Hydroquinone (HQ), catechol (CC) and resorcinol (RE) are three dihydroxybenzene isomers, which are widely used in dyes, cosmetics, tanning, pesticides, flavoring agents, medicines, antioxidant and photography chemicals.1-4 Because of their high toxicity and low degradability in the ecological environment, HQ, CC and RE are considered as environmental pollutants by the US Environmental Protection Agency (EPA) and the European Union (EU).5 During the application and manufacturing process of these compounds, some of them are inevitably released into the environment to contaminate rivers and ground waters. Thus, industrial effluents and sanitary wastewater exists a large number of dihydroxybenzene isomers.6-8 Furthermore, due to their similar characteristics and structures, HQ, CC and RE usually coexist and interfere with each other during their identification. Therefore, it is significant to develop a simultaneous, simple, and rapid analytical method to detect dihydroxybenzene isomers. Nowadays, the determination of dihydroxybenzene isomers is commonly performed by fluorescence quenching,9 liquid chromatography,10 gas chromatography/mass spectrometry,11 capillary electrophoresis12 and electrochemical8,13 methods. Among them, the electrochemical methods have attracted considerable attentions due to their advantages such as fast response, cheap instrument, low cost, simple operation, time saving, and free of complicated sample pre-treatments.14 For electrochemical methods, more attentions are focused on the simultaneous determination of CC and HQ2,15-25 and only a few works addressed on the simultaneous determination of HQ, CC and RE.7,26
Compared to the parallel single-analyte methods, it is well-known that simultaneous detection of HQ, CC and RE means using fewer samples, reducing overall cost per assay and improving assay efficiency.27 However, the oxidation potentials of these electroactive species were interfered with other compounds at bare glassy carbon (GC) electrode, which results in poor sensitivity and selectivity.20 The key factor to solve this problem is the choosing the appropriate electrode material which has excellent electrocatalytic activity. Many materials have been used to overcome these problems, such as polymer film,28 carbon materials29,30 and enzyme.31 Among these materials, carbon materials have attracted researchers' attention because of their low cost and good electrocatalytic properties.20,26,29,32
On the other hand, many kinds of nitrogen (N)-doped carbon materials, such as N-doped graphene,33-35 N-doped carbon nanotubes (CNTs),36 and N-doped hollow carbon microsphere,37 were reported and showed high sensitivity in biosensing applications and excellent electrocatalytic activity toward oxygen reduction reaction. Recently, N-doped porous carbon nanopolyhedrons (N-PCNPs) were synthesized in our groups through the direct carbonization of ZIF-8 nanopolyhedrons.38 The N-PCNPs exhibit good characteristics such as uniform morphology, narrow pore-size distribution centered at 3.7 nm, high surface area (2221 m2 g−1) and good analytical performance for the simultaneous detection of dopamine (DA), ascorbic acid (AA) and uric acid (UA). However, the preparation reproducibility of the N-PCNPs modified electrode might suffer from their poor dispersibility in aqueous solution. It is necessary to increase the dispersibility of N-PCNPs in aqueous solution.
On the other hand, it was reported that carbon nanotubes (CNTs) could be used in the simultaneous detection of CC and HQ with good peak separation due to the good electrocatalytic properties towards the oxidation of CC and HQ.26,39,40 However, the electrocatalytic activity of CNTs towards the oxidation of RE was relatively low, which would limit the simultaneous detection of CC, HQ and RE.
In this work, the N-PCNPs-MWCNTs hybrid materials were synthesized and used to simultaneously detect CC, HQ and RE. Due to the intertwist action between N-PCNPs and MWCNTs, the obtained N-PCNPs-MWCNTs hybrid materials exhibited excellent film forming ability compared with the single materials (N-PCNPs or MWCNTs), and the N-PCNPs-MWCNTs hybrid materials modified glassy carbon (GC) electrode showed excellent electrocatalytic activities towards the oxidation of CC, HQ and RE. Furthermore, the NPCNPs-MWCNTs/GC electrode was successfully applied to determine HQ, CC and RE in local river water samples.
Reagents and Materials. CC, HQ, RE, zinc acetate dihydrate and 2-methylimidazole were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. Other chemicals were of analytical grade. Pristine multiwalled CNTs (diameter 20-40 nm) were purchased from Shenzhen Nanotech Port Co. Ltd., China. Further purification was accomplished by sonicating CNTs in a mixture of concentrated sulfuric acid-nitric acid (3:1, v/v) for about 12 h. The treated CNTs were filtered and washed with ultrapure water, and then dried in a vacuum at 60 ℃. All solutions were prepared using ultrapure water having a resistivity ≥ 18 Ω cm (Milli-Q). The experiments were carried out at room temperature (25 ± 2 ℃).
Apparatus and Measurements. A scanning electron microscopy (SEM, Hitachi S-4800, Japan) was used to characterize the morphology of the N-PCNPs-MWCNTs hybrid materials. All electrochemical measurements were performed on a CHI 1440A electrochemical workstation (Chenhua Instrument Company of Shanghai, China) with a conventional three-electrode system with a platinum wire as an auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode, and a modified GC electrode (diameter 3 mm) as the working electrode. All the potentials in this paper were in respect to SCE.
Preparation of N-PCNPs-MWCNTs Hybrid Materials. ZIF-8 nanopolyhedrons (ZIF-8 NPs) were prepared according to the literature.41-43 N-PCNPs were synthesized by carbonization of ZIF-8 NPs according to our recent work and the nitrogen content in the N-PCNPs was about 2.8 wt %.38
To prepare the N-PCNPs-MWCNTs hybrid materials, 2.5 mg N-PCNPs and 2.5 mg MWCNTs were mixed completely in 5 mL ultrapure water by successive ultrasonic for 2 h and stirred continuously for 5 h at room temperature. Finally, the obtained black solution was collected and the N-PCNPs-MWCNTs hybrid materials were obtained.
Preparation of N-PCNPs-MWCNTs Modified GC Electrode. For the preparation of the modified electrode, the GC electrode was carefully polished to a mirror-like surface with 0.3 and 0.05 μm alumina slurries and then washed ultrasonically in ultrapure water and ethanol, and then dried with N2.
The N-PCNPs-MWCNTs/GC electrode were prepared by casting 6 μL N-PCNPs-MWCNTs suspension (0.5 mg mL−1) onto the surface of the GC electrode (the loading mass is 42.3 μg cm−2) and dried at room temperature. As comparison, the MWCNTs modified GC (MWCNTs/GC) and N-PCNPs modified GC electrodes were also prepared according to the same procedure.
For real sample analysis, a local rover water sample was diluted 40 times with 0.1 M PBS (pH = 7.0) without any pretreatment.
Results and Discussion
Characterization of N-PCNPs-MWCNTs Hybrid Materials. The surface morphologies of N-PCNPs and N-PCNPs-MWCNTs were investigated by SEM. As shown in Figure 1(a), the N-PCNPs show the typical rhombic dodecahedron morphology with a remarkably narrow size distribution (about 200 nm), which is in agreement with that reported previously.38 Figure 1(b) shows the morphology of N-PCNPs-MWCNTs hybrid materials. It is noted that the N-PCNPs was intertwined homogeneously by MWCNTs and the 3D network structure will be beneficial to the transport of reactants and products. In addition, MWCNTs can function as electrical bridges that interconnect the separated N-PCNPs, thus generating good electrical networks. Noteworthy is that the formation of the N-PCNPs-MWCNTs hybrid materials would prevent aggregation of N-PCNPs and induce stable N-PCNPs-MWCNTs suspensions in water due to the good dispersibility of the acid-treated MWCNTs, which is also confirmed by the results shown in Figure 2.
Figure 1.SEM images of N-PCNPs (a) and N-PCNPs-MWCNTs (b).
Figure 2.Photographs of N-PCNPs (a, 0.5 mg mL−1) and N-PCNPs-MWCNTs (b, 1 mg mL−1) in water after 2.0 h storage.
Figure 2 shows the photographs of 0.5 mg mL−1 N-PCNPs (a) and 1 mg mL−1 N-PCNPs-MWCNTs (b) in water after 2 h storage. It can be observed that the dispersion ability of the N-PCNPs in aqueous solution is rather poor due to the strong attractive interaction between the hydrophobic sidewalls of the N-PCNPs. However, for the N-PCNPs-MWCNTs, a dark and homogeneous solution was observed and no obvious precipitates appeared. This implies that the N-PCNPs-MWCNTs hybrid materials have a satisfactory dispersibility in water. This should result from the good dispersibility of the acid-treated MWCNTs in aqueous solution and intertwist action between N-PCNPs and MWCNTs. It is important for efficiently inhibiting their aggregation and thus enhancing the utilization of nanohybrids, as well as being beneficial towards improving the preparation reproducibility of the modified electrode.
Electrochemical Properties of the N-PCNPs-MWCNTs Hybrid Materials. Fe(CN)6 3−/4− is always used as an electrochemical probe to evaluate the electrochemical properties of the electrode. Figure 3(A) shows the cyclic voltammograms (CVs) obtained at the N-PCNPs-MWCNTs/GC (a), N-PCNPs/GC (b) and bare GC electrodes (c) in Fe(CN)6 3−/4− (5.0 mM) + KCl (0.1 M) aqueous solution. It can be observed that the difference in potential between the anodic and cathodic peaks (ΔEp) is 88 mV for the N-PCNPs-MWCNTs/GC (curve a, Fig. 3(A)), 94 mV for the N-PCNPs/GC (curve b, Fig. 3(A)) and 112 mV for the bare GC (curve c, Fig. 3(A)) electrodes. As ΔEp is the function of the electron transfer rate, the lower ΔEp, the higher electron transfer rate. Therefore, the electron transfer at the N-PCNPs-MWCNTs/GC electrode is easier than that at the bare GC and NPCNPs/GC electrodes. Moreover, the redox peak currents at the N-PCNPs-MWCNTs/GC electrode are also much larger than that at the other two electrodes. The smaller value of ΔEp and the higher redox peak currents indicate that the NPCNPs- MWCNTs/GC electrode has better electrochemical properties than the bare GC and N-PCNPs/GC electrodes.
Figure 3.CVs (A) and EIS (B) obtained for 5.0 mM Fe(CN)6 3−/4− (1:1) + 0.1 M KCl at N-PCNPs-MWCNTs/GC (a), N-PCNPs/GC (b) and bare GC (c) electrodes. Scan rate for CVs, 100 mV s−1. The electrochemical impedance spectra were recorded within the range of 100 KHz-0.1 Hz at the formal potential of 0.2 V and AC amplitude of 5 mV. Insets: the electrochemical impedance spectrum of the N-PCNPs-MWCNTs/GC electrode (a).
The electron transfer process of Fe(CN)6 3−/4− at different electrodes are also investigated with electrochemical impedance measurements and the results are shown in Figure 3(B). It is noted that the order of the value of charge transfer resistance (Rct) for different electrodes is as follows: bare GC > N-PCNPs/GC > N-PCNPs-MWCNTs/GC. This result is in accordance with that observed in Figure 3(A). All these results indicate that the N-PCNPs-MWCNTs/GC electrode should have better electrocatalytic properties towards the oxidation of HQ, CC and RE than the N-PCNPs/GC and bare GC electrodes due to the good electrochemical properties of N-PCNPs and MWCNTs, and the synergistic effect between N-PCNPs and MWCNTs.
Figure 4.CVs of N-PCNPs-MWCNTs/GC (a), N-PCNPs/GC (b), MWCNTs/GC (c) and bare GC electrodes (d) in 0.1 M PBS (pH 7.0) containing: 0.5 mM HQ + 0.5 mM CC + 0.5 mM RE (curve c). Insets: CVs of the bare GC electrode (d). CV scan rate, 100 mV s−1.
Electrocatalytic Oxidation of HQ, CC and RE. Figure 4 shows the CVs of the mixture of 0.5 mM HQ, 0.5 mM CC and 0.5 mM RE at bare GC, MWCNTs/GC, N-PCNPs/GC and N-PCNPs-MWCNTs/GC electrodes in 0.1 M PBS (pH 7.0). At the bare GC electrode (curve d), two broad oxidation peaks appear at 241 mV and 630 mV, respectively. The peak at 630 mV can be attributed to the oxidation of RE. And the peak at 241 mV can be attributed to the fact that the oxidation peaks of CC and HQ merge into a large peak. This indicates that these peaks cannot be separated at the bare GC electrode. Therefore, HQ, CC and RE can not be simultaneously determined at the bare GC electrode. At the both NPCNPs/GC (curve b) and MWCNTs/GC (curve c) electrodes, the oxidation peaks of HQ, CC and RE are observed, although the oxidation peak of RE is small and not clear. Furthermore, at the N-PCNPs/GC electrode, the oxidation peak current of HQ is higher than that of CC. However, for the MWCNTs/GC electrode, the oxidation peak current of HQ is smaller than that of CC. For the N-PCNPs-MWCNTs/GC electrode, three oxidations peaks of HQ, CC and RE can be observed obviously at 101, 212 and 520 mV, respectively. It is noted that the oxidation peak of RE at the N-PCNPs-MWCNTs/GC electrode is much higher than that at both N-PCNPs/GC and MWCNTs/GC electrodes, due to the synergistic effect between the N-PCNPs and MWCNTs. Furthermore, the peak potential separation for HQ-CC is 111 mV, which is superior to that at the MWCNTs/GC electrode (78 mV), NPCNPs/GC electrode (74 mV) and comparable to other electrodes (100 mV,3,7 109 mV44). Additionally, the peak separation for CC-RE at the N-PCNPs-MWCNTs/GC electrode (308 mV) is comparable to that at the MWCNTs/GC (319 mV). Furthermore, the oxidation peak current of HQ (CC or RE) at the N-PCNPs-MWCNTs/GC electrode is 92 μA (86 μA or 41 μA) and 4.8 times (3.3 times or 3.1 times), 3.2 times (3.7 times or 2.9 times) and 15.3 times (12.3 times or 7.6 times) higher than that at the MWCNTs/GC (19 μA (26 μA or 13 μA)), N-PCNPs/GC (28 μA (23 μA or 14 μA)) and bare GC (6 μA (7 μA or 6 μA)) electrodes, respectively. These results demonstrate that the NPCNPs-MWCNTs hybrid materials have excellent electrocatalytic properties towards the oxidation of HQ, CC and RE. The reasons may be as follows: (1) the nitrogen atoms in N-PCNPs-MWCNTs may interact with the target molecules via hydrogen bonds, which can activate the hydroxyl and accelerate the charge transfer kinetics of the target molecules at N-PCNPs-MWCNTs;45 (2) N-PCNPs-MWCNTs have high surface area and narrow pore-size distribution centered at 3.7 nm originated from N-PCNPs, which are suitable for quick mass transfer of target molecules in aqueous electrolyte.46 In other words, the excellent electrocatalytic activities of NPCNPs-MWCNTs may be due to their mesoporous microstructure, the high surface area and properties originating from the nitrogen-doping effect. All these results indicate that simultaneous determination of HQ, CC and RE is feasible at the N-PCNPs-MWCNTs/GC electrode.
Optimization of the Determination Conditions. In most cases, the solution pH is an important influence factor to the electrochemical detection. The effect of solution pH value on the oxidation peak current of HQ (CC or RE) at the N-PCNPs-MWCNTs/GC electrode in 0.1 M PBS solutions was investigated. As shown in Figure 5(a), for all HQ, CC and RE, the related oxidation peak currents increase from pH 5.5 to 7.0 and then decrease from 7.0 to 9.0. The maximum currents are observed at pH 7.0. Therefore, pH 7.0 is selected as the optimum pH value in the simultaneous electrochemical determination of HQ, CC and RE.
Figure 5.Effects of the pH value (a) and the amount of N-PCNPs-MWCNTs (b) on the oxidation peak currents of 0.5 mM HQ (0.5 mM CC or 0.5 mM RE) at the N-PCNPs-MWCNTs/GC electrode. Scan rate, 100 mV s−1.
On the other hand, the effect of the amount of N-PCNPs-MWCNTs loaded on the GC electrode on the oxidation peak current of HQ (CC or RE) was also investigated and the corresponding results are shown in Figure 5(b). For all HQ, CC and RE, the oxidation peak currents increase gradually with the increase of the amount of N-PCNPs-MWCNTs and the maximum values are observed at 42.3 μg cm−2. When the amount of N-PCNPs-MWCNTs loaded on the GC electrode is more than 42.3 μg cm−2, the oxidation peak current of HQ (CC or RE) decreases. The reasons may be as follows: the thickness of the N-PCNPs-MWCNTs layer increases with the increase of the amount of N-PCNPs-MWCNTs loaded on the GC electrode. The thick layer of N-PCNPs-MWCNTs will obstruct the diffusion of the reactants and products, resulting in the inhibition of electrochemical oxidation process of HQ (CC or RE). Therefore, 42.3 μg cm−2 is selected as the optimum loading mass of N-PCNPs-MWCNTs on the electrode.
Simultaneous Determination of HQ, CC and RE with Differential Pulse Voltammetry. Differential pulse voltammetry (DPV) has much higher current sensitivity and better resolution compared to cyclic voltammetry, so the simultaneous determination of HQ, CC and RE is carried out by using the DPV method. The individual determination of HQ, or RE, or CC in their mixtures was first investigated when the concentration of one species changed, whereas those of other two species remained constant.
For HQ (Figure 6(a)), the linear relationship between the oxidation peak current and HQ concentration is obtained in the range of 0.2-455 μM, and the linear regression equation is calibrated as IHQ (μA) = 3.8044 + 0.2014CHQ (μM) with the correlation coefficient of R = 0.9995. In Figure 6(b), the oxidation peak current of CC increases with the increase of CC concentration from 0.7 μM to 440 μM. The corresponding linear function is ICC (μA) = 5.7971 + 0.2001CCC (μM) (R = 0.9964). Similarly, the oxidation peak current of RE increases linearly with the increase of the RE concentration with the linear function IRE (μA)= 1.4227 + 0.2398CRE (μM) (R = 0.999), and the linear range is 3.0-365 μM. The detection limits (S/N=3) for HQ, CC and RE are 0.03 μM, 0.11 μM and 0.38 μM, respectively. These results demonstrate that simultaneous determination of HQ, CC and RE can be achieved with excellent analytical performance at the N-PCNPs-MWCNTs/GC electrode. Furthermore, the present results are compared with those reported in literatures (Table 1). It can be seen that the analytical parameters including linear range and detection of limit obtained on the N-PCNPs-MWCNTs/GC electrode are much better than that obtained on some graphene or CNTs-based materials modified electrodes.
Figure 6.DPV responses of N-PCNPs-MWCNTs/GC electrode in 0.1 M PBS (pH 7.0) (a) containing 10.0 μM CC, 10.0 μM RE and different concentrations of HQ. a) 0.2 μM, b) 3.0 μM, c) 10.0 μM, d) 30.0 μM, e) 50.0 μM, f) 90.0 μM, g) 150.0 μM, h) 250.0 μM, i) 455.0 μM; (b) containing 10.0 μM HQ, 10.0 μM RE and different concentrations of CC. a) 0.7 μM, b) 5.0 μM, c) 10.0 μM, d) 20.0 μM, e) 40.0 μM, f) 70.0 μM, g) 120.0 μM, h) 220.0 μM, i) 440.0 μM; (c) containing 10.0 μM HQ, 10.0 μM CC and different concentrations of RE. a) 3.0 μM, b) 5.0 μM, c) 10.0 μM, d) 20.0 μM, e) 30.0 ?M, f) 60.0 μM, g) 100.0 μM, h) 180.0 μM, i) 365.0 iM. Insets. The relationship between the DPV response peak current (IP) of the N-PCNPs-MWCNTs/GC electrode and the concentration of HQ (CC or RE).
For further evaluating the feasibility of the N-PCNPs-MWCNTs/GC electrode in the simultaneous determination of HQ, CC and RE, the modified electrode was applied to detect HQ, CC and RE by simultaneously changing their concentration. As shown in Figure 7, The DPV results indicate that the simultaneous determination of HQ, CC and RE with a well-distinguished three anodic peaks, corresponding to the oxidation HQ, CC and RE, respectively, could be achieved at the N-PCNPs-MWCNTs/GC electrode. The oxidation peak currents of HQ, CC and RE increase linearly with the concentration of their own in the range of 5.0 μM to 60 μM (IHQ (μA) = 1.1277 + 0.1365CHQ, R = 0.9938) for HQ, 5.0 μM to 60 μM (ICC (μA) = 1.1592 + 0.1203CCC, R = 0.9914) for CC and 5.0 μM to 60 μM (IRE (μA) = 1.1164 + 0.1196CRE, R = 0.9945) for RE. Thus, the selective and sensitive determination of HQ, CC and RE was achieved simultaneously at this modified electrode. The relative standard deviations (R.S.D.) were 2.52% for HQ, 2.71% for CC and 3.15% for RE, respectively.
Table 1.Comparison of the N-PCNPs-MCNTs/GC electrode for HQ, CC and RE detection with other electrodes
Figure 7.DPV responses of the N-PCNPs-MWCNTs/GC electrode in 0.1 M PBS (pH 7.0) containing different concentrations of HQ, CC and RE (the concentrations of HQ, CC and RE are same.): a) 5.0 μM, b) 15.0 μM, c) 25.0 μM, d) 40.0 μM, e) 60.0 μM. Insets: The relationship between the DPV response peak current (IP) of the N-PCNPs-MWCNTs/GC electrode and the concentration of HQ (CC or RE).
Interference Studies. The interferences of other species on the sensor response were investigated by analyzing a solution containing 0.5 mM HQ, 0.5 mM CC and 0.5 mM RE in existence of interfering species (Figure 8). The results showed that 50 mM Ca2+, Cu2+, K+, Zn2+, SO4 2−, NO3− and Cl− virtually had no obvious interference to the DPV signals of the targets at the N-PCNPs-MWCNTs/GC electrode, indicating the proposed sensor shows a high selectivity and good anti-interference ability.
Figure 8.Interferences of some common ions (100-fold excess for each) on the determination of dihydroxybenzene isomers. (A–G): (Ca2+, Cu2+, K+, Zn2+, SO42−, NO3−, and C1−). The concentrations of HQ, CC and RE are the same and 0.5 mM.
Reproducibility and Stability. The fabrication reproducibility for ten N-PCNPs-MWCNTs/GC electrodes was investigated by comparing the oxidation peak currents of 0.5 mM HQ, 0.5 mM CC and 0.5 mM RE in a mixed solution. The relative standard deviation (RSD) is 4.31% for HQ, 6.22% for CC and 5.46% for RE, indicating an acceptable reproducibility. After the modified electrode was stored for 20 days, only a small decrease of the oxidation peak current was observed with the signal change of 7.52% for HQ, 9.46% for CC and 5.84% for RE, which sufficiently proves that the sensor possesses acceptable stability.
Table 2.aRecovery = (Found (μM) – Diluted water sample (μM))/Spiking (μM)
Real Sample Analysis. The developed sensor was further applied for the simultaneous determination of HQ, CC and RE in the local river (Xiangjiang River) water samples using the standard addition method and the results are shown in Table 2. It is noted that the recoveries are in the range between 94.7% and 103.7%. Therefore, the developed sensor has promising application in the simultaneous determination of HQ, CC and RE in environmental samples.
The N-PCNPs-MWCNTs hybrid materials were prepared for the first time and used to simultaneously electrochemical detect three dihydroxybenzene isomers (HQ, CC and RE). Due to the excellent catalytic activity, good electrical conductivity, high surface area and porous structure of the hybrid materials, the N-PCNPs-MWCNTs/GC electrode showed excellent electrochemical performance for the oxidation of HQ, CC and RE. The simultaneous determination of HQ, CC and RE at the N-PCNPs-MWCNTs/GC electrode was achieved with a larger oxidation peak separation and higher peak currents. The linear range and detection of limit obtained at the N-PCNPs-MWCNTs/GC electrode are much better than that obtained on some graphene or CNTs-based materials modified electrodes. All these results indicate that NPCNPs-MWCNTs hybrid materials have a bright future in analytical applications.
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