Screen-Printed Electrode Modified with 3-D Nanoporous Nickel for the Determination of Narirutin in Wastewater from Citrus Industry

Yellow Water (YW) is the term given to liquid residues produced from the washing of orange fruits, distilling activities of distilleries, and the production of essential oil from orange; the handling and disposal of these residues are a matter of great concern to orange industries worldwide. An average size orange plant produces about 100,000 liters of YW daily. This wastewater has a considerable amount of organic compounds with high polluting power. Citrus processing industries are commercially interested in recovering significant quantities of some usefully relevant compounds present in YW. Among the compounds that have high added value, citrus flavonoids show an important biological activities, highlighting the molecule narirutin - a flavanone glucoside, which has been shown to present interesting therapeutic properties against cancer, Alzheimer's disease, and more recently, effects against the coronavirus disease 2019 (COVID-19). So, this work reports for the first time the electrochemical determination of narirutin using three-dimensional nanostructured porous nickel with high surface area on screen-printed electrode (3DnpNi/SPE), since the literature reports only chromatographic methods for the detection and quantification of narirutin. The modified electrode was successfully developed by the dynamic hydrogen bubble template method. This method involves the evolution of hydrogen bubbles together with the metal deposition process. Thereby, these bubbles become a dynamic template for the electrochemical deposition. Compared to the non-modified SPE, the 3DnpNi/SPE presented better electrochemical performance in terms of the oxidation of narirutin. The 3DnpNi/SPE was characterized by scanning electron microscopy (SEM), proving the formation of nickel nanopores with homogeneous distribution on the electrode surface, with a diameter varying between 70 - 120 nm. The energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) showed the presence of nickel and nickel oxide in the nanostructure. The EIS technique was applied for the electrochemical characterization of the 3DnpNi/SPE. The electron-transfer resistance was analyzed using a solution of 5.0×10−3 mol L-1 [Fe(CN)6]3-/4- in 0.1 mol L-1 KCl. The results obtained were discussed in terms of Rct (charge-transfer resistance) from the equivalent circuits. It was can observe the presence of a semicircle for the unmodified SPE with Rct of 2.3 KΩ. When the SPE was modified with nanoporous nickel, the electrochemical response appeared to be close to a straight line, and the Rct value was notably lower, 39 Ω; this shows that the modification of the electrode facilitated the transfer of electrons between the redox probe and the electrode surface. The cyclic voltammetry (CV) results showed that the 3DnpNi/SPE exhibited a peak current 3.3 times greater than that of bare SPE. The oxidation process involved 1 electron and 1 proton in a single irreversible step, with the formation of phenoxyl radical, which was adsorbed on the electrode surface due to the irreversible reaction (dimerization), to form dimer products. The quantification of narirutin was conducted by differential pulse voltammetry (DPV), which showed a wide concentration range (1.0×10-7 – 1.0×10-5 mol L-1), with low detection limit (3.9×10-8 mol L-1), and excellent sensitivity (0.31 A L mol-1). The good electrochemical performance of the proposed electrode was attributed to the high density of the nanostructured porous nickel on the SPE. The inter-electrode repeatability (n=5) was investigated by CV using the concentration of 5.0×10-6 mol L-1 of narirutin in 0.1 mol L-1 citrate buffer solution (pH 4.0). The average oxidation current obtained was 2.2 µA, with RSD of 5.9 %. In addition, the 3DnpNi/SPE was stored for 10 days in contact with air at room temperature (25 °C). The current response relative to narirutin detection was found to be 94.2 % of its initial peak current value. These results show that the 3DnpNi/SPE has good repeatability and long-term stability. The standard addition method was adopted for determining the concentration of narirutin in the sample. The sample was spiked with known concentrations of standard narirutin in the range of 2.0×10-6 mol L-1 to 6.0×10-6 mol L-1. A linear relationship was observed between the current peak and the added concentration of narirutin. The regression equation was as follows: y = 0.56 x + 2.3×10-6 (R = 0.999). From the standard addition curve, and based on the extrapolation of the straight line and the sample dilution (20-fold), the concentrations of narirutin found in the yellow water sample was (8.3 ± 0.39)×10-5 mol L-1 (n=3). To determine the analytical accuracy of the proposed method, recovery experiments were performed in standard solutions. The recovery rates obtained ranged from 97.1% to 100.9%, with RSD lower than 7.0 %. The concentration of narirutin in the YW sample was also determined by HPLC and the concentration value obtained was (8.6 ± 0.25)×10-5 mol L-1. Thereby, the RSD obtained between the two techniques was 0.16 %. These results demonstrate a good agreement between the proposed method and the comparative method (HPLC); essentially, this points to the reliability of the method proposed in this work. Thus, the findings of the present study show that the proposed 3DnpNi/SPE-based electroanalytical method can certainly be used for the determination of narirutin in yellow water. Figure 1