{'fraction_non_alphanumeric': 0.036766809728183115, 'fraction_numerical': 0.01855030996661898, 'mean_word_length': 4.761263736263737, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 1, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Developing the sensing features of copper electrodes as an environmental friendly detection tool for chemical oxygen demand

The chemical oxygen demand (COD) of water bodies is an essential indicator of organic contaminants. The majority of current testing methods have the drawbacks of requiring multiple processes, being time-consuming, and requiring the use of harmful and hazardous reagents. In this work, a low-cost copper wire (Cu-wire) electrode was designed and fabricated to be used as a sensing electrode for the detection of chemical oxygen demand in water. The sensing features were developed by electrodeposition of copper nanoparticles (nano-Cu) that were prepared by fast-scan cyclic voltammetry (FSCV) deposition at the optimum preparation conditions. For improving the adherence and stability of the deposited nano-Cu thin layer, the Cu-wire electrode was scratched to increase the surface roughness. The surface morphology of the prepared nano-Cu/Cu-wire electrode was investigated by scanning electron microscope (SEM). Energy-dispersive X-ray spectrometer (EDX) was used for elemental analysis characterization. The non-modified and the nano-copper modified electrode were utilized and optimized for electrochemical assay of COD using glycine as a standard in 0.075 M NaOH as an electrolyte solution. The calibration curves (COD, mg L−1vs. I, mA) were plotted from linear sweep voltammetry (LSV) and chronoamperometry (I–t) curves for a wide range of COD under the optimized conditions. It shows that the electroanalytical features of the proposed nano-Cu-based COD sensor exhibit a linear range from 2 to 595 mg L−1 and a lower limit of detection (LOD) of 2.6 mg L−1 (S/N = 3). The established electrochemical method demonstrated a high tolerance level to Cl− ions where 1.0 M Cl− exhibited a negligible influence. The sensor was employed for detecting the COD in diverse real water samples and the attained results were validated using the standard dichromate method. The obtained results could open the window toward using simple and cost effective tools in order to monitor the water quality.


Introduction
With the advancement of contemporary industry, the preservation of water resources from pollution by waste effluents is becoming increasingly important. Organic molecules are a prevalent pollutant found in urban garbage, agricultural and livestock output, and industrial discharge. The chemical oxygen demand is an important measure for assessing the level of contamination caused by general organic chemicals, as it is a speedier alternative to the biochemical oxygen demand (BOD), which requires precise extensive estimating durations. COD is described as the amount of molecular oxygen (in mg of O 2 ) necessary to breakdown all of the organic substances in 1 liter of aqueous environment to CO 2 and H 2 O via chemical oxidation. In many countries, measuring COD with potassium dichromate as the oxidizer in H 2 SO 4 media is a well-established and standardized method. However, because there is a reux from 2 to 4 h in the process, and several expensive (Ag), corrosive (H 2 SO 4 and Cr 2 O 7 2À ), and highly toxic (Hg and Cr(VI)) reagents are needed during the procedure, this approach is time-consuming, expensive, and ecologically dangerous. 1 To overcome the limitations of the traditional method, some novel trials and approaches have been investigated, some of which include the use of sensors. For the detection of COD, Teixeira designed a unique photochemiresistor sensor utilizing m-bismuth vanadate semiconductor. 2 To develop a quick approach, Kong and Wu used UV spectroscopy to determine COD values in printing and dyeing wastewater effluents. 3 Depending on a potassium permanganate-glutaraldehyde, Cheng and coworkers devised an elevated chemiluminescence a Holding Company for Water and Wastewater (HCWW), Cairo, Egypt approach for the determination. 4 The COD concentrations of effluents were also determined using photocatalytic and photoelectrocatalytic techniques. 5, 6 Wang et al. 7 has developed a portable photoelectrochromic visualization sensor for COD monitoring.
The challenge for electrochemical detection is to nd a suitable simple technique, environmentally friend, highly sensitive, short analysis time and low cost. These factors deliver uses of solid electrodes in sensing and sensors with promising advantages as high sensitivity, accuracy, stability and reproducibility, low detection, and quantication limits. 8,9 Solid electrodes become one the most appropriate electrodes for electroanalytical research for a wide range of applications in industry, water and wastewater quality monitoring, food and drug quality control. 10,11 However, the solid electrode material selection depends on the redox behavior of the investigated electroactive species and the background current over the potential region required for the detection of the investigated analyte. The requirements imposed on electrodes are as follows: electrochemical inertness over a broad interval of potentials; high overvoltage of H 2 and O 2 evolution; low residual current; and easiness of surface treatment for good stability of applied material modied electrode. 8,9,12 To realize the direct detection of COD electrochemically, many materials have been tested as electrocatalysts, including Cu-electrodes, 8,9,[12][13][14][15] CuO-AgO, 16 Ni-Cu, 17 nano-Ni, 18 CoO, 19 TiO 2 , 5,20-37 PbO 2 , 38 Pt-ring, 39 nickel nanoparticle/naon-graphene oxide (GO), 40 boron doped diamond (BDD), 41 Ti/Sb-SnO 2 / PbO 2 , 34 WO 3 /W, 42 palladium-graphene. 43 COD analysis was developed over time starting from a simple titrimetric method to the novel sensing methods till now. The main concept of this development is to overcome some of the disadvantages as environmental impact, cost control and operation easiness. 12 Herein, a modied copper electrode proposed as COD sensor for water quality monitoring. The proposed modied Cu electrode was prepared by FSCV technique and used for investigating a variety of water samples (synthetic, real). Which could enable the use of COD sensors as an online analyzer tool for monitoring the water resources and pollution control.

Reagents and materials
Chemicals were purchased with high purity of minimum assay 99.9% and used as received without further purication. Double distilled water was used in the preparation of all solutions. Ag 2 SO 4 , H 2 SO 4 , HgSO 4 , CuSO 4 $5H 2 O, K 2 Cr 2 O 7 , NaOH and (NH 4 ) 2 Fe(SO 4 ) 2 were purchased from Panreac (Spain), Sigma-Aldrich (USA) and glycine. Potassium Hydrogen Phthalate (KHP) as standard organic compounds was purchased from ADWIC ne chemicals (Egypt).

Electrochemical preparation of nano-Cu sensing lm
Electrochemical preparation of nano-Cu lms was performed on a potentiostat/galvanostat (EG&G Princeton Applied Research, Model 273A). A conventional three-electrode system was employed. The working electrode was 1.5 mm diameter of commercially available/household electrical Cu-wire, the reference electrode was Ag/AgCl/(3.0 M KCl), the counter electrode was a platinum wire and the electrolyte solution was 0.075 M NaOH.
Before electrodeposition, Cu-wire electrodes (6 electrodes) were treated mechanically by grit emery paper 60 then thoroughly rinsed in bi distilled water. The Cu-wire sensing lm was electrodeposited from 4 mM CuSO 4 /1 mM H 2 SO 4 by FSCV, the potential scan window was from 0.1 to À1.3 V for different (n) cycles at 100 mV s À1 . The resulting electrodes coated with a thin lm of CuNPs were rinsed with double distilled water to remove any adsorbed species before use.

Characterization of the prepared Cu electrodes
Field Emission Scanning Electron Microscopy (FESEM; Zeiss, Sigma 500 VP) was used for studying the surface morphology of the prepared Cu lm on the surface of Cu wire. The elemental analysis of the obtained lm was evaluated using the energy dispersive X-ray spectrometer detector (EDX; BRUKER) xed on the SEM.

Detection of COD using standard dichromate method
The closed reux method was used to measure the COD values of real water samples taking into consideration the sampling (composite, grab) for sewage and wastewater samples that have high COD values and long term of interferences (Cl À , NO 2 À ) as reported in the literature. Briey, 10 mL water sample and 10 mL 0.04 M K 2 Cr 2 O 7 solution were added into a 250 mL round-bottomed ask and reuxed for 2 h at 150 C. Then the excess dichromate was determined using titration against 0.025 M Fe(NH 4 ) 2 (SO 4 ) 2 $6H 2 O and (2 to 3 drops) ferroin indicator. The value of COD was calculated according to the literature procedures. 44

COD detection by the proposed sensing method
Before measurements, the real samples (water and wastewater) were ltrated using (GAST, USA) ltration apparatus, equipped with cellulose nitrate (pore size $0.2 mm) lter paper. The oxidation current obtained from LSVs was determined with a standard addition method. Standard COD solutions of different concentrations in the range 20-40 mg L À1 were added to a series of 100 mL asks, each containing 5 mL of a ltered fresh real water sample, the volume of each ask complete to 100 mL using 0.075 M NaOH. The testing solution was placed into an electrochemical cell to measure the oxidation current where the coated copper wire was used as a working electrode. The oxidation current is proportional to the concentration of COD in the samples. All measurements were performed at room temperature ($25 C).

Characterization of Cu electrode surface morphology
Before surface treatment of the working electrode, mechanical treatment for Cu electrodes has been done to enhance the adhesion with the substrate surface and the lm stability. Vigorous surface polishing of the prepared thin Cu lm modi-ed electrode may destroy the surface properties. Various sizes of CuNPs were electrodeposited on the Cu electrode via FSCV by varying the deposition time (e.g. scan rate, potential range and number of cycles). SEM images were taken for the free Cu-wire electrode and the prepared nano-Cu/Cu-wire electrode by FSCV at n ¼ 30 cycles prepared previously at the optimum condition of preparation. It is clear that the non-modied Cu electrode has smooth surface features as seen in Fig. 1(A) and the magnied image indicated that there is no any identied particles at the surface as clear in the inset image of Fig. 1(A). However, aer treatment of Cu electrode, there are many irregular shaped nanoparticles of copper are formed at the surface Fig. 1(B). The formed nanoparticles are monodispersed and distributed over the whole surface of Cu electrode.
The chemical composition of the Cu-wire and nano-Cu/Cuwire electrodes were analyzed by EDX ( Fig. 1(C and D)) to determine the surface elements (%). It is cleare that the elemental composition of the non-modied Cu wire is Cu and some impurities of Al and C of low quantities Fig. 1(C).
The modication of Cu-electrode by nano-Cu based lm that is composed of (Cu) as a bulk and minor impurities as (C), (Al) and (O) comes from the atmosphere as adsorbed CO 2 or due to oxidation of the of some Cu particles on the surface as shown in Fig. 1(D). The formation of CuO enhance the electrocatalytic activity and the stability of the proposed nano-Cu modied Cu-wire electrode.

Electrocatalytic activity of Cu electrodes in alkaline medium
When Cu electrodes activated by cycling in an alkaline medium, it can electrocatalytically oxidize a wide range of organic species (synthetic, natural) responsible for COD. Cycling voltammetry has been adapted over the potential window from À1 to +0.7 V as indicated in Fig. 2. The oxidation peaks at (1) & (2) at À0.26 & À0.1 V respectively are corresponds to the formation of Cuoxide species Cu(I)/Cu(II). At the potential of 0.70 V, the Cu species were oxidized and Cu(II)/Cu(III) was formed. In the reverse scan it is clearly seen that the reduction peaks (3) & (4) at À0.59 & À0.88 V respectively correspond to Cu-oxide species of Cu(II)/Cu(0) reduction reactions. 8 A solution of 0.075 M NaOH is used as an electrolyte to enhance the catalytic activity by formating hydroxyl radical (OHc). This radical has a high oxidation potential and enhances the formation of Cu(III)OOH and Cu(OH) 4 . At high NaOH concentrations (>0.1 M) a large background noise was obtained. In addition, a lot of gas bubbles were observed on the modied electrode surface, causing difficulty in the measurement of the low current signal and decline in the sensitivity towards low COD concentrations. Thus, 0.075 M was chosen as the optimal concentration of COD detection.
Cu(III) is central in the electrocatalytic oxidation of organic compounds in the basic medium. 3 The expected electrocatalytic oxidation of organic compounds (e.g., glycine) at the copper electrodes in the basic medium as follows:  Fig. 3. The resulting cathodic peak potential (E pc ) is reached when all the substrate at the surface of the electrode has been reduced. In an stirred solution, mass transport of the analyte (Cu ++ ) to the electrode surface occurs by diffusion and continual decrease on the current i pc due to unstirred Cu electrode. From CV and LSV experiments (current vs. potential), the potential swept between two values are oen used to study the kinetics of electron transfer reactions including catalysis. The peak current response through the redox reaction by the electroactive species is recorded. It is noted that the maximum cathodic peak current at a potential À1.1 V is repetitive. The optimum condition of preparation is investigated by FSCV deposition for Cu-wire electrodes at a different number of cycles  and determines the highest oxidation response by LSVs. Fig. 4(A) plotting the last CV curves obtain at different (n) cycles. Fig. 4(B) shows the relation between the (n) and the value of i pc . As clearly seen, with increasing the number of electrodeposition cycles, i pc was reaching the maximum at n ¼ 30. For these prepared Cu-wire electrodes at n ¼ 30, the LSVs were measured in 75 mg L À1 gly/0.075 M NaOH electrolyte as presented in Fig. 4(C). It was found that there is oxidation peak appeared at a potential of 0.6 V due to oxidation of glycine at (Cu electrode) as a catalyst and with increasing the surface area at Cu particles thin lm modied Cu-wire electrode the electrocatalytic oxidation was increased. The electrode prepared by CV for n ¼ 30 cycles shows the highest oxidation peak current response (Fig. 4(D)) and was selected as the optimal condition of preparation.
LSV experiments were carried out with the Cu electrode modied nano-Cu lm to establish the effect of scan rate at a constant concentration (75 mg L À1 ) of glycine in 0.075 M NaOH. The anodic oxidation process can be described by Randles-Sevčik equation (eqn (4)) where n is the number of electron transfers, a is the electron transfer coefficient, n a is the number of electrons involved in the rate-determining step, A is the electrode area, C b is glycine bulk concentration 75 mg L À1 and D is the diffusion coefficient of glycine. According to eqn (4) a plot of the peak current I p against the square root of scan rate should give a linear relationship for diffusion-controlled process. 45 Indeed, a linear relation was observed for glycine oxidation in 0.075 M NaOH, with a linear regression equation of R 2 ¼ 0.9909. The electrocatalytic oxidation process is a diffusioncontrolled process. LSV experiment (Fig. 5(A)) was performed at different scan rates for 75 mg L À1 gly/0.075 M NaOH, it is noted a linear relationship between the oxidation current and square root of scan rate with a positive shi with increasing scan rate as shown in Fig. 5(B). Furthermore, the electrocatalytic oxidation was examined at a different temperature; the oxidation response increased by temperature via increasing the diffusion rate, mass transport and charge transfer between the activated species (Cu(III) & organics) Fig. 5(C and D). Thus, we must take part in the effect of applied room temperature in running some of the specic features of sensors that require running the experiment at a different time as reversibility, reproducibility, stability, and lifetime of the proposed COD sensors.

Amperometric detection of COD
Chronoamperometry (I-t) response curve for Cu-wire electrode and nano-Cu based COD sensor has been used for the investigation (Fig. 6(A)). The electrolyte solution and the applied potential that is suitable for background current and detection of low COD concentrations have been selected. The effect of applied potential on (I-t) response curve was examined. It is found by running the amperometric detection at different applied potentials, the oxidation current signal of glycine remarkably increases with raising the detected potential from 0.6 to 0.9 V. The higher oxidation potential causes faster electron transfer and certainly enhances the oxidation current. When we further change the detected potential from 0.8 to 1 V, the oxidation current of glycine increases slightly, and the background current signicantly increases. So, 0.075 M NaOH electrolyte solution and 0.75 V as the applied potential for suitable background current for detection of a wide range of COD values. [46][47][48] Analytical spike addition of COD concentrations (i.e., glycine), well-dened oxidation current signals was observed at Cu-wire electrode and nano-Cu based sensor. Background current decreases dramatically and attains a steady state at about 250 s. Upon increasing the COD concentrations, the oxidation current signals at Cu-wire and nano-Cu/Cu-wire linearly increases. With further increasing the additions of COD, the increment in oxidation current signal decreases (Highest Detection Limit, HDL) then inversely relationship as indicated in Fig. 6(A). The proposed sensors exhibited a fast response time of about 1 s at low COD values and 2 s for COD values higher than 300 mg L À1 .

Analysis of real samples and recovery study
The developed COD sensor reliability was tested practically by analyzing different real water samples and synthetic samples simultaneously by both the classical dichromate method and the developed sensing method (nano-Cu/Cu-electrode). By calculating the relative error between the values of the two methods, it was found that the data changed from À0.6 to 12.6% and exhibited appropriate recovery as indicated in Table  1, in addition to the good correlation between the two methods, it showed linear regression coefficient: COD nano-Cu sensor (mg L À1 ) ¼ 0.862 COD dichromate (mg L À1 ) + 6.18 (R 2 ¼ 0.995) and COD nano-Cu sensor (mg L À1 ) ¼ 0.979 COD dichromate (mg L À1 ) À 0.768 (R 2 ¼ 0.997) for real water samples and synthetic samples, respectively as indicated in Fig. 7 with good evidence for using the new method for water quality and pollution monitoring.

Comparison with reported COD sensors
In comparison with the previously reported COD sensors based on electrode modication (e.g., electrodeposition of metallic nanomaterial, 3-5 electrode modied lm, 19 composite electrodes, 49 polymerization, 50 activated electrodes, 51 electrodes recently proposed with new electroanalytical features). 26,40,50 The optimized nano-Cu/Cu-electrode prepared by FSCV exhibited lower LOD and high stability with high tolerance level towards inorganic interferences in comparison with listed electrochemical sensors in Table 2.

Conclusion
A sensing electrode for the detection of chemical oxygen demand in water was fabricated easily from available household Cu-wire. The Cu wire surface was modied by copper nanoparticles (nano-Cu) using FSCV electrodeposition technique. The modied Cu electrode COD sensor was found to be environmentally nd and with economic cost. It characterized by its fast electroanalytical detection response time and stable and reproducible response over a period of time. The modied nano-Cu electrode showed an excellent trend for a wide range of COD concentrations. In addition, the developed sensor shows excellent sensitivity toward COD with a lower detection limit of 2 mg L À1 with a linear range of 2 to 595 mg L À1 . Moreover, it possesses a high tolerance level towards inorganic interferences Cl À , NO 2 À of about 1.0 M, 0.1 M respectively. Analysis of real water samples by the newly developed nano-Cu modied COD sensor showed a good agreement (good repeatability) compared to the standard dichromate method, with low relative standard deviation and good accuracy with acceptable blank recovery results. This is assuring that using the new nano-Cu/Cu-wire electrode in the routine analysis of different kinds of water resources including surface water (e.g., drinking water, lakes and rivers) with low COD value up to industrial wastewater with high COD content, will help in enhancing water quality monitoring.

Conflicts of interest
There are no conicts to declare.