polyaniline fibrous nanocomposite supported glassy carbon electrode for selective and ultrasensitive detection of nitrite

Sensors & Actuators: B. Chemical 309 (2020) 127763

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Interfacial polymerized RGO/MnFe2O4/polyaniline fibrous nanocomposite supported glassy carbon electrode for selective and ultrasensitive detection of nitrite

T

S. Sahooa,1, P.K. Sahoob,c,1, A. Sharmab, A.K. Satpatia,d,* a

Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India c Department of Mechanical Engineering, Siksha ‘O’ Anusandhan, Deemed to be University, Bhubaneswar-751030, India d Homi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nitrite detection Reduced graphene oxide Manganese ferrite PANI Nano composite Electroanalysis

In the present investigation fibrous nanocomposite of reduced graphene oxide, manganese ferrite and polyaniline (RGO/MnFe2O4/Polyaniline) has been synthesized through a facile two-step greener approach by a solvent less thermolysis synthesis procedure. Long-chain amine is used as the solvent, reducing agent and surface functionalizing agent. Morphology and chemical functionalities of the nanocomposites are characterized using microscopic and spectroscopic measurements. Modified electrode is fabricated using the nanocomposite material and the electrochemical property of nitrite has been investigated using cyclic voltammetry measurements, an analytical method of detection of nitrite is developed using the modified electrode based on differential pulse voltammetry. Under optimum conditions, RGO/MnFe2O4/PANI fibrous nanocomposite based modified electrode have shown the linear dynamic range from 0.05 to 12000 μM with a detection limit of 0.015 μM for the detection of nitrite. The nanocomposite modified electrode showed good anti-interference ability against various common metal ions and some organic interfering agents. The nanocomposite modified electrodeis applied to determine the nitrite level in tap water. The indicative performance of the RGO/MnFe2O4/PANI fibrous nanocomposite modified electrode widens the scope of applications of RGO/MnFe2O4/PANI fibrous nanocomposite materials for on-site monitoring of nitrite.

1. Introduction Development of analytical method of detection of nitrite has been of important research interest among the analytical scientist’s due to their detrimental effect on the environment. Environment is contaminated with nitrite due to various anthropogenic activities like, agricultural actvities (by using nitrogen based fertilizer), biological denitrification and waste generated from industry [1–6]. The permissible limit of nitrite concentration in the drinking water is 3 mg L−1 as per the world health organization guidelines [7]. Nitrite has been declared as classified hazardous species due to its toxicity to human. It combines with the blood pigments to produce meta-hemoglobin, which is results in the depletion of the oxygen in the tissue,the excess amount of nitrite in the blood leads to the irreversible oxidation of the hemoglobin. In addition to that, the presence of nitrite in stomach interacts with amines and amides to form N-nitrosamine compounds, which are carcinogens [8,9].

It is therefore essential to develop sensitive, fast, accurate, reliable and field applicable method for detection of nitrite. The detection of nitrite has been reported using various techniques such as spectrophotometry [10,11], chromatography [12,13], atomic absorption spectroscopy, chemiluminescene [14] and electrochemistry [15–18]. Electrochemical techniques have been very popular among others as it provides sensitive fastand a low cost method of detection and most importantly it can provide field applicability of the analytical method developed. Application of the nano composite materials modified electrode improves the sensitivity and selectivity further and leads to the development analytical methods [19–26] suitable for the application in real sample. The electrochemical detection of nitrite would be possible both through the oxidation and reduction of nitrite [27–31]. Oxidation of nitrite is associated with high over-potential at unmodified electrodes. Further, the interferences effect from some commonly occurring interfering agents leads to the analytical method unsuitable for real samples

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Corresponding author at: Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India. E-mail address: [email protected] (A.K. Satpati). 1 Equal contribution. https://doi.org/10.1016/j.snb.2020.127763 Received 16 August 2019; Received in revised form 3 January 2020; Accepted 20 January 2020 Available online 21 January 2020 0925-4005/ © 2020 Elsevier B.V. All rights reserved.

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solvent and reducing agent and oleic acid was used as a stabilizer. In the synthesis procedure of MnFe2O4 nanoparticles a stoichiometric amount of MnCl2.4H2O and FeCl2.4H2O were added to 30 mL of oleylamine and 20 mL of oleic acid and the mixture was taken in a three neck round bottom flask. The mixture solution was stirred constantly for 1 h at 50 °C. Then the solution was heated in a box furnace for 30 min at 130 °C. Further, the solution was heated for 4 h at 210 °C with a heating rate of 5 °C min. Finally, the black-brown product was collected through magnetic separation from the reaction solution and successively washed several times with a mixture of cyclohexane and acetone to remove unbounded amine molecules.

on unmodified electrodes. Additionally, the slow kinetics of the redox reaction over the unmodified electrode [32,33] leads to the decrease in sensitivity. All these detrimental factors makes analytical determination over bare electrode unsuitable for high sensitive analytical determination. The modification of electrode with suitable modifier resolves most of the issues and generates analytical method for high sensitive and selective detection of nitrite through the enhancement in the redox kinetics and also selective preconcentration of nitrite. Nanoparticles modified electrodes are being used for the detection of nitrite due their favorable surface charge density, conductivity and the enhanced surface area. Copper sulphonated phthalocyanine with alternate layers of iron(III)tetra-(N-methyl-4-pyridyl)-porphyrin, Pd nanoparticles, Leadruthenate pyrochlore modified Nafion, series of inorganic materials and enzymes modified electrodes were used for the determination of nitrite using various electrochemical techniques [34–38]. The electrical conductivity and surface area of reduced graphene oxide is helpful to enhance the catalytic activity when present in composite with metal oxides such as Cu2O, Co3O4 and SnO2 or metal nano particles (Au, Ag, Pd and Cu) [39–41]. Non precious metal based ferrites MnFe2O4 is basically n-type metal oxides with narrow band gap (Eg = 2.2 eV) and it has various applications in catalysts, pigments, biosensor, gas sensors, magnetic materials and lithium ion battery due to its low cost, nontoxic, easy production and easy storage and handle [42–46], most importantly it has has good electrocatalytic property. Polyaniline acts as building agent without reduction in conductivity of the catalytic material, it also provides high surface area by suitably embeding the graphene and manganese ferrite composite. The increase in the electrical conductivity of the nanocomposite is due to the ᴨ-ᴨ interaction between the RGO and PANI, which is responsible to facilitate the fast electron transfer property within the moiety, additionally the favorable surface charge of PANI will be helpful in Preconcentration of nitrite ions. It would thus be interesting to investigate the combined property of graphene, manganese ferrite and polyaniline in the composite form in electrochemical sensing of nitrite. A facile two-step greener approach is adopted for the first time to synthesize graphene, manganese ferrite and polyaniline nano composite which is named as RGO/MnFe2O4/PANI. The performance of the RGO/ MnFe2O4/PANI fibrous nanocomposite has been investigated in the sensing application of the nitrite at trace/ultra trace level using differential pulse voltammetry techniques. Mechanism of interaction between the nano composite and nitrite has been evaluated in the line of explaining the electrochemical oxidation of nitrite. it remains a challenge for lowering the overpotential of electrochemical oxidation of NO2− particularly for the development of non-precious catalysts, which has been intended in the present manuscript. Finally the modified electrode was successfully applied for the determination of nitrite in the rain water and recovery test was carried out in tap water samples.

2.3. Synthesis of Graphene/MnFe2O4 binary nanocomposite Graphite Oxide (GO) was prepared from graphite powder by a modified Hummers method [48]. In a typical synthesis procedure, 100 mg of as synthesized GO was added to 30 mL of oleylamine and 20 mL of oleic acid in a beaker and ultra-sonicated for 1 h to get a stable dispersion of graphene oxide. To this graphene dispersion, a stoichiometric amount of MnCl2.4H2O and FeCl2.4H2O were added and stirred for 1 h at 50 °C. Afterwards the same heating, separation and washing procedures were employed as incase of the synthesis of bare MnFe2O4. 2.4. Synthesis of Graphene/MnFe2O4/ PANI ternary nanocomposite An insitu chemical oxidative polymerization method was adapted for the synthesis of Graphene/MnFe2O4/ PANI ternary nanocomposite. In this synthesis, a desired amount of MnFe2O4 binary nanocomposite was dispersed in 10 ml of 1 M HCl in a three neck round bottom flask under and followed by sonication for 1 h. Then, 0.125 mL of aniline and 80 mL of 1 M HCl were added to the above mixture solution under ice bath. The whole mixture was stirred well for few min. After that, 10 mL of 1 M HCl having 375 mg ammonium peroxydisulfate (APS) was added dropwise into the above mixture solution under stirring. After addition of APS, the color of the solution turns to dark green which is an indication of PANI formation in the ternary nanocomposite. Then, the stirring was continued for 12 h. The final product was collected by centrifugation and then successively washed with 1 M HCl, distilled water and ethanol, the material and was then dried at 45 °C for 12 h under vacuum. 2.5. Instrumentation and measurements Conventional three-electrode system was used for the electrochemical measurements, a saturated calomel electrode (SCE) as reference electrode, a platinum rod as counter electrode and glassy carbon (GC) modified with ternary composite as working electrode. Electrochemical experiments were carried out using Potentiostat/ Galvanostatic model Autolab-302 N supported with GPES 4.9 software (from Metrohm The Netherland). All the potentials were reported in this study are with respect to the SCE reference electrode under thermostatic condition of 298 K. For the deaeration of the test solution it was purged with high purity nitrogen gas for 10 min prior to analysis. Determination of nitrite was carried out by standard addition method. The surface morphology of the materials were examined using field emission scanning electron microscope (FE-SEM), JEOL Model JSM7600 F, which was equipped with an energy-dispersive X-ray spectroscopy (EDS). The high resolution transmission electron microscopy (HRTEM) images were obtained by using the Phillips-CM 200 electron microscope, AFM investigations were carried out using Flex AFM system from Nanosurf, Switzerland. The bare GC electrode was cleaned mechanically by polishing with alumina powder suspension of pore size 0.3 and 0.5 μM respectively and then the electrode was washed thoroughly with double distilled water and methanol, followed by drying under IR lamp at 50 °C. After that, 1 mg quantity of ternary composite material was dispersed in 1 mL of 1:1 methanol water medium and ultra

2. Experimental methods 2.1. Chemicals and reagents Electrochemical experiments were carried out using Anal R grade chemicals. All acids used in the experiments were of supra pure grade. Phosphate buffer solution (PBS, pH 7) was prepared from Na2HPO4 and KH2PO4. The standard stock solution of NO2− was prepared from NaNO2 of analytical grade and obtained from sigma Aldrich. All these reagents were weighed in the appropriate quantities and dissolved in deionised water to made up to the desired volumes. The analyte solution was degassed with high purity nitrogen gas (from Indian Oxygen Ltd.) to remove dissolved oxygen prior to the measurement (Scheme 1). 2.2. Synthesis of MnFe2O4 nanoparticles The bare MnFe2O4 nanoparticles were synthesized by solvent less thermolysis technique [47]. Long-chain oleylamine was used as a 2

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Scheme 1. The synthesis procedure adopted to synthesize the nano composite material.

sonicated for 10 min, 10 μL of the dispersed solution (aliquot) was drop casted on the surface of the GC electrode and allowed the electrode to dry completely in the ambient condition. Modifications using the other materials like MnFe2O4 and RGO/MnFe2O4 were carried out using the same procedure.

and RGO/MnFe2O4/PANI nano composites suggesting the reduction of GO and prevention of restacking of GO sheets due to incorporation of MnFe2O4 nanoparticles into these sheets [50]. The other diffraction peaks near the 2θ values of 16.9, 30.5, 35.6, 43.2, 52.1, 57.1 and 62.5° corresponds to the formation of cubic phase of MnFe2O4. The additional broad diffraction peak at around 2θ value of 15 to 25° in RGO/ MnFe2O4/PANI nanocomposite, correspond to (200) crystal plane of pure PANI. It reveals that PANI had been successfully synthesized by oxidative polymerization method. Raman spectroscopy is a powerful technique for characterizing the carbonaceous materials on the basis of electronic structures and analyzing the defects in the sp2 carbon materials. The characteristic Raman spectra of GO, RGO/MnFe2O4 and RGO/MnFe2O4/PANI nanocomposite are shown in Fig. 1B. Both GO and RGO/MnFe2O4 nanocomposite have D band corresponding to the defects in the curved graphene sheets at around 1345 cm−1and G band related to stretching mode of crystalline graphite at around 1594 cm-1 [51]. The D to G band intensity ratio (ID/ IG) is a measure of average size of sp2 domains [52]. The increase in ID/ IG from 0.89 for GO to 1.22 for RGO/MnFe2O4 confirms that there is a successful reduction of GO. In addition, a weak peak at 613 cm-1 in the RGO/MnFe2O4 nanocomposite can be assigned to the MnFe2O4 nanoparticles [53]. Apart from the G (1578 cm−1) and D (1328 cm−1) bands, the observation of characteristic peaks for PANI at 1478 cm−1,

3. Results and discussion 3.1. Characterization of the composite materials The crystal structure of MnFe2O4, GO, RGO/MnFe2O4 and RGO/ MnFe2O4/PANI nanocomposite were investigated by XRD. Fig. 1A. (a) shows the XRD pattern of MnFe2O4. Diffraction peaks at 2θ values of 16.9, 30.5, 35.6, 43.2, 52.1, 57.1 and 62.5°are indexed to (111), (220), (311), (400), (422), (333) and (440) crystal planes of cubic phase with inverse spinel ferrite for MnFe2O4 JCPDS no. 742403. The diffraction peaks are slightly broadened without showing the presence of any impure phases. Therefore, the broadening of the XRD peaks indicates that nano-crystalline nature of the as synthesize MnFe2O4. The XRD pattern of GO as shown in Fig. 1A. (b), reveals an intense and sharp peaks at 2θ values of 10.8 and 43°, which corresponding to the (002) plane of GO and (100) plane of hcp structure of carbon respectively [49]. However, the (002) plane nearly disappears in the RGO/MnFe2O4

Fig. 1. A. XRD patterns of (a) MnFe2O4, (b) GO, (c) RGO/MnFe2O4 nanocomposite and (d) RGO/MnFe2O4/PANI nanocomposite.(B) Raman spectra of (a) GO (b) RGO/MnFe2O4 nanocomposite and (c) RGO/MnFe2O4/PANI nanocomposite. 3

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Fig. 2. (a)Wide scan survey XPS spectrum of GO and RGO/MnFe2O4/PANI nano-composite, (b–d) High-resolution Fe 2p, Mn 2p and N 1s spectrum of RGO/ MnFe2O4/PANI nano-composite, (e) High-resolution deconvoluted C 1s core level XPS spectra of GO, and (f) High-resolution deconvoluted C 1s core level XPS spectra of RGO/MnFe2O4/PANI nano-composite.

1210 cm−1, 1158 cm−1, 767 cm−1, 573 cm−1 and for MnFe2O4 at 622 cm−1 indicate the presence of RGO, PANI and MnFe2O4 in the nanocomposite [54]. The lower shifting of D and G bands in RGO/ MnFe2O4 nanocomposite may be due to the charge transfer between the GO nano-sheets and MnFe2O4 nanoparticles, while in case of RGO/ MnFe2O4/PANI nanocomposite, it is due to the π-π interaction between PANI and RGO sheets to form stable nanocomposite [54,55]. The chemical states of elements present in the GO and RGO/ MnFe2O4/PANI nanocomposite were investigated by XPS analysis. The XPS spectra of GO and RGO/MnFe2O4/PANI nanocomposite are shown in Fig. 2. The XPS survey spectra of GO (Fig. 2(a) consist of C1 s and O1 s peaks, where as RGO/MnFe2O4/PANI nanocomposite exhibits additional Fe2p, Mn2p and N1 s peaks along with C1 s and O1 s peaks. This suggests that MnFe2O4 nanoparticles and PANI are formed and deposited on the surface of RGO sheets. It has also been noticed that the intensity of C1 s peak is higher than O1 s peak in case of RGO/ MnFe2O4/PANI nanocomposite as compared to GO. This indicates that GO was successfully deoxygenated and reduced to form graphene. The high-resolution XPS spectra were performed to know the chemical states of N, Fe and Mn in RGO/MnFe2O4/PANI nanocomposite. The Fe2p spectrum in Fig. 2(b) shows two peaks centered nearly at the binding energy of 711.1 and 724.6 eV, which can be ascribed to the Fe 2p3/2 and Fe 2p1/2 of Fe3+ in MnFe2O4, respectively [56]. In Fig. 2(c), the doublet Mn2p peaks located at binding energy of 641.2 and 653.2 eV are corresponding to Mn 2p3/2 and Mn 2p1/2 of Mn2+ in MnFe2O4 [57]. Fig. 2(d) presents the deconvoluted N1 s core level spectrum of RGO/MnFe2O4/PANI nanocomposite. N1s core level spectrum consists of three major components, which are corresponding to three different electronic states. The three major components are benzenoid amine (eNHe) at binding energy 399.3 eV, quinonoid amine (]Ne) at binding energy 398.7 eV and nitrogen cationic radical (N+.) at binding energy 401.4 eV, respectively [57]. These results indicate

MnFe2O4 nanoparticles and PANI are formed and attached with RGO sheets in RGO/MnFe2O4/PANI nanocomposite. The deconvolution of the N(1s) core level spectrum presented here is comparable to the typical XPS N(1 s) deconvolution of PANI reported in the literature [58]. In Fig. 2(e & f), the high resolution C1 s spectrum of GO and RGO/ MnFe2O4/PANI nanocomposites are shown. The C1 s spectrum of GO shows the signature of different oxygenic functional groups such as -CO at 286.6 eV, eC]O at 286.9 eV, eCOO at 288.4 eV along with sp2 carbon at 284.5 eV and sp3 carbon at 285.1 eV [50]. The C1 s spectrum of RGO/MnFe2O4/PANI nanocomposite shows the presence of same oxygenic functional groups and carbon peaks (sp2 and sp3) along with CeN peak. The broad peak at binding energy of 285.1 eV corresponding to the combined peak of CeN and sp3 carbon peaks [57]. Additionally, it has been noticed Fig. 2(f) that the intensities of various oxygenic functional groups and the intensity of the sp3 carbon peak (285.1 eV) are effectively reduced as compared to intensity of the sp2 carbon peak (284.5 eV). The effective reduction of intensities of oxygenic functional groups confirms that GO is successfully reduced to RGO in the RGO/ MnFe2O4/PANI nanocomposite [59]. The surface morphology, microstructures and particle size of the MnFe2O4, RGO/MnFe2O4 nanocomposite and RGO/MnFe2O4/PANI nanocomposite were evaluated by FEG-SEM and HRTEM. As seen from Fig. 3, MnFe2O4 nanoparticles are spherical in shape however, severe agglomeration is observed and spherical shaped small particles are observed to be agglomerated to bigger particles. In comparison, the agglomeration of MnFe2O4 nanoparticles are prevented to some extents in the RGO/MnFe2O4 nanocomposite and the MnFe2O4 nanoparticles are uniformly distributed over the RGO sheets Fig. 3(a). This is ascribed to be due to the strong interaction between the bare MnFe2O4 nanoparticles and RGO sheets, which prevents the agglomeration of MnFe2O4 nanoparticles in presence of RGO to some extents and enhances the mechanical stability of the nanocomposite. Additionally, the 4

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Fig. 3. FEG-SEM images of (a) RGO/MnFe2O4 nanocomposite, (b) RGO/MnFe2O4/PANI nanocomposite, and (c) Higher magnification image of RGO/MnFe2O4/PANI nanocomposite; TEM images of (d) RGO/MnFe2O4 nanocomposite, and (f) RGO/MnFe2O4/PANI nanocomposite; HRTEM image of RGO/MnFe2O4/PANI nanocomposite.

further electrochemical measuremnets. To reason out the enhanced electrochemical activity of the modified electrode, electrochemical impedance spectroscopic (EIS) measurements were carried out to investigate the electronic and interfacial properties of the ternary composite modified GC electrode. The impedance results are recorded at an applied potential of 0.3 V in the frequency region of 0.01 Hz to 50,000 Hz and the results are shown in Fig. 4. The semicircle portion of the impedance spectra at higher frequency region relating to the interfacial charge transfer resistance and the linear portion in the lower frequency region corresponds to the diffusion process. The Nyquist plot was fitted with the equivalent circuits, which is placed as Fig. 4C, where the R1 is the series resistance, mainly associated with resistance of the solution, R2 is the resistance due the materials and R3 is the charge transfer resistance, Q1 and Q2 are the two imperfect capacitive elements used in the fitting, W is the Warburg element. However, the Nyquist plot has been discussed qualitatively to compare the electrochemical performance of various modified electrodes using potassium ferrocyanide as the redox probe, hence the absolute values of various parameters are not tabulated. As shown from the Nyquist plot, the interfacial charge transfer resistance of the materials is decreased from bare GC, to MnFe2O4 modified GC to RGO/MnFe2O4 modified GC and finally the resistance has been reduced significantly in the ternary composite material RGO/MnFe2O4/PANI modified GC electrode. The bare glassy carbon electrode does not show the diffusion nature in the Nyquist plot, diffusivity has been pronounced only with the modified electrodes. Cyclic voltammetry experiment was also carried out at different scan rates in 1 mM K4[Fe(CN)6] solution and the plots are shown in Fig. 5(a). The peak current values are measured and plotted with respect to the square root of scan rates of the measurements, the resulted correlation is fitted with the Randle Sevick’s equation as below.

RGO also helps to accelerate the electron transport process from the RGO sheets to the MnFe2O4 nanoparticles [60]. In case of RGO/ MnFe2O4/PANI nanocomposite, besides the spherical shaped MnFe2O4, fiber-like network structure of PANI are observed to be distributed on the RGO sheets Fig. 3(b). The diameter of these fibrous PANI is calculated from the high magnification SEM image of RGO/MnFe2O4/ PANI nanocomposite as 20−40 nm Fig. 3(c). The morphology of the RGO/ MnFe2O4 and RGO/MnFe2O4/PANI nanocomposites were further analyzed by TEM. In case of RGO/MnFe2O4 nanocomposite as shown in Fig. 3(d), MnFe2O4 nanoparticles are appeared as dark sphere and homogenously distributed throughout the RGO sheets. The average diameter of the MnFe2O4 nanoparticles on RGO/MnFe2O4 nanocomposite is 19 nm. TEM image of RGO/MnFe2O4/PANI nanocomposite confirms the presence of spherical shaped MnFe2O4 nanoparticles, fiber shaped PANI and sheet like RGO structure Fig. 3(e), Further, the PANI nano fibers are appeared to be interconnected with each other to form network like structure. The lattice spacing of MnFe2O4 nanoparticle in Fig. 3(f) is 0.254 nm, which corresponds to the (311) plane of MnFe2O4. This is in good agreement with the d-spacing value obtained from the XRD measurements. 3.2. Electrochemical characteristics of the composite modified electrode Electrochemical activity of three materials were investigated using cyclic voltammetry measurements in 0.1 M K4[Fe(CN)6] solution using the composite materials MnFe2O4, RGO/MnFe2O4, and RGO/MnFe2O4/ PANI modified electrode, the scan rate of the measurements was taken as 10 mVS−1 and the results are shown in Fig. 4(a). The peak current for the redox reaction in K4[Fe(CN)6] is found to be higher in the case of ternary composite in comparison to both MnFe2O4 and RGO/MnFe2O4. The significant increase in the peak current in ternary compound is due to the enhanced conductivity and surface area of the ternary composites, arises due to the synergetic effect from the presence of all the three compounds i.e. MnFe2O4, RGO and PANI, together in the composite, the contribution from the enhanced surface area and the improved conductivity is discussed in the following sections based on

Ip = 2.68 x l05 n2/3AD1/2v1/2 Co

(1)

Where ip is peak current, A is the surface area of the electrode, D is diffusion coefficient, ν is scan rate and Co is reactant concentration. Experiments are carried out in 1 mM Fe(CN)64− in 0.1 M aqueous 5

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Fig. 4. (a) Cyclic Voltammetric plot of Ferrocyanide using electrodes modified with different modifier in 0.1 M phosphate buffer of pH 7 (b) Nyquist plot recorded at the excitation potential of 0.3 V using electrodes modified with different modifier in 0.1 M phosphate buffer of pH 7 (C) Equivalent circuit used for the fitting of the Impedance plot.

Fig. 5. (a) CV of Ferrocyanide at different scan rates on Inset: Corresponding peak current vs. square root of scan rate plot (b) CV of Nitrite ions at various scan rates Inset: Corresponding peak current vs. square root of scan rate plot (c) Peak current vs. Scan rate plot from the CV measurements of plot (B), (d) Peak current vs. log of scan rate plot from the CV measurements of plot (B); experiments are carried out in 0.1 M phosphate buffer of pH 7. 6

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electrode over the other electrodes, the results of the zeta potential measurements are shown in Fig. 6 (b). It is observed that the zeta potential value of RGO is negative due to the presence of dissociated acidic functional groups. The pure MnFe2O4 material has shown the zeta potential of −10 mV at the neutral pH solution, incorporation of RGO into the MnFe2O4 has undergone expected shift towards negative direction and the zeta potential is obtained as −16 mV. PANI itself has zeta potential positive and when in composite with RGO and MnFe2O4, it has shown the zeta potential of 36 mV [64]. Therefore, the ternary composite RGO/MnFe2O4/PANI has zeta potential suitable for the preconcentration of negatively charged nitrite ions through electrostatic interaction. Further, the catalytic activity of manganese ferrite nano structure would enhance the electrochemical oxidation of nitrite ions. The strong interaction with the negatively charged ion containing epoxy, carbonyl and hydroxyl group of the graphene sheets with PANI and ferrites makes a good composite which makes the uninterrupted flow of charge while oxidation of nitrite. It has been reported previously that the holes in the valence band (VB) of MnFe2O4 migrate easily to the π-orbital of PANI because of the directed electric fields of the two materials. Due to such facilitated migration of holes from the MnFe2O4 to the PANI matrix the oxidation of NO2− ions will be facilitated [65]. It has been revealed from the XRD measurements that the MnFe2O4 material present as reverse spinel form and Mn2+ occupies both the tetrahedral and octahedral vacancies and Fe3+ occupies the octahedral vacancy. During the electrochemical scanning the Mn2+ is oxidized to Mn3+, which oxidizes NO2− ions to NO3− ions and itself reduced back to Mn2+. Therefore, the probable mechanism of the electrochemical oxidation process is summarized as below [66,67].

solution of KCl. The active surface area of the ternary composite material modified and bare glassy carbon electrode were obtained as 8.14 cm2/sec and 0.07 cm2/sec respectively. Thus, the surface area of the ternary composite material observed to be enhanced by around two orders of magnitude compared to the bare glassy carbon electrode. After having the electrochemical investigation of the modified electrode using the standard redox probe and obtaining the indication about the superior electrochemical activity of the ternary composite modified materials, electrochemical behavior of NO2− ion over the ternary composite modified electrode has been investigated pertaining to the sensitive detection of NO2− ions in aqueous solution. Cyclic voltammetry and differential pulse voltammetry investigations are carried out in NO2− ion containing solution. Fig. 5(b), shows the cyclic voltammogram of the NO2− at different scan rates in 0.1 M PBS buffer electrolyte medium at pH 7 using the ternary composite (RGO/ MnFe2O4/PANI) modified glassy carbon electrode in the potential range from 0.5 V to 1.2 V. The oxidation peak is observed at around 0.86 V and the peak current is increased with increase in the scan rates of the measurements. The variation in the peak current is plotted against the scan rates and square root of scan rate of measurements and the results are shown in Fig. 5(c). The peak current is increased linearly with the square root of the scan rate of measurements indicating diffusion controlled nature of the electrochemical oxidation process of nitrite over the ternary composite modified electrode surface. The peak potential of the nitrite oxidation peak is shifted with the scan rate of the measurements, indicating quasi-reversible to irreversible nature of the oxidation process. The peak potential values are plotted with respect to the logarithm of scan rates of the measurements and the plot is shown in Fig. 5(d). The results were analyzed using the Laviron equation as in Eq. (2) [61].

Ep = E0+ (RT ∕αnF)[ln(RTks/αnF)− lnν]

RGO/Mn(II)Fe2 (III)O4 /PANI → RGO/Mn(III)Fe2 (III)O4 /PANI RGO/Mn(III)Fe2 (III)O4 /PANI+NO−2

(2)

→ RGO/Mn(II)Fe2 (III) O4 /PANI+NO−3

Where, Ep is the peak potential, E0 is the formal potential, R is the universal gas constant (8.314 J/ mol. K.), T is the absolute temperature, α is the electron transfer coefficient, n is electron transfer number, F is Faraday’s constant 96485 C mol−1), ks is standard rate constant of the reaction, ν is scan rate (V/s). The (αn) value is obtained using the correlation. E0 of Cf is obtained from the intercept of the correlation between peak potential with respect to the scan rate of the measurements (plot is not shown). The value of E0 obtained in this way is 0.81 V. From the slope and intercept of the plot in Fig. 5(d), the (αn) and the Ks values are obtained as 0.22 and 14.2 s−1 respectively. The electron transfer rate constant obtained using the Laviron analysis procedure is reasonably fast, indicating enhanced electron transfer process on the ternary composite modified electrode surface. After having the qualitative information about the oxidation process and obtaining good electrochemical signal in cyclic voltammetry measurements, investigations were carried out to develop the analytical method for the determination of nitrite in water solution and differential pulse anodic stripping voltammetry (DPASV) technique has been employed for recording the signals. The voltammetric signals using three materials are shown in Fig. 6(a), it is observed that the oxidation current of nitrite is highest, when the RGO/MnFe2O4/PANI composite materials is used for the modification of the electrode. The over-potential of the RGO/MnFe2O4/PANI composite modified electrode is lowest and the peak is observed at 0.82 V. Major issue with the electrochemical oxidation of NO2− ions is the associated over-potential and there are previous reports about reduction of over-potential of the oxidation process [37,38,62,63]. Lin Cui et al. reported the copper calcined layered double hydroxide and gold nano particles providing suitable microenvironment for oxidation of NO2− ions. In the present material also the combination of better adsorption and the enhance charge transfer property is reveled from the cyclic voltammetry and the impedance measurements using the ferro cyanide redox probe. Zeta potential measurements were carried out to reason out the superior electrochemical activity of the ternary composite modified

Where Mn(II) is oxidized to Mn(III) over the electrode surface, this oxidized Mn(III) in chemical oxidation processed reduced back to Mn (III) and reduces NO2− ion to NO3− ion. 3.3. Effect of pH of the measurements Voltammetric scans were recorded in solutions of different pH values, the peak potential remained unchanged with the variation in pH of the solution however, the peak current has shown significant variation with the pH of the solution. The variation of the peak current with the pH of the solution is plotted and shown in Fig. 6(c). It is observed that the oxidation peak current of nitrite is increased with pH of the solution from 3 to 7 after which the peak current is reduced significantly with increase in the pH. Thus, the oxidation of nitrite ions has shown highest possibility at the pH range between 6–7 and outside this region the oxidation peak current reduces significantly. At higher acidic solution the nitrite ions undergo the following disproportionation reactions [68]

2H++3NO−2 → 2NO+NO−3 + H2 O Due to which the NO2− ions do not remain available for the oxidation and the peak current reduces. The maximum peak current was obtained at the neutral pH region (pH 6–7). When pH is increased above 7, the peak current is reduced. The oxidation of NO2− ions is pH independent process therefore there is no reason for decrease in peak current at higher pH. It has been reported that at higher pH, the oxide film formation at the surface of the working electrode is promoted thus the electro-oxidation peak due to the oxidation of nitrite is decreased with increase of pH [69,70]. In a previous report the maximum current for the oxidation of NO2− ions was obtained at pH 4, when gold nano particle modified carbon paste electrode used as the modified electrode [71]. The charge distribution in the present material is different compared to the previous materials, which resulted in the highest electrochemical response at different pH values compared to the materials 7

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Fig. 6. (a) DPV plots of Nitrite ions using different modified electrode (b) Results of the Zeta potential measurements (c) Peak current Vs. pH of the measurement.

with the linear correlation shown in Fig. 7(b), the method is having wide linear dynamic range of 0.05 μM to 12000 μM. The detection limit of the analytical method is obtained as 0.015 μM for the signal to noise ratio S/N = 3. The inter-electrode reproducibility of the modified electrode has been tested by preparing the electrode 6 time and recording the signal at 30 μM concentration, the results are shown in Fig. 7(c). The percentage deviation of the current signal is obtained as 3.7 %, which can be accepted for the present type of analysis. Similartly, the stability iof the electrode over time has been tested and the test results are shown in Fig. S1 of the supporting information, the electrode observed to be remained stable after 30 days with about 7 % decrease in the performance from the original plot. Analytical performance of the modified electrode was also tested through the chronoamperometric measuremnets, measuremnets are carried out under strirring conditrion placing a magnetic stirrer rorating at 500 rpm. The results are shown in Fig. S1 of the supporting information. The current response from the chronoamperometric measuremnets at 10 μM concentration remained stable for over 10 addition.

reported earlier, this has been reflected in the zeta potential measurements as discussed in the previous section. Thus from the pH dependent investigation the optimized pH range chosen for the present investigation is around neutral pH range of 6–7. All subsequent experiments were carried out at pH 7. 3.4. Analytical features of the method 3.4.1. Detection sensitivity The current response was measured on successive addition of standard concentration of NO2−. The experiments were carried out at the accumulation potential of 0.8 V and the accumulation time of 100 s. Corresponding voltammetric plots with successive addition of NO2− concentration is shown in Fig. 7(a), the voltammetric current observed to be increased with increase in the concentration of NO2- ions in the test solution. The variation of the peak current with the concentration of the NO2- ions is shown in the inset of Fig. 7(a). The peak current is observed to be increased linearly with increase in the concentration of NO2−ions. The variation of peak current with concentration is fitted

Fig. 7. (a) DPV with varying concentration of nitrite using the RGO/MnFe2O4/PANI nano-composite (b) DPV plots of nitrite ions using the RGO/MnFe2O4/PANI nano-composite showing the wide concentration range of its application (C) Voltammetric measurements recorded using 6 sets of modified electrodes, scans were recorded at 30 μM nitrite concentration and all measuremnets are carried out at pH 7. 8

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Fig. 8. Interference study of the analytical measurements (a) Interference due to Cu2+ (b) Interference due to Mg2+ (c) Interference due to Na+ (d) Interference due to Ca2+ (e) Interference due to Urea (f) Interference due to glucose. Table 1 Recovery test result of nitrite in rain water samples. Sample number

Spiked Nitrite in the sample in mgL−1

Recovery of nitrite In mgL−1

Percentage of recovery of nitrite

Tap-1 Tap-2

0.75 0.35

0.76 ± 0.038 0.34 ± 0.017

101 % 98.8 %

Table 3 Comparison of different chemically modified electrodes for the determination of nitrite with RGO/MnFe2O4/PANI modified electrode.

3.4.2. Interference test of the analytical method Interferences on the analytical determination of NO2− ions using the ternary composite modified electrode has been investigated using some of the commonly occurring interfering agents. Interference from Na+, Mg2+, Ca2+, K+, NO3-, SO42-, Cl-, PO43-, CO32-, I-, urea, ascorbic acid, Methanol, Ethanol and glucose are investigated. The result of the interference test with Na+, Mg2+ K+ and Cu2+ are shown in Fig. 8(a) (b) (c) and (d). No deviation in the voltammetric peak of NO2- ion is observed with the addition of Na+, Mg2+ and K+ up to 500 mg L-1 concentration, and Cu2+up to100 mgL-1 in the test solution. Furthermore, the interference of urea and glucose with nitrite was also investigated in the same experimental condition and results are shown in Fig. 8(e) and (f). The peak current of nitrite was marginally increased with addition of Urea in the solution.

Electrode Material

Linear dynamic range (μM)

Detection limit (μM)

Ref.

PdFe Alloy Fe2O3–CoO Fe2O3/rGO IL-SWCNT PEDOT/AuNP POA β-MnO2 Au/ZnO/MWCNTs Cu-NDs/rGO Pd/rGO PDDA/P2W17V-CNTs Fe-HNPs Cu-nano/CNTs/CS RGO/MnFe2O4/PANI

500–25000 200–16200 0.05–780 1.0–12,000 3.0–300 2.0–50 0.29–26090 0.78–400 1.25–13000 0.04–108 0.05–2130 9.0–3000 0.1–2500 0.05–12000

0.8 0.1 0.015 0.1 0.1 1.05 0.29 0.40 0.40 0.015 0.0367 2.60 0.024 0.015

[30] [31] [66] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] present work

Division for the analysis of nitrite content, 2 mL of the samples and 8 mL of deionized were taken in the voltammetric cell for electrochemical measurements and pH of the solution was adjusted to 7 and the voltammetric scan was recorded. The analysis results of nitrite were validated with ion chromatography and the results are shown in Table 1. The recovery test was also carried out in two tap water samples

3.4.3. Analysis of NO2− ion in processed water samples Two rain water samples were received in the Analytical Chemistry

Table 2 Analysis results of the water samples carried out using the modified electrode and also using ion chromatography. SL No

Sample Types

Sample Code

Nitrite by Ion chromatography technique (in ppm)

Nitrite by present method (in ppm)

1 2 3 4 5 6

Kerala Flood Water Kerala Flood Water Kerala Flood Water Tumurapali Bore Well, AP Sangli, Bore Well Tap Water Anushakti Nagar Rain Water Anushakti Nagar

ACD-Kl-1 ACD-KL-2 ACD-KL-3 ACD-Tm-1 ACD-Sn-1 ACD-AN-1

4.4 ± 0.3 15.2 ± 1.1 0.35 ± 0.03 1.9 ± 0.2 1.2 ± 0.1 Not detected

4.2 ± 0.3 14.6 ± 1.5 0.29 ± 0.05 2.1 ± 0.2 1.2 ± 0.1 Not detected

ACD-AN-2

0.22 ± 0.01

0.21 ± 0.02

7

9

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to verify the reproducibility of the developed method and the results are shown in the Table 2. The recovery of nitrite in the Tap water-1 sample is found to be 101 % and for the Tap water-2 sample is 98.8 %. The performance of the present analytical method is compared with the performance reported in the literature and the comparison table is placed in Table 3, the detection limit of the present method is comparable or better compared to the methods reported and the dynamic range is better compared to the similar methods reported [72,81].

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Srikant Sahoo working as Technical Officer in Analytical Chemistry Division, Bhabha Atomic Research Centre. His research interest is on the development of analytical methods using modified electrode, electrochemical investigations of biomolecules. He has expertise in the preparation of modified carbon paste electrode which is being used as the substrate electrode for the determination of many metal ions and organic molecules. Carbon paste electrode was modified with mercury thin films/ gold nano particles/ Bi-Au nano composite films to improve sensitivity of determination of metal ions and important organic molecules. Shri Sahoo is presently working on the development of mixed oxide and sulphide nano composite materials with different form of carbon for the supercapacitor applications. Shri sahoo has expertise in electrodeposition of thin metallic films, metallic and non-metallic nano particle with carbon composites, reduced graphene oxide modification on to the electrode surface. He is teaching and technically guiding project students from various universities.

Prasanta Kumar Sahoo, an associate professor at the Siksha ‘O’ Anusandhan, Deemed to be University, Bhubaneswar, India. He obtained his PhD degree under the joint venture of IIT Bombay, India and Monash University, Australia. His Ph.D thesis is entitled as “Design of Graphene-Metal Nano-composites for Electrochemical Sensors and Catalytic Applications”, for which he got “Best PhD Thesis Award” in 2016. He was also awarded Deutscher Akademischer Austauschdienst (DAAD) scholarship in his Master degree at IIT Bombay, India. He was a postdoctoral fellow at the National Taiwan University, Taiwan for one year. He was working on the project “Synthesis and architectural design of morphology, structure, and composition of two-dimensional (2D) nanomaterials for Biosensors and Catalysis applications. His research focus on design and development of 2D based nanomaterials for sensing, electrocatalysis and energy-storage applications.

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S. Sahoo, et al. Anshul Kumar Sharma completed his Master of Technology in Nanotechnology at Amity University, Delhi in 2015. He started working extensively on graphene based materials at IIT Bombay during his Master’s dissertation and continued his work at IIT Bombay after completion of his degree. In 2016, he joined Log 9 Materials Scientific Pvt Ltd, a R & D company working on commercializing graphene - based products. Currently, he is working as Head of R & D at Log 9 Materials and is leading a team which is working extensively to develop products in energy, air filtration and water filtration domains. He is involved in the present work during his Master in IIT Bombay.

them for the determination of metal ions at trace and ultra-trace levels. Development of modified electrode based on carbon paste and carbon nano-tube, thio-compound and DNA. Electrodeposition of thin metallic films. He is teaching electrochemistry and analytical chemistry in post graduate level since 2004. He is recipient of the following awards and recognition. “Young Scientist Award-2008” from Department of Atomic Energy, Government of India. Fulbright fellowship to carry out research work with Prof. Allen J. Bard in University of Texas Austin, USA during 2011–2012, Young associate of Maharashtra Academy of Sciences 2015 and member of National Academy of Sciences India (NASI) in 2018.

Ashis Kumar Satpati is presently working as Scientific Officer in Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai. His research interest includes electrochemical investigation of thin films, biomolecules and interfaces. Fabrication of ultramicro electrodes for scanning electrochemical microscopy. He has been working on the development of composite materials for efficient supercapacitors and electrochemical investigation of materials for supercapacitors. He has investigated electron Transfer (ET) reactions in homogeneous and micro-heterogeneous media. Different series of coumarin and quinone dyes were used as the electron acceptors at their excited state and amines of different nature and structure as the donors, at their ground state. He has been working on the development of analytical methods using voltammetry and developed many electro analytical methods and used

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