Development of polyindole/tungsten carbide nanocomposite-modified electrodes for electrochemical quantification of chlorpyrifos

The present investigation deals with the development of a novel polymer nanocomposite (PNCs) electrodes for simple, selective and sensitive detection of chlorpyrifos (CHL). PNCs were developed using surfactant facilitated polymerization of indole using different concentrations (wt%) of WC ranging 5–30. Formation of PNCs was ascertained through diversified analytical methods. Electrodes were derived from PNCs over stainless steel substrate for electrochemical quantification of CHL. With concentration of WC, the DC conductivity (10−2 × S/cm) of electrodes was increased ranging 3.54–0.75 at 313 K. Electrochemical impedance spectroscopy reveals well stability of electrodes in phosphate buffer (PBS, 0.1 M) at pH 7.4. The performance of electrodes towards detection and quantification of CHL was investigated through square wave voltammetry. Study reveals that detection and quantification of CHL were dependent on concentration of WC in nanocomposites. Square wave voltammetry reveals that the electrode derived from PNCs with 5 wt% of WC has rendered highest limits of detection and quantification of CHL (10−8 mol/L) up to 5.94 and 18. This work describes a viable method of preparation of synergistic blend of WC in PIN matrix having high electrical conductivity, rapid electron shift, huge surface area and enhanced stability for fast and précised electrochemical detection of CHL.


Introduction
Organophosphates (OPs) are the esters, amide and thiol derivatives of phosphorous-based acid [1][2][3]. Low water solubility and high absorption coefficient impart long residual effect of OPs in soil. This allows the entry of OPs into food chain causing neurotoxicity to animals due to irreversible inhibition of acetyl cholinesterase (AChE) [4][5][6]. For such reasons, development of novel methods of detection of OPs has been the subject of investigation over decades. In this context, various chromatographic, mass spectrometric, electrophoretic, [7][8][9][10] and electrochemical [11][12][13][14] methods were developed for detection of OPs.
Chlorpyrifos (CHL, o,o-diethyl-o-(3,5,6-trichloro-2-pyridinyl)phosphorothioate) belongs to the family of OPs, used for crop protection as insecticide and nematicide. Realizing health hazards imposed over ecosystem, there has been growing concern on advancement of selective, precise, rapid and reliable approach for CHL detection. In this context, the quantification of CHL through chromatographic [15,16] and electrochemical [17,18] methods has been well established.

Preparation of nanocomposites
PIN along with various wt% fractions of WC ranging 5-30 [I-III] was synthesized by cationic surfactant method placed in two-necked glass reaction vessel implemented with mechanical stirrer and dropping funnel. To this, CTAB (2.7 × 10 −3 mol) was added and the content was stirred @500 rpm over 6 h. To initiate the polymerization process, a solution of freshly prepared FeCl 3 (3.5 M) was added to content @1 mL/min keeping stirring to be continued over 24 h. A dark brown precipitate was produced, that was subsequently filtered and successively washed with deionized water till the filtrate became free from chloride ions. Isolated PNCs was left for 6 h at room temperature and then dried at 40 °C at 400 mmHg for additional 8 h. PIN was also prepared under similar reaction conditions in the absence of WC [61,62].

Preparation of electrodes
Working electrodes (WE) were prepared through depositing PNCs over stainless steel plates of 1 cm 2 area. Prior to deposition of PNCs, the plates were thoroughly finished with emery paper followed by ultrasonicated cleaning with acetone. A suspension of PNCs was prepared through ultrasonicating the composition of SPS (7.5 × 10 −3 g) with graphite (5.0 × 10 −3 g) and ammonium per sulfate (80 μM) in the presence of PNCs in NMP (1.5 mL) over 2 h. The suspension (125 μL) was applied over electrode and left at room temperature for 8 h, thereafter dried at 40 °C/400 mm Hg for additional 48 h (Scheme 1). The electrodes with mass thickness ranging 1.80-46.15 (× 10 −3 g) were obtained and employed for characterization [63].

Instrumentations
SEM images were recorded on JEOL, JSM 6610 LV at 0.2 KX (7 µm) and 15 kV. For this purpose, the electrodes were prepared via mentioned procedure. The SEM images were scanned under identical conditions for comparable results. The conductivity data were recorded over Keithley fourpoint probe conductivity nanovoltmeter with current (6221 A) and voltage source (2182 V) in the range of 313-373 K.
The activation energy (E a ) was deduced through Arrhenius equation: where σ is electrical electrodes conductivity, σ o is pre-exponential factor k is Boltzmann constant and T is absolute temperature [64].
The electrochemical studies of prepared electrodes were performed in PBS (0.1 M) over IVIUM Potentiostat-Galvanostat using a triple-electrode cell assembly. WE were fabricated through depositing PNCs over SS plates. Pt foil (1 cm 2 ) and Ag/AgCl were used as auxiliary and reference electrodes. All peak currents (µA) and peak potentials (V) were expressed in due units. Calibration curves were obtained from SWV by plotting maximum peak current and respective increasing concentration of CHL.
Limit of detection (LOD) and limit of quantification (LOQ) were calculated from calibration curve using formula: LOQ = 10 s/m and LOD = 3.3 s/m, where s is intercept and m is slope of the calibration curve. Stability of electrodes was investigated through electrochemical impedance spectra (EIS) and circuit simulation was conducted using Randles diagrams. WC has shown by the low-frequency fingerprint region at 665.82 cm −1 [66].

Results and discussion
[III] shows all the characteristic absorption bands of PIN. EM shows absorptions corresponding to vN-H (3137.12), vC-H (3017.8) and symmetric stretches of WC near the low-frequency fingerprint region (655.84) and out-of-plane Scheme 1 Preparation of electroactive material and modification of electrode using PNCs for CHL sensing   10-22 nm [67]. Diffraction peaks at 31.68° (2.82), 35

Surface morphology of electrodes
Morphology of fabricated electrodes along with quantitative elemental composition was analyzed through SEM-EDX study. To compare the morphology, all electrodes were imaged under identical conditions at 1 KX, 10 μM and camera width of 10 ± 1 nm. EDX spectrum shows the presence of W and high C content in the EM as well as quantity of other elements. EDX represents qualitative detection of elements present in EM. To simulate the emission of X-rays, high-energy beam of charged particles was focused on to the EM. The difference between energies of higher and lower energy shell is released in the form of X-rays which is measured by spectrometer.
SEM and EDX of WC, PIN and PNCs are sequentially shown in Fig. 3a1-a3 and b1-b3, respectively. The dark and bright phases in SEM images represent the morphology of PIN and WC, respectively. Electrodes derived from PIN show a co-continuous morphology without any distinct phase separation. SEM of PIN-coated electrode showed granular structure with rough morphology, which concludes that the nature of particles formed was irregular. Blending of PIN with WC, has afforded with rough morphology. It is observed from SEM images that intercalation of WC into PIN matrix also increases, which leads to roughness in electrode surface. The synergy between the constituents may be weak or strong, which is regulated by the technique used for synthesis of the composite [69]. The reason of surface roughness may be due to the non-homogenized distribution of WC into polymer phase during the procedure of electrode preparation. The morphological structure of PNCs is found to be different from PIN and reveals homogeneous dispersion of WC nanoparticles into PIN matrix.

Thermal stability
Information regarding thermal stability of PIN as well as [III] is provided by thermal studies (Fig. 4). The weight loss caused by evaporation of physically adsorbed moisture and residual solvents is observed in the curves at a temperature lower than 200 °C. The weight residue (Wr) corresponding to TG is expressed as %w/w. DTA signals and rate of degradation of samples in DTG are expressed as mV and mg/ Cel, respectively. The heat of fusion data revealed through DTA has been expressed in mJ/mg. TG onset shows thermal stability of WC up to 384 °C. WC shows weight gain of 104.90% due to oxidation at 528 °C [22]. Decomposition of WC was progressed at the rate of − 21.97 × 10 −3 mg/°C at 552 °C with DTA signal (0.22 mV) at 548 °C Decomposition of WC was concluded at 597 °C leaving char residue of 114.70 wt% (Fig. 4a).
PIN was decomposed with TG onset at 337 °C leaving 79.4% Wr. Decomposition of PIN was progressed at the rate of 197.6 × 10 −3 mg/°C at 559 °C with DTA signal (0.65 mV) at 543 °C. Decomposition of PIN was concluded at 573 °C  (Fig. 4b).
[III] shows single-step decomposition with TG onset at 328 °C leaving 85% Wr. The collective weight loss of 15.0% wt indicates that due to filling of WC, the thermal stability of PIN was compromised. Decomposition of [III] was progressed at the rate of 0.858 mg/°C at 519 °C with DTA signal (0.61 mV) at 520 °C. TG endset of [III] was appeared at 538 °C leaving 36.2% char residue. DTA reveals fusion of [III] with ∆H f of − 7.44 × 10 3 mJ/mg (Fig. 4c). Thermal data reveal that due to addition of WC, the thermal stability of [III] was compromised.

Electrical conductivity
The linear variation in I-V (Fig. 5a) at room temperature implies the ohmic conduction behaviour of WC in PIN [70]. The increased conductivity attributes to high conductivity of WC as well as interaction between PIN and WC which favored charge transfer process [71]. Effect of WC on σDC of electrodes was studied under various voltages ranging 1-100 V at room temperature (Fig. 5b). It was found that the conductivity of the [III] increased with concentration of WC and maximum for 100 V which is 3.54 × 10 −2 Scm −1 . The σDC measurements were also performed at temperature range 313-373 K (Fig. 5c). It was observed that increasing value of σDC with temperature defines the semiconducting behaviour of PNCs and maximum σDC was observed for [III]. Increase in σDC with temperature attributes to enhanced charge mobility across PNCs. Using theoretical Arrhenius model, the calculated activation energies for

Cyclic voltammetry
CV is the most popular continuous wave technique employed to investigate the redox behavior and electron transfer kinetics of molecules over electrochemically active surface. CV presents the set of anodic (E pa ) and cathodic peak (E pc ) potentials along with respective anodic (i pa ) and cathodic (i pc ) peak currents. Potential applied across WE moves back and forth past the formal potential. This leads to current flow across the electrode that renders the redox behavior of analyte [72,73].
Redox behavior was investigated through scanning the CV of CHL in PBS (0.1 M, pH 7.4) @ 40 mV s −1 over WE derived from PNCs [I] and [II] (Fig. 6). Voltammograms were recorded till fivefold degeneracy in potential window of − 0.11 to + 0.55.
[I] has rendered a well-defined redox behavior with peaks peak potentials (− V) ranging 0.089-0.418 V. Under identical conditions, [II] has shown the peak potentials (− V) of 0.093 V and 0.43. Increase in peak current attributes to the formation of electron-conducting tunnel during redox reaction of CHL over [II].The electron transfer mechanism during redox behavior f CHL over [II] has been presented in Fig. 7. Based on peak potential deuced from CV of [II], the redox behavior of CHL over electrodes was found to lead according to one electron transfer mechanism between electrode surface and CHL generating synergy on EC properties [74].

Effect of scan rate on CV
Frequent elevation in scan rate has raised the Ipa (µA) of [I] from 0.084 to 0.279 (Fig. 8a). The coinciding anodic peak potential was observed ranging from − 0.089 to − 0.013 V. At the same time, there was reduction in Ipc (− µA)   (Fig. 8b). However, no redox behavior was shown by [III] irrespective of scan rate (Fig. 8c). Figure 9 shows linear regression curve for [II] between Ipa and square root of scan rate. A comparative account reveals higher electrochemical behavior of [II] over [I].

Square wave voltammetry
The SWV experiments have been performed in the potential window (V) from − 2.0 to 2.0 at pulse amplitude 2 mV, scan increment 20 mV and frequency 25 Hz. A comparative SWV voltammograms of electrodes in 0.1 M PBS (pH 7.4) at varying concentrations of CHL are sketched in Fig. 10a-c. The presence of WC in the PIN matrix has raised the peak current that attributes to increase in the sensitivity towards PNCs. With concentration of CHL, the peak was increased over [I] from 6.204 to 7.808 (Fig. 10a). Under identical conditions, the peak currents of [II] and [II] were raised in the range of 7.699-8.99 and 9.892-11.90 (Fig. 10b, c). Linear range of proposed CHL sensor was found to be 25 × 10 −7 to 225 × 10 −7 mol L −1 and calibration curves obtained between maximum peak current and concentration of CHL added in SWV were used to calculate LOD and limit of quantification LOQ.
The proposed sensor demonstrates acceptable quantification of CHL with significantly low LOD values.

Electrochemical impedance spectroscopy
The impedance data have been expressed in Hz and spectrum was examined by equivalent electrical circuit model (Fig. 11). The elements of the circuit are general electrical components, which include polarization resistance or charge transfer, solution resistance and a double-layer capacitor. EIS measurements were managed by FRA. Nyquist plots at higher and lower frequency range appeared as semicircle attributing to electron transfer resistance (Ret). High impedance in the form of semicircular loop was observed at the surface of electrodes covered with coating of PNCs; whereas, loops were opened within 24 h representing the destruction of coating.
[III] has shown R p of 2.669 × 10 2 and |Z| appeared at 2.998 × 10 2 . With the fabrication of electrode with PNCs, the diameter of semicircle decreased, which shows that PNCs possess high electrical conductivity with low charge transfer resistance. It is observed from  Fig. 11 that the diameter of the semicircles was successively decreased in the order from [I] to [III], that indicates successive increase in electron transfer at the surface of PNCs-coated electrode. Best semicircular loop in [III] represents highest charge storage and capacitive behavior of [III]. In case of EIS studies [II], opening of semicircular loop indicates the diffusion of electrolyte and that is also responsible for cracking of electrode surface as shown in SEM images (Fig. 12a, b).

Conclusion
A series of electrochemical polymer nanocomposite (PNC)based electrode was developed by an easy and effective chemical oxidative polymerization method using various concentrations of WC ranging 0-30 wt%. Electrodes from PNCs were developed over SS plates and used for electrochemical quantification of chlorpyrifos (CHL) in phosphate buffer (PBS, 0.1 M) at 7.4. Electroanalytical methods based on square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) reveal immense feasibility of electrodes towards quantification of CHL with stability in PBS. The electrical and electrochemical behavior of electrodes was found synergistic with concentration of WC in PIN. DC conductivity of electrodes was found in increasing order with concentration of WC. Electrodes with 5 wt% of WC have shown enhanced redox behavior of CHL with limits of detection and quantification (10 −8 mol L −1 ) up to 5.94 and 18. Study reveals that incorporation of WC in PIN provides a novel nanohybrid electrode coating material for efficient detection of CHL.
Acknowledgements Financial support by the Ministry of Defence No. ERIP/ER/0703649/M/01 is hereby acknowledged.

Conflict of interest No potential conflict of interest was reported by the author.
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