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Preparation and application of sunset yellow imprinted ionic liquid polymer − ionic liquid functionalized graphene composite film coated glassy carbon electrodes
Electrochimica Acta 115 (2014) 247–254
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and application of sunset yellow imprinted ionic liquid polymer − ionic liquid functionalized graphene composite film coated glassy carbon electrodes Lijuan Zhao, Faqiong Zhao ∗ , Baizhao Zeng Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei Province, P. R. China
a r t i c l e
i n f o
Article history: Received 13 June 2013 Received in revised form 5 October 2013 Accepted 20 October 2013 Available online 7 November 2013 Keywords: Sunset yellow 1-(␣-Methyl acrylate)-3-allylimidazolium bromide Molecularly imprinted polymer Graphene Ionic liquid
a b s t r a c t A novel water-compatible molecularly imprinted ionic liquid polymer − ionic liquid functionalized graphene composite film coated glassy carbon electrode (MIP − rGO-IL/GCE) is presented. It is fabricated by coating a GCE with amine-terminated ionic liquid functionalized graphene (rGO-IL) and then with water-compatible MIP suspension. The water-compatible MIP is prepared by free radical polymerization in methanol-water system using sunset yellow (SY) as template and ionic liquid 1-(␣-methyl acrylate)3-allylimidazolium bromide (1-MA-3AI-Br) as functional monomer, which can interact with SY through -, hydrogen-bonding and electrostatic interaction. The resulting MIP − rGO-IL/GCE shows good performance when it is used for the differential pulse voltammetric determination of SY. Under the optimized conditions (i.e. pH 7.5, 0.1 M phosphate buffer, preconcentration under open-circuit for 570 s), the peak current is linear to SY concentration in the ranges of 0.010 M − 1.4 M and 1.4 M − 16 M with sensitivities of 5.0 A/M mm2 and 1.4 A/M mm2 respectively; the detection limit is 4 nM (S/N = 3). The electrode has been successfully applied to the determination of SY in some soft drinks, and the recoveries for the standards added are 95% − 107%. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Sunset yellow (SY) is a highly used synthetic dye which is usually added in foods such as beverages, candies and bakery products [1,2]. It can not only improve the appearance and texture of foods, but also make the foods to maintain the natural colour during process and storage [3,4]. However, SY has harmful effects on human beings if excessively consumed [5,6]. Therefore, the use of SY in food products is strictly controlled. In some countries, the permitted maximum content of SY is 100 g mL−1 (individually or in combination) in nonalcoholic beverages with added juices or flavours [7]. Considering the food safety, the detection of SY is important. Various methods have been developed for the detection of SY, including high-performance liquid chromatography (HPLC) [6], capillary electrophoresis [8], spectrophotometry [9] and electrochemical method [10]. Among these methods, the electrochemical method has recently become more attractive and a number of electrochemical detection methods have been reported for SY. For example, Song et al. [11] used a multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode (GCE) to detect SY;
∗ Corresponding author. Tel.: +86 27 68752701; fax: +86 27 68754067. E-mail address: [email protected] (F. Zhao). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.10.181
the linear detection range was 2.6 M − 0.59 mM and the detection limit was 1.1 M. Gan et al. [10] demonstrated a graphene (GN) layer-wrapped phosphotungstic acid (PTA) hybrid electrode for the simultaneous determination of SY and tartrazine (TT), the linear detection range was 2.2 nM − 0.66 M and the detection limit was 1.1 nM. Ghoreishi et al. [7] constructed a carbon-paste electrode (CPE) modified with gold nanoparticles for the simultaneous determination of SY and TT, and a linear detection range of 0.10 M − 2.0 M was obtained. These electrochemical detection methods were quite sensitive, but their selectivity was not enough good. To improve the selectivity of electrochemical methods, recognition elements should be introduced [12,13]. Among multifarious recognition elements, molecularly imprinted polymers (MIPs), called as ‘plastic antibody’, are biomimetic recognition materials. They have advantages of high stability, low cost and good selectivity [14,15]. These properties make MIPs attractive in many fields such as sensors [16], antibody mimics [17] and solid phase extraction [18]. However, current technology often generates MIPs for the recognition of target compounds by using methacrylic acid (MAA) or 4-vinyl pyridine (4-VP) as functional monomer, and aprotic or low polar organic solvents as porogen [19]. When polar solvents are present, the interaction between the template and the monomer will be inevitably weakened or disrupted [20]. Thus the recognition
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capacity of MIPs to strong hydrophilic target molecules is generally poor. In addition, when MIPs are prepared in organic solvents, they usually show ‘solvent memory’ in aqueous media [21]. To overcome the shortcoming, water-compatible MIPs should be synthesized. Ionic liquids (ILs) are molten salts, consisting of relatively large asymmetric organic cation and inorganic or organic anion. They have many unique characters such as non-volatility, nonflammability, high ion density and high ionic conductivity [22,23], hence they are very useful in many fields including solid-phase microextraction [24] and polymers [25]. Recently, a few molecularly imprinted ionic liquid polymers were reported [19,20,26,27]. As some IL monomers can interact with hydrophilic template molecules, IL-based water-compatible MIPs could be prepared. Thus, the recognition capacity of MIPs to hydrophilic target molecules in aqueous environment could be enhanced. In general, MIPs are nonconductive. To fabricate MIPs based electrochemical sensors, conductive materials should be introduced. Graphene (GR) displays superior electronic and chemical properties, hence it is suitable for preparing electrochemical sensors [28,29]. Further more, when IL is combined with GR, the resulting composite material presents improved conductivity, compatibility and stability [30]. Thus, GR-IL hybrid is expected to show good performance in constructing electrochemical sensors. In this paper, a novel IL-based water-compatible MIP and ILfunctionalized GR are used to construct a SY sensor. Owing to their synergic effect, the resulting electrochemical sensor exhibits good selectivity, high sensitivity and fast response towards SY. The sensor has been applied to the detection of SY in soft drinks. 2. Experimental 2.1. Reagents and materials Sunset yellow, Amaranth, Brilliant blue G, Allura red and Tratarzine (Fig. 1) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China) and their stock solutions (0.010 M) were prepared with water and stored in a refrigerator. 2,2‘Azobis-(isobutyronitrile) (AIBN) was obtained from Shanghai Shisihewei Chemical Industry Limited Company (China) and employed after twice recrystallization. Methacrylic acid (MAA) came from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Ethyleneglycol dimethacrylate (EDMA) was purchased
from Energy-Chemical Company (Shanghai, China). MAA and EDMA were distilled under reduced pressure to remove inhibitors. The ionic liquid 1-(3-aminopropyl)-3-methylimidazolium bromide (IL-NH2 ) (purity: 99%) was provided by Lanzhou Institute of Chemical Physics (Lanzhou, China) and used as received; hydrazine hydrate (85%) and N-hydroxy succinimide (NHS) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China); 1-ethyl-3-(3-dimethylaminopropyl)-carbodimide hydrochloride (EDC) was from Shanghai Medpep Co. Ltd. 1-(␣-Methyl acrylate)3-allylimidazolium bromide (1-MA-3AI-Br) was synthesized in our laboratory and its structure was confirmed by 1 H NMR, 13 C NMR and IR analysis (see Supporting Information). All other chemicals used were of analytical reagent grade. The water used was redistilled. 2.2. Apparatus Cyclic voltammetric and differential pulse voltammetric experiments were performed with a CHI 620D electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional three-electrode system was adopted. The working electrode was a modified GCE (diameter: 2 mm), and the auxiliary and reference electrodes were a Pt wire and a saturated calomel electrode (SCE), respectively. The scanning electron microscope (SEM) images were obtained using a Hitachi X-650 SEM (Hitachi Co., Japan). Ultraviolet visible (UV-Vis) absorption spectra were recorded by a U-3900 spectrometer (Hitachi Co., Japan). The Fourie transform infrared (FTIR) absorption spectra were recorded with a model Nexus-670 spectrometer (Nicolet, USA). Atomic force microscopy (AFM) images were obtained using a digital Nanoscope IIIa atomic force microscope in tapping mode. Transmission electron microscopy (TEM) images were obtained using JEOL Ltd. All experiments were carried out at room temperature. 2.3. Preparation of ionic liquid functionalized graphene (rGO-IL) rGO-IL was synthesized according to the literature with minor modification [31]. Briefly, 20 mg graphene oxide (GO) was dispersed in 20 mL water by sonicating for 1 h, then 25 mg EDC and 7 mg NHS were added in the suspension. After sonication for 30 min, 10 mg IL-NH2 was introduced and the mixture was stirred for 24 h at room temperature. The resulted mixture was centrifuged
Fig. 1. Molecular structure of sunset yellow and some analogues.
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and the precipitate (GO-IL) was washed with water. The GO-IL colloid (0.5 mg mL−1 ) was then reduced with 16.5 L hydrazine hydrate at 95 ◦ C for three hours. The product (i.e. rGO-IL) was centrifuged, washed with distilled water and dried in air. 2.4. Preparation of molecularly imprinted polymer Three kinds of MIPs were prepared by non-covalent approach with different monomers (i.e. 1-MA-3AI-Br, MAA and 4-VP). Briefly, SY (i.e. template, 0.5 mmol), monomer (2 mmol) and 50 mL porogenic solvent (i.e. methanol-water, V/V: 4:1) were added into a one-neck roundbottom flask (100 mL) successively. After 12 h pre-polymerization process at room temperature, AIBN (30.0 mg, 0.18 mmol) as initiator and EDMA (1.9820 g, 10.0 mmol) as crosslinker were added to the mixture. Then it was purged with nitrogen for 15 min and sealed in nitrogen atmosphere. Next, the mixture was placed in a water bath at 60 ◦ C and let to polymerize for 24 h. When the polymerization was completed, the product was washed thoroughly with methanol-ammonia solution (V/V: 9:1) and collected after centrifugation for 10 min at 10000 rpm. These procedures were repeated for several times until the product was free of template molecule, which was detected by UV-Vis absorption spectrometry. Subsequently, the product was washed with methanol to remove residual ammonia and dried at 50 ◦ C. The nonimprinted polymer (NIP) was synthesized in the same way except that the template molecule was absent. 2.5. Preparation of MIP and NIP modified electrodes The bare GCE was polished with slurry alumina (˚ = 0.5 m) and washed with water, with the aid of ultrasonication. Then 10 L rGOIL suspension (0.4 mg mL−1 in DMF) was dropcast on the cleaned GCE, and after the solvent evaporated under an infrared lamp 6 L MIP suspension (2 mg mL−1 in DMF) was dropped onto the resulting rGO-IL/GCE and let to evaporate in air. Thus, a MIP − rGOIL film coated GCE electrode (MIP − rGO-IL/GCE) was obtained. Prior to measurements, the electrode was conditioned by repeating potential scan between -0.2 V and 0.6 V in a phosphate buffer solution (PBS) until a stable voltammetric curve was obtained. The NIP − rGO-IL/GCE and other electrodes were prepared in a similar way.
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3. Results and discussion 3.1. Characterization of GO and rGO-IL rGO-IL was synthesized through the linkage reaction of EDC and NHS. The -NH2 group of the IL and the -COOH group located at the edges of GO sheet were covalently attached via amide bond formation. The covalently linked GO-IL was subsequently chemically reduced to rGO-IL (Scheme S1). Fig. 2 presents the typical TEM images of GO and as-prepared rGO-IL. Different from GO with rippled structure, rGO-IL displays flake-like shape and many folds occur probably due to lattice defects, which is the intrinsic feature of graphene sheets. Meanwhile, the corresponding selected area electron diffraction yields well-defined hexagonal lattice pattern, matching that expected for the ordered graphitic crystalline nature [32]. GO, IL-NH2 and rGO-IL were also characterized by FTIR (Fig. 3). The GO sheets showed a strong absorption band at 1718 cm−1 corresponding to C = O, and a band at 1620 cm−1 for the aromatic C = C stretching frequency. In the FTIR spectra of rGO-IL, CH3 (N) stretching, CH2 (N) stretching and ring in-plane asymmetric stretching were observed at 1168 cm−1 , which was produced by IL-NH2 . The new peak at 1354 cm−1 corresponded to the N connected to C in conjugated system. The FTIR spectra indicated the covalent reaction between amidocyanogen of IL-NH2 and the epoxy group of GO sheets [33]. 3.2. Preparation and morphology of 1-MA-3AI-Br − SY MIPs As can be seen in Scheme 1, SY molecule has two -SO3 − groups, one -OH and a benzene ring, thus, 1-MA-3AI-Br can form - stacking and hydrogen-bonding with it and they also show strong
2.6. Determination of adsorption amount The equilibrium adsorption amounts of various MIPs and NIP were determined as follows: 5 mg MIP (or NIP) was added to 5 mL SY solution (concentration: 20 M to 0.16 mM). After stirring for 5 h at room temperature, the mixture was centrifuged and the solution was collected and determined by UV-Vis absorption spectrometry. The adsorption amount was calculated according to the formula: Q = V (c0 - cS )/m, where V represents the volume of solution, c0 and cS are the SY concentrations before and after adsorption, m is the mass of MIP (or NIP). 2.7. Electrochemical measurements Proper volumes of SY and PBS (pH = 7.5) solutions were transferred to a 10 mL cell, and the electrode-system was installed on it. After accumulation for 570 s under open-circuit, cyclic voltammograms (CVs) or differential pulse voltammograms (DPVs) were recorded in the same solution. The potential scan range was 0.3 V − 1.0 V. After every measurement, the electrode was rinsed with methanol-ammonia solution (V/V: 9:1) to remove SY for reuse.
Fig. 2. TEM images of GO sheets (A) and rGO-IL composite (B). Inset is the selected area electron diffraction pattern (SAED) of rGO-IL.
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MAA − SY MIP and 4-VP − SY MIP particles, so they have larger specific surface. The MAA − SY MIP and 4-VP − SY MIP particles exhibit rather rough shape and wide size distribution (diameter: 0.5 m − 1 m). This is related to the nature of different polymers. In general, the MIP with larger specific surface is favorable for molecular recognition. 3.3. Adsorption curves
Fig. 3. FTIR spectra of GO, IL-NH2 and rGO-IL.
electrostatic attraction. Therefore, the combination between SY and 1-MA-3AI-Br is tight. However, 4-VP interacts with SY just through hydrogen-bonding and electrostatic attraction; MAA and SY only have hydrogen-bonding interaction, so their combination is weak. Furthermore, the absorbance of 1-MA-3AI-Br − SY is stable in aqueous solutions, meaning that the combination of 1-MA-3AI-Br and SY is quite strong in aqueous solutions. In addition, 1-MA-3AI-Br possesses two groups suitable for polymerization. Consequently, 1MA-3AI-Br − SY MIP can be prepared more easily and it may show higher specific recognition ability than MAA − SY MIP and 4-VP − SY MIP. Fig. 4 exhibits the SEM images of 1-MA-3AI-Br − SY MIP, MAA − SY MIP and 4-VP − SY MIP. The 1-MA-3AI-Br − SY MIP particles are smaller (mean diameter: 0.2 m) in comparison with the
The adsorption curves of SY on 1-MA-3AI-Br − SY MIP and 1MA-3AI-Br NIP are shown in Fig. 5. It is clear that the adsorption amount of SY on the MIP changes with its concentration more rapidly compared with that on the NIP. When SY concentration exceeds 0.14 mM, the adsorption amount is almost unchanged. The maximum adsorption amount of MIP is about 91 mol g−1 . Similarly, when SY concentration exceeds 0.10 mM, the adsorption curve of SY exhibits a platform on the NIP. This means that a saturated adsorption is achieved and the adsorption amount of NIP is about 20 mol g−1 , which is much smaller than that of MIP. Obviously, through molecular imprinting, the adsorption amount of SY is greatly enhanced. In addition, the adsorption amounts of 4-VP − SY MIP (72 mol g−1 ) and MAA − SY MIP (60 mol g−1 ) are determined and they are lower than that of 1MA-3Al-Br − SY MIP (Fig. 5). Therefore, the 1-MA-3AI-Br − SY MIP is selected for the following experiments. 3.4. Voltammetric behavior of SY Fig. 6 shows the cyclic voltammograms of SY at different modified electrodes. At the rGO-IL/GCE, a pair of redox peaks at 0.66 V and 0.62 V are observed for SY, meaning that the electrochemical
Scheme 1. Schematic illustration of the preparation of 1-MA-3AI-Br − SY MIP.
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Fig. 5. Saturation rebinding curves of SY on 1-MA-3AI-Br − SY MIP, 4-VP − SY MIP, MAA − SY MIP and 1-MA-3AI-Br NIP.
reason is that the MIP can provide many specific sites for SY. SY produces small peaks at the MIP/GCE, which can be ascribed to the poor conductivity and small effective surface area of MIP/GCE. The small background signal can support this point to some extent. 3.5. Optimization of parameters affecting the determination of SY 3.5.1. The amount of rGO-IL and MIP The effect of the amount of rGO-IL was studied (Fig. 7A). Results showed that the peak current of SY increased with increasing the volume of rGO-IL suspension up to 10.0 L, then it decreased slowly. This is related to the change of electrode surface area and electron transfer resistance. When the rGO-IL amount was too much, the electrode surface area kept almost unchanged, but the resistance increased. Therefore, 10.0 L of rGO-IL suspension was adopted for further study. The amount of MIP was also varied to examine its influence on the peak current of SY. It was found when the MIP amount was less (e.g. 2.0 or 4.0 L, 2 mg mL−1 ) the peak current was bigger, but the selectivity was poor; when the amount of MIP was too much (e.g. 8.0 L) the MIP particles easily peeled off the electrode surface. It was found that 6.0 L MIP suspension was optimal. 3.5.2. Solution pH Next, the influence of pH on the peak current of SY was investigated (Fig. 7B). The peak current increased with increasing pH
Fig. 4. SEM images of MIP particles. (A) 1-MA-3AI-Br − SY MIP, (B) MAA − SY MIP, (C) 4-VP − SY MIP.
reaction of SY is quasi-reversible. As the oxidation peak is higher, it is selected for the determination of SY. To improve the selectivity of rGO-IL/GCE, MIP is introduced. At the resulting MIP − rGO-IL/GCE, the peak current of SY decreases slightly because the additional MIP film hinders the electron transfer to some extent. It should be pointed out that the peak current of SY at the MIP − rGO-IL/GCE is much bigger than at the MIP − rGO/GCE, meaning that the IL plays an important role. We think it is related to the conductivity and preconcentration of IL. Compared with the NIP − rGO-IL/GCE, the MIP − rGO-IL/GCE exhibits more sensitive response to SY, indicating that the MIP also plays an important role in sensing SY. The
Fig. 6. Cyclic voltammograms of rGO-IL/GCE (a), MIP − rGO-IL/GCE (b), MIP − rGO/GCE (c), NIP − rGO-IL/GCE (d) and MIP/GCE (e) in 0.1 M PBS (pH 7.5) containing 50 M SY. Scan rate: 50 mV s−1 ; accumulation time: 570 s (under open circuit).
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Fig. 8. (A) Differential pulse voltammograms of SY at MIP − rGO-IL/GCE. SY concentration: 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.40, 0.60, 0.80, 1.0, 1.2, 1.4, 1.6, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0 M (from a to t). (B) Calibration curves for SY at MIP − rGO-IL/GCE and NIP − rGO-IL/GCE. Other conditions as in Fig. 6.
3.5.3. Preconcentration time When the preconcentration time was changed from 210 s to 570 s, the peak current of a 0.50 M SY solution increased rapidly (Fig. 7C). However, when it exceeded 570 s, the peak current kept almost unchanged, indicating that the adsorption equilibrium of SY at the MIP − rGO-IL/GCE was achieved. For a 5.0 M SY solution, the peak current reached the maximum around 450 s (see the inset in Fig. 7C). Herein, 570 s was selected for SY determination.
Fig. 7. Optimization of different conditions affecting the determination of 0.50 M SY solutions. (A) Influence of the amount of rGO-IL used. (B) Influence of solution pH; inset: the plot of peak potential versus pH. (C) Influence of accumulation time on peak currents of 0.50 M SY and 5.0 M SY (inset). Other conditions as in Fig. 6.
up to 7.5, then gradually decreased. Thus, pH 7.5 phosphate buffer solution was selected as supporting electrolyte. At the same time, the peak potential (Ep ) decreased linearly with increasing pH (see the inset in Fig. 7B). The linear relationship could be expressed as follows: Ep (V) = - 0.0031 pH + 0.852, with a correlation coefficient of 0.9900 and a slope of 31.0 mV pH−1 . According to the Nernstian equation: E ∝ 59.16 m/n pH (m: number of proton transferred; n: number of electron transferred), we can see that the number of electron transferred is twice as the number of proton transferred in the electrochemical reaction.
3.6. Calibration curves Fig. 8 shows the DPVs of SY under the optimized experimental conditions. It can be seen that the peak current increases with increasing SY concentration, and they present good linear relationship in the ranges of 10 nM − 1.4 M and 1.4 M − 16 M. The regression equations are: Ip (A) = 15.8 c (M) + 0.74 (R2 = 0.996) and Ip (A) = 4.28 c (M) + 19.58 (R2 = 0.997), with sensitivities of 5.0 A/M mm2 and 1.4 A/M mm2 , respectively. When SY concentration is above 16 M, the calibration curve gradually deviates from the straight line, indicating that the saturated adsorption is gradually reached. The limit of detection is ca. 4.0 nM (S/N = 3). Compared with other sensors reported for SY [7,11,34,35], the MIP − rGO-IL/GCE offers a relatively wide linear range and a low detection limit. The NIP − rGO-IL/GCE shows a smaller linear range and lower sensitivity than the MIP − rGO-IL/GCE. This can be explained by the lack of specific binding sites on the NIP film.
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Table 1 Determination results of SY in samples by using a MIP − rGO-IL/GCE (n = 3). Samples
SY added (M)
SY expected (M)
SY found (M)
Recovery (%)
Fruit juice
0 0.50 2.00 5.00 0 0.50 2.00 5.00 0 0.50 2.00 5.00
0.50 2.00 5.00 7.07 8.57 11.57 8.37 9.87 12.87
0.48 2.07 5.03 6.57 7.06 8.70 11.66 7.87 8.38 9.76 12.93
96 103 101 98 107 102 102 95 101
Mirinda drink
Orange juice
Fig. 9. Influence of coexistent substance on the electrochemical response of MIP − rGO-IL/GCE to SY. Solution composition: (a) 5.0 M SY + 0.1 M PBS (pH 7.5), (b) a + 50.0 M Brilliant blue G, (c) a + 50.0 M Amaranth, (d) a + 50.0 M Tratarzine. (e) a +50.0 M sodium salicylate. Other conditions as in Fig. 6.
3.7. Selectivity, reproducibility and stability of MIP − rGO-IL/GCE The selectivity of the MIP − rGO-IL/GCE was evaluated by testing the electrochemical response of 5.0 M SY in the presence of some analogues and potential interfering substances, such as Brilliant blue G, Amaranth, Tratarzine and sodium salicylate. As shown in Fig. 9, when their concentrations were 10-fold of that of SY the peak current of SY changed by 2% − 10%. When SY concentration was 0.50 M similar result was obtained (Fig. S1). This indicated that the MIP − rGO-IL/GCE had good selectivity. However, Allura red showed serious influence on the determination of SY. When 1fold of Allura red was present, the peak current of SY increased by 24%, meaning that Allura red could be bound by the molecularly imprinted site to some extent and could produce redox peaks around the peak potential of SY. This indicated that their chemical structures were too similar to be well discriminated by the MIP. To evaluate the reproducibility five MIP − rGO-IL/GCEs were prepared by the same way and a 0.50 M SY solution was determined. As a result, the relative standard deviation (RSD) of the peak current was 7.2% (n = 5). The repeatability was investigated by monitoring a 0.50 M SY solution using one modified electrode, and the RSD of peak current was 4.5% (n = 5). After used successively for 30 assays the current response of the sensor still remained 94% of its initial value. The inter-day RSD was 5.3% (n = 5). After stored for 5 days in a refrigerator, the MIP − rGO-IL/GCE retained 93% of its initial current response for 0.50 M SY; after two week-storage it still retained 87% of its initial current response. These reflected the good reproducibility and stability of MIP − rGOIL/GCE. 3.8. Analytical application of MIP − rGO-IL/GCE In order to test its application feasibility, the MIP − rGO-IL/GCE was applied to the determination of SY in several soft drinks (i.e. fruit juice, Mirinda drink and orange juice from a local market). Prior to determination, the soft drinks were diluted from 2.0 mL to 10 mL with 0.1 M phosphate buffer solution (pH 7.5). Results showed that in the Mirinda drink SY concentration was ca. 32.5 M and in the orange juice it was ca. 39.5 M (RSD < 5%). But in the fruit juice sample SY was not detected. SY standard solutions were added in the sample solutions to estimate the recoveries and they were 95 − 107% for different samples (Table 1). This indicated that the proposed method was reliable.
4. Conclusions A novel molecularly imprinted ionic liquid polymer − ionic liquid functionalized graphene composite film coated glassy carbon electrode is fabricated for SY sensing. The MIP is water-compatible and it has high recognition capacity to hydrophilic SY; the ILfunctionalized grapheme has good conductivity and large surface area. Thus the modified electrode exhibits high selectivity, reproducibility and sensitivity, and it can be applied to the determination of SY in real samples. This work provides a new way for constructing novel MIP based sensors. Acknowledgements The authors appreciate the financial support of the National Natural Science Foundation of China (Grant Nos.: 21075092, 21277105). 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. 2013.10.181. References [1] Y.S. Al-Degs, Determination of three dyes in commercial soft drinks using HLA/GO and liquid chromatography, Food Chemistry 117 (2009) 485–490. [2] Y. Ni, Y. Wang, S. Kokot, Simultaneous kinetic spectrophotometric analysis of five synthetic food colorants with the aid of chemometrics, Talanta 78 (2009) 432–441. [3] M.M. Ghoneim, H.S. El-Desoky, N.M. Zidan, Electro-Fenton oxidation of Sunset Yellow FCF azo-dye in aqueous solutions, Desalination 274 (2011) 22–30. [4] N. Yoshioka, K. Ichihashi, Determination of 40 synthetic food colors in drinks and candies by high-performance liquid chromatography using a short column with photodiode array detection, Talanta 74 (2008) 1408–1413. [5] S.M. Ghoreishi, M. Behpour, M. Golestaneh, Simultaneous voltammetric determination of Brilliant Blue and Tartrazine in real samples at the surface of a multi-walled carbon nanotube paste electrode, Analytical Methods 3 (2011) 2842–2847. [6] K.S. Minioti, C.F. Sakellariou, N.S. Thomaidis, Determination of 13 synthetic food colorants in water-soluble foods by reversed-phase high-performance liquid chromatography coupled with diode-array detector, Analytica Chimica Acta 583 (2007) 103–110. [7] S.M. Ghoreishi, M. Behpour, M. Golestaneh, Simultaneous determination of Sunset yellow and Tartrazine in soft drinks using gold nanoparticles carbon paste electrode, Food Chemistry 132 (2012) 637–641. ´ P. Vrábel, P. Krásensky, ´ J. Preisler, Sensitive [8] M. Ryvolová, P. Táborsky, determination of erythrosine and other red food colorants using capillary electrophoresis with laser-induced fluorescence detection, Journal of Chromatography A 1141 (2007) 206–211. [9] N.E. Llamas, M. Garrido, M.S. Di Nezio, B.S. Fernandez Band, Second order advantage in the determination of amaranth, sunset yellow FCF and tartrazine by UV-vis and multivariate curve resolution-alternating least squares, Analytica Chimica Acta 655 (2009) 38–42. [10] T. Gan, J.Y. Sun, S.Q. Cao, F. Gao, Y.X. Zhang, Y.Q. Yang, One-step electrochemical approach for the preparation of graphene wrapped-phosphotungstic acid
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and application of sunset yellow imprinted ionic liquid polymer − ionic liquid functionalized graphene composite film coated glassy carbon electrodes Lijuan Zhao, Faqiong Zhao ∗ , Baizhao Zeng Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei Province, P. R. China
a r t i c l e
i n f o
Article history: Received 13 June 2013 Received in revised form 5 October 2013 Accepted 20 October 2013 Available online 7 November 2013 Keywords: Sunset yellow 1-(␣-Methyl acrylate)-3-allylimidazolium bromide Molecularly imprinted polymer Graphene Ionic liquid
a b s t r a c t A novel water-compatible molecularly imprinted ionic liquid polymer − ionic liquid functionalized graphene composite film coated glassy carbon electrode (MIP − rGO-IL/GCE) is presented. It is fabricated by coating a GCE with amine-terminated ionic liquid functionalized graphene (rGO-IL) and then with water-compatible MIP suspension. The water-compatible MIP is prepared by free radical polymerization in methanol-water system using sunset yellow (SY) as template and ionic liquid 1-(␣-methyl acrylate)3-allylimidazolium bromide (1-MA-3AI-Br) as functional monomer, which can interact with SY through -, hydrogen-bonding and electrostatic interaction. The resulting MIP − rGO-IL/GCE shows good performance when it is used for the differential pulse voltammetric determination of SY. Under the optimized conditions (i.e. pH 7.5, 0.1 M phosphate buffer, preconcentration under open-circuit for 570 s), the peak current is linear to SY concentration in the ranges of 0.010 M − 1.4 M and 1.4 M − 16 M with sensitivities of 5.0 A/M mm2 and 1.4 A/M mm2 respectively; the detection limit is 4 nM (S/N = 3). The electrode has been successfully applied to the determination of SY in some soft drinks, and the recoveries for the standards added are 95% − 107%. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Sunset yellow (SY) is a highly used synthetic dye which is usually added in foods such as beverages, candies and bakery products [1,2]. It can not only improve the appearance and texture of foods, but also make the foods to maintain the natural colour during process and storage [3,4]. However, SY has harmful effects on human beings if excessively consumed [5,6]. Therefore, the use of SY in food products is strictly controlled. In some countries, the permitted maximum content of SY is 100 g mL−1 (individually or in combination) in nonalcoholic beverages with added juices or flavours [7]. Considering the food safety, the detection of SY is important. Various methods have been developed for the detection of SY, including high-performance liquid chromatography (HPLC) [6], capillary electrophoresis [8], spectrophotometry [9] and electrochemical method [10]. Among these methods, the electrochemical method has recently become more attractive and a number of electrochemical detection methods have been reported for SY. For example, Song et al. [11] used a multiwalled carbon nanotube (MWCNT) modified glassy carbon electrode (GCE) to detect SY;
∗ Corresponding author. Tel.: +86 27 68752701; fax: +86 27 68754067. E-mail address: [email protected] (F. Zhao). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.10.181
the linear detection range was 2.6 M − 0.59 mM and the detection limit was 1.1 M. Gan et al. [10] demonstrated a graphene (GN) layer-wrapped phosphotungstic acid (PTA) hybrid electrode for the simultaneous determination of SY and tartrazine (TT), the linear detection range was 2.2 nM − 0.66 M and the detection limit was 1.1 nM. Ghoreishi et al. [7] constructed a carbon-paste electrode (CPE) modified with gold nanoparticles for the simultaneous determination of SY and TT, and a linear detection range of 0.10 M − 2.0 M was obtained. These electrochemical detection methods were quite sensitive, but their selectivity was not enough good. To improve the selectivity of electrochemical methods, recognition elements should be introduced [12,13]. Among multifarious recognition elements, molecularly imprinted polymers (MIPs), called as ‘plastic antibody’, are biomimetic recognition materials. They have advantages of high stability, low cost and good selectivity [14,15]. These properties make MIPs attractive in many fields such as sensors [16], antibody mimics [17] and solid phase extraction [18]. However, current technology often generates MIPs for the recognition of target compounds by using methacrylic acid (MAA) or 4-vinyl pyridine (4-VP) as functional monomer, and aprotic or low polar organic solvents as porogen [19]. When polar solvents are present, the interaction between the template and the monomer will be inevitably weakened or disrupted [20]. Thus the recognition
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capacity of MIPs to strong hydrophilic target molecules is generally poor. In addition, when MIPs are prepared in organic solvents, they usually show ‘solvent memory’ in aqueous media [21]. To overcome the shortcoming, water-compatible MIPs should be synthesized. Ionic liquids (ILs) are molten salts, consisting of relatively large asymmetric organic cation and inorganic or organic anion. They have many unique characters such as non-volatility, nonflammability, high ion density and high ionic conductivity [22,23], hence they are very useful in many fields including solid-phase microextraction [24] and polymers [25]. Recently, a few molecularly imprinted ionic liquid polymers were reported [19,20,26,27]. As some IL monomers can interact with hydrophilic template molecules, IL-based water-compatible MIPs could be prepared. Thus, the recognition capacity of MIPs to hydrophilic target molecules in aqueous environment could be enhanced. In general, MIPs are nonconductive. To fabricate MIPs based electrochemical sensors, conductive materials should be introduced. Graphene (GR) displays superior electronic and chemical properties, hence it is suitable for preparing electrochemical sensors [28,29]. Further more, when IL is combined with GR, the resulting composite material presents improved conductivity, compatibility and stability [30]. Thus, GR-IL hybrid is expected to show good performance in constructing electrochemical sensors. In this paper, a novel IL-based water-compatible MIP and ILfunctionalized GR are used to construct a SY sensor. Owing to their synergic effect, the resulting electrochemical sensor exhibits good selectivity, high sensitivity and fast response towards SY. The sensor has been applied to the detection of SY in soft drinks. 2. Experimental 2.1. Reagents and materials Sunset yellow, Amaranth, Brilliant blue G, Allura red and Tratarzine (Fig. 1) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China) and their stock solutions (0.010 M) were prepared with water and stored in a refrigerator. 2,2‘Azobis-(isobutyronitrile) (AIBN) was obtained from Shanghai Shisihewei Chemical Industry Limited Company (China) and employed after twice recrystallization. Methacrylic acid (MAA) came from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Ethyleneglycol dimethacrylate (EDMA) was purchased
from Energy-Chemical Company (Shanghai, China). MAA and EDMA were distilled under reduced pressure to remove inhibitors. The ionic liquid 1-(3-aminopropyl)-3-methylimidazolium bromide (IL-NH2 ) (purity: 99%) was provided by Lanzhou Institute of Chemical Physics (Lanzhou, China) and used as received; hydrazine hydrate (85%) and N-hydroxy succinimide (NHS) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China); 1-ethyl-3-(3-dimethylaminopropyl)-carbodimide hydrochloride (EDC) was from Shanghai Medpep Co. Ltd. 1-(␣-Methyl acrylate)3-allylimidazolium bromide (1-MA-3AI-Br) was synthesized in our laboratory and its structure was confirmed by 1 H NMR, 13 C NMR and IR analysis (see Supporting Information). All other chemicals used were of analytical reagent grade. The water used was redistilled. 2.2. Apparatus Cyclic voltammetric and differential pulse voltammetric experiments were performed with a CHI 620D electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional three-electrode system was adopted. The working electrode was a modified GCE (diameter: 2 mm), and the auxiliary and reference electrodes were a Pt wire and a saturated calomel electrode (SCE), respectively. The scanning electron microscope (SEM) images were obtained using a Hitachi X-650 SEM (Hitachi Co., Japan). Ultraviolet visible (UV-Vis) absorption spectra were recorded by a U-3900 spectrometer (Hitachi Co., Japan). The Fourie transform infrared (FTIR) absorption spectra were recorded with a model Nexus-670 spectrometer (Nicolet, USA). Atomic force microscopy (AFM) images were obtained using a digital Nanoscope IIIa atomic force microscope in tapping mode. Transmission electron microscopy (TEM) images were obtained using JEOL Ltd. All experiments were carried out at room temperature. 2.3. Preparation of ionic liquid functionalized graphene (rGO-IL) rGO-IL was synthesized according to the literature with minor modification [31]. Briefly, 20 mg graphene oxide (GO) was dispersed in 20 mL water by sonicating for 1 h, then 25 mg EDC and 7 mg NHS were added in the suspension. After sonication for 30 min, 10 mg IL-NH2 was introduced and the mixture was stirred for 24 h at room temperature. The resulted mixture was centrifuged
Fig. 1. Molecular structure of sunset yellow and some analogues.
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and the precipitate (GO-IL) was washed with water. The GO-IL colloid (0.5 mg mL−1 ) was then reduced with 16.5 L hydrazine hydrate at 95 ◦ C for three hours. The product (i.e. rGO-IL) was centrifuged, washed with distilled water and dried in air. 2.4. Preparation of molecularly imprinted polymer Three kinds of MIPs were prepared by non-covalent approach with different monomers (i.e. 1-MA-3AI-Br, MAA and 4-VP). Briefly, SY (i.e. template, 0.5 mmol), monomer (2 mmol) and 50 mL porogenic solvent (i.e. methanol-water, V/V: 4:1) were added into a one-neck roundbottom flask (100 mL) successively. After 12 h pre-polymerization process at room temperature, AIBN (30.0 mg, 0.18 mmol) as initiator and EDMA (1.9820 g, 10.0 mmol) as crosslinker were added to the mixture. Then it was purged with nitrogen for 15 min and sealed in nitrogen atmosphere. Next, the mixture was placed in a water bath at 60 ◦ C and let to polymerize for 24 h. When the polymerization was completed, the product was washed thoroughly with methanol-ammonia solution (V/V: 9:1) and collected after centrifugation for 10 min at 10000 rpm. These procedures were repeated for several times until the product was free of template molecule, which was detected by UV-Vis absorption spectrometry. Subsequently, the product was washed with methanol to remove residual ammonia and dried at 50 ◦ C. The nonimprinted polymer (NIP) was synthesized in the same way except that the template molecule was absent. 2.5. Preparation of MIP and NIP modified electrodes The bare GCE was polished with slurry alumina (˚ = 0.5 m) and washed with water, with the aid of ultrasonication. Then 10 L rGOIL suspension (0.4 mg mL−1 in DMF) was dropcast on the cleaned GCE, and after the solvent evaporated under an infrared lamp 6 L MIP suspension (2 mg mL−1 in DMF) was dropped onto the resulting rGO-IL/GCE and let to evaporate in air. Thus, a MIP − rGOIL film coated GCE electrode (MIP − rGO-IL/GCE) was obtained. Prior to measurements, the electrode was conditioned by repeating potential scan between -0.2 V and 0.6 V in a phosphate buffer solution (PBS) until a stable voltammetric curve was obtained. The NIP − rGO-IL/GCE and other electrodes were prepared in a similar way.
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3. Results and discussion 3.1. Characterization of GO and rGO-IL rGO-IL was synthesized through the linkage reaction of EDC and NHS. The -NH2 group of the IL and the -COOH group located at the edges of GO sheet were covalently attached via amide bond formation. The covalently linked GO-IL was subsequently chemically reduced to rGO-IL (Scheme S1). Fig. 2 presents the typical TEM images of GO and as-prepared rGO-IL. Different from GO with rippled structure, rGO-IL displays flake-like shape and many folds occur probably due to lattice defects, which is the intrinsic feature of graphene sheets. Meanwhile, the corresponding selected area electron diffraction yields well-defined hexagonal lattice pattern, matching that expected for the ordered graphitic crystalline nature [32]. GO, IL-NH2 and rGO-IL were also characterized by FTIR (Fig. 3). The GO sheets showed a strong absorption band at 1718 cm−1 corresponding to C = O, and a band at 1620 cm−1 for the aromatic C = C stretching frequency. In the FTIR spectra of rGO-IL, CH3 (N) stretching, CH2 (N) stretching and ring in-plane asymmetric stretching were observed at 1168 cm−1 , which was produced by IL-NH2 . The new peak at 1354 cm−1 corresponded to the N connected to C in conjugated system. The FTIR spectra indicated the covalent reaction between amidocyanogen of IL-NH2 and the epoxy group of GO sheets [33]. 3.2. Preparation and morphology of 1-MA-3AI-Br − SY MIPs As can be seen in Scheme 1, SY molecule has two -SO3 − groups, one -OH and a benzene ring, thus, 1-MA-3AI-Br can form - stacking and hydrogen-bonding with it and they also show strong
2.6. Determination of adsorption amount The equilibrium adsorption amounts of various MIPs and NIP were determined as follows: 5 mg MIP (or NIP) was added to 5 mL SY solution (concentration: 20 M to 0.16 mM). After stirring for 5 h at room temperature, the mixture was centrifuged and the solution was collected and determined by UV-Vis absorption spectrometry. The adsorption amount was calculated according to the formula: Q = V (c0 - cS )/m, where V represents the volume of solution, c0 and cS are the SY concentrations before and after adsorption, m is the mass of MIP (or NIP). 2.7. Electrochemical measurements Proper volumes of SY and PBS (pH = 7.5) solutions were transferred to a 10 mL cell, and the electrode-system was installed on it. After accumulation for 570 s under open-circuit, cyclic voltammograms (CVs) or differential pulse voltammograms (DPVs) were recorded in the same solution. The potential scan range was 0.3 V − 1.0 V. After every measurement, the electrode was rinsed with methanol-ammonia solution (V/V: 9:1) to remove SY for reuse.
Fig. 2. TEM images of GO sheets (A) and rGO-IL composite (B). Inset is the selected area electron diffraction pattern (SAED) of rGO-IL.
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MAA − SY MIP and 4-VP − SY MIP particles, so they have larger specific surface. The MAA − SY MIP and 4-VP − SY MIP particles exhibit rather rough shape and wide size distribution (diameter: 0.5 m − 1 m). This is related to the nature of different polymers. In general, the MIP with larger specific surface is favorable for molecular recognition. 3.3. Adsorption curves
Fig. 3. FTIR spectra of GO, IL-NH2 and rGO-IL.
electrostatic attraction. Therefore, the combination between SY and 1-MA-3AI-Br is tight. However, 4-VP interacts with SY just through hydrogen-bonding and electrostatic attraction; MAA and SY only have hydrogen-bonding interaction, so their combination is weak. Furthermore, the absorbance of 1-MA-3AI-Br − SY is stable in aqueous solutions, meaning that the combination of 1-MA-3AI-Br and SY is quite strong in aqueous solutions. In addition, 1-MA-3AI-Br possesses two groups suitable for polymerization. Consequently, 1MA-3AI-Br − SY MIP can be prepared more easily and it may show higher specific recognition ability than MAA − SY MIP and 4-VP − SY MIP. Fig. 4 exhibits the SEM images of 1-MA-3AI-Br − SY MIP, MAA − SY MIP and 4-VP − SY MIP. The 1-MA-3AI-Br − SY MIP particles are smaller (mean diameter: 0.2 m) in comparison with the
The adsorption curves of SY on 1-MA-3AI-Br − SY MIP and 1MA-3AI-Br NIP are shown in Fig. 5. It is clear that the adsorption amount of SY on the MIP changes with its concentration more rapidly compared with that on the NIP. When SY concentration exceeds 0.14 mM, the adsorption amount is almost unchanged. The maximum adsorption amount of MIP is about 91 mol g−1 . Similarly, when SY concentration exceeds 0.10 mM, the adsorption curve of SY exhibits a platform on the NIP. This means that a saturated adsorption is achieved and the adsorption amount of NIP is about 20 mol g−1 , which is much smaller than that of MIP. Obviously, through molecular imprinting, the adsorption amount of SY is greatly enhanced. In addition, the adsorption amounts of 4-VP − SY MIP (72 mol g−1 ) and MAA − SY MIP (60 mol g−1 ) are determined and they are lower than that of 1MA-3Al-Br − SY MIP (Fig. 5). Therefore, the 1-MA-3AI-Br − SY MIP is selected for the following experiments. 3.4. Voltammetric behavior of SY Fig. 6 shows the cyclic voltammograms of SY at different modified electrodes. At the rGO-IL/GCE, a pair of redox peaks at 0.66 V and 0.62 V are observed for SY, meaning that the electrochemical
Scheme 1. Schematic illustration of the preparation of 1-MA-3AI-Br − SY MIP.
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Fig. 5. Saturation rebinding curves of SY on 1-MA-3AI-Br − SY MIP, 4-VP − SY MIP, MAA − SY MIP and 1-MA-3AI-Br NIP.
reason is that the MIP can provide many specific sites for SY. SY produces small peaks at the MIP/GCE, which can be ascribed to the poor conductivity and small effective surface area of MIP/GCE. The small background signal can support this point to some extent. 3.5. Optimization of parameters affecting the determination of SY 3.5.1. The amount of rGO-IL and MIP The effect of the amount of rGO-IL was studied (Fig. 7A). Results showed that the peak current of SY increased with increasing the volume of rGO-IL suspension up to 10.0 L, then it decreased slowly. This is related to the change of electrode surface area and electron transfer resistance. When the rGO-IL amount was too much, the electrode surface area kept almost unchanged, but the resistance increased. Therefore, 10.0 L of rGO-IL suspension was adopted for further study. The amount of MIP was also varied to examine its influence on the peak current of SY. It was found when the MIP amount was less (e.g. 2.0 or 4.0 L, 2 mg mL−1 ) the peak current was bigger, but the selectivity was poor; when the amount of MIP was too much (e.g. 8.0 L) the MIP particles easily peeled off the electrode surface. It was found that 6.0 L MIP suspension was optimal. 3.5.2. Solution pH Next, the influence of pH on the peak current of SY was investigated (Fig. 7B). The peak current increased with increasing pH
Fig. 4. SEM images of MIP particles. (A) 1-MA-3AI-Br − SY MIP, (B) MAA − SY MIP, (C) 4-VP − SY MIP.
reaction of SY is quasi-reversible. As the oxidation peak is higher, it is selected for the determination of SY. To improve the selectivity of rGO-IL/GCE, MIP is introduced. At the resulting MIP − rGO-IL/GCE, the peak current of SY decreases slightly because the additional MIP film hinders the electron transfer to some extent. It should be pointed out that the peak current of SY at the MIP − rGO-IL/GCE is much bigger than at the MIP − rGO/GCE, meaning that the IL plays an important role. We think it is related to the conductivity and preconcentration of IL. Compared with the NIP − rGO-IL/GCE, the MIP − rGO-IL/GCE exhibits more sensitive response to SY, indicating that the MIP also plays an important role in sensing SY. The
Fig. 6. Cyclic voltammograms of rGO-IL/GCE (a), MIP − rGO-IL/GCE (b), MIP − rGO/GCE (c), NIP − rGO-IL/GCE (d) and MIP/GCE (e) in 0.1 M PBS (pH 7.5) containing 50 M SY. Scan rate: 50 mV s−1 ; accumulation time: 570 s (under open circuit).
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Fig. 8. (A) Differential pulse voltammograms of SY at MIP − rGO-IL/GCE. SY concentration: 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.40, 0.60, 0.80, 1.0, 1.2, 1.4, 1.6, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0 M (from a to t). (B) Calibration curves for SY at MIP − rGO-IL/GCE and NIP − rGO-IL/GCE. Other conditions as in Fig. 6.
3.5.3. Preconcentration time When the preconcentration time was changed from 210 s to 570 s, the peak current of a 0.50 M SY solution increased rapidly (Fig. 7C). However, when it exceeded 570 s, the peak current kept almost unchanged, indicating that the adsorption equilibrium of SY at the MIP − rGO-IL/GCE was achieved. For a 5.0 M SY solution, the peak current reached the maximum around 450 s (see the inset in Fig. 7C). Herein, 570 s was selected for SY determination.
Fig. 7. Optimization of different conditions affecting the determination of 0.50 M SY solutions. (A) Influence of the amount of rGO-IL used. (B) Influence of solution pH; inset: the plot of peak potential versus pH. (C) Influence of accumulation time on peak currents of 0.50 M SY and 5.0 M SY (inset). Other conditions as in Fig. 6.
up to 7.5, then gradually decreased. Thus, pH 7.5 phosphate buffer solution was selected as supporting electrolyte. At the same time, the peak potential (Ep ) decreased linearly with increasing pH (see the inset in Fig. 7B). The linear relationship could be expressed as follows: Ep (V) = - 0.0031 pH + 0.852, with a correlation coefficient of 0.9900 and a slope of 31.0 mV pH−1 . According to the Nernstian equation: E ∝ 59.16 m/n pH (m: number of proton transferred; n: number of electron transferred), we can see that the number of electron transferred is twice as the number of proton transferred in the electrochemical reaction.
3.6. Calibration curves Fig. 8 shows the DPVs of SY under the optimized experimental conditions. It can be seen that the peak current increases with increasing SY concentration, and they present good linear relationship in the ranges of 10 nM − 1.4 M and 1.4 M − 16 M. The regression equations are: Ip (A) = 15.8 c (M) + 0.74 (R2 = 0.996) and Ip (A) = 4.28 c (M) + 19.58 (R2 = 0.997), with sensitivities of 5.0 A/M mm2 and 1.4 A/M mm2 , respectively. When SY concentration is above 16 M, the calibration curve gradually deviates from the straight line, indicating that the saturated adsorption is gradually reached. The limit of detection is ca. 4.0 nM (S/N = 3). Compared with other sensors reported for SY [7,11,34,35], the MIP − rGO-IL/GCE offers a relatively wide linear range and a low detection limit. The NIP − rGO-IL/GCE shows a smaller linear range and lower sensitivity than the MIP − rGO-IL/GCE. This can be explained by the lack of specific binding sites on the NIP film.
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Table 1 Determination results of SY in samples by using a MIP − rGO-IL/GCE (n = 3). Samples
SY added (M)
SY expected (M)
SY found (M)
Recovery (%)
Fruit juice
0 0.50 2.00 5.00 0 0.50 2.00 5.00 0 0.50 2.00 5.00
0.50 2.00 5.00 7.07 8.57 11.57 8.37 9.87 12.87
0.48 2.07 5.03 6.57 7.06 8.70 11.66 7.87 8.38 9.76 12.93
96 103 101 98 107 102 102 95 101
Mirinda drink
Orange juice
Fig. 9. Influence of coexistent substance on the electrochemical response of MIP − rGO-IL/GCE to SY. Solution composition: (a) 5.0 M SY + 0.1 M PBS (pH 7.5), (b) a + 50.0 M Brilliant blue G, (c) a + 50.0 M Amaranth, (d) a + 50.0 M Tratarzine. (e) a +50.0 M sodium salicylate. Other conditions as in Fig. 6.
3.7. Selectivity, reproducibility and stability of MIP − rGO-IL/GCE The selectivity of the MIP − rGO-IL/GCE was evaluated by testing the electrochemical response of 5.0 M SY in the presence of some analogues and potential interfering substances, such as Brilliant blue G, Amaranth, Tratarzine and sodium salicylate. As shown in Fig. 9, when their concentrations were 10-fold of that of SY the peak current of SY changed by 2% − 10%. When SY concentration was 0.50 M similar result was obtained (Fig. S1). This indicated that the MIP − rGO-IL/GCE had good selectivity. However, Allura red showed serious influence on the determination of SY. When 1fold of Allura red was present, the peak current of SY increased by 24%, meaning that Allura red could be bound by the molecularly imprinted site to some extent and could produce redox peaks around the peak potential of SY. This indicated that their chemical structures were too similar to be well discriminated by the MIP. To evaluate the reproducibility five MIP − rGO-IL/GCEs were prepared by the same way and a 0.50 M SY solution was determined. As a result, the relative standard deviation (RSD) of the peak current was 7.2% (n = 5). The repeatability was investigated by monitoring a 0.50 M SY solution using one modified electrode, and the RSD of peak current was 4.5% (n = 5). After used successively for 30 assays the current response of the sensor still remained 94% of its initial value. The inter-day RSD was 5.3% (n = 5). After stored for 5 days in a refrigerator, the MIP − rGO-IL/GCE retained 93% of its initial current response for 0.50 M SY; after two week-storage it still retained 87% of its initial current response. These reflected the good reproducibility and stability of MIP − rGOIL/GCE. 3.8. Analytical application of MIP − rGO-IL/GCE In order to test its application feasibility, the MIP − rGO-IL/GCE was applied to the determination of SY in several soft drinks (i.e. fruit juice, Mirinda drink and orange juice from a local market). Prior to determination, the soft drinks were diluted from 2.0 mL to 10 mL with 0.1 M phosphate buffer solution (pH 7.5). Results showed that in the Mirinda drink SY concentration was ca. 32.5 M and in the orange juice it was ca. 39.5 M (RSD < 5%). But in the fruit juice sample SY was not detected. SY standard solutions were added in the sample solutions to estimate the recoveries and they were 95 − 107% for different samples (Table 1). This indicated that the proposed method was reliable.
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