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Technical Notes
Gold Electrodes from Recordable CDs Lu´cio Angnes,*,† Eduardo M. Richter,† Ma´rcio A. Augelli,‡ and Gustavo H. Kume†
Departmento de Quimica Fundamental, Instituto de Quı´mica, Universidade de Sa˜o Paulo, Av. Prof. Lineu Prestes, 748-CEP 05508 900, Sa˜o Paulo SP, Brazil, and Faculdade de Engenharia de Guaratingueta´ , UNESP, Guaratingueta´ , SP, Brazil
Gold electrodes are widely used in electrochemistry and electroanalytical chemistry. The notable performance when used in stripping analysis of many ionic species and the extraordinary affinity of thio compounds for its surface make these electrodes very suitable for many applications. This paper reports a simple and novel way to construct gold electrodes (CDtrodes) using recordable CDs as the gold source. The nanometer thickness of the gold layer of recordable disks (50-100 nm) favors the construction of band nanoelectrodes with areas as small as 10-6 cm2. The plane surface can be easily used for the construction of conventional-sized gold electrodes for batch or flow injection analysis or even to obtain electrodes as large as 100 cm2. The low price of commercial recordable CDs allows a “one way use”. The evaluation and applicability of these electrodes in the form of nanoelectrodes, in batch and associated with flow cells, are illustrated in this paper. Gold and platinum are the most widely used metallic solid electrodes for electrochemical and electroanalytical purposes. The preference for these metals is attributed to their high purity, their ease of machining, and the “inertness” in the presence of almost all reagents. Until recently, platinum electrodes were preferred, despite the larger overpotential of gold for hydrogen reduction. This preference was attributed to tradition and to the fact that gold is much more difficult to seal in glass than platinum.1,2 Recently, gold electrodes have gained increasing preference due to two important applications: for stripping analysis and for studies involving surface modifications by self-assembling. For stripping analysis of many metallic ions, gold electrodes give larger and sharper peaks than platinum, attaining remarkable low detection limits.2-6 Gold electrodes modified with self-assembled monolayers of thiols can be tailored in order to gain selectivity,7,8 to minimize underpotential deposition of metals in stripping studies,9 for the * To whom correspondence should be addressed: (e-mail)
[email protected]; (fax) 0055 11 815 5579. † Universidade de Sao Paulo. ‡ UNESP. (1) Sawyer, D. T.; Roberts, J. L., Jr. Experimental Electrochemistry for Chemists; John Willey & Sons: New York, 1974. (2) Wang, J. Stripping Analysis: Principles, Instrumentation and Applications; VCH Publishers: Deerfield Beach, 1985. (3) Forsberg, G.; Laughlin, L. W.; Magargle, R. G.; Koirtuohann, S. R. Anal. Chem. 1975, 47, 1586-1592. (4) Wang, J.; Tian, B.; Lu, J.; Wang, J.; Luo, D.; MacDonald D. Electroanalysis 1998, 10, 399-402. 10.1021/ac000437p CCC: $19.00 Published on Web 09/30/2000
© 2000 American Chemical Society
immobilization of enzymes or antibodies on the surface of the electrode,10-12 or to study many aspects of large molecules such as DNA13,14 or cytochrome c.15 These studies are usually done on commercial gold electrodes, with an amorphous or orientated surface. However, the complete removal of the thiol from the electrode surface after the measurements requires drastic treatments, such as strong abrading of the electrode with sandpaper or very cumbersome work with many portions of abrasive slurries. In our laboratory, alternative methods for construction of gold microelectrode arrays were already developed, starting from split electronic chips16 followed by a thin-layer flow cell, using a electronic chip sanded on its top to expose the gold microwires. These arrays were used for analysis of metallic ions in beverages,17 ascorbic acid in beer and soft drinks,18 and ascorbic and uric acids in urine.19 In the present work, we describe a new and very versatile way of construction of gold electrodes using recordable CDs, an almost inexhaustible supply of gold thin layers. The low cost of this “new” source of gold electrodes favors the “one way use” of each electrode, a very desirable condition in many situations, especially when surface modifications with thiols are explored. Additionally, (5) Richter, E. M.; Kume, G. H.; Augelli, M. A.; Angnes, L. Book of abstracts, Mercury as a Global Pollutant-5th International Conference, Rio de Janeiro, Brazil, May 23-28, 1999; p 6. (6) Richter, E. M.; Augelli, M. A.; Kume, G. H.; Mioshi, R.; Angnes L. Fresenius J. Anal. Chem. 2000, 366 (5), 444-448. (7) Wang, J.; Wu, H.; Angnes, L. Anal. Chem. 1993, 65, 1893-1896. (8) Li, J.; Kaifer, A. E. Langmuir 1993, 9 (2), 591-596. (9) Shen, H.; Mark, J. E.; Seliskar, C. J.; Mark, H. B.; Heineman, W. R. J. Solid State Electrochem. 1997, 1 (3), 241-247. (10) Ruan, C. M.; Yang, R.; Lei, F.; Deng, J. Q. Anal. Chem. 1998, 70 (9), 17211725. (11) Murthy, A. S. N.; Sharma, J. Anal. Chim. Acta 1998, 363 (2-3), 215-220. (12) Park, I. S.; Kim, N. Biosens. Bioelectron. 1998, 13 (10), 1091-1097. (13) Steel, A. B.; Herne, T. M.; Tarlov, M. J. Anal. Chem. 1998, 70 (22), 46704677. (14) Kelley, S. O.; Barton, J. K.; Jackson, N. M.; McPherson, L. D.; Potter, A. B.; Spain, E. M.; Allen, M. J.; Hill, M. G. Langmuir 1998, 14 (24), 67816784. (15) Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438 (1-2), 91-97. (16) Nascimento, V. B.; Augelli, M. A.; Pedrotti, J. J.; Gutz, I. G. R.; Angnes, L. Electroanalysis 1997, 9 (4), 335-339. (17) Augelli, M. A.; Nascimento, V. B.; Pedrotti, J. J.; Gutz, I. G. R.; Angnes, L. Analyst 1997, 122, 843-847. (18) Matos, R. C., Augelli, M. A.; Pedrotti, J. J.; Lago, C. L.; Angnes, L. Electroanalysis 1998, 10 (12), 1-4. (19) Matos, R. C., Augelli, M. A.; Pedrotti, J. J.; Lago, C. L.; Angnes, L. Anal. Chim. Acta 2000, 404, 151-157.
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Figure 1. Composition of a recordable compact disk: (a) polycarbonate base; (b) dye-photosensitive layer; (c) gold reflective layer; (d) one or two layers of polymeric films.
the very thin gold layers (50-100 nm of thickness20) utilized in recordable CDs can be explored for the construction of band nanoelectrodes. This paper describes many aspects involving the construction of gold electrodes with conventional and nanometric dimensions employing recordable CDs. The electrochemical response of the nanoelectrodes is explored, the high sensibility of planar electrodes used for the quantification of mercury and copper in the low-microgram per liter region is demonstrated, and the utility of ring electrodes associated with flow injection analysis is illustrated. EXPERIMENTAL SECTION Apparatus and Reagents. Constant-current potentiometric stripping analysis and cyclic voltammetry were performed with an Electrochemical System, model Autolab PGSTAT 20 (Eco Chemie, Utrecht, The Netherlands) connected to a Pentium 200MHz computer. A peristaltic pump (MS REGLO, Ismatec, Zurich, Switzerland) was used to feed the system. All solutions were prepared with deionized (18 MΩ) water obtained from a Milli-Q system (Millipore). Potassium ferricyanide, potassium chloride, and sodium nitrate solutions were prepared with analytical grade reagents (Merck), without further purification. The mercury and copper stock solutions (1000 mg L-1 atomic absorption standard solution, Aldrich) were diluted as required, just before their use. Hydrochloric acid solutions, used as supporting electrolyte, were prepared by dilution of Suprapur grade concentrated acid (Merck). Recordable CDs from Nipponic and Basf brand were used in this work. Experiments using new and discarded disks produced similar results. The comparative studies during the characterization of the gold electrodes (CDtrodes) were made using a Metrohm (model 6.1204.140) gold electrode. Composition of a Recordable Compact Disk. Recordable compact disks are generally constructed from four or five layers of material, as depicted in Figure 1. Each disk contains a polycarbonate base (a), which provides the mechanical support to the unit. A very thin layer (b) of photodegradable polymer, which is sensitized during the recording process is placed on that base. This layer can be constituted with phthalocyanine, azo
(20) http://www.fadden.com/cdrfaq/faq02.html.
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groups, or metal-stabilized cyanine.21 The next layer (c) is a nanometric metal layer, which contains very thin lines to be tracked during the recording process. The thickness of this mirrorlike film is situated between 50 and 100 nm (a median value of 80 nm was found by Rutherford backscattering measurements), and the total area is ∼100 cm2. This metallic layer is again covered by one or two polymeric films (d): the first one is a lacquer that protects the reflective film, and the second one is sometimes referred to as the data shield by the manufacturers. Recordable CDs with photodegradable phthalocyanine have a gold or greenish gold color when the reflective layer is gold or a greenish-yellowish silver when the reflective layer is silver. The photodegradable polymer containing cyanine produces an emerald green color when the reflective layer is gold and cobalt blue when silver is used. Preliminary Preparation of CDtrodes. The electrodes used throughout this work were constructed with small parts of recordable disks. Each compact disk can supply many “electrodes”. To cut the recordable CD in many parts, a large scissors or a paper guillotine can be employed. The disks were usually cut in slices “like a pizza”, since this design provides more electrodes from each disk. To construct the CDtrodes, it was necessary to remove the protective film from its surface. The exposition of the gold layer was achieved after chemical attack to these films. Many reagents were tested, and the most effective result (for all CDs tested, purchased from different suppliers) was obtained with concentrated nitric acid. Usually, the chemical attack to the protective films requires just a few minutes and after that, the remaining material can be easily removed with a water jet only. RESULTS AND DISCUSSION Studies with Planar Electrodes . To construct conventionally sized CDtrodes (for batch or flow injection analysis), the surface of the gold layer was exposed by chemical attack to the protective film, as described in the previous section. The next step consists of covering the central region of the CD slice with insulating material and fixing a wire (with conductive material) to the other extremity to provide the electrical contact. Many materials were tested as insulators, and a variety of geometries can be adopted. Epoxy resin and enamel based on nitrocellulose were found to be very favorable; these materials give uniform recovery and have good stability, easy application, and low cost. A disadvantage would be the incompatibility with many organic solvents. Alternatively, the connection between gold electrode and wire can be done in a easy way, by just positioning the wire on the CD slice and firmly wrapping it with Teflon (plumbing) tape. To characterize the electrochemical behavior of the new CDtrode, studies involving cyclic voltammetry were performed in 0.2 mol L-1 H2SO4. Figure 2 compares the cyclic voltammograms of a commercial electrode (A) and a CDtrode (B). The cyclic voltammograms obtained with these two electrodes presented practically the same shape. The two main peaks observed, one anodic (situated near to 1.25 V) and the other cathodic (at ∼0.90 V), attributed to oxide formation and its reduction, respectively, are in very good agreement with the ones presented in the literature.1 (21) http://www.kopyrite.com/cd/theory/read_cd.html.
Figure 4. Voltammograms of increasing concentrations (from 0 to 9 × 10-3 mol L-1) of ferrocyanide using a band nanoelectrode (dimensions, 8 mm × 80 nm). Scan rate, 10 mV/s; electrolyte, KCl 0.1 mol L-1
Figure 2. Comparative cyclic voltammograms with (A) commercial gold electrode (area, 3.15 mm2) and (B) CDtrode (area, 2.83 mm2) in 0.2 mol L-1 H2SO4; scan rate, 20 mV s-1.
Figure 3. Stripping potentiograms for increasing concentrations of copper (a) from 5 to 60 µg L-1 and mercury (b) from 10 to 120 µg L-1, using a planar working electrode (A ) 0.05 cm2). Collection time, 60 s; deposition potential, -0.1 V; stripping current, 1 µA; electrolyte, HCl 0.1 mol L-1. Calibration plots contains the correspondent analytical curves.
Figure 3 illustrates the potentiality of planar electrodes for application in stationary cells. A series of potentiometric stripping responses for copper (a) and mercury (b), obtained over the ranges 5-60 (a) and 10-120 µg L-1 (b) are presented. The working electrode utilized was a planar unit with 0.05-cm2 area. Each experiment was performed after a collection time of 60 s, under stirring conditions, at -0.100 V deposition potential, and using a stripping current of 1 µA. These experiments were performed in 0.10 mol L-1 HCl, and after each run the electrode was maintained for 15 s at +0.700 V to ensure the complete stripping of the deposited metals. The respective calibration plots presented excellent linearity: r ) 0.9994 and sd ) 0.013 for copper analysis and r ) 0.9991 and sd ) 0.016 for the series of measurements of mercury. Band Nanoelectrodes. The first motivation to explore the CDtrodes was the possibility of the construction of nanoband electrodes. During this work, many assays were made to learn how to successfully construct these nanoelectrodes. In all cases, only a relatively small percentage of these extremely thin electrodes were well behaved. The most effective way was the construction of sandwiched electrodes. To build these electrodes, the gold layer can be fixed
between the polycarbonate support and a glass slide or between two glass slides with epoxy resin. Before fixing the parts, all surfaces must be cleaned with concentrated nitric acid and well rinsed with pure water. In this study, the first way was preferred and no influence of the photodegradable material (situated between the gold layer and the polycarbonate support) was observed. In cases where the photodegradable material can catalyze electrochemical reactions, this material must be removed and a second slide must be fixed on the opposite side of the gold layer. A series of voltammograms obtained with a sandwiched electrode is presented in Figure 4 for increasing concentrations of ferrocyanide. For the band nanoelectrode used in these experiments (80 nm width × 8 mm length), a current density of 4 A mol-1 cm-2 was calculated from the inset in Figure 4. This current density is larger than the ones obtained with disk microelectrodes in the presence of the same analyte.22 This result is in agreement with the equations well discussed by Oldham,23 who demonstrated that, in a inlaid microelectrode, the current depends not only upon the area of the electrode but also on its shape () perimeter). When the relation perimeter/area is larger, the nondiffusional current is favored. In present case, the perimeter of the electrode used is 178 times larger than the perimeter of a disk electrode with similar area. Flow Analysis with CDtrodes. Flow injection analysis is a very suitable technique for many applications, such as for routine use or for the development of new analytical methodologies. A functional and simple thin-layer flow cell, utilizing CDs as the gold electrode source is in use in our laboratory and was described previously.5 This flow cell was used with success in quantification of mercury in seawater and drinking water.6 The detection limit obtained with synthetic samples was 250 ng L-1. In the present work, different ways of constructing a flow cell with a ring nanoelectrode were explored. Our attempts to make perfect holes in the middle of CDs slices were not successful, but these efforts resulted in an alternative design, based in a flow cell containing a ring electrode, such as the one presented in Figure 5A. There, a CD slice (c) is fixed between two polyethylene spacers (b, d) and two acrylic pieces (a, e). A hole with a 3.2-mm diameter was drilled perpendicularly across the middle of the (22) Mori, V.; Bertotti, M. Talanta 1998, 47, 651-658. (23) Oldham, K. B. J. Electroanal. Chem.,1981, 122, 1-17.
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detector exhibits well-defined concentration dependence and highly reproducible peaks. The proportionality of the analyte concentration, the very effective washout, and the low noise are favorable characteristics of this new cell. Experiments involving injections of high and low concentrations of the analyte (not shown) confirm the absence of carryover effect in this system. The calibration plot for the series of measurements (relating peak area vs concentration) gives a highly linear response (r ) 0.9999, sd ) 0.010). The reproducibility calculated for each group of injections of Figure 5C varied from 0.6 to 2.4%. For a series of 30 repetitive injections of 50 µmol L-1 ferrocyanide, (not shown) the standard deviation was just 0.9%. These results attest to the excellent performance of this flow cell.
Figure 5. Representation of the flow cell with the gold ring electrode. (A) parts of the flow cell: (a, e) acrylic pieces; (b, d) spacers; (c) CD “slice”; (f) reference electrode; (g) auxiliary electrode; (h) resulting ring electrode. (B) Side view of the central region of the cell. (C) Repetitive injections of 100 µL of potassium ferrocyanide solutions containing 12.5-62.5 µmol L-1 (a-e) in NaNO3 0.1 M. Measurements made with a gold ring electrode with 0.07 cm2, using a flow rate of 2.4 mL min-1. Working potential, +0.36 V (vs Ag/AgCl).
acrylic pieces and the spacers. The CD slice was then positioned in the cell and a hole was made with a 1.0-mm-diameter drill in the center of the orifice. The assembling of the five parts (see Figure 5A) produces a cell with a gold ring electrode with a 0.07cm2 active area. The solutions were carried through Teflon tubes (1.5-mm diameter). A miniaturized Ag/AgCl reference electrode (developed in our laboratory24) and an auxiliary electrode (a stainless steel tube) were positioned in the outflow side of the cell. Figure B presents details of the central region of this same cell when assembled. Figure 5C contains the amperometric response for potassium ferrocyanide injections over the range 12.5-62.5 µmol L-1, with 0.1 mol L-1 NaNO3 electrolyte solution. The response of the
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CONCLUSIONS The CDtrodes are characterized by a remarkable versatility, great availability, good reproducibility, and very low price. The versatility of these new electrodes developed in our group is noteworthy. The simplicity of construction of planar electrodes favors the use of these sensors as disposable electrodes. The extremely thin gold layer permits the construction of band nanoelectrodes with areas as small as 10-6 cm2 (or smaller, because the thickness of the gold layer is only 80 nm). Going to the opposite extreme, each disk can supply electrodes with 100 cm2. This means a remarkable flexibility of electrodes area, with a factor larger than 108 times between the smallest and the largest electrode. The characteristics of the CDtrodes also favor the construction of flow cells. Many other applications can be envisioned for this electrodes: The use of disposable units for studies involving surface modification with thiols are in course in our laboratory. The very large area of the CDtrodes will be soon tested as a collector for mercury vapor from the atmosphere. Flow cells for spectroelectrochemistry and for coulometric generation of reagents are also underway and will be presented in the future. ACKNOWLEDGMENT The authors acknowledge the scholarship and grants received from Conselho Nacional de Pesquisa (CNPq) and Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP). We thank Milton S. Rocha for providing many discarded recordable CDs. Received for review April 18, 2000. Accepted August 1, 2000. AC000437P (24) Pedrotti, J. J.; Angnes, L.; Gutz, I. G. R. Electroanalysis 1996, 8, 673-675.
