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Ultrasensitive electrochemical sensor for simultaneous determination of cadmium and lead ions based on one-step co-electropolymerization strategy
Sensors & Actuators: B. Chemical 284 (2019) 414–420
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Ultrasensitive electrochemical sensor for simultaneous determination of cadmium and lead ions based on one-step co-electropolymerization strategy
T
⁎
Yishan Fanga,b, , Bo Cuia,b, Jianzhi Huangc, Lishi Wangc a
State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353, China School of Food Science and Engineering, Qilu University of Technology Technology, Shandong Academy of Sciences, Jinan, 250353, China c School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510641, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemical sensor Polyfurfural Nanoporous silica Ultrasensitive Cadmium Lead ions
A convenient and novel electrochemical strategy for simultaneous detection of cadmium ion (Cd2+) and lead ion (Pb2+) was established in this work. Well identifiable stripping peaks for Cd2+ and Pb2+ were achieved based on a three-dimensional nanoporous silica incorporated with polyfurfural modified glassy carbon electrode (GCE) by one-step co-electropolymerization. The polyfurfural was merged into nanoporous silica in order to enhance the sensor performance through improving the electrode surface area, supplying more binding sites for coordinating with metal ions. The prepared sensor exhibits more excellent electroactive surface area and excellent analytical properties than those just polyfurfural film modified GCE or just nanoporous silica on GCE. Comparing the traditional method, the proposed strategy displays high stability, reliability, favorable sensitivity, and outstanding selectivity for the simultaneous detecting Cd2+ and Pb2+. Impressively, this proposed strategy possessed a wide linear range of 1.5 ng/L to 6200 μg/L and a low limit of detection (LOD) of 0.9 ng/L (S/N = 3) for Cd2+, while linear response range with 0.05 ng/L to 5200 μg/L and LOD of 0.02 ng/L (S/N = 3) for Pb2+. In particular, the fabricated sensor had splendid analytical performance for detection of target ions in potable water, and satisfactory results have been obtained in practice.
1. Introduction Nowadays, heavy metals have been considered as one of the most dangerous water pollutants, which had represented a rising environmental problem and had caused high attention by several international organizations. Heavy metal ions are not biodegradable, highly destructive for nature, and injurious to human health [1–4]. Thus, determining the presence of the heavy metal ions in natural and drinking water has attracted widespread consideration and is of paramount importance. Cd2+ and Pb2+ are the most serious risk factors in heavy metal ions. For instance, the virulence of Cd2+ is mainly caused by affecting normal gene expression, inducing oxidative stress, inhibiting DNA damage repair, and other factors [5–7]. While Pb2+ does harm to many organs and physiological functions of human bodies. It can cause nervous system damage by peripheral neuritis, resulting in motor and abnormal sensation [8–11]. In addition, within the rapid industrialization, Cd2+ and Pb2+ are generally widespread and commonly found in groundwater and soil [6,12]. They have often been
coexistence in various environmental samples, and are potentially dangerous to aquatic organisms as well as human health through the process of bioaccumulation and the food chain [13–15]. Therefore, the development of accurate quantitative analysis methods for sensitive and selective monitoring Cd2+ and Pb2+ are very necessary and challenging. High sensitivity and latest technologies such as inductively coupled plasma mass spectrometry (ICP-MS), ultraviolet absorption spectrum (UV), atomic absorption spectrometry (AAS) are widely used in research institutions for heavy metal ions detection. However, they are not suitable for point-of-care detection because of their bulky and expensive instruments and complicated sample preparation processes [16–19]. At present, some new-type techniques for Cd2+ and Pb2+ detection based on oligonucleotide, optical methods, electrochemistry and colorimetry have been developed [20–28]. Among these methods, electrochemical technology is outstanding in various fields for its low cost, high sensitivity, good reliability, and high selectivity [12,29–31]. Nanomaterial-modified electrodes are the most popular strategies to enhance the detection sensitivity as sensor platforms. High performance
⁎ Corresponding author at: State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353, China. E-mail address: [email protected] (Y. Fang).
https://doi.org/10.1016/j.snb.2018.12.148 Received 19 October 2018; Received in revised form 20 December 2018; Accepted 28 December 2018 Available online 29 December 2018 0925-4005/ © 2018 Published by Elsevier B.V.
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2. Experimental
of electrochemical sensors were developed for synchronous monitoring metal ions based on various nanomaterial, such as carbon nanotubes (CNTs) [32,33], gold nanoparticles [34], bismuth nanoparticle [35–37], silica nanoparticles [38], quantum dots [39,40], graphene oxide (GO) [32,41], and so on. For example, based on carbon quantum dots/molybdenum sulfide/magnetic ferrite nanohybrid composites with large amounts of amino and oxygen-containing groups on the surface, Wang et al provided efficient opportunities for efficient Pb2+ removal [40]. You et al also constructed an electrochemical strategy for simultaneous analysis of Cd2+ and Pb2+ based on GO and carbon quantum dots, owing to the lots of oxygen contained functional groups on the surface of nanocomposites [39]. By means of plating bismuth film in situ on reduced GO and CNTs composites, Xuan et al. fabricated a sensor for cadmium and lead ions detection [42]. Inspired by the binding capacity of organic nanomaterials to Cd2+ and Pb2+, a sensitive electrochemical method was developed by combination of PDMcT and multi-walled CNTs for detecting Cd2+ and Pb2+, which limit detection was low to 0.01 μg/L [43]. However, most of these methods are complex in preparation, poor in reproducibility and sensitivity. Therefore, it is desirable to establish new strategies to overcome these limitations [13,43]. Compared with nanomaterial modified electrodes, polymer modified electrodes have also attracted wide attention in recent years due to their good stability, multi active sites, good electrocatalytic activity, uniformity of electrochemical deposition and strong adhesion to the surface of the electrode [44–48], including polyaniline, poly(amidoamine), poly (naphthol green B), poly(ophenylenediamine), polytyramine, thiadiazole, polypyrrole, et al [49–54]. As a few recent reports [55,56], polyfurfural modified electrodes has good adherence and electrical conductivity, especially excellent selectivity. Noteworthy, polyfurfural with abundant functional groups on the surface, can provide extremely abundant binding sites for coordination with heavy metal ions [39,40,57,58]. Nevertheless, to our best knowledge, there are few reports about electrochemical sensors based on polyfurfural for simultaneous analysis of heavy metal ions. Recently, our group has successfully constructed a polyfurfural film/MWCNTs/GCE sensor for simultaneous determination of heavy metal ions, which were demonstrated that polyfurfural can act as an excellent coordination substrates to efficiently capture heavy metals through strong coordination bonds between the ionic unoccupied orbitals and the conjugated π-electrons of the polyfurfural [56]. Herein, we prepare three dimensional nanoporous polyfurfural/silica nanocomposites (3D-PF/SA) via one-step coelectropolymerization, which is different from that of our previous work and much simpler in preparation and remarkably more sensitive in detection for heavy metal ions. Silica nanomaterials (SiO2), the most commonly used nanomaterial, has also been made extensive use in electrochemical sensors for its monodispersed size, high surface area, low-temperature encapsulation, and good biocompatibility. Especially, its numerous active hydroxyl group (−OH) group on its surfaces make it easy to be functionalized and offer a promising candidate to be applied in heavy metal removal and detection [57,59,60]. Thus, co-electricpolymer of three-dimensional nanoporous polyfurfural-silica on glassy carbon electrode was carried out in this paper, which was used as an immobilization platform to distinguish and coordinating with metal ions, and cadmium and lead ions were used as model heavy metal ions to investigate the analytical performance. This proposed sensor exhibited excellent analytical performance, showed wide linear range, high selectivity, excellent stability, and good sensitivity. Furthermore, the established sensor was performed using drinking water as actual samples to expound and demonstrate the feasibility of its applicability, and satisfactory testing recoveries were received. It represented great potential of the established strategy for determination of trace Cd2+ and Pb2+ or other heavy metal ions in environmental applications.
2.1. Chemicals and reagents Cadmium acetate, lead acetate, and sodium hexafluorosilicate were purchased from J&K Chemical (Beijing, China). Furfural (PF, ≥99%), sodium perchlorate were obtained from National drug chemical reagents CO., Ltd. Potassium ferricyanide, potassium ferricyanide (ACS reagent), acetic acid sodium (NaAc) and acetic acid (HAc) were obtained from Sigma Aldrich. 0.1 M acetate buffer solution (ABS) with various pH was prepared by mixing NaAc and HAc solution. 0.1 M phosphate buffer solution (PBS, pH7.0) was prepared by mixing K2HPO4 and KH2PO4 solution. All other reagents were of analytical grade. 2.2. Apparatus SEM with Energy Dispersive X-Ray Spectroscopy (EDS) measurements was carried out on Hitachi Regulus 8220 electron microscope (Japan) for the morphology characterizations. Raman spectroscopy (inVia, U.K) was obtained with various vibrational modes. Electrochemical measurements were performed on electrochemical workstation (CHI 660E, Shanghai, China), including anodic stripping voltammetry (ASV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and differential pulse voltammetry (DPV). 2.3. Fabrication of the electrochemical sensor Prior to modification, bare GCE was successively polished with 0.3 and 0.05 um Al2O3 powder, and washed with absolute alcohol and doubly distilled water for 5 min of ultrasound. According to the previous reports with some modifications [55,61], co-electropolymerization on the cleaned GCE was carried out with CV scans ranging from -0.9 to 2.7 V at 100 mV/s for 10 cycles in a stirred acetonitrile aqueous solution (50%), containing 0.1 M sodium hexafluorosilicate, 0.06 M sodium perchlorate, and 0.05 M furfural. Then the modified electrode was washed to eliminate any physically adsorbed materials. For comparison, the polyfurfural/GCE (PF/GCE) was also prepared through the similar route. The preparation of stepwise procedure is showed in Scheme 1. 2.4. Electrochemical measurements procedure To analyze heavy metal ions, ASV was carried out with a common
Scheme 1. Schematic illustration for the fabrication of the sensor. 415
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the silica matrix in situ, which would offer more abundant oxygencontaining functional groups for heavy metal ions chelating coordination. Elemental compositions of PF and 3D-PF/SA on GCE were examined by EDS. In Fig. 1C, C and Au elementals for PF and 3D-PF/SA can be clearly found, and the presence of Au was due to the treatment by spraygold with all samples. Noticeably, O and Si elements were detected in 3D-PF/SA, caused by the silica in 3D-PF/SA. This result further affirmed the successful decoration of 3D-PF/SA on the GCE, which was identical with the SEM analysis. Raman absorption spectroscopy was used for further testifying the successful preparation of 3D-PF/SA/GCE in Fig. 1D, which shows the Raman spectra of PF and 3D-PF/SA. The Raman spectrum of furfural at 1369 cm−1 displays a HeCeC/O bending mode [62], while C]C stretching modes at 1575 cm−1 [62,63] was found for PF and 3D-PF/ SA. Furthermore, significantly stronger signal of furfural was present in the 3D-PF/SA than that in PF, which may result from the large specific surface area of porous silica nanomaterials. For 3D-PF/SA, a slightly broad band at about 455 cm−1 was shown corresponds to the symmetrical Si-O-Si bending mode, while the narrower bands situated around 495 and 605 cm−1, known as SiO2 network, are characteristic for the symmetric stretching modes of Si-O bonds [64–66]. Meanwhile, the signal around 976 cm−1 is attributed to Si-(OH) groups [64–66]. All these results further proved the successful preparation of 3D-PF/SA modified electrode.
three-electrode cell. Firstly, the modified electrode was immersed in sodium acetate buffer solution consisting of different concentrations of cadmium acetate and lead acetate, and preconcentration was performed at -1.2 V for 350 s. After stabilization for 15 s, the ASV was recorded at the potential range of -1.2 V to -0.45 V, and the electrode was cleaned at -0.2 V with 90 s to carry out the next measurement. EIS was obtained with the frequency ranging from 0.01 to 100 kHz in 0.1 M potassium chloride solution with 5 mM K4[Fe(CN)6]/K3[Fe(CN)6]. 2.5. Treatment of water sample The samples were from our domestic water. They were firstly purified through centrifugation with 8000 rpm for 20 min. Then, the supernatant was filtered with 0.45 μm membrane. Lastly, the samples were appropriate diluted by using the ABS solution, and then used for detection by the provided 3D-PF/SA/GCE using a standard addition method. 3. Results and discussion 3.1. Characterization of 3D-PF/SA/GCE, PF /GCE In Fig. 1, the as-prepared PF/GCE, 3D-PF/SA/GCE were characterized by SEM. The SEM image of PF/GCE clearly reveals a turquoise and uniform film for PF/GCE (Fig. 1A), which is similar to the previous reports [55,56,61]. When electrodeposition duration continues to 10 cycles through cyclic voltammetry in sodium hexafluorosilicate and furfural solution, much smaller cavities interconnected by silica incorporated polyfurfural covered all over the surface of GCE (Fig. 1B). This may attributable to the viscous force of hydrogen and oxygen bubbles with silica during the electrodeposition, which limits the deposition of silica along the gas/liquid interface and ultimately leads to three-dimensional nanoporous structures. Such structure can efficient provide larger specific surface area for more polyfurfural entrapped in
3.2. CV and EIS characterization Fig. 2A represents CVs with different electrodes in 0.1 M pH 7.0 PBS. As seen from Fig. 2A, there are no peaks appearing at bare GCE, PF/GCE, and 3D-PF/SA/GCE (curve a, b, c). However, larger background current was obtained at the PF/GCE (curve b), while the 3D-PF/ SA/GCE had the largest among them (curve c). This suggests that the 3D-PF/SA/GCE has higher surface area than PF/GCE and bare GCE, it
Fig. 1. SEM images of the PF (A), and 3D-PF/SA on GCE (B). EDS of the PF and 3D-PF/SA on GCE (C). Raman spectra of the PF and 3D-PF/SA hybrid on GCE (D). 416
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Fig. 2. (A) CV of (a) bare GCE, (b) PF/GCE, (c) 3D-PF/SA/GCE in pH 7.0 PBS with 0.1 M potassium chloride at 50 mV/s.; (B) EIS of (a) bare GCE, (b) PF/GCE, (c) 3DPF/SA/GCE in 0.1 M potassium chloride with 5 mM K4[Fe(CN)6]/K3[Fe(CN)6]; (C) DPV of (a) bare GCE, (b) SA/GCE, (c) PF/GCE, (d) 3D-PF/SA/GCE in pH 5.0 ABS solution with 50 ng/mL Cd2+ and Pb2+ at 50 mV/s.
of electrochemical sensing platform. In order to estimate the effect of pH value on Cd2+ and Pb2+ detetion in 0.2 M ABS, the sensors were measured in a variety of solutions within pH from 3.0 to 8.0. As shown in Fig. 3B, promptly increase of current signals can be noticed from 3.0 to 5.0, and then had a decrease down to pH 8.0. This phenomenon may result from the complexation mechanism and electrostatic interaction by the positive charge of heavy metal ions and the numerous oxygen groups of 3D-PF/SA [39,40]. Whereas, the decrease of current responds at higher pH value may relate with the participation of the heavy metal ions in hydrolysis reaction. Thus, pH 5.0 was chose as the optimal pH value to detect Cd2+ and Pb2+. Deposition potential is a significant factor that would greatly influence the sensitivity of Cd2+ and Pb2+ analysis. Studies were performed using 3D-PF/SA as detection platform at the potential range from -0.8 to -1.5 V (Fig. 3C), the measurement was recorded in 0.1 M pH 5.0 ABS and the deposition time is 350 s. When moving from -0.8 to -1.2 V, the response current of the stripping peak of Pb2+ or Cd2+ increases gradually and the highest current signal is received at -1.2 V. Subsequently, the stripping currents for metal ions were greatly dropped as the deposition potential becomes more negative, the reason for that is the evolution of hydrogen is occurred on the electrode surface that affect the active surface area, resulting in a decrease in the amount of Cd2+ and Pb2+ on 3D-PF/SA/GCE [32,39]. Therefore, -1.2 V was chosen as the best deposition potential for next experiments. The impact of the preconcentration time on the current signal of the proposed strategy was investigated from 50 s to 500 s in 0.1 M pH 5.0 ABS with 50 ug/L Pb2+ or Cd2+ at -1.2 V. The stripping peak currents increased upon the increasing preconcentration time from 50 s to 350 s, due to the significantly enhanced metal ions reduced on the surface of 3D-PF/SA/GCE. Concretely, these results further verifies that the 3DPF/SA nanocomposite has a great accumulating ability for the reduced metal ions because of its large active surface area. However, longer incubation time did not improve the response, which shows the electrode surface has reached saturate tendency. Then 350 s of preconcentration time was chosen as a compromise between a suitable time and a high signal for the experiments. The 3D nanoporous silica entrapped polyfurfural modified glassy carbon electrode (GCE) was carried out by the one-step co-electropolymerization with cyclic voltammograms. In the electropolymerization process, an oxidative peak was appeared at 1.9–2.3 V, and the peak current rose steadily with each scan from one to ten circles (Fig. 3D). This process is considered to be related to the oxidation and polymerization of furfural. Compared with just electropolymerization of polyfurfural film with 5 circles at its strongest signal, 10 circles were performed in this study, this may be owing to their large surface area of nanoporous silica on the electrode surface.
also verifies the successful construction of the 3D-PF/SA/GCE. Fig. 2B displays the EIS curves of various modified electrodes in 0.1 M potassium chloride solution with 5 mM K4[Fe(CN)6]/K3[Fe(CN)6]. The resistance with the modification of polyfurfural film obviously increased comparison with that obtained on bare GCE (curve a, b), and the resistance dramatically increased on 3D-PF/SA/GCE (curve c), which has been used to further confirm that 3D-PF/SA were successfully co-electrodeposited on the electrode surface. Fig. 2C represents the DPV curves of different electrodes in 0.1 M pH 5.0 ABS with 50 ug/L of Cd2+ and Pb2+. Two weak stripping current signals for Cd2+ and Pb2+ were observed for the bare GCE (curve a), while two slightly larger peak currents were appeared on the porous silica nanomaterials electrodeposited GCE (curve b). When polyfurfural film was doped onto the bare GCE surface, it resulted in two increasing peak currents for Cd2+ and Pb2+ (curve c), which suggests that the polyfurfural has electrocatalytic activity toward cadmium and lead ions. After co-electrodeposition of 3D-PF/SA on the bare electrode, two further significantly current signals emerged for Cd2+ and Pb2+ (curve d) at -0.89 V and -0.64 V, which are approximately 5 times as much as that of PF/GCE, respectively. These suggested that the 3D-PF/SA exhibited larger electroactive surface area and leaded to the signal amplification for the proposed sensor. To research the effective surface area of the 3D-PF/SA/GCE, PF/ GCE and the bare GCE, these electrodes were evaluated in 1 mM K3Fe (CN)6 by CVs at different scan rates. For a reversible process, the activity areas of the electrode surface are obtained through the Randles–Sevcik formula [67,68], it can be calculated to be 0.298 cm2 for 3D-PF/SA/GCE, 0.157 cm2 for PF/GCE, and 0.070 cm2 for GCE, which was about twice as much as that of PF/GCE and 4.2 times of GCE, ascribing to the much small cavities of the modified nanomaterials on the GCE surface and its high surface-to-volume ratio properties. 3.3. Optimization of experimental conditions To achieve the best electrochemical signal for detection of Cd2+and Pb2+, the effect of furfural concentration was studied. With the increase of furfural concentration from 0.01-0.05 M, the peak responses of Cd2+and Pb2+ increased gradually, and then decreased. Higher concentration of furfural would reduce the electrocatalytic surface area of 3D-PF/SA hybrid because of the large aggregation. Meanwhile, the ratio of furfural to sodium hexafluorosilicate is also crucial for the effective adsorption and desorption of Cd2+and Pb2+ on the 3D-PF/SA hybrid surface. As shown in Fig. 3A, various molar ratios of furfural and sodium hexafluorosilicate including 2:1, 1:1, 1:2, 1:4, 1:5, and 1:6 (M/ M) were used for the preparation of 3D-PF/SA hybrid, and the maximum value of peak currents was obtained at 1:2. Higher or lower concentration of sodium hexafluorosilicate as matrix would lead to the poor electrocatalytic performance of the assembled 3D-PF/SA hybrid and reduce the complexation towards metal ions. Thus, 0.05 M furfural and 0.1 M sodium hexafluorosilicate were chosen for the construction 417
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Fig. 3. Effect of (A) the mole ratios of furfural/sodium hexafluorosilicate, (B) pH value of buffer solution, (C) deposition potential, and (D) scanning circles on the stripping peak currents of Cd2+and Pb2+.
3.4. Simultaneous detection of Cd2+ and Pb2+
high resolution for selective detection of Cd2+ and Pb2+ simultaneously. In Fig. 4B, the excellent corresponding linear responses were acquired over the Cd2+ and Pb2+. For analysis of Cd2+, the linear equation was I(μA) = -0.01381c (μg/L) - 0.7813 (n = 16, R2 = 0.999) from 1.5 ng/L to 6200 μg/L, with the detection limit of 0.9 ng/L. For analysis of Pb2+, the linear equation was I(μA) = -0.009962C (μg/L) 0.3425 (n = 16, R2 = 0.995) in the range of 0.05 ng/L to 5200 μg/L, which detection limit was down to 0.02 ng/L (Fig. 4B). Compared with the previous studies for simultaneous detection of Cd2+ and Pb2+ in Table 1, the as-proposed sensor had exhibited simpler operation, better stability, wider linear region, and lower detection
To assess the quantitative range for Cd2+ and Pb2+ detection simultaneously at the 3D-PF/SA/GCE, different Cd2+ and Pb2+ concentrations were performed at the optimal conditions. The response of the proposed sensor was recorded in 0.1 M ABS (pH 5) by applying an accumulation potential at −1.20 V for 350 s. Fig. 4A displays that the current responds had significantly enhanced within the increasing concentrations of Cd2+ and Pb2+, where two individual peaks were recorded at around -0.89 V for Cd2+ and -0.64 V for Pb2+, respectively. There is about 250 mV between the peaks separation, which reveals a
Fig. 4. (A) ASV for Cd2+and Pb2+simultaneous detection (from a to p: 1 × 10−6, 1 × 10−5, 1 × 10−4, 1 × 10−3,1 × 10−2, 0.1, 0.25, 0.4, 0.5, 1, 2, 3.5, 5, 6.2 μg/ mL for Cd2+; 1 × 10−7, 1 × 10−6, 1 × 10−5, 1 × 10−4, 1 × 10−3,1 × 10−2, 0.1, 0.25, 0.4, 0.5, 1, 2, 3.5, 5 μg/mL for Pb2+). (B) Calibration curves of the sensor for Cd2+and Pb2+simultaneous detection. 418
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Table 1 Comparison of analytical performance of the proposed method with other methods for the simultaneous determination of Cd2+and Pb2+. Method
Linear range (μg/L)
Antimony film carbon paste electrode Polysulfoamino anthraquinone solid ionophore Bismuth nanoparticle-porous carbon paste electrodes Electrochemical method with active pharmaceutical ingredient Bismuth nanoparticles integration as modifiers of screen-printed carbon electrodes Microfabricated on-chip planar bismuth electrode Antimony film electrode Microfabricated three-electrode on-chip device with polydimethylsiloxane cover Bismuth/glassy carbon composite electrode Disposable multiwalled carbon nanotubes modified screen-printed electrode Reduced graphene oxide modified disposable bismuth film electrode Micropatterned reduced graphene oxide with carbonnanotube and bismuth composite electrodes N-doped carbon quantum dots-graphene oxide hybrid Polyfurfural film/multi-walled carbon nanotube modified electrode One-step co-electropolymerization Of 3D nanoporous silica incorporated with polyfurfural on glassy carbon electrode
Detection limit (μg/L)
Reference
4.0-150.0 (Cd, Pd) 0.5-30 (Cd, Pd) 1-100 (Cd, Pd) 50-250 (Cd); 5-200 (Pb) — 0-280 (Cd); 25-400 (Pb) 20-140 10-80 (Cd, Pd) 20-100 (Cd,Pd) 0.5-80(Cd); 0.05-100(Pb) 1-60 (Cd, Pd) 20-200 (Cd, Pd)
0.8 (Cd); 0.2 (Pb) 0.2 (Pb); 0.1 (Cd) 0.65 (Pb); 0.81 Cd) 2 (Cd); 1.9 (Pb) 2 (Pb); 5 (Cd) 9.3 (Cd); 8 (Pb)
[69] [70] [71] [72] [36] [73]
0.7 (Cd); 0.9 (Pb) 0.11 (Cd); 0.25 (Pb) 1.5 (Cd); 2.3 (Pb) 0.1 (Cd); 0.07 (Pb) 0.5 (Cd); 0.8 (Pb) 0.6 (Cd); 0.2 (Pb)
[74] [75] [37] [33] [13] [32]
11.24-11241 (Cd); 20.72-10360 (Pb) 0.5–15 (Cd); 0.1–15 (Pb) 1.5×10−3 -6200 (Cd); 5×10-5 -5200 (Pb)
7.45 (Cd); 1.17 (Pb) 0.03 (Cd); 0.01 (Pb) 9×10−4 (Cd); 2 ×10−5 (Pb)
[39] [56] This work
4.3% for Cd2+ and 4.9% for Pb2+, respectively. Similarly, the average RSDs of the measurements were 3.7% for Cd2+ and 4.1% for Pb2+ in interassay, respectively. This fact displays that the established sensor had good reproducibility.
limit. The reasons for its low detection limit may result from the excellent electrical conductivity and charge transfer ability of 3D-PF/SA with metal ions. Meanwhile, the construction of 3D-PF/SA have larger electroactive surface areas with rich oxygen containing groups, which offer more active sites to effectively adsorb heavy metal ions through electrostatic interaction, leading to the higher sensitivity of the established strategy.
3.6. Determination of Cd2+ and Pb2+ in real water To evaluate the feasibility of the as-prepared strategy for Cd2+ and Pb detection, different concentrations of Cd2+ and Pb2+ solutions were added in real water and determined by the 3D-PF/SA/GCE. The water was obtained from our university drinking water, and the results were showed and summed up in Table 3. Excellent recoveries were in the range of 94–105% and RSD was less than 5% for Cd2+ and Pb2+ detection, which indicates this proposed method provide a promising potential in practical applicability. 2+
3.5. Specificity, stability, and reproducibility of the sensors To further study the influence of substances that may interfere with the signals for Cd2+ and Pb2+ detection, the possible interfere ions such as K+, Na+, Cu2+, Ni2+, Zn2+, Mn2+, Hg2+, Mg2+, Ca2+ and Fe3+ were used in this study (Table 2). In comparison with the current signals acquired from Cd2+ and Pb2+ without interference, it showed minimal difference in current in Cd2+ and Pb2+ solution containing interfering ions (30-fold mass ratio), where the average relative standard deviations (RSD) were less than 7% for both Cd2+and Pb2+ change (Table 2). This result affirmed the good specificity of the asprepared 3D-PF/SA/GCE. Stability is another significant parameter in its application for 3DPF/SA/GCE. When the 3D-PF/SA/GCE was stored at 4 °C for 15 and 30 days, the peak response reduced less than 5% and 9% compared with their initial responses, respectively. Evidently, the proposed sensor has acceptable long-term stability. The reproducibility of the 3D-PF/SA/GCE was studied by interassay and intra- assay coefficients of variation with five replicate measurements. By simultaneous determining Cd2+and Pb2+ at one concentration level with 100 ng/mL, the RSDs of interassay for five sensors were
4. Conclusion In summary, 3D nanoporous silica nanostructure incorporated with polyfurfural modified glassy carbon electrode was firstly applied for the simultaneous detection of Cd2+ and Pb2+ through one-step co-electropolymerization. The presentation of oxygen containing groups offered abundant anchor sites for adsorption of Cd2+ and Pb2+ on the 3DPF/SA surface. Moreover, by combination of the advantages of polyfurfural and silica nanocomposites, 3D-PF/SA provides large effective surface area as well as good electrical conductivity for Cd2+ and Pb2+, displaying excellent electrochemical performance for Cd2+ and Pb2+ detection simultaneously. Compared with other methods, this proposed Table 3 Simultaneous determination of Cd2+ and Pb2+ in a drinking water sample.
Table 2 Interference study of other ions in Cd2+and Pb2+ detection. Interfering ions
K+ Na+ Cu2+ Ni2+ Zn2+ Mn2+ Hg2+ Mg2+ Ca2+ Fe3+
Sample
Added (ng/mL, n = 3)
Found (ng/mL, n = 3)
Recovery (%)
Relative deviation (%)
A1 A2 A3 A1 A2 A3 B1 B2 B3 B1 B2 B3
0 (Cd2+) 10 (Cd2+) 50 (Cd2+) 0 (Pb2+) 10 (Pb2+) 50 (Pb2+) 0 (Cd2+) 10 (Cd2+) 50 (Cd2+) 0 (Pb2+) 10 (Pb2+) 50 (Pb2+)
0 10.25 52.36 0 9.68 51.19 0 9.88 52.01 0 9.55 47.33
– 102.5 104.72 – 96.8 102.38 – 98.8 104.02 – 95.5 94.66
– 3.99 4.68 – 4.36 5.03 – 4.05 3.77 – 4.27 4.81
Relative deviation (%) Cd2+
Pb2+
−0.51 −0.74 −6.5 −4.3 −0.36 2.2 −2.9 −1.5 −2.7 1.7
−0.70 −0.63 −5.3 3.9 0.45 −1.9 −3.3 −1.6 −2.2 2.1
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strategy exhibits simpler preparation, lower detection limit, wider linear range, higher selectivity, excellent reproducibility and stability. Impressively, the established sensor was successfully performed in real water for detection of Cd2+ and Pb2+, and satisfactory testing recoveries were received, exhibiting great potential for determination of various trace toxic metallic ions in practical applications.
[39] [40]
Acknowledgments
[41]
[35] [36] [37] [38]
[42]
We are deeply grateful for the support of National Natural Science Foundation of China (No. 21705084), and the Natural Science Foundation of Shandong Province of China (No. ZR2017BB074), National Training Program of Innovation and Entrepreneurship for Undergraduates (No. 201710431040), and Special Funds for Taishan Scholars Project.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Ultrasensitive electrochemical sensor for simultaneous determination of cadmium and lead ions based on one-step co-electropolymerization strategy
T
⁎
Yishan Fanga,b, , Bo Cuia,b, Jianzhi Huangc, Lishi Wangc a
State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353, China School of Food Science and Engineering, Qilu University of Technology Technology, Shandong Academy of Sciences, Jinan, 250353, China c School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510641, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemical sensor Polyfurfural Nanoporous silica Ultrasensitive Cadmium Lead ions
A convenient and novel electrochemical strategy for simultaneous detection of cadmium ion (Cd2+) and lead ion (Pb2+) was established in this work. Well identifiable stripping peaks for Cd2+ and Pb2+ were achieved based on a three-dimensional nanoporous silica incorporated with polyfurfural modified glassy carbon electrode (GCE) by one-step co-electropolymerization. The polyfurfural was merged into nanoporous silica in order to enhance the sensor performance through improving the electrode surface area, supplying more binding sites for coordinating with metal ions. The prepared sensor exhibits more excellent electroactive surface area and excellent analytical properties than those just polyfurfural film modified GCE or just nanoporous silica on GCE. Comparing the traditional method, the proposed strategy displays high stability, reliability, favorable sensitivity, and outstanding selectivity for the simultaneous detecting Cd2+ and Pb2+. Impressively, this proposed strategy possessed a wide linear range of 1.5 ng/L to 6200 μg/L and a low limit of detection (LOD) of 0.9 ng/L (S/N = 3) for Cd2+, while linear response range with 0.05 ng/L to 5200 μg/L and LOD of 0.02 ng/L (S/N = 3) for Pb2+. In particular, the fabricated sensor had splendid analytical performance for detection of target ions in potable water, and satisfactory results have been obtained in practice.
1. Introduction Nowadays, heavy metals have been considered as one of the most dangerous water pollutants, which had represented a rising environmental problem and had caused high attention by several international organizations. Heavy metal ions are not biodegradable, highly destructive for nature, and injurious to human health [1–4]. Thus, determining the presence of the heavy metal ions in natural and drinking water has attracted widespread consideration and is of paramount importance. Cd2+ and Pb2+ are the most serious risk factors in heavy metal ions. For instance, the virulence of Cd2+ is mainly caused by affecting normal gene expression, inducing oxidative stress, inhibiting DNA damage repair, and other factors [5–7]. While Pb2+ does harm to many organs and physiological functions of human bodies. It can cause nervous system damage by peripheral neuritis, resulting in motor and abnormal sensation [8–11]. In addition, within the rapid industrialization, Cd2+ and Pb2+ are generally widespread and commonly found in groundwater and soil [6,12]. They have often been
coexistence in various environmental samples, and are potentially dangerous to aquatic organisms as well as human health through the process of bioaccumulation and the food chain [13–15]. Therefore, the development of accurate quantitative analysis methods for sensitive and selective monitoring Cd2+ and Pb2+ are very necessary and challenging. High sensitivity and latest technologies such as inductively coupled plasma mass spectrometry (ICP-MS), ultraviolet absorption spectrum (UV), atomic absorption spectrometry (AAS) are widely used in research institutions for heavy metal ions detection. However, they are not suitable for point-of-care detection because of their bulky and expensive instruments and complicated sample preparation processes [16–19]. At present, some new-type techniques for Cd2+ and Pb2+ detection based on oligonucleotide, optical methods, electrochemistry and colorimetry have been developed [20–28]. Among these methods, electrochemical technology is outstanding in various fields for its low cost, high sensitivity, good reliability, and high selectivity [12,29–31]. Nanomaterial-modified electrodes are the most popular strategies to enhance the detection sensitivity as sensor platforms. High performance
⁎ Corresponding author at: State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences, Jinan, 250353, China. E-mail address: [email protected] (Y. Fang).
https://doi.org/10.1016/j.snb.2018.12.148 Received 19 October 2018; Received in revised form 20 December 2018; Accepted 28 December 2018 Available online 29 December 2018 0925-4005/ © 2018 Published by Elsevier B.V.
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2. Experimental
of electrochemical sensors were developed for synchronous monitoring metal ions based on various nanomaterial, such as carbon nanotubes (CNTs) [32,33], gold nanoparticles [34], bismuth nanoparticle [35–37], silica nanoparticles [38], quantum dots [39,40], graphene oxide (GO) [32,41], and so on. For example, based on carbon quantum dots/molybdenum sulfide/magnetic ferrite nanohybrid composites with large amounts of amino and oxygen-containing groups on the surface, Wang et al provided efficient opportunities for efficient Pb2+ removal [40]. You et al also constructed an electrochemical strategy for simultaneous analysis of Cd2+ and Pb2+ based on GO and carbon quantum dots, owing to the lots of oxygen contained functional groups on the surface of nanocomposites [39]. By means of plating bismuth film in situ on reduced GO and CNTs composites, Xuan et al. fabricated a sensor for cadmium and lead ions detection [42]. Inspired by the binding capacity of organic nanomaterials to Cd2+ and Pb2+, a sensitive electrochemical method was developed by combination of PDMcT and multi-walled CNTs for detecting Cd2+ and Pb2+, which limit detection was low to 0.01 μg/L [43]. However, most of these methods are complex in preparation, poor in reproducibility and sensitivity. Therefore, it is desirable to establish new strategies to overcome these limitations [13,43]. Compared with nanomaterial modified electrodes, polymer modified electrodes have also attracted wide attention in recent years due to their good stability, multi active sites, good electrocatalytic activity, uniformity of electrochemical deposition and strong adhesion to the surface of the electrode [44–48], including polyaniline, poly(amidoamine), poly (naphthol green B), poly(ophenylenediamine), polytyramine, thiadiazole, polypyrrole, et al [49–54]. As a few recent reports [55,56], polyfurfural modified electrodes has good adherence and electrical conductivity, especially excellent selectivity. Noteworthy, polyfurfural with abundant functional groups on the surface, can provide extremely abundant binding sites for coordination with heavy metal ions [39,40,57,58]. Nevertheless, to our best knowledge, there are few reports about electrochemical sensors based on polyfurfural for simultaneous analysis of heavy metal ions. Recently, our group has successfully constructed a polyfurfural film/MWCNTs/GCE sensor for simultaneous determination of heavy metal ions, which were demonstrated that polyfurfural can act as an excellent coordination substrates to efficiently capture heavy metals through strong coordination bonds between the ionic unoccupied orbitals and the conjugated π-electrons of the polyfurfural [56]. Herein, we prepare three dimensional nanoporous polyfurfural/silica nanocomposites (3D-PF/SA) via one-step coelectropolymerization, which is different from that of our previous work and much simpler in preparation and remarkably more sensitive in detection for heavy metal ions. Silica nanomaterials (SiO2), the most commonly used nanomaterial, has also been made extensive use in electrochemical sensors for its monodispersed size, high surface area, low-temperature encapsulation, and good biocompatibility. Especially, its numerous active hydroxyl group (−OH) group on its surfaces make it easy to be functionalized and offer a promising candidate to be applied in heavy metal removal and detection [57,59,60]. Thus, co-electricpolymer of three-dimensional nanoporous polyfurfural-silica on glassy carbon electrode was carried out in this paper, which was used as an immobilization platform to distinguish and coordinating with metal ions, and cadmium and lead ions were used as model heavy metal ions to investigate the analytical performance. This proposed sensor exhibited excellent analytical performance, showed wide linear range, high selectivity, excellent stability, and good sensitivity. Furthermore, the established sensor was performed using drinking water as actual samples to expound and demonstrate the feasibility of its applicability, and satisfactory testing recoveries were received. It represented great potential of the established strategy for determination of trace Cd2+ and Pb2+ or other heavy metal ions in environmental applications.
2.1. Chemicals and reagents Cadmium acetate, lead acetate, and sodium hexafluorosilicate were purchased from J&K Chemical (Beijing, China). Furfural (PF, ≥99%), sodium perchlorate were obtained from National drug chemical reagents CO., Ltd. Potassium ferricyanide, potassium ferricyanide (ACS reagent), acetic acid sodium (NaAc) and acetic acid (HAc) were obtained from Sigma Aldrich. 0.1 M acetate buffer solution (ABS) with various pH was prepared by mixing NaAc and HAc solution. 0.1 M phosphate buffer solution (PBS, pH7.0) was prepared by mixing K2HPO4 and KH2PO4 solution. All other reagents were of analytical grade. 2.2. Apparatus SEM with Energy Dispersive X-Ray Spectroscopy (EDS) measurements was carried out on Hitachi Regulus 8220 electron microscope (Japan) for the morphology characterizations. Raman spectroscopy (inVia, U.K) was obtained with various vibrational modes. Electrochemical measurements were performed on electrochemical workstation (CHI 660E, Shanghai, China), including anodic stripping voltammetry (ASV), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and differential pulse voltammetry (DPV). 2.3. Fabrication of the electrochemical sensor Prior to modification, bare GCE was successively polished with 0.3 and 0.05 um Al2O3 powder, and washed with absolute alcohol and doubly distilled water for 5 min of ultrasound. According to the previous reports with some modifications [55,61], co-electropolymerization on the cleaned GCE was carried out with CV scans ranging from -0.9 to 2.7 V at 100 mV/s for 10 cycles in a stirred acetonitrile aqueous solution (50%), containing 0.1 M sodium hexafluorosilicate, 0.06 M sodium perchlorate, and 0.05 M furfural. Then the modified electrode was washed to eliminate any physically adsorbed materials. For comparison, the polyfurfural/GCE (PF/GCE) was also prepared through the similar route. The preparation of stepwise procedure is showed in Scheme 1. 2.4. Electrochemical measurements procedure To analyze heavy metal ions, ASV was carried out with a common
Scheme 1. Schematic illustration for the fabrication of the sensor. 415
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the silica matrix in situ, which would offer more abundant oxygencontaining functional groups for heavy metal ions chelating coordination. Elemental compositions of PF and 3D-PF/SA on GCE were examined by EDS. In Fig. 1C, C and Au elementals for PF and 3D-PF/SA can be clearly found, and the presence of Au was due to the treatment by spraygold with all samples. Noticeably, O and Si elements were detected in 3D-PF/SA, caused by the silica in 3D-PF/SA. This result further affirmed the successful decoration of 3D-PF/SA on the GCE, which was identical with the SEM analysis. Raman absorption spectroscopy was used for further testifying the successful preparation of 3D-PF/SA/GCE in Fig. 1D, which shows the Raman spectra of PF and 3D-PF/SA. The Raman spectrum of furfural at 1369 cm−1 displays a HeCeC/O bending mode [62], while C]C stretching modes at 1575 cm−1 [62,63] was found for PF and 3D-PF/ SA. Furthermore, significantly stronger signal of furfural was present in the 3D-PF/SA than that in PF, which may result from the large specific surface area of porous silica nanomaterials. For 3D-PF/SA, a slightly broad band at about 455 cm−1 was shown corresponds to the symmetrical Si-O-Si bending mode, while the narrower bands situated around 495 and 605 cm−1, known as SiO2 network, are characteristic for the symmetric stretching modes of Si-O bonds [64–66]. Meanwhile, the signal around 976 cm−1 is attributed to Si-(OH) groups [64–66]. All these results further proved the successful preparation of 3D-PF/SA modified electrode.
three-electrode cell. Firstly, the modified electrode was immersed in sodium acetate buffer solution consisting of different concentrations of cadmium acetate and lead acetate, and preconcentration was performed at -1.2 V for 350 s. After stabilization for 15 s, the ASV was recorded at the potential range of -1.2 V to -0.45 V, and the electrode was cleaned at -0.2 V with 90 s to carry out the next measurement. EIS was obtained with the frequency ranging from 0.01 to 100 kHz in 0.1 M potassium chloride solution with 5 mM K4[Fe(CN)6]/K3[Fe(CN)6]. 2.5. Treatment of water sample The samples were from our domestic water. They were firstly purified through centrifugation with 8000 rpm for 20 min. Then, the supernatant was filtered with 0.45 μm membrane. Lastly, the samples were appropriate diluted by using the ABS solution, and then used for detection by the provided 3D-PF/SA/GCE using a standard addition method. 3. Results and discussion 3.1. Characterization of 3D-PF/SA/GCE, PF /GCE In Fig. 1, the as-prepared PF/GCE, 3D-PF/SA/GCE were characterized by SEM. The SEM image of PF/GCE clearly reveals a turquoise and uniform film for PF/GCE (Fig. 1A), which is similar to the previous reports [55,56,61]. When electrodeposition duration continues to 10 cycles through cyclic voltammetry in sodium hexafluorosilicate and furfural solution, much smaller cavities interconnected by silica incorporated polyfurfural covered all over the surface of GCE (Fig. 1B). This may attributable to the viscous force of hydrogen and oxygen bubbles with silica during the electrodeposition, which limits the deposition of silica along the gas/liquid interface and ultimately leads to three-dimensional nanoporous structures. Such structure can efficient provide larger specific surface area for more polyfurfural entrapped in
3.2. CV and EIS characterization Fig. 2A represents CVs with different electrodes in 0.1 M pH 7.0 PBS. As seen from Fig. 2A, there are no peaks appearing at bare GCE, PF/GCE, and 3D-PF/SA/GCE (curve a, b, c). However, larger background current was obtained at the PF/GCE (curve b), while the 3D-PF/ SA/GCE had the largest among them (curve c). This suggests that the 3D-PF/SA/GCE has higher surface area than PF/GCE and bare GCE, it
Fig. 1. SEM images of the PF (A), and 3D-PF/SA on GCE (B). EDS of the PF and 3D-PF/SA on GCE (C). Raman spectra of the PF and 3D-PF/SA hybrid on GCE (D). 416
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Fig. 2. (A) CV of (a) bare GCE, (b) PF/GCE, (c) 3D-PF/SA/GCE in pH 7.0 PBS with 0.1 M potassium chloride at 50 mV/s.; (B) EIS of (a) bare GCE, (b) PF/GCE, (c) 3DPF/SA/GCE in 0.1 M potassium chloride with 5 mM K4[Fe(CN)6]/K3[Fe(CN)6]; (C) DPV of (a) bare GCE, (b) SA/GCE, (c) PF/GCE, (d) 3D-PF/SA/GCE in pH 5.0 ABS solution with 50 ng/mL Cd2+ and Pb2+ at 50 mV/s.
of electrochemical sensing platform. In order to estimate the effect of pH value on Cd2+ and Pb2+ detetion in 0.2 M ABS, the sensors were measured in a variety of solutions within pH from 3.0 to 8.0. As shown in Fig. 3B, promptly increase of current signals can be noticed from 3.0 to 5.0, and then had a decrease down to pH 8.0. This phenomenon may result from the complexation mechanism and electrostatic interaction by the positive charge of heavy metal ions and the numerous oxygen groups of 3D-PF/SA [39,40]. Whereas, the decrease of current responds at higher pH value may relate with the participation of the heavy metal ions in hydrolysis reaction. Thus, pH 5.0 was chose as the optimal pH value to detect Cd2+ and Pb2+. Deposition potential is a significant factor that would greatly influence the sensitivity of Cd2+ and Pb2+ analysis. Studies were performed using 3D-PF/SA as detection platform at the potential range from -0.8 to -1.5 V (Fig. 3C), the measurement was recorded in 0.1 M pH 5.0 ABS and the deposition time is 350 s. When moving from -0.8 to -1.2 V, the response current of the stripping peak of Pb2+ or Cd2+ increases gradually and the highest current signal is received at -1.2 V. Subsequently, the stripping currents for metal ions were greatly dropped as the deposition potential becomes more negative, the reason for that is the evolution of hydrogen is occurred on the electrode surface that affect the active surface area, resulting in a decrease in the amount of Cd2+ and Pb2+ on 3D-PF/SA/GCE [32,39]. Therefore, -1.2 V was chosen as the best deposition potential for next experiments. The impact of the preconcentration time on the current signal of the proposed strategy was investigated from 50 s to 500 s in 0.1 M pH 5.0 ABS with 50 ug/L Pb2+ or Cd2+ at -1.2 V. The stripping peak currents increased upon the increasing preconcentration time from 50 s to 350 s, due to the significantly enhanced metal ions reduced on the surface of 3D-PF/SA/GCE. Concretely, these results further verifies that the 3DPF/SA nanocomposite has a great accumulating ability for the reduced metal ions because of its large active surface area. However, longer incubation time did not improve the response, which shows the electrode surface has reached saturate tendency. Then 350 s of preconcentration time was chosen as a compromise between a suitable time and a high signal for the experiments. The 3D nanoporous silica entrapped polyfurfural modified glassy carbon electrode (GCE) was carried out by the one-step co-electropolymerization with cyclic voltammograms. In the electropolymerization process, an oxidative peak was appeared at 1.9–2.3 V, and the peak current rose steadily with each scan from one to ten circles (Fig. 3D). This process is considered to be related to the oxidation and polymerization of furfural. Compared with just electropolymerization of polyfurfural film with 5 circles at its strongest signal, 10 circles were performed in this study, this may be owing to their large surface area of nanoporous silica on the electrode surface.
also verifies the successful construction of the 3D-PF/SA/GCE. Fig. 2B displays the EIS curves of various modified electrodes in 0.1 M potassium chloride solution with 5 mM K4[Fe(CN)6]/K3[Fe(CN)6]. The resistance with the modification of polyfurfural film obviously increased comparison with that obtained on bare GCE (curve a, b), and the resistance dramatically increased on 3D-PF/SA/GCE (curve c), which has been used to further confirm that 3D-PF/SA were successfully co-electrodeposited on the electrode surface. Fig. 2C represents the DPV curves of different electrodes in 0.1 M pH 5.0 ABS with 50 ug/L of Cd2+ and Pb2+. Two weak stripping current signals for Cd2+ and Pb2+ were observed for the bare GCE (curve a), while two slightly larger peak currents were appeared on the porous silica nanomaterials electrodeposited GCE (curve b). When polyfurfural film was doped onto the bare GCE surface, it resulted in two increasing peak currents for Cd2+ and Pb2+ (curve c), which suggests that the polyfurfural has electrocatalytic activity toward cadmium and lead ions. After co-electrodeposition of 3D-PF/SA on the bare electrode, two further significantly current signals emerged for Cd2+ and Pb2+ (curve d) at -0.89 V and -0.64 V, which are approximately 5 times as much as that of PF/GCE, respectively. These suggested that the 3D-PF/SA exhibited larger electroactive surface area and leaded to the signal amplification for the proposed sensor. To research the effective surface area of the 3D-PF/SA/GCE, PF/ GCE and the bare GCE, these electrodes were evaluated in 1 mM K3Fe (CN)6 by CVs at different scan rates. For a reversible process, the activity areas of the electrode surface are obtained through the Randles–Sevcik formula [67,68], it can be calculated to be 0.298 cm2 for 3D-PF/SA/GCE, 0.157 cm2 for PF/GCE, and 0.070 cm2 for GCE, which was about twice as much as that of PF/GCE and 4.2 times of GCE, ascribing to the much small cavities of the modified nanomaterials on the GCE surface and its high surface-to-volume ratio properties. 3.3. Optimization of experimental conditions To achieve the best electrochemical signal for detection of Cd2+and Pb2+, the effect of furfural concentration was studied. With the increase of furfural concentration from 0.01-0.05 M, the peak responses of Cd2+and Pb2+ increased gradually, and then decreased. Higher concentration of furfural would reduce the electrocatalytic surface area of 3D-PF/SA hybrid because of the large aggregation. Meanwhile, the ratio of furfural to sodium hexafluorosilicate is also crucial for the effective adsorption and desorption of Cd2+and Pb2+ on the 3D-PF/SA hybrid surface. As shown in Fig. 3A, various molar ratios of furfural and sodium hexafluorosilicate including 2:1, 1:1, 1:2, 1:4, 1:5, and 1:6 (M/ M) were used for the preparation of 3D-PF/SA hybrid, and the maximum value of peak currents was obtained at 1:2. Higher or lower concentration of sodium hexafluorosilicate as matrix would lead to the poor electrocatalytic performance of the assembled 3D-PF/SA hybrid and reduce the complexation towards metal ions. Thus, 0.05 M furfural and 0.1 M sodium hexafluorosilicate were chosen for the construction 417
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Fig. 3. Effect of (A) the mole ratios of furfural/sodium hexafluorosilicate, (B) pH value of buffer solution, (C) deposition potential, and (D) scanning circles on the stripping peak currents of Cd2+and Pb2+.
3.4. Simultaneous detection of Cd2+ and Pb2+
high resolution for selective detection of Cd2+ and Pb2+ simultaneously. In Fig. 4B, the excellent corresponding linear responses were acquired over the Cd2+ and Pb2+. For analysis of Cd2+, the linear equation was I(μA) = -0.01381c (μg/L) - 0.7813 (n = 16, R2 = 0.999) from 1.5 ng/L to 6200 μg/L, with the detection limit of 0.9 ng/L. For analysis of Pb2+, the linear equation was I(μA) = -0.009962C (μg/L) 0.3425 (n = 16, R2 = 0.995) in the range of 0.05 ng/L to 5200 μg/L, which detection limit was down to 0.02 ng/L (Fig. 4B). Compared with the previous studies for simultaneous detection of Cd2+ and Pb2+ in Table 1, the as-proposed sensor had exhibited simpler operation, better stability, wider linear region, and lower detection
To assess the quantitative range for Cd2+ and Pb2+ detection simultaneously at the 3D-PF/SA/GCE, different Cd2+ and Pb2+ concentrations were performed at the optimal conditions. The response of the proposed sensor was recorded in 0.1 M ABS (pH 5) by applying an accumulation potential at −1.20 V for 350 s. Fig. 4A displays that the current responds had significantly enhanced within the increasing concentrations of Cd2+ and Pb2+, where two individual peaks were recorded at around -0.89 V for Cd2+ and -0.64 V for Pb2+, respectively. There is about 250 mV between the peaks separation, which reveals a
Fig. 4. (A) ASV for Cd2+and Pb2+simultaneous detection (from a to p: 1 × 10−6, 1 × 10−5, 1 × 10−4, 1 × 10−3,1 × 10−2, 0.1, 0.25, 0.4, 0.5, 1, 2, 3.5, 5, 6.2 μg/ mL for Cd2+; 1 × 10−7, 1 × 10−6, 1 × 10−5, 1 × 10−4, 1 × 10−3,1 × 10−2, 0.1, 0.25, 0.4, 0.5, 1, 2, 3.5, 5 μg/mL for Pb2+). (B) Calibration curves of the sensor for Cd2+and Pb2+simultaneous detection. 418
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Table 1 Comparison of analytical performance of the proposed method with other methods for the simultaneous determination of Cd2+and Pb2+. Method
Linear range (μg/L)
Antimony film carbon paste electrode Polysulfoamino anthraquinone solid ionophore Bismuth nanoparticle-porous carbon paste electrodes Electrochemical method with active pharmaceutical ingredient Bismuth nanoparticles integration as modifiers of screen-printed carbon electrodes Microfabricated on-chip planar bismuth electrode Antimony film electrode Microfabricated three-electrode on-chip device with polydimethylsiloxane cover Bismuth/glassy carbon composite electrode Disposable multiwalled carbon nanotubes modified screen-printed electrode Reduced graphene oxide modified disposable bismuth film electrode Micropatterned reduced graphene oxide with carbonnanotube and bismuth composite electrodes N-doped carbon quantum dots-graphene oxide hybrid Polyfurfural film/multi-walled carbon nanotube modified electrode One-step co-electropolymerization Of 3D nanoporous silica incorporated with polyfurfural on glassy carbon electrode
Detection limit (μg/L)
Reference
4.0-150.0 (Cd, Pd) 0.5-30 (Cd, Pd) 1-100 (Cd, Pd) 50-250 (Cd); 5-200 (Pb) — 0-280 (Cd); 25-400 (Pb) 20-140 10-80 (Cd, Pd) 20-100 (Cd,Pd) 0.5-80(Cd); 0.05-100(Pb) 1-60 (Cd, Pd) 20-200 (Cd, Pd)
0.8 (Cd); 0.2 (Pb) 0.2 (Pb); 0.1 (Cd) 0.65 (Pb); 0.81 Cd) 2 (Cd); 1.9 (Pb) 2 (Pb); 5 (Cd) 9.3 (Cd); 8 (Pb)
[69] [70] [71] [72] [36] [73]
0.7 (Cd); 0.9 (Pb) 0.11 (Cd); 0.25 (Pb) 1.5 (Cd); 2.3 (Pb) 0.1 (Cd); 0.07 (Pb) 0.5 (Cd); 0.8 (Pb) 0.6 (Cd); 0.2 (Pb)
[74] [75] [37] [33] [13] [32]
11.24-11241 (Cd); 20.72-10360 (Pb) 0.5–15 (Cd); 0.1–15 (Pb) 1.5×10−3 -6200 (Cd); 5×10-5 -5200 (Pb)
7.45 (Cd); 1.17 (Pb) 0.03 (Cd); 0.01 (Pb) 9×10−4 (Cd); 2 ×10−5 (Pb)
[39] [56] This work
4.3% for Cd2+ and 4.9% for Pb2+, respectively. Similarly, the average RSDs of the measurements were 3.7% for Cd2+ and 4.1% for Pb2+ in interassay, respectively. This fact displays that the established sensor had good reproducibility.
limit. The reasons for its low detection limit may result from the excellent electrical conductivity and charge transfer ability of 3D-PF/SA with metal ions. Meanwhile, the construction of 3D-PF/SA have larger electroactive surface areas with rich oxygen containing groups, which offer more active sites to effectively adsorb heavy metal ions through electrostatic interaction, leading to the higher sensitivity of the established strategy.
3.6. Determination of Cd2+ and Pb2+ in real water To evaluate the feasibility of the as-prepared strategy for Cd2+ and Pb detection, different concentrations of Cd2+ and Pb2+ solutions were added in real water and determined by the 3D-PF/SA/GCE. The water was obtained from our university drinking water, and the results were showed and summed up in Table 3. Excellent recoveries were in the range of 94–105% and RSD was less than 5% for Cd2+ and Pb2+ detection, which indicates this proposed method provide a promising potential in practical applicability. 2+
3.5. Specificity, stability, and reproducibility of the sensors To further study the influence of substances that may interfere with the signals for Cd2+ and Pb2+ detection, the possible interfere ions such as K+, Na+, Cu2+, Ni2+, Zn2+, Mn2+, Hg2+, Mg2+, Ca2+ and Fe3+ were used in this study (Table 2). In comparison with the current signals acquired from Cd2+ and Pb2+ without interference, it showed minimal difference in current in Cd2+ and Pb2+ solution containing interfering ions (30-fold mass ratio), where the average relative standard deviations (RSD) were less than 7% for both Cd2+and Pb2+ change (Table 2). This result affirmed the good specificity of the asprepared 3D-PF/SA/GCE. Stability is another significant parameter in its application for 3DPF/SA/GCE. When the 3D-PF/SA/GCE was stored at 4 °C for 15 and 30 days, the peak response reduced less than 5% and 9% compared with their initial responses, respectively. Evidently, the proposed sensor has acceptable long-term stability. The reproducibility of the 3D-PF/SA/GCE was studied by interassay and intra- assay coefficients of variation with five replicate measurements. By simultaneous determining Cd2+and Pb2+ at one concentration level with 100 ng/mL, the RSDs of interassay for five sensors were
4. Conclusion In summary, 3D nanoporous silica nanostructure incorporated with polyfurfural modified glassy carbon electrode was firstly applied for the simultaneous detection of Cd2+ and Pb2+ through one-step co-electropolymerization. The presentation of oxygen containing groups offered abundant anchor sites for adsorption of Cd2+ and Pb2+ on the 3DPF/SA surface. Moreover, by combination of the advantages of polyfurfural and silica nanocomposites, 3D-PF/SA provides large effective surface area as well as good electrical conductivity for Cd2+ and Pb2+, displaying excellent electrochemical performance for Cd2+ and Pb2+ detection simultaneously. Compared with other methods, this proposed Table 3 Simultaneous determination of Cd2+ and Pb2+ in a drinking water sample.
Table 2 Interference study of other ions in Cd2+and Pb2+ detection. Interfering ions
K+ Na+ Cu2+ Ni2+ Zn2+ Mn2+ Hg2+ Mg2+ Ca2+ Fe3+
Sample
Added (ng/mL, n = 3)
Found (ng/mL, n = 3)
Recovery (%)
Relative deviation (%)
A1 A2 A3 A1 A2 A3 B1 B2 B3 B1 B2 B3
0 (Cd2+) 10 (Cd2+) 50 (Cd2+) 0 (Pb2+) 10 (Pb2+) 50 (Pb2+) 0 (Cd2+) 10 (Cd2+) 50 (Cd2+) 0 (Pb2+) 10 (Pb2+) 50 (Pb2+)
0 10.25 52.36 0 9.68 51.19 0 9.88 52.01 0 9.55 47.33
– 102.5 104.72 – 96.8 102.38 – 98.8 104.02 – 95.5 94.66
– 3.99 4.68 – 4.36 5.03 – 4.05 3.77 – 4.27 4.81
Relative deviation (%) Cd2+
Pb2+
−0.51 −0.74 −6.5 −4.3 −0.36 2.2 −2.9 −1.5 −2.7 1.7
−0.70 −0.63 −5.3 3.9 0.45 −1.9 −3.3 −1.6 −2.2 2.1
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strategy exhibits simpler preparation, lower detection limit, wider linear range, higher selectivity, excellent reproducibility and stability. Impressively, the established sensor was successfully performed in real water for detection of Cd2+ and Pb2+, and satisfactory testing recoveries were received, exhibiting great potential for determination of various trace toxic metallic ions in practical applications.
[39] [40]
Acknowledgments
[41]
[35] [36] [37] [38]
[42]
We are deeply grateful for the support of National Natural Science Foundation of China (No. 21705084), and the Natural Science Foundation of Shandong Province of China (No. ZR2017BB074), National Training Program of Innovation and Entrepreneurship for Undergraduates (No. 201710431040), and Special Funds for Taishan Scholars Project.
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