Sensitive and Simultaneous Determination of Hydroquinone and Catechol in Water Using an Anodized Glassy Carbon Electrode with Polymerized 2-(Phenylazo) Chromotropic Acid
Hydroquinone (HQ) and catechol (CT) are considered as environmental pollutants with high toxicity. We have developed a simple electrochemical sensor using an anodized glassy carbon electrode modified with a stable 2-(phenylazo) chromotropic acid- (CH-) conducting polymer (PCH/AGCE). The PCH/AGCE sensor showed good electrocatalytic activity and reversibility towards the redox of HQ and CT in phosphate buffer solution (PBS, pH 7.0). The cyclic voltammetry (CV) in mixed solution of HQ and CT showed that the oxidation peaks of them became well resolved with a peak separation of 0.1 V. The detection limits of HQ and CT were 0.044 and 0.066 μM, respectively, in a wide linear response range of 1–300 μM for both. Moreover, the sensor displayed an excellent selectivity in the presence of common interferences. This study provided a simple, sensitive, and high recovery method for simultaneous and quantitative determination of HQ and CT in aqueous medium.
HQ (1,4-benzenediol) and CT (1,2-benzenediol) have been widely used in cosmetics, pesticides, tanned leather, spices, medicines, and photography chemicals [1–3]. However, the release of HQ and CT into industrial wastewater  or smoke [5, 6] could cause severe pollution due to their high toxicity and low degradability [7–9]. Therefore, it is very important for the monitoring of excessive amount of HQ and CT. One of the main barricades is their coexistence and mutual interference because of their similar structures and behaviors . Several analytical approaches including fluorescence , chromatography , chemiluminescence , spectrophotometry  have been applied for the simultaneous determination of HQ and CT. However, the aforementioned methods possess disadvantages such as the requirement of sophisticated laboratories, well-trained instrument operators, an expensive instrument, complicated analysis procedures, and time-consuming preparations . Meanwhile, various electrochemical methods have attracted increasing attention due to their high sensitivity, simple operation, fast response, and low cost [4, 15–17]. Many modifiers such as mesoporous Pt , RGO-MWCNT , graphene/AuNPs/chitosan , Pt-MnO2 , carbon nanofragment , and conducting polymers (CPs)  have been introduced to enhance the electrode performance. Especially, CPs are preferential in view of their easy preparation and surface homogeneity as well as good reproducibility and stability . Azobenzene compounds that consist of the azo group (−N=N−) and large π-conjugated ring system have been adopted to prepare CPs for the detection of biomolecules such as dopamine [25, 26]. Nevertheless, the electrochemical ability raised by the functional groups demands extensive investigation.
In this study, we presented a sensitive and simple method for simultaneous determination of HQ and CT based on the electropolymerization of CH, whose chemical structure is shown in Figure 1. CH is one of the azo dyes containing two OH and SO3H functional groups [27, 28], which make it a good electron donor for efficiently catalyzing the redox reactions of target molecules . During the experiment, several experimental factors were optimized, such as the number of CVs sweeping for the polymerization and pH to obtain high selectivity and sensitivity . The sensor exhibited good catalytic activity for the detection of HQ and CT, which has a low detection limit and wide dynamic range. We will try to implement outdoor field testing. The specific experimental is in the supplementary information (available here).
2. Results and Discussion
2.1. Formation and Characterization of the PCH Film
As shown in Figure 2(a), the electropolymerization of CH onto the AGCE was stimulated by taking the advantages of anodization process [29–31] and confirmed by the continuous growth of redox peaks in CVs. However, there was no polymer formed at the bare GCE, as shown in the inset. Herein, the CH molecules react with each other via –OH– bridge to form polymer  as shown in Figure S1.
We also studied the effect of pH in the range of 4.0–9.0 on the PCH/AGCE. From Figure 2(b), we can see the anodic peak potential (Epa) shifted negatively with the increase of the pH and Epa was linear against pH with the slope of −32 mV/pH. This value was verge on the half of theoretical value of −59 mV/pH obtained from the Nernst equation, indicating that the ratio of protons and the transferred electrons involved in the PCH film was 1 : 2. Additionally, as displayed in Figure 2(c), the peak currents (Ipa and Ipc) increased with the scan rate grown and it was proportional to the scan rate in the range of 10–400 mV/s with the linear regression equations: Ipa (μA) = 3.902ν (mV/s) + 9.605 (r2 = 0.998) and Ipc (μA) = −1.606ν (mV/s) − 8.203 (r2 = 0.996), respectively. The result indicated that the redox process of PCH was surface-controlled. The PCH film could be undergoing a redox process between naphthoquinone and naphthol [15, 32–34].
The EDX spectra displaying the peaks of C, N, O, Na, and S elements further prove the formation of PCH and SEM image showed an even surface with well-distributed nanoflakes (Figure 3).
2.2. Electrochemical Behavior of HQ and CT
The number of sweeping cycles was firstly optimized by means of peak current towards HQ and CT (100 μM of each) in PBS, which clearly displayed that 20 cycles exhibited the highest currents for both HQ and CT, as shown in Figure S2. Then, the electrochemical behavior of PCH/AGCE was studied using CV (Figure 4(a)) in PBS and a mixture of HQ and CT to compare with bare GCE and AGCE. Bare GCE showed a broad peak at 0.3 V, as mentioned in many reports . Both AGCE and PCH/AGCE could discriminate the oxidation signals of HQ and CT with the same peak-to-peak potential separation (ΔEp) of 0.10 V, which was sufficient for the simultaneous detection of them. Moreover, PCH/AGCE clearly showed higher currents resulting in a better catalytic sensitivity. It was also in coincidence with their DPV responses in Figure 4(b).
2.3. Effect of Scan Rate and pH
The effect of the scan rates on the electrochemical signals of HQ and CT was investigated by CV (Figure 5(a)) at the PCH/AGCE. The redox peak currents increased with the increasing scan rate. Both Ipa and Ipc of HQ and CT were linear with the square root of scan rate () in a range of 10–500 mV/s suggesting the diffusion-controlled process with the equations of linear regression: Ipa (μA) = 4.09 (mV/s) + 8.38 (r2 = 0.99) and Ipa (μA) = 1.12 (mV/s) + 1.16 (r2 = 0.97) for HQ and CT , respectively.
The CVs of HQ and CT in PBS with different pH values were also displayed in Figures 5(b) and 5(c). The peak potentials shifted negatively with increasing pH from 4.0 to 9.0; when the pH was between 4.0 and 6.0, the hydroxyl in dihydroxy benzenes will not ionized, which will reduce the adsorption capacity of the two dihydroxybenzene isomers; when the pH was increased from 7.0 to 9.0, the increase in hydroxyl ion in the solution might also reduce the adsorption capacity of the two dihydroxybenzene isomers . Considering the sensitivity of the measurement, pH 7.0 was chosen as the optimal experimental condition and is exactly between 4 and 9. The Epa showed linear response towards pH with the corresponding regression equations: Epa (V) = 0.568 – 0.059 pH (r2 = 0.99) and Epa (V) = 0.671 – 0.058 pH (r2 = 0.98) for HQ and CT, respectively. The slopes of −59 and −58 mV/pH clearly indicated that the number of involved electron and proton was equal in oxidation of HQ and CT .
2.4. Chronoamperometric Measurement for HQ and CT
The diffusion coefficient and catalytic rate constant of HQ and CT was calculated from chronoamperometry. The plots of current showed a good linear relationship with the reciprocal of the square root of time  (Figure 6(a)). The slope of the linear equation could be obtained by using the Cottrell equation:where n is the number of transferred electrons; F is the Faraday constant; A is the proportion of the electrode; D is the diffusion coefficient of active substance; C is the initial molar concentration; and t is the running time [37, 38].
From the resulting slope, the D value of the HQ and CT was obtained to be 1.121 × 10−5 and 9.170 × 10−6 cm2 s−1, respectively.
Chronoamperometry was also be used to measure the catalytic rate constants  from the following equation:where Icat and Id were the currents of the PCH/AGCE in the presence and absence of HQ and CT; γ = kCt is the error function; k is the catalytic rate constant; C is the concentration of HQ and CT, and t is the running time (s) .
From the slope of the Icat/Id vs. t1/2 plot, as shown in Figure 6(b), the k value was obtained to be 1.57 × 102 and 1.51 × 102 cm3·mol−1·s−1 of HQ and CT in the concentration range of 0.1 to 0.4 mM, respectively.
2.5. Simultaneous and Selective Detection of HQ and CT at the PCH/AGCE
The prominent electrocatalytic performance of the PCH/AGCE was expected for the selective detection of HQ and CT. Figure 7(a) showed DPVs at different concentrations of HQ on the PCH/AGCE in the presence of a constant concentration of CT (50 μM). The peak current of the HQ increased linearly with increasing concentration while the CT remained constant. Similarly, when the HQ concentration was fixed, the Ipa of CT increases with increasing concentration, while the Ipa of HQ remains almost constant (Figure 7(b)). The standard curve of Ipa vs. [HQ] and Ipa vs. [CT] is shown in the insets of Figures 7(a) and 7(b), respectively, corresponding to the equations of linear regression: Ipa (μA) = 8.17 × [HQ] (μM) + 9.26 (r2 = 0.93) and Ipa (μA) = 4.78 × [CT] (μM) + 3.46 (r2 = 0.98), respectively. The detection limit (S/N = 3) of HQ and CT was calculated to be 0.044 and 0.066 μM, respectively.
Figure 7(c) showed the DPVs for detection of HQ and CT at PCH/AGCE simultaneously. The Ipa of both HQ and CT at the PCH/AGCE sensor increased as their concentrations increased, but the peak potential hardly changed. The dynamic linear ranges of both HQ and CT were 20–300 μM, with following equations of linear regression: Ipa (μA) = 3.68 × [HQ] (μM) + 1.18 (r2 = 0.99) and Ipa (μA) = 3.538 × [CT] (μM) + 0.48 (r2 = 0.99), respectively. The detection limits were 0.052 and 0.073 μM of HQ and CT, respectively.
These demonstrated that the PCH/AGCE sensor can be successfully adapted to the detection of HQ and CT simultaneously and can rival the analytical performance of other reported sensors; the summary is shown in Table 1. Therefore, the PCH/AGCE sensor is advantageous for detecting both HQ and CT without any significant mutual interference.
2.6. Repeatability, Stability, and Interference Analysis
Electrochemical repeatability of PCH/AGCE was measured by CV for 30 consecutive cycles with a range of −0.1 to +0.5 V in the mixture of HQ and CT (0.1 mM each, PBS, pH 7.0). The relative standard deviation (RSD) of the oxidation currents of both HQ and CT was about 2.56% and 3.03%, respectively, which proved the excellent repeatability of the PCH/AGCE sensor. Additionally, the oxidation peak currents of HQ and CT at the PCH/AGCE sensor was decreased by only 3.5% and 4.0% of their initial values even though the PCH/AGCE sensor was stored in PBS (pH 7.0) for a week at 4°C in a refrigerator. This clearly shows that the chemical composition of the PCH/AGCE sensor is highly stable.
The interference studies were examined with some organic compounds and inorganic ions. We measured the oxidation peaks of the mixture of HQ and CT (100 μM each) in the absence and presence of more than 10-fold excess of resorcinol, 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, and phenol  and 100-fold 4-acetamidophenol, bisphenol A, NH4+, Na+, SO42−, Cl−, and NO3− ions in PBS. The oxidation potential range of resorcinol, 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol was 0.5–1.0 V, which was higher than those of HQ and CT (Figure S3). The ratios of the anodic peak current of HQ and CT before and after adding interferences were measured and summarized in Table 2. This clearly implied that there was no significant interference in the simultaneous and quantitative detection of HQ and CT.
2.7. Real Sample Analysis
Real sample analysis was performed to investigate the practical application of the PCH/AGCE sensor. The recovery experiments were conducted by DPV, and known concentrations of HQ and CT were added in local tap water and lake water samples (Figure S4). As summarized in Table 3, the recoveries of HQ and CT were 97.3–100.3% and 96.5–104.2%, respectively. It indicated the practical applicability and reliability of the proposed method.
In this work, we have developed a simple and sensitive electrochemical sensor for simultaneous detection of HQ and CT on the basis of PCH/AGCE. The characteristic behavior was tested by CV and the DPV; it clearly certifies that PCH/AGCE has a better electrocatalytic performance for both CT and HQ. The detection limits were 0.044 and 0.066 μM for HQ and CT, respectively. In comparison with GCE, the PCH/AGCE showed good stability, reproducibility, and practicability in the real sample. The obtained results revealed that PCH could be a promising candidate for the development of biosensing and other electrochemical applications.
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
Miao Zhang and Chuang-ye Ge contributed equally to this work.
This work was financially supported by the Program for Science and Technology Innovation Talents in Universities of Henan Province (18HASTIT037), Henan Technology System for Conventional Freshwater Fish Industries (S2018-10-G2), the open fund of the State Key Laboratory of Luminescent Materials and Devices in South China University of Technology (no. 2018-skllmd-09), and the Key Research Projects for Institutions of Higher Education from the Department of Education, Henan Province (no. 17A150010).
Figure S1: eletropolymerization process and redox reaction of PCH. Figure S2: dependency of the oxidation peak currents of both HQ and CT with the variation of the number of CV cycles (i.e., the thickness of PCH film) obtained from the CV measurement of the mixture of HQ and CT (100 μM each) in PBS), scan rate 100 mV/s. Figure S3: CVs of mixture containing HQ and CT (100 μM) with 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, phenol, and resorcinol (1 mM each) at PCH/AGCE in PBS (pH 7.0), scan rate 100 mV/s. Figure S4: DPV profiles of tap water (A) and local lake water (B) containing CT and HQ with different concentrations (0, 20, 40, and 60 μM). (Supplementary Materials)
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