Anal. Chem. 1988, 60, 2163-2165
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Ultramicrodisk Electrode Ensembles Prepared by Incorporating Carbon Paste into a Microporous Host Membrane I. F r a n c i s Cheng a n d Charles R. Martin*
Department of Chemistry, Texas A&M University, College Station, Texas 77843 Because ultramicroelectrodes offer electrochemists a myriad of opportunities that are not possible at electrodes of conventional dimensions, ultramicroelectrodes are of considerable current research interest (1-5). We have recently described a novel procedure for preparing ensembles of ultramicrodisk electrodes (2). This procedure is based on electrochemical deposition of Pt into the pore structure of a commercial (Nuclepore) microporous host membrane. Nuclepore membrane contains linear, cylindrical pores of equivalent pore diameter. The pores of the host act as templates for the elements of the ultramicroelectrode ensemble; ensembles composed of uniformly sized ultramicrodisks are obtained (2). This electrochemical procedure for preparing ultramicroelectrode ensembles (UME's) is rapid, simple, and relatively inexpensive (2). Furthermore, we have prepared ultramicrodisk ensembles with the smallest element radii to be reported in the literature to date (1000.&) using this technique. We have, however, recently developed an even quicker, simpler, and less expensive method for preparing UMEs. In this new method, the elements of the UME are formed by simply filling the pores of a Nuclepore membrane with carbon paste. Thus, this new procedure eliminates the electrochemical deposition step (and the precious metal) required in the previous method (2). It is clear that the inability to prepare ultramicroelectrode arrays and ensembles with very small, dense, and well-defined elements is hampering progress in this important research area (5). Because these new carbon paste UME's offer a posssible solution to this problem, we report the procedure for preparation, and the results of preliminary electrochemical characterizations, of these new ensembles in this note. EXPERIMENTAL SECTION Materials and Reagents. Ferrocenylmethyltrimethylammonium hexafluorophosphate was prepared as described previously (6). Carbon paste was prepared from carbon powder (Carbopack C, 80/100 mesh, Supelco) and high vacuum grease (Dow Corning). Highly purified water was obtained by passing house-distilled water through a Milli-Q water purification system. The Nuclepore membranes were obtained from the Nuclepore Corp. In this note, we describe results of preliminary electrochemical investigations of UME's prepared from Nuclepore membranes with pore diameters of 12 and 8 pm. Specifications for these membranes are presented in Table I. The fractional pore area (Table I) is an especially important parameter since this parameter will determine the ratio of the active to geometric areas of the UME (2). Electrochemical Instrumentation and Cell. Cyclic voltammograms were obtained by using a PARC Model 175 programmer in conjunction with a PARC 173 potentiostat and either a Soltec VP-64245 X-Y recorder or a Nicolet 206 digital oscilloscope. A three-electrodecell consisting of a carbon paste working electrode (either macrosized or UME, see below), a Ag/AgCl reference, and a Pt flag counter electrode was used. NaCl (0.2 M) served as the supporting electrolyte. Electrode Preparation. Carbon paste was prepared by intimately mixing 0.42 g of Carbopack C carbon powder with 0.20 g of vacuum grease with a glass mortar and pestle. In addition to mixing, this procedure reduced the sizes of the carbon particles. The as-received particles had diameters of 150-180 pm; electron micrographs show that 0.1-0.3 pm-diameter particles were obtained after the mixing/grinding procedure (Figure 1). The ease with which the Carbopack C particles were broken and the rel0003-2700/88/0360-2163$01.50/0
Table I. Nuclepore Membrane Specifications
pore dia: pm
pore density: pores cm-2
fractional pore areab
av distance
between pores: om
8.0 12.0
1.0 x 105
0.050
1.0 x 105
15.8 15.8
0.11
OFrom Nuclepore Corp. (Pleasanton, CA) 1984 Catalog, p 17. Precision of pore diameter = +O to -20%. Precision of pore density = *15%. bCalculated ratio of pore surface area to total geometric area. See text. cCalculated as per eq 21, ref 10. Table 11. Experimentally Measured Capacitances and Active Area Ratios electrode"
capacitance, pF
ratio of active to geometric areasb
0.38 0.020 0.047
0.068 0.12
macro 8-pm UME 12-pm UME
1.0
"All electrodes had a geometric area of 0.072 cm2. *Should be eauivalent to fractional Dore area in Table I (see text). ~~
~
atively uniform size distributionof the resulting fragments suggest, to us, that the original particles are in fact loosely bound conglomerates of the smaller particles. The UMEs were prepared as follows: As-receivedNuclepore membrane is coated with poly(vinylpyrrolidone), which acts as a wetting agent. This poly(vinylpyrro1idone) was removed by ultrasonicating the membranes for 20 min in glacial acetic acid (7). The sonicated membranes were then repeatedly rinsed with purified water. Nuclepore membrane has a polished (shiny)and an unpolished (dull) face. Carbon paste was applied to the dull face and the pestle was used to rub the paste into the pore and through the membrane. This procedure was continued until carbon paste began to leak out from the opposite (shiny)side of the membrane. The membrane was then turned over and this procedure was repeated on the shiny side of the membrane. Excess carbon paste was then removed by gently rubbing each face with tissue paper. The carbon-paste-impregnatedmembrane was then applied (dull side down) to the surface of a macrosized carbon paste electrode (8) and held in place with a rubber O-ring. At this point, capacitive currents at the UME's were measured and used to evaluate the electrochemicallyactive surface areas. A cyclic voltammetric method, described in detail in ref 2, was used. This method requires a value for the double layer capacitance of carbon paste (2);this value was obtained by conducting analogous experiments at macrosized carbon paste electrodes (8). The capacitance data, and the electrochemically active surface areas obtained from these data, are reported in Table 11. RESULTS AND DISCUSSION Table I1 shows that the experimentally measured area ratios are identical to the fractional pore areas presented in Table I (note precisions on pore diameters and densities, Table I). This suggests that all of the pores in the host membrane produce active UME elements and, more importantly, that the sealing problem observed for the Pt UMEs (2) is not observed here. However, this agreement between experimental and calculated areas could, in principle, be fortuitous. For example, there could be fewer elements than expected, but these elements might be imperfectly sealed (2). This would 0 1988 American Chemical Society
2164
ANALYTICAL CHEMISTRY. VOL. 60. NO. 19, OCTOBER 1. 1988
.
n
A
4 I
Figure 1. Scanning electron micrograph of carbon particles used to prepare the UMEs
"*.AOIAPCI
peot.ntio~
Figure 3. Cyclic voltammograms for 1 mM ferrocenylmethyfblmethylamnmnium, 0.1 M KCI (aqueous): (A) comparison of voltam mograms for UME and carbon paste macrosked electrode (of Oquhralent geometric area, 0.072 cm2). Scan rate 100 V s-'; (6) UME voltammogram, scan rate 0.500 V s-': (C)"parison of witan" grams at 5 mV s-'. Electrodes as per part A. above.
Photomicrograph of the surface of a carbon paste UME prepared from 12 Fm pore diameter Nuclepore membrane. 260X magnification. Flgun 2.
cause the exposed surface area of each element to be greater than anticipated; this combination of fewer elements with higher areas per element could cause fortuitous agreement between the measured and calculated areas. A photomicrograph of the surface of a typical carbon paste UME is shown in Figure 2. This photomicrograph clearly shows that all of the pores of the host are filled with carbon paste. Furthermore, Figure 2 shows that the UME element diameter is, as expected, defined by the pore diameter of the host membrane. Because all of the pores are filled, the apeement between the measured and calculated areas (Tables I and 11) cannot be fortuitous. Therefore, the element sealing
problem, observed for the F't UME's (Z), is not observed here. It is not surprising that better seals between the elements and the host membranes are obtained with these UME's than with the Pt UMEs (2). Seals were formed, in the Pt UMEs, by exposing the UME to moltant polyethylene (PE) and then allowing the P E to harden. Because P E contracts upon solidification,imperfect seals are formed (2). Vacuum grease forms the seal between the element and the host membrane in the carbon paste UMEs. Because vacuum grease does not undergo a liquid-to-solid transformation, contxaction does not occur and high-quality seals are formed. The shape of the cyclic voltammogram at an UME depends on the time scale (scan rate) of the experiment; three distinct limiting cases should, theoretically, be observed ( 2 4 ) . The first limiting case is obtained a t very high scan rates, where the diffusion layers at the UME elements are thin, hear, and completely isolated. This linear diffusion situation produces a conventional, peak-shaped voltammogram; currents are proportional to the electmchemicallyactive surface area (2-4). The second limiting case occurs a t lower scan rates, where the diffusion layers take on radial character but remain totally isolated from each other. These isolated radial diffusion fields should yield a sigmoidal voltammogram (24). The final limiting case occurs a t very low scan rates where the radial diffusion layers overlap, yielding a net linear diffusion field.
Anal. Chem. 1988, 60, 2165-2167
Because linear diffusion obtains, a peak-shaped voltammogram is again observed; however, in this “totaloverlap” limiting case, currents are proportional to the entire geometric area of the UME (2-4). Figure 3 shows voltammograms for ferrocenylmethyltrimethylammonium, at various scan rates, for an UME and for a macrosized electrode of equivalent geometric area. The voltammograms in Figure 3A show that the first (linear diffusion to the active area) limiting case can be achieved at the UME. (Note that the current sensitivity is higher for the UME voltammogram than for the macrosized electrode voltammogram.) The voltammogram in Figure 3B shows that the second (radial diffusion) limiting case can be approached but not achieved; given the geometric characteristics of the membrane, this was the anticipated result (9). Finally, the voltammograms in Figure 3C show that the third (totaloverlap) limiting case can also be achieved at this UME. (Note that the current sensitivities for the UME and macrosized electrodes are the same.) CONCLUSIONS A new procedure for preparing UMEs was described. The UMEs obtained yielded voltammetric responses that agreed with predictions of established electrochemical theory (2-4). Furthermore (and perhaps of most importance) the capacitive currents at these UME’s were reduced to values predictable from the known geometry of the host membrane. This diminution in background current, coupled with the ability to achieve the “total overlap” limiting case, suggests that these
2165
UME’s should yield electroanalytical detection limits that are lower than detection limits obtained at the corresponding macrosized electrode (2). We are currently investigating this possibility. Finally, this approach for preparing UME’s can only be used when the diameter of the carbon particles (Figure 1) is appreciably smaller than the diameter of the pores in the host membrane. We have used this approach to prepare UME’s with element diameters as small as 3 pm (9). If this technique is to be pushed to smaller element diameters, smaller carbon particles will be required. Registry No. Nuclepore, 12673-61-9;carbon, 7440-44-0. LITERATURE CITED (1) Fleischmann, M.; Pons, S.;Rolison, D. R.; Schmidt, P. P. U/tramlcroelectrodes; Datatech Systems, Inc.: Morganton, NC. 1987. (2) Penner, R. M.; Martin, C. R. Anal. Chem. 1907, 59, 2625. (3) Amatore, C.; Saveant, J. M.; Tessier, D. J. J . Electroanal. Cbem. Interfaclal Electrochem. IS03, 147, 39. (4) Cassidy, J.; Ghoroghchan, J.; Sarfarazl, F.; Smlth, J. J.; Pons, S.Electrochim. Acta 1888, 3 1 , 629. (5) Thorman, W.; Bixier, J. W. Presented at the 173rd Meeting of the Electrochemlcal Society, Atlanta, GA, May 16, 1988. (6) Martin, C. R.; Dollard, K. A. J . Electroanal. Chem. lg83, 159, 127. (7) Prothro, J., Nuclepore Corp.. personal communication, Sept 1987. (8) Dryhurst, G.; McAllister, D. L. I n Laboratoty Tecbn@wsin Electroanalytical Chemistry;Klssinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984; pp 294-301. (9) Cheng, I.F., unpublished results, Texas A&M Unlverslty, Jan 1988. (10) Scharifker, 0. R. J. Electroanal. Cbem. l90& 240, 61.
RECEIVED for review March, 8,1988. Accepted June 3,1988. This work was supported by the Office of Naval Research and by the Dow Chemical Co.
Utilization of an Inorganic Phosphor as a Reference Signal in Solid-Surface Room-Temperature Phosphorimetry Andr6s D. Campiglia’ and Clausius G. de Lima* Departamento de Quimica, Universidade de Brasilia,
Brasilia, D.F. 70910, Brazil
Solid-surfaceroom-temperature phosphorimetry (SSRTP) is a technique that has been suggested for the trace determination of organic compounds (1-4). The phenomenon was discovered by Roth in 1967 (1)and rediscovered 5 years later by Schulman and Walling (2). The technique is extremely simple to use, principally due to the fact that the utilization of a Dewar and cryogenic fluids or of a cryostat is not necessary. Several solid matrices, such as silica gel, sodium acetate, filter or chromatographic paper, etc., have been studied, with paper being the substrate that has been most thoroughly investigated (3, 4). The utilization of a standard or reference for the wavelength calibration or intensity correction is frequent in room temperature solution fluorescence measurements. An example is the classical use of quinine sulfate in 0.1 N sulfuric acid medium. From the point of view of phosphorescence, standards prepared by embedding polycyclic aromatic hydrocarbons (such as coronene, triphenylene, phenanthrene, or chrysene) in poly(methy1 methacrylate) have been suggested by Melhuish (5). This solid solution may be used at either 77 K or room temperature. However, according to Melhuish, as the phosphorescence signal is strongly quenched by oxygen (which Present address: Department of Chemistry University of Florida, Gainesville, FL, 32611. *Author t o whom correspondence should be sent.
diffuses through the matrix), the plastic has to be vacuum sealed in a Pyrex tube. In the absence of such arrangement, the phosphorescencequantum efficiency of coronene at 77 K decreased, after having been exposed to air for several days, from 0.30 to 0.12. Despite that fact, according to Miller (6),aromatic hydrocarbons (such as coronene) in acrylic matrix can be used as a standard in room-temperature phosphorimetry and are commercially available. A solid plastic (poly(methy1methacrylate)) matrix of europium(II1) thenoyltrifluoroacetate (1 X lo4 M) (7)is also commercially available for room-temperature phosphorescence intensity and lifetime measurements. Although extremely convenient to use, its short lifetime (ca. 0.300 ms) precludes the use in instruments with a rotating-can phosphoroscopeassembly, such as the type used in our and other laboratories. The typical designs of such standards are solid cuvettes with usual dimensions or narrow cylinders, the latter for use in Dewar flasks (6-8). In the search for another room-temperaturephosphorescent secondary standard or a reference signal source, a commercial phosphor based on zinc and cadmium sulfide embedded in a plastic film matrix, which is described in the present work, was examined. Besides other inorganic luminescent compounds, the synthesis and the phosphorescent properties of metal sulfides (calcium, barium, magnesium, or zinc) have been known since the beginning of this century (9). In the case of the products obtained with the mixed crystals of zinc
0003-2700/88/0380-2165$01.50/00 1988 American Chemical Society
