A Novel Electrochemical Sensor for Detection of Nicotine in Tobacco Products Based on Graphene Oxide Nanosheets Conjugated with (1,2-Naphthoquinone-4-Sulphonic Acid) Modified Glassy Carbon Electrode

A simple electrochemical sensor for nicotine (NIC) detection was performed. The sensor based on a glassy carbon electrode (GCE) was modified by (1,2-naphthoquinone-4-sulphonic acid)(Nq) decorated by graphene oxide (GO) nanocomposite. The synthesized (GO) nanosheets were characterized using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), transmission electron microscope (TEM), FT-IR, and UV-Visible Spectroscopy. The insertion of Nq with GO nanosheets on the surface of GCE displayed high electrocatalytic activity towards NIC compared to the bare GCE. NIC determination was performed under the optimum conditions using 0.10 M of Na2SO4 as a supporting electrolyte with pH 8.0 at a scan rate of 100 mV/s using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV). This electrochemical sensor showed an excellent result for NIC detection. The oxidation peak current increased linearly with a 6.5–245 µM of NIC with R2 = 0.9999. The limit of detection was 12.7 nM. The fabricated electrode provided satisfactory stability, reproducibility, and selectivity for NIC oxidation. The reliable GO/Nq/GCE sensor was successfully applied for detecting NIC in the tobacco product and a urine sample.

Hence, great attention has been paid to the determination of NIC, especially in medicine, toxicology, and the tobacco industry. To date, several analytical techniques have been reported for the measurement of NIC, including high-performance liquid chromatography [14], gas chromatography [15], spectrophotometry [16], spectrofluorimetry [17],

Apparatus
The voltammetric experimental conditions have been described in detail in previous literature [32]. Briefly, EG&G Princeton applied research potentiostat/galvanostat model 263 (USA) linked with a PC using 352 corrosion software [32,33]. A conventional threeelectrode cell consisted of a GO/Nq/GCE as the working electrode, a saturated Ag/AgCl reference electrode, and an auxiliary electrode made of a bright platinum wire. A pH meter (Euteoh-India) was employed for the pH measurements. The synthesized GO was characterized by powder X-ray diffraction (XRD, X'Pert3 Powder, PA Analytical, Netherlands), Raman spectroscopy, a field-emission scanning electron microscope (SEM, QUANTA FEG250, Tokyo, Japan), transmission electron microscope (TEM, JEOL 2100 HRTEM 200V, Tokyo, Japan), FT-IR and UV-Visible spectroscopy (Perkin Elmer, Waltham, WA, USA). In this study, all measurements were carried out at room temperature.

Synthesis of GO
Synthesis of GO nanosheets were prepared according to the modified Hummers' method [34][35][36]. Specifically, in an ice-water bath, 1.0 g of natural graphene powder was mixed with the concentrated H 2 SO 4 (30 mL) and NaNO 3 (3g). Then, 3.0 g of KMnO 4 was slowly added to keep the oxidation reaction temperature between 10 and 25 • C (Scheme 1). Subsequently, 100 mL of deionized water and 60 mL of 35% H 2 O 2 were carefully added to reduce residual KMnO 4 and MnO 2 . Afterward, the mixture was stirred for 30 min and then centrifuged. Finally, the product was filtered and dried in the oven at 50 • C overnight. According to the literature, GO nanosheets were synthesized from the resulting graphene oxide by magnetic stirring and heating [37,38]. Briefly, 0.50 g of dried graphene oxide nanocomposite powder was stirred with 50 mL of deionized water at 70 • C using magnetic stirring at 400 r.p.m for 12 h [38][39][40]. HRTEM 200V, Tokyo, Japan), FT-IR and UV-Visible spectroscopy (Perkin Elmer, Waltham, WA, USA). In this study, all measurements were carried out at room temperature.

Synthesis of GO
Synthesis of GO nanosheets were prepared according to the modified Hummers' method [34][35][36]. Specifically, in an ice-water bath, 1.0 g of natural graphene powder was mixed with the concentrated H2SO4(30 mL) and NaNO3(3g). Then, 3.0 g of KMnO4 was slowly added to keep the oxidation reaction temperature between 10 and 25 °C (Scheme 1). Subsequently, 100 mL of deionized water and 60 mL of 35% H2O2 were carefully added to reduce residual KMnO4 and MnO2. Afterward, the mixture was stirred for 30 min and then centrifuged. Finally, the product was filtered and dried in the oven at 50 °C overnight. According to the literature, GO nanosheets were synthesized from the resulting graphene oxide by magnetic stirring and heating [37,38]. Briefly, 0.50 g of dried graphene oxide nanocomposite powder was stirred with 50 mL of deionized water at 70 °C using magnetic stirring at 400 r.p.m for 12 h [38][39][40].

GO/Nq/GCE Fabrication
Before modification, a GCE (1 mm diameter) was sequentially polished with alumina powder (3 µm) to a mirror finish surface and ultrasonically cleaned with water: ethanol for 2 min and rinsed with deionized water. The modification of GCE by a layer of Nq can be obtained by electrodeposition on the cleaned surface of GCE. This was performed by dipping GCE in 0.10 M PBS containing 3.33 mM of Nq monomer and was conditioned by cyclic sweeping from −1.25 to 1.3 V vs. Ag/AgCl for 15 cycles (Section 3.2) [41]. The Nq/GCE was carefully cleaned with deionized water and dried in the air. On the other hand, the Nq/GCE was decorated by prepared GO nanosheets by dissolving the GO nanosheets in deionized water to form a suspended solution subjected to an ultrasonic device for 30 min to form a homogenous suspended solution. Then, 5.0 µL of the homogenous suspended solution (2.0 mg/mL) was cast on the surface of Nq/GCE to form GO/Nq/GCE. Finally, the GO/Nq/GCE was carefully cleaned with deionized water and dried in air. To calculate the activate surface of the different electrodes, the cyclic voltammetry of 5.0 mM of K3Fe(CN)6as the redox at the surface of bare GCE, Nq/GCE, andGO/Nq/GCE at different scan rates (from 50 to 300 mV/s) were recorded. There is a direct correlation between peak current and scan rate. By increasing the scan rate, the peak current increased for each electrode. By deriving the relationship between Ip and the square root of scan rate for different electrodes. Straight lines were obtained. Moreover,by compensation of the slope in the following Randles-Sevcik equation used [41]. Scheme 1. Synthesis of GO using modified Hummers' method.

GO/Nq/GCE Fabrication
Before modification, a GCE (1 mm diameter) was sequentially polished with alumina powder (3 µm) to a mirror finish surface and ultrasonically cleaned with water: ethanol for 2 min and rinsed with deionized water. The modification of GCE by a layer of Nq can be obtained by electrodeposition on the cleaned surface of GCE. This was performed by dipping GCE in 0.10 M PBS containing 3.33 mM of Nq monomer and was conditioned by cyclic sweeping from −1.25 to 1.3 V vs. Ag/AgCl for 15 cycles (Section 3.2) [41]. The Nq/GCE was carefully cleaned with deionized water and dried in the air. On the other hand, the Nq/GCE was decorated by prepared GO nanosheets by dissolving the GO nanosheets in deionized water to form a suspended solution subjected to an ultrasonic device for 30 min to form a homogenous suspended solution. Then, 5.0 µL of the homogenous suspended solution (2.0 mg/mL) was cast on the surface of Nq/GCE to form GO/Nq/GCE. Finally, the GO/Nq/GCE was carefully cleaned with deionized water and dried in air. To calculate the activate surface of the different electrodes, the cyclic voltammetry of 5.0 mM of K 3 Fe(CN) 6 as the redox at the surface of bare GCE, Nq/GCE, andGO/Nq/GCE at different scan rates (from 50 to 300 mV/s) were recorded. There is a direct correlation between peak current and scan rate. By increasing the scan rate, the peak current increased for each electrode. By deriving the relationship between Ip and the square root of scan rate for different electrodes. Straight lines were obtained. Moreover, by compensation of the slope in the following Randles-Sevcik equation used [41]. Ip = 0.446 nFAC(nFDν/RT) 1/2 (1) where R is the universal gas constant (J/mol.K), n is the number of electrons, C is the concentration of the electroactive species (mM), F is the Faraday constant (C/mol), ν is the scan rate (mV/s), T is the temperature in Kelvin, Ip is the peak current, D isthe diffusion coefficient (cm 2 /s) and A(cm 2 ) is the electroactive surface area of the different types of electrodes. The accurate value of D is 5.6 × 10 −6 cm 2 /s [42].So the values of active surface area(A cm 2 ) were estimated for bare GCE, Nq/GCE, andGO/Nq/GCE as 0.0126 cm 2 , 0.033 cm 2 ,and0.0425 cm 2 , respectively. So GO/Nq/GCE provides superior conductivity to other electrodes according to the active surface area data.

Sample Preparation
For the cigarette samples analysis, leaves were boiled for 5 min in 50 mL of deionized water. The mixture was later cooled and filtered to remove suspended impurities, followed by centrifugation. Decanting the supernatant centrifuged solution resulted in a clean dark brown colored solution. A human urine sample was provided by a project team member and diluted 100 times with 0.10 M Na 2 SO 4 at pH 8.0. Figure 1A displays the XRD patterns of the GO, which demonstrates that GO was successfully synthesized. Thus, the GO exhibits a broad diffraction peak at 2θ of 11.58 • due to the interlayer distance of the GO [34]. The Raman spectrum of the synthesized GO has been shown in Figure 1B to confirm the structural properties of GO. Two peaks were observed at 1345 cm −1 (D band) and 1581 cm −1 a (G band). The D (related to structural defects and amorphous carbon) to G (associated with the stretching of in-plane sp2carbons) intensity ratio (I D /I G ) was 1.05. The Raman spectrum of the GO is consistent with previous reports [42,43]. Ip = 0.446 nFAC(nFDν/RT) 1/2 (1) where R is the universal gas constant (J/mol.K), n is the number of electrons, C is the concentration of the electroactive species (mM), F is the Faraday constant (C/mol), ν is the scan rate (mV/s), T is the temperature in Kelvin, Ip is the peak current, D isthe diffusion coefficient (cm 2 /s) and A(cm 2 ) is the electroactive surface area of the different types of electrodes. The accurate value of D is 5.6 × 10 −6 cm 2 /s [42].So the values of active surface area(A cm 2 ) were estimated for bare GCE, Nq/GCE, andGO/Nq/GCE as 0.0126 cm 2 , 0.033 cm 2 ,and0.0425 cm 2 , respectively. So GO/Nq/GCE provides superior conductivity to other electrodes according to the active surface area data.

Sample Preparation
For the cigarette samples analysis, leaves were boiled for 5 min in 50 mL of deionized water. The mixture was later cooled and filtered to remove suspended impurities, followed by centrifugation. Decanting the supernatant centrifuged solution resulted in a clean dark brown colored solution. A human urine sample was provided by a project team member and diluted 100 times with 0.10 M Na2SO4 at pH 8.0. Figure 1A displays the XRD patterns of the GO, which demonstrates that GO was successfully synthesized. Thus, the GO exhibits a broad diffraction peak at 2θ of 11.58° due to the interlayer distance of the GO [34]. The Raman spectrum of the synthesized GO has been shown in Figure 1B to confirm the structural properties of GO. Two peaks were observed at 1345 cm −1 (D band) and 1581 cm −1 a (G band). The D (related to structural defects and amorphous carbon) to G (associated with the stretching of in-plane sp2carbons) intensity ratio (ID/IG) was 1.05. The Raman spectrum of the GO is consistent with previous reports [42,43]. On the other hand, the SEM image of the GO is depicted in Figure 2A to examine its structural morphology. As expected, a typical wrinkled, thin sheet-like morphology is shown for the surface of GO. For further support, the features of morphology and structure of the GO can be observed by high-resolution TEM. Figure 2B clearly shows the single-layer sheets containing flake-like wrinkles and rough surfaces. This microstructure gives the stability of GO nanosheets and prevents the collapse back into graphene structure, which provides a high surface area [44]. On the other hand, the SEM image of the GO is depicted in Figure 2A to examine its structural morphology. As expected, a typical wrinkled, thin sheet-like morphology is shown for the surface of GO. For further support, the features of morphology and structure of the GO can be observed by high-resolution TEM. Figure 2B clearly shows the single-layer sheets containing flake-like wrinkles and rough surfaces. This microstructure gives the stability of GO nanosheets and prevents the collapse back into graphene structure, which provides a high surface area [44].   FT-IR spectroscopy is vital for describing functional moieties involved on GO's surface. Figure 4 shows the site of absorption bands of functional groups in GO. The existence of broad stretching peaks at 3409 cm −1 , 2925 cm −1 , and 2857 cm −1 are stretching asymmetric and symmetric a hydroxyl (O-H) and C-H groups of GO matrix [38][39][40]45], respectively. Moreover, the C=O stretching peak of the carboxylic (COOH) group at 1724 cm −1 , stretching band at 1626 cm −1 of aromatic C=C matrix of GO, and phenolic C-O deformation,stretching, and epoxy C-O-C stretching were recognized at 1387, 1224 and 1050 cm −1 , respectively [38][39][40]46,47].     FT-IR spectroscopy is vital for describing functional moieties involved on GO face. Figure 4 shows the site of absorption bands of functional groups in GO. The ex of broad stretching peaks at 3409 cm −1 , 2925 cm −1 , and 2857 cm −1 are stretching asym and symmetric a hydroxyl (O-H) and C-H groups of GO matrix [38][39][40]45], respe Moreover, the C=O stretching peak of the carboxylic (COOH) group at 1724 cm −1 , s ing band at 1626 cm −1 of aromatic C=C matrix of GO, and phenolic C-O mation,stretching, and epoxy C-O-C stretching were recognized at 1387, 1224 an cm −1 , respectively [38][39][40]46,47]. FT-IR spectroscopy is vital for describing functional moieties involved on GO's surface. Figure 4 shows the site of absorption bands of functional groups in GO. The existence of broad stretching peaks at 3409 cm −1 , 2925 cm −1 , and 2857 cm −1 are stretching asymmetric and symmetric a hydroxyl (O-H) and C-H groups of GO matrix [38][39][40]45], respectively. Moreover, the C=O stretching peak of the carboxylic (COOH) group at 1724 cm −1 , stretching band at 1626 cm −1 of aromatic C=C matrix of GO, and phenolic C-O deformation, stretching, and epoxy C-O-C stretching were recognized at 1387, 1224 and 1050 cm −1 , respectively [38][39][40]46,47]. Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 19  Additionally, by increasing the number of cycles, the continuous growth of Nq film was obtained. The peak separation potential ΔEp = (Epa − Epc) was calculated to be 0.36 V (was higher than 0.059 V) as expected for a quasi-reversible system [48]. Then, the Nq/GCE behavior in 0.10 M PBS (pH 7.0) was examined under various scan rates ranging from 25 to 500 mV/s, as displayed in Figure 6. As can be observed, both anodic and cathodic peak currents gradually increased with a raised scan rate. Furthermore, the oxidative and reductive peak potentials shifted to more positive and negative values. This behavior indicated a surface-controlled process of the electrode reactions process for the organic film of Nq formation [41,49].   Additionally, by increasing the number of cycles, the continuous growth of Nq film was obtained. The peak separation potential ∆E p = (E pa − E pc ) was calculated to be 0.36 V (was higher than 0.059 V) as expected for a quasi-reversible system [48]. Then, the Nq/GCE behavior in 0.10 M PBS (pH 7.0) was examined under various scan rates ranging from 25 to 500 mV/s, as displayed in Figure 6. As can be observed, both anodic and cathodic peak currents gradually increased with a raised scan rate. Furthermore, the oxidative and reductive peak potentials shifted to more positive and negative values. This behavior indicated a surface-controlled process of the electrode reactions process for the organic film of Nq formation [41,49].   Additionally, by increasing the number of cycles, the continuous growth of Nq film was obtained. The peak separation potential ΔEp = (Epa − Epc) was calculated to be 0.36 V (was higher than 0.059 V) as expected for a quasi-reversible system [48]. Then, the Nq/GCE behavior in 0.10 M PBS (pH 7.0) was examined under various scan rates ranging from 25 to 500 mV/s, as displayed in Figure 6. As can be observed, both anodic and cathodic peak currents gradually increased with a raised scan rate. Furthermore, the oxidative and reductive peak potentials shifted to more positive and negative values. This behavior indicated a surface-controlled process of the electrode reactions process for the organic film of Nq formation [41,49].   The interpretation of the negative shift in the peak potential is due to the presence of both GO nanosheets and Nq films, which acted as electrocatalysts toward NIC oxidation, which accelerated the electron transfer rate. On the other hand, the usage of GO/Nq/GCE led to the enhancement and increase in the oxidation peak current of NIC by four-fold, compared to that which appeared at bar GCE. This is because the unique conjugation between the Nq film and nanosheets of GO increased the electroactive surface area that catalyzed NIC oxidation [26][27][28]. The absence of voltammetric response (peak) in the cathodic scan is attributed to the irreversibility of NIC oxidation as supported by the literature [24,50].   Figure 7 (curves b and c). The interpretation of the negative shift in the peak potential is due to the presence of both GO nanosheets and Nq films, which acted as electrocatalysts toward NIC oxidation, which accelerated the electron transfer rate. On the other hand, the usage of GO/Nq/GCE led to the enhancement and increase in the oxidation peak current of NIC by four-fold, compared to that which appeared at bar GCE. This is because the unique conjugation between the Nq film and nanosheets of GO increased the electroactive surface area that catalyzed NIC oxidation [26][27][28]. The absence of voltammetric response (peak) in the cathodic scan is attributed to the irreversibility of NIC oxidation as supported by the literature [24,50].

Effect of pH
The pH of the supporting electrolyte is considered an essential factor for the peak current (I p ). By examining the change of pH values from (2.0 to 9.0), it was observed that there are no current responses of NIC at a pH lower than 6.0. However, by increasing the pH value, the anodic peak of NIC increased until a maximum value was observed at pH 8.0, and then it decreased, as shown in Figure 8. This agrees with previous papers that prove that a neutral or slightly alkaline media were appropriate in the voltammetric determination of NIC [5,24]. On the other hand, the potential shifted towards less positive values by increasing the pH values, demonstrating proton participation in the oxidation process [51]. A plot of the oxidation peak potential vs. pH has three almost linear segments, and one is linear over the pH, ranging from 8.0 to 9.0 (inset Figure 8)

Effect of pH
The pH of the supporting electrolyte is considered an essential factor for the peak current (Ip). By examining the change of pH values from (2.0 to 9.0), it was observed that there are no current responses of NIC at a pH lower than 6.0. However, by increasing the pH value, the anodic peak of NIC increased until a maximum value was observed at pH 8.0, and then it decreased, as shown in Figure 8. This agrees with previous papers that prove that a neutral or slightly alkaline media were appropriate in the voltammetric determination of NIC [5,24]. On the other hand, the potential shifted towards less positive values by increasing the pH values, demonstrating proton participation in the oxidation process [51]. A plot of the oxidation peak potential vs. pH has three almost linear segments, and one is linear over the pH, ranging from 8.0 to 9.0 (inset Figure 8) The calculated slope of 60 mV is very close to the theoretical value (59 mV) of the Nernst equation. Therefore, the oxidation process of NIC on the surface of GO/Nq/GCE involves the same number of protons and electrons [52][53][54]. This segment of the curves (∼pH 8.0-9.0) is close to the pKa1 due to changes in protonation of the acid-base functions in the pyrrolidine moiety. As you know, NIC has pKa1 = 8.02, corresponding to the protonation of pyrrolidine nitrogen, and pKa2 = 3.12 of pyridine nitrogen [55]. The NIC oxidation process is pH-dependent, and subsequent measurements were carried out in a buffer with pH 8.0.

Effect of Scan Rate
To achieve the optimum conditions, the influence of scan rate on the peak current of 3.5 mM of NIC in 0.10 M Na2SO4 (pH 8.0)on the surface of the GO/Nq/GCE was investigated. The results demonstrated a linear increase in the peak current of NIC with the increase in scan rate from 5.0 to 100 mV/s, as illustrated in Figure 9. It can be observed that the potential peak shifts toward more positive values with increasing scan rate. Of course, this behavior was characteristic of an irreversible reaction [5,24]. To assess the oxidation process of NIC at the GO/Nq/GCE, a plot of the anodic peak current of NIC vs. square root of the scan rate was recorded (inset Figure 9). A straight linear relationship was ob- The calculated slope of 60 mV is very close to the theoretical value (59 mV) of the Nernst equation. Therefore, the oxidation process of NIC on the surface of GO/Nq/GCE involves the same number of protons and electrons [52][53][54]. This segment of the curves (∼pH 8.0-9.0) is close to the pK a1 due to changes in protonation of the acid-base functions in the pyrrolidine moiety. As you know, NIC has pK a1 = 8.02, corresponding to the protonation of pyrrolidine nitrogen, and pK a2 = 3.12 of pyridine nitrogen [55]. The NIC oxidation process is pH-dependent, and subsequent measurements were carried out in a buffer with pH 8.0.

Effect of Scan Rate
To achieve the optimum conditions, the influence of scan rate on the peak current of 3.5 mM of NIC in 0.10 M Na 2 SO 4 (pH 8.0)on the surface of the GO/Nq/GCE was investigated. The results demonstrated a linear increase in the peak current of NIC with the increase in scan rate from 5.0 to 100 mV/s, as illustrated in Figure 9. It can be observed that the potential peak shifts toward more positive values with increasing scan rate. Of course, this behavior was characteristic of an irreversible reaction [5,24]. To assess the oxidation process of NIC at the GO/Nq/GCE, a plot of the anodic peak current of NIC vs. square root of the scan rate was recorded (inset Figure 9). A straight linear relationship was obtained, indicating the oxidation reaction of NIC under a diffusion-controlled process and can be expressed as: For more support, a linear relationship with a slope of 0.54 was obtained when plotting the log peak current versus log scan rate (inset Figure 9) and can be expressed by: The slope of the above relation (α = 0.54) is close to the theoretical value calculated, 0.5, as expected for a diffusion-controlled process [56][57][58].
On the other hand, the relation between Ip versus scan rate was studied, a linear relationship was found with correlation co efficient R 2 = 0.996 Moreover, Ep and Log scan rate was determined to inform the number of electron that contributed in the oxidation process of NIC on the surface ofGO/Nq/GCE as seen in Figure 10a,b. For more support, a linear relationship with a slope of 0.54 was obtained when plotting the log peak current versus log scan rate (inset Figure 9) and can be expressed by: log I p = 0.54 log υ + 0.681 (R 2 = 0.996) (4) The slope of the above relation (α = 0.54) is close to the theoretical value calculated, 0.5, as expected for a diffusion-controlled process [56][57][58].
On the other hand, the relation between Ip versus scan rate was studied, a linear relationship was found with correlation co efficient R 2 = 0.996 Moreover, Ep and Log scan rate was determined to inform the number of electron that contributed in the oxidation process of NIC on the surface ofGO/Nq/GCE as seen in Figure 10a,b. According to the following equation, a linear relation was obtained with correlation co-efficient R 2 = 99.6. where α is (the electron transfer coefficient) and n (refers to number of electrons). The calculated number of electrons were found to be (2.1). That is mean two electron was participated in the oxidation process of NIC. So the expected mechanism (Scheme 2) for the oxidation of NIC at the GO/Nq/GCE in alkaline media which involves substitution of CH3 group by OH at the nitrogen of pyrrolidine ring with formation of methanol by two protons and two electrons transfer [5,59].

Scheme 2.
Probable electrochemical oxidation mechanism of NIC.

Effect of Concentration and Calibration Curve
The DPV technique is the most appropriate technique used to examine the correlation between the peak current and the standard adding of NIC concentration, characterized by accurate electrochemical determination with a low detection limit and background current [59]. The parameters of DPV were optimized, and the influence of various NIC concentrations at the GO/Nq/GCE in 0.1M Na2SO4(pH = 8.0) was examined. Figure 11 represents the increase in the peak current of NIC by successive standard additions of NIC concentration over the wide range from 6.5 to 245 µM. An excellent linear dependence of the peak current of NIC on its concentration in the same range is shown in Figure 11 According to the following equation, a linear relation was obtained with correlation co-efficient R 2 = 99.6. Ep = 0.00127 + 0.414log υ (R 2 = 0.997) The number of electrons involved was calculated from the Laviron equation: ∆E vs. ∆log(v) = 59/nα (7) where α is (the electron transfer coefficient) and n (refers to number of electrons). The calculated number of electrons were found to be (2.1). That is mean two electron was participated in the oxidation process of NIC. So the expected mechanism (Scheme 2) for the oxidation of NIC at the GO/Nq/GCE in alkaline media which involves substitution of CH 3 group by OH at the nitrogen of pyrrolidine ring with formation of methanol by two protons and two electrons transfer [5,59]. According to the following equation, a linear relation was obtained with correlation co-efficient R 2 = 99.6. where α is (the electron transfer coefficient) and n (refers to number of electrons). The calculated number of electrons were found to be (2.1). That is mean two electron was participated in the oxidation process of NIC. So the expected mechanism (Scheme 2) for the oxidation of NIC at the GO/Nq/GCE in alkaline media which involves substitution of CH3 group by OH at the nitrogen of pyrrolidine ring with formation of methanol by two protons and two electrons transfer [5,59].

Scheme 2.
Probable electrochemical oxidation mechanism of NIC.

Effect of Concentration and Calibration Curve
The DPV technique is the most appropriate technique used to examine the correlation between the peak current and the standard adding of NIC concentration, characterized by accurate electrochemical determination with a low detection limit and background current [59]. The parameters of DPV were optimized, and the influence of various NIC concentrations at the GO/Nq/GCE in 0.1M Na2SO4(pH = 8.0) was examined. Figure 11 represents the increase in the peak current of NIC by successive standard additions of NIC concentration over the wide range from 6.5 to 245 µM. An excellent linear dependence of the peak current of NIC on its concentration in the same range is shown in Figure 11. The following regression line equation can represent the obtained calibration curve: Probable electrochemical oxidation mechanism of NIC.

Effect of Concentration and Calibration Curve
The DPV technique is the most appropriate technique used to examine the correlation between the peak current and the standard adding of NIC concentration, characterized by accurate electrochemical determination with a low detection limit and background current [59]. The parameters of DPV were optimized, and the influence of various NIC concentrations at the GO/Nq/GCE in 0.1 M Na 2 SO 4 (pH = 8.0) was examined. Figure 11 represents the increase in the peak current of NIC by successive standard additions of NIC concentration over the wide range from 6.5 to 245 µM. An excellent linear dependence of the peak current of NIC on its concentration in the same range is shown in Figure 11. The following regression line equation can represent the obtained calibration curve: I p (µA) = 0.016 (µM) + 0.573 (R 2 = 0.9999) (8) The limit of detection (LOD) and quantification of NIC at the GO/Nq/GCE can be determined according to LOD = 3SD/b and LOQ = 10SD/b, respectively, where b is the slope of the calibration curve. SD is the standard deviation of I p of NIC, estimated from the following equation [60]. SD = 1/(n − 2)Σ(I exp − I cal ) 2 (9) where I exp is the experimental value of I p , and I cal is the corresponding calculated value at the same concentration. The calculated LOD and LOQ of NIC at the proposed sensor were 12.7 and 42.4 nM, respectively.
Nanomaterials 2022, 12, x FOR PEER REVIEW 12 of 19 The limit of detection (LOD) and quantification of NIC at the GO/Nq/GCE can be determined according to LOD = 3SD/b and LOQ = 10SD/b, respectively, where b is the slope of the calibration curve. SD is the standard deviation of Ip of NIC, estimated from the following equation [60].
where Iexp is the experimental value of Ip, and Ical is the corresponding calculated value at the same concentration. The calculated LOD and LOQ of NIC at the proposed sensor were 12.7 and 42.4 nM, respectively. Under the optimum conditions, the sensitivity of our modified sensor was tested. The comparison of the LOD and linearity range obtained in this work with previous reports with different modified electrodes was recorded. The comparison results are epitomized in Table 1. The lower LOD and good linearity range were achieved from the presented data, reflecting the efficiency and sensitivity our modified electrode toward lower concentration of NIC.  Under the optimum conditions, the sensitivity of our modified sensor was tested. The comparison of the LOD and linearity range obtained in this work with previous reports with different modified electrodes was recorded. The comparison results are epitomized in Table 1. The lower LOD and good linearity range were achieved from the presented data, reflecting the efficiency and sensitivity our modified electrode toward lower concentration of NIC.

Chronoamperometric Measurements
The diffusion coefficient of NIC at the GO/Nq/GCE can be determined using the chronoamperometry method. Chronoamperograms were applied by setting the working electrode potential at 0.6 V with various concentrations of NIC (14-44 µM) in 0.10 M Na 2 SO 4 (pH = 8.0) as represented in Figure 12. Plotting of I for various NIC concentrations against t −1 is depicted in Figure 12A. Then, the slopes of the resulting straight lines were plotted versus the different concentrations of NIC that were constructed inset Figure 12B. Using the obtained slope and applying the Cottrell equation [69], where C b (mol cm −3 ) is the bulk concentration, and D (cm 2 s −1 ) is the diffusion coefficient, the other symbols have their usual meaning. The D value of NIC at the GO/Nq/GCE was determined to be 4.02 × 10 −7 cm 2 /s is considered a satisfactory result.

Chronoamperometric Measurements
The diffusion coefficient of NIC at the GO/Nq/GCE can be determined us chronoamperometry method. Chronoamperograms were applied by setting the w electrode potential at 0.6 V with various concentrations of NIC (14-44 µM) in Na2SO4 (pH = 8.0) as represented in Figure 12. Plotting of I for various NIC concen against t −1 is depicted in Figure 12A. Then, the slopes of the resulting straight lin plotted versus the different concentrations of NIC that were constructed inset Figu Using the obtained slope and applying the Cottrell equation [69], where Cb (mol cm −3 ) is the bulk concentration, and D (cm 2 s −1 ) is the diffusion coe the other symbols have their usual meaning. The D value of NIC at the GO/Nq/G determined to be 4.02 × 10 −7 cm 2 /s is considered a satisfactory result.

Stability and Reproducibility
The viability of the GO/Nq/GCE could be evaluated by their stability and re

Stability and Reproducibility
The viability of the GO/Nq/GCE could be evaluated by their stability and reproducibility towards the NIC determination. Therefore, Figure 13A shows the CVs of 3.5 mM NIC in 0.10 M Na 2 SO 4 (pH 8.0) for 14 days, while the modified electrode was kept at 25 • C. The results indicate no significant change in the peak potential, and the peak current remains at 97.71% of its initial current response. Additionally, Figure 13B shows the reproducibility of the GO/Nq/GCE, taking five repetitive CVs' measurements via the same optimized protocol. It was concluded that the relative standard deviation of RSD = 0.80% in peak current. The results indicated that the fabricated electrode exhibited good stability and high reproducibility.

Interference Study
An important parameter for a developed electrode is its selectivity towards the target analyte. The influence of some possible interferences on the determination of 3.5 mM NIC in 0.1 M Na2SO4 (pH 8.0) at the GO/Nq/GCE was examined. The tolerance limit was calculated as less than 5% of the relative error in this study. Figure 14 represents the results investigated from the current changes comparing the peak current of NIC only and the signal of NIC with the interfering substances. It is clear that 1000-fold concentration ratios of Na + , K + , Fe 2+ , Cl − , NO3 − , and SO4 2− , 100-fold concentration ratios of glucose and L-cysteine, and 5-fold concentration ratios of pyridine, dopamine, ascorbic acid, and uric acid did not interfere with the measurement of NIC. Additionally, there is no significant variation in the current response after adding 50-fold concentration ratios of urea, caffeine, and cotinine, a primary metabolite of NIC. These results prove the excellent selectivity of the developed sensor for the electrochemical determination of NIC in presences of other interferences.

Interference Study
An important parameter for a developed electrode is its selectivity towards the target analyte. The influence of some possible interferences on the determination of 3.5 mM NIC in 0.1 M Na 2 SO 4 (pH 8.0) at the GO/Nq/GCE was examined. The tolerance limit was calculated as less than 5% of the relative error in this study. Figure 14 represents the results investigated from the current changes comparing the peak current of NIC only and the signal of NIC with the interfering substances. It is clear that 1000-fold concentration ratios of Na + , K + , Fe 2+ , Cl − , NO 3 − , and SO 4 2− , 100-fold concentration ratios of glucose and L-cysteine, and 5-fold concentration ratios of pyridine, dopamine, ascorbic acid, and uric acid did not interfere with the measurement of NIC. Additionally, there is no significant variation in the current response after adding 50-fold concentration ratios of urea, caffeine, and cotinine, a primary metabolite of NIC. These results prove the excellent selectivity of the developed sensor for the electrochemical determination of NIC in presences of other interferences.

Real Sample Analysis
Next, the content of NIC in a tobacco product and a urine sample was estimated using the GO/Nq/GCE under optimized DPV. Figure 15 depicts DPVs recorded for different concentrations of NIC standard solution, which was added to actual samples in 0.10 M Na 2 SO 4 . At the same time, the final NIC concentrations of 100, 110, 130, and 150 µM and the obtained results are shown in Table 2. As observed, good recoveries were obtained in the range of 99.44-100.33%. Therefore, the proposed sensor is capable of detecting NIC in actual and real samples. not interfere with the measurement of NIC. Additionally, there is no significant variation in the current response after adding 50-fold concentration ratios of urea, caffeine, and cotinine, a primary metabolite of NIC. These results prove the excellent selectivity of the developed sensor for the electrochemical determination of NIC in presences of other interferences.

Real Sample Analysis
Next, the content of NIC in a tobacco product and a urine sample was estima ing the GO/Nq/GCE under optimized DPV. Figure 15 depicts DPVs recorded for di concentrations of NIC standard solution, which was added to actual samples in Na2SO4. At the same time, the final NIC concentrations of 100, 110, 130, and 150 µ the obtained results are shown in Table 2. As observed, good recoveries were obta the range of 99.44-100.33%. Therefore, the proposed sensor is capable of detecting actual and real samples.

Conclusions
In this work, we provide an electrochemical sensor used for the first time for NIC determination. The behavior of NIC was studied under optimum conditions. Using different voltammetric techniques such as Cyclic voltammetry. Differential pulse voltammetry and chronoamperometry. The sensor was built on GCE, modified by a layer film of Nq decorated by GO nanosheets. The GO nanosheets were prepared by the Hummers' method and characterized by different techniques. The fabricated electrode provided a remarkable electrochemical activity toward the oxidation of NIC compared with either bar GCE and GCE modified by Nq. The unique conjugation between Nq and nanosheets of GO has significantly increased the electroactive surface area of the electrode toward NIC.
Additionally, the GO/Nq/GCE showed high stability, reproducibility, a lower detection limit of 12.7 nM, and a linear response range from 6.5 to 450 µM with R 2 = 0.9999. The oxidation process of NIC is described as control diffusion. The reliable GO/Nq/GCE sensor was successfully applied for detecting NIC in the tobacco product and a urine sample.

Conflicts of Interest:
The authors declare no conflict of interest and that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.