Developing an electrochemical sensor based on a carbon paste electrode modified with nano-composite of reduced graphene oxide and CuFe2O4 nanoparticles for determination of hydrogen peroxide

Materials Science and Engineering C 75 (2017) 1435–1447

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Developing an electrochemical sensor based on a carbon paste electrode modified with nano-composite of reduced graphene oxide and CuFe2O4 nanoparticles for determination of hydrogen peroxide Ali Benvidi ⁎, Mohammad Taghi Nafar, Shahriar Jahanbani, Marzieh Dehghan Tezerjani, Masoud Rezaeinasab, Sudabeh Dalirnasab Department of Chemistry, Faculty of Science, Yazd University, Yazd, Iran

a r t i c l e

i n f o

Article history: Received 14 August 2016 Received in revised form 16 February 2017 Accepted 9 March 2017 Available online 10 March 2017 Keywords: Hydrogen peroxide Voltammetric sensor Chronoamperometry CuFe2O4 High conductive platform

a b s t r a c t In this paper, a highly sensitive voltammetric sensor based on a carbon paste electrode with CuFe2O4 nanoparticle (RGO/CuFe2O4/CPE) was designed for determination of hydrogen peroxide (H2O2). The electrocatalytic reduction of H2O2 was examined using various techniques such as cyclic voltammetry (CV), chronoamperometry, amperometry and differential pulse voltammetry (DPV). CuFe2O4 nanoparticles were synthesized by co-precipitation method and characterized with scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) techniques. Then, a high conductive platform based on a carbon paste electrode modified with RGO and CuFe2O4 nanoparticles was prepared as a suitable platform for determination of hydrogen peroxide. Under the optimum conditions (pH 5), the modified electrode indicated a fast amperometric response of b 2 s, good linear range of 2 to 200 μM, low detection limit of 0.52 μM for determination of hydrogen peroxide. Also, the peak current of differential pulse voltammetry (DPV) of hydrogen peroxide is increased linearly with its concentration in the ranges of 2 to 10 μM and 10 to 1000 μM. The obtained detection limit for hydrogen peroxide was evaluated to be 0.064 μM by DPV. The designed sensor was successfully applied for the assay of hydrogen peroxide in biological and pharmaceutical samples such as milk, green tea, and hair dye cream and mouthwash solution. © 2017 Published by Elsevier B.V.

1. Introduction As known, the fast and easy determination of H2O2 has paid more attention due to its important role. Recently a lot of researches have been performed to measure H2O2 concentration which is existed in the samples. Determination of H2O2 is important in various fields such as environmental and clinical fields, the food, chemical and pharmaceutical industries, and the biological and medical sciences [1–8]. Also, it exists in some commercial products like cosmetics and pharmaceutical applications [9,10] and its determination is important as a production of enzymatic reactions in the field of bio sensing [11–17]. So, hydrogen peroxide as an essential mediator is used for biochemical analyses of cellular pathology and the sensitive and accurate detection of H2O2 has a practical importance [18]. To the best of our knowledge, it has been developed various approaches for H2O2 determination such as titrimetry [19], spectrophotometry [20], fluorimetry [21], chemiluminescence [22], chromatography [23] and electrochemical methods [24]. Among the ⁎ Corresponding author. E-mail address: [email protected] (A. Benvidi).

http://dx.doi.org/10.1016/j.msec.2017.03.062 0928-4931/© 2017 Published by Elsevier B.V.

mentioned techniques, electrochemical detection of H2O2 has some advantages such as high sensitivity, simplicity, selectivity, relatively low cost, and fast response. In the electrochemistry field, the modified electrodes have been widely used for sensitive and selective determination of the environmental, clinical, and biotechnical targets [25–30]. Carbonbased conductive substrates like graphite, glassy carbon, carbon nanotube, and graphene have been used for modification of electrodes due to their good conductivity and flexible modification. Modification of nanostructure with carbon nanomaterials enhances their charge separation efficiency [31]. Carbon nanotubes (CNTs) assemblies have a potential to become a conductive material of the future for electrical and electronic devices. These nanostructures have high surface area-to-volume ratio, and excellent chemical, thermal, mechanical and optical properties [32–35]. Graphene is a two dimensional, atomistically thick planar sheet built on sp2 hybridized carbon atoms with superior properties; high thermal and electrical conductivity, and a high modulus. It is regarded as the fundamental building block for all sp2 carbon allotropes including graphite in which large numbers of single layer graphene sheets are stacked together by weak van der Waals forces [36–38]. Graphene is able to accommodate ionic charges readily from electrostatic interactions, and can act as a current collector for rapid electron

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transport from faradaic charge transfer reactions of anchored pseudocapacitive polymer material. This dual action helps enhance the overall electrochemical performance of a hybrid composite [39]. Graphene can be peeled mechanically from graphite; however, this method is not suitable for large-scale production of graphene due to its low productivity. Therefore, graphenes are prepared normally by chemical reduction of graphite oxide (GO) dispersed in a solvent because GO can be easily exfoliated into single- or few-layer GO in a solvent. Graphenes can also be produced effectively in bulk by rapid heating of GO powders because CO2 gas is generated through thermal decomposition of the oxygencontaining groups of GO [40]. Carbon nanostructure base is a versatile coating material due to its advantages such as highly stable even under extremely chemical, and physical environments, excellent electrical conductivity, numerous areas, high thermal stability, catalysis and possessing various hydrophilic groups such as hydroxylic and carboxylic groups to enhance the dispensability of hybrid NPs in water, emerging on the surface of carbon [41–44]. Carbon paste is a convenient substrate for making composite electrode by simply mixing all components (including reactive materials) together [45]. During the past several decades, electrochemical reactions catalyzed by transition metal complexes have paid more attention and metal complexes due to their excellent electrocatalytic properties are used more [46,47]. Up to now, lots of researches for modified non-enzymatic sensors with various metal nanoparticles such as Pt NPs [48], Au NPs [49], Ag NPs [50] and Pd NPs [51] have been reported. Nanostructured CuFe2O4 has been used for H2O2 sensors due to its large specific surface area, satisfactory electrochemical activity and small overpotential for electron-transfer reactions, but the applications of previous H2O2 sensors based on copper nanocomposites were limited by cumbersome fabrication and expensive equipment [52,53]. Additionally, the CuNPs reveal excellent electrocatalytic ability to the reduction of hydrogen peroxide (H2O2) according to their high electrical conductivity, high specific surface area, low water solubility at the electrode surface and also good corrosion resistance under oxidizing conditions [54]. In the present work, the combination of reduced graphene oxide (RGO) and CuFe2O4 was used in the fabrication of RGO/CuFe2O4/CPE sensor for detection of H2O2. Modification of electrode with RGO can lead to increasing the surface area of electrode and its catalytic properties [55]. Thus, the nano composite of RGO and CuFe2O4 was used for determination of hydrogen peroxide due to its unique properties. Some novelties of this research are: 1- Increasing the effective surface of electrode surface by using nano composite of CuFe2O4 and reduced graphene oxide, 2- The trace determination of H2O2 (the detection

limit of 0.064 μM and linear ranges of 2–10, 10–1000 μM) using two methods of amperometry and differential pulse voltammetry and 3Possessing a god potential of the fabricated sensor for determination of H2O2 in various real samples of green tea, milk, shaving cream, hair dye cream and mouthwash solution The obtained observations indicate that this designed sensor has many advantages including good stability, good repeatability, excellent reproducibility, high surface charge transfer rate constant and technical simplicity in the electrocatalytic detection of H2O2. To evaluate the analytical applicability of the fabricated sensor, it was used for the voltammetric determination of H2O2 in biological and pharmaceutical samples. 2. Experimental 2.1. Apparatus and chemicals In this research, the electrochemical measurements were performed with a potentiostat/galvanostat (Autolab PGSTAT101) coupled with a personal computer. The fabricated modified electrode (RGO/CuFe2O4/ CPE) was applied as a working electrode. An Ag/AgCl (KCl, sat.) electrode and a platinum electrode were used as the reference and auxiliary electrodes, respectively. All the potentials in this research were reported with respect to the reference electrode by using a Metrohm model 691 pH/mV meter. Phosphate buffer solutions (0.1 M) were prepared from 0.1 M H3PO4, while pH was regulated with 0.1 M HCl or NaOH solutions. H2O2 (30%), graphite fine powder, and viscous paraffin were obtained from Merck Company and used as received. All the other chemicals also purchased from Merck Company in an analytical grade and were used without any further purification. Doubly distilled water was used in the experiments. The solutions were prepared just prior to use, and all the experiments were carried out at the ambient temperature of the laboratory (about 25 °C). 2.2. Preparation of the CuFe2O4 nanoparticles Nano-CuFe2O4 was prepared with a modified method according to the literature [56]. At first, 1.51 g Cu(NO3)2·3H2O and 5.04 g Fe(NO3)3·9H2O were dissolved in 100 mL distilled water, then it was added to 50 mL of 4 mol L−1 NaOH solution which was followed by heating at 90 °C for 2 h. The obtained product was centrifuged, washed by water and dried at 80 °C overnight. Finally, the powder was calcined at 800 °C for 2 h. The prepared CuFe2O4 nanoparticles were characterized with scanning electron microscopy (SEM), Transmission electron

Scheme 1. Schematic representation of the modified electrochemical biosensor based on RGO/CuFe2O4/CPE platform.

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microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) techniques. 2.3. The synthesis process of reduced graphene oxide The reduced graphene oxide nano sheets were synthesized according to the procedure offered in the literature [57]. Briefly, a mixture of graphite/KMnO4 (3:18 g) and a mixture of H2SO4/H3PO4 (360:40 mL) were prepared at 50 °C. Two mixtures were added by shaking and then stirring for 12 h. The reaction product cooled down to 25 °C and was then transferred into ice bath containing 6 mL of H2O2 (30%). The obtained solution was centrifuged and then filtered, and the obtained precipitate was washed with water, HCl (30%), and finally washed twice with 200 mL of ethanol. After sonication for 3 h, a colloidal suspension of graphene oxide nano sheets were obtained in purified

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water (150 mg/50 mL). Graphene oxide nano sheets (GO) were reduced chemically using hydrazine and ammonia solutions to obtain nano sheets of reduced graphene oxide (RGO). To synthesize RGO, 50 μL of hydrazine solution (98%) with 200 μL of ammonia solution (30% in water) were added to the graphene oxide suspension. The obtained solution was refluxed at 90 °C for 12 h and cooled down to room temperature. After that, this solution was centrifuged, and the precipitated material (RGO) was washed with water and then it was dried at 60 °C in vacuum for 24 h. 2.4. Preparation of the designed sensor To obtain the best conditions in the fabrication of RGO/CuFe2O4/CPE sensor, the ratio of CuFe2O4, and RGO was optimized, and consequently the maximum peak current intensity of analyte (H2O2) was observed in

Fig. 1. SEM images of (A) CPE (B) RGO/CPE, (C) CuFe2O4/CPE (D) RGO/CuFe2O4/CPE, TEM images of (E) CuFe2O4 and (F) CuFe2O4/RGO.

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Fig. 2. XRD patterns of (A) CuFe2O4/CPE and RGO/CuFe2O4/CPE and (B) FTIR spectra of GO, CuFe2O4/CPE and RGO/CuFe2O4/CPE composite.

the optimum condition. To prepare RGO/CuFe2O4/CPE sensor, a mixture of 0.455 g of graphite powder, 0.025 g of CuFe2O4, 0.02 g of RGO and ~0.4 mL of paraffin oil was blended by hand, mixed in a mortar and pestle and was then inserted in the bottom of a glass tube (internal radius: 2 mm and 10 cm long). Electrical contact was performed by pushing a conductive copper wire into the end of the glass tube to stick to the carbon paste. When it is necessary to have a fresh electrode surface, the new surface was generated by extruding a small plug of the paste with a stainless steel rod rapidly and smoothing the resulting surface on the white paper. The fabrication processes of RGO/CuFe2O4/CPE sensor are indicated in Scheme 1.

2.5. Preparation of real sample Milk, green tea, hair dye cream and mouthwash solution were separately transferred to a flask and diluted to volume of 100.0 mL with twice distilled water. 0.3 mL portion of each solution was diluted in a voltammetric cell to 10.0 mL of a 0.1 mol L−1 phosphate buffer solution (pH 5.0), and the differential pulse voltammograms were recorded.

3. Results and discussion 3.1. Characterization and electrochemical properties of RGO/CuFe2O4/CPE In this experiment, the morphology of bare CPE, RGO/CPE, CuFe2O4/ CPE and RGO/CuFe2O4/CPE was studied by SEM (Fig. 1). As shown in Fig. 1A, there is no spherical and amorphous structure in the SEM image of CPE. Fig. 1B reveals the SEM image of RGO/CPE indicating petal-like graphene nano sheets with sharp edges, and random directions which makes the nest-like structure with a large surface area. Fig. 1C indicates the SEM image of CuFe2O4/CPE as shown in this figure, Cu nanoparticles have spherical shape and modification of electrode surface with these nanoparticles increases the electrode surface. Fig. 1D exhibits the SEM image of RGO/CuFe2O4/CPE and due to this figure, nest-like shapes and spherical shape reveal that the modification of electrode surface has been done well and leads to providing a large surface area. The CuFe2O4 and CuFe2O4/RGO nanocomposite was prepared and their shape, average diameter and crystal structure were observed by TEM. The TEM images of CuFe2O4 clearly show the presence of well dispersed homogenous particles, with an average size of 47 nm (Fig. 1E).

Fig. 3. (A) The UV–vis spectra of GO and RGO confirming the restoration of electronic conjugation after reduction. (B) The Raman spectra of GO, RGO and RGO/CuFe2O4 samples.

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The TEM photo in Fig. 1F shows that RGO sheets are smooth and neartransparent. RGO sheets were decorated with a large quantity of CuFe2O4 NPs and the edges of RGO are clearly seen. The NPs were not observed outside the RGO sheets indicating very good interactions between NPs and RGO sheets CuFe2O4 nanoparticles with a shape of cuboid are homogeneously dispersed on the RGO sheets [58–62]. To further characterize CuFe2O4/CPE and CuFe2O4/RGO/CPE nanocomposites, XRD pattern was determined as shown in Fig. 2A. The XRD diffraction patterns of the as-prepared CuFe2O4, and CuFe2O4/ RGO composite are shown in Fig. 2A. For the CuFe2O4/RGO/CPE composite, all the diffraction peaks can be indexed as spinel CuFe2O4 while no typical diffraction peak of RGO is observable. It is speculated that the RGO in the CuFe2O4/RGO/CPE composite was exfoliated due to the crystal growth of CuFe2O4 NPs between the interlayer of RGO sheets. The diffraction peaks at 2θ = 30.8, 35.66, 38.36, 42.7, 52.48, 57.36, 64.41, 73.1, and 74.5° can be indexed to the (220), (311), (320), (400), (422), (511), (440), (533) and (442) planes of cubic CuFe2O4 (JCPDS 25-0283). The indices were consistent with those determined from the XRD measurements, and were assigned to cubic structure of CuFe2O4. The obtained results are in good agreement with the literature [58,62–64]. The vibration of atoms at the surface of the sample and functional groups of the CuFe2O4, GO and RGO, has been analyzed by the FTIR measurement. FTIR spectra of CuFe2O4/CPE and the RGO/CuFe2O4/CPE composite are shown in Fig. 2B. The peak at 1620 cm−1 can be assigned to the aromatic skeletal C_C stretching vibration of the unoxidized

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graphitic domains. In the spectrum of GO at 3415 cm−1 are attributed to the stretching vibrations of O\\H. The bands at 1055, 1220 and 1716 cm−1 correspond to the C\\O stretching vibration, the C\\O\\H deformation vibration, and the C_O stretching vibration, respectively. Compared with that of GO, new prominent absorption bands at about 570 and 478 cm−1 appeared in the FTIR spectrum of the CuFe2O4/CPE and CuFe2O4/RGO/CPE composite, which can be assigned to CuFe2O4 NPs. Compare to the spectrum of CuFe2O4 NPs, the CuFe2O4/RGO composite showed weaker peaks which can be due to the nucleation and growth of CuFe2O4 NPs in to the layered RGO sheets. After the chemical reaction, the GO peaks became disappear in CuFe2O4/RGO nanocomposite to imply that there is reduced GO in the CuFe2O4/RGO composite. The peaks centered at 2856 and 2923 cm−1 in the GO spectrum also remain prominent in RGO, CuFe2O4/CPE and CuFe2O4/RGO/CPE can be assigned to\\CH2 stretching vibrations of grapheme and graphite sheets. As expected, the FTIR spectrum of GO is in good agreement with those from previous works [58,65]. Further the characterization of GO and RGO was done by the UV–vis spectroscopy. The ultraviolet–visible (UV–vis) spectra of dispersed GO and RGO in double ionized water were shown in Fig. 3A. As this figure shows the π → π* transition peak for C_C of GO is detected at ∼230 nm. The appearance of an additional peak at 304 nm is ascribed to the n → π* transition of C_O, confirming the presence of oxygen containing functional groups in GO. In contrast, both of these peaks are disappeared in the RGO and a sharp peak appears at ∼295 nm indicating the restoration of graphitic structure after reduction depicts a strong

Fig. 4. Cyclic voltammograms of RGO/CuFe2O4/CPE sensor in a 0.1 M phosphate buffer solution (pH = 5.0), at various scan rates: the numbers 1–17 correspond to 10 mV s−1 to 450 mV s−1 scan rates, respectively. Insets: (A) Variations of Ip versus scan rates (B) Variation of Ep versus the logarithm of the scan rate. (C) Magnification of the same plot for high scan rates.

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sharp peak at around 295 nm suggests almost removal of oxygen containing functional groups and establishment of C_C conjugated graphene structure [66,67]. Raman spectroscopy is a powerful technique in characterization of structural properties of carbon based materials. Fig. 3B represents the Raman spectra of GO, RGO and CuFe2O4/RGO. As shown in Raman spectrum, GO displays two prominent peaks at 1366 cm−1 and 1591 cm−1 corresponding to D and G bands of carbon respectively. It is well known that D band can be assigned to disordered structure or defects in graphene sheets while G band is associated with E2g mode observed for sp2 carbon domains. As this figure shows with reduction of GO the ratio of ID/IG increases from 0.88 to 1.12 because of enrichment of conjugated sp2 carbon. The small shifting was observed in D and G bands of RGO and CuFe2O4/RGO indicating increase in disorderness and removing functional oxygen groups from graphene oxide sheets. The calculated intensity ratio of D and G bands for CuFe2O4/RGO (ID/IG = 1.17) is slightly higher than RGO (ID/IG = 1.12) which suggests that conjugated graphene network (sp2) is re-established after loading of CuFe2O4 NPs on RGO sheets resulting increased ID/IG value for nanocomposites. [68,69]. 3.2. Electrochemical behavior of RGO/CuFe2O4/CPE RGO/CuFe2O4/CPE was constructed and its electrochemical properties investigated in an aqueous solution (pH = 5.0). As known, one of the main advantages of CuFe2O4 is its insolubility in aqueous media, so it can be attached to carbon paste without leaching from its surface. Fig. 4 shows the obtained cyclic voltammograms of RGO/CuFe2O4/CPE sensor in a 0.1 M phosphate buffer solution (pH = 5.0). According to inset A of this figure, a reproducible, well-defined anodic and cathodic peaks (with Epa = − 20 mV, Epc = − 210 mV at scan rate of 10 mV s− 1) for CuFe2O4 as a modifier was obtained. The obtained peak separation potential (ΔEp = (Epa − Epc) is − 180 mV) which is more than excepted value for a reversible system (ΔEp ~ 59/n, n = 1). Thus, due to these observations the redox couple of CuFe2O4 as a modifier in the structure of RGO/CuFe2O4/CPE has a quasi-reversible behavior in an aqueous medium. According to sharp equation [70], the intensity of peak current is related to the surface concentration of the electroactive species (Γ). Ip ¼ n2 F 2 AΓν=4RT

slope of 2.303RT/(1 − αa)nαF and 2.303RT/αcnαF for the anodic peaks and cathodic peaks respectively, the average value of electron transfer coefficient (α) can be calculated (αc = 0.47). Based on Laviron equation (Eq. 2), the apparent electron transfer rate constant (ks) between the modifier (CuFe2O4) and the electrode surface can be calculated. logks ¼ α logð1−α Þ þ ð1−α Þ logα− log ðRT=nα FνÞ−α ð1−α Þnα FΔEp =2:3RT

ð2Þ

The value of ks was estimated to be 1.59 s−1 using Eq. (2). 3.3. Electrocatalytic reduction of H2O2 at RGO/CuFe2O4/CPE sensor The reduction of H2O2 at the various electrodes was investigated with or without H2O2 (2 μM). Fig. 5 exhibits the obtained cyclic voltammograms of CPE electrode without H2O2 (curve a), CPE with H2O2 (curve b), RGO/CPE with H2O2 (curve c), CuFe2O4/CPE without H2O2 (curve d), CuFe2O4/CPE with H2O2 (curve e), and RGO/CuFe2O4/CPE with H2O2 (curve f) in a 0.1 mol L−1 phosphate buffer solution (pH = 5.0) at the scan rate of 20 mV s−1. As shown in this figure, the unmodified CPE does not show scarcely reduction of H2O2. However, the designed sensor (RGO/CuFe2O4/CPE) shows excellent electrocatalytic behavior for reduction of H2O2 at the potential of −0.226 V. The RGO/ CuFe2O4/CPE sensor indicates a highly enhanced cathodic peak for reduction of H2O2 at less overpotential of − 0.344 V (onset potential at − 0.57 V). According to these observations, it is inferred that RGO/ CuFe2O4/CPE sensor displays a significantly improved electrocatalytic ability compared with other electrodes (CPE, CuFe2O4/CPE) in the presence of analyte. The excellent synergy between RGO and CuFe2O4 facilitated the electrocatalytic ability of the composite. RGO act as support for CuFe2O4 and also the existence of numerous edge planes like in the nano sheets of RGO leads to providing additional catalytic sites to access

ð1Þ

where n reveals the number of electrons involved in the reaction (n = 1), A is the geometric surface area (0.096 cm2) of the designed electrode, Γ (mol cm−2) is the surface coverage, and the other symbols (F, R, T) have their usual meanings. From the slope of the anodic peak currents versus the scan rate (inset B), the surface coverage of RGO/CuFe2O4 at the surface of designed sensor (RGO/CuFe2O4/CPE) is calculated to be 3.76 × 10−9 mol cm−2. Also inset B of Fig. 4 reveals the effect of potential of various scan rates on the electrochemical properties of the fabricated sensor (RGO/ CuFe2O4/CPE) in a 0.1 M phosphate buffer solution. The plots of the anodic and cathodic peak currents (Ip) (which are indicating in inset A) have a linearly dependent on the scan rate (ʋ) from 10 to 450 mV s−1. The obtained results indicated that the immobilization of modifier (CuFe2O4) at the carbon paste electrode was performed well and the nature of the redox process is controlled in diffusion less manner [71]. Some kinetic parameters such as apparent charge transfer rate constant (ks) and the charge transfer coefficient (α) of CuFe2O4 at the electrode surface can be calculated due to the variation of anodic and cathodic peak potentials versus logarithm of the scan rate and using Laviron method [72], (see inset C). The inset C of Fig. 4 indicates a variation of Ep versus the logarithm of scan rate which indicates that in the potentials of higher than − 90 mV the separation of anodic and cathodic peak currents are increased due to the limitation of charge transfer kinetic. So, respect to the linear segments of E vs. Log ν (inset C) and the

Fig. 5. The CV results indicated the responses at the electrodes: CPE without H2O2 (curve a), CPE with 2.0 μM H2O2 (curve b), RGO/CPE with 2.0 μM H2O2 (curve c), CuFe2O4/CPE without H2O2 (curve d), CuFe2O4/CPE with 2.0 μM H2O2 (curve e), and RGO/CuFe2O4/ CPE with 2.0 μM H2O2 (curve f) in a 0.1 mol L−1 phosphate buffer solution (pH 5.0) at the scan rate of 20 mV s−1.

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H2O2. The possible electrocatalytic reduction process of H2O2 at the modified electrode surface can be described by the EC′ mechanism.

surface. The charge transfer coefficient (α) of the electrode process can be calculated from the slope of the cathodic Tafel plot and using Eq. (5) [70] if nα = 1.

Fe2 O4 CuðIIÞ  CPE þ e− ⟶Fe2 O4 CuðIÞ  CPE

ð3Þ

Cathodic Tafel slope ¼ −αnαF=2:3RT

2Fe2 O4 CuðIÞ  CPE þ H2 O2 þ 2Hþ ⟶2Fe2 O4 CuðIIÞ  CPE þ 2H2 O

ð4Þ

According to the obtained results and from the slopes of the Tafel plot in Fig. 6D, the cathodic charge transfer coefficient, α, was evaluated to be 0.52.

As shown in Fig. 6, by increasing potential scan rates, the peak potentials corresponding to the electroreduction of H2O2 are shifted to more negative potentials, which suggest the kinetic limitation in the reaction between the redox active sites of the modified electrode (RGO/CuFe2O4/ CPE) and H2O2. However, the catalytic peak current is increased linearly with the square root of the potential scan rate (see inset A of Fig. 6), and this fact suggests that the reaction is diffusion limited at a sufficient overpotential. Also, the plot of the scan rate normalized current (Ipν−1/2) versus the potential scan rate exhibits a shape typical of an EC′ catalytic process (see inset B of Fig. 6). To obtain information which are related to the rate determining step, the Tafel plot was drawn using the data from the rising part of the cyclic voltammograms (the Tafel region) recorded at different potential scan rates (see Fig. 6D). This part of the voltammogram is affected by the electron transfer kinetic between analyte (H2O2) and RGO/ CuFe2O4/CPE. The obtained results can be used to calculate the kinetic parameters of H2O2 electrocatalytic reduction at the modified electrode

ð5Þ

3.4. Optimization of experimental parameters To increase the sensitivity of electrochemical sensor (RGO/CuFe2O4/ CPE), some experimental parameters such as the percentages of CuFe2O4, RGO present in CPE and the pH of solution were optimized. The influence of CuFe2O4 present in the CPE structure on the oxidation peak current of H2O2 was studied in the range of 0.5 to 7% w/w at the scan rate of 20 mV s−1 was examined. The observations indicate that, by increasing the percentages of CuFe2O4 in the structure of CPE, up to 5.0% w/w the peak currents are increased, then by increasing the percentages of CuFe2O4 the peak current values are constant, therefore, 5.0% w/w of CuFe2O4 was selected as the optimal percentage of CuFe2O4 in the structure of CPE (see Fig. 7A). The second parameter for optimization was the percentage of RGO nano-material which is present in the CPE structure. According to Fig. 7B, the optimum percentage of RGO nanoparticles in the carbon

Fig. 6. Cyclic voltammograms of RGO/CuFe2O4/CPE in a 0.1 M phosphate buffer solution (pH 5.0) containing 0.4 μM H2O2 at different scan rates. The curves from down to up correspond to 10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, and 80.0 mV s−1 scan rates, respectively. Insets: (A) Variation of electrocatalytic peak currents vs. ʋ1/2, (B) Variation of scan rate-normalized peak current (Ip/ʋ1/2) versus scan rate (ʋ). (C) The selected region for plotting Tafel diagram in scan rate of 10 mV s−1 for electrocatalytic oxidation of H2O2 at RGO/CuFe2O4/CPE. (D) Tafel plot derived from the rising part of the current potential curve recorded at the scan rate of 10 mV s−1.

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Fig. 7. Optimization of operating conditions: (A) the percentages of CuFe2O4 in CPE, (B) percentages of RGO nanoparticles in CuFe2O4/CPE suspension, (C) E vs. pH, (D) Variation of anodic peak current of DHPMB vs. pH in the presence of H2O2 in a phosphate buffer solution and scan rate 20 mV s−1, the repetitive number of experiments is five (n = 5).

paste was selected to be 4.0% w/w. The obtained results show that by increasing the percentage of RGO nanoparticles in the carbon paste up to 4.0% w/w, the peak current values are increased and after that by increasing the percentages of RGO the peak current values are constant. This phenomenon can be related to the unique properties of RGO (increasing the surface area and conductivity). The effect of solution pH on the obtained CVs of the RGO/CuFe2O4/ CPE was examined at the different pH. The slope of a typical plot of E1/2 vs. pH was a Nernstian value of − 52 mV/decade indicating that the redox reaction involving Cu2 +/Cu+ species occurs with 1H+/1e− participation. The strong pH-dependent property of the RGO/CuFe2O4/ CPE reveals the involvement of proton accessible species (Fig. 7C). The observations indicated that the stability of the designed sensor was pH dependent, and there was higher stability at pH = 5.0 comparing to other pH values. Therefore, the electrochemical behavior and electrocatalytic activity of the modified electrode were studied in a phosphate buffer solution (0.1 mol L−1, pH = 5.0). The electrochemical behavior of RGO/CuFe2O4/CPE was studied by using cyclic voltammetry method in a phosphate buffer solution (0.1 mol L−1, pH = 2.0–11.0) at the potential scan rate of 20 mV s−1 (Fig. 7D).

3.5. Chronoamperometric investigation The electrocatalytic reduction of H2O2 by RGO/CuFe2O4/CPE sensor was also studied by chronoamperometry. Fig. 8 shows the obtained chronoamperograms at the potential step of −350 mV.

Inset A of this figure reveals the experimental plot of I versus t−1/2 with the best fits for different concentrations of H2O2. The inset B of Fig. 8 indicates the drawn plot from the slopes of the resulting straight lines versus the H2O2 concentration. Using the mentioned slopes the Cottrell equation [70] a diffusion coefficient was calculated to be 8.87 × 10−6 cm2 s−1. I ¼ nFAD1=2 Cbπ−1 =2t −1=2 While in this equation, I is the current controlled by the diffusion of H2O2 from the bulk solution to the electrode/solution interface. The obtained slope in Fig. 8B was used for calculation of diffusion coefficient (D) of analyte (H2O2) and the value of D was evaluated to be 8.87 × 10− 6 cm2 s−1 under the working conditions. This value is in good agreement with the reported values by other researches [73,4]. Also, chronoamperometry was also used for determination of the catalytic rate constant (kcat). At intermediate times, when the current is dominated by the rate of the electrocatalyzed reduction of H2O2: 1=2

Icat =IL ¼ ðkcat πCtÞ

where IL is the current of phosphate buffer solution in the absence of H2O2 and Icat is the catalytic current due to the addition of H2O2 (2 μM). After a limited time, the values of Icat/IL were linearly dependent on t1/2, and from its slope kcat can be calculated. The obtained values in this work are 1.32 × 103 M−1 s−1 which are in good agreement with data found in the literature [74]. This obtained value for kcat explains

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Fig. 8. The obtained chronoamperograms at RGO/CuFe2O4/CPE in a 0.1 M phosphate buffer solution (pH 5.0) for different concentration of H2O2 by setting potential step at −350 mV. The numbers 1–6 are correspond to 0.0, 2, 4, 6, 10, and 14 μM of H2O2. Insets: (A) Plots of I vs. t−1/2/s−1/2 obtained from chronoamperograms 2–6 and (B) plot of the slope of the straight lines against the H2O2 concentration. (C) Dependence of Icat/Il on t1/2/s1/2 derived from the data of chronoamperograms.

the sharp feature of the catalytic peak observed for the catalytic oxidation of H2O2 at the surface of RGO/CuFe2O4/CPE sensor, too.

sensor responds rapidly to the changes of H2O2 concentration and reaches the steady-state current within 7 s.

3.6. Amperometric response of H2O2 3.7. Differential pulse voltammetry investigation For the measurement of H2O2, the reduction currents of H2O2 were recorded with applied potential of − 0.15 V and plotted versus H2O2 concentration (Fig. 9). As shown in Fig. 9A, a well-defined response was observed during the successive additions of 2 to 200 μM, of H2O2. According to calibration curve with an equation of I(μA) = −0.8238 C (μA) − 22.812 and a regression equation of 0.9957, a linear range and a detection limit of 2 to 200 μmol L−1 and 5.2 × 10−7 M for H2O2 determination were obtained, respectively. Some obtained analytical characteristics in this research (linear range, detection limit and pH) are compared with several similar literatures in Table 1. From Table 1, it is clear that the detection limit, of RGO/CuFe2O4/CPE sensor is comparable and even better than those obtained by other modified electrodes [75– 83]. These results reveal a stable and efficient catalytic property of RGO/ CuFe2O4/CPE sensor. Also, it can be observed that the RGO/CuFe2O4/CPE

The modified electrode (RGO/CuFe2O4/CPE sensor) was applied for determination of H2O2 by using differential pulse voltammetry as a very sensitive and selective method with a sub-micromolar detection limit. Fig. 10 shows the obtained DPVs for the oxidation of different concentrations of H2O2 in a 0.1 M of phosphate buffer solution (pH = 5.0). Inset A of Fig. 10 indicates the dependence of the peak current on the H2O2 concentration which contains two linear range (2 to 10 μM and 10 to 1000 μM) with a detection limit of 6.4 × 10−8 M for determination of H2O2 (based on S/N = 3). Insets B and C of Fig. 10 reveal two different calibration segments individually (2 to 10 μM and 10 to 1000 μM, respectively). It is clear that the detection limit, of RGO/CuFe2O4/CPE sensor is comparable and even better than those obtained by other modified electrodes (Table 1).

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not affected by a 100-fold increase in the concentration of above compounds for H2O2 determination. The investigation of ascorbic acid showed that the peak current of H2O2 was not affected by a 5-fold increase in its concentration for H2O2 determination. 3.9. Repeatability, reproducibility and stability of the designed sensor The repeatability of the modified electrode was studied by recording the differential pulse voltammograms of H2O2 for 10 measurements with the concentration of 2.0 μM. The relative standard deviations (RSD) of the anodic currents were calculated to be 2.8% which indicates the suitable repeatability of RGO/CuFe2O4/CPE sensor for determination of H2O2. The reproducibility of the constructed sensor was checked by preparing the five modified electrode, separately. The DPV responses for each of these electrodes in a solution of H2O2 (2.0 μM) were recorded. The calculated RSD for peak currents is about 3.5% which indicates that reproducibility of RGO/CuFe2O4/CPE sensor is suitable. The last measured parameter of the designed sensor was its stability. To obtain this aim, RGO/CuFe2O4/CPE sensor was stored at room temperature and after various periods (5 days, 1 week and 2 weeks), the reduction DPV currents for H2O2 were obtained 99.1, 97.2 and 95.8% of the initial responses, respectively. The obtained results indicate the high stability of the suggested sensor. Fig. 9. Amperometric response at −0.350 V by increasing H2O2 concentration and using RGO/CuFe2O4/CPE sensor. Inset: Relation between the amperometric response and H2O2 concentration.

3.8. Interference study As known, one of the most important problems in the application of sensors is their responses to interfering species existing in real samples. As known, the tolerance limit was explained as the maximum concentration of foreign substances that makes an error of less than ±5% for the determination of 10.0 μmol L−1 of analyte (H2O2). To study the effect of different potential interferences (the selectivity of RGO/ CuFe2O4/CPE sensor), the effect of some materials were examined. Under the optimum conditions, the interference effect of glucose, fructose, uric acid, dopamine, sodium chloride, sodium sulfate, sodium sulfite, sodium nitrite, sodium hydrogen carbonate, stearic acid and ammonium acid carbonate in the presence of 10.0 μmol L−1 H2O2 was investigated. The results showed that the peak current of H2O2 was

Table 1 Comparison of some used electrochemical procedures for determination of H2O2 using RGO/CuFe2O4/CPE sensor. Electrode

Method Linear range (μM)

Detection limit (μM)

Ref.

[PFeW11O39]4− polyoxoanion Nanonickel oxide/thionine SiO2-pro-NH2 SPCE/GS-Nafion/Fe3O4–Au-HRP PNMA (SDS)/Co Bis(N-2 methylphenylsalicyldenaminato) copper(II)/CPE CV-activated Pt-Black PDDA/ERGO–ATP–Pd/GCE MP-11/Nafion/MWCNTs–BPPF6/GCE CuFe2O4/RGO

Amp Amp Amp Amp DPV DPV

10–200 5–20,000 5.14–1250 20–2500 5–48 1–10, 10–300

7.4 1.67 0.85 12 3.0 0.63

[75] [76] [77] [78] [79] [80]

C.A. Amp CV Amp

0. 01–300 100–10,000 0.05–0.7 2–200

0.01 0.016 0.0038 0.52

CuFe2O4/RGO

DPV

2–10, 10–1000

0.064

[81] [82] [83] This work This work

AMP = amperometry CV = cyclic voltammetry

3.10. Analysis of real samples In order to evaluate the applicability of the fabricated sensor (RGO/ CuFe2O4/CPE), the constructed sensor was used for detection of H2O2 in milk, green tea, hair dye cream and mouthwash solution samples using DPV technique. A specific concentration of H2O2 containing different samples was spiked into the phosphate buffer solution for the real sample analysis. The calculation of recovery of H2O2 in samples and the recovery of H2O2 was performed and the obtained results are listed in Table 2. The obtained results validates that the designed sensor possesses a satisfactory recovery towards H2O2 in milk, green tea, hair dye cream samples and mouthwash solution. 4. Conclusion In this paper, a sensitive electrochemical sensor (RGO/CuFe2O4/CPE) for determination of hydrogen peroxide (H2O2) was designed. The characterization of used CuFe2O4 and CuFe2O4/RGO nanoparticles in the structure of RGO/CuFe2O4/CPE sensor was performed with various techniques such as scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR). The electrochemical reduction of hydrogen peroxide (H2O2) at the RGO/CuFe2O4/CPE sensor was studied by different techniques such as cyclic voltammetry, differential pulse voltammetry, amperometry and chronoamperometry. The obtained results revealed that the modification of a carbon paste electrode with reduced graphene oxide, and nanoparticles of CuFe2O4 can lead to decreasing over potential of H2O2. The reduction of H2O2 is catalyzed by RGO/CuFe2O4 as a heterogeneous mediator at the surface of RGO/CuFe2O4/CPE sensor. The designed modified electrode revealed some advantages such as being fast (i.e. b 1 min per sample solution), ease of preparation, possessing high selectivity and sensitivity for determination of H2O2 using DPV and amperometry techniques. In addition, under the optimum conditions the fabricated electrochemical sensor (RGO/CuFe2O4/CPE) was applied successfully for determination of hydrogen peroxide in biological and pharmaceutical samples. Acknowledgement We gratefully acknowledge the support of this study by Yazd University (50-982) research council.

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Fig. 10. Plot of differential pulse voltammograms versus H2O2 concentrations, Insets: (A) differential pulse voltammograms versus different concentrations of H2O2 at RGO/CuFe2O4/CPE in a 0.1 M phosphate buffer solution (pH = 5.0), (B) the first linear range of calibration curve in inset A and (C) the second linear range of calibration curve in inset A.

Table 2 Determination of H2O2 in the real samples using RGO/CuFe2O4/CPE and the standard addition method. Sample

Original (μM)

Spiked (μM)

Found (μM)

Recovery (%)

RSD (%)

Hair coloring cream Mouthwash solution Milk

882

0

880

99.7

1.42

300

0

299

99.6

1.21

0

Green tea

0

20 50 100 5 15 25

19 48 99 4.9 15.1 25

95 96 99 98 100.66 100

2.25 1.23 1.42 1.27 1.71 0.9

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