Preparation of a novel poly-(di-ionic liquid)/BDD-modified electrode for the detection of tetrachloro-p-benzoquinone

Graphical abstract

Abstract

During the COVID-19 pandemic, the release of toxic disinfection by-products (DBPs) has increased due to the intensive, large-scale use of disinfectants. Halogenated benzoquinones (HBQs) are among the most toxic DBPs, but there is no rapid, convenient, and economical detection method. In this study, a novel PDIL/BDD-modified electrode was prepared in a mixed solvent of dimethyl sulfoxide (DMSO) and acetonitrile (ACN) by electrochemical polymerization with a di-ionic ionic liquid containing alkenyl groups as the monomer. The electrochemical behavior of tetra-chloro-p-benzoquinone (TCBQ) on the modified electrode was studied. By studying the cyclic voltammetry behavior of TCBQ on the PDIL/BDD electrode, it was concluded that the electrode reactions of TCBQ included the reduction of TCBQ to TCBQH₂ (C1) and the reduction of bis-quinhydrone imidazole π-π type charge transfer complex to TCBQH₂ (C2). By studying the SWV responses of TCBQ in the concentration range of 1–100 ng/L on the PDIL/BDD electrode, it was found that the reduction peak current (I_pa) had a linear relationship with the concentration. The electrochemical SWV technique was used to detect the concentration of trace TCBQ in water and is expected to be used for the detection of other HBQs in drinking water and swimming pool water.

1. Introduction

Disinfection is an important process for inactivating pathogenic microorganisms and reducing the risk of diseases in domestic water [1]. During disinfection, disinfectants (e.g., chlorine, chlorine dioxide, chloramines, or ozone) react with some natural organic compounds in raw water to produce a series of disinfection by-products (DBPs) [2]. Especially during the current COVID-19 pandemic, large-scale intensive disinfection measures have increased the release of toxic DBPs. Epidemiological studies have found that long-term exposure to chlorinated disinfection pool water may induce respiratory diseases such as asthma, reproductive system diseases, cancer, and even genetic mutations [3].

Halogenated benzoquinones (HBQs) are DBPs that were first detected in disinfected drinking water in 2010. Although the concentration of HBQs is generally at the ng/L level, which is lower than other DBPs (μg/L or mg/L level), it is much more toxic than non-halogenated DBPs. HBQ is cytotoxic and may also be carcinogenic and carcinogenic [4]. Therefore, HBQ has attracted more attention than other DBPs, and there have been many studies on its generation mechanism, biological toxicity, detection method, and treatment methods. Twelve kinds of HBQs have been reported, whose names and abbreviations are shown in Table 1 [5]. Due to the low solubility and low concentration of HBQs, conventional detection methods, such as gas chromatography-mass spectrometry (GC–MS), high-performance liquid chromatography (HPLC), ultra-high performance liquid chromatography (UPLC), and liquid chromatography-mass spectrometry (LC-MS), have difficulty directly detecting it. Thus, these methods require pre-processing techniques to concentrate samples [6]. In particular, tetra-chloro-p-benzoquinone (TCBQ) is a recently detected HBQ [7]. Although its concentration range is at the ng/L level in water, its molecular structure contains four chlorine atoms, so its toxicity is very high. Due to its low solubility and concentration, it is difficult to detect, and there have been very few reports about it. Zhao et al. developed an SPE-LC-MS/MS method for the characterization and determination of DCBQ, DCMBQ, TCBQ, and DBBQ in chlorinated drinking water with concentrations ranging from 0.5 to 165 ng/L, but the samples must be concentrated using solid phase extraction before determination [8]. Therefore, it is important to develop a convenient, economical, and sensitive TCBQ detection technique.

Table 1.

List of names and abbreviations of HBQs.

Name	Abbreviation
2,5-dichloro-1,4-benzoquinone	2,5-DCBQ
2,6-dichloro-1,4-benzoquinone	2,6-DCBQ
2,5-dibromo-1,4-benzoquinone	2,5-DBBQ
2,6-dibromo-1,4-benzoquinone	2,6-DBBQ
2,6-dichloro-3-methyl-1,4-benzoquinone	DCMBQ
2,3,6-trichloro-1,4-benzoquinone	Tri CBQ
2,3,5,6-tetrachloro-1,4-benzoquinone	Tetra CBQ
3,4,5,6-tetrachloro-1,2-benzoquinone	Tetra C-1,2-BQ
2,3,5,6-tetrabromo-1,4-benzoquinone	Tetra BBQ
3,4,5,6-tetrabromo-1,2-benzoquinone	Tetra B-1,2-BQ
2,3-dibromo-5,6-dimethyl-1,4-benzoquinone	DBDMBQ
2,5-diiodo-1,4-benzoquinone	DIBQ

As an important technology for the determination of environmental pollutants, electrochemical analysis has a high sensitivity, wide detection range, and low detection limit compared with traditional analytical methods and does not require complex derivatization or pretreatment methods [9], [10], [11], [12]. Therefore, electrochemical methods are suitable for the analysis of HBQs. In recent years, boron-doped diamond (BDD) thin-film electrodes have become promising electrode materials for electrochemical analysis due to their wide electrochemical potential window, low capacitance, and electrochemical stability [13], [14], [15], [16]. However, few reports have focused on the use of BDD electrodes for the determination of trace HBQs in water.

Ionic liquids (ILs) are organic salts consisting entirely of discrete anions and cations with a melting point of less than 100 °C [17]. When the number of cations or anions in the IL molecule is greater than 3, it is called a poly(ionic) liquid (PIL), which is a polymer containing at least one ion center and a repeat unit similar to the structure of the mono-ionic liquid (MIL) in the polymer chain [18]. PILs can combine the properties of both polymers and ionic liquids because ILs have several excellent physicochemical properties, including negligible vapor pressure, thermal stability, inflammability, high ionic conductivity, and wide electrochemical stabilization window. These properties can be transferred to the PIL polymer chains after polymerization [19]. If ILs are electrodeposited on BDD electrodes instead of conventional metal deposition, such as platinum and gold, the covalent attachment of polymers will greatly increase the stability of the polymer-BDD interface, while functional groups in the polymer structure will help improve the electrochemical performance of the electrodes [20], [21], [22].

In this study, we developed a novel poly-(alkenyl-based di-ionic IL)-modified BDD (PDIL/BDD) electrode for the rapid electrochemical detection of TCBQ. The PDIL/BDD-modified electrode was prepared by electrochemical polymerization using an alkenyl-based di-ionic IL (DIL) as the monomer, acetonitrile/dimethyl sulphone as the solvent, and sodium perchlorate as the supporting electrolyte. The structure of the alkenyl IL monomer is shown in Scheme 1 .

Scheme 1.

The structure of alkenyl-based di-ionic IL (DIL).

2. Experimental

2.1. Main reagents and instrumentation

2,3,5,6-Tetrachloro-1,4-benzoquinone (TCBQ); acetic acid (HAc); anhydrous dimethyl sulfoxide (DMSO); anhydrous acetonitrile (ACN); sodium perchlorate (NaClO₄); anhydrous ethyl alcohol (EtOH); potassium ferricyanide (K₃[Fe(CN)₆]); ferrous potassium cyanide (K₄[Fe(CN)₆]). The experimental reagents were analytically pure. No reagents were further purified before use except TCBQ, which was purified by acetone recrystallization at a low temperature before use. The water used in the experiment was ultra-pure (conductivity <0.07 μS/cm).

2.2. Preparation of PDIL/BDD electrode

The synthesis process of DIL is shown in reference [23]. A CHI660E electrochemical workstation was used, with BDD as the working electrode, a SCE reference electrode, and Pt auxiliary electrode. In 0.5 M H₂SO₄ aqueous solution, cyclic voltammetry was used to scan for stability in the potential range of −0.3–1.3 V at a scan rate of 0.1 V/s. The activated BDD electrode was rinsed with water several times and placed in a vacuum drying oven at 50 °C for later use. The BDD electrode was placed in a mixture of ACN and DMSO (volume ratio 1: 1) containing [VC₄(Vim)₂]Cl₂ monomer (0.1 mol∙L⁻¹) and sodium perchlorate (0.1 mol∙L⁻¹). The electrolyte was deoxygenated through N₂ for 10 min before electropolymerization. The PDIL/BDD-modified electrode was obtained by cyclic voltammetry with a scan rate of 0.1 V/s for 50 cycles in the range of −1.2–1.5 V. The modified electrode was washed several times with ethanol and water successively and placed in a vacuum drying oven at 50 °C for 24 h. The preparation process of PDIL/BDD electrode is shown in Scheme 2 .

Scheme 2.

The preparation process of PDIL/BDD electrode.

2.3. Electrochemical characterization of the PDIL/BDD-modified electrode

Both BDD and PDIL/BDD electrodes were scanned to assess their stability over the potential range of −0.3–1.3 V by cyclic voltammetry at a scan rate of 0.1 V/s in 0.5 M H₂SO₄ solution before use. The cyclic voltammetry (CV) responses and square wave voltammetry (SWV) responses of the PDIL/BDD electrode were studied in HAc solution containing TCBQ with an electrochemical workstation using a SHE reference electrode and Pt auxiliary electrode. After each test, the organic matter adsorbed on the surface of the electrode was removed for continuous cyclic voltammetry tests in 0.01 M HAc blank solution at a scanning rate of 0.1 V/s for 10 cycles. Electrochemical impedance spectroscopy (EIS) was carried out in a 5.0 × 10⁻³ mol/L K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution containing 0.1 M KCl. All electrolytes were fed with N₂ for 15 min before analysis to remove oxygen.

The CV responses of BDD and PDIL/BDD electrodes were studied in 100 ng/L TCBQ HAc (0.01 M) solution with enrichment time of 180 s, rest time of 2 s, scan rate of 0.1 V/s, and temperature of 25 °C.

The CV responses of the PDIL/BDD electrode were studied in 100 ng/L TCBQ HAc (0.01 M) solution at different scan rates (0.01–0.12 V/s), enrichment time of 180 s, rest time of 2 s, and temperature of 25 °C.

The SWVs response of different concentrations of TCBQ (1–100 ng/L) on the PDIL/BDD electrode was studied. The enrichment time was 180 s, the rest time was 2 s, the initial potential was 1.2 V, the termination potential was −0.4 V, the potential increment was 0.005 V, the amplitude was 0.05 V, and the frequency was 10 Hz.

EIS was carried out in 5.0 × 10⁻³ mol/L K₃[Fe(CN)₆]/ K₄[Fe(CN)₆] solution containing 0.1 M KCl, with a frequency range of 10⁵– 0.01 Hz and a sinusoidal signal amplitude of 0.005 V.

In order to study the interference of Na⁺, Fe²⁺ and Ca²⁺, 0.1 mg/L NaCl, FeCl₂ and CaCl₂ were added into 100 ng/L TCBQ HAC (0.01 M) solution, to study the SWVs response on the PDIL/BDD electrode, enrichment time of 180 s, rest time of 2 s, scanning speed of 0.1 V/s, and temperature of 25 °C.

3. Results and discussion

3.1. Analysis of the electropolymerization process of [VC₄(Vim)₂]Cl₂ on the BDD electrode

Fig. 1 shows the CV curve of [VC₄(Vim)₂]Cl₂ on the BDD electrode at a scan rate of 0.1 V/s in the range of −1.2–1.5 V. In the first cycle, a weak anodic oxidation peak appeared at 0.86 V in the forward scan, and the oxidation peak current decreased upon increasing the number of scanning cycles. A cathodic reduction peak appeared at −0.60 V in the reverse scan, and the peak current increased gradually upon increasing the number of scanning cycles. When the scanning potential range was set to 0 – −1.2 V, there was no reduction peak at the cathode, indicating that the oxidation peak at 0.86 V of the anode was due to the initiation of the olefin alkenyl polymerization. That is, ClO₄ ⁻ lost electrons and became ClO₄• [24], and the cathodic peak at − 0.60 V was due to the reduction of the alkenyl radical polymerization [25]. The electro-initiated polymerization of [VC₄(Vim)₂]Cl₂ occurs via a radical mechanism, and the initiator is ClO₄•, which was added to the unsaturated carbon–carbon double bond of [VC₄(Vim)₂]Cl₂. The chain propagation process involved a further reaction of the free radical with the unsaturated double bond of the surrounding [VC₄(Vim)₂]Cl₂. This was finally stopped by bimolecular termination of polymer free radicals or the trapping of these free radicals in the polymer matrix. This free-radical polymerization mechanism is shown in Fig. 2 .

Fig. 1.

Cyclic voltammograms of [VC₄(Vim)₂]Cl₂ on the BDD electrode.

Fig. 2.

Electropolymerization mechanism of [VC₄(Vim)₂]Cl₂ on the BDD electrode.

3.2. Electrochemical behaviors of TCBQ on the PDIL/BDD-modified electrode

The CV curves of TCBQ on the PDIL/BDD and BDD electrodes are shown in Fig. 3 . TCBQ displayed a pair of redox peaks on the BDD electrode, with a reduction peak potential (E _pc) of 0.364 V and oxidation peak potential (E _pa) of 0.499 V, corresponding to TCBQ reduction to generate TCBQH₂. However, after PDIL modification, the CV response of TCBQ on the PDIL/BDD electrode was greatly enhanced, and two reduction peaks (C1: E _pc1 = 0.196 V, C2: E _pc2 = −0.124 V) appeared at the cathode, along with two oxidation peaks (A1: E _pa1 = 0.530 V, A2: E _pa2 = 0.394 V).

Fig. 3.

CV responses of TCBQ on the PDIL/BDD and BDD electrodes.

To further study the electrocatalytic reaction of TCBQ on the PDIL/BDD electrode, the CV responses of TCBQ on the PDIL/BDD electrode were tested at scan rates (ν) from 0.01 V/s to 0.12 V/s, and the results are shown in Fig. 4 . Upon increasing the scan rate, the reduction peak current (I _pc1 and I _pc2) of TCBQ increased, the reduction peak potential (E _pc1 and E _pc2) shifted negatively, the oxidation peak current (I _pa1) increased, and the oxidation peak potential (E _pa1) shifted positively. The reduction peak currents (I _pc1 and I _pc2) had a linear relationship with ν (Fig. 5 (a)), indicating that under the experimental conditions, the reduction reactions C1 and C2 of TCBQ were controlled by adsorption. The oxidation peak current (I _pa1) had a good linear relationship with ν and ν^1/2 (Fig. 5(b)), and the correlation coefficients r ² were 0.972 and 0.988, respectively, indicating that I _pa1 had a better linear correlation with ν^1/2. Therefore, under the experimental conditions, the oxidation reaction A1 of TCBQH₂ on the PDIL/BDD electrode was controlled by diffusion in solution.

Fig. 4.

CV responses of TCBQ on the PDIL/BDD electrode at scan rates of 0.01–0.12 V/s.

Fig. 5.

The relationship between ν ∼ I_pc1,ν ∼ I_pc2, and ν,ν^1/2 ∼ I_pa1 in the range of 0.01–0.12 V/s.

lnν ∼ E _pc1, lnν ∼ E _pc2, and lnν ∼ E _pa1 also had linear relationships (Fig. 6 ), and the linear equations were as follows:

$$E_{pc1}=0.1522-0.0143\ast \ln \nu ,r^2=0.982$$ (1)

$$E_{pc2}=-0.0848-0.0086\ast \mathrm{l}\mathrm{n}\nu ,r^2=0.973$$ (2)

$$E_{pa1}=0.6212+0.0324\ast \mathrm{l}\mathrm{n}\nu ,r^2=0.990$$ (3)

$$|E_{\text{pc2}}\text{-}{E}_{\text{pc2}/\text{2}}\text{| =}64mV$$ (4)

Fig. 6.

Relationships between (a) lnν ∼ E_pc1, lnν ∼ E_pc2, and (b) lnν ∼ E_pa1 at scan rates of 0.01–0.12 V/s.

According to the Laviron equations [26]:

$$E_{\text{p}}\text{=}{E}^0\text{-}\frac{\mathit{RT}}{\beta nF}\text{ln}\frac{\mathit{RT}{K}^0}{\beta nF}\text{+}\frac{\mathit{RT}}{\beta nF}\ln \nu \text{,}\beta \text{= 1 -}\alpha ,$$ (5)

$$E_{\text{pa}}=E^{\theta }+\frac{\mathit{RT}}{(1-\alpha )nF}\text{ln}\frac{\mathit{RT}{\kappa }_{\text{s}}}{(1-\alpha )nF}+\frac{\mathit{RT}}{(1-\alpha )nF}\text{ln}\nu$$ (6)

$$E_{\text{pc}}=E^{\theta }+\frac{\mathit{RT}}{\alpha nF}\text{ln}\frac{\mathit{RT}{\kappa }_{\text{s}}}{\alpha nF}-\frac{\mathit{RT}}{\alpha nF}\text{ln}\nu$$ (7)

It can be derived:

$$I_{pc2}(\mu A)=-0.4222--0.0612\ast C_{\mathit{TCBQ}}\left(\mathit{ng}/L\right),r^2=0.984\frac{\mathit{RT}}{{\beta }_1{n}_{1}F}=0.03241,{\beta }_1=1-{\alpha }_1$$ (8)

$$\frac{\mathit{RT}}{\left(1-{\beta }_2\right)n_{2}F}=0.00861\frac{\text{47.7}}{{\beta }_2{n}_2}=64$$ (9)

where α ₁ and β ₁ are the electron transfer coefficients of the oxidation peak A1 and reduction peak C1 respectively, β ₂ is the electron transfer coefficient of the reduction peak C2, and n ₁ and n ₂ are the electron transfer numbers of C1 and C2 respectively. Further, n ₁ = 2.4≈2, n ₂ = 3.7≈4, α₁ = 0.694, β₁ = 0.306, and β₂ = 0.21 were obtained. Therefore, the electron transfer number of reaction C1 was 2 and that of C2 was 4. It can be inferred that reaction C1 was the reduction of TCBQ to TCBQH₂, as shown in Scheme 3 .

Scheme 3.

Reduction reaction C1 of TCBQ on the PDIL/BDD electrode.

TCBQ is an electron receptor that easily forms a charge transfer complex (CT). If the electron recipient provides π electrons, it is called a π-π type CT [27]. The CT formed with hydroquinone is commonly known as quinhydrone [28]. Therefore, TCBQ and TCBQH₂ can form a π-π type CT, as shown in Fig. 7 [29]. However, the real reason for the formation of quinhydrone was not the hydrogen bond between BQ and BQH₂, but rather the electron effect between the two molecules. Electron transfer and hydrogen bonds formed between molecules only played a synergistic role in stabilizing the molecules. Therefore, some aromatic heterocyclic compounds with higher π electron cloud density than benzoquinones can also be used as electron receptors to form a π-π type CT. Because the ¹³C NMR chemical shift (δ _C) of aromatic compounds is inversely related to their π-electron cloud density (ρ _π), the magnitude of ρ _π of aromatic compounds can be analyzed by combining δ _C [30]. According to ChemDrawʹs calculated δ _C of [VC₄(Vim)₂]Cl₂, TCBQ, and TCBQH₂ (Fig. 8 ), it can be concluded that: ρ_{π([VC4(Vim)2]Cl2)} > ρ_π(TCBQH2) > ρ_π(TCBQ). Therefore, imidazole ring cation has a stronger π electron-donating ability than TCBQH₂, and the H atom of the C(2) atom of the imidazole ring can engage in strong hydrogen bonding, which can stabilize the formed π-π type CT.

Fig. 7.

Resonance structure of π-π type CT formed by TCBQ and TCBQH2.

Fig. 8.

The δ_C of TCBQ, TCBQH₂, and [VC₄(Vim)₂]Cl_2.

Based on the above theoretical support, the C1 and C2 reactions of TCBQ on the PDIL/BDD electrode were deduced, as shown in Fig. 9 . Firstly, due to the hydrogen bonding of imidazole cations on the polymer chain of the PDIL/BDD electrode film, some TCBQ was adsorbed on the electrode surface (TCBQ^…EPVDIL/BDD). Some was located near the electrode surface, and then the TCBQ near the electrode surface obtained 2e⁻ to generate [TCBQ]²⁻. TCBQ adsorbed on the electrode surface obtained 2e⁻ to generate [TCBQ]^{2− …}PDIL/BDD. Then, [TCBQ]²⁻ and [TCBQ]^{2− …}PDIL/BDD respectively obtained 2H⁺ to produce TCBQH₂, which was a C1 reduction reaction. The C1 reduction reaction of TCBQ, whether near the electrode surface or adsorbed on the electrode, involved 2 electrons and 2 protons. Secondly, after some C1 product TCBQH₂ and [TCBQ]²⁻ formed quinhydrone CT, more bis-quinhydrone imidazole π-π type CT complex formed [TCBQ]^{2− …}PDIL/BDD and obtained 4e⁻ and 4H⁺ to generate TCBQH₂. This is the C2 reduction reaction, which involved 4 electrons and 4 protons. It can be seen that reactions C1 and C2 were competitive, but C1 was a prerequisite for reaction C2.

Fig. 9.

Reactions C1 and C2 of TCBQ on the PDIL/BDD electrode.

3.3. SWV responses of TCBQ on the PDIL/BDD electrode

Fig. 10(a) shows the SWVs responses of TCBQ in the concentration range of 1–100 ng/L. Upon decreasing the TCBQ concentration, the peak current (I _pc1) and (I _pc2) both decreased. When the TCBQ concentration was very low, I _pc2 showed almost no response, indicating that the C1 reaction took precedence over the C2 reaction. I _pc1 and I _pc2 had the following linear relationship with the TCBQ concentration (C _TCBQ), respectively (Fig. 10(b) and (c)):

$$I_{pc1}\left(\mu A\right)=-1.1319--0.0830\ast C_{\mathit{TCBQ}}\left(\mathit{ng}/L\right),r^2=0.982$$ (10)

$$I_{pc2}\left(\mu A\right)=-0.4222--0.0612\ast C_{\mathit{TCBQ}}\left(\mathit{ng}/L\right),r^2=0.984$$ (11)

Fig. 10.

(a) SWV responses of 1–100 ng/L TCBQ on PDIL/BDD electrode; (b) fitting curve of I_pc1 and I_pc2 with C_TCBQ; (c) fitting curve of I_pc2 and C_TCBQ.

This shows that the SWV method can be used to detect the concentration of trace TCBQ in water. Compared with conventional methods such as GC–MS, LC-MS, and gas chromatography-photoionization, the extraction pretreatment step was omitted, and the detection error was smaller. The operation was also more convenient and the detection speed was faster [31].

3.4. EIS analysis of the BDD and PDIL/BDD electrode

Fig. 11 shows the Nyquist plots of the BDD and PDIL/BDD electrodes in Fe(CN)₆ ^3−/4−. The illustration shows the equivalent circuit model fitted according to the EIS data. Table 2 lists the fitted equivalent circuit data. The Nyquist plots of both BDD and PDIL/BDD electrodes were semicircular in the high-frequency region and straight in the low-frequency region. The results showed that the reaction of Fe(CN)₆ ^3−/4− on the electrodes was controlled by electrochemical reactions and diffusion.

Fig. 11.

Nyquist plots of EIS of the BDD and PDIL/BDD electrodes.

Table 2.

The equivalent circuit fitting data of the BDD and EPVDIL/BDD electrodes.

	Rs	Cdl-T	Cdl-P	Rct	W-R	W-T	W-P
BDD	9.965	5.207 × 10−5	0.7803	381.0	10,300	192.3	0.4924
EPVDIL/BDD	9.893	5.665 × 10−5	0.7386	328.7	12,012	288.0	0.5026

In the high-frequency region, the semicircle diameter of the Nyquist plot of the PDIL/BDD electrode was slightly smaller than that of the BDD electrode. This was because PIL is a conductive polymer with better conductivity than BDD. Due to the thin polymer film thickness, the conductivity of the PDIL/BDD electrode was not significantly improved. According to Table 2, the R _ct of PDIL/BDD was 328.7 Ω, while that of BDD was 381 Ω, indicating that electrons were more easily transferred at the electrode interface of PDIL/BDD. Therefore, the peak current of the PDIL/BDD electrode was significantly higher than that of the BDD electrode.

The low-frequency region of the Nyquist plots of the BDD and PDIL/BDD electrodes were straight lines, which are diffusion-control regions. The slope of the PDIL/BDD electrode was slightly higher than that of the BDD electrode, indicating that the surface roughness of the PDIL/BDD electrode was lower than that of the BDD electrode (see Table 3 ).

Table 3.

Percentage of deviation of I_pc1 and I_pc2.

Solutions	Ipc1 (%)	Ipc2 (%)
TCBQ	0	0
TCBQ + NaCl	0.4773	0.515
TCBQ + FeCl2	2.0955	3.0609
TCBQ + CaCl2	2.1080	4.8270

3.5. Surface morphology of the PDIL/BDD electrode

Fig. 12 shows the SEM images of the BDD and PDIL/BDD electrode surfaces. As can be seen from Fig. 12(a), (b), and (c), the crystal shape of the BDD film was clear, the structure was dense, and the grain size was in the range of 2–4 nm. After electropolymerization, a very thin film covered the surface of the BDD substrate, but it was not thick enough to cover the morphology of the crystal grain (Fig. 12 (d)). This was due to the limited number of active sites on the electrode surface during electropolymerization. The steric hindrance of the molecular structure of DIL and electrostatic repulsion led to a relatively low degree of polymerization and film thickness [32]. The magnified image showed that due to the coating of the film, the sharpness of the crystal grains on the surface of the BDD decreased (Fig. 12(e)). It can be seen from Fig. 12(f) that some EPVDIL existed on the film with a cubic grain structure. The SEM images show that the surface roughness of the PDIL/BDD electrode was lower than that of the BDD electrode, which is consistent with the conclusions derived from EIS.

Fig. 12.

SEM images of the BDD and PDIL/BDD electrode surfaces.

3.6. Interference analysis of NaCl, FeCl₂ and CaCl₂ on TCBQ detection

Fig. 13 shows the interference of NaCl, FeCl₂ and CaCl₂ on TCBQ detection. It can be seen that Na⁺, Fe²⁺ and Ca²⁺ have little influence on TCBQ detection results, which are 0.4773 %, 2.0955 % and 2.1080 % for I _pc1, and 0.515 %, 3.0609 % and 4.8270 % for I _pc2, respectively.

Fig. 13.

CV responses of TCBQ, TCBQ + NaCl, TCBQ + FeCl₂ and TCBQ + CaCl₂ on the PDIL/BDD electrode.

3.7. Stability of the PDIL/BDD electrode

In order to detect the stability of the PDIL/BDD electrode, the freshly prepared electrode was stored in a dryer for 4 weeks and then removed. The CV behaviors of TCBQ on the placed electrode and the freshly prepared electrode were investigated respectively. The results are shown in Fig. 14 . The peak currents I _pa1, I _pc1 and I _pc2 of the PDIL/BDD electrode after 4 weeks of storage were only 0.2338 μA, 0.2975 μA and 0.2981 μA lower than that of the freshly prepared electrode, respectively, indicating that the PDIL/BDD electrode prepared by electropolymerization has good stability.

Fig. 14.

CV behaviors of TCBQ on the placed electrode and the freshly prepared electrode.

4. Conclusion

A PDIL/BDD-modified electrode was prepared in a mixed solvent of DMSO and ACN by electrochemical polymerization with a di-ionic ionic liquid containing alkenyl groups as the monomer. The electrochemical behavior of TCBQ on the modified electrode was studied. SEM results showed that the film layer on the PDIL/BDD electrode was very thin, and the surface roughness of the PDIL/BDD electrode was lower than that of the BDD electrode. The EIS results showed that the R _ct of the PDIL/BDD electrode was 328.7 Ω, which was less than that of BDD (R _ct = 381 Ω), indicating that electrons were more easily transferred at the PDIL/BDD electrode interface. CV showed that the two reduction reactions C1 and C2 of TCBQ occurred on the PDIL/BDD electrode, corresponding to the reduction of TCBQ to TCBQH₂ and the reduction of bis-quinhydrone imidazole π-π type CT to TCBQH₂, respectively. By studying the SWV responses of TCBQ in the concentration range of 1–100 ng/L on the PDIL/BDD electrode, it was found that the reduction peak current (I _pa) had a good linear relationship with the concentration.

Therefore, the electrochemical SWV technique can be used to detect the concentration of trace TCBQ in water and is expected to be used to detect other HBQs in drinking water and swimming pool water. The electropolymerization conditions need to be optimized to improve the detection performance of the electrode. Despite this, the method reported in this paper is more convenient, faster, and cheaper than other conventional technologies.

Funding

The study was financially supported by Science and Technology Innovation Project of colleges and universities in Shanxi Province, China, project no. 2022L512.

CRediT authorship contribution statement

Yanni Guo: Conceptualization, Methodology, Writing – original draft, Supervision, Project administration, Resources. Deliang He: Writing – review & editing, Software.

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.

Data availability

The data that has been used is confidential.