Electrochemical sensing of caffeic acid antioxidant in wine samples using carbon paste electrode amplified with CdO/SWCNTs http://dx.doi.org/10.5599/jese.1701 1 J. Electrochem. Sci. Eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1701 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper Electrochemical sensing of caffeic acid antioxidant in wine samples using carbon paste electrode amplified with CdO/SWCNTs Zahra Arab1, Sara Jafarian1, Hassan Karimi-Maleh2,, Leila Roozbeh Nasiraie1 and Mohammad Ahmadi3 1Department of Food Science and Technology, Nour Branch, Islamic Azad University, Nour, Iran 2Department of Chemical Engineering, Quchan University of Technology, Quchan 9477177870, Iran 3Department of Food Hygiene, Ayatollah Amoli Branch, Islamic Azad University, Amol 4615143358, Iran Corresponding authors: sara_jafary2002@yahoo.com; ,h.karimi.maleh@gmail.com Tel.: 0098 9123110253 Received: February 11, 2023; Accepted: May 22, 2023; Published: June xx, 2023 Abstract An electrochemical sensor was introduced as an analytical tool for monitoring caffeic acid in food samples. This analytical tool was amplified by cadmium oxide decorated on single wall carbon nanotubes as a new catalyst and showed a powerful ability to sensing of caffeic acid in food products. The presence of cadmium oxide decorated on single wall carbon nanotubes catalyst improved the oxidation signal of caffeic acid about 2.4 times at optimum conditions. The pH investigation confirmed that the redox reaction of caffeic acid was pH dependent and showed maximum sensitivity at pH 7.0. The paste electrode amplified with cadmium oxide decorated on single wall carbon nanotubes was successfully monitored caffeic acid in the concentration range 0.02–200 µM with a detection limit of 9.0 nM, respectively. The standard addition strategy showed a recovery range of 97.96 – 102.59 % to the measurement of caffeic acid in fruit juice, white and red wine that was acceptable for the fabrication of a new analytical tool in food monitoring. Keywords Food sensor, nanocomposite, electroanalysis, modified electrodes, real sample analysis Introduction Antioxidants, as one of the most famous substances used in food, prevent the adverse effects of free radicals [1,2]. Antioxidants can be found as natural or artificial and are important in food safety [3,4]. Although the body directly confronts some free radicals, the wide range of radicals makes it important to get antioxidants through food [5]. Caffeic acid is one of the famous natural antioxidants found in various beverages and wine samples [6,7]. Caffeic acid can cause mild effects on insomnia http://dx.doi.org/10.5599/jese.1701 http://dx.doi.org/10.5599/jese.1701 http://www.jese-online.org/ mailto:sara_jafary2002@yahoo.com mailto:h.karimi.maleh@gmail.com J. Electrochem. Sci. Eng. 00(0) (2023) 000-000 SENSING OF CAFFEIC ACID ANTIOXIDANT IN WINE 2 and excessive consumption is not recommended [8]. Therefore, monitoring of caffeic acid in food products was investigated by some analytical methods such as HPLC [9], LC-MS/MS [6], TLC-densito- metry [10] and electrochemical sensors. Electrochemical sensors are a useful and powerful strategy for sensing food products and especially food antioxidants [11-14]. Easy modification to create highly selective and sensitive tools in food products analysis is the main advantage of electrochemical methods compared to other analytical strategies in sensing electrochemically active materials [15-20]. Different modifiers were suggested for the amplification of electrochemical sensors, such as 2D nanomaterials, nano- composites [21], ionic liquids, composites, conductive polymers, biological compounds, etc. [22-24]. Between them, nanomaterials showed more advantages due to easy diversity and high electrical conductivity [25,26]. Many research efforts focused on the modification of electrochemical sensors to trace-level analysis in complex matrixes [27-30]. Nanotechnology changed the approach to pure science and creating unique properties in new materials [31-37]. Nanomaterials showed unbelievable properties and a wide range of applications in different branches of science [38-42]. The application of nanomaterials in the fabrication of modified sensors is increasing significantly [43]. This issue relates to high electrical conductivity and easy modification of nanomaterials with different functionalized groups. In this study, a nanostructure-based sensor (cadmium oxide decorated on single wall carbon nanotubes (CdO/SWCNTs) modified paste electrode (PE) (CdO/SWCNTs/PE) in this case) was suggested as a simple and economical type of sensor for easy monitoring of caffeic acid in food products and results showed the high ability of this sensor. Experimental Materials Cadmium acetate, SWCNTs-COOH, and sodium hydroxide were purchased from Sigma-Aldrich Company and used to synthesize CdO/SWCNTs nanocomposite with the recommended procedure by Cheraghi and Taher [44]. Graphite powder (99.99 %) and paraffin oil were purchased from Merck Company and used to fabricate paste electrodes. Phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate anhydrous, and trisodium phosphate were purchased from Across company and used to prepare phosphate buffer solution (PBS). Instruments All electrochemical signals were recorded by an Ivium-Vertex machine connected to Ag/Ag/KClsat, Pt wire, and CdO/SWCNTs/PE as reference, counter, and working electrodes, respectively. A pH meter model 780 pH Meter, Metrohm was used to prepare PBS solution. Fabrication of CdO/SWCNTs/PE The working electrode (CdO/SWCNTs/PE in this case) was fabricated after optimization of CdO/SWCNTs compared to graphite powder in the presence of caffeic acid in pH 7.0 as the optimum condition. The maximum sensitivity was obtained in the presence of 9.0 wt.% CdO/SWCNTs nanocomposite at the surface of the paste electrode (Figure 1). Therefore, 910 mg + 90 mg CdO/SWCNTs was mixed in mortar and pestle and converted to paste using paraffin oil and hand mixing for 30 min. Zahra Arab et al. J. Electrochem. Sci. Eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1701 3 Mass of nanocatalyst in electrode matrix, mg Figure 1. The oxidation current of caffeic acid at surface of paste electrode modified with different values of CdO/SWCNTs in carbon paste matrix Real sample preparation 10 mL of white, fruit juice and red wines were selected and centrifuged at 4000 rpm for 15 min and, after filtration, diluted into 10 mL PBS (pH 7.0) and transferred to an electrochemical cell for the analysis of caffeic acid using the standard addition method. Results and discussion pH optimization Caffeic acid is a phenolic antioxidant and according to reported papers and Scheme 1 [45], its oxidation in signal changes with the changes in the pH of the solution, and optimization of this factor is so important in the first step. Scheme 1. Redox reaction of caffeic acid Therefore, linear sweep voltammograms of caffeic acid were recorded in the pH range of 5.0 – 9.0 and the results are shown in Figure 2 inset. Results showed that maximum oxidation current to http://dx.doi.org/10.5599/jese.1701 J. Electrochem. Sci. Eng. 00(0) (2023) 000-000 SENSING OF CAFFEIC ACID ANTIOXIDANT IN WINE 4 caffeic acid was observed at pH = 7.0 (Figure 2), and this condition was selected as the optimal condition for monitoring caffeic acid in food samples. Catalytic activity of senor The oxidation signal of 500 µM caffeic acid at the surface of PE (Figure 3, curve a) and CdO/SWCNTs/PE (Figure 3, curve b) was recorded and results showed oxidation currents of 7.97 and 19.2 µA at oxidation potentials of 563 and 460 mV, respectively. Figure 2. Current-pH curve to electrooxidation of 500 µM caffeic acid. Inset) Linear sweep voltammograms of 500 µM caffeic acid in the pH range 5.0 to 9.0. Figure 3. LS voltammograms 500 µM caffeic acid at surface of a) PE and b) CdO/SWCNTs/PE. Inset) Current density diagram relative to LS voltammogarms Zahra Arab et al. J. Electrochem. Sci. Eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1701 5 Comparing these values clearly showed that the presence of CdO/SWCNTs at the surface of PE plays a catalytic role in the redox behavior of caffeic acid. On the other hand, the active surface area of PE and CdO/SWCNTs/PE were determined to be about 0.09 and 0.17 cm2, respectively. The active surface was determined by the recording of cyclic voltammograms of [Fe(CN)6]3-/4- in the presence of 0.1 M KCl and Randles–Sevcik equation.. This confirms CdO/SWCNTs increase the active surface area of the electrode. The current densities relative to two electrodes was shown in Figure 3 inset. Scan rate investigation The role of scan rate on the potential and current of caffeic acid signal was investigated and results showed in Figure 4 inset. The results showed a positive shift in the oxidation potential of caffeic acid and also increase in the oxidation current at the surface of CdO/SWCNTs/PE at the scan rate range 5 – 80 mV s-1. The linear relationship between current of caffeic acid and 1/2 with an equation of I = 1.61971/2 +4.8775 (R2= 0.9944) confirms a diffusion-controlled process. Also, a positive shift in oxidation potential indicates a kinetic limitations of the redox reaction of caffeic acid at the surface of CdO/SWCNTs/PE. Analytical parameters The limit of detection and linear dynamic range are two main analytical factors in the design of new types of analytical methods. The linear dynamic range between 0.02 to 200 µM with equation I = 0.0481C + 0.7410 (R2 = 0.9911) was determined. A detection limit of 9.0 nM was determined to for this method of caffeic acid at the surface of CdO/SWCNTs/PE at optimum conditions using equation YLOD = 3σ/m. Figure 4. I - 1/2 curve to electro-oxidation 450 µM caffeic acid at surface of CdO/SWCNTs/PE. Inset) LSV 450 µM caffeic acid at scan rates a) 5; b) 20; c) 40; d) 60 and e) 80 mV s-1 Selectivity of CdO/SWCNTs/PE to sensing of caffeic acid The selectivity of CdO/SWCNTs/PE as a new catalyst for sensing of 15.0 µM caffeic acid was investigated at pH 7.0 as the optimum condition. The results were reported with an acceptable error of 5 % in Table 1. Data were repeated five times and confirmed the high selectivity of CdO/SWCNTs/PE as a new sensor for sensing caffeic acid in an aqueous solution. http://dx.doi.org/10.5599/jese.1701 J. Electrochem. Sci. Eng. 00(0) (2023) 000-000 SENSING OF CAFFEIC ACID ANTIOXIDANT IN WINE 6 Table 1. Interference study to sensing 15.0 µM caffeic acid using CdO/SWCNTs/PE Species Tolerance limit (Interference mass, g / caffeic acid mass, g) Li+, K+, Br- and Na+ 1000 Sucrose, glucose, fructose and lactose 600 Vitamin B9 and glycine 400 Starch Saturated Determination of caffeic acid in real samples The fruit juice and wine samples were prepared according to the reported procedure in section 2.4 and transferred to an electrochemical cell for electrochemical monitoring by standard addition methods. The results were repeated three times and compared with one published sensor and data shown in Table 2. F-test and t-test were used to investigate accuracy data and results clearly confirmed CdO/SWCNTs/PE's high ability to sense caffeic acid in real samples. Table 2. Determination of caffeic acid using CdO/SWCNTs/PE (n=3) Sample Content of caffeic acid, M Recovery, % Ftab Fexp ttab texp Added Founded Founde by HPLC White wine --- 3.11±0.13 2.96±0.13 --- --- --- --- --- 10.00 13.45±0.34 13.67±0.55 102.59 19.0 6.7 3.8 1.5 Red wine --- 2.85±0.22 2.94±0.13 --- --- --- --- --- 5.00 7.69±0.45 8.05±0.49 97.96 19.0 7.3 3.8 1.8 Fruit juice ---