An efficient electrochemical sensing of hazardous catechol and hydroquinone at direct green 6 decorated carbon paste electrode

In this proposed work, direct green 6 (DG6) decorated carbon paste electrode (CPE) was fabricated for the efficient simultaneous and individual sensing of catechol (CA) and hydroquinone (HY). Electrochemical deeds of the CA and HY were carried out by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) at poly-DG6-modfied carbon paste electrode (Po-DG6-MCPE). Using scanning electron microscopy (SEM) studied the surface property of unmodified CPE (UCPE) and Po-DG6-MCPE. The decorated sensor displayed admirable electrocatalytic performance with fine stability, reproducibility, selectivity, low limit of detection (LLOD) for HY (0.11 μM) and CC (0.09 μM) and sensor process was originated to be adsorption-controlled phenomena. The Po-DG6-MCPE sensor exhibits well separated two peaks for HY and CA in CV and DPV analysis with potential difference of 0.098 V. Subsequently, the sensor was practically applied for the analysis in tap water and it consistent in-between for CA 93.25–100.16% and for HY 97.25–99.87% respectively.


Scientific Reports
| (2021) 11:15064 | https://doi.org/10.1038/s41598-021-93749-w www.nature.com/scientificreports/ HY and CA procured from Sigma-Aldrich and standard solutions (25 × 10 -4 M) was prepared in double distilled water. DG6, sodium dihydrogen phosphate (NaH 2 PO 4 ), disodium hydrogen phosphate (Na 2 HPO 4 ) were gained from Merck chemicals of AR grade and all aqueous solution were prepared with double distilled water. All the reagents are utilized in this study with analytical grade and used as received.

Results and discussion
Preparation of UCPE and Po-DG6-MCPE. The UCPE was fabricated by blending of graphite powder and silicone oil in the ratio 70:30 (w/w) for about 30 min and get homogeneous mixture. The gotten blend was then filled with homemade Teflon cavity consuming 3 mm internal diameter and cooper wire was utilized for the electrical contact. The pre-treated carbon paste electrode (PCPE) was constructed by electrochemical oxidised by cycling the potential between − 0.6 to 1.0 V in 0.1 M NaOH with speed rate of 0.05 V s -1 at 10 multiple cycle.
Electro-polymerisation manner was applied for the constructed fabricated electrode. DG6 (1.8 mM) was carried on the surface of UCPE using cyclic voltammetry in the existence of 0.1 M NaOH (supporting media) and cyclic the potential scanned between − 0.6 to 1.0 V with speed rate 0.05 V s -1 for 10 polymerization cycles as elucidates in Fig. 1a. As witnessed from the figure, the peak current gradually boosted with increasing the polymer cycles this endorses the growth of polymeric films on UCPE 37 . The deposition of DG6 on UCPE was carried by changing the polymer series 5 to 25 cycles (Fig. 1b) and applied to identify the electrochemical reactions towards CA in 0.2 M PBS of pH 7.4. By perceiving the Fig. 1b, the Ipa was enhanced upto ten cycles and after 15 cycles it decreases slightly and sudden increment in Ipa for 20 and 25 cycles. Therefore, ten cycles of electro-polymerization were selected and optimised for the fabrication of modified electrode.   where A is the area of working electrode (cm 2 ), C 0 is the concentration of the electroactive species (mol/cm 3 ), n is the electrons transformed, ν is the sweep rate (V/s), D is the diffusion coefficient (cm 2 s −1 ), Ip is the peak current, Γ (M/cm 2 ) is the surface average concentration and R, F, T are the physical constants.

Characteristics and surface property of
Electrocatalytic performance of CA at Po-DG6-MCPE. The sensing proficiency of Po-DG6-MCPE towards the CA was scanned by CV technique as most sensitive and precise voltammetric technique. In Fig. 4 the dotted line (green), dashed line (pink) and hard line (red) depicts CVs for UCPE, PCPE and Po-DG6-MCPE for CA (10 μM) in 0.2 M PBS with speed rate of 0.05 V s −1 . The UCPE provides CVs with reduced and broad response and at PCPE it gave voltammetric response with poor response. Compared UCPE and PCPE the designed electrode signifies ultimate enrichment in peak current with very precise sensitivity with sharp peaks. The peak potential difference (ΔEp = Epa-Epc) was acquired at 0.153 V (BCPE), 0.095 V (PCPE) and 0.017 V (Po-DG6-MCPE) individually. At tailored electrode the electron transfer was easier than UCPE and PCPE, because where ΔEp value is inferior and transfer of electron rate will be greater. Therefore, the Po-DG6-MCPE (1) Ip = 2.69 × 10 5 n 3/2 AD 1/2 Co ν 1/2 Effect of speed rate and concentration study of CA at Po-DG6-MCPE. The Po-DG6-MCPE process was assessed by changing the speed rate. Figure 5a elucidated the CVs for CA (10 μM) in presence of supporting media with altered speed rates. As seeming in figure, the redox (oxidation and reduction) peak current of CA was successively boosted as increase in the speed rate (60 to 220 mV s −1 ) and minute move of their peak potential to positive and negative side. The linear correlation between anodic peak current (Ipa) versus speed rate (ν) and Ipa versus square root of the speed rate was drawn in Fig. 5b and c. The gotten plot gave very fine straight line with correlation coefficient value (R 2 ) was initiate at 0.9991 and 0.9937 correspondingly. Consequently, by spotting the overhead practical result the kinetic property of Po-DG6-MCPE was originated at adsorption controlled process 42,43 . Heterogeneous rate constant (k 0 in s −1 ) were computed by Eq. 3 and attained results are tabulated in Table 1.  www.nature.com/scientificreports/ where, k 0 , ΔEp, ν, is the heterogeneous rate constant in s -1 , difference in peak potential, speed rate respectively. The revealing of detection sensitivity at modified electrode was assessed by utilizing CV and DPV performances. Figure 6a and b epitomizes the obtained CVs (5-45 μM) and DPVs (10-45 μM) for altered concentrations of CA in presence of pH 7.4 with speed rate of 0.05 V s −1 . These figures evidently portray that the peak current was boosted noticeably when the analyte concentration was rises. Inset Fig. 6a and b indicates the correlation between Ipa and concentration of analyte with good linearity values of R 2 : 0.9992 and 0.9993. Utilizing the slope value (M) and standard deviation (S) of the Ipa (acquired from inset Fig. 6a and b) assessed the LLOD and LOQ employing Eqs. (4) and (5) Fig. 7a, it obviously exposed that as the pH solution was changed then the peak potential of CA was moved to more negative side. This accomplished result was proof for the directly participation of proton in the electrochemical reaction. The linear connection between Epa and varied pH of CA was clarified in inset Fig. 7b. The attained regression eqn. is expressed for CA as Epa (V) = − 0.049 pH + 0.445 (R 2 = 0.9961). The acquired slope value 49 mV pH −1 was very close to the Nernstian theoretical value (59 mV), so it clearly advises the equal number of electron and proton was involved in the electrode reaction 43,45 . Figure 7c portrays the plot of Ipa versus altered pH solution.    Where ΔEp is lower than electron transfer rate will be greater, hence at the constructed electrode electron transfer is easier than UCPE. Thus, the Po-DG6-MCPE achieved as good sensor and prominent for the investigation of HY and redox mechanism was signifying in Scheme 2.

Impact of speed rate and concentration variation on HY at Po-DG6-MCPE. The speed rate study
offered the essential confirmation about the electrode process. Figure 9a exhibits the gotten CVs for HY in the existence of 0.2 M PBS (pH 7.4) with various speed rate (0.06 to 0.24 V s −1 ). By witnessing the Fig. 9a, as the speed rates elevated the redox peak current is improved subsequently with tiny move in their peak potential to positive and negative potentials. The relationship between Ipa versus speed rate and Ipa versus square root of speed rate was plotted in Fig. 9b and c correspondingly. The plotted graph gave straight linearity with R 2 value was originate at 0.9997 and 0.9984 respectively and electrode process was governed by adsorption controlled process. The LLOD and LOQ was calculated utilizing Eqs. (4) and (5) for HY by applying the CV and DPV performance. Figure 10a and b epitomizes the obtained CVs (5-40 μM) and DPVs (10-45 μM) for altered concentration   Fig. 10a and b, the oxidation peak current of HY was growths linearly as the concentration raises and peak potential tiny swing towards negative and positive direction. The inset Fig. 10a and b portrays the connection between concentration of HY and Ipa and it gave very adequate linearity with R 2 value 0.9984 and 0.9979. The LLOD and LOQ was establish at 0.11 and 0.36 μM correspondingly. The Po-DG6-MCPE offered low LLOD for CA and HY than other fabricated sensor and displayed in Table 2. Here also (Fig. 11b), at UCPE it fails to separate the oxidation peaks for CA and HY but at the designed electrode it clearly depicts two well distinguish sharp peaks (0.105 and 0.014 V) with improvement in their peak current than UCPE. Therefore, the designed Po-DG6-MCPE was excellent capability for the simultaneous recognition of CA and HY.

Simultaneous sensing capability of
Selectivity and stability of CA and HY at Po-DG6-MCPE. The efficiency and selectivity detection of CA and HY was testified at Po-DG6-MCPE by applying the DPV technique. Figure 12a represents the tracing of CA by kept the concentration of HY (20 μM) was constant. As we perceived, the peak current of CA was boosted by increasing the concentration of CA in the range 20-160 μM. Inset the Fig. 12a implies the linearity graph of Ipa versus altered concentration of CA. Likewise for HY, the concentration was altered in the range 20 to 160 μM and CA (20 μM) concentration was kept constant and signified in Fig. 12b. By witnessing the above consequence, as the concentration of analyte increased the peak current was boosted gradually but there was no variation in peak potential and peak current of constant analytes. Inset the Fig. 12b displays the linearity The stability of the Po-DG6-MCPE was assessed for CA (10 µM) in presence of 0.2 M PBS for 10 cycles with speed rate of 50 mV s −1 by utilizing CV system. As depicted in Fig. 13 the redox current remains steady and after accomplishment 10 multiple cycles the small reductions in their redox current was 4.57%. The percentage degradation was assessed by eqn. % degradation = Ip n /Ip 1 47, 48 , where Ip 1 and Ip n are the 1st and nth cycle Ipa respectively. The retained steadiness of the Po-DG6-MCPE was 95.43% and this confirms the established electrode have magnificent stability.
Interference and practical analysis. The anti-interference capability of Po-DG6-MCPE were tested for CA and HY in presence of some coexisting compounds such as resorcinol, uric acid, serotonin, NaCl and KCl and CaCl 2 . There was no considerable interference was perceived for the analysis. Thus, the proposed Po-DG6-MCPE sensor reveals the excellent anti-interference ability.   www.nature.com/scientificreports/ Finally, the fashioned Po-DG6-MCPE was testified for the practical applicability of CA and HY in a local tap water sample using standard addition method 49,50 . This advises the recommended electrode exposes satisfactory recovery for each sample addition and attained results are recorded in Table 3. Therefore, this result clarifies the accuracy of constructed electrode for the revealing CA and HY in tap water.

Conclusion
The proposed work reports Poly-DG6-MCPE was accomplished as a sensor for the detection of CA and HY. The surface morphology of UCPE and Po-DG6-MCPE was characterized by SEM examination. This proposed electrode exposed the strong electrocatalytic activity, high sensitivity, stability and gave improved electron transfer response than UCPE with respect to the oxidation of CA and HY. The impact of pH study, concentration variation and speed rate study was tested at fabricated electrode. The Po-DG6-MCPE involves adsorption-controlled procedure and displayed low detection limit value compared to other reported electrodes. Simultaneous firmness of CA and HY was tracked by CV method. The constructed modified electrode results in good stability, selectivity and offers satisfactory recovery of analytes. Therefore, the mentioned Po-DG6-MCPE was substantial potentiality for the specific and simultaneous investigation and this constructed electrode was utilized for the further analysis of other electro-active biomolecules.