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. 2020 Aug 6;5(32):20664–20673. doi: 10.1021/acsomega.0c03095

A Molecularly Imprinted Fluorescence Sensor Based on the ZnO Quantum Dot Core–Shell Structure for High Selectivity and Photolysis Function of Methylene Blue

Rui Wang †, Ming Guo †,‡,*, Yinglu Hu †, Jianhai Zhou §, Ronghui Wu ‡, Xuejuan Yang ‡
PMCID: PMC7439697  PMID: 32832820

Abstract

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ZnO quantum dots and CuFe2O4 nanoparticles were synthesized by chemical precipitation. The ZCF composite was created by the solvothermal method. A new molecularly imprinted fluorescence sensor (ZCF@MB-MIP) with unique optical properties and specific MB recognition was successfully generated. ZCF@MB-MIPs were characterized by Fourier-transform infrared spectroscopy, transmission electron microscopy, and X-ray diffraction and were applied for the selective detection of methylene blue (MB). The optimal working time of ZCF@MB-MIPs was 15 min, and the optimal working concentration was 37 mg·L–1. The fluorescence intensity was linearly quenched within the 0–100 μmol·L–1 MB range, and the detection limit was 1.27 μmol·L–1. The imprinting factor of the sensor (IF, KMB-MIPs/N-MIPs) was 5.30. At the same time, a real-time monitoring system was established for the photodegradation process of MB, which had the effect of reflecting the degradation degree of MB at any given time. Hence, ZCF@MB-MIPs are a promising candidate for use in MB monitoring, and they also provides a new strategy for constructing a multifunctional fluorescence sensor with a high selectivity and photolysis function.

1. Introduction

Nano-CuFe2O4 is a strong magnetic semiconductor catalytic material. Compared with other catalytic materials, such as Cu2O1 and Cu–SBA-15,2 nano-CuFe2O4 has an advantage of low-cost preparation, good environmental performance,3 and a surface rich in charge and valence elements. Nano-CuFe2O4 exhibits good thermal stability, surface adsorption for catalysis,4 photocatalytic activity5 and other characteristics. Nano-CuFe2O4 as a p-type semiconductor material has a relatively narrow band gap of 1.4 eV,6 can be effectively stimulated by visible light, and is widely used in environmental governance. For example, nano-CuFe2O4 was proved to have high photocatalytic activities against atrazine, and surface hydroxyl groups of CuFe2O4 played an important role in the generation of radicals.7 However, the nano-CuFe2O4 catalytic band gap is limited to the visible-light band and cannot make full use of the energy provided by natural light. In addition, nano-ZnO quantum dots (QDs) are nontoxic and pollution free, containing the good physical and chemical properties of a semiconductor;8 this material has a band gap energy of 3.37 eV, can be effectively stimulated by UV light,9 shows good photocatalytic activity,10 and is widely used in various fields, such as cement,11 solar cells,12 photodegradation methylene blue,13 and so on. In summary, nano-ZnO QDs and nano-CuFe2O4 could catalyze the degradation of pollutants in different bands, and they can complement each other; solar energy could be used more effectively. Therefore, studying CuFe2O4 and ZnO QDs is very meaningful to the field of catalytic degradation of organic pollutants.

In addition to their good ultraviolet catalytic degradation performance, QDs also have the advantages of a small size, large specific surface area, and high biocompatibility.14 ZnO QDs contain abundant defects, and the suspended particle surface is the key15 to easily access surface area defects for spontaneous visible irradiation of the compound center, which then produces excellent fluorescence;16 consequently, it is favored by many researchers in the field of fluorescent sensors.17 The ZnO quantum dot sensor based on a fluorescence quenching mechanism performs well in detecting the content of specific molecules in a solution. In recent years, the construction of molecularly imprinted fluorescent sensors has been fully developed,18 which can effectively improve sensitivity and solve the problem of poor selectivity.19

Molecularly imprinted polymers (MIPs) are a kind of polymer prepared by molecularly imprinted technology and have specific recognition for a template molecule20−22 with the characteristic of recognition specificity and application universality,23 and fluorescence sensor-based MIPs have been studied and applied.24 In recent years, surface molecular imprinting has been applied to solve some limits of MIPs.25 The combination of molecularly imprinted polymers with ZnO photocatalytic technology and the high catalytic activity of CuFe2O4 is expected to effectively achieve a ZnO/CuFe2O4 composite substrate for the selective photocatalytic degradation of organic pollutants.26Through the simultaneous use of the fluorescent properties of ZnO quantum dots to detect the degree of specific photodegradation, the efficiency of the fluorescence sensor, in terms of identification and degradation, is greatly improved, and the problems associated with magnetic material separation and identification selection are effectively solved.

Methylene blue (MB) is a golden red shiny or flashing bronze powder, soluble in water, and its solution is blue, widely used in various fields; MB is used to treat methemoglobinemia, cystitis, urethritis, and so on; as a representative of aromatic heterocyclic sulfur dyes,30 it was a commonly used synthetic dye in the industry. However, MB also has serious harm to the environment and human body. For example, MB can absorb light of a certain wavelength, thereby affecting the propagation of light and the photosynthesis of plants;27,28 due to its high water solubility29 and high chromaticity, MB can also cause large areas of water to be stained,31,32 affecting the beauty of the water. In addition, MB harms the human skin, which can cause nausea when inhaled or ingested.33 Therefore, MB is a serious pollutant, and it has received extensive attention in environmental pollution research. Many methods have been explored to remove the hazards of MB, such as physical adsorption,34,35 chemisorption,36,37 biodegradation,38 and so on, as the most widely used method in the MB processing. However, the current method of adsorption/degradation of MB is not very effective. Therefore, it is very meaningful to find an environmentally friendly and efficient method for degrading MB; the use of solar energy to degrade methylene blue is very necessary.

In the present study, ZnO QDs and CuFe2O4 nanoparticles were synthesized by chemical precipitation, and they were modified by amino and double bonds, respectively. Then, the core–shell structure (ZCF) was synthesized by nanoparticle surface chemical bonds via Michael addition. With ZCF as the matrix, a ZCF@MB-MIP fluorescent sensor was prepared by the imprinted technology on the surface of the nanoparticles. Finally, a real-time degradation monitoring system was established for the simultaneous detection of MB molecule degradation and content in a solution. Compared with other research, such as Yu25 et al. and Wang26 et al., we successfully degraded pollutants. In this paper, the materials we used were nonpolluting, nano-ZnO QDs and nano-CuFe2O4 could complement each other in the use of solar energy, and the degree of adsorption and degradation can be reflected by the obvious fluorescence characteristics. The experimental results indicated that the synthetic molecularly imprinted fluorescence sensor exhibited high special recognition performance and degradation efficiency; this was the first time that zinc oxide quantum dots, ferrites, and molecularly imprinted polymers have been combined to adsorb methylene blue. Furthermore, in this study, the real-time degradation monitoring system was first established and monitored MB molecular degradation successfully, providing a new idea for related degradation research in the future. Compared with traditional adsorbents and fluorescence sensors, ZCF@MB-MIPs exhibited a more porous and open structure, specific recognition, high repeatability, and fluorescence sensitivity.

2. Experimental Section

2.1. Chemicals

Anhydrous ethanol, zinc acetate dihydrate, sodium hydroxide, cuprous chloride, and iron chloride were purchased from Sigma-Aldrich. Polyethylene glycol (PEG) 400, ethylene glycol, MB, neutral red, methyl red, and basic red were obtained from Saen Chemical Technology (Shanghai) Co., Ltd. Polyacrylamide (PAM), azobisisobutyronitrile (AIBN), epichlorohydrin (ECH), vinyltriethoxysilane (VTEO), and 3-aminopropyl triethoxysilane (APTES) were purchased from Shanghai Aladdin Biotechnology Co., Ltd. Each reagent and solvent adopted in this experiment was analytically pure, which was utilized as received.

2.2. Equipment

FTIR spectra of the products were recorded by an IR Prestege-21 type Fourier infrared spectrometer (Shimadzu, Japan). An XRD-6000 X-ray diffractometer (Shimadzu, Japan) was used to obtain crystal data of the magnetic nanoparticles. Transmission electron microscopy (TEM) images were taken using a Tecnai F30 G2 transmission electron microscope (FEI, USA). HPLC spectra of the degradation experiment were measured by an Agilent 1200 high-performance liquid chromatograph (Agilent Technologies, China). Fluorescence emission measurements were recorded on an F-7000 02 fluorescence spectrophotometer (Shimadzu, Japan). UV–visible absorption spectra were obtained from a UV-2550 UV spectrophotometer (Shimadzu, Japan).

Synthesis of ZnO@NH2 QDs (APTES-coated ZnO QDs) is as follows: To prepare ZnO@NH2 QDs, Zn(OAc)2 (1.86 mmol, 341 mg) was dissolved into 10.0 mL of anhydrous ethanol and heated at 60 °C for 1.5 h. A colorless solution was obtained. Once cool, a stirred colorless solution of Zn(OAc)2 in anhydrous ethanol was dropwise added to a solution of NaOH (4.11 mmol, 164 mg) in anhydrous ethanol (5.0 mL). The reaction mixture was stirred at room temperature for 1 h. Then, a solution of APTES (0.34 mmol, 75 mg) in anhydrous ethanol (5.0 mL) was added to the mixture followed by the rapid addition of deionized water (0.25 mL). The mixture was stirred 2 h at room temperature. Finally, the precipitate was centrifuged, and the as-obtained product was washed twice with toluene (20 mL) and ethanol (10 mL), respectively, to remove the unconverted reactants, affording the product ZnO@NH2 QDs as a white solid (200 mg). The white product was dispersed in 10 mL of anhydrous ethanol to make white sample solution, which was reserved for subsequent analysis and test.

Synthesis of VCF (CuFe2O4 nanoparticles modified by double bond) is as follows: To prepare CuFe2O4, CuCl2·2H2O (2.5 mmol, 426 mg) and FeCl3·6H2O (5 mmol, 1351 mg) were dissolved into ethylene glycol (40 mL). Under stirring, 44 mmol of NaOAc and 0.75 g of polyethylene glycol (PEG 400) were added, affording a green solution. The green reaction solution was sealed in a high-pressure reactor (60 mL) and reacted at 200 °C for 11 h. Upon cooling to room temperature, the black precipitate was collected under an external magnetic field. The black precipitate was washed five times with deionized water and anhydrous ethanol. Next, the product was placed in a vacuum drying oven at 60 °C for 6 h. The product CuFe2O4 was obtained (840 mg).39

Then, 400 mg of CuFe2O4 was dissolved in 20 mL of water and sonicated for 10 min. Hydrochloric acid (1 mL, 0.012 mol·L–1) and vinyl triethoxysilane (VTEO) (0.2 mL) were added. The mixture reaction was stirred at room temperature for 24 h. The product VCF was collected under an external magnetic field, washed three times with ethanol and DW, and dried under vacuum. The VCF was obtained as black solid (400 mg).

2.3. Synthesis of ZCF (Core–Shell Structure ZCF Composite)

In a N2 atmosphere, the vinyl modified VCF (400 mg) was dispersed in anhydrous ethanol (20 mL) by sonicating. The solution of ZnO@NH2 QDs (200 mg) in anhydrous ethanol (10 mL) was added. The mixture was stirred at 60 °C for 10 h. The product ZCF was washed five times with anhydrous ethanol and dried at 60 °C, affording a brown solid in 568 mg.

Synthesis of molecularly imprinted polymers ZCF@MB-MIPs and ZCF@N-MIPs is as follows: To prepare ZCF@MB-MIPs, ZCF (400 mg), MB (0.4 mmol, 128 mg), and polyacrylamide (300 mg) were dissolved into methanol (15 mL). The solution was heated and sonicated at 40 °C for 3 h. Next, 1.3 g of the cross-linker epichlorohydrin (ECH) and 0.3 g of the initiator azobisisobutyronitrile (AIBN) were added under stirring. The reaction mixture was transferred to an ampoule. Then, the ampoule was continuously purged with N2, sealed, heated and sonicated at 50 °C and 180 r·min–1 for 6 h. The solvent was evaporated. The as-obtained product was eluted with anhydrous ethanol to remove MB and dried under vacuum at 60 °C. ZCF@MB-MIPs with the template molecules of MB were obtained as a brown solid (756 mg). For comparison, the non-imprinted magnetic nanoparticles (ZCF@N-MIPs) were prepared in the same manner as ZCF@MB-MIPs but without the addition of the template molecule.

The synthesis of the molecularly imprinted polymers is shown in Figure 1.

Figure 1.

Figure 1

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Preparation of ZCF@MB-MIPs and ZCF@N-MIPs.

In this work, the ZCF composite material served as the matrix, MB as a model molecule, PAM as a functional monomer, ECH as a cross-linking agent, and AIBN as a radical initiator. Surface-imprinted polymerization was used to prepare the imprinted polymers (ZCF@MB-MIPs).

Characterization of the starting material and molecularly imprinted composite fluorescence sensor is as follows: A Fourier infrared spectrometer was used to confirm the structure of the product (the method was KBr tableting, and the scanning wavenumber range was 4000–500 cm–1). A transmission electron microscope was used to observe the morphology of the products. The sample was dispersed in ethanol by sonicating, added to the copper mesh, and adsorbed for 20–30 min. Upon drying, the sample was observed and analyzed by transmission electron microscopy, X-ray diffraction was used to analyze the sample crystal structure (scanning mode: step scan; voltage/current: 35 kV/30 mA; scanning speed: 2°·min–1; step size: 0.02°; Cu target, Kα1 radiation (λ = 0.15406 nm); and scanning range: 2θ = 20–80°). A fluorescence spectrophotometer was used to determine the fluorescence intensity. The slit width for excitation and emission was 10 nm, the photomultiplier tube voltage was 750 V (the excitation wavelength was 360 nm, and the scanning wavelength range was 450–700 nm).

Optimization of the conditions for fluorescence detection is as follows: First, the optimal working time of ZCF@MB-MIPs for the detection of MB was investigated. The fluorescence intensity of ZCF@MB-MIPs (concentration: 20.0 mg·L–1) without MB was measured at room temperature. After adding MB (20 μmol·L–1), the fluorescence intensity of ZCF@MB-MIPs (concentration: 20.0 mg·L–1) was recorded at different times.

Next, the optimal working concentration of ZCF@MB-MIPs, different MB concentrations (4, 8, 12, 16, 20, 24, 28, 32, 36, and 40 mg·L–1) for the detection of MB was explored. The fluorescence intensity of different concentrations of ZCF@MB-MIPs was measured without MB. After adding MB (40 μmol·L–1), the fluorescence intensity of different ZCF@MB-MIP concentrations was measured.

Finally, the effects of pH on the fluorescence detection were studied. The pH range of 4.5–8.0 was tested (concentration: 37 mg·L–1).

Quantitative detection experiments of MB were carried out. The same amount of ZCF@MB-MIPs and ZCF@N-MIPs was weighed individually. After adding different MB concentrations (0–130 μmol·L–1), the fluorescence intensities of the ZCF@MB-MIP and ZCF@N-MIP sensor solutions (37 mg·L–1) were measured. For example, ZCF@MB-MIP (2 mL) was added to a 10 mL cuvette with different volumes (0, 64, 128, 192, 256, 320, 384, 448, 512, 576, 640, 704, and 768 μL) of the MB standard solution (20 μmol·L–1). Then, double distilled water (DDW) was added to the solution and adjusted to a total of 5.0 mL. The test solution was uniformly mixed and then remained static at room temperature for 15 min. Portions of the different test solutions were sequentially transferred to a cuvette for determining and recording the fluorescence intensity of the solution.

Selectivity performance and interference recovery test are as follows: To determine the selectivity of ZCF@MB-MIPs for the detection of MB, another three similar dyes, neutral red (BR5), methyl red (AR2), and basic red (BR2), were explored.

To investigate the effect of common anions and cations and other similarly structured dyes on the adsorption of MB by ZCF@MB-MIPs. The fluorescence intensity of the mixture solution of ZCF@MB-MIPs (37 mg·L–1, 100 mL) and MB (20 μmol·L–1, 100 mL) was measured. Next, the same concentrations (1 μmol·L–1) of Fe(NO3)3, CuSO4, Na2CO3, and KCl were added to the mixture solution. Then, the changes in fluorescence were determined.

The real-time degradation monitoring system method is as follows: The MB solution (100 mL, 30 μmol·L–1) was prepared. The ZCF@MB-MIP solution (37 mg·L–1) was sonicated for 30 min, and then, the solution was static at room temperature for 15 min. Next, the mixed solution was placed by a xenon lamp, which simulates natural light. The photocatalytic degradation was carried out in a photoreactor. A portion of the solution (3 mL) was taken in a cuvette and examined by a fluorescence spectrophotometer every 30 min. The corresponding data were recorded.

3. Results and Discussion

3.1. Structural Characterization Analysis

TEM experiments were carried out to investigate the particle size and distribution of the products. ZnO@NH2 QDs, which were APTES-coated ZnO QDs, have good dispersion and a relatively uniform particle size distribution (Figure 2a). The average particle size was approximately 5 nm. The ZCF that was VCF coated with ZnO@NH2 QDs had a particle size of approximately 200 nm and had a relatively flat surface (Figure 2d). The coating structures on the surfaces of ZCF@MB-MIPs and ZCF@N-MIPs were clearly observed (Figure 2b,c), and these data confirm the coating of the core with the imprinted polymer and non-imprinted polymer, respectively. The size distribution diagrams of these nanoparticles are shown in Figure S1.

Figure 2.

Figure 2

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(a) HRTEM spectrum of ZnO@NH2 QDs. (b) TEM spectrum of ZCF@MB-MIPs. (c) TEM spectrum of ZCF@N-MIPs. (d) TEM spectrum of ZCF. XRD patterns of (e) ZCF@MB-MIPs and (f) CuFe2O4.

As shown in Figure 2e, the diffraction peaks of ZCF@MB-MIPs at angles 2θ = 31.9, 34.39, 36.29, 47.60, 56.70, and 62.89° correspond to the (100), (002), (101), (102), (110), and (103) planes ((hkl) values) of the sample crystal, respectively. The data were almost consistent with the JCPDS card standard values (JCPDS36-1451). The X-ray diffraction pattern shows that ZCF@MB-MIP is the hexagonal wurtzite structure. As shown in Figure 2f, the diffraction peaks of CuFe2O4 at angles 2θ = 30.2, 35.6, 53.6, 57.1, 62.8, and 74.5° correspond to the (220), (311), (511), (440), and (533) planes of the sample crystal, respectively. The data were consistent with the single SPAR structure of the CuFe2O4 crystal of the standard card (JCPDS77-0010). Combined with the results from the transmission electron microscopy, these data showed that ZCF@MB-MIPs were successfully synthesized, and the modified polymer on the ZCF@MB-MIP surface had no effect on the crystal structure of the product.

For the FTIR analysis, the FTIR spectra of ZnO@NH2 QDs, VCF, ZCF@MB-MIPs, and ZCF@N-MIPs are shown in Figure 3. The peak at 483 cm–1 is the characteristic absorption peak of the Zn–O bond. The stretching vibration peak for −NH2 in APTES was observed at 3437 cm–1. The bending vibration peak for −NH2 in APTES was observed at 1561 cm–1. The peaks at 1020 and 1094 cm–1 were attributed to Si–O. These FTIR data indicated that the ZnO QD surface was successfully modified by APTES, affording ZnO@NH2 QDs. In the MB–MIP curves, the Cu–O bond and Fe–O bond absorption peak of copper ferrite was observed at 588 cm–1. The absorption peak of the O–H in polyethylene glycol coated on the surface of copper ferrite was observed at 3437 cm–1. These data indicated the successful generation of CuFe2O4. The position and shape of the ZCF@MB-MIP and ZCF@N-MIP absorption peaks are very similar. This result demonstrated that the model molecule MB was removed completely from the molecularly imprinted polymer. The stretching bands for C=O and C–O were observed at 1650 and 1121 cm–1, respectively. The peak at 2975 cm–1 was attributed to −CH2– stretching, and the peak bond at 1482 cm–1 was assigned to −CH2– bending. The out-of-plane bending vibration peak of C–H was observed at 826 cm–1. These FTIR data illustrated that the ZCF@MB-MIPs and ZCF@N-MIPs were successfully synthesized and that the cross-linking agent ECH and the functional monomer existed in the ZCF@MB-MIPs and ZCF@N-MIPs.

Figure 3.

Figure 3

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FTIR spectra of ZnO@NH2 QDs, VCF, ZCF@MB-MIPs, and ZCF@N-MIPs.

3.2. Fluorescence Analysis of ZnO QDs and ZCF@MB-MIPs and the Optimal Detection Conditions

The solutions were prepared by dissolving the ZnO QDs or ZCF@MB-MIPs in anhydrous ethanol (concentration: 20.0 mg·L–1) and sonicating to allow for the dispersion of ZnO QDs or ZCF@MB-MIPs. The fluorescence emission spectra of ZnO QDs and ZCF@MB-MIPs are shown in Figure 4. The fluorescence peak of ZnO QDs was at 540 nm (Figure 4a), and the fluorescence peak of ZCF@MB-MIPs was at 554 nm (Figure 4b). There is little difference between the two dates, and this is due to the inevitable error obtained by machine measurement. Next, a solution was obtained by dissolving MB in anhydrous ethanol (concentration: 0.4 mg·L–1), and the ultraviolet absorption experiment of MB was carried out. From the UV spectrum, the ultraviolet absorption peaks of MB occurred at 617 and 665 nm. From Figure 4, we clearly observed that there was an overlap between the absorption spectrum of MB (Figure 4c) and the emission spectrum of ZCF@MB-MIPs in the range of 569–617 nm. This overlap means that the energy emitted from ZCF@MB-MIPs after being excited by light can be transferred to and adsorbed by MB; the fluorescence of ZCF@MB-MIPs was quenched by radiant energy transfer (RET). In this work, the RET theory was applied for the detection of MB in the aqueous phase.

Figure 4.

Figure 4

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Fluorescence emission spectra of (a) ZnO QDs and (b) ZCF@MB-MIPs. (c) Ultraviolet absorption spectrum of MB.

The fluorescence stability of ZCF@MB-MIPs and ZCF@N-MIPs, when serving as fluorescence sensors, was investigated (Figure 5). The solutions were prepared by dissolving ZCF@MB-MIPs or ZCF@N-MIPs in anhydrous ethanol (concentration: 20.0 mg·L–1). The fluorescence intensities of both ZCF@MB-MIPs and ZCF@N-MIPs decreased. The reason for this decrease is that as the defect count of ZnO decreases, the electron mobility increases continuously, resulting in an increase in electron holes and a decrease in fluorescence.40 Based on the fluorescence intensity curves of the ZCF@MB-MIP and ZCF@N-MIP, the fitting equations were obtained as follows: y1 = 863.38 – 2.00x1, and y2 = 591.96 – 2.02x2, respectively. According to the slopes of the two equations, the reductions in the fluorescence intensity of ZCF@MB-MIPs and ZCF@N-MIPs are almost identical. This result suggests that the fluorescence intensities of ZCF@MB-MIPs and ZCF@N-MIPs are relatively stable within a certain range.

Figure 5.

Figure 5

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Fluorescence stability trend of ZCF@MB-MIPs and ZCF@N-MIPs.

Based on the stability of the fluorescence, the effect of absorption time was investigated to determine the optimal working time of ZCF@MB-MIPs for MB (Figure 6A). After adding MB to the prepared ZCF@MB-MIP solution, the fluorescence intensity was quenched quickly within the first 2 min and weakened slowly over the next 2–6 min. After 6 min, the fluorescence intensity remained almost unchanged. At the same time, the effect of the absorption time of the ZCF@N-MIP sensor for MB was investigated (Figure 6A). After adding MB to the prepared ZCF@N-MIP solution, the fluorescence intensity was quenched in the first 15 min. After 15 min, the fluorescence intensity was nearly constant. These observations suggest that the sensor response time of ZCF@MB-MIPs was twice as fast as that of ZCF@N-MIPs. We speculated that the mesoporous structure increased the volume of the pores and that the mass transfer rate of MB in ZCF@MB-MIPs was accelerated. Therefore, the optimal working time of the sensors for detecting MB was 15 min.

Figure 6.

Figure 6

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(A) Effect of the time on the quenching of the ZCF@MB-MIP and ZCF@N-MIP solutions after adding MB. (B) Effect of the MB concentration on the ZCF@MB-MIP-based detection of MB (F0 is the fluorescence intensity of ZCF@MB-MIPs without MB, and F is the fluorescence intensity after the interaction with MB (40 μmol·L–1). Curve a shows the response of the fluorescence quenching rate (F0 – F)/F0 versus the ZCF@MB-MIP concentration. Curve b is the response of the relative fluorescence intensity versus the ZCF@MB-MIP concentration.

Next, the effect of concentration on ZCF@MB-MIPs for the detection of MB was surveyed (Figure 6B). First, the fluorescence intensity of ZCF@MB-MIPs was investigated without MB. At the same time, the fluorescence intensity of different ZCF@MB-MIP concentrations was determined when the concentration of MB was 40 μmol·L–1. The ZCF@MB-MIP concentration range was varied from 4.0 to 40 mg·L–1. The fluorescence quenching rate (F0 – F)/F0 increased until 0.312 and then decreased (Figure 6B, curve a). Furthermore, the relative fluorescence intensity increased linearly (Figure 6B, curve b). To ensure a high detection sensitivity and wide linear range, the optimal working concentration of the ZCF@MB-MIPs was 37 mg·L–1.

The effect of pH on fluorescence quenching efficiency was investigated in Figure S2. As shown in Figure S2, when the solution pH increased from 4.5 to 7.5, fluorescence quenching efficiency increased obviously, and the quenched fluorescence intensity is found to decrease when the pH is higher than 7.5. In the range of pH 6.0–7.5, fluorescence quenching efficiency of ZCF@MB-MIPs tends to stabilize. Hence, a neutral condition was used as an optimum reaction condition for further analytical experiments.

3.3. Quantitative Detection of MB

The fluorescence intensity of the ZCF@MB-MIP solution (37 mg·L–1) was investigated after adding different MB concentrations (Figure 7A). While the MB concentration (CMB) was varied from 0 to 100 μmol·L–1, the fluorescence intensity of ZCF@MB-MIPs (37 mg·L–1) decreased gradually. However, after CMB > 100 μmol·L–1, the fluorescence intensity decreased slowly. This behavior suggests that adsorption saturation was approached when the concentration was higher than 100 μmol·L–1. Therefore, the detection range of the ZCF@MB-MIP solution is CMB ≤ 100 μmol·L–1. The fluorescence intensity of the ZCF@N-MIP sensor solution (37 mg·L–1) was also investigated (Figure 7B). As shown in Figure 7C,D, the relative fluorescence intensity (F0/F) of the sensors increased linearly when the MB concentration (CMB) was increased. The relative fluorescence intensity (F0/F) was calculated using the Stern–Volmer equation (eq 1).

3.3. 1

Figure 7.

Figure 7

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(A) Fluorescence spectra of ZCF@MB-MIPs in the presence of different MB concentrations. (B) Fluorescence spectra of ZCF@N-MIPs in the presence of different MB concentrations. (C) Plot shows the linear response of the relative fluorescence intensity F0/F of ZCF@MB-MIPs versus the MB concentration. (D) Linear response of the relative fluorescence intensity F0/F of ZCF@N-MIPs versus the MB concentration. (E) Fluorescence intensity of ZCF@MB-MIPs after adding and removing MB.

where F0 is the fluorescence intensity of ZCF@MB-MIPs without MB, F is the fluorescence intensity after the interaction with MB, KSV is the quenching constant, and CMB is the concentration of MB in the sensor solution. A linear relationship exists between the relative fluorescence intensity F0/F and CMB.

According to the detection of MB by the sensor solution, the related parameters of the Stern–Volmer equation were obtained (Table 1). The ZCF@MB-MIP quenching constant (KMB-MIPs) is 0.0212 L·mol–1 with R2 being 0.9740. The ZCF@N-MIP quenching constant (KN-MIPs) is 0.0040 L·mol–1 with R2 being 0.9855. The imprinting factor of the sensor (IF, KMB-MIPs/N-MIPs) was 5.30, and the LOD was 1.27 μmol·L–1 (LOD = 3 σ/k, where k is the slope of the calibration line, and σ is the standard deviation of a blank measure; n = 10). The reusability of ZCF@MB-MIPs was investigated for a single MB concentration (80 μmol·L–1) and adsorption/removal time (15 min). As shown in Figure 7E, while the MB was repeatedly adsorbed and removed, the fluorescence intensity of ZCF@MB-MIPs continued to be stable. Compared with other reports on the removal of MB, Yang et al.41 constructed high-performance nanofiltration membranes via LBL assembly toward highly efficient membrane-based water energy and achieved an efficient rejection to MB; Chen et al.42 explored the synthesis of biomass porous carbon based on Euonymus japonicus leaves, proved its efficient adsorption of methylene blue, and studied its adsorption mechanism. Asman et al.43 reports the synthesis of a molecularly imprinted polymer for removal of MB, and the MB-MIP membranes are able to remove MB rapidly with high binding capacity. It can be seen form Table 2 that the ZCF@MB-MIPs we report in this paper have similar properties to the abovementioned materials in terms of detection limit, reusability, and stability, and they also have specific adsorption and photodegradation characteristics. In addition, The ZCF@MB-MIPs have excellent degradation properties, and their processing potential for methylene blue is much higher than other materials. Therefore, the characteristic of all aspects of ZCF@MB-MIPs is good.

Table 1. Stern–Volmer Equation and Related Parameters of the Fluorescence Sensors.

fluorescence sensor calibration curve KSV R2
ZCF@MB-MIPs F0/F = 0.0212CMB + 0.8730 0.0212 0.9740
ZCF@N-MIPs F0/F = 0.0040CMB + 0.9959 0.0040 0.9855
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Table 2. Comparison of Processing Capacity of Different Materials for MB.

sample absorption capacity/rejection references
PI (polyimide)/TA (tannic acid) mussel-inspired nanofiltration 92.3% (39)
PI (polyimide) /PEI (polyethylenimine) mussel-inspired nanofiltration 99.8% (39)
MB-MIP membranes 84.56 mg·g–1 (41)
polymerizing acrylic acid MIPs 3603 mg·g–1 (44)
a molecularly imprinted fluorescence sensor the degradation rate increases linearly with time this work
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3.4. Construction of the Real-Time Degradation Monitoring System

When the degradation time was varied from 0 to 180 min, the fluorescence intensity of the sensor solution increased correspondingly (Figure 8A). The inset shows the linear response of the relative fluorescence intensity (F0/F) of ZCF@MB-MIPs versus the degradation time (t), which was described using eq 2.

3.4. 2
3.4. 3

Figure 8.

Figure 8

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(A) Fluorescence emission spectra of ZCF@MB-MIP degrading MB and the linear response of the relative fluorescence intensity of ZCF@MB-MIPs versus degradation time. (B) Linear response of the degradation rate (η%) versus the degradation time (t)

The degradation rate (η%) was described using eq 3 where F0 is the fluorescence intensity of ZCF@MB-MIPs without MB, F1 is the fluorescence intensity just after adding MB, Ft is the fluorescence intensity after interaction with MB at t. Combined with the Stern–Volmer equation (eq 1), the degradation rate (η%) versus the degradation time (t) represents the linear response (Figure 8B). Within 3 h, the degradation rate of the MB solution (100 mL, 30 μmol·L–1) obtained by ZCF@MB-MIPs was 35%.

At the same time, the concentration of the degradation solution that was detected by the fluorescence sensor was compared with the concentration that was determined by high-performance liquid chromatography (HPLC) (Table 3). Reviewing the results acquired from fluorescence spectrometry and HPLC, the error rate was within the 0.77–6.39% range. This error means that the results of fluorescence spectrometry are almost consistent with those of HPLC. Therefore, the sensor can be considered sufficient for the detection of MB at any given time.

Table 3. Comparative Analysis of the Fluorescence Results of the Real-Time ZCF@MB-MIP Degradation of MB and the HPLC Results (c = 30 μmol·L–1, V = 100 mL).

degradation time detection result of MB-MIP (μmol·L–1) detection result of HPLC (μmol·L–1) error rate (%)
0 30.001 ± 0.011 30.511 ± 0.002 1.67
30 28.010 ± 0.013 29.240 ± 0.003 4.21
60 25.816 ± 0.015 26.612 ± 0.001 0.77
90 24.019 ± 0.010 25.651 ± 0.001 6.39
120 22.982 ± 0.011 23.705 ± 0.003 3.04
150 20.933 ± 0.012 21.892 ± 0.002 4.39
180 19.505 ± 0.010 20.534 ± 0.002 5.02
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3.5. Selection Performance and Interference Experiment Analysis

The experiments for the selective adsorption of methylene blue (MB), neutral red (BR5), methyl red (AR2), and basic red (BR2) by the ZCF@MB-MIP solution was investigated. The selected adsorption effect of ZCF@MB-MIPs/ZCF@N-MIPs for different dyes and the effect of other dyes on ZCF@MB-MIP detection of MB are shown in Figure 9A,B, respectively. As shown in Figure 9A, the IF value of MB (IF = 5.30) is higher than that of BR5 (3.41), AR2 (3.23), and BR2 (3.14). This difference suggests that MB can be selectively recognized by ZCF@MB-MIPs. These results are also consistent with the mechanism of adsorption by ZCF@MB-MIP for the template molecules. At the same time, the selective adsorption experiments with ZCF@N-MIPs were also investigated. Because there is no space left on the surface of ZCF@N-MIPs, the adsorption capacity of ZCF@N-MIPs for MB is similar to that for the other three dyes. Thus, ZCF@MB-MIPs, which served as an MIP fluorescent probe, can selectively detect MB.

Figure 9.

Figure 9

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(A) Selected adsorption effect of ZCF@MB-MIPs and ZCF@N-MIPs for different dyes. (B) Effect of other dyes on ZCF@MB-MIP detection of MB.

The effect of other dyes on ZCF@MB-MIPs for the detection of MB was investigated (Figure 9B). First, the fluorescence intensity of the ZCF@MB-MIP solution was recorded. After successively adding neutral red (BR5), methyl red (AR2) and alkaline red (BR2), the fluorescence intensity of the ZCF@MB-MIP solution decreased slowly. Upon adding MB, the fluorescence intensity was quenched significantly. When the BR5, AR2 and BR2 were added again, the fluorescence intensity insignificantly decreased. However, the fluorescence intensity quenching effect was also evident when MB was added again. Therefore, the influence of other dye molecules on the adsorption of MB by ZCF@MB-MIPs is negligible.

In general, many coexisting ions (Fe3+, Cu2+, Na+, K+, NO3–, SO42–, Cl–, CO32–, etc.) in the water may affect the quantitative detection of MB by ZCF@MB-MIP fluorescence. Next, the effect of general ions on ZCF@MB-MIPs was surveyed. As shown in Table 4, the experimental results suggest that Fe(NO3)3, CuSO4, Na2CO3, and KCl have no influence on the fluorescence intensity of ZCF@MB-MIPs. Thus, coexisting ions do not need to be considered in practical applications.

Table 4. Effect of General Cations and Anions on the Fluorescence Intensity of ZCF@MB-MIPs.

coexisting substance change in fluorescence intensity (%)
Fe(NO3)3 +1.7%
CuSO4 +0.5%
Na2CO3 –2.1%
KCl +1.9%
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4. Conclusions

This study used chemical precipitation to synthesize ZnO quantum dots and CuFe2O4 nanoparticles. The employment of the ZnO/CuFe2O4 (ZCF) composite material strengthened the photodegradation of the substrate. The degradation performance for the pollutant molecule methylene blue (MB), as the template molecule, was examined, and the surface imprinting method was used to synthesize a material with the function of photolysis and a high recognition performance. The more efficient and new molecularly imprinted fluorescent sensors (MB-MIPs) were characterized by using the TEM, XRD, and FTIR methods to determine the sensor structure. Fluorescence spectrometry was used to optimize the detection conditions of the sensor MB, while the MB content in the solution was detected in real time by the constructed degradation monitoring system. The results showed that the MB-MIP fluorescence sensor was successfully synthesized. The optimal static adsorption time for MB molecules in an aqueous solution was 15 min, and the optimal detection concentration of the sensor was 37 mg·L1. Under these conditions, the fluorescence sensitivity and linear range of the sensor were the best. The sensor presents a good detection in the concentration range of MB molecules from 0 to 100 μmol·L–1. The linear equation is F hing a real-time monitoring system, and the MB-MIPs were realized as a catalyst and fluorescent sensor at the same time. The high selectivity for photodegradation served as one of the functions. Fluorescence detection was combined with the Stern–Volmer equation to calculate the different concentrations of MB and then compared with the high-performance liquid chromatography (HPLC) results to determine the degradation time. The two kinds of detection methods deviate between 0.77 and 6.39%, which shows that the sensor for the real-time monitoring of the degradation of MB has a good application value.

Acknowledgments

The Zhejiang Public Welfare Technology Application Research Project (LGN20B070001) and the Natural Science Foundation of Zhejiang Province (LY18B070003) are thanked for their financial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03095.

  • Size distribution diagrams of all nanoparticles and effects of pH on the fluorescence detection experiments of MB (PDF)

Author Contributions

∥ M.G. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

ao0c03095_si_001.pdf (410.4KB, pdf)

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ao0c03095_si_001.pdf (410.4KB, pdf)

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