ZnS

Article pubs.acs.org/JAFC

Development of Fluorescence Sensing Material Based on CdSe/ZnS Quantum Dots and Molecularly Imprinted Polymer for the Detection of Carbaryl in Rice and Chinese Cabbage Can Zhang,*,† Hanyu Cui,† Jianrong Cai,† Yuqing Duan,† and Yuan Liu‡ †

School of Food & Biological Engineering, Jiangsu University, Zhenjiang 212013, P. R. China Zhenjiang Institute for Drug Control, Zhenjiang, 212000, P. R. China

‡

ABSTRACT: A fluorescence sensing material based on quantum dots with excellent optical properties and molecularly imprinted polymer (QDs@MIP) with specific recognition has been developed. First the surface of CdSe/ZnS QDs was modified with ionic liquids (ILs) by electrostatic interaction. The fluorescence sensing material was constructed from anchoring the MIP layer on IL modified CdSe/ZnS QDs by copolymerization, which had been developed for the detection of carbaryl in rice and Chinese cabbage. The MIP fluorescence was more strongly quenched by carbaryl than the non-imprinted polymer (NIP) fluorescence, which indicated that the QDs@MIP could selectively recognize the corresponding carbaryl. Furthermore, the developed QDs@MIP method was validated by HPLC and ELISA respectively, and the results of these methods were well correlated (R2 = 0.98). The fluorescence sensing material had obvious advantages, such as being easily prepared and having specific recognition and photostability. The developed method was simple and effective for the detection of carbaryl. And, it could also provide the technical support for the rapid detection in food safety fields. KEYWORDS: quantum dots, molecularly imprinted polymer, carbaryl, fluorescence sensing material

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INTRODUCTION Molecular imprinted polymers (MIPs) are prepared by the template molecule, functional monomer, cross-linker, and initiator with a heat or light trigger. After the template molecules are eluted from the MIPs, they have a cavity match, in such aspects as size and shape in space and chemical action, which can specifically combine with the template molecules. For the structure−activity reservation, specific recognition and extensive applicability, MIPs are being researched and developed in many fields such as solid-phase extraction,1−4 chromatography,5,6 and sensor technology.7,8 Quantum dots (QDs) are semiconductor nanoparticles composed of groups II−VI or III−V elements with unique optical properties. QDs have unique quantum size and surface effect, showing good spectral characteristics and chemical stability. Consequently, they have been widely applied as fluorescence labels for biomolecules and as optoelectronic devices for sensitive detection of analytes.9 On the basis of quantum dots with excellent optical properties and an MIP with specific recognition, fluorescence sensing material (QDs@MIP) have been developed and have drawn considerable attention. Many researchers have constructed fluorescence materials based on QDs for determination of analytes. It is reasonably believed that the molecular imprinting technique will be a powerful tool to improve the selectivity of the optical detection. For example, Wang et al. proposed MIPs coated with QD fluorescence materials for highly selectivity and sensitive determination of ractopamine,10 vitamin E,11 and tocopherol.12 Xie et al.13 developed a high affinity dipheolic acid (DPA)-MIPs-QD sensor using DPA as a dummy template, for selective determination of trace tetrabromobisphenol A in water and soil samples. Li et al.14 © 2015 American Chemical Society

reported a simple procedure for the synthesis of molecularly imprinted silica nanosphere-embedded mercaptosuccinic acidcoated CdTe quantum dots for selective recognition of λcyhalothrin. To illustrate the usefulness of the MIPs combined with QD materials, carbaryl was chosen as a target. Carbaryl (1-naphtholN-methylcarbamate) was the first successful carbamate insecticide used due to its broad-spectrum efficacy to control insecticide pests. It is widely used in Chinese agricultural systems to combat amounts of pests. Through inhibiting the activity of cholinesterase and the accumulation of acetylcholine in the organization, the normal secretion of biological systems was disrupted.15−17 Various methods have been explored to detect carbaryl, such as chromatographic techniques, 18 biosensors,19 enzyme-linked immunosorbent assay,20,21 et al. MIP-capped QD fluorescence sensing material for analytical applications is still in rapid developed. As far as we know QDs@MIP fluorescence material for the quantitative analysis of carbaryl has not been reported. In this study, we synthesized fluorescence material for detection of carbaryl. The sensing material is based on fluorescence quenching when carbaryl are selectively recognized and combined with QDs@MIP. The participation of QDs exhibits a Stern−Volmer quenching behavior.22 This method is prospected to realize a fast and efficient detection method for carbaryl without derivation. Received: Revised: Accepted: Published: 4966

February 27, 2015 April 7, 2015 May 6, 2015 May 6, 2015 DOI: 10.1021/acs.jafc.5b01072 J. Agric. Food Chem. 2015, 63, 4966−4972

Article

Journal of Agricultural and Food Chemistry

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carried out as follows: The carbaryl (1 mmol) and MAA (4 mmol) were added. The mixture was stirred overnight before adding crosslinker (EDGMA; 20 mmol) and AIBN (40 mg). After purging with nitrogen 5 min, the mixture was placed in a water bath at 60 °C for 24 h. When the polymerization reaction finished, the template molecules were removed by Soxhlet extraction with methanol and acetic acid (200 mL, v:v = 9:1) until no analyte was detected by UV−vis spectrophotometry. Finally, the polymers were dried in a vacuum oven at 55 °C. And, the QDs@MIP was obtained. The QDs@NIP was prepared without the template molecules. Fluorescence Analysis. In the adsorption experiments, QDs@ MIP and QDs@NIP nanoparticles (20 mg, 200 mesh screens) were dispersed in a 15 mL plug centrifuge tube, containing carbaryl standard solution (10 mL) with the concentration of 0−100 mg L−1. After shaking for a certain time at room temperature, the templates were bound to the QDs@MIP, causing fluorescence quenching of the QDs. To further evaluate the analytical performance of the fluorescence sensing material, the detection limit and linear range were investigated. The detection limit (LOD) was calculated as the concentration of carbaryl that quenched three times the standard deviation of the blank signal divided by the slope of the standard curve. Metolcarb and isoprocarb were used as structural analogues in selectivity experiments. In the experiment, all the FL detections were performed under the same conditions: the excitation wavelength was set at 280 nm with a recording emission range of 450−650 nm, and the slit widths of the excitation and emission were both 2.5 nm. Then the mixture was mixed thoroughly and quickly scanned by the Cary Eclipse spectrophotometer.

MATERIALS AND METHODS

Materials and Reagents. Carbaryl, metolcarb, and isoprocarb were purchased from J&K Chemicals (Shanghai, China) (shown in Figure 1). CdSe/ZnS QDs (catalog Q1565) were purchased from

Figure 1. Structures of carbaryl, metolcarb, and isoprocarb. Jiayuan (Wuhan, China). The 1-vinyl-3-hexyllimidazoliumhexafluorophosphate ILs was purchased from the Chinese Academy of Sciences (Lanzhou, China). Whatman filter paper no. 1 was purchased from GE Healthcare Worldwide. The functional monomer methacrylic acid (MAA), the cross-linker ethylene glycol dimethacrylate (EDGMA, 98%), and the initiator 2, 2-azobis(isobutyronitrile) (AIBN, 99%) were purchased from J&K Chemicals (Shanghai, China). The MAA and EDGMA were distilled under reduced pressure and AIBN was recrystallized in ethanol before use. All of the reagents were analytical grade. Instruments. Fluorescence (FL) spectrophotometry was performed on an FLR Varian Cary Eclipse (Varian, USA) equipped with a quartz cell (1 cm × 1 cm). Ultraviolet−visible (UV−vis) spectra (200−800 nm) were recorded on a Cary 100-Bio UV spectrometer (Victoria, Australia). Scanning electron microscopy (S-4800, JEOL) was used to observe the surface morphologies of QDs@MIP and fluorescence material based on quantum dots and non-imprinted polymer (QDs@NIP). Fourier-transform infrared (FT-IR) spectra (4000−400 cm−1) were recorded using a Thermo Scientific Nicolet IS 50 (Thermo Fisher Scientific, USA). Synthesis of Fluorescence Material QDs@MIP. ILs are considered to be a green reaction media because of their nonvolatility, nonflammability, high ionic density, and high ionic conductivity, and they are widely used in areas such as synthesis, separation, and catalytic chemistry. In this study, ILs were used as the medium in the preparation of MIPs. They were anchored on the surface of QDs by electrostatic interaction. Under dark conditions, CdSe/ZnS QDs (50 μmol) were dissolved in chloroform (10 mL), mixed with 1-vinyl-3hexyllimidazolium hexafluorophosphate IL (3 mmol), and stirred for 3 h at room temperature. Then, synthesis of molecularly imprinted fluorescence material based on IL-modified CdSe/ZnS QDs was

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RESULTS AND DISCUSSION Preparation and Characterization of QDs@MIP and QDs@NIP. The general scheme for the synthesis of QDs@MIP and QDs@NIP was illustrated in Figure 2. First, the CdSe/ZnS QDs were modified with the ILs through electrostatic interactions. Then the ILs-modified CdSe/ZnS QDs were reacted with MAA through covalent bonding and the carbaryl was reacted with the MAA by hydrogen-bond interactions. The CC bonds of ILs, anchored on the surface of QDs, participated in the polymerization process of MIPs. The introduction of ILs could increase the surface functional groups of QDs and improve the fluorescence stability of QDs. After the template molecules were extracted from the composites by decomposing the hydrogen bonds, the identification of specific cavity sites was made in the composites and the QDs@MIP was obtained. Before removal of the

Figure 2. Schematic illustration of the preparation of QDs@MIP. 4967

DOI: 10.1021/acs.jafc.5b01072 J. Agric. Food Chem. 2015, 63, 4966−4972

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Journal of Agricultural and Food Chemistry template molecules, the FL intensity of the QDs@MIP was about 24.4% of that of the QDs@NIP, while after removal of it, the FL intensity of the QDs@MIP increased to 96.6% of that of the QDs@NIP (Figure 3). This result indicated that the template molecule (carbaryl) was removed cleanly from the composites.

Figure 5. FT-IR spectra of (a) QDs@MIP and (b) IL-QDs.

Furthermore, the characteristic CC peaks of 1666 cm−1 in IL-QDs disappeared markedly from the QDs@MIP, which showed the covalent interactions between the ILs and MAA. All these results indicated that the QDs@MIP was successfully grafted on the surface of the IL-coated CdSe/ZnS QDs. Adsorption Capacity of the QDs@MIP and the QDs@ NIP. In order to investigate the binding performance of the QDs@MIP and QDs@NIP, an equilibrium binding analysis was carried out using 10 mg L−1 carbaryl. First, to compare the effects of different solvents on the adsorption, acetonitrile, ethanol, methanol, and chloroform were selected. The difference in FL intensity between the QDs@MIP and the QDs@NIP was obvious when acetonitrile as solvent, showing that acetonitrile was a more suitable solvent for adsorption of carbaryl than the others. Therefore, acetonitrile was chosen as the adsorption solvent in the following experiments. Effect of Adsorption Time. The influence of adsorption time on the fluorescence intensity investigated at different times was showed in Figure 6. It was found that the mixture showed a rapid decrease in fluorescence intensity after the carbaryl was added. The reaction reached equilibrium within 30 min, and the fluorescence intensity was kept constant with the rise of adsorption time beyond 30 min (Figure 6a). In addition, the UV−vis spectra of carbaryl (Figure 6b) decreased gradually until no carbaryl was detected after 30 min. This result was in accordance with the FL result. This further indicated that the optimal adsorption time was 30 min. Optosensing of Carbaryl by QDs@MIP. QDs@MIP was used in this study as a sensing material to detect carbaryl. As shown in the absorption spectrum of the carbaryl and QDs@ MIP (Figure 7), the UV absorption band of carbaryl is close to the band gap of the QDs@MIP and there is no spectral overlap

Figure 3. FL spectra of QDs@NIP, QDs@MIP (after wash), and QDs@MIP (before wash).

Characterization of QDs@MIP and QDs@NIP. The surface morphologies of QDs@MIP and QDs@NIP were observed by SEM (Figure 4). The QDs@ MIP (Figure 4a) had better uniformity and spherical structure and a rougher surface than QDs@NIP (Figure 4b), indicating that the template molecules’ sites were left in the QDs@MIP. FT-IR spectroscopy is performed to further characterize the QDs@MIP (Figure 5). The characteristic peaks of IL-QDs were shown in Figure 5b: the bands at 3173 and 3113 cm−1were the stretching vibrations of aromatic C−H, the 2963 and 2940 cm−1 were the stretching vibrations of saturated C− H, the strong bands at 1666 cm−1 were the stretching vibrations of CC, the bands at 1554, 1465, and 1178 cm−1 were the stretching vibrations of CN, aromatic CC, and C−N, respectively. The characteristic peaks of QDs@MIP were shown in Figure 5a: the peaks at 2981 and 2958 cm−1 were the stretching vibrations of saturated C−H, the bands at 1456 and 1159 cm−1 were respectively the stretching vibrations of aromatic CC and C−N in ILs. The strong peak at around 1731 cm−1 of CO, indicating that the cross-linker (EDGMA) had been successfully incorporated into the polymers.

Figure 4. SEM images of (a) QDs@MIP and (b) QDs@NIP. 4968

DOI: 10.1021/acs.jafc.5b01072 J. Agric. Food Chem. 2015, 63, 4966−4972

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Journal of Agricultural and Food Chemistry

Figure 6. Dynamic adsorption of QDs@MIP for carbaryl (10 mg L−1): (a) fluorescence; (b) UV−visible.

Figure 7. UV−vis spectra of carbaryl and QDs@MIP.

between the absorption spectrum of the carbaryl and the emission spectrum of the QDs@MIP (Em 565 nm). On the basis of former studies,23,24 the mechanism of the decrease in fluorescence intensity may be charge transfer instead of energy transfer from the QDs to carbaryl. The QD valence band is full of charges; when a beam of light shines on the QDs, the charges tranfer to the conduction band. Charges in the conduction band are usually instable and can go back to the valence band, emitting photons and causing fluorescence. When carbaryl was presented, the charges of the conductive bands of the QDs could transfer to the lowest unoccupied molecular orbital of the UV band of carbaryl. They could not go back to the valence band, and there was no more fluorescence. That was FL quenching between carbaryl and QDs. The FL quenching in this system followed the Stern− Volmer equation. F0/F = 1 + KSV[Q ]

(1)

F0 and F were the FL intensity of the QDs in the absence and presence of carbaryl, respectively, KSV was the Stern−Volmer constant, and [Q] was the concentration of carbaryl. This equation was used to quantify the different quenching constants in this research, and the ratio of the KSV values of the MIP and NIP (KSV,MIP/KSV,NIP) was defined as the imprinting factor to evaluate the selectivity of the materials. Selectivity Adsorption of QDs@MIP. Metolcarb and isoprocarb were selected as structural analogues to evaluate the selectivity of QDs@MIP (Figure 8). In Figure 8a and b, the FL changes of QDs@MIP were obvious between carbaryl and its structural analogues but less obvious in QDs@NIP. There are customized recognition sites in QDs@MIP after removing templates molecule that can specifically adsorb carbaryl, while

Figure 8. Selectivity of QDs@MIP (a) and QDs@NIP (b) for carbaryl, metolcarb, and isoprocarb and quenching constant of QDs@ MIP and QDs@NIP (c) for carbaryl, metolcarb, and isoprocarb.

metolcarb and isoprocarb are not easy to identify. And there are no customized recognition sites formed in the QDs@NIP. This demonstrated the recognition mechanism of QDs@MIP is 4969

DOI: 10.1021/acs.jafc.5b01072 J. Agric. Food Chem. 2015, 63, 4966−4972

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Journal of Agricultural and Food Chemistry

Figure 9. FL stability of QDs@ MIP and QDs@ NIP: (a) room temperature away from light for 60 days, (b) continuous UV irradiation for 10 h.

Figure 10. (a) FL spectra of QDs@MIP with increasing concentration of carbaryl. (b) Effect of carbaryl concentration on the FL intensity of QDs@ MIP.

linear fluorescent quenching for carbaryl in the concentration range from 4.97 × 10−7 to 3.98 × 10−4 mol L−1, with a correlation R2 = 0.9954, as shown in Figure 10b. The LOD for carbaryl was 1.47 × 10−7 mol L−1. The precision for five replicate detections of carbaryl (10 mg L−1) was 2.4% (relative standard deviation). The reproducibility of QDs@MIP was also investigated. Three replicate measurements were carried out under the same conditions. The relative standard deviation was 3.8%.This indicates that the FL sensing method with QDs@ MIP could be used for simple, rapid determination of carbaryl with efficiency and sensitivity. Application of the QDs@MIP to the Detection of Carbaryl in Real Samples. The ability of the developed FL method to quantify concentration of carbaryl in a real sample was examined. Samples of rice and Chinese cabbage were purchased from a supermarket. No response corresponding to carbaryl was observed by HPLC. Furthermore, three levels of carbaryl at 5, 10, and 20 mg L−1 were spiked. The sample pretreatment procedures were performed as follows: 2 g of spiked sample was treated by mixing 6 mL of methanol and shaking at 200 rpm on the rotary shaker for 30 min. The extracts were purified by filter paper. The filtrate was concentrated to near dryness and redissolved with acetonitrile

based on the interactions of the structure and function of the templates. As shown in Figure 8c, the QDs@MIP exhibited a larger KSV than the QDs@NIP. And the KSV of carbaryl was much higher than its structural analogues. The ratio of KSV, MIP to KSV,NIP was the highest for carbaryl, at 6.78, indicating efficient imprinting. Stability of QDs@MIP. The stability of QDs@MIP was also studied. The FL stabilities of QDs@MIP and QDs@NIP are shown in Figure 9 under the conditions of room temperature away from light for 60 days and with continuous UV irradiation for 10 h, respectively. It could be seen that for QDs@MIP in Figure 9a, FL decreased less than 5% at room temperature away from light for 60 days. For UV irradiation, the FL change was not obvious within 6 h. After 10 h, the FL of QDs@MIP reduced by 5.8%, and QDs@NIP was down 14.4%. It was known that the shelf life of bare QDs was very short and the solution turned to opaque black easily without surfacemodification. However, the sensing material of QDs@MIP had better photostability. It could be explained that the package of an MIP layer outside might protect the photo stability of QDs. QDs@MIP for the FL Detection of Carbaryl. In this system, the FL quenching followed the Stern−Volmer equation. Under optimum conditions, the QDs@MIP exhibited 4970

DOI: 10.1021/acs.jafc.5b01072 J. Agric. Food Chem. 2015, 63, 4966−4972

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Journal of Agricultural and Food Chemistry

had attractive advantages, such as being easily prepared and highly selective and photostable. The high selectivity of the analytical method based on QDs@MIP as the sensing material was successfully applied to analyze carbaryl in rice and Chinese cabbage samples. The study could also provide the technical support for the rapid detection in food safety fields.

before analysis. A summary of the spiked concentration for samples and the corresponding analysis results is shown in Table 1. The concentrations of carbaryl in the spiked samples Table 1. Spiked Recovery Results for the Determination of Carbaryl in Rice and Chinese Cabbage Samples by QDs@ MIP, HPLC, and ELISA sample rice

Chinese cabbage

spiked level (mg kg−1)

QDs@MIP (mean ± SD; n = 3, mg kg−1) ± ± ± ±

5 10 20 5

4.22 8.14 15.72 4.01

0.30 0.65 1.04 0.35

10 20

8.56 ± 0.79 14.19 ± 1.09

ELISA (mg kg−1) 4.36 8.47 16.10 4.14

± ± ± ±

0.15 0.24 0.50 0.17

8.87 ± 0.27 14.94 ± 0.35

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HPLC (mg kg−1) 4.27 8.70 16.29 4.08

± ± ± ±

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel.: +86 0511 88780201.

0.10 0.13 0.28 0.14

Notes

The authors declare no competing financial interest.

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8.65 ± 0.20 14.78 ± 0.27

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos.31000783), the Jiangsu Planned Projects for Postdoctoral Research Funds (No.1202010B), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Zhenjiang Social Developing Fund Program (SH2014019), and the Talent Foundation of Jiangsu University (No.09JDG052).

determined by the developed FL method based on QDs@MIP were in good agreement with those of carbaryl spiked. The recoveries of carbryl were between 74% and 88%. The detected results were also validated by HPLC and ELISA, respectively. And the correlation between these methods was good (Figure 11, R2 = 0.98). In summary, we here reported an optosensing material which is a simple and effective method to detect carbaryl. The material

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DOI: 10.1021/acs.jafc.5b01072 J. Agric. Food Chem. 2015, 63, 4966−4972