Anal. Chem. 1988, 60,2385-2392
2385
On the Nature of Poly(chlorotrifluoroethy1ene) Composite Electrode Surfaces Steven
L.Petersen, Duane E. Weisshaar,’ a n d Dennis E. Tallman*
Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105 Roland K. Schulze a n d J o h n F. Evans
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Scott E. DesJarlais and Royce C. Engstrom
Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069
A variety of composlte materlals have been used as electrodes for electrochemlcal measurement. Such materlals lnclude varlous blends of an electrically Insulating polymer (e.g., an epoxy, polyethylene, Teflon, Kel-F, or Nujol) and a powdered conductor (e.g., graphlte, carbon black, or preclous metal), as well as porous materlals such as graphite or retlculated vitreous carbon Impregnated with an Insulating polymer such as a wax, polystyrene, or an epoxy. I n spite of the widespread use of such materials in electrochemlstry, relatively llttle effort has been devoted to understandlng the surface morphology of composite electrode materials and the relationships between surface morphology and electrochemical behavlor. I n this work we employ four techniques that probe various aspects of the surface microstructure of Kel-F composite electrodes: scannlng electron microscopy In conjunction wlth X-ray element mapping, which provides Information on the spatlal distribution of polymer and conductor across the composite surface; X-ray photoelectronspectroscopy, which ylelds Information on the chemical Interactions between polymer and conductor as well as on the distrlbutlon of conductor partlcles between electrically conducting and electrically lnsulatlng surface regions; electrogenerated chemllumlnescence Imaging, which provides an optical map of electron-transfer activity across the composlte electrode surface; and double-layer capacitance measurements, which probe the degree of mlcrometer dlmenslon roughness or porosity within the electroactlve surface reglons. The results of these experiments have enabled us to formulate a conceptual model for KeCF composlte electrode surfaces, certain features of which may be applicable to other composite electrode materlals as well.
Conducting composite materials represent a versatile approach to the fabrication of new electrodes for electroanalytical measurements. Typically, these materials are fabricated by suspending a powdered conductor in a polymeric matrix, analogous to the formation of the popular carbon paste (composite) electrode ( I ) , and have included composites fabricated from various epoxies and graphite (2),Teflon and graphite (3),polyethylene and carbon black (4),silicone rubber and graphite (5),Kel-F and graphite (6),and Kel-F and silver (7). Alternatively, a porous conductor can be impregnated with an insulating polymer, and this approach has produced the wax- (8),the styrene- (9) and the epoxy-resin-impregnated (10) graphite electrodes as well as the epoxy-impregnated reticulated vitreous carbon electrode (11).
* To whom correspondence should be addressed.
Present address: Department of Chemistry,Augustana College, Sioux Falls, SD 57197.
The initial motivation for our work in this area was the desire to develop a carbon-based electrode material that would be mechanically robust and chemically stable like glassy carbon, yet would be inexpensive, easy to fabricate, and perhaps more reproducible from electrode to electrode, these latter attributes often being associated with carbon paste electrodes. Of the electrode materials we have developed and others we have used, those based on Kel-F (a 3M Co. trade name for poly(chlorotrifluoroethy1ene)or PCTFE) have shown the most promise for fulfilling these early expectations. Additionally, Kel-F composite electrodes based on graphite (the Kelgraf electrode) (6,12) and on silver (the Kelsil electrode) (7) exhibit behavior typical of microelectrode ensembles (7, 13),including enhanced current density compared to that of the corresponding solid carbon or silver electrode, and this in turn can lead to a signal-to-noise advantage in many types of electroanalytical measurements, both in quiescent (7, 13) solution and in flowing streams (14, 15). Precious metal composite electrodes, for which the Kelsil electrode is a prototype, offer the additional advantages of lower density and lower cost than their solid electrode counterparts (7). Chemical modification of a Kelgraf electrode has recently been demonstrated (16),providing further evidence of the versatility of polymer composites for designing new electrode systems. A better understanding of the electrochemical behavior of Kel-F composite electrodes requires a more detailed picture of the surface of these materials. In particular, we need to be able to answer such questions as: How are conductor particles distributed across the surface of the composite material? Does this distribution change significantly with the type of surface preparation (polishing)? Are there significant numbers of electrically isolated regions of conductor particles on the surface at which electron transfer cannot take place? How do the regions of conductor and Kel-F interface with each other? Are the conducting regions (which consist of aggregates of conductor particles ( 7 ) )microscopically rough or porous or does the Kel-F polymer tend to fill in any voids between the conductor particles? These and other questions are addressed in this work by employing the techniques of scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDXA), X-ray photoelectron spectroscopy (XPS or ESCA), electrogenerated chemiluminescence (ECL) imaging, and capacitance measurements. It can be argued with some justification that we do not yet fully understand the surfaces of “simple”electrode materials such as solid carbon and silver, and therefore, attempts to obtain a detailed understanding of the more complex surfaces of composite materials will be fruitless. However, investigations of the type presented here do provide insight into the microscopic organization of polymer and conductor on the Kel-F composite electrode surfaces and aid in our interpretation of the electrochemical response of these and perhaps other composite electrodes.
0003-2700/88/0360-2385$01.50/00 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
EXPERIMENTAL SECTION Reagents and Materials. All chemicals were of reagent grade unless otherwise noted. Aqueous solutions were prepared from Milli-Q water (Millipore Corp.). Kelgraf and Kelsil composite electrodes and glassy carbon and silver disk electrodes were prepared as previously described (7, 13). Apparatus. Scanning electron micrographs were obtained on an InternationalScientific Instruments (Milpitas, CA) Model lOOB scanning electron microscope detecting secondary electron emission; element maps were generated with a Princeton Gamma Technology (Princeton, NJ) System 1000 X-ray analyzer. XPS analyses were carried out on a Perkin-Elmer/Physical Electronics (Eden Prairie, MN) Model 555 electron spectrometer equipped with an Al Ka X-ray source operated at 3W-400 W and 14 kV. No surface cleaning using argon ion sputtering or other means (except for solvent cleaning in an ultrasonic bath) was employed prior to the acquisition of XPS spectra. In all cases spectra were recorded with and without the use of a low-energy electron flood gun (5 V bias, 0.5 mA emission current, optimized by examination of peak position) to compensate for surface charging. This procedure allowed for the differential analysis of electrically isolated regions vs conducting regions that were in intimate electrical contact with the sample holder, made through the back side of the composite electrodes (as is the case for electrochemical experiments). Survey scans (1000-0 eV of binding energy) were taken at 100-eV pass energy, while a pass energy setting of 25 eV was used for the acquisition of high-resolution spectra. All binding energies are referenced to graphitic and hydrocarbon C(1s) at 284.6 eV. Surface elemental ratios were quantified from the high-resolution spectra by using peak area measurements in conjunction with sensitivity factors provided by the manufacturer of the spectrometer. Compositional homogeneity was assumed in the volume element sampled by XPS. Electrogenerated chemiluminescence (ECL) imaging was carried out as described previously (17, 18). Light produced at the electrode surface in the ECL reaction sequence of rubrene (19) was magnified with a 55-mm “macro” lens and recorded photographically. A 3-min exposure onto Kodak Tri-X film (Rochester, NY) was used. Capacitance measurements were made by superimposing a low-amplitude triangular waveform (5-10 mV from a Krohn-Hite Model 5200A function generator) onto a base potential (supplied by a PAR Model 175 universal programmer). These signal sources were fed to the summing amplifier input stage of an in-house constructed potentiostat. The resulting output waveform from the current follower was monitored with a Tektronix Type 549 storage oscilloscope. The auxiliary electrode was a large cylindrical platinum gauze electrode situated symmetrically around the working electrode. The reference electrode was Ag/AgC1/3.5 M KC1 and was separated from the cell by a salt bridge containing the supporting electrolyte. Salt bridge solutions were replaced daily. Measurements were performed at room temperature (20-23 “C) in 70-80 mL of 0.10 M NaC104 solution. Solutions were deoxygenated with purified, solvent presaturated nitrogen for at least 15 min prior t o each run, and the solution was blanketed with nitrogen during equilibration and measurement. Prior to each run, electrodes were hand-polished with 0.3-pm alumina on kitten-ear polishing cloth (Buehler), polished on bare cloth to remove residual alumina, etched for 1 min in 10% HC104, thoroughly rinsed with water before introduction (wet) into the cell, cycled between -0.1 and -0.9 V at 100 mV/s for 10 min, and equilibrated for 1 h at a base potential of -0.500 V. This pretreatment is the same as that employed by Hampson et al. (20), except for the potentiodynamic step, which was included because it resulted in better reproducibility from run to run. The POtentiodynamic treatment was omitted in the case of Kelgraf. Electrodes employed for SEM, XPS, and ECL studies were polished with a 1-pm-alumina/deionied water slurry on polishing cloth, and for XPS analysis also were sonicated in methanol. RESULTS AND DISCUSSION A distinguishing feature of Kel-F composite electrodes (as fabricated in our group) that sets them apart from other composite electrode materials (for example, carbon paste ( I ) , carbon black/polyethylene ( 4 ) ,or graphite/epoxy (2)) is the
ability to achieve high conductivity even with rather low conductor content, as low as 5 % by volume (13,14). This appears to be a consequence of the particle size difference between the Kel-F resin particles (on the order of 100 pm in dimension) and the conductor particles (on the order of 1pm in dimension) (7). It is also important to note that during the compression-molding step we do not permit the Kel-F polymer to remain in the molten state long enough for the conductor particles to randomize throughout the Kel-F matrix. Unlike other conducting composite materials that contain percolating clusters or randomly distributed conductor particles (21),our Kel-F composite materials appear to contain a rather ordered network of agglomerated conductor particles pervading the Kel-F matrix (7). Therefore, each active region (or microelectrode) on the surface of our Kel-F composite materials consists of many aggregated conductor particles and may be separated from neighboring active (conducting) regions by considerable distances (7,13). Depending on geometric area, a Kel-F composite electrode surface may contain several thousand such regions or microelectrodes (7). As the conductor content in the composite increases, the surface should more closely approximate that of the corresponding solid conductor, although the mechanical integrity of the composite material is normally lost well before this limit is reached (22). Both the percentage of conductor and the ratio of Kel-F particle to conductor particle size can influence the electrical resistance of the material as well as the distribution of active regions on the surface (7, 13, 14). Microscopy and X-ray Analysis. Our earlier attempts to study the surfaces of Kelgraf electrodes by optical and electron microscopies met with limited success. Optical microscopy was unrevealing because we were unable to distinguish surface graphite from graphite residing beneath the surface of the transparent Kel-F. Furthermore, SEM revealed three distinct regions on the Kelgraf surface (13),but we were not able to resolve individual graphite particles, possibly due to the similar densities and electron scattering efficiencies of graphite and Kel-F. Our recently described Kebil electrode displays neither of the above difficulties. Surface and subsurface silver are readily distinguished by optical microscopy, and individual surface silver particles can be identified by SEM (7). Three regions (gray, black, and white) similar in appearance to those observed at Kelgraf can be seen in the SEM micrograph of Kelsil (Figure la). With EDXA, elemental maps have been generated that permit unambiguous identification of each of these three regions. Figure l b shows the silver map for the same region displayed in Figure la. Inspection reveals that the gray regions of the SEM micrograph correspond to conducting surface silver. Closer comparison of Figures l a and 1b indicates that only a very small amount of surface silver charges under the influence of the electron beam and appears white in the SEM micrograph, suggesting that very little surface silver is isolated from the conducting network of silver within the composite. Of particular interest are the black regions that surround each of the (gray) conducting silver regions (Figure la). The chlorine map of Figure ICc o n f i i s that these black regions are Kel-F polymer, as are the white regions of Figure la. The white regions are Kel-F that develops a negative charge under the influence of the electron beam, thereby providing a very high secondary electron contribution to the overall detected signal. The black regions at the edge of the patches identified as silver by EDXA are attributed to regions where a thin (up to several micrometers) overlayer of Kel-F is present on the silver aggregate. This is based on the fact that the low atomic number constituents of Kel-F are poor scattering centers for the primary beam and thus generate few secondary electrons unless the surface has accumulated
ANALYTICAL CHEMISTRY, VOL. 60. NO. 21. NOVEMBER 1, 1988
a
1
.
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F IKLLI
Ag 3d
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b
-1000-900
- 8 0 0 - 7 0 0 - 6 0 0 -500 -400 -300 - 2 0 0 -100
0
0lNDlNG ENERGY. EV
F b r e 2 XPS survey spectra of a 14% KeWl electmde (tap) and a 14% Kelgraf electrode (boitom).
C
Figure 1. SEM micrograph and element maps of a 14% Kelsil electrode surface: (a)SEM micrographat ZOOX, (b) silver X-ray map of the same region displayed in (a): (c) chlorine X-ray map of the same region displayed in (a).
negative charge. As such, the primary beam readily penetrates to the subsurface silver in these regions, where the beam current may be easily conducted to the sample holder without charging the polymer surface significantly or causing the emission of significant numbers of secondary electrons. As the Kel-F overlayer thickens, the penetration of the beam to a cluster of suburface silver particles is increasingly less likely. The type of surface polishing (coarse or smooth) appears to have little effect on the SEM appearance of these regions. With Kelgraf electrodes there is a reversal in the appearance of the black and gray regions, with the black regions being surrounded by the gray (13). Recent EDXA data (not shown) supports our original assignment of the blackest regions of the SEM micrographs of Kelgraf to conducting carbon and the gray regions to Kel-F in intimate contact with conducting carbon (13). The nature of the conductor particles appears to strongly influence the appearance of these regions in the SEM micrographs. X-ray Photoelectron Spectroscopy. XFS ' survey spectra of Kelsil and Kelgraf electrodes are presented in Figure 2.
Aside from the bands expected there is only one major contribution, oxygen, that cannot be attributed to the elements comprising the Kel-F and conductor phases present in the composites. In the case of Kelsil, we attribute the primary source of oxwen to uartiallv oxidized hydrocarbons and/or surface s i t e s - h are associated with the Kel-F phase. The O(1s) feature is more pronounced in the case of the Kelgraf material. As elaborated below there are various plausible sources of the oxygen. In the case of the Kelgraf, a higher oxygen content in the conducting phase (graphite) would be expected due to the ease with which cleaved graphite edge planes undergo reaction with the atmcephere u, yield a variety of oxygen-containing functional groups (23,24).As such. then, to a first approximation both the conducting and nonconducting regions would be expected to contribute some oxygen signal. A detailed evaluation of the high-resolution data (vide post) clarifies the sources of oxygen for both the Kelgraf and Kelsil surfaces. Normally the surface charging that is found in the XPS analysis of nonconductors is a nuisance. However, in sptems that are composites of materials or phases of differing conductivity, this may be turned to one's advantage, because it provides a means of differentiating the origin of various observed photoemission lines. Such is the case with the composite electrode materials studied here. By carrying out high-resolution XPS first under conditions where no attempt is made to compensate for the surface charging, followed by analysis under conditions in which a source of low-energy electrons (flood gun) is employed to dissipate the positive surface charge, one may attrihute those bands that am shifted by the use of the flood gun to the nonconducting or more poorly conducting medium. The nonspecialist is reminded that surface charge accumulates as photoemission occu~sfrom the nonconducting regions and represents an additional retarding field to the ejected electrons. Thus, electrons originating in the nonconducting material register at higher binding energy (lower kinetic energy) in the acquired spectrum. The high-resolution XPS spectrum of the Ag(3d) region of a Kelsil composite taken with and without charge neutralization appears in Figure 3. It is clear that there is no a p preciable smearing of this conducting phase into the Kel-F as a result of the polishing procedure because the spectra are the same with and without charge compensation. Close examination of the other high-resolution data, which are sum-
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ANALYTICAL CHEMISTRY, VOL. 60, NO.
21, NOVEMBER 1, 1988
Table I. Summary of High-Resolution Peak Positions under Compensated end Uncompensated Conditions sample Kelsil (14% Ag)
neutralization no
yes
Kelgraf (14% graphite)
band
Ag(3d6/*) 367.5 FUs) 693.0 687.6 Cl(2p) 206.2 197.3 C(1S) 295.8 291.2 284.6 O(1S) 537.5 531.5 Ag(3d,,2) 367.5 F(W 687.4 Cl(2p) 200.6 197.3 COS) 290.6 284.6 O(lS) 531.4
no F(ls) Cl(2p) 00s) yes
BE, eV
C(1s) F(ls) Cl(2p) O(1s)
295.4 289.5 284.6 692.8 688.8 205.9 201.6 536.4 532.7 290.3 284.6 687.4 200.8 531.6
assignments Ago Kel-F AgF (3% of Ago) Kel-F AgCl (5% of Ago) Kel-F partially oxidized hydrocarbon on Ago ads hydrocarbon on Ago oxidized Kel-F partially oxidized hydrocarbon on A$ Ago Kel-F and AgFn Kel-F AgCl Kel-F and partially oxidized hydrocarbon on Ago ads hydrocarbon on Ag oxidized Kel-F and partially oxidized hydrocarbon on A$" (slight broadening to higher binding energy noted) Kel-F electrically isolated graphite graphite (conducting) Kel-F graphite (low-intensity shoulder) Kel-F graphite (low-intensity shoulder) oxygen-containing functionalities on Kel-F and electrically isolated graphite oxygen-containing functionalities on graphite (conducting) Kel-F graphiteb Kel-F Kel-F broad band of more than one constituent
OUnresolved contributions from more than one chemical state. *There are a number of other carbon states present lying between these extremes in binding energy (see Figure 4). The more prominent of these are carbonyl and carboxylate functional groups on graphite, as discussed in the text. marized in Table I, indicates that there is some minor extent of reaction of a small amount of the silver with constituents of the Kel-F matrix. This is an expected result of the chemistry (bond rupture and subsequent reaction) that accompanies the removal of material in a polishing procedure. From the high-resolution XPS spectrum (Ag, F, and C1 regions, not shown) there is evidence of the formation of a small amount of silver chloride and fluoride on the conducting silver particles. There are trace levels of oxygen indicated on the surface of the Kel-F regions. This may be attributed to the atmospheric oxidation of the polymer backbone radicals, which are formed by the same bond-breaking process (homolytic C-Cl and C-F bond rupture) that liberates the halogens. The majority (ca.80%)of the oxygen arises from partially oxidized hydrocarbons adsorbed on the silver particles (see Table I). This is also not surprising in light of the use of silver as an oxidation catalyst. Unreacted adsorbed hydrocarbon is also found on the silver particles. The high-resolution spectra for Kelgraf with and without charge compensation are even more informative with regard to the consequences of the polishing procedure on the surface chemistry and physical intermixing of the two phases. The C(1s) and Cl(2s) bands under these experimental conditions are shown in Figure 4. At first glance, the interpretation of these spectra is straightforward. The indication from the uncompensated spectrum (Figure 4, lower panel) is that the highest binding energy carbon band is attributable to the carbon in the Kel-F matrix and the lowest binding energy carbon band arises from graphitic carbon, with oxidized graphite the obvious assignment for the middle carbon band. However, under closer scrutiny it is apparent that this interpretation is flawed in that there is too much intensity in the middle band to allow for its assignment only to surface
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Flgure 3. High-resolution XPS spectrum of a 14% Kelsil electrode in the Ag(3d) region under charge-compensated and uncompensated conditions.
oxides on the graphite even in light of the rather intense O(1s) feature typically observed on this material (Figure 2). In addition, the uncompensated O(1s) spectnun shows two bands with the higher binding energy band being the more predominant one by a factor of ca. 2 (see Table I). Because these O(1s) bands merge into one broad band when the flood gun
ANALYTICAL CHEMISTRY, VOL. 80, NO. 21, NOVEMBER 1, 1988
>
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BINDING ENERGY,EV
Figure 4. High-resolutionXPS spectrum of a 14 % Keigraf electrode in the C(1s) region under charge-compensated and uncompensated conditions. Graphitic carbon and KeFF polymer carbon are denoted by Cg and Cp, respectively.
is employed to neutralize the surface charge, we must conclude that the majority of the surface oxygen is associated with the insulating phase (Kel-F). Moreover, the binding energy for the middle C(1s) band (289.5 eV) is too high to be assigned to the surface oxides of the nonconducting graphite phase: carbonyl groups (287.4 eV) and carboxyl groups (288.8 eV) (23-27). These species contribute to the high binding energy shoulder on the graphitic peak. We are left with two possible explanations: (1)the middle C(1s) band (uncompensated) is due to the reaction of fluorine with the graphite during the polishing of the composite or (2) there is a considerable quantity of graphitic material that has been physically dislodged as small particles during the polishing procedure and is embedded in the Kel-F phase. The binding energy a t the band maximum for the central peak is consistent with either hypothesis. Unshifted C-F carbon would be found a t this binding energy, but electrically isolated graphite (284.6 eV) charge-shifted by the amount observed here for the F(1s) band (5.1eV) would also appear within 0.1 eV of the peak position for the middle band. We favor the latter hypothesis for several reasons. Firstly, although there is some evidence for fluorination (and chlorination) of the graphite accompanying polishing (see Table I), this proceeds to a much less extent than for the Kelsil material. Consequently, it is difficult to account for the magnitude of the middle band even though the binding energy is consistent with unshifted C-F. Secondly, close comparison of the entire C(1s) region with and without charge compensation reveals that it is the lowest binding energy C(1s) band that shows a significant increase in intensity with charge neutralization. Therefore, there is either graphite or hydrocarbon on the insulating regions of the surface, which upon proper compensation appears at 284.6 eV. Lastly, there is the matter of the surface concentration of oxygen being higher on the Kelgraf. As discussed in the preceding paragraph, the
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major component of this surface oxygen is associated with a nonconducting phase. There is a subtle inconsistency in attributing this observation to Kel-F being more prone to oxidation during polishing of the Kelgraf composite than for the Kelsil material, particularly in light of the fact that silver is a good catalyst for oxidation of hydrocarbon while graphite is rather inert. A more plausible argument is that the graphite that is embedded in the Kel-F phase during polishing consists of small particles with high surface area. These particles may be expected to readily undergo air oxidation, thus yielding surface oxygen which would be charge-shifted because it is present as surface oxides on these electrically isolated particles. To summarize, from XPS analysis there is evidence of a small extent of polishing-induced chemistry in both the Kebil and Kelgraf materials. This is manifested as fluorine and chlorine reacting with the respective conducting phases to yield at most a 3% modification of the composition of the conducting phases. In that the nonconducting phase (Kel-F) is the source of these halogens, a correspondingly small fraction of this phase undergoes air oxidation. The XPS results are consistent with the anticipated relative surface areas predicted from consideration of the weight percentages of the silver or graphite and Kel-F selected for the preparation of the composite (7,13).The two materials differ subtly in that whereas for Kelsil all of the surface silver is conductive, in Kelgraf as much as 5% of the surface graphite is present as electrically isolated small particles. This is likely to be the result of the polishing process but may also indicate that there is a small percentage of graphite in the bulk that is not conductive. Electrogenerated Chemiluminescence. The results of the preceding sections have provided useful information regarding the distribution of conductor and polymer across Kel-F composite electrode surfaces. However, an important question remains. What fraction of the conductor regions on the composite surface actually supports electron transfer? The spatial distribution of current density on electrode surfaces has recently been characterized by imaging the light produced in an ECL reaction (17,18). By use of this technique, the potential of a Kelsil electrode was stepped to -1.4 V vs SCE in the presence of 0.43 mM rubrene, 0.43 mM benzoyl peroxide, and 0.1 M tetraethylammonium perchlorate in DMF. The rubrene undergoes reduction to the radical anion, which is then chemically oxidized by benzoyl peroxide, producing an excited state of rubrene (19). The excited rubrene luminesces, producing a yellow light that forms an image of electron-transfer activity across the composite electrode surface. The location, size, and shape of individual active regions can then be determined by magnifying and recording the image photographically (17,18). We assume that the sequence of steps (following electron transfer) leading to chemiluminescence is of sufficiently short duration that no luminescence occurs at significant distance from an active region on the electrode surface. For the rubrene system, the results presented below support this assumption. Figure 5b shows a photograph of the ECL pattem obtained from the same Kelsil electrode (in the same orientation and under the same magnification) shown under reflected light in Figure 5a. Careful inspection of enlarged versions of Figure 5 reveals a direct correspondence between the position and shape of the individual regions of luminescence of Figure 5b and the blackest regions (representing surface silver (7)) of Figure 5a. We can now estimate that greater than 98% of the surface silver identified in the optical and scanning electron micrographs (7) is capable of supporting electron transfer. This is consistent with the XPS data, which suggests that very little silver on the surface of Kelsil electrodes is electrically isolated from the conducting matrix. In contrast, a significant fraction of the surface graphite of Kelgraf elec-
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 21. NOVEMBER 1, 1988
250 Q
a
4 b
Flgun 5. Electrogenerated chemiluminescence from a 14% Keisil electrode: (a) optical micrograph under reflected light; (b) ECL image
for the same electrode (same orientation) displayed in (a).
trodes appears to he electrically isolated (as evidenced by the XPS data) and presumably cannot support electron transfer. However, since optical microscopy does not permit the unambiguous distinction between surface graphite (conducting or not) and graphite lying just beneath the polymer surface, the ECL experiment would not yield useful information on this question of graphite distribution. Figure 5h clearly shows the variation in size and shape of the electroactive regions on the surface of Kelsil. Particularly noteworthy are the number of sites having rather high perimeter to area ratio, a feature conducive to current density enhancement at short times by convergent diffusion (or the edge effect) (28). The boundaries of the active regions are sharply delineated, indicating a rather well-defined interface between the active and inactive surface regions. If the chemical steps leading to chemiluminescence were sufficiently slow to permit significant diffusion of the excited-state ruhrene and/or the ruhrene radical anion away from an active electron-transfer region, then rather fuzzy or gradual transitions between light and dark regions would he expected. We conclude, therefore, that the map of Figure 5h accurately reflects active electron-transfer regions on the Kelsil electrode surface. Close scrutiny of the projected (negative) images of Figure 5 rev& a fascinating observation. Chemiluminescence arises from a few regions on the composite surface that are clearly polymer overlying subsurface silver. In a few cases, such emissions are observed between closely adjacent hut nonconnected surface silver regions, while in other cases the emissions appear to originate from polymer immediately surrounding certain of the surface silver regions. We do not believe diffusion is responsible for these ohservations, since
I
'
" " " " 10
10'
10'
FREQUENCY (Hz)
Figure 6. Apparent capacitance fj~Flcm?) vs !og frequency curves 101 lhree ebclrodes in 0.10 M NaCIo, at -0.50 V (vs AglAgCV3.5 M KCO. Each point is an average of the number of measurements indicated in p a r m e s s : (0)14% KdsB elecbode (12));(X) silver disk elecbode (6): ( 0 )14% Kelgraf electrode (3).
in the majority of cases emission is confined to surface silver regions and the emission boundaries are sharply delineated, as noted earlier. We have previously reported that a t Kelgraf electrodes there appears to be a somewhat greater area available for electron transfer to a less polar solute such as 1,l'-his(hydroxymethy1)ferrocene than to a highly charged solute such as ferricyanide (13). We have noticed other instances where solute polarity influences electrochemical hehavior a t these composite electrodes (6). The nature of the interactions that lead to these ohservations is a mystery a t this time. We speculate that very thin Kel-F films overlaying silver may be responsible, perhaps involving very short range diffusion of the solute through the film or (as suggested hy a reviewer of this paper) diffusion of the solute into micrometer dimension pores or holes in the film leading to the subsurface silver. In any event, the OhseNatiOnS are sufficiently interesting to warrant further studies into their origin. Capacitance Measurements. A comparison of the double-layer capacitance measured a t a composite electrode with that m e a s d at the corresponding solid electrcde in the same electrolyte can reveal fundamental differences in the surface states of the electrode materials as well as differences in the microscopic surface roughness or porodiity of the two electrodes. In the following discussion, each capacitance value has been normalized with respect to the octiue (conductor) area of the respective electrode surface. At a measurement frequency of 100 Hz,the capacitance at a 14%Kelgraf electrode immersed in 1.0 M KC1 varied from 40 pF/cm2 a t a potential of 0.6 V to 60 pF/cm2 a t -0.6 V. Under the same conditions, the capacitance at a glasay carbon electrode varied from 70 to 120 pF/cm2. The potential dependence of the capacitance throughout the above potential range (values were obtained a t 0.1-V intervals) was similar for the two types of electrodes, suggesting considerable similarity in the interfacial regions of these electrodes. Lower capacitance is consistently observed a t Kelgraf electrodes and may reflect Kel-F filming of the graphite during polishing. Altogether, the rather large differences in electrode surface morphology have relatively minor influence on the douhlelayer capacitance. The frequency dependence (dispersion) of the capacitance at Kelgraf is rather small (Figure 6),being similar to that observed a t glassy carbon (not shown).
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
The apparent capacitance at 80 Hz of an 11% Kelsil electrode in 0.10 M NaC10, varied from 70 pF/cm2 at -1.0 V to 95 pF/cm2 at -0.4 V to 80 pF/cm2 at 0.0 V, whereas the silver disk displayed values ranging from 39 to 62 to 43 MF/cm2, respectively, at the same potentials. Again the variation in capacitance with potential was very similar for the two electrodes. With Kelsil, however, a much greater frequency dependence is observed, the capacitance increasing ca. 7-fold upon decreasing the measurement frequency from ca. 500 to 5 Hz (Figure 6). This dispersion is much greater than observed for either the silver disk or the Kelgraf electrode. At higher frequencies, the Kelsil capacitance appears to converge on that observed for the silver disk, ca. 30 pF/cm2 (Figure 6). The capacitance we observe at the silver disk is very similar to that reported by Sevast’yanov et al. (29) in 0.10 M NaC10, (32 pF/cm2 at 210 Hz and -0.5 V). The difference in the frequency dependence of the capacitance at the Kelgraf and Kelsil electrodes apparently reflects a difference in microscopic surface roughness or porosity of the respective active regions. Examination by SEM of individual carbon and silver particles from the powders used to prepare the composites reveals a significant difference in their shapes. The graphite particles are very thin, flat platelets, ranging in dimension from ca. 5 to ca. 1pm. The silver particles, on the other hand, are more spherical in shape, spanning a size range from ca. 3 to smaller than 0.3 pm. It is reasonable to suggest that the graphite platelets on the Kelgraf composite surface become oriented during polishing (and possibly to some extent during fabrication) so that their planar shapes are largely parallel to the electrode surface, thereby reducing the degree of surface roughness and porosity within each graphite region. Furthermore, the platelets could readily slide over one another, even to the extent of becoming dislodged completely from the graphite region and becoming embedded in the surrounding polymer. This could explain the significant amount of nonconducting graphite detected on the Kelgraf surface by the XPS experiments. On the other hand, the SEM and XPS experiments reveal that very few of the rather spherical silver particles of the Kelsil composite are dislodged from the conducting silver regions during polishing. High-resolution SEM (7) reveals the particles within a silver region to be tightly packed, but with voids between particles, as expected for particles of nearly spherical geometry. Solution can penetrate these voids, thereby increasing the interfacial area contributing to the observed capacitance. A t higher frequency (or faster scan rates) the increased charging current results in substantial iR drop within the interparticle voids, due to the highly resistant solution path within such very small volume elements. This in turn reduces the potential change at the interface within these voids, therefore reducing the effective area and apparent capacitance of the electrode. At higher frequencies, the observed capacitance should decrease and approach more closely that observed at the silver disk electrode, consistent with the data of Figure 6. A model for the capacitance at a microscopically porous electrode will be presented elsewhere (30). The frequency dependence of the Kelsil capacitance could also originate from microscopic cracks between the Kel-F polymer and the surface silver regions (microelectrodes), analogous to that postulated for other microelectrode assemblies (31). Such cracks might develop during fabrication or surface polishing, a result of differences in the coefficients of thermal expansion of Kel-F and silver. However, the rather high coefficient of linear thermal expansion of Kel-F (ca. 60 X lo4 C-I) is actually closer to that of silver (ca. 19 X lo4 C-l) than to that of carbon (ca.2.4 X lo4 C-l), and the development of such cracks should, therefore, be more prevalent with Kelgraf electrodes. The capacitance data of Figure 6 does not
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support this conclusion. Furthermore, even if microscopic cracks do exist (and we do not see any evidence of such cracks under high-resolution SEM), the hydrophobic character of Kel-F would likely preclude the penetration of aqueous solution to any significant depth within the cracks. We conclude that the difference in the low frequency dispersion of the capacitance of Kelsil and Kelgraf composites reflects a fundamental difference in the microscopic structure of the surface conducting regions of these two composite materials. We further conclude that the Kel-F polymer does not significantlypermeate these conductor regions or otherwise fill the interparticle voids within these regions.
CONCLUSIONS With the results reported here, we are beginning to obtain a more complete understanding of the surface morphology of Kel-F composite electrodes and the influence of surface morphology on electrochemical behavior. The extent to which the conclusions reached in this work apply to other types of composite electrode materials remains to be seen. Truly the surfaces of these composite materials are complex, with extensive possibiIities for physical and chemical interactions between the conductor and insulating polymer phases. Nevertheless, composite electrodes will continue to find extensive use in electroanalytical measurement, and research in the Tallman group will continue to explore and exploit the unique features of these versatile materials.
ACKNOWLEDGMENT We wish to thank Steven Meyer and Janice Glasgow of 3M Co. and Samuel Newman of the Fargo USDA Laboratory for obtaining the element maps of the composite electrode surfaces. We are also grateful to 3M Co. for donating the KelF-81 used in this work. Registry No. PCTFE,9002-83-9; Ag, 7440-22-4; graphite, 7782-42-5.
LITERATURE CITED (1) Klsslnger, P. T.; Refshauge, C.; Drelling, R.; Adams, R. N. Anal. Left. 1973,6,465. (2) Anderson, J. E.; Tallman, D. E. Anal. Chem. 1976,48. 209. (3) Klatt, L. N.; Connell. D. R.; Adams, R. E.; Honlgberg, I. L.; Price, J. C. Anal. Chem. 1975,47, 2470. (4) Armentrout, D. N.; McLean, J. D.; Long, M. W. Anal. Chem. 1979, 5 1 , 1039. ( 5 ) Nagy, G.; Feher, 2s.;Pungor, E. Anal. Chim. Acta 1970,5 2 , 47. (6) Anderson, J. E.; Tallman, D. E.; Chesney, D. J.; Anderson, J. L. Anal. Chem. 1978,5 0 , 1051. (7) Petersen, S. L.; Tallman, D. E. Anal. Chem. 1988,60, 62. (8) Morris, J. B.; Schempf, J. M. Anal. Chem. 1959. 3 1 , 286. (9) Clem, R. G.; Sciamanna, A. F. Anal. Chem. 1975,47, 276. (10) Sykut, K.; Cukrowskl. I.; Cukrowska, E. J . Eiecfroanal. Chem. Interfacial Electrochem. 1880, 115, 137. (11) Sleszynski, N.; Osteryoung, J.; Carter, M. Anal. Chem. lB84, 5 6 , 130. (12) Chesney, D. J.; Anderson, J. L.; Weisshaar, D. E.; Tallman, D. E. Anal. Chim. Acta 1981, 124, 321. (13) Weisshaar, D. E.; Tallman, D. E. Anal. Chem. 1983,5 5 , 1146. (14) Tallman, D. E.; Welsshaar, D. E. J . Liq. Chrometogr. 1983, 6 , 2157. (15) Cope, D. K.; Tallman, D. E. J . Elecfroanal. Chem. Interfac/ai€/ecfrochem. 1986,205, 101. (16) Cox, J. A.; Kulkarnl, K. R. Talanta 1986. 33,911. (17) Engstrom, R. C.; Johnson, K. W.; DesJarlais, S. Anal. Chem. 1987, 59, 670. (18) Engstrom, R. C.; Pharr, C. M.; Koppang. M. D. J . Electroanal. Chem. Interfacial Nectrochem. 1967,221. 251. (19) Aklns, D. L.; Birke, R. L. Chem. f h y s . Left. 1974,2 9 , 428. (20) Hampson, N. A.; Larkln, D.; Morley, J. R. J . Electrochem. SOC.1967, 114, 817. (21) Chen, 1.4.; Johnson, W. B. J . Mater. Sci. 1986,2 7 , 3162. (22) Anderson, J. E. Ph.D. Dissertation, North Dakota State University. 1979. (23) Evans, J. F.; Kuwana, T. Anal. Chem. 1977,49, 1632. (24) Evans, J. F.; Kuwana, T. Anal. Chem. 1979,5 1 , 358. Sci., Polym. Chem. (25) Clark, D. T.; Comarty, B. J.; Dllks, A. J . Po”. Ed. 1978, 16, 3173. (26) Clark, D. T.; Harrison, A. J . Polym. Sci., Polym. Chem. Ed. 1981, 19, 1945. (27) Evans, J. F.; Gibson, J. H.; Moulder, J. F.; Hammond, J. S.;Goretskl, H. Fresenius’ 2.Anal. Chem. 1984,319, 841. (26) Oldham, K. B. J . Elecfroanal. Chem. Inteffacisi Electrochem. 1981, 122, 1.
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(29) Sevast’yanov, E. S.; Ter-Akopyan, M. N.; Chubarova, V. K. Nekrrokhim@ 1980, 76, 432. (30) Shepherd, G.; MacKellar, W. J.; Petersen, S. L.; Tallman, D. E., North Dakota State Universlty, 1988, unpublished work. (31) Wehmeyer, K. R.; Wightman, R. M. J . Nectroanal. C h m . Interfacial Electrochem. 1985, 196, 417.
RECEIVEDfor review February 8,1988. Accepted July 11,1988. This work was supported by the U.S. Department of the
Interior/Geological Survey through Grant 14-080001-Gl441, administered through the North Dakota Water Resources Research Institute as Project No. 6582 (D.E.T.), and by the National Science Foundation, Grants fJHE-g411000 (R.C.E.) and CHE-8711595 (D.E.T.). A portion of this work was performed a t the Regional Instrumentation Facility for Surface Science, originally funded by the National Science Foundation under Grant CHE-7916206.
Potentiometric Adenosine Triphosphate Polyanion Sensor Using a Lipophilic Macrocyclic Polyamine Liquid Membrane Yoshio Umezawa,* Masamitsu Kataoka, a n d Wako Takami
Department of Chemistry, Faculty of Science, and Division of Environmental Conservation, Graduate School of Environmental Science, Hokkaido University, Sapporo 060, Japan Eiichi Kimura,* T o h r u Koike, a n d Hiroko Nada
Institute of Pharmaceutical Science, Hiroshima University School of Medicine, Kasumi, Minami-ku, Hiroshima 734, Japan
A llpophlllc macrocycllc polyamine, 15-hexadecyl1,4,7,10,13-pentaaracyclohexadecane (C,,H,,-[ 16]aneN5), was used to construct a potentlometric llquld membrane sensor for adenosine triphosphate polyanlon (ATP4- and/or HATP3-). The membrane potential was strongly dependent on the pH of the solutlon due to the proton uptake of [le]aneN, at the membrane surface. This proton uptake seems prerequlslte for the potential response for polyanlons. The M linearity of the log C vs E curve ranges from lo-’ to wlth a slope of -14.5 mV/decade In a HEPES buffer (pH 6.7). The extent of Nernstlan or sub-Nernstlan slopes of the present electrode toward ATP4- and/or HATP3-, ADP3-, and AMP*was experimentally correlated wlth the association constants of each nucleotlde wlth [16]aneN5 and also wlth a solvent extraction sequence for the same system. Observed selectivities agalnst ADP”, AMP2-, HPO?-, and monovalent anions were evaluated. The response mechanism of the present electrode was briefly discussed In terms of the formation of strong Ion palrs between polyanlons and the protonated [16]aneN5 at the membrane surface.
Recognition and binding of ionic substrates by organic host molecules are of vital importance in analytical chemistry. Most studies so far have centered on the recognition of alkali and alkaline-earth metal cations by such host compounds as macrocyclic polyethers and other types of ionophores ( I , 2). Recently, certain kinds of macrocyclic polyamines were found to behave like “receptors” of biological polyanions such as adenosine triphosphate (ATP4- or HATP3-), adenosine diphosphate (ADP”), adenosine monophosphate (AMP2-), and polycarboxylates occurring in the catabolic tricarboxylate cycle (3, 4 ) .
In the present study, we have prepared a liquid membrane incorporated with a lipophilic macrocyclic polyamine for a potentiometric sensor for these biological polyanions, particularly for ATP4- and/or HATP3-. Most liquid membrane ion-selective electrodes (ISEs) for anions are the so-called ion-exchanger type, and few studies have been performed on
neutral carrier and charged carrier type anion sensors. Recently, some interesting neutral carrier anion sensors based on an organotin compound (5) and trifluoroacetyl-p-butylbenzene (6) have been reported. In addition, charged carrier type sensors using lipophilic derivatives of vitamin B12(7,8) and metalloporphyrin compounds (9) have been reported for nitrite and thiocyanate ions. To our knowledge, the present paper marks the first time that lipophilic macrocyclic polyamines have been studied as active components for ISEs. Compared to macrocyclic polyethers, a fundamentally different feature of macrocyclic polyamines is their ability of proton uptake to form polycation macrocycles. This unique characteristic of macrocyclic polyamines would promise designing a variety of anion receptor sensors if the ring size and the charge density of the protonated polyamine host molecules are properly tailored to meet the desired host-guest interactions. EXPERIMENTAL SECTION Reagents. The lipophilic 16-membered macrocyclic penta(abamine, 15-hexadecyl-1,4,7,10,13-pentaazacyclohexadecane breviated as C16H33-[16]aneN,) (4) was synthesized as follows: C,6H33-substituted malonate 1 (18.8 g, 53 mmol) and linear pentaamine 2 (10 g, 53 mmol) were refluxed in dry MeOH (1L) for ca. 3 weeks. Evaporation of the MeOH solvent in vacuo and recrystallization of the residue from CH3CN yielded Cl6HS3-dioxo[16]aneN5(3) (yield 14 g, 55%): mp 131-132 “C; IR (KBr) 1660 cm-’ (vc=o). ‘H NMR (in CDC13, 35 OC, Me4Si reference) 6 0.7-1.0 (br, 3 H, -CH,), 1.0-2.0 (br, 30 H, C-CH,-), 2.0-2.2 (br, 3 H, -NH-), 2.4-2.8 (br, 12 H, N-CH,-), 2.8-3.1 (br, 1H, -CH=), 3.2-3.4 (br, 4 H, CON-CH,-), 7.0-7.3 (br, 2 H, -CONH-). The desired compound 4 was obtained by reducing 3 with B,& in THF (refluxing for overnight). The reaction mixture was treated with 6 N HCl and made into alkaline solution with NaOH. 4 was extracted into n-hexane and purified by recrystallization from CH3CN-MeOH: yield 40%; mp 134-135 OC; ‘H NMR (in CDC13, 35 O C , Me4% reference) 6 0.7-1.0 (br, 3 H, -CH3), 1.0-1.4 (br, 30 H, C-CH,-), 1.8-2.1 (br, 5 H, -NH-), 2.2-2.8 (br, 21 H, N-CH2-, -CH=). Commercially available salts and buffer reagents were purchased from Wako Pure Chemicals Co. (Tokyo, Japan). Adenosine triphosphate (ATP,pK5 = 6.5), adenosine diphosphate (ADP, pK4 = 6.3), and adenosine monophosphate (AMP, pK3 = 6.2) stock solutions were prepared and stored in a refrigerator.
0003-2700/88/0360-2392$01.50/00 1988 American Chemical Society
