and the aqueous phase. The time required to achieve adsorption equilibrium depends on the mass transfer rate of the target analyte itself, as well as factors such as the type and properties of the coating. In this experiment, the influence of adsorption time on the adsorption performance of BPA was investigated within the range of 20–120 min. The results are shown in Figure 6. It can be observed that the target analyte BPA reached adsorption equilibrium after 100 min; therefore, 100 min was selected as the optimal adsorption time for subsequent experiments.

3.4.2. Influence of Adsorption Temperature

Temperature is an important factor to consider in the adsorption process, as increasing the temperature can accelerate mass transfer between molecules and shorten the time required to reach adsorption equilibrium. In this experiment, five different temperatures were set, 15 °C, 20 °C, 25 °C, 30 °C, and 35 °C, to investigate their effect on adsorption efficiency. The results are shown in Figure 7. It can be observed that as the temperature increases from 15 °C to 25 °C, there is a significant improvement in the removal rate of BPA. However, further increasing the temperature does not show a noticeable change in the removal rate. Considering the energy consumption associated with heating, the optimal adsorption temperature for this experiment is determined to be 25 °C, which is room temperature.

3.4.3. Influence of Stirring Speed and PH

In order to shorten the time required to reach adsorption equilibrium, the molecular mass transfer rate can be accelerated through stirring. This experiment investigated the effect of stirring speed on the adsorption efficiency of BPA within the range of 500–900 rpm. The results are shown in Figure 8. The target analyte BPA in this experiment contains phenolic hydroxyl groups, and the pH of the solution can affect its chemical form, thereby impacting the adsorption efficiency of the molecularly imprinted coating towards the target analyte. Therefore, this experiment examined the effect of solution pH on the adsorption efficiency of BPA within the range of 4–8. The results are shown in Figure 9. It can be observed that regarding the adsorption efficiency of the target analyte BPA, the effect of agitation speed on the adsorption efficiency is not significant. The situation is similar with regard to PH, so PH has little effect on the adsorption effect.

3.4.4. Influence of Desorption Time

The selected mixture was used as the desorption solvent, and the desorption efficiency within the range of 5–25 min of desorption time was investigated. The results are shown in Figure 10. It can be observed that within the range of 5–20 min, the desorption efficiency increases with increasing desorption time. After 20 min, the desorption reaches a steady state. Therefore, the desorption time is selected as 20 min.

3.5. Selective Adsorption of Molecularly Imprinted Stir Bars

In order to better study the adsorption characteristics of BPA-MIPs-SBSE and BPA-NIPs-SBSE for BPA in aqueous solutions, the equilibrium experimental data of BPA adsorbed by BPA-MIPs-SBSE and BPA-NIPs-SBSE were fitted to the Langmuir adsorption isotherm equation, and the correlation coefficient (R2) was used to evaluate the fitting degree of each adsorption process to the equation. The fitting results are shown in Figure 11.
From Figure 11 and Table 1, it can be observed that both BPA-MIPs-SBSE and BPA-NIPs-SBSE exhibit good fitting to the Langmuir adsorption isotherm for BPA (with R2 values of 0.960 and 0.941, respectively). The Langmuir adsorption coefficients were determined to be 0.031 and 0.015 L/μmol for BPA-MIPs-SBSE and BPA-NIPs-SBSE, respectively. The maximum adsorption capacity of BPA-MIPs-SBSE was found to be 35.19 μmol/g, while for a BPA aqueous solution of 500 μg/L, the maximum adsorption capacity was 3.545 μmol/g. On the other hand, the maximum adsorption capacity of BPA-NIPs-SBSE was determined as 18.92 μmol/g, with a maximum adsorption capacity of 1.963 μmol/g for a BPA aqueous solution of 500 μg/L.
In conclusion, based on the correlation coefficient R2, it can be seen that within the studied concentration range, BPA-MIPs-SBSE and BPA-NIPs-SBSE exhibit the best fit to the Langmuir adsorption isotherm equation for BPA. This indicates that the adsorption capacity of BPA-MIPs-SBSE and BPA-NIPs-SBSE for BPA is uniform across their surfaces, suggesting a monolayer adsorption process. This also indirectly suggests that the imprinting sites on the surface of BPA-MIPs-SBSE are evenly distributed. When the adsorbate concentration on its surface does not continue to increase, it can be determined that adsorption has reached saturation. The maximum adsorption capacity of BPA-MIPs-SBSE for BPA is twice that of BPA-NIPs-SBSE. Therefore, it can be observed that BPA-MIPs-SBSE exhibits good binding ability and selective adsorption characteristics towards BPA. Comparison with non-imprinted materials confirms that the imprinting process indeed enhances the adsorption performance of BPA-MIPs-SBSE for BPA.

3.6. Selectivity of Molecularly Imprinted Magnetic Stir Bar for Adsorption of Bisphenol A

Relevant research has shown that molecularly imprinted stirring rods have a certain adsorption effect on compounds that are similar in chemical structure and properties to the template molecules [29,30,31]. In order to further investigate the selective adsorption properties of the bisphenol A molecularly imprinted magnetic stirring rod, three phenolic substances, phenol, 2,4,6-trichlorophenol (2,4,6-TCP), and tetrabromobisphenol A (TBBPA), were selected as interfering substances in the experiment. The competitive adsorption of the three phenols on BPA was analyzed in single system, binary mixed system, and ternary mixed system. The adsorption effects of bisphenol A molecularly imprinted stir bar (BPA-MIPs-SBSE) and non-molecularly imprinted stir bar (BPA-NIPs-SBSE) on four phenols (BPA, phenol, 2,4,6-TCP, and TBBBPA) with the same initial concentration were tested, as shown in Figure 12 and Figure 13. Under the same conditions, the removal rates of BPA, phenol, 2,4,6-TCP, and TBBPA by BPA-MIPs-SBSE were 86.2%, 15.34%, 36.78%, and 34.7%, respectively, and the adsorption capacities were 3.48 μmol/g, 0.58 μmol/g, 0.15 μmol/g, and 1.19 μmol/g. The removal rates of BPA, phenol, 2,4,6-TCP, and TBBPA by BPA-NIPs-SBSE were 24.7%, 7.88%, 36.09%, and 21.6%, respectively, and the adsorption capacities were 1.89 μg/g, 0.49 μg/g, 1.83 μg/g, and 0.49 μg/g, respectively. By comparing the adsorption effects of BPA-MIPs-SBSE and BPA-NIPs-SBSE on four phenols, it was found that the removal rate and adsorption capacity of BPA-MIPs-SBSE for BPA were much higher than those of the other three phenols, and also higher than those of BPA-NIPs-SBSE for the four phenols. This suggests that there are sites on the surface of BPA-MIPs-SBSE that can recognize the template molecules, thereby improving its adsorption effect for BPA.

3.7. Water Sample Analysis

There are currently many analytical methods for the determination of bisphenol A in actual water samples [32,33,34]. The practicality of the BPA-MIPs-SBSE-HPLC analysis method was evaluated by determining the concentration of BPA in real water samples. Under optimized conditions, the BPA content in the real water samples, including deionized water, tap water, bottled water, and reservoir water, was measured using the BPA-MIPs-SBSE-HPLC analysis method. The results showed that BPA was not detected in any of the four water samples. To evaluate the accuracy of the method, spike recovery experiments were conducted by adding BPA to the four types of water samples to create spiked samples at concentrations of 500 μg/L and 2000 μg/L. After the BPA-MIPs-SBSE adsorption process, HPLC analysis was performed, and the results are shown in Table 2. The recovery rates for the two concentrations in the four water samples ranged from 71% to 93%, with relative standard deviations ranging from 4.5% to 8.7%. These experimental results demonstrate the reliability of the method for the analysis of BPA in complex samples.

4. Conclusions

A novel magnetic molecularly imprinted polymer (MIP)-coated stir bar was prepared in this study and combined with high-performance liquid chromatography (HPLC) for the detection and analysis of trace amounts of bisphenol A (BPA) in real water samples. Under optimal conditions, the maximum adsorption capacity of BPA-MIPs-SBSE was twice that of BPA-NIPs-SBSE. This indicates that BPA-MIPs-SBSE exhibits superior binding affinity and selective adsorption characteristics compared to non-imprinted coatings. In conclusion, the method proposed in this study is simple, highly sensitive, selective, environmentally friendly, and particularly suitable for the determination of trace amounts of BPA in complex samples.

Author Contributions

J.W., Z.W., Z.C., P.Y., J.S. and J.K. contributed to the conception and design of the study; data curation, J.W.; formal analysis, J.W., Z.W. and P.Y.; funding acquisition, J.K. and P.Y.; investigation, J.K.; methodology, J.W. and J.K.; project administration, Z.C.; supervision, J.S., Z.C. and P.Y.; writing—original draft, J.W. and Z.W.; writing—review and editing, J.S., Z.C., P.Y. and J.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was jointly supported by the National Key Research and Development Pro-gram of China (Grant No. 2022YFD3203701), The National Natural Science Foundation of China (Grant No. 51608148), Heilongjiang Province Postdoctoral Fund (Grant No. LBH-Z22142), the Heilongjiang Touyan Innovation Team Program (Grant No. HIT-SE-01), the Science Foundation of Heilongjiang Academy of Science (No. KY2022ZR04), the Science Foundation of Heilongjiang Provincial Institute (CZKYF2023-1-B036), and the Central Government Guides Local Science and Technology Development Projects (No. ZY20B15).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The list of symbols in the manuscript.
Serial NumberAcronyms/Math SymbolsExplain with Notes
1BPAbisphenol A
22,4,6-TCP2,4,6-trichlorophenol
3TBBPAtetrabromobisphenol A
4MIPsmolecularly imprinted polymer
5NIPsnon-imprinted polymers
6BPA-MIPsbisphenol A molecularly imprinted polymer
7BPA-NIPsbisphenol A non-imprinted polymer
8SBSEstir bar sorptive extraction
9SEMscanning electron microscopy
10FT-IRFourier transform infrared spectroscopy
11BETBrunauer–Emmett–Teller analysis
12MIPs-SBSEmolecularly imprinted stir bar sorptive extraction
13HPLChigh-performance liquid chromatography
14Qmmaximum apparent binding capacity
15Kddissociation constant
16qeamount of adsorption per unit mass of adsorbent at equilibrium.
17qmsaturation adsorption capacity of a monolayer
18KLadsorption coefficient
19Ceamount of residual BPA adsorbate in the solution at equilibrium

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Water 15 03361 g001
Figure 1. SEM images of BPA-MIPs (a) and BPA-NIPs (b).
Figure 1. SEM images of BPA-MIPs (a) and BPA-NIPs (b).
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Water 15 03361 g002
Figure 2. SEM images of BPA-MIPs-SBSE (a) and BPA-NIPs-SBSE (b).
Figure 2. SEM images of BPA-MIPs-SBSE (a) and BPA-NIPs-SBSE (b).
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Water 15 03361 g003
Figure 3. Fourier transform infrared spectrum.
Figure 3. Fourier transform infrared spectrum.
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Figure 4. BET analysis of the coating of the bisphenol A molecular imprinting stirring rod.
Figure 4. BET analysis of the coating of the bisphenol A molecular imprinting stirring rod.
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Water 15 03361 g005
Figure 5. Scatchard model fitting curve.
Figure 5. Scatchard model fitting curve.
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Water 15 03361 g006
Figure 6. The influence of adsorption time on adsorption efficiency.
Figure 6. The influence of adsorption time on adsorption efficiency.
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Figure 7. The effect of adsorption temperature on adsorption efficiency.
Figure 7. The effect of adsorption temperature on adsorption efficiency.
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Water 15 03361 g008
Figure 8. The effect of stirring speed on adsorption efficiency.
Figure 8. The effect of stirring speed on adsorption efficiency.
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Water 15 03361 g009
Figure 9. The effect of adsorption pH on adsorption efficiency.
Figure 9. The effect of adsorption pH on adsorption efficiency.
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Figure 10. The impact of desorption time on desorption efficiency.
Figure 10. The impact of desorption time on desorption efficiency.
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Water 15 03361 g011
Figure 11. Fitted curves of the Langmuir adsorption isotherm model.
Figure 11. Fitted curves of the Langmuir adsorption isotherm model.
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Water 15 03361 g012
Figure 12. Removal rates of several phenols by BPA-MIPs-SBSE and BPA-NIPs-SBSE.
Figure 12. Removal rates of several phenols by BPA-MIPs-SBSE and BPA-NIPs-SBSE.
Water 15 03361 g012
Water 15 03361 g013
Figure 13. Adsorption capacities of BPA-MIPs-SBSE and BPA-NIPs-SBSE for several phenols.
Figure 13. Adsorption capacities of BPA-MIPs-SBSE and BPA-NIPs-SBSE for several phenols.
Water 15 03361 g013
Table 1. The parameters and correlation coefficients of the Langmuir adsorption isotherm model.
Table 1. The parameters and correlation coefficients of the Langmuir adsorption isotherm model.
Model ParametersBPA-MIPs-SBSEBPA-NIPs-SBSE
qe (μmol/g)3.5451.963
qm (μmol/g)35.1918.92
KL (L/μmol)0.0310.015
R20.9600.941
Table 2. The average recovery rates of spiked real water samples.
Table 2. The average recovery rates of spiked real water samples.
SamplesSpiking
Concentration (μg/L)		Recovery
Concentration (μg/L)		Recovery Rate
(%)		RSD
(%)
Deionized water500465.5934.5
20001768.8885.7
Tap water500434875.5
20001677.8846.4
Bottled water500397.6805.0
20001516.8768.2
Reservoir water500379768.3
20001410.6718.7
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MDPI and ACS Style

Wang, J.; Wang, Z.; Chen, Z.; Yan, P.; Shen, J.; Kang, J. Preparation of Molecularly Imprinted Magnetic Stir Bar for Bisphenol A and Its Analysis on Trace Bisphenol A in Actual Water Samples. Water 2023, 15, 3361. https://doi.org/10.3390/w15193361

AMA Style

Wang J, Wang Z, Chen Z, Yan P, Shen J, Kang J. Preparation of Molecularly Imprinted Magnetic Stir Bar for Bisphenol A and Its Analysis on Trace Bisphenol A in Actual Water Samples. Water. 2023; 15(19):3361. https://doi.org/10.3390/w15193361

Chicago/Turabian Style

Wang, Jing, Zhehao Wang, Zhonglin Chen, Pengwei Yan, Jimin Shen, and Jing Kang. 2023. "Preparation of Molecularly Imprinted Magnetic Stir Bar for Bisphenol A and Its Analysis on Trace Bisphenol A in Actual Water Samples" Water 15, no. 19: 3361. https://doi.org/10.3390/w15193361

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