Anal. Chem. 1999, 71, 4642-4648
Hydrodynamic Modulation Voltammetry with an Oscillating Microjet Electrode Julie V. Macpherson* and Patrick R. Unwin
Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
A new method of hydrodynamic modulation voltammetry (HMV) is introduced, based on the microjet electrode (MJE) with an oscillating nozzle position. In MJE-HMV a jet of solution is fired at high velocities from a nozzle (with a typical diameter in the range 25-50 µm) onto the surface of a disk ultramicroelectrode (UME). The mass transport rate to the electrode is modulated by oscillating the lateral position of the jet between two different coordinates: one where the jet impinges directly on the electrode surface and the other where the flowing stream (largely) misses the electrode. The resulting modulated (transport-limited) current in phase with the moving jet is quantitative and discriminates effectively against background processes. Studies of iridium (III) hexachloride (IrCl63-) oxidation at a Pt MJE serve to demonstrate the general capabilities of the technique. For this system, detection limits are estimated to be ∼5 × 10-9 mol dm-3. In its present form, modulation frequencies of as much as 20 Hz can be successfully employed without serious attenuation of the current signal, and there is scope for further improvement through the use of smaller nozzles and electrodes and piezoelectric positioners with improved frequency responses. It has long been recognized that hydrodynamic modulation voltammetry (HMV) represents one way of extending electroanalytical detection limits.1,2 In HMV, the mass transfer rate at an electrode surface is periodically oscillated by either varying the solution flow rate over a static electrode or modulating the motion of an electrode with respect to the solution. Subsequent extraction of the alternating current component of the signal enables masstransport-controlled processes to be separated from those which are not, including, for example, capacitative background processes, electrode surface reactions, and solvent decomposition. Much of the past work in this area has been concerned with the rotating disk electrode (RDE). In the simplest approach, the rotation rate of a RDE was switched between two well-separated speeds3,4 (for stopped rotation voltammetry the lower speed was zero4), and the steady-state current difference, largely free from background contributions, was measured in dc mode only. Using (1) Wang, J. Talanta 1981, 28, 369. (2) Williams, D. E.; Macpherson, J. V. in Comprehensive Chemical Kinetics; Compton, R. G., and Hancock, G., Eds.; Elsevier: Amsterdam, 1999; Vol. 37, pp 369-438. (3) Blaedel, W. J.; Engstrom, R. C. Anal. Chem. 1978, 50, 476. (4) Wang, J. Anal. Chem. 1981, 53, 1528.
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this procedure, detection limits of 1 × 10-8 mol dm-3 were reported for Fe(CN)64- in a phosphate buffer.3 Bruckenstein and Miller employed phase-sensitive detection to measure the in-phase current component of the oscillating signal from a RDE, which was sinusoidally modulated about a fixed central rotation frequency.5,6,7 For these studies, the modulation amplitude was only some 1-10% of the central rotation frequency, dictating that the measured in-phase current was only a small component of the transport-controlled current signal, predicted in the absence of modulation. Detection levels as low as 5 × 10-8 mol dm-3, for nitrobenzene in perchloric acid, were reported with this technique.5 Flowing solutions offer many desirable features for electroanalysis, such as low sample consumption, multisample analysis, and on-line detection.8 Hydrodynamic modulation has thus found some use as a method to improve concentration detection limits in flow systems. In these studies, the solution flow rate, through a tubular or channel electrode, was pulsed9,10,11 between a maximum and minimum value and the steady-state current difference measured in dc mode only. Experimental detection limits of 1 × 10-7 mol dm-3 were typically achieved. Unfortunately, as a result of the fairly low mass transport rates, inherent in these early systems, it was necessary to employ lengthy cycling times, often tens of seconds, to attain the maximum steady-state currentdifference signal. Moreover, experimental constraints prevented the rapid change of volume flow rate, precluding the use of such devices as detection systems for flow injection analysis or high performance liquid chromatography. In later work, measurement times were enhanced by an order of magnitude, by adopting a thin-layer channel flow geometry,12,13 which served to decrease the distance, adjacent to the electrode, over which the concentration boundary layer relaxed. Using a hydrodynamically modulated radial flow microring electrode (RFMRE),14,15 coupled with phase sensitive detection of the oscillating current signal, we have recently achieved further improvements in detection times in flowing systems.16 This was (5) Miller, B.; Bruckenstein, S. Anal. Chem. 1974, 46, 2026. (6) Miller, B.; Bruckenstein, S. J. Electrochem. Soc. 1974, 121, 1558. (7) Miller, B.; Bellavance, M. I.; Bruckenstein, S. Anal. Chem. 1972, 44, 1983. (8) (a) Heineman, W. R.; Kissinger, P. T. Anal. Chem. 1980, 52, 138R. (b) Ruzicka, J.; Hansen, H. E. Flow Injection Analysis; Wiley: New York, 1981. (c) Stulik, K.; Pacakova, V. Electroanalytical Measurements in Flowing Liquids; Ellis Horwood, Chichester, England, 1987. (9) Blaedel, W. J.; Boyer, S. L. Anal. Chem. 1971, 43, 1538. (10) Blaedel, W. J.; Wang, J. Anal. Chem. 1979, 51, 799. (11) Blaedel, W. J.; Iverson, D. G. Anal. Chem. 1977, 49, 1563. (12) Blaedel, W. J.; Yim, Z. Anal. Chem. 1980, 52, 564. (13) Blaedel, W. J.; Wang, J. Anal. Chem. 1981, 53, 78. 10.1021/ac990515c CCC: $18.00
© 1999 American Chemical Society Published on Web 09/18/1999
possible as a result of the extremely high mass transfer rates achievable with the device, as much as 2 cm s-1, presently the highest steady-state mass transfer rate of any hydrodynamic voltammetric technique. The RFMRE arrangement consisted of a very thin ring ultramicroelectrode (UME) deployed in a thinlayer radial flow cell. Modulation of the mass transfer rate was achieved through the periodic oscillation of the cell height. Although the in-phase current signals recorded with this technique were far in excess of the equivalent dc currents measured at a ring UME in quiescent solution, they were still only some 2-20% of the dc current measured, and predicted, with the RFMRE at constant cell height under convective-diffusive conditions. The microjet electrode (MJE) is a further example of a recently introduced hydrodynamic UME, characterized by high mass transport rates.17,18,19,20,21 In the MJE, a jet of solution is fired at high velocities from a nozzle (with a typical diameter in the range 25-100 µm) onto the surface of a disk UME. With the jet over the electrode, mass transfer is high as a result of convective diffusion. However, as the jet moves awaysin a direction parallel to the surface of the disk UMEsthe rate eventually falls until a value for diffusive mass transfer in the absence of forced convection is reached.18 In this paper, we demonstrate that by simply modulating the lateral position of the jet between a position of maximum and low mass transfer rate and employing phasesensitive detection of the resulting oscillating current signal, it is possible to achieve the lowest concentration detection limits of any hydrodynamically modulated technique involving a flow system. An important feature of this approach, which distinguishes it from other HMV techniques employing phase-sensitive detection, is that the mass transport rate is modulated over a wide range, from a maximum level to a value which is a small component of the maximum. This greatly enhances the magnitude of the modulated (phase-sensitive) current. EXPERIMENTAL SECTION Chemicals. All solutions were prepared from Milli-Q (Millipore Corp.) reagent water. Stock solutions of potassium iridium (III) hexachloride (Sigma-Aldrich) at a concentration of 5 × 10-4 mol dm-3 were prepared and diluted on the day, as appropriate, and used with 0.2 mol dm-3 potassium nitrate (Aldrich, AR grade), which served as a supporting electrolyte. For mass transfer imaging experiments, potassium iridium (III) hexachloride was used at a concentration of 1 × 10-2 mol dm-3 with 0.5 mol dm-3 of potassium nitrate. Apparatus. The main features of the MJE-HMV arrangement were as described previously for the MJE,17,18 although there were some modifications. As shown in Figure 1, the MJE-HMV setup comprised a nozzle, with a diameter e50 µm, connected to micropositioners, which enabled the nozzle to be moved in all three axial directions, and most importantly, oscillated in a plane parallel to the surface of a 25-µm diameter disk UME. The nozzle of the MJE was constructed by drawing a borosilicate glass capillary (Clark Electromedical, Reading, U.K.; 2.0(14) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 1998, 70, 2914. (15) Macpherson, J. V.; Jones, C. E.; Unwin, P. R. J. Phys. Chem. B 1998, 102, 9891. (16) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 1999, 71, 2939. (17) Macpherson, J. V.; Marcar, S.; Unwin, P. R. Anal. Chem. 1994, 66, 2175. (18) Macpherson, J. V.; Beeston, M. A.; Unwin, P. R. J. Chem. Soc., Faraday Trans. 1995, 91, 899.
Figure 1. (a) Video microscopy image and (b) schematic representation of the hydrodynamically modulated MJE arrangement. The capillary nozzle, dn ) 42 µm, is located at a height, H, of 100 µm above the surface of a 25-µm diameter electrode, RG ) 1.6. With the jet turned on, solution is forced onto the surface of the electrode. Oscillation of the lateral position of the jet with respect to the underlying UME, using a piezoelectric positioner, modulates the mass transport rate of electroactive species to the electrode. Typical modulation frequencies and nozzle modulation amplitudes of 1-40 Hz and 30 µm, respectively, are employed.
mm o.d., 1.2-mm i.d.) to a fine point using a Narishighe (Tokyo, Japan) PB7 vertical micropipet puller. The tip of the nozzle was cut with a knife, typically to reveal a capillary with an inner diameter in the range of 25-50 µm. The nozzle was then polished flat using a succession of finer grade diamond impregnated polishing pads (9 µm, 6 µm, and 1 µm; Buehler, Coventry, U.K.), which were fixed to a home-built polishing wheel, fabricated from a PC hard disk.14 The general procedure for constructing UMEs, by sealing a fine metal wire in a glass capillary, was as described elsewhere.22 However, for these experiments, it was essential that the majority of the glass insulator surrounding the electrode was removed, by careful polishing, until the ratio of the radius of the probe (electrode plus insulating glass sheath), rp, to that of the electrode itself, a, was as close to one as possible. The value of RG ) rp/a was measured directly using an optical microscope (Olympus BH2). The MJE cell consisted of a fully detachable Teflon base, a cylindrical glass body, and a Teflon lid, with a total volume of 25 cm3. The glass body contained an outlet pipe to prevent solution overflow in the cell and an optical window (15-mm diameter) so that video microscopy could be used to monitor the position of the capillary nozzle relative to the UME. The zoom microscope was equipped with a CCD camera attachment that offered maximum on-screen resolution of 2.2 µm per pixel. The camera was attached to a video recorder and video capture card (model Win-TV Celebrity, Hauppage, NY), which allowed images to be transferred to a PC. The cell base contained a centrally located hole through which the UME could be secured vertically and Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
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positioned such that it sat below the level of the solution outlet. A tap in the base of the cell facilitated the ready change of solution in the MJE cell, when necessary. Flow of electrolyte through the MJE nozzle was achieved with a Gilson (Villiers-Le-Bel, France) model 305 HPLC pump equipped with a 25 W Ti pump head. Instrumentation. Two types of positioning instrumentation were employed. For the MJE-HMV experiments, the position of the nozzle relative to the UME was controlled with micrometer resolution by mounting the nozzle on a Newport Corp. (Fountain Valley, CA) model 416 x, y, z stage. The z axis (normal to the UME surface) was controlled by a differential micrometer (model DM-13), and the x and y axes with AJS-1 fine-adjustment screws. High resolution remote positioning in the x direction was achieved by mounting a piezoelectric translator, of maximum travel 60 µm, on the stages. The piezoelectric positioner was oscillated sinusoidally, in open loop mode (P-267 controller, Physik Instrumente, Waldbronn, Germany), with typically amplitudes and frequencies of vibration of 30 µm and 1-40 Hz, respectively, using the waveform generator from a Stanford Research Systems (Sunnyvale, CA) lock-in amplifier (model SR810 DSP). The x, y, z stage was located on a vibrationally isolated Newport CSD series breadboard and the cell shielded using a home-built Faraday cage. The current in phase with the position of the oscillating MJE was detected using the lock-in amplifier. Both the raw and inphase current-potential characteristics were acquired with an Eco-Chemie (Utrecht, The Netherlands) Autolab Electrochemical Workstation, incorporating a preamplifier (model ECD) for lowcurrent measurements. All voltammetric measurements were made in a two-electrode arrangement with an Ag wire serving as a quasi-reference electrode (AgQRE) and the Pt disk UME serving as the working electrode. For mass transfer imaging experiments, a modified scanning electrochemical microscope (SECM)18,23 was used to scan the nozzle over the UME, in a series of unidirectional scans, at constant volume flow rate and nozzle-electrode separation. Images were obtained by measuring the transport-limited current as a function of the jet position. The nozzle was typically scanned at a velocity of 20 µm s-1. Images comprised ∼26 lines with data acquired at 101 equally spaced points per line, and the datum at each position was the mean of 100 current readings. In these experiments, the position of the nozzle was controlled with three TSE-75 (x, y, and z) stages from Burleigh Instruments (Fischer, NJ) linked to a 6200-3-3 controller, which itself was interfaced to a PC via a Burleigh 660 interface card. The stages were mounted on a vibrationally isolated bench and the cell shielded, as described above. The current was measured in a simple twoelectrode mode with a home-built current follower (gains of 10-510-9 AV-1). Data from the current follower were acquired using a National Instruments (Austin, TX) Lab-PC card. (19) Martin, R. D.; Unwin, P. R. J. Electroanal. Chem. 1995, 397, 325. (20) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 1997, 69, 5045. (21) Alden, J. A.; Hakoura, S.; Compton, R. G. Anal. Chem. 1999, 71, 806. (22) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 267. (23) For reviews see, for example: (a) Bard, A. J.; Fan, F.-R. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. O.; Zhou, F. Science (Washington, D.C.) 1991, 254, 68.; (b) Unwin, P. R.; Macpherson, J. V. Chem. Ind. (London) 1995, 21, 874-879. (c) Mirkin, M. V. Anal. Chem. 1996, 68, 177A.; (d) Barker, A. L.; Gonsalves, M.; Macpherson, J. V.; Slevin, C. J.; Unwin, P. R. Anal. Chim. Acta 1999, 385, 223.
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Procedure. For the MJE-HMV experiments, with the jet turned on, the nozzle was first aligned in both the x and y axes, such that the current for the transport-limited electrolysis of the mediator (5 × 10-4 mol dm-3 IrCl63-) was just at its maximum value. This task was greatly aided by the use of video microscopy, which was also employed to set the distance between the nozzle and the electrode surface to ∼100 µm. The nozzle was then moved ∼30 µm away in a plane parallel to the electrode surface (using the piezoelectric positioner). Modulation of the nozzle position ((30 µm, the maximum vibration amplitude allowed with the piezoelectric translator employed) about this central location resulted in the greatest change in the mass transfer rate to the UME in this jet configuration and hence the largest possible value for the measured in-phase current signal. For mass transfer imaging measurements, the procedure involved setting the position of maximum mass-transfer-limited current to the center of the scan image (coordinates 0 µm, x scan; 0 µm, y step). For low-concentration (
