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3D nitrogen-doped porous graphene aerogel as high-performance electrocatalyst for determination of gallic acid
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3D nitrogen-doped porous graphene aerogel as high-performance electrocatalyst for determination of gallic acid Nabilah Al-Ansi , Abdulwahab Salah , Mbage Bawa , Salah Adlat , Iram Yasmin , Ayman Abdallah , Bin Qi PII: DOI: Reference:
S0026-265X(19)32314-8 https://doi.org/10.1016/j.microc.2020.104706 MICROC 104706
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Microchemical Journal
Received date: Revised date: Accepted date:
27 August 2019 5 January 2020 5 February 2020
Please cite this article as: Nabilah Al-Ansi , Abdulwahab Salah , Mbage Bawa , Salah Adlat , Iram Yasmin , Ayman Abdallah , Bin Qi , 3D nitrogen-doped porous graphene aerogel as highperformance electrocatalyst for determination of gallic acid, Microchemical Journal (2020), doi: https://doi.org/10.1016/j.microc.2020.104706
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3D nitrogen-doped porous graphene aerogel as high-performance electrocatalyst for determination of gallic acid Nabilah Al-Ansi1, 3, Abdulwahab Salah1, 3*, Mbage Bawa1, Salah Adlat2, Iram Yasmin1, Ayman Abdallah1, and Bin Qi 1* 1.
Faculty of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, Jilin
Province 130024, P.R. China 2.
Key Laboratory of Molecular Epigenetics of MOE, School of Life Science, Northeast Normal
University, 130024 Changchun, Jilin Province, China 3.
Faculty of Education, Department of Chemistry, Sana’a University, Yemen
*E-mail address of corresponding author: [email protected] (A. Salah) [email protected] (B. Qi)
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Abstract In this work, three-dimensional nitrogen-doped porous graphene aerogel (NPGA) was easily synthesized via one-step hydrothermal reduction by mixing graphene oxide (GO) with pphenylenediamine (PPD) and ammonia solution and then followed by freeze-dried. The NPGA sample was studied by different techniques such as SEM, TEM, nitrogen adsorption-desorption isotherms, XRD, XPS, and electrochemical measurements. NPGA composite exhibits a microporous with 3D structure, a large specific surface area and exposes edge plane likesites/defects which lead to high electrical conductivity and facilitate electrons transfer towards the probe. The appropriate experimental conditions such as supporting electrolyte, pH value and scan rate were optimized before applying NPGA electrode to determine gallic acid (GAL) using differential pulse voltammetry (DPV) method. NPGA electrode shows superior electrochemical performance compared to the previous studies with regard to the limit of detection (0.067 μmol/L) and linear range (2.5 - 1000 μmol/L). The NPGA electrode provides a new way for GAL concentration determination with high selectivity, excellent stability, and good reproducibility. In addition, it is highly suitable for the detection of GAL concentration in real samples with excellent reliability.
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Keywords: Gallic acid; Graphene aerogel; Nitrogen-doping; Hydrothermal reduction
1. Introduction Gallic acid (GAL) chemically known as 3,4,5-trihydroxybenzoic acid is considered as one of the phenolic compounds [1, 2], which plays a main role in human health and can be used as an antimicrobial [3], antioxidant [4], depression of hypertension [5], cholesterol reduction, protection against cancer and cardiovascular diseases [6]. GAL can be found in noteworthy concentrations in different foodstuffs such as tea leaves, grapes, orange, apples, lemon, blueberries, walnuts, and hot cocoa [5, 7]. Since the pharmacological significance reported of GAL, several methods such as highpressure liquid chromatography (HPLC) [8], resonance light scattering [9], flow injection analysis [10, 11] and thin-layer chromatography [12] have been used for the determination of it. Despite that these methods provided good sensitivity and selectivity, they have some shortcoming which include expensive material and equipment that are needed as well as a lot of time-consuming with a series of procedures [2, 13]. At present, electrochemical techniques have received growing attention and have been developed to determine GAL with several advantages such as cheap cost, simplicity, high sensitivity, and selectivity. In addition, they consume a short time for their analysis compared to other methods [1, 2, 13, 14]. In the electrochemical methods, the direct oxidation process takes place on the surfaces of the electrodes, so choosing the chemically modified electrode is an important and critical point to get the best results. In the recent decade, graphene materials have aroused significant attention due to their exceptional possession of unique properties and several potential applications [15]. To alleviate irreversible aggregations as a result of the strong π–π bonds between graphene layers, several 3
functional modifications on graphene such as preparation of three-dimensional (3D) porous graphene aerogel have been achieved [16]. The porous and interconnected 3D structure provides a large specific surface area, sufficient channels for conduction between the electrolyte ion and electrode surface, and strong adhesion to the catalyst particles, which lead to the fast diffusion of electrolyte ions [17-19]. 3D porous graphene is synthesized through three essential methods: template-assisted, direct deposition, and self-assembly [20]. Among these synthesis methods, the hydrothermal approach is the most commonly used self-assembly method because of a large yield of its product and easiness of the reaction conditions [21, 22]. In addition, graphene assembly is an effective method to control and functionalize the assembled of graphene via arranging carbon-carbon bonds in the planar framework of graphene [23]. Graphene doping heteroatoms (e.g. P, S, B, and N) strategies can also be used to reduce the aggregation of graphene nanosheets and promote the catalytic performance by offering a lot of exposed basal plane and edge plane like-sites/defects with unlimited numbers of the boundaries, which have an extremely important role in gaining a rapid electrons transfer at graphene surface and increase oxidation of the probe [24-26]. Nitrogen-doped porous graphene (NPG) attracted growing attention due to the features mentioned above as well as a nitrogen atom has a large size compared to a carbon atom and five valence electrons that are available to form strong bonds with carbon atoms, which strengthen graphene conductivity and density of charge carriers [27, 28]. Therefore, NPG with these properties could probably enhance the chemical and biochemical performance of sensors [29]. Many studies have reported using nitrogen-doped porous graphene aerogels as electrocatalysts, which were prepared under different chemical conditions by using several nitrogen sources. For instance, Zhu-Yin Sui et al. [30] reported the preparation of nitrogen-doped graphene aerogel (NGA) with high porosity through a simple hydrothermal
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reaction by using ammonia and graphene oxide (GO) as sources of nitrogen and carbon, respectively. The NGA showed a large specific surface area, excellent electrical conductivity, and high nitrogen content. Also, Lei Zhao et al. [31] successfully synthesized graphene aerogel via an assembly-assisted method by mixing melamine and cyanuric acid with GO. This catalyst exhibited excellent catalytic activity and high stability for electrooxidation of methanol. On the other hand, there were many previous studies that have been focused on the electrochemical methods for detection GAL (as listed in Table 1). Although the previous electrochemical studies have provided good results for GAL detection, but the goal of researchers studies is the focus of looking for new, effective, easy and low-cost methods with high sensitivity and selectivity to determine GAL in a wide linear range of concentrations, So, this study was carried out to achieve this goal. To the best of our knowledge, no study has focused on the electrochemical detection of GAL with the porous graphene aerogel. In this study, three-dimensional nitrogen-doped porous graphene aerogel (NPGA) was synthesized by a facile and effective approach via a one-step hydrothermal reduction of GO, which was mixed with p-phenylenediamine (PPD) and ammonia solution (NH3·H2O), then followed by freeze-dried for 24 h. A few drops of NH3·H2O were added into PPD to adjust pH value to 10.6 and modify the internal structure of the 3D graphene, enhancing its performance as a catalyst [32]. GO acted as the graphene precursor, while PPD and NH3·H2O were served as nitrogen doping sources. Herein, nitrogen doping plays a role in promoting electrocatalytic activity for GAL oxidation at NPGA surface. The nitrogen atoms which were inserted in graphene aerogel are favorable to offer more defective sites and lead to high electrical conductivity and facilitate electrons transfer towards the probe, as well as nitrogen doping also contributes to the greatly enhances long-term stability for GAL oxidation. The appropriate 5
experimental conditions such as supporting electrolyte, pH value and scan rate were optimized before applying the NPGA electrode to determine GAL using differential pulse voltammetry (DPV) method. The fabricated NPGA electrode was applied to determine of GAL in the 0.1 mol/L phosphate-buffered saline (PBS) solution pH 2 and showed an excellent electrocatalytic performance towards GAL oxidation. In addition, the NPGA electrode was successfully employed for determination GAL concentration in some foodstuffs such as fresh lemon, green and black tea.
2. Experimental 2.1. Instruments and reagents The instruments and reagents that were used in this work are in the supporting information. 2.2. Synthesis of NPGA The modified Hummers’ method was used to prepare GO from flaky graphite powder [33]. In the beginning, GO suspension (2 mg/mL) was prepared by dispersion 10 mL of GO solution (10 mg/mL) into 40 mL deionized water and sonicated for about 2 h. Hydrothermal method was used to prepare NPGA sample according to the article [32], as follows: Firstly, 30 ml of GO suspension (2 mg/mL) was mixed with PPD (0.03 g) in a small beaker and stirred for 10 min. Secondly, the pH value was adjusted to 10.6 by using NH3·H2O. Thirdly, the mixture was placed in a Teflon-lined stainless steel hydrothermal autoclave and kept at the temperature of 180 ᵒC for 12 h. Finally, the as-prepared nitrogen-doped porous graphene hydrogel was washed with twice distilled water and followed by freeze-dried for 24 h for further characterization. The sample which was prepared by mixing GO suspension and PPD with
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NH3·H2O at pH 10.6 was named as NPGA, while the sample synthesized using only GO suspension was labeled as reduced graphene oxide (RGO). The preparation pathway of NPGA is demonstrated in Scheme 1. 2.3. Fabrication of NPGA and RGO electrodes Firstly, 3 mg of RGO and NPGA samples were dispersed and sonicated into 1 mL of diluted Nafion solution (0.5 wt. %) for 40 min. Secondly, a glassy carbon electrode (GCE) was polished with Al2O3 powder and ultrasonic cleaning, followed by rinsing with deionized water. Finally, 5 µL of each sample was dropped onto the GCE surface and then placed under an infrared lamp to dry it. Also, clean and dry bare GCE was used to compare its electroanalytical performance towards GAL oxidation with RGO and NPGA electrodes. 2.4. Preparation of real samples 2.4.1 Preparation of tea samples Tea samples (green and black) were prepared according to the article [34]. 2.0 g of each sample was weighed and soaked in 30 mL of boiling water for 30 min. After filtering the samples, transferred to 100 mL flask. The extraction solutions were diluted to 100 mL with PBS (0.1 mol/L, pH 2) and kept them as the stock solution in a refrigerator at 4 °C. 200 μL of each sample was taken for electrochemical determination. 2.4.2 Preparation of orange juice and lemon samples According to the article [35], orange juice and lemon samples were prepared. Fresh orange and lemon grains were squeezed, collected their juice, filtrated by filter papers and then preserved in a refrigerator at 4 °C. PBS solution (0.1 mol/L, pH 2) was used to dilute juice samples 100 folds before analysis directly.
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3. Results and discussion 3.1. Morphology and structural characterization GO sheets consist of hydrophobic basal planes and hydrophilic edges, so it is possible to self-assemble in the aqueous medium by π–π interactions, hydrogen bonding, coordination, and electrostatic interactions to configure 3D networks. Fig. 1(a, b) shows digital photographs of RGO and NPG hydrogel samples. They exhibit well-defined 3D graphene hydrogel assemblies with cylindrical structures. The morphology and microstructure of RGO and NPGA were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM). Fig. 1(c) shows the SEM image of RGO nanosheets, containing many cracks randomly distributed and broken holes created in the sheet-like structure of RGO. SEM image of NPGA nanosheets in Fig. 1(d) shows that the accumulation of graphene sheets effectively decreased after the addition of PPD and NH3·H2O (NPGA sample) and generated many pores with the pillared structure that were created due to the interference of graphene nanosheets with each other. Since the replacement of some of the water with ammonia solution inside the graphene hydrogel, the freezing point of the solution in graphene hydrogel had decreased and facilitated the generation more pores during the freezedrying process [32]. C–N covalent bonds were configured between the inter-sheets of graphene under the synergy process of PPD and NH3·H2O, leading to the enhancement of the strength of the NPGA sample, which exhibits well-defined and interlinked 3D porous structures with numerous pores. Fig. 1(e, f) shows the TEM images of RGO and NPGA. RGO sample illustrates gauzelike structures, crimped edges and heavy folds because of the defects existent and weak dispersions on the surface and edges of RGO sample. After the addition of PPD and NH3·H2O, 8
the wrinkled degree of the graphene surface enhanced due to the excellent disperse ability with large thin pores walls formed, making it favorable for the electrons transmission and diffusion of supporting electrolyte ions. The porous microstructure of NPGA and RGO samples were detected by nitrogen adsorption-desorption isotherms. As indicated in Fig. S1(a), the adsorption isotherm of NPGA is corresponding to mesoporous adsorbents (type IV) with the characteristic H2 hysteresis loop [36, 37], which appeared at P/P0 = 0.45 – 1.0. The specific surface areas of NPGA and RGO were calculated to be 596 and 172 m2/g, respectively, by using Brunauer, Emmett, and Teller (BET) method. Fig. S1(b) exhibits the pore size distribution plots of NPGA and RGO samples. The pore size distribution of RGO was ignored in comparison with NPGA distribution which concentrated at around 0.59 nm. This result confirms that the addition of PPD and NH3·H2O increased the BET surface area and pore sizes distribution of graphene nanosheets, and thus led to reducing agglomeration and stacking between its layers, as well as assisting for the diffusion of electrolyte ions and electrons transmission between reactants/electrolytes and electrode surface. The compositions of NPGA, RGO, and GO were revealed by X-ray diffraction (XRD), as shown in Fig. S2. A broad diffraction peak of graphite carbon, which appeared at around 24.8 – 25.5ᵒ for all samples, corresponding to the diffraction of graphite carbon (002) with a d-spacing of 0.34 – 0.37 nm. In addition to the peak of graphite carbon, one clear peak appeared near 11.2ᵒ in GO sample, which was attributed to the crystalline plane of the GO (001) and correspond to an interlayer spacing of nearly 1.06 nm [32, 38, 39]. After the addition of PPD and NH3·H2O, the diffraction peak of GO (001) disappeared in the RGO and NPGA peaks because the removal of oxygen functional groups, demonstrating that the GO was reduced by hydrothermal reaction. 9
X-ray photoelectron spectroscopy (XPS) measurements were utilized to explain the structural bonding model and the elemental composition of GO, RGO, and NPGA samples. As shown in Fig. 2(a), the C 1s peak of GO can be deconvoluted into four fitting peaks, which are attributed to O-C=O, C=O, C-O, and C-C bonding that correspond to the binding energies that appeared at 289.1, 287.6, 286.2, and 284.3 eV, respectively [40, 41]. After the reduction of GO to RGO or NPGA, the peaks related to O-C=O, C=O bonding decreased and the peak of C-C bonding becomes predominant, as shown in Fig. 2(b, c). This indicates the effective removal of oxygen-containing groups in GO by hydrothermal reduction. Fig. 2(d) illustrates the N 1s peak of NPGA, which is related to the doping nitrogen and can be split into four major peaks corresponding to nitrogen sources (pyridine N-oxide, pyrrolic N, graphitic N, and pyridinic N ) with binding energies located near 403, 401, 399 and 398 eV, respectively [25, 42]. The contents of N, C, and H were measured by an elemental analyzer instrument and estimated to be 8.81, 85.54 and 4.29 wt. %, respectively. Also, the oxygen element was evaluated to be 1.35% by calculating the difference between the total constituents from the total value of 100 %, assuming that the remaining element is oxygen. 3.2. Electrochemical characterizations Electrochemical impedance spectroscopy (EIS) measurements were used to evaluate the electron transfer abilities and electrical conductivity between electrolytes and electrode surface. Fig. 3(a) shows the Nyquist plots of NPGA and RGO electrodes which were recorded in 5 mmol/L Fe(CN)6 4-/ 3- containing 0.1 mol/L KCl solution, the biased potential was +0.25 V versus Ag/AgCl with amplitude of 5 mV and the spectra of electrochemical impedance were recorded in the frequency range of 0.1 Hz to 10.0 kHz. The complex impedance is presented as the sum of the real (Z'-real) and the imaginary (Z''-imaginary) components established mainly from the 10
resistance and capacitance of the cell, respectively [43]. The EIS curve consists of the semicircle at high frequency and linear portion parts at low frequency. These two parts represent the electron transfer resistance and limiting diffusion process, respectively [44]. Randles equivalent electrical circuit model (semicircle diameter) was used to analyze the EIS data and calculate the electron transfer resistance (Rct). Rct values of NPGA and RGO electrodes could be estimated to be 6.44 and 63.53 Ω, respectively. This result indicates that the electron transfer between the electrolytes and electrode surface can be greatly accelerated when used NPGA as the electrode. Cyclic voltammetry (CV) curves were carried out in 5 mmol/L Fe(CN)6
4-/ 3-
containing
0.1 mol/L KCl solution to determine the electrochemically active surface areas of NPGA, RGO, and GEC electrodes, as shown in Fig. 3(b). The electrochemical surfaces areas of NPGA, RGO and GCE electrodes were calculated to be 0.076, 0.068 and 0.059 cm2, respectively, according to the following Randles Sevcik equation (1) [45, 46]: Ip= 2.69 ×105 A D1/2 n3/2 1/2 C
(1)
Where Ip, n, A, D, C and 1/2 are representing the peak current value (cathodic or anodic, Amp), electrons number of the redox reaction, electroactive surface area (cm2), diffusion coefficient (7.6 × 10−6 cm2 s−1) [46, 47], concentration of the electroactive species (5 ×10-6 mol cm−3) and square root of the scan rate (V s-1), respectively. This result confirms that the NPGA electrode has a large active surface area and better electrochemical reacting ability compared to other electrodes. 3.3. Electrochemical behavior of GAL at different electrodes CV and differential pulse voltammetry (DPV) were used to investigate the catalytic efficiency of GAL electrochemical behavior at NPGA, RGO and GCE electrodes in 5 mL PBS 11
(0.1 mol/L, pH 2). As shown in Fig. 4(a, b), the NPGA electrode displays a high oxidation peak in the CV and DPV curves compared to other electrodes, indicating that the NPGA electrode has superior electrochemical performance and high activity towards the GAL oxidation. This result was expected because the measurements of both nitrogen adsorptions-desorption isotherm and CV clarified that the NPGA electrode has a large specific and electroactive surface area which provides adequate active sites (e.g. edge plane like-sites and defective sites) for GAL oxidation. Chronoamperometry (CA) measurements were utilized to determine the steady-state at NPGA, RGO, and GCE electrodes. As shown in Fig. 4(c), after a transient period of time, the NPGA electrode reached the steady-state before other electrodes. CA curves reveal that the NPGA electrode has higher initial current density and more stable current with a higher catalytic activity than those of other electrodes. Fig. 4(d) displays Tafel plots of NPGA, RGO, and GCE which were used to obtain the Tafel slopes. It can be seen that the Tafel slope for the NPGA electrode (45 mV/dec) is smaller than those for the RGO electrode (57 mV/dec) and GCE (73 mV/dec). The lower value of the Tafel slope indicates a higher reaction and higher catalytic activity [48]. Thus, the NPGA electrode has higher catalytic activity for GAL oxidation, which is highly consistent with the CV and DPV results with regard to the high oxidation of GAL at the NPGA electrode. The oxidation mechanism of GAL at the NPGA electrode in the PBS (0.1 mol/L, pH 2) can be clarified by evaluating the difference of electrode response before and after adding the GAL to the supporting electrolyte. As shown in Fig. 5(a), in the absence of GAL, the CV curve did not demonstrate any oxidation peak both positive and negative potential scan, but after adding 200 μmol/L GAL, two oxidation peaks appeared at the positive potential scan. The first peak (Q1) is probably attributed to the generation of the semiquinone radical, and then followed
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by its oxidation to formation the quinone which appears as the second peak (Q2) [1]. As reported in the literature [1, 49-52], the first peak (Q1) which appeared around 0.53 V is most likely due to the galloyl group oxidation, while the second peak (Q2) appeared at 0.86 V was perhaps happened because of the oxidation of third OH group of the galloyl moiety. The oxidation mechanism process of GAL at NPGA electrode in the PBS (0.1 mol/L, pH 2) is summarized in Scheme 2. 3.4. Optimization of supporting electrolyte, pH and scan rate CV measurements were used to study the effect of the supporting electrolyte nature, change value of pH and scan rate on the electrochemical behavior of GAL at NPGA electrode. Fig. 5(b) shows the current response of NPGA electrode in 5 mL of BR buffer solution (0.4 mol/L, pH 2) (red line) and 5 mL of PBS (0.1 mol/L, pH 2) (black line) containing 200 μmol/L GAL at a scan rate of 50 mV s-1. NPGA electrode shows a higher current response in PBS (0.1 mol/L, pH 2) than BR buffer solution (0.4 mol/L, pH 2). For this reason, PBS (0.1 mol/L, pH 2) was selected as the supporting electrolyte solution for all tests in this study. Fig. 5(c) shows the influence change of pH value of 0.1 mol/L PBS from 1.5 to 8.0 on the electrochemical behavior of NPGA electrode containing 200 μmol/L GAL. The oxidation peak of GAL decreased gradually while increasing pH value from 1.5 to 8.0, and the oxidation peak potential of GAL shifted towards more negative values. As shown in Fig. 5(c), the peak current values corresponding to pH 1.5 and 2.0 are almost similar, but the value of the potential at pH 2 is lower than pH 1.5. Therefore, pH 2.0 of 0.1 mol/L PBS was chosen for all the electroanalytical examinations of GAL. Scan rate is considered as one of the analytical parameters which affect the electrooxidation of several compounds. Herein, the scan rates measurements were carried out to 13
evaluate whether the oxidation process of GAL on the NPGA electrode is diffusion-controlled or surfaced. Fig. 5(d) shows several scan rates in the range from 50 to 500 mV s−1. Inset Fig. 5(d) demonstrates a linear relationship between the oxidation peak of GAL and the square root of the scan rate (ʋ1/2). This obtained result indicates that the electrochemical oxidation of GAL at NPGA electrode is a diffusion-controlled process. 3.5. Determination of GAL at NPGA electrode DPV method was used to determine GAL in different concentrations at NPGA electrode surface versus Ag/AgCl in PBS (0.1 mol/L, pH 2). As shown in Fig. 6(a), when increasing the concentration of GAL, the oxidation peak is increased. In addition, it can be noticed that the NPGA electrode is able to sense low concentrations of GAL. This suggested that there is a high electrocatalytic activity of the NPGA electrode towards GAL oxidation. Fig. 6(b) shows the calibration curve of GAL concentrations vs. current responses. It can be observed that there is a linear relationship between the peak current oxidation and GAL concentration in the wide range from 2.5 to 1000 μmol/L according to this linear equation of I (μA) = 0.2363 C + 23.6146 with correlation coefficient (R2) of 0.9987, where C is the concentration of GAL (μmol/L). From this obtained calibration curve, the limit of detection (LOD) was estimated to be 0.067 μmol/L by using the following equation: LOD = 3 N/S, where S and N represent the slope of the calibration curve and noise of blank, respectively. Table 1. displays the comparison of electrochemical performances of NPGA electrode with the previous electroanalytical methods at different electrodes for GAL detection. As seen in Table 1, the NPGA electrode which used in this study shows superior analytical behavior for GAL detection compared to the electrodes those used in the previous studies.
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3.6. Interferences, reproducibility, and stability The selectivity is considered one of the important analytical parameters for the analytical procedure. NPGA electrode selectivity was tested in the presence of 30 μmol/L GAL with the same concentration of some interfering reagents such as ascorbic acid (AA), dopamine (DA), Uric acid (UA) and 4-acetamidophenol (AP). The presence of these interfering reagents is probably modifying the registered current signals of GAL. As indicated in Fig. 6(c), although new small currents peaks appeared after adding these interfering reagents, the oxidation peak of GAL is stilling enough for its detection with good reliability. This result endorses that the NPGA electrode has excellent selectivity towards GAL oxidation. Stability and reproducibility are also two fundamental analytical parameters that must be estimated to evaluate the efficiency of the NPGA electrode. By measuring the response of the NPGA electrode for 21 days, the electrode kept at least 93.14 % of its premiere value, as seen in Fig. 6(d). This result confirms that the NPGA electrode has excellent long-term stability. The reproducibility of the NPGA electrode was assessed by recording the responses of current to 50 μmol/L GAL in 5 mL of PBS (0.1 mol/L, pH 2) at 5 different NPGA electrodes which were prepared in the same conditions. The relative standard deviation was calculated to be 0.602 %, referring that the NPGA electrode has excellent reproducibility. 3.7. Real samples analysis NPGA electrode was employed for detection of GAL concentration in some foodstuffs samples such as orange juice, lemon, green and black tea by using the standard addition method. The values of mean recoveries of GAL in the foodstuffs samples are listed in Table 2. This result confirms that the NPGA electrode can be utilized to determine GAL concentration in the real samples with good precision and reliability. 15
4. Conclusion In this study, a facile preparation pathway was used to prepared NPGA sample via a onestep hydrothermal reduction of GO which was mixed with PPD and NH3·H2O and then followed by freeze-dried. Experimental results of nitrogen adsorption-desorption isotherms and XPS measurements confirm the presence of pores and nitrogen-doped atoms in the NPGA structure, respectively. The NPGA electrode provides a new way for determination GAL and shows superiority in the analytical behavior compared to the most electrodes in the previously reported studies with regard to the very low LOD and a wide linear range. In addition to high sensitivity and selectivity, the NPGA electrode has excellent long-term stability, good reproducibility as well as it can be applied to determine the GAL concentration in the real samples with excellent reliability.
Acknowledgment The authors acknowledge with many thanks to support this work by Jilin Province Development and Reform Commission (Grant No. 2019C040-8) and Ministry of Public Security, P. R. China (2018GABJC38).
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Author statement All persons who meet author criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Microchemical Journal.
Category 1 Conception and design of study:
Nabilah Al-Ansi, Abdulwahab Salah, and Bin Qi. Acquisition of data:
Nabilah Al-Ansi, Abdulwahab Salah, Iram Yasmin and Ayman Abdallah. Analysis and/or interpretation of data:
Nabilah Al-Ansi, Abdulwahab Salah, Mbage Bawa, Salah Adlat.
Category 2 Drafting the manuscript:
Nabilah Al-Ansi, Abdulwahab Salah, Iram Yasmin and Ayman Abdallah. Revising the manuscript critically for important intellectual content: Nabilah Al-Ansi, Abdulwahab Salah, Mbage Bawa, Salah Adlat and Bin Qi. .
Category 3 Approval of the version of the manuscript to be published: Nabilah Al-Ansi, Abdulwahab Salah, Mbage Bawa, Salah Adlat, Iram Yasmin, Ayman
Abdallah and Bin Qi.
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Conflict of interest We, the above-mentioned authors declare that this manuscript contains original, unpublished results that are not currently being considered for publication elsewhere while for all reused data the agreement of the publisher is attached. We confirm that the manuscript has been read and approved by all the authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the corresponding author is the sole contact for the Editorial process. The corresponding authors are responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs.
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Figure captions
Scheme 1. Illustration of the synthesis pathway of NPGA.
Scheme 2. Oxidation mechanism process of GAL.
23
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Fig. 1. Digital photographs of (a) RGO and (b) NPG hydrogel samples. SEM images of (c) RGO and (d) NPGA samples. TEM images of (e) RGO and (f) NPGA samples.
Fig. 2. C1s XPS spectra of (a) GO, (b) RGO, and (c) NPGA. (d) N 1s XPS spectra of NPGA.
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Fig. 3. (a) EIS diagrams and (b) CV curves of NPGA and RGO electrodes in 5 mmol/L Fe(CN)6 4-/ 3-
containing 0.1 mol/L KCl at a scan rate of 50 mV s-1.
26
Fig. 4. (a) CV curves at scan rate of 50 mV s-1, (b) DPV curves, (c) CA curves and (d) Tafel plots of NPGA, RGO, and GCE electrodes in 5 mL PBS (0.1 mol/L, pH 2), involving different concentrations of GAL (CV: 200 μmol/L GAL, DPV: 30 μmol/L GAL, CA: 30 μmol/L GAL and Tafel plots: 200 μmol/L).
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Fig. 5. (a) CV curves of NPGA electrode in 5 mL of 0.1 mol/L PBS pH 2 without (red line) and with (black line) 200 μmol/L GAL at a scan rate of 50 mV s-1. (b) CV curves of NPGA electrode in 5 mL of 0.4 mol/L BR buffer solution pH 2 (red line) and 5 mL of 0.1 mol/L PBS pH 2 (black line) containing 200 μmol/L GAL at a scan rate of 50 mV s-1. (c) CV curves of NPGA electrode containing 200 μmol/L GAL in 5 mL of 0.1 mol/L PBS with different values of pH (1.5 to 8) at a
28
scan rate of 50 mV s-1. (d) CV curves of NPGA electrode containing 200 μmol/L GAL in 5 mL of PBS (0.1 mol/L, pH 2) at several scan rates (50, 100, 150, 200, 250, 300, 400, and 500 mV s-1.
Fig. 6. (a) DPV curves of GAL with different concentrations from 0 to 1000 μmol/L in 5 mL of PBS (0.1 mol/L, pH 2) at NPGA electrode. (b) Calibration plots of the peak currents of GAL oxidation with previous concentrations. (c) DPV curves of 30 μmol/L GAL in 5 mL of PBS (0.1 mol/L, pH 2) with 30 μmol/L AA, 30 μmol/L DA, 30 μmol/L UA, and 30 μmol/L AP at NPGA electrode. (d) The long-term stability of the NPGA electrode for 21 days.
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Table 1. Comparison of the electrochemical performances of NPGA electrode with the previous electroanalytical methods at different electrodes for GAL detection Detection Linear range Working electrode
of limit
Method
Reference
(μmol/L) (μmol/L) NPGA-GCE
DPV
GCE/LDHf
DPV
CPE/SiO2
2.5 - 1000
0.067
This work
4 - 600
1.6
[5]
0.25
[53]
0.8 - 100
DPV SWV
0.51- 46.46
0.087
[54]
Amperometry
0.60 -625.8
0.28
[55]
Amperometry
0.5 -150
0.3
[1]
1 - 17
0.33
[56]
100 - 1300
0.14
[57]
LSV
0.58 -58.78
0.41
[34]
DPV
2.5 -150
0.94
[49]
DPV
4.5 - 76
1.1
[2]
GCE–Tyr–nAu
DPV
25 - 900
0.7
[58]
PmPD/Pal-IL/GCE
DPV
1 - 300
0.28
[35]
CS–fFe2O3– ERGO/GCE MOF -801/MC 3/GCE
DPV
1 - 50
0.15
[59]
DPV
0.2 -100
RGO-GCE AgNP/Delph/GCE MCPE/CNT
Poly-Glu/rGO GCE/MWCNT PEI-rGO/GCE TiO2/CPE TNrGO-GCE
Amperometry Chronoamperometry
30
0.15
[60]
Table 2. Determination of GAL concentrations in the real samples (n = 4). Sample
Found
Added
After added
Recovery
R.S.D.
(µmol/L)
(µmol/L)
(µmol/L)
(%)
(%)
Green tea
10.1
10
20.45 ± 0.043
103.50
3.33
Black tea
5.85
10
15.92 ± 0.057
100.70
3.78
Orange juice
0
10
9.83 ± 0.042
98.30
2.95
0.90
10
10.68 ± 0.035
97.80
3.89
Lemon
31
3D nitrogen-doped porous graphene aerogel as high-performance electrocatalyst for determination of gallic acid Nabilah Al-Ansi , Abdulwahab Salah , Mbage Bawa , Salah Adlat , Iram Yasmin , Ayman Abdallah , Bin Qi PII: DOI: Reference:
S0026-265X(19)32314-8 https://doi.org/10.1016/j.microc.2020.104706 MICROC 104706
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
27 August 2019 5 January 2020 5 February 2020
Please cite this article as: Nabilah Al-Ansi , Abdulwahab Salah , Mbage Bawa , Salah Adlat , Iram Yasmin , Ayman Abdallah , Bin Qi , 3D nitrogen-doped porous graphene aerogel as highperformance electrocatalyst for determination of gallic acid, Microchemical Journal (2020), doi: https://doi.org/10.1016/j.microc.2020.104706
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3D nitrogen-doped porous graphene aerogel as high-performance electrocatalyst for determination of gallic acid Nabilah Al-Ansi1, 3, Abdulwahab Salah1, 3*, Mbage Bawa1, Salah Adlat2, Iram Yasmin1, Ayman Abdallah1, and Bin Qi 1* 1.
Faculty of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun, Jilin
Province 130024, P.R. China 2.
Key Laboratory of Molecular Epigenetics of MOE, School of Life Science, Northeast Normal
University, 130024 Changchun, Jilin Province, China 3.
Faculty of Education, Department of Chemistry, Sana’a University, Yemen
*E-mail address of corresponding author: [email protected] (A. Salah) [email protected] (B. Qi)
1
Abstract In this work, three-dimensional nitrogen-doped porous graphene aerogel (NPGA) was easily synthesized via one-step hydrothermal reduction by mixing graphene oxide (GO) with pphenylenediamine (PPD) and ammonia solution and then followed by freeze-dried. The NPGA sample was studied by different techniques such as SEM, TEM, nitrogen adsorption-desorption isotherms, XRD, XPS, and electrochemical measurements. NPGA composite exhibits a microporous with 3D structure, a large specific surface area and exposes edge plane likesites/defects which lead to high electrical conductivity and facilitate electrons transfer towards the probe. The appropriate experimental conditions such as supporting electrolyte, pH value and scan rate were optimized before applying NPGA electrode to determine gallic acid (GAL) using differential pulse voltammetry (DPV) method. NPGA electrode shows superior electrochemical performance compared to the previous studies with regard to the limit of detection (0.067 μmol/L) and linear range (2.5 - 1000 μmol/L). The NPGA electrode provides a new way for GAL concentration determination with high selectivity, excellent stability, and good reproducibility. In addition, it is highly suitable for the detection of GAL concentration in real samples with excellent reliability.
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Keywords: Gallic acid; Graphene aerogel; Nitrogen-doping; Hydrothermal reduction
1. Introduction Gallic acid (GAL) chemically known as 3,4,5-trihydroxybenzoic acid is considered as one of the phenolic compounds [1, 2], which plays a main role in human health and can be used as an antimicrobial [3], antioxidant [4], depression of hypertension [5], cholesterol reduction, protection against cancer and cardiovascular diseases [6]. GAL can be found in noteworthy concentrations in different foodstuffs such as tea leaves, grapes, orange, apples, lemon, blueberries, walnuts, and hot cocoa [5, 7]. Since the pharmacological significance reported of GAL, several methods such as highpressure liquid chromatography (HPLC) [8], resonance light scattering [9], flow injection analysis [10, 11] and thin-layer chromatography [12] have been used for the determination of it. Despite that these methods provided good sensitivity and selectivity, they have some shortcoming which include expensive material and equipment that are needed as well as a lot of time-consuming with a series of procedures [2, 13]. At present, electrochemical techniques have received growing attention and have been developed to determine GAL with several advantages such as cheap cost, simplicity, high sensitivity, and selectivity. In addition, they consume a short time for their analysis compared to other methods [1, 2, 13, 14]. In the electrochemical methods, the direct oxidation process takes place on the surfaces of the electrodes, so choosing the chemically modified electrode is an important and critical point to get the best results. In the recent decade, graphene materials have aroused significant attention due to their exceptional possession of unique properties and several potential applications [15]. To alleviate irreversible aggregations as a result of the strong π–π bonds between graphene layers, several 3
functional modifications on graphene such as preparation of three-dimensional (3D) porous graphene aerogel have been achieved [16]. The porous and interconnected 3D structure provides a large specific surface area, sufficient channels for conduction between the electrolyte ion and electrode surface, and strong adhesion to the catalyst particles, which lead to the fast diffusion of electrolyte ions [17-19]. 3D porous graphene is synthesized through three essential methods: template-assisted, direct deposition, and self-assembly [20]. Among these synthesis methods, the hydrothermal approach is the most commonly used self-assembly method because of a large yield of its product and easiness of the reaction conditions [21, 22]. In addition, graphene assembly is an effective method to control and functionalize the assembled of graphene via arranging carbon-carbon bonds in the planar framework of graphene [23]. Graphene doping heteroatoms (e.g. P, S, B, and N) strategies can also be used to reduce the aggregation of graphene nanosheets and promote the catalytic performance by offering a lot of exposed basal plane and edge plane like-sites/defects with unlimited numbers of the boundaries, which have an extremely important role in gaining a rapid electrons transfer at graphene surface and increase oxidation of the probe [24-26]. Nitrogen-doped porous graphene (NPG) attracted growing attention due to the features mentioned above as well as a nitrogen atom has a large size compared to a carbon atom and five valence electrons that are available to form strong bonds with carbon atoms, which strengthen graphene conductivity and density of charge carriers [27, 28]. Therefore, NPG with these properties could probably enhance the chemical and biochemical performance of sensors [29]. Many studies have reported using nitrogen-doped porous graphene aerogels as electrocatalysts, which were prepared under different chemical conditions by using several nitrogen sources. For instance, Zhu-Yin Sui et al. [30] reported the preparation of nitrogen-doped graphene aerogel (NGA) with high porosity through a simple hydrothermal
4
reaction by using ammonia and graphene oxide (GO) as sources of nitrogen and carbon, respectively. The NGA showed a large specific surface area, excellent electrical conductivity, and high nitrogen content. Also, Lei Zhao et al. [31] successfully synthesized graphene aerogel via an assembly-assisted method by mixing melamine and cyanuric acid with GO. This catalyst exhibited excellent catalytic activity and high stability for electrooxidation of methanol. On the other hand, there were many previous studies that have been focused on the electrochemical methods for detection GAL (as listed in Table 1). Although the previous electrochemical studies have provided good results for GAL detection, but the goal of researchers studies is the focus of looking for new, effective, easy and low-cost methods with high sensitivity and selectivity to determine GAL in a wide linear range of concentrations, So, this study was carried out to achieve this goal. To the best of our knowledge, no study has focused on the electrochemical detection of GAL with the porous graphene aerogel. In this study, three-dimensional nitrogen-doped porous graphene aerogel (NPGA) was synthesized by a facile and effective approach via a one-step hydrothermal reduction of GO, which was mixed with p-phenylenediamine (PPD) and ammonia solution (NH3·H2O), then followed by freeze-dried for 24 h. A few drops of NH3·H2O were added into PPD to adjust pH value to 10.6 and modify the internal structure of the 3D graphene, enhancing its performance as a catalyst [32]. GO acted as the graphene precursor, while PPD and NH3·H2O were served as nitrogen doping sources. Herein, nitrogen doping plays a role in promoting electrocatalytic activity for GAL oxidation at NPGA surface. The nitrogen atoms which were inserted in graphene aerogel are favorable to offer more defective sites and lead to high electrical conductivity and facilitate electrons transfer towards the probe, as well as nitrogen doping also contributes to the greatly enhances long-term stability for GAL oxidation. The appropriate 5
experimental conditions such as supporting electrolyte, pH value and scan rate were optimized before applying the NPGA electrode to determine GAL using differential pulse voltammetry (DPV) method. The fabricated NPGA electrode was applied to determine of GAL in the 0.1 mol/L phosphate-buffered saline (PBS) solution pH 2 and showed an excellent electrocatalytic performance towards GAL oxidation. In addition, the NPGA electrode was successfully employed for determination GAL concentration in some foodstuffs such as fresh lemon, green and black tea.
2. Experimental 2.1. Instruments and reagents The instruments and reagents that were used in this work are in the supporting information. 2.2. Synthesis of NPGA The modified Hummers’ method was used to prepare GO from flaky graphite powder [33]. In the beginning, GO suspension (2 mg/mL) was prepared by dispersion 10 mL of GO solution (10 mg/mL) into 40 mL deionized water and sonicated for about 2 h. Hydrothermal method was used to prepare NPGA sample according to the article [32], as follows: Firstly, 30 ml of GO suspension (2 mg/mL) was mixed with PPD (0.03 g) in a small beaker and stirred for 10 min. Secondly, the pH value was adjusted to 10.6 by using NH3·H2O. Thirdly, the mixture was placed in a Teflon-lined stainless steel hydrothermal autoclave and kept at the temperature of 180 ᵒC for 12 h. Finally, the as-prepared nitrogen-doped porous graphene hydrogel was washed with twice distilled water and followed by freeze-dried for 24 h for further characterization. The sample which was prepared by mixing GO suspension and PPD with
6
NH3·H2O at pH 10.6 was named as NPGA, while the sample synthesized using only GO suspension was labeled as reduced graphene oxide (RGO). The preparation pathway of NPGA is demonstrated in Scheme 1. 2.3. Fabrication of NPGA and RGO electrodes Firstly, 3 mg of RGO and NPGA samples were dispersed and sonicated into 1 mL of diluted Nafion solution (0.5 wt. %) for 40 min. Secondly, a glassy carbon electrode (GCE) was polished with Al2O3 powder and ultrasonic cleaning, followed by rinsing with deionized water. Finally, 5 µL of each sample was dropped onto the GCE surface and then placed under an infrared lamp to dry it. Also, clean and dry bare GCE was used to compare its electroanalytical performance towards GAL oxidation with RGO and NPGA electrodes. 2.4. Preparation of real samples 2.4.1 Preparation of tea samples Tea samples (green and black) were prepared according to the article [34]. 2.0 g of each sample was weighed and soaked in 30 mL of boiling water for 30 min. After filtering the samples, transferred to 100 mL flask. The extraction solutions were diluted to 100 mL with PBS (0.1 mol/L, pH 2) and kept them as the stock solution in a refrigerator at 4 °C. 200 μL of each sample was taken for electrochemical determination. 2.4.2 Preparation of orange juice and lemon samples According to the article [35], orange juice and lemon samples were prepared. Fresh orange and lemon grains were squeezed, collected their juice, filtrated by filter papers and then preserved in a refrigerator at 4 °C. PBS solution (0.1 mol/L, pH 2) was used to dilute juice samples 100 folds before analysis directly.
7
3. Results and discussion 3.1. Morphology and structural characterization GO sheets consist of hydrophobic basal planes and hydrophilic edges, so it is possible to self-assemble in the aqueous medium by π–π interactions, hydrogen bonding, coordination, and electrostatic interactions to configure 3D networks. Fig. 1(a, b) shows digital photographs of RGO and NPG hydrogel samples. They exhibit well-defined 3D graphene hydrogel assemblies with cylindrical structures. The morphology and microstructure of RGO and NPGA were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM). Fig. 1(c) shows the SEM image of RGO nanosheets, containing many cracks randomly distributed and broken holes created in the sheet-like structure of RGO. SEM image of NPGA nanosheets in Fig. 1(d) shows that the accumulation of graphene sheets effectively decreased after the addition of PPD and NH3·H2O (NPGA sample) and generated many pores with the pillared structure that were created due to the interference of graphene nanosheets with each other. Since the replacement of some of the water with ammonia solution inside the graphene hydrogel, the freezing point of the solution in graphene hydrogel had decreased and facilitated the generation more pores during the freezedrying process [32]. C–N covalent bonds were configured between the inter-sheets of graphene under the synergy process of PPD and NH3·H2O, leading to the enhancement of the strength of the NPGA sample, which exhibits well-defined and interlinked 3D porous structures with numerous pores. Fig. 1(e, f) shows the TEM images of RGO and NPGA. RGO sample illustrates gauzelike structures, crimped edges and heavy folds because of the defects existent and weak dispersions on the surface and edges of RGO sample. After the addition of PPD and NH3·H2O, 8
the wrinkled degree of the graphene surface enhanced due to the excellent disperse ability with large thin pores walls formed, making it favorable for the electrons transmission and diffusion of supporting electrolyte ions. The porous microstructure of NPGA and RGO samples were detected by nitrogen adsorption-desorption isotherms. As indicated in Fig. S1(a), the adsorption isotherm of NPGA is corresponding to mesoporous adsorbents (type IV) with the characteristic H2 hysteresis loop [36, 37], which appeared at P/P0 = 0.45 – 1.0. The specific surface areas of NPGA and RGO were calculated to be 596 and 172 m2/g, respectively, by using Brunauer, Emmett, and Teller (BET) method. Fig. S1(b) exhibits the pore size distribution plots of NPGA and RGO samples. The pore size distribution of RGO was ignored in comparison with NPGA distribution which concentrated at around 0.59 nm. This result confirms that the addition of PPD and NH3·H2O increased the BET surface area and pore sizes distribution of graphene nanosheets, and thus led to reducing agglomeration and stacking between its layers, as well as assisting for the diffusion of electrolyte ions and electrons transmission between reactants/electrolytes and electrode surface. The compositions of NPGA, RGO, and GO were revealed by X-ray diffraction (XRD), as shown in Fig. S2. A broad diffraction peak of graphite carbon, which appeared at around 24.8 – 25.5ᵒ for all samples, corresponding to the diffraction of graphite carbon (002) with a d-spacing of 0.34 – 0.37 nm. In addition to the peak of graphite carbon, one clear peak appeared near 11.2ᵒ in GO sample, which was attributed to the crystalline plane of the GO (001) and correspond to an interlayer spacing of nearly 1.06 nm [32, 38, 39]. After the addition of PPD and NH3·H2O, the diffraction peak of GO (001) disappeared in the RGO and NPGA peaks because the removal of oxygen functional groups, demonstrating that the GO was reduced by hydrothermal reaction. 9
X-ray photoelectron spectroscopy (XPS) measurements were utilized to explain the structural bonding model and the elemental composition of GO, RGO, and NPGA samples. As shown in Fig. 2(a), the C 1s peak of GO can be deconvoluted into four fitting peaks, which are attributed to O-C=O, C=O, C-O, and C-C bonding that correspond to the binding energies that appeared at 289.1, 287.6, 286.2, and 284.3 eV, respectively [40, 41]. After the reduction of GO to RGO or NPGA, the peaks related to O-C=O, C=O bonding decreased and the peak of C-C bonding becomes predominant, as shown in Fig. 2(b, c). This indicates the effective removal of oxygen-containing groups in GO by hydrothermal reduction. Fig. 2(d) illustrates the N 1s peak of NPGA, which is related to the doping nitrogen and can be split into four major peaks corresponding to nitrogen sources (pyridine N-oxide, pyrrolic N, graphitic N, and pyridinic N ) with binding energies located near 403, 401, 399 and 398 eV, respectively [25, 42]. The contents of N, C, and H were measured by an elemental analyzer instrument and estimated to be 8.81, 85.54 and 4.29 wt. %, respectively. Also, the oxygen element was evaluated to be 1.35% by calculating the difference between the total constituents from the total value of 100 %, assuming that the remaining element is oxygen. 3.2. Electrochemical characterizations Electrochemical impedance spectroscopy (EIS) measurements were used to evaluate the electron transfer abilities and electrical conductivity between electrolytes and electrode surface. Fig. 3(a) shows the Nyquist plots of NPGA and RGO electrodes which were recorded in 5 mmol/L Fe(CN)6 4-/ 3- containing 0.1 mol/L KCl solution, the biased potential was +0.25 V versus Ag/AgCl with amplitude of 5 mV and the spectra of electrochemical impedance were recorded in the frequency range of 0.1 Hz to 10.0 kHz. The complex impedance is presented as the sum of the real (Z'-real) and the imaginary (Z''-imaginary) components established mainly from the 10
resistance and capacitance of the cell, respectively [43]. The EIS curve consists of the semicircle at high frequency and linear portion parts at low frequency. These two parts represent the electron transfer resistance and limiting diffusion process, respectively [44]. Randles equivalent electrical circuit model (semicircle diameter) was used to analyze the EIS data and calculate the electron transfer resistance (Rct). Rct values of NPGA and RGO electrodes could be estimated to be 6.44 and 63.53 Ω, respectively. This result indicates that the electron transfer between the electrolytes and electrode surface can be greatly accelerated when used NPGA as the electrode. Cyclic voltammetry (CV) curves were carried out in 5 mmol/L Fe(CN)6
4-/ 3-
containing
0.1 mol/L KCl solution to determine the electrochemically active surface areas of NPGA, RGO, and GEC electrodes, as shown in Fig. 3(b). The electrochemical surfaces areas of NPGA, RGO and GCE electrodes were calculated to be 0.076, 0.068 and 0.059 cm2, respectively, according to the following Randles Sevcik equation (1) [45, 46]: Ip= 2.69 ×105 A D1/2 n3/2 1/2 C
(1)
Where Ip, n, A, D, C and 1/2 are representing the peak current value (cathodic or anodic, Amp), electrons number of the redox reaction, electroactive surface area (cm2), diffusion coefficient (7.6 × 10−6 cm2 s−1) [46, 47], concentration of the electroactive species (5 ×10-6 mol cm−3) and square root of the scan rate (V s-1), respectively. This result confirms that the NPGA electrode has a large active surface area and better electrochemical reacting ability compared to other electrodes. 3.3. Electrochemical behavior of GAL at different electrodes CV and differential pulse voltammetry (DPV) were used to investigate the catalytic efficiency of GAL electrochemical behavior at NPGA, RGO and GCE electrodes in 5 mL PBS 11
(0.1 mol/L, pH 2). As shown in Fig. 4(a, b), the NPGA electrode displays a high oxidation peak in the CV and DPV curves compared to other electrodes, indicating that the NPGA electrode has superior electrochemical performance and high activity towards the GAL oxidation. This result was expected because the measurements of both nitrogen adsorptions-desorption isotherm and CV clarified that the NPGA electrode has a large specific and electroactive surface area which provides adequate active sites (e.g. edge plane like-sites and defective sites) for GAL oxidation. Chronoamperometry (CA) measurements were utilized to determine the steady-state at NPGA, RGO, and GCE electrodes. As shown in Fig. 4(c), after a transient period of time, the NPGA electrode reached the steady-state before other electrodes. CA curves reveal that the NPGA electrode has higher initial current density and more stable current with a higher catalytic activity than those of other electrodes. Fig. 4(d) displays Tafel plots of NPGA, RGO, and GCE which were used to obtain the Tafel slopes. It can be seen that the Tafel slope for the NPGA electrode (45 mV/dec) is smaller than those for the RGO electrode (57 mV/dec) and GCE (73 mV/dec). The lower value of the Tafel slope indicates a higher reaction and higher catalytic activity [48]. Thus, the NPGA electrode has higher catalytic activity for GAL oxidation, which is highly consistent with the CV and DPV results with regard to the high oxidation of GAL at the NPGA electrode. The oxidation mechanism of GAL at the NPGA electrode in the PBS (0.1 mol/L, pH 2) can be clarified by evaluating the difference of electrode response before and after adding the GAL to the supporting electrolyte. As shown in Fig. 5(a), in the absence of GAL, the CV curve did not demonstrate any oxidation peak both positive and negative potential scan, but after adding 200 μmol/L GAL, two oxidation peaks appeared at the positive potential scan. The first peak (Q1) is probably attributed to the generation of the semiquinone radical, and then followed
12
by its oxidation to formation the quinone which appears as the second peak (Q2) [1]. As reported in the literature [1, 49-52], the first peak (Q1) which appeared around 0.53 V is most likely due to the galloyl group oxidation, while the second peak (Q2) appeared at 0.86 V was perhaps happened because of the oxidation of third OH group of the galloyl moiety. The oxidation mechanism process of GAL at NPGA electrode in the PBS (0.1 mol/L, pH 2) is summarized in Scheme 2. 3.4. Optimization of supporting electrolyte, pH and scan rate CV measurements were used to study the effect of the supporting electrolyte nature, change value of pH and scan rate on the electrochemical behavior of GAL at NPGA electrode. Fig. 5(b) shows the current response of NPGA electrode in 5 mL of BR buffer solution (0.4 mol/L, pH 2) (red line) and 5 mL of PBS (0.1 mol/L, pH 2) (black line) containing 200 μmol/L GAL at a scan rate of 50 mV s-1. NPGA electrode shows a higher current response in PBS (0.1 mol/L, pH 2) than BR buffer solution (0.4 mol/L, pH 2). For this reason, PBS (0.1 mol/L, pH 2) was selected as the supporting electrolyte solution for all tests in this study. Fig. 5(c) shows the influence change of pH value of 0.1 mol/L PBS from 1.5 to 8.0 on the electrochemical behavior of NPGA electrode containing 200 μmol/L GAL. The oxidation peak of GAL decreased gradually while increasing pH value from 1.5 to 8.0, and the oxidation peak potential of GAL shifted towards more negative values. As shown in Fig. 5(c), the peak current values corresponding to pH 1.5 and 2.0 are almost similar, but the value of the potential at pH 2 is lower than pH 1.5. Therefore, pH 2.0 of 0.1 mol/L PBS was chosen for all the electroanalytical examinations of GAL. Scan rate is considered as one of the analytical parameters which affect the electrooxidation of several compounds. Herein, the scan rates measurements were carried out to 13
evaluate whether the oxidation process of GAL on the NPGA electrode is diffusion-controlled or surfaced. Fig. 5(d) shows several scan rates in the range from 50 to 500 mV s−1. Inset Fig. 5(d) demonstrates a linear relationship between the oxidation peak of GAL and the square root of the scan rate (ʋ1/2). This obtained result indicates that the electrochemical oxidation of GAL at NPGA electrode is a diffusion-controlled process. 3.5. Determination of GAL at NPGA electrode DPV method was used to determine GAL in different concentrations at NPGA electrode surface versus Ag/AgCl in PBS (0.1 mol/L, pH 2). As shown in Fig. 6(a), when increasing the concentration of GAL, the oxidation peak is increased. In addition, it can be noticed that the NPGA electrode is able to sense low concentrations of GAL. This suggested that there is a high electrocatalytic activity of the NPGA electrode towards GAL oxidation. Fig. 6(b) shows the calibration curve of GAL concentrations vs. current responses. It can be observed that there is a linear relationship between the peak current oxidation and GAL concentration in the wide range from 2.5 to 1000 μmol/L according to this linear equation of I (μA) = 0.2363 C + 23.6146 with correlation coefficient (R2) of 0.9987, where C is the concentration of GAL (μmol/L). From this obtained calibration curve, the limit of detection (LOD) was estimated to be 0.067 μmol/L by using the following equation: LOD = 3 N/S, where S and N represent the slope of the calibration curve and noise of blank, respectively. Table 1. displays the comparison of electrochemical performances of NPGA electrode with the previous electroanalytical methods at different electrodes for GAL detection. As seen in Table 1, the NPGA electrode which used in this study shows superior analytical behavior for GAL detection compared to the electrodes those used in the previous studies.
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3.6. Interferences, reproducibility, and stability The selectivity is considered one of the important analytical parameters for the analytical procedure. NPGA electrode selectivity was tested in the presence of 30 μmol/L GAL with the same concentration of some interfering reagents such as ascorbic acid (AA), dopamine (DA), Uric acid (UA) and 4-acetamidophenol (AP). The presence of these interfering reagents is probably modifying the registered current signals of GAL. As indicated in Fig. 6(c), although new small currents peaks appeared after adding these interfering reagents, the oxidation peak of GAL is stilling enough for its detection with good reliability. This result endorses that the NPGA electrode has excellent selectivity towards GAL oxidation. Stability and reproducibility are also two fundamental analytical parameters that must be estimated to evaluate the efficiency of the NPGA electrode. By measuring the response of the NPGA electrode for 21 days, the electrode kept at least 93.14 % of its premiere value, as seen in Fig. 6(d). This result confirms that the NPGA electrode has excellent long-term stability. The reproducibility of the NPGA electrode was assessed by recording the responses of current to 50 μmol/L GAL in 5 mL of PBS (0.1 mol/L, pH 2) at 5 different NPGA electrodes which were prepared in the same conditions. The relative standard deviation was calculated to be 0.602 %, referring that the NPGA electrode has excellent reproducibility. 3.7. Real samples analysis NPGA electrode was employed for detection of GAL concentration in some foodstuffs samples such as orange juice, lemon, green and black tea by using the standard addition method. The values of mean recoveries of GAL in the foodstuffs samples are listed in Table 2. This result confirms that the NPGA electrode can be utilized to determine GAL concentration in the real samples with good precision and reliability. 15
4. Conclusion In this study, a facile preparation pathway was used to prepared NPGA sample via a onestep hydrothermal reduction of GO which was mixed with PPD and NH3·H2O and then followed by freeze-dried. Experimental results of nitrogen adsorption-desorption isotherms and XPS measurements confirm the presence of pores and nitrogen-doped atoms in the NPGA structure, respectively. The NPGA electrode provides a new way for determination GAL and shows superiority in the analytical behavior compared to the most electrodes in the previously reported studies with regard to the very low LOD and a wide linear range. In addition to high sensitivity and selectivity, the NPGA electrode has excellent long-term stability, good reproducibility as well as it can be applied to determine the GAL concentration in the real samples with excellent reliability.
Acknowledgment The authors acknowledge with many thanks to support this work by Jilin Province Development and Reform Commission (Grant No. 2019C040-8) and Ministry of Public Security, P. R. China (2018GABJC38).
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Author statement All persons who meet author criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Microchemical Journal.
Category 1 Conception and design of study:
Nabilah Al-Ansi, Abdulwahab Salah, and Bin Qi. Acquisition of data:
Nabilah Al-Ansi, Abdulwahab Salah, Iram Yasmin and Ayman Abdallah. Analysis and/or interpretation of data:
Nabilah Al-Ansi, Abdulwahab Salah, Mbage Bawa, Salah Adlat.
Category 2 Drafting the manuscript:
Nabilah Al-Ansi, Abdulwahab Salah, Iram Yasmin and Ayman Abdallah. Revising the manuscript critically for important intellectual content: Nabilah Al-Ansi, Abdulwahab Salah, Mbage Bawa, Salah Adlat and Bin Qi. .
Category 3 Approval of the version of the manuscript to be published: Nabilah Al-Ansi, Abdulwahab Salah, Mbage Bawa, Salah Adlat, Iram Yasmin, Ayman
Abdallah and Bin Qi.
17
Conflict of interest We, the above-mentioned authors declare that this manuscript contains original, unpublished results that are not currently being considered for publication elsewhere while for all reused data the agreement of the publisher is attached. We confirm that the manuscript has been read and approved by all the authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the corresponding author is the sole contact for the Editorial process. The corresponding authors are responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs.
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Figure captions
Scheme 1. Illustration of the synthesis pathway of NPGA.
Scheme 2. Oxidation mechanism process of GAL.
23
24
Fig. 1. Digital photographs of (a) RGO and (b) NPG hydrogel samples. SEM images of (c) RGO and (d) NPGA samples. TEM images of (e) RGO and (f) NPGA samples.
Fig. 2. C1s XPS spectra of (a) GO, (b) RGO, and (c) NPGA. (d) N 1s XPS spectra of NPGA.
25
Fig. 3. (a) EIS diagrams and (b) CV curves of NPGA and RGO electrodes in 5 mmol/L Fe(CN)6 4-/ 3-
containing 0.1 mol/L KCl at a scan rate of 50 mV s-1.
26
Fig. 4. (a) CV curves at scan rate of 50 mV s-1, (b) DPV curves, (c) CA curves and (d) Tafel plots of NPGA, RGO, and GCE electrodes in 5 mL PBS (0.1 mol/L, pH 2), involving different concentrations of GAL (CV: 200 μmol/L GAL, DPV: 30 μmol/L GAL, CA: 30 μmol/L GAL and Tafel plots: 200 μmol/L).
27
Fig. 5. (a) CV curves of NPGA electrode in 5 mL of 0.1 mol/L PBS pH 2 without (red line) and with (black line) 200 μmol/L GAL at a scan rate of 50 mV s-1. (b) CV curves of NPGA electrode in 5 mL of 0.4 mol/L BR buffer solution pH 2 (red line) and 5 mL of 0.1 mol/L PBS pH 2 (black line) containing 200 μmol/L GAL at a scan rate of 50 mV s-1. (c) CV curves of NPGA electrode containing 200 μmol/L GAL in 5 mL of 0.1 mol/L PBS with different values of pH (1.5 to 8) at a
28
scan rate of 50 mV s-1. (d) CV curves of NPGA electrode containing 200 μmol/L GAL in 5 mL of PBS (0.1 mol/L, pH 2) at several scan rates (50, 100, 150, 200, 250, 300, 400, and 500 mV s-1.
Fig. 6. (a) DPV curves of GAL with different concentrations from 0 to 1000 μmol/L in 5 mL of PBS (0.1 mol/L, pH 2) at NPGA electrode. (b) Calibration plots of the peak currents of GAL oxidation with previous concentrations. (c) DPV curves of 30 μmol/L GAL in 5 mL of PBS (0.1 mol/L, pH 2) with 30 μmol/L AA, 30 μmol/L DA, 30 μmol/L UA, and 30 μmol/L AP at NPGA electrode. (d) The long-term stability of the NPGA electrode for 21 days.
29
Table 1. Comparison of the electrochemical performances of NPGA electrode with the previous electroanalytical methods at different electrodes for GAL detection Detection Linear range Working electrode
of limit
Method
Reference
(μmol/L) (μmol/L) NPGA-GCE
DPV
GCE/LDHf
DPV
CPE/SiO2
2.5 - 1000
0.067
This work
4 - 600
1.6
[5]
0.25
[53]
0.8 - 100
DPV SWV
0.51- 46.46
0.087
[54]
Amperometry
0.60 -625.8
0.28
[55]
Amperometry
0.5 -150
0.3
[1]
1 - 17
0.33
[56]
100 - 1300
0.14
[57]
LSV
0.58 -58.78
0.41
[34]
DPV
2.5 -150
0.94
[49]
DPV
4.5 - 76
1.1
[2]
GCE–Tyr–nAu
DPV
25 - 900
0.7
[58]
PmPD/Pal-IL/GCE
DPV
1 - 300
0.28
[35]
CS–fFe2O3– ERGO/GCE MOF -801/MC 3/GCE
DPV
1 - 50
0.15
[59]
DPV
0.2 -100
RGO-GCE AgNP/Delph/GCE MCPE/CNT
Poly-Glu/rGO GCE/MWCNT PEI-rGO/GCE TiO2/CPE TNrGO-GCE
Amperometry Chronoamperometry
30
0.15
[60]
Table 2. Determination of GAL concentrations in the real samples (n = 4). Sample
Found
Added
After added
Recovery
R.S.D.
(µmol/L)
(µmol/L)
(µmol/L)
(%)
(%)
Green tea
10.1
10
20.45 ± 0.043
103.50
3.33
Black tea
5.85
10
15.92 ± 0.057
100.70
3.78
Orange juice
0
10
9.83 ± 0.042
98.30
2.95
0.90
10
10.68 ± 0.035
97.80
3.89
Lemon
31