Electrochemical determination of vanillin in food samples by using pyrolyzed graphitic carbon nitride

Materials Chemistry and Physics 242 (2020) 122462

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Electrochemical determination of vanillin in food samples by using pyrolyzed graphitic carbon nitride Li Fu a, *, Kefeng Xie b, **, Dihua Wu a, Aiwu Wang c, Huaiwei Zhang a, Zhenguo Ji a a

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, PR China School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, 730070, PR China c Center for Advanced Material Diagnostic Technology, Shenzhen Technology University, Shenzhen, 518118, PR China b

H I G H L I G H T S

� A high sensitive electrochemical based vanillin detection method was proposed. � A simple pyrolysis method has been proposed for graphitic carbon nitride preparation. � The electrochemical sensor has been successfully applied for vanillin detection in food sample. � Sensor should a linear vanillin detection from 20 nM to 10 μM and 15 μM–200 μM. A R T I C L E I N F O

A B S T R A C T

Keywords: Vanillin C3N4 Electroanalytical chemistry Sensor Pyrolysis

Rapid detection of vanillin in food samples is important in food safety. In this work, lamellar C3N4 sheets were prepared by simple pyrolysis of melamine. The as-prepared C3N4 sheets were used to fabricate a vanillin elec­ trochemical sensor. C3N4 was also successfully used to modify glassy carbon electrode and exhibited excellent sensing performance. Under the optimized condition, the prepared electrochemical sensor could linearly detect vanillin between 20 nM to 10 μM and 15 μM–200 μM with a low detection limit of 4 nM. In addition, the vanillin sensor was applied to detect vanillin in milk tea and biscuits.

1. Introduction Vanillin is the most important aroma compound in vanilla. The vanillin content in dried vanilla pod can reach 2%. In addition, vanillin is found in orchids of Paraguay and southern Brazil and red pine grown in China. Low concentrations of vanillin can enhance the aroma char­ acteristics of food, giving the food a mixture of butter and fruit aroma. Adding vanillin into aging technology can provide a special flavor for many wines. Vanillin is mainly used in condiments for food, especially sweets. Vanillin (75%) is used to make ice cream, chocolate, cakes, and wines. In addition, vanillin is used as an additive to conceal the un­ pleasant smell of other substances in medicines, feedstuffs, and de­ tergents. Vanillin can also be used as a general coloring agent for development of thin-layer chromatography plates to help monitor the components of mixtures in chemical reactions. However, vanillin can cause allergic reactions [1] and aggravate the condition of migraine sufferers. The United Nations Food and Agriculture Organization

stipulates that the daily intake of vanillin should be less than 10 mg/kg. Therefore, determination of vanillin in food is necessary [2,3]. To date, many technologies have been developed for determination of vanillin. For example, Waliszewski and co-workers demonstrated an advanced high-performance liquid chromatography method for vanillin detection [4]. Duran and co-workers reported an optical fluorescence sensor for vanillin detection based on CdSe/ZnS quantum dots modified with β-cyclodextrin [5]. Ni and co-workers reported a UV–vis spectros­ copy method for vanillin detection [6]. These analytical methods can accurately separate and determine vanillin content. However, the in­ struments needed are expensive, not miniaturized, and unsuitable for field testing. In recent years, the rapid development of electrochemical sensing has provided a new option for detection of vanillin [7–20]. For example, Li and co-workers synthesized an Ag–Pd bimetallic nanocomposite-decorated graphene for electrode surface modification [21]. The modified electrode was applied for electrochemical determi­ nation of vanillin. Wang et al. [22] adopted a molecularly imprinted

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Fu), [email protected] (K. Xie). https://doi.org/10.1016/j.matchemphys.2019.122462 Received 27 July 2018; Received in revised form 26 September 2019; Accepted 16 November 2019 Available online 19 November 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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Materials Chemistry and Physics 242 (2020) 122462

strategy for electrochemical sensor construction. The components of the sensor include graphene oxide, carboxylated multiwalled carbon nano­ tube, ionic liquid, and gold nanoparticles. These electrochemical methods are effective in detecting vanillin content and have been suc­ cessfully applied to various foods. However, a complex electrode modification process requires meticulous operation. At the same time, some nanomaterials, such as graphene and noble metal nanoparticles, are expensive, thereby increasing the price of sensors. Therefore, developing an electrochemical sensor with high sensitivity to detect vanillin by a simple electrode modification is necessary. C3N4 is considered to be one of the oldest synthetic compounds. Its history can be traced back to 1834 as first reported by Berzelius and Liebig. In 1922, Franklin [23] obtained an amorphous carbon nitride by pyrolysis of precursors, such as Hg(CN)2 and Hg(SCN)2. He also pro­ posed its possible structure. In 1937, Pauling and Sturdivant [24] first proposed that C3N4 is a multicluster compound with three mean three azines as the basic structural unit and proved by X-ray crystallography. C3N4 can be obtained by a variety of preparation methods through a variety of nitrogen-rich precursors, such as dicyandiamide, urea, mel­ amine, and thiourea. C3N4 has a very suitable band edge position to meet the thermodynamic requirements for the photolysis of aquatic hydrogen; thus, the recent research focused on its photocatalytic per­ formance [25–30]. Compared with the traditional TiO2 photocatalyst, C3N4 can effectively activate molecular oxygen and produce superoxide radicals for the photocatalytic transformation of organic functional groups and the photocatalytic degradation of organic pollutants. Very recently, the electrochemical behavior of C3N4 has been provided considerable attention [31–35]. C3N4 has been commonly used as sub­ strate for electrocatalyst loading [36]. Limited works focused on the direct use of C3N4 for sensing [37–39]. In this work, we used an extremely simple pyrolysis method to convert melamine into C3N4. The synthesized C3N4 was directly used to modify glassy carbon electrode (GCE) and detect vanillin. The C3N4-modified GCE greatly enhanced the electrochemical oxidation current of vanillin. The assembled electro­ chemical sensor detected vanillin in a wide range of concentration and was successfully applied to detect vanillin in biscuits and milk tea.

the sample was recorded using a Nicolet iS5 FTIR spectrometer. All electrochemical experiments were conducted using a CHI 760E elec­ trochemical workstation with a conventional three-electrode system comprising platinum wire as the auxiliary electrode, 3 M Ag/AgCl as the reference electrode, and GCE as the working electrode. 3. Results and discussion Fig. 1 shows the morphology of the synthesized C3N4. Notably, the C3N4 synthesized by pyrolysis has a layered morphology. A gap is observed between the lamellae, facilitating the diffusion of small mol­ ecules and is conducive to electrochemical analysis and detection. Fig. 2A shows the FTIR diagram of C3N4. As shown in the spectrum, a series of peaks can be found between 1200 and 1600 cm 1, corre­ – N vibration modes [43]. A heptazine sponding to C–N and C– arrangement peak can be found in 811 cm 1, which is a characteristic peak belonging to C3N4. Besides, the surface NH2 and NH groups related to breathing modes can be found between 3000 and 3400 cm 1 [40,44]. Fig. 2B shows the XRD pattern of the synthesized C3N4. Remarkably, a characteristic peak was located at 28.1� , which corresponds to the stacking of the conjugated aromatic structure. A wide peak between 10� and 20� was observed, suggesting the interlayer structural packing be­ tween C3N4 sheets [45]. The XRD pattern matches the previous report [46]. Therefore, both XRD and FTIR results confirmed the successful formation of C3N4 from melamine. [Fe(CN)6]3 /4 was used as an electrochemical probe to characterize the electrode performance after C3N4-modified modification. As shown in Fig. 3A, cyclic voltammetry results showed that the peak–peak sep­ aration of bare GCE and C3N4/GCE were 97.76 mV and 109.87 mV, respectively, suggesting the C3N4 modification could slightly affect the electrode conductivity. Moreover, the C3N4/GCE showed a higher cur­ rent response than that of the bare GCE, indicating that the C3N4/GCE could provide more electroactive surface areas (EASAs) than that of the bare GCE. Based on these voltammograms, the EASAs of the bare GCE and C3N4/GCE can be calculated according to the Randles–Sevcik equation [47,48]: Ip ¼ ð2:69 � 105 Þn3=2 AD1=2 C* v1=2

2. Experimental

The EASA values of the bare GCE and C3N4/GCE were calculated to be 0.0577 and 0.0851, respectively. Therefore, lamellar C3N4 can pro­ vide a large effective area for electrochemical reaction. Electrochemical impedance spectroscopy is a very effective technique for characterizing the internal properties of electrodes. Fig. 3B shows the plots of the bare GCE and C3N4/GCE. The plots of bare GCE and C3N4/GCE both contain a semicircle and a straight line, representing the electron transfer re­ striction and diffusion limit, respectively. Notably, the bare GCE has a slightly larger semicircle than C3N4/GCE, suggesting that the charge transfer resistance between C3N4 and electrochemical probe is reduced. Therefore, C3N4 modification on the surface of GCE can promote the electron transfer rate and increase the sensitivity of the electrode [49]. The electrochemical behavior of vanillin at bare GCE and C3N4/GCE

All chemicals were of analytical grade. C3N4 was synthesized using a modified pyrolysis method using melamine [40–42]. Typically, 10 g of melamine was added into a quartz tube with a lid into a furnace for 5 h at 600 � C at temperature rising speed of 5 � C per min. Solid C3N4 can be obtained after cooling down to room temperature. For GCE surface modification, 1 mg/mL C3N4 ethanol dispersion containing 1% Nafion was first prepared. Then, a certain amount of the dispersion was drop on the GCE surface and dried naturally. The modified GCE was denoted as C3N4/GCE. Scanning electron microscopy (SEM, LEO 1530VP) was used for morphology characterization. X-ray diffraction (XRD) (D8-Advanced, Bruker) was used for characterizing crystal information of the synthe­ sized C3N4. Fourier-transform infrared spectroscopy (FTIR) spectrum of

Fig. 1. SEM images of C3N4 at (A) low and (B) resolution. 2

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Materials Chemistry and Physics 242 (2020) 122462

Fig. 2. (A) FTIR spectrum and (B) XRD pattern of C3N4.

Fig. 3. (A) Cyclic voltammograms and (B) representative impedance spectra of bare GCE and C3N4/GCE in 5 mM [Fe(CN)6]3

is shown in Fig. 4. As shown in the figure, a well-defined oxidation peak can be noticed in bare GCE at 0.68 V at a current of 3.34 μA. By contrast, C3N4/GCE shows an oxidation peak with a slight lower potential with much district response. Specifically, the oxidation current of 10 μM vanillin at C3N4/GCE can reach up to 31.2 μA. The low background current when the absence of vanillin and a significant improvement of oxidation signal with a lower overpotential suggested the C3N4 exhibi­ ted an electrocatalytic response towards the vanillin. Fig. S1 shows the mechanism of vanillin oxidation based on C3N4. To achieve high sensitive and selective detection of vanillin, some main parameters in electrochemical detection were optimized. The pH

/4

.

value of electrolyte can affect the current response of electrochemical oxidation of vanillin. As shown in Fig. 5A, the oxidation current of vanillin first rises with the increase of pH and reaches the maximum at pH 7. Subsequently, with the increase of pH, the oxidation current began to decrease. Therefore, pH 7 was selected as the best pH state for vanillin detection. The amount of electrode modifier is also an important parameter. Insufficient amount of modifier cannot result in a sensitive electrochemical sensing platform, whereas too much modifier will make the modified layer too thick to obstruct the conduction of the electrons from the outside to the inside. Fig. 5B shows the effect of different amounts of modifiers on the electrochemical oxidation of vanillin. As shown in the figure, the electrochemical oxidation current of vanillin increases rapidly with the increase of amount of the C3N4 from 1 μL to 4 μL. However, several C3N4 modifications cause a decrease in current. Therefore, 4 μL of C3N4 dispersion is selected as the optimal quantity of the modifier. Finally, adsorption time has a great influence on the C3N4 toward vanillin detection. As shown in Fig. 5C, the electrochemical oxidation current of vanillin increases with the increase of adsorption time. However, when the adsorption time exceeds 80 s, the rise of the current tends to be gentle. Considering the convenience of detection, we choose 80 s as the adsorption time before detection. Differential pulse voltammetry (DPV) has been used to study the linear detection range and feasibility of C3N4/GCE due to its high sensitivity. Fig. 6A shows the DPV curves of the C3N4/GCE toward vanillin from 20 nM to 300 μM in 0.1 M PBS. The anodic peak responses of the vanillin were found to be proportional to its concentration over two linear ranges. As shown in Fig. 6B, the C3N4/GCE shows linear detection range at 20 nM to 10 μM and 15 μM–200 μM with linear regression equations of I ¼ 0.10492LnC(μM) þ 0.60461 and I ¼ 0.896 LnC (μM) þ 1.214, respectively. The limit of detection can be found to be 4 nM based on the signal to noise of 3. This analytical performance has been compared with previous reports. As summarized in Table 1, the

Fig. 4. (A) Cyclic voltammograms of bare GCE and C3N4/GCE toward 10 μM vanillin in 0.1 M phosphate-buffered saline (PBS) (pH 7). 3

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Materials Chemistry and Physics 242 (2020) 122462

Fig. 5. Effect of (A) pH, (B) amount of C3N4, and (C) adsorption time on the C3N4/GCE toward 10 μM vanillin in 0.1 M PBS.

Fig. 6. (A) Differential pulse voltammograms of C3N4/GCE toward vanillin from 20 nM to 300 μM in 0.1 M PBS (pH 7). (B, C) Plots of vanillin concentrations against peak currents.

proposed C3N4/GCE showed a superior detection performance than many other electrochemical sensors. Considering the quick and simple modification, the proposed C3N4/GCE can be considered as an excellent candidate for vanillin detection. Reproducibility, repeatability, and stability are important factors in electrochemical sensors [55,56]. Five independent C3N4/GCEs were used to measure the reproducibility of electrodes. The results showed that five independent C3N4/GCEs had very similar detection results for 5 μM vanillin. A relative standard deviation (RSD) of 3.4% of was observed. The repeatability of C3N4/GCE was studied by an electrode for five successive detections. A current decline of 7.5% was observed in the second test, and only 47% of the current remained in the fifth test. As

noticed in the cyclic voltammogram, vanillin electrochemical oxidation is an irreversible process and its oxidation products are likely to adsorb on the electrode surface, thereby reducing the electrochemical active area. Therefore, the C3N4/GCE is not a repeatable electrochemical sensor. The long-term stability of the C3N4/GCE was tested by storing the electrode at room temperature for one month. No clear electro­ chemical sensing performance changes can be noticed because the C3N4 is very stable. The selectivity of C3N4/GCE has also been verified. Common inor­ ganic ions, such as Cu2þ, Fe3þ, Al3þ, Zn2þ, C2O24 , NO3 , and SO24 , of 50folds do not affect the vanillin detection. Some organic interference species were also tested including vanillic acid, vanillic alcohol, and phydroxybenzoic acid. The results indicated that the 5-folds of vanillic acid, vanillic alcohol, and p-hydroxybenzoic acid also showed no effect on vanillin detection. We further conducted the selectivity test of the proposed electrochemical sensor towards heliotropin, cinnamaldehyde and lilial. These common aldehyde-based food additives showed no oxidation response in the scan range, suggesting the vanillin can be determined in food samples without interference. For practical application, the feasibility of the vanillin detection in real samples was investigated. Milk tea and biscuit extract were used as real samples. Standard addition method has been utilized. As shown in Table 2, the results indicated the milk tea contains no vanillin content, whereas the biscuit showed 12.4 μg/g. For comparison purpose, the vanillin content in both samples were also determined by using UV spectrophotometer. No valine was found in milk tea, whereas the biscuit showed 11.7 μg/g. A well recovery performance was noticed, suggesting that the proposed C3N4/GCE can be applied for vanillin test in real sample.

Table 1 Comparison of previously reported vanillin electrochemical sensor with this work. Electrode

Detection method

LR

LOD

Ref

Graphene nanoflakes/ GCE CoS nanorods/GCE G-QD@Nafion/AuNPSPCE Lysine/CPE Nitrogen-doped graphene Low-defect graphene electrodes AgNP/graphene/GCE

DPV

0.01 to 53

[50]

DPV DPV

0.5–56 μM 0.66–33 μM

0.0124 μM 0.07 μM 0.32 μM

[51] [52]

DPV SWV

10–100 μM 0.01–10 μM

2.88 μM 3.3 nM

[53] [54]

DPV

0.2–40 μM

10 nM

[48]

SWV

2–100 μM

[8]

AuNP-PAH/GCE

SWV

0.9–15 μM

C3N4/GCE

DPV

0.02–10 μM/ 15–200 μM

0.332 μM 0.055 μM 4 nM

[9]

4. Conclusion

This work

Lamellar C3N4 sheets were obtained from a simple pyrolysis of 4

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Materials Chemistry and Physics 242 (2020) 122462

Table 2 Electrochemical determination of vanillin in biscuit and milk tea. Milk tea

Added (μM)

Found (μM)

RSD (%)

Recovery (%)

1 2 3

0 1.00 3.00

0 1.07 2.86

0 4.21 2.20

– 107.00 95.30

0 1.00 3.00

0.45 1.41 3.43

1.47 3.33 2.68

– 97.24 99.42

[14]

[15]

Biscuit 1 2 3

[16] [17]

melamine. The C3N4 nanostructure can be used to improve the effective electrochemical area of GCE, thereby enhancing the electrochemical performance of the electrode. We found that C3N4 could adsorb vanillin effectively. Thus, the electrochemical oxidation of vanillin showed a great improvement in C3N4-modified GCE than in bare GCE. Under optimized conditions, C3N4/GCE can detect vanillin in 20 nM to 10 μM and 15 μM–200 μM linearly with a detection limit of 4 nM.

[18]

[19]

[20]

Acknowledgments

[21]

This research was supported by the Zhejiang Province Natural Sci­ ence Foundation of China (LQ18E010001), China Postdoctoral Science Foundation (2018M640523).

[22]

Appendix A. Supplementary data [23] [24]

Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122462.

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