Magnetic imprinted electrochemical sensor combined with magnetic imprinted solid-phase extraction for rapid and sensitive detection of tetrabromobisphenol S

Journal of Electroanalytical Chemistry 777 (2016) 58–66

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Magnetic imprinted electrochemical sensor combined with magnetic imprinted solid-phase extraction for rapid and sensitive detection of tetrabromobisphenol S Fang Long a, Jing Wang a, Zhaohui Zhang a,b,c,⁎, Liang Yan a a b c

College of Chemistry and Chemical Engineering, Jishou University, Hunan 416000, China Key Laboratory of Mineral Cleaner Production and Exploit of Green Functional Materials in Hunan Province, Jishou University, Jishou 416000, PR China State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, PR China

a r t i c l e

i n f o

Article history: Received 8 April 2016 Received in revised form 20 July 2016 Accepted 24 July 2016 Available online 25 July 2016 Keywords: Combination detection system Imprinted sensor Magnetic solid-phase extraction Tetrabromobisphenol S

a b s t r a c t A novel magnetic molecularly imprinted electrochemical sensor (MMIP-sensor) combined with magnetic molecularly imprinted solid-phase extraction (MMISPE) was constructed for rapid and sensitive detection of tetrabromobisphenol S (TBBPS) in water samples. The magnetic molecularly imprinted polymer based on magnetic graphene/multi-walled carbon nanotubes hybrid composite (MMIP/Gr-MWCNTs) was prepared as MMISPE sorbet to separate and enrich TBBPS from complex system. Adsorption results showed that the MMIP/ Gr-MWCNTs possessed excellent selective adsorption toward TBBPS with a maximum adsorption capacity of 17.83 mg g−1. The magnetic imprinted sensor based on the magnetic Gr-MWCNTs composite modified electrode was prepared by electropolymerization of dopamine monomer. The high sensitivity of the MMIP-sensor was attributed to the large specific surface area and excellent electrical conductivity of the magnetic Gr-MWCNTs composites. The detection limit of the MIP-sensor toward TBBPS was calculated as 2.1 × 10−11 mol L−1 (S/N = 3). Under the optimum conditions, the MISPE-MMIP-sensor combination system provided 10–12 fold preconcentration than that of MMIP-sensor alone determination. The MISPE-MMIP-sensor was successfully used for detection of TBBPS in drinking water, rain water, lake water and tap water with recoveries of 88.4–98.8%. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Tetrabromobisphenol S (TBBPS), 4, 4-sulphonylbis (2, 6dibromophenol), a new type of brominated flame retardant, has been widely used for preparation of various heat-resistant products [1]. The environmental released TBBPS is highly hazardous to the humans [2]. Studies showed trace TBBPS can result in carcinogenic effect, hepatotoxicity, and disruption of endocrine system [3]. Currently, some approaches involving fluorescence spectroscopy, high performance liquid chromatography (HPLC), gas chromatography and electrochemical sensor [4–6] have been developed for detection of TBBPS in complex samples. Nevertheless, spectroscopy analysis and chromatography method need expensive instrumentation and complicated pretreatment process [4,5]. The detection limit of the direct electrochemical determination can not achieve the trace level of TBBPS in the environmental [6]. Hence, a rapid preconcentration method was required for electrochemical determination of trace TBBPS. Molecularly imprinted polymers (MIP) are cross-linked synthetic polymers by copolymerizing functional monomer with cross-linker in ⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Jishou University, Hunan 416000, PR China. E-mail address: [email protected] (Z. Zhang).

http://dx.doi.org/10.1016/j.jelechem.2016.07.027 1572-6657/© 2016 Elsevier B.V. All rights reserved.

the presence of template molecule [7]. Currently, the MIP has been widely utilized for chromatography [8], sensor [9], drug-controlled release [10] and solid-phase extraction [11] because of their excellent stability and high selectivity. Notably, the combination of molecularly imprinted sensor (MIP-sensor) and molecularly imprinted solid phase extraction (MISPE) technique has been proven a high-efficiency separation and determination technique owing to the high preconcentration in MISPE procedure and the high sensitivity of the MIP-sensor [12,13]. However, the reported MISPE method mainly relied on imprinted bar and fiber extraction which showed a lower extraction efficiency [12– 15]. In addition, our group has also developed an imprinted sensor combined with magnetic imprinted solid-phase extraction using imprinted polymer based on magnetic silicon sphere as SPE material for detection of dibutyl phthalate [16]. Magnetic molecularly imprinted solid-phase extraction (MMISPE) is one of rapid sample separation and enrichment technique because the magnetic adsorbents can be easily separated by an external magnet [17]. However, to the best of our knowledge, no article on MIP-sensor combined with MMISPE was reported for detection of TBBPS. In this study, a novel magnetic molecularly imprinted polymer based on magnetic graphene/multi-walled carbon nanotubes hybrid composite (MMIP/Gr-MWCNTs) was prepared as solid-phase extraction material with dopamine as the functional monomer. Adsorption results

F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66

showed that the MMIP/Gr-MWCNTs possessed special selectivity and high adsorption capacity toward TBBPS with a maximum adsorption capacity of 17.83 mg g−1. The magnetic imprinted sensor was prepared based on the magnetic Gr-MWCNTs composite modified electrode to improve its sensitivity. The MMIP-sensor combined with MMISPE detection system was applied to detect traces TBBPS in water sample successfully. 2. Experimental 2.1. Apparatus and reagents Graphite powder was purchased from Shanghai Carbon Plant (Shanghai, China). Carboxylic multi-wall carbon nanotubes (MWCNTs-COOH) were purchased from Shenzhen Nanotech Port Co., Ltd. Dopamine (DA) was obtained from Aladdin Reagents Company (Shanghai, China). TBBPS, tetrabromobisphenol A (TBBPA), tetrabromobisphenol S bis-(2,3-dibromopropyl ether) (TBBPS-DBPE), and bisphenol A (BPA) were purchased from Alfa Aesar Company (Tianjin, China). N-hydroxy succinimide (NHS), ethylenediamine (EDA), N, N ′-dicyclohexylcarbodiimide (DCC) and potassium ferricyanide were all of analytical grade and obtained from Beijing Chemical Reagents Company. Tetrabutylammonium tetrafluoroborate (Bu4NBF4), methanol and acetic acid were purchased from Chengdu Jinshan Chemical Reagent Co. Ltd., China. All other chemicals were of analytical reagent grade. Double-distilled water was used throughout the experiment. All electrochemical experiments including cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed on CHI660B electrochemical workstation (Chenhua Instruments Co. Ltd., Shanghai, China). A three-electrode system was consisted of a platinum plate electrode as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode and home-made carbon electrode as the working electrode. Fe(CN)36 −/4 − with concentration of 5.0 × 10−3 mol L−1 was acted as probe in the electrochemical experiment. Morphology characterization was conducted with an electron microscope (SEM, Zeiss-SIGMA HD; TEM, JEM-2010F). Infrared spectra were recorded on a Fourier transform infrared spectrometer (FT-IR, Nicolet iS10). HPLC analysis was performed using a Shimadzu (Japan) LC-2010A HPLC. 2.2. Preparation of imprinted polymers based on magnetic Gr-MWCNTs composite 2.2.1. Preparation of amino functionalized graphene (GO-NH2) Firstly, graphene oxide (GO) was prepared from graphite powder by a modified Hummers' method [18]. Then, 0.35 g of GO was dispersed into 200 mL of distilled water uniformly under sonication for 1 h. Next, 2.5 g of NHS, 4.1 g of DCC and 1.25 g of H3BO3 were added into the dispersion solution. The mixture was stirred for 1 h at 60 °C. After that, 30 mL of EDA was added dropwise into the mixture and stirred for 5 h. Washed in succession with ethanol and water, the GO-NH2 was dried overnight under vacuum at 60 °C. 2.2.2. Preparation of magnetic MWCNTs grafted dopamine (MWCNTs-CO-DA) Magnetic MWCNTs-COOH was prepared according to the previous reported literature [19]. Briefly, 0.3 g of DA, 0.1 g of DCC and 0.5 g of magnetic MWCNTs-COOH were added into 45 mL of DMF and the mixture was stirred under 65 °C for 24 h. Collected by an external magnet, the product was washed respectively with ethanol and ultra pure water. Finally, the product was dried under vacuum for 12 h to obtain magnetic MWCNTs-CO-DA. 2.2.3. Preparation of magnetic Gr-MWCNTs imprinted polymer Briefly, 0.1 g of TBBPS and 0.15 g of DA were dissolved in 10 mL of 0.2 mol L− 1 sodium hydroxide solution. Then, 0.18 g of GO-NH2,

59

0.12 g of magnetic MWCNTs-CO-DA and 50 mL of acetonitrile were added into the solution under sonication for 2 h. Next, 0.05 g of AIBN and 2.5 mL of EGDMA were added into the above mixture under stirring for 1 h. The mixture was sealed in a Teflon-lined stainless steel autoclave and maintained at 60 °C under nitrogen atmosphere for 24 h. Then, the template molecule TBBPS was successively removed from the magnetic imprinted polymer with methanol/acetic acid (8:2, v/v) mixture solution until no TBBPS molecule was detected in the extraction solution by a UV–vis spectrometer at 210 nm. Finally, the magnetic imprinted polymer based on Gr-MWCNTs (MMIP/Gr-MWCNTs) was dried in a vacuum oven at 60 °C overnight. For comparison, the magnetic non-imprinted polymer based on GrMWCNTs (MNIP/Gr-MWCNTs) was prepared by the same procedures only without addition of the template TBBPS in the polymerization process. 2.3. Adsorption experiment Kinetic adsorption experiment was carried out as follows. A series of 40.0 mg of the MMIP/Gr-MWCNTs and MNIP/Gr-MWCNTs were suspended respectively into 10.0 mL of 30.0 mg L−1 TBBPS solution. Incubated at RT under shaking, the polymers were taken out at defined time intervals of 5, 10, 15, 20, 25 and 30 min. The amount of TBBPS adsorbed by the polymers was detected by HPLC. The condition of the HPLC for determination TBBPS is following. The injection volume is 10.0 μL. The HPLC separation is carried out on a C18 column at 40 °C. The mobile phase is methanol and water (85:15, v/v) mixture solution with a flow rate of 1.0 mL min− 1. The wavelength of UV detector is set at 210 nm. Static adsorption experiment was performed by adding 40.0 mg of the MMIP/Gr-MWCNTs and MNIP/Gr-MWCNTs into 10.0 mL of TBBPS solution with different concentrations ranged from 10.0 to 100.0 mg L−1. Incubated at RT for 12 h, the suspension was separated by the external magnet and the eluent concentration was analyzed by HPLC. The amount of TBBPS adsorbed by the polymers was obtained by subtracting the eluent concentration from initial concentration of TBBPS. The equilibrium adsorption capacity (Qe, mol g−1) of the MMIP/GrMWCNTs and MNIP/Gr-MWCNTs toward TBBPS was calculated by Eq. (1): Qe ¼

ðC 0 −C e Þ  V m

ð1Þ

where C0 and Ce are the initial and equilibrium concentration (mol L−1) of TBBPS in the eluant solution, respectively; V (mL) is the solution volume; m (mg) is the weight of polymers. 2.4. Preparation of TBBPS magnetic imprinted electrochemical sensor The prepared procedure of TBBPS imprinted sensor was illustrated in Scheme 1. Prior to surface modification, a home-made bare carbon electrode (4 × 8 mm) was polished to a mirror-like surface with 1.0 and 0.3 μm alumina slurry and washed thoroughly with ultrapure water. 4-Nitrobenzenediazonium tetrafluoroborate salt (NBT) was prepared according to the previous literature [6]. The NBT modified carbon electrode (NBT/CE) was completed by CV scanning under the potential of −1.8 to −0.5 V for 5 cycles at a scanning rate of 0.05 V s−1 in 25.0 mL of acetonitrile solution containing 1.0 × 10−3 mol L− 1 NBT and 0.1 mol L−1 Bu4NBF4. After that, the reduction of nitrophenyl of the NBT/CE was performed by CV scanning under the potential of −1.8 to −0.2 V vs. SCE at a scanning rate of 0.05 V s−1 in 10% ethanol solution for 3 cycles. Rinsed with water, the NBT/CE was immersed in 25.0 mL of 0.1 mol L− 1 BBS (pH = 10.0) dispersion solution containing 1.0 mg mL−1 magnetic Gr-MWCNTs at a scanning rate of 0.025 V s−1 for 10 cycles between − 1.5 and 0.6 V vs. SCE to obtain magnetic

60

F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66

Scheme 1. Preparation process for MMIP-sensor combined with MMISPE.

Gr-MWCNTs modified electrode (magnetic Gr-MWCNTs/CE). Finally, the magnetic Gr-MWCNTs/CE was washed with distilled water and dried at RT. The TBBPS imprinted film electropolymerized onto the magnetic GrMWCNTs/CE surface (MMIP/Gr-MWCNTs/CE) was prepared using CV scanning between 0 and 0.8 V vs. SCE with a scanning rate of 0.05 V s−1 for 9 cycles in 25.0 mL of 0.2 mol L−1 DA PBS (pH = 7.0) containing 1.0 × 10−7 mol L−1 TBBPS. Non-imprinted sensor (MNIP/ Gr-MWCNTs/CE) was prepared identically with the imprinted procedure but without addition of TBBPS. Finally, the imprinted electrode was washed with methanol/acetic acid (8:2, v/v) mixture solution for 30 min to remove TBBPS molecule. 2.5. Combination MMIP-sensor with MMISPE detection system The detection system of MMIP-sensor combined with MMISPE was shown in Scheme 1. Firstly, 30.0 mg of the MMIP/Gr-MWCNTs was dispersed in 25.0 mL TBBPS solution for 20 min. Then, the MMIP/GrMWCNTs were separated by an external magnet. Next, the MMIP/GrMWCNTs were washed with 2.0 mL of methanol/acetic acid (8:2, v/v) mixture solution for 30 min. After that, the MMIP-sensor was incubated into the extraction solution for 14 min. Finally, the MMIP-sensor was measured by DPV in 25.0 mL PBS (pH = 7.0) containing . 5.0 × 10−3 mol L−1 Fe(CN)3−/4− 6 2.6. Samples pretreatment The drinking water and tap water samples were collected from the laboratory. The lake water sample was collected from Fengyu Lake in Jishou University. The rain water sample was collected in Jishou. All those samples were filtered through a 0.45 μm microporous membrane before use.

3. Results and discussion 3.1. Preparation and characterization of MMIP-sensor 3.1.1. Preparation of MMIP-sensor Scheme 1 represents the preparation procedure of MMIP-sensor detection system combined with the MMISPE. For stable immobilization of magnetic Gr-MWCNTs composite on the CE surface, NBT was firstly electrodeposited onto the bare CE surface using CV method. As shown in Fig. S1A, there is an irreversible peak at − 1.2 V during the first cycle, which is attributed to the formation of nitrophenyl from diazonium salt on the CE surface [20]. Then, the nitro groups were electrochemically reduced to amine groups. As shown in Fig. S1B, a broad irreversible peak at − 0.9 V was observed in the first cycle, and the peak disappears in the further scans, which corresponds to the irreversible reduction of the grafted nitrophenyl groups in a multielectron and multiproton pathway to aminophenyl [21]. Next, the magnetic GrMWCNTs composites were electrodeposited on the NBT/CE surface using CV scanning to obtain magnetic Gr-MWCNTs/CE. Finally, the imprinted DA film was polymerized onto the magnetic Gr-MWCNTs/ CE surface by electropolymerization technique to obtain MMIP/GrMWCNTs/CE. 3.1.2. Electrochemical characterization The CVs studies of bare and modified CE were conducted from −0.3 PBS at a scanning to 0.8 V vs. SCE in 5.0 × 10−3 mol L−1 Fe(CN)3−/4− 6 rate of 0.05 V s−1. As shown in Fig. 1(a), a pair of well-defined reversible redox peaks for the bare CE was observed. When the Gr-MWCNTs were modified onto the CE surface, the peak current of the Gr-MWCNTs/CE in Fig. 1(b) was higher than that of the bare CE, which was attributed to the synergistic effect of the MWCNTs hybrid Gr sheets with high surface area and excellent electrical conductivity [22]. Interestingly, when the

F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66

were also investigated. As shown in Fig. 1(d), the peak current of the imprinted sensor containing TBBPS was lower than that of the magnetic Gr-MWCNTs/CE, because the imprinted film hindered the electron to the electrode surface. Interestingly, after extransfer of Fe(CN)3−/4− 6 traction of TBBPS from the MMIP/Gr-MWCNTs/CE, the peak current shown in Fig. 1(e) was higher than that of the MMIP/Gr-MWCNTs/CE containing the template molecules. This is attributing to the formation of porous imprinted cavities allowing Fe(CN)36 −/4 − to pass through the imprinted film. As shown in Fig. 1(f), the MNIP/Gr-MWCNTs/CE presents the lower peak current than that of the MMIP/Gr-MWCNTs/ CE because no imprinted cavities were generated in the DA film.

e b

Current/mA

1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 -0.25 -0.50 -0.75 -1.00 -1.25 -1.50 -1.75 -0.4

c f

d

a

-0.2

0.0

0.2

0.4

0.6

61

0.8

Potential/V vs·SCE Fig. 1. Cyclic voltammograms of bare CE (a), Gr-MWCNTs/CE (b), magnetic Gr-MWCNTs/ CE (c), MMIP-sensor before removed of TBBPS (d), MMIP-sensor after removed of TBBPS (e) and MNIP-sensor (f) in 5.0 × 10−3 mol L−1 Fe(CN)63−/4− PBS (pH = 7.0) at a scanning rate of 0.05 V s−1.

magnetic Gr-MWCNTs composites were electrodeposited on the CE surface, the peak current of the magnetic Gr-MWCNTs/CE shown in Fig. 1(c) was higher than that of the Gr-MWCNTs/CE due to the Fe3O4 nanoparticles decorated Gr-MWCNTs composite providing more electrochemical active sites and larger surface area. Additionally, the Fe3O4 nanoparticles decorated Gr-MWCNTs composites can prevent the graphene sheets from aggregating and restacking. The electrochemical performances of the imprinted sensor containing and extracted TBBPS

3.1.3. Morphology characterization The morphologies of the CE at different modification procedures were characterized by scanning electron microscope in this study. As shown in Fig. 2A, a smooth surface was observed for the bare CE. When the magnetic Gr-MWCNTs composites were modified on CE surface, a dense network structure with ultrathin Gr sheets between magnetic MWCNTs entangled and overlapped was observed in Fig. 2B. Since both Gr and magnetic MWCNTs have large surface area and excellent electrical conductivity, the magnetic Gr-MWCNTs hybrid composites could serve as fast electronic conducting channels for the modified electrode [23]. Fig. 2C shows a dense imprinted layer coated on the magnetic Gr-MWCNTs surface, which indicated the MMIP/GrMWCNTs/CE was prepared successfully. As shown in Fig. 2D, when TBBPS was removed from the imprinted film with methanol/acetic acid (8:2, v/v) mixture solution, the surface of MMIP/Gr-MWCNTs/CE became roughness. 3.1.4. Optimization of the detection conditions of MMIP-sensor 3.1.4.1. Effect of solution pH value. The effect pH value of the supporting electrolyte solution on the response current of the MMIP-sensor

Fig. 2. SEM images of (A) bare CE; (B) magnetic Gr-MWCNTs/CE; (C) MMIP-sensor before and after removed TBBPS (D).

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F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66

was studied by DPV in 5.0 × 10−3 mol L−1 Fe(CN)3−/4− PBS containing 6 of 1.0 × 10−7 mol L−1 TBBPS with different pH values ranged from 5.0 to 8.0. As shown in Fig. S2(A), the current responses of the MMIP-sensor toward TBBPS (ΔIR, ΔIR = Iblank − Ip) increased with the increment of pH values in the pH range of 5.0–7.0 and then decreased when the pH value exceeded 7.0, which demonstrated that the PBS solution with pH of 7.0 can facilitate the MMIP/Gr-MWCNTs/CE to bind TBBPS. Thus, PBS with 7.0 was selected as the supporting electrolyte solution. 3.1.4.2. Incubation time. The incubation time of the imprinted sensor is an important parameter during the electrochemical measurement. In this study, the effect of the incubation time ranged from 1 to 20 min on the response currents of the MMIP-sensor was evaluated in PBS (pH = 7.0) containing 1.0 × 10− 7 mol L− 1 of TBBPS by DPV. As shown in Fig. S2(B), the response currents (ΔIR) of the MMIP-sensor toward TBBPS increased within the incubation time of 14 min. When the incubation time was over 14 min, the response currents leveled off slowly, indicating that the imprinted combination of the MMIP-sensor toward TBBPS tends to equilibrium state. So the incubation time of 14 min was chosen in the following experiments. 3.1.5. Calibration curve DPV was used to investigate the electrochemical performance of the MMIP-sensor toward TBBPS. The MMIP/Gr-MWCNTs/CE was immersed in TBBPS solution with different concentrations ranged from 1.0 × 10− 10 to 1.0 × 10− 7 mol L− 1 for 14 min and the results were shown in Fig. 3. A peak current at 0.18 V was obtained and the peak currents (Ip) decreased with the increasing TBBPS concentrations. The decrement of the peak currents of the MMIP-sensor was derived from the more TBBPS molecules combined by the imprinted cavities, which crossing through and taking place redox reaction hinder Fe(CN)3−/4− 6 on the CE surface. The MMIP-sensor showed a linear relationship between the response currents (ΔIR, ΔIR = Iblank − Ip) and the negative logarithm of TBBPS concentrations (− logС[TBBPS]) range of 1.0 × 10−10 − 1.0 × 10−7 mol L−1. As shown in the inset of Fig. 3, the corresponding regression equation was as follows: h
i ΔIR ðmAÞ ¼ 2:59 þ 0:254 log C mol L−1 R2 ¼ 0:992

ð2Þ

where ΔIR and Ip are response current and peak current of the MMIPsensor toward TBBPS solution, and the Ibalnk is the peak current of the MMIP-sensor toward the blank solution. The detection limit (LOD) of the MMIP-sensor toward TBBPS was calculated as 2.1 × 10−11 mol L−1 (S/N = 3). In order to investigate the imprinted

1.4

A

3.1.6. Selectivity and stability of MMIP-sensor DPV responses of the MMIP/Gr-MWCNTs/CE and MNIP/GrMWCNTs/CE toward 1.0 × 10−7 mol L−1 TBBPS and structural analog interferences including TBBPA, TBBP-DBPE, and BPA were compared to investigate the selectivity of the MMIP-sensor. As shown in Fig. S3, the response current of the MMIP-sensor toward TBBPS was highest, which indicated that the MMIP/Gr-MWCNTs/CE possessed special selectivity toward TBBPS. In addition, the change of response currents of the MNIP/Gr-MWCNTs/CE toward the different targets is unobvious because no imprinting cavities were existed on the MNIP/Gr-MWCNTs/CE surface. Although these structural analogues have the same functional groups with the template molecule, the recognition effect can depend on the distinct template size and chemical structure. The stability of the MMIP-sensor was investigated by DPV measurement of 1.0 × 10−7 mol L−1 TBBPS. The results showed that the current response decayed by 10.0% ± 2.0% after the MMIP/Gr-MWCNTs/CE was stored at 4 °C for a month, which confirmed that the MMIP-sensor had good stability. 3.2. Characterization and application of MMIP/Gr-MWCNTs 3.2.1. FT-IR spectra analysis FT-IR spectrum was carried out to characterize the chemical structure of the magnetic MWCNTs-COOH, MWCNTs-CO-Dopa, GO-NH2 and MMIP/Gr-MWCNTs. As shown Fig. 4(A), a board stretching vibration peak for O\\H at 3330–3500 cm−1 was recorded (curve a). The peaks at 1632 and 1387 cm−1 were assigned to C_C stretching bands for aromatic rings and C\\OH stretching [24]. The peak at 533 cm−1 corresponds to Fe\\O group, which was consistent with the characteristic adsorption peak of the magnetic particles [25]. For the MWCNTs-CODA (curve b), the stretching vibration peaks at 1653, 1590 and 1131 cm−1 correspond to the C_O, N\\H and C\\N, which are caused by the reaction of amides (− NH-C = O) [26]. The results indicated that DA was coated onto the magnetic MWCNTs-COOH surface successfully. For the GO-NH2 (curve c), the peak at 1623 cm−1 corresponds the C_C stretching vibration and the bands at 2926 and 2844 cm−1 are ascribed to the asymmetric\\CH2 and symmetric\\CH2 stretching of the

0.90

blank -10

1.0 10 mol L

MMIP-sensor MNIP-sensor

0.75

1.0 0.8 0.6 -7

B

-1

IR/mA

Current/mA

1.2

performance of the imprinted sensor, the imprinting factor which is defined as the the ratio of the slope of the calibration plot of TBBPS for the MMIP-chemosensor to that for NIP has been calculated as 3.78. Compared with previous methods for determination of TBBPS [4–6], as shown in Table 1, the developed MMIP-sensor in this paper exhibited better sensitivity toward TBBPS. Importantly, the detection limit of the proposed MMIP-sensor was lower than that of previous methods.

0.60 0.45 R

= 0.254logC + 2.59

2

-1

1.0 10 mol L

0.30

R = 0.990

0.4 0.15

0.2

R

0.00

0.0 -0.4

-0.2

0.0

0.2

0.4

Potential/V vs·SCE

0.6

0.8

= 0.067logC + 0.73

2

R = 0.994

7.0

7.5

8.0

8.5

9.0

9.5

-1

10.0

-log{[TBBPS]/mol L }

Fig. 3. (A) DPV responses of the MMIP-sensor toward different concentrations of TBBPS (1.0 × 10−7, 5.0 × 10−8, 1.0 × 10−8, 5.0 × 10−9, 1.0 × 10−9, 5.0 × 10−10 and 1.0 × 10−10 mol L−1, ; (B) inset of calibration curve of the MMIP-sensor and MNIP-sensor toward different concentrations of TBBPS respectively) in pH 7.0 PBS containing 5.0 × 10−3 mol L−1 Fe(CN)3−/46 solution.

F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66

63

Table 1 Comparison of different methods for determination of TBBPS. Detection methods

Linear range

LOD

Recoveries (%)

Samples

References

FSa SPME-HPLCb MIP-sensorc LC-APPI-MSd MISPE-MMIPsensore

1–10 μmol L−1 5.0–1000 μg L−1 5.0 × 10−10–1.0 × 10−5 mol L−1 – 1.0 × 10−10–1.0 × 10−7 mol L−1

– 0.4–0.9 μg L−1 1.3 × 0−10 mol L−1 1.28 ng g−1 2.1 × 10−11 mol L−1

– 86.5–103.6 100.3–104.0 – 89.1–101.5

– Water Water Chicken egg Water

[4] [5] [6] [28] This work

a b c d e

Fluorescence spectroscopy. Solid-phase microextraction coupled with high performance liquid chromatography. Imprinted electrochemical sensor. Liquid chromatography-atmospheric pressure photoionization-tandem mass spectrometry. Magnetic imprinted electrochemical sensor coupled with magnetic imprinted solid-phase extraction.

C\\H bond, respectively [27]. The peaks at 1590 and 1248 cm−1 correspond to the NH\\bending vibration and C\\N stretching vibration. The peaks at 895 and 1579 cm−1 correspond to in plane bending vibration and out of plane being vibration band of -NH2 group, respectively. The above characteristic peaks were also appeared in the spectrum of the MMIP/Gr-MWCNTs (curve d), but these peaks became weak, which can be attributed to the interactions between TBBPS and the MMIP/ Gr-MWCNTs. Thus, it can be concluded that the MMIP/Gr-MWCNTs composite was prepared successfully.

A 1632

a 533

1131

b

4000

3500

2500

2000

1500

1000

643 622 648

1232 1064 1020

1710 1626 1520

3000

895

1440 1307 1248 1085

c

1623 1579

2926 2844

3386

3325

1590

1653

1387

3430

1387

d

500

3.2.2. Magnetism property analysis A vibrating sample magnetometer (VSM) was employed to investigate the magnetism properties of the magnetic Gr-MWCNTs and MMIP/Gr-MWCNTs. As shown in Fig. 4B, the saturation magnetization of the magnetic Gr-MWCNTs and MMIP/Gr-MWCNTs was calculated as 28.62 and 19.25 emu g−1, respectively. The decrement of saturation magnetization of the MMIP/Gr-MWCNTs compared to the magnetic Gr-MWCNTs was attributing to the formation of imprinting layer on the magnetic Gr-MWCNTs surface. As shown in the insert of Fig. 4B, in the absence of an external magnet, a black homogeneous MMIP/GrMWCNTs dispersion existed. When an external magnet was closed, the black MMIP/Gr-MWCNTs could be easily attracted to the wall of vial rapidly (20 s). 3.2.3. Morphology analysis The morphologies of the magnetic MWCNTs-COOH, magnetic GrMWCNTs composites and MMIP/Gr-MWCNTs were characterized with TEM. Fig. 5a reveals the spherical shape Fe3O4 nanoparticles were grafted on MWCNTs-COOH surface successfully. Fig. 5b shows the ultrathin Gr sheets were adhered to the surface of magnetic MWCNTs-COOH with a diameter of 10–20 nm, indicating that the Gr hybrid magnetic MWCNTs-COOH composites were formed successfully. After imprinting polymerization, the size of MMIP/Gr-MWCNTs increased obviously. Compared to the magnetic Gr-MWCNTs (shown in Fig. 5b), the average thickness of imprinted layer of the MMIP/Gr-MWCNTs was calculated as 30–40 nm (shown in Fig. 5c).

-1

Magnetization(emu g-1)

W ave number(cm ) 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30

B

MMIP/Gr-MWCNTs magnetic Gr-MWCNTs

-20000-15000-10000-5000

0

5000 10000 15000 20000 25000

Magnetic field(Oe) Fig. 4. (A) FT-IR of magnetic MWCNTs-COOH (a), MWCNTs-CO-DA (b), GO-NH2 (c) and MMIP/Gr-MWCNTs (d); (B) VSM magnetization curves of magnetic Gr-MWCNTs and MMIP/Gr-MWCNTs. The inserted photograph shows the dispersion (left) and magnetic separation (right) process of the MMIP/Gr-MWCNTs in solution.

3.2.4. Adsorption property of MMIP/Gr-MWCNTs The static adsorption of the MMIP/Gr-MWCNTs and MNIP/GrMWCNTs toward TBBPS were investigated and the results were shown in Fig. S4(A). The MMIP/Gr-MWCNTs exhibited a higher adsorption capacity than that of the MNIP/Gr-MWCNTs, which can be attributed to the fact that no imprinted sites were existed in the MNIP/Gr-MWCNTs during the preparation process. The adsorption capacity of the MMIP/Gr-MWCNTs toward TBBPS increased with the TBBPS concentration increasing. This was ascribed to that a large number of imprinted sites were existed on the MMIP/Gr-MWCNTs surface, which results in the specific imprinted adsorption toward TBBPS easily. When the imprinted sites were filled up, the adsorption rate dropped and the adsorption process achieved equilibrium state. The equilibrium adsorption capacity (Qe, mol L−1) of the MMIP/Gr-MWCNTs toward TBBPS was calculated by Eq. (1). The theoretical maximum adsorption capacity (Qm) was obtained according to Langmuir adsorption isotherm Eq. (3): Ce 1 Ce ¼ þ Q e KdQ m Q m

ð3Þ

where Qe (mg g− 1) is the amount of TBBPS bound by the MMIP/GrMWCNTs at equilibrium; Ce (mg L− 1) is the elution concentration of

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F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66

Fig. 5. TEM images of magnetic MWCNTs-COOH (a), magnetic Gr-MWCNTs (b) and MMIP/Gr-MWCNTs (c).

TBBPS in equilibrium; Qm (mg g− 1) is the theoretical maximum adsorption capacity of MMIP/Gr-MWCNTs toward TBBPS, and K d (L mg− 1 ) is the Langmuir adsorption equilibrium constant of the

90

MMIP/Gr-MWCNTs toward TBBPS. The values of Kd and Qm were calculated from the slope and intercept of the linear line plotted in Ce/Qe versus Ce. As shown in Fig. S4(B), the linear regression equation was

A

90

B

85

Recovery(%)

Recovery(%)

85 80 75 70 65

80 75 70

60 65

55

60

50 10 90

20

30

40

50

60

5

10

15

20

25

30

Adsorption time/min

MMIP amount (mg)

C

Recovery(%)

85 80 75 70 65 60 55 50

5/5

6/4

7/3

8/2

9/1

Vmethanol:Vacetic acid Fig. 6. The effect of MMIP/Gr-MWCNTs amount (A), adsorption time (B) and elute solvent (C) on the recovery of TBBPS.

F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66 1.4 1.2

y = 0.056 × + 1.13, R2 = 0.990. Thus, the values of Qm and Kd were calculated as 17.83 mg g− 1 and 0.05 L mg− 1, respectively.

A

1.0

MMIP-sensor MMISPE-MMIP-sensor blank

0.8

3.2.5. Optimization of MSPE procedure The extraction conditions of magnetic imprinted solid-phase extraction were optimized by analyzing 25.0 mL of 1.0 × 10−7 mol L−1 TBBPS water sample. Several parameters including the MMIP/Gr-MWCNTs amount, adsorption time, and elute solvent were investigated with HPLC technique to evaluate the applicability of the MMIP/Gr-MWCNTs for extraction of TBBPS in real samples. When one parameter was changed, the other parameters were fixed at their optimized values.

Drinking water

0.6 0.4 0.2 1.4-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0.0

0.2

0.4

0.6

0.8

0.0

0.2

0.4

0.6

0.8

1.2

Current/mA

1.0 0.8 0.6

Rain water

0.4 0.2 1.4-0.4

-0.2

3.2.5.1. MMIP/Gr-MWCNTs amount.. Different amounts of the MMIP/GrMWCNTs ranged from 10.0 to 50.0 mg were applied to extract 25.0 mL of 1.0 × 10–7 mol L− 1 TBBPS water sample. As shown in Fig. 6A, when the amount of MMIP/Gr-MWCNTs was 30.0 mg, the highest recovery of TBBPS (86.2%) was obtained. Further increasing the amount of MMIP/Gr-MWCNTs, no improvement was obtained for recovery of TBBPS.

1.2 1.0 0.8 0.6

65

Lake water

0.4 0.2 1.4-0.4

-0.2

1.2 1.0

3.2.5.2. Adsorption time.. milligrams of the MMIP/Gr-MWCNTs was dispersed in TBBPS sample solution for different adsorption time range of 5–30 min to optimize the best adsorption time. Fig. 6B shows that the recoveries of TBBPS adsorbed by the MMIP/Gr-MWCNTs increased with the increment of adsorption time prior to 20 min. Further increase the adsorption time, the recoveries of TBBPS tend to equilibrium. Therefore, 20 min was chosen as the best adsorption time for following studies.

0.8 0.6

Runing water

0.4 0.2

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Potential/V vs·SCE

a

B

20000

TBBPS original sample

Signal intensity/mV

10000

3.2.5.3. Eluting solvent.. In order to obtain the highest recoveries of TBBPS, a series of 2.0 mL methanol/acetic acid mixture solutions with different volume ratios (5:5, 6:4, 7:3, 8:2, 9:1, v:v) were used to optimize the elution condition. As shown in Fig. 6C, the highest recovery was obtained using 2.0 mL of methanol/acetic (8:2, v/v) mixture solution as eluting solution.

0

b

20000 10000

MMIP-SPE extract

0

3.3. Evaluation of the combination system of MMISPE-MMIP-sensor

c

20000 10000

To evaluate the superiority of the combination system of MMISPEMMIP-sensor, TBBPS in water samples was separated and enriched as described in Section 2.5. Then, the TBBPS concentration in solution was detected with MMIP-sensor. Fig. 7A illustrates the peak current of the MMIP-sensor toward TBBPS sample solution before and after treated by the MMISPE. A high peak current was obtained for the MMIP-sensor to directly measure TBBPS in water sample. However, after TBBPS water samples were enriched with the MMISPE, a low peak current compared with the MMIP-sensor directly detection was observed. The results indicated that the combined system was enough sensitive to measure trace TBBPS in water samples based on dual preconcentration of TBBPS on the MMISPE and the MMIP-sensor. Compared with the current peak of the MMIP-sensor alone, the current peak of MMISPE-

Eluting sample solution

0 0

2

4

6

8

Time/min Fig. 7. (A) DPV peak current of using only MMIP-sensor and the combination system of MMISPE and MIP-sensor in four water samples containing of 1.0 × 10−7 mol L−1 TBBPS; (B) Chromatogram of TBBPS original sample (a); TBBPS extracted solution with MMIP/ Gr-MWCNTs (b) and eluting solution of MMISPE (c).

Table 2 Recovery of actual samples spiked different TBBPS concentrations. Samples

Spiked (×10−7 mol L−1)

Founda (×10−7 mol L−1)

Recoverya RSD%

Drinking water

10.0 10.0 20.0 20.0 50.0 50.0 80.0 80.0

7.85 80.6 17.9 17.6 44.3 45.1 71.2 72.4

78.5 80.6 89.9 88.3 88.6 90.2 89.0 90.5

Rain water Lake water Tap water a b

HPLC detection. Imprinted electrochemical sensor coupled with magnetic solid-phase extraction detection.

3.9 4.0 3.8 4.3 3.6 3.1 3.2 3.8

Foundb (×10−7 mol L−1)

Recoveryb RSDb%

9.1 9.2 18.9 18.8 44.2 45.6 78.2 79.0

90.8 91.5 94.5 93.8 88.4 91.2 97.8 98.8

4.2 3.8 3.3 3.1 3.5 3.4 4.4 3.9

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F. Long et al. / Journal of Electroanalytical Chemistry 777 (2016) 58–66

MMIP-sensor toward the same TBBPS solution increased to 0.25 mA. According with the regression Eq. (2), the TBBPS concentration in the solution before and after treated by the MMISPE was calculated. The determination system of MMISPE combined with MMIP-sensor provided up 10–12 folds preconcentration than that of the MMIP-sensor directly toward TBBPS. In order to further validate the reliability of the combination system of MMIP-sensor coupled with MMISPE to extract and detect TBBPS from water samples, the extraction solutions were also detected by HPLC. Fig. 7B shows the chromatograms of TBBPS in water samples before and after treated by MMISPE. After the sample solution was extracted with MMIP/Gr-MWCNTs, the peak area of TBBPS in Fig. 7B(b) was smaller than that of TBBPS original sample shown in Fig. 7B(a), which indicated that the MMIP/Gr-MWCNTs possessed of high adsorption capacity toward TBBPS. The chromatogram of TBBPS elution solution was shown in Fig. 7B(c). The peak area of TBBPS in the elution solution was larger than that of TBBPS in original sample. The data further confirmed that the MMIP/Gr-MWCNTs exhibited higher selectivity and enrichment ability toward TBBPS. The results were in accord with that the MMIP-sensor coupled with MMISPE method to detect traces TBBPS. Four TBBPS-spiked water samples involving drinking water, rain water, lake water, and tap water were detected by the MMISPEMMIP-sensor, and the results were summarized in Table 2. The recoveries of TBBPS for the MMISPE-MMIP-sensor were range of 90.8–98.8%, which was consistent with the results of HPLC. The results confirmed that the proposed MMISPE-MMIP-sensor is feasible, sensitive and highly selective for TBBPS analysis. 4. Conclusions A novel detection system combined MMIP-sensor with MMISPE was developed for rapid, sensitive and selective detection of traces TBBPS in water samples. The MMIP/Gr-MWCNTs exhibited high selectivity toward TBBPS with a maximum adsorption capacity of 17.83 mg g− 1. The MMISPE-MMIP-sensor combination detection system provided 10–12 folds preconcentration than that of the imprinted sensor directly determination toward TBBPS. The MMISPE-MMIP-sensor was used for the analysis of TBBPS in drinking water, rain water, lake water and tap water successfully with recoveries of 90.8–91.5, 93.8–94.5, 88.4–91.2 and 97.8–98.8%, respectively. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 21267010 and 21565014), the Research Innovation

Project Fund for Graduate Students in Hunan Province (No. CX2015B540), the Program of Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province (Environment and Energy Materials and Deep Processing of Mineral Resources in Wuling Mountain). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2016.07.027. References [1] I. Liagkouridis, A.P. Cousins, I.T. Cousins, Sci. Total Environ. 524–525 (2015) 416–426. [2] A. Papachlimitzou, J.L. Barber, S. Losada, P. Bersuder, R.J. Law, J. Chromatogr. A 1219 (2012) 15–28. [3] F.T. Dettmer, H. Wichmann, J. de Boer, M. Bahadir, Chemosphere 39 (1999) 1523–1532. [4] K. Ding, H. Zhang, H. Wang, X. Lv, L. Pan, W. Zhang, S. Zhuang, J. Hazard. Mater. 299 (2015) 486–494. [5] X. Wang, J. Liu, A. Liu, Q. Liu, X. Du, G. Jiang, Anal. Chim. Acta 753 (2012) 1–7. [6] H. Chen, Z. Zhang, R. Cai, W. Rao, F. Long, Electrochim. Acta 117 (2014) 385–392. [7] L. Figueiredo, N. G.L. Erny, L. Santos, A. Alves, Talanta 146 (2016) 754–765. [8] Y. Zhang, Y. Li, Y. Hu, G. Li, Y. Chen, J. Chromatogr. A 1217 (2010) 7337–7344. [9] A. Nezhadali, M. Mojarrab, J. Electroanal. Chem. 744 (2015) 85–94. [10] H. Rosy, R.N.G. Chasta, Talanta 125 (2014) 167–173. [11] A. Sarafraz-Yazdi, N. Razavi, Trends Anal. Chem. 73 (2015) 81–90; E. Turiel, J.L. Tadeo, A. Martin-Esteban, Anal. Chem. 79 (2007) 3099–3104. [12] B.B. Prasad, A.S.M.P. Tiwari, J. Colloid Interface Sci. 396 (2013) 234–241. [13] B.B. Prasad, K. Tiwari, M. Singh, P.S. Sharma, A.K. Patel, S. Srivastava, J. Chromatogr. A 1198–1199 (2008) 59–66. [14] B.B. Prasad, M.P. Tiwari, R. Madhuri, P.S. Sharma, Anal. Chim. Acta 662 (2010) 14–22. [15] B.B. Prasad, M.P. Tiwari, R. Madhuri, P.S. Sharma, J. Chromatogr. A 1217 (2010) 4255–4266. [16] Z. Zhang, L. Luo, R. Cai, H. Chen, Biosens. Bioelectron. 49 (2013) 367–373. [17] D. Xiao, P. Dramou, N. Xiong, H. He, H. Li, D. Yuan, H. Dai, J. Chromatogr. A 1274 (2013) 44–53. [18] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [19] B. Jia, L. Gao, J. Sun, Carbon 45 (2007) 1476–1481. [20] O. Arias de Fuentes, T. Ferri, M. Frasconi, V. Paolini, R. Santucci, Angew. Chem. 123 (2011) 3519–3523. [21] Z. Nasri, E. Shams, Electrochim. Acta 112 (2013) 640–647. [22] R. Wang, J. Sun, L. Gao, C. Xu, J. Zhang, Chem. Commun. 47 (2011) 8650–8652. [23] R. Kou, Y. Shao, D. Wang, M.H. Engelhard, J.H. Kwak, J. Wang, V.V. Viswanathan, C. Wang, Y. Lin, Y. Wang, I.A. Aksay, J. Liu, Electrochem. Commun. 11 (2009) 954–957. [24] S.C. Wang, J. Yang, X.Y. Zhou, J. Xie, L.L. Ma, B. Huang, J. Electroanal. Chem. 722–723 (2014) 141–147. [25] H.Y. Zhu, R. Jiang, L. Xiao, W. Li, J. Hazard. Mater. 179 (2010) 251–257. [26] C.T. Hsieh, H. Teng, W.Y. Chen, Y.S. Cheng, Carbon 48 (2010) 4219–4229. [27] T. Ramanathan, F.T. Fisher, R.S. Ruoff, L.C. Brinson, Chem. Mater. 17 (2005) 1290–1295. [28] R.J. Letcher, S. Chu, Environ. Sci. Technol. 44 (2010) 8615–8621.