Molecular Imprinting in Fluorescent Particle Stabilized Pickering

Article pubs.acs.org/JPCC

Molecular Imprinting in Fluorescent Particle Stabilized Pickering Emulsion for Selective and Sensitive Optosensing of λ‑Cyhalothrin Chunbo Liu,*,† Zhilong Song,† Jianming Pan,*,† Xiao Wei,† Lin Gao,† Yongsheng Yan,† Linzi Li,‡ Juan Wang,† Rui Chen,§ Jiangdong Dai,∥ and Ping Yu† †

School of Chemistry and Chemical Engineering, ‡School of the Environment, and ∥School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, People’s Republic of China § School of Biology and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang, 212013, People’s Republic of China S Supporting Information *

ABSTRACT: A red luminescent europium (Eu) complex covalently modified mesoporous silica spheres (mSiO2− Eu(TTA)3Bpc; TTA, 2-thenoyltrifluoroacetone; Bpc, 2,2′bipyridine-4,4′-dicarboxylic acid) were designed. Then the molecularly imprinted polymers combined with fluorescence sensor were first prepared via mSiO2−Eu(TTA)3Bpc stabilized Pickering emulsion polymerization, and were further applied for selective and sensitive optosensing of λ-cyhalothrin (LC). The as-prepared mSiO2−Eu(TTA)3Bpc@MIPs were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and elemental analyzer, the results demonstrated the formation of monodisperse and uniform-sized LCimprinted spheres. Meanwhile, fluorescence spectroscopy was used to evaluation of optical stability, effect of pH, and selective and sensitive determination of LC. The fluorescent intensity of mSiO2−Eu(TTA)3Bpc@MIPs decreased in the first 5 days and then kept stable for nearly a month. Under optical conditions, mSiO2−Eu(TTA)3Bpc@MIPs were successfully applied to selectively and sensitively detect LC in water, and a linear relationship could be obtained covering the concentration range of 10−1000 μM with a correlation coefficient of 0.9986. Moreover, LC could quench the luminescence of mSiO2−Eu(TTA)3Bpc@ MIPs in a concentration-dependent manner, which was best described by a Stern−Volmer-type equation.

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INTRODUCTION In agricultural situations, pyrethroids are used for the control of a wide range of pests.1 Recently, λ-cyhalothrin (LC), one of pyrethroids, has been increasingly used in replace of highly toxic insecticides such as organochlorines and organophosphorus insecticides because of its relatively low mammalian toxicity and environmental persistence.2,3 However, LC has been identified to be related to the disruptions in endocrine system, which results in negative effects on the reproduction and sexual development, as well as the immune system. Thus, a rapid, sensitive, and selective analytical technique to monitor LC is crucial in order to safeguard public health, and it is now challenging to develop novel and efficient analysis platforms. Until now, pyrethroids detecting techniques mostly focus on chromatography separation, such as high performance liquid chromatography (HPLC),4 gas chromatography (GC), 5 coupled column liquid chromatography with mass spectrophotometry (LC/MS),6 and so forth. Despite their enhanced sensitive and specificity, these methods are time-consuming and require a tedious sample pretreatment. In addition, whenever matrix effects are not appropriately overcome in the HPLC (GC) analytical schemes, the quality of the analytical results will be impaired and the lifetime of the chromatographic © 2013 American Chemical Society

columns will be also severely deteriorated. Therefore, the great priority has been given to the development of novel molecular recognition and selective separation techniques. Over the past decades, molecularly imprinted polymers (MIPs), which provide artificial receptor-like recognition sites selective for template molecules, have drawn great attention and have been widely developed to improve the separation efficiency and selectivity to biological and chemical analytes. Although the molecular imprinting technology can be introduced into the preconcentration and microextraction of trace amounts of analytes prior to the developed assay methods, there still exit some limitations. For instance, conventional methods to prepare MIPs, including suspension polymerization,7 emulsion polymerization,8 and distillation precipitation polymerization,9 all require large amounts of surfactants to maintain the stability of the system before polymerization. As a result, the surfactant will certainly cause new pollution in the preparation process. On the other hand, it is still difficult for these conventional methods to obtain MIPs with high stability Received: January 15, 2013 Revised: April 3, 2013 Published: April 30, 2013 10445

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stabilized oil-in-water Pickering emulsion. In our synthesis procedure, mesoporous silica particles were first covalently modified by Eu(TTA)3Bpc (TTA, 2-thenoyltrifluoroacetone; Bpc, 2,2′-bipyridine-4,4′-dicarboxylic acid), then MIPs-based fluorescence sensor were obtained by Pickering emulsion polymerization using these particles as stabilities (mSiO2− Eu(TTA)3Bpc@MIPs). Owing to the incorporation of Eu complex, when the templates (LC) were removed by solvent extraction, the mSiO2−Eu(TTA)3Bpc@MIPs were capable of selectively rebinding and sensitively optosensing the target molecules LC. The characterization, evaluation of optical stability, effect of pH, selective and sensitive determination of LC were investigated. Finally, mSiO2−Eu(TTA)3Bpc@MIPs were used as the sensor for selective and sensitive optosensing LC from environmental water samples.

and good monodispersity. Additionally, template molecules rebound to the recognizing sites stained in the MIPs must be removed from the MIPs and then detected via expensive and complicated instruments such as HPLC, GC, and LC/MS. Therefore, removing the templates from the MIPs calls for plenty of organic solvents, thus, causing more environmental burden. Overall, it is still a difficult but promising task, great endeavors should be done to address these issues. As we know, Pickering emulsion polymerization, in which dispersed liquid droplets are stabilized by solid particles instead of conventional surfactants, is a new interfacial nano and molecular imprinting approach to prepare MIPs.10 The stabilizing particles are located at the interface between the two immiscible liquids, thereby preventing coalescence of the droplets.11,12 Depending on the surface tension of the solid particles, Pickering emulsions may be prepared as either oil-inwater or water-in-oil systems.13 Compared with conventional polymerizations, Pickering emulsion polymerization has the following advantages: (i) the formed inner polymer shell can interlock the outer building blocks, enhancing the mechanical robustness; (ii) much fewer particles are required to produce good stability, thereby leading to a reduction in the use of hazardous surfactants and their environmental consequences;14,15 (iii) it can obtain uniform polymer layer, which offers fast molecular binding and releasing kinetics for the templates. As we know, Shen and Ye10 first reported the preparation of MIPs via Pickering emulsion polymerization using templatemodified colloidal particles. However, there is rarely report about preparation of MIPs by Pickering emulsion polymerization, especially MIPs based fluorescence sensors. MIPs-based fluorescence sensors can be applied to specifically recognize and directly quantify target analytes, independent of extracting the templates from the MIPs network and further time-consuming and complicated analysis. The incorporation of a fluorescent signal greatly enhances the utility of the MIPs sensors assay approach without sacrificing the accuracy or the efficiency.16−22 Based on the energy transfer from the excitation of the fluorescent donor to the absorption of templates (acceptor), the MIPs based fluorescence sensors were great quenched as the templates rebound into the recognition cavities. Among optical materials investigated for sensors, organic fluorescent compounds,23 and quantum dots (QDs)24,25 are the most widely used fluorescent signal. Unfortunately, both of them have inherent limitations. For example, organic fluorescent compounds typically undergo rapid photobleaching,26 whereas QDs are less chemically stable, potentially toxic,27 and show fluorescence intermittence.28,29 These problems would hinder their applications from biological detections and medical diagnosis. Therefore, it is essentially necessary to find substitutes for current luminescent materials. Lanthanide ions have been well-known for their unique luminescence properties.30−36 Their complexes also show narrow band photoluminescence and have a high luminescence quantum yield, which make them interesting candidates for luminescence applications such as biochemical sensors and fluoroimmunoassays.37 However, as we know, combining lanthanide complex as fluorescent signal with molecular imprinting technique has not been explored for the detection of analytes. Herein, our group make an attempt to explore europium (Eu, one of the lanthanide ions) complex based MIPs sensor for the detection of template molecules. In this work, MIPs-based fluorescence sensor were first achieved in Eu complex modified mesoporous silica particles

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EXPERIMENTAL SECTION Materials. λ-Cyhalothrin (LC), β-cyfluthrin (BC), fenvalerate (FE), and bifenthrin (BI) were purchased from Yingtianyi standard sample company (Beijing, China). Cetyltrimethylammonium bromide (CTAB ≥99%), ammonia solution (NH3·H2O; 25−28%), and 2-ethoxyethanol (≥99.5%) were purchased from Sinopharm Chemical Reagent Co. 2,2′Bipyridine-4,4′-dicarboxylic acid (Bpc), 2-thenoyltrifluoroacetone (TTA), 2-[N-morpholino] ethane sulfonic acid (MES), and 1-ethyl-3-(3-dimethyl amino propyl) carbodiimide hydrochloride (EDC) were purchased from Alfa Aesar. Tetraethoxysilane (TEOS ≥99.9%), (3-aminopropyl) triethoxysilane (KH550 ≥99.0%), N-hydroxysulfosuccinimide sodium salt (sulfoNHS), N,N-dimethylformamide (DMF, 99.5%), methacrylic acid (MAA), ethylene glycol dimethacrylate (EGDMA), and 2,2′-azobis(2-methylpropionitrile) (AIBN) were obtained from Aladdin-reagent. Doubly distilled water was used for preparing all aqueous solutions and cleaning processes. Preparation of Amino-Modified Mesoporous Silica Spheres (mSiO2−NH2). The silica spheres were synthesized by using CTAB as the porogen and 2-ethoxyethanol as the cosolvent.38 In a typical procedure, 0.5 g of CTAB was dissolved in 70 mL of H2O, followed by addition of 0.5 mL of ammonia solution (25−28%) and 50 mL of 2-ethoxyethanol. After 2.5 mL of TEOS was added to the solution, the mixture was vigorously stirred in a closed vessel for 24 h. A white precipitate was obtained, filtered, washed with pure water, and dried at 60 °C. CTAB and other organic components were removed by calcinations in air at 550 °C for 5.0 h. The yields of mesoporous silica spheres were above 90%. To grow the obtained mesoporous silica spheres with amino groups, 1.5 mL of KH-550 was added dropwise to the mesoporous silica spheres toluene (50 mL) solution and stirred for 12 h at 80 °C. The resulting aminated particles (mSiO2−NH2) were separated and further washed with ethanol. Preparation of Bipyridine Modified Mesoporous Silica Spheres (mSiO2−Bpc). First, 50 mg of mSiO2−NH2 particles were dissolved in 2.0 mL of water containing 0.2 mL of DMF. A solution of 40 mg Bpc in 4.0 mL of 100 mM MES (pH = 6.0) was activated with a solution of 30 mg EDC and 30 mg sulfoNHS for 1.0 h. Following the activation step, the pH was raised to 8.0 and subsequently added to the mSiO2−NH2 solution. The resulting mixture was stirred for 24 h at room temperature and washed with water by centrifugation. The white precipitate was redispersed in DMSO and filtered through a 0.22 μm polycarbonate membrane. This procedure was repeated two times each with DMSO and CH2Cl2/MeOH (10:1) mixture to 10446

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Scheme 1. Schematic Representation of mSiO2−Eu(TTA)3Bpc

remove the excess of Bpc. After the final washing with ethanol, the white particles (mSiO2−Bpc) were dried under vacuum. Synthesis of Reference Eu Complex (Eu(TTA)3Bpc). Eu(TTA)3Bpc was synthesized according to the conventional route as follows:39 TTA (120 mg) and Bpc (40 mg) were dissolved in ethanol (10 mL) under stirring at room temperature. Then five drops of 2.0 M NaOH were added to adjust the pH level to 8.0. Afterward, a EuCl3·6H2O (51 mg) solution in ethanol (3.5 mL) was added dropwise. After complete addition, the solution was stirred for 1.0 h to ensure a complete precipitation at room temperature. The precipitate was filtered out, washed repeatedly with ethanol and water, and then dried overnight under vacuum. Synthesis of Eu Complex Modified Mesoporous Silica Spheres (mSiO2−Eu(TTA)3Bpc). First, a precursor complex Eu(TTA)3(H2O)2 was prepared according to the standard procedure. In general, the synthesis was very similar to the method described above for Eu(TTA)3Bpc complex. The only difference was that no Bpc was added to the reaction mixture. The mSiO2−Eu(TTA)3Bpc particles were obtained via ligand exchange reactions. Eu(TTA)3(H2O)2 (20 mg) was codissolved with mSiO2−Bpc (40 mg) in 40 mL of CHCl3. To ensure completion of the reaction, the reagents were stirred for 24 h at room temperature. Then the solutions were concentrated in vacuum to dryness (on a rotary evaporator), and the residues were washed with ethanol and dried in a vacuum desiccator to yield Eu complex modified mesoporous silica spheres (mSiO2− Eu(TTA)3Bpc). Synthesis routes of mSiO2−Eu(TTA)3Bpc were shown in Scheme 1. Preparation of Molecularly Imprinted Polymers by Pickering Emulsion Polymerization (mSiO 2 -Eu(TTA)3Bpc@MIPs). mSiO2−Eu(TTA)3Bpc (40 mg) particles were added into a mixture of MAA (0.17 mL) and water (5.0 mL), and the mixture was sonicated for 5.0 min to produce a stable suspension of colloidal particles in water. After addition the mixture solution of EGDMA (0.94 mL) and LC (0.5 mol, molar ratio of LC/MAA = 1:4, Supporting Information, Figure S1) containing AIBN (30 mg), the pH value of the mixture was adjusted to 5.5 using 3.0 M NaOH. The mixture was again sonicated for 5.0 min and shaken vigorously for about 1.0 min by hand to give a stable Pickering emulsion, where no coalescence of the oil droplets could be observed within 2.0 h. The resulting particle-stabilized emulsion was transferred into a three-neck flask, after purging oxygen with nitrogen gas, the temperature was increased to 70 °C, and kept for 18 h without agitation. Following this polymerization procedure, composite polymer/silica particles were obtained. After decantation, the solid particles were washed with ethanol and water. After this step, the solid polymer microspheres were eluted with methanol/acetonitrile (v/v, 8:2) to removal of the templates (LC), followed by pure ethanol, and then dried in a vacuum chamber. Nonimprinted nanoparticles (NIPs) as a control to evaluate the molecular recognition properties of imprinted materials were synthesized same as MIPs without addition of LC. mSiO2−Eu(TTA)3Bpc@MIPs prepared by Pickering

emulsion polymerization were designed and synthesized according to Scheme 2. Scheme 2. Schematic Illustration of the Fabrication of mSiO2−Eu(TTA)3Bpc@MIPs

Determination of LC by Fluorescence Measurement. The fluorescence spectra analysis of the samples were carried out by adding 10 mg of MIPs in 5.0 mL tubes, then 5.0 mL of LC solutions with concentrations ranging from 0 to 1000 μmol/L were added. The samples were mixed end-over-end at room temperature for 6.0 h; afterward, the sorbent was dried at room temperature and measured with a QuantaMaster and TimeMaster spectrofluorometer under a 373 nm excitation light source with slit width of 3.0 nm. Characterizations. Infrared spectra (4000−400 cm−1) were collected on a Nicolet NEXUS-470 FT-IR apparatus (U.S.A.) using KBr disks. Scanning electron microscopy (SEM) images were recorded by JSM-7001-F. Transmission electron microscopy (TEM) images were recorded by a JEM-2100 (HR) electron microscope. A Vario EL elemental analyzer (Elementar, Hanau, Germany) was employed to investigate the surface elemental composition of the prepared composites. The thermogravimetric analysis (TGA) of samples were measured using a Diamond TG/DTA Instruments (STA 449C Jupiter, Netzsch, Germany) under a nitrogen atmosphere up to 600 °C with a heating rate of 5.0 °C/min. Fluorescence spectra was taken on a QuantaMaster and TimeMaster spectrofluorometer (Photon Technology International, Inc.).

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RESULTS AND DISCUSSION Synthesis and Characterization of mSiO 2 −Eu(TTA)3Bpc, mSiO2−Eu(TTA)3Bpc@MIPs, and mSiO2−Eu(TTA)3Bpc@NIPs. Luminescent Eu complex covalently modified mesoporous silica spheres (mSiO2−Eu(TTA)3Bpc) were synthesized through the sequence of reactions (Scheme 1). First, mesoporous silica spheres (mSiO2) were obtained by a novel combination of stabilizing condensation and dynamic self-assembly using CTAB as the porogen and 2-ethoxyethanol as the cosolvent. After modification with KH-550 to form mSiO2−NH2, 2,2′-bipyridine-4,4′-dicarboxylic acid (Bpc) was activated using standard coupling reagents, and then mSiO2− 10447

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Figure 1. SEM and TEM images of silica spheres before (a, b) and after (c, d) calcination at 550 °C, SEM and TEM images of mSiO2− Eu(TTA)3Bpc spheres (e, f).

Bpc was offered by further condensing with mSiO2−NH2. Finally, Eu complex Eu(TTA)3(H2O)2, acted as a precursor in the ligand exchange, was complexed with mSiO2−Bpc to form the mSiO2−Eu(TTA)3Bpc. The morphology of silica spheres was observed by SEM and TEM. Typical SEM and TEM images of silica spheres before (a, b) and after (c, d) calcination at 550 °C were shown in Figure 1. The mean diameter of the spherical particles was estimated to be 400 nm. After calcination at 550 °C, it could be clearly observed that channel-like pores were present inside the spheres (Figure 1d). In additional, mSiO2−Eu(TTA)3Bpc were also characterized by SEM and TEM, which were exhibited in Figure 1e and f, respectively. Pendent complexes were located on the surface of silica spheres and almost no independent Eu complex agglomerates were observed. Furthermore, it was in sharp contrast with that of just a mixture of mesoporous silica spheres with Eu(TTA)3Bpc, further indicating that Eu complex modified mesoporous silica spheres were successfully synthesized. The corresponding EDS spectrum 1 inset in Figure 1e (Supporting Information, Figure S2) showed the presence elements of Eu(TTA)3Bpc, confirming that the spheres were successfully modified with Eu(TTA)3Bpc. Spectroscopic properties of mSiO2, mSiO2−NH2, mSiO2− Eu(TTA)3Bpc, and Eu(TTA)3Bpc were also investigated. It could be seen that mSiO2−Eu(TTA)3Bpc had the same absorbance as Eu(TTA)3Bpc (Figure 2). Their fluorescence

Figure 2. UV−visible absorption spectra of mSiO2 (black), mSiO2− NH2 (royal), mSiO2−Eu(TTA)3Bpc (red), and Eu(TTA)3(Bpc) itself (green) in ethanol solution at room temperature.

spectra in ethanol showed the characteristic sharp emission lines in the red light emitting region that were typical for Eu ions (Figure 3, left). Five apparent bands in the fluorescence spectra centered at 579, 591, 615, 653, and 700 nm could be attributed to the 5D0-7F0, 5D0-7F1, 5D0-7F2, 5D0-7F3, and 5D07F4 transitions for the Eu3+ ion, respectively.40−44 But the mentioned peaks above were absent for mSiO2 and mSiO2− NH2. Direct proof of successful grafting could be found in the 10448

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1142, 1548, 1611, and 1682 cm −1 of Eu(TTA) 3 Bpc (Supporting Information, Figure S3) were also observed in the FT-IR spectrum of mSiO2−Eu(TTA)3Bpc, suggesting the attachment of the Eu(TTA)3Bpc to mSiO2 surface.45 To prepare oil-in-water Pickering emulsions, mSiO2−Eu(TTA)3Bpc spheres were employed as the stabilizing particles. A mixture of MAA and H2O was used as the water phase, EGDMA, and the template LC containing the initiator (AIBN) formed the oil phase, After being sonicated and shaken for a while to give a stable oil-in-water Pickering emulsion, mSiO2− Eu(TTA)3Bpc@MIPs were obtained when the temperature increased to 70 °C and retained for 18 h without agitation. The FT-IR of MIPs and NIPs possessed of the same absorption bands around 1729, 1262, and 1161 cm−1, which were assigned to CO stretching vibration of carboxyl (MAA), C−O asymmetric and symmetric stretching vibration of ester (EGDMA), respectively.46 The medium peaks at 1637 cm−1, which corresponded to the CC stretching mode of methacrylic vinyl groups, suggested that less than 100% of bonded EGDMA molecules were cross-linked in the MIPs. The initial EGDMA/MAA molar ratio was 2.5, so that higher molar mounts of EGDMA compared to MAA were expected crosslinked.47 Meanwhile, the absorption band at 3446 cm−1 of the MIPs could be attributed to the stretching vibration of O−H bonds from MAA molecules. All the results confirmed that the cross-linking reaction was successfully initiated in the presence of AIBN. The size and surface morphologies of the microspheres before and after polymerization were examined using optical micrographs and SEM, respectively. It could be seen clearly (Figure 5a) that the drops around (in Pickering emulsions) were surrounded by some particles and the drops diameter was about 50 μm. After polymerization, as shown in Figure 5b (under optical micrograph), monodisperse and uniform-sized spheres were obtained by the Pickering emulsions polymerization. SEM image shown in Figure 5c further confirmed the size and morphology of the assynthesized mSiO2−Eu(TTA)3Bpc@MIPs. It was clearly visible that the particle diameter was approximately the same as the results in Figure 5a,b. Figure 5c (inset) revealed that some silica particles were located on the surface of the microspheres, which proved that MIPs were successfully synthesized by Pickering emulsions polymerization. Elemental analysis was employed to ascertain each modification. The results were shown in Table 1. Compared with mSiO2, the carbon and nitrogen compositions of mSiO2− NH2 increased to 13.94 and 4.557%, respectively, suggesting that about 3.255 mmol of amino groups were successfully grafted to the surface of 1.0 g mSiO2.48 After covalently modified by the Eu complex, the carbon and nitrogen

Figure 3. (Left) Emission spectra of mSiO2 (magenta), mSiO2−NH2 (black), mSiO2−Eu(TTA)3Bpc (blue), and Eu(TTA)3Bpc itself (red) in ethanol solution at room temperature. (Right) Photographs of (1) mSiO2, (2) mSiO2−NH2, and (3) mSiO2−Eu(TTA)3Bpc in ethanol solution taken under daylight lamp (top) and 365 nm UV light (bottom).

following designed experiments. As shown in Figure 3, right, well-dispersed aqueous mSiO2−Eu(TTA)3Bpc particles emitted bright-red light originating from the characteristic emission of Eu3+ upon UV light irradiation. The infrared spectra of mSiO2, mSiO2−Eu(TTA)3Bpc, and Eu(TTA)3Bpc also confirmed covalent modifications that occurred. In Figure 4, the absorption peaks at 1096 cm−1 and

Figure 4. FT-IR spectra of (a)mSiO2 (black), mSiO2−Eu(TTA)3Bpc (red), MIPs (blue), and NIPs (cyan).

3437 cm−1 could be attributed to Si−O−Si asymmetric stretching vibration and Si−OH stretching vibration, respectively. Additionally, the characteristic absorption bands at 688,

Figure 5. Optical micrographs and digital photographs of Pickering emulsions before (a) and after (b) polymerization, SEM images of molecularly imprinted polymers (MIPs; c) by Pickering emulsion polymerization. 10449

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Table 1. Elemental Composition of the Particles from Elemental Analysis particle type

C (%)

H (%)

N (%)

S (%)

mSiO2 mSiO2−NH2 mSiO2−Eu(TTA)3Bpc MIPs NIPs

0 13.94 23.80 58.20 55.52

3.081 3.655 3.087 5.565 6.708

0 4.557 5.185 0.2018 0.2316

0 0 0.7210 0 0

compositions of mSiO2−Eu(TTA)3Bpc increased to 23.80 and 5.185%, respectively, and the sulfur increased to 0.721%, while the hydrogen composition was slightly reduced. Due to the polymerization process, it could also be observed that carbon and hydrogen compositions increased in MIPs and NIPs. The elemental compositions of the MIPs were different from those of the NIPs, which might be attributed to the different degrees of polymerization, or residual imprinted LC. The degree of functionalization for mSiO2, mSiO2−NH2, and mSiO2−Eu(TTA)3Bpc, MIPs and NIPs were evaluated by TGA under N2 atmosphere (Figure 6). The TGA curves of mSiO2−Eu-

Figure 7. Photostability of MIPs (circles) and NIPs (squares).

Figure 8. Effect of pH on luminescence of the MIPs (circles) and NIPs (squares).

Selectivity and Sensitivity Determination. The fluorescence response of MIPs toward various pesticides was conducted to examine the selectivity, and the initial concentration of each pesticide was 500 μmol L−1. The selectivity analyses of the MIPs in single solute were exhibited in Figure 9. As was evident from Figure 9, the quench amount [(Imax − I)/Imax] of MIPs for the four compounds followed the order LC > BC > FE > BI. By calculation, the differences in the quench amount of MIPs and NIPs were 0.294, 0.265, 0.115, and 0.0523 for LC, BC, FE, and BI, respectively. The results suggested that MIPs were specific to LC but nonspecific to other pesticides. Due to the distinct sizes, structures, and functional groups of the template, different binding forces could form between the pesticides and MAA, resulting in a different recognition effect. Moreover, BC has almost the same structure as LC except for the different position of the substituted chlorine and fluorine functional group, but the removal rate of MIPs for BC was still lower than that for LC, suggesting that the memory of specific functional groups also played an important role in the fabrication of tailored stereo binding sites.50 To further investigate the competitive quench amount of BC, FE, and BI, three competitive pesticides were added into LC solution in turn to form dual solute solutions, and the concentrations of both LC and the competitive pesticides were 500 μmol L−1. There was no obvious effect on FL intensity of MIPs by the three competitive pesticides, and the interference of BC, BI, and FE were too weak to be ignored (Figure 10). But for NIPs, shown in inset of Figure 10, it was necessary to consider the significant influence by the three competitive pesticides. The effects of various ions (K+, Na+,

Figure 6. Thermogravimetric analysis curves of mSiO2 (black), mSiO2−NH2 (red), mSiO2−Eu(TTA)3Bpc (green), MIPs (cyan), NIPs (magenta).

(TTA)3Bpc showed a gradual weight loss of about 34.52% at 600 °C, as compared to 17.12% of mSiO2−NH2 and 16.04% of the mSiO2, respectively. The significant weight losses of MIPs (98.92%) and NIPs (96.53%) were assigned to the decomposition of the imprinted layer, respectively, which was consistent with the carbon composition of MIPs and NIPs in Table 1. Evaluations of Optical Stability. The optical stabilities of NIPs and MIPs were estimated at room temperature, as shown in Figure 7. It was found that the FL intensity of the MIPs (red circles) was decreased gradually in 5.0 days. After that, the FL intensity almost kept in the same order of magnitude for a mouth. The similar phenomenon was observed for the photostability of NIPs (black squares). Effect of pH. The effect of pH in a range between 4.0 and 12 was studied for MIPs (red circles) and NIPs (black squares) in Figure 8. The FL intensity of MIPs at the range of 4.0 to 6.0 was considerably stable. When the pH = 7.0, the FL intensity reached to the maximum. As the pH increased from 8.0 to 12, the FL intensity decreased quickly. This was probably because the coordination compound of Eu3+ decomposed in strong alkaline media, which weakened the energy transfer from TTA to Eu3+ ions.49 As a result, a pH of 7.0 was selected for the further experiments. 10450

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selective analysis of LC in the presence of other commonly interfering ions and pyrethroids. It was rational that the imprinted binding sites (cavities) in MIPs played an important role in selective response to LC. Determination of Pyrethroids. The FL intensity of the MIPs turned out to be decreased sensitively in the presence of LC in 50% (v/v) ethanol−water solution. The relationship between the FL intensity and the concentration of quenching LC can be described by the Stern−Volmer equation:51,52

Imax /I = 1 + KSV ·[c]

(1)

where I and Imax are the FL intensities of the MIPs at a given related concentration of LC and in a LC-free solution, respectively. Ksv is the Stern−Volmer quenching constant, and [c] is the concentration of LC. The dependence of Imax/I as a function of [c] was shown in Figure 11a. The Ksv was found to

Figure 9. Top: quenching amount of MIPs and NIPs by different kinds of 500 μmol L−1 pyrethroids (A, LC; B, BC; C, BI; D, FE). Bottom: the molecular structure of different pyrethroids.

Figure 10. Test for the interference of different pyrethroids on the fluorescence response toward LC of MIPs. (a) MIPs, (b) MIPs + LC, (c) MIPs + LC + BC, (d) MIPs + LC + BI, (e) MIPs + LC + FE. Inset: the interference for NIPs. (a) NIPs, (b) NIPs + LC, (c) NIPs + LC + BC, (d) NIPs + LC + BI, (e) NIPs + LC + FE (500 μmol L−1 of LC and competitive pyrethroids were used).

Figure 11. Fluorescence spectra of the MIPs (a) and NIPs (b) with the increasing concentrations of LC. Inset: Stern−Volmer-type description of the data showing a linear fit throughout the LC concentration range, with a correlation coefficient R2 = 0.9986 for MIPs and R2 = 0.9982 for NIPs.

Ca2+, Mg2+, Cl−, NO−3 , CO2− 3 ) were listed in Table 2, the results also proved that various ions mentioned above had no effect on FL intensity of MIPs. Therefore, MIPs could be used for highly Table 2. Test for the Interference of Different Substances on the Fluorescence of MIPs and NIPs

coexisting substance

coexisting concentration (10−7 mol L−1)

change of fluorescence intensity (%) for MIPs

change of fluorescence intensity (%) for NIPs

K+ Na+ Ca2+ Mg2+ Cl− NO−3 CO2− 3

500 500 200 200 100 200 100

0.90 −1.2 1.8 1.80 −4.4 −2.2 0.44

−1.3 0.96 2.1 2.6 3.8 4.4 −3.3

be 9.953 × 10−4 L μmol−1. The linear range of the calibration curve was 10−1000 μmol L−1 with a correlation coefficient of 0.9986. To demonstrate the imprinting effect, as a control experiment, the fluorescence response of NIPs to the template molecule was investigated. As shown in Figure 11b, the Ksv was 5.074 × 10−4 L μmol−1, the linear range for LC was also 10− 1000 μmol L−1 but with a correlation coefficient of 0.9982. The Ksv of the MIPs and NIPs was important data to evaluate the selectivity and sensitivity of the materials we obtained. According to the obtained results, the MIPs have a better selectivity and sensitivity than those of NIPs. Therefore, the FL analysis was more suitable for the on-site and rapidly detect analysis. 10451

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The Journal of Physical Chemistry C Application to Water Sample Analysis. Surface river water samples were collected from local rivers. The samples were filtered through 0.45 μm Supor filters and stored in precleaned glass bottles. As no pyrethroids in the collected water samples were detectable by the proposed method, a recovery study was carried out on the samples spiked with 1.0− 10.0 μmol L−1 LC to evaluate the developed method, and the corresponding results were listed in Table 3. It could be found

LC LC LC LC LC LC

found (μmol L−1)

recovery (%)

10.0 8.0 5.0 4.0 2.0 1.0

9.95 8.04 5.08 3.89 2.15 1.34

99.5 100.5 101.6 97.3 107.5 134.0

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CONCLUSIONS A new molecular imprinting concept has been obtained based on Pickering emulsion polymerization. In this work, mSiO2− Eu(TTA)3Bpc particles were used to establish a stable oil-inwater emulsion, followed by radical polymerization of the functional and cross-linking monomer in the oil phase. When the templates were removed by solvent extraction, imprinted binding sites were left in the particle materials that could be capable of selective rebinding the target molecules. Based on the fluorescence quenching via template analytes (LC) rebinding into the recognition cavities in the polymer matrixes, mSiO2−Eu(TTA)3Bpc@MIPs were successfully applied to the direct fluorescence quantification of LC, independent of extracting templates from the MIPs spheres, as well as further complicated and time-consuming assays. Our attempt to synthesize Eu complex based MIPs sensor was not only proved to be a novel and sensitive luminescence probe for optical recognition of LC, but also improved the potential application of molecularly imprinted polymers. In addition, the present studies provide a new strategy to fabricate other multifunctional (luminescent and magnetic) inorganic−organic MIPs with potential application in the recognition and sensitive sensing of analytes. ASSOCIATED CONTENT

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AUTHOR INFORMATION

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REFERENCES

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that this method was not suitable to detect LC at the low concentration below 4.0 μmol L−1. However, it was found sufficiently to obtain a quantitative recovery (99.5−101.6%) of spiked LC from 5.0−10.0 μmol L−1 and to use a simple aqueous standard solution for the accurate quantification of LC.

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ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Nos. 21107037 and 21176107), Ph.D. Programs Foundation of Ministry of Education of China (No. 20110205110014), and Natural Science Foundation of Jiangsu Province (BK2011461).

Table 3. Recovery of LC in Water Samples with LC Solution at Different Concentration Levels concentration taken (μmol L−1)

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Article

* Supporting Information S

Additional analytical details, figures, and references. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail: [email protected] (C.L.); [email protected] (J.P.). Tel.: +86-0511-88790683 (C.L.). Fax: + 86-0511-88791800 (C.L.). Notes

The authors declare no competing financial interest. 10452

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The Journal of Physical Chemistry C

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