Fabrication of a Food Nano-Platform Sensor for Determination of Vanillin in Food Samples

Herein, we describe the fabrication of NiO decorated single wall carbon nanotubes (NiO-SWCNTs) nanocomposites using the precipitation method. The synthesized NiO-SWCNTs nanocomposites were characterized by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Remarkably, NiO-SWCNTs and 1-butylpyridinium hexafluorophosphate modified carbon paste electrode (CPE/NiO-SWCNTs/BPrPF6) were employed for the electrochemical detection of vanillin. The vanillin sensor showed an ultra-high sensitivity of 0.3594 μA/μM and a low detection limit of 0.007 μM. In the final step, the NiO-SWCNTs/BPrPF6 was used as the suitable tool for food analysis.

Vanillin is a natural phenolic product with a great smell that is extensively used in food and pharmaceutical products. This phenolic product can be synthesized by chemical methods. The high level of vanillin in food or pharmaceutical products can cause an increased risk of allergic reactions and so the control of its level is very important in food and pharmaceutical samples [46].
In this research, a CPE/NiO-SWCNTs and 1-butylpyridinium hexafluorophosphate modified carbon paste electrode (CPE/NiO-SWCNTs/BPrPF 6 ) is employed for the electrochemical detection of vanillin in food samples. The analytical ability of CPE/NiO-SWCNTs/BPrPF6 to determine the quantity of vanillin is compared to that of recently developed technologies which use electrochemical sensors (see Table 1). In addition, the proposed sensor showed other advantages compared to previous suggested sensors such as easy preparation, low cost, and high sensitivity.

Materials and Methods
Vanillin, mineral oil, nickel nitrate hexahydrate, graphite powder, sodium hydroxide, single wall carbon nanotubes-COOH, phosphoric acid, diethyl ether, and sulfuric acid were obtained from Sigma-Aldrich. For experimental investigation, a stock standard solution of vanillin (10 mM) was prepared daily by dissolving 0.038 g vanillin in 25 mL water solution.
The electrochemical study was performed using the PGSTAT 302 N system. TEM (Philips CM30, 300 kV) and X-ray powder diffraction instruments were used for the investigation of NiO-SWCNTs structure and morphology.
The NiO-SWCNTs were synthesized according to our previous recommended procedure-the chemical precipitation method with SWCNTs-COOH, nickel nitrate hexahydrate, and sodium hydroxide as precursors [52].

Preparation of CPE/NiO-SWCNTs/BPrPF 6
CPE/NiO-SWCNTs/BPrPF 6 were prepared by mixing 0.95 g of graphite powder and 0.05 g of NiO-SWCNTs in the presence of an appropriate amount of mineral oil and 1-butylpyridinium hexafluorophosphate until a uniformly wetted paste was obtained. The paste was input into the end of a glass tube in the presence of copper wire as a conductive binder.

Preparation of Real Sample
Coffee, milk, biscuit, and chocolate, were purchased and used for checking the ability of NiO-SWCNTs/BPrPF 6 to perform vanillin analysis in real samples. Ten real samples were obtained from the local market and were ground using a mortar and pestle. Half a gram of powder or 0.5 mL coffee was transferred in 5 mL ethanol solution and then sonicated for 1.0 h. The obtained samples, including the vanillin extract, were centrifuged (3000× g rpm) for 50 min and directly used for determination of vanillin by standard addition method.

NiO-SWCNTs Morphological and Structure Investigation
The XRD pattern of NiO-SWCNTs are presented in Figure 1 and the results confirmed the FCC structure for the NiO nanoparticle with a spherical shape and also the presence of a layer with miller index (002) at 2 •~2 6 • confirmed the presence of single wall carbon nanotubes. The TEM image of NiO-SWCNTs matches the XRD results. The NiO nanoparticle decorated the surface of single wall of carbon nanotubes (Figure 1 insert).

Electrochemical Behavior of Vanillin at the Surface of the Proposed Sensor
The electrochemical behavior of vanillin at different pH values was investigated by the linear sweep voltammetric method (Figure 2 insert). The oxidation potential shifted to a negative value with increasing pH and the plot of E vs. pH showed a linear relation with the equation of E = −0.0639 pH + 1.1064. As can be seen, the slope of E vs. pH is near to the Nernst equation for equal value of electron and proton (see the Scheme 1).

Electrochemical Behavior of Vanillin at the Surface of the Proposed Sensor
The electrochemical behavior of vanillin at different pH values was investigated by the linear sweep voltammetric method (Figure 2 insert). The oxidation potential shifted to a negative value with increasing pH and the plot of E vs. pH showed a linear relation with the equation of E = −0.0639 pH + 1.1064. As can be seen, the slope of E vs. pH is near to the Nernst equation for equal value of electron and proton (see the Scheme 1).

Electrochemical Behavior of Vanillin at the Surface of the Proposed Sensor
The electrochemical behavior of vanillin at different pH values was investigated by the linear sweep voltammetric method (Figure 2 insert). The oxidation potential shifted to a negative value with increasing pH and the plot of E vs. pH showed a linear relation with the equation of E = −0.0639 pH + 1.1064. As can be seen, the slope of E vs. pH is near to the Nernst equation for equal value of electron and proton (see the Scheme 1).    The maximum value of current for electro-oxidation of vanillin occurred at pH = 6.0 and this condition was selected for the next steps.
The linear sweep voltammograms of vanillin at the surface of the CPE/NiO-SWCNTs/BPrPF 6 (curve a), CPE/BPrPF 6 (curve b), NiO-SWCNTs (curve c), and CPE (curve d) was recorded (Figure 3). With moving of CPE to NiO-SWCNTs/BPrPF 6 , the oxidation signal of vanillin increased and the oxidation potential of vanillin decreased. This phenomenon can be attributed to the presence of NiO-SWCNTs and CPE/BPrPF 6 at a surface of the carbon paste electrode. The NiO-SWCNTs and CPE/BPrPF 6 improved the oxidation current of vanillin~11.9 times and decreased the oxidation overpotential of vanillin by approximately 50 mV.     Using the slopes from Figure 5B and Cottrell equation (Equation (1)), we determined the value of D ~ 3.57 × 10 −6 cm 2 s −1 . I = nFAD 1/2 C π 1/2 t 1/2 (1) The square wave voltammetric method was used for investigation of the linear dynamic range and limit of detection of vanillin at a surface of CPE/NiO-SWCNTs/BPrPF6 (Figure 6 inset). We detected a linear dynamic range 0.01-350 μM with a detection limit of 0.007 μM (LOD = 3SB/m) for vanillin at a surface of CPE/NiO-SWCNTs/BPrPF6 ( Figure 6).   Using the slopes from Figure 5B and Cottrell equation (Equation (1)), we determined the value of D ~ 3.57 × 10 −6 cm 2 s −1 .
The square wave voltammetric method was used for investigation of the linear dynamic range and limit of detection of vanillin at a surface of CPE/NiO-SWCNTs/BPrPF6 (Figure 6 inset). We detected a linear dynamic range 0.01-350 μM with a detection limit of 0.007 μM (LOD = 3SB/m) for vanillin at a surface of CPE/NiO-SWCNTs/BPrPF6 ( Figure 6). Using the slopes from Figure 5B and Cottrell equation (Equation (1)), we determined the value of D~3.57 × 10 −6 cm 2 s −1 . I = nFAD 1/2 C π 1/2 t 1/2 (1) The square wave voltammetric method was used for investigation of the linear dynamic range and limit of detection of vanillin at a surface of CPE/NiO-SWCNTs/BPrPF 6 ( Figure 6 inset). We detected a linear dynamic range 0.01-350 µM with a detection limit of 0.007 µM (LOD = 3S B /m) for vanillin at a surface of CPE/NiO-SWCNTs/BPrPF 6 ( Figure 6). The selectivity of CPE/NiO-SWCNTs/BPrPF6 for determination of vanillin was checked by an acceptable error of 5% in current (the obtained currents were compared before and after the addition of interference). The 1000-fold of K + , Na + , Cl − , glucose, and 300-fold of folic acid, vitamin B6, vitamin B1, and tartrazine had no influence on the determination of vanillin.
The ability of CPE/NiO-SWCNTs/BPrPF6 was checked for determination of vanillin in coffee milk, biscuit, and chocolate samples. The results are presented in Table 2. According to the results in Table 1, the CPE/NiO-SWCNTs/BPrPF6 was suggested as a powerful sensor for vanillin analysis in food samples.

Conclusions
This work described fabrication of a highly sensitive and new sensor for determination of vanillin in food samples. The presence of NiO-SWCNTs and BPrPF6 at a surface of a carbon paste electrode improved the ability of the sensor for analysis of vanillin at the nanomolar level. The NiO-SWCNTs and CPE/BPrPF6 improved the oxidation current of vanillin ~11.9 times and decreased the oxidation overpotential of vanillin by ~50 mV. The CPE/NiO-SWCNTs/BPrPF6 showed a powerful ability for determination of vanillin in food samples such as coffee milk, biscuit, and chocolate.  The selectivity of CPE/NiO-SWCNTs/BPrPF 6 for determination of vanillin was checked by an acceptable error of 5% in current (the obtained currents were compared before and after the addition of interference). The 1000-fold of K + , Na + , Cl − , glucose, and 300-fold of folic acid, vitamin B 6 , vitamin B 1 , and tartrazine had no influence on the determination of vanillin.
The ability of CPE/NiO-SWCNTs/BPrPF 6 was checked for determination of vanillin in coffee milk, biscuit, and chocolate samples. The results are presented in Table 2. According to the results in Table 1, the CPE/NiO-SWCNTs/BPrPF 6 was suggested as a powerful sensor for vanillin analysis in food samples.

Conclusions
This work described fabrication of a highly sensitive and new sensor for determination of vanillin in food samples. The presence of NiO-SWCNTs and BPrPF 6 at a surface of a carbon paste electrode improved the ability of the sensor for analysis of vanillin at the nanomolar level. The NiO-SWCNTs and CPE/BPrPF 6 improved the oxidation current of vanillin~11.9 times and decreased the oxidation overpotential of vanillin by~50 mV. The CPE/NiO-SWCNTs/BPrPF 6 showed a powerful ability for determination of vanillin in food samples such as coffee milk, biscuit, and chocolate.