Fabrication of E ﬃ cient and Selective Modiﬁed Graphene Paste Sensor for the Determination of Catechol and Hydroquinone

: An electrochemical sensor, based on a graphene paste electrode (GPE), was modiﬁed with a polymerization method, and the electrochemical behavior of catechol (CC) and hydroquinone (HQ) was investigated using electroanalytical methods like cyclic voltammetry (CV) and di ﬀ erential pulse voltammetry (DPV). The e ﬀ ect of CC at the modiﬁed electrode was evidenced by the positive shift of the oxidation peak potential of CC at the poly (rosaniline)-modiﬁed graphene paste electrode (PRAMGPE) and the nine-fold enhancement of the peak current, as compared to a bare graphene paste electrode (BGPE). The sensitivity of CC investigated by DPV was more sensitive than CV for the analysis of CC. The DPV method showed the two linear ranges of 2.0 × 10 − 6 –1.0 × 10 − 5 M and 1.5 × 10 − 5 –5 × 10 − 5 M. The detection limit and limit of quantiﬁcation were determined to be 8.2 × 10 − 7 and 27.6 × 10 − 7 M, respectively. The obtained results were compared successfully with respect to those obtained using the o ﬃ cial method. Moreover, this sensor is applied for the selective determination of CC in the presence of HQ. The high sensitivity, good reproducibility, and wide linear range make the modiﬁed electrode suitable for the determination of CC in real samples. The practical application of the sensor was demonstrated by determining the concentration of CC in water samples with acceptable recoveries (97.5–98%).


Introduction
Isomeric benzenediol compounds, for example, hydroquinone (HQ) and catechol (CC), show notable antagonistic impacts on human health and the environment. Such compounds, for the most part, begin from creating solutions in the photography, paper, and rubber industries [1]. From these industrial routes, in addition to some other natural sources, hydroquinone and catechol discover their way to the environment [2]. The poisonousness of phenols produced from bioremediation, for example, fertilizing the soil, can likewise welcome on bothersome environmental impacts and genuinely harm evacuation efficiencies [3][4][5][6][7]. These modern uses of the two compounds not withstanding their poisonousness in the environment demand simple, accurate, precise, sensitive, and selective determination of the two compounds. A few methods have been proposed to determine CC and HQ such as spectrophotometric [8,9], liquid chromatography [10,11], synchronous fluorescence [12], chemiluminescence [13], capillary electrophoresis [14], and gas chromatography/mass spectrometry [15]. Those methods are time-consuming and expensive, they also have low sensitivity. Electrochemical methods are extensively used due to their good sensitivity, reproducibility, and low cost [16][17][18][19][20][21][22][23][24].

Chemicals
Graphene powder was attained from Tokyo Chemicals, Tokyo, Japan. CC and HQ were purchased from Sisco Research Laboratories, Mumbai, India. Silicone oil was bought from Nice Chemical, Cochin, India. Rosaniline (RA), Disodium hydrogen phosphate, and monosodium dihydrogen phosphate were procured from Himedia, Bangalore, India. The analytes 0.1 mM CC and 0.1 M HQ were prepared by suspending known quantities of the analytes in distilled water. PBS was ready by mixing an appropriate quantity of 0.1 M disodium hydrogen phosphate and 0.1 M monosodium dihydrogen phosphate in a known amount of distilled water.

Apparatus
The electro-analysis of CC was done through a tri-electrode arrangement attached to a CHI 6038E electrochemical workstation (CH Instruments, Austin, TX, USA). Here, the tri-electrode system was made up ofa saturated calomel electrode (reference electrode), platinum electrode (counter electrode), bare graphene paste electrode (BGPE), and poly (rosaniline)-modified graphene paste electrode (Poly(RA)MGPE) (working electrode). The EQ-610 digital pH tool was used for the preparation of different pH PBS. A VITSIL-VBSD/VBDD water purifier was utilized to get distilled water. FE-SEM was obtained with a device from DST-PURSE Laboratory, Mangalore University, Konaje, India).

Electrode Preparation
BGPE was equipped by combining graphene powder (60%) and silicone oil (40%) in an agate-mortar through hand mixing till a homogeneous paste was achieved. Afterward, a bit of homogenous paste was packed into the hole of a Teflon tube and electrical connection was completed by a copper wire. The electrode surface was smoothed with a weighing paper and rinsed carefully with distilled water initial to each experiment. Poly(RA)MGPE was prepared by electropolymerization of 1.0 mM RA on the surface of BGPE by running ten cyclic voltammetry (CV) cycles at the potential gap of −0.2 to 1.0 V with a 0.1 V/s scan rate in 0.01 M sodium hydroxide via pre-treatment with sulphuric acid. Then, the prepared electrodes were just washed with distilled water.

FE-SEM Inspection
The superficial properties of BGPE and Poly(RA)MGPE were explicated by field emission scanning electron microscopy (FE-SEM) analysis. The inset in Figure 1 picturizes the surface morphology of BGPE ( Figure 1a) and Poly(RA)MGPE (Figure 1b). In Figure 1a, the unsystematically distributed flakes of graphene represent the surface of the BGPE material. Conversely, Figure 1b displays the regularly arranged electrochemical polymer layers on the surface of graphene flakes, characterizing the Poly(RA)MGP material. The Poly(RA) layer increases the catalytic activity, active sites, and rate of electron transfer during the redox behavior of CC.
Surfaces 2020, 3 FOR PEER REVIEW 3 to 1.0 V with a 0.1 V/s scan rate in 0.01 M sodium hydroxide via pre-treatment with sulphuric acid. Then, the prepared electrodes were just washed with distilled water.

FE-SEM Inspection
The superficial properties of BGPE and Poly(RA)MGPE were explicated by field emission scanning electron microscopy (FE-SEM) analysis. The inset in Figure 1 picturizes the surface morphology of BGPE ( Figure 1a) and Poly(RA)MGPE (Figure 1b). In Figure 1a, the unsystematically distributed flakes of graphene represent the surface of the BGPE material. Conversely, Figure 1b displays the regularly arranged electrochemical polymer layers on the surface of graphene flakes, characterizing the Poly(RA)MGP material. The Poly(RA) layer increases the catalytic activity, active sites, and rate of electron transfer during the redox behavior of CC.

Electro-Polymerization of RA at BGPE through Pre-Treatment Process
The electro-polymerization of rosaniline on the exterior of BGPE needs two stages [45]. In the primary stage, the BGPE was pre-treated with 1.0 mM RA in 0.1 M Sulphuric acid by ten CV cycles in the potential gap of −0.2 to 1.0 V with a 0.1 V/s scan rate. In the second stage, electro-polymerization of RA was achieved on the pre-treated BGPE surface using 1.0 mM rosaniline in 0.01 M sodium hydroxide. Figure 2 displays the ten cyclic voltammograms (CVs) for the electro-polymerization of RA in 0.01 M sodium hydroxide at BGPE with a −0.2 to 1.0 V potential gap at a 0.1 V/s scan rate. During the electrochemical polymerization, the rosaniline monomer was transformed to an RA polymer and was deposited on the exterior of BGPE, and also the peak current of RA intensified at some certain CV cycles. Subsequently, the peak current of the RA polymer declined due to the saturation of the electrode surface and hindered electron transfer rate, hence the number of CV cycles was fixed to ten. The obtained Poly(RA)MGPE was rinsed with distilled water to eradicate the impurities.

Electro-Polymerization of RA at BGPE through Pre-Treatment Process
The electro-polymerization of rosaniline on the exterior of BGPE needs two stages [45]. In the primary stage, the BGPE was pre-treated with 1.0 mM RA in 0.1 M Sulphuric acid by ten CV cycles in the potential gap of −0.2 to 1.0 V with a 0.1 V/s scan rate. In the second stage, electro-polymerization of RA was achieved on the pre-treated BGPE surface using 1.0 mM rosaniline in 0.01 M sodium hydroxide. Figure 2 displays the ten cyclic voltammograms (CVs) for the electro-polymerization of RA in 0.01 M sodium hydroxide at BGPE with a −0.2 to 1.0 V potential gap at a 0.1 V/s scan rate. During the electrochemical polymerization, the rosaniline monomer was transformed to an RA polymer and was deposited on the exterior of BGPE, and also the peak current of RA intensified at some certain CV cycles. Subsequently, the peak current of the RA polymer declined due to the saturation of the electrode surface and hindered electron transfer rate, hence the number of CV cycles was fixed to ten. The obtained Poly(RA)MGPE was rinsed with distilled water to eradicate the impurities.

Impact of Sustaining Electrolyte pH
The influence of sustaining electrolyte pH on the redox action of 0.1 mM CC was described on the surface of Poly(RA)MGPE in altered pH in the range from 6.0 to 8.0 of 0.1 M PBS by CV having

Impact of Sustaining Electrolyte pH
The influence of sustaining electrolyte pH on the redox action of 0.1 mM CC was described on the surface of Poly(RA)MGPE in altered pH in the range from 6.0 to 8.0 of 0.1 M PBS by CV having the potential window of −0.3 to 0.6 V at a 0.1 V/s scan rate. Figure 3a discloses that the escalation of sustaining electrolyte pH shifts the redox peak potential of CC towards a less positive side. On the basis of the achieved result, the anodic peak potential V/s solution pH graph was plotted ( Figure 3b) and presents a decent linear response with a linear regression relation of E pa (V) = 0.5855-0.049 pH (V/pH) and the correlation coefficient (R 2 ) of 0.980. Here, the slope of 0.049 V/pH is approximately nearer to the theoretical value, which describes that the electrode redox reaction mechanism of CC is done with a like number of protons and electrons [46]. Further, Figure 3b shows that the peak current of CC is low at pH 6.0, 6.5, 7.5, and 8.0 with a lower rate of electron transfer, but pH 7.0 provides a high current response with elevated stability and electron transfer rate. Therefore, pH 7.0 was selected as an optimum electrolyte pH for this analysis.

Impact of Sustaining Electrolyte pH
The influence of sustaining electrolyte pH on the redox action of 0.1 mM CC was described on the surface of Poly(RA)MGPE in altered pH in the range from 6.0 to 8.0 of 0.1 M PBS by CV having the potential window of −0.3 to 0.6 V at a 0.1 V/s scan rate. Figure 3a discloses that the escalation of sustaining electrolyte pH shifts the redox peak potential of CC towards a less positive side. On the basis of the achieved result, the anodic peak potential V/s solution pH graph was plotted (Figure 3b) and presents a decent linear response with a linear regression relation of Epa (V) = 0.5855-0.049 pH (V/pH) and the correlation coefficient (R 2 ) of 0.980. Here, the slope of 0.049 V/pH is approximately nearer to the theoretical value, which describes that the electrode redox reaction mechanism of CC is done with a like number of protons and electrons [46]. Further, Figure 3b shows that the peak current of CC is low at pH 6.0, 6.5, 7.5, and 8.0 with a lower rate of electron transfer, but pH 7.0 provides a high current response with elevated stability and electron transfer rate. Therefore, pH 7.0 was selected as an optimum electrolyte pH for this analysis.   On BGPE, the oxidation current of CC was lower with a high oxidation potential of 0.244 V, which is due to the less catalytic nature of BGPE and shorter rate of electron transfer in the CC redox reaction. Nonetheless, at Poly(RA)MGPE, CC designates an enhanced oxidation current of 13.35 µA at 0.185 V of peak potential. Further, the modified electrodes do not disclose any electrochemical action in the absence of the analyte CC. These outcomes reveal that the modified Poly(RA)MGPE is very stable and specific for the CC redox nature and also it reduces the over-potential and fouling nature of the electrode material. The dynamic surface area of Poly(RA)MGPE and BGPE is calculated by following the Randles-Sevcik equation [47,48]: where "I p " designates the peak current of the analyte, "n" describes the number of electrons, "A" designates the active surface area, "D" defines the diffusion coefficient, "C" entitles the concentration of the analyte, and "ν" designates the scan rate. The deliberated values of the dynamic surface areas for BGPE and Poly(RA)MGPE were found to be 0.0204 and 0.0445 cm 2 , respectively. Ip = 2.69 × 10 5 n 3/2 A D 1/2 C ν 1/2 (1) where "Ip" designates the peak current of the analyte, "n" describes the number of electrons, "A" designates the active surface area, "D" defines the diffusion coefficient, "C" entitles the concentration of the analyte, and "ν" designates the scan rate. The deliberated values of the dynamic surface areas for BGPE and Poly(RA)MGPE were found to be 0.0204 and 0.0445 cm 2 , respectively.

Scan Rate Impact on Electrode Reaction, Peak Current, and Peak Potential of CC
The scan rate influence on the electrode reaction, peak current, and peak potential for 0.1 mM CC on Poly(RA)MGPE was examined at optimal circumstances.  (Figure 5b) illuminates that the peak current of CC was amplified by amplifying the scan rate with a high rate of electron transfer in each increment and shows a fine linear relationship and the linear regression relation of Ipa (µA) = 6.044 + 68.68 ν (V/s) (R 2 = 0.997). These data disclose that the redox reaction mechanism of CC at Poly(RA)MGPE was accomplished through adsorption-controlled phenomena [49]. Figure 5c denotes the fine linear relationship among the anodic potential and the logarithmic value of the scan rate with a linear regression relation of Epa(V) = 0.2910 + 0.0908 log ν (V/s) (R 2 = 0.995), and it displays that the electro-

Scan Rate Impact on Electrode Reaction, Peak Current, and Peak Potential of CC
The scan rate influence on the electrode reaction, peak current, and peak potential for 0.1 mM CC on Poly(RA)MGPE was examined at optimal circumstances.  (Figure 5b) illuminates that the peak current of CC was amplified by amplifying the scan rate with a high rate of electron transfer in each increment and shows a fine linear relationship and the linear regression relation of I pa (µA) = 6.044 + 68.68 ν (V/s) (R 2 = 0.997). These data disclose that the redox reaction mechanism of CC at Poly(RA)MGPE was accomplished through adsorption-controlled phenomena [49]. Figure 5c denotes the fine linear relationship among the anodic potential and the logarithmic value of the scan rate with a linear regression relation of E pa (V) = 0.2910 + 0.0908 log ν (V/s) (R 2 = 0.995), and it displays that the electro-catalytic redox reaction of CC ensues through the electron transfer step. The obtained slope of 0.0908 was applied for the assessment of the participated electrons number in the redox reaction of CC by following Laviron's relationship [50,51]: where "n" indicates the number of electrons, "E 0 " designates the standard potential, "α" indicates the charge transfer coefficient, "E p " defines the peak potential, "k 0 " specifies the heterogeneous rate constant, and the remaining terms represent their respective physical meanings. The value of the charge transfer coefficient was measured by the following equation: where "E p/2 " denotes the peak potential at exactly half peak current.
where "Ep/2" denotes the peak potential at exactly half peak current. The premeditated value of α is 0.70 and the transferrable electrons in the CC redox reaction at Poly(RA)MGPE are 2.19 (approximately 2.0). The possible redox reaction mechanism of CC at Poly(RA)MGPE is presented in Scheme 1.
where "Ep/2" denotes the peak potential at exactly half peak current. The premeditated value of α is 0.70 and the transferrable electrons in the CC redox reaction at Poly(RA)MGPE are 2.19 (approximately 2.0). The possible redox reaction mechanism of CC at Poly(RA)MGPE is presented in Scheme 1.

Limit of Detection and Quantification
The analyzing capability of the modified senor was investigated by the differential pulse voltammetry (DPV) method at a much lower CC concentration ([CC]) range. The differential pulse voltammograms (DPVs) for altered [CC] are in the range from 2.0 to 50 µM in 0.1 M PBS (7.0 pH) (Figure 6a). Further, all the attained DPVs show an enhanced peak current as there is an increase in [CC].  Table 1 [52][53][54][55][56]. The tabulated results conclude that the proposed Poly(RA)MGPE and DPV method deliver an improved sensitivity with a lower LOD than listed reports. Surfaces 2020, 3, 34 6 of 10 = 0.998). The LOD and LOQ for CC at Poly(RA)MGPE were calculated by LOD = 3 S/M and LOQ = 10 S/M relations (where S is the standard deviation of the blank and M is the slope of the calibration plot). The calculated LOD and LOQ values were found to be 0.82 and 2.76 µM, respectively. The comparison of the presently achieved LOD values with previously reported works are tabulated in Table 1 [52][53][54][55][56]. The tabulated results conclude that the proposed Poly(RA)MGPE and DPV method deliver an improved sensitivity with a lower LOD than listed reports.

Oxidative Response of CC at Poly (RA)MGPE and BGPE via DPV
The oxidative response of CC in 0.1 M PBS (pH 7.0) on the surface of Poly(RA)MGPE and BGPE was scrutinized through DPV having the potential gap of −0.05 to 0.35 V. Figure 7 displays the DPVs for CC oxidation at Poly(RA)MGPE (curve b) and BGPE (curve a). The proposed Poly(RA)MGPE specifies the fine and distinctive oxidation CC peak at the peak current and peak potentials of 14.10 µA and 0.146 V, respectively. Nonetheless, BGPE reveals a less sensitive oxidation peak for CC at the peak potential of 0.154 V. This achieved outcome deduces that the projected Poly(RA)MGPE imparts higher sensitivity and a more rapid response than BGPE. Additionally, Poly(RA)MGPE shows some significant properties over BGPE such as a fast rate of electron transfer, its antifouling nature, and reduced over-potential.    Figure 7 displays the DPVs for CC oxidation at Poly(RA)MGPE (curve b) and BGPE (curve a). The proposed Poly(RA)MGPE specifies the fine and distinctive oxidation CC peak at the peak current and peak potentials of 14.10 µA and 0.146 V, respectively. Nonetheless, BGPE reveals a less sensitive oxidation peak for CC at the peak potential of 0.154 V. This achieved outcome deduces that the projected Poly(RA)MGPE imparts higher sensitivity and a more rapid response than BGPE. Additionally, Poly(RA)MGPE shows some significant properties over BGPE such as a fast rate of electron transfer, its antifouling nature, and reduced over-potential.

Selective DPV Analysis of CC with HQ at Poly(RA)MGPE
The selectivity of the projected Poly(RA)MGPE and BGPE towards the electro-oxidation of CC (0.1 mM) with HQ (0.1 mM) in 0.1 M PBS (7.0 pH) was evaluated by cycling DPV in the potential range of −0.05 to 0.7 V. Figure 8 parades the DPV curves for CC in the presence of HQ on the exterior of Poly(RA)MGPE (curve b) and BGPE (curve a). Here, BGPE shows weak electro-oxidation peaks for CC and HQ at the peak potentials of 0.10 and 0.54 V, respectively. Nevertheless, at Poly(RA)MGPE, CC and HQ molecules exhibit distinctive and fine resolved electro-oxidation peaks with the oxidation currents of 7.92 and 5.31 µA at the oxidation potentials of 0.138 and 0.535 V, respectively. The attained result clarifies that the highly sensitive electro-catalytic activity of the modified sensor for the oxidation of CC in the appearance and nonappearance of HQ is virtually identical, therefore the oxidation nature of CC and HQ at Poly(RA)MGPE is different and consequently selective.

Selective DPV Analysis of CC with HQ at Poly(RA)MGPE
The selectivity of the projected Poly(RA)MGPE and BGPE towards the electro-oxidation of CC (0.1 mM) with HQ (0.1 mM) in 0.1 M PBS (7.0 pH) was evaluated by cycling DPV in the potential range of −0.05 to 0.7 V. Figure 8 parades the DPV curves for CC in the presence of HQ on the exterior of Poly(RA)MGPE (curve b) and BGPE (curve a). Here, BGPE shows weak electro-oxidation peaks for CC and HQ at the peak potentials of 0.10 and 0.54 V, respectively. Nevertheless, at Poly(RA)MGPE, CC and HQ molecules exhibit distinctive and fine resolved electro-oxidation peaks with the oxidation currents of 7.92 and 5.31 µA at the oxidation potentials of 0.138 and 0.535 V, respectively. The attained result clarifies that the highly sensitive electro-catalytic activity of the modified sensor for the oxidation of CC in the appearance and nonappearance of HQ is virtually identical, therefore the oxidation nature of CC and HQ at Poly(RA)MGPE is different and consequently selective.

Reproducibility, Repeatability, and Stability
The reproducibility, repeatability, and stability of the projected Poly(RA)MGPE were analyzed by the CV method at optimal circumstances. The Poly(RA)MGPE exposes fine reproducibility during

Reproducibility, Repeatability, and Stability
The reproducibility, repeatability, and stability of the projected Poly(RA)MGPE were analyzed by the CV method at optimal circumstances. The Poly(RA)MGPE exposes fine reproducibility during five successive CV cycles having 3.8% of relative standard deviation (RSD). Further, the repeatability was checked by running five sequential CV cycles and provides a fine RSD of 1.28%. Furthermore, the steadiness of Poly(RA)MGPE was revealed by cycling thirty continual CV sequences with 95.25% of the retained initial peak current. These achieved data reveal that the adopted sensor shows acceptable reproducibility, repeatability, and stability for the detection of CC.

CC Analysis in Water Samples
The examined voltammetric technique was implemented for the inspection of CC in water samples at Poly(RA)MGPE. The concentration, quantity, and recovery rate of CC in water samples (in 0.1 M PBS of 7.0 pH) at Poly(RA)MGPE were detected by a standard addition procedure. The obtained result revealed that the equipped electrochemically stable Poly(RA)MGPE proves a robust electro-catalytic activity towards the redox nature of CC, even in the existence of further electroactive molecules or ions in water samples. The percentage recovery of CC shows that Poly(RA)MGPE is the best alternative for CC detection. Table 2 gives the information about the recovery of CC in water samples.