Environ. Sci. Technol. 2008, 42, 4877–4882
Underpotential Deposition-Anodic Stripping Voltammetric Detection of Copper at Gold Nanoparticle-Modified Ultramicroelectrode Arrays JAHIR OROZCO,* ´ SAR FERNA ´ NDEZ-SA ´ NCHEZ, AND CE ´ NEZ-JORQUERA CECILIA JIME Instituto de Microelectronica de Barcelona (CNM-IMB), CSIC, Campus UAB, 08193 Bellaterra, Spain
Received March 4, 2008. Revised manuscript received April 25, 2008. Accepted April 28, 2008.
The sensitive detection of copper (II) at gold nanoparticlemodified ultramicroelectrode arrays (UMEAs) is reported. Gold nanoparticles were electrodeposited onto the UMEAs surface by applying a constant positive potential of 1.6 V for 20 min in a 20-nm gold nanoparticle solution. This process significantly increases the electrode area without losing the UMEAs analytical features. Underpotential deposition-anodic stripping voltammetry of copper (II) with such modified UMEAs was performed and showed a high increase in sensitivity (25.9 ( 1.3 nC · µM-1) and a broader linear range of response (0-10 µM) compared with those values obtained using bare UMEAs (7.5 ( 0.6 nC · µM-1 and 0-2 µM, respectively). The copper content of acid extracts of contaminated soils was successfully determined with the modified UMEAs and results are in good agreement with those obtained using the ICPAES standard method. Overall, this work shows an alternative easy-to-use novel miniaturized device for the rapid and reliable determination of copper in soil samples whose application could be readily extended to other heavy metals of environmental interest.
Introduction Detection of toxic metals is an important issue in environmental monitoring. Soil analysis is almost exclusively performed using conventional laboratory-based techniques, mainly atomic spectroscopy (1, 2), which are expensive and time-consuming. However, large efforts have been made for the development of analytical systems able to provide fast, inexpensive, and reliable detection of a wide variety of pollutants, including metals. In this context, the application of electroanalytical techniques combined with suitable miniaturized solid electrodes appears to be a feasible alternative. Previous reports address the use of anodic stripping voltammetry (ASV) combined with under potential deposition (UPD) techniques for trace-metal determination (3) and specifically the determination of copper in soil matrices (4–7). On the other hand, miniaturized solid electrodes such as ultramicroelectrode arrays (UMEAs) have been interrogated using different electroanalytical techniques * Corresponding author tel: +34 935947700; fax: + 34 935801496; e-mail:
[email protected]. 10.1021/es8005964 CCC: $40.75
Published on Web 06/03/2008
2008 American Chemical Society
for the detection or screening of heavy metals in samples of diverse nature (8, 13. The main part of these works involve the generation of a mercury layer, on top of the electrode, suitable for the preconcentration of heavy metals and further voltammetric stripping detection (9, 11–13). Using UPD-ASV techniques with UMEA devices enable the easy and sensitive detection of heavy metals and in turn circumvent the use of mercury films. UMEA devices have experienced significant attention for routine analytical applications (14, 21) and their advantages over conventional electrodes are widely documented (22, 23). UMEAs various designs have partially solved the low density current obtained with single ultramicroelectrodes (UMEs) (24). However, special attention has been paid to further increase the current density measured when either extreme high sensitivity or improvement of the detection limits are required. In this context, nanostructured materials are offering new opportunities in the development of highly sensitive sensors in the field of electroanalytical chemistry. Nanostructured materials provide excellent prospects for the fabrication of chemical transducers thanks to their unique physical and chemical properties (25), as well as their potential application in interdisciplinary fields (26). They are very attractive materials for the development of (bio)chemical sensors, electrocatalysts, and also for the design of a new generation of devices with novel functions and uses. Gold nanoparticles have received considerable attention due to their relatively high surface area-to-volume ratio and their interface dominant properties, which significantly differ from those of their bulk counterparts (27). The literature about gold nanoparticle-modified electrodes is scarce. The detection of arsenic (III) has been performed with a gold nanoparticle-modified glassy carbon electrode (28, 29) and gold nanoparticles deposited onto iridium tin oxide film coated glass (30). Gold-amalgam nanoparticle-coated glassy carbon electrodes have shown high sensitivity toward oxidation of some heavy metals (31). Also composites have been successfully applied to the detection of tallium (I) and other target analytes (32). Different methodologies have been used for the anchorage of gold nanoparticles on electrode surfaces, which include tethering by electrostatic interaction, covalent linkage, and electrochemical deposition. In a previous work reported by the authors, gold nanoparticles were electrodeposited on the surface of UMEAs, which resulted in an increase of their active area and an improvement of the UMEA performance, while keeping their microelectrode features (33). In this work, the sensitive detection of copper (II) using gold nanoparticle-modified UMEAs is described. Gold nanoparticles were easily electrodeposited onto the UMEAs surface by applying a constant positive potential. Using such modified electrode devices, UPD-ASV detection of the target analyte was carried out. The performance of the sensor was evaluated in a complex matrix coming from soil extracts and results were validated by standard ICP-AES method.
Experimental Section Reagents and Solutions. Gold nanoparticle (20-nm) stock solution, arsenic standard solution, and Cu(NO3)2 and Pb(NO3)2 analytical-grade salts were all purchased from Sigma-Aldrich (Spain). Cu and Pb stock solutions were made in deionized water. All other chemicals were of analytical grade. Solutions were prepared using 18 MΩ · cm deionized water. Apparatus and Materials. Voltammetric experiments were performed at room temperature using a type III µ-Autolab VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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potentiostat (Ecochemie), controlled with a GPES 4.7 software package. Measurements were carried out with a threeelectrode cell. This cell consisted of an UMEA working electrode and an on-chip counter electrode. An Ag/AgCl/ 10% (w/v) KNO3 reference electrode (Metrohm 0726 100) completed the setup. A Ag/AgCl miniaturized pseudoreference electrode RC6, 50 mm, 1.5 mm external diameter (World Precision Instruments) was used in electrodeposition experiments. Microelectrode Fabrication. Au microelectrode arrays were fabricated at the CNM-IMB by standard photolithographic techniques using Si/SiO2/metal structures as described elsewhere (33). Au microelectrodes with three different geometries were tested. A first array (UMEA-10) consisted of 100 disk-shaped microelectrodes, with 10 µm diameter, 100 µm of interelectrode distance, and 7 × 10-5 cm2 area. The second and third arrays consisted of 400 and 1600 discs, 5 µm in diameter (UMEA-5A and 5B respectively; see Figure S1 in the Supporting Information). The interelectrode distances were 100 and 50 µm and the areas were 7.05 × 10-5 cm2 and 3.14 × 10-4 cm2, respectively. All arrays included a counter gold electrode on chip with an area of 0.7 cm-2 separated 0.5 µm from the array. Experimental Procedure. An initial chemical electrochemical cleaning process was carried out as described elsewhere (33). The background electrolyte for the detection of Cu (II) was selected by recording cyclic voltammograms in a potential window between +0.1 and +0.6 V in 10 mM HNO3, 0.1 M H2SO4, and 10 mM H2SO4 solutions, at a scan rate of 0.1 V · s-1. Deposition of Cu was performed by applying a potential of -0.1 V in a 1 µM Cu aqueous sulfuric acid solution, varying the preconcentration time between 0 and 400 s, at a scan rate of 0.1 V · s-1. The electrodeposition of gold nanoparticles on the electrode surface was carried out as follows. Ten µL of commercial gold nanoparticles were dropped over the surface of the array and a constant potential of +1.6 V was applied for 20 min. Cyclic voltammetric scans in a potential window between +1.4 and -0.2 V in 0.1 M H2SO4 /0.1 M KNO3 at a scan rate of 0.1 V · s-1 was carried out to estimate the microscopic surface area of the UMEAs. Response of both bare and modified UMEAs to changes in copper concentration was evaluated by UPD-ASV, at a scan rate of 0.1 V · s-1, after addition of different volumes of 10 mM Cu stock solution. Digestion of samples of contaminated soils was carried out in concentrated HNO3 using a microwave. Soil acid extracts were analyzed by ICP-AES and data were provided by a certificated laboratory. Copper concentration of samples was estimated by the standard addition method. The sample was diluted in 10 mM H2SO4 solution and two consecutive additions of the Cu stock solution were carried out.
Results and Discussion Electrochemical Detection of Copper at Bare Gold UMEAs. Initial experiments were conducted in order to choose the suitable supporting electrolyte for the detection of copper. Figure 1 shows cyclic voltammograms recorded in a 10 mM H2SO4 solution containing increasing amounts of Cu. On the forward scan (negative direction) a broad not-well-defined wave at +0.040 V corresponds to the formation of the underpotential deposit of copper on the gold surface. On the reverse scan, the anodic peak at +0.225 V is due to the oxidation (stripping) of the deposited copper atoms. The charge under the copper stripping peak tends to level off as the copper concentration in solution increases. Experiments performed in 10 mM HNO3 and 100 mM H2SO4 solutions, respectively, do not lead to noticeable improvements in terms 4878
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FIGURE 1. Cyclic voltammograms recorded in 10 mM H2SO4 and 10 mM HNO3 solutions containing increasing amounts of copper. Scan rate: 0.1 V · s-1. of peak shape, potential, or charge. Hence, 10 mM H2SO4 was chosen as the supporting electrolyte for further experiments. The electrodeposition of metals on the electrode surface by means of UPD proceeds via the formation of a submonolayer of the target at a potential more positive that the Nernst Potential (4, 6, 7). The subsequent metal determination is carried out by the voltammetric stripping of the metal deposit. To verify the influence of the three different UMEA designs on the deposition process, a series of UPD-ASV experiments using the three UMEA geometries was carried out. It was observed that as the deposition time was longer, an increase of the peak current measured at + 0.225 V was observed, which is related to the expected increase of the amount of Cu deposited onto the electrode surface. No changes in the shape and stripping peak potential were evidenced in any case. UMEA-10 and 5A units showed similar behavior but the current densities obtained with these latter devices were higher compared to those from the former ones (see Figure S2, Supporting Information). UMEA-5B showed peaks considerably higher compared to those recorded with the other two UMEA designs. However, the capacitive current recorded with the UMEA-5B is significantly higher than that obtained with the other geometries, this being a limiting factor for the highly sensitive determination of copper. Thus, UMEA-5A was chosen for further experiments. The quantity of Cu deposited onto the electrode can be evaluated from the charge of the stripping voltammetric peaks, which is used to estimate the surface coverage (Γ) according to the following equation: Γ)
Q nFA
(1)
where Q is the charge (C) measured from the copper stripping peak; n is the number of electrons exchanged during the stripping process; F is the Faraday constant (in C · mol-1), and A is the geometrical surface area of the electrode (in cm2). In addition to Γ, the sensitivity of each array was calculated with the aim of evaluating and comparing the influence of the different UMEAs geometries. Here, the sensitivity is defined as the rate at which the peak charge increases with deposition time. This sensitivity is given by the slope of the stripping peak charge versus deposition time plots for deposition times between 0 and 60 s (with n ) 3). This time range was chosen taking into account that the surface coverage value reached a maximum when the deposition time applied is sufficiently long to generate a full monolayer. The theoretical value (Γtheoretical) for a monolayer of metal on a substrate, reported in refs (6) and (7), is 2 × 10-9 mol · cm-2. Thus, as Table 1 depicts, the value of Γ for a deposition time of 60 s (Γt ) 60s) indicates that saturation of the electrode
TABLE 1. Values of Sensitivity, Surface Coverage, and Saturation Time for UMEAS with Different Geometriesa UMEA-10
TABLE 2. Values of the Gold UMEA Surface Areas before and after Gold Nanoparticle Electrodepositiona area/cm2
UMEA-5A UMEA-5B
sensitivity (103 nC · cm-2.s-1) 3.6(0.0) 5.2(0.0) 4.5(0.1) Γt ) 60s (10-9 mol · cm-2) 1.0(0.0) 1.1(0.0) 1.4(0.1) saturation time (s) 116 74 85 Values of standard deviation for n ) 3 are between brackets. Values were calculated using the theoretical geometric area of the UMEAs. a
UMEA
before electrodeposition
after electrodeposition
area increase times
10 5A 5B
7.85 × 10-5 7.85 × 10-5 3.14 × 10-4
3.4 × 10-3 (0.1)* 3.5 × 10-3 (0.2)** 37.4 × 10-3 (1.8)*
43 44 119
a Standard deviation between brackets with *n ) 5; **n ) 10.
FIGURE 2. Response of a UMEA-5A unit to changes in copper concentration using a 10 mM H2SO4 background solution. Concentration increases between 0 and 2.0 µM Cu, deposition time 15 s, and scan rate of 0.1 V · s-1.
FIGURE 3. Dependence of the peak current measured against the deposition time of Cu by UPD-ASV for a bare (9) and a gold nanoparticle-modified (2) UMEA-5A unit. Experiments were carried out in a 1 µM Cu in 10 mM H2SO4 solution. Scan rate: 0.1 V · s-1.
surface was not reached. Table 1 also shows the saturation time for each UMEA geometry, calculated using the ratio between Γtheoretical and Γ. A detailed analysis of the estimated surface coverage values highlighted that, for UMEAs with a constant interelectrode distance 20 times the radius, a decrease in the critical dimension (radius) resulted in an increase in sensitivity. This reaches a maximum value of 5.2 × 103 nC cm-2 · s-1 (SD ) 0.0) for UMEA-5A units. Furthermore, by decreasing the ratio between the interelectrode distance and the electrode radius for a constant radius, the sensitivity appears to decrease. This reaches a minimum value of 4.5 × 103 nC cm-2 · s-1 (SD ) 0.02) for UMEA-5B devices. Although these results can not be categorical, because the comparison was performed based on just three different geometries, all of them are in good agreement with those reported in the literature (6, 10. On the other hand, when quantitative determinations are performed, deposition time has to be appropriately set. Deposition time can be chosen establishing a compromise between short times that permit working in a regime of UPD and sufficient quantities of metal deposited to get high sensitivity. Therefore, results described above indicate that UMEA-5A devices would show better for the UPD-ASV determination of copper. Thus, the UMEAs5A and a deposition time of 15 s were chosen for further experiments. In the next step, the use of UMEAs for copper analysis was assessed by measuring the concentration of copper in standard solutions using a preconcentration time of 15 s. In the first experiment the copper concentration was changed between 0 and 40 µM in a 10 mM H2SO4 solution. The copper stripping peak charge increased linearly with copper concentration up to 2 µM. Therefore, a range between 0 and 2 µM was estimated as the working linear interval where sensitivity, repeatability, and reproducibility were tested. Figure 2 illustrates the UPD-ASV of copper in this concentration range. The linear response (slope 7.5 ( 0.6 nC · µM-1, intercept 3.5 ( 0.2 nC · µM-1, and r2 0.9995, with n ) 3) shows the high reproducibility achieved with these sensors. The
LOD was estimated to be 0.12 µM (7.4 µg · L-1), using the IUPAC (3Sb/m) criterion. Electrochemical Detection of Copper at Gold Nanoparticle-Modified UMEAs. Improvement of electrochemical techniques, development of new materials and devices as well as different electrode modified strategies for enhancing analytical selectivity and sensitivity, offer new opportunities in the field of electroanalytical chemistry. We previously reported that electrodeposition of gold nanoparticles on the UMEAs surface not only enabled the estimation of the yielding of their fabrication process, but also increased their surface microscopic area while keeping the microelectrode features (33). Gold nanoparticles exhibit a net negative charge induced in their preparation process, which was used to deposit them over the UMEAs surface by applying a positive potential high enough to induce the discharge of the nanoparticles by anodic oxidation. During the electrodeposition process at +1.6 V a temporary decrease in the current was recorded, which can be explained as an oscillating electrode surface during the growth of the nanostructured gold layer. The electrodeposition was stopped when a minimum and constant current was reached, which means that deposition of as many gold nanoparticles as possible is guaranteed. The time and charge necessary to reach such stage was evaluated with 10 UMEAs and values were 20 min and 2.41 mC (SD ) 0.16), respectively. Based on cyclic voltammetric curves obtained in 0.1 mM H2SO4 solution, an estimation of the surface microscopic area was achieved. This was conducted based on the amount of charge consumed during the reduction of the gold surface oxide monolayer. The reported value of 400 µC · cm-2 was used for the calculations (34). Table 2summarizes the values of the surface microscopic area and the electrochemical surface area of the UMEAs with and without electrodeposited gold nanoparticles, respectively. It is shown that the areas obtained electrochemically are in agreement with the geometrical ones. Furthermore, a high area increase was obtained after gold nanoparticle electrodeposition, which was similar (more than 43 times) to those UMEA units sharing the same geometric area (UMEA-10 and 5A units). However an increase VOL. 42, NO. 13, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Response of a UMEA-5A unit to changes in copper concentration in 10 mM H2SO4 solution and deposition time 90 s. (a) UPD-ASV signal obtained for Cu concentrations between 0 and 10 µM. Scan rate 0.1 V · s-1. (b) Resultant calibration plot up to 40 µM. Error bars were calculated from the standard deviation of the values obtained with three different electrodes. higher than 100 times was obtained with UMEA-5B units, which have a geometric area 4 times higher than the other two geometries. Electrodeposition of gold nanoparticles was confirmed by AFM (see Figure S3 and explanation, Supporting Information). The next set of experiments was conducted in order to test the feasibility of using gold nanoparticle-modified UMEAs for copper detection. The peak current of the stripping voltammetric process of Cu in a 1 µM Cu in 10 mM H2SO4 solution was studied as a function of the preconcentration time between 0 and 420 s. Results in Figure 3 reveal an almost linear dependence between peak current and time in the entire time interval. This figure also shows the results obtained with a bare UMEA unit of the same geometry. The plot is linear up to 60 s deposition time. For longer times, the voltammetric peak current keeps increasing and levels off at times longer than 240 s due to saturation of the electrode surface. This fact demonstrates the improved performance of the modified UMEAs for the electrodeposition of a higher amount of copper, which, a priori, will result in higher sensitivity values and longer linear range intervals. This is in concordance with the increase in surface roughness achieved by gold nanoparticle electrodeposition, as explained above. To compare the performance of bare and modified UMEAs for UPD-ASV of Cu, various sets of experiments were carried out for a preconcentration time of 15 s. For this deposition time the increase of sensitivity expected with the modified UMEAs is hardly noticeable with respect to the bare ones in a linear range between 0 and 3.2 µM Cu. However, for a preconcentration time of 90 s, the differences in sensitivity are significant (sensitivity 25.9 ( 1.3 nC · µM-1; intercept 19.9 ( 9.7 nC · µM-1, r2 0.992, n ) 3). This is almost three times higher than the sensitivity obtained with the bare UMEAs (7.5 ( 0.6 nC · µM-1). The linear range was also greatly extended from 0 up to 10 µM Cu concentration (0-2 µM with the bare UMEAs). Figure 4a shows the voltammetric stripping peak of copper for different copper concentrations. Figure 4b depicts the corresponding calibration plot for copper concentration up to 40 µM. The LOD was estimated to be 0.30 µM (19.2 µg · L-1), using the IUPAC (3Sb/m) criterion. Undesirable capacitive currents are directly proportional to the electrochemical active area of the electrode. The increase of capacitive current when measurements were performed with gold nanoparticle-modified UMEAs led to a higher LOD than that obtained with bare electrodes. This however could be minimized by using voltammetric techniques other than cyclic voltammetry. Here, no experiments were carried out in this sense since the limit of detection is still very low and the significant increase in sensitivity and linear range are really noteworthy from the analytical point of view. Preconcentration times longer than 90 s were studied and the sensitivity of the device was clearly enhanced (36.7 nC · µM-1, 4880
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at 180 s of preconcentration time). However the linear range was shorter at lower Cu concentrations (0-6 µM) due to saturation of the electrode. Thus, 90 s preconcentration time was set for further experiments. Matrix Effects. Together with Cu, soil extract samples used in this work were mainly contaminated with Pb and in minor quantity with a number of other heavy metals such as As. Considering that As exhibits a voltammetric process at a potential close to the potential of the Cu stripping peak, both Pb and As were identified as the principal interferences in copper determination by UPD-ASV. Then, a study of the detection of Cu in the presence of these two heavy metals was carried out. First, it was observed that the peak potential, peak current, and peak width of copper were unaffected by the presence of a 1000-fold excess of Pb2+. The precision of the determination of copper in a 6 µM copper in 10 mM H2SO4 solution containing such a large excess of Pb2+ did not vary. Only a 10 000-fold excess of Pb2+ led to an 11 mV shift in the potential stripping peak and a 5% decrease in the peak charge (data not shown). This may be caused by the formation of a precipitate of PbSO4 observed in the bulk of the solution. Second, UPD-ASV of Cu in the presence of As was carried out. A second stripping peak at a potential around +0.04 V appears and is related to the stripping voltammetric process of As (see Figure S4, Supporting Information). The Cu stripping peak was nearly unaffected when Cu and As were present in solution at concentrations of the same order of magnitude. Nevertheless, it was just shown a slight shift of the Cu peak potential of 11 mV toward more negative values and a decrease of its peak charge of less than 5% when the As concentration was up to almost ten times higher than the Cu one. However, these slight changes did not affect the reliable detection of Cu in a real matrix, as is shown below. Additionally, As stripping peak (at +0.040 V) tends to enhance as As concentration in solution increases, which suggests that both analytes (Cu and As) could be determined simultaneously in a matrix using the modified UMEAs presented here. This fact opens new possibilities to the modified UMEAs for multianalyte detection that should be explored in further experiments. Analysis of Soil Extract Samples. The efficiency of four gold nanoparticle-modified UMEAs for the UPD-ASV detection of copper in soil extracts was tested. The analysis of five acid soil extract samples was carried out using the standard additions calibration method (see Figure S5, Supporting Information). The extracts were analyzed by UPD-ASV at -0.1 V for 90 s in a 10 mM H2SO4 unstirred solution. Results for the five soil extract samples are shown in Table 3. The electrochemical results were compared with those obtained by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) method (adapted from 3051 EPA)
TABLE 3. Heavy Metals Concentration of Soil Extract Samples Determined by ICP-AES and Comparison with Those Obtained for Copper with Gold Nanoparticle-Modified UMEAs ionic concentration by ICP (µM)
Cu concentration (µM)
sample
As
Pb
Cu
UPD-ASV UMEAsa
RE (%)
1 2 3 4 5
12.5 6.4 9.0 42.0 2.5
53.4 60.6 36.8 406.2 12.4
35.2 24.6 20.7 143.8 23.8
34.9 (2.8) 24.7 (1.4) 20.8 (0.5) 128.5 (6.4) 23.5 (3.0)
0.8 -0.6 -0.6 10.6 1.2
a Between brackets: standard deviation of four gold nanoparticle-modified UMEAs.
(35), after acid digestion of the samples in concentrated HNO3. Standard deviations among the stripping peaks of copper and their percentages of relative error with respect to the values obtained by ICP-AES standard method for all samples are quite satisfactory. All these results are presented in Table 3. Statistical analysis of results carried out using T-student criteria of the differences with 95% confidence clearly demonstrated that the differences obtained by the ICP-AES and UPD-ASV methods are not statistically relevant, except for sample four. This fact must be explained by the high content of a number of other heavy metals present in this sample. Overall, we can conclude that the whole results here presented validate the nanoparticle-modified UMEA devices for the accurate UPD-ASV of copper in soil extract samples.
Acknowledgments This work has been supported by the DPI2003-08229-C03 project from the Spanish Ministery of Science and Education (MEC). J.O. and C.F.-S. thank MEC for the award of a FPI studentship and a Ramo´n y Cajal research contract, respectively. Soil extract samples were provided by Prof J. Alonso from Universidad Auto´noma de Barcelona, which is gratefully acknowledged.
Supporting Information Available SEM and AFM images of the devices, some results about copper deposition time experiments, studies of As as interference in copper detection, and the UPD-ASV peak of a soil extract sample. This information is available free of charge via the Internet at http://pubs.acs.org.
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