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Electrochemical Detection of Low Concentrations of Mercury in Water Using Gold Nanoparticles Noga Ratner, and Daniel Mandler Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 19 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015
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Analytical Chemistry
Electrochemical Detection of Low Concentrations of Mercury in Water Using Gold Nanoparticles Noga Ratner and Daniel Mandler* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel Keywords: Mercury, gold nanoparticles, electrochemistry, glassy carbon, indium tin oxide Abstract: The electrochemical detection of mercury in aqueous solutions was studied at glassy carbon (GC) and indium-tin oxide (ITO) electrodes modified by gold nanoparticles (AuNPs). Two methods of modification were used; electrochemical reduction of HAuCl4 and electrostatic adsorption of AuNPs stabilized by citrate. We found that the AuNPs modified surfaces yielded higher sensitivity and sharper and more reproducible stripping peaks of Hg as compared with the bare electrodes. The effect of the modification by AuNPs on the stripping potential was examined. Interestingly, the stripping of Hg on GC and ITO modified by AuNPs occurred at the same potential as on bare GC and ITO, respectively. Only, the full coverage of ITO by either electrochemically deposited Au for a long time or by vapor deposition shifted the stripping potential more positive by ca. 0.4 V to that observed on a bare Au electrode. These and further experiments led us to conclude that the AuNPs served as nucleation sites for the deposition of Hg, whereas the GC or ITO are superior for the stripping of mercury. Hence, a combination of well-defined AuNPs on ITO or GC were found ideal for the electrochemical detection of Hg. Indeed, we achieved a remarkable detection limit of 1 µm.L-1 of Hg using an ITO surface modified by electrostatically adsorbed AuNPs. According to WHO - the world's health organization; mercury is one of the ten most dangerous chemicals to public health.1 Mercury is a heavy metal that contaminates the water and is often accumulated throughout the food chain, thus posing a serious threat to the environment, animals and humans by causing damage to the brain, heart, kidneys, lungs and the immune system.1,2 Due to these life threatening hazards; developing protocols for sorting, identifying and quantifying mercury are of utmost importance. Accordingly, numerous studies using various approaches, such as atomic absorption spectrometry3 colorimetric detection4 and cold vapor atomic fluorescence spectrometry (CVAFS)5 have been published aiming at monitoring mercury, amongst which electrochemistry is a particular appealing method.6 The advantages of the latter are its high inherent sensitivity, low-cost, simple operation, fast analysis and on-site out-door applicability. As a matter of fact, even the EPA recommends using electrochemistry for the detection of mercury.6 Indeed, substantial efforts have been spent on the development of high sensitive electrochemical methods, mostly based on voltammetric determination using solid electrodes. Gold and glassy carbon (GC) have been the most common working electrodes applied for Hg electrochemical determination. Surveying these studies reveals that 0.07 ppb Hg can be determined in tap, river and sea waters on a vibrating gold microwire electrode7. At the same time, most studies employing GC electrodes tend to modify their surface in order to obtain level of detection (LOD) in the ppb range8-10. The lowest LOD by a bare GC electrode is, to the best of our knowledge, ca. 0.1 µm.L-1 on a glassy carbon vessel macroelectrode using chronopotentiometric stripping analysis with 10 min. deposition.11 Comparing the pros and cons of Au vs. GC suggests that the determination of Hg on a gold electrode is highly reproducible and always results in a broad oxidation peak (which is used for Hg determination) at ca. 0.57 V vs. Ag/AgCl. This wide peak may overlap with some other oxidation waves, and in general, is not very suitable for detection of very low levels. An additional drawback is the formation of amalgam and the high diffusion of Hg in Au, which fouls the electrode especially at higher Hg concentrations. On the other hand, GC is not a
well-defined surface, which very often yields very sharp peaks that are very convenient for Hg oxidation at 0.15 V vs. Ag/AgCl. Moreover, the reduction and deposition of Hg onto GC is highly affected by the pretreatment of the electrode surface, causing the analysis of Hg by GC to be sometimes irreproducible. Nevertheless, Hg can be efficiently removed from the GC electrode upon oxidation. Hence, it would be highly beneficial to combine the advantages of both Au and GC electrodes for the detection of Hg, which is the essence of this work. We envisioned that by modifying a solid electrode, such as GC and indium tin oxide (ITO) with Au nanoparticles, AuNPs, we will introduce nucleation sites for Hg reduction and precipitation and at the same time eliminate fouling of the electrodes by the deposit. Moreover, we believed that the sharp peaks obtained upon Hg oxidation on GC will remain on these nanoparticles modified surfaces. AuNPs are widely used in various research areas and applications. They can easily be prepared with narrow size distribution and are highly stable. It should be noticed that there are a few studies where Hg was electrochemically determined using AuNPs. Abollino et al. reported the detection of Hg and CH3-Hg using a GC electrode modified with AuNPs.12 The latter were applied on the electrode surface by dipping the electrode into a HAuCl4 solution and applying a potential of −0.80 V for 6 min. They reached a detection limit of 0.2 ppb upon two min. of deposition. The oxidation peak they reported was at 0.57 V vs. Ag/AgCl. Hezard et al. also described the detection of Hg using a GC electrode modified by AuNPs.13 The AuNPs on the modified GC electrodes were obtained by cyclic voltammetry scanning from an open-circuit potential for different number of scans. Their reported detection limit was 0.42 µm.L-1, achieved upon deposition Hg for 5 min. and applying square wave anodic stripping voltammetry (SWASV). The oxidation peak was also at 0.57 V vs. Ag/AgCl. More recently Behzad et al. claimed to reach a detection limit of 0.5 pM (equals to 0.1 µm.L-1), however, the oxidation peak was at −0.63 V, a value that is inconsistent with any known process of oxidation-reduction of mercury.14 A study conducted by Zhou et al. on an ITO electrode modified with 5-methyl-2-thiouracil, graphene oxide and AuNPs reached a LOD of 0.2 ppb in bottled water.15
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Here we describe a detailed study in which we examine two different methods for the formation of GC and ITO electrode modified with AuNPs. The first method comprised the electrochemical deposition of Au onto these electrodes to form AuNPs. The second approach involved the adsorption of presynthesized well-defined AuNPs stabilized by citrate, which were electrostatically attached onto the solid electrodes modified by a very thin layer of polyethylenimine (PEI). We found that the nanoparticles enhanced the deposition of Hg and as a result increased the reproducibility of the electrodes towards Hg detection and their sensitivity. The adsorbed AuNPs showed better performance in terms of Hg determination and enabled the detection of 1 µm.L-1 of Hg. Finally, a mechanism is proposed to account for the effect of the AuNPs on GC and ITO on the enhanced detection of Hg. Materials and Methods Instrumentation Voltammetry was recorded using both CHI 750B computercontrolled electrochemical analyzer (CH Instruments, TX, USA) and a PGSTAT10 Autolab potentiostat (EcoChemie, Utrecht, The Netherlands) using GPES software (version 4.9). All measurements were performed at room temperature with a standard 10 mL three-electrode electrochemical cell. Indium tin oxide (ITO) electrodes were purchased from Delta Technologies (CG-60IN-CUV, CO, USA). Glassy carbon (GC) and gold electrodes were assembled by sealing a GC (3 mm diameter, purchased from Atomergic Chemetals Corp., Farmingdale, NY, USA or Tokai Carbon Co., Ltd., Tokio, Japan) or gold rod (99.99%, Holland-Moran, Israel) in a Teflon tube under pressure. Pt wire (0.5 mm diameter, 99.99%) and a commercial Ag|AgCl (3 M KCl, purchased from CH Instruments, TX, USA) were used as counter and reference electrodes, respectively. A thin film deposition system (Vacuum Coater, VST TFDS-141E, VST products, Delhi, India) was used to evaporate gold on ITO plates. Extra high-resolution scanning electron microscopy (XHR SEM) was carried out with Magellan 400L (FEI company, The Netherlands) equipped with a large area EDS silicon drift detector Oxford X-Max (Oxford Instruments, UK). Chemicals Mercury standard solution (Hg(NO3)2 in HNO3 2 M) 1001±2 mg/L and ethanol were purchased from Merck (Darmstadt, Germany), chloroauric acid (HAuNPs ≥97%), polyethylenimine 50% (w/v) and potassium chloride (≥99.9995%) were obtained from Fluka (Buchs, Switzerland). Trisodium citrate (99%) was purchased from BDH (PA, USA), acetone (AR) was obtained from Gadot (Israel). Gold (99.999%) for vapor deposition was purchased from Kurt J. Lesker (PA, USA). All solutions were prepared using deionized water (18.3 MΩ.cm, EasyPure UV, Barnstead). Procedures GC and Gold electrodes were first polished with silicon carbide grinding papers (down to 1200 grit, Buehler, IL, USA) followed by alumina slurry (1 and 0.05 µm, Buehler, IL, USA) and finally sonicated in water for five minutes. The gentle polishing and cleaning was performed occasionally. ITO electrodes were first sonicated in an equal volume solution of H2O2 (30% wt) and NH4OH for 10 minutes. Then, the electrodes were sonicated in ethanol (10 min) and acetone (10
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min). Finally, the electrodes were sonicated in deionized water for additional 10 minutes and dried with clean nitrogen. Two protocols were used for the deposition of AuNPs onto GC and ITO electrodes. The first protocol comprised the electrochemical reduction of AuCl4−. Specifically, a negative potential (0.3 V vs. Ag/AgCl) was applied to the electrode in a stirred 1 mM HAuCl4 and 0.1 M KCl for a given time. The second involved the reduction of AuCl4- by sodium citrate as previously described16. The AuNPs were 14.2 ± 2.3 nm. To electrostatically adsorb the AuNPs on the electrodes surface, we first dipped them in polyethylenimine (PEI, 0.05% wt in water) for 5 minutes. The electrodes were thoroughly washed with water for 10 min. Then they were dipped into the AuNPs solution for a given time. Coating of ITO electrodes with a precise thickness of Au was carried out by first evaporating a thin, ca. 5 nm thick chromium layer, on which a gold layer was deposited. The thickness of the gold coating was 20, 40, 60 and 80 nm. Gold was removed from ITO surfaces upon applying 1.5 V in 0.1 M HCl. Most of the electrochemical experiments were performed using linear sweep voltammetry (LSV). Background was acquired for both the bare and the modified electrodes, using 0.1 M KCl solution as the electrolyte. After Hg(II) was added to the solution in the tested concentration, mercury deposition was accomplished by applying a negative potential of −0.3 V (specific deposition time will be noted for each experiment). The removal of mercury from the working electrodes was achieved by applying 0.7 V for 60 s. Results and Discussion The electrochemistry of Hg2+ on GC, ITO and Au electrodes The electrochemical detection of low levels of mercury in aqueous solutions is significantly affected by the nature of the working electrode. Figure 1 shows the linear sweep voltammetry (LSV) of 1 mg.L-1 of mercury using gold, glassy carbon (GC) and indium tin oxide (ITO) electrodes. The electrolyte was 0.1 M KCl and the scan rate 0.1 V/s. In all electrodes a clear oxidation wave of the pre-deposited mercury can be observed. However, while the peak potential of mercury oxidation on Au electrode was ca. 0.57 V (Figure 1A-B), the oxidation peaks on GC (Figure 1C-D) and ITO (Figure 1E-F) were at 0.18 V. The significant difference in the potential of mercury oxidation must be related to the electrode material and its interaction with mercury. The presence of chloride is also important as it associates with the oxidized forms of mercury, i.e. Hg22+ and Hg2+ to form a variety of mercurous and mercuric chloride species, amongst which Hg2Cl2 is dominant under our experimental conditions. The peak potentials on GC and ITO electrodes are close to the redox potential of mercury to form the insoluble species Hg2Cl2 (eq. 1). Yet there is a shift of ca. 0.15 V between the peak potentials (Figure 1C-F) and that of eq. 1. (eq. 1) Hg 2 Cl 2 + 2 e − → 2 Hg + 2Cl − E 0 = 0 .268 V vs . NHE 17 The oxidation of Hg to form mercuric chloride (eq. 2) is also plausible. The redox potential of eq. 2 under our experimental conditions, i.e., 0.1 M KCl, is ca. 0.42 V vs. NHE, which is ca. 0.21 V vs. Ag/AgCl (3 M KCl). HgCl2 can basically form a passivation layer; however, not under our experimental conditions because of the low coverage of Hg. (eq. 2) HgCl 2 + 2 e − → Hg + 2Cl −
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On the other hand, the peak potential on Au at more positive potential can be explained by the increased stability of the amalgam that is formed. Hence, we assign the oxidation of Hg on Au to the anodic dissolution of the amalgam (eq. 3) for which we could not find the standard redox potential. (eq. 3) Hg 2 Cl 2 + 2 e − → 2 Hg ( Au ) + 2Cl − It is worth mentioning that Abruna, who studied the deposition of Hg on Au(111),18 attributed a stripping peak at the same potential to the oxidation of calomel to Hg2+ (eq. 4). We cannot exclude this possibility; which means that mercury is oxidized at less potentials to Hg2Cl2. (eq. 4) Hg 2 Cl 2 → 2 Hg 2 + + 2Cl − + 2 e − Figure 1A-B shows the effect of potential and time of deposition on the oxidation wave of mercury on Au. Clearly, the oxidation peak increased as the potential was made more negative and time of deposition was increased, which was due to the accumulation of mercury on the electrode surface. The same behavior was found for the GC and ITO electrodes (Figures 1C and 1E). Yet, whereas the reproducibility of repetitive measurements on GC is poor (Figure 1D) that on ITO is satisfactory (Figure 1F). We attribute the poor reproducibility on GC to the nucleation of the mercury, vida infra.
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Another difference between the oxidation peaks on Au vs. GC and ITO concerns their shape. While the mercury oxidation wave on Au is broad the shapes of the peaks on GC and ITO are much narrower, which is advantageous for the detection of low levels of mercury in the presence of other electroactive species.
Detection of Hg2+ by GC and ITO electrodes modified by AuNPs With the aim of combining the superior features for sensing mercury; good kinetics of mercury reduction on gold, sharp mercury oxidation peaks on GC and high reproducibility of stripping peaks on ITO, we decided to integrate gold nanoparticles at the interface of GC and ITO electrodes. GC and ITO are non-porous and do not form amalgam with mercury, contrary to gold. Our hypothesis was that AuNPs will serve as nucleation sites for depositing and oxidizing mercury on GC and ITO, and at the same time, the formation of amalgam will be eliminated and the oxidation signals will be sharp and highly reproducible. In other words by combining the characteristics of two surfaces we believed that the determination of Hg can be significantly enhanced and improved. It is worth noticing that the AuNPs are to become an integral part of the working electrode, and hence, should be used repetitively. Indeed, the removal of mercury from ITO or GC is accomplished at 0.7 V, while Au is oxidized at a more positive potential of ca. 0.9 V, and therefore, the careful removal of mercury should not oxidize the AuNPs. A
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Figure 1B shows also the effect of the applied potential on the stripping of Hg. It can be seen that the effect of potential is relatively small, i.e. the current almost did not increase by applying potentials that were more negative than −0.1 V. Hence, it is conceivable that under this potential, mercuric ions were reduced under diffusion-controlled conditions. Clearly, it is beneficial to apply a deposition potential that is as positive as possible to avoid deposition of other species. The effect of deposition is shown in Figures 1A, C and E. For all cases, as the time of deposition increased the current increased as well. The main disadvantage of GC electrodes is shown in Figure 1D, which is lack of reproducibility in Hg stripping. Performing five repetitive experiments resulted in continuously increasing of the oxidation wave of Hg, which suggests that either Hg is not removed efficiently after deposition or that deposition becomes more efficient. Since the blank background conducted after each experiment did not show mercury traces, we attributed the increase of the peaks (Figure 1D) to facilitating the reduction of Hg2+ as a result of its deposition. Evidently, this problem needs to be solved for the successful application of the GC electrode for Hg determination. On the other hand, highly reproducible waves were obtained on ITO (Figure 1F). It should be noted that ITO was only scarcely applied for the detection of Hg15.
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Figure 2. AuNPs after electrodeposition of 1 mM HAuCl4- for 30 seconds at 0.3 V. A-B: SEM images (200,000). A- GC, B- ITO. C-D: CV of the AuNPs modified surfaces (C- GC and D-ITO) in a 0.1 M KCl solution. The scans commenced at 0.7 V to positive potentials and then back. Scan rate was 0.01 V/s. A
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Two methods for integrating AuNPs on GC and ITO surfaces were examined. The first method involved the electrochemical reduction of AuCl4- (0.3 V for 30 s in 1 mM AuCl4- and 0.1 M KCl) similarly to the previously described procedure19. Evidence for the deposited gold can be seen in Figure 2, which shows the SEM images and CVs of the GC and ITO surfaces after deposition of the gold. The gold deposit is evident (also confirmed by EDS analysis) on both surfaces, however, the density of AuNPs is substantially higher on GC than on ITO. Moreover, the size of the AuNPs that are electrochemically formed is non-uniform and was determined as 18.2 ± 7.0 nm and 13.0 ± 4.2 nm for GC and ITO, respectively. Further confirmation of the electrochemical deposition of Au is found in the CV (Figure 2C-D). The potential was scanned (in the same solution where gold was deposited) from 0.7 V to positive potentials after deposition. A clear oxidation wave of Au to form AuCl4- at 0.96 V for both GC and ITO, is evident. On the reverse scan the reduction of AuCl4- can also be seen. Notice that the current flux of the GC electrode is significantly larger than that of ITO, which is in agreement with the density of AuNPs formed on these surfaces. The second method for integrating AuNPs onto the GC and ITO comprised the electrostatic attachment of pre-synthesized AuNPs stabilized by citrate.16 To electrostatically adsorb the negatively charged AuNPs we treated the GC and ITO surfaces with PEI. The surfaces were thoroughly washed after the adsorption of PEI to remove the excess of the polymer. Then, the AuNPs were adsorbed for 10 min. Figure 3 shows, as before, the SEM images and CVs of the surfaces after adsorption. The AuNPs in this case are obviously identical in size on both surfaces ca. 14.2 ± 2.3 nm; however, the density of the particles is higher on ITO than on GC. The distribution of the AuNPs on ITO is uniform and the particles do not aggregate. The CVs conducted in 0.1 M KCl clearly show the oxidation of the AuNPs and the absence of a reduction wave of AuCl4-, which is understandable taking into account that the gold complex, which is formed upon oxidation, diffused to the solution. Both CVs are plotted in the same scale to enable comparison the fluxes. It can be seen, that indeed the oxidation wave on ITO is larger than that on GC supporting the SEM images. The next step was to study the electrochemical deposition of Hg on these four surfaces, namely, on GC and ITO electrodes on which Au was electrochemically deposited, and on GC and ITO on which AuNPs were electrostatically adsorbed. Previous studies showed20 that the reduction of Hg2+ onto ITO involved the formation of Hg22+ as an intermediate or as a disproportionation product (eq. 4-6). Electrochemical deposition of Hg from Hg+ and Hg2+ onto GC was also studied and it was concluded that the rates of nucleation were governed by surface energies and deposit–substrate interactions.21 (eq. 4) 2 Hg 2 + + 2e − → Hg 22 +
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Figure 3. Electrostatically absorbed AuNPs on bare electrodes. The electrodes were cleaned, dipped in PEI for 5 minutes, rinsed for 10 minutes and then dipped in AuNPs for 10 minutes. A-B: SEM images (200,000 magnification). A- GC. B- ITO. C-D: CV of the AuNPs modified surfaces (C- GC, D- ITO) in 0.1 M KCl solution. The scans commenced at 0.7 V to positive potentials and then back. Scan rate was 0.01 V/s.
Figure 4 focuses on GC on which Au was electrochemically deposited. Figure 4A shows the effect of different times of Au electrochemical deposition on the oxidation waves of mercury. As can be seen, for the same mercury concentration (10 ppb) and Hg reduction time (30 s), the longer we deposited Au the larger the mercury oxidation peaks were. Since a clear signal of Hg oxidation was obtained for deposition of Au for 30 s, we used this deposition time for further experiments. Figure 4B shows the effect of deposition time of Hg on its stripping peak. As expected, the longer we electrochemically reduced the Hg ions the higher the oxidation peaks were. Figure 4C illustrates the reproducibility of our measurements. Five consecutive analyses were performed with the same electrode and under exactly the same conditions. Mercury was removed between the analyses by applying a constant potential of 0.7 V for 30 s. It can be seen that the reproducibility was fairly high (std. dev. ~±4%) yet careful examination shows that the peaks constantly increased, suggesting that the electrochemical deposition was facilitated upon previous Hg deposition. This limited the application of the electrochemical deposition of Au on GC for the detection of very low levels of Hg. We attempted to determine 1 ppb of Hg using these electrodes; however the peaks were not very sharp and clear and therefore we decided to abandon this approach. It is worth mentioning, that the oxidation peaks of Hg were at 0.15 V, which is similar to that obtained upon oxidizing mercury on a bare GC electrode. This means that Au is not significantly involved in the oxidation of predeposited Hg on GC modified by electrochemically deposited AuNPs.
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Figure 4. Determination of Hg on GC modified by electrochemically deposited Au. LSV of a GC electrode after electrochemically depositing Au, recorded in 10 ppb Hg and 0.1 M KCl with scan rate of 0.05V/s. A – Effect of Au deposition time (0.3 V deposition potential): black-no Au, red-30 s, blue-60 s and green-120 s. Hg was electrochemically reduced −0.3 V for 30 s. B – Effect of Hg deposition times (in sec, −0.3 V) after Au deposition for 30 s: black-10, red-30, blue-60 and green-120. C – Consecutive analyses of Hg stripping after deposition at −0.3 V for 30 s (Au was deposited for 30 s).
Figure 5A-B shows the stripping peaks of Hg (10 ppb) carried out similarly to the previous section; however on ITO. Au was electrochemically deposited under the same conditions, i.e., at 0.3 V from 1 mM AuCl4. Figure 5A shows the effect of Au deposition time on the stripping peak of Hg. It is evident that the deposition of Hg is very efficient even in the absence of Au as the stripping peaks are clear and sharp. Thus, increasing the time of Au deposition increased only moderately the stripping of Hg. As before, we decided to carry out further experiments with ITO electrodes that were modified by deposition of Au for 30 s. Figure 5B illustrates the oxidation peaks of mercury as a function of its deposition time. As expected, the longer we deposited mercury on ITO, the larger the oxidation peaks became. The reproducibility of consecutive analyses were extremely high (not shown) with a standard deviation of 1%. This allowed us to determine lower concentrations of Hg as shown in Figure 5C, which represents the stripping peaks of 1 ppb of Hg as a function of Hg deposition time. Our attempts to use this modification process for the determination of lower Hg concentrations, failed. This encouraged us to switch to the second method of modification, i.e., electrostatically adsorption of AuNPs, as a means of improving the sensitivity of our approach.
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Figure 5. Determination of Hg on ITO modified by electrochemically deposited Au. A-B: LSV of an ITO electrode after electrochemically depositing Au, recorded in 10 ppb of Hg and 0.1 M KCl with scan rate of 0.05 V/s. A – Effect of Au deposition time (0.3 V deposition potential): black-no Au, red-10 s, blue-30 s pink- 60 s and green-120 s. Hg was electrochemically reduced at −0.3 V for 30 s. B – Effect of Hg deposition time (potential of deposition of Hg −0.3 V) after Au was deposited for 30 s: black10 s, red-30 s, green-60 s and blue-120 s. C – As B but the concentration of Hg was 1 ppb. Black-10 s, red-30 s, green-60 s, blue-120 s, turquoise- 300 s and pink 600 s.
Figure 6 shows the stripping peaks obtained on GC after AuNPs were electrostatically adsorbed. Three LSV are shown in Figure 6A: of a bare GC electrode, after adsorption of AuNPs without PEI and after adsorption of AuNPs onto a PEI modified surface. It is evident that Hg sensing was enhanced substantially when using the combination of the positively charged PEI polymer with AuNPs. Figure 6B shows the effect of the adsorption duration of the AuNPs on GC, on the stripping peak of Hg. Increasing the adsorption time of AuNPs amplified the oxidation of Hg, suggesting that the nanoparticles play indeed an important role in the deposition and stripping of Hg. In spite of the fact that a plateau was not reached after 16 min, we decided to use a dipping time of 10 minutes for further experiments. Figure 6C depicts the oxidation peaks of 10 ppb Hg for various deposition times of Hg. As expected, the longer we deposited mercury on GC activated with AuNPs, the larger the oxidation peaks became. The reproducibility of successive LSV of 10 ppb of Hg is shown in Figure 6D. As opposed to Hg determination on GC electrode, where Au was electrochemically deposited the reproducibility here was significantly higher (SD 0.5%). Yet, the determination of lower levels of Hg, i.e. 1 ppb using these type of electrodes was not successful. The next obvious step was to examine the determination of Hg on ITO modified by electrostatically attached AuNPs (Figure 7).
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Analytical Chemistry
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Figure 6. Determination of Hg on GC modified by electrostatically adsorbed AuNPs. A-D: LSV of Hg stripping (potential of deposition −0.3 V) in 0.1 M KCl and scan rate of 0.05V/s. A – Comparison between bare (black), AuNPs modified without PEI (red) . -1 and with PEI (blue) in 23 mg L Hg (time of deposition 30 s). B – Effect of time (in sec) of AuNPs adsorption on the stripping of 1 mg.L-1 of Hg (black-0.25, red-1, blue-4 and green-16. C and D – Effect of Hg deposition time (in sec, −0.3 V, black-30, red-60, blue-120, pink- 240 and green-300; and reproducibility (time of deposition 120 s), respectively, on the stripping of 10 ppb of Hg.
Figure 7A shows the effect of ITO modification. In spite of the positive charge that is likely to attract negatively charged Hg complexes, e.g. HgCl42-, no stripping of Hg was detected when the ITO surface was modified with PEI only. This was presumably due to the sluggish kinetics of Hg(II) reduction on this surface. The fact that AuNPs adsorbed on ITO in the absence of PEI is evident by the small stripping peak that is seen (red curve). A significant improvement of Hg detection was obtained as a result of adsorbing AuNPs on an ITO surface on which PEI was previously adsorbed (curve blue). Figure 7B shows the effect of AuNPs adsorption time (after PEI adsorption for 5 min and washing for 10 min) on the Hg oxidation peak. The results described are for a solution containing 1 mg.L-1 of Hg. As expected, the longer we adsorbed AuNPs on the ITO surface, the higher the Hg oxidation wave. A plateau was reached after ca. 1 hr, yet to shorten the preparation we used a duration of 10 min of adsorption. The reproducibility of this approach was extremely high (not shown, i.e., better than 0.3%), which enabled us to reduced significantly the detection limit. Figure 7C shows the LSV obtained in a solution of 1 µm.L-1 of Hg. Clearly, the introduction of AuNPs on either GC or ITO electrodes has a significant impact on Hg detection. The AuNPs must interact with the mercury and facilitate its deposition and stripping. The effect of Au on GC and ITO on the stripping potential Yet, the stripping potential on the modified GC and ITO electrodes (0.18 V, Figures 4-7) is still substantially different than that observed on an Au electrode (0.55 V, Figure 1A-B). This puzzled and motivated us to carry out some more work.
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Figure 7. Determination of Hg on ITO modified by electrostatically adsorbed AuNPs. A-C: LSV of Hg stripping (potential of deposition −0.3 V) in 0.1 M KCl and scan rate of 0.05V/s. A – Comparison between ITO modified with PEI (black), with AuNPs without PEI (red) and with both AuNPs and PEI (blue) in 23 mg.L-1 Hg (time of deposition 30 s). B – Effect of time (in min) of . -1 AuNPs adsorption on the stripping of 1 mg L of Hg (red-0.25, blue-1, pink-4, green-16 and purple 64). C – Effect of Hg deposition time (in sec, −0.3 V, black- 0, red-10, blue-30, pink 60, green-120 and navy-300) on the stripping of 1 µm.L-1 of Hg. A
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Figure 8. AuNPs after electrodeposition of 5 mM HAuCl4- at +0.3 V on ITO. A-B: SEM images after 5 min of deposition. A- Magnification 25,000. B –Magnification 100,000. C-D: SEM images after 60 min of deposition. C- Magnification 25,000. D – . -1 Magnification 100,000. E – LSV of ITO 1 mg L Hg in 0.1 M KCl solution, after 120 s Hg deposition at −0.3 V. Scan rate was 0.01 V/s. HAuCl4- was reduced at +0.3 V for: black – 30 and red 60 min. Baseline was subtracted in order to fit the scale of different measured electrodes.
Figure 8 shows the SEM images and the LSV of ITO on which Au was electrochemically deposited for long times. We ex-
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pected that the stripping peak potential will eventually shift from 0.18 to 0.55 V as a thick enough Au film will be formed. It can be seen that although after depositing Au for 30 min (Figure 8A-B) the ITO is covered with significantly more Au than before (Figure 2B) the stripping peak potential (Figure 8E, black curve) did not change. Increasing the deposition time to 60 min (Figures 8C-D), resulted in much more significant Au coverage and a second stripping peak appeared (Figure 8E, red curve) at 0.58 V in addition to that at 0.20 V. We can conclude that mercury preferentially strips from ITO (than from Au) since the surface must be covered almost completely by Au to obtain the characteristics of an Au surface. At the same time, the presence of Au enhances the detection of Hg by possibly serving as nucleation sites for its electrodeposition. Therefore, an ideal electrode would comprise of a very thin homogeneous layer of Au on either ITO or GC. Such films can be formed by depositing Au from the gaseous phase, i.e., by vapor deposition. Accordingly, we vapor deposited Au on ITO surfaces for different times forming continuous layers of Au with a thickness of 20, 40 and 80 nm. A
Then, we decided to gently remove the Au film by electrochemical oxidation and study its effect on Hg stripping. Figure 9B shows the SEM image of the ITO coated surface after 30 s of applying 1.5 V in 0.1 M HCl. Some ITO islands are noticeable as a result of the partial dissolution of the Au. This had indeed a clear effect on the stripping of deposited Hg as shown in figure 9E (red curve). The Hg oxidation wave at 0.55 V decreased dramatically whereas an Hg oxidation wave appeared at 0.18 V. Further dissolution of the gold by applying another pulse of 30 s twice (total of 60 and 90 s, respectively) left only Au islands on the ITO surface (Figure 9C-D). Accordingly, the stripping peak of Hg at 0.55 V further decreased and eventually diminished while the stripping peak of Hg at 0.18 V further increased (Figure 9E, blue and pink curves). Interestingly, as the ITO was almost "gold free", the stripping peak of Hg at 0.18 V became much larger and sharper. 60 50 40 30 20
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Figure 10. LSV of an ITO electrode after vapor depositing 20 nm of gold. The solution consisted of 100 µm.L-1 Hg and 0.1 M KCl and the scan rate was 0.05V/s. Effect of Hg deposition times (in sec, −0.3 V): black-10, red-30, blue-60, pink-120 and green 300. C
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Figure 10 shows the LSV of 100 µm.L-1 Hg acquired with an ITO electrode coated with a 20 nm vapor deposited gold film. Clearly the stripping peaks are at 0.55 V, which is expected for a fully Au coated surface. The stripping peaks increased with increasing deposition time. The peaks that are obtained on this electrode were much sharper than recorded with a gold macroelectrode. Finally, we were able to use such electrode to determine Hg reproducibly with a LOD that reached 100 µm.L1 .
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Figure 9. Removal of gold from a 20 nm gold evaporated ITO electrode. A-D: SEM images of ITO surface after different gold removal times (in sec) at 1.5 V in 0.1 M HCl solution. Magnification 100,000. A-0, B-30, C-60 and D-90. E- LSV of ITO in 0.1 M . -1 KCl solution and 1 mg L Hg, after 30 sec Hg deposition at −0.3 V. Hg was removed from the electrode at +0.7 V for 60 s. Scan rate was 0.01 V/s. Black – before gold removal, red – removal of gold for 30 s, blue – removal of gold for 60 s and pink – removal of gold for 120 s. Baseline was subtracted in order to fit the scale of different measured electrodes
Figure 9A shows the SEM of a 20 nm thick Au layer vapor deposited on ITO. A homogeneous complete coverage is seen. Figure 9E (black curve) shows the LSV of mercury stripping after deposition at −0.3 V for 30 s from a solution containing 1 mg.L-1. The Hg oxidation wave appears at 0.55 V as if it were a gold macroelectrode.
Conclusions The electrochemical detection of Hg on GC and ITO electrodes modified with AuNPs was studied. Two methods for modifying the GC and ITO surfaces with AuNPs were examined. The first comprised the electrochemical reduction of HAuCl4 under constant potential, while the second approach involved the electrostatic adsorption of negatively charged AuNPs. A significant improvement in mercury detection was obtained for all cases, i.e., for GC and ITO coated by AuNPs either electrochemically deposited or adsorbed as compared with bare GC and ITO. In addition to increasing the sensitivity, modification by AuNPs resulted in sharper and more reproducible Hg stripping peaks. A remarkable lower detection limit of 1 µm.L-1 of Hg was accomplished using electrostatically adsorbed AuNPs on ITO. Such detection limit implies that such electrodes could be used for the detection of mercury in various aquatic environments. Clearly, some more adjustment is required and in particular studying the possible interference by various substances. The effect of the AuNPs on the stripping potential of Hg was studied as a means of understanding the role played by the AuNPs. We found that the stripping potential of Hg on GC and ITO was almost unaffected by the amount of deposited
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AuNPs. This led us to the second part of the study where we investigated the effect of gold density (on the electrode surface) on the oxidation potential of mercury. Only after depositing Au on ITO for a long time, we observed two stripping peaks, one associated with Hg on ITO (0.18 V vs. Ag/AgCl) and the other was attributed to the oxidation of Hg on Au (0.57 V vs. Ag/AgCl). Similar results were obtained by vapor depositing Au on ITO followed by its partial electrochemical dissolution. That is, Hg was entirely stripped on a thin vapor deposited Au film on ITO at 0.57 V vs. Ag/AgCl, while a second stripping peak at 0.18 V vs. Ag/AgCl appeared after part of the gold was electrochemically removed to expose the bare ITO surface. Moreover, we found that the oxidation waves of mercury on such surfaces were highly reproducible and much sharper than those on bare gold electrode. This led us to conclude that mercury preferentially strips from ITO (than from Au) and at the same time, the presence of low levels of Au enhances the detection of Hg by possibly serving as nucleation sites for its deposition. In other words, the components of the surface, i.e., the gold and the ITO or GC play important role in the detection of Hg. This is schematically shown in figure 11. We still do not know whether Hg diffuses inside the gold and is oxidized at the Au/ITO or Au/GC interface. Moreover, we believe that such process is enhanced by applying a potential, which suggests that initially Hg is reduced and deposited onto the AuNPs and upon applying positive potential it migrates and is oxidized on the ITO or GC. Our results clearly imply that depositing AuNPs by vapor deposition or adsorption on ITO is ideal for Hg detection. We believe that further research could lead to the construction of commercial electrodes, which will be an inexpensive alternative, easy to use and provide reliable sensing of low levels of mercury in waters. 2+
Hg
-
+2e
Hg
2+
Hg
Au ITO or GC Figure 11. Schematics of the suggested mechanism for the electrochemical deposition and stripping of Hg on ITO or GC modified with Au nanoparticles.
AUTHOR INFORMATION Corresponding Author * Tel: +972 2 6585831; Fax: +972 2 6585319; Email:
[email protected] Funding Sources This project was partially supported by the Israel Science Foundation (1150/11).
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Pharmaceuticals is acknowledged for supporting N Ratner. This work was partially supported by the Israel Science Foundation (1150/11).
REFERENCES (1) WHO-http://www.who.int/phe/news/Mercury-flyer.pdf. (2) Clarkson, T. W.; Magos, L. Critical Reviews in Toxicology 2006, 36, 609-662. (3) Hatch, W. R.; Ott, W. L. Analytical Chemistry 1968, 40, 2085-&. (4) Lee, J. S.; Han, M. S.; Mirkin, C. A. Angewandte Chemie-International Edition 2007, 46, 4093-4096. (5) Bloom, N.; Fitzgerald, W. F. Analytica Chimica Acta 1988, 208, 151-161. (6) Martín-Yerga, D.; González-García, M. B.; CostaGarcía, A. Talanta 2013, 116, 1091-1104. (7) Alves, G. M. S.; Magalhaes, J.; Salaun, P.; van den Berg, C. M. G.; Soares, H. Analytica Chimica Acta 2011, 703, 1-7. (8) Turyan, I.; Mandler, D. Nature 1993, 362, 703-704. (9) Turyan, I.; Mandler, D. Electroanalysis 1994, 6, 838843. (10) Shahar, T.; Tal, N.; Mandler, D. Journal of Solid State Electrochemistry 2013, 17, 1543-1552. (11) Svarc-Gajic, J.; Stojanovic, Z.; Suturovic, Z.; Marjanovic, N.; Kravic, S. Desalination 2009, 249, 253259. (12) Abollino, O.; Giacomino, A.; Malandrino, M.; Marro, S.; Mentasti, E. Journal of Applied Electrochemistry 2009, 39, 2209-2216. (13) Hezard, T.; Fajerwerg, K.; Evrard, D.; Collière, V.; Behra, P.; Gros, P. Journal of Electroanalytical Chemistry 2012, 664, 46-52. (14) Behzad, M.; Asgari, M.; Shamsipur, M.; Maragheh, M. G. Journal of the Electrochemical Society 2013, 160, B31-B36. (15) Zhou, N.; Chen, H.; Li, J.; Chen, L. Microchimica Acta 2013, 180, 493-499. (16) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discussions of the Faraday Society 1951, 11, 55-75. (17) Allen J Bard, R. P., Joseph Jordan. standard potentials in aqueous solutions; ; marcel dekker, inc,, 1985. (18) Herrero, E.; Abruna, H. D. Langmuir 1997, 13, 44464453. (19) Abollino, O.; Giacomino, A.; Malandrino, M.; Piscionieri, G.; Mentasti, E. Electroanalysis 2008, 20, 7583. (20) Maghasi, A. T.; Conklin, S. D.; Shtoyko, T.; Piruska, A.; Richardson, J. N.; Seliskar, C. J.; Heineman, W. R. Analytical Chemistry 2004, 76, 1458-1465. (21) Serruya, A.; Mostany, J.; Scharifker, B. R. Journal of Electroanalytical Chemistry 1999, 464, 39-47.
ACKNOWLEDGMENT The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University are acknowledged. TEVA
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Analytical Chemistry
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