Electrochemical determination of 2,4-dichlorophenol at β-cyclodextrin functionalized ionic liquid modified chemical sensor: voltammetric and amperometric studies

Abstract

A highly effective approach was developed for the specific detection of 2,4-dichlorophenol (2,4-DCP) in real samples, based on a cyclodextrin functionalized ionic liquid modified carbon paste electrode (β-CD-BIMOTs/CPE). First, systematic optimization procedures were carried out for β-CD-BIMOTs/CPE by cyclic voltammetry (CV) and chrono-amperometry methods. Second, the optimized methods were applied for the determination of 2,4-DCP in environmental samples. A large increase in the peak currents was observed in CV and chrono-amperometry of 2,4-DCP when using β-CD-BIMOTs/CPE compared to native cyclodextrin modified carbon paste electrode (β-CD/CPE) and bare carbon paste electrode (CPE). A comparison of voltammetric behavior between CPE, β-CD/CPE and β-CD-BIMOTs/CPE indicated that the combination of ionic liquid and cyclodextrin in the composition of the carbon paste significantly improved the conductivity and compatibility of the sensor. The introduction of β-CD-BIMOTs as a modifier results in clearly enhanced sensitivity and selectivity towards 2,4-DCP over a wide concentration range of 4 μmol L⁻¹ to 100 μmol L⁻¹, with a detection limit of 1.2 μmol L⁻¹. The interferences from foreign substances in the peak current response were also studied and caused only minor changes in the signals (<4.6%). Good repeatability was achieved because of the stability of the modified electrode. The proposed method was applied to the determination of 2,4-DCP in leachates from landfill, mineral water and lake water with recoveries of (85.46–117.68%), (98.72–107.91%) and (89.25–107.46%), respectively.

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1. Introduction

Chlorophenols have caused great concern during recent years due to their environmental pollutant properties. The use of chlorophenols as intermediates for dyes, biocides and in leather- and wood-preservation industries as well as chemical raw materials is of concern because of its direct or indirect effects on aquatic ecosystems and animal populations.^1,2 Chlorophenol transportation occurs in water, soil, and sediment depending on the pH and number of chlorine atoms in the systems. These semivolatile organochlorine compounds can be found in nature, such as surface and ground waters as well as bottom sediments, due to their persistence in the receiving aquatic environments.³ The United States Environmental Protection Agency (US EPA) has classified 2,4-dichlorophenol (2,4-DCP) at the top of the list of priority pollutants.³ Recently, 2,4-DCP has been suspected as an endocrine disruptor.^4–7

The inhalation of 2,4-DCP irritates the respiratory tract and detrimentally affects the kidneys, liver and blood-forming organs.^8–10 In addition, permanent impairment of vision or blindness and severe injury of the upper respiratory tract have been observed in humans and animals exposed to 2,4-DCP.¹¹ The toxic characteristics of 2,4-DCP necessitate the development of simple, fast, sensitive and accurate analytical methods for its detection and quantification. Several methods have been developed and validated for this purpose, such as gas chromatography,^12–15 liquid chromatography,^4,16,17 UV-spectrophotometry¹⁸ and chemiluminescence.^19,20 These methods offer good limits of detection (LODs) and wide working concentration ranges. However, the above methods are inconvenient and require tedious sample preparation for in situ measurement.²¹ In contrast, the high sensitivity, simple instrumentation, low production cost and promising response speed of sensor devices based on electrochemical principles can offer on-the-spot testing of environmental samples.^22–25

In recent years, an ever-increasing number of analytes, especially phenolic compounds, have been detected by means of electrochemical and bio-sensors.^26–35 Carbon paste electrodes (CPE) have become a popular tool in electroanalysis,³⁶ mainly due to their renewability, stable response, low cost, low ohmic resistance and ease of modification when compared to other solid electrodes.^37,38 Recently, ionic liquid-based carbon paste electrodes (ILCPEs), or carbon ionic liquid electrodes, have attracted much attention and become the most popular CPEs, due to the use of room temperature ionic liquids (ILs) with good electrochemical and thermal stability, negligible vapour pressure, low viscosity, high ionic conductivity, and biocompatibility.^39,40 In addition, cyclodextrin as a modifier can also improve the sensitivity and selectivity of the carbon paste sensor due to its high molecular recognition.^41,42 In the past few years, the abilities of ionic liquids and cyclodextrin have been merged together, wherein the cyclodextrin-ionic liquid was used for sensor applications.^41,43,44

To our knowledge, only limited work has reported the utilization of β-CD functionalized ionic liquids in the sensing field. This work is the first attempt at the modification of a carbon paste electrode with a β-CD functionalized ionic liquid, which extends the earlier work into the utilization of a β-CD-IL (mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodextrintosylate (β-CD-BIMOTs)) in a sensor application.⁴⁵ In this work, we describe the preparation of β-CD-BIMOTs and investigate its performance for the voltammetric determination of 2,4-dichlorophenol in environmental samples.

2. Experimental

2.1 Reagents

All chemicals used were of analytical reagent grade. 1-Benzylimidazole (BIM), 2,4-dichlorophenol (2,4-DCP) and paraffin oil were purchased from Sigma-Aldrich while N,N-dimethylformamide (DMF) and β-cyclodextrin were purchased from Merck. Graphite was purchased from HmbG Chemicals. 0.1 mol L⁻¹ phosphate at pH 7.2 was used (sodium dihydrogen phosphate and sodium monohydrogen phosphate, both from Sigma). All aqueous solutions were prepared with ultrapure water (UPW; Milli-Q water purification system, Millipore, Billerica, MA, USA). All measurements were performed at room temperature.

2.2 Apparatus and procedure

The field emission scanning electron microscope (FESEM) analysis was performed using an FEG Quanta 450 (FEI, Eindhoven, the Netherlands). All electrochemical experiments were performed on a computer-controlled Autolab potentiostat (PGSTAT 101). A platinum wire and Ag/AgCl/3 M KCl were used as counter and reference electrodes, respectively. The electrodes were inserted into the cell through holes in its Teflon cover. Cyclic voltammetry experiments were performed at 0.10 V s⁻¹. The amperometric experiments were carried out by applying the desired potential.

2.3 Fabrication of CPE and the modified electrode

Graphite powder and paraffin oil were mixed at the ratio of 3 : 1 (w/w) in an agate mortar. The mixture was firmly packed into a syringe (3.0 mm id syringe tip), with copper wire as the electrical contact. The synthesis procedure of mono-6-deoxy-6-(3-benzylimidazolium)-β-cyclodextrintosylate (β-CD-BIMOTs) was similar to the methods previously reported.⁴⁵ The β-CD-BIMOTs/CPE was prepared according to the optimized ratio by mixing 0.225 g of graphite powder, 0.075 g of β-CD-BIMOTs and 0.135 ml of the liquid paraffin. Then the mixture was mixed well and filled firmly into the syringe. Prior to use, the unmodified/modified electrode was dried at 60 °C for 1 h and the electrode surface was polished with a piece of weighing paper and rinsed with UPW. Briefly, the bare CPE was dipped into phosphate buffer solution (PBS) (pH = 7.2) containing 2,4-DCP (100 μmol L⁻¹) and CV was performed in the potential range of −0.4 to 1.0 V at 0.10 V s⁻¹. Finally, the β-CD-BIMOTs/CPE was carefully washed with UPW for further use. For comparison, β-CD/CPE was also prepared.

2.4 Real samples analysis

The real samples (lake water, mineral water, and leachate from landfill) were filtered and stored in a refrigerator immediately after collection. All samples were adjusted to pH 7.2 using PBS pH 7.2. Each solution was transferred into a voltammetric cell to be analyzed without any further pre-treatment. Later, 25, 50 and 80 μmol L⁻¹ of 2,4-DCP were spiked into the real sample solutions, and amperometry voltammograms were recorded.

3. Results and discussion

3.1 Surface morphological study and characterization of CPE, β-CD/CPE and β-CD-BIMOTs/CPE

FESEM images were recorded to characterize the surface morphology of the different carbon pastes prepared. Fig. 1 shows the FESEM images of modified/unmodified CPEs under the same conditions. Fig. 1(a) displays a typical image of the surface of a CPE, which consists of isolated and unorganized flake graphite with clearly distinguished layers. On the surface of β-CD/CPE, a more dense and solid surface structure with a slightly blurry shape was seen (Fig. 1(b)). As expected from the BET analysis of β-CD-BIMOTs by Raoov et al. (2013), a uniform solid surface structure of β-CD-BIMOTs/CPE was seen (Fig. 1(c)).⁴⁶ In this case, the IL might act as an ion carrier between carbon layers, leading to the improved conductive performance of β-CD-BIMOTs/CPE.⁴⁷

Fig. 1 FESEM images of (A) CPE, (B) β-CD/CPE and (C) β-CD-BIMOTs/CPE.

The electrochemical behaviours of all the electrodes in 60 μmol L⁻¹ K₃[Fe(CN)₆] in 0.1 mol L⁻¹ PBS pH 7.2 solution were investigated by cyclic voltammetry. As shown in Fig. 2, the peak current (I_p) at β-CD-BIMOTs was much larger than that at CPE, while no significant difference was found at β-CD/CPE compared with CPE. The reversibility of the ionic liquid functionalized cyclodextrin modified electrode process was improved, as indicated by the decrease in the difference between peak potentials (ΔE_p). It is obvious that the functionalization of 1-benzylimidazole (1-BIM) onto cyclodextrin caused an increase in the conductivity of the composite electrode, which is connected with the larger electro-active area.⁴⁸ There was no significant increase of the capacitive current, which was beneficial for a larger magnitude of the faradaic signal to noise ratio.

Fig. 2 Cyclic voltammograms of (A) CPE, (B) β-CD/CPE and (C) β-CD-BIMOTs/CPE in the presence of 60 μmol L⁻¹ K₃[Fe(CN)₆] at pH 7.2; respectively. Conditions: 0.1 mol L⁻¹ PBS (pH 7.2); scan rate of 0.10 V s⁻¹.

3.2 Electrochemical behavior of 2,4-DCP at modified CPE

The electrochemical behavior of 2,4-DCP at various electrodes was studied by CV. Fig. 3 shows the cyclic voltammograms of the oxidation of 2,4-DCP in 0.1 mol L⁻¹ PBS pH 7.2 at various electrodes in the potential range of −0.4 to +1.0 V. The scan rate employed was 0.10 V s⁻¹. It was observed that the single oxidation peaks obtained during the scans were broad for CPE, β-CD/CPE and β-CD-BIMOTs/CPE. From Fig. 3, the bare CPE (A) with a concentration of 100 μmol L⁻¹ of 2,4-DCP gave an oxidation peak at 0.673 V and peak current of 0.7 μA. Similarly, for the same concentration (100 μmol L⁻¹ of 2,4-DCP), the method gave oxidation peaks at 0.7153 V and 0.7251 V with peak currents of 1.1 μA and 1.6 μA at β-CD/CPE (B) and β-CD-BIMOTs/CPE (C), respectively. The peak current was 2.2-fold as high at β-CD-BIMOTs/CPE compared with bare CPE but only 1.5 fold for β-CD/CPE. The highest oxidation peak current of 2,4-DCP, which was found at β-CD-BIMOTs/CPE, demonstrates that this material was responsible for a greater enhancement of the oxidation peak current compared to native CD.

Fig. 3 Cyclic voltammograms of (A) CPE, (B) β-CD/CPE and (C) β-CD-BIMOTs/CPE in the presence of 100 μmol L⁻¹ 2,4-DCP at pH 7.2; respectively. Conditions: 0.1 mol L⁻¹ PBS (pH 7.2); scan rate of 0.1 V s⁻¹.

The peak current improvement might be due to the characteristics of β-CD-BIMOTs. In our previous studies, it was shown that 2,4-DCP has a strong binding affinity towards β-CD-BIMOTs; the interactions occur via inclusion complexes, electrostatic and π–π interactions.⁴⁵ The hydrophobic inner cavity of cyclodextrin provides room for the formation of a stable host–guest inclusion complex with the guest analyte. At the same time, the presence of an ionic liquid with an imidazolium ring provides π–π interactions with the 2,4-DCP, which can increase the selectivity of the supramolecule towards 2,4-DCP. The positive charge of the imidazolium ring promotes the access of 2,4-DCP to the electrode surface, and hence improves the electron transfer rate of 2,4-DCP. In addition, the larger capacitive current of β-CD-BIMOTs/CPE, compared to bare CPE and β-CD/CPE, may be caused by the electroactivity of a fraction of graphite particles being indirectly in contact with the IL.⁴⁸ The formation of an inclusion complex slightly increases the activation energy of the oxidation reaction in the β-CD cavity, and thus the peak potential shifts positively by about 40 mV and 50 mV with respect to β-CD/CPE and β-CD-BIMOTs/CPE.⁴⁹

3.3 Optimization of experimental parameters

In order to evaluate the effect of the modifier amount on the anodic peak current of 2,4-DCP, β-CD-BIMOTs with different weight percentages (11–37%) were examined and the best signal was achieved with 18.9% of β-CD-BIMOTs. The percentage ratio of graphite powder/mineral oil/β-CD-BIMOTs of 54.1 : 27.0 : 18.9 was selected for further investigation.

In order to increase the sensitivity of determination, the effect of the pH of the supporting electrolyte phosphate buffer solution (PBS) (0.1 mol L⁻¹, pH range from 5.8 to 7.6) was investigated using CV (Fig. 4(A)). For the PBS solution in the pH range 5.8–7.6, the oxidation peak current reached the maximum value at pH 7.2, while the oxidation peak potential shifted negatively with the increased pH. The pH value of the solution may affect the form in which 2,4-DCP exists. The pK_a of 2,4-DCP is 7.89, and when the pH < 7.6, 2,4-DCP is non-dissociated, while at higher pH values, 2,4-DCP mainly exists in the form of ions. When the pH > pK_a the ionized form of CP is dominant and consequently no protons participate in the reaction. Meanwhile, at basic pH, a higher concentration of hydroxyl ions co-exists, which may replace the 2,4-DCP molecules in the adsorption sites of the β-CD-BIMOTs/CPE surface. Therefore, pH 7.2 was selected as the optimal pH in the following experiments.

Fig. 4 (A) The effect of pH value on the oxidation current of 60 μmol L⁻¹ 2,4-DCP in 0.1 mol L⁻¹ PBS on the β-CD-BIMOTs. Inset: peak current with respect to pH. (B) Peak potential with respect to pH. Scan rate 0.1 V s⁻¹. The error bar length accounts for the relative standard deviations for 3 measurements.

The regression equation of the peak potential (E_pa) and pH can be expressed as: E_pa (V) = 0.0627 V + 1.1693 (R = 0.9955) (Fig. 4(B)). The value of the slope (−62.7 mV per pH) was close to the theoretical Nerstian value of 59 mV per pH, which indicated that the total number of electrons and protons taking part in the oxidation mechanism were the same, i.e. 1 : 1.⁵⁰

The kinetic process at β-CD-BIMOTs/CPE was investigated by CV. The scan rate (v) versus the electrochemical response of 60 μmol L⁻¹ 2,4-DCP in PBS (pH 7.2) is shown in Fig. 5(A). The anodic peak intensity increased continuously with increased scan rate. The regression equation is I_pa (μA) = 0.1102 V (mV s⁻¹) + 0.1593 (R = 0.9974). A good linear relationship between the peak current and the square root of the scan rate (v^1/2) is illustrated in Fig. 5(B), suggesting that the oxidation process of 2,4-DCP at β-CD-BIMOTs/CPE is a typical diffusion controlled process.⁵¹

Fig. 5 (A) Cyclic voltammograms of 60 μmol L⁻¹ 2,4-DCP in 0.1 mol L⁻¹ PBS (pH 7.2) on β-CD-BIMOTs/CPE at various scan rates. Curve numbers (from a to i) represent: 0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.20, 0.25 and 0.30 V s⁻¹, respectively. (B) The plot shows the linear relationship of I and v^1/2. The inset expresses the relationship of E_pa with respect to ln v. The error bar length accounts for the relative standard deviations for 3 measurements.

According to the literature, the electrooxidation process of 2,4-DCP is a two electron and two proton process. The reaction process of 2,4-DCP on the electrode is represented in Scheme 1.⁵¹

Scheme 1 The oxidation reaction mechanism of 2,4-dichlorophenol.

Due to its high sensitivity and wide applicability, the amperometric method was selected for the electrochemical detection of 2,4-DCP. The detection potential was investigated and optimized as shown in Fig. 6(A) and (B). The hydrodynamic voltammogram of 2,4-DCP was optimized as shown in Fig. 6(A). The anodic current signal of 2,4-DCP significantly increased as the detection potential increased (line a); however, the background current also increased (line b). For that reason, the hydrodynamic of the signal-to-background ratios was investigated instead of the current signal, as shown in Fig. 6(B). The signal-to-background ratio measured at 0.75 V showed the highest sensitivity for 2,4-DCP; therefore, 0.75 V was selected as the amperometric detection potential for further experiments.

Fig. 6 (A) Hydrodynamic voltammogram of 9.0 μmol L⁻¹ 2,4-DCP (line a), and background (line b) in 0.1 μmol L⁻¹ PBS pH 7.2 at 35 s sampling time measured on β-CD-BIMOTs/CPE and (B) hydrodynamic voltammogram of the signal-to-background ratios extracted from the data in (A).

3.4 Analytical performance of β-CD-BIMOTs/CPE sensor

Under the optimum conditions, the linearity plotted by the peak current (I_pa) of 2,4-DCP versus its concentration can be expressed as: I_pa (μA) = 0.009c (μmol L⁻¹) + 0.2011 (R² = 0.9959) (Fig. 7). The linear relationship between the peak current I_pa and 2,4-DCP concentration was in the range of 4–100 μmol L⁻¹ and the limit of detection was estimated to be 1.2 μmol L⁻¹.

Fig. 7 The calibration curve of amperometric response of 2,4-DCP with the concentration of 10, 20, 30, 50, 70, 90 and 100 μmol L⁻¹; respectively. The error bar length accounts for the relative standard deviations for 3 measurements.

3.5 Lifetime, reproducibility and interference of the sensor

The lifetime of the β-CD-BIMOTs/CPE was evaluated by measuring the amperometric response of 2,4-DCP, with the electrode being stored at 4 °C in a refrigerator when not in use. The current retained 96–110% of its original response after continuous use for 10 days, indicating a good long-term stability of the sensor. The relative standard deviations (RSDs%) for intra-day and inter-day sensing of 100 μmol L⁻¹ 2,4-DCP were measured as 0.92% and 0.89% (n = 7), respectively, showing a reasonable precision. For the evaluation of the batch-to-batch reproducibility, a series of (seven) electrodes with optimum composition was prepared and the RSD% of the amperometric responses of this sensor to 2,4-DCP was 1.36%. After 10 consecutive days, the peak current of 100 μmol L⁻¹ 2,4-DCP retained >96% of the initial response, which indicated that the modified electrode has good stability.

In order to assess the possible analytical application of the proposed electrochemical sensor, the effects of some common anions and cations, phenol and other chlorophenols were examined in 0.1 mol L⁻¹ PBS (pH 7.2) containing 100 μmol L⁻¹ 2,4-DCP. The results in Table 1 show that all of the species had almost no influence on the signals of 2,4-DCP, with signal changes below 5%. Thus, the procedure was able to assay 2,4-DCP in the presence of these studied interference compounds, and hence it can be considered specific.

Table 1 The influence of some possible interfering substances on the determination of 100 μmol L⁻¹ 2,4-DCP

Interfering substances	Signal change (%)
LiCl	1.06
NaCl	1.76
NH4Cl	3.83
MgCl2	0.68
KCl	2.81
MgSO4	1.08
Na2SO4	2.78
K2SO4	4.66
NH4NO3	1.75
KNO3	4.44
LiOH	0.96
NaOH	3.65
Phenol	4.46
2-Chlorophenol	2.83
3-Chlorophenol	4.41
4-Chlorophenol	1.92

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3.6 Determination of 2,4-DCP in environmental samples

The developed sensor was used for the determination of 2,4-DCP from water samples under optimized conditions (Table 2). The water samples were collected from lake water, leachate from landfill and mineral water; the pH of the water was adjusted to 7.2 with phosphate buffer solution. The mineral water was purchased in plastic bottles. The mineral water, as well as the collected lake water and leachate from the landfill, were filtered through a 0.22 μm filter to remove any impurities. The determination was performed by using the proposed sensor via a recovery study.

Table 2 Determination of 2,4-DCP in environmental samples^a

Sample	Added (μmol L^−1)	Found (μmol L^−1)	Recovery%	RSD%
a The results are expressed as mean values and the ±SD is based on three replicates.
Leachate	25	29	117.68	0.95
	50	51	102.01	3.13
	80	68	85.46	0.06
Lake water	25	25	98.72	3.72
	50	49	99.02	0.89
	80	86	107.91	0.73
Mineral water	25	26	107.25	3.19
	50	47	89.25	1.85
	80	94	107.46	0.25

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Each sample was analysed in triplicate, and the determination was repeated under optimum experimental conditions. The recoveries from the samples were measured by spiking samples with known amounts of 2,4-DCP. A quantitative analysis was carried out by adding the standard solution of 2,4-DCP into the desired sample, and the peaks linearly increased in height. The calibration graph was used for the determination of the spiked 2,4-DCP in all samples. The detection results of all samples were in the range of 85.46–117.68%, indicating that the proposed method had great potential for practical sample analysis.

Compared with the other electrochemical methods in Table 3, the proposed β-CD-BIMOTs/CPE showed higher sensitivity, a wide linear range and a simple electrode fabrication process. For the same detection technique, β-CD-BIMOTs/CPE offers higher sensitivity and provides a comparable linear range of detection to those of tyrosine/MWCNTs/GCE,⁵¹ HRP/MWNT/GCE,⁵² MIP/chitosan/Nafion modified GCE⁵³ and MIP/GCE.⁵⁴ PVA/F108/AuNPs/Lac/GCE shows higher sensitivity but gives a shorter linear range of detection. The comparison confirmed that β-CD-BIMOTs/CPE was an appropriate material for the electrochemical sensing of 2,4-DCP. More importantly, this cheap preparation procedure with its competitive sensitivity represents a new platform for designing environmentally-friendly electrochemical sensors.

Table 3 Comparison of recently published electrochemical methods in the determination of 2,4-DCP

Electrode	Technique	Sensitivity (μA/μM)	Detection limit (μM)	Linear range (μM)	References
Tyrosine/MWCNTs/GCE	Amperometry	0.00425	0.66	2–100	51
HRP/MWNT/GCE	Amperometry	0.00005	0.38	1.0–100	52
MIP/chitosan/Nafion modified GCE	Amperometry	0.0067	1.6	5–100	53
MIP/GCE	DPV	—	1.6	5–100	54
PVA/F108/AuNPs/Lac/GCE	Amperometry	0.0371	2.70	5–25	55
MB-AG/GCE	Amperometry	0.00174	2.06	12.5–208	56
CS/CDs-CTAB/GCE	DPV	—	0.01	0.04–8	57
β-CDBIMOTs/CPE	Amperometry	0.0090	1.2	4–100	This work

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4. Conclusions

The voltammetric oxidation of 2,4-DCP at β-CD-BIMOTs/CPE in phosphate buffer solution under optimum conditions at pH = 7.2 has been investigated. The peak current was linear with 2,4-DCP concentration over a certain range, under the selected conditions. This allows the voltammetric determination of 2,4-DCP as low as 4.0 μmol L⁻¹ (LOQ value) and can be used successfully to assay this endocrine disruptor compound in environmental water samples. The method achieves a low LOD (1.2 μmol L⁻¹) and high percentage recovery (85.46–117.68%), and the study of interference shows that the method is not compromised by the selected interference compounds. In addition, the results obtained in the analysis of 2,4-DCP in spiked environmental samples demonstrated the applicability of the method in real samples analysis. The β-CD-BIMOTs fabrication process is a promising method to improve the CPE material in electrochemical analysis and extend its application scope in electrochemical sensing.

Acknowledgements

The authors appreciate the support from the UMRG Grant RP020A-16SUS and PPP Grant PG050-2014B. The researcher named Fairuz Liyana binti Mohd Rasdi would like to thank Ministry of Higher Education Malaysia for its financial support through My-PhD scholarship.

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