Synthesis of acid resistant Fe₂V₄O₁₃-polypyrrole nanocomposite: its application towards the fabrication of disposable electrochemical sensor for the detection of As(III)

The amorphous Fe₂V₄O₁₃ was prepared according to the previously reported method [9]. Briefly, NH₄VO₃ (0.467 g) was dissolved in 20 ml of distill water at 60 °C. To this, Fe(NO₃)₃·9H₂O (0.808 g) was added and stirred for 3 h. The resulting precipitate was retrieved by centrifugation and then washed with water and ethanol followed by dried at 60 °C. Finally, the resulting product was calcined at 300 °C/6 h to get amorphous Fe₂V₄O₁₃.

The prepared amorphous Fe₂V₄O₁₃ (0.10 g) was dispersed in 20 ml of distilled water under sonication. To this, 0.1 ml of pyrrole dissolved in 10 ml of water was added at 10 °C. Then, 1.0 g ammonium persulphate dissolved in 20 ml distill water was added drop wise at temperature below 10 °C. Finally, the resulting amorphous Fe₂V₄O₁₃-polypyrrole nanocomposite was retrieved by centrifugation and washed with water and ethanol followed by dried at 60 °C.

Powder x-ray diffraction patterns (PXRD) of amorphous Fe₂V₄O₁₃ and its polypyrrole composite were collected by using PANalytical X'pert PRO x-ray diffractometer. The FTIR spectra of Fe₂V₄O₁₃ and its composite with polypyrrole were collected using IS5-800 Fourier transform infrared Nicolet spectrometer. The surface morphology of Fe₂V₄O₁₃ and its polypyrrole composite were studied by using JEOL-5600LV scanning electron microscopy. The electrochemical studies, such as cyclic voltammetry (CV), differential pulse anodic stripping voltammetry (DPASV) have been conducted using biologic SP150 potentiostat. As(III) content present in the synthetic water solution was estimated using G.B.C. make, Avanta model, Atomic Absorption Spectrophotometer with AA Hydride System HG 3000.

The prepared amorphous Fe₂V₄O₁₃-polypyrrole nanocomposite (2.0 mg) was dispersed in 2 ml of distilled water under sonication. The 10 μl of the resulting suspension was drop casted on SPE (working electrode area, 3 mm) and then dried under infra-red light. Finally, dried SPE was used for electrochemical measurements. The schematic representation of the fabrication of Fe₂V₄O₁₃-polypyrrole nano-composite modified SPE is presented in scheme 1.

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All experiments were carried out at room temperature, without removing oxygen from the solution. The amorphous Fe₂V₄O₁₃-polypyrrole nanocomposite modified SPE was immersed in the electrochemical cell containing As(III) along with the supporting electrolyte 0.1 M HCl (scheme 2). The CV experiments were recorded between the potential −0.5 to 0.5 V at scan rate of 50 mV per second. The stripping measurements were carried out by sweeping the potential in the positive direction from −0.3 V to +0.5 V without stirring.

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100 ppm of As(III) standard solution was prepared by using NaAsO₂ in double distilled water and diluted to 1000 ml. So far, ground water containing arsenic has not been reported in southern states of India. Hence, synthetic water was prepared according to the previous literature [10, 11] where known quantities of As(III) standard solution was added to the drinking water, supplied by Bangalore water supply and sewerage board, Bangalore, Karnataka, India. Finally, As(III) content was estimated, using atomic absorption spectrophotometer, based on calibrated hydride generation method in accordance with the protocol suggested [12].

The powder XRD patterns of the prepared Fe₂V₄O₁₃ and its composite were shown in figures 1(a) and (b) respectively, where the presence of broad peak between 20 and 30° confirms the amorphous nature of both Fe₂V₄O₁₃ and its composite with polypyrrole. The FTIR spectrum of the amorphous Fe₂V₄O₁₃ (figure 1(c)) reveals the presence of characteristic bands at 523, and 850 cm⁻¹, wherein the band appeared at 523 cm⁻¹ is assigned to V-O-V deformation and 850 cm⁻¹ is assigned to bridging V-O-Fe mode. The FTIR spectrum of the composite shown in figure 1(d) exhibits a series of characteristic bands corresponding to the Fe₂V₄O₁₃ and polypyrrole, indicating the formation of the Fe₂V₄O₁₃-polypyrrole nano-composite. The peaks at 960 cm⁻¹, 784, and 682 cm⁻¹ are respectively accredited to the in-plane C=C bending and out-of-plane C-H bending in polypyrrole. The peaks appeared at 1094 cm⁻¹ and 1040 cm⁻¹ assigned to N⁺H₂ in-plane vibration and combination of C-H in-plane ring bending and C=C–N deformation of the five-membered ring respectively. The peaks observed at 1550 and 1478 cm⁻¹ are due to the pyrrole ring vibration [13–15]. Additionally, the bands 523 and 760 and 850 cm⁻¹ appeared for Fe₂V₄O₁₃ were shifted towards higher wavenumber in the composite, indicating the significant interaction between polypyrrole and Fe₂V₄O₁₃.

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The formation of Fe₂V₄O₁₃-polypyrrole nanocomposite is further confirmed by the SEM analysis. The morphology of the amorphous Fe₂V₄O₁₃ and its composite with the polypyrrole, is presented in the figure 2. As observed from the figures 2(a) and (b), the amorphous Fe₂V₄O₁₃ is made up of fine particles with porous structure, while the Fe₂V₄O₁₃-nanocomposite, shown in the figures 2(c) and (d), exhibits the different morphology confirming the formation of composite where it is made of mono-dispersed nanoparticles with diameter of about 100–200 nm, indicating the formation of nanocomposite.

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As shown in the figure 3, the CV response of the nanocomposite modified SPE towards As(III) detection exhibits well resolved sharp peak at 0.06 V in HCl and broad peak at around 0.0 V in H₂SO₄ electrolyte. The observed sharp/broad peak is due to the oxidation of As(0) → As(III) [16]. Whereas, no oxidation peak was observed in HNO₃ electrolyte, indicating the prepared nanocomposite is sensitive towards As(III) only in HCl and H₂SO₄ electrolyte. Further, the nanocomposite modified SPE shows higher peak current in HCl compared to the H₂SO₄ and therefore further quantification studies were conducted in HCl electrolyte. The high peak current in HCl is attributed to the complexation of Cl^- ions with As(III) [17].

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The similar CV experiments were conducted using polypyrrole modified SPE and Fe₂V₄O₁₃ modified SPE separately to understand the individual effect on As(III) detection and the observed results were presented in the figure 4, where no obvious peak corresponds to the oxidation of As(0) → As(III) was observed for polypyrrole modified SPE in HCl, H₂SO₄ and HNO₃ electrolytes. Further, no data obtained with Fe₂V₄O₁₃ modified SPE since Fe₂V₄O₁₃ is soluble in the acid electrolytes.

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Thus, the presence of polypyrrole in the composite enhances the stability of Fe₂V₄O₁₃ and thereby resists the solubility in acid electrolyte. Therefore, the observed response for As(III) is attributed due to the cooperative effect of polypyrrole and Fe₂V₄O₁₃ where Fe₂V₄O₁₃ provides electroactive surface and polypyrrole enables the required conducting path for the fast electron transport. Hereafter, the differential pulse anodic stripping voltammetry (DPASV) was used to quantify the As(III) in order to get better analytical response.

The optimized ratio of Fe₂V₄O₁₃ and polypyrrole was selected based on the CV result obtained with the Fe₂V₄O₁₃-polypyrrole nanocomposite prepared using different amount of pyrrole. Figure 5 shows the CV results obtained with the Fe₂V₄O₁₃-polypyrrole nanocomposite prepared using 0.7 mmol, 1.0 mmol and 1.44 mmol pyrrole where we observed that the Fe₂V₄O₁₃-polypyrrole nanocomposite prepared using 1.44 mmol pyrrole gives highest current density for As(III) in HCl electrolyte. Therefore, the Fe₂V₄O₁₃-polypyrrole nanocomposite prepared using 1.44 mmol pyrrole was selected for further work.

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The applied potential significantly affects the current and thus the effect of applied potential on the Fe₂V₄O₁₃- polypyrrole modified SPE towards the reduction of As(III) to As(0) has been carried out. The deposition potential is varied from the potential value of −0.3 to 0.8 V to evaluate the optimized value. As the potential increases from −0.1 to −0.4 V, the peak current increases without change in the shape of the peak (figure 6). However, further increase in potential also increases the peak current with change in the shape of the peak. Therefore, −0.4 V is considered as optimum value for the detection of As(III).

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Similar to the deposition potential, the deposition time also plays an important role in the accumulation of As(III) from the bulk of the electrolyte onto the modified electrode (figure 7). Therefore, the deposition time has been varied from 30 s to 360 s where the peak current increases from 30 s to 300 s and thereafter decreases. Thus, the deposition time of 300 s has been considered as optimized value.

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Then, the DPASV response of the proposed sensor at different concentrations of As(III) was carried out under optimized conditions of deposition potential (−0.4 V) and time (300 s), where the stripping peak current increases linearly with increase in As(III) concentration up to 500 ppb without change in the peak position (figure 8(a)). The correlation coefficient for the linear fit is found to be 0.985 (figure 8(b)), indicating reasonably a good fitting.

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The limit of detection (3σ method) is found to be 0.3 ppb, which is well below the value of 10 ppb prescribed by WHO for As(III) [18]. Further, the observed LOD and the linear range have been compared with the electrochemical methods, reported for the detection of As(III), shown in table 1 [8, 19–22], where the lower LOD and wide linear range makes the proposed sensor very attractive.

The reproducibility, repeatability and stability of the nanocomposite modified SPE are also important for practical applications. The reproducibility of the proposed electrode was examined by using the same electrode for 10 repetitive measurements in 1 M HCl electrolyte containing 50 ppb of As (III) under identical and optimized conditions. The relative standard deviation was found to be ±4.4%, indicating that the analytical performance of the fabricated electrode is highly reproducible. Further, the stripping voltammograms utilizing one single electrode were measured over a period of 30 days and the maximum deviation obtained was 4.5%, indicating the excellent stability and repeatability of the proposed electrode. Owing to this high selectivity and sensitivity with low detection limit and wide linear range the proposed sensor could be used for real sample analysis.

In order to evaluate the proposed analytical method, the Fe₂V₄O₁₃-polypyrrole nanocomposite sensor has been successfully applied for the detection of As(III) present in synthetic water. The results obtained by the proposed method are in good agreement with results obtained by atomic absorption spectrophotometry (AAS) data shown in table 2. Thus, the observed result shown in the table 2 reveals that the proposed sensor could be used as a potential sensor for As(III) detection.