Electrochemiluminescent graphene quantum dots ...

Electrochimica Acta 228 (2017) 107–113

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Electrochemiluminescent graphene quantum dots enhanced by MoS2 as sensing platform: a novel molecularly imprinted electrochemiluminescence sensor for 2-methyl-4chlorophenoxyacetic acid assay Yukun Yanga,b , Guozhen Fanga,* , Xiaomin Wanga , Fuyuan Zhanga , Jingmin Liua,c, Wenjie Zhengd , Shuo Wanga,c,* a

Key Laboratory of Food Nutrition and Safety, Ministry of Education of China, Tianjin University of Science and Technology, Tianjin 300457, China School of Life Science, Shanxi University, Taiyuan 030006, China c Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Technology and Business University, Beijing 100048, China d Animal, Plant and Food Stuffs Inspection and Quarantine Center, Tianjin Exit-Entry Inspection and Quarantine Bureau, Tianjin 300461, China b

A R T I C L E I N F O

Article history: Received 18 October 2016 Received in revised form 5 January 2017 Accepted 7 January 2017 Available online 9 January 2017 Keywords: graphene quantum dots molybdenum disulfide electrochemiluminescence sensor molecularly imprinted polymers 2-methyl-4-chlorophenoxyacetic acid

A B S T R A C T

The ECL properties and application of a novel luminescent material molybdenum disulfide-graphene quantum dots (MoS2-GQDs) hybrid nanocomposite was reported for the first time. The hybridization of MoS2 and GQDs endowed nanocomposite with structural and compositional advantages for boosting the ECL performance of GQDs. Impressively, the ECL could be remarkable enhanced using MoS2-GQDs hybrid nanocomposite, which was 13, 185 and 596-folds larger than the ECL intensity of GQDs, MoS2 modified electrodes and bare electrode, respectively. Subsequently, as a sensing platform, the MoS2GQDs hybrid nanocomposite was applied to fabricate molecularly imprinted electrochemiluminescence sensor for the ultrasensitive and selective determination of 2-methyl-4-chlorophenoxyacetic acid. Under optimal conditions, the detection limit of the prepared sensor was 5.5 pmol L1 (S/N = 3) within a linear concentration range of 10 pmol L1–0.1 mmol L1. The developped sensor exhibited high sensitivity, good selectivity, reproducibility and stability, suggesting the potential for detecting pesticides and veterinary drugs at trace levels in food safety and environmental control. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Molecularly imprinted polymers (MIPs) are synthetic polymers with recognition sites which are spatially and chemically complementary to the template [1–4]. Molecularly imprinted electrochemiluminescence (ECL) sensor (MIECLS) combined the numerous advantages of both ECL analysis and MIPs, demonstrating high sensitivity and selectivity, good controllability, rapidity, simplified optical set-up, low background noise, ease of preparation, reusability and low cost [5]. Since the first MIECLS was reported in 2012 [6], MIECLS have received considerable attentions in analytical fields. Along with the developments of MIECLS, a

* Corresponding authors at: No. 29, 13th Avenue, Tianjin Economic and Technological Development Area (TEDA), Tianjin 300457, China. Tel.: (+86 22) 60912493, Fax: (+86 22) 60912493. E-mail addresses: [email protected] (G. Fang), [email protected] (S. Wang). http://dx.doi.org/10.1016/j.electacta.2017.01.043 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

variety of ECL-emitting species, including ruthenium compounds [7,8], luminol [6,9–11], and nanomaterials [12–14] have been extensively reported. Despite the outstanding developments of MIECLS were achieved, to meet the requirements of rapid expansion, intense research still focused on exploiting the innovative, nontoxic, stable and highly efficient ECL luminescent materials. Graphene quantum dots (GQDs), carbon fragments derived from graphene with diameters less than 100 nm, are considered as quasi-zero-dimensional material with novel physical and chemical properties [15]. Compared with traditional ECL luminescent materials (ruthenium compounds and luminol) and semiconductor quantum dots, GQDs possesses low toxicity, better biocompatibility, large specific surface area, flexible chemical modification and resistance to photobleaching, making it as highly promising luminescent nanomaterial in establishing ECL sensors [16–18]. Interestingly, it has been reported that GQDs hybridized with other

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nanomaterials exhibit fascinating properties and unexpected characteristics [19–23]. Being a nontoxic, environment-friendly two-dimensional (2D) transition metal disulfides, molybdenum disulfide (MoS2) have been the focus of fundamental research and technological applications because of their specific 2D layered feature, electronic properties and unique physical and chemical properties [24,25]. Furthermore, the integration of other nanomaterials with this layered material to form hybrid nanocomposite system would be anticipated to obtain superior properties [26–28]. It should be noted that, the GQDs/MoS2 heterostructure was first investigated about its unique electronic and optical properties in 2015 [29]. And the MoS2-GQDs hybridization for the use of photodetectors [30], electrochemical biosensor [31] and lithium storage performance [32] were reported very recently. Though the excellent elctronic and optical properties of MoS2-GQDs hybrid nanocomposite were investigated very recently, its ECL properties owing promising perspect were not fully investigated yet. Herein, for the first time, a novel ECL luminescent material MoS2-GQDs hybrid nanocomposite was obtained, which features remarkable enhanced ECL. MoS2 nanosheets were used as carrier for GQDs with structural and compositional advantages for boosting the ECL performance of GQDs. ECL performance and possible ECL mechanism were investigated and discussed. The MoS2-GQDs hybrid nanocomposite was subsequently integrated within a MIECLS for the ultrasensitive and selective determination of 2-methyl-4-chlorophenoxyacetic acid (MCPA), which is widely used to control broadleaf weeds in various crop fields. With the increasing usage, residence time of MCPA in the environment and crops become longer, which make it potentially hazardous to human health. The maximum residue limit (MRL) of MCPA in grain and fruits are 0.05 mg/kg and 0.1 mg/kg, respectively in the national food safety standard of China. In 2005, Majzik. et al. tested 55 surface and 110 ground water samples using the established SPE-LC-MS-MS method with a limit of quantitation (LOQ) of 0.01 mg L1. Detectable phenoxy acid compounds were found in 37 samples and the concentration ranged from 0.01 to 0.35 mg L1 [33].

Ltd. (Beijing, China). K2S2O8 were purchased from Sigma-Aldrich (St. Louis, USA). N,N-dimethylformamide (DMF) were obtained from Alfa Aesar (Tianjin, China). All chemicals were at least analytical grade. Milli-Q purified water was used for all experiments. 2.2. Modification of the electrodes

2. Experimental

Prior to modification, bare GCE was polished with 0.3 and 0.05 mm alumina slurry successively followed by rinsing thoroughly with doubly distilled water (DDW) between each polishing step until a mirror-like surface was obtained. The GCE was scanned by cyclic voltammetry (CV) from 0.2 to 0.6 V in aqueous solution consisting of 1 mmol L1 K3[Fe(CN)6] and 0.1 mol L1 KCl until standard cyclic voltammograms of K3[Fe(CN)6] appeared. The electrodes were washed with DDW and dried in air before use. A layer of MoS2-GQDs hybrid nanocomposite modified GCE (MoS2-GQDs/GCE) was obtained as follows: MoS2 (6 mg), GQDs (4 mg) and DMF/H2O solution (1:1, v:v, 10 mL) were mixed in a glass bottle with the help of ultrasonic treatment for 3 h to form a homogeneous suspension. Then, the GCE was coated with 10 mL of the resulting MoS2-GQDs suspension solution and allowed to dry at room temperature. As control, MoS2 suspension (0.6 mg mL1) and GQDs suspension (0.4 mg mL1) were prepared to construct MoS2/GCE and GQDs/GCE respectively. Simple superposition of MoS2 and GQDs modified electrodes were obtained as follows: 10.0 mL of 0.4 mg mL1 GQDs was coated on the GCE surface firstly. After the GQDs suspension was dried, 10.0 mL of 0.6 mg mL1 MoS2 was coated and allowed to dry at room temperature (MoS2/GQDs/GCE). GQDs/MoS2/GCE was modified using the same procedure but changing the modification order of 0.4 mg mL1 GQDs and 0.6 mg mL1 MoS2. The MIPs modified MoS2-GQDs/GCE was electrochemically synthesized by repetitive CV scans in a deoxygenated acetate buffer solution (pH 5.2) containing 5 mmol L1 oPD and 5 mmol L1 MCPA at a scan rate of 50 mV s1 from 0 to 0.8 V for 5 cycles. Extraction of the template was performed by shaking the electrodes in a methanol/acetic acid (5:1, v/v) solution for 10 min with magnetically stirring. Similarly, the non-imprinted polymers (NIPs) modified MoS2-GQDs/GCE was also fabricated under the same conditions in the absence of MCPA.

2.1. Instruments and Reagents

2.3. Sample preparation

All the electrochemical experiments were carried out using an electrochemiluminescence analyzer LK5100 (Tianjin Lanlike Chemical and Electronic High Technology Co., Ltd., China). A conventional three-electrode system was employed, consisting of a bare or modified glassy carbon electrode (GCE, 4 mm diameter) as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum column electrode as the auxiliary electrode. Transmission electron microscope (TEM, JEM2010FEF, JEOL, Japan) was employed to observe morphology of the nanoparticles. Fluorescence measurements were performed on a Lumina fluorescence spectrometer (Thermo, USA). ECL detection: the ECL sensors with different concentrations of MCPA were tested in PBS (0.01 mol L1, 5 mL, pH = 7.4) containing K2S2O8 (100 mmol L1), scanning from 0 V to 1.4 V with a scan rate of 100 mV s1, the voltage of photomultiplier was set at 800 V. Oxidized graphene quantum dots (GQDs) and MoS2 nanosheets were purchased from XFNANO Materials Tech Co., Ltd. (Nanjing, China). 2-methyl-4-chlorophenoxyacetic acid (MCPA, 98%) and its structural analogs 2,4-dichlorophenoxyacetic acid (99%), 4-chlorophenoxyacetic acid (99%), phenoxyacetic acid (98%), 2,4-dichlorobenzaldehyde (98%), 2,4-dichlorophenol (99%) and ophenylenediamine (oPD) (99.5%) were obtained from J&K Scientific

Different sample matrices, enviromental samples including tap water (Tianjin, China), lake water (Tianjin University of Science and Technology) and food sample oat, were sampled nearby or purchased from a local market, and spiking and recovery studies were used to evaluate the application capability of the developed MIECLS. The water samples were filtered with 0.45 mm nylon filter and the food samples were turned into powder using grinder before the extraction process. Each sample (5.00 0.01 mL, 5.00 0.01 g,) was packed into a 100 mL polypropylene centrifuge tube, and spiked with MCPA at three levels: 5.0 104, 10.0 104 and 50.0 104 mg L1 or mg kg1. After overnight of rest in the dark, ethyl acetate (20.0 mL) with NaCl (1.25 g) was added and the mixture was vortex mixed for 5 min. For liquid samples, the supernatant were transferred into another 100 mL polypropylene centrifuge tube. For solid sample, the supernatant was transferred into another 100 mL polypropylene centrifuge tube after centrifugation at 7,000 rpm for 10 min. The above process was then repeated, and the supernatants were combined. Then the supernatant was dried under nitrogen at 40 C. Finally, the residue was dissolved with phosphate buffer solution (PBS, 0.01 mol L1, pH = 7.4, 5.0 mL) containing 100 mmol L1 K2S2O8 for the ECL analysis.


3. Results and discussion 3.1. Characterization of MoS2-GQDs hybrid nanocomposite and its ECL performance In our expected nanoarchitecture, MoS2 nanosheets are used as carrier for the GQDs with structural and compositional advantages, which not only leads to increased electronic conductivity, but also improves ECL performance. The transmission electron microscopy (TEM) images of GQDs (A), MoS2 (B) and MoS2-GQDs nanocomposite (C) were showed in Fig. 1. Fig. 1A showed the uniformly dispersed GQDs with an average diameter of 5 nm. The MoS2 nanosheets appeared transparent, filmy and folded structure, displaying a suitable carrier structure. While in the image of MoS2GQDs hybrid nanocomposite, MoS2 nanosheets were the carrier of GQDs with GQDs attached on its surface. Meanwhile, the MoS2, GQDs and MoS2-GQDs owned excellent dispersibility in N, Ndimethylformamide (DMF)/H2O (1:1, v:v) solution (Fig. 1D), and showed dark green, brown and brown green, respectively. Different concentrations of MoS2 and GQDs dispersion were shown in Fig. S1 with excellent dispersibility. Moreover, the obtained MoS2-GQDs dispersion was stable, exhibiting a homogeneous phase without any precipitation or agglomeration at room temperature for at least 1 month. Fluorescence spectrum of GQDs was shown in Fig. S2, the maximum excitation wavelength and maximum emission wavelength of the GQDs were 467 and 536 nm, respectively. To investigate the effect of MoS2-GQDs ratio on the ECL intensity, the ECL curves of MoS2-GQDs nanocomposite modified electrodes varying the ratio of MoS2: GQDs (1.0: 0, 0.9: 0.1, 0.8: 0.2, 0.7: 0.3, 0.6: 0.4, 0.5: 0.5, 0.4: 0.6, 0.3: 0.7, 0.2: 0.8, 0.1: 0.9, 0: 1.0 mg mL1, the total concentration was fixed at 1.0 mg mL1) were performed and compared. As shown in Fig. 2A, when the MoS2 was modified on the electrode independently (the ratio of MoS2: GQDs was 1.0: 0 mg mL1), the ECL intensity was the lowest one. As GQDs was modified on the electrode independently (the ratio of MoS2: GQDs was 0: 1.0 mg mL1), the ECL was slightly enhanced, indicating that GQDs was the luminescent material that

Fig. 1. TEM images of GQDs (A), MoS2 (B) and MoS2-GQDs hybrid nanocomposite (C). The dispersion contrast (D) of MoS2, GQDs and MoS2-GQDs in DMF/H2O (1:1, v: v).

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participated in the ECL process. The ECL of GQDs was greatly enhanced as the MoS2 and GQDs were hybrided together. The largest ECL intensity was obtained when the ratio of MoS2: GQDs was 0.6: 0.4 mg mL1, exhibiting higher ECL enhancement and better stability (data not provided). It could be seen that the ECL of GQDs was enhanced as the proportion of MoS2 was increased in the MoS2-GQDs hybrid nanocomposite. When the amount of MoS2 was larger than 0.6 mg mL1, the ECL intensity was decreased gradually, indicating that the moderate hybridization with MoS2 could enhance the ECL of GQDs. Meanwhile, effect of different modification order of MoS2 and GQDs on ECL was also investigated and compared with the MoS2GQDs nanocomposite to further illustrate the importance of nanocomposite for excellent ECL performance. The electrodes were modified with MoS2 and GQDs in turn (MoS2/GQDs/GCE and GQDs/MoS2/GCE) by changing the modification order of 0.6 mg mL1 MoS2 and 0.4 mg mL1 GQDs. As shown in Fig. S3, the ECL intensity of MoS2/GQDs/GCE and GQDs/MoS2/GCE were much lower than that of the hybrid nanocomposite modified electrode (MoS2-GQDs/GCE), confirming that the structure changes of hybrid nanocomposite may remarkably boost its ECL properties. Combining the results of TEM, the MoS2-GQDs hybrid nanocomposite may increase the contact area between MoS2 and GQDs as compared to simple superposition of the two materials. The increased contact area will promote the charge transfer at the MoS2-GQDs interface, thereby tuning the ECL properties [34]. The ECL curves of different modified electrodes (MoS2/GCE, GQDs/GCE, MoS2-GQDs/GCE) and bare GCE in the same ECL system (PBS (0.01 mol L1, pH = 7.4) containing 100 mmol L1 K2S2O8) were compared in Fig. 2B. The ECL intensity of bare GCE and MoS2/GCE were tiny, while GQDs/GCE displayed relatively high ECL. The ECL intensity of GQDs/GCE was 46-folds higher that of bare GCE, which was attributed to the ECL behavior of the GQDs. Impressively, the ECL of GQDs was greatly enhanced on MoS2GQDs/GCE, which was 13, 185 and 596-folds larger than the ECL intensity of GQDs/GCE, MoS2/GCE and bare GCE, respectively. The possible ECL mechanism was described as follows [35]: firstly, the ground-state GQDs and co-reactant S2O82 were electrochemically reduced to negatively charged GQDs and SO4 radicals. Then, excited state (noted as GQDs*) was produced by the electron-transfer annihilation between SO4 and GQDs radicals. Finally light was emitted by GQDs* which came back to the ground-state. The corresponding equations were proposed as follows: GQDs þ ne ! nGQDs

ð1Þ

S2 O8 2 þ e ! SO4 2 þ SO4

ð2Þ

GQDs þ SO4 ! GQDs þ SO4 2

ð3Þ

GQDs ! GQDs þ hv

ð4Þ

The remarkable ECL enhancement on the MoS2-GQDs/GCE may be attributed to that MoS2-GQDs hybrid nanocomposite was favorable for the electron transport in the ECL process. Moreover, the MoS2 nanosheets in the nanocomposite improved the stability of radical species and promoted the formation of GQDs in ECL reaction process [36]. In order to assess the possible effect of MoS2 in the ECL process, CVs of the different modified electrodes in phosphate buffer solution (PBS, 0.01 mol L1, pH = 7.4) containing 100 mmol L1 K2S2O8 were carried out (shown in Fig. 2C). It was observed that

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Fig. 2. (A) The effects of MoS2-GQDs ratio on the ECL intensity. (B) ECL curves of bare GCE (a), MoS2/GCE (b), GQDs/GCE (c), MoS2-GQDs/GCE (d). (C) CVs of ECL process on the different modified electrodes in PBS (0.01 mol L1, pH = 7.4) containing 100 mmol L1 K2S2O8. (D) Schematic diagram of possible electron transfer of MoS2-GQDs hybrid nanocomposite.

the onset of reduction potential of MoS2-GQDs/GCE shifted positively compared with GQDs/GCE, MoS2/GCE and bare GCE. The results suggested that the electron transfer from GQDs to MoS2 may reduce the electron density of GQDs, promoting the reduction of GQDs. And hybridization with MoS2 reduced the potential obstruction of Eq. (1). The schematic diagram of possible electron transfer of MoS2-GQDs hybrid nanocomposite in the ECL process was shown in Fig. 2D. Meanwhile, this persulfate electrochemical reduction occured at lower cathodic potential also indicated that MoS2-GQDs deposited on the electrode surface owned excellent electronic characteristics. Therefore, we may concluded that the hybridization with MoS2 improved the quantity of reduced GQDs (GQDs) and excited GQDs (GQDs*), enhancing the ECL intensity of GQDs significantly [17]. To further explain this, electrochemical impedance spectroscopy (EIS) spectrum of MoS2-GQDs and MoS2 modified electrodes and bare GCE were obtained in Fig. S4. The charge-transfer impedance (RCT) of MoS2-GQDs/GCE was 360 V, which was lower than the value of 540 V of MoS2/GCE and 430 V of bare GCE. The lower RCT of MoS2-GQDs/GCE was due to the hybridization of MoS2 and GQDs, suggesting faster kinetics of the electrochemical transfer process and an improved rate performance. In view of the remarkable quantum-confinement and edge effects of GQDs, hybridization of MoS2 and GQDs could drastically alter their electronic characteristics, promote the

charge transport and offer more active sites. Thus, MoS2-GQDs hybrid nanocomposite could enhance the electrical conductivity and ECL performance via tuning the electronic structures [37]. From the above discussion, the remarkable enhanced ECL properties might be assigned to the following factors: (1) hybridization of GQDs and MoS2 improved the electrical conductivity and thus facilitate the charge transfer in the ECL process; (2) a moderate amount of MoS2 might serve not only a carrier to improve the structure of MoS2-GQDs, but also a booster and stablizer to obtain the enhanced ECL; (3) the MoS2-GQDs hybrid nanocomposite possessed complementary properties of the individual components with a synergistic effect [29,32]. 3.2. Characterization of MIPs sensor and NIPs sensor Subsequently, the MoS2-GQDs hybrid nanocomposite was used as a sensing platform to establish a MIELCS for the determination of MCPA. ECL (Fig. 3A) and EIS (Fig. 3B) were used to characterize the successful fabrication of the imprinted polymers modified electrodes. As shown in Fig. 3A, the ECL curves of different modified electrodes were compared. The ECL intensity of initially prepared imprinted polymers modified MoS2-GQDs/GCE was drastically decreased (curve a) compared with MoS2-GQDs/GCE, which was the same with situation of NIPs coated MoS2-GQDs/GCE


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Fig. 3. (A) ECL curves of MIPs/MoS2-GQDs/GCE (a) before and (b) after template removal (c) after rebinding in 5 1010 mol L1 MCPA, NIPs/MoS2-GQDs/GCE (d) before and (e) after elution. (B) EIS curves of (a) bare GCE, (b) MoS2-GQDs/GCE, MIPs/MoS2-GQDs/GCE (c) before and (d) after template removal (the concentration of the MCPA for curve e was 1.0 1010 mol L1). All data were taken in 0.1 M KCl aqueous solution. The impedance spectra of frequency range 100 mHz–100 kHz were recorded under an alternating current (AC) amplitude of 10 mV. (e) MIPs/MoS2-GQDs/GCE after rebinding, in a solution containing 0.2 mol L1 KCl and 2.5 mmol L1 Fe(CN)63/Fe(CN)64. The impedance spectra of frequency range 100 mHz–100 kHz were recorded under an alternating current (AC) amplitude of 10 mV.

(curve d). It was because that the ECL reaction between lightemitting material GQDs on the electrode surface and the coreactant K2S2O8 was hindered by the nonconducting MIPs and NIPs. After the template removal, imprinted cavities in the MIPs were appeared to be the channels for the coreactant contacting and electron transport, resulting in the obviously increasing of ECL intensity (curve b). Meanwhile, because the NIPs had no cavities after elution, the ECL intensity recovered very little (curve e). After template rebinding, the cavities of MIPs was blocked by the rebound analytes, resulting in the decreased ECL (curve c). The rebinding of MCPA may be due to spatial structure and hydrogen bonds in the MIPs matrix. Furthermore, the changes in the surface features of the modified electrodes in the assembly process were demonstrated by EIS. As shown in Fig. 3B, the charge-transfer resistance for MoS2GQDs/GCE (curve b) showed a decrease compared with bare GCE (curve a) due to the electron transfer acceleration ability of MoS2GQDs hybrid nanocomposite. The resistance increased obviously (curve c) when the surface of MoS2-GQDs/GCE was coated by nonconductive MIPs. However, after template molecule was eluted from the MIPs, imprinted cavities were formed and acted as the electron transfer channels, revealing the decreased resistance (curve d). When the MIPs was incubated with the template molecule again (the concentration of the MCPA was 1.0 1010 mol L1), MCPA that rebound into the imprinted cavities blocked the channels of electron transfer and the resistance increased (curve e). The EIS results were in accordance with that of ECL analysis. 3.3. ECL response of the MIECLS towards MCPA Under the optimal conditions, we investigated the relationship between the MCPA concentration and the ECL response of the established MIECLS by testing a series of standard solutions containing different MCPA concentrations. As observed in Fig. 4A, ECL intensity was decreased with the increasing MCPA concentrations. This was attributed that the generated imprinted cavities during the imprinting process could specifically fit the target molecule (MCPA) in terms of spatial structure and functional groups. When MCPA concentration was greater than 0.1 mmol L1, the ECL response remained constant owing to the saturation of MIPs with MCPA. A good linear relationship between the quenching value/initial value of the ECL intensity (F0F/F0) and

the logarithm of MCPA concentration was obtained within the MCPA concentrations ranging from 10 pmol L1 to 0.1 mmol L1 with a correlation coefficient of 0.9943 (n = 9, Fig. 4B). The detection limit was calculated to be 5.5 pmol L1 (S/N = 3), which was lower than other detection methods such as HPLC (1.0 nmol L1 in water samples, 2.0 nmol L1 in urine samples), GC-MS-MS (0.05 nmol L1) and SPE-LC-MS-MS (5.0 nmol L1) [33,38,39]. The calibration curve was described as follows: ðF 0 F Þ=F 0 ¼ 0:3182logC MCPA þ 3:6515

ð5Þ

A competitive adsorption test was performed to evaluate the selectivity of the proposed MIECLS. Five other structural analogs including 2,4-dichlorophenoxyacetic acid (a), 4-chlorophenoxyacetic acid (b), phenoxyacetic acid (c), 2,4-dichlorobenzaldehyde (d), 2,4-dichlorophenol (e) were chosen (Fig. S5). 3-folds concentration of these interferents were added to the 5 1010 mol L1 MCPA. As shown in Fig. 4C, ECL intensities of MIPs/MoS2-GQDs/ GCE had no obvious change, indicating the MIECLS had high selectivity for the determination of MCPA. Furthermore, the selectivity can be evaluated intuitively by calculating the imprinting factor (IF): IF = (F0F/F0)MIPs/(F0F/F0)NIPs. The IFs for MCPA, 2,4-dichlorophenoxyacetic acid (a), 4-chlorophenoxyacetic acid (b), phenoxyacetic acid (c), 2,4-dichlorobenzaldehyde (d), 2,4dichlorophenol (e) were 3.74, 1.22, 1.15, 0.89 and 1.05, respectively. This could be attributed to the thin MIPs membrane on the MIPs/ MoS2-GQDs/GCE with selective cavities, which matched with the shape of the template and specifically interacted with the template MCPA. To evaluate the stability of the sensor, the consecutive ECL signals of the sensor for 10 cycles were shown in Fig. 4D. Obviously, the changes of the response signal for 1 1010 mol L1 MCPA detections were about 2.64%. Further experiments revealed that DECL decreased 5.6% for 5 1010 mol L1 MCPA detection after the imprinted electrode was used 5 times with subsequent washing and measuring operations (Fig. S6). These results indicated that the sensor had good operational stability. All these results indicated that the prepared imprinted sensor exhibited good sensitivity, selectivity and reproducibility. 3.4. Application of the ECL sensor in real samples Using the established sample pretreatment method, the concentration of MCPA in tap water, lake water and oat samples,

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Fig. 4. (A) ECL responses of the proposed MIECLS in the presence of various MCPA concentrations (0, 1.0 1011, 5.0 1011, 1.0 1010, 5.0 1010, 1.0 109, 5.0 109, 1.0 108, 5.0 108, 1.0 107 mol L1). (B) Calibration curves of MIECLS for MCPA determination. (C) Competive tests of the MIECLS to MCPA and its analogues (2,4dichlorophenoxyacetic acid (a), 4-chlorophenoxyacetic acid (b), phenoxyacetic acid (c), 2,4-dichlorobenzaldehyde (d), 2,4-dichlorophenol (e)). 3-folds concentration of these interferents were added to the 5 1010 mol L1 MCPA. (D) Stability of ECL emission under continuous cyclic potential scans for 10 cycles from MIECLS for 1 1010 mol L1 MCPA determination.

which were spiked at three concentrations (5.0 104, 10.0 104 and 50.0 104 mg L1 or mg kg1), was measured using the established MIECLS. As shown in Table 1, the recoveries for the measurements ranged from 87.1% to 98.6% with SDs below 5.1%, indicating that the prepared MIECLS could be used in practical application to accurately determine MCPA.

4. Conclusion The current study presented a novel ECL luminescent material MoS2-GQDs hybrid nanocomposite for the first time, exhibiting remarkably enhanced ECL. Impressively, the ECL intensity of solidstate ECL emitting electrode (MoS2-GQDs/GCE) was 13, 185 and

Table 1 Results of ECL measurements of MCPA content in real samples using the proposed MIECLS. Sample

Added/(104 mg L1/mg kg1)

Found/(104 mg L1/mg kg1)

Tap water

0 5.0 10.0 50.0

0 4.72 9.55 48.55

94.4 3.5 95.5 2.9 97.1 2.6

0 5.0 10.0 50.0

0 4.62 9.42 49.30

92.4 3.9 94.2 2.1 98.6 2.7

0 5.0 10.0 50.0

0 4.36 9.12 46.60

87.1 5.1 91.2 3.3 93.2 4.1

Lake water

Oat

Recovery % (mean SD) (n = 3)


596-folds larger than GQDs/GCE, MoS2/GCE and bare GCE, respectively. The excellent ECL performance was attributed to superior properties of the MoS2-GQDs hybrid nanocomposite. Subsequently, the MoS2-GQDs hybrid nanocomposite was used as a sensing platform to establish a MIECLS for the determination of MCPA. In view of the satisfactory performance of the established MIECLS, we believe that the findings of the current study could be extended further to advance progress in MIECLS. Acknowledgments This work was supported by International Science and Technology Cooperation Program of China (Project No. 2014DFR30350) and Beijing Municipal Commission of Science and Technology, China (project No. Z151100001215002) and the PhD Training Foundation of Tianjin University of Science and Technology (project No. 201401 and No. 201501). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.01.043. References [1] Y. Pan, L. Shan g, F. Zhao, B. Zeng, A novel electrochemical 4-nonyl-phenol sensor based on molecularly imprinted poly (o-phenylenediamine-co-otoluidine)-nitrogen-doped graphene nanoribbons-ionic liquid composite film, Electrochim. Acta 151 (2015) 423–428. [2] H. Li, H. Guan, H. Dai, Y. Tong, X. Zhao, W. Qi, S. Majeed, G. Xu, An amperometric sensor for the determination of benzophenone in food packaging materials based on the electropolymerized molecularly imprinted poly-ophenylenediamine film, Talanta 99 (2012) 811–815. [3] B. Rezaei, M.K. Boroujeni, A.A. Ensafi, Fabrication of DNA o-phenylenediamine, and gold nanoparticle bioimprinted polymer electrochemical sensor for the determination of dopamine, Biosens. Bioelectron. 66 (2015) 490–496. [4] Y. Yang, G. Fang, X. Wang, M. Pan, H. Qian, H. Liu, S. Wang, Sensitive and selective electrochemical determination of quinoxaline-2-carboxylic acid based on bilayer of novel poly (pyrrole) functional composite using one-step electro-polymerization and molecularly imprinted poly (ophenylenediamine), Anal. Chim. Acta 806 (2014) 136–143. [5] M.M. Richter, Electrochemiluminescence (ecl), Chem. Rev. 104 (2004) 3003– 3036. [6] S. Li, H. Tao, J. Li, Molecularly imprinted electrochemical luminescence sensor based on enzymatic amplification for ultratrace isoproturon determination, Electroanal 24 (2012) 1664–1670. [7] B. Wu, Z. Wang, Z. Xue, X. Zhou, J. Du, X. Liu, X. Lu, A novel molecularly imprinted electrochemiluminescence sensor for isoniazid detection, Analyst 137 (2012) 3644–3652. [8] Q. Wang, M. Chen, H. Zhang, W. Wen, X. Zhang, S. Wang, Solid-state electrochemiluminescence sensor based on RuSi nanoparticles combined with molecularly imprinted polymer for the determination of ochratoxin A, Sensor. Actuat. B–Chem. 222 (2016) 264–269. [9] J. Li, S. Li, X. Wei, H. Tao, H. Pan, Molecularly imprinted electrochemical luminescence sensor based on signal amplification for selective determination of trace Gibberellin A3, Anal. Chem. 84 (2012) 9951–9955. [10] X. Li, J. Li, W. Yin, L. Zhang, Clopyralid detection by using a molecularly imprinted electrochemical luminescence sensor based on the gate-controlled effect, J. Solid State Electr. 18 (2014) 1815–1822. [11] J. Li, F. Ma, X. Wei, C. Fu, H. Pan, A highly selective molecularly imprinted electrochemiluminescence sensor for ultra-trace beryllium detection, Anal. Chim. Acta 871 (2015) 51–56. [12] S. Chen, A. Li, L. Zhang, J. Gong, Molecularly imprinted ultrathin graphitic carbon nitride nanosheets–Based electrochemiluminescence sensing probe for sensitive detection of perfluorooctanoic acid, Anal. Chim. Acta 896 (2015) 68–77. [13] Y. Yang, G. Fang, X. Wang, G. Liu, S. Wang, Imprinting of molecular recognition sites combined with p-donor–acceptor interactions using bis-anilinecrosslinked Au-CdSe/ZnS nanoparticles array on electrodes: Development of electrochemiluminescence sensor for the ultrasensitive and selective detection of 2-methyl-4-chlorophenoxyacetic acid, Biosens. Bioelectron. 77 (2016) 1134–1143. [14] Q. Wang, M. Chen, H. Zhang, W. Wen, X. Zhang, S. Wang, Enhanced electrochemiluminescence of RuSi nanoparticles for ultrasensitive detection of ochratoxin A by energy transfer with CdTe quantum dots, Biosens. Bioelectron. 79 (2016) 561–567.

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