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    Shandong Binzhou Zhiyuan Biotechnology Co.,Ltd

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    Email: info@cydextrin.com

    Add: Boxing Economic Development Zone, Binzhou, Shandong, China

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A Versatile Environmental Impedimetric Sensor For Ultrasensitive Determination Of Persistent Organic Pollutants (POPs) And Highly Toxic Inorganic Ions

1 Introduction

Electrochemical methods, which can offer excellent merits in terms of high sensitivity, easy-use, and low cost, have been widely employed in clinical, industrial, environmental, and agricultural applications.[1] Especially in environmental science, a series of great results have been obtained in the electroanalysis and quantification of toxic micropollutants,[2] which was achieved mainly based on the redox reactions of target analytes. However, as for such ubiquitous persistent organic ­pollutants (POPs) as polychlorinated biphenyls (PCBs) existing in the environment, their direct electrochemical determination has rarely been reported owing to their chemical inertness and insulating properties,[3] which was recognized to be the limitation in the development of electrochemical techniques. Besides, in the case of such inorganic ions as Cr(VI), As(III), and As(V), their electrochemical determination was strongly dependent on the using of noble metal nanomaterials and their structures and morphologies of (e.g., gold, silver, and platinum). Gold nanofilm, Au(111)-like Au electrode, 3D gold nanodendrite network, silver nanoparticles, polymer film platinum nanocomposite, and highly ordered platinum-nanotube array electrode were therefore designed and fabricated in a complicated way aiming at a more sensitive electroanalysis of such inorganic ions.[4] Meanwhile, strongly acidic media (e.g., HCl, H2SO4, HClO4, and HNO3) were mainly employed as electrolytes,[4e],[5] which could cause the problems of hydrogen evolution and ­undesirable corrosion. Considering the severe adverse toxic effects of persistent toxic substances (PTS) including POPs and highly toxic inorganic ions in environment and human health, such as genotoxicity, tumor promotion, lung cancer, skin allergy, and arsenicosis,[6] it is critically challenging and necessary to explore the novel and simple electrochemical method on their determination.

Alternatively, electrochemical impedance spectroscopy (EIS) is currently attracting a great deal of attention, which is a powerful and efficient tool for analyzing the electrode-solution interface and sensitively detecting that change in complex electrical resistance.[7] Recently, EIS technique has been developed for studying the fundamental and applied electrochemistry and materials science, including characterization of materials,[8] biosensors in sensing of bacterial, DNA or protein,[9] corrosion science,[10] fuel cell, and batteries.[11] Despite these abroad applications, only a few studies have reported on the electrochemical detection of POPs at μm level by our group,[12] which could not satisfy the demand of environmental science. In combination with the excellent properties of microelectrode and nanoelectrode, we expect that the ultrasensitive analysis of POPs and highly toxic inorganic ions could be achieved based on EIS technique. To the best of our knowledge, such interesting and meaningful work has not been developed.

Herein, we demonstrate an impedimetric sensor for PTS determination by introducing mercapto-β-cyclodextrin (β-CD) self-assembly monolayers (SAMs) onto 2 mm, 25 μm, and 400 nm diameter gold electrodes. Cyclodextrins (CDs) explored as molecular hosts are capable of including small hydrophobic molecules inside their cavities in aqueous media.[13] The sensing mechanism is based on the formation of guest–host complexes or the hydrogen bonding between Cr(VI), As(III), and As(V), and β-CD under neutral conditions (pH 7.4), as well as the formation or dissociation of Mm(OH)n−β-CD complexes (M: Cu(II), Zn(II), Cd(II), Pb(II), and Mn(II)) through a reversible pH-stimulation. The complexes cause the specific inhibition of electron transfer of the redox probe, Fe(CN)63−/4−, in the sensing interface, leading a change in electron-transfer resistance before and after sampling. The results suggest that such a simple and smart platform with mercapto-β-CD modified gold microelectrode is expected to be useful in the impedance analysis of PTS (including POPs and highly toxic inorganic ions) with ultrasensitivity.

2 Results and Discussion

Figure 1 briefly illustrates our strategy, in which an impedimetric sensor was fabricated for the determination of POPs that generally based on electron-transfer-blockage. Herein mercapto-β-cyclodextrin (β-CD) monolayer was self-assembled via Au–S bond onto macro, micro, and nanogold electrodes (that is, 2 mm, 25 μm, and 400 nm diameter, Figure 1a). Cyclodextrin is characterized as a conical cylinder with a hydrophobic inner cavity and a hydrophilic exterior.[14] As depicted in Figure1b, a PCB-77 molecule, as a representative in POPs, with appropriate size can serve as a guest molecule and be captured into the internal cavity of β-CDs based on the hydrophobic interactions, thus forming stable host–guest inclusion complexes.[13c],[15] Because of the insulating property of PCB-77, an electrical barrier was constructed on the surface of electrode, which further increased the electron-transfer-blockage in this system. Changes in the electron-transfer resistance before and after sampling gave a quantitative amount of POPs. The more increase in electron-transfer resistance occurred, the more amount of POPs would be given, which can be used for detection and quantification of POPs.

Figure 1.

 

a) Idealized view of mercapto-β-CD-decorated macro, micro, and nanogold electrodes before and after their interaction with POPs. Changes in the electron-transfer resistance before and after sampling, expressed as a relative increase, and gave a quantitative amount of POPs. The more increase in electron-transfer resistance occurred, the more amount of POPs would be given. b) How β-CDs interact with guest molecules (POPs) when they are being sensed in a solution.

 

We first characterized the different dimension of working electrodes (Figure S1, Supporting Information). Due to the different dimension of working gold electrodes, different cyclic voltammograms (CVs) and Nyquist diagrams of EIS were presented. As the electrode dimension reduced, the absolute current decreased while the corresponding resistance increased, which was attributed to the differential mass diffusion profiles.[16] After modification with mercapto β-CD, it can be observed that the electron-transfer resistance (Ret,mod, semicircle domain in EIS) increased by different degrees in comparison with the 2 mm, 25 μm, and 400 nm diameter bare gold electrode, indicating that mercapto-β-CD SAMs had been decorated onto the working electrode surface. Assuming that when the mercapto-β-CD was 100% covered onto the bare electrode the Ret,mod was much higher than the Ret,bare. The Ret in EIS for three-kind of electrodes before and after mercapto-β-CD modification were calibrated (Figure S2, Supporting Information). The electrode surface coverage θ modified with mercapto-β-CD SAMs was calculated as 0.921 ± 0.008, 0.807 ± 0.029, and 0.388 ± 0.039 for 2 mm, 25 μm, and 400 nm diameter gold ­electrodes, respectively.

Subsequently, the efficiency of mercapto-β-CD SAMs modified 2 mm, 25 μm, and 400 nm diameter gold electrodes toward POPs was tested. With immersion into the solutions containing 2 × 10−15 mPCB-77 for 1 h, their impedance behaviors toward ultratrace PCB-77 were presented in Figure 2. No change in the EIS response was observed at mercapto-β-CD 2-mm-diameter electrode before and after interacting with PCB-77 (Figure 2a), revealing the weak resolution of the modified electrode toward PCB-77. With reducing the dimension of the electrode, Ret increased and the resolution toward PCB-77 was more obvious (Figure 2b,c), where the resolution was defined as ΔRet/Ret,modRet = Ret,PCB – Ret,mod). The discernible ΔRet of 1.43 and 885 kΩ, as well as the resolution of ΔRet/Ret,mod ratio of 0.134 and 0.272, toward PCB-77 were observed at mercapto-β-CD modified 25 μm and 400 nm diameter electrode, respectively, which were more apparent than that at the modified 2 mm diameter electrode (ΔRet: 5Ω, ΔRet/Ret,mod: 0.008) (Figure 2d). It was worth pointing out that the modified micro and nanoelectrode possessed the effective capture capacity and high resolution in the analysis of the ultratrace target molecules PCB-77.

Figure 2.

 

Nyquist diagram of EIS responses at mercapto-β-CD modified a) 2 mm, b) 25 μm, and c) 400 nm diameter gold electrodes before and after preconcentration of 2 × 10−15 mPCB-77 for 1 h. d) Comparison of Ret change and its increasing rate (ΔRet/Ret,mod) on three kinds of electrode. Data were collected from panels (a-c).

 

Furthermore, we explored the impedance behaviors of mercapto-β-CD modified 2 mm, 25 μm, and 400 nm dia­meter gold electrodes toward PCB-77 over a concentration range (Figure 3a–c and optimized condition of preconcentration time in Figure S3, Supporting Information). The electron-transfer resistances were found to continually increase at the three kind of mercapto-β-CD modified gold electrode as the PCB-77 concentrations were increased. Interestingly, it should be noted that with the reduction in dimension of the modified electrode, the limit of quantitation was decreased, that is, the determination of PCB-77 can be realized varying from nm to fm level with the electrode down to nanoscale. Figure 3d gives the linear relationship between ΔRet and the logarithmic value of PCB-77 concentration at mercapto-β-CD modified 2 mm, 25 μm, and 400 nm diameter gold electrodes over a different concentration range. The obtained sensitivities (ΔRet/logc) were 2.89 kΩ per log nm, 7.80 kΩ per log pm, and 15.7 MΩ per log pm at modified macroelectrode, microelectrode, nanoelectrode, respectively (Figure 3e). The limits of detection (a signal-to-noise ratio (S/N) of 3) of 4 × 10−12 m, 0.249 × 10−12 m, and 0.209 × 10−15 m were correspondingly achieved. The amazing results suggested that the ultrasensitive determination of PCB-77 can be implemented with the electrochemical impedance technique based on electron-transfer blockage. The reduction in dimension of modified electrode led to the higher sensitivity and lower detection limit, which was attributed to the different diffusion models.[17] Especially, in comparison with the modified 2 mm gold electrode, mercapto-β-CD modified 25 μm, and 400 nm diameter gold electrodes can be applied in the determination of ultratrace PCB-77 at pm and fm level.

Figure 3.

 

Nyquist diagram of EIS responses at mercapto-β-CD modified a) 2 mm, b) 25 μm, and c) 400 nm diameter gold electrode with different concentrations of PCB-77 in PBS saline solution containing 0.1 m KCl and 5 × 10−3 m Fe(CN)63-/4−, respectively. d) Calibration plots of ΔRet against the logarithmic value of PCB-77 in different concentration ranges. Data were collected from panels (a–c). e) Sensitivity (ΔRet/logc) comparison of mercapto-β-CD modified 2 mm, 25 μm, and 400 nm diameter gold electrodes toward PCB-77. f) Detection of nine POP molecules at mercapto-β-CD modified 25 μm diameter gold electrode. Data were collected from corresponding electrochemical impedance spectra (Figure S4, Supporting Information).

In the case of the modified 400-nm-diameter gold electrode, although ultrahigh resolution toward PCB-77 can be obtained, it inevitably suffered from the complicated fabrication procedures and sophisticated instrument. Furthermore, referring to the self-assemble process, the surface coverage θ of modified nanoelectrode (0.388 ± 0.039) was far less than that modified microelectrode (0.807 ± 0.029). Remarkably, the modified microelectrode fabricated in a simple way was good enough to develop in analysis of ultratrace PCB-77 with high sensitivity. Next, we evaluate the efficiency of mercapto-β-CD modified 25 μm diameter gold electrode in the determination of POPs, nine POPs with different sizes, namely, PCB-29, PCB-77, ­PCB-101, PCB-153, PCB-187, lindane, pentachlorobenzene (PeCB), hexachlorobenzene (HCB), and coronene, were further investigated in a phosphate-buffered saline (PBS) solution (pH 7.4). EIS responses and corresponding calibration plots toward the target molecules were shown in Figure S4, Supporting Information. Figure 3f presents a comparison of sensitivities for individual analysis of these target molecules. Clearly, there were the similar sensitivities toward the four PCBs, which were slightly lower than that of chlorobenzenes (PeCB and HCB) and lindane. Moreover, in comparison with previous reports, this platform realized the detection of PCBs and chlorobenzenes with the highest sensitivity and lowest limit of detecition (LOD).[18] However, in the analysis of coronene that was a relatively larger molecule, no obvious change in Ret as well as a negligible sensitivity were observed even with the addition of coronene up to 20 000 × 10−12 m. We suggest that the different impedimetric sensing may be attributed to the size matching effect between host and gust molecules.[19]

A scientific understanding on the different impedimetric sensing was further demonstrated. As known, β-CD possesses the special properties and unique structure of a rather rigid hydrophobic cylinder with cavity depth about 7.9 Å and diameter about 6.0–6.5 Å (Figure 1a),[13a] which was quite suitable to serve as molecular receptor and tended to interact with some guest molecules. Generally, the principle of size matching was one of important factors in the formation of host–guest inclusion complexes,[20] which was of benefit for proceeding with the examination of our results. In combination with DFT method, the 1D parameters of the target molecules were calculated andFigure 4 illustrates the corresponding information in detail. Two of 1D sizes of PCB-29 (4.3 Å, 5.5 Å), PCB-77 (4.9 Å, 4.9 Å), PCB-101 (5.5 Å, 5.5 Å), PCB-153 (5.5 Å, 5.5 Å), and PCB-187 (5.5 Å, 5.5 Å) were smaller than the diameter of β-CD. Therefore, these five guest molecules can easily enter the cavity of β-CD to form inclusion complexes, which made it possible to block the electron transfer and accordingly increase the electrochemical resistance, Ret. In the case of target molecules with planar structure, dimensional parameters of PeCB (5.5 Å, 6.3 Å) and HCB (5.4 Å, 6.3 Å) and lindane (5.6 Å, 5.7 Å) were all smaller than that of β-CD, which increased the possibility of target molecules to enter into the cavity of β-CD. Relatively better sensitivities were achieved, especially toward lindane. As for coronene, its dimension sizes (9.2 and 9.3 Å) were larger than that of β-CD, leading a difficulty for coronene to enter into the cavity of β-CD. Thus the electron-transfer resistance, Ret, was almost similar before and after preconcentration of coronene over a concentration range (Figure S4, Supporting Information). Based on the results, it was clear that the above analysis was in good agreement with our experimental data. Notably, herein β-CD strictly performed the rule of size matching and possessed the selectivity to form the host–guest inclusion complexes during the molecular process.[13c],[19a] Depending on the size matching effect between β-CD and POPs, it may be difficult to discriminate the POPs molecules from another in this class because of the similar sizes. Thus, the impedimetric sensor was suggested to be capable to quantify total concentrations of all PCBs with this size range.[19a],[21]

Figure 4.

 

Molecular models and calculated dimensions of POPs predicted by DFT method using Gaussian 03 software package.

Since the impedimetric sensing of POPs was developed, we found that the sensing performance toward Cr(VI), As(III), and As(V) can be realized in different concentration ranges. Taking Cr(VI) sensing as an example, a sensitivity of 4934.1 kΩ per log nm over concentration range 2 × 10−12 m to 32 × 10−12 m was obtained on mercapto-β-CD modified nanoelectrode. Whereas the response range of microelectrode and macroelectrode was in nm and μm, respectively. The sensitivities on micro and nanoelectrode were almost 300 times and 8000 times higher than that on modified macroelectrode, respectively (Figure 5a). Based on the same consideration, a mercapto-β-CD modified 25 μm diameter gold was selected for the investigation of the representative inorganic ions (e.g., As(III), As(V), Cu(II), Zn(II), Hg(II), Cd(II), Pb(II), and Mn(II)) (see Figure S5, Supporting Information for EIS data). Figure 5b gives a comparison of the sensitivities of these target ions at mercapto-β-CD modified 25 μm diameter gold electrode, which was derived from three independent experiments. As depicted, the sensitivities for Cr(VI), As(III), As(V) were much higher than that of Cu(II), Zn(II), Hg(II), Cd(II), Pb(II), and Mn(II), which was recognized as the remarkable interaction between Cr(VI), As(III), As(V), and β-CD.[22] The impedimetric sensing performance toward As(III) (sensitivity: 7.24 kΩ per log nm; LOD: 0.26 × 10−9 m) and Cr(VI) (sensitivity: 17.5 kΩ per log nM; LOD: 0.24 × 10−9 m) under mild and neutral conditions was much better than those based on electrochemical redox reaction under strong acidic or harsh conditions (Tables S1 and S2, Supporting Information). Inset in Figure 5b illustrates the interaction between CrO42− and β-CD via hydrogen bonding. It has been reported that different species of these high valence ions were existed in neutral solution: CrO42− for Cr(VI), As(OH)3 for As(III) and HAsO32−, and AsO43− for As(V).[13c],[23] The visual molecular models and calculated dimensions of Cr(VI), As(III), and As(V) in neutral solution are shown in Figure 5c. The presence of Cr(VI), As(III), and As(V) in the cavity of β-CD would hinder the electrochemical process of redox probe, Fe(CN)63−/4−, thus leading to the increase in the electron-transfer resistance, Ret. Besides, no obvious inference can be observed in the presence of such high-valence inorganic ions as Fe(III), Al(III), and Co(III) because of the occurrence of precipitation under pH 7.4 (pKsp for Fe(OH)3 Al(OH)3, Co(OH)3: 37.40, 32.34, 43.80).[24]

Figure 5.

 

a) Calibration plots of ΔRet against the logarithmic value of Cr(VI) concentrations on mercapto-β-CD modified 2 mm, 25 μm, and 400 nm diameter gold electrode. Data were collected from electrochemical impendence spectra. b) Comparison of sensitivity of Cr(VI), As(III), and As(V) in neutral solution on mercapto-β-CD modified 25 μm diameter gold electrode. Data were collected from corresponding EIS responses (Figure 5a and Figure S5, Supporting Information). Inset: the interaction between β-CD and CrO42− through H-bond. c) Molecular models of Cr(VI), As(III), and As(V) in neutral solution and calculated dimensions. The structures were predicted by DFT method using Gaussian 03 software package.

On the basis of the above results, it was recognized that our strategy, the impedimetric sensor can be efficiently developed for the ultrasensitive determination of such highly toxic ions. However, in respect to such bivalent metal ions as Cu(II), Zn(II), Cd(II), Pb(II), and Mn(II), their predominant species mainly existed as M2+ in neutral aqueous solution at low concentration. Because of lack of the appropriate binding site, such free ion, M2+, could not interact with β-CD. Thus even the concentration of Cu(II), Zn(II), Cd(II), Pb(II), and Mn(II) up to 20 × 10−6 

 

 

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Shandong Binzhou Zhiyuan Biotechnology Co.,Ltd
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