Zinc Nanocomposite Supported Chitosan for Nitrite Sensing and Hydrogen Evolution Applications

Nanoparticles of ZnO-Chitosan (Zn-Chit) composite were prepared using precipitation methods. Several analytical techniques, such as scanning electron microscope (SEM), transmitted electron microscope (TEM), powder X-ray diffraction (XRD), infrared spectroscopy (IR), and thermal analysis, were used to characterize the prepared composite. The activity of the modified composite was investigated for nitrite sensing and hydrogen production applications using various electrochemical techniques. A comparative study was performed for pristine ZnO and ZnO loaded on chitosan. The modified Zn-Chit has a linear range of detection 1–150 µM and a limit of detection (LOD) = 0.402 µM (response time ~3 s). The activity of the modified electrode was investigated in a real sample (milk). Furthermore, the anti-interference capability of the surface was utilized in the presence of several inorganic salts and organic additives. Additionally, Zn-Chit composite was employed as an efficient catalyst for hydrogen production in an acidic medium. Thus, the electrode showed long-term stability toward fuel production and enhanced energy security. The electrode reached a current density of 50 mA cm−2 at an overpotential equal to −0.31 and −0.2 V (vs. RHE) for GC/ZnO and GC/Zn-Chit, respectively. Electrode durability was studied for long-time constant potential chronoamperometry for 5 h. The electrodes lost 8% and 9% of the initial current for GC/ZnO and GC/Zn-Chit, respectively.


Introduction
Nitrite is a crucial element for food additives, concrete admixtures, and the cycling of soil [1,2]. The World Health Organization (WHO) has classified nitrite as a highly hazardous substance because of its potential to threaten human health (such as through carcinogenesis) and cause water and soil contamination over time [3]. Furthermore, nitroaromatic hydrocarbons, nitramines, and nitrates, which comprise the most used high-energy explosives, are mostly degraded by nitrite, which is one of the key components of improvised explosives [4]. The urgent requirement for on-site detection of trace nitrite is driven by the need to ensure sustainable environmental development and safeguard public health and security [5][6][7][8]. Many attempts have been undertaken to measure nitrite below one micromolar, such as by high-performance liquid chromatography, surface-enhanced Raman spectroscopy, fluorescence spectroscopy, and electrochemical detection [9,10].
Although most of these techniques perform well in lab settings, electrochemical methods are very sensitive and selective for detecting drugs and contaminated water [11][12][13][14]. Various benefits include affordability, reaction speed, simplicity, and reproducibility [15,16]. Using nanomaterials has led to the development of different electrochemical systems, Using a hydrothermal process, hexagonal nanocubes of zinc oxide were created. In a typical experiment, 1.2 g of Zn(NO 3 ) 2 ·6H 2 O were dissolved in double-distilled water, followed by the slow addition of 0.1 M of KOH. This resulted in a mixture of 100 mL, which was then agitated for 20 min while being dispersed in an ultrasonic water bath for 30 min. The mixture was then placed into a Teflon-lined stainless-steel autoclave, which was then dried in an oven before undergoing a 12-h hydrothermal treatment at 180 • C. The item was filtered, cleaned with ethanol and water, and then dried in an oven at 60 • C.

Preparation of Zn-Chit Composite
Zn-Chit composite was prepared by a mixture of chitosan solution mixed with ZnO nanoparticles: 0.5 g of chitosan (Sigma Aldrich, St. Louis, MO, USA, 50,000-190,000 Da (based on viscosity), and 75-85% deacetylated) was added to 40 mL absolute ethanol in a 250 mL beaker, and then the temperature was raised gently with stirring. 0.5 g of ZnO nanoparticles were added to the mixture. The solution temperature was cooled to room temperature. The addition of the chitosan to the chitosan solution led to crosslinking and the encapsulation of ZnO nanoparticles. Chitosan in acidic media becomes a polyelectrolyte because of the protonation of the -NH 2 groups. The following equilibrium reaction describes the state of ionization. Therefore, about 1 mL of acetic acid (10%) was added to the mixture and stirred until the solution turned viscous. After ten minutes, the mixture underwent filtration and was rinsed with distilled water. The product was dried in an oven set at 80 • C for 3 h.

Fabrication of Electrode
A glassy carbon electrode with a diameter of 3 mm (0.0707 cm 2 ) was used as the working electrode. It was first polished with gentle emery paper and cleaned with ethanol and double-distilled water. After that, the cast solution was created by dispersing 20 mg of the catalyst powder (ZnO or Zn-Chit) in 0.5 mL of ethanol and 0.5 mL of 5 wt% Nafion using an ultrasonic bath for 1 h. The modified electrode was made in the following manner: GC/ZnO and GC/Zn-Chit are accomplished by a drop cast of 20 µL of catalyst solution onto electrode's surface, allowing it to dry for 4 h at 50 • C. The Autolab PGSTAT128N was used to perform all electrochemical experiments. The electrochemistry program NOVA (Version 2.1, Metrohm Autolab, Utrecht, The Netherlands) fits the impedance spectrum. Ag/AgCl/KCl (sat.) and Pt wire were employed as reference and counter electrodes.
GC/ZnO and GC/Zn-Chit were also used as working electrodes. All potential values in this work were compared to a reference electrode made of Ag/AgCl/sat.KCl for nitrite electrochemical sensor application. Furthermore, the potential was referenced to a reversible hydrogen electrode (RHE) for hydrogen evolution reaction (HER) applications. The electrochemical impedance spectroscopy measurements were adjusted to a constant AC voltage value using an AC voltage amplitude of 10 mV and a frequency range of 1 × 10 4 Hz to 0.1 Hz.
The potential was referenced to the reversible hydrogen electrode (RHE) regarding the following equations [37]: Electrochemical experiments were carried out in a 0.5 M supporting electrolyte solution with H 2 SO 4 . The potential was standardized to a hydrogen electrode that is reversible as follows:

Catalyst Characterization
The structure of modified Zn-Chit materials was determined using X-ray diffraction, as illustrated in Figure 1 [38]. Furthermore, the observed peak at a 2θ value of approximately 20 degrees can be attributed to the crystallographic plane with Miller indices of {110} for chitosan, as reported in previous studies [39]. Moreover, the homogeneous and consistent dispersion of ZnO onto chitosan substrate promotes the adsorption of nitrite onto the electrode interface. Transmission electron microscopy (TEM) was employed as the conventional technique for determining the dimensions of ZnO nanoparticles. The mean particle size of zinc oxide was estimated to be around 70 nm. (Figure 1b).
The scanning electron microscope (SEM) was utilized to characterize the surface morphology of Zn-Chit, as illustrated in Figure 1c,d. The scanning electron microscopy (SEM) images of ZnO nanoparticles incorporated within chitosan sheets revealed the presence of hexagonal nanocubes.
Energy-dispersive X-ray spectroscopy (EDX), a technique for elemental analysis, was employed to identify the existence of elements, including Zn, C, N, and O (see Figure 1e). The atomic ratios of Zn and O suggest the existence of ZnO and the lack of adulteration from other constituents in the specimens. Additionally, the presence of the carbon and nitrogen on surface attributed to the chemical structure of chitosan.
The scanning electron microscope (SEM) was utilized to characterize the surface morphology of Zn-Chit, as illustrated in Figure 1c,d. The scanning electron microscopy (SEM) images of ZnO nanoparticles incorporated within chitosan sheets revealed the presence of hexagonal nanocubes.
Energy-dispersive X-ray spectroscopy (EDX), a technique for elemental analysis, was employed to identify the existence of elements, including Zn, C, N, and O (see Figure 1e). The atomic ratios of Zn and O suggest the existence of ZnO and the lack of adulteration from other constituents in the specimens. Additionally, the presence of the carbon and nitrogen on surface attributed to the chemical structure of chitosan. Zn-Chit samples were characterized using FT-IR, revealing a range of absorption bands that facilitate identifying specific functional groups. Figure 2a illustrates the infrared spectra of the Zn-Chit sample. The stretching vibrations observed at wave number 3460 cm −1 were attributed to O-H and N-H bond-stretching while the stretching of C-H Zn-Chit samples were characterized using FT-IR, revealing a range of absorption bands that facilitate identifying specific functional groups. Figure

Zn-Chit Composite for Nitrite Detection
The present study investigated the efficacy of various modified GC electrodes, specifically GC/Chitosan, GC/ZnO, and GC/Zn-Chit, in detecting nitrites through the cyclic voltammetry (CV) technique. The experiment was utilized in a solution of 0.05 M phosphate buffer (pH = 7) in the presence of 50 µM of nitrite, with a sweep rate of 10 mV s −1 . One oxidation peak was observed in the potential range 0.8~ 1.0 V (vs. Ag/AgCl). For instance, the estimated Eps were observed to be 1.01, 0.94, 1.02, and 0.99 V for GC, GC/Chitosan, GC/ZnO, and GC/Zn-Chit, respectively. The forward peak indicates the transformation of N 2 − into N 3 − . The electrochemical detection of nitrite is deemed irreversible due to an undefined backward peak and the lack of reduction peaks. Figure 3 shows the activity of the Zn-Chit surface toward nitrite detection compared to pristine ZnO counterparts. However, the well distribution of ZnO on chitosan sheets enhanced the detection process by increasing the compatibility between the ZnO and the glassy carbon electrode.
Furthermore, chitosan facilitates the adsorption of ions from bulk to surface. However, several chitosan composites were reported in the literature as efficient surfaces for nitrite detection in an aqueous medium, such as graphene/polypyrrole/chitosan, Au-chitosan, and Au@RGO-Chitosan [43][44][45]. Therefore, adding the chitosan to ZnO nanoparticles enhanced the anodic oxidation current of the nitrite from 58 to 92 µA. Moreover, the activity of bare glassy carbon and chitosan was studied for nitrite detection. The adsorption step is essential for the electrochemical process [46]. Furthermore, chitosan showed lower activity toward nitrite detection than ZnO due to the absence of active centers for the charge transfer process. Additionally, the skeletal vibrations involving C-O stretching were predicted to occur at 1084 and 1023 cm −1 [40,41]. The spectrum of synthesized ZnO nanoparticles reveals a fundamental vibration mode at 3425 cm −1 , identified as O-H stretching and deformation. This mode is associated with the adsorption of water on the metal surface. The tetrahedral coordination of Zn is accountable for its absorption at 873 cm −1 . The vibrational stretching of ZnO nanoparticles is indicated by the observed bands within the range of 732 to 609 cm −1 .
A thermal analytical technique, namely TGA, was utilized to estimate the thermal durability and chemical decomposition. As depicted in Figure 2b, the thermogravimetric analysis curve of Zn-Chit is presented. The composite material exhibited four distinct thermal transitions at 82, 277.2, 388.4, and 454.2 • C. The thermal transition observed at approximately 85 • C is commonly attributed to the water removal. Additionally, it has been reported that the thermal decomposition of chitosan occurs through a two-stage process [42]. It is anticipated that the degradation of chitosan will result in thermal decomposition peaks at 277.2 and 388.4 • C. The fourth thermal transition within the temperature range of 454.2 • C is associated with ZnO.

Zn-Chit Composite for Nitrite Detection
The present study investigated the efficacy of various modified GC electrodes, specifically GC/Chitosan, GC/ZnO, and GC/Zn-Chit, in detecting nitrites through the cyclic voltammetry (CV) technique. The experiment was utilized in a solution of 0.05 M phosphate buffer (pH = 7) in the presence of 50 µM of nitrite, with a sweep rate of 10 mV s −1 . One oxidation peak was observed in the potential range 0.8~1.0 V (vs. Ag/AgCl). For instance, the estimated Eps were observed to be 1.01, 0.94, 1.02, and 0.99 V for GC, GC/Chitosan, GC/ZnO, and GC/Zn-Chit, respectively. The forward peak indicates the transformation of NO − 2 into NO − 3 . The electrochemical detection of nitrite is deemed irreversible due to an undefined backward peak and the lack of reduction peaks. Figure 3 shows the activity of the Zn-Chit surface toward nitrite detection compared to pristine ZnO counterparts.
However, the well distribution of ZnO on chitosan sheets enhanced the detection process by increasing the compatibility between the ZnO and the glassy carbon electrode. Since the electrode surface's sensitivity toward nitrite is considered as an essential parameter for the study, the calibration curve was studied for different surfaces in the presence of least amount of nitrite. The limit of detection is a method of analysis that can calculate the smallest amount of analyte detected in a sample [47]. Contrarily, the limit of quantization is the lowest drug concentration that can be quantitatively detected with the claimed accuracy and precision [48]. Various values (from 1 µM to 150 µM) were used to produce the calibration curve for nitrite ion detection and sensitivity determination. The chronoamperometric method was used to determine the limit of detection and linear dynamic range by gradually introducing nitrite into drinking water. The electrochemical behavior of the GC/ZnO-and GC/Zn-Chitosan-modified electrodes was investigated by observing their constant potential chronoamperograms. The experiments were conducted in PBS with a pH of 7 and a concentration of 0.05 M at a constant potential of 0.9 V (vs Ag/AgCl), as illustrated in Figure 4a,b. Figure 4c,d illustrates the relationship between the nitrite concentrations and oxidation current within a concentration range of 1-150 µM nitrite. Furthermore, the slope of the calibration curve was used to assess the limits of quantization and detection.
Equations (3) and (4) represent the linear ranges for the modified electrodes GC/ZnO and GC/Zn-Chit, as follows: i(µA) = 1.27 CZn-Chit (µM) + 4.16 (4) Additionally, using the equations LOD = 3 s/m and LOQ = 10 s/m, respectively, the detection and quantization limits are computed [15], where m is the slope of the calibration curve and s is the standard deviation. The LOD and LOQ for electrodes were Furthermore, chitosan facilitates the adsorption of ions from bulk to surface. However, several chitosan composites were reported in the literature as efficient surfaces for nitrite detection in an aqueous medium, such as graphene/polypyrrole/chitosan, Au-chitosan, and Au@RGO-Chitosan [43][44][45]. Therefore, adding the chitosan to ZnO nanoparticles enhanced the anodic oxidation current of the nitrite from 58 to 92 µA. Moreover, the activity of bare glassy carbon and chitosan was studied for nitrite detection. The adsorption step is essential for the electrochemical process [46]. Furthermore, chitosan showed lower activity toward nitrite detection than ZnO due to the absence of active centers for the charge transfer process.
Since the electrode surface's sensitivity toward nitrite is considered as an essential parameter for the study, the calibration curve was studied for different surfaces in the presence of least amount of nitrite. The limit of detection is a method of analysis that can calculate the smallest amount of analyte detected in a sample [47]. Contrarily, the limit of quantization is the lowest drug concentration that can be quantitatively detected with the claimed accuracy and precision [48]. Various values (from 1 µM to 150 µM) were used to produce the calibration curve for nitrite ion detection and sensitivity determination. The chronoamperometric method was used to determine the limit of detection and linear dynamic range by gradually introducing nitrite into drinking water. The electrochemical behavior of the GC/ZnO-and GC/Zn-Chitosan-modified electrodes was investigated by observing their constant potential chronoamperograms. The experiments were conducted in PBS with a pH of 7 and a concentration of 0.05 M at a constant potential of 0.9 V (vs. Ag/AgCl), as illustrated in Figure 4a,b. Figure 4c,d illustrates the relationship between the nitrite concentrations and oxidation current within a concentration range of 1-150 µM nitrite. Furthermore, the slope of the calibration curve was used to assess the limits of quantization and detection.    Equations (3) and (4) represent the linear ranges for the modified electrodes GC/ZnO and GC/Zn-Chit, as follows: i (µA) = 1.27 C Zn-Chit (µM) + 4.16 (4) Additionally, using the equations LOD = 3 s/m and LOQ = 10 s/m, respectively, the detection and quantization limits are computed [15], where m is the slope of the calibration curve and s is the standard deviation. The LOD and LOQ for electrodes were determined as 0.689 µM and 2.3 µM for GC/ZnO, 0.402 µM, and 1.34 µM for GC/Zn-Chit, respectively.
The modified GC/ZnO and GC/Zn-Chit electrodes' linear dynamic range and limit of detection for determining nitrite were compared to those of other results reported in the published literature, and the results are shown in Table 1.
For a complete understanding of nitrite sensing, kinetic parameters were estimated for nitrite oxidation over the modified electrodes. Figure 5a Figure 6a shows the chronoamperograms of GC/Zn-Chit for different nitrite concentrations at constant potential of 900 mV (vs. Ag/AgCl). however, the constant potential chronoamperometery technique was employed to estimate the diffusion coefficients for nitrite toward the modified electrode.
The Cottrell equation [70][71][72] was used to determine the average diffusion coefficient of nitrite molecules at a GC/Zn-Chit-modified electrode at a potential of 0.85 V (vs. Ag/AgCl) in a solution of 0.05 M PBS and several nitrite concentrations, as follows:  (5) where, i is the nitrite's oxidation current, n is the number of electrons (n = 1), A is the electrode's surface area, D is the analyte diffusion coefficient, C o is the analyte concentration, and ν is the scan rate. The diffusion coefficient was estimated using Randles-Sevick by constructing the linear relation between the nitrite oxidation current versus the square root of the scan rate (see Figure 5c). The provided diffusion coefficients are 6.3 × 10 −5 and 2.4 × 10 −4 cm 2 s −1 for GC/ZnO and GC/Zn-Chit electrodes, respectively. A higher diffusion coefficient value for a chitosan-based surface regards the chitosan's higher ability to adsorb the nitrite. Figure 5d represents a linear relation between peak potential versus the logarithm of scan rate for different modified surfaces. Thus, reversibility can be confirmed by the shift of the Ep positively as the scan rate increases [63]. The shift in the peak potential's position was observed by increasing the scan rate values according to the Laviron equation for irreversible reactions [63][64][65][66][67]: E pa is the peak potential, R is the universal gas constant, E • is the formal potential, T is the temperature, n is the number of electrons, v is the scanning rate, and F is the Faraday constant.
The transfer coefficient (α) is the kinetic parameter that infers the reaction's propensity for moving in the oxidation/reduction direction. When (α) is smaller than 0.5, the direction of oxidation is preferred. Considering the Laviron relation, the transfer coefficients were calculated using the linear relationship between Log (v) and Epa as 0.32 and 0.61 for modified GC/ZnO and GC/Zn-Chit, respectively. However, the symmetry factor, charge transfer coefficient (α), indicates that the oxidation of nitrite upon GC/Zn-Chit is more favorable compared to its GC/ZnO counterparts. The interaction between electrode and nitrite was established by linear relation between E p vs. Log(i), and I p vs. scan rate. (see Figure 5e,f). Whereas, the linear relation reflects the adsorption of nitrite on electrode surfaces [68,69]. Additionally, reaction is considered to be controlled by mixed adsorption/desorption process. Figure 6a shows the chronoamperograms of GC/Zn-Chit for different nitrite concentrations at constant potential of 900 mV (vs. Ag/AgCl). however, the constant potential chronoamperometery technique was employed to estimate the diffusion coefficients for nitrite toward the modified electrode. The activity of both electrodes was utilized by EIS. Figure 7 represents Nyquist plot for the GC/ZnO and GC/Zn-Chit surfaces at a constant potential of 0.85 V (vs. Ag/AgCl). However, EIS data was fitted using NOVA software. The inset figure represents the fitting circuits of the corresponding Nyquist results. However, circuit number one is attributed The Cottrell equation [70][71][72] was used to determine the average diffusion coefficient of nitrite molecules at a GC/Zn-Chit-modified electrode at a potential of 0.85 V (vs. Ag/AgCl) in a solution of 0.05 M PBS and several nitrite concentrations, as follows: where, i is the current, n is the number of oxidizing electrons (n = 1), F is the Faraday constant and A is the electrode surface area, and t is the operating time. As represteneted in Figure 6b, diffusion coefficient estimated using the slope of the Cottrell relation. Whereas, the provided diffusion values for nitrite equaled 6.1 × 10 −4 , 3.07 × 10 −4 , 4.7 × 10 −4 , 6.2 × 10 −4 , and 5.4 × 10 −4 cm 2 mol −1 for 10, 20, 30, 40, and 50 µM, respectively. The activity of both electrodes was utilized by EIS. Figure 7 represents Nyquist plot for the GC/ZnO and GC/Zn-Chit surfaces at a constant potential of 0.85 V (vs. Ag/AgCl). However, EIS data was fitted using NOVA software. The inset figure represents the fitting circuits of the corresponding Nyquist results. However, circuit number one is attributed to the GC/ZnO electrode that included solution resistance (Rs) connected in series with a cell of charge transfer resistance (Rc) connected to a constant phase element (CPE). On the other hand, Nyquist data for GC/Zn-Chit fitted by circuit number two includes solution resistance (Rs) connected to two cells of resistance and a constant phase element. Whereas the presence of two cells is attributed to two layers of surface behavior. However, the Nyquist plot's diameter reflects the electrode's activity. The lower diameter represents the higher charge transfer process and respectable activity toward nitrite sensing. The charge transfer resistances of 2480.8 and 1192.3 Ω were evaluated for GC/ZnO and GC/Zn-Chit electrodes, respectively. Table 2 contains the reported fitting data for the modified electrodes.   Real samples were employed to determine the electrodes' activity in real-time applications. Thus, three types of samples were prepared for electrochemical analysis in milk. In the pre-treatment process of milk, a mixture of 15 g milk and 2 mL semi-saturated ammonium sulfate solution was subjected to a temperature of 65 °C for 15 min to induce  Real samples were employed to determine the electrodes' activity in real-time applications. Thus, three types of samples were prepared for electrochemical analysis in milk. In the pre-treatment process of milk, a mixture of 15 g milk and 2 mL semi-saturated ammonium sulfate solution was subjected to a temperature of 65 • C for 15 min to induce protein precipitation. Subsequently, the acquired supernatant was diluted to a volume of 45 mL using a 0.1 M K 2 S 2 O 8 /PBS (pH 6.0) solution and administered [73]. Figure 8a shows the DPV of the different spiked nitrite concentrations in milk samples at a wide range of nitrite concentrations (10 to 500 µM). The calibration curve was established and attributed to the milk sample, as illustrated in Figure 8b. Linear dynamic regions were detected at 10-500 µM with corresponding limit of detections equal to 8.82 µM. In order to perform electrochemical detection for the real samples, the standard addition technique was employed. This involved the addition of various concentrations of nitrite (20,80,150,230, and 300 µM) to the treated extracts from milk. Table 3 shows the real sample detection recovery.  To examine the anti-interference capabilities of the sensor, a GC/Zn-Chit sensor was utilized to measure a variety of potentially interfering substances, including inorganic ions such as CuSO4, MgCl2, CaCl2, NaCl, and NH4Cl, as well as organic substances such as ascorbic acid, uric acid, glucose, and dopamine. Figure 9 depicts the chronoamperogram of the modified electrode GC/Zn-Chit. Firstly, nitrite solution was spiked to the solution, whereas the electrode response was observed as an increase in current. Then, the interfering solutions were added to the solution with only a slight change in the measured current. Finally, the reactivity of the electrode was investigated by spiking different concentrations of nitrite in the solution. Thus, the observed current increased again, reflecting the electrode surface's regeneration and the electrode's high anti-interference ability.  To examine the anti-interference capabilities of the sensor, a GC/Zn-Chit sensor was utilized to measure a variety of potentially interfering substances, including inorganic ions such as CuSO 4 , MgCl 2 , CaCl 2 , NaCl, and NH 4 Cl, as well as organic substances such as ascorbic acid, uric acid, glucose, and dopamine. Figure 9 depicts the chronoamperogram of the modified electrode GC/Zn-Chit. Firstly, nitrite solution was spiked to the solution, whereas the electrode response was observed as an increase in current. Then, the interfering solutions were added to the solution with only a slight change in the measured current. Finally, the reactivity of the electrode was investigated by spiking different concentrations of nitrite in the solution. Thus, the observed current increased again, reflecting the electrode surface's regeneration and the electrode's high anti-interference ability.

Zn-Chit Composite for Hydrogen Production
Hydrogen evolution reactions on GC/ZnO-and GC/Zn-Chit-modified surfaces were investigated. Figure 10a represents the LSV of the modified electrodes in a solution of 0.5 M H2SO4 at a scan rate of 10 mV s −1 . However, the presence of chitosan and ZnO composite enhanced the efficiency of the hydrogen production process. Whereas, the mechanism of hydrogen evolution mainly consists of adsorption steps, as follows [74,75]: Tafel step : 2 → 2 (9) Heyrovsky step : Regarding the previously mentioned equations, the first stage of the HER process requires the adsorption of hydrogen ions (Volmer step) on the electrode surface. Next, two hydrogen ions that have been adsorbed on the surface are either recombined (Tafel step) or a direct bond is formed between an adsorbed hydrogen atom and a hydrated proton in the medium (Heyrovsky step).
Accordingly, chitosan was reported to enhance the catalyst's ability to generate more hydrogen. For instance, a combination between metal oxides was found to shift the overpotential of hydrogen production toward a more positive value that indicates the reaction became more thermodynamically favored, such as CdS-Chitosan, CoNPs-Chitosan, and Pt-Chitosan-TiO2 [76][77][78]. By comparing pristine and chitosan-modified electrodes, both electrodes reached a current density of 50 mA cm −2 at over potential of −0.31 and −0.2 V (vs. RHE) for GC/ZnO and GC/Zn-Chit, respectively. On the other hand, a Tafel slopes curve was established for different surfaces to determine each electrode's thermodynamic favorability (see Figure 10b). Consequently, the Tafel slopes provided for GC/ZnO and GC/Zn-Chit electrodes are 103 and 126 mV dec −1 , respectively. Figure 10c shows the long-

Zn-Chit Composite for Hydrogen Production
Hydrogen evolution reactions on GC/ZnO-and GC/Zn-Chit-modified surfaces were investigated. Figure 10a represents the LSV of the modified electrodes in a solution of 0.5 M H 2 SO 4 at a scan rate of 10 mV s −1 . However, the presence of chitosan and ZnO composite enhanced the efficiency of the hydrogen production process. Whereas, the mechanism of hydrogen evolution mainly consists of adsorption steps, as follows [74,75]: Tafel step : 2H ads → H 2 (9) Heyrovsky step : Regarding the previously mentioned equations, the first stage of the HER process requires the adsorption of hydrogen ions (Volmer step) on the electrode surface. Next, two hydrogen ions that have been adsorbed on the surface are either recombined (Tafel step) or a direct bond is formed between an adsorbed hydrogen atom and a hydrated proton in the medium (Heyrovsky step).
Accordingly, chitosan was reported to enhance the catalyst's ability to generate more hydrogen. For instance, a combination between metal oxides was found to shift the overpotential of hydrogen production toward a more positive value that indicates the reaction became more thermodynamically favored, such as CdS-Chitosan, CoNPs-Chitosan, and Pt-Chitosan-TiO 2 [76][77][78]. By comparing pristine and chitosan-modified electrodes, both electrodes reached a current density of 50 mA cm −2 at over potential of −0.31 and −0.2 V (vs. RHE) for GC/ZnO and GC/Zn-Chit, respectively. On the other hand, a Tafel slopes curve was established for different surfaces to determine each electrode's thermodynamic favorability (see Figure 10b). Consequently, the Tafel slopes provided for GC/ZnO and GC/Zn-Chit electrodes are 103 and 126 mV dec −1 , respectively. Figure 10c shows the long-term stability of HER upon modified GC/ZnO and GC/Zn-Chit electrodes for 5 h. The chronoamperometry was performed in a solution of 0.5 M H 2 SO 4 at a potential of −0.3 V (vs. RHE). High electrode stability was observed, and the outcome current density decreased by 8% and 9% for GC/ZnO and GC/Zn-Chit electrodes, respectively. Electrochemical impedance spectroscopy (EIS) was employed to characterize HER upon modified GC/ZnO and GC/Zn-Chit electrodes. As represented in Figure 10d, Nyquist plot of modified GC/ZnO and GC/Zn-Chit electrodes in a solution of 0.5 M H 2 SO 4 at constant potential equaled −0.3 V (vs. RHE). The EIS data were fitted by NOVA software. The inset of Figure 10d illustrates the fitting circuit for the modified electrode. The equivalent circuit for HER consisted of solution resistance (Rs) connected in parallel with two similar resistance circuits and a constant phase element (CPE). The presence of a constant phase element corresponds to the surface roughness of the electrode. The following are essentially identical parameters for the constant phase element and capacitance [79]: where, constant phase element and capacitance are equaled (α = 1). However, the charge transfer resistances for GC/ZnO and GC/Zn-Chit are 216 and 110 Ω, respectively. The fitted EIS data is reported in Table 4 for GC/ZnO and GC/Zn-Chit. term stability of HER upon modified GC/ZnO and GC/Zn-Chit electrodes for 5 h. The chronoamperometry was performed in a solution of 0.5 M H2SO4 at a potential of −0.3 V (vs. RHE). High electrode stability was observed, and the outcome current density decreased by 8% and 9% for GC/ZnO and GC/Zn-Chit electrodes, respectively. Electrochemical impedance spectroscopy (EIS) was employed to characterize HER upon modified GC/ZnO and GC/Zn-Chit electrodes. As represented in Figure 10d, Nyquist plot of modified GC/ZnO and GC/Zn-Chit electrodes in a solution of 0.5 M H2SO4 at constant potential equaled −0.3 V (vs. RHE). The EIS data were fitted by NOVA software. The inset of Figure  10d illustrates the fitting circuit for the modified electrode. The equivalent circuit for HER consisted of solution resistance (Rs) connected in parallel with two similar resistance circuits and a constant phase element (CPE). The presence of a constant phase element corresponds to the surface roughness of the electrode. The following are essentially identical parameters for the constant phase element and capacitance [79]: where, constant phase element and capacitance are equaled (α = 1). However, the charge transfer resistances for GC/ZnO and GC/Zn-Chit are 216 and 110 Ω, respectively. The fitted EIS data is reported in Table 4 for GC/ZnO and GC/Zn-Chit.

Conclusions
The chitosan-based composite was successfully prepared for nitrite sensing and hydrogen production applications. Comparative studies between pristine and chitosan-based ZnO showed a synergistic effect between chitosan and ZnO nanoparticles. The modified GC/Zn-Chit electrodes showed efficient activity toward nitrite detection with a linear detection range of 1 to 150 µM and a detection limit of 0.402 µM. Therefore, a synergistic effect between ZnO nanoparticles and chitosan membrane could be described by the outstanding activity of modified GC/Zn-Chit toward nitrite detection. The electrode showed antiinterference ability in the presence of several metal ions and organic additives. The ability of nitrite detection was studied for milk as real samples. Furthermore, the modified surfaces found efficient activity toward hydrogen production. The electrode reached 50 mA cm −2 at overpotentials of −0.31 and −0.2 V (vs. RHE) for GC/ZnO and GC/Zn-Chit, respectively.