Electrochemical sensor based on PVP coated cobalt ferrite/graphite/PANI nanocomposite for the detection of hydrazine

In this study, we developed a multi-layered electrode as an efficient nanocomposite electrochemical sensor for detecting carcinogenic hydrazine in water. Nano-cobalt ferrite (CoFe2O4) was prepared using poly (vinylpyrrolidone) (PVP), mixed with an optimum quantity of graphite and pasted on a stainless-steel current collector. The nanocomposite was further hybridised by electrodepositing with polyaniline (PANI). The obtained composite was characterized using XRD, FTIR, SEM, and electrochemical techniques. XRD analysis shows the successful formation of composite from individual precursors. According to SEM, wrinkled and layered morphology for graphite and aggregate clusters for cobalt with spike or tubular structure for polyaniline were observed. When subjected to amperometry current, the prepared electrode showed different peaks for different concentrations of hydrazine, such as 1 μM to 100 μM. Cyclic voltammetry studies showed an increase in oxidation and reduction peaks. These studies will lead to a new platform for their potential applications in detecting toxic materials in real samples such as water, plastic bottle, water etc.,


Introduction
Hydrazine is widely used as a foaming agent, preparing polymer foams, agrochemicals (pesticides and insecticide), storable propellent for spacecraft, rocket fuel, disinfectants, fuel cells, and reducing agents [1].Being a toxic chemical that leads to skin burns and eye damage, it also has corrosive properties and is carcinogenic to humans.The International Agency for Research on Cancer proved that high levels of hydrazine can cause cancer.Based on IDLH (Immediately Dangerous to Life or Health Concentrations), respirators are to be worn for inhaling hydrazine concentrations above 0.03 ppm.Currently, OSHA (Occupational Safety and Health Administration) standard concentration limit is 1ppm (1.3 mg m −3 ) to exposure [2].The toxicity of such material can be dangerous to human health and the environment.To treat such problems, consequently, sensors have been developed to detect toxic materials and satisfy goal 6, clean water and sanitation.Being an electrochemically active compound, an electroanalytical technique can be used for effective detection of hydrazine using electrodes.
Cobalt ferrite (CoFe 2 O 4 ) nanoparticles are well-known as magnetic material because ferrite exhibits a form of magnetism.Along with physical and chemical stability, high-coercivity cobalt ferrite have been widely used in sensors, solar cells, recording devices, and magnetic drug delivery.Cobalt ferrite improved detection ability as reported by Ying He [3] in a review article.One of articles discussed an electrode loaded with BiNPs and CoF 2 O 4 nanocomposite acts as sensor that can detect Pb +2 and Cd +2 simultaneously in water samples.T Mekuria [4] reported cobalt ferrite (CFO) and carbon nanotube (CNT) composite for detection of Cr-IV in water wherein a change in the structure of CFO/CNTs was observed when compared to a sample without Cr-IV.The change indicates that Cr is incorporated into the CFO structure and hence, enhances the detection limit.The CoFe 2 O 4 lacks conductivity, therefore it needs to be hybridized with conducting material.One such material is conducting polymer.Among all the conducting polymers, polyaniline is easy to synthesize and has a good binding ability.
Polyaniline is known as an organic conducting polymer, and composite with inorganic metal (Co, Cu) and carbon material enhances their conductivity and wettability [5].Developing a sensor for hydrazine PANI i.e., conducting polymer has gained the attention of scientists because of its low cost, unique redox behaviour, good chemical stability, and high conductivity ease of blending with other materials.Sadia Ameen [6], using a modified electrode based on an electrochemically synthesized PANI and graphene composite sensor was able to detect hydrazine from 0.01μm to 0.01 M. As concentration increased the peak of cyclic voltammetry enhanced indicating increased detection ability of the sensor.Balakumar Vellaichamy et al synthesized copper-PANIgraphene nanocomposite which detected 10 μM to 90 μM of the hydrazine using CV and chronoamperometry.Due to the electrooxidation of hydrazine, the current peaks of cyclic voltammetry between 0.0 to 1.1 V increased with increasing in concentration of hydrazine [7].Furthermore, to enhance the connectivity and conductivity of CoFe 2 O 3 and conducting polymer, graphite was mixed to get the cobalt ferrite/graphite/PANI composite electrode.The carbon-based metal i.e., graphite is a non-metallic conductor of electricity and withstand high temperatures.It is cheap, insoluble in water, acid, base.Most important it is noncorrosive material.For chemical sensors, the presence of graphite will be able to detect specifically one molecule of a potentially dangerous substance.MFe 2 O 4 @GO [M(metal) = Cu, Co, or Ni] was synthesized by Allen et al for the removal of dye particles from aqueous solutions.Maximum sensing capacities of the metal ferrite were 25.81, 50.15, and 76.34 mg g −1 [8] Sankararao Mutyala (2015) synthesized graphene nanoflakes from graphite for the detection of hydrazine.GNF-modified glassy carbon electrode shows hydrazine sensing at lower potential with high selectivity and stability and response time less than 3 s [9].The works of literatures survey show that article related to cobalt ferrite based electrochemical sensor hybridised with conducting polymer is rare.Along with that hydrazine specificity based on the structural morphology of the electrode surface has been less discussed.The study on the stability of electrode materials which can adhere to is inadequate to describe the importance of conducting polymer.
This report aims to develop a simple and effective sensor to detect hydrazine concentration.Materials that are used have low cost, high stability, nontoxic.Sensors show short response in time and very low detection of hydrazine.The potential of PANI/Graphite/Cobalt ferrite sensors studied through changes in electrical conductivity upon the addition of hydrazine.The novel combo of electrochemical double layer from graphite; redox behaviour from PANI and structural support from PVP coated CoFe boosts the sensitivity of hydrazine.The stability, reliability, and reproducibility of the composite material are studied and tested for real water sample.

Synthesis of PVP coated cobalt ferrite nanoparticles
About 9%(w/v) of the PVP solution was made using ultra-pure water.To the PVP solution, 0.01 M cobalt chloride and 0.02 M ferric chloride (FeCl 3. 6H 2 O), about 1:2 ratio, and iron precursor were added and stirred continuously to obtain a homogeneous mixture [10].0.1 M NaOH solution was added dropwise until pH 12 was obtained.The solution was further ultrasonicated for about 20 min and then the contents were transferred to 250 ml round bottom flask.The solution was refluxed at 160 °C for 16 h with the stirring a rate of 100 rpm.The solution was cooled to room temperature and centrifugated at a rate of 100 rpm.The precipitate was washed several times with water and ethanol.The was heated (calcinated) at a high temperature of 400 °C and allowed to cool.The powder obtained is labelled as CFP and stored in an airtight container for further experimental purposes.

Preparation of working modified electrode
Stainless steel is used as the base for coating of composite (graphite and cobalt ferrite) followed by electrodeposition of polyaniline.1:1 cm length of stainless steel was polished with the help of aluminium powder and scratched using sandpaper and washed with distilled water.For further process, 0.5 g of each graphite and cobalt ferrite (1:1) was weighted and mixed properly using a binder made into a paste.The paste was applied uniformly on the surface of the stainless-steel electrode using the doctor blade method.This modified electrode was kept in oven at 60 °C for 24 h.

Electrodeposition of PANI on modified electrode
Cobalt ferrite and graphite composite-based dried electrode was used for electrodeposition of PANI using cyclic voltammetry.In the cleaned beaker about 30 ml of 0.1 M sulphuric acid and 0.01 g of surfactant were added.0.1 M of aniline was added dropwise and dissolved with continuously stirred condition.Furthermore, threeelectrode system (modified CF/G/PANI electrode as working electrode, platinum wire as counter electrode, and calomel electrode as reference electrode) were dipped in the above electrolytic solution, using cyclic voltammetry of potential about 50 mV s −1 , PANI was deposited via electro oxidative reaction.Green colour coating appeared uniformly on the surface of the modified electrode.

Detection hydrazine
Detection of hydrazine was carried out by cyclic voltammetry and amperometry method. 1 μM to 100 μM hydrazine solution were prepared for CV detection.In a cleaned beaker about 30 ml PBS solution were added, and three-electrode system were dipped in the beaker.Similarly, the amperometry method was used to detect the concentration from 1 μM to 100 μM hydrazine solution.

Characterisation techniques
Scanning electron microscopic (SEM) analysis (EVO MA18) was done to study the surface morphology of CF/ G/PANI nanocomposite using a spectrophotometer.x-ray diffraction (XRD) was performed using x-ray diffractometer (Rigaku Miniflex 600 (5th gen)) scan rate about 0.75 per second.FTIR spectra were obtained from the FTIR spectrophotometer ( Schimadzu 400 MHz Bruker spectrometer) of CF/G/PANI in the range of 400-4000 cm −1 used to determine functional groups.Electrochemical measurements were performed by cyclic voltammetry and chronoamperometry with three electrode system.Modified CF/G/PANI electrode as working electrode, platinum wire as counter electrode, and calomel electrode as a reference electrode.

Results and discussion
Cyclic voltammetry (CV) is used to deposit polyaniline on modified CF/G electrode figure 1 at a scan rate of 50 mVs −1 between −0.2 to 0.8 for 25 cycles.However, as the number of cycles is increasing, the oxidation peak is shifting towards right direction (anodic direction) and reduction peak is shifting towards left direction (cathodic direction) with an increase in the current [11].This condition shows that polyaniline is deposited on a modified electrode with uniform distribution and good adhesion.CV curve of PANI has four specific peaks.Two are anodic current peaks, and two are cathodic current peaks.First anodic and cathodic peaks are independent of pH because no proton is involved in the reaction.Whereas the second anodic peak and cathodic peak shifts on the left side as the pH increases because proton concentration increases in the solution.Figure 1(b) shows the comparison between the CV curve of PANI deposited modified electrode and only modified electrode (CF/G).The modified electrode (CF/G) showed pseudocapacitive behaviour with a low current window, while the The FTIR analysis provides an information regarding the interaction of composites and the FTIR spectrum of electrode materials is shown in figure 4. FTIR analysis of the synthesized cobalt ferrite sample shows a 422 cm −1 band due to intrinsic Co-O stretching vibration.877 cm −1 and 1155 cm −1 attribute Fe-Co alloy system [12].3378 cm −1 and 1639 cm −1 correspond to O-H and C=O respectively.The bands are mainly because of the absorption of water molecules [13].Similarly, CF/G/PANI FTIR analysis 1137 cm −1 to  1220 cm −1 absorption bands C-N stretching, 1291 cm −1 to 1339 cm −1 bands corresponds to C-N aromatic amine stretching [5].That indicates the successful coating of polyaniline on CF/G/PANI electrode surface.CF/ G composite shows an absorption band at 1461 cm −1 that indicates C=C aromatic vibration.Graphite comprises various oxygen functional groups that attribute O-H stretching vibration at 3378 cm −1 [14].At 442 cm −1 band corresponds to cobalt metal [13].
Figure 5 shows the XRD spectra of CF, CF/G, and CF/G/PANI, respectively.XRD results of cobalt ferrite show diffraction peaks at angle 2θ = 31, 36, 43, 58, and 63 corresponding to the plane (220), (311), (400), (422), (440,) respectively [15].It indicates the formation of nanocrystalline cobalt ferrite, and the weak intensity of crystalline peaks explains the successful coating of PVP [16].In figure 5 CF/G, (modified electrode), graphite shows a high intense peak at 2θ =26.7°and a slight peak at 2θ =54°corresponding to the plane (002), (004) and d-placing 3.5 A°and 1.9A°.Compared to graphite, cobalt ferrite has very small size so failed to detect diffraction pattern [5].In CF/G/PANI electrode, the intensity of the graphite diffraction pattern is decreased because the coating of PANI decreases the crystallinity of graphite on the surface of the electrode [17].From this result, we confirmed that PANI is uniformly coated on the surface of the modified electrode.

SEM analysis
The surface morphology of the modified electrode was studied under a scanning electron microscope.The image of cobalt ferrite nanoparticles (figure 6(a)) shows a spongy surface due to the presence of PVP. Figure 6(b) shows both graphite and cobalt ferrite together on the surface of a graphite cluster of small spherical particles, showing the presence of cobalt ferrite nanoparticles.They are always present in aggregated forms due to magnetic dipolar interaction.Figure 6(c) shows graphite particles having irregular, flat, plate-like, flaky morphology [5,16].Figure 6(d) represents the deposition of PANI on the CF/G electrode.PANI shows a spongy, grainy layer, having high porosity flower-like microstructure [18].It formed a compact layer on the electrode surface and adhered firmly.Even the presence of nodule-like structure of PANI having short nanorods is visible due to high nucleation in these regions [18].Hence, graphite provides a high surface area, PANI with electric conductivity, and cobalt ferrite has magnetic properties.

Electrochemical detection of hydrazine
The pH value of an aqueous solution influences the electrochemical behaviour of hydrazine.Hence, optimization of pH was performed for electrocatalytic detection of hydrazine.In figures 7(a) and (b), the peak current obtained from CV response increases up to pH 7 and shifts towards lower current values with increase in pH value.While peak potential shifts towards negative slightly up to pH 7 and later sharply.It implies higher oxidation potential of hydrazine and it is easier to oxidize at pH 7. Hence, pH 7.0 of PBS buffer was used in the present study.
Detection of hydrazine was tested by using cyclic voltammetry (figure 8).The potential is fixed at the range from 0.0 to 1.1 V and scanning rate 50 mVs −1 [7].30 ml of 0.1 M PBS solution is taken in a beaker and dipped in three electrodes [19].increases proportional to the increasing hydrazine concentration.The reason is that after the addition of hydrazine into the electrolytic solution, N 2 H 4 gets oxidized to N 2 H 5 + .This hydrazine ion releases the proton and gets oxidised on the electrode surface.A potential of 0.22 V was selected as the oxidation potential of hydrazine for achieving good sensitivity in measurements.

Chronoamperometry
The sensing capability of CF/G/PANI electrode was investigated further using the amperometric current-time curve technique.The applied potential of + 0.22 V was chosen based on the CV data.In figure 9 (a), the amperometric response of CF/G/PANI enhances along with the concentration of hydrazine from 1 to 100 μM.
The linear detection range of CF/G/PANI is from 1 μM to 100 μM with the linear regression equation of I (μA) = y = 16.5962+ 0.2209x (R 2 = 0.9893), as shown in the figure 9 (b).
The detection limit of the electrochemical sensor based on CF/G/PANI has been estimated to be 2.24 μM using the formula, LOD = 3S b /S where S b is the standard deviation and S is the slope of the calibration plot [20].The applied potential was + 0.22 V. Successive addition of 1 μM to 100 μM hydrazine at regular interval shows fluctuations of current drift as the hydrazine concentration which is sufficient for the sensing [21][22][23].

Square wave voltammetry (SWV)
The SWV signal depends on pH and the electroanalytical parameter [24].The current showed a linear relationship with concentration (concentration range of 1.0−10.0μM; Ipa = 0.635x + 5.503; R 2 = 0.9975 and  10.0−100 μM: Ipa =0.07033x + 18.74; R 2 = 0.9901).As the hydrazine concentration increased, the I (μA) of the hydrazine also gradually increased as shown in figure 10.A clear oxidation peak was observed around + 0.25 V which is having almost good agreement with cyclic voltammetry results.

Selectivity, stability and reproducibility of the sensor
The interfering agents usually show up during electrochemical sensing of hydrazine in real samples.The interference/selectivity studies were performed using SWV at 0.25 V with 50 μM hydrazine solution in 0.1 M phosphate buffer at pH 7 in the presence of 50 μM various ions (Mg 2+ , K + , Zn 2+ , Fe 2+ , Na + ).The figure 11 shows no prominent change with relative standard error less than 7% in the presence of other interfering ions, which indicates that CF/G/PANI sensor is capable of sensing hydrazine without much influence of interfering ions.After SWV analysis the electrode were stored in PBS buffer solution and a decrease in current response of 1.6 (± 0.05) % after one week was observed.Further, change in current of 5.6 (± 0.09) % after two weeks was recorded.Four different sets of CF/G/PANI were prepared to check the reproducibility of the sensor.The electrodes were tested using 40 μM of hydrazine repeatedly for 25 cycles.The response of all the four electrodes showed RSD of 2 (± 0.05) %, which indicates the good reproducibility of CF/G/PANI electrode.

Real sample analysis
The validity of CF/G/PANI sensor was performed with the collected water sample from outlet of metal and refining industry near Gurupura river, which is located at Baikampady, Mangaluru (12.947796, 74.833865).We employed standard addition method wherein known amounts of aqueous samples were mixed with real samples  in 0.1 M PBS and detected using CF/G/PANI electrode.Table 1 shows that present modified electrode was able to detect the presence of hydrazine in real water samples.Hence, CF/G/PANI electrode is reliable for electrochemical sensing of highly polluted water.

Mechanism
The PVP stabilized the aggregation of cf Moreover, the presence of O and N in PVP extended the affinity of these particles to interact with graphite-layered particles.CF nanoparticles did not wholly block the sight of conductive graphite, as pseudocapacitive behaviour was observed in the CV of the CG/G electrode [25].Furthermore, the dispersed CF nanoparticles provided nucleation sites for the growth of PANI on the surface of graphite.Hence, uniform coating was observed with protonated electrode material.When hydrazine was   Various concentrations of hydrazine were detected by the same method and electrode [26].The advantage of our sensor is that after several cycles the PANI surface deteriorates exposing the CF/G layer.This CF/G layer can be redeposited with PANI giving fresh surface.Comparison of electrocatalytic oxidation of hydrazine using various electrode materials is shown in table 2.

Conclusion
In summary, we synthesised an advanced and effective CF/G/PANI sensor for the detection of carcinogenic hydrazine.Compared to other studies of sensors, CF/G/PANI sensor, the material which is used has low cost, high stability, and non-toxic.Modified electrode materials were characterized using CV, SEM, FTIR, and XRD.Sensors show short response in time with very low detection of hydrazine and are also easy to manufacture.Hence, this method is a promising chemical sensing probe with a ternary mixture of metal oxide, carbon, and conducting polymer to detect environmental toxins.

Figure 2 .
Figure 2. (a) CV response of prepared electrodes in 0.1 M PBS solution, (b) Nyquist plot of prepared electrodes.
Figure 8(a)  shows CV behaviour after each addition from 1-100 μM hydrazine, and for better clarity CVs of 10-90 μM (figure8(b)) was plotted.The CVs show that the intensity of anodic peak

Figure 7 .
Figure 7. Effect of pH on (A) peak current and (B) peak potential, for the oxidation of 40 μM hydrazine in 0.1 M PBS (pH = 7.0).

Figure 9 .
Figure 9. (a) Amperometric response of the CF/G/PANI electrode towards sequential addition of hydrazine at +0.22 V in PBS solution.(b) calibration curve representing the response of CF/G/PANI electrode with different concentrations of hydrazine in a three-electrode system.

Figure 10 .
Figure 10.Square wave voltammetry (SWV) response of the CF/G/PANI electrode towards sequential addition of hydrazine at + 0.22 V in PBS solution.

Figure 11 .
Figure 11.Variation in the response current of CF/G/PANI in the presence combined interferences.

Figure 12 .
Figure 12.Most probable mechanism of hydrazine sensing on electrode surface.

Table 1 .
Real sample analyses by CF/G/PANI with SWV method.introduced, the special cage-like structure, as shown in figure12, trapped them and got oxidized to N 2 H 5 + .

Table 2 .
Comparison of electrocatalytic oxidation of hydrazine using various electrode materials.