Detection of nonfluorescent cyhalothrin in honey by a spheral SiO₂-based particle coating with thin fluorescent molecularly imprinted polymers film

Abstract

In this study, we report a general protocol for making core–shell SiO₂@KH570-MIP based on the surface modification of SiO₂ beads for the selective detection of ultra trace cyhalothrin. We first prepared the fluorescent surface molecularly imprinted polymer (SMIP) spheres via copolymerization of acrylamide with allyl fluorescein in the presence of cyhalothrin to form recognition sites. The experimental results showed that the fluorescence quenching of SiO₂@KH570-MIP for cyhalothrin was much higher than that of the structural analogue composite, which illustrated good recognition capacity and selectivity of SiO₂@KH570-MIP for the template. In addition, a linear relationship could be obtained at a lower concentration range of 0–2.5 nM with a correlation coefficient of 0.99698 described by the Stern–Volmer equation. The results of practical detection suggest that the developed method satisfactorily determines cyhalothrin in honey samples. This study therefore demonstrated the potential of SiO₂-based SMIP spheres for the recognition and detection of cyhalothrin in food.

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1. Introduction

Pyrethroid insecticides are widely used for pest-control in agriculture areas because of relatively low mammalian toxicity and high activity against a broad spectrum of insect pests. Nevertheless, for their widespread usages and less solubility, pyrethroid insecticides are usually removed by solid sorbent, resulting in low residue concentration in the environment and enrichment of agricultural products. Some of the pyrethroid insecticides are not only known to cause disturbed consciousness and seizures by affecting the central nervous system of humans, but they are also suspected to have endocrine-disrupting effects, which can adversely affect reproduction and sexual development, as well as the immune system.^1–5 Thus, sensitive analytical methods that involve a small amount of solvent and are sensitive to trace levels of pesticide residues in food must be employed in monitoring pyrethroid insecticides. Usually, pyrethroid insecticides are determined by gas chromatography coupled with electron-capture detection (GC-ECD), mass spectrometry (GC-MS), or liquid chromatography-electrospray ionization mass spectroscopy (LC-MS).^6–8 The MS instruments are very expensive but exhibit high selectivity and sensitivity.⁹ These methods are also complicated to operate, time-consuming and require a tedious sample pretreatment. The method in this study not only has the advantages of high sensitivity, simplicity, efficiency, speed, and convenience but also could deal with complex samples. The interference of coexisting substances in the specimen material can be eliminated by this coupling technology, and the selectivity of MIPs can be combined with the high sensitivity of fluorescence detection.

An optosensing system, especially applying fluorescence technology, is a useful method for detecting environmental pollutants¹⁰ because it has the advantages of high sensitivity, low detection limit, and low cost.¹¹ In view of these properties, fluorescence technology would be appropriate for detecting pyrethroid pesticides at low concentrations. However, not all the analytes can be directly detected by fluorescence technology. Phosphors with strong fluorescence could be used to recognize the targets with no fluorescence through chemical reactions. It is well known that fluorescein and its derivatives are well known detector as a fluorescent probe in food testing, which are generally used to detect heavy metal ions and analyze environmental pollution based on the variations of fluorescence,^12,13 because this type of compound has a high extinction coefficient and high fluorescent quantum yield in an aqueous or organic solution.¹⁴ In addition, the Stoke's shift¹⁵ is big enough to avoid self-absorption, which improves the validity of fluorescence detection. However, fluorescein and its derivatives are not very selective in recognizing target molecules and are not able to distinguish between the proper target and analogues.¹⁶ Recently, considerable effort has been devoted to develop functional receptors with synthetic counterparts as recognition elements for the simple, rapid, selective, and sensitive detection of pesticides.

Molecular imprinting technique (MIT) has been known as an attractive method to develop artificial receptors, which are used to prepare molecularly imprinted polymers (MIPs).^17,18 MIPs are frequently used to separate certain molecules as a type of established and powerful particle with specific recognition sites. After extraction of the template, the spatial cavity that is complimentary in shape for the desired analyte is formed in addition to a combination of hydrogen bonding, hydrophobic and/or electronic interactions. MIPs looking like a recognition system by nature are characterized by their capability of recognizing and binding target molecules with high affinity and selectivity.¹⁹ By virtue of the super-cross-linked nature, MIPs can tolerate extreme physical and chemical treatment such as high temperatures, pressures, extreme pH values, organic solvents, acids, and bases.²⁰ To date, the simplest method that can be used for the production of MIP in the laboratory involves conventional free-radical solution polymerization. A highly cross-linked monolithic polymer is obtained upon copolymerization of functional monomer(s) with an excess of cross-linking agent in the presence of a template. When the polymer particles are discrete, grinding and sieving of the monolith is necessary. Unfortunately, the extraction of template molecules from the thick polymer network is considerably difficult due to the high cross linking nature of MIPs, which results in low binding kinetics and small binding capacity.²¹ By coating the MIPs film onto a solid support, the surface-imprinting technique provides an alternative way to improve mass transfer and reduce permanent entrapment of the template. To date, SiO₂ spheres, which possess a vast surface area, physical robustness and thermal stability, have been widely used in the surface imprinting process.^22,23

In this study, we proposed a novel fluorescent SiO₂@KH570-MIP by the surface-imprinting technique for the optical detection of ultra trace cyhalothrin. Fluorescent SiO₂@KH570-MIP was obtained using cyhalothrin as template, acrylamide (AM) as functional monomer, allyl fluorescein as fluorophore, ethylene glycol dimethacrylate (EGDMA) as crosslinker, azobisisobutyronitrile (AIBN) as initiator and acetonitrile as porogen. The characterization of the fluorescent SiO₂@KH570-MIP spheres was performed through Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fluorescence spectra were obtained on a fluorescence spectrophotometer. The selectivity of the fluorescent SiO₂@KH570-MIP particles and detection of target molecules were elucidated by the different fluorescent intensities of the cyhalothrin and the similar pyrethroid pesticides. Desired pyrethroid pesticide fluorescent SiO₂@KH570-MIP particles were obtained for the quantitative analysis of ultra trace pesticide residue.

2. Experimental section

2.1 Materials

Cyhalothrin, cyfluthrin, fenvalerate and bifenthrin were purchased from Yingtianyi standard sample company (Beijing, China). N,N-Dimethylformamide (DMF, 99.5%), acrylamide (AM), ethylene glycol dimethacrylate (EGDMA), 2,2′-azobis (2-methylpropionitrile) (AIBN), tetraethyl orthosilicate (TEOS) and γ-methacryloxypropyltrimethoxysilane (KH570) were obtained from Aladdin-reagent. Double distilled water was prepared in-house and used for cleaning processes. All other chemicals used were of analytical grade and obtained commercially.

2.2 Fluorescence measurement

The samples for the spectral measurements were prepared by dispersing 100.0 mg of fluorescent-MIP particles in 100.0 mL alcohol solution. 5.0 mL of the solutions were then poured into various amounts of cyhalothrin (0–10 nM) in 5.0 mL of ethanol. The mixture was stirred thoroughly before measurement. Fluorescence spectra were obtained on a Cary Eclipse fluorescence spectrophotometer under a 450 nm excitation light source.

2.3 Characterization

Infrared spectra (4000–400 cm⁻¹) were obtained on a Nicolet NEXUS-470 FT-IR apparatus (USA) using KBr disks. Scanning electron microscopy (SEM) images were recorded by a JSM-7001-F. The energy dispersive spectrometer was an Inca Energy-350 X-ray photoelectron spectrometer (Oxford, Britain). Fluorescence intensity was measured by Cary Eclipse fluorescence spectrophotometer (Varian, USA). Transmission electron microscopy (TEM) images were recorded by a JEM-2100 (HR) electron microscope.

2.4 Synthesis

2.4.1 Preparation of allyl fluorescein. Allyl fluorescein was prepared according to a procedure from the literature.²⁴ In a typical synthetic procedure (Scheme S1†), a mixture of fluorescein (2.00 g, 6.0 mmol), allyl bromide (2.42 g, 20.0 mmol), K₂CO₃ (4.97 g, 36.0 mmol), hydroquinone and iodine (trace), and dry DMF (60 mL) was heated and stirred at 71 °C in the dark under N₂ for 25 h. The solvent was removed under reduced pressure. The crude product was recrystallized from carbon tetrachloride, and the resulting product was separated by column chromatography on silica.

2.4.2 Preparation of SiO₂@KH570 beads. First, SiO₂ beads were prepared according to the Stöver process. Typically, 2 mL of ammonium hydroxide was dissolved in 25 mL ethanol and 25 mL double distilled water by sonication for 15 min in a 100 mL round-bottomed flask. Then, 2.0 mL of TEOS was added into the flask. The mixture was reacted for 24 h at room temperature with continuous stirring. Second, 1 mL of KH570 was added into the flask, and the mixture was reacted for another 24 h. The resulting product was separated from the solvent by centrifugation, and washed five times with ethanol and double distilled water. Finally, the SiO₂@KH570 beads obtained were dried under vacuum overnight at 40 °C.

2.4.3 Preparation of fluorescent SiO₂@KH570-MIP particles. 150 mg SiO₂@KH570 beads were added to a 100 mL round-bottomed flask and dispersed in 60 mL acetonitrile by sonication for 30 min. Then, cyhalothrin, AM, allyl fluorescein and EGDMA were dissolved in the round-bottomed flask. The solution was degassed in an ultrasonic bath for 5 min then sparged with oxygen-free nitrogen for 10 minutes. The flask was then submerged in a thermostatically controlled oil bath and then the mixture was stirred. The temperature was increased from room temperature to 60 °C over 2 h. Then, free-radical initiator AIBN was added to the flask, and the flask was maintained at 60 °C for further 24 h. Accurately measured amounts of the reactants were mixed in the correct order, as shown in Table S1.† At the end of the reaction, the particles were collected from the reaction medium by centrifugation, and were then cleaned successively with methanol/acetic acid (100 mL, 90/10 v/v) to remove the templates by Soxhlet extraction. Finally, the product was dried in vacuo overnight at 40 °C. Fluorescent SiO₂@KH570-NIP were prepared under nominally duplicate conditions to SiO₂@KH570-MIP in the absence of the cyhalothrin template. By gravimetric analysis, the yields of fluorescent-MIP and fluorescent-NIP were found to be 62% and 58%, respectively.

2.5 The adsorption experiments of fluorescent SiO₂@KH570-MIP particles

The samples for the spectral measurements were prepared by dispersing 100.0 mg of fluorescent SiO₂@KH570-MIP particles in 100.0 mL alcohol solution. 5.0 mL of the solutions were then poured into various concentration of the cyhalothrin (0–10.0 nM) in 5.0 mL of ethanol. The mixture was stirred thoroughly before measurement. The fluorescence intensity was detected with the fluorescence spectrophotometer. The fluorescence quenching efficiency of the fluorescent SiO₂@KH570-MIP particles with cyhalothrin was calculated using the Stern–Volmer equation (eqn (1)). A linear relationship could be described by plotting [(I₀/I) − 1] as a function of concentration (nM). The same procedure was performed for the fluorescent SiO₂@KH570-NIP.

(I₀/I) − 1 = K_SV × C (1)

I₀ is the initial fluorescence intensity in the absence of analyte, I is the fluorescence intensity in the presence of related concentrations of cyhalothrin, K_SV is the Stern–Volmer quenching constant, and C (nM) is the concentration of cyhalothrin.

2.6 Selectivity and interference experiments

To further evaluate the selectivity of the fluorescent SiO₂@KH570-MIP beads, the potential interference of several structurally related compounds upon the determination of cyhalothrin were studied and compared. The fluorescent SiO₂@KH570-MIP beads were added to 10 mL of methanol solutions containing 1.0 nM L⁻¹ of cyfluthrin, fenvalerate or/and bifenthrin. The mixture was agitated for 4 h at room temperature. Then, the fluorescence intensity was detected with the fluorescence spectrophotometer and the [(I₀/I) − 1] values were calculated from eqn (1) with the fluorescence data. The fluorescent SiO₂@KH570-NIP was also evaluated with the same procedure.

3. Results and discussion

3.1 Characterization of fluorescent SiO₂@KH570-MIP

FTIR spectra. The FTIR spectra of SiO₂@KH570 and fluorescent SiO₂@KH570-MIP are shown in Fig. 1. As shown in Fig. 1a, the observed feature around 1094 cm⁻¹ is assigned to the asymmetric stretching vibration of Si–O–Si bonds. The peaks at 953 and 800 cm⁻¹ are attributed to the stretching vibrations of Si–O–H and Si–O, respectively. The peak at 1634 cm⁻¹ proves that SiO₂ beads had been modified successfully by KH570. In Fig. 1b, the characteristic peaks of EGDMA at 1731 cm⁻¹ are attributed to the C=O stretch. The typical peaks at 3439 cm⁻¹ represent the stretching vibration of secondary amine groups of AM. At the same time, the band appearing at 1683 cm⁻¹ is attributed to the C=O stretching of carbonyl group of AM. Furthermore, the characteristic peak at 1387 cm⁻¹ is attributed to the stretch of C–N of AM. The characteristic peaks verified the successful polymerization of the functional monomer (AM) with the crosslinker (EGDMA). These FT-IR spectra suggest that the fluorescent SiO₂@KH570-MIP particles had been successfully prepared.

Fig. 1 FT-IR spectra of (a) SiO₂@KH570 and (b) SiO₂@KH570-MIP particles.

SEM and TEM images. The morphological structure and particle size distribution of SiO₂@KH570 and fluorescent SiO₂@KH570-MIP are provided by SEM and TEM. As shown in Fig. S1,† the SEM image of the SiO₂@KH570 beads reveals a relatively uniform size distribution with a mean diameter of about 260 nm. A thin imprinting layer and much smaller core–shell particles can be obtained on this basis. The SiO₂@KH570-MIPs particles have a regular spherical morphology with a decrease in the amount of monomer and crosslinker, as shown in Fig. S2.† It is also obvious that the particles are uniformly dispersed. Fig. S3† shows the TEM image of SiO₂@KH570-MIP with the decrease in the amount of monomer and crosslinker. It is clearly observed that the particles have a regular spherical morphology under the optimal conditions. A 10 nm imprinting layer on the surface of SiO₂@KH570 core can also be seen. Thus, the SiO₂@KH570-MIP was successfully prepared. The morphology of fluorescent SiO₂@KH570-MIP was evaluated by SEM and TEM from the resulting patterns shown in Fig. S2 and S3.† They clearly show that the thickness of the shell film of core–shell structure is about 10 nm under the optimal conditions. Thus, it is confirmed that the fluorescent SiO₂@KH570-MIP particles with a core–shell structure have been successfully prepared. The size of the fluorescent SiO₂@KH570-MIP was about 270 nm. Thus, fluorescent SiO₂@KH570-MIP has a large surface to selectively recognize the target molecule.

3.2 Fluorescence properties and analytical applications of the fluorescent SiO₂@KH570-MIPs

The adsorption experiments were carried out using fluorescent SiO₂@KH570-MIP or fluorescent SiO₂@KH570-NIP in varying concentrations of cyhalothrin in methanol at room temperature. The fluorescence spectrophotometer was used to monitor the fluorescence intensity of values at an excitation wavelength of 450 nm. Different fluorescent spectra could be obtained by varying the fluorescence intensity of the SiO₂@KH570-MIP particles with the concentration of cyhalothrin. To further study the fluorescence property of the particles, the fluorescence emission spectra of allyl fluorescein and fluorescent SiO₂@KH570-MIP particles were recorded (Fig. 2). It was observed that the peak position of fluorescence emission for fluorescent SiO₂@KH570-MIP particles was about 517 nm, which is a blue shift of 35 nm as compared to the free allyl fluorescein (552 nm) in ethanol. This blue shift in the fluorescence spectra might be responsible for the difference of the allyl fluorescein environments in ethanol and in the polymer matrix as a result of the change in the dielectric environment of allyl fluorescein molecules upon loading into the polymer matrix. Quantitative analysis of cyhalothrin was performed by monitoring the fluorescence emission intensity at 517 nm of the ethanol dispersion.

Fig. 2 Fluorescence spectra of allyl fluorescein and SiO₂@KH570-MIPs.

To investigate the fluorescence quenching mechanism of SiO₂@KH570-MIP particles with cyhalothrin, the quenching efficiency of SiO₂@KH570-MIP particles was evaluated by the Stern–Volmer equation, as follows:

(I₀/I) − 1 = K_SV × C

where I₀ is the initial fluorescence intensity in the absence of analyte, I is the fluorescence intensity in the presence of C, and K_SV is quenching constant with cyhalothrin.

As shown in Fig. 3, the fluorescence intensity decreased with increasing cyhalothrin concentration. In Fig. 3, it is noticed that the fluorescence intensity of fluorescent SiO₂@KH570-MIP (Fig. 3a) is quenching with the increasing cyhalothrin concentration, and the reduced degree of fluorescent SiO₂@KH570-MIP was notably higher than that of the fluorescent SiO₂@KH570-NIP (Fig. 3b). It is illustrated that the spatial adsorption sites could be incorporated into the SiO₂@KH570-MIP matrix, but none in SiO₂@KH570-NIP. Therefore, it is also confirmed that the SiO₂@KH570-MIP particles had been successfully prepared.

Fig. 3 Response of (a) SiO₂@KH570-MIP and (b) SiO₂@KH570-NIP to cyhalothrin in the concentration range from 0 to 10.0 nM.

As shown in the inset of Fig. 4a, the good linearity of the method was investigated by the relationship of the fluorescence intensity against the concentration of cyhalothrin in the range from 0 to 2.5 nM. The linear equation of SiO₂@KH570-MIP particles was (I₀/I) − 1 = 0.26795C_c + 0.01604 (where C is the concentration of cyhalothrin in nM, and (I₀/I) − 1 is the relative fluorescence intensity), and the corresponding correlation coefficients (R²) were R² = 0.99698. The limit of detection was evaluated using 3s/S, and is found to be 0.004 nM, where s is the standard deviation of the blank signal, and S is the slope of the linear calibration plot. The linear Stern–Volmer relationship suggested that the quenching arises from either a static mechanism by the quenching of a bound complex or a dynamic mechanism by the quenching of a bimolecular collision of the excited states. However, as shown in the inset of Fig. 4b, the relationship of the fluorescence intensity against the concentration of cyhalothrin in the range from 0 to 0.5 nM was worse with R² = 0.99382. In the case of SiO₂@KH570-MIP particles, a specific binding effect of the recognition site to cyhalothrin results in the better affinity of cyhalothrin. The obtained high quenching efficiency of the SiO₂@KH570-MIP resulted from specific recognition sites for a template molecule that was created during the course of imprinting.

Fig. 4 Dependence of fluorescence quenching efficiency on cyhalothrin concentration. Insets show the linear equations of (a) SiO₂@KH570-MIP and (b) SiO₂@KH570-NIP.

3.3 Selective recognition of the fluorescent SiO₂@KH570-MIP

To further study the selective recognition properties of the SiO₂@KH570-MIP particles, cyfluthrin, fenvalerate and bifenthrin were selected to act as the competitors; their structures are similar to that of cyhalothrin. The experiment was performed by adding SiO₂@KH570-MIP ethanol dispersion into each solution (1.0 nM), and the fluorescence intensity was analyzed.

Fig. 5a shows that the fluorescence quenching efficiency of SiO₂@KH570-MIP for cyhalothrin was considerably higher than that of the structural analogues. The results showed that none of the competitors being evaluated lead to any significant fluorescence quenching. To further investigate the competitive quenching amount of cyfluthrin, fenvalerate and bifenthrin, three competitive pesticides were added into cyfluthrin solution in turn to form blend solutions, and the concentrations of both cyfluthrin and the competitive pesticides were 1.0 nM. As can be seen from Fig. 5b, the reduction in the fluorescence intensity of SiO₂@KH570-MIP particles was not obvious with the three competitive pesticides, and the competitors being evaluated did not give any significant interference. It can be proven that the SiO₂@KH570-MIP particles provided high selectivity to cyhalothrin, which shows that the higher selectivity for cyhalothrin results from its specific binding affinity of cyhalothrin due to an efficient imprinting effect. The fluorescence quenching of SiO₂@KH570-NIP particles was invisible because of non-specific recognition sites in the SiO₂@KH570-NIP particles.

Fig. 5 Quenching amount of SiO₂@KH570-MIP and SiO₂@KH570-NIP by different types of 1.0 nM pyrethroids (a); test for the interference of different pyrethroids on the fluorescence response (b).

3.4 Application to honey sample analysis and reuse

To assess the applicability of SiO₂@KH570-MIP particles to a practical food treatment, 5.0 g honey sample dissolved into cyhalothrin solution (0–10.0 nM) was analyzed by the equation (I₀/I) − 1 = 0.26795C_c + 0.01604, and the corresponding results are listed in Table 1. The results clearly demonstrate that the SiO₂@KH570-MIP particles can produce good recovery in the range from 0 to 2.5 nM and can be effectively applied in the detection of cyhalothrin from honey samples.

Table 1 Recovery of cyhalothrin in honey samples with cyhalothrin solution at different concentration levels

	Concentration taken (nM)	Found (nM)	Recovery (%)
Cyhalothrin	0.00	0.001	—
Cyhalothrin	0.10	0.094	94
Cyhalothrin	0.25	0.261	104
Cyhalothrin	0.50	0.572	114
Cyhalothrin	1.00	1.021	102
Cyhalothrin	2.50	2.496	100
Cyhalothrin	5.00	2.687	54
Cyhalothrin	10.00	2.853	29

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To appraise the regeneration for SiO₂@KH570-MIP particles, 1.0 nM cyhalothrin solutions were first prepared. 100 mg of SiO₂@KH570-MIP particles were added into 100 mL of cyhalothrin solution and incubated for 2.0 h before measurement. After the test, the SiO₂@KH570-MIP particles containing cyhalothrin were washed with methanol–acetic acid solution (100 mL, 90/10 v/v) for 24 hours using a Soxhlet extractor, collected by centrifugation, and rinsed 3 times with ethanol. As shown in Fig. S4,† the SiO₂@KH570-MIP particles certify that they can be reused for not less than five times without a remarkable loss in signal intensity.

4. Conclusions

A core–shell nanostructure fluorescent SiO₂@KH570-MIP based on the surface of SiO₂ beads was synthesized and investigated for selective detection of cyhalothrin. The adsorption of fluorescent SiO₂@KH570-MIP was detected with a fluorescence spectrophotometer. The fluorescence intensity was analyzed using the Stern–Volmer equation. The experimental results showed that the fluorescence quenching of fluorescent SiO₂@KH570-MIP for cyhalothrin was considerably higher than that of the structurally analogue composite and the fluorescent SiO₂@KH570-NIP, which illustrates good adsorption capacity and selectivity of fluorescent SiO₂@KH570-MIP for the template. To sum up, it is noteworthy that the fluorescent SiO₂@KH570-MIP not only has a fluorescence property, but also has rapid detection and high selective recognition for the target molecules. Thus, molecular imprinting with fluorescence spectrum analysis was established to determine cyhalothrin.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (No. 21107037, No. 21407057, No. 21407064, No. 21176107 and No. 21277063), Natural Science Foundation of Jiangsu Province (No. BK2011461, No. BK2011514), National Postdoctoral Science Foundation (No. 2013M530240), Postdoctoral Science Foundation funded Project of Jiangsu Province (No. 1202002B) and Programs of Senior Talent Foundation of Jiangsu University (No. 12JDG090), Ph.D. Innovation Programs Foundation of Jiangsu Province (No. CXZZ13_0681).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17851g

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