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Sensitive electrochemical detection of Hg(II) via a FeOOH modified nanoporous gold microelectrode
Sensors & Actuators: B. Chemical 287 (2019) 517–525
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Sensitive electrochemical detection of Hg(II) via a FeOOH modified nanoporous gold microelectrode Zhonggang Liua, Emily Puumalab, Aicheng Chena, a b
T
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Electrochemical Technology Centre, Department of Chemistry, University of Guelph, 50 Stone Road East, Guelph, Ontario, N1G 2W1, Canada Department of Chemistry, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: FeOOH nanoflakes Nanoporous gold microelectrode Electrochemical detection Square wave voltammetry Hg(II)
As a global scale pollutant, mercury poses serious risks for human health and the environment; thus, it is of critical importance to develop a simple and efficient sensing protocol to achieve the precise detection of Hg(II). Here we report on the facile fabrication of electro-synthesized FeOOH nanoflakes on nanoporous gold (NPG) microwires. The resulting FeOOH/NPG microelectrode was applied to the electrochemical detection of Hg(II) via square wave voltammetry (SWV). Nanoporous gold microwires with 3D network structures can provide a large specific surface area, facilitate analyte transport and electron transfer, and enhance the electro-catalytical activity of FeOOH-Au. Meanwhile FeOOH nanoflakes, with high adsorption capacities can accumulate analytes onto the electrode surface. Under the optimized conditions, excellent electrochemical performance was achieved with a high sensitivity of 123.5 μA μM−1 cm-2 and low detection limit of 7.81 nM, respectively. In addition, no obvious interference from common ions (e.g., Cu(II), Pb(II), Cd(II)) was observed, and the FeOOH/NPG microelectrode demonstrated exceptional stability. Furthermore, the fabricated electrochemical sensor could be utilized for the detection of Hg(II) in tap water and lake water samples.
1. Introduction Mercury (Hg) has been recognized as a global pollutant due to its widespread presence, and bioaccumulative and toxic nature [1]. Mercury is a neurotoxin that can damage multiple physiological systems, including neurological, immune, genetic, enzyme, cardiovascular, respiratory, and gastrointestinal [2]. Since dietary seafood is the primary route of exposure for most residents of North America, elevated levels of mercury in fish has been a well-known environmental issue in North America [3]. Increasingly stringent regulations have been implemented in the U.S. and Canada over the last few decades, with the aim of reducing environmental mercury emissions in North America. Additionally, various initiatives have been engaged to reduce mercury emissions from automobile switches and the coal-fired electricity generation, which comprises the largest remaining anthropogenic source of mercury in Canada [3]. Thus, there is an urgent need to explore a sensitive and simple analytical method for the detection and monitoring of Hg(II) in water. To date, several common analytical techniques have been employed for the detection of Hg(II), such as atomic absorption spectroscopy (AAS) [4], inductively coupled plasma-mass spectrometry (ICP-MS) [5], inductively coupled plasma optical emission spectrometry (ICP-OES)
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[6], inductively coupled plasma-atomic emission spectrometry (ICPAES) [7], fluorescence spectroscopy [8–10], colorimetry [11–13], and surface-enhanced Raman scattering. (SERS) [14,15]. However, there are several drawbacks related to these traditional laboratory-based analytical techniques, including the requirement of sophisticated instrumentation, time consuming processes, multi-step sample preparation, and complex analytical procedures, which limit the application of rapid on-site measurements. Alternatively, electrochemical techniques have been adopted by the environmental sciences due to their merits of high sensitivity, low cost, easy operation, rapid analysis, portability, and applicability, which are feasible for the monitoring of environmental samples [16]. Up to now, some excellent work has emerged for the electrochemical sensing of Hg (II). In general, electrochemical performance is strongly impacted by the effectiveness of electrode materials. Noble metal nanoparticles and carbon-based materials (e.g., graphene, carbon nanotubes (CNTs), carbon fibers, porous carbon) have been employed for the detection of Hg(II) [17–20]. Among these materials, gold-based materials are highly attractive for the electrochemical detection of Hg(II) [21–24]. For example, Mandler et al. investigated the electrochemical reduction of HAuCl4 and the electrostatic adsorption of Au NPs, for the electrochemical detection of Hg(II), and studied the effects of Au NPs. It was
Corresponding author. E-mail address: [email protected] (A. Chen).
https://doi.org/10.1016/j.snb.2019.02.080 Received 1 December 2018; Received in revised form 17 February 2019; Accepted 18 February 2019 Available online 19 February 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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2.3. Instrumentation
revealed that gold nanoparticles could serve as nucleation sites for the deposition of Hg, thus facilitating its detection [25]. Chen et al. fabricated field-effect transistor (FET) sensors based on a hybrid sensing platform (rGO and functionalized Au NPs) for the detection of Hg(II), whereby a detection limit of 25 nM was achieved [26]. Gold electrodes with different morphologies were also applied as efficient substrates for the determination of Hg(II) [21,27,28]. Nanostructured metal oxides/ hydroxides (NMOs/HOs) have promising applications as catalysts in electrochemical sensors for the monitoring of environmental contamination due to their unique electrical and electrocatalytic activity, chemical and photochemical stability, as well as enhanced electrontransfer and adsorption capacities [29–31]. In consideration of the advantages of gold-based nanomaterials and nanostructured metal oxides/hydroxides, we endeavored to develop an efficient electrochemical sensor based on gold and NMO/HO materials, for the detection of Hg(II) in environmental samples. Herein, we report on the facile fabrication of electro-synthesized FeOOH nanoflakes on nanoporous gold (NPG) microwires. The prepared FeOOH/NPG microelectrode was further advanced for the electrochemical detection of Hg(II) using square wave voltammetry (SWV). The nanoporous structure of gold microwires can provide a large specific surface area, facilitate the transport of analytes and electron transfer, while enhancing the electrocatalytic capacity of FeOOH/Au. The stability and interference resistance of the developed microelectrode were examined as well. Furthermore, the fabricated electrochemical sensor was investigated for its potential to detect Hg(II) in environmental samples.
Electrochemical experiments were carried out using a CHI 660B computer-controlled potentiostat (CH Instrument Inc., USA). A standard three-electrode system was utilized for the electrochemical measurements, where prior to and following modification, a gold microwire was employed as the working electrode; an Ag/AgCl (1 M KCl) electrode as the reference electrode, and a platinum coil as the auxiliary electrode. The structure and morphology of the gold microwire before and after modification were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi SU-70), with energy dispersive X-ray spectrometry (EDS). The crystal structures were characterized by X-ray diffraction (XRD) via a PhilipsX’Pert Pro X-ray diffractometer (Cu Kα radiation, 1.5406 Å). 2.4. Electrochemical experiments Square wave voltammetry (SWV) was employed for Hg(II) detection under optimal experimental conditions. In the detection process, a deposition potential of -0.40 V was initially applied for 150 s under stirring. The SWV responses were recorded in a potential range from -0.40 to 0.80 V (vs. Ag/AgCl, 1 M KCl), with a step potential of 4 mV, amplitude of 25 mV, and frequency of 15 Hz. Finally, a desorption potential of 0.80 V (vs. Ag/AgCl, 1 M KCl) was applied for 150 s to remove residual metals and acivate the working electrode. All of the electrochemical experiments were conducted at room temperature. The detection of Hg(II) in tap water or lake water samples was performed using a standard addition method. Tap water was collected from local venues and the lake water was from the Lake Superior. During testing, the real water samples were spiked with a 0.1 M PBS solution at a ratio of 1:9 without any further treatment.
2. Experimental 2.1. Chemical reagents
3. Results and discussion Gold wire (Ø127 μm, 99.99%) was purchased from Alfa Aesar. Analytical grade ferrous ammonium sulfate ((NH4)2Fe(SO4)2), sodium acetate (NaAc), Hg(NO3)2, Cu(NO3)2∙3H2O, Pb(NO3)2, Cd(NO3)2∙4H2O, Zn(NO3)2∙6H2O, MgSO4, KNO3, NaH2PO4, Na2HPO4, and benzyl alcohol (BA) were obtained from Sigma-Aldrich. Phosphate buffer saline (PBS, 0.1 M) solutions were prepared using 0.1 M NaH2PO4, 0.1 M Na2HPO4, and 0.1 M NaCl. All solutions were obtained using water from a Nanopure® Diamond™ UV water purification system (18.2 MΩ cm).
3.1. Electrochemical Hg(II) ion detection principle Scheme 1 illustrates the facile fabrication process of the FeOOH/ NPG microelectrode and the detection strategy for Hg(II) ions. As shown, the nanoporous gold microwire possessed a 3D interconnected network structure, and the FeOOH nanoflakes uniformly covered the surface of the gold microwire. The unique structure was of great benefit for the fabrication of the electrochemical sensor. As is well-known, for the electrochemical sensing of analytes, the oxidation peak current, ip, is directly proportional to the concentration of the analyte in solution under defined experiment conditions. The effective pre-concentration of the analyte onto the working electrode surface is important. In this detection process, a significant quantity of metal ions could be adsorbed onto the surfaces of the nanomaterials, and then released to the working electrode surface. The more metal ions that are adsorbed to the surface of nanomaterial, the more metal ions are released; hence, a stronger peak current may be obtained.
2.2. Fabrication of FeOOH modified nanoporous gold (NPG) microelectrode A nanoporous gold microwire electrode was custom-fabricated in our laboratory using an electrochemical alloying/dealloying method in a three-electrode system as shown in Scheme 1. The gold microwire served as the working electrode, whereas Zn foil and Zn wire were employed as the counter electrode and reference electrode, respectively [32]. The synthesis was carried out in a mixture of benzyl alcohol (BA) and 1.5 M ZnCl2 via cyclic voltammetry at a scan rate of 10 mV s−1 and 110 °C, where the potential was swept between -0.70 V and +1.80 V (vs. Zn) for 50 cycles. Prior to modification, the cleaning/activation of the NPG microelectrode was carried out in 0.1 M H2SO4 by recording 10 cycles in the potential range from 0.0 to 1.5 V (vs. Ag/AgCl, 1 M KCl) at 20 mV s−1. Subsequently, a facile electrochemical process was employed to prepare the modified microelectrode. The FeOOH nanoparticles were electro-synthesized in situ on the NPG microelectrode according to a technique modified from the literature, which was achieved by applying 0.7 V (vs. Ag/AgCl, 1 M KCl) for 1800s in a solution of 0.1 M NaAc, containing 10 mM ferrous ammonium sulfate [33]. The fabricated FeOOH/NPG microelectrode was subsequently rinsed with water and used for further measurements.
3.2. Surface characterization of the FeOOH/NPG microelectrode Figs. 1a and b depict SEM images of the gold microwire being treated via the alloying/dealloying method in a benzyl alcohol/1.5 M ZnCl2 electrolyte. As seen in Fig. 1a, a hierarchical porous structure was formed on the surface of the gold microwire. A 3D interconnected network with pore dimensions from ˜100 to ˜300 nm was observed under a high-magnification SEM image (Fig. 1b). Sequentially, the FeOOH nanoflakes were directly grown onto the surface of gold microelectrode (Fig. 1c and d) via a facile electrochemical deposition process. It may be seen from Fig. 1c that large quantities of FeOOH nanoflakes uniformly covered over the surface of the gold microwire. Fig. 1d presents a high-magnification SEM image, where the FeOOH nanoflakes were assembled with a narrow thickness distribution 518
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Scheme 1. Schematic illustration of the effective electrochemical detection of Hg(II) using the FeOOH/NPG microelectrode.
electrodeposition of the FeOOH nanoflakes. The calculated average crystallite size of FeOOH nanoflakes was ˜79.0 nm from the (110) diffraction line.
between 20 and 40 nm. Fig. 2a displays the EDS spectra of the formed as-prepared nanoporous gold microelectrode and FeOOH/NPG microelectrode. The strong peak for Au at ˜2.1 keV and weak peaks at ca. 9.7 and 11.5 keV are observed in the EDS spectrum of the nanoporous gold microelectrode. Followed the electrodeposition of FeOOH, distinct peaks for Fe at ca. 0.7, 6.4, and 7.0 keV appeared, which confirmed the successful electrodeposition of FeOOH nanoflakes on the surface of the nanoporous gold microelectrode. The NPG microelectrode and the fabricated FeOOH/NPG microelectrode were characterized by XRD, and their typical XRD patterns are presented in Fig. 2b. The peaks centred at ca. 38.3°, 44.4°, 64.6°, 77.6°, and 81.7° could be indexed to the (111), (200), (220), (311), and (222) planes of gold (face centered cubic (fcc), JCPDS no. 01-1172), respectively [34]. The average crystallite sizes of the nanoporous gold microelectrode were calculated using the DebyeScherrer equation, to be ˜67.4 nm, based on the (111) diffraction line. Following the electrodeposition of FeOOH, a well-defined peak at ca 21.3° appeared, which corresponded to the (110) plane of α-FeOOH (JCPDS no. 81-0462), respectively [35,36], which verified the
3.3. Electrochemical behaviors of the FeOOH/NPG microelectrode The electrochemical behaviors of the nanoporous gold microelectrode were investigated, prior to and following the electrodeposition of FeOOH. Fig. 3 shows the square wave voltammetric (SWV) responses of 0.40 μM Hg(II) on the nanoporous gold microelectrode and the FeOOH/ NPG microelectrode in 0.1 M PBS solution (pH 5.0). Only a small oxidation peak of Hg(II) was observed at the NPG microelectrode (red line, Fig. 3a). However, in the case of the FeOOH modified NPG microelectrode, a well-defined response of Hg(II) at 0.560 V (vs. Ag/AgCl, 1 M KCl) appeared (blue line, Fig. 3b), which was ascribed to the re-oxidation of Hg(0) to Hg(II). The dashed line represents the SWV curve in the absence of Hg(II). For comparison, the electrochemical active surface areas (EASAs) of the NPG microelectrode before and after the insitu electro-synthesis of FeOOH were calculated; thus, the current
Fig. 1. SEM images of (a, b) nanoporous gold (NPG) microwire and (c, d) FeOOH-modified NPG microwire. 519
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Fig. 3. SWV responses of 0.40 μM Hg(II) at (a) the NPG microelectrode and (b) the FeOOH/NPG microelectrode in 0.1 M PBS solution (pH 5.0). The dotted lines refer to the baselines. Deposition potential -0.40 V; deposition time 150 s; SWV setting parameters: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
Fig. 2. (a) EDS spectra of the formed as-prepared nanoporous gold microwire and FeOOH/NPG microwire. (b) XRD pattern of NPG microwire (black line) and FeOOH/NPG microwire (red line) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
examined in the analysis of 0.4 μM Hg(II), with a deposition potential of -0.40 V (vs. Ag/AgCl, 1 M KCl) in 0.1 M PBS solution (pH 5.0). As shown in Fig. 4b, the peak current densities increased with longer deposition times, from 90 to 150 s, and then slowly increased with deposition times beyond 150 s. Therefore, a deposition time of 150 s was selected in order to attain a lower detection limit and wider range of response. The effect of pH on the electrochemical performance of the FeOOH/ NPG microelectrode was also evaluated. Fig. 4c displays the current response of 0.4 μM Hg(II) in a 0.1 M PBS solution wherein the pH values varied from 4.0 to 6.0. As is clearly shown in Fig. 4c, the peak current densities of Hg(II) increased when the pH values changed from 4.0 to 5.0. However, further increases in pH resulted in lower current densities. This may have been associated with the hydrolysis of Hg(II), which decreased of the activity of Hg(II) [37,38]. It is worth noting that the pHpzc for the FeOOH has been reported as 6.74 ± 0.31 [39,40], and that the FeOOH surface is positively charged when the pH value is less than pHpzc. In the presence of 0.1 M NaCl, HgCl3− was predominated in the electrolyte; thus the FeOOH with the multi-nanoflake structure could be benefit for the adsorption of the negatively charged Hg(II) species, leading to the improved electrochemical sensing of Hg (II) [41]. Thus, based on the aforementioned results, pH 5.0 was employed for further measurements.
density (j) was determined. The measurements were carried out in a 0.1 M KCl solution that contained 5 mM Fe(CN)63−/4−, and the EASAs were calculated based on the Randles–Sevcik equation. The EASAs of the NPG microelectrode prior to and following modification were calculated to be 7.69 ± 0.27 mm2 and 3.87 ± 0.22 mm2, respectively. Herein, it may be observed in Fig. 3 that the peak current density of Hg (II) obtained at the FeOOH-modified NPG microelectrode was increased by more than seven-fold over that of at the NPG microelectrode.
3.4. Optimization of experimental conditions Since several experimental parameters might influence the electrochemical performance of the FeOOH/NPG microelectrode toward Hg (II), the experimental parameters (deposition potential, deposition time, pH values) were optimized and discussed. Fig. 4a shows the peak current densities of 0.4 μM Hg(II) on the FeOOH/NPG microelectrode by varying the deposition potential between -0.70 and -0.30 V (vs. Ag/ AgCl, 1 M KCl). As the potential shifted negatively, from -0.30 V to -0.40 V (vs. Ag/AgCl, 1 M KCl), the current densities increased significantly, which was due to the enhanced kinetics, that is, Hg(II) could be easily reduced at more negative deposition potentials. The current densities attained a maximum at a deposition potential of -0.40 V (vs. Ag/AgCl, 1 M KCl). However, when the deposition potential continued to negatively shift, from -0.40 V to -0.70 V (vs. Ag/AgCl, 1 M KCl), the current densities decreased, which was attributed to the interference of hydrogen evolution. Thus, the deposition potential of -0.40 V (vs. Ag/ AgCl, 1 M KCl) was selected as optimal. Different deposition times of 90, 120, 150, 180, and 210 s were
3.5. Electrochemical detection of Hg(II) with SWV Fig. 5a presents SWV curves for the electrochemical detection of Hg (II) at the FeOOH/NPG microelectrode in a PBS solution (0.1 M, pH 5.0) based on optimized conditions. The well-defined peak for Hg(II) may be clearly observed at +0.560 V with higher concentrations of Hg(II). The 520
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Fig. 4. Dependence of (a) deposition potential, (b) deposition time and (c) pH values on the voltammetric response of 0.40 μM Hg(II). Data were collected from the SWV response of 0.40 μM Hg (II) at the FeOOH/NPG microelectrode in a 0.1 M PBS solution (pH 5.0). The setting parameters in the SWV technique: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
adsorption sites; Hg(II) ions in the aqueous solution were initially adsorbed and accumulated on the surfaces of the FeOOH nanoflakes during the electrochemical sensing process. The adsorbed Hg(II) ions could easily diffuse from the FeOOH nanoflake surface onto the interconnected nanoporous gold, where the Hg(II) ions could be effectively reduced and stripped off from the surface, leading to the highly sensitive detection of Hg(II).
3.6. Interference and stability measurements Interference from common ions on the electrochemical analysis of Hg(II) was further investigated. Fig. 6 depicts the voltammetric responses of 0.40 μM Hg(II) in the absence, or presence of ions, including Cu(II), Cd(II), Pb(II), Zn(II), Mg(II), K(I), Ca(II), NO3−, SO42-. It may be observed that even though the concentrations of these ions were 10 times higher than that of Hg(II), there was no obvious interference (within ± 1.81%) on the response of Hg(II). Among all of the potential interference ions, since Cu(0) had a slightly more negative potential than did Hg(0), it may have caused competitive oxidation on the electrode, leading to potential interference. Herein, the effect of Cu(II) on the electrochemical sensing of Hg(II) was emphasized. Fig. 7 displays the SWV curves of 0.40 μM Hg(II) at the FeOOH/NPG microelectrode in the presence of Cu(II), where the concentration of Cu(II) ions was in the range of from 0 to 4.0 μM. The inset shows the SWV curves of 0.40 μM Hg(II) in the absence of Cu(II). As shown in this study, distinct and well-defined oxidation peaks for Hg(II) and Cu(II) may be clearly observed, as the Cu(II) was increased up to 4.0 μM. The current responses of Cu(II) were increased with the addition of Cu(II), while the current responses of 0.40 μM Hg(II) remained almost constant. The peak current densities of 0.40 μM Hg(II) were estimated to be 67.1 ± 3.71 μA cm-2 in the presence of different concentrations of Cu (II) ions, even when their concentrations were 10 times higher than that of Hg(II) (0.40 μM). Further, the coexisting Cu(II) did not affect the electrochemical analysis of Hg(II). These results indicated that the FeOOH/NPG microelectrode possessed an efficient anti-interference capacity. Repetitive measurements of the detection of Hg(II) were performed to estimate the stability of the FeOOH-modified NPG microelectrode, with the corresponding results presented in Fig. 8a. This was realized by recording the current responses of 0.40 μM Hg(II) at the FeOOH/NPG microelectrode over 20 cycles using SWV (Fig. 8a inset). No obvious change in the SWV responses of Hg(II) was observed with its continuous sensing. The relative standard deviation (RSD) was calculated to be 1.10%, based on the 20-cycle current responses. Further, Fig. 8b presents the extended stability (30 days) of the FeOOH/NPG microelectrodes, which was evaluated by the current response of 0.40 μM Hg(II) in a 0.1 M PBS solution with SWV. As shown, the SWV responses of Hg (II) at the FeOOH/NPG microelectrodes were almost constant over 30 days, with a RSD of 2.63%. These results demonstrated that the FeOOH/NPG microelectrode demonstrated excellent reproducibility under repeated measurements, along with prolonged stability.
Fig. 5. (a) Typical SWV responses of Hg(III) on the FeOOH/NPG microelectrode over a concentration range of from 0.02 to 2.2 μM in a PBS solution (pH 5.0). The dotted line refers to the baseline. (b) The corresponding linear calibration plot. Deposition potential -0.40 V; deposition time 150 s; SWV setting parameters: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
calibration curve of the FeOOH/NPG microelectrode for Hg(II) detection presented in Fig. 5b, shows a sensitivity of 123.5 μA μM−1 cm-2, which gave a correlation coefficient of 0.995 over a linear range of from 0.02 μM to 2.2 μM. The calculated detection limit was 7.81 nM, which was lower than a guideline value of 30 nM defined by the World Health Organization (WHO) [42]. Furthermore, a comparison of electrochemical performance for the detection of Hg(II) with other previously reported electrochemical sensors is summarized in Table 1. Based on the listed results, it was indicated that the FeOOH/NPG microelectrode exhibited favorable electrochemical performance for Hg(II) detection with high sensitivity and a low detection limit. As illustrated in Scheme 1 and Fig. 1, the synergetic effects of the designed electrochemical sensor for the sensitive detection of Hg(II) can be attributed to the excellent adsorption capacity of the FeOOH nanoflakes and the high catalytic properties of the nanoporous gold. The electro-synthesized FeOOH with the multi-nanoflake structure could provide numerous 521
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Table 1 Comparison of performance for the voltammetric determination of Hg(II) at modified electrodes. Electrodes
Technique
Linear range (μM)
Sensitivity (μA μM−1)
LOD (nM)
Ref.
ssDNA/Au electrode Gold ultramicroelectrode arrays Screen-printed gold electrode On-chip integrated Au–Ag–Au three-electrode polypyrrole/rGO/GCE Fe3O4-chitosan/GCE Bio-ssDNA/Fe3O4-SA/MGCE HNTs-Fe3O4–MnO2/CPE Au-SPCE CB-15-crown-5/GEC SPGE CalixareneSPCCEs Au-NPs/-SH/rGO/GCE Au–TiO2NPGs/Chit/gold electrode Hydroxyapatite/GCE NiCo2O4/Ni foam ZnO/Nafion/Au electrode
DPV LSV SWV DPV SWV SWV DPV DPV SWV DPV SWV ASV DPV DPV SWV SWV LSV CA SWV
0.1 – 2.0 0.01 – 1.0 0.025 – 0.15 0.05 – 5.0 0 – 0.1 0.4 – 1.1 0.001 – 0.1 0.0025 – 0.75 0.005 – 0.5 0.5 – 1.0 0.1 – 1.4 0.453 – 11.9 1 – 10 0.005 – 0.4 0.2 – 210 0.8 – 2.8 25 – 250 0.025 – 0.25 0.02 – 2.2
– 0.11 nC/ppb 7.16 1.32 124 9.65 13.55logc
0.0237 2.2 9.17 0.55 3.13 0.31 23.7
100 16 5.5 15 15 95.7 0.33 1 5 60 3.5 239 200 1.0 141 9.91
[43] [44] [45] [46] [47] [48] [49] [29] [50] [51] [27] [52] [22] [38] [53] [54] [55]
0.92 μA μM−1 cm-2 123.5 μA μM−1 cm-2
25 7.81
FeOOH/NPG microelectrode
This work
MGCE: magnetic glassy carbon electrode; SA: streptavidin; Bio-ssDNA: biotin labeled T-enriched single-stranded DNA; HNTs-Fe3O4–MnO2: halloysite nanotubes-iron oxide–manganese oxide nanocomposite; SPGE: screen-printed gold electrode; SPCCEs screen-printed electrodes; ASV: anode stripping voltammetry; CPE: carbon paste electrode; CA: Chronoamperometry.
It was further demonstrated that the developed electrochemical sensor based on the FeOOH/NPG microelectrode could offer potential practical Hg(II) detection in real samples.
3.7. Environmental sample analysis We further investigated the feasibility of the fabricated FeOOH/NPG microelectrode in practical applications. The electrochemical detection of Hg(II) in tap water and lake water (S1, S2, S3) samples was implemented using the standard addition method. No voltammetric response of Hg(II) was observed in a 0.1 M PBS solution spiked with real samples, which indicated that the concentrations of Hg(II) in tap water and lake water were lower than the LOD. Subsequently, three different concentrations of Hg(II) (0.04, 0.10, 0.20 μM) were added to the spiked samples, respectively. The current responses were recorded with the standard addition of Hg(II), and the recovery was calculated based on the corresponding linear plots. The obtained results are presented in Table 2. The results derived from the tap water and lake water samples showed the good recovery values in the range of from 98.1% and 106%.
4. Conclusions In summary, uniform FeOOH nanoflakes were successfully coated onto nanoporous gold (NPG) microwires via a simple electro-synthesis method. The fabricated FeOOH/NPG microelectrode was further developed for the electrochemical sensing of Hg(II) by square wave voltammetry (SWV). Through the integration of the large combined surface areas of the FeOOH nanoflakes and nanoporous gold, high electron-transfer of gold, and high adsorption capacity of the FeOOH nanoflakes, the FeOOH/NPG microelectrode showed excellent electrochemical performance with high sensitivity and a low detection limit of
Fig. 6. Interference studies of Hg(II) (0.40 μM) on the FeOOH/NPG microelectrode in the presence of 4.0 μM Cu(II), Pb(II), Zn(II), Cd(II), Mg(II), K(I), Ca(II), NO3−, and SO42-, respectively. 522
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Fig. 7. SWV responses of 0.4 μM Hg(III) on the FeOOH/NPG microelectrode in the presence of Cu(II) in a concentration range of from 0 to 4.0 μM in a PBS solution (pH 5.0). The inset shows the SWV responses of 0.4 μM Hg(III) in the absence of Cu(II). Deposition potential -0.40 V; deposition time 150 s; SWV setting parameters: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
Table 2 SWV detection of Hg(II) in real tap water and lake water samples. Real sample
Added (μM)
Found (μM)
Recovery (%)
Tap water
0.04 0.10 0.20 0.04 0.10 0.20 0.04 0.10 0.20 0.04 0.10 0.20
0.0407 0.0981 0.209 0.0402 0.106 0.204 0.0403 0.0992 0.203 0.0405 0.103 0.198
102 98.1 105 101 106 102 101 98.1 102 101 103 99.0
Lake Superior water S1
Lake Superior water S2
Lake Superior water S3
used for the detection of Hg(II) in real tap water and lake water samples. These results demonstrated that this simple and efficient FeOOH/ NPG microelectrode sensor could provide a feasible protocol for environmental analysis. Acknowledgements This work was supported by a Discovery grant from the Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN2015-06248). A.C. acknowledges NSERC and the Canada Foundation for Innovation (CFI) for the Canada Research Chair Award in Electrochemistry and Nanoscience. References [1] C.T. Driscoll, R.P. Mason, H.M. Chan, D.J. Jacob, N. Pirrone, Mercury as a global pollutant: sources, pathways, and effects, Environ. Sci. Technol. 47 (2013) 4967–4983. [2] N. Gandhi, S.P. Bhavsar, R.W.K. Tang, G.B. Arhonditsis, Projecting fish mercury levels in the Province of Ontario, Canada and the implications for fish and human health, Environ. Sci. Technol. 49 (2015) 14494–14502. [3] N. Gandhi, R.W.K. Tang, S.P. Bhavsar, G.B. Arhonditsis, Fish mercury levels appear to be increasing lately: a report from 40 years of monitoring in the Province of Ontario, Canada, Environ. Sci. Technol. 48 (2014) 5404–5414. [4] H. Shirkhanloo, A. Khaligh, H.Z. Mousavi, M.M. Eskandari, A.A. Miran-Beigi, Ultratrace arsenic and mercury speciation and determination in blood samples by ionic liquid-based dispersive liquid-liquid microextraction combined with flow injectionhydride generation/cold vapor atomic absorption spectroscopy, Chem. Pap. 69
Fig. 8. (a) Stability test of the FeOOH/NPG microelectrode for the analysis of 0.40 μM Hg(II) in a 0.1 M PBS solution (pH 5.0). Data were collected from the SWV response at ˜0.578 V (vs. Ag/AgCl, 1 M KCl) shown in the inset. (b) Longterm stability measurement at the FeOOH/NPG microelectrode for 0.40 μM Hg (II).
123.5 μA μM−1 cm-2 and 7.81 nM, respectively. Additionally, no obvious interference from common ions (e.g. Cu(II), Cd(II), Pb(II)), was observed, and the FeOOH/NPG microelectrode exhibited remarkable stability. Furthermore, the fabricated electrochemical sensor could be 523
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Zhonggang Liu received his PhD in electroanalytical chemistry from University of Science and Technology of China (USTC) in 2015. Then he joined Prof. Aicheng Chen’s research team as a postdoctoral fellow. His research interests are in the areas of synthesis of metal and metal oxide nanomaterials, micro- and nano-electrodes, design of electrochemical sensors/biosensor, and electroanalysis of heavy metal ions and biological molecules.
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Associate Professor in 2005 and to Full Professor in 2010. In 2017, he joined the University of Guelph as a Professor of Chemistry and the Director of the Electrochemical Technology Centre. His research interests span the areas of electrochemistry, biosensors, green chemistry, materials science and nanotechnology. He has published over 200 book chapters and peer-reviewed journal articles. He is a Tier 1 Canada Research Chair in Electrochemistry and Nanoscience, a Fellow of the Chemical Institute of Canada, a Fellow of the Royal Society of Chemistry (UK) and a Fellow of the International Society of Electrochemistry.
Emily Puumala received her MSc from Durham University, UK in 2018. She worked as a summer research assistant under the supervision of Dr. Aicheng Chen at Lakehead University, Canada in 2016 and 2017. She is pursuing her PhD degree at University of Toronto. Her interests include molecular microbiology and electrochemical sensors. Aicheng Chen received his MSc from Xiamen University in 1992 and his PhD in electrochemistry from the University of Guelph in 1998. Subsequent to working in the chemical industry as a Research Scientist and Electrochemical Specialist for four years, he joined Lakehead University in 2002 as an Assistant Professor, where he was promoted to
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Sensitive electrochemical detection of Hg(II) via a FeOOH modified nanoporous gold microelectrode Zhonggang Liua, Emily Puumalab, Aicheng Chena, a b
T
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Electrochemical Technology Centre, Department of Chemistry, University of Guelph, 50 Stone Road East, Guelph, Ontario, N1G 2W1, Canada Department of Chemistry, Lakehead University, Thunder Bay, Ontario, P7B 5E1, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: FeOOH nanoflakes Nanoporous gold microelectrode Electrochemical detection Square wave voltammetry Hg(II)
As a global scale pollutant, mercury poses serious risks for human health and the environment; thus, it is of critical importance to develop a simple and efficient sensing protocol to achieve the precise detection of Hg(II). Here we report on the facile fabrication of electro-synthesized FeOOH nanoflakes on nanoporous gold (NPG) microwires. The resulting FeOOH/NPG microelectrode was applied to the electrochemical detection of Hg(II) via square wave voltammetry (SWV). Nanoporous gold microwires with 3D network structures can provide a large specific surface area, facilitate analyte transport and electron transfer, and enhance the electro-catalytical activity of FeOOH-Au. Meanwhile FeOOH nanoflakes, with high adsorption capacities can accumulate analytes onto the electrode surface. Under the optimized conditions, excellent electrochemical performance was achieved with a high sensitivity of 123.5 μA μM−1 cm-2 and low detection limit of 7.81 nM, respectively. In addition, no obvious interference from common ions (e.g., Cu(II), Pb(II), Cd(II)) was observed, and the FeOOH/NPG microelectrode demonstrated exceptional stability. Furthermore, the fabricated electrochemical sensor could be utilized for the detection of Hg(II) in tap water and lake water samples.
1. Introduction Mercury (Hg) has been recognized as a global pollutant due to its widespread presence, and bioaccumulative and toxic nature [1]. Mercury is a neurotoxin that can damage multiple physiological systems, including neurological, immune, genetic, enzyme, cardiovascular, respiratory, and gastrointestinal [2]. Since dietary seafood is the primary route of exposure for most residents of North America, elevated levels of mercury in fish has been a well-known environmental issue in North America [3]. Increasingly stringent regulations have been implemented in the U.S. and Canada over the last few decades, with the aim of reducing environmental mercury emissions in North America. Additionally, various initiatives have been engaged to reduce mercury emissions from automobile switches and the coal-fired electricity generation, which comprises the largest remaining anthropogenic source of mercury in Canada [3]. Thus, there is an urgent need to explore a sensitive and simple analytical method for the detection and monitoring of Hg(II) in water. To date, several common analytical techniques have been employed for the detection of Hg(II), such as atomic absorption spectroscopy (AAS) [4], inductively coupled plasma-mass spectrometry (ICP-MS) [5], inductively coupled plasma optical emission spectrometry (ICP-OES)
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[6], inductively coupled plasma-atomic emission spectrometry (ICPAES) [7], fluorescence spectroscopy [8–10], colorimetry [11–13], and surface-enhanced Raman scattering. (SERS) [14,15]. However, there are several drawbacks related to these traditional laboratory-based analytical techniques, including the requirement of sophisticated instrumentation, time consuming processes, multi-step sample preparation, and complex analytical procedures, which limit the application of rapid on-site measurements. Alternatively, electrochemical techniques have been adopted by the environmental sciences due to their merits of high sensitivity, low cost, easy operation, rapid analysis, portability, and applicability, which are feasible for the monitoring of environmental samples [16]. Up to now, some excellent work has emerged for the electrochemical sensing of Hg (II). In general, electrochemical performance is strongly impacted by the effectiveness of electrode materials. Noble metal nanoparticles and carbon-based materials (e.g., graphene, carbon nanotubes (CNTs), carbon fibers, porous carbon) have been employed for the detection of Hg(II) [17–20]. Among these materials, gold-based materials are highly attractive for the electrochemical detection of Hg(II) [21–24]. For example, Mandler et al. investigated the electrochemical reduction of HAuCl4 and the electrostatic adsorption of Au NPs, for the electrochemical detection of Hg(II), and studied the effects of Au NPs. It was
Corresponding author. E-mail address: [email protected] (A. Chen).
https://doi.org/10.1016/j.snb.2019.02.080 Received 1 December 2018; Received in revised form 17 February 2019; Accepted 18 February 2019 Available online 19 February 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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2.3. Instrumentation
revealed that gold nanoparticles could serve as nucleation sites for the deposition of Hg, thus facilitating its detection [25]. Chen et al. fabricated field-effect transistor (FET) sensors based on a hybrid sensing platform (rGO and functionalized Au NPs) for the detection of Hg(II), whereby a detection limit of 25 nM was achieved [26]. Gold electrodes with different morphologies were also applied as efficient substrates for the determination of Hg(II) [21,27,28]. Nanostructured metal oxides/ hydroxides (NMOs/HOs) have promising applications as catalysts in electrochemical sensors for the monitoring of environmental contamination due to their unique electrical and electrocatalytic activity, chemical and photochemical stability, as well as enhanced electrontransfer and adsorption capacities [29–31]. In consideration of the advantages of gold-based nanomaterials and nanostructured metal oxides/hydroxides, we endeavored to develop an efficient electrochemical sensor based on gold and NMO/HO materials, for the detection of Hg(II) in environmental samples. Herein, we report on the facile fabrication of electro-synthesized FeOOH nanoflakes on nanoporous gold (NPG) microwires. The prepared FeOOH/NPG microelectrode was further advanced for the electrochemical detection of Hg(II) using square wave voltammetry (SWV). The nanoporous structure of gold microwires can provide a large specific surface area, facilitate the transport of analytes and electron transfer, while enhancing the electrocatalytic capacity of FeOOH/Au. The stability and interference resistance of the developed microelectrode were examined as well. Furthermore, the fabricated electrochemical sensor was investigated for its potential to detect Hg(II) in environmental samples.
Electrochemical experiments were carried out using a CHI 660B computer-controlled potentiostat (CH Instrument Inc., USA). A standard three-electrode system was utilized for the electrochemical measurements, where prior to and following modification, a gold microwire was employed as the working electrode; an Ag/AgCl (1 M KCl) electrode as the reference electrode, and a platinum coil as the auxiliary electrode. The structure and morphology of the gold microwire before and after modification were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi SU-70), with energy dispersive X-ray spectrometry (EDS). The crystal structures were characterized by X-ray diffraction (XRD) via a PhilipsX’Pert Pro X-ray diffractometer (Cu Kα radiation, 1.5406 Å). 2.4. Electrochemical experiments Square wave voltammetry (SWV) was employed for Hg(II) detection under optimal experimental conditions. In the detection process, a deposition potential of -0.40 V was initially applied for 150 s under stirring. The SWV responses were recorded in a potential range from -0.40 to 0.80 V (vs. Ag/AgCl, 1 M KCl), with a step potential of 4 mV, amplitude of 25 mV, and frequency of 15 Hz. Finally, a desorption potential of 0.80 V (vs. Ag/AgCl, 1 M KCl) was applied for 150 s to remove residual metals and acivate the working electrode. All of the electrochemical experiments were conducted at room temperature. The detection of Hg(II) in tap water or lake water samples was performed using a standard addition method. Tap water was collected from local venues and the lake water was from the Lake Superior. During testing, the real water samples were spiked with a 0.1 M PBS solution at a ratio of 1:9 without any further treatment.
2. Experimental 2.1. Chemical reagents
3. Results and discussion Gold wire (Ø127 μm, 99.99%) was purchased from Alfa Aesar. Analytical grade ferrous ammonium sulfate ((NH4)2Fe(SO4)2), sodium acetate (NaAc), Hg(NO3)2, Cu(NO3)2∙3H2O, Pb(NO3)2, Cd(NO3)2∙4H2O, Zn(NO3)2∙6H2O, MgSO4, KNO3, NaH2PO4, Na2HPO4, and benzyl alcohol (BA) were obtained from Sigma-Aldrich. Phosphate buffer saline (PBS, 0.1 M) solutions were prepared using 0.1 M NaH2PO4, 0.1 M Na2HPO4, and 0.1 M NaCl. All solutions were obtained using water from a Nanopure® Diamond™ UV water purification system (18.2 MΩ cm).
3.1. Electrochemical Hg(II) ion detection principle Scheme 1 illustrates the facile fabrication process of the FeOOH/ NPG microelectrode and the detection strategy for Hg(II) ions. As shown, the nanoporous gold microwire possessed a 3D interconnected network structure, and the FeOOH nanoflakes uniformly covered the surface of the gold microwire. The unique structure was of great benefit for the fabrication of the electrochemical sensor. As is well-known, for the electrochemical sensing of analytes, the oxidation peak current, ip, is directly proportional to the concentration of the analyte in solution under defined experiment conditions. The effective pre-concentration of the analyte onto the working electrode surface is important. In this detection process, a significant quantity of metal ions could be adsorbed onto the surfaces of the nanomaterials, and then released to the working electrode surface. The more metal ions that are adsorbed to the surface of nanomaterial, the more metal ions are released; hence, a stronger peak current may be obtained.
2.2. Fabrication of FeOOH modified nanoporous gold (NPG) microelectrode A nanoporous gold microwire electrode was custom-fabricated in our laboratory using an electrochemical alloying/dealloying method in a three-electrode system as shown in Scheme 1. The gold microwire served as the working electrode, whereas Zn foil and Zn wire were employed as the counter electrode and reference electrode, respectively [32]. The synthesis was carried out in a mixture of benzyl alcohol (BA) and 1.5 M ZnCl2 via cyclic voltammetry at a scan rate of 10 mV s−1 and 110 °C, where the potential was swept between -0.70 V and +1.80 V (vs. Zn) for 50 cycles. Prior to modification, the cleaning/activation of the NPG microelectrode was carried out in 0.1 M H2SO4 by recording 10 cycles in the potential range from 0.0 to 1.5 V (vs. Ag/AgCl, 1 M KCl) at 20 mV s−1. Subsequently, a facile electrochemical process was employed to prepare the modified microelectrode. The FeOOH nanoparticles were electro-synthesized in situ on the NPG microelectrode according to a technique modified from the literature, which was achieved by applying 0.7 V (vs. Ag/AgCl, 1 M KCl) for 1800s in a solution of 0.1 M NaAc, containing 10 mM ferrous ammonium sulfate [33]. The fabricated FeOOH/NPG microelectrode was subsequently rinsed with water and used for further measurements.
3.2. Surface characterization of the FeOOH/NPG microelectrode Figs. 1a and b depict SEM images of the gold microwire being treated via the alloying/dealloying method in a benzyl alcohol/1.5 M ZnCl2 electrolyte. As seen in Fig. 1a, a hierarchical porous structure was formed on the surface of the gold microwire. A 3D interconnected network with pore dimensions from ˜100 to ˜300 nm was observed under a high-magnification SEM image (Fig. 1b). Sequentially, the FeOOH nanoflakes were directly grown onto the surface of gold microelectrode (Fig. 1c and d) via a facile electrochemical deposition process. It may be seen from Fig. 1c that large quantities of FeOOH nanoflakes uniformly covered over the surface of the gold microwire. Fig. 1d presents a high-magnification SEM image, where the FeOOH nanoflakes were assembled with a narrow thickness distribution 518
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Scheme 1. Schematic illustration of the effective electrochemical detection of Hg(II) using the FeOOH/NPG microelectrode.
electrodeposition of the FeOOH nanoflakes. The calculated average crystallite size of FeOOH nanoflakes was ˜79.0 nm from the (110) diffraction line.
between 20 and 40 nm. Fig. 2a displays the EDS spectra of the formed as-prepared nanoporous gold microelectrode and FeOOH/NPG microelectrode. The strong peak for Au at ˜2.1 keV and weak peaks at ca. 9.7 and 11.5 keV are observed in the EDS spectrum of the nanoporous gold microelectrode. Followed the electrodeposition of FeOOH, distinct peaks for Fe at ca. 0.7, 6.4, and 7.0 keV appeared, which confirmed the successful electrodeposition of FeOOH nanoflakes on the surface of the nanoporous gold microelectrode. The NPG microelectrode and the fabricated FeOOH/NPG microelectrode were characterized by XRD, and their typical XRD patterns are presented in Fig. 2b. The peaks centred at ca. 38.3°, 44.4°, 64.6°, 77.6°, and 81.7° could be indexed to the (111), (200), (220), (311), and (222) planes of gold (face centered cubic (fcc), JCPDS no. 01-1172), respectively [34]. The average crystallite sizes of the nanoporous gold microelectrode were calculated using the DebyeScherrer equation, to be ˜67.4 nm, based on the (111) diffraction line. Following the electrodeposition of FeOOH, a well-defined peak at ca 21.3° appeared, which corresponded to the (110) plane of α-FeOOH (JCPDS no. 81-0462), respectively [35,36], which verified the
3.3. Electrochemical behaviors of the FeOOH/NPG microelectrode The electrochemical behaviors of the nanoporous gold microelectrode were investigated, prior to and following the electrodeposition of FeOOH. Fig. 3 shows the square wave voltammetric (SWV) responses of 0.40 μM Hg(II) on the nanoporous gold microelectrode and the FeOOH/ NPG microelectrode in 0.1 M PBS solution (pH 5.0). Only a small oxidation peak of Hg(II) was observed at the NPG microelectrode (red line, Fig. 3a). However, in the case of the FeOOH modified NPG microelectrode, a well-defined response of Hg(II) at 0.560 V (vs. Ag/AgCl, 1 M KCl) appeared (blue line, Fig. 3b), which was ascribed to the re-oxidation of Hg(0) to Hg(II). The dashed line represents the SWV curve in the absence of Hg(II). For comparison, the electrochemical active surface areas (EASAs) of the NPG microelectrode before and after the insitu electro-synthesis of FeOOH were calculated; thus, the current
Fig. 1. SEM images of (a, b) nanoporous gold (NPG) microwire and (c, d) FeOOH-modified NPG microwire. 519
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Fig. 3. SWV responses of 0.40 μM Hg(II) at (a) the NPG microelectrode and (b) the FeOOH/NPG microelectrode in 0.1 M PBS solution (pH 5.0). The dotted lines refer to the baselines. Deposition potential -0.40 V; deposition time 150 s; SWV setting parameters: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
Fig. 2. (a) EDS spectra of the formed as-prepared nanoporous gold microwire and FeOOH/NPG microwire. (b) XRD pattern of NPG microwire (black line) and FeOOH/NPG microwire (red line) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
examined in the analysis of 0.4 μM Hg(II), with a deposition potential of -0.40 V (vs. Ag/AgCl, 1 M KCl) in 0.1 M PBS solution (pH 5.0). As shown in Fig. 4b, the peak current densities increased with longer deposition times, from 90 to 150 s, and then slowly increased with deposition times beyond 150 s. Therefore, a deposition time of 150 s was selected in order to attain a lower detection limit and wider range of response. The effect of pH on the electrochemical performance of the FeOOH/ NPG microelectrode was also evaluated. Fig. 4c displays the current response of 0.4 μM Hg(II) in a 0.1 M PBS solution wherein the pH values varied from 4.0 to 6.0. As is clearly shown in Fig. 4c, the peak current densities of Hg(II) increased when the pH values changed from 4.0 to 5.0. However, further increases in pH resulted in lower current densities. This may have been associated with the hydrolysis of Hg(II), which decreased of the activity of Hg(II) [37,38]. It is worth noting that the pHpzc for the FeOOH has been reported as 6.74 ± 0.31 [39,40], and that the FeOOH surface is positively charged when the pH value is less than pHpzc. In the presence of 0.1 M NaCl, HgCl3− was predominated in the electrolyte; thus the FeOOH with the multi-nanoflake structure could be benefit for the adsorption of the negatively charged Hg(II) species, leading to the improved electrochemical sensing of Hg (II) [41]. Thus, based on the aforementioned results, pH 5.0 was employed for further measurements.
density (j) was determined. The measurements were carried out in a 0.1 M KCl solution that contained 5 mM Fe(CN)63−/4−, and the EASAs were calculated based on the Randles–Sevcik equation. The EASAs of the NPG microelectrode prior to and following modification were calculated to be 7.69 ± 0.27 mm2 and 3.87 ± 0.22 mm2, respectively. Herein, it may be observed in Fig. 3 that the peak current density of Hg (II) obtained at the FeOOH-modified NPG microelectrode was increased by more than seven-fold over that of at the NPG microelectrode.
3.4. Optimization of experimental conditions Since several experimental parameters might influence the electrochemical performance of the FeOOH/NPG microelectrode toward Hg (II), the experimental parameters (deposition potential, deposition time, pH values) were optimized and discussed. Fig. 4a shows the peak current densities of 0.4 μM Hg(II) on the FeOOH/NPG microelectrode by varying the deposition potential between -0.70 and -0.30 V (vs. Ag/ AgCl, 1 M KCl). As the potential shifted negatively, from -0.30 V to -0.40 V (vs. Ag/AgCl, 1 M KCl), the current densities increased significantly, which was due to the enhanced kinetics, that is, Hg(II) could be easily reduced at more negative deposition potentials. The current densities attained a maximum at a deposition potential of -0.40 V (vs. Ag/AgCl, 1 M KCl). However, when the deposition potential continued to negatively shift, from -0.40 V to -0.70 V (vs. Ag/AgCl, 1 M KCl), the current densities decreased, which was attributed to the interference of hydrogen evolution. Thus, the deposition potential of -0.40 V (vs. Ag/ AgCl, 1 M KCl) was selected as optimal. Different deposition times of 90, 120, 150, 180, and 210 s were
3.5. Electrochemical detection of Hg(II) with SWV Fig. 5a presents SWV curves for the electrochemical detection of Hg (II) at the FeOOH/NPG microelectrode in a PBS solution (0.1 M, pH 5.0) based on optimized conditions. The well-defined peak for Hg(II) may be clearly observed at +0.560 V with higher concentrations of Hg(II). The 520
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Fig. 4. Dependence of (a) deposition potential, (b) deposition time and (c) pH values on the voltammetric response of 0.40 μM Hg(II). Data were collected from the SWV response of 0.40 μM Hg (II) at the FeOOH/NPG microelectrode in a 0.1 M PBS solution (pH 5.0). The setting parameters in the SWV technique: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
adsorption sites; Hg(II) ions in the aqueous solution were initially adsorbed and accumulated on the surfaces of the FeOOH nanoflakes during the electrochemical sensing process. The adsorbed Hg(II) ions could easily diffuse from the FeOOH nanoflake surface onto the interconnected nanoporous gold, where the Hg(II) ions could be effectively reduced and stripped off from the surface, leading to the highly sensitive detection of Hg(II).
3.6. Interference and stability measurements Interference from common ions on the electrochemical analysis of Hg(II) was further investigated. Fig. 6 depicts the voltammetric responses of 0.40 μM Hg(II) in the absence, or presence of ions, including Cu(II), Cd(II), Pb(II), Zn(II), Mg(II), K(I), Ca(II), NO3−, SO42-. It may be observed that even though the concentrations of these ions were 10 times higher than that of Hg(II), there was no obvious interference (within ± 1.81%) on the response of Hg(II). Among all of the potential interference ions, since Cu(0) had a slightly more negative potential than did Hg(0), it may have caused competitive oxidation on the electrode, leading to potential interference. Herein, the effect of Cu(II) on the electrochemical sensing of Hg(II) was emphasized. Fig. 7 displays the SWV curves of 0.40 μM Hg(II) at the FeOOH/NPG microelectrode in the presence of Cu(II), where the concentration of Cu(II) ions was in the range of from 0 to 4.0 μM. The inset shows the SWV curves of 0.40 μM Hg(II) in the absence of Cu(II). As shown in this study, distinct and well-defined oxidation peaks for Hg(II) and Cu(II) may be clearly observed, as the Cu(II) was increased up to 4.0 μM. The current responses of Cu(II) were increased with the addition of Cu(II), while the current responses of 0.40 μM Hg(II) remained almost constant. The peak current densities of 0.40 μM Hg(II) were estimated to be 67.1 ± 3.71 μA cm-2 in the presence of different concentrations of Cu (II) ions, even when their concentrations were 10 times higher than that of Hg(II) (0.40 μM). Further, the coexisting Cu(II) did not affect the electrochemical analysis of Hg(II). These results indicated that the FeOOH/NPG microelectrode possessed an efficient anti-interference capacity. Repetitive measurements of the detection of Hg(II) were performed to estimate the stability of the FeOOH-modified NPG microelectrode, with the corresponding results presented in Fig. 8a. This was realized by recording the current responses of 0.40 μM Hg(II) at the FeOOH/NPG microelectrode over 20 cycles using SWV (Fig. 8a inset). No obvious change in the SWV responses of Hg(II) was observed with its continuous sensing. The relative standard deviation (RSD) was calculated to be 1.10%, based on the 20-cycle current responses. Further, Fig. 8b presents the extended stability (30 days) of the FeOOH/NPG microelectrodes, which was evaluated by the current response of 0.40 μM Hg(II) in a 0.1 M PBS solution with SWV. As shown, the SWV responses of Hg (II) at the FeOOH/NPG microelectrodes were almost constant over 30 days, with a RSD of 2.63%. These results demonstrated that the FeOOH/NPG microelectrode demonstrated excellent reproducibility under repeated measurements, along with prolonged stability.
Fig. 5. (a) Typical SWV responses of Hg(III) on the FeOOH/NPG microelectrode over a concentration range of from 0.02 to 2.2 μM in a PBS solution (pH 5.0). The dotted line refers to the baseline. (b) The corresponding linear calibration plot. Deposition potential -0.40 V; deposition time 150 s; SWV setting parameters: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
calibration curve of the FeOOH/NPG microelectrode for Hg(II) detection presented in Fig. 5b, shows a sensitivity of 123.5 μA μM−1 cm-2, which gave a correlation coefficient of 0.995 over a linear range of from 0.02 μM to 2.2 μM. The calculated detection limit was 7.81 nM, which was lower than a guideline value of 30 nM defined by the World Health Organization (WHO) [42]. Furthermore, a comparison of electrochemical performance for the detection of Hg(II) with other previously reported electrochemical sensors is summarized in Table 1. Based on the listed results, it was indicated that the FeOOH/NPG microelectrode exhibited favorable electrochemical performance for Hg(II) detection with high sensitivity and a low detection limit. As illustrated in Scheme 1 and Fig. 1, the synergetic effects of the designed electrochemical sensor for the sensitive detection of Hg(II) can be attributed to the excellent adsorption capacity of the FeOOH nanoflakes and the high catalytic properties of the nanoporous gold. The electro-synthesized FeOOH with the multi-nanoflake structure could provide numerous 521
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Table 1 Comparison of performance for the voltammetric determination of Hg(II) at modified electrodes. Electrodes
Technique
Linear range (μM)
Sensitivity (μA μM−1)
LOD (nM)
Ref.
ssDNA/Au electrode Gold ultramicroelectrode arrays Screen-printed gold electrode On-chip integrated Au–Ag–Au three-electrode polypyrrole/rGO/GCE Fe3O4-chitosan/GCE Bio-ssDNA/Fe3O4-SA/MGCE HNTs-Fe3O4–MnO2/CPE Au-SPCE CB-15-crown-5/GEC SPGE CalixareneSPCCEs Au-NPs/-SH/rGO/GCE Au–TiO2NPGs/Chit/gold electrode Hydroxyapatite/GCE NiCo2O4/Ni foam ZnO/Nafion/Au electrode
DPV LSV SWV DPV SWV SWV DPV DPV SWV DPV SWV ASV DPV DPV SWV SWV LSV CA SWV
0.1 – 2.0 0.01 – 1.0 0.025 – 0.15 0.05 – 5.0 0 – 0.1 0.4 – 1.1 0.001 – 0.1 0.0025 – 0.75 0.005 – 0.5 0.5 – 1.0 0.1 – 1.4 0.453 – 11.9 1 – 10 0.005 – 0.4 0.2 – 210 0.8 – 2.8 25 – 250 0.025 – 0.25 0.02 – 2.2
– 0.11 nC/ppb 7.16 1.32 124 9.65 13.55logc
0.0237 2.2 9.17 0.55 3.13 0.31 23.7
100 16 5.5 15 15 95.7 0.33 1 5 60 3.5 239 200 1.0 141 9.91
[43] [44] [45] [46] [47] [48] [49] [29] [50] [51] [27] [52] [22] [38] [53] [54] [55]
0.92 μA μM−1 cm-2 123.5 μA μM−1 cm-2
25 7.81
FeOOH/NPG microelectrode
This work
MGCE: magnetic glassy carbon electrode; SA: streptavidin; Bio-ssDNA: biotin labeled T-enriched single-stranded DNA; HNTs-Fe3O4–MnO2: halloysite nanotubes-iron oxide–manganese oxide nanocomposite; SPGE: screen-printed gold electrode; SPCCEs screen-printed electrodes; ASV: anode stripping voltammetry; CPE: carbon paste electrode; CA: Chronoamperometry.
It was further demonstrated that the developed electrochemical sensor based on the FeOOH/NPG microelectrode could offer potential practical Hg(II) detection in real samples.
3.7. Environmental sample analysis We further investigated the feasibility of the fabricated FeOOH/NPG microelectrode in practical applications. The electrochemical detection of Hg(II) in tap water and lake water (S1, S2, S3) samples was implemented using the standard addition method. No voltammetric response of Hg(II) was observed in a 0.1 M PBS solution spiked with real samples, which indicated that the concentrations of Hg(II) in tap water and lake water were lower than the LOD. Subsequently, three different concentrations of Hg(II) (0.04, 0.10, 0.20 μM) were added to the spiked samples, respectively. The current responses were recorded with the standard addition of Hg(II), and the recovery was calculated based on the corresponding linear plots. The obtained results are presented in Table 2. The results derived from the tap water and lake water samples showed the good recovery values in the range of from 98.1% and 106%.
4. Conclusions In summary, uniform FeOOH nanoflakes were successfully coated onto nanoporous gold (NPG) microwires via a simple electro-synthesis method. The fabricated FeOOH/NPG microelectrode was further developed for the electrochemical sensing of Hg(II) by square wave voltammetry (SWV). Through the integration of the large combined surface areas of the FeOOH nanoflakes and nanoporous gold, high electron-transfer of gold, and high adsorption capacity of the FeOOH nanoflakes, the FeOOH/NPG microelectrode showed excellent electrochemical performance with high sensitivity and a low detection limit of
Fig. 6. Interference studies of Hg(II) (0.40 μM) on the FeOOH/NPG microelectrode in the presence of 4.0 μM Cu(II), Pb(II), Zn(II), Cd(II), Mg(II), K(I), Ca(II), NO3−, and SO42-, respectively. 522
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Fig. 7. SWV responses of 0.4 μM Hg(III) on the FeOOH/NPG microelectrode in the presence of Cu(II) in a concentration range of from 0 to 4.0 μM in a PBS solution (pH 5.0). The inset shows the SWV responses of 0.4 μM Hg(III) in the absence of Cu(II). Deposition potential -0.40 V; deposition time 150 s; SWV setting parameters: step potential 5 mV; amplitude 25 mV; frequency 15 Hz.
Table 2 SWV detection of Hg(II) in real tap water and lake water samples. Real sample
Added (μM)
Found (μM)
Recovery (%)
Tap water
0.04 0.10 0.20 0.04 0.10 0.20 0.04 0.10 0.20 0.04 0.10 0.20
0.0407 0.0981 0.209 0.0402 0.106 0.204 0.0403 0.0992 0.203 0.0405 0.103 0.198
102 98.1 105 101 106 102 101 98.1 102 101 103 99.0
Lake Superior water S1
Lake Superior water S2
Lake Superior water S3
used for the detection of Hg(II) in real tap water and lake water samples. These results demonstrated that this simple and efficient FeOOH/ NPG microelectrode sensor could provide a feasible protocol for environmental analysis. Acknowledgements This work was supported by a Discovery grant from the Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN2015-06248). A.C. acknowledges NSERC and the Canada Foundation for Innovation (CFI) for the Canada Research Chair Award in Electrochemistry and Nanoscience. References [1] C.T. Driscoll, R.P. Mason, H.M. Chan, D.J. Jacob, N. Pirrone, Mercury as a global pollutant: sources, pathways, and effects, Environ. Sci. Technol. 47 (2013) 4967–4983. [2] N. Gandhi, S.P. Bhavsar, R.W.K. Tang, G.B. Arhonditsis, Projecting fish mercury levels in the Province of Ontario, Canada and the implications for fish and human health, Environ. Sci. Technol. 49 (2015) 14494–14502. [3] N. Gandhi, R.W.K. Tang, S.P. Bhavsar, G.B. Arhonditsis, Fish mercury levels appear to be increasing lately: a report from 40 years of monitoring in the Province of Ontario, Canada, Environ. Sci. Technol. 48 (2014) 5404–5414. [4] H. Shirkhanloo, A. Khaligh, H.Z. Mousavi, M.M. Eskandari, A.A. Miran-Beigi, Ultratrace arsenic and mercury speciation and determination in blood samples by ionic liquid-based dispersive liquid-liquid microextraction combined with flow injectionhydride generation/cold vapor atomic absorption spectroscopy, Chem. Pap. 69
Fig. 8. (a) Stability test of the FeOOH/NPG microelectrode for the analysis of 0.40 μM Hg(II) in a 0.1 M PBS solution (pH 5.0). Data were collected from the SWV response at ˜0.578 V (vs. Ag/AgCl, 1 M KCl) shown in the inset. (b) Longterm stability measurement at the FeOOH/NPG microelectrode for 0.40 μM Hg (II).
123.5 μA μM−1 cm-2 and 7.81 nM, respectively. Additionally, no obvious interference from common ions (e.g. Cu(II), Cd(II), Pb(II)), was observed, and the FeOOH/NPG microelectrode exhibited remarkable stability. Furthermore, the fabricated electrochemical sensor could be 523
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Zhonggang Liu received his PhD in electroanalytical chemistry from University of Science and Technology of China (USTC) in 2015. Then he joined Prof. Aicheng Chen’s research team as a postdoctoral fellow. His research interests are in the areas of synthesis of metal and metal oxide nanomaterials, micro- and nano-electrodes, design of electrochemical sensors/biosensor, and electroanalysis of heavy metal ions and biological molecules.
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Associate Professor in 2005 and to Full Professor in 2010. In 2017, he joined the University of Guelph as a Professor of Chemistry and the Director of the Electrochemical Technology Centre. His research interests span the areas of electrochemistry, biosensors, green chemistry, materials science and nanotechnology. He has published over 200 book chapters and peer-reviewed journal articles. He is a Tier 1 Canada Research Chair in Electrochemistry and Nanoscience, a Fellow of the Chemical Institute of Canada, a Fellow of the Royal Society of Chemistry (UK) and a Fellow of the International Society of Electrochemistry.
Emily Puumala received her MSc from Durham University, UK in 2018. She worked as a summer research assistant under the supervision of Dr. Aicheng Chen at Lakehead University, Canada in 2016 and 2017. She is pursuing her PhD degree at University of Toronto. Her interests include molecular microbiology and electrochemical sensors. Aicheng Chen received his MSc from Xiamen University in 1992 and his PhD in electrochemistry from the University of Guelph in 1998. Subsequent to working in the chemical industry as a Research Scientist and Electrochemical Specialist for four years, he joined Lakehead University in 2002 as an Assistant Professor, where he was promoted to
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