Anal. Chem. 2008, 80, 7392–7400
Dielectrophoretic and Electrothermal Effects at Alternating Current Heated Disk Microelectrodes Aliaksei Boika and Andrzej S. Baranski* Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, Canada, S7N 5C9 When a disk microelectrode is polarized with an alternating potential of very high frequency (0.1-2 GHz) and a high amplitude (up to 2.8 Vrms), the electrode is heated up, and at the same time, a very intense electric field is created around the electrode (>106 V/m for electrodes 1 µm in radius). This strong electric field gives rise to positive or negative dielectrophoretic effects. Positive dielectrophoretic effects can be used to assemble nanowires from nanoparticles at the electrode edge. On the other hand, a negative dielectrophoretic effect is probably responsible for “jet boiling” observed at overheated microelectrodes. In addition, a combination of a high temperature gradient and a high potential gradient generates an intense electrothermal flow of solution which very strongly enhances the mass transport and is responsible for intense convection in such systems. The electrothermal flow and dielectrophoretic forces can be generated directly on a microelectrode employed in electrochemical detection because the high frequency ac polarization of the electrode does not interfere with the acquisition of analytical signals. Heated (or hot) electrodes have a surface temperature significantly higher than the bulk solution temperature. Four basic ways of heating the electrode have been reported: by using a laser beam,1 by passing an ac current through a microwire serving as a cylindrical (or semicylindrical) electrode,2 by employing a focused microwave radiation,3 or by modulating the electrode potential with high frequency and high amplitude alternating voltage.4 The laser beam heating method is the oldest one; it was initially applied in the form of pulses to modulate the temperature of a standard sized rotating disk electrode.5 Temperature modulation and impedance measurements have been combined in a thermoelectrochemical impedance method,6,7 and this method was found useful in physicochemical studies of various interfacial processes.7,8 The temperature of the electrode surface can also be raised by electrical heating. The simplest approach was proposed by Gru¨ndler.2 In his original design, two ends of a cylindrical * Corresponding author. Phone (306) 9664701, fax (306) 9664730, e-mail
[email protected]. (1) Miller, B. J. Electrochem. Soc. 1983, 130, 1639–1640. (2) Gruendler, P.; Zerihun, T.; Kirbs, A.; Grabow, H. Anal. Chim. Acta 1995, 305, 232–240. (3) Compton, R. G.; Coles, B. A.; Marken, F. Chem. Commun. 1998, 2595– 2596. (4) Baranski, A. S. Anal. Chem. 2002, 74, 1294–1301. (5) Valdes, J. L.; Miller, B. J. Phys. Chem. 1989, 93, 7275–7280.
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microwire electrode (typically 25 µm in diameter) are connected to an ac power source (∼100 kHz). The contact between such an electrode and the potentiostat has to be made in the center of the heated microwire (symmetrically between two power line contacts) to avoid signal distortion associated with the ohmic polarization of the microwire. In Gru¨ndler’s approach, permanent heating2 and pulsed heating9 are used. The main application of such heated electrodes is in the area of electrochemical detection, for example, for the stripping analysis of Cu(II), Hg(II), and As(III), the label-free detection of DNA hybridization, and the nonisothermal operation of amperometric glucose biosensors. Various studies performed by this research group have been presented in a recent review.10 Electrodes can also be heated with microwave3 and radio frequency (rf)11 radiation. In the case of microwaves, an electrode acts as an “antenna” and heating of the solution occurs due to a much stronger electric field near the electrode than in the bulk of solution. Radio frequency radiation, in turn, causes the appearance of eddy currents which heat the electrode itself and therefore the solution near it. Microwave heating in ionic liquids can cause a tremendous increase in the temperature of the region near the electrode, values of 600 K and even higher have been reported.12 These methods of heating have been applied to the study of electrode processes at elevated temperatures; the determination of diffusion coefficients of redox species, activation energies of diffusion, and standard rate constants have been reported. A summary of research progress in this field can be found in recent reviews.13,14 An alternative approach to the heating of microelectrodes was developed in our laboratory.4 In this case a high frequency (100 MHz to 2 GHz) and high amplitude (up to a few Vrms) sinusoidal waveform is superimposed on a potential waveform normally used in electrochemical measurements. Consequently, a relatively large ac current flows through the electrode (and the solution resistance) causing Joule heating in the electrolyte surrounding the (6) Olivier, A.; Merienne, E.; Chopart, J. P.; Aaboubi, O. Electrochim. Acta 1992, 37, 1945–1950. (7) Rotenberg, Z. A. Electrochim. Acta 1996, 42, 793–799. (8) Climent, V.; Garcia-Araez, N.; Compton, R. G.; Feliu, J. M. J. Phys. Chem. B 2006, 110, 21092–21100. (9) Gruendler, P.; Kirbs, A.; Zerihun, T. Analyst 1996, 121, 1805–1810. (10) Gruendler, P.; Flechsig, G.-U. Microchim. Acta 2006, 154, 175–189. (11) Qiu, F.; Compton, R. G.; Coles, B. A.; Marken, F. J. Electroanal. Chem. 2000, 492, 150–155. (12) Sur, U. K.; Marken, F.; Coles, B. A.; Compton, R. G.; Dupont, J. Chem. Commun. 2004, 2816–2817. (13) Wildgoose, G. G.; Giovanelli, D.; Lawrence, N. S.; Compton, R. G. Electroanalysis 2004, 16, 421–433. (14) Marken, F.; Sur, U. K.; Coles, B. A.; Compton, R. G. Electrochim. Acta 2006, 51, 2195–2203. 10.1021/ac801094s CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
electrode. In the case of disk electrodes, a significant solution resistance develops only in close proximity of the electrode, roughly in a distance comparable with the radius of the electrode; therefore, the size of the hot zone is also comparable with the radius of the electrode. The ac generator used in this case can be easily miniaturized and the heating method can be easily used in various electrochemical measurements using microelectrodes in various geometrical arrangements. In general, our heating method allows for very easy control of the electrode temperature and for very rapid change of temperature when required. The goal of this paper is to study convectional mass transport at ac heated microelectrodes. Convectional mass transport on hot microelectrodes was reported by the Gru¨ndler15 and Compton16,17 research groups. The convection was assumed to be driven by gravity (buoyancy)15,16 or by jet boiling of the heated solution.17 We have observed a very intense convection similar to the one recently reported by Compton and co-workers,17 and we realized that this convection cannot possibly be driven by buoyancy but rather by an electrothermal effect arising from a combination of a high temperature gradient with a high potential gradient.18,19 We also observed very strong (negative and positive) dielectrophoretic effects18 at ac heated microelectrodes. These phenomena have been studied for the last 10 years in microfluidic systems, but they have not been reported for a single working electrode used in electrochemical detection systems. EXPERIMENTAL SECTION Reagents. All solutions were prepared using Millipore water and ACS grade chemicals. Methyl viologen (Sigma-Aldrich) as well as all other chemicals were used without any further purification. Gold powder used in video experiments was purchased from Alfa Aesar; the average particle size was 0.5-0.8 µm. Electrochemical measurements were done without the removal of dissolved oxygen. Electrochemical Cell. In most experiments, a standard threeelectrode electrochemical cell was used. However, controlled temperature isothermal experiments were carried out with a cell equipped with a water jacket and a circulating water bath (Haake, Germany). The auxiliary and pseudoreference electrodes were made of a platinum wire ∼0.3 mm in diameter (Alfa Aesar). In some cases, a standard Ag|AgCl|KCl(sat.) reference electrode was employed. The disk working electrodes (with a diameter of 10 µm or larger) were prepared by sealing platinum or gold microwires (Goodfellow Metals Ltd.) into glass tubing (World Precision Instruments). A lead was made by connecting a copper wire with a microwire with the aid of a small piece of Pb-Sn solder. Then the electrode tip was cut, and the electrode was polished with 3- and 0.3-µm finishing films on a Micropipette Beveller (WPI, model 48000) to achieve a mirrorlike surface. In some experiments special microelectrodes, such as a thermocouple microelectrode and electrochemically sharpened elec(15) Zerihun, T.; Gruendler, P. J. Electroanal. Chem. 1996, 404, 243–248. (16) Marken, F.; Tsai, Y.-C.; Coles, B. A.; Matthews, S. L.; Compton, R. G. New J. Chem. 2000, 24, 653–658. (17) Ghanem, M. A.; Thompson, M.; Compton, R. G.; Coles, B. A.; Harvey, S.; Parker, K. H.; O’Hare, D.; Marken, F. J. Phys. Chem. B 2006, 110, 17589– 17594. (18) Ramos, A.; Morgan, H.; Green, N. G.; Castellanos, A. J. Phys. D: Appl. Phys. 1998, 31, 2338–2353. (19) Cao, J.; Cheng, P.; Hong, F. J. Microfluid. Nanofluid. 2008, 5, 13–21.
Figure 1. Side view of the tip of the thermocouple microelectrode.
trodes, were used. Preparation of these electrodes is described below. Thermocouple Microelectrode. Direct determination of the electrode temperature can be achieved by using a microthermocouple as the working electrode. The joint of the two microwires (25 µm in diameter) melted together serves as a disk microelectrode, so both electrochemical and temperature measurements can be taken at the same time. Thermocouple microelectrodes used in the experiments were fabricated in the following way: A piece of an ordinary microscope glass slide (4 mm × 15 mm) was used as a support of the whole assembly. A thermocouple (Pt/ 13% Rh-Pt, OMEGA part number P13R-001) was placed on the glass surface, and the joint of the two microwires was then carefully positioned very close to one of the edges. In order to isolate microwires from one another and to secure their position on the glass slide, a UV curable adhesive was used (Norland Products, NOA 81). To make a contact between each of the microwires and a copper wire, a piece of tin solder was carefully melted using a butane torch. After that the adhesive was used once again to isolate wires and joints and to provide enough rigidity to the electrode. As a final step, the side of the glasspolymer electrode body was polished to expose the disk of the thermocouple joint. This step was very crucial in the whole fabrication process as it is quite easy to polish off the joint of the two microwires and thus destroy the electrode. A side view of one of the thermocouple electrodes prepared in our laboratory is shown in Figure 1. Electrochemically Sharpened Microelectrodes. Pt electrodes with a diameter smaller than 10 µm were obtained by the electrochemical etching of Pt wire based on a procedure described in the literature.20 The 25 µm Pt wire was sharpened by periodical dipping (for 1-2 s) of one of its ends into a 14% CaCl2 solution. The ac signal applied between a microwire and a counter electrode placed in the solution was 7 Vrms, 60 Hz. After sealing such a sharp microwire in glass, it is very important to polish it well so that only the very tip of the wire gets exposed. To simplify the polishing (20) Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2004, 76, 6229–6238.
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Figure 2. An interface between a potentiostat, an ac generator, and an electrochemical cell (W. is the working, Aux. is the auxiliary, and Ref. is the reference electrode). C1 and C3 are 1 nF capapacitors; C2, C4, and C5 are 10 nF capacitors; R2 and R1 are 100 Ω resistors; R3 is a 100 kΩ resistor; and L1 is a 33 nH inductor. The area enclosed by a dotted line border indicates the ac amplitude monitoring circuit with a high speed Schottky diode (SD).
task, we used a method recently reported by Zhang et al.21 that employs electronic monitoring of the electrode resistance; with this approach, electrodes as small as 1 µm in diameter were prepared. The exact value of an electrode radius was calculated from the limiting current recorded with such an electrode. Electronic Circuit. The block diagram of the electronic setup used in this work is shown in Figure 2. The high-frequency sinusoidal waveform was produced by a Hewlett-Packard signal generator (model HP8648B). In typical experiments, frequencies ranging from 100 MHz to 2 GHz and voltage amplitudes ranging from 0 to 2.8 Vrms were used. The higher frequency signal was passing from the generator output through capacitor C1 (1 nF), resistor R1 (100 Ω), and capacitor C4 (10 nF) to the generator ground. At frequencies larger than 100 MHz capacitors, C1 and C2 present negligible impedance; hence, resistor R1 was polarized with an alternating voltage. Since the working electrode input of the potentiostat is at a virtual ground potential, point B shown in Figure 2 was also at a ground potential, and the potential of the working electrode oscillated about the ground potential following the polarization changes of resistor R1. The magnitude of these oscillations was measured at point A with a simple rectifying circuit, based on a high speed Schottky diode (Infineon BAT6202LS), connected to a 12-bit analog-to-digital converter (ADC). Alternating polarization of the working electrode caused a flow of significant currents (up to about 3 mArms) through the solution resistance of the working electrode and produced Joule heating; these currents sank to ground through the auxiliary and reference electrodes and capacitor C5. Low-pass LC filters were used on all three lines connecting the cell with the potentiostat (details of the filters are shown in Figure 2) to prevent high-frequency currents from entering the potentiostat. The low-pass filter on the working electrode line is absolutely essential (insufficient filtering can cause, for example, frequency dependent offset currents). Resistor R2 (100 Ω) is needed to reduce the noise level for currents recorded with the potentiostat. It should be noted that the dc currents passing between the Aux. and W. terminals of the potentiostat were always very small (
