Facile synthesis of nitrogen-doped graphene

Solid State Sciences 86 (2018) 6–11

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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie

Facile synthesis of nitrogen-doped graphene aerogels for electrochemical detection of dopamine

T

Shun Aia, Yuxin Chena, Yulan Liua, Qiao Zhanga, Lijun Xionga, Huabo Huanga, Liang Lia,∗, Xianghua Yua, Lai Weib a

Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, PR China b School of Physical Science and Technology, Yili Normal University, Yining 835000, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Nitrogen-doped graphene Aerogel Detection Dopamine

Nitrogen-doped graphene aerogels with three-dimensional network structures are fabricated using hydrothermal method which includes the reduction of graphene oxide by organic amine and self-assembly of reduced graphene oxide. The effect of amine-containing compounds including aniline, 2-aminoethanol, ethylenediamine, melamine and chitosan on the assembly of nitrogen-doped graphene aerogel is investigated. The microstructure and chemical composition of nitrogen-doped graphene aerogels are characterized. The results reveal that nitrogendoped graphene aerogel prepared using aniline as nitrogen source possesses a large specific surface area, high nitrogen content, good mechanical strength and excellent electrical conductivity. Based on these features, the asprepared nitrogen-doped graphene aerogel shows high performance in electrochemical detection of dopamine in the presence of uric acid and ascorbic acid. Given the facile and scalable processability of aerogels, the proposed nitrogen-doped graphene aerogels are expected to have potential applications in sensors and other related devices.

1. Introduction Aerogels are three-dimensional solid materials with porous structures and usually obtained from processing or assembling of nanoscale building blocks, followed by replacing solvents in the wet gels with air [1]. Some interesting features of high porosity, low density, high mechanical strength, high surface area enable them to be used in the fields of construction, catalyst supports, environmental remediation, sensing, and electronics [2–6]. Polymeric aerogels can be prepared from crosslinked hydrogels. When the starting materials are metal oxides or other inorganic materials, sol-gel method is generally adopted to prepare the corresponding aerogels [7–10]. Carbonaceous aerogels with different carbon nanostructures have been synthesized in various methods, such as carbonaceous nanofiber aerogels fabricated by hydrothermal carbonization, carbon nanotube sponges prepared by chemical vapor deposition (CVD), carbonaceous polymeric or biomass aerogels created by the pyrolysis treatment at high temperature [11–16]. Among them, graphene-based aerogels have been widely investigated because they not only possess large specific surface area and pore volume, but also inherit the intrinsic of chemical stability, electrical and thermal conductivity from graphene [17–21]. ∗

Recently, chemically heteroatom-doping in graphene can result in the local change of elemental composition of graphene, and optimize the surface structure and electronic properties of graphene [22–24]. Nitrogen is widely used to dope the graphene-based materials. Nitrogen-doped graphene (NG) has attracted more attention owing to its high electrical conductivity and good chemical stability [25,26]. Therefore, it is expected that the introduction of nitrogen element into graphene aerogels can adjust the chemical nature and electrical property to some extent and thus enhance the performance of these aerogel materials. Up to now, NG aerogels have been successfully prepared by the different routes. For example, three-dimensional NG was prepared through CVD by using porous nickel foam as the substrate [27]. However, the sophisticated CVD method involves the expensive facilities and restricts the massive fabrication of NG aerogels in large scale. Zhao et al. prepared the ultralight NG framework by the hydrothermal treatment of graphene oxide (GO) aqueous solution mixed with 5 vol% pyrrole [28]. Xie et al. demonstrated the fabrication of pore-rich NG aerogel using the sonicated GO nanosheets with dopamine as precursors [29]. NG aerogels with good capacitive behavior and high carbon dioxide uptake capacity were also prepared using ammonia as the

Corresponding author. E-mail address: [email protected] (L. Li).

https://doi.org/10.1016/j.solidstatesciences.2018.09.014 Received 27 June 2018; Received in revised form 2 September 2018; Accepted 29 September 2018 Available online 02 October 2018 1293-2558/ © 2018 Elsevier Masson SAS. All rights reserved.


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nitrogen source during the hydrothermal process [30]. Although the pioneer works about the synthesis of NG aerogels are exciting, it is still a research hotspot about the fabrication and application of NG aerogels owing to their unique characteristics. To the best of our knowledge, little work has reported the effects of amines on selfassembly of NG aerogels in the hydrothermal reaction and on their performance of electrochemical detection of dopamine. Dopamine (DA) belongs to the family of neurotransmitter of catecholamine and is distributed in the human brain [31]. The level of DA has been associated with human motivation, emotion, and attention [32]. Several methods, such as chemiluminescence, capillary electrophoresis, fluorescence, and ion chromatography, have been developed for detection of DA [33–36]. However, most of these methods have drawbacks, such as high cost, time consumption, or complicated equipment. The electrochemical method is an ideal alternative owing to the rapid, simple, and sensitive determination of analytes. In this work, we present the preparation of NG aerogels by using different amines as the nitrogen source, reducing agent and cross-linker. Aniline is beneficial for the fabrication of NG aerogels with higher nitrogen content, larger specific surface area, higher electrical conductivity, and better electrocatalytic detection of dopamine.

Fig. 1. Photographs of (a) NG-1, (b) NG-2, (c) NG-3, (d) NG-4 and (e) NG-5.

lamp.

2. Experimental

3. Results and discussion

2.1. Materials

It has been reported that the hydrothermal treatment can produce graphene sheets through the reduction of GO and promote self-assembly of graphene sheets via π-π stacking interactions to form a graphene hydrogel even without the addition of reducing agents [38]. The presence of reducing agents can greatly accelerate this process. In this study, five different reagents containing amine groups were added into GO solution through the hydrothermal treatment at 120 °C for 4 h, followed by annealing at 800 °C. It was interesting to note that these asprepared samples exhibit totally different macroscopic appearance. When aniline, ethylenediamine or 2-aminoethanol was chosen, the corresponding NG aerogel existed as a bulk aerogel with a stable shape, as shown in Fig. 1a-c. On the contrary, the hydrothermal reaction only yielded black and flocculated precipitate and just powders of NG were obtained in the case of melamine and chitosan (Fig. 1d and e). In a typical preparation process, the black hydrogel precursors of NG-1, NG2 and NG-3 with cylinder shape were formed after the hydrothermal treatment at 120 °C for 4 h (Fig. S1). The as-prepared hydrogels contained lots of water. The NG aerogels were prepared through freezedrying and pyrolysizing the hydrogel precursors. The accompanied volume shrinkage was likely ascribed to the overlapping of graphene sheets with the self-aggregation during the drying process. It was further worthy of noticing the difference among NG-1, NG-2 and NG-3. In the case of aniline, the obtained NG-1 maintained a three-dimensional cylindrical morphology with more regular integrated whole after freeze-drying and pyrolysis. Moreover, NG-1 aerogel could support 50 g counterpoise without obvious deformation of its size and shape, which was about 658 times its own weight (Fig. S2). However, the other aerogels could not bear such weight. It indicated that aniline was beneficial for the fabrication of NG aerogel with three-dimensional monolithic architecture and high mechanical strength, suggesting that aniline not only served as the nitrogen source, but also as a strength intensifier perhaps due to the interaction between aniline and graphene during the process of aerogel self-assembly. The structure and morphology of the as-prepared NG aerogels were studied by XRD, SEM, and Raman spectroscopy. As indicated in Fig. 2, GO had a very strong diffraction peak at 10.5° according to the (001) planes. After the hydrothermal and annealing treatment, this peak completely disappeared, while a new broad peak appeared at about 26° in the XRD patterns of NG-1, NG-2 and NG-3, corresponding to the reduction of GO. Compared with GO, the reduced interlayer spacing in NG aerogels indicated that most oxygen-containing functional groups in GO were removed and the structure of graphene were mostly restored

GO was prepared based on the modified Hummers method [37]. Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. 2.2. Fabrication of NG aerogels In a typical procedure for the fabrication of NG aerogels, 100 mg of aniline was added to 30 mL of GO solution (4.0 mg mL−1) and the mixture was subjected to sonication for 1 h. The mixture was then transferred into Teflon-lined autoclaves at 120 °C for 4 h to form a nitrogen-containing hydrogel. After naturally cooled down to room temperature, the black monolith was taken out, dialyzed by ultrapure water and freeze-dried. Then, NG aerogel was obtained after heating at 800 °C for 1 h under Ar atmosphere. It was designated as NG-1. For comparison, ethylenediamine, 2-aminoethanol, melamine or chitosan was added into GO solution and the synthesized samples were designated as NG-2, NG-3, NG-4 or NG-5, respectively. 2.3. Characterization X-ray diffraction (XRD) patterns were recorded by X-ray powder diffraction instrument (XRD, Shimadzu) with monochromatic Cu Kα radiation. The morphologies of the aerogels were observed by a JSM5510LV scanning electron microscope (SEM). Raman spectra were measured on Thermo Scientific Raman Microscope. The specific surface area was analyzed using Brunauere-Emmette-Teller (BET, Micromeritics ASAP 2010 M, Micromeritics Inc.). Conductivities of the samples were measured by a standard four-probe method. X-ray photoelectron spectroscopy (XPS) was carried out on an AXIS Ultra spectrometer. The water contact angles were analyzed using DSA10 contact angle goniometer (Krüss, Germany). Differential pulse voltammetry (DPV) were preformed in a conventional three-electrode system on CHI660D electrochemical workstation (Shanghai CH Instrument Company, China). An Ag/AgCl electrode and a platinum foil were used as the reference electrode and the counter electrode, respectively. A glassy carbon electrode was polished with 0.3 and 0.05 μm Al2O3 powder and rinsed with distilled water and anhydrous ethanol and dried by high purity N2. 5.0 mg of the sample was dispersed in 5.0 mL of DMF by sonication. Then, 10 μL of the suspension were casted onto the surface of glassy carbon electrode and left to dry under a near-infrared 7


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Fig. 2. XRD patterns of GO, NG-1, NG-2 and NG-3.

Fig. 4. Raman spectra of GO, NG-1, NG-2 and NG-3.

after the hydrothermal and annealing process. The SEM images of the NG aerogels (Fig. 3), especially NG-1 and NG-2, exhibited the hierarchical porous network, which was composed of randomly oriented graphene sheets with pore size from sub-micrometer to several micrometers. It was similar to those of graphene aerogels [39]. The highly porous architectures were well preserved after such annealing treatment at 800 °C, which could greatly prevent the aggregation of graphene nanosheets caused by conjugated interaction and van der Waals force and should be beneficial for the electrolyte to rapidly transport in the electrochemical process. Raman spectroscopy is an important technology for the characterization of graphene. Raman spectra of GO, NG-1, NG-2 and NG-3 were shown in Fig. 4. It was clearly observed that there were two prominent peaks for all of the samples. The G peak at 1600 cm−1 was attributed to the bond stretching of sp2 carbon atoms in the hexagonal carbon framework. The D peak at 1348 cm−1 originated from the breathing mode of sp3-hybridized carbon atoms or the defects on graphene basal plane. The intensity ratio of D peak to G peak (ID/IG) was generally used to evaluate the defects density in the carbon samples. The increased defects due to the decomposition of surface functionalities could be confirmed by the increase of ID/IG ratio in NG-1 (1.18) compared with that of GO (0.92). The type and content of nitrogen species of NG-1, NG-2 and NG-3 were further characterized by XPS. As shown in Fig. 5, nitrogen atoms were doped in graphene basal plane in three configurations. They could be deconvolved into pyridinic N at binding energy of 398.7 eV, pyrrolic N at 399.9 eV, and quaternary N at 401.8 eV. Different nitrogen species in molecular skeleton of graphene led to the different chemical or electronic environments of adjacent carbon atoms, which could enhance the electrocatalytic activity of NG aerogels [40]. The nitrogen content of NG-1 was higher than those of NG-2 and NG-3 (Table 1). The NG-1 also exhibited a higher BET specific surface area of 870 m2 g−1, compared with those of NG-2 and NG-3. It was larger than those of most graphene-based porous materials, for example, nitrogen-doped graphene sponge [28] and graphene aerogel [41]. High specific surface area enabled more active nitrogen atoms to be exposed to solutions. To investigate the wettability properties, water contact angle measurements were conducted. As shown in Table 1, the surface wettability was increased with the increase of the nitrogen content and specific surface area in the aerogel because the nitrogen atoms in the graphene skeleton effectively tune local electronic structure and facilitate the access of aqueous solution to the graphene surface [42]. The high nitrogen content, large specific area, and high electrical conductivity indicated that NG-1 should have the best electrocatalytic activity to be confirmed later. Dopamine (DA), as one of the most significant catecholamine

Fig. 3. SEM mages of NG-1, NG-2 and NG-3.

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Fig. 5. N1s XPS spectra of NG-1, NG-2 and NG-3. Table 1 Nitrogen contents, BET specific surface areas, electrical conductivity and water contact angel of NG-1, NG-2 and NG-3. Sample

Nitrogen content (%)

BET specific surface area (m2 g−1)

electrical conductivity (S cm−1)

Contact angel (°)

NG-1 NG-2 NG-3

7.86 5.93 4.27

870 560 410

19.3 15.2 11.6

98 110 116

Fig. 7. (A) DPV plots for different concentrations of DA with the coexistence of 0.80 mmol L−1 AA and 0.10 mmol L−1 UA for NG-1 electrode. (B) Plots of peak current against the concentrations of DA for NG-1electrode.

have close oxidation potentials with DA, can cause an overlapping voltammetric response and lead to poor sensitivity and selectivity in its detection [32]. Differential pulse voltammetry (DPV) is a commonly used technique for the improvement of specificity and sensitivity in electrochemical analysis. DPV plots for AA, DA and UA for electrodes of NG-1, NG-2 and NG-3 were shown in Fig. 6. Three distinguishable oxidation peaks, belonging to AA, DA, and UA, respectively, could be observed. It is known that the oxidation peaks of AA, DA, and UA for the pure glassy carbon electrode (GCE) are completely overlapped and cannot totally be distinguished. It indicated that the sensitivity and selectivity of electrochemical sensors have been enhanced with NG aerogels. The oxidation peaks of AA and DA is associated with the oxidation of hydroxyl groups. UA is firstly oxidized to quinonoid, and then changed to a carboxylic acid or a tertiary alcohol by the reaction with water [43]. Compared with AA and UA, there was higher π electron density in DA because of the difference in their chemical structures (Fig. S3). So, the larger peak current intensity of DA than that of AA or UA could be attributed to stronger π-π interaction between the phenyl ring of DA and the graphene basal plane of NG aerogel, which was beneficial for DA to absorb on the NG surfaces and induce the enhanced voltammetric response. On the other hand, AA and UA possessed

Fig. 6. DPV plots of the (a) NG-1, (b) NG-2 and (c) NG-3 electrodes in 0.10 mol L−1 PBS (pH 7.0) with 1.0 mmol L−1 AA, 0.15 mmol L−1 DA and 0.20 mmol L−1 UA.

neurotransmitters, plays a key role in biological organisms. The abnormal level of DA results in neurological diseases, such as Parkinsonism and schizophrenia [31]. Therefore, a simple, reliable technique for the detection of DA is greatly required for the clinical diagnosis. Although electrochemical methods have been applied to detect DA, the coexisting ascorbic acid (AA) and uric acid (UA), which 9


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sensor, which might be contributed to its hierarchical structure, nitrogen-doping and other properties.

Table 2 Electrochemical detection of DA at different materials modified electrodes. Electrode

Interferents

Graphene/polypyrrole Graphene/polyaniline Nitrogen-doped graphene aerogel Nitrogen-doped graphene Graphene/chitosan Graphene/chitosan NG-1

Linear range (μM)

Detection limit (μM)

Ref.

AA AA, UA –

0.5–100 1–14 0.5–160

0.1 0.5 0.2

[44] [45] [46]

AA, UA

3–100

0.001

[27]

AA, UA AA, UA AA, UA

1.0–24 5–200 1–250

1 – 0.1

[47] [48] this work

4. Conclusions The NG aerogels have been successfully fabricated by a facile hydrothermal reaction followed by annealing, in which organic amine is adopted as nitrogen source. Although the selected amines can reduce GO, only aniline, 2-aminoethanol or ethylenediamine further facilitates the reduced GO nanosheets to self-assemble into NG aerogels with porous three-dimensional architectures. The morphology and composition of the resulting NG aerogels are characterized by XRD, SEM, Raman, XPS, specific surface area, water contact angle and conductivity measurements, and electrochemical analysis. When aniline is used as the nitrogen source, reducing agent and cross-linker, it enables the fabrication of corresponding aerogel with highest nitrogen content, largest specific surface area, best mechanical strength and electrocatalytic activity. On the basis of these characteristics, this prepared NG aerogel can be used in construction of a sensing platform for the determination of dopamine as healthcare monitoring and biomedical devices.

Table 3 Recovery of DA from the human blood serum sample by NG-1. Sample

Spiked (μM)

Found (μM)

Recovery (%)

RSD (%)

1 2 3 4

20 30 40 50

22.18 31.92 41.08 50.41

110.9 106.4 102.7 100.8

4.12 3.63 2.89 3.25

Acknowledgement weaker adsorbed interactions than DA because of the absence of phenyl group in their molecular structures. Therefore, the sensor exhibited high selectivity to DA molecules in the presence of interferents. It was further worth noting that the peak potential for NG-1 electrode was slightly smaller than that for NG-2 and NG-3 electrodes, while the peak current intensity for NG-1 electrode was obviously larger than that for NG-2 and NG-3 electrodes. It could be ascribed to the higher nitrogen content, the larger specific surface area, the higher electrical conductivity of NG-1 (see Fig. 7). DPV of selective determination of DA in the presence of AA and UA was also performed by using NG-1 electrode. When the concentrations of AA and UA were kept constant, the peak current of DA was almost linearly proportional to its concentration ranging from 1 μm to 200 μm with the correlation coefficient of 0.991. The detection limit of DA, assuming a signal-to-noise ratio of 3, was estimated from the slope value of linear calibration plot. Table 2 showed the electrochemical detection of DA at different graphene-based electrodes. It could be seen that the NG-1 electrode possessed comparable or higher detection performance than the other graphene-based electrodes. This might be due to the perfect three-dimensional structure, large surface area and high conductivity of NG-1, which was beneficial for the enhancement of electroactivity to the detection of DA. The coexistence of AA and UA had no obvious interference for the determination of DA. Moreover, when the three parallel NG-1 electrodes were scanned by continuous DPV, there was no obvious difference in the peak current. The relative standard deviation (RSD) of the current response was evaluated to be 3.02%, indicating the fabrication method was reproducible. The longterm stability of the NG-1 electrode was further investigated for 10 days. When the fabricated electrode was measured, a slight current drop of 1.78% was observed within the first two days, and finally 91.2% of the initial current remained after 10 days. The determination of DA in human serum was carried out to evaluate the application of the sensor in real samples. The serum samples were diluted 1:10 with 0.01 M PBS (pH 7.0) and then were detected. Then, the pre-determined concentrations of DA solution were spiked into the human serum and the current responses for dopamine were measured and the corresponding concentrations were estimated by the calibration graphs. It was measured 3 times. The good recovery from 100.8% to 111.9% with RSD ≤4.12% (Table 3) of the electrode indicates that the prepared sensor can be used as a suitable electrode for the detection of DA in real samples. These results showed the remarkable stability and reproducibility of the three-dimensional NG-1 aerogel-based electrode as DA

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