Hydroquinone (1,4-dihydroxybenzene, HQ) and catechol (1,2-dihydroxybenzene, CT) are two isomers of dihydroxybenzene and often coexist in environmental samples as pollutants with high toxicity [1]. HQ and CT are harmful to environment including people, animals, and plants even at very low concentrations [2]. The acceptable emission of phenolic compounds according to the national standard of China (GB 8978-1996) is 0.5 mg/L (for dihydroxybenzene, 0.00454 M) [3]. Therefore, it is necessary to develop a simultaneous, simple, and rapid analytical method for the determination of dihydroxybenzene isomers. So far, numerous methods have been established for their determination, such as liquid chromatography [4], synchronous fluorescence [5], chemiluminescence [6], spectrophotometry [7], pH-based flow injection analysis [8], and electrochemical methods [9], [10] and [11]. Among them, electrochemical methods have attracted increasing attentions owing to the advantages of low cost, fast response, excellent selectivity, and high sensitivity. However, a number of challenges for the simultaneous determination of HQ and CT isomers by electrochemical methods still exist [12]. A significant obstacle for the conventional solid electrodes is that the voltammetric peaks corresponding to the oxidation/reduction of the dihydroxybenzene isomers are largely overlapped in many cases. Second, the competitive adsorption of phenolic isomers on the electrode surface in the mixtures makes the relationship between the voltammetric response and isomers concentration nonlinear. In order to overcome these problems, many chemically modified electrodes have been prepared for simultaneous determination of HQ and CT. For instance, Saleh Ahammad et al. reported that poly(thionine)-modified glassy carbon electrode (GCE) can separate the oxidation peaks of HQ and CT; however, the detection limits of dihydroxybenzene isomers were very low [13]. Canevari et al. developed a sensitive technique for the simultaneous detection of HQ and CT in the presence of resorcinol using a SiO2/C electrode spin-coated with a thin film of Nb2O5[14]. Qi et al. used multiwalled carbon nanotubes (MWCNTs)-modified GCE for the simultaneous determination of hydroquinone and catechol with good electrochemical performances [15].
MWCNTs have been widely used in electrochemistry due to their unique one-dimensional (1D) structural, electronic, and physical properties. In the field of chemically modified electrode, one of the most important characteristics of MWCNTs is their reported ability to promote electron-transfer process [16]. Graphene (GR) has attracted tremendous attention because of its unique nanostructure and extraordinary electrocatalytic properties such as high surface area, excellent conductivity, and high mechanical strength associated with its two-dimensional (2D) structure [17]. Based on its properties, GR is considered as an ideal electrode material for electrochemical and biosensing. GR electrodes have been successfully applied as biosensors for sensing glucose [18], NADH [19], hydrogen peroxide [20], etc. However, the excellent properties of GR emerge only in a planar direction. Recently, Yang et al. reported a new method to reduce the stacking of GR by introducing 1D carbon nanotubes to form a three-dimensional (3D) nanohybrid [21]. The properties of MWCNTs emerge in the axial direction while providing current density, high specific surface area, and thermal conductivity. Thus, a GR-MWCNTs hybrid that combines the unique properties of the two carbon allotropes in all directions and provides a high surface area per unit volume for increased catalyst loading could be an ideal electrode material [22]. Therefore, the long and tortuous MWCNTs bridged adjacent to GR efficiently inhibited their aggregation, thus enhancing the utilization of GR-based composites. Room temperature ionic liquids (RTILs) have attracted a lot of attention because of their unique electrochemical properties such as high ionic conductivity, low vapor pressure, low melting temperature, no requirement for additional supporting electrolytes, and thermal stability [23]. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), a type of RTIL, has attracted widespread attention as a nonaqueous polar, moderately hydrophobic, nonvolatile, chemically and thermally stable ionic liquid [24]. Recently, the RTIL-GR-modified electrodes have been reported for detecting many targets [25], [26] and [27]. For instance, Liu et al. developed a GR/BMIMPF6 as the nanocomposite modified electrode for the simultaneous determination of HQ and CT, which exhibited a wide linear range and low detection limit [28].
In this study, a 3D carbon electrode was fabricated by combining 1D MWCNTs and 2D GRs that further advance the utilization of GR-based composites, BMIMPF6 ionic liquid may improve the dispersibility and stability of the 3D carbon electrode. This GR/MWCNTs/BMIMPF6 nanocomposite-modified electrode was formed and its application potential was evaluated for the simultaneous determination of HQ and CT. The separation of anodic peak potentials for HQ and CT was 122 mV, which makes it easier for the simultaneous determination of HQ and CT. The detection limits (S/N = 3) for HQ and CT for the GR/MWCNTs/BMIMPF6/GCE were 0.1 μM and 0.06 μM, respectively. Moreover, the proposed sensor was applied for the simultaneous determination of HQ and CT in water samples with high selectivity