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A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor
G Model JIEC 4631 No. of Pages 10
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A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor Mohamed A. Yassina , Bishnu Kumar Shresthaa , Joshua Leea , Ju Yeon Kima , Chan Hee Parka,b,* , Cheol Sang Kima,b,* a Department of Bionanosystem Engineering, Graduate School, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea b Division of Mechanical Design Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 March 2019 Received in revised form 12 June 2019 Accepted 22 June 2019 Available online xxx
A novel nanostructure of three-dimensional graphene hydrogel nanotubes (3DGHNTs) is successfully synthesized for the purpose of sensing non-enzymatic H2O2 in alkaline solution. The 3DGHNTs were fabricated using manganese dioxide nanotubes (MnO2 NTs) as the effective sacrificial template and without the use of any acids or a high temperature process. 3DGH with different percentages of MnO2 NTs ranging from 5 to 30% are prepared via a hydrothermal method. When the loading percentage of MnO2 NTs is 10%, the obtained 3DGHNTs-Mn10 nanocomposite exhibits a large specific surface area with high porosity, which enhance the electrochemical properties for H2O2 detection. The developed biosensor exhibits excellent sensitivity (220.4 mA mM1 cm2) with a wide linear detection range (25 m M–22.57 mM) and a low detection limit (4 mM). The biosensor also shows a fast response time (less than 5 s) and good selectivity as well as reproducibility and long-term stability. Hence, the prepared 3DGHNTs-Mn10 nanocomposite can be considered a promising electrode material for the detection of H2O2 in real sample. © 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: 3D graphene nanotubes Sacrificial template Electrochemical sensors Hydrogen peroxide
Introduction Hydrogen peroxide (H2O2), an important reactive oxygen species, not only plays a major role in normal cellular growth and proliferation, but it can also be considered a strong oxidizing agent [1–3]. Further, it serves a vital role in various fields such as food processing, wood pulp bleaching, clinical diagnostics, environmental analysis, and many other fields [4,5]. However, an excessive level of H2O2 in a biological system can lead to diabetes, aging, cancer, and neurodegeneration [6]. Therefore, developing a sensitive, accurate, and rapid technique with which to detect H2O2 is necessary. Many techniques for the detection of H2O2 have been investigated, such as spectrophotometry [7], chemiluminescence [8], fluorescence [9], colorimetric [10], chromatography [11], and electrochemistry [12]. Among these techniques, electrochemical methods have been shown to be powerful tools due to their desirable properties such as low cost,
* Corresponding authors at: Department of Bionanosystem Engineering, Graduate School, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea. E-mail addresses: [email protected] (C.H. Park), [email protected] (C.S. Kim).
operational simplicity, higher sensitivity, and selectivity [13–15]. There are two types of electrochemical sensors for the detection of H2O2, which are either based on the existence of enzymes or based on the absence of enzymes. However, there are certain practical drawbacks (complicated fabrication, high cost, and lack of stability) resulting from the inherent nature of the enzyme that is highly affected by temperature, humidity, pH value, and toxic chemicals [16]. Therefore, considerable research efforts have gone toward developing a simple, cheap, highly stable, and eco-friendly non-enzymatic H2O2 sensor [17–19]. In recent years, due to its unique properties such as large theoretical surface area, superior electronic conductivity, remarkable mechanical strength, and high flexibility, graphene has been successfully applied in different material sciences fields [20–22]. However, 3D graphene hydrogels and aerogels are attracting attention as an ideal solution for the agglomeration dilemma of graphene during the fabrication process [23]. Therefore, many techniques have been reported to synthesize 3D graphene hydrogel and aerogels including the hydrothermal method [24], chemical reduction method [25], and template-directed CVD method [26]. Due to the fact that its fabrication process is low cost, involves a one-pot facile method, and is large scalable, numerous 3D graphene hydrogel materials have been prepared for a variety of
https://doi.org/10.1016/j.jiec.2019.06.045 1226-086X/© 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: M.A. Yassin, et al., A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.06.045
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applications through the hydrothermal process [27–29]. In addition, it is convenient to enhance graphene materials through the doping of other chemical elements giving rise to highly excellent electrocatalytic properties which significantly increase the sensitivity and stability of electrochemical biosensors. For example, Ruiyi et al. [30] developed a new electrochemical biosensor based on a significant synergy of gold nanostar and nitrogen-doped graphene aerogel for the ultrasensitive detection of circulating-free DNA in human serum with a low detection limit of 3.9 1022 g mL1. More recently, using low temperature hydrothermal with a simultaneous short time, Qi et al. [31] fabricated 3D sulfur/nitrogen co-doped graphene hydrogel in the absence of a freeze-drying process for the detection of catechol and hydroquinone with low detection limits of 0.28 and 0.15 mM, respectively. Further studies in a wide range of applications, such as water desalination [32], photocatalytic [33], solar cell [34], supercapacitors [35,36], and biosensors [37–39], have underscored the importance of effective assembly between heteroatom dopants and graphene materials, which would improve the electrocatalytic behavior of graphene by creating abundant defective/active sites. On the other hand, a sacrificial template growth process is another important strategy for designing a new morphology of material science applications [40]. Therefore, the fabrication of new structures and morphologies using a sacrificial template method has attracted increased attention due to the significant effect of morphology on the materials behavior [41]. Additionally, controlling the morphology through this process would improve the specific surface area, electrical properties, and pore structures, as well as decrease the self-aggregation of active material sites [42,43]. Yang et al. [44] used ZnO nanorods as a template to synthesize hierarchical NiCo2O4 hollow nanorods in order to detect glucose with a high sensitivity of 1685.1 mA mM1 cm2. Recently, Wang et al. [45] proposed a novel method for the fabrication of nickel silicate nanotubes using mesoporous SiO2 nanorods as a sacrificial template via the hydrothermal process for wastewater treatment. However, most of the current research has focused on the removal of template using acid or thermal treatment, which inevitably leads to a high cost, complicated, multi-step procedure as well as harmful waste. Motivated by the above considerations, we have reported here for the first time the synthesis of threedimensional graphene hydrogel nanotubes for nonenzymatic hydrogen peroxide detection through a facile two-step hydrothermal method. The fabrication process of three-dimensional graphene hydrogel nanotubes (3DGHNTs) is illustrated in Fig. 1. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption (BET) were used to investigate the structures and morphologies of the 3DGHNTs-Mn nanocomposites. The
results showed that 3DGHNTs-Mn10 nanocomposite displays a wide linear range (from 25 mM to 22.57 mM), high sensitivity (220.4 mA mM1 cm2), a low detection limit (4 mM at an S/N of 3), and a response time of 5 s. Experimental section Chemicals and reagents Graphite powder, Potassium permanganate (KMnO4), Sodium nitrate (NaNO3), Sulfuric acid (H2SO4), hydrogen peroxide (H2O2), and hydrochloric acid (HCl) were purchased from Sigma-Aldrich, Korea. D-Glucose, Dopamine (DA), uric acid (UA), and ascorbic acid (AA) were purchased from Tokyo chemical industry Co. Ltd. and from Bioshop Canada Inc. NaCl, Na2HPO4, KH2PO4, and KCl were purchased from Samchun Pure Co. Ltd. (Korea). All of the chemicals were of analytical grade and used as received without any further purification. Phosphate buffer solution (PBS, 0.1 M, pH 7.4) was freshly prepared in ultrapure water purified using a Millipore-Q system. Characterization instruments The X-ray diffraction (XRD) measurements were recorded using an X-ray diffractometer (Rigaku, Japan) with high-intensity monochromatic Cu-Kα radiation as an incident beam (l = 1.54 Å) over a Bragg’s angle range from 10 to 90 . In addition, Raman spectroscopy measurements were conducted using a Nanofinder 30 (Tokyo Instruments Co., Japan). Field emission scanning electron microscopy (FE-SEM) images of the as-prepared materials were obtained in Carl Zeiss SUPRA 40 V P, Germany. Highresolution transmission electron microscopy (HR-TEM) patterns were obtained with H-7650 Hitachi Ltd., Japan. The surface structure and chemical valence states of the prepared samples were characterized by X-ray photoelectron spectroscopy (XPS; Axis Ultra DLDX-ray photoelectron spectrometry, Kratos, UK). Moreover, the specific surface area and porosity of the as-prepared samples were characterized through the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively, using ASAP 2420 (Micromeritics, USA) at 77 K under low-pressure volumetric gas N2 adsorption/desorption. Preparation of graphene oxide (GO) Graphene Oxide (GO) was synthesized from graphite powder according to the modified Hummer's method [46,47]. Briefly, 0.6 g of graphite powder was added to 50 mL of concentrated H2SO4 solution, followed by the addition of 0.3 g of NaNO3 in a round bottom flask settled in an ice bath. Next, 2 g of KMnO4 was slowly
Fig. 1. Schematic illustration for the fabrication of 3DGHNTs nanocomposites.
Please cite this article in press as: M.A. Yassin, et al., A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.06.045
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added to the mixture under vigorous stirring for 1 h, then transferred to a preheated oil bath at 35 C under stirring for 6 h. Afterwards, 70 m of deionized water was gradually added to the mixture in an ice bath. Subsequently, 2 mL of 35% H2O2 dissolved in 100 mL of deionized water was added to the mixture under stirring for 1 h. Finally, the mixture was washed with 5% of HCl, then deionized water, and then freeze-dried for 48 h to obtain graphene oxide. Preparation of MnO2 NTs MnO2 nanotubes were prepared by the hydrothermal method according to the methods described in the literature [48]. In a typical procedure, 0.506 g of KMnO4 was dissolved in 120 mL of deionized water under magnetic stirring at room temperature. Afterwards, 2.25 mL of HCl (37%) was added dropwise into the mixture, then the mixture was transferred to a 150 mL Teflon-lined autoclave and heated in an oven at 140 C for 12 h. The final products were centrifuged and washed several times with deionized water and dried under vacuum at 60 C to obtain MnO2 NTs. Preparation of 3DGHNTs/Mn nanocomposites The 3DGHNTs/Mn nanocomposite was prepared by the hydrothermal method, as described in Fig.1. In a typical process, as-prepared MnO2 NTs (6 mg) and 60 mg of prepared GO were immersed in 40 mL deionized water under prop-sonication for 1.5 h. The mixture was then transferred to a 75 mL Teflon-lined autoclave and heated in an oven at 180 C for 12 h. Following normal cooling to room temperature, the product named 3DGHNTs-Mn10 was then washed several times with deionized water and finally freeze-dried. This procedure was repeated for the formation of 3DGHNTs-Mn5 (3 mg of MnO2 NTs), 3DGHNTs-Mn20 (12 mg of MnO2 NTs), and 3DGHNTs-Mn30 (18 mg of MnO2 NTs) Sensor fabrication and measurements A bare glassy carbon electrode (GCE, D = 3 mm) was polished using alumina suspension, then sonicated in ethanol and deionized water for 10 min, respectively. Next, 2 mg of 3DGHNTs-Mn10 was mixed with 25 mL of a 5% Nafion solution and 400 mL of isopropanol under sonication in order to obtain homogeneous solution. Afterwards, 15 mL of the prepared homogeneous solution
3
was coated on a bare glassy carbon electrode. Finally, the prepared electrode was dried at 60 C for 30 min. For comparison, 3DGHNTsMn5, 3DGHNTs-Mn20, and 3DGHNTs-Mn30 electrodes were prepared by following the same procedure. The electrochemical workstation ZIVE SP1 (WonATech Co. Ltd. Seoul, Korea) was used to perform all electrochemical analysis including cyclic voltammetry (CV), amperometry, and electrochemical impedance spectroscopy (EIS). The electrochemical measurements were conducted using a conventional three-electrode system with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode, and a modified GCE as the working electrode. The measurements were conducted at room temperature in 10 mL of 0.1 M PBS (pH 7.4) solution with or without the presence of hydrogen peroxide and purged with high purity nitrogen for 10 min prior to each measurement. Results and discussion Structural characterizations In order to further understand the crystallized structures of obtained samples, X-ray diffraction (XRD) patterns were investigated. As shown in Fig. 2a, a single broad peak was observed at 2u = 24.6 (002) in the diffraction pattern of 3DGH, indicating the weak-ordered restacking of graphene sheets [49]. In addition, all of the diffraction peaks can be definitively indexed to α-MnO2 tetragonal structure (JCPDS Card No. 44-0141) with lattice constants a = b =9.78 Å and c = 2.86 Å [space group: I4/m] [50,51]. The significant sharp peaks demonstrating the high purity of tetragonal crystalline α-MnO2 were fabricated in a preponderance. After self-assembling among 3DGH and α-MnO2 nanotubes, two significant broad peaks can be seen at about 2u = 24.6 and 43.8, which were assigned to the (002) and (100) planes, respectively, and indicated the few-layer-stacked graphene nanosheets of their frameworks [52,53]. In addition, only one small peak appearing at about 2u = 41.8 might be indexed to the doping amounts of MnO2, and the remaining peaks of MnO2 NTs are clearly absent in the XRD pattern of 3DGHNTs-Mn10, meaning that the new morphology of graphene contains a tiny amount of MnO2 NTs. Moreover, Raman spectra provided worthy information for the crystallinity and microstructure of the carbon based materials. According to Fig. 2b, 3DGH clearly exhibits two characteristic peaks centered at 1345 cm1 and 1583 cm1 which were indicative of the D and G band, respectively. The first band (D) is assigned to the structural defects
Fig. 2. (a) XRD patterns of 3DGH, MnO2 NTs and 3DGHNTs/Mn10. (b) Raman spectra of 3DGH and 3DGHNTs/Mn10.
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from the vibrations of sp3 carbon atoms and the next band (G) is corresponds to the vibration of sp2 hybridized carbon atoms in the graphitic 2D hexagonal lattice [54]. However, after self-assembling between 3DGH and α-MnO2 NTs, both D and G peaks positively shift to 1350 cm1 and 1600 cm1, respectively, which indicated that the new morphology of 3DGHNTs were successfully achieved. Moreover, it has been adopted the peak area ratio of the D and G band (ID/IG) for evaluating the ordered and disordered degree of carbonaceous materials [55]. A significant increase of the intensity ratio (ID/IG) is observed for 3DGHNTs-Mn10 (0.98) as compared to 3DGH (0.91), demonstrating that the disordering of carbon increases after 3DGH is modified by α-MnO2 NTs. Field emission scanning electron microscopy (FE-SEM) is used to investigate the morphologies and microstructures of the asprepared materials. The bulk MnO2 NTs has regular 1-D nanostructured crystals with an average outer diameter of 70 nm and an average length of several microns [56], as shown in Fig. 3a. Further, it can clearly be seen that the 3DGHNTs-Mn10 nanocomposite has unique pore sizes which vary from tens of nanometers to several hundred nanometers (Fig. 3b). This indicates that the porosity properties of graphene hydrogel can be improved through the self-assembly of MnO2 NTs via the hydrothermal process. More importantly, the high-magnification FE-SEM images in Fig. 3c, d clearly show that the surface of 3DGH contained a new morphology of graphene channels nanotubes following the growth of a few layers of graphene as a shell around MnO2 NTs as a core and the dissolution of most MnO2 NTs under high temperature and pressure. Further evidence for the structure of the 3DGHNTs materials is provided by TEM analysis. As shown in Fig. 4a, b there is an abundance of black channels nanotubes, which is consistent with the FE-SEM observation results. In order to analyze the chemical composition and distribution of elements in the newly-developed material, the 3DGHNTs is subjected to energy dispersive X-ray spectroscopy (EDS). As shown in Fig. 4c, the C content is the major amount (80%) followed by O content (22%), while the Mn content decreased by about 80% of its initial amount via self-assembly with 3DGH, which can be ascribed to the degradation of MnO2 NTs during the hydrothermal process. Fig. 4d
depicts the elemental mapping analysis for 3DGHNTs. Notably, it can be clearly observed that carbon and oxygen have high densities. Meanwhile, the random presence and low density of manganese demonstrated the hypothesis of our study. The surface structure and chemical valence states of 3DGH and 3DGHNTsMn10 were investigated through XPS. As shown in Fig. 5(a), the survey spectrum of 3DGH is mainly composed of C and O. The C1s high-resolution spectrum (Fig. 5b) shows four peaks at the binding energies of 284.7 eV, 285.9 eV, 287.6 eV, and 289.4 eV, which correspond to C¼C/C C, C–O, C C, and O C = O, respectively [57]. As shown in Fig. 5c, the O1s high-resolution spectrum displays three peaks at the binding energies of 531.7 eV, 532.5 eV, and 533.4 eV, which are assigned to the C–O (epoxy and hydroxyl), C¼O (carbonyl), and O = C–O (carboxyl) groups, respectively. The XPS spectrum of 3DGHNTs-Mn10 nanocomposite is shown in Fig. 5a. Clearly, a new peak of the Mn2p spectrum appears at the range of binding energies from 630 eV to 660 eV. The Mn2p spectrum (Fig. 5d) can be split into two peaks related to the Mn 2p3/2 and Mn 2p1/2 spin-orbit peaks of MnO2 at binding energies of 641.8 eV and 653.4 eV, respectively [58]. Additionally, the atomic percentages of elements in our prepared samples are presented in Table S1. We observed that a small amount of Mn of 0.67 atom% has been detected, which is consistent with the EDS results. Nitrogen adsorption/desorption isotherms and Barrett–Joyner– Halenda (BJH) pore size distribution analysis were recorded in order to determine the specific surface area, porosity, and pore size distribution of the as-prepared samples, as shown in Figures (6 and S2). It can clearly be seen that all of the isotherm curves show a wide relative pressure range (0.4–1.0), which reflects a typical IUPAC type-IV characteristic for the mesoporous structure of synthesized 3D graphene hydrogel composites. The pore size distributions curve of 3DGH in Fig. 6c shows the existence of mesopores at 8 nm and 14 nm and a low amount at 19 nm, as well as a complete absence of micropores. However, there is a wide range of pore size distribution from supermicropores (1–2 nm) to mesopores (2–50 nm) in 3DGHNTs-Mn10 (Fig. 6d), which can be attributed to the effects of MnO2 NTs as a sacrificial template in the generation of a mesopores structure with a wide range.
Fig. 3. (a) FE-SEM image of the prepared MnO2 nanotubes. (b, c, and d) Higher and lower magnification FE-SEM images of 3DGHNTs-Mn10 nanocomposite.
Please cite this article in press as: M.A. Yassin, et al., A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.06.045
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Fig. 4. (a, b) TEM image of the as-prepared 3DGHNTs-Mn10 nanocomposite. (c, d) TEM EDX analysis and TEM mapping of the 3DGHNTs-Mn10 for the following elements: C, O, and Mn.
Fig. 5. (a) XPS survey spectra of 3DGH and 3DGHNTs-Mn10; (b–d) High-resolution XPS spectra of C1s, O1s and Mn2p of 3DGHNTs-Mn10.
Please cite this article in press as: M.A. Yassin, et al., A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.06.045
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Fig. 6. (a, b) Nitrogen adsorption and desorption isothermals at 77 K as well as (c, d) pore size distributions of 3DGH and 3DGHNTs-Mn10.
As compared to 3DGH and other composites, the 3DGHNTs-Mn10 composite exhibits the highest BET specific surface area (95.89 m2/ g) as well as the largest total pore volume (0.54 cm3/g), as presented in Table S2. Therefore, the large specific surface area and high total pore volume can enhance the electron transfer and provide additional electroactive sites, which could increase the reduction of H2O2. Electrochemical properties of the 3DGHNTs-Mn modified electrodes Electrochemical impedance spectroscopy The electronic transfer properties of four 3DGHNTs-Mn nanocomposites were evaluated in aqueous solution consisting of 5 mM [Fe(CN)6]3/4 and 0.1 M KCl through electrical impedance spectroscopy (EIS) analysis with a frequency range from 1 Hz to 1 MHz at an amplitude of 5 mV. The Nyquist plot of impedance spectra has a semicircle arc section at higher frequencies and a straight-line section at low frequencies. Additionally, the semicircle arc represents the electron transfer limited process, which determines the electron transfer resistance (Rct) based on the diameter of the semicircle section [59]. With a decrease in the diameter of the semicircle, the charge transfer resistance will be decreased accordingly, thus facilitating electron transfer on the electrode. As shown in Fig. 7, the values of charge transfer resistance (Rct) for 3DGHNTs-Mn5, 3DGHNTs-Mn10, 3DGHNTsMn20, and 3DGHNTs-Mn30 were 145 V, 105 V, 120 V, and 185 V, respectively. Therefore, the semicircle diameter in the highfrequency region associated with the 3DGHNTs-Mn10 is lower than that of other nanocomposites electrodes, demonstrating the higher electrochemical reaction rate of 3DGHNTs-Mn10 nanocomposite, which further facilitates the electron transfer and
Fig. 7. Nyquist plots of four modified electrodes in 5.0 mM K3Fe[CN]6 containing 0.1 M KCl from 0.1 Hz to 100 kHz at an AC amplitude of 5 mV.
hence increases the responding signals of H2O2 [60]. Furthermore, the 3DGHNTs-Mn10 electrode shows the highest slope of the straight line at low frequency, indicating the ideal capacitive characteristics of the new composite electrode. Electrocatalytic properties toward H2O2 reduction The electrochemical performance of the different modified electrodes was measured in 0.1 M PBS solution (PH = 7.4) via CV at a
Please cite this article in press as: M.A. Yassin, et al., A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.06.045
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Fig. 8. CVs response curves of studied electrodes in the (a) absence and (b) presence of 4 mM H2O2 in 0.1 M PBS (pH-7.4) at 50 mV/s.
scan rate of 50 mV/s in both the absence and presence of 4 mM H2O2. As shown in Fig. 8a, no obvious responses were observed in the absence of H2O2 for any of the electrodes. By contrast, in the presence of 4 mM H2O2 in PBS solution, as shown in Fig. 8b, no reduction peak is observed on bare/GCE and MnO2 NTs/GCE electrodes while a weak reduction peak is present for 3DGH/GCE.
However, a remarkable reduction peak at about 0.35 V is observed for 3DGHNTs-Mn different composite electrodes. Among the four 3DGHNTs-Mn composites, the 3DGHNTs-Mn10 show the highest reduction peak, which means that the 3DGHNTs-Mn10 composite had substantially better electrocatalytic activity toward H2O2 than the other composites. This was attributed to the
Fig. 9. (a) CV curves of the 3DGHNTs-Mn10/GCE electrode at different scan rates in N2 saturated 0.1 M PBS solution containing 4 mM H2O2. (b) Plot of reduction peaks current vs. square root of scan rates. (c) CV curves of the 3DGHNTs-Mn10/GCE electrode in 0.1 M PBS with different concentrations of H2O2 at a scan rate of 50 mV s1. (d) Plot of reduction peaks current vs. concentrations of H2O2.
Please cite this article in press as: M.A. Yassin, et al., A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.06.045
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optimum 10% of MnO2 NTs preventing the restacking of graphene hydrogel, which produces the highest surface area (95.89 m2/g) and thus enhances the conductivity between the modified electrode and the H2O2 molecules. In addition, the reduction current peak decrease with the increase of MnO2 NTs above 10% due to the poor conductivity of MnO2, which suppresses electron transfer ability, as shown in the EIS measurement results (Fig.7). In order to confirm the electrochemical behavior of 3DGHNTs-Mn10 towards H2O2 reduction, different scan rates ranging from 20 to 120 mV/s were further applied in PBS solution containing 4 mM H2O2. As shown in Fig. 9a, the reduction current peak clearly increases with the increase in scan rate value, accompanied with the negative shift in peak potential. Moreover, Fig. 9c shows the relationship between the current reduction peak of 3DGHNTsMn10 at different concentrations of H2O2 ranging from 1 to 4 mM. It can be seen that the reduction current peak appeared at 1 mM of H2O2 and that this peak increases with the increase of H2O2 concentration. In addition, good linearity was observed between the peak of current reduction and concentration of H2O2 (Fig. 9d). These superior analytical behaviors of the 3DGHNTs-Mn10 can be attributed to the effects of MnO2 NTs as a sacrificial template in the generation of outstanding pore size distribution ranges from supermicropores (1–2 nm) to a wide range of mesopores (2–50 nm), leading to effective electron/ion transport channels and excellent H2O2 sensing.
Amperometric H2O2 detection on the 3DGHNTs-Mn10 Further, in order to confirm the notable CV results of 3DGHNTsMn10 as an electrode material in the detection of H2O2, chronoamperometric measurements were observed at an applied potential of 0.35 V with successive increments of H2O2 at different concentrations into continuously stirred N2 saturated 0.1 M PBS solution (Fig.10a). the results showed that the current response of the developed sensor increased rapidly and reached 95% of the steady state current before 5 s, indicating a fast electrocatalytic response on as-synthesized electrode surface. Fig.10b. shows a calibration curve for the H2O2 sensor which was created by adding a successive certain concentration of H2O2 solution while observing the current density of the developed sensor. It can clearly be seen that the response current of H2O2 increased linearly with increasing H2O2 concentration with a wide linear range (25 mM–22.57 mM) at a correlation coefficient of 0.9996. Further, based on the slope of the linear equation, Ipa ðmAÞ ¼ 3:24615:429c ðmMÞ, the high sensitivity of the proposed sensor was calculated as 220.4 mA mM1 cm2 with the lower detection limit of 4 mM (signal-to-noise ratio of 3). A comparison of our result with the previously reported nonenzymatic H2O2 sensors is shown in Table 1. The as-prepared electrode has improved electrocatalytic active behavior with high sensitivity, a wide linear range, and a low detection limit towards H2O2 detection. Therefore, these results demonstrate that the
Fig. 10. (a) Amperometric response of 3DGHNTs-Mn10/GCE on successive additions of H2O2 into 0.1 M PBS at 0.35 V. Inset shows a blown-up image of the low-concentration region. (b) Calibration curve between H2O2 concentration and amperometric response. (c) Interference test of the 3DGHNTs-Mn10/GCE electrode in 0.1 M PBS at 0.35 V with 0.5 mM H2O2 and other interferents including 1 mM of NaCl, AA, UA, DA, Urea, KCl, Citric acid, and Glucose. (d) Stability test of the 3DGHNTs-Mn10/GCE for 21 days. Inset shows the reproducibility study for five electrodes of 3DGHNTs-Mn10/GCE.
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Table 1 Comparison of the as-developed our sensor with the other reported electrochemical H2O2 sensors. Electrode
Linear range (mM)
Sensitivity (mA mM1 cm2)
Detection limit (mM)
Ref.
GO/MnO2 Fe3O4/RGO PDDA/graphene O-MoS2/graphene Graphene/CNTs Ag–MnO2–MWCNTs RGO/tyrosine Cu-MOF/graphene Ag–HNTs–MnO2 MnO2-Ag@C 3DGHNTs-Mn10
5–600 0.5–3000 0.5–500 250–16000 20–2100 5–10400 100–2100 10–11,180 2–4710 0.5–5700 25–22570
38.2 22.27 140.8 269.7 32.91 82.5 69.07 57.73 11.9 127.2 220.41
0.8 0.18 0.1 0.12 9.4 1.7 80 2 0.7 0.17 4
[61] [62] [63] [60] [17] [64] [65] [66] [67] [68] This work
3DGHNTs-Mn10 modified electrode has significant electrocatalytic activity towards H2O2. Selectivity, reproducibility, stability and real sample analysis The selectivity test has become predominant in assessing the performance of electrochemical biosensors. The effects of different electroactive agents were observed using an amperometric technique in 0.1 M PBS (pH 7.4) solution at an applied potential of 0.35 V (vs. Ag/AgCl) with the initial addition of 0.5 mM H2O2 and then the subsequent addition of 1 mM of each electroactive agent, i.e., NaCl, AA, UA DA, Urea, KCl, Citric Acid, and glucose, as shown in Fig.10c. An obvious rapid current change has been observed with the addition of 0.5 mM H2O2 for our proposed electrode. Compared with other interfering species, the current response did not show any change in the presence of other species on 3DGHNTs-Mn10 electrode, suggesting a significant contribution of the developed electrode in anti-interference efficiency and selectivity towards the detection of H2O2. In order to further prove the excellent electrochemical behavior of the as-synthesized electrode, five modified glassy carbon electrodes were prepared separately to examine the reproducibility of the new composite material by measuring the response of current in the same PBS solution containing 2 mM H2O2. The results showed that the reduction peak currents of 2 mM H2O2 were almost equal values, as shown in the inset of Fig. 10d, with a relative standard deviation (RSD) of 1.47%. In addition, the longterm stability of 3DGHNTs-Mn10 nanocomposite electrode was also investigated by monitoring the reduction current peak every two days for 3 weeks, as shown in Fig.10d. The developed biosensor exhibits excellent stability, with 96.2% of its initial reduction peak value being maintained after three weeks. The high reproducibility and long-term stability indicated that the proposed electrode is a suitable applicable biosensor for the detection of H2O2 with a wide linear range and a high sensitivity. According to the impressive results discussed above, the new developed H2O2 electrochemical biosensor relying on a 3DGHNTs-Mn10 nanocomposite electrode was used to detect the concentration of H2O2 in a yogurt real sample. The sample of yogurt was diluted in 0.1 M PBS (pH 7.4) solution. Next, the standard addition method was used to explore the recovery and the accuracy of the as-prepared electrode for sensing H2O2. As shown in Table 2, the measurement of H2O2 recovery percentage ranged from 97.6% to 101.3% with a relative
Table 2 Detection of H2O2 in yogurt samples using 3DGHNTs/Mn10/GCE. Sample
Added (mM)
Found (mM)
Recovery (%)
RSD (%) n = 3
1 2 3 4
20 35 50 80
19.68 34.16 50.65 78.32
98.4 97.6 101.3 97.9
1.04 0.86 0.91 1.12
standard deviation (RSD) under 1.12%, indicating the excellent operational properties of the proposed sensor for the real-time detection of H2O2 in daily life products. Conclusion In this study, we used a facile and cost effectiveness method to produce a novel morphology of three-dimensional graphene hydrogel nanotubes (3DGHNTs) for use as a biosensor electrode for the detection of H2O2. Various physicochemical studies including XRD, Raman, FE-SEM, HR-TEM, XPS, and BET were adopted in order to characterize the structural and morphological properties. Cyclic voltammetry and amperometric techniques were used to investigate the electrochemical properties of the 3DGHNTs nanocomposites for detecting H2O2 in phosphate buffer solution. The optimized nanocomposite showed excellent electrocatalytic activity towards H2O2 reduction. This is attributed to its highly specific surface area and excellent mesoporous and nonporous structure which lead to highly improved electron transport and ion diffusion in the electrolyte. Remarkably, the results presented here reveal that the optimized 3DGHNTs-Mn10 can detect H2O2 in a wide linear range (25 mM–22.57 mM) with high sensitivity (220.4 mA mM1 cm2) and a low detection limit (4 mM). Considering the good selectivity, reproducibility, and longterm stability of the proposed electrode, it can be further explored in real samples of our daily life. Acknowledgements The research reported in this work has been supported by grant from the Basic Science Research Program through National Research Foundation of Korea (NRF) by Ministry of Education, Science and Technology (Project no. 2018R1D1A1B07044717) and ministry of SEMs and startups (Project no. C0504566). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.06.045. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
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A novel morphology of 3D graphene hydrogel nanotubes for high-performance nonenzymatic hydrogen peroxide sensor Mohamed A. Yassina , Bishnu Kumar Shresthaa , Joshua Leea , Ju Yeon Kima , Chan Hee Parka,b,* , Cheol Sang Kima,b,* a Department of Bionanosystem Engineering, Graduate School, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea b Division of Mechanical Design Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 March 2019 Received in revised form 12 June 2019 Accepted 22 June 2019 Available online xxx
A novel nanostructure of three-dimensional graphene hydrogel nanotubes (3DGHNTs) is successfully synthesized for the purpose of sensing non-enzymatic H2O2 in alkaline solution. The 3DGHNTs were fabricated using manganese dioxide nanotubes (MnO2 NTs) as the effective sacrificial template and without the use of any acids or a high temperature process. 3DGH with different percentages of MnO2 NTs ranging from 5 to 30% are prepared via a hydrothermal method. When the loading percentage of MnO2 NTs is 10%, the obtained 3DGHNTs-Mn10 nanocomposite exhibits a large specific surface area with high porosity, which enhance the electrochemical properties for H2O2 detection. The developed biosensor exhibits excellent sensitivity (220.4 mA mM1 cm2) with a wide linear detection range (25 m M–22.57 mM) and a low detection limit (4 mM). The biosensor also shows a fast response time (less than 5 s) and good selectivity as well as reproducibility and long-term stability. Hence, the prepared 3DGHNTs-Mn10 nanocomposite can be considered a promising electrode material for the detection of H2O2 in real sample. © 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: 3D graphene nanotubes Sacrificial template Electrochemical sensors Hydrogen peroxide
Introduction Hydrogen peroxide (H2O2), an important reactive oxygen species, not only plays a major role in normal cellular growth and proliferation, but it can also be considered a strong oxidizing agent [1–3]. Further, it serves a vital role in various fields such as food processing, wood pulp bleaching, clinical diagnostics, environmental analysis, and many other fields [4,5]. However, an excessive level of H2O2 in a biological system can lead to diabetes, aging, cancer, and neurodegeneration [6]. Therefore, developing a sensitive, accurate, and rapid technique with which to detect H2O2 is necessary. Many techniques for the detection of H2O2 have been investigated, such as spectrophotometry [7], chemiluminescence [8], fluorescence [9], colorimetric [10], chromatography [11], and electrochemistry [12]. Among these techniques, electrochemical methods have been shown to be powerful tools due to their desirable properties such as low cost,
* Corresponding authors at: Department of Bionanosystem Engineering, Graduate School, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea. E-mail addresses: [email protected] (C.H. Park), [email protected] (C.S. Kim).
operational simplicity, higher sensitivity, and selectivity [13–15]. There are two types of electrochemical sensors for the detection of H2O2, which are either based on the existence of enzymes or based on the absence of enzymes. However, there are certain practical drawbacks (complicated fabrication, high cost, and lack of stability) resulting from the inherent nature of the enzyme that is highly affected by temperature, humidity, pH value, and toxic chemicals [16]. Therefore, considerable research efforts have gone toward developing a simple, cheap, highly stable, and eco-friendly non-enzymatic H2O2 sensor [17–19]. In recent years, due to its unique properties such as large theoretical surface area, superior electronic conductivity, remarkable mechanical strength, and high flexibility, graphene has been successfully applied in different material sciences fields [20–22]. However, 3D graphene hydrogels and aerogels are attracting attention as an ideal solution for the agglomeration dilemma of graphene during the fabrication process [23]. Therefore, many techniques have been reported to synthesize 3D graphene hydrogel and aerogels including the hydrothermal method [24], chemical reduction method [25], and template-directed CVD method [26]. Due to the fact that its fabrication process is low cost, involves a one-pot facile method, and is large scalable, numerous 3D graphene hydrogel materials have been prepared for a variety of
https://doi.org/10.1016/j.jiec.2019.06.045 1226-086X/© 2019 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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applications through the hydrothermal process [27–29]. In addition, it is convenient to enhance graphene materials through the doping of other chemical elements giving rise to highly excellent electrocatalytic properties which significantly increase the sensitivity and stability of electrochemical biosensors. For example, Ruiyi et al. [30] developed a new electrochemical biosensor based on a significant synergy of gold nanostar and nitrogen-doped graphene aerogel for the ultrasensitive detection of circulating-free DNA in human serum with a low detection limit of 3.9 1022 g mL1. More recently, using low temperature hydrothermal with a simultaneous short time, Qi et al. [31] fabricated 3D sulfur/nitrogen co-doped graphene hydrogel in the absence of a freeze-drying process for the detection of catechol and hydroquinone with low detection limits of 0.28 and 0.15 mM, respectively. Further studies in a wide range of applications, such as water desalination [32], photocatalytic [33], solar cell [34], supercapacitors [35,36], and biosensors [37–39], have underscored the importance of effective assembly between heteroatom dopants and graphene materials, which would improve the electrocatalytic behavior of graphene by creating abundant defective/active sites. On the other hand, a sacrificial template growth process is another important strategy for designing a new morphology of material science applications [40]. Therefore, the fabrication of new structures and morphologies using a sacrificial template method has attracted increased attention due to the significant effect of morphology on the materials behavior [41]. Additionally, controlling the morphology through this process would improve the specific surface area, electrical properties, and pore structures, as well as decrease the self-aggregation of active material sites [42,43]. Yang et al. [44] used ZnO nanorods as a template to synthesize hierarchical NiCo2O4 hollow nanorods in order to detect glucose with a high sensitivity of 1685.1 mA mM1 cm2. Recently, Wang et al. [45] proposed a novel method for the fabrication of nickel silicate nanotubes using mesoporous SiO2 nanorods as a sacrificial template via the hydrothermal process for wastewater treatment. However, most of the current research has focused on the removal of template using acid or thermal treatment, which inevitably leads to a high cost, complicated, multi-step procedure as well as harmful waste. Motivated by the above considerations, we have reported here for the first time the synthesis of threedimensional graphene hydrogel nanotubes for nonenzymatic hydrogen peroxide detection through a facile two-step hydrothermal method. The fabrication process of three-dimensional graphene hydrogel nanotubes (3DGHNTs) is illustrated in Fig. 1. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption (BET) were used to investigate the structures and morphologies of the 3DGHNTs-Mn nanocomposites. The
results showed that 3DGHNTs-Mn10 nanocomposite displays a wide linear range (from 25 mM to 22.57 mM), high sensitivity (220.4 mA mM1 cm2), a low detection limit (4 mM at an S/N of 3), and a response time of 5 s. Experimental section Chemicals and reagents Graphite powder, Potassium permanganate (KMnO4), Sodium nitrate (NaNO3), Sulfuric acid (H2SO4), hydrogen peroxide (H2O2), and hydrochloric acid (HCl) were purchased from Sigma-Aldrich, Korea. D-Glucose, Dopamine (DA), uric acid (UA), and ascorbic acid (AA) were purchased from Tokyo chemical industry Co. Ltd. and from Bioshop Canada Inc. NaCl, Na2HPO4, KH2PO4, and KCl were purchased from Samchun Pure Co. Ltd. (Korea). All of the chemicals were of analytical grade and used as received without any further purification. Phosphate buffer solution (PBS, 0.1 M, pH 7.4) was freshly prepared in ultrapure water purified using a Millipore-Q system. Characterization instruments The X-ray diffraction (XRD) measurements were recorded using an X-ray diffractometer (Rigaku, Japan) with high-intensity monochromatic Cu-Kα radiation as an incident beam (l = 1.54 Å) over a Bragg’s angle range from 10 to 90 . In addition, Raman spectroscopy measurements were conducted using a Nanofinder 30 (Tokyo Instruments Co., Japan). Field emission scanning electron microscopy (FE-SEM) images of the as-prepared materials were obtained in Carl Zeiss SUPRA 40 V P, Germany. Highresolution transmission electron microscopy (HR-TEM) patterns were obtained with H-7650 Hitachi Ltd., Japan. The surface structure and chemical valence states of the prepared samples were characterized by X-ray photoelectron spectroscopy (XPS; Axis Ultra DLDX-ray photoelectron spectrometry, Kratos, UK). Moreover, the specific surface area and porosity of the as-prepared samples were characterized through the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively, using ASAP 2420 (Micromeritics, USA) at 77 K under low-pressure volumetric gas N2 adsorption/desorption. Preparation of graphene oxide (GO) Graphene Oxide (GO) was synthesized from graphite powder according to the modified Hummer's method [46,47]. Briefly, 0.6 g of graphite powder was added to 50 mL of concentrated H2SO4 solution, followed by the addition of 0.3 g of NaNO3 in a round bottom flask settled in an ice bath. Next, 2 g of KMnO4 was slowly
Fig. 1. Schematic illustration for the fabrication of 3DGHNTs nanocomposites.
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added to the mixture under vigorous stirring for 1 h, then transferred to a preheated oil bath at 35 C under stirring for 6 h. Afterwards, 70 m of deionized water was gradually added to the mixture in an ice bath. Subsequently, 2 mL of 35% H2O2 dissolved in 100 mL of deionized water was added to the mixture under stirring for 1 h. Finally, the mixture was washed with 5% of HCl, then deionized water, and then freeze-dried for 48 h to obtain graphene oxide. Preparation of MnO2 NTs MnO2 nanotubes were prepared by the hydrothermal method according to the methods described in the literature [48]. In a typical procedure, 0.506 g of KMnO4 was dissolved in 120 mL of deionized water under magnetic stirring at room temperature. Afterwards, 2.25 mL of HCl (37%) was added dropwise into the mixture, then the mixture was transferred to a 150 mL Teflon-lined autoclave and heated in an oven at 140 C for 12 h. The final products were centrifuged and washed several times with deionized water and dried under vacuum at 60 C to obtain MnO2 NTs. Preparation of 3DGHNTs/Mn nanocomposites The 3DGHNTs/Mn nanocomposite was prepared by the hydrothermal method, as described in Fig.1. In a typical process, as-prepared MnO2 NTs (6 mg) and 60 mg of prepared GO were immersed in 40 mL deionized water under prop-sonication for 1.5 h. The mixture was then transferred to a 75 mL Teflon-lined autoclave and heated in an oven at 180 C for 12 h. Following normal cooling to room temperature, the product named 3DGHNTs-Mn10 was then washed several times with deionized water and finally freeze-dried. This procedure was repeated for the formation of 3DGHNTs-Mn5 (3 mg of MnO2 NTs), 3DGHNTs-Mn20 (12 mg of MnO2 NTs), and 3DGHNTs-Mn30 (18 mg of MnO2 NTs) Sensor fabrication and measurements A bare glassy carbon electrode (GCE, D = 3 mm) was polished using alumina suspension, then sonicated in ethanol and deionized water for 10 min, respectively. Next, 2 mg of 3DGHNTs-Mn10 was mixed with 25 mL of a 5% Nafion solution and 400 mL of isopropanol under sonication in order to obtain homogeneous solution. Afterwards, 15 mL of the prepared homogeneous solution
3
was coated on a bare glassy carbon electrode. Finally, the prepared electrode was dried at 60 C for 30 min. For comparison, 3DGHNTsMn5, 3DGHNTs-Mn20, and 3DGHNTs-Mn30 electrodes were prepared by following the same procedure. The electrochemical workstation ZIVE SP1 (WonATech Co. Ltd. Seoul, Korea) was used to perform all electrochemical analysis including cyclic voltammetry (CV), amperometry, and electrochemical impedance spectroscopy (EIS). The electrochemical measurements were conducted using a conventional three-electrode system with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode, and a modified GCE as the working electrode. The measurements were conducted at room temperature in 10 mL of 0.1 M PBS (pH 7.4) solution with or without the presence of hydrogen peroxide and purged with high purity nitrogen for 10 min prior to each measurement. Results and discussion Structural characterizations In order to further understand the crystallized structures of obtained samples, X-ray diffraction (XRD) patterns were investigated. As shown in Fig. 2a, a single broad peak was observed at 2u = 24.6 (002) in the diffraction pattern of 3DGH, indicating the weak-ordered restacking of graphene sheets [49]. In addition, all of the diffraction peaks can be definitively indexed to α-MnO2 tetragonal structure (JCPDS Card No. 44-0141) with lattice constants a = b =9.78 Å and c = 2.86 Å [space group: I4/m] [50,51]. The significant sharp peaks demonstrating the high purity of tetragonal crystalline α-MnO2 were fabricated in a preponderance. After self-assembling among 3DGH and α-MnO2 nanotubes, two significant broad peaks can be seen at about 2u = 24.6 and 43.8, which were assigned to the (002) and (100) planes, respectively, and indicated the few-layer-stacked graphene nanosheets of their frameworks [52,53]. In addition, only one small peak appearing at about 2u = 41.8 might be indexed to the doping amounts of MnO2, and the remaining peaks of MnO2 NTs are clearly absent in the XRD pattern of 3DGHNTs-Mn10, meaning that the new morphology of graphene contains a tiny amount of MnO2 NTs. Moreover, Raman spectra provided worthy information for the crystallinity and microstructure of the carbon based materials. According to Fig. 2b, 3DGH clearly exhibits two characteristic peaks centered at 1345 cm1 and 1583 cm1 which were indicative of the D and G band, respectively. The first band (D) is assigned to the structural defects
Fig. 2. (a) XRD patterns of 3DGH, MnO2 NTs and 3DGHNTs/Mn10. (b) Raman spectra of 3DGH and 3DGHNTs/Mn10.
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from the vibrations of sp3 carbon atoms and the next band (G) is corresponds to the vibration of sp2 hybridized carbon atoms in the graphitic 2D hexagonal lattice [54]. However, after self-assembling between 3DGH and α-MnO2 NTs, both D and G peaks positively shift to 1350 cm1 and 1600 cm1, respectively, which indicated that the new morphology of 3DGHNTs were successfully achieved. Moreover, it has been adopted the peak area ratio of the D and G band (ID/IG) for evaluating the ordered and disordered degree of carbonaceous materials [55]. A significant increase of the intensity ratio (ID/IG) is observed for 3DGHNTs-Mn10 (0.98) as compared to 3DGH (0.91), demonstrating that the disordering of carbon increases after 3DGH is modified by α-MnO2 NTs. Field emission scanning electron microscopy (FE-SEM) is used to investigate the morphologies and microstructures of the asprepared materials. The bulk MnO2 NTs has regular 1-D nanostructured crystals with an average outer diameter of 70 nm and an average length of several microns [56], as shown in Fig. 3a. Further, it can clearly be seen that the 3DGHNTs-Mn10 nanocomposite has unique pore sizes which vary from tens of nanometers to several hundred nanometers (Fig. 3b). This indicates that the porosity properties of graphene hydrogel can be improved through the self-assembly of MnO2 NTs via the hydrothermal process. More importantly, the high-magnification FE-SEM images in Fig. 3c, d clearly show that the surface of 3DGH contained a new morphology of graphene channels nanotubes following the growth of a few layers of graphene as a shell around MnO2 NTs as a core and the dissolution of most MnO2 NTs under high temperature and pressure. Further evidence for the structure of the 3DGHNTs materials is provided by TEM analysis. As shown in Fig. 4a, b there is an abundance of black channels nanotubes, which is consistent with the FE-SEM observation results. In order to analyze the chemical composition and distribution of elements in the newly-developed material, the 3DGHNTs is subjected to energy dispersive X-ray spectroscopy (EDS). As shown in Fig. 4c, the C content is the major amount (80%) followed by O content (22%), while the Mn content decreased by about 80% of its initial amount via self-assembly with 3DGH, which can be ascribed to the degradation of MnO2 NTs during the hydrothermal process. Fig. 4d
depicts the elemental mapping analysis for 3DGHNTs. Notably, it can be clearly observed that carbon and oxygen have high densities. Meanwhile, the random presence and low density of manganese demonstrated the hypothesis of our study. The surface structure and chemical valence states of 3DGH and 3DGHNTsMn10 were investigated through XPS. As shown in Fig. 5(a), the survey spectrum of 3DGH is mainly composed of C and O. The C1s high-resolution spectrum (Fig. 5b) shows four peaks at the binding energies of 284.7 eV, 285.9 eV, 287.6 eV, and 289.4 eV, which correspond to C¼C/C C, C–O, C C, and O C = O, respectively [57]. As shown in Fig. 5c, the O1s high-resolution spectrum displays three peaks at the binding energies of 531.7 eV, 532.5 eV, and 533.4 eV, which are assigned to the C–O (epoxy and hydroxyl), C¼O (carbonyl), and O = C–O (carboxyl) groups, respectively. The XPS spectrum of 3DGHNTs-Mn10 nanocomposite is shown in Fig. 5a. Clearly, a new peak of the Mn2p spectrum appears at the range of binding energies from 630 eV to 660 eV. The Mn2p spectrum (Fig. 5d) can be split into two peaks related to the Mn 2p3/2 and Mn 2p1/2 spin-orbit peaks of MnO2 at binding energies of 641.8 eV and 653.4 eV, respectively [58]. Additionally, the atomic percentages of elements in our prepared samples are presented in Table S1. We observed that a small amount of Mn of 0.67 atom% has been detected, which is consistent with the EDS results. Nitrogen adsorption/desorption isotherms and Barrett–Joyner– Halenda (BJH) pore size distribution analysis were recorded in order to determine the specific surface area, porosity, and pore size distribution of the as-prepared samples, as shown in Figures (6 and S2). It can clearly be seen that all of the isotherm curves show a wide relative pressure range (0.4–1.0), which reflects a typical IUPAC type-IV characteristic for the mesoporous structure of synthesized 3D graphene hydrogel composites. The pore size distributions curve of 3DGH in Fig. 6c shows the existence of mesopores at 8 nm and 14 nm and a low amount at 19 nm, as well as a complete absence of micropores. However, there is a wide range of pore size distribution from supermicropores (1–2 nm) to mesopores (2–50 nm) in 3DGHNTs-Mn10 (Fig. 6d), which can be attributed to the effects of MnO2 NTs as a sacrificial template in the generation of a mesopores structure with a wide range.
Fig. 3. (a) FE-SEM image of the prepared MnO2 nanotubes. (b, c, and d) Higher and lower magnification FE-SEM images of 3DGHNTs-Mn10 nanocomposite.
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Fig. 4. (a, b) TEM image of the as-prepared 3DGHNTs-Mn10 nanocomposite. (c, d) TEM EDX analysis and TEM mapping of the 3DGHNTs-Mn10 for the following elements: C, O, and Mn.
Fig. 5. (a) XPS survey spectra of 3DGH and 3DGHNTs-Mn10; (b–d) High-resolution XPS spectra of C1s, O1s and Mn2p of 3DGHNTs-Mn10.
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Fig. 6. (a, b) Nitrogen adsorption and desorption isothermals at 77 K as well as (c, d) pore size distributions of 3DGH and 3DGHNTs-Mn10.
As compared to 3DGH and other composites, the 3DGHNTs-Mn10 composite exhibits the highest BET specific surface area (95.89 m2/ g) as well as the largest total pore volume (0.54 cm3/g), as presented in Table S2. Therefore, the large specific surface area and high total pore volume can enhance the electron transfer and provide additional electroactive sites, which could increase the reduction of H2O2. Electrochemical properties of the 3DGHNTs-Mn modified electrodes Electrochemical impedance spectroscopy The electronic transfer properties of four 3DGHNTs-Mn nanocomposites were evaluated in aqueous solution consisting of 5 mM [Fe(CN)6]3/4 and 0.1 M KCl through electrical impedance spectroscopy (EIS) analysis with a frequency range from 1 Hz to 1 MHz at an amplitude of 5 mV. The Nyquist plot of impedance spectra has a semicircle arc section at higher frequencies and a straight-line section at low frequencies. Additionally, the semicircle arc represents the electron transfer limited process, which determines the electron transfer resistance (Rct) based on the diameter of the semicircle section [59]. With a decrease in the diameter of the semicircle, the charge transfer resistance will be decreased accordingly, thus facilitating electron transfer on the electrode. As shown in Fig. 7, the values of charge transfer resistance (Rct) for 3DGHNTs-Mn5, 3DGHNTs-Mn10, 3DGHNTsMn20, and 3DGHNTs-Mn30 were 145 V, 105 V, 120 V, and 185 V, respectively. Therefore, the semicircle diameter in the highfrequency region associated with the 3DGHNTs-Mn10 is lower than that of other nanocomposites electrodes, demonstrating the higher electrochemical reaction rate of 3DGHNTs-Mn10 nanocomposite, which further facilitates the electron transfer and
Fig. 7. Nyquist plots of four modified electrodes in 5.0 mM K3Fe[CN]6 containing 0.1 M KCl from 0.1 Hz to 100 kHz at an AC amplitude of 5 mV.
hence increases the responding signals of H2O2 [60]. Furthermore, the 3DGHNTs-Mn10 electrode shows the highest slope of the straight line at low frequency, indicating the ideal capacitive characteristics of the new composite electrode. Electrocatalytic properties toward H2O2 reduction The electrochemical performance of the different modified electrodes was measured in 0.1 M PBS solution (PH = 7.4) via CV at a
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Fig. 8. CVs response curves of studied electrodes in the (a) absence and (b) presence of 4 mM H2O2 in 0.1 M PBS (pH-7.4) at 50 mV/s.
scan rate of 50 mV/s in both the absence and presence of 4 mM H2O2. As shown in Fig. 8a, no obvious responses were observed in the absence of H2O2 for any of the electrodes. By contrast, in the presence of 4 mM H2O2 in PBS solution, as shown in Fig. 8b, no reduction peak is observed on bare/GCE and MnO2 NTs/GCE electrodes while a weak reduction peak is present for 3DGH/GCE.
However, a remarkable reduction peak at about 0.35 V is observed for 3DGHNTs-Mn different composite electrodes. Among the four 3DGHNTs-Mn composites, the 3DGHNTs-Mn10 show the highest reduction peak, which means that the 3DGHNTs-Mn10 composite had substantially better electrocatalytic activity toward H2O2 than the other composites. This was attributed to the
Fig. 9. (a) CV curves of the 3DGHNTs-Mn10/GCE electrode at different scan rates in N2 saturated 0.1 M PBS solution containing 4 mM H2O2. (b) Plot of reduction peaks current vs. square root of scan rates. (c) CV curves of the 3DGHNTs-Mn10/GCE electrode in 0.1 M PBS with different concentrations of H2O2 at a scan rate of 50 mV s1. (d) Plot of reduction peaks current vs. concentrations of H2O2.
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optimum 10% of MnO2 NTs preventing the restacking of graphene hydrogel, which produces the highest surface area (95.89 m2/g) and thus enhances the conductivity between the modified electrode and the H2O2 molecules. In addition, the reduction current peak decrease with the increase of MnO2 NTs above 10% due to the poor conductivity of MnO2, which suppresses electron transfer ability, as shown in the EIS measurement results (Fig.7). In order to confirm the electrochemical behavior of 3DGHNTs-Mn10 towards H2O2 reduction, different scan rates ranging from 20 to 120 mV/s were further applied in PBS solution containing 4 mM H2O2. As shown in Fig. 9a, the reduction current peak clearly increases with the increase in scan rate value, accompanied with the negative shift in peak potential. Moreover, Fig. 9c shows the relationship between the current reduction peak of 3DGHNTsMn10 at different concentrations of H2O2 ranging from 1 to 4 mM. It can be seen that the reduction current peak appeared at 1 mM of H2O2 and that this peak increases with the increase of H2O2 concentration. In addition, good linearity was observed between the peak of current reduction and concentration of H2O2 (Fig. 9d). These superior analytical behaviors of the 3DGHNTs-Mn10 can be attributed to the effects of MnO2 NTs as a sacrificial template in the generation of outstanding pore size distribution ranges from supermicropores (1–2 nm) to a wide range of mesopores (2–50 nm), leading to effective electron/ion transport channels and excellent H2O2 sensing.
Amperometric H2O2 detection on the 3DGHNTs-Mn10 Further, in order to confirm the notable CV results of 3DGHNTsMn10 as an electrode material in the detection of H2O2, chronoamperometric measurements were observed at an applied potential of 0.35 V with successive increments of H2O2 at different concentrations into continuously stirred N2 saturated 0.1 M PBS solution (Fig.10a). the results showed that the current response of the developed sensor increased rapidly and reached 95% of the steady state current before 5 s, indicating a fast electrocatalytic response on as-synthesized electrode surface. Fig.10b. shows a calibration curve for the H2O2 sensor which was created by adding a successive certain concentration of H2O2 solution while observing the current density of the developed sensor. It can clearly be seen that the response current of H2O2 increased linearly with increasing H2O2 concentration with a wide linear range (25 mM–22.57 mM) at a correlation coefficient of 0.9996. Further, based on the slope of the linear equation, Ipa ðmAÞ ¼ 3:24615:429c ðmMÞ, the high sensitivity of the proposed sensor was calculated as 220.4 mA mM1 cm2 with the lower detection limit of 4 mM (signal-to-noise ratio of 3). A comparison of our result with the previously reported nonenzymatic H2O2 sensors is shown in Table 1. The as-prepared electrode has improved electrocatalytic active behavior with high sensitivity, a wide linear range, and a low detection limit towards H2O2 detection. Therefore, these results demonstrate that the
Fig. 10. (a) Amperometric response of 3DGHNTs-Mn10/GCE on successive additions of H2O2 into 0.1 M PBS at 0.35 V. Inset shows a blown-up image of the low-concentration region. (b) Calibration curve between H2O2 concentration and amperometric response. (c) Interference test of the 3DGHNTs-Mn10/GCE electrode in 0.1 M PBS at 0.35 V with 0.5 mM H2O2 and other interferents including 1 mM of NaCl, AA, UA, DA, Urea, KCl, Citric acid, and Glucose. (d) Stability test of the 3DGHNTs-Mn10/GCE for 21 days. Inset shows the reproducibility study for five electrodes of 3DGHNTs-Mn10/GCE.
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Table 1 Comparison of the as-developed our sensor with the other reported electrochemical H2O2 sensors. Electrode
Linear range (mM)
Sensitivity (mA mM1 cm2)
Detection limit (mM)
Ref.
GO/MnO2 Fe3O4/RGO PDDA/graphene O-MoS2/graphene Graphene/CNTs Ag–MnO2–MWCNTs RGO/tyrosine Cu-MOF/graphene Ag–HNTs–MnO2 MnO2-Ag@C 3DGHNTs-Mn10
5–600 0.5–3000 0.5–500 250–16000 20–2100 5–10400 100–2100 10–11,180 2–4710 0.5–5700 25–22570
38.2 22.27 140.8 269.7 32.91 82.5 69.07 57.73 11.9 127.2 220.41
0.8 0.18 0.1 0.12 9.4 1.7 80 2 0.7 0.17 4
[61] [62] [63] [60] [17] [64] [65] [66] [67] [68] This work
3DGHNTs-Mn10 modified electrode has significant electrocatalytic activity towards H2O2. Selectivity, reproducibility, stability and real sample analysis The selectivity test has become predominant in assessing the performance of electrochemical biosensors. The effects of different electroactive agents were observed using an amperometric technique in 0.1 M PBS (pH 7.4) solution at an applied potential of 0.35 V (vs. Ag/AgCl) with the initial addition of 0.5 mM H2O2 and then the subsequent addition of 1 mM of each electroactive agent, i.e., NaCl, AA, UA DA, Urea, KCl, Citric Acid, and glucose, as shown in Fig.10c. An obvious rapid current change has been observed with the addition of 0.5 mM H2O2 for our proposed electrode. Compared with other interfering species, the current response did not show any change in the presence of other species on 3DGHNTs-Mn10 electrode, suggesting a significant contribution of the developed electrode in anti-interference efficiency and selectivity towards the detection of H2O2. In order to further prove the excellent electrochemical behavior of the as-synthesized electrode, five modified glassy carbon electrodes were prepared separately to examine the reproducibility of the new composite material by measuring the response of current in the same PBS solution containing 2 mM H2O2. The results showed that the reduction peak currents of 2 mM H2O2 were almost equal values, as shown in the inset of Fig. 10d, with a relative standard deviation (RSD) of 1.47%. In addition, the longterm stability of 3DGHNTs-Mn10 nanocomposite electrode was also investigated by monitoring the reduction current peak every two days for 3 weeks, as shown in Fig.10d. The developed biosensor exhibits excellent stability, with 96.2% of its initial reduction peak value being maintained after three weeks. The high reproducibility and long-term stability indicated that the proposed electrode is a suitable applicable biosensor for the detection of H2O2 with a wide linear range and a high sensitivity. According to the impressive results discussed above, the new developed H2O2 electrochemical biosensor relying on a 3DGHNTs-Mn10 nanocomposite electrode was used to detect the concentration of H2O2 in a yogurt real sample. The sample of yogurt was diluted in 0.1 M PBS (pH 7.4) solution. Next, the standard addition method was used to explore the recovery and the accuracy of the as-prepared electrode for sensing H2O2. As shown in Table 2, the measurement of H2O2 recovery percentage ranged from 97.6% to 101.3% with a relative
Table 2 Detection of H2O2 in yogurt samples using 3DGHNTs/Mn10/GCE. Sample
Added (mM)
Found (mM)
Recovery (%)
RSD (%) n = 3
1 2 3 4
20 35 50 80
19.68 34.16 50.65 78.32
98.4 97.6 101.3 97.9
1.04 0.86 0.91 1.12
standard deviation (RSD) under 1.12%, indicating the excellent operational properties of the proposed sensor for the real-time detection of H2O2 in daily life products. Conclusion In this study, we used a facile and cost effectiveness method to produce a novel morphology of three-dimensional graphene hydrogel nanotubes (3DGHNTs) for use as a biosensor electrode for the detection of H2O2. Various physicochemical studies including XRD, Raman, FE-SEM, HR-TEM, XPS, and BET were adopted in order to characterize the structural and morphological properties. Cyclic voltammetry and amperometric techniques were used to investigate the electrochemical properties of the 3DGHNTs nanocomposites for detecting H2O2 in phosphate buffer solution. The optimized nanocomposite showed excellent electrocatalytic activity towards H2O2 reduction. This is attributed to its highly specific surface area and excellent mesoporous and nonporous structure which lead to highly improved electron transport and ion diffusion in the electrolyte. Remarkably, the results presented here reveal that the optimized 3DGHNTs-Mn10 can detect H2O2 in a wide linear range (25 mM–22.57 mM) with high sensitivity (220.4 mA mM1 cm2) and a low detection limit (4 mM). Considering the good selectivity, reproducibility, and longterm stability of the proposed electrode, it can be further explored in real samples of our daily life. Acknowledgements The research reported in this work has been supported by grant from the Basic Science Research Program through National Research Foundation of Korea (NRF) by Ministry of Education, Science and Technology (Project no. 2018R1D1A1B07044717) and ministry of SEMs and startups (Project no. C0504566). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jiec.2019.06.045. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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