Electrochemical sensor based on green-synthesized iron oxide nanomaterial modified carbon paste electrode for Congo red electroanalysis and capacitance performance

ABSTRACT In this study, a facile protocol was used to convert non-valuable orange peels (OP) waste into a new sensing iron oxide orange-peel nanomaterial (FeOP). The presence of iron oxide nanoparticles in the modified OP was confirmed by physicochemical characterisations including Fourier-transform infrared spectroscopy, X-ray diffractometry, thermogravimetry, and scanning electron microscopy-energy dispersive X-ray. FeOP was used to modify a carbon paste electrode (CPE/FeOP) which displayed a significant increase in specific capacitance of 2939 F.g−1, two folds higher than that obtained with CPE at 10 m.s−1 in NaCl. The electroanalysis of Congo red (CR) in aqueous solutions using CPE/FeOP displayed detection limits of 2.8 × 10−7 mol.L−1 and 8.2 × 10−7 mol.L−1 respectively in deionised and spring waters, in the linear range of 5 to 55 µM. CPE/FeOP electrochemical sensor is therefore suitable for the determination of Congo red in wastewater.


Introduction
Iron nanomaterials have received great attention nowadays due to their unique properties including high catalytic activity, high magnetism, low toxicity and microwave absorption ability [1]. The range of applications include catalysis [2], water treatment [3], drug delivery [4], cancer treatment [5], hyperthermia [6], ferrofluid [7] and antimicrobial agents [8]. Several chemical and physical methods are used for the synthesis of nanomaterials, including electrochemical methods, hydrothermal approach, laser ablation, lithography, microwave and thermal decomposition [9]. However, these methods are costly due to high energy used and special equipment [3,10]. Moreover, sodium borohydride, hydrazine hydrate and other compounds generally used as reducing agents are toxic and hazardous on humans and the environment [11]. In order to overcome these drawbacks, great efforts have been made to use environmentally friendly methods for the synthesis of iron nanomaterials.
Recently, plant-mediated green synthesis of iron nanomaterials has extensively been reviewed [1,3,9,11,12]. Buds, fruits, seeds, stems or leaf extracts of several plants have been used as bioreducing agents for green synthesis of iron-based nanomaterials [11]. Furthermore, waste materials from citrine juice (orange, lime, lemon and mandarin) have also been employed [13].
Agro food-based economy generates an abundance of waste residues (husks, roots, shells, peels, etc.) which are largely under-utilised (principally used as animal feed). Moreover, poor management of these residues leads to several environmental problems. Indeed, when dumped near production sites or in landfills, they lead to severe impacts on air, soil and water [14]. Therefore, turning these residues into nanomaterials is an important way forward that can be applied for the development of high-quality electrode materials [15,16]. An eco-friendly and economical green method has been recently reported for the synthesis of iron-oxide (Fe 2 O 3 ) nanoparticles using orange peel extract as a stabilising agent, followed by the evaluation of the antibacterial activity of the obtained nanoparticles [17]. Moreover, raw and modified orange peel have shown good performances for the adsorption of heavy metals and organic compounds (dyes, pesticides) [18][19][20][21][22]. The sorbent properties of orange peels open a new insight in the field of electrode modification to produce efficient electrochemical sensors. Moreover, composite materials containing metal nanoparticles have shown good performance when used as electrode modifier [23][24][25].
Congo red (CR) is one of the most abundant azo dye in wastewater, released from many sources such as textile, paper, and leather industries [26]. Wastewater containing CR is phytotoxic, resistant to biological treatment and harmful for flora and fauna. CR causes allergic reactions, skin damages, respiratory tract and mucous membrane irritation [27,28]. Considering its negative impacts on human life and environment, it is necessary to develop analytical techniques for the fast, efficient and effective detection and removal of CR in wastewater. To the best of our knowledge, only one study recently published by Shetti and co-workers [26] is devoted to the development of an electrochemical sensor for the detection of CR in water samples.
In this work, green synthesis of iron nanoparticles is carried out with orange peels powder acting as supporting, reducing and stabilising agent. The obtained nanoparticles are characterised using XRD, TGA, SEM, EDX and FTIR in order to analyse their size, morphology, composition, and structure. The synthesised nanomaterial is used to modify a carbon paste electrode. After the electrochemical characterisations and the evaluation of capacitive properties, the resulting sensor is applied for the electroanalysis of Congo red in aqueous solutions. In addition to the advantages of green synthesis, such composite material could combine adsorption properties of orange peels [29] and catalytic properties of iron nanomaterials [30].

Collection of waste material
Orange-peels were collected at Mokolo market in Yaoundé. They were washed thoroughly with deionised water to eliminate dust on their surface, dried at room temperature, grounded and sieved to obtain particles of 40 µm (named OP). Thereafter, 15 g of OP powder were boiled in 250 mL of deionised water at 80°C for 1 h in order to increase the adsorption ability by elimination of extractible compounds. The obtained product was vacuum filtered and the paste was stored at 4°C [31].

Synthesis of nanomaterial
The synthesis of iron orange-peel nanomaterial (FeOP) was done by adding the previously obtained paste in 50 mL of 0.1 M Fe (II) solution at room temperature and constantly stirred for 30 min. A black colour appeared immediately indicating the reduction of Fe (II) [31] and the prepared nanomaterial was collected by vacuum filtration.

Preparation of carbon paste electrode
The modified carbon paste electrodes (CPE/OP and CPE/ FeOP) were prepared by hand-mixing the carbon powder, silicone oil and OP or FeOP in the proportion 66:30:4. For the unmodified electrode, carbon powder was mixed with silicone oil in the proportion 70:30. The resulting pastes were packaged firmly into the cavity (3 mm diameter) of a Teflon holder. The electric contact was established via a stainlesssteel rod connected to the paste. A new surface was obtained by smoothing the electrodes onto a weighing paper.

Electrochemical procedures
Electrochemical measurements were performed in a conventional three-electrodes cell, with CPE modified or unmodified as working electrode, Ag/AgCl as reference electrode, and platinum wire as counter electrode. These electrodes were connected to a PGSTAT 12 Autolab (EcoChemie) Potentiostat controlled by the GPES (General Purpose Electrochemical System) software.
Electrochemical impedance spectroscopy (EIS) measurements were recorded by applying a current-voltage of 5 mV in a frequency range of 0.01-10 Hz. The measurements were done in 0.1 M KCl containing a mixture of 10 −3 mol.L −1 ferricyanide and ferrocyanide and the specific capacities were estimated by integrating the full cyclic voltammogram to determine the average value according to the following relationship [32][33][34]: where Cs (F.g −1 ) is the specific capacitance, i (A) is the response current, V (V) is the potential, υ (V.s −1 ) is the potential scan rate, and m (g) is the mass of the material (OP or FeOP) in the electrode. A frequency response analysis (FRA) software was used to control EIS measurements. The ion-exchange and permselectivity abilities of nanomaterials were examined by multi-sweep cyclic voltammetry (MSCV), using K 3 [Fe(CN) 6 ] and [Ru(NH 3 ) 6 ]Cl 3 as redox probes and 0.1 M KCl electrolytic solution. Differential pulse voltammetry (DPV) was used for the electroanalysis of CR in 0.1 M phosphate buffer. The DPV signals were recorded with a modulation time of 0 s, an interval time of 0.1 s, a step potential of 10 mV and a modulation amplitude of 100 mV.

Characterisation of materials
X-ray diffraction (XRD) analysis was performed using a Bruker diffractometer system EMPYREAN with Cu-Kα radiation (λ = 1.542 Å) using a generator with a voltage of 110 V and controlled by a PAN analytical X'Pert High Score software. The measurement was done at a scan of 24.765 s and a step size of 0.0170 degree.
The morphologies of the materials were examined using a JEOL-IT 300 SEM instrument coupled with EDX. Before analysis, the samples were placed on the double-sided carbon conductive tape and were double coated with the carbon layer using Quorum Q150R ES instrument to prevent accumulation charge during measurement.
TGA measurements were conducted with a TG-DSC instrument (SETARAM KEP Technologies SETSYS Evolution) in a temperature range from 20 to 1000°C at 10° C.min− 1 heating rate in N 2 atmosphere.
FTIR spectra were obtained using attenuated total reflectance (ATR) mode at room temperature using a spectrophotometer alpha-P of Bruker with a resolution of 4 cm −1 .

Physicochemical characterisation of materials
The SEM images of orange peel (OP) and synthesised iron nanomaterial (FeOP) are shown in Figures 1a and b, respectively. The samples presented a compact morphology with relatively high porosity and particles of different sizes and shapes. A high agglomeration of particles was observed in the case of FeOP. Moreover, the surface of FeOP seemed to be loaded compared to that of OP. This result showed that iron oxide nanoparticles were chemically bonded and/or physically adsorbed on the surface of OP [35].
The SEM elemental mapping images of OP ( Figure  1c) and FeOP (Figure 1d), associated to their EDX spectra (Figures 1e and f) showed an increase in iron and oxygen contents from OP to FeOP. These observations are good indications of the formation of iron oxide nanoparticles during the synthesis process. Furthermore, the oligo-elements present in both OP and FeOP revealed the heterogeneous structure of lignocellulosic materials [36].
The FTIR spectrum of OP (Supplementary Material Fig.  S1) showed a band at 3400 cm −1 corresponding to the stretching vibration of O-H of alcohols and phenols functions [37]. The absorption bands at 2950 cm −1 and 2900 cm −1 can be attributed to the stretching vibrations of C-H of methyl, methylene and methoxy groups [28,38]. The vibration band around 1600 cm −1 can be assigned to the stretching vibrations of C = O of carboxylate functions [39][40][41]. The absorption band at 1000 cm −1 can be assigned to the stretching vibration of C-OH of alcohols and carboxylic acids [42]. All these bands are characteristics of OP. They also appeared on the spectrum of FeOP but were less intense compared to OP due to the modification. For the band attributed to the stretching vibration of hydroxyl groups around 3400 cm −1 , the loss of intensity is the evidence that fewer free R-OH units were present, which is the consequence of the loss of some functional groups of hemicellulose and cellulose during the treatment [39]. The absorption band at 1500 cm −1 corresponds to the vibrations of carboxylate groups and C = C bonds in aromatic compounds [43]. The absorption band around 500 cm −1 indicate the presence of metals and can be assigned to Fe-O stretching vibration, which confirms the presence of iron oxide in the composite material [38,44,45]. This analysis showed that some functional groups (-COOH and -OH) were successfully modified by chemical treatment, resulting in the adsorption or grafting of iron oxide.
The XRD pattern of FeOP (Figure 2a) depicts an intense peak at 27° 2θ indicating the presence of iron oxide (d 220 value of 3.34 Å). This is further confirmed by the presence of five other characteristic peaks of Fe 3 O 4 at 22°, 30°, 43°, 47° and 50° 2θ corresponding, respectively, to the 022, 220, 400, 110 and 422 reflections [38]. The peaks at 43° and 47° 2θ corresponding to γ-Fe 2 O 3 also indicate the presence of iron oxides in the composite FeOP material. Thus, the XRD analysis confirms the presence of iron oxide as Fe 3 O 4 and γ-Fe 2 O 3 [46].
The TG and DTG curves of FeOP ( Figure 2b) display a progressive mass loss of about 15% occurring at 122°C and 150°C, attributed to the elimination of adsorbed water molecules from the surface of FeOP nanomaterial. The DTG presents peaks centred at 220°C, 256°C, 297°C and 353°C which are correlated to the decomposition of the organic part of the material, mainly epoxy and carboxyl groups of cellulose [47]. The peak observed at about 686°C can be attributed to the exfoliation in the FeOP nanomaterials during the high-temperature treatment.

Electrochemical characterisations of materials
In order to investigate the permeability and ions exchange properties of FeOP nanomaterial, the modified carbon paste electrode (CPE/FeOP) was characterised by multisweep   (Figure 3a) reveal a slight increase in the peak current from the 1 st to the 80 th cycle. This increase is attributed to a progressive accumulation of [Fe(CN) 6 ] 3ions in the modified electrode. This accumulation, attributed to the porosity of the materials is low due to a poor affinity between anionic compounds and lignocellulosic materials [48]. Indeed, some hydroxyl functions of OP are deprotonated, causing electrostatic repulsions with the targeted analyte. The signal obtained at saturation was more intense and well defined on CPE/FeOP compared to CPE and CPE/OP (Figure 3b). This is the proof that iron oxide increases the adsorption capacity of OP.
The successive voltammograms registered on CPE/FeOP for 10 −3 mol.L −1 [Ru(NH 3 ) 6 ] 3+ in 0.1 mol.L −1 KCl (Figure 3c) showed a well-defined and reversible signal appearing at 0.0 V for the oxidation and -0.6 V for the reduction. The peak currents increased with the number of cycles, until the 80 th scan. This predictable result reflects the gradual accumulation of cations on the adsorption sites of FeOP nanomaterial. As a matter of fact, many works in the literature report the accumulation of cationic species on lignocellulosic materials [48][49][50][51]. This accumulation is generally due to electrostatic interactions between the cationic probe and the deprotonated hydroxyl and carboxylate groups present at the surface of the material.
The signals obtained on CPE/OP and CPE remained reversible and well defined but were less intense compared to that obtained on CPE/FeOP. At saturation, the signal obtained on CPE/FeOP was 2.4 and 8.4 times higher, respectively, compared to those obtained on CPE/ OP and CPE (Figure 3d). This result can be attributed to the presence of iron oxide incorporated in the synthesised nanomaterial which improves the adsorption properties of the electrode. The high current intensities obtained on the modified electrodes can also be explained by natural affinity between lignocellulosic materials and cationic compounds [48].

Evaluation of capacitive properties
In order to explore the potentiality of FeOP as electrode materials in supercapacitors, the pseudo-capacitive properties of the electrodes were investigated by CV and electrochemical impedance spectroscopy (EIS). Figures 4a and c show the voltammograms of CPE and CPE/FeOP electrodes in 0.1 mol.L −1 NaCl at different scan rates. Non-noticeable redox peaks or faradic reactions were observed, indicating an ideal electric double-layer capacitive behaviour of the electrodes [34,52]. This suggests that the ions are mainly adsorbed on the surface of the electrodes through coulombic interactions rather than electrochemical oxidation/reduction reaction [52,53]. This result was expected, because the ions present in solution are not electroactive.  Furthermore, the peaks were located at the two poles of the potential window, due to the low polarisation [52]. Obviously, at high scan rates, the CV curves revealed a slight deviation from a rectangular shape to oval shape for CPE (Figures 4a and b) and this deviation could be considered as a result of the inherent resistivity of the salt solutions. It is widely known that an increase in surface area under the CV curve is a good indication of ion adsorption capacity and suggests high specific capacitance [52]; this is not the case for CPE/FeOP (Figures 4c and d).
The specific capacitance values of the fabricated electrodes, which provide an efficient way to determine the performance of electrode materials were calculated from CV results. Cyclic voltammetry (CV) was used to evaluate the potential of the materials and specific capacitances were estimated from I-V cycles according to equation 1 [34]. The values obtained were used to plot Figure 5a which displays the CV profiles of the electrode used for all measurements.
A high electrosorptive capacitance at lower scan rates was found, owed to the diffusion of ions from the solution to access easily at the electrode surface, leading to a more adsorption/desorption of ions at the surface [34,54]. However, at high scan rates, the effective inner-surface adsorption of ions was accordingly reduced. As shown in Figure 5a, the corresponding specific capacitance of the introduced CPE/FeOP electrode showed an obvious increase compared to CPE. Typically, at a scan rate of 10 mV/s, the observed specific capacitances were 2939 and 2090 F/g for CPE/FeOP and 1640 and 969 F/g for CPE in NaCl and KH 2 PO 4 respectively. This is attributed to the presence of incorporated iron oxide nanoparticles in orange-peel material (FeOP) that enhance its specific surface [34].
The electrochemical impedance spectroscopy (EIS) was conducted to investigate the charge and electronic conductivity of the materials. A combination of semicircle at high frequency and linear region at low frequency was observed for each electrode. The Nyquist plots in Figure 5b revealed charge transfer resistances of 420 Ω, 640 Ω and 2000 Ω, respectively for CPE, CPE/FeOP and CPE/OP. The high increase of the charge transfer resistance from CPE to CPE/ OP is the result of the very low conductivity of OP. Indeed, as shown from the electrochemical characterisation (section 3.2), there is an electrostatic repulsion between negatively charged OP and ferricyanide ions. The high decrease of the charge transfer resistance from CPE/OP to CPE/FeOP suggested a high electronic conductivity of FeOP compared to OP. Indeed, the presence of iron nanoparticles significantly increased the electronic conductivity of OP. However, the electronic conductivity of CPE/FeOP remained smaller than that of CPE. This result is probably due to a low percentage of nanoparticles in FeOP material. Moreover, the Nyquist profile of CPE/OP showed an overlap of two semi-circles in the high-frequency region (inset Figure 5b). This behaviour may be attributed to the heterogeneity of the electrode which combine the conductive carbon powder and the non-conductive raw OP.

Electrochemical behaviour of Congo red on CPE
The electrochemical behaviour of Congo red (CR) at modified and unmodified CPE were evaluated by cyclic voltammetry (CV) measurements, using 5 × 10 −4 mol.L −1 CR in 0.1 mol.L −1 phosphate buffer solution (pH 6.65) at a potential scan rate of 50 mV.s −1 . As shown in Figure 6a, the voltammogram obtained on each electrode displayed an anodic peak characteristic of the irreversible oxidation of CR, involving one electron [26,55]. The peaks appeared at 0.761, 0.742 and 0.702 V with intensities of 3.10, 5.13 and 6.32 µA, respectively for CPE/OP, CPE and CPE/FeOP. Compared to CPE, the signal was less intense on CPE/OP due to a decrease in conductivity of the electrode. Indeed, OP is a non-conductive material. In the case of CPE/FeOP, there was an increase of the peak current. This result is probably due to the presence of iron oxide nanoparticles which bring new active sites that facilitate the adsorption of CR at the electrode surface. Moreover, CR is easily oxidised at the electrode in the presence of iron oxide nanoparticles which act as catalyst, as evidenced by a decrease of 40 mV on the oxidation peak potential. Similar results were obtained by Shetti and co-workers during the electrochemical detection of CR at graphene oxide modified glassy carbon electrode [26].

Influence of the scan rate
In order to clarify the mechanism of the electrochemical reaction of CR at CPE/FeOP, CV measurements were performed at different scan rates ranging from 25 to 300 mV. s −1 in 0.1 mol.L −1 phosphate buffer solution containing CR 5.10 −4 mol.L −1 . The voltammograms obtained (Figure 6b) showed that the peak current increased with the scan rate, due to an increase in concentration gradient of CR between the bulk solution and the electrode surface. A plot of the peak current as a function of the square root of the scan rate exhibited a linear dependence (inset Figure 6b). This result indicates that the oxidation of CR at the modified electrode is governed by a diffusion-controlled mechanism [56]. Moreover, the plot of log(Ip) versus log(Vp) was linear, with a slope value of 0.552 (Figure 6c), which confirms that diffusion is the predominant mode of transport of CR during the electrochemical reaction. However, for a perfect diffusion-controlled mechanism, the slope is supposed to be equal to 0.5. The value obtained (between 0.5 and 1) shows that an adsorption process is associated with the electron transfer at the electrode surface [56,57]. The diffusion coefficient of CR, D, was calculated according to the Randles-Sevcik equation (equation 2) [56], using the slope of the regression equation of I versus v 1/2 (inset Figure 6b), and the value obtained was 2.38 � 10 −6 cm 2 .s −1 . This value of the diffusion coefficient of CR is of the same order of magnitude as that obtained by Iwunze (1.72 � 10 −6 cm 2 .s −1 ) [55]. From the above results, it appears that FeOP used to modify the CPE may be suitable to build an electrochemical sensor for CR.
Where I (A) is the peak current, n (1) is the number of electrons involved in the electrochemical oxidation, A (0.0707 cm 2 ) is the electrode surface, C (5.0 � 10 −7 mol. cm −3 ) is the molar concentration of CR in bulk solution, D (cm 2 .s −1 ) is the diffusion coefficient of CR and v (V.s −1 ) is the scan potential rate. In order to determine the charge transfer coefficient, α, a cyclic voltammogram of 0.5 × 10 −4 mol.L −1 CR in 0.1 mol.L −1 phosphate buffer solution (pH 6.65) was recorded on CPE/ FeOP at a scan rate of 10 mV.s −1 . The slope of the Tafel plot obtained from this voltammogram (Figure 6d) was used for the determination, using equation 2 [56]. The value of α obtained was 0.862.
Where n (1) is the number of electrons involved in the electrochemical oxidation, F (96,485 C.mol −1 ) is the Faraday's constant, R (8.314 J.mol −1 K −1 ) is the ideal gas constant and T (298 K) is the absolute temperature.

Optimisation of detection parameters
For further experiments, differential pulse voltammetry was used due to its high sensitivity compared to cyclic voltammetry [56]. The experimental parameters (frequency of 50 Hz, step potential of 5 mV/s and amplitude of 20 mV) were initially optimised to perform the analysis in the best conditions.

Effect of the quantity of modifier.
The effect of the quantity of modifier (FeOP) in the carbon paste electrode was investigated in the range of 2% to 10%, the percentage of silicone oil being maintained at 30%. The results (Figure 7a) show that the peak current increased with the quantity of FeOP from 2 to 4%. This increase in peak current is attributed to an increase in the number of binding sites at the surface of the electrode and confirms the success of the synthesis process of the modified material. Above 4%, the oxidation peak current gradually falls resulting from a decrease in the electrode conductivity. Indeed, the results of EIS investigations (paragraph 3.3) showed that the CPE/ FeOP has a lower electronic conductivity than the CPE. 4% is therefore chosen as the optimum percentage of the material within the carbon paste.

Influence of the pH of the electrolytic solution.
The charge of CR (pKa around 4) can be modified by varying the pH of the solution which may lead to a change in adsorption ability of the material, with a significant effect on the peak current. The influence of the pH was evaluated by analysing CR 10 −4 mol.L −1 in phosphate buffer 0.1 mol.L −1 at various pH ranging from 2 to 8.5. The corresponding peak currents plotted as a function of pH (Figure 7b) reveal that the higher intensities were observed in the pH range of 5 to 7. In basic (pH > 7) or highly acidic (pH ˂ 5) solutions, the peak currents were very low. This result is probably due to strong electrostatic repulsions observed between CR and the material. Indeed, FeOP and CR are both positively charged in highly acidic medium, resulting from the protonation of the hydroxyl groups present at the surface of FeOP and the protonation of the amino groups and neutralisation of sulphonates of CR [58]. Also, FeOP and CR are both negatively charged in basic medium due to the deprotonation of the functional groups. The highest peak current was observed at pH 6 which was thus taken as the optimal pH of the electrolytic solution. Similar results were recently obtained by Farias et al. during their work on the adsorption of CR onto aminofunctionalised silica gel [58].

Calibration curve
After optimisation of experimental parameters, the influence of the concentration of CR on the peak current was investigated. Results obtained in the concentration range of 5 to 55 µmol.L −1 in deionised water containing phosphate buffer 0.1 mol.L −1 at pH 6 are depicted in Figure 7c. When the concentration of CR increase in solution, the peak current intensities also increase due to oxidation of progressively high amounts of CR at the electrode surface. The calibration curve plotted (Figure 7d) shows excellent linearity [R 2 > 0.99]. This result indicates that in the concentration range used, the current intensity is proportional to the concentration of CR. The detection limit obtained for a signal/ noise ratio of 3 is 2.8 × 10 −7 mol.L −1 , with a sensitivity of 0.021 µA.µM −1 and a background current of 0.0020 µA. The detection limit obtained here is in the same range of that obtained by 26. The proposed electrochemical sensor can be applied for the detection of CR in environmental polluted media.

Repeatability of the sensor and analytical application in spring water sample
Before applying an electrochemical sensor for measurements on real samples, it is important to investigate its repeatability. In order to evaluate the repeatability of the modified electrode (CPE/FeOP), eight consecutive signals were recorded for 50 µM CR in 0.1 mol.L −1 phosphate buffer solution at pH 6.0. The signals obtained showed an average current of 1.26 ± 0.06 µA, corresponding to a repeatability of 95.3%. This result shows that the electrochemical sensor can be used for the determination of CR in environmental samples. The performance of the electrochemical sensor in a real milieu was evaluated using spring water. For this to be achieved, differential pulse voltammograms were recorded in spring water containing phosphate buffer 0.1 mol.L −1 at pH 6 and CR at various concentrations ranging from 5 to 40 µM. The results obtained (not shown), revealed that the peak current increases with the concentration of CR as observed previously in deionised water. The calibration curve plotted (Figure 7d) shows a linear relationship between the peak current and CR concentration (R 2 > 0.98). The detection limit calculated on the basis of a signal/noise ratio of 3 was 8.2 × 10 −7 mol.L −1 , with a sensitivity of 0.015 µA.µM −1 and a background current of 0.0041 µA. The detection limit obtained is only 4 times higher than that obtained in deionised water but it remained in the same range (10 −7 M). This is the proof that even with the presence of interfering species in spring water, the electrochemical sensor elaborated in this work still displays good performances. This electrochemical sensor may thus be used for the quantification of pollutants in environmental samples.

Conclusion
This study was focused on the electroanalysis of CR, using for the first time, a carbon paste electrode modified by iron oxide-orange peels nanomaterials (FeOP) manufactured from low-cost agricultural waste. FeOP was successfully synthesised from orange peel powder mixed with iron sulphate. FTIR, SEM, EDX, DRX and TG/DTA characterisations techniques were applied to confirm the presence of iron oxide highly dispersed on OP. The carbon paste electrode modified with FeOP (CPE/FeOP) showed an improvement in the electrosorption capacity and revealed higher specific capacitance compared to the unmodified carbon paste electrode (CPE). This new type of modified CPE was successfully applied for the electroanalysis of Congo red. The detection limits of 2.8 × 10 −7 mol.L −1 and 8.2 × 10 −7 mol.L −1 were achieved in deionised water and spring water, respectively. The orange peels can be used as a low-cost material in the fabrication of electrochemical sensors for the detection of complex pollutants in wastewater.