Anal. Chem. 1994,66, 798-801 This Research Contribution is in Commemoration of the Life and Science of I. M. Kolthoff (1894- 1993).
Radio Frequency Detectors Based on Microelectrodes John J. O'Dea, Robert A. Osteryoung,' and Janet G. Osteryoung Department of Chemistty, North Carolina State Univers& Ralei'gh, North Carolina 27695-8204
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Microelectrodes commonly used in electrochemistry display a rectification effect that has escaped current notice. The application of microelectrodes as radio frequency detectors dates to 1899. Rectification at 60 Hz and 1 MHz is demonstrated. Commonly available commercial microelectrodes are inlaid disks by construction. Fibers of carbon, platinum, gold, or other metal typically 1-100 Nm in diameter are sealed in glass or plastic. The exposed cross section of the fiber is polished flat and acts as the active area of the electrode. Microelectrodes are widely used for electrochemical detection in amperometry and voltammetry. For amperometry, the potential of the electrode is held constant and current is measured. On the time scale of seconds nonplanar diffusion dominates mass transport of depolarizers to the electrode surface in quiescent solution. The measured current is directly proportional to the bulk concentration of analyte and is independent of time. This behavior is the most celebrated characteristic of microelectrodes. Microelectrodes are also used for high-speed voltammetry. If the electrode potential is swept back and forth rapidly enough, nonplanar diffusion can be ignored. This yields a normal cyclic voltammogram. Since the potential is changing continuously, however, charging of the electrical double layer can be a significant interference. Problems with charging currents can be ameliorated by using pulse waveforms instead of a simple ramp. Current measurements are made at the end of potential pulses after charging currents have decayed. Differenced pulse currents provide additional discrimination against background currents. Peak-shaped voltammograms are obtained using square wave voltammetry with microelectrodes over a wide range of time scales.' Microelectrodes are used for detection in flowing systems.2 Voltammograms are obtained in real time as analytes are swept past the detector. Microelectrodes can, under many circumstances, function as classical detectors of Hertzian waves. In modern terms one would say that microelectrodes are capable of demodulating radio frequency carrier waves. Reviews on microelectrode applications and electrochemical detection are totally The purpose of this correspondence is silent on this
d Flgurr 1. Radio clrcuk with electrdytlc detector: (a) antenna, (b) eiectroiytic detector, (c) earth ground, (d) local battery, (e) pdarlzlng potentiometer, (f) telephone receiver.
to bring this effect to the attention of electroanalysts and to suggest applications of microeiectrodes based on detection by high-frequency rectification. Historical Background. High-frequency electrolytic rectification at microelectrodes was first reported by Pupin in 1899.8 An electrolytic detector for radio waves was subsequently developed by Fessenden9 and independently by Schloemilchloin 1903. Radio applications of microelectrodes predate resonant frequency tuning (syntony), crystal detectors, vacuum tubes, transistors, and point contact diodes. Surprisingly enough, electrolytic detection can be used today to receive modern continuous-wave amplitude-modulated broadcasts. The detector consists of a cell containing electrolyte and one electrodeofvery small area, usually a fine wireof platinum, and another larger electrode of platinum or some other metal. A radio circuit using the device is shown schematically in Figure 1. We have used a circuit of this type to receive local radio stations broadcasting at frequencies near 1 MHz. The operation of this circuit as understood in 1909 is described as follows. The detector is polarized by means of a local battery (d) and potentiometer (e). When connected to the antenna (a), rapidly oscillating electric currents are made to traverse ~
(1) ODea, J.; Wojciehowski. M.; Osteryoung, J.; Aoki, K. Anal. Chem. 1985.57,
954. (2) Stulik, K. Analyst 1989, 114, 1519. (3) Montenegro, M. I.; Queiros, M. A.; Daschback, I. L., Eds. Microelectrodes: Theory and Applications; NATO ASI Ser. E 1991, 197. (4) Wightman, R. M.; Wipf, D. 0. In Electroanalyricul Chemistry; Bard, A. J., Ed.;Marcel Dekker: New York, 1989; Vol. 15, pp 267-353. (5) Fleischman, M.; Pons, S.; Rolison, D. R.;Schmidt, P. P. Ultramicrwlectrodes; Datatech Systems, Inc.: Morgantown, NC, 1987.
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(6) Weber, S. G. In Detectors for Uquid Chromatography; Yeung, E. S.,Ed.; John Wiley: New York, 1986; pp 229-291. (7) Kissinger, P. T. In tiquid Chromarography Detectors; Vickrey, T. M., Ed.; Marccl Dekker: New York, 1983; pp 125-164. (8) Pupin. M. I. Elecrr. World 1899, 34, 743. (9) Aitken, H. G. J. The Contlnuous Wnoe: Technology and American Radfo, 1900-1932; Princeton University Press: Princeton, NJ 1985; p 55, 191. (10) Pierce, G. W. Principles of Wireless Telegraphy; McGraw-Hill Book Co.: New York. 1910; p 201.
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F l g m Suspendedwire electrolyticdetector: (a) auxiliary c.-ctrode, (b) adjusting screw, (c) fine wire, and (d) electrolyte.
the cell of the detector (b). These rapidly oscillating currents momentarily disrupt the polarization of the cell. This causes an increased conduction through the local circuit of the battery, detector, and telephone receiver (f). The result is that sound is produced in the telephone receiver with a period determined by the train frequency of the incident electric waves. The advantage of the electrolytic detector over other radio wave detectors of its time was its capability of responding to radio waves over a continuous range of amplitudes. Hence, it was able to receive voice as well as the simple clicks of telegraphy. It was more reliable in operation than "poor contact" detectors by virtue of its liquid construction and did not require constant adjustment. The device is self-restoring. Strong bursts of static due to lightening, which destroyed other types of detectors, left the electrolytic detector unaffected after a few seconds. Because of these features, the microelectrode detector saw commercial service in radio from 1906 to 1913. With the advent of crystal detectors it was abandoned and remains nearly forgotten today. Engineered forms of the electrolytic detector range from the simple to the elaborate. The early wireless telegraphy literaturelOJ1-19 describes a variety of electrode materials and electrolytes. The electrode designs fall into two types, partially immersed thin wire and those which employ only the cross section of the wire, Le., inlaid disks. A typical wire detector is shown in Figure 2. It consists of a cell containing electrolyte, above which is suspended a partially immersed wire. Manual adjustment of the depth of immersion is provided by the fine screw. The electrolyte is 20% sulfuric or nitric acid. The (1 1) Sci. Am. Suppl. 1905.59 (1537),24626. (12) Kennelly, A. E. Wireless Telegraphy; Moffatt, Yard and Co.: New York, 1906; pp 137-142. (13) Fasenden, R. A. Proc. Am. Insr. Elecrr. Eng. 1908,27,1325. Also reprinted in The Developmenr of Wireless to 1920; Shiers, G., Ed.; Arno Press, New York. 1977. (14) Maver, W. Wireless Telegraphy and Telephony; Maver Publishing Co.: New York. 1910 D 232. 312. (15) Marchant, W. H.Wireless Telegraphy;Whittaker and Co.;New York, 1914; p 65. (16) Fleming, J. A. An Elemenrory Manual ofRadiorelegraphy andRadiorelephony; Longmans, Green, and Co.: London, 1916; p 216. (17) Fleming, J. A. The Principles of Electric Wave Telegraphy and Telephony; Longmans, Green, and Co.: London, 1916; p 509. (18) Bucher, E. E. Pracrical Wireless Telegraphy, 2nd 4.;Wireless Press Inc.: New York, 1917; p 178. (19) Phillips, V. J. Ear/y Radio Wave Detectors; Peter Peregrinus Ltd.: London, 1980; pp 65-84.
Figure 3. IniaM dlsk electrolytlc detector: (a) auxiliary electrode,(b) glass tube with sealed wire, and (c) electrolyte.
wire is platinum, 0.005-0.000 06 in. in diameter. The most sensitive detectors employ the exposed platinum core of Wollaston wire. The fine wire is usually polarized as the anode, although the device may function with the opposite polarization or without external polarization at all. Thesecond larger electrode in the cell may be platinum or lead. A second more robust design has the fine platinum wire sealed in a glass tube with only the end of the wire exposed. The tube is simply dipped into the electrolyte, the depth of immersion no longer being critical. The second electrode may be any convenient size or shape. The distance between the two electrodes also makes no appreciable difference. This simple arrangement is shown in Figure 3. Kennelly12observed that this formof electrolytic receiver was probably the simplest of all receivers in wireless telegraphy (radio). This is still true today. The operation of the electrolytic detector has been studied by several workers.2G24The evidence points to a polarization mechanism associated with the small size of the electrode, perhaps involving chemisorbed oxygen. IvesZ3found that platinizing the electrodes suppressesthe rectification. Pierce24 confirmed that on platinum the detection is a polarization phenomenon and not a thermal phenomenon, as suggested by Fessenden. Phillips25complained that as of 1980 the physical principles of electrolytic detection remain obscure. However, at 1 MHz on a 10-pm-diameter electrode, the diffusion is substantially planar so the phenomenon of radial diffusion near the steady state cannot be essential to the explanation. Phillips also wondered about reports of self-rectification involving capillary electrometers and other devices which resemble modern static mercury drop electrodes. More recently Geddes26reported rectification in physiologicalsalineat 100-10 000Hz withO.01-in.-diameterstainless (20) Schloemilch, W. Sci. Absrr. 1904, 7A, 78 (Abstr. 211). (21) Reich, M. Sei. Absrr. 1904, 7A, 544 (Abstr. 1756). (22) Rothmund, V.; Lessing, A. Sci. Absrr. 1904, 7A. 896 (Abstr. 2971). (23) Ives, J. E. Electr. World Eng. 1904, 44, 995. (24) Pierce, G. W. Phys. Rev. 1909, 28, 56. (25) Reference 19, pp 143-148. (26) Geddes, L.A,; Foster, K. S.;et al. IEEE Trans. Biomed. Eng. 1987, BME-34, 669.
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Flgure 4. Electrolyticdetection demonstration: (a)waveformgenerator, (b) microelectrode cell, and (c) amplified speaker or oscilloscope.
steel disks. Rectification was not observed until the sinusoidal current density exceeded certain frequency-dependent critical values. These experiments were performed with electrodes 100 times larger in area than those considered here. The phenomenon of radio frequency demodulation is easily demonstrated with modern commercially available microelectrodes.
EXPERIMENTAL SECTION Instrumentation. A 33 12A Hewlett Packard function generator was used to generate an amplitude-modulated radio frequency. Waveforms were captured with a 100-MHz Tektronix 2430 oscilloscope. Audio frequency signals were rendered into sound by a Radio Shack 32-2040 miniamplified speaker (200-12 000 Hz). Materials. Diluted (20%) Fisher reagent grade sulfuric acid was used as the electrolyte. Cell and Electrodes. The cell was a 5-mL beaker left open to the atmosphere. Microelectrodes used were either a Cypress Systems EE016 10-pm-diameter platinum disk or a EG&G PARC GC 10-pm-diameter glassy carbon disk. Microelectrodes were polished with 0.05-pm alumina before use. The counter electrode was a 13 X 25 mm bright platinum flag. Procedure. The cell is connected with the waveform generator and speaker as shown in Figure 4. Connection to the speaker is made through the auxiliary jack. The 47-kQ resistor allows electrode polarization by a dc bias applied from the function generator. The function generator is adjusted to produce a 1 MHz carrier 100% amplitude-modulated at 500 Hz with a peak-to-peak amplitude of 1.2 V. A bias value of -1.0 V is used to polarize the electrode. Care must be taken to ensure that the radio frequency waveform is not clipped at a positive or negative limit. Clipping will produce audio frequency artifacts. Immersion of the microelectrode into the electrolyte completes the circuit. Rectification across the microelectrode demodulates the radio frequency carrier and produces an audible tone. The exact strength of the audio signal depends on many factors, including the previous polarization history of the electrode. Experiments at lower frequencies, similar to those performed by Pierce24in 1909, allow direct visualization of the effect. Figure 5, curve A, shows a biased 60-Hz sinusoid applied across the cell. The voltage drop across the 47-kQ resistor is shown in Figure 5, curve B. The nonlinear transfer characteristic of the microelectrode is causing a partial rectification of the applied waveform. In effect, the cell is acting as a diode. DISCUSSION The experimental demonstration of rectification at microelectrodes suggests several areas for investigation. 000
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Figure 5. Waveforms: (a) 80-Hz excitation and (b) voltage measured across 47-kQ resistor.
(1) Is this effect simply faradaic rectification as studied extensively by Oldham,z7 Barker et a1.,28 D e l a h a ~ and ,~~ A g a r ~ a l ?Are ~ ~ there practical advantages in using microelectrodes for such studies? Does the characteristically large current density of microelectrodes facilitate the measurement of the dc component of the response to excitation by alternating current? Is a more holistic approach to the zeroth order harmonic response such as that used in impedance spectroscopy31 required? (2) At radio frequencies the skin depth of conductors can be comparable to the dimensions of the electrode and the diffusion field around it. Are electrodynamic effects such as the skin e f f e ~ t 3important? ~ (3) Are there analytical applications of the effect? If the rectification involves an oxide film, can it be used to monitor the concentration of biomolecules such as carbohydrate^^^ by reaction with the film? Would the action of such a detector be self-restoring, thereby obviating the use of electrodeconditioning protocols? Would the speed of such detection fall into the audio frequency range? Would other electrode materials (e.g., palladium, copper, silver, gold, carbon, etc.) provide chemical selectivity? (4) Is the effect observed with the mercury microelectrode? These have the advantage of a small and continuously adjustable surface area.34 Can the magnitude of the effect be predicted quantitatively from the area of the electrode? (5) What is the effect of adsorbed monolayers at the interface (electroactive or not)? These are all questions which can be answered with the help of modern electroanalytical techniques. Perhaps it is time for modern electrochemistry tocatch up with 19thcentury radio technology. The investigation of this curious phenomenon may lead to improved understanding of the complex structures and processes associated with interfaces and may also lead to specialized techniques for chemical analysis. (27) Oldham, K. B. Tram. Faraday SOC.1957,53, 80. (28) Barker, G. C.; Faircloth, R. L.; Gardner, A. W. Nature 1958, 181, 247. (29) Delahay, P. In Advances in Electrochemistry and Electrochemical Engineering Delahay, P., Ed.; Interscience Publishers: New York, 1961; Vol. 1, p 265. (30) Agarwal, H. P. Electroanalytical Chemistry; In Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, pp 161-271. (31) Macdonald, J. R., Ed. Impedance Spectroscopy; John Wiley & Sons: New York, 1987; pp 267-214. (32) Schwartz, S.E. Electromagneticsfor Engineers; Saunders College Publishing: Philadelphia, 1990; p 218. (33) Johnson, D. C.; Lacourse, W. R. Anal. Chem. 1990,62, 589A. (34) Colyer, C. L.; Luscombe, D.; Oldham, K. B. J. Electroanal. Chem. 1990,283, 379.
ACKNOWLEDGMENT The authors thank Dr. Louise Mahoney for technical assistance. This work was supported in part by the National Science Foundation under Grants CHE 92-08987 and CHE 92-96280. Scientific Parentage ofthe Authors. John O'Dea: Ph.D. under R. A. Osteryoung, Ph.D. under H. A. Laitinen, Ph.D.
under I. M. Kolthoff. Janet G. Osteryoung: Ph.D. under Fred C. Anson, Ph.D. under J. J. Lingane, Ph.D. under I. M. Kolthoff. Recelved for review September 10, 1993. Accepted December 179 1993-" Abstract published in Advance ACS Absrracrs. February 1, 1994.
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