Original Research PaperA novel preparation of water-dispersed graphene and their application to electrochemical detection of dopamine
Graphical abstract
Introduction
Since its introduction in 2004, graphene has become a hot topic in business and academia due to its excellent properties as well as to its potential applications in numerous fields [1]. At present, the methods for preparing graphene are divided into two categories: 1) bottom-up, 2) top-down. The bottom-up methods include chemical vapor deposition [2] and epitaxial growth [3], which have rigorous requirements for operating conditions. The top-down methods include mechanical exfoliation [4], reduction of graphene oxide [5], [6], liquid-shear-exfoliation [7], [8], and intercalation and expansion of graphite [9]. In all these ways, the reduction of graphene oxide and liquid-shear-exfoliation are two the core routes for large-scale preparation of graphene. The chemical and thermal reduction of graphene oxide are cheap. However, the reduced graphene oxide still has many defects due to partial recovery of sp3 structure in reduction process, which hinders application of graphene in electronic field. In addition, liquid-shear-exfoliation in solvents can obtain high-quality graphene on a large scale. However, this method still exists some limitations in preparing graphene with less layers and small size.
Electrochemical exfoliation is facile, low cost, environmentally friendly, and controllable [10], [11], [12], [13], [14]. Electrochemical exfoliation can be divided into anodic and cathodic exfoliation. Anodic exfoliation is most common because it can easily produce large amount of thin graphene in a short time. The electrolytes of anodic exfoliation include ionic liquids [15] and aqueous solution [12], [16]. The former usually results in a low yield of graphene with a small lateral size. At the same time, graphene is often functionalized with the ionic liquids, which disrupts the electronic properties of graphene [17]. The latter can produce large amount of graphene with a large lateral size. However, graphene prepared by this method contains a lot of oxygen-containing functional groups due to excessive oxidation of graphite [12], [14], [18]. In order to further improve the carbon–oxygen ratio of graphene, melamine [19] or antioxidants [20] have been introduced to prevent oxidation of graphene during anodic exfoliation. However, such approach has two disadvantages: the use of expensive additives and difficulty in removing adsorbed additives from the graphene. Moreover, the restacking of graphene in water is another fatal drawback, which can cause a severe decrease of the electrochemically accessible surface area. So the good dispersion and the low oxidation degree of graphene prepared by anodic exfoliation still have some challenges.
DA is a kind of brain secretion, which is the most abundant catecholamine neurotransmitter in the brain. It can regulate many physiological functions of the central nervous system [21], [22]. Changes in the concentration of DA lead to neurological diseases [23], [24], [25], [26], [27], [28]. Thus, sensitive and accurate method for detection of DA is highly essential. Due to simple operation, fast response, low-cost, high sensitivity and excellent selectivity, electrochemical method can be effectively used for detection of DA [29]. However, green, economic, highly sensitive and selective sensing materials still need to be developed.
In this work, alternating voltage was employed to exfoliate high purity graphite plates in inorganic sodium sulfate aqueous solution. Note that the positive voltage activates the exfoliation and oxidizes the graphene sheets. When the voltage is switched to negative voltage, the produced functional groups are reduced. The use of two graphite plates as electrodes contributes to the improvement of exfoliation rate, and the use of alternating voltage reduces the oxidation of graphite. In the preparation process, a large number of GQDs were found among graphene sheets, and their sizes were about 1–4 nm. The existence of GQDs helped to alleviate the agglomeration of graphene and facilitated the dispersion of graphene in water. In addition, the potential value of as-prepared materials in detection of DA was also explored. The experimental results showed that the sensor based on the graphene-GQDs composites possessed a good sensitivity and selectivity for electrochemical detection of DA.
Section snippets
Materials
High purity graphite plates (99.99%, 100 × 50 × 2 mm) were bought from Beijing Jinglong Te Carbon Graphite Plant. Sodium sulfate (Na2SO4), N-Methyl pyrrolidone (NMP), ethanol (99.8%), isopropylalcohol (IPA), sodium dihydrogen phosphate (NaH2PO4·2H2O) and Disodium hydrogenorthophosphate (Na2HPO4·12H2O) were purchased from Beijing Chemical Plant. Dopamine hydrochloride (98%), uric acid (UA, 99%), and ascorbic acid (AA, 99%) were from Shanghai Macklin Biochemical Co., Ltd.
Preparation of graphene using alternating voltage
Graphite plates were
Results and discussion
In this paper, the electrochemical exfoliation of graphite plate was performed in 1 M Na2SO4 solution using alternating voltage (generated by a homemade setup). The effects of different voltages, alternating frequency and the distance between electrodes on the products were studied.
Conclusion
In this paper, a novel method of alternating electrochemistry combining ultrasonic treatment was used to exfoliate graphite. The results showed that the quality of the product from alternating voltage is better than that of direct voltage. The obtained product is the composite of graphene and GQDs, which can be dispersed in water stably. The existence of GQDs can hinder the agglomeration of graphene and facilitate the dispersion of graphene in water. Graphene-GQDs were used as an
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 21676023).
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