In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY-Cortland Cortland, NY 13045
Laboratory-Made Electrochemical Sensors for Adsorptive Stripping Voltammetry Teresa Goscinska School of Biological and Chemical Sciences, Deakin University, Geelong, Victoria 3217, Australia
There is a need to develop sensitive electrochemical sensors for the selective and quantitative determination of organic and organometallic compounds at trace levels in small samples. Research into sensitive electrochemical sensors has been conducted by many laboratories around the world and has branched in several directions. One of these is the development of novel sensors by modifying (1–3) or miniaturizing existing electrodes to create microelectrodes (4, 5). A second direction is the production of disposable screen-printed sensor strips used widely in medicine (6 ). A third is the integration of chemically sensitive membranes with solid-state electronics to create ion-sensitive field-effect transistors known as ISFETs (7 ). One field that offers great potential for future applications is that of chemically modified electrodes. Because carbon paste electrodes (CPEs) provide a convenient matrix for modifiers, CPEs have become a flourishing field of research (2). Interest in the CPE has also been revived because of the usefulness of this type of electrode in adsorptive stripping voltammetry (AdSV). The theoretical and practical aspects of AdSV following non-Faradaic preconcentration with a medium exchange have been reported by Wang (8). It is expected that at low concentrations this analytical method will provide better performance than traditional polarography. As was shown in previous papers (9, 10), electrochemistry is a convenient method for testing and analyzing small amounts of product. The purpose of this paper is to (i) describe the construction of sensitive electrochemical sensors in the chemistry lab by students and (ii) illustrate the benefits of AdSV with a medium exchange preconcentration procedure in chemical analysis of small samples of organic or organometallic compounds. A facile procedure for the incorporation of clay (kaolin) into the carbon paste electrode is described and the advantage of the sensor for the particular purpose over the conventional carbon paste electrode is emphasized.
Experimental Procedure
Electrode Preparation The carbon paste electrode was prepared, as was reported previously (11), by packing the carbon paste into Teflon tubing (3-mm diameter) with a copper wire on the opposite side. Clay–carbon paste was prepared by thoroughly mixing 2 g of 1.5-µm graphite powder (Aldrich), 1 g of hydrated kaolin (Riedel-de Haen), and 1 mL of Nujol (Merck). For hydration, 2 g of kaolin was immersed in 15 mL of double-distilled water for two hours, after which the excess of water was decanted. The clay-modified carbon-paste electrode (a sensor) was prepared in exactly the same way as the CPE (11). The working surface of each electrode was smoothed on a piece of filter paper before the test. A fresh surface was revealed each time by cutting off a piece of the electrode with a scalpel. Apparatus Electrochemical tests were performed on a BAS 100B electrochemical analyzer with a printer. A three-electrode cell with a platinum wire as the counter electrode and the reference electrode (Ag/AgCl in a 0.5 M aqueous solution of sodium perchlorate) was used. The CPE or the sensor was used as the working electrode. For all experiments, a 0.5 M aqueous solution of NaClO4 was used as the electrolyte. All solutions were prepared using double-distilled water. Each test was performed under a nitrogen blanket.
Preconcentration and Analysis Procedure Ferrocene solutions were prepared by dissolving the required weight of ferrocene (AR Aldrich) in absolute ethanol (Aldrich). For the preconcentration the electrode was immersed for the desired time (1, 5, 15 or 50 min) in the small vial with 1 mL of the analyte solution. During this interval, the solution was stirred at a constant rate by a magnetic stirrer. After preconcentration, the immobilized electrode was Continued on page 1039
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Journal of Chemical Education • Vol. 75 No. 8 August 1998 • JChemEd.chem.wisc.edu
In the Laboratory
Figure 1. Cyclic voltammograms of 10᎑4 M ferrocene at (1) CPE, (2) sensor. Scan rate was 250 mV/s.
Figure 2. The oxidation peak (DPV) for 10᎑5 M ferrocene at (1) baseline, (2) CPE, (3) sensor. Scan rate was 50 mV/s.
rinsed thoroughly with double-distilled water to remove excess organic solvent and immersed in the electrochemical cell, and the potential was scanned in either cyclic (CV) or differential pulse (DPV) mode. For DPV, a 50-mV pulse amplitude, 50-ms pulse width, and 200-ms pulse period were used. Blank experiments were performed by the same procedure as above, except that the electrode was not immersed in the analyte solution.
ferrocene strongly and extended the detection level from 10᎑5 M down to 10᎑7 M (not shown). The electrochemical response increased minimally with the extending of the preconcentration time (see table). It appears that 15 min is the optimal preconcentration time for this case.
Results and Discussion An increase of the electrochemical signal from ferrocene at the modified electrode was seen by comparing the cyclic voltammograms at the two electrodes. Figure 1 presents cyclic voltammograms of 10᎑4 M ferrocene at the CPE and the sensor (the analyte was preconcentrated for 1 min). At the sensor the electrochemical response increased strikingly. In addition, this electrode became saturated and produced a broad oxidation peak and a broad reduction peak, which were shifted in the positive and negative directions, respectively, giving the impression that the process is irreversible. Saturation is a particularly severe problem. When the electrode becomes saturated, the analytical signal no longer bears any relation to the concentration of the species of interest. In some cases shorter preconcentration times can eliminate the saturation problem. If this does not help, a more dilute sample should be used. Figure 2 compares the electrochemical signals for 10᎑5 M ferrocene at the CPE and the sensor. The table illustrates an influence of the preconcentration time on the electrochemical signal for 10᎑5 M ferrocene at the sensor. Influence of Preconcentration Time on Electrochemical Signal for 10 –5 M Ferrocene at the Sensor Preconcentration time/min
1
5
15
50
Current/µA
0.8
1.0
1.2
1.25
The mechanism by which kaolin increases the electrochemical signal is unclear and was not tested. Clays represent an important class of solid inclusion compounds (12) that can function as catalyst supports and cation exchangers. They can also discriminate among analytes on the basis of size and charge. The most important fact here is that the addition of kaolin increased the electrochemical response from
Conclusions Carbon paste was shown to be a convenient matrix for the incorporation of modifiers in a suitable and responsive form. The clay-modified sensors show sensitivity enhanced 100-fold. These sensors are inexpensive and simple to make in the laboratory. A wide variety of analytical possibilities exists, based on the choice of modifier and control of experimental conditions. The experiment is simple and enhances student interest in modern electrochemistry. Using a medium exchange for absorbing organic compounds by the sensor and coupling it with stripping voltammetry is a promising approach in analyzing small samples in microscale chemistry. Acknowledgments This work was supported by Deakin University Fellowship Scheme. I am grateful to Enrico Mocellin for providing access to the electrochemical instrumentation, to Simon Lewis for the constructive comments on the first version of the paper, and to Richard Russell for his encouragement. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Bard, J. J. Chem. Educ. 1983, 60, 302–304. Gilmartin, M. A. T.; Hart, J. P. Analyst 1995, 120, 1029–1045. Arrigan, D. W. M. Analyst 1944, 119, 1953–1966. Fletcher, S. Chem. Aust. 1994, 61(2), 80–82. Foster, R. J. Chem. Soc. Rev. 1994, 289–297. Wang, J.; Macca, C. J. Chem. Educ. 1996, 73, 797–800. Wang, J. Analytical Electrochemistry; VCH: New York, 1993. Wang, J. In Electrochemical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1989; Vol. 16, Chapter 1. Mocellin, E.; Ravera, M.; Russell, R. A.; Hynson, T. J. Chem. Educ. 1996, 73, A99–A104. Mocellin, E.; Russell, R. A.; Ravera, M. J. Chem. Educ. 1998, 75, 773–775. Mocellin, E.; Goscinska, T. J. Chem. Educ. 1998, 75, . Lipkowski, J.; Ross, P. N. Electrochemistry of Novel Materials; VCH: New York, 1994.
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