Determine methylene blue based on carbon paste electrode modified with nanoparticles of nickel oxide-nitrogen carbon quantum dots and carbon structures by cyclic voltammetry

electrode is used as a working electrode. The analytical method used is cyclic voltammetry (CV), The oxidation-reduction curve of methylene blue was shown using this electrode. It is a quasi-reversible curve, and it works at (pH =1) and the best acid used is HCl a concentration of (0.1M). It was also found that the linear range is within the range of (7.99-31.98 mg L -1 ), where the oxidation equation for it can be described by the equation I OX =1.508C MB +229.5 and while the redaction equation is I Red =-2.236C MB -232.5 where is the correlation coefficient (R²=0.9823) and (R²=0.9722) for both oxidation and reduction respectively. The standard deviation (SD) and relative standard deviation (RSD%) were obtained at (0.361 mg L -1 and 0.294 mg L -1 ) and (4.52% and 3.68%) for both oxidation and reduction respectively. Retrospective, the limit of quantitative (LOQ) and limit of detection (LOD) were achieved at (99.65%; 99.70%), (0.24 mg L -1 ; 0.13 mg L -1 ), and (0.071 mg L -1 ; 0.039 mg L -1 ) for both oxidation and reduction respectively. Methylene blue was analyzed by UV-Vis spectrophotometry at (663 nm).

All Equations are shown in below Table 1.

Determine methylene blue by cyclic voltammetry and spectrophotometry
Table1.The equations and parameters of kinetic and mechanistic information of Cyclic voltammetry(CV).v: Scan rate R: Gas constant

Table1. The equations and parameters of kinetic and mechanistic information of Cyclic voltammetry(CV)
It is defined by the ratio of standard rate constant (k 0 ) to mass transfer.(K0=i0/F , C: It is the electrostatic interaction energy for the initially formed ion pair, generally, this symbol refers to the energy gap law).This research aims to determine the concentration of methylene blue (MB) in aqueous media and its electrochemical behavior in a volt-ampere cell by CV by manufacturing an electrode from carbon paste modified with nanomaterials (NiO-NCQD, rGO, G-C 3 N 4 ).Many parameters related to the behavior of this pollutant were calculated using cyclic voltammetry, such as diffusion coefficient (D), mass transport (mTrans), electrochemical reversibility (Λ), Gibbs free energy (ΔG), interface trap density (Dit), constant (K 0 ), highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular Orbital (LUMO), thermodynamic equilibrium constants (K th ), Gibbs free energy (ΔG), Electronegativity (χ), Electronic Chemical (μ), Chemical hardness (), the electrophilicity index (ω), the maximum transferred charge capacity (ΔNmax), Softness (σ), electron affinity (A), and the ionization energy (I).In addition, a comparison was made between the two methods using carbon paste modified by CV and UV-Vis spectrometer.The results showed that there is no difference between the two methods in terms of accuracy.

Instrumental
The voltammetry system is used for trace analysis and education.The accessories with VA computer software and all electrodes for a complete measurement system: Multi-Mode Electrode pro (MME pro), Ag/AgCl reference electrode, and Pt as auxiliary electrode were used.In this study, a modern voltammetric was connected to a PC based on a USB port (Metrohm797; volt-amperometry analyzer with analyzer cell, USA).
A spectrophotometric device D-Lab model SP-UV1000 was used.Sartorius pH meter type PB-11 was used from Data Weighing System Company (pH meter and mV meter; DWS Inc.,).

Sample Preparation and Procedure
The preparation of electrode and standard solutions is as follows:

2.4.1.Manufacture of the electrode body
The electrode body was made of a glass tube cut to a length of 50∓0.5 mm so that it is open from both ends.Inside it is a copper wire whose lower end is connected with the modified carbon paste, and its upper end is connected to the device, the modified carbon paste consists of nickel oxide nitrogen carbon quantum dots (NiO-NCQD) of (10%) graphenecarbon nitride G-C 3 N 4 powder (10%), reduced graphene oxide (10%), graphite powder (30%), paraffin oil (40%) as a plasticizer.This electrode is used as a working electrode, it is symbolized by the symbol (NiO-NCQD/g-C 3 N 4 /rGO/CPME) the measuring cell is a platinum auxiliary electrode and the comparison electrode is silver chloride silver with a constant potential of 0.222V.

2.4.2.Preparation of standard solutions
First, the standard solution of MB was prepared based on 0.032 g of MB powder which was transferred completely into a volumetric flask of 100 mL distilled water to the capacity mark solution with a concentration of 10 mM.A solution of monosodium phosphate modified with phosphorous acid based on a buffer solution of (0.1M) monosodium phosphate NaH 2 PO 4 was prepared and a solution of phosphorous acid (H 3 PO 4 ; 1.08 M) was added to it until the PH changed to pH=1.For the preparation of the sulphur acid solution, a solution of (0.1M) sulphur acid is prepared by taking 0.2776 mL of concentrated sulfuric acid with a purity of 98% and a density of 1.84 g cm -3 for a volume of 50 mL distilled water (DW) up to the capacity mark, and we get the pH of 1.For modified Britton-Robinson Buffer solution (BRB), a buffer solution is prepared from BRB, which consists of H 3 PO 4 (0.2076M) with acetic acid (CH 3 COOH; 0.04M) and boron acid (H 3 BO 3 ; 0.04M) concentrations on the pH value of 1.For the preparation of phosphorous acid solution, a solution of 1.08M of phosphorous acid was prepared by taking 3.64 mL of concentrated phosphorous acid with a purity is 85% and density of 1.71 g cm -3 for a volume of 50 mL and it was supplemented with distilled water (DW) until the capacity mark reached to the pH value 1.Finally, a solution of hydrochloric acid 0.1M was prepared by taking 0.414 mL of concentrated hydrochloric acid based on a purity of 37% and density of 1.19 g cm -3  in 50 mL of distilled water (DW) up to the capacity mark to get a pH value 1.

2.4.3.Test solution for electrode
Weigh approximately 0.0329 g of K 3 [Fe(CN) 6 ] and 0.186 g of KCl.Then, transfer completely to a 25mL of DW to a concentration solution of K 3 [Fe(CN) 6 ] (5Mm) and 0.1 M of KCl.

2.4.4.Preparation of solution for spectrophotometric measuring
First, 0.025 g of MB powder was transferred completely to a volumetric flask of 25 mL capacity and the volume was completed with distilled water (DW), so the concentration is 1000 mg L -1 (1.0 g L -1 ) was made then, the different standard solution samples of 1, 2, 4, 6, 8, 10 mgL -1 was prepared from it by dilution with DW.

Results and Discussion
The prepared electrode was tested by test solution, to ensure the work of the prepared electrode before starting the measurement procedure, as shown in Figure 3A.It was found that the electrode worked and was reliable, identified the contaminant methylene blue, and determined the optimal conditions for work, including pH, scanning speed, and scanning rate.

The effect of pH
The effect of the pH value was studied within the pH range from 1 to 10, where the scanning was done within a potential scanning range (-1 to +1) V.At a range of pH (5-10) has no peak, while a redaction peak appeared within the pH range of 1 to 4. Also, an oxidation peak has appeared within the range of pH  The previous curve shows the oxidation peak at a potential value of 0.585V and a redaction peak at a potential value of -0.21V, so the practical potential difference is ∆E=0.795V,which is a value greater than ∆E=0.059/2=0.0295Vusing the manufactured electrode (NiO-NCQD/g-C 3 N 4 /rGO/CPME), it is a semireversible curve subjected to oxidation equation in the middle (pH=1), according to the following oxidation and redaction Equation.The effect of the type of medium was evaluated.Several media were used at pH 1, which are hydrochloric acid, sulfuric acid, phosphorous acid, modified peritoneal, and monosodium phosphate modified with phosphorous acid which is shown in Figures 3L and 3M.It was found from the previous curve that the best acid used was 0.1M hydrochloric acid.

The effect of scan rate
The scan rate was studied within the range of 10, 30, 50, 70, and 100 mv sec -1 on the peak current I (p) as shown in Figures 4A and 4B, and it was observed that it is the best scan rate is100 mv sec -1 .From the scanning rate in Figures 3 and 4, it is concluded that the relationship between the potential scanning rate (mv sec -1 ) and the peak current intensity I (µA) for both oxidation and reduction is linear.Also, it showed a directly proportional to the value of the oxidation peak current intensity, and inversely proportional to the value of the reduction peak current intensity.Whereas the value of the correlation coefficient was obtained at R²=0.9938 and R²=0.9019 for both oxidation and reduction, respectively.Also, the equation of the line for each of them is y=5.0082x+80.574and y=-3.116x-194.37for both oxidation and reduction, respectively.
The standard curve for the oxidation and reduction of methylene blue in terms of concentration and the current strength of the oxidation and reduction peaks is shown in Figure 5B.
Due to Figure 5B, it was found that the concentration of methylene blue can be determined within the linear range (25-100) µM or (7.99-31.98)mg L -1 , where the oxidation equation was I Ox =1.508C MB +229.5 and the correlation coefficient (R²=0.9823),while the redaction equation is I Red =-2.236CMB -232.5 and the correlation coefficient (R²=0.9722).

Studying the behavior of methylene blue in aqueous solutions
The behavior of methylene blue has been studied in aqueous solutions for a range of concentrations of methylene blue (25-50-75-100) µM using the proposed electrode and at (pH=1) using (0.1M) hydrochloric acid.In this study, diffusion coefficient (D), mass transport (m trans ), constant (K 0 ), electrochemical reversibility (), and interface trap density (D it ) were shown in Table 2.The values in Table 2 are plotted and shown in graphical curves in Figure 6.Effect of the concentration of methylene blue using electrode (NiO-NCQD/g-C 3 N 4 /rGO/ CPME) on Diffusion coefficient (D), Mass transport (m trans ), Constant (K 0 ), Electrochemical reversibility (Λ), and interface trap density (D it ) were shown in Figure 6.The diffusion coefficient and the mass transport to the electrode surface decreased with the increase in the concentration of methylene blue in aqueous media.Also, some physical and chemical properties were calculated in Table 3.
It is concluded from the value of the chemical voltage that the electrons in the molecule (MB) need an energy of (3.8842 eV) in order to exit the equilibrium system, and the molecule receives an additional electronic charge from its surroundings within its studied system of (-12.905eV) from the value of the electrophilicity index, the molecule has a resistance to charge transfer of (-1.7107 eV) from the value of chemical hardness, and the compound is an electron acceptor since the value of the maximum charge capacity is positive, and the compound has an energy difference between the vacuum energy level and the minimum level of the conduction band, its value is (-2.17351eV) of electronic affinity, and the reaction is spontaneous from the free energy of gypsum, this indicates that the oxidation of (MB) Spontaneous oxidation in the studied solution in the presence of an electric current (generated by the auxiliary stream), and the solution of this pollutant is thermodynamically balanced at Laboratory temperature and normal pressure, knowing that the energy gap value is equal to (3.42149 eV) .

Comparison of methylene blue analysis using the proposed electrode with the Cyclic voltammetry method and the spectrophotometric method
It is studied spectroscopically in the visible field (VIS), where standard solutions of methylene blue solutions including 1, 2, 4, 6, 8, and 10 mg L -1 were prepared and then scanned spectroscopically as in Figure 7A.It appears from the previous curve that the maximum absorption value of Methylene blue was observed at 663nm, which has a high colour intensity, and the absorption curve in terms of concentration was shown in Figure 7B.The proposed method was applied to a standard sample of 7.99 mg L -1 concentration in two ways, comparing of spectroscopic method with the CV method based on NiO-NCQD/G-C 3 N 4 / rGO/ MCPE, to ensure the validity and accuracy of the proposed method by calculating the statistical parameters which was shown in Table 4.It was also calculated by the value of F ex .So, the value of F ex was 2.04 for oxidation and 1.35 for reduction which is smaller than the value of F tab =19.0 as a confidence level of 95% with α = 0.05 and n = 3.Therefore, there is no significant difference between the two methods.

Conclusions
This paper deals with an electrochemical method for the determination of methylene blue (MB) by fabrication of an electrode (NiO-NCQD/G-C 3 N 4 / rGO/CPME), This electrode is used as a working electrode.The oxidation-reduction curve of methylene blue was shown using this electrode.It is a quick-reversible curve, and it works at (pH=1) and the best acid used is HCl a concentration of (0.1M).It is found that the linear range is within the range of 25-100 µM.Several constants were studied to determine the behavior of methylene blue within the electrochemical cell.The CV method was compared to spectrophotometry at 663nm and there is no difference between the determination of MB by the CV and spectrophotometry methods.

Fig. 2 .Fig. 1 .
Fig. 2. Distribution diagrams of the protonated species of methylene blue LMB a with pH LMB Leuco-Methylene Blue, LMBH Mono-Protonated form of LMB

0 F
-------------E1/2 : Half-oxidation potential for peak q: electron charge ∆V: flat-band voltage shift It is defined by the ratio of standard rate constant (k 0 ) to mass transfer.K 0 = i .(C: It is the electrostatic interaction energy for the initially formed ion pair, generally, this symbol refers to the energy gap law).

7 F
(1-2).The results of pH based on NiO-NCQD/g-C 3 N 4 /rGO/CPME electrode are shown in Figures 3A-3M.A) Curve Graph 5mM of and 0.1M of KCl B) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH =10 C) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = 9 D) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = 8 E) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = ) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = 6 G) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = 5 H) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = 4 I) Oxidation curve of methylene blue oxidation and reduction of 0.5m M methylene blue at pH = 3 J) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = 2 K) Effect of pH on the value of the oxidation current strength and the return of a 0.5 mM MB L) Oxidation curve of methylene blue oxidation and reduction of 0.5mM methylene blue at pH = 1 Anal.Methods Environ.Chem.J. 7 (1) (2024) 17-29 M) Effect of the type and nature of the medium used at pH = 1 on the oxidation and redaction current of a 0.25 mM methylene blue solution where, A: monosodium phosphate with phosphorous acid, B: 0.1M sulfur acid, C: modified (BRB), D: Phosphorous acid, E: Hydrochloric acid 0.1M.