sensors Article

A Zinc Oxide Nanoflower-Based Electrochemical Sensor for Trace Detection of Sunset Yellow Yu Ya 1,2 , Cuiwen Jiang 1,2 , Tao Li 1,2 , Jie Liao 1,2 , Yegeng Fan 1,2 , Yuning Wei 1,2 , Feiyan Yan 1,2 and Liping Xie 1,2, * 1

2

*

Institute for Agricultural Product Quality Safety and Testing Technology, Guangxi Academy of Agricultural Sciences, Nanning 530007, China; [email protected] (Y.Y.); [email protected] (C.J.); [email protected] (T.L.); [email protected] (J.L.); [email protected] (Y.F.); [email protected] (Y.W.); [email protected] (F.Y.) Quality Inspection and Test Center for Sugarcane and Its Product, China Ministry of Agriculture (Nanning), Nanning 530007, China Correspondence: [email protected]

Academic Editor: W. Rudolf Seitz Received: 20 January 2017; Accepted: 6 March 2017; Published: 8 March 2017

Abstract: Zinc oxide nanoflower (ZnONF) was synthesized by a simple process and was used to construct a highly sensitive electrochemical sensor for the detection of sunset yellow (SY). Due to the large surface area and high accumulation efficiency of ZnONF, the ZnONF-modified carbon paste electrode (ZnONF/CPE) showed a strong enhancement effect on the electrochemical oxidation of SY. The electrochemical behaviors of SY were investigated using voltammetry with the ZnONF-based sensor. The optimized parameters included the amount of ZnONF, the accumulation time, and the pH value. Under optimal conditions, the oxidation peak current was linearly proportional to SY concentration in the range of 0.50–10 µg/L and 10–70 µg/L, while the detection limit was 0.10 µg/L (signal-to-noise ratio = 3). The proposed method was used to determine the amount of SY in soft drinks with recoveries of 97.5%–103%, and the results were in good agreement with the results obtained by high-performance liquid chromatography. Keywords: zinc oxide nanoflower; sunset yellow; electrochemical sensor; trace detection

1. Introduction Sunset yellow (SY) (Scheme 1) is a synthetic dye that is commonly added into food products and soft drinks to make them more visually appealing [1]. SY is legitimately used as a food additive in China; however, if it is excessively consumed, it may cause asthma, immunosuppression, eczema, and anxiety migraines [2]. Due to the potential negative side-effects of excess SY, it is imperative to control the amount of SY in food. Consequently, a simple, rapid, and highly sensitive method for SY detection is critical to monitor SY levels. Many techniques have been utilized for the determination of SY, such as high performance liquid chromatography [3–5], UV-visible, fluorescence, and Raman spectrometry [6–9], and electrochemical techniques [10–17]. Recently, due to the low cost, operational simplicity, high sensitivity, and fast response, electrochemical techniques have gained attention and have exhibited promising applications in SY analysis. Based on the unique properties such as high electrical and thermal conductivity, extremely high surface area/volume ratio, high mechanical strength, and even excellent catalytic properties [18], nanomaterials have been employed as outstanding electrode modifiers for the electrochemical detection of SY. Some examples of nanomaterial-modified electrodes developed for the electrochemical determination of SY include porous carbon-modified glassy carbon electrode (GCE) [10], gold nanoparticle-modified carbon paste electrode (CPE) [11], gold nanoparticle/graphene-modified GCE [12], Au–Pd and reduced graphene Sensors 2017, 17, 545; doi:10.3390/s17030545

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oxide nanocomposite-modified GCE [13], and polypyrrole-modified oxidized single-walled carbon oxide nanocomposite-modified nanotubes-modified GCE [14]. GCE [13], and polypyrrole-modified oxidized single-walled carbon nanotubes-modified GCE [14].

Scheme 1. Electrochemical oxidation of sunset yellow (SY). Scheme 1. Electrochemical oxidation of sunset yellow (SY).

More recently, metal oxide nanoparticles have received much attention in the field of More recently, oxide nanoparticles have received much attention in the stability, field of electroanalysis due tometal the high surface area-to-volume ratios, biocompatibility, chemical electroanalysis to the high surfaceelectron area-to-volume biocompatibility, chemical stability, surface reactiondue activity, and adjustable transportratios, properties [19,20]. Zinc oxide is a common surface reaction activity, and adjustable electron transport properties [19,20]. Zinc oxide is a common metal oxide used in the electroanalysis of analytes. To date, many zinc oxide nanostructures have metal oxide used in the electroanalysis of including analytes. To date, many nanowires, zinc oxide nanorods, nanostructures been utilized as electrochemical sensors, nanoparticles, and have been utilized as electrochemical sensors, including nanoparticles, nanowires, nanorods, and nanosheets [21–24], but nanoflower-like zinc oxide is seldom used. In this paper, we describe a nanosheets [21–24], nanoflower-like zinc oxide is seldom In this paper, we using describe a simple, simple, rapid, and but sensitive electrochemical method for used. the detection of SY zinc oxide rapid, and sensitive method forelectrochemical the detection of SY usingofzinc oxidestudied. nanoflower nanoflower (ZnONF)electrochemical as sensing materials. The behaviors SY were The (ZnONF) as sensing materials. The electrochemical behaviors of SY were studied. The result result showed that the electrochemical response of SY was significantly improved at the ZnONFshowed the(ZnONF/CPE) electrochemical response of SY was significantly improved at the ZnONF-modified modifiedthat CPE when compared to the electrochemical response at the bare CPE. CPE (ZnONF/CPE) when the electrochemical response at drink the bare CPE. and Finally, Finally, the ZnONF/CPE wascompared applied totodetermine the amount of SY in soft samples, the the ZnONF/CPE was applied to determine the amount of SY in soft drink samples, and the results results were then validated by comparison with high-performance liquid chromatography. were then validated by comparison with high-performance liquid chromatography. 2. Experimental 2. Experimental 2.1. Reagents and Apparatus 2.1. Reagents and Apparatus 5.000 mg/mL standard stock solutions of SY were obtained from the National Institute of 5.000 mg/mL standard stock solutions of SY were obtained from the National Institute of Metrology (Beijing, China). Zinc acetate dihydrate [Zn(CH3COO)2·2H2O], graphite powder, paraffin Metrology (Beijing, China). Zinc acetate dihydrate [Zn(CH3 COO)2 ·2H2 O], graphite powder, paraffin oil of spectral grade, sodium hydroxide, potassium hydroxide, and phosphoric acid were purchased oil of spectral grade, sodium hydroxide, potassium hydroxide, and phosphoric acid were purchased from Sinopharm Chemical Reagent Company (Shanghai, China). All other reagents were of analytical from Sinopharm Chemical Reagent Company (Shanghai, China). All other reagents were of grade and were used as received. All aqueous solutions were prepared in deionized water. analytical grade and were used as received. All aqueous solutions were prepared in deionized water. Electrochemical experiments were performed on a CHI760E electrochemical workstation (Chenhua Electrochemical experiments were performed on a CHI760E electrochemical workstation (Chenhua Instrument Co. Ltd., Shanghai, China). A conventional three-electrode system was used for all Instrument Co. Ltd., Shanghai, China). A conventional three-electrode system was used for all electrochemical experiments. A bare or modified CPE of 3 mm diameter was the working electrode, electrochemical experiments. A bare or modified CPE of 3 mm diameter was the working electrode, a saturated calomel electrode was used as the reference electrode, and a platinum wire electrode was a saturated calomel electrode was used as the reference electrode, and a platinum wire electrode was used as the auxiliary electrode. Scanning electron microscopy (SEM) characterization was conducted used as the auxiliary electrode. Scanning electron microscopy (SEM) characterization was conducted with a S-3400N scanning electron microscope (Hitachi Co., Tokyo, Japan). X-ray diffraction (XRD) with a S-3400N scanning electron microscope (Hitachi Co., Tokyo, Japan). X-ray diffraction (XRD) studies were carried out using a X’Pert PRO diffractometer using Cu Kα radiation (PANalytical B.V., studies were carried out using a X’Pert PRO diffractometer using Cu Kα radiation (PANalytical B.V., Almelo, Netherlands). High-performance liquid chromatography determination of SY was carried Almelo, Netherlands). High-performance liquid chromatography determination of SY was carried out with a Waters 2695 liquid chromatograph coupled with an UV-vis detector. Chromatographic out with a Waters 2695 liquid chromatograph coupled with an UV-vis detector. Chromatographic conditions were developed employing previously described conditions [10]. conditions were developed employing previously described conditions [10]. 2.2. Preparation 2.2. Preparation of of ZnONF ZnONF In aa typical experiment, an an ethanol ethanol solution solutionof of0.1 0.1M M Zn(CH Zn(CH3COO) COO)2··2H 2O (100 mL) was prepared In typical experiment, 3 2 2H2 O (100 mL) was prepared using mild stirring. Then, 50 mL of 1.0 M sodium hydroxide was slowly added dropwise into the using mild stirring. Then, 50 mL of 1.0 M sodium hydroxide was slowly added dropwise into the above above solution the mixture wasfor stirred for a1 white h until a white suspension wasThe obtained. The solution and theand mixture was stirred 1 h until suspension was obtained. suspension suspension was then immersed in a preheated water bath for 6 h at 65 °C. After removing ◦ was then immersed in a preheated water bath for 6 h at 65 C. After removing the suspension from the the suspension from the water bath and cooling to room temperature, the resulting precipitates were centrifuged and washed with absolute ethanol and distilled water three times. Finally, the product was dried at 60 °C for 24 h to afford ZnONF.

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water bath and cooling to room temperature, the resulting precipitates were centrifuged and washed with absolute ethanol and distilled water three times. Finally, the product was dried at 60 ◦ C for 24 h to afford ZnONF. 2.3. Preparation of the Modified Electrode The conventional CPE was prepared by thoroughly mixing graphite powder and paraffin oil in a ratio of 70:30 until a uniform wetted paste was obtained. The resulting paste 3was Sensors 2017, 17,(w/w) 545 of 8 firmly packed into the cavity (3.0 mm diameter) of a polytetrafluoroethylene cylindrical tube fitted with a 2.3. Preparation the Modified Electrode copper piston, whichofserved as an inner electrical contact, and the surface was polished on a smooth The(10.0 conventional CPE was prepared by thoroughly graphite oil in paper. ZnONF mg) was added into 5.0 mL of ethanolmixing solution, and powder after 30and minparaffin of ultrasonication, ratio of 70:30with (w/w)a until a uniform wetted paste waswas obtained. The resulting wasoffirmly a stableasuspension concentration of 2.0 mg/mL obtained. Finally,paste 5.0 µL the ZnONF packed into the cavity (3.0 mm diameter) of a polytetrafluoroethylene cylindrical tube fitted with a suspension was coated on the CPE surface and dried in air to obtain ZnONF/CPE. copper piston, which served as an inner electrical contact, and the surface was polished on a smooth paper. ZnONF (10.0 mg) was added into 5.0 mL of ethanol solution, and after 30 min of 2.4. Analytical Procedure ultrasonication, a stable suspension with a concentration of 2.0 mg/mL was obtained. Finally, 5.0 μL of the otherwise ZnONF suspension coated on the CPE surface and driedwere in air conducted to obtain ZnONF/CPE. Unless stated,was electrochemical measurements in a conventional

electrochemical cell containing 10 mL of 1/15 mol/L phosphate buffer solution (PBS) with a pH 2.4. Analytical Procedure of 5.0, which contained a certain concentration of SY. After an accumulation time of 360 s under open Unless square otherwise stated, electrochemical were measurements conducted a conventional circuit conditions, wave voltammograms recordedwere in the potentialinrange from 0.40–1.0 V, electrochemical cell containing 10 mL of 1/15 mol/L phosphate buffer solution (PBS) with a pH of 5.0, and the oxidation peak current at 0.72 V was measured as the analytical signal for SY. Square wave which contained a certain concentration of SY. After an accumulation time of 360 s under open circuit voltammetry (SWV) conditions employed awere frequency an amplitude 25 mV, and a step conditions, square wave voltammograms recordedof in 15 theHz, potential range from of 0.40–1.0 V, and potential 4 mV. peak current at 0.72 V was measured as the analytical signal for SY. Square wave theofoxidation voltammetry (SWV) conditions employed a frequency of 15 Hz, an amplitude of 25 mV, and a step

3. Results and Discussion potential of 4 mV. 3. Results andof Discussion 3.1. Characterization ZnONF and Modified Electrode

Figure 1a,b shows the typical images of the as-synthesized ZnONF. As observed in the 3.1. Characterization of ZnONF andSEM Modified Electrode SEM images, the ZnONF nanostructures present a flower-like shape consisting of many whisker-like Figure 1a,b shows the typical SEM images of the as-synthesized ZnONF. As observed in the SEM nanopetals. Undoubtedly, such a structure is highly beneficial for consisting maintaining a high surface area and images, the ZnONF nanostructures present a flower-like shape of many whisker-like numerous adsorption sites on the foris use in the electrochemical detection of SY. The nanopetals. Undoubtedly, suchelectrode a structure highly beneficial for maintaining a high surface area surface and numerous adsorption sites on the electrode for use in the electrochemical detection of SY. The 1 c,d). morphologies of the bare CPE and the ZnONF/CPE were also characterized by SEM (Figure surface morphologies of the bare CPE and the ZnONF/CPE were also characterized by SEM (Figure As indicated by the SEM images, the surface of bare CPE is smooth and with many small gaps. 1 c,d). As indicated by the SEM images, the surface of bare CPE is smooth and with many small gaps. After ZnONF was added to the surface of CPE, the ZnONFs were distributed on the surface of the After ZnONF was added to the surface of CPE, the ZnONFs were distributed on the surface of the electrode with a well-defined flower-like shape,shape, indicating that that the ZnONFs were successfully modified electrode with a well-defined flower-like indicating the ZnONFs were successfully on the CPE surface. modified on the CPE surface.

Figure 1. SEM images of (a,b) zinc oxide nanoflower (ZnONF); (c) bare carbon paste electrode (CPE);

Figure 1. SEM images of (a,b) CPE zinc(ZnONF/CPE). oxide nanoflower (ZnONF); (c) bare carbon paste electrode (CPE); and (d) ZnONF-modified and (d) ZnONF-modified CPE (ZnONF/CPE). The crystalline structure of ZnONF was characterized by XRD, as shown in Figure 2. Nine peaks of the prepared sample were present corresponding to (100), (002), (101), (102), (110), (103), (200),

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The crystalline structure of ZnONF was characterized by XRD, as shown in Figure 2. Nine peaks Sensors 2017, 17, 545 4 of 8 of the prepared sample were present corresponding to (100), (002), (101), (102), (110), (103), (200), Sensors 2017, 17, 545 4 of 8 (112), and (201) crystal face, which agree well with the standard XRD pattern of ZnO (JCPDS Card (112), and (201) crystal face, which agree well with the standard XRD pattern of ZnO (JCPDS Card No. 36-1451) [25,26]. The strong and narrow diffraction peaks indicate that the prepared ZnONF is (112),36-1451) and (201) crystal face, which welldiffraction with the standard XRD pattern ZnO (JCPDS Card No. [25,26]. The strong andagree narrow peaks indicate that theofprepared ZnONF is well crystallized. No characteristic peaks from impurities were detected, indicating that the product is No. 36-1451) [25,26]. strong and narrow the prepared ZnONF is well crystallized. No The characteristic peaks fromdiffraction impuritiespeaks were indicate detected,that indicating that the product rather pure. well crystallized. No characteristic peaks from impurities were detected, indicating that the product is rather pure. is rather pure.

Figure 2. XRD pattern of ZnONF. Figure 2. XRD pattern of ZnONF. Figure 2. XRD pattern of ZnONF.

3.2. Cyclic Voltammetric Behavior of SY 3.2. Cyclic Voltammetric Voltammetric Behavior 3.2. Cyclic Behavior of of SY SY The electrochemical behaviors of SY at different electrodes were investigated using cyclic The behaviors of of SY at electrodes were investigated using cyclic The electrochemical electrochemical behaviors SY= 5.0). at different different electrodes investigated using cyclic voltammetry (CV) in PBS (1/15 mol/L, pH As shown in Figurewere 3, there were no redox peaks in voltammetry (CV) in PBS (1/15 mol/L, pH = 5.0). As shown in Figure 3, there were no redox voltammetry (CV) PBSwithout (1/15 mol/L, pH =a). 5.0). As shown Figureinto 3, there wereano redox peaks in the ZnONF/CPE ininPBS SY (curve When SY wasinadded the PBS, pair of reversible peaks in the ZnONF/CPE in PBS without SYWhen (curveSY a).was When SY into wasthe added into theofPBS, a pair the ZnONF/CPE PBS without (curve a). added PBS, a pair reversible redox peaks werein observed in theSY cyclic voltammograms obtained at the bare CPE with a poor current of reversible redox peaks were observed in the cyclic voltammograms obtained at the bare CPE redox peaks wereb). observed in thethe cyclic voltammograms at the bare CPE a poor current response (curve In contrast, redox peaks of SY obtained at the ZnONF/CPE werewith greatly enhanced with a poor current (curve redox b). In contrast, of SY at were the ZnONF/CPE were response b). response In contrast, peaks of the SY redox at thepeaks ZnONF/CPE enhanced (curve c), (curve indicating that ZnONFthe can provide superior electrocatalytic activity togreatly SY. This may be greatly enhanced (curve c), indicating that ZnONF can provide superior electrocatalytic activity to (curve c),toindicating that ZnONF can provide superior to SY. This may SY. be ascribed the good adsorption ability and large surface electrocatalytic area of ZnONF,activity which increase the loading This may be ascribed to the good adsorption ability and large surface area of ZnONF, which increase ascribed of to the large surface area of ZnONF, which increase the loading amount SY good onto adsorption the surfaceability of theand modified electrode. Therefore, a remarkable increase of the loading of SY onto the of surface of the modified electrode. Therefore, a remarkable increase amount of amount SY onto thethemodified electrode. Therefore, a remarkable increase of electrochemical signalthe wassurface observed at ZnONF/CPE. of electrochemical signal was observed ZnONF/CPE. electrochemical signal was observed at at thethe ZnONF/CPE.

Figure 3. Cyclic voltammograms of (a) the ZnONF/CPE in pH 5.0 phosphate buffer solution (PBS) without3.SY; (b) the 40 μg/L SY in pH 5.0the PBSZnONF/CPE at the bare CPE; and the ZnONF/CPE. Figure Cyclic voltammograms of (a) in pH 5.0(c)phosphate buffer solution (PBS) Figure 3. Cyclic voltammograms of (a) the ZnONF/CPE in pH 5.0 phosphate buffer solution (PBS) without SY; (b) the 40 μg/L SY in pH 5.0 PBS at the bare CPE; and (c) the ZnONF/CPE. without SY; (b) the 40 µg/L SY in pH 5.0 PBS at the bare CPE; and (c) the ZnONF/CPE.

3.3. Influence of Scan Rate 3.3. Influence of Scan Rate The influence of scan rates on the oxidation current response of SY was studied by linear sweep The influence scanof rates on the oxidation responsethat of SY studied by linear sweep voltammetry in theof range 50–500 mV/s. Figure current 4 demonstrates thewas current responses increased voltammetry therate, range of 50–500 mV/s. Figure 4 demonstrates that current as responses linearly with in scan and consequently, the linear equation can bethe expressed follows:increased ip (μA) = linearly with scan rate, and +consequently, the (r linear equation be expressed ip (μA) = (0.01479 ± 0.00042) v (mV/s) (0.3501 ± 0.0099) = 0.999). Thesecan results show thatas thefollows: electrochemical (0.01479 0.00042) v (mV/s) + (0.3501is±an 0.0099) (r = 0.999). These process. results show that the electrochemical oxidation± of SY at the ZnONF/CPE adsorption-controlled oxidation of SY at the ZnONF/CPE is an adsorption-controlled process.

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3.3. Influence of Scan Rate The influence of scan rates on the oxidation current response of SY was studied by linear sweep voltammetry in the range of 50–500 mV/s. Figure 4 demonstrates that the current responses increased linearly with scan rate, and consequently, the linear equation can be expressed as follows: ip (µA) = (0.01479 ± 0.00042) v (mV/s) + (0.3501 ± 0.0099) (r = 0.999). These results show that the electrochemical Sensors 2017, 17, 545oxidation of SY at the ZnONF/CPE is an adsorption-controlled process. 5 of 8 Figure 4. (a) Linear sweep voltammograms of 40 μg/L SY at the ZnONF/CPE in pH 5.0 PBS with different scan rates (from 1 to 9: 50, 100, 150, 200, 250, 350, 400, 450, and 500 mV/s); (b) Linear relationship between the current response and the scan rate.

3.4. Optimization of Detection Variables The effect of the concentration of ZnONF on current response of SY was investigated, and the results are shown in Figure 5a. As the concentration of ZnONF increased from 0.50 to 2.0 mg/mL, the current response of SY increased greatly. When the amount of ZnONF was increased, increasing amounts of SY were adsorbed on the surface of ZnONF/CPE, which resulted in a remarkable current response enhancement. As the current response of SY changed slightly when the concentration of ZnONF was4.greater than 2.0 mg/mL, the optimal ZnONF was to be 2.0 Figure (a) Linear sweep voltammograms of 40concentration μg/L SY at thefor ZnONF/CPE in determined pH 5.0 PBS with Figure 4. (a) Linear sweep voltammograms of 40 µg/L SY at the ZnONF/CPE in pH 5.0 PBS with mg/mL. different scan scan rates rates (from (from 11 to to 9:9: 50, 50, 100, 100, 150, 150, 200, 200, 250, 250, 350, 350, 400, 400, 450, 450, and and 500 500 mV/s); mV/s); (b) (b) Linear Linear different Figure 5b displays the current effect of pH onand the the current response of SY. When the pH was increased relationship between response scan rate. relationship between the current response and the scan rate. from 2.0 to 5.0, the current response likewise increased, and a plateau appeared between pH 5.0 to 3.4. Optimization ofgreater Detection Variables 6.0. For pH valuesof than 6.0, the current response decreased as pH further increased. Therefore, 3.4. Optimization Detection Variables 1/15 mol/L PBS with pH of 5.0 was used as the electrolyte in this work. The effect effect of of the the concentration concentration of of ZnONF ZnONF on on current current response response of of SY SY was was investigated, investigated, and and the the The The accumulation efficiency of ZnONF/CPE to SY was studied under open-circuit conditions. results are in 5a. As concentration ofof ZnONF increased from 0.50 to 2.02.0 mg/mL, the results are shown shown in Figure Figure Asthe the concentration ZnONF increased from 0.50 mg/mL, As Figure 5c illustrates, the5a. current response of SY was dramatically improved by to increasing the current response of SY increased greatly. When the amount of ZnONF was increased, increasing the current response of SY 0increased Whenthat the accumulation amount of ZnONF was increased, increasing accumulation time from to 360 s,greatly. indicating is effective in improving the amounts of of SY SY were were adsorbed adsorbed on on the the surface surface of of ZnONF/CPE, ZnONF/CPE, which resulted in current amounts which resulted in aa remarkable remarkable detection sensitivity. When the accumulation time was further increased beyond 360 s, the current current response enhancement. enhancement. As current response of SY changed changed slightly slightly when when the the concentration of of response As the the This current of saturation SY response was almost constant. mayresponse be due to of the amount of SY concentration adsorbed on the ZnONF was greater than 2.0 mg/mL, the optimal concentration for ZnONF was determined to be 2.0 ZnONF greater than 2.0 mg/mL, optimal concentration for ZnONF was determined be surface was of ZnONF/CPE. Thus, 360 sthe was selected as the optimal accumulation time fortothe mg/mL. 2.0 mg/mL. of SY. determination Figure 5b displays the effect of pH on the current response of SY. When the pH was increased from 2.0 to 5.0, the current response likewise increased, and a plateau appeared between pH 5.0 to 6.0. For pH values greater than 6.0, the current response decreased as pH further increased. Therefore, 1/15 mol/L PBS with pH of 5.0 was used as the electrolyte in this work. The accumulation efficiency of ZnONF/CPE to SY was studied under open-circuit conditions. As Figure 5c illustrates, the current response of SY was dramatically improved by increasing the accumulation time from 0 to 360 s, indicating that accumulation is effective in improving the detection sensitivity. When the accumulation time was further increased beyond 360 s, the current response was almost constant. This may be due to saturation of the amount of SY adsorbed on the surface of ZnONF/CPE. Thus, 360 s was selected as the optimal accumulation time for the determination of SY.

Figure 5. The effect of the (a) concentration of ZnONF; (b) pH; and (c) accumulation time on the Figure 5. The effect of the (a) concentration of ZnONF; (b) pH; and (c) accumulation time on the current current response of 40 μg/L SY. response of 40 µg/L SY.

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Figure 5b displays the effect of pH on the current response of SY. When the pH was increased from 2.0 to 5.0, the current response likewise increased, and a plateau appeared between pH 5.0 to 6.0. For pH values greater than 6.0, the current response decreased as pH further increased. Therefore, 1/15 mol/L PBS with pH of 5.0 was used as the electrolyte in this work. The accumulation efficiency of ZnONF/CPE to SY was studied under open-circuit conditions. As Figure 5c illustrates, the current response of SY was dramatically improved by increasing the accumulation time from 0 to 360 s, indicating that accumulation is effective in improving the detection sensitivity. When the accumulation time was further increased beyond 360 s, the current response was almost constant. This may be due to saturation of the amount of SY adsorbed on the surface of ZnONF/CPE. Thus, 360 s was selected as the optimal accumulation time for the determination of6 of SY.8 Sensors 2017, 17, 545 3.5. 3.5. Analytical Analytical Properties Properties The The linear linear range range and and detection detection limit limit for for SY SY were were examined examined using using SWV SWV under under the the optimized optimized conditions. As can be seen from Figure 6, the current response of SY is linearly related to conditions. As can be seen from Figure 6, the current response of SY is linearly related to SY SY concentration a range of 0.50–10 and 10–70 The regression linear regression equation concentration overover a range of 0.50–10 μg/Lµg/L and 10–70 μg/L. µg/L. The linear equation can be can be expressed as follows: i (µA) = (0.2245 ± 0.0047) C (µg/L) + (0.3015 ± 0.0063) and± p expressed as follows: ip (μA) = (0.2245 ± 0.0047) C (μg/L) + (0.3015 ± 0.0063) and ip (μA) = (0.08952 i0.00188) (0.08952 0.00188) C (µg/L) + (1.925 ± 0.0404)coefficients with the correlation coefficients of 0.999 and p (µA) = C (μg/L) ± + (1.925 ± 0.0404) with the correlation of 0.999 and 0.995. The detection 0.995. Theestimated detection to limit to be 0.10 µg/L ratio =of3).ZnONF/CPE The comparison limit was be was 0.10 estimated μg/L (signal-to-noise ratio(signal-to-noise = 3). The comparison with of ZnONF/CPE with other typeselectrodes of material electrodesare forlisted SY determination listed in other types of material modified formodified SY determination in Table 1. As are indicated in Table in Table 1, the ZnONF/CPE exhibited highdetection sensitivity and low toward Table1.1,As theindicated ZnONF/CPE exhibited high sensitivity and low toward thedetection electrochemical the electrochemical determination of SY. determination of SY.

Figure 6. (a) Square wave voltammetry (SWV) curves of SY at the ZnONF/CPE in pH 5.0 PBS with Figure 6. (a) Square wave voltammetry (SWV) curves of SY at the ZnONF/CPE in pH 5.0 PBS with various concentrations. Curves 1 to 8 correspond to 0.50, 2.50, 5.0, 10, 20, 40, 60, and 70 μg/L of SY, various concentrations. Curves 1 to 8 correspond to 0.50, 2.50, 5.0, 10, 20, 40, 60, and 70 µg/L of SY, respectively; (b) The current response as a function of the SY concentration. respectively; (b) The current response as a function of the SY concentration. Table 1. Comparison of different modified electrodes for SY determination. GCE: glassy carbon Table 1. Comparison of different modified electrodes for SY determination. GCE: glassy carbon electrode. electrode. Electrode

Electrode

Porous carbon-modified GCE Gold nanoparticles-modified GCE Porous carbon-modified GCE Nanoparticles/graphene-modified GCE Gold nanoparticles-modified GCE Nanoparticles/graphene-modified GCE Au-Pd and reduced graphene oxide Nanocomposites-modified GCE Au-Pd and reduced graphene oxide Polypyrrole-modified oxidized single-walled Nanocomposites-modified GCE Carbon Nanotubes-modified GCE Polypyrrole-modified oxidized single-walled Poly (L-cysteine)-modified GCE Carbon Nanotubes-modified GCE Bismuth film-modified GCE Poly (L-cysteine)-modified GCE Functionalized montmorillonite-modified CPE Bismuth film-modified GCE ZnONF/CPE Functionalized montmorillonite-modified CPE ZnONF/CPE

Linearity Range (µg/L) Linearity Range

Detection LimitLimit (µg/L) Detection

Reference

Reference

2.5–500 (μg/L)

1.4 (μg/L)

23–723 2.5–500

0.90 1.4

[11] [10]

0.90 0.90

[12] [11]

0.90 0.68

[13]

0.9–49,372 23–723

0.9–49,372 310–150,048 310–150,048

0.68

[10]

[12]

[13]

2.3–452

0.32

[14]

3.6–317 2.3–452

1.8 0.32

[15] [14]

4.4–87

3.6–317 4.4–87 0.50–10 and 10–70 1.1–90 0.50–10 and 10–70 1.1–90

1.0

1.8 1.0 0.10 0.32 0.10 0.32

[16]

[15] [16] This work [17] This work [17]

The reproducibility of the ZnONF/CPE was examined using a PBS containing 20 μg/L SY, and the relative standard deviation (RSD) of the current response was determined to be 4.22% by using six independently-prepared ZnONF/CPE. The reproducibility of one ZnONF/CPE was estimated by recording the responses of 20 μg/L SY in 11 successive measurements, and the RSD was calculated to

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The reproducibility of the ZnONF/CPE was examined using a PBS containing 20 µg/L SY, and the relative standard deviation (RSD) of the current response was determined to be 4.22% by using six independently-prepared ZnONF/CPE. The reproducibility of one ZnONF/CPE was estimated by recording the responses of 20 µg/L SY in 11 successive measurements, and the RSD was calculated to be 5.31%. The sample-to-sample reproducibility was also examined, and the RSD was 2.57% for three time measurement. The stability of the ZnONF/CPE was investigated after long-term storage in a refrigerator at 4 ◦ C for one week, and 93.7% of the original oxidation peak current was retained. From these results, it is evident that the ZnONF/CPE exhibits good reproducibility and stability. The influence of various foreign species on the determination of 10 µg/L SY was investigated by SWV using the above-optimized conditions. For this work, the tolerance limit was defined as the molar ratio of foreign species/SY that caused the ±5.0% change in the current response of SY. The result showed that 500-fold of K+ , Na+ , Ca2+ , Mg2+ , Cu2+ , Zn2+ , Fe3+ , sucrose, glucose, citric acid, and vitamin C; 50-fold of Sudan red, ponceau 4R, allura red, and amaranth; and 20-fold of tartrazine did not interfere in the determination of SY. These results demonstrate that ZnONF/CPE has good selectivity for SY. The analytical applicability of the ZnONF/CPE for the determination of SY was examined using the SWV method under optimized experimental conditions. Ten microliters of the soft drink sample was added to 10 mL of PBS and used for the determination of SY. A standard addition method was adopted to estimate the accuracy of the proposed method. The results are given in Table 2. The recoveries of the standards added ranged from 97.5% to 103%. The concentration of SY was also detected using HPLC, and the results obtained by HPLC and the proposed method were in good agreement, indicating that the proposed method is reliable. Table 2. Determination and recovery of SY in soft drink sample. Original (mg/L)

Added (mg/L)

Found (mg/L)

Recovery (%)

By HPLC (mg/L)

Relative Error (%)

13.57

20.00 30.00 40.00

33.07 42.25 54.71

97.5 95.6 103

31.42 45.05 53.12

−4.99 6.63 −2.91

4. Conclusions In this work, we have designed an ultrasensitive electrochemical sensor for the detection of SY based on the ZnONF/CPE. The ZnONF with unique nano-structure exhibited a large surface area and outstanding binding capacity, which greatly improved the electrochemical oxidation signals of SY. The fabricated sensing system demonstrated advantages including high sensitivity, good selectivity, simple fabrication, and low cost, indicating that the ZnONF/CPE could be employed as a highly promising tool for electrochemical sensing. Acknowledgments: The authors gratefully acknowledge the financial support by the Guangxi Natural Science Foundation (No. 2015GXNSFBA139037) and the Guangxi Academy of Agricultural Sciences Research Fund (No. 2015YT94). Author Contributions: Yu Ya and Liping Xie designed the experiments, analyzed the data and wrote the paper; Cuiwen Jiang, Tao Li, Jie Liao, Yegeng Fan, Yuning Wei and Feiyan Yan helped the experiments. Conflicts of Interest: The authors declare no conflict of interest.

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A Zinc Oxide Nanoflower-Based Electrochemical Sensor for Trace Detection of Sunset Yellow.

Zinc oxide nanoflower (ZnONF) was synthesized by a simple process and was used to construct a highly sensitive electrochemical sensor for the detectio...
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