Surface Characterizations of Mercury-Based Electrodes with the

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J. Phys. Chem. C 2010, 114, 9049–9055

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Surface Characterizations of Mercury-Based Electrodes with the Resulting Micro and Nano Amalgam Wires and Spheres Formations May Reveal Both Gained Sensitivity and Faced Nonstability in Heavy Metal Detection Gemma Aragay,†,‡ Anna Puig-Font,† Miquel Cadevall,† and Arben Merkoc¸i*,†,§ Nanobioelectronics & Biosensors Group, Institut Catala` de Nanotecnologia, CIN2(ICN, CSIC), 08193, Bellaterra, Spain, Department of Chemistry, UniVersitat Auto`noma de Barcelona, 08193, Bellaterra, Spain, and ICREA, Bellaterra (Barcelona), Spain ReceiVed: March 9, 2010; ReVised Manuscript ReceiVed: April 10, 2010

Surface characterizations of mercury-coated screen-printed electrodes with interest for electrochemical stripping analysis of heavy metals detection are presented. SEM images and microanalysis data of the working electrode surface show a homogeneous distribution of mercury film prior to sensor application in heavy metals detection in seawater. The response stability during the 16 days measuring period in terms of RSD (relative standard deviation) was 33%, 24%, and 7.7% for Cd2+, Pb2+, and Cu2+, respectively. Detection limits lower than 7.0 µg L-1 in a seawater matrix modified with HCl are achieved for the three metals. The microscopic and microanalysis studies revealed some damages of the mercury film after a long period of use (continuous measuring) of the SPE. The decrease of the sensitivity as well as splitting of the peaks is related to the deposition and stripping of heavy metals in mixed mercury and “mercury free” areas. The application of this sensor for a longer period would need the use of a less acidic medium so as to avoid the damage of the working electrode. This would need further optimizations so as to find a compromise between sensitivity and stability. The frequency of measurements with interest for application in an automatic control system where the sensor may be integrated and applied in the future should also be considered. Moreover, the deposition of metals into the surface of the electrode was evaluated. Surface characterizations of mercury-based electrodes with the resulting micro and nano amalgam wires and spheres formations may in addition reveal both gained sensitivity and faced nonstability while being applied in real water samples. Introduction There is an increasing concern about the determination of heavy metal content in rivers, lakes, and waste waters all over the world. Heavy metals are a threat to the environment and human health due to the fact that they are not biodegradable and therefore they remain indefinitely in the ecological systems and in the food chain exposing top-level predators to very high levels of pollution.1 Consequently, monitoring of trace heavy metals is vital due to the potential health and ecological hazard they present. Such monitoring systems have to be accurate and have to be able to detect heavy metals in low concentrations. In addition, the new monitoring technologies could give automatic self-maintaining systems for continuous in situ measurements which could detect heavy metal concentrations in water with the possibility of early warning.2,3 Miniaturization of heavy metal detection systems is crucial for their further application in the field. Electrochemical methods, and more particularly stripping techniques, combined with the use of screen-printed electrodes (SPEs) offer several advantages related to cost, simplicity, and miniaturization of these systems. These techniques enhance selectivity and sensitivity by combining separation, preconcentration, and determination in one process.4–7 * To whom correspondence should be addressed. E-mail: arben. [email protected]. † Institut Catala` de Nanotecnologia. ‡ Universitat Auto`noma de Barcelona. § ICREA, Barcelona, Spain.

Mercury-coated carbon SPEs have been used recently for trace heavy metal detection in connection with voltammetric stripping analysis.8,9 The usage of mercury-based electrodes is critical due to its toxicity. For this reason the development of carbon-based material,10 boron-doped diamond electrodes,11 bismuth-modified electrodes,12 and other environmental friendly materials is being studied. However, mercury film electrodes are still being used. This kind of electrode in comparison to conventional ones used so far (i.e., hang drop mercury electrode, HDME) reduces the amount of mercury used in the analysis.13,14 Although some studies have been presented using this kind of sensor in batch systems, most works tend to the use of disposable SPEs.15 Our group has recently presented the long-term application of this type of sensor in a batch system.16 As an extension of these studies, the aim of this work is to get insight into the electrochemical response of the sensors through their morphological characterization while being used in heavy metal detection. This would explain the loss of response after a longterm measuring period. The analytical performance of the SPEs is also presented showing low detection limits for lead, cadmium, and copper in spiked seawater samples including the possibility of their simultaneous detection. The SPEs show a long-term stability (up to 16 days) for measuring heavy metals in seawater while being used in a flow-through system. The obtained results show that these devices show good promise for their integration into automatic control systems for future applications in seawater monitoring.

10.1021/jp102123w  2010 American Chemical Society Published on Web 04/27/2010

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Figure 1. SEM images and microanalyses for mercury-coated SPE before activation (a) and after activation (b). Experimental details are described in the text.

In addition to the analytical studies, morphological studies have also been performed in order to find the deposition behavior of each metal onto the mercury film. Interesting results related to different amalgam shapes are shown. Experimental Methods General Information. Heavy metals solutions were prepared by diluting AAS grade (Panreac) Cd2+, Pb2+, and Cu2+ standard solutions. HCl (32%) was purchased from Panreac. The pH when required was adjusted by using a 1 M HCl solution. The pH values were determined with a Crisol Model GLP 22 pH meter. The SPE (from University of Florence, available from Palm Instruments, http://www.palmsens.com) consisted of a screenprinted electrochemical cell with three electrodes in one single strip: a graphite working electrode modified with mercury acetate mixed with a plasticizer, a graphite counter electrode, and a Ag/AgCl pseudoreference electrode.9 A computer-controlled Autolab PGSTAT-12 (302N-High performance, potentiostat/galvanostat) with a general purpose electrochemical software operating system (GPES version 4.9.007) from Eco Chemie B.V., The Netherlands, was used for electrochemical measurements. The integrated threeelectrode strips were connected to the Autolab PGSTAT-12 with a specially adapted electrical edge connector. A scanning electron microscope (SEM) Jeol JSM-6300 (Jeol Ltd., Japan) coupled to an energy dispersive X-ray (EDX) spectrophotometer ISIS 200 (Oxford Instruments, England) was used to characterize the working electrode surface of the SPEs after gold sputtering. Flow-Through System. The flow-through system consists of a peristaltic pump (PERIMAX 12) used for pumping the measuring solutions to a flow cell adapted to the SPEs. The flow cell enables the use of SPEs for flow-through applications. The SPE is inserted into the slit/opening of the cell and tightened by a screw. The cell assures the wall-jet flow around the working electrode.

Experimental Conditions. Heavy metal concentrations were determined by square wave voltammetry (SWV). The voltammetric parameters for the experiments were as follows: conditioning potential -0.15 V for 60 s, deposition potential -1.1 V for 120 s, equilibration time 30 s, SW amplitude 28 mV, step potential 3 mV, and frequency 15 Hz. Flow-through conditions were used during the conditioning and the accumulation step, whereas the equilibration step and the square wave scan were performed in a quiescent solution. No oxygen removal was performed. Before using a new SPE, a pretreatment step was necessary to activate the mercury film applying a potential of -1.1 V during 300 s under flow-through conditions. Repeated blank measurements in seawater were carried out (usually ten times) up to obtain a stable background current (see Figure S1 at Supporting Information). A new sensor was used for each experiment, except for the study of long-term stability. Results and Discussion SPEs Morphological Characterization. The morphological characterization of the electrodes is of great importance in order to fully understand the obtained results. A better knowledge of the effect of the applied potential upon the surface morphology of the working electrode surface during each step of the analytical performance is crucial to get insight into the phenomenon that directly affects the sensor stability and lifetime.17,18 It is well-known that the performance of the SPE is strongly affected by the working electrode material. The composition of the working electrode and the distribution of its components (i.e., graphite, binding matrix of the used carbon ink, mercury film) affect the stripping peaks of the heavy metals. In addition, this characterization could also be useful to compare the homogeneity between different batches of electrodes so as to ensure adequate reproducibility during their applications. Scanning electron microscopic (SEM) images of the surface of the mercury-coated SPEs were taken to explore the surface

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TABLE 1: Analytical Performance Parameters for Mercury-Coated SPEs for Heavy Metals Detection in a Spiked Seawater Sample Parameter 2

eq (r ) CV/% LOD/µg L-1 LOQ/µg L-1 repeabilitya LDCa

CdII

PbII

CuII

y ) 0.012x - 0.079 (0.987) 7.8 7.0 8.2 7.0 × 10-3 (20) 3.9 × 10-2 (80) 2.1 × 10-2 (20) 1.2 × 10-1 (80)

y ) 0.032x - 0.0023 (0.999) 1.8 0.31 0.86 1.7 × 10-2 (20) 3.3 × 10-2 (80) 5.2 × 10-2 (20) 9.9 × 10-2 (80)

y ) 0.024x + 0.052 (0.999) 11 0.53 6.7 2.1 × 10-2 (20) 6.0 × 10-2 (80) 6.4 × 10-2 (20) 1.8 × 10-1 (80)

a The values were obtained from the standard deviation of six repetitive measurements of a solution of 20 and 80 ppb of each metal, respectively.

distribution of the mercury film deposited onto the carbon ink printed area. Figure 1a depicts typical SEM images of the working electrode surface before the SPE pretreatment (mercury acetate mixed with a plasticizer). A poor homogeneity of the deposited mercury film onto the working electrode surface can be observed. Although two different areas can be distinguished (bright and dark areas) microanalyses data of both areas indicate the same mercury distribution. The observed difference is probably related to the mercury agglomeration in certain areas provoking the brighter areas. The enlargement of the image reveals a high porosity structure of the material indicating a large electroactive surface that involves fast electron transfer rates.19,20 However, the rough surface can also induce a decrease on the homogeneity and reproducibility of the measurements performed between and within batches of sensors. After the electrode activation (Figure 1b), the mercury distribution onto the working electrode surface becomes more homogeneous owing to the reduction of mercury acetate to metallic mercury while applying a negative potential during the pretreatment step (-1.1 V during 300 s). Although some cracks on the electrode surface are observed, the microanalysis again reveals that mercury is well distributed onto the whole surface. Heavy Metal Detection in Seawater. Study of the pH Effect. The pH of the measuring media affects the electrochemical detection of heavy metals. The effect of pH on the peak signal obtained for different heavy metals has been studied. The pH values were varied between 2 and 8 by adjusting the seawater samples with a 1 M HCl solution. Higher sensitivities for lead and copper were obtained at pH 2, whereas for cadmium a pH between 2 and 4 could be selected. As a trade-off between the optimum conditions for these metals, pH 2 was the one selected for further studies due to the welldefined and higher peak current obtained during the performed measurements (results not shown). Analytical Performance Parameters. To evaluate the performance of the mercury-coated SPEs analytical parameters such as the linear range of response, coefficient of variation (CV), limit of detection (LOD), limit of quantification (LOQ), repeatability, lowest detectable change (LDC), and the calibration method have been determined. The obtained values for these parameters are given in Table 1. The data reported in Table 1 are referred to a separate analysis of each metal and have been obtained from the calibration curves shown in Figure 2. Detection limits (estimated as three times the standard deviation of six consecutive measurements of 5 µg L-1 heavy metal solution)21 were 7.0, 0.31, and 0.53 µg L-1 for Cd2+, Pb2+, and Cu2+, respectively. Quantification limits (estimated as ten times the standard deviation of six consecutive measurements of 5 µg L-1 heavy metal solution)21 were 8.2, 0.86, and 6.7 µg L-1 for Cd2+, Pb2+, and Cu2+, respectively. Considering the limit

Figure 2. Calibration curves and linear range of response for cadmium (a), lead (b), and copper (c) in the range of 0 to 100 ppb in seawater matrix, using single mercury-coated SPE for the three metals. Experimental details are described in the text.

values in seawater recommended by WHO and EPA (10 and 50 µg L-1 for Cd2+ and Pb2+, respectively) and taking into account the obtained analytical performance, the proposed system can be a good choice for monitoring heavy metals in seawater.22 The reproducibility of a single sensor was verified at two different concentrations with six repetitive measurements of 20 and 80 µg L-1 of each heavy metal (results shown in Table 1).

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Figure 3. Calibration curves (a) and linear range of response (b) for the multidetection of cadmium, lead, and copper in the range of 0 to 500 ppb in a seawater matrix, using a single mercury-coated SPE. Experimental details are described in the text.

Relative standard deviations between 2.5% and 8.3% were obtained for the studied metals. While good peak shapes and linear relations were obtained for the three metals, the sensitivity against Cd2+ was lower than that for Cu2+ and Pb2+ being the response for Pb is the highest (Figure 2). This has already been reported in other works and it is explained by different facts such as the different diffusivity and the difference on the intermetallic compounds formed among others.23 Figure 3 shows simultaneous detection of Cd2+, Pb2+, and Cu2+ in seawater and the corresponding calibration curves at a concentration range from 5 to 500 µg L-1. From these data it can be deduced that the linear range of response for Cd2+ and Pb2+ is observed to be from 0 to 150 µg L-1 while the linearity range for Cu2+ is increased up to 500 µg L-1. The linearity ranges can be changed by changing the deposition times. Standard Addition Method. On the basis of the complexity of seawater matrices, i.e., diversity of anions and cations and conductivity among other physical and chemical parameters,

Aragay et al. the standard addition method for the analysis of the samples was used. Three replicate determinations of 50 ppb of Cd2+, Pb2+, and Cu2+ in a spiked seawater sample were carried out. Three different electrodes were used and recoveries of 95%, 91%, and 140% with a RSD of 17%, 15%, and 14% between electrodes for Cd2+, Pb2+, and Cu2+, respectively were obtained. The obtained results show that the proposed flow-through system with mercury-coated SPEs has promising potential for applications in heavy metal monitoring of real samples. The comparison of the analytical performance parameters of the described Hg-based SPE with other electrodes described in the literature and used in heavy metals detection in seawater points out that although having a similar LOD, the ones described in this paper can be used for simultaneous detection of three heavy metals (Cd2+, Pb2+, and Cu2+) with a higher stability. The described SPEs show a long-term usage possibility in comparison to other reported electrodes, most of which have been of single use. Long-Term Stability. Preliminary results revealed that mercurycoated screen-printed electrodes were damaged if a continuous potential was not applied during their whole operational cycle.16 This phenomenon is related to the fact that at noncontrolled potential conditions at an open circuit and moreover in the presence of oxygen the mercury film can be oxidized. To avoid this effect the measurements done for the lifetime study were performed while keeping the SPE potential under control even when not in use. In the study, a solution of 100 µg L-1 of Pb2+ and Cd2+ and 550 µg L-1 of Cu2+ in seawater was measured every day during a 16-days period. To avoid the electrode damage a stand-by potential of -0.15 V was applied when the SPE was not in use but just immersed into seawater (pH 8) overnight. The stability of the SWV peak height with interest in the lifetime study using seawater (pH 8) for stand-by conditions is shown at Figure 4. The peak height values for each heavy metal show good reproducibility within the measurements performed during 1 day (see Figure S2 in the Supporting Information). As the day-to-day repeatability is not as good as during a single day, for the measurement of continuous monitoring of a real seawater sample the use of the standard addition method is applied. The RSD of the peak height in a single measurement per day was 5.9%, 2.1%, or 1.6% for Cd2+, Pb2+, and Cu2+, respectively, using the same electrode for all the measurements. The response stability during the 16-days measuring period in terms of RSD was 33%, 24 %, and 7.7% for Cd2+, Pb2+, and Cu2+, respectively. The evaluation of the metal recoveries for a spiked seawater sample of 100 ppb of Cd2+ and Pb2+ and 550 ppb of Cu2+ was also performed for the same measuring period. The results obtained are shown at Figure S3 in the Supporting Information.

Figure 4. Long-term stability study for a 16-days checking period measuring a Cd (green), Pb (red), and Cu (blue) solution in a seawater matrix. Experimental details are described in the text.

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Figure 5. (a) Typical voltammetric curves showing the damage on the mercury film. (b) TEM images of a damaged mercury-coated SPE with the corresponding microanalyses for each area. Experimental details are described in the text.

From these studies it can be observed that the errors in the recovery calculations are higher than that for the peak height homogeneity due to the additional errors coming from the standard addition method. The interference effect between the different heavy metals in solution (see Figure S3 in the Supporting Information) might also have affected the recoveries. After 16 days of measurements the long-term and even the daily stabilities dramatically decrease (results not shown). In addition, peak splitting, typical for long-term uses, appeared as shown in Figure 5a. Figure 5b shows the changes on the surface of the working electrode that happen to occur after a long-term use even within a day. Such a deterioration of the electrode surface after longterm use might have happened also with the electrode after 16 days of measurement. The observed mercury film distribution after electrode activation during the first day of measurement (Figure 1a,b) was significantly affected due to the damage during the long-term use (Figure 5b). These images clearly show mercury agglomeration formed in some areas of the working electrode. This is due to the repeated stripping cycles that destroy the film leaving some electrode areas without mercury. Microanalyses data also reveal that mercury is not present in the dark areas while still being present in the brighter ones. Other metals like Na, Mg, and Ba among others can be observed on the microanalyses due to their presence in the seawater used during the measurements. The splitting of the curve is strongly related to the homogeneity of mercury film distribution. At a deteriorated electrode, heavy metals can be deposited in two

Figure 6. (a) Scheme of the heavy metal deposition onto a homogeneous Hg surface. (b) Scheme of the heavy metal deposition onto a damaged Hg-SPE with two different areas of deposition (Hg and Hgfree areas).

different areas leading to two different responses (splitting of the stripping peak): mercury agglomerates area and “mercury free” area as shown schematically in Figure 6. The combined deposition mechanism onto two different areas causing the peak

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Figure 7. (a) SEM images for mercury-coated SPE after lead deposition. (b) SEM images for mercury-coated SPE after cadmium deposition. (c) SEM images and microanalyses for mercury-coated SPE after cadmium, lead, and copper deposition. The presence of Au is related to the Au sputtering during SEM sample preparation so as to obtain a better image resolution.

splitting has directly affected the reproducibility of the measurements. In addition, the sensor performance should have also been affected by noncontrolled cracking observed after the electrode activation (Figure 1b). Both effects need better attention while considering these sensors for further applications in automatic control of heavy metal contamination. In addition to the effect of the homogeneity of the mercury film after a long-term use, we have studied the different deposition modes of each metal onto the Hg film. Surprisingly, SEM images for Hg-coated SPE after the deposition of Cd2+, Pb2+, and Cu2+ without the stripping step (in high concentrations due to the poor sensibility of the microanalysis technique) show that each metal presents a different deposition profile (Figure 7). These profiles were identified by using EDX microanalysis. While Cu is homogeneously distributed forming a film along the whole working electrode surface, Pb and Cd are only deposited in certain isolated areas. Moreover, we found that both Pb and Cd adopt certain unexpected shapes while depositing onto the mercury film that can be well observed in Figure 7. Cadmium is deposited forming an amalgam with Hg in wire shape (of around 100 to 500 nm diameter and longitude of few micrometers) while lead is deposited forming small sphere amalgams (of around 20 to 100 nm diameter) with Hg. This morphology of heavy metal deposition through amalgam formation has never been reported so far. It can be directly related to the nonhomogeneity of the mercury film including the presence of discovered graphite particles (from screenprinted ink) in the middle of the mercury pool. This area rich in compositions led to an uncontrolled depositions/growing of metal amalgams in different shapes and forms. This complex deposition of the different metals onto the Hg surface can also lead to irreproducibility of the corresponding electrochemical stripping curves.

Conclusions Screen-printed electrodes modified with mercury are shown to be a powerful tool for fast and efficient measurement of heavy metals in flow-through measuring systems. The detection of cadmium, lead, and copper in seawater modified with HCl and based on electrochemical stripping using screen-printed electrodes is of high sensitivity. The sensor shows detection limits of 7.0, 0.31, and 0.53 µg L-1 for Cd2+, Pb2+, and Cu2+, respectively, for single measurements. The simultaneous detection of Cd2+, Pb2+, and Cu2+ also has been achieved and similar performances as for the single heavy metal detection have been obtained. Linear ranges of response from 5 to 150 ppb for Cd2+ and Pb2+ and from 5 to 500 ppb for Cu2+ have been obtained for heavy metal multidetection. The sensor shows a stable response during a 16-days measuring period with a RSD of 33%, 24%, or 7.7% for Cd2+, Pb2+, and Cu2+, respectively. Decreases of the stripping signals including the splitting of lead peaks have been observed during the long continuous use of the sensor. SEM images and microanalysis results have revealed the mechanism of sensor damage responsible for the restricted lifetime of the sensor. This damage of the sensor surface is mostly related to the relatively long period of its contact with HCl-modified sample. In addition, damages of the sensor surface even right after the electrode activation have been observed and probably should be considered to be responsible of stability problems. Unexpected forms and shapes that correspond to amalgams of Cd, Pb, and Cu formed onto the surface of the mercury electrode have been observed and identified by SEM images in connection to EDX microanalysis.

Surface Characterizations of Hg-Based Electrodes Although we have shown the potential application of mercurymodified screen-printed sensor in seawater monitoring, further studies and optimizations so as to avoid problems related to sensor surface damage are necessary prior their applications in long-term heavy metal measurements with interest for environmental control with automatic analyzers. The use of less acidic medium (i.e., acetate buffer etc.) should be another alternative so as to avoid the sensor damage. Nevertheless, a compromise between the sensors sensitivity and long-term application should be necessary prior to applications. Acknowledgment. The financial support by the Spanish Ministry of Science and Innovation through project MAT200803079/NAN and EU for WARMER project FP6-034472 are acknowledged. G.A. thanks the Generalitat de Catalunya for a predoctoral fellowship (FI 2009). Supporting Information Available: Figures showing blank measurements behavior, six repetitive measurements of a solution of 80 ppb of Cd, Pb, and Cu, and metal recoveries for a 16-days period. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Banks, E. C.; Ferreti, L. E.; Shucard, D. W. Neurotoxicology 1997, 18, 237. (2) Yanyasee, W.; Lin, Y.; Hongsirikarn, K.; Fryxell, G. E.; Addleman, R.; Timchalk, C. EnViron. Health Perspect. 2007, 115, 1683. (3) Fini, J. B.; Pallud-Mothre´, S.; Le Me´vel, S.; Palmier, K.; Havens, C. M.; Le Brun, M.; Mataix, V.; Lemkine, G. F.; Demeneix, B. A.; Turque, N.; Johnson, P. E. EnViron. Sci. Technol. 2009, 43, 8895. (4) Honeychurch, K. C.; Hart, J. P. Trends Anal. Chem. 2003, 22, 456.

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