Anal. Chem. 1995, 67,2586-2591
Characterization of Thick-Layer Graphite Disposable Voltammetric Electrodes Kh. 2. Braininat and A. M. Bond* School of Chemistty, La Trobe University, Bundoora, Victoria 3083, Australia
Graphite thick-film disposable electrodes which may be used for sensitive analytical determinations by the techniques of conventional and stripping voltammetry have been characterized by electron scanning microscopy, optical spectroscopy,and electrochemical methods. Studies on the reversible Cc- e- * Cc (Cc is cobaltocene) reduction process in acetonilrile confirmed the inlaid and recessed disk microelectrode random may-typebehavior expected at a rough surface on the basis of examination of the electrode surface by electron scanning microscopy. Anodic stripping voltammograms of copper, lead, and cadmium in 0.1 M HCl at an in situ mercury-plated disposable graphite electrode demonstrate the excellent analytical performance that may be achieved for trace analysis even at unusually fast scan rates in the volts per second range. The microelectrode array properties also mean that oxygen removal and stirring of the solution can be avoided when the electrodes are used for stripping voltammetry.
+
Numerous publications related to the use of carbon electrodes have appeared in the last few years.’J In analytical voltammetric studies, graphite, glassy carbon, carbon ceramic, and carbon paste electrodes have been employed in their natural state, after modification of the surface or mixing with a nonconducting reagent. To achieve acceptable analytical performance, the requirements imposed on carbon electrodes are demanding because all the following features are desirable: (a) electrochemical inertness over a wide range of potentials; (b) high hydrogen and oxygen evolution overvoltages; (c) low background currents; (d) high electrical conductivity;and (e) simple regeneration of the electrode surface. The above factors, when present in combination, would provide the voltammetrically ideal situation of high accuracy, sensitivity, and reproducibility of analytical determinations as well as low detection limits for a broad range of analytes. However, unfortunately, there are no carbon electrode materials and pretreatment procedures which meet all these criteria. In practice, surface regeneration is the most complicated and irreproduciblestage of voltammetric procedures based on carbon electrodes. At present, a favored way to address the difficulty of adequate electrode regeneration is to eliminate this stage of the analytical procedure via the use of the so-called “disposable”carbon electrodes which may be manufactured at very low cost (tens of cents per electrode) ‘Work undertaken while on leave from the Urals Institute of National Economy, 8th of March St., Building 62. 620219 Ekaterinburg, Russia. (1) Khanina, R. M.; Tataurov,V. P.; Brainina, Kh. 2. Zavod. Lab. 1988,54, 1. (2) McCreety, R. L. Electroanal. Chem. 1991,17, 221 and references cited therein.
2586 Analytical Chemistry, Vol. 67, No. 15, August 1, 1995
via large-scale production procedure^.^-^ These electrodes are therefore referred to as disposable in the economic sense. The present paper describes the results of an investigation of the characterization and use of a mass-produced form of a graphite thick-film disposable electrode (GDE) which has the analytically desirable properties a-d noted abovelo but which is termed disposable on the basis of the low manufacturing cost. Thus, the need for point e may be avoided by only requiring a single use electrode. However, it must be emphasised that these electrodes should not be considered to be disposable in every sense of the word. The physical disposal of any electrode, particularly after the electrode has been used in trace metal analysis, naturally still requires careful consideration and should be based on relevant occupational health and environmental issues. EXPERIMENTALSECTION Instrumentation. A Cypress Systems Model CS 1087A electrochemical system was used for experiments involving cyclic voltammetry and chronoamperometry. Other techniques such as anodic stripping voltammetry, which required a preset potential to be used between measurements, were implemented with a Bioanalytical Systems Model BASlOOA electrochemical analyzer. A conventional three-electrode cell was used for voltammetric investigations in acetonitrile with a GDE or a platinum inlaid microdisk working electrode (see below), an aqueous Ag/AgCl (saturated KC1) reference electrode, and a platinum wire auxiliary electrode. A three-compartment cell developed by IVA (8th of March Street, Building 62,620219 Ekaterinberg, Russia) was used for stripping voltammetry in aqueous solutions. The GDE was the working electrode, with a Ag/AgCl (saturated KC1) electrode as the referknce and graphite or glassy carbon rods as the auxiliary electrode. An ETEC Autoscan (20 kV accelerator voltage) scanning electron microscope was used to characterize the disposable graphite electrodes after gold plating in the standard manner with a Balzers sputter unit. Optical microscopy studies were performed using a Nikon Epiphot inverted metallurgical microscope fitted with a longworking-distance objective lens. The microscope was connected (3) Craston, D. H.; Jones, C. P.; Williams, D. E.; El Murr, N. Talanta 1991, 38, 17 and references cited therein. (4) (a) Wang, J.; Tian, B. Anal. Chem. 1992,64, 1706 (b) Anal. Chim.Acta 1993,274, 1; (c) Anal. Chem. 1993,65, 1529 (d) Electroanalysis 1993, 5. 809. (5) Sprules, S. D.; Hart, J. P.; Wring, S. A; Pittson, R Analyst 1994,119, 253. (6) Hart, J. P.; Hartley, I. C. Analyst 1994,119, 259. (7) Wring, S. A; Hart, J. P.; Bracey. L.; Birch, B. J. Anal. Chim. Acta 1990, 231, 203. (8) Wring, S. A; Hart, J. P. Analyst 1992,117, 1281. (9) Wang, J.; Lu, J.; Tian, B.; Yarnitzky, C. J. Electroanal Chem. 1993,361, 77. (10) Brainina. Kh.; Neyman, E. Electrochemical Stripping Methods in Chemical Analysis; John Wiley: New York, 1993; p 51.
0003-2700/95/0367-2586$9.00/0 0 1995 American Chemical Society
to a chargecoupled device which allowed observation of the electrode surface on a television screen. For these experiments, the electrochemical cell was fitted with a flat quartz section that enabled a clear image of the surface to be obtained. Working Elechodes. The fabrication method for the GDEs used in this work involves the low-cost mass production method of vacuum plating a rectangular aluminum contact layer (-12 mm x 4 mm) onto the top part of a rectangular ceramic support (-40 mm x 5 mm), followed by addition of a thick rectangular (-30 mm x 3 mm) layer of graphite-epoxy resin mixture. Masking technology is then applied to leave part of the aluminum surface at the top of the electrode available as an electrical contact and a small rectangular graphite-epoxy resin section at the other end which forms the electrode. In an electrochemical experiment, the graphite-epoxy resin mixture which is in contact with the solution represents the electrode surface. The disposable electrodes have an apparent geometricworking electrode area of 0.075 cm2 and are available from IVA Electron scanning micrographs of the surface and the edge of an electrode are shown in Figure 1. Graphite particles (average size,-5Opm) randomly distributed and randomly oriented in the epoxy resin can be seen in the micrograph,with the average distance between the particles being approximately equal to the particle size. However, additionally, as seen in Figure 1, the surface is relatively rough. On the basis of information obtained from electron microscopy, the electrode behavior would therefore be predicted to be related to that expected from a random array of both recessed and inlaid microdisk electrodes. Furthermore, the roughness is likely to inhibit overlap of diffusion layers that is othenvise expected to occur in long-timedomain experiments. The model of a rough surface consisting of randomly placed electroactive sites and nonelectroactive particles will be confirmed by other evidence obtained by voltammetric studies and optical microscopy. An analog of this kind of electrode would be the composite electrodes based on carbonized poly(acrylonitri1e) foams described by Wang?' A platinum inlaid microdisk electrode having a radius of 10 pm and fabricated according to procedures described in ref 12 was used to provide voltammograms and chronoamperogramsthat could be compared with those obtained at the GDEs and thereby confirm some aspects of their random array microelectrodetype properties. Chemicals and Reagents. Reduction of a 5 x M solution of CcPF6 (Aldrich) [Cc+is the cobaltocenium cation [Co( + C & l ~ ~ l +in] CH&N (Mallinkrodt, HPLC grade) containing 0.1 M EhNBF4 (Southwestern Analytical, electrochemical grade) as the electrolytewas used for voltammetric and chronoamperometric experiments aimed at characterizing the GDE. The temperature used for all electrochemical experiments was 25 "C. To remove oxygen, solutions were degassed with (CIG) high-purity nitrogen gas for 10 min before the experiments. The
-
cc++ e- cc
(1)
electrode process (Cc is cobaltocene) represents an example of an ideal reversible electrode process in organic solvents.'3 A 0.1 M HCI @DH, analytical reagent grade) solution (25 "C) containing the required concentrations of C d O , PbUI), and
Er., , .. Figure 1. Scanning electron micrographs of (a, lop) the top and (b, bonom) the edge of a GDE.
Cu(II) ions @DH, atomic absorption spectrometry standards) was used to evaluate the application of the analytically sensitive stripping voltammetric technique at a GDE. To this solution was added 1 x 10-4 M Hg(NO& so that simultaneous in situ preparation of a mercnry-zoated GDE could be undertaken along with deposition of Cd, Pb, and Cu metals as their amalgams. Removal of oxygen by degassing with nitrogen was not done in the stripping voltammetry experiments. For the mercury and copper electrodeposition studies coupled with optical microscopy, 10-4 M HgiJ9 or CuiJ9 was added to the 0.1 M HCI electrolyte.
(11) Wang. 1.: Brennsteiner. A Sylwester, A P. Arol. Chem. 1990, 62,1102
and references cited therein. (12) Koppenol, M;.Bond, A M.; Cooper. I. B. A n Ln6. 1994,26 (11). 25. (13) Stojmovic. R S.: Bond. A M. And. Chrm. 1993. 65.56 and references
cited therein.
RESULTSAND DISCUSSION
GDE Charactekation. A cyclic voltammogramobtained for reduction of Cc+ io acetonitrile (eq 1) at a GDE (very slow scan Analylical Chemisiv, Vol. 67, No. 15, August 1, 1995 2587
‘1
12.01
Q 3
16.0314.0v I
E 12.0-
4.
5
0
1 10.05 1 -700
-800
.
-900 -1000 Potential (mV)
’
-If00
I
’
-1200
g
6.0-
Figure 2. Cyclic voltammogram (scan rate, 2 mV s-l for reduction of 5 x M Cc+ in acetonitrile (0.1 M Et4NBF4) at a GDE.
rate of 2 mV s-l) is shown in F w r e 2. Apart from the current magnitude, the voltammogram is similar to that observed at a 10 pm radius platinum inlaid disk microelectrode (not shown) in that both curves are sigmoidal shaped. However, more hysteresis is evident in the GDE response than in the voltammograms obtained at the platinum inlaid microdisk electrode. The hysteresis observed at the GDE is consistent with a small contribution from linear diffusion, although radial diffusion appears to be dominant, since a near-steady-stateresponse is observed. A small contribution from convection also may be associated with the response of the GDE at the very slow scan rate of 2 mV s-l. The platinum inlaid microdisk electrode is theoretically expected to give a sigmoidal-shaped current-voltage curve for the reversible diffusion (radial)-controlled process at slow scan rates.14 If the GDE were a conventional macroelectrode in which the entire surface was electroactive, and convection was not important, then an asymmetric peak-shaped curve would be expected since mass transport would occur predominantly by linear rather than radial (steady-state)diffusion. Alternatively, if the GDE had a completely flat surface and the graphite particles provided the equivalent of an array of closely spaced inlaid disk microelectrodes, then overlap of diffusion layers would occur at slow scan rates and give rise to linear diffusion at an electrode with an apparent area equal to the geometric area. That is, a microarray electrode may behave as a macroelectrode at long time domains where complete overlap of diffusion layers occurs. The fact that half-wave potential (Ell2) values of -0.87 f 0.02 VVs &/&cl and E V s h[(id - 9 / i ] slopes of 65 f 5 mV (E, potential; i, current; id, diffusion-controlled limiting current) are observed with either the GDE or the 10 pm radius platinum inlaid microdisk electrode under the conditions of Figure 2 implies that a steady-state or near-steady-statetype response is observed at very slow scan rates with the GDE. A predominantly sigmoidal-shaped response with a scan rate independent limitingcurrent is in fact observed at the GDE over the scan rate range of 2-10 mV s-1 (Figure 5). Assuming that this steady-type response at slow scan rates does not result solely from the presence of convection leads to the conclusion that at least some array-like properties exist and that complete overlap of diffusion layers does not occur at the rough surface, where characteriztics of both inlaid and recessed disk microelectrodes are expected. However, further proof of this feature can be obtained from other kinds of experiments which define the electrode area (see below). (14) Bond, A M.; Oldham, IC B.;Zoski, C. G . Anal. Chim.Acta 1989,216,177 and references cited therein.
2588 Analytical Chemistry, Vol. 67,
No. 15, August 1, 1995
8.0-
0
4.0-
\
2.0-
-760
-900 Potential (mV)
-800
- 1000
Figure 3. Differential pulse voltammogram for reduction of 5 x M Cc+ at a GDE in acetonitrile (0.1 M Et4NBF4). Pulse amplitude, -50 mV; pulse width, 40 ms; sample time, 35 ms.
Potential (mV)
-80.0
.
Potential (mV)
1.1 40.01
-700 .800 -900 -1000 -1100-1200 Potential (mv)
1
-700 -800 -900 -1000 -1100 -1200 Potential (mV)
Figure 4. Cyclic voltammograms for reduction of 5 x 10-4 M Cc+ in acetonitrile (0.1 M Et4NBF4) at a GDE using scan rates of (a) 50 mV s-l, (b) 500 mV s-l, (c) 10 V s-’, and (d) 100 V s-l.
Conventional reversible-shape differential pulse voltamme grams are observed on the GDE as seen in Figure 3, confirming that the electron transfer step is reversible and that slow electron transfer or another mechanistic complication is not the origin of the sigmoidal- rather than peak-shaped response observed at slow scan rates. Cyclic voltammograms obtained for the Cc+ + eCc process over the scan rate range 50 mV ssl to 100 V (considerably higher scan rates than used in Figure 2) are presented in Figure 4, and a change in the shape toward that expected when linear diffusion is the dominant mode of mass transport is evident. However, at the highest scan rates examined, background current terms become signscant and lead to uncertainty in measurement of the faradaic current (see Figure 4d for example). A log-log (limiting or peak current versus scan rate) plot is linear in the fast scan rate region (Figure 5), with a slope that is characteristic of linear diffusion. As noted above, at very low scan rates, the limiting current is independent of scan rate (Figure 5), as expected for the steady-state regime. +
The fast scan rate voltammetric and short time chronoampere metric data can be qualitatively understood if it is assumed that the GDE represents a rough surface with conducting (electroactive) graphite particles dispersed throughout the electrode in a nonconducting (electroinactive)epoxy medium. That is, theoretically, the GDE can be considered to be an electrodewith a partially blocked surface. However, because of the rough nature of the surface, the electroactive sites consist of a mixture of what effectively are inlaid and recessed microdisk electrodes that are randomly spaced and randomly oriented. Furthermore, the rough + + surface minimizes the overlap of diffusion layers, and at short time domains it can be assumed that minimal overlap of diffusion zones 0.6 associated with the electroactive sites occurs and that linear .3.0 -2.0 .1.0 0.0 1.o 2.0 3.0 diffusion is the dominant mode of mass transport. If the model Log v (v in V s.1 ) Figure 5. Plot of log(limitingor peak current) versus log(scan rate) of ref 15 is used, in which r = nrl (where r is the half-distance M Cc+ in acetonitrile (0.1 M EhNBF4) at for the reduction of 5 x between the centers of the particles and r1 is their radii), then a GDE. the blocked part (electroinactive) of the electrode surface is given by Q = (n2r12- r ~ ~ ) / n or ~ r Ql ~ = (n2 - l ) / n 2 , and the electroactive part is given by Q = l/n2. If 2r = 3r1,which is the (a' approximate result deduced from Figure 1, then the part of the 400 surface covered with electroactive graphite particles is equal to 44%. The theoretical slope16of the Zversus t-'l2 plot at short times for mass transport by linear diffusion is nFSD1/2C/n1/2, where S is the electroactive surface area, D the diffusion coefficient, C the concentration, F the Faraday constant, and n the number of electrons transferred in the charge transfer process. Substituting cm2 s-I,17 n = 1, and C = 5 x M gives a D = 1.28 x slope of 0.096s. The experimental slope is 0.0286 (Figure 6b), so the calculated value of S is therefore -0.03 cm2. Since the total electrode surface area is 0.075 cm2,the electroactive part of 10 the electrode surface therefore is calculated to be 40%, which is 0 25 50 75 100 in excellent agreement with the value of 44% estimated on the Time (ms) basis of measurements made by electron microscopy. This result -1.01 I c o d r m sthat little overlap of diffusion layers is encountered at short experimental times. At slow scan rates, radial diffusion at an inlaid disk microelectrode may become important. For a single sensing element, the radial dfision contribution is expressed in terms of the sphericity of the electrode. The sphericity factor, 6, is given18-20by the expression
1
6 = l/rl (DRT/nFu)1'2
-1.6'
(2)
where rl is the radius, v is the scan rate, and the other parameters are as defined previously or else have their usual significance. that It has been suggested in the
6
=. 40
