Anal. Chem. 2004, 76, 1466-1473
Spectroelectrochemical Sensing Based on Attenuated Total Internal Reflectance Stripping Voltammetry. 3. Determination of Cadmium and Copper Tanya Shtoyko,† Sean Conklin,† Anne T. Maghasi,† John N. Richardson,‡ Aigars Piruska,† Carl J. Seliskar,*,† and William R. Heineman*,†
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, and Department of Chemistry, Shippensburg University, 1871 Old Main Drive, Shippensburg, Pennsylvania 17257
The optical and electrochemical properties of metallic films on ITO surfaces resulting from deposition of copper and cadmium were monitored by stripping voltammetryattenuated internal reflectance spectroscopy. The voltammetric or optical responses of both metals were examined with respect to solution conditions such as pH and presence of dissolved oxygen. The morphologies of these films were also examined using environmental scanning electron microscopy, and the microscopic electrodeposition patterns were found to influence the optical response. The wavelength dependence of the optical response of deposited copper was determined and compared with calculations; optimal performance was at 400 nm for copper. A linear calibration curve was obtained over a range of 1 × 10-7-1 × 10-4 M for copper and compared with that of cadmium. The simultaneous determination of cadmium and copper was demonstrated, and the mechanism of co-deposition is discussed. Anodic stripping voltammetry is a well-known analytical technique that provides simultaneous multielement trace metal determination at low cost.1-4 Stripping voltammetry (SV) consists of two steps: the preconcentration step, in which the analyte accumulates on the working electrode by reduction of the metal at a certain potential, and the stripping step, in which the potential is swept or stepped in the positive direction to oxidize the deposited metal.1-3 Mercury electrodes (the hanging mercury electrode and mercury film electrode) have historically been very popular for * To whom correspondence should be addressed. E-mail: William.
[email protected]. Tel: (513) 556-9210. Fax: (513) 556-9239. E-mail: Carl.
[email protected]. Tel: (513) 556-9213. Fax: (513) 556-9239. † University of Cincinnati. ‡ Shippensburg University. (1) Wang, J. Stripping Analysis: Principles, Instrumentation, and Applications; VCH Publishers: Deerfield Beach, FL, 1985. (2) Wang, J. In Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1996. (3) Wang, J. In Analytical Electrochemistry, 2nd ed.; John Wiley & Sons: New York, 2000; pp 75-84. (4) Heineman, W. R.; Mark, H. B., Jr.; Wise, J. A. In Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984; pp 499-538.
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SV. Mercury-platinum optically transparent electrodes (OTE) were coupled with stripping analysis some time ago.5-7 Optical and electrochemical characteristics of the mercury-platinum OTE were evaluated during deposition of metals into the thin mercury film. It was demonstrated that light passing through the Hg-PtOTE during the diffusion-controlled reduction of metal ions was attenuated by the accumulation of electrodeposited metal in the thin mercury film. The degree of attenuation was related to the molar absorptivity of the metal in mercury, concentration of the metal ion, and deposition time. Solid electrodes such as graphite, platinum, and silicon have also been used for SV and give good results under certain conditions.8 We have recently reported9 deposition and anodic stripping coupled to optical spectroscopy employing indium tin oxide (ITO) OTEs. The electrodeposition and subsequent stripping of lead and cadmium on ITO were monitored individually by attenuated total internal reflectance (ATR). It was shown that attenuation was proportional to the concentration of metal ions, deposition time, and optical properties of the deposited metal. The wavelength dependence of the optical attenuation of deposited Pb and Cd was determined; optimal performance was at 750 nm for Pb and at 400 nm for Cd. Calibration curves were obtained over the range of 1 × 10-7-5 × 10-5 and 1 × 10-9-1 × 10-5 M for Pb and Cd, respectively, using change in absorbance that accompanied deposition and subsequent stripping of the electrodeposited metal from the ITO. The purpose of this paper is to further evaluate ATR-SV as a technique to measure Cu deposited on ITO and compare it to Cd measurements. This paper also focuses on optimization of sensor performance for Cu and Cd through an improved understanding of the interplay between the nature of metal deposition and the magnitude of the optical ATR signal through microscopic evaluation of the ITO surfaces onto which Cu and Cd have been individually deposited. We also address the behavior of co(5) Heineman, W. R.; DeAngelis, T. P.; Goelz, J. F. Anal. Chem. 1975, 47, 13641369. (6) Heineman, W. R.; Kuwana, T. Anal. Chem. 1971, 43, 1075-1078. (7) Heineman, W. R.; Kuwana, T. Anal. Chem. 1972, 44, 1972-1978. (8) Brainina, K. A. Talanta 1971, 18, 513. (9) Shtoyko, T.; Maghasi, A. M.; Richardson, J. N.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2003, 75, 4585-4590. 10.1021/ac034829i CCC: $27.50
© 2004 American Chemical Society Published on Web 01/31/2004
deposition of two metals, Cu and Cd, onto the same surface, both spectroelectrochemically and using microscopic techniques. Similar studies for Hg and Pb are reported in a companion paper that also demonstrates advantages of measuring an optical signal over the usual current signal.10 It was reported previously that during the preconcentration step the metal deposition depends on the purity of the electrolyte used, temperature, and current density.11 Both optical microscopy and electron diffraction have been used to study these films, and the influence of the bath conditions on the structure of the substrate is reasonably well understood.11 Two types of crystal growth were distinguished. In one case, growth occurs outwardly from nuclei on the substrate, with negligible deposition between these nuclei, while in the second case, growth takes place laterally so that deposition occurs over the whole of the substrate surface. The first type of growth is favored by high current density, low ion concentration, low bath temperature, and low circulation rate of the electrolyte. These conditions all tend to produce local depletion of the strength of the electrolyte on the surface so that growth occurs on nuclei and not over the entire substrate area. Conditions that tend to maintain high ion concentration over the whole surface favor the second type of deposition and result in smooth deposits contouring the substrate.11 Studies of the structures and orientations of electrodeposits by electron diffraction have shown clearly how the mode of growth is influenced by the substrate structure.11 When deposition of a metal occurs on a single crystal of the same metal under conditions such that lateral growth takes place, the electrodeposit is found to continue the lattice of the substrate.11 However, when electrodeposition takes place on a polycrystalline substrate, the structure of the initial layers is determined by that of the substrate. At large thicknesses, the crystal structure is determined by the bath conditions, with the substrate influence being lost. The smaller the crystal size of the substrate, the more easily is the deposit structure influenced by the bath conditions.11 Electrochemical thin-film growth on silicon12-19 and polypyrrole films20 has been studied before. The effects of organic additives, bath temperature, and pH on the nanostructure of copper deposition were also studied.21 It was reported that the electrode surface morphology strongly influences the nucleation and growth of copper on Si(111) from 1 mM CuSO4 and 0.1 M H2SO4 solution.18 The deposition occurs through progressive nucleation followed by three-dimensional diffusion-limited growth on both (10) Maghasi, A. T.; Conklin, S. D.; Shtoyko, T.; Piruska, A.; Richardson, J. N.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 2004, 76, 1458-1465. (11) Heavens, O. S. In Optical Properties of Thin Films; Dover Publications: New York, 1991; pp 43-44. (12) Ziegler, J. C.; Wielgosz, R. I.; Kolb, D. M. Electrochim. Acta 1999, 45, 827. (13) Ziegler, J. C.; Reitzle, A.; Bunk, O.; Zegenhagen, J.; Kolb, D. M. Electrochim. Acta 2000, 45, 4599-4605. (14) Stiger, R. M.; Gorer, S.; Craft, B.; Penner, P. M. Langmuir 1999, 15, 790. (15) Rashkova, B.; Guel, B.; Potzschke, R. T.; Staikov, G. Electrochim. Acta 1998, 43, 3021. (16) Krumm, R.; Guel, B.; Schmitz, C.; Staikov, G. Electrochim. Acta 2000, 45, 3255. (17) Oskam, G.; Searson, P. C. J. Electrochem. Soc. 2000, 147, 2199. (18) Ji, C.; Oskam, G.; Searson, P. C. Surf. Sci. 2001, 492, 115. (19) Oskam, G.; van Heerden, D.; Searson, P. C. Appl. Phys. Lett. 1998, 73, 3241. (20) Sarkar, D. K.; Zhou, X. J.; Tannous, A.; Leung, K. T. J. Phys. Chem. 2003, 107, 2879. (21) Natter, H.; Hempelmann, R. J. Phys. Chem. 1996, 100, 19525.
Si(111) and miscut Si(111) over a wide potential range with nucleation occurring preferentially at step edges. EXPERIMENTAL SECTION Chemicals, Materials, and Instruments. All chemicals were used as received without further purification. Potassium nitrate, sodium acetate, acetic acid, and cadmium sulfate were purchased from Fisher Scientific. Cupric sulfate was supplied by Mallinckrodt Chemical Works. Cadmium sulfate solutions were prepared by dissolving the appropriate amount in 0.1 M sodium acetate (pH 7.1) or 0.1 M acetic acid/sodium acetate buffer (pH 5.5). Copper(II) sulfate solutions were made by dissolving the appropriate amount in 0.1 M KNO3 (pH 7.3) or 0.1 M acetic acid/sodium acetate buffer (pH 5.5). ITO-coated glass (11-50 Ω/0, 150-nm-thick ITO layer over tin float or 1737F glass) was supplied by Thin Film Devices and diced into 1 × 3 in. slides. An A-2 Harrick plasma cleaner was used to clean ITO slides. A Philips XL 30 environmental scanning electron microscope (ESEM) was used for surface imaging and energy-dispersive X-ray surface analysis. Preparation of ITO-OTEs. ITO slides were washed with Alconox soap, rinsed with deionized water, wiped with ethanol, and rinsed with deionized water again. The dried ITO slides were then Ar plasma cleaned for at least 30 min. Attenuated Total Reflection Measurements. The instrument used for ATR spectroelectrochemical measurements has been previously described.22,23 A Xe arc lamp was used as a light source. ATR-SV measurements were performed as also described previously.9 The current and corresponding optical signal were digitized and stored on a personal computer. Data records were manipulated and analyzed using commercially available spreadsheet programs. Absorbance values were obtained by recording the light intensity through the prism-coupled slide when exposed to pure supporting electrolyte (Io) and the intensity after being exposed to an analyte solution (I). Sensor absorbance in the multiple reflection arrangement was defined as A ) log(Io/I). During multiple-wavelength measurements, optical detection was accomplished using an Ocean Optics PC2000 multichannel CCD spectrometer. During metal deposition, the solution was continuously circulated through the ATR cell at a rate of 50 mL/min using a Barnant model 7520-35 peristaltic pump. RESULTS AND DISCUSSION Copper and Cadmium Deposition and Stripping at Bare ITO-OTE. The electrochemical behavior of CuSO4 in 0.1 M acetic acid/sodium acetate buffer (pH 5.5) is shown in Figure 1A (curve 1). Copper acetate complex forms in buffer and reduces to copper metal at a potential of -0.35 V (vs Ag/AgCl). Presumably, copper oxidizes back to Cu2+ at 0.15 V, and Cu2+ is stabilized by formation of copper acetate. The cathodic (deposition) wave is well defined, but the anodic (stripping) wave, while having a sharp peak, displays a gradually decaying shoulder on the more positive potential side. The absorbance change measured concurrently (22) Shi, Y.; Slaterback, A. F.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 3679-3686. (23) Shi, Y.; Seliskar, C. J.; Heineman, W. R. Anal. Chem. 1997, 69, 48194827.
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Figure 1. (A1) Cyclic voltammogram recorded at a bare ITO-OTE in 1 × 10-3 M CuSO4 in 0.1 M acetic acid/sodium acetate buffer (pH 5.5). The voltammogram was obtained by cycling the potential in the range from 1.5 to -0.5 V at 25 mV/s (E/V vs Ag/AgCl ref). (A2) Sensor absorbance measured at 400 nm taken concurrently with the cyclic voltammogram shown in (A1). (B1) Cyclic voltammogram recorded with bare ITO-OTE in 1 × 10-3 M CdSO4 in 0.1 M acetic acid/sodium acetate buffer at pH 5.5. The voltammogram was recorded by cycling the potential in the range from 0.5 to -1.0 V at 25 mV/s (E/V vs Ag/AgCl ref). (B2) Sensor absorbance measured at 400 nm taken concurrently with the cyclic voltammogram shown in (B1). (C1) Cyclic voltammogram recorded with bare ITO-OTE in 1 × 10-3 M CuSO4 in 0.1 M sodium nitrate. The voltammogram was recorded by cycling the potential in the range from 1.5 to -0.5 V at 25 mV/s (E/V vs Ag/AgCl ref). (C2) Sensor absorbance measured at 400 nm taken concurrently with the cyclic voltammogram shown in (C1). (D1) Cyclic voltammogram recorded with bare ITO-OTE in 1 × 10-3 M CdSO4 in 0.1 M sodium acetate (pH 7.3). The voltammogram was recorded by cycling the potential in the range from 0.5 to -1.0 V at 25 mV/s (E/V vs Ag/AgCl ref). (D2) Sensor absorbance measured at 400 nm taken concurrently with the cyclic voltammogram shown in (D1).
with deposition and stripping of copper is shown in Figure 1A (curve 2). Sensor absorbance increases at -0.4 V, in tandem with the onset of Cu2+ reduction and peaks at -0.1 V, concurrent with the onset of stripping. The absorbance then decreases, gradually approaching its initial value. This behavior mimics the gradual decay of the anodic stripping wave and is indicative of a gradual stripping process. Figure 1B (curve 1) is the voltammetric response for 1 mM Cd2+ in 0.1 M acetic acid/ sodium acetate buffer (pH 5.5) showing a reduction peak at -0.9 V (vs Ag/AgCl) corresponding to reduction to metallic cadmium. Absorbance measured at 400 nm increased concurrently with cadmium deposition (Figure 1B, curve 2) and decreased at -0.6 V where cadmium was stripped off the electrode surface. The electrochemical and optical signals are again quite well defined, following each other closely. Careful inspection of the absorbance signal shows a small amount of Cd remaining on the surface between -0.5 and 0.0 V. This is stripped off at 0.0 V as evidenced by absorbance going to baseline and a very small current stripping peak. We assume this is a monolayer of Cd on the ITO surface. The electrochemical deposition and stripping of cadmium and copper at ITO from unbuffered electrolyte solutions were also studied, primarily because of the potential application of the ATR sensor for remote sensing where buffering may not be possible. A detailed description of their voltammetric and optical characteristics during deposition and stripping follows. Electrochemical and optical responses of Cu2+ in unbuffered 0.1 M KNO3 at ITO are shown in Figure 1C. In curve 1, there are 1468 Analytical Chemistry, Vol. 76, No. 5, March 1, 2004
three cathodic peaks for Cu2+ reduction to Cu0 due to different deposition behaviors corresponding to Cu0 deposition (1) at the ITO surface and (2) at the copper-copper interface and to various hydroxyl complexes of Cu2+ that can exist depending on pH. Presumably, the initial cathodic peak (-0.15 V vs Ag/AgCl) corresponds to deposition of a thin layer of Cu onto ITO, while the peak at more negative potential corresponds to deposition of Cu2+ onto Cu. The corresponding increase in absorbance in Figure 1C (curve 2) at 400 nm correlates with copper deposition on the ITO electrode surface. There are also multiple anodic peaks for the oxidation of Cu0 (curve 1), with decreases in absorbance associated with each one. As reported previously24 the first anodic peak (∼0.1 V in our case) was attributed to formation of a surface Cu2O layer. The second peak (at 0.3 V in our case) was attributed to oxidation of bulk copper to Cu2O.24 The concurrent decrease in absorbance during copper oxidation to Cu2O was reported before24 and is due to electrochromism in copper oxide thin films. The dominant stripping peak occurring at 1.35 V (vs Ag/AgCl) corresponds to the stripping of Cu2O from the ITO surface; the magnitude of this peak scales linearly with both deposition time and Cu2+ concentration. The electrochemical behavior of Cd2+ in 0.1 M sodium acetate at ITO has been investigated previously9 and is shown in Figure 1D (curve 1). Cadmium ion undergoes a 2e- reduction at -0.9 V (vs Ag/AgCl), which leads to metal deposition on ITO-OTE. The corresponding increase in absorbance shown in Figure 1D (curve (24) Richardson, T. J.; Slack, J. L.; Rubin, M. D. Electrochim. Acta 2001, 46, 2281.
2) coincides with cadmium deposition on the electrode surface. At ∼-0.6 V (vs Ag/AgCl), cadmium was stripped off the electrode surface producing a much less defined stripping peak than in buffered solution. A concurrent decrease in absorbance9 accompanies the stripping process, exhibiting a smaller change than in buffered solution. The effect of dissolved oxygen on the voltammetric response was also studied since deoxygenation may not be applicable in remote sensing situations. Deoxygenation of the unbuffered 1 mM Cu2+ solution led to a larger and better defined current stripping peak (results not shown) compared to the nondeoxygenated solution. However, in the case of unbuffered 1 mM Cd2+ solution, the effects of deoxygenation were minimal. We emphasize, however, that the presence of dissolved oxygen does not appear to hinder the measured absorbance difference appreciably in the case of either metal. Dependence of the Attenuated Total Reflection on Wavelength. Most metals behave as neutral density filters, absorbing light at all visible wavelengths; however, some dependence of molar extinction coefficient on wavelength does occur.6,7 The deposition of copper was monitored in the visible region using ATR in order to determine the wavelength of most sensitive response, as illustrated in Figure 2, panel A. A 5 × 10-5 M solution of Cu2+ was used for copper deposition, and spectra were collected every 2 min for 30 min. It is apparent that copper has a more favorable optical response at 400 nm and decreases in absorbance at longer wavelengths. The optical properties of metal films are determined by the real and imaginary parts of the refractive index as n′ ) n + ik. The imaginary part of the refractive index, k, is related to the extinction coefficient by R ) 4πk/λ. These constants11,25,26 can be used to calculate the reflectance and transmittance (or absorbance) of a thin film at any angle of interrogation and wavelength. For the sake of modeling the expected wavelength dependence of the absorbance under ATR conditions for copper deposited on ITO, we have used the optical constants for a 130-nm-thick ITO film that we have independently determined27 and the bulk material constants of copper metal.28 For the purposes of modeling the metal-coated ITO-glass system, we have assumed a simple multilayer system composed of four uniform isotropic layers (glass, ITO, deposited metal, and liquid solvent (water)). The reflectance of light from this four-layer system was then calculated at individual wavelengths using the appropriate optical constants and the Fresnel relationships. From the calculated reflectance values, sensor absorbance for one reflection was then calculated. Figure 2B shows the results from a simulation of the absorbance under ATR conditions of a 5-nm-thick uniform layer of copper metal deposited on 130-nm of ITO on Corning 1737F glass for one reflection (dotted curve); experimental data are shown as a solid curve and represent the total absorbance of 8-10 reflections in the spectroelectrochemical cell. From the results shown, it can be seen that the general wavelength dependence of the absor(25) Born, M.; Wolf, E. In Principles of Optics, 6th ed.; Pergamon Press: New York, 1980; pp 611-664. (26) Paquin, R. A. In Properties of Metals; McGrow Hill: New York, 1995; Vol. II, pp 35.31-35.78. (27) Zudans, I.; Seliskar, C. J.; Heineman, W. R. Thin Solid Films 2003, 426, 238-245. (28) Handbook of Optical Constants of Solids II; Palik, E. D., Ed.; Academic Press: Boston, 1991.
Figure 2. (A) ATR spectra for copper deposited on ITO-OTE. Copper was deposited from a solution of 5 × 10-5 M CuSO4 in 0.1 M sodium acetate by stepping the potential from open circuit to -0.5 V and holding for 30 min; spectra were taken at 2-min intervals. (B) The computed (theoretical; dotted curve) absorbance for one reflection from a uniform thin film of copper metal (5-nm thickness) is shown with experimental data for copper absorbance vs wavelength. The experimental curve (solid curve) was acquired as the total absorbance of 8-10 reflections in the spectroelectrochemical cell of a Cu film on an ITO surface deposited from 5 × 10-5 M CuSO4 in 0.1 M KNO3 by stepping the potential from 1.5 to -0.5 V (vs Ag/AgCl ref.) and holding for 12 min. The theoretical absorbance curve was calculated using reflectance coefficients from a stratified uniform layer system with and without a metal layer. The optical constants n and k for all layers were taken as standard literature values.
bance observed is modeled adequately. Indeed, the results appear to be too good given the limitations of the model applied. For example, the applied simple model does not simulate surface roughness, nonuniform surface coverage, and the specific crystalline morphology of the copper film. It is interesting to note in Figure 2 that a peak in the absorbance at ∼600-nm wavelength grows in as copper deposition increases. In turn, this corresponds to copper nanocrystals growing in size as the deposition proceeds in time (vide infra, Figure 3 and associated discussion). These crystals reach a size of more than 100 nm on an edge. Further computations of the absorbance (not shown) as a function of wavelength at larger film thicknesses of copper metal clearly indicate that the peak observed at ∼600 nm is due to the plasmon resonance of copper metal. In addition, it is well known that the optical constants of thin metal layers tend to differ from those of bulk metal. Therefore, we conclude that comparison of calculated data to experimental data is only qualitative and should be limited only to the obvious similarity of shapes of the experimental and theoretical curves. The wavelength dependence of the absorbance of cadmium was examined previously in the region from 400 to 800 nm and Analytical Chemistry, Vol. 76, No. 5, March 1, 2004
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Figure 3. SEM images of an ITO-OTE with deposited copper. The metal deposition at -0.5 V was done using 5 × 10-5 M CuSO4 in 0.1 M sodium acetate for 2 (A), 10 (B), and 30 min (C). The solution with total volume 250 mL was cycled through the cell at a flow rate of 50 mL/min during deposition.
Figure 4. SEM images of ITO-OTE with deposited cadmium. The metal deposition at -1.0 V was done using 5 × 10-5 M CdSO4 in 0.1 M sodium acetate for 2 (A), 10 (B), and 30 min (C). The solution with total volume 250 mL was cycled through the cell at a flow rate of 50 mL/min during deposition.
also gave the highest response at 400 nm.9 The simulation of absorbance under ATR conditions for the cadmium thin film was not consistent with the experimental data, most likely because of limitations of the model applied as discussed later. SEM Analysis of Deposited Copper and Cadmium. Electron micrographs of deposited copper and cadmium on ITO surfaces are shown in Figures 3 and 4, respectively. Figure 3 shows the deposition of 5 × 10-5 M CuSO4 in 0.1 M sodium acetate for deposition times of 2 (A), 10 (B), and 30 min (C). By 2 min, cube-shaped copper crystals of approximately uniform size (∼150 nm) are dispersed across the entire electrode area (Figure 3A). By 10 min, the density of crystals has increased to fill the entire electrode area, but crystal size remains relatively constant (∼100 nm) (Figure 3B). At longer deposition time (Figure 3C), copper crystals evenly grow in size to form a uniform film.
Potassium nitrate and sodium acetate were used as supporting electrolytes. We have found that the choice of supporting electrolyte does not change the deposition behavior of copper. Figure 4 illustrates the deposition of 5 × 10-5 M CdSO4 in 0.1 M sodium acetate. After 2 min of deposition, cadmium forms crystalline features of ∼100-200 nm in diameter distributed evenly across the electrode surface (Figure 4A). By 10 min, the crystals become larger (∼250-300 nm) with tiny weblike structures connecting them (Figure 4B). After 30 min of deposition, cadmium crystals grow larger yet (∼400-800 nm) forming a nonuniform layer of cadmium features, again with weblike structures radiating out from them (Figure 4C). These evenly distributed weblike structures are believed to contribute significantly to the sensor absorbance.
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Figure 5. Calibration ATR spectra of (A) (1) 1 × 10-7, (2) 5 × 10-7, (3) 1 × 10-6, (4) 5 × 10-6, (5) 1 × 10-5, (6) 5 × 10-5, and (7) 10-4 M CuSO4, all in 0.1 M KNO3 on a bare ITO-OTE. The potential was stepped from 1.5 to -0.5 V (vs Ag/AgCl ref.) and held for 30 min. (B) is a calibration curve for Cu.
Figure 6. (A) Simultaneous determination of 0.1 mM CdSO4 and 0.1 mM CuSO4 in 0.1 M sodium acetate at pH 7.3. The potential was held at -0.5 V for 30 s and was then stepped to -1.0 V and held for 30 s. Then the potential was swept from -1.0 to 1.5 V (vs Ag/AgCl ref.) at a potential sweep rate of 10 mV/s (A1). The corresponding absorbance change during the potential sweep is shown in (A2). Also shown is the ATR absorbance at 400 nm (B2) taken during the metals deposition from the same solution, but resulting from the potential steps shown in (B1).
Energy-dispersive X-ray fluorescence data (not shown here) verified that copper and cadmium were deposited in each case. Comparison of copper and cadmium deposition suggests that copper deposits much more uniformly by covering the electrode surface completely and only then contributing to increased film thickness. Cadmium deposits somewhat differently by forming large crystals, but not covering the electrode surface uniformly. For this reason, the simulation of absorbance under ATR conditions (Figure 2 B) gave adequate results for copper (more uniform film) but not for cadmium (nonuniform surface). Evaluation of ATR-SV Sensors for Individual Determination of Cu2+ and Cd2+. The ATR-SV sensor performance on a bare ITO-OTE for Cu is illustrated in Figure 5. Different concentrations of metals were deposited by stepping the applied potential from 1.5 to -0.5 V and holding for 30 min (with stirring by recirculating solution through the flow cell at 50 mL/min) on bare ITO and then stripped with a positive potential step. Optical changes concurrent with deposition and stripping were monitored at 400 nm and quantified as reported previously;9 absorbance due to modulation of the ITO alone was similarly obtained in electrolyte solution and was subtracted from the absorbance profile obtained at each concentration for Cu deposition. The linear dependence of absorbance on concentration was observed from 5 × 10-7 to 1 × 10-4 M copper (Figure 5B) and can be explained by uniform deposition of the metal on the ITO surface (Figure 3). Depositing for 30 min made possible detection of 1 × 10-7 M copper in standard solutions. To achieve greater sensitivity, the deposition time could be lengthened since the amount of metal deposited depends on the concentration of the
metal in the solution and the deposition time. As reported previously, cadmium yielded a linear dependence of absorbance on concentration from 1 × 10-9 to 1 × 10-6 M cadmium.9 Comparison of calibration curves for Cu and Cd9 shows that the sensor absorbance difference for Cu deposited from the same concentration solution is approximately two and a half times smaller than that for Cd, thus suggesting that this method should yield lower limits of detection for Cd. The difference in sensor absorbance is probably due to differences in the optical properties of the two metal films. Simultaneous Determination of Cd2+ and Cu2+ by ATRSV. Cadmium and copper, each at 1 × 10-4 M in 0.1 M sodium acetate, were co-deposited from a binary mixture onto an ITOOTE to demonstrate the concept of using one ITO slide as a single sensor for simultaneous determination of multiple metals. For solid films thicker than a few monolayers formed on a solid electrode, the underlying metal cannot undergo oxidation at the interface during the stripping step until the overlying metal has been stripped. This can cause less than 100% stripping efficiency, as well as overlapping of stripping peaks when mixtures of metals are deposited on the surface of a solid electrode.4 In Figure 6A, copper and cadmium were deposited sequentially by holding at a less negative potential (-0.5 V for 30 s) to deposit copper first and then sweeping the potential sufficiently negative (-1.0 V for 30 s) to deposit cadmium, which is more difficult to reduce. In this case, the metal film is partially layered in an order that enables the last metal deposited (cadmium) to be the first one stripped. Use of this technique enables sensing of multiple metals using only one OTE. As illustrated in Figure 6A (curve 1), the potential was swept positively at 10 mV/s in order to strip the metals Analytical Chemistry, Vol. 76, No. 5, March 1, 2004
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Figure 7. SEM images of ITO-OTE with deposited cadmium and copper. The metal deposition at -1.0 V was done using 1 × 10-5 M CdSO4 and 1 × 10-5 M CuSO4 in 0.1 M sodium acetate for 2 (A), 10 (B), and 30 min (C), all at 20000× magnification. The solution was cycled through the cell at a flow rate of 50 mL/min during deposition.
individually. Cadmium was stripped first at -0.7 V. At this potential, the absorbance at the OTE decreased (Figure 6A, curve 2) due to cadmium stripping. As the potential was swept further in the positive direction, multiple anodic peaks for copper appeared at -0.15, 0.15, and 1.35 V, as described before. There was a decrease in absorbance associated with each anodic peak for copper in Figure 6A (curve 2). Another approach is to deposit and strip the metals by stepping the potential instead of scanning it; this is a more efficient process because the deposition or stripping potential is accessed immediately, resulting in sharper deposition and stripping profiles. The same mixture of cadmium and copper used in the previous discussion was again used for the stepped potential experiment shown in Figure 6B. When the potential was held at 1.5 V (profile 1), no metal deposited on the ITO-OTE; there was also no 1472 Analytical Chemistry, Vol. 76, No. 5, March 1, 2004
Figure 8. SEM images of ITO-OTE with deposited cadmium and copper at -1.0 V. The metal deposition was done using 1 × 10-5 M CdSO4 and 1 × 10-5 M CuSO4 in 0.1 M sodium acetate for 30 min (A-C) at 25×, 1000×, and 10000× magnification, respectively. The solution was recycled through the cell at a flow rate of 50 mL/min during deposition.
absorbance change at that potential (curve 2). When the potential was stepped to -0.5 V to deposit copper, an absorbance increase was noted (curve 2). After 1 min of copper deposition, the potential was stepped to -1.0 V to initiate deposition of cadmium. Absorbance of the sensor continued to increase due to co-deposition of both metals (curve 2). When the potential was stepped back to -0.5 V, the absorbance decreased due to stripping of cadmium. However, the absorbance still increased slightly with time because copper was still depositing. Finally, the potential was stepped to 1.5 V to strip copper, resulting in an absorbance drop to zero (curve 2). The stepped potential experiment allows simplification of the quantification of multiple stripping peaks of copper by automatic integration of the optical response.
In the interest of elucidating the mechanism of co-deposition and the morphology of the resulting metallic film, the ITO surface after the deposition of metals from 1 × 10-5 M CuSO4 and 1 × 10-5 M CdSO4 in 0.1 M sodium acetate at -1.0 V for 2, 10, and 30 min was examined by SEM at 20000× magnification, as shown in Figure 7A-C, respectively. After 2 min of deposition, cadmium and copper form a low density of ∼50-100-nm-diameter crystals distributed evenly across the electrode surface (Figure 7A). By 10 min, the crystals are uniformly distributed with higher density and larger size (Figure 7B). After 30 min of deposition, cadmiumcopper crystals grow larger (∼200-250 nm), forming a nonuniform layer with tiny weblike structures connecting the crystals (Figure 7C). The ITO slide after 30 min of co-deposition was examined at different magnifications as shown in Figure 8. As described previously, there are two types of crystal growth: (1) when the growth occurs outwardly from nuclei with negligible deposition between these nuclei and (2) when the deposition occurs over the whole of the substrate surface. In the case of codeposition of copper and cadmium, both mechanisms of crystal growth were observed simultaneously. At 25× magnification, a large area of the OTE is shown in Figure 8A. There are larger (∼10-30 µm) and smaller (
