1549
Anal. Chem. 1988, 60, 1549-1553
Irreversible Self-Assembly of Monomolecular Layers of a Cobalt(I I) Hexadecyltetrapyridylporphyrin Amphiphile at Gold Electrodes and Its Catalysis of Oxygen Reduction Dean A. Van Galen' and Marcin Majda* Department of Chemistry, University of California, Berkeley, Berkeley, California 94720
An amphiphiiic cobalt( I I)-tetrapyridyiporphyrin derivative (C&otPyP) was synthesized and its adsorption on gold surfaces from acidic aqueous electrolytes was Investigated by a spectrophotometric assay. A limiting coverage of (8.6 f 0.6) X moi/cm2 was determined and interpreted based on the pressure vs surface area isotherms from a Langmuir trough, as a close-packed monolayer of the porphyrin rings oriented parallel to the electrode surface. A hydrophobic layer consisting of the hexadecyi chains of the amphiphiie is formed between the adsorbed porphyrin moleties and the electrolyte solutlon. We postulate that the presence of this hydrophobic region induces the observed irreversibility of the C,,CoTPyP adsorption by hindering the kinetics of the adsorption/desorption processes. The nonamphiphilic analogues of C,,CoTPyP do not adsorb under the same conditions. Rotating disk electrode measurements revealed full catalytic activity of the adsorbed amphiphile in the eiectroreduction of O2 to H20,. A rate constant of ca. 1 X 105 ~ - 5-1 1 was obtained, in agreement with the literature data for other cobalt-porphyrin derivatives adsorbed at electrodes.
There have been many reports of the electrocatalysis of dioxygen reduction in aqueous solutions by immobilized metalloporphyrins at electrode surfaces (see for example ref 14-17 for the review of earlier work). In the cases involving adsorption of a metalloporphyrin, the choice of the electrode material was limited to either pyrolytic graphite or glassy carbon electrodes apparently due to a unique interaction between metalloporphyrins and carbon surfaces. Kuwana and co-workers suggested that this may involve the interaction of oxygen functionalities on the carbon surface with the axial position of the adsorbed metalloporphyrin (18),while Anson and co-workers have demonstrated the high affinity of aromatic molecules for graphite surfaces (15). Our approach is unique in demonstrating irreversible adsorption of a catalytically active metalloporphyrin on gold electrodes. More importantly, on the basis of this research and other examples of the formation of stable surfactant monolayers (7, 8), the self-assembly of amphiphiles seems to be a general method of reagent immobilization in organized monolayers at electrodes.
EXPERIMENTAL SECTION 5-(l-Hexadecyl-
Syntheses, Reagents, a n d Materials.
pyridinium-4-yl)-l0,15,20-tris(4-pyridyl)-21~,23~-porphine-
Deliberate attachment of reagents to surfaces via covalent binding and irreversible adsorption are well-established methods of chemical modification of electrodes ( I ) . A special class of these types of methods involves Langmuir-Blodgett techniques where a well-organized monolayer film of an amphiphile is transferred from the water/air interface of a Langmuir trough, where it was initially spread and compressed under controlled surface pressure, to the electrode surface (2-6). Well-ordered, monomolecular layers of amphiphiles can be also formed at solid surfaces by the self-assembly process where mere exposure of a surface to a solution of an amphiphile is involved (7,8). The significance of the amphiphilic monolayers at interfaces in biomimetic research, surface science, and several other areas of technological importance (e.g. lubrication, friction, and microelectronics) has generated renewed interest in the self-assembly phenomenon. Self-assembly of aliphatic trichlorosilanes (Bll),carboxylates (12),and disulfides (13)from hexadecane and other nonpolar solvents has been a subject of many investigations. In most cases, the extent of ordering in the self-assembled layers resembles that of well-organized Langmuir-Blodgett films. In this study, we report on the self-assembly of a hexadecyl derivative of cobalt tetrapyridylporphyrin (c&OTPyP) on gold electrodes in aqueous solutions. The self-assembly leads to an irreversible immobilization of Cl6CoTPyP a t the electrode surface and preserves its full catalytic activity with respect to oxygen reduction.
* Author to whom correspondence should be addressed.
Present address: Division of Science, Northeast Missouri State University, Kirksville, MO 63501.
cobalt(I1) bromide monohydrate (CI6CoTPyP)was prepared by using a modification of literature procedures as follows (19). The monoquaternization of meso-tetrapyridylporphyrin(TPyP) was accomplished by refluxing 600 mg of TPyP and 340 mg of 1bromohexadecane in 600 mL of DMF under N2for 4 h. The DMF was then removed by vacuum distillation and the residue was washed with ether ( 5 20-mL portions) to remove unreacted 1bromohexadecane and dried under vacuum at 40 "C overnight. The solid was dissolved in 10 mL of methanol and the solution was vacuum filtered three times t o remove unreacted TPyP. Purification was accomplished by gel fitration chromatography using a Sephadex LH-20 column with methanol as the eluting solvent. Two bands appeared upon elution. The first band, which was believed to be a chlorin due to its green color, was discarded. The second band, burgundy in color, was collected. The methanol was removed, and the product was dried under vacuum. Metalation of the hexadecyl derivative of TPyP was carried out following the procedure described by Adler and co-workers for the preparation of metalloporphyrins (20). Accordingly, 20 mL of DMF containing 50 mg of CIGTPyPand 70 mg of CO(NO~)~.~H,O was refluxed for 7 h under argon. The reaction was monitored by UV-vis spectrophotometry. The reaction was cooled in an ice bath and 20 mL of chilled water was slowly added. After the mixture was stirred for several minutes, the product was collected by vacuum fitration, washed with water, and dried under vacuum at 40 "C overnight. The identity of the resulting material was confirmed as the bromide salt of CI6CoTPyP.H20by fast-atom bombardment mass spectrometry (experimental m / e 916 of the parent ion). Elemental analysis data produced only an approximate confirmation of the compound's purity, probably due to the presence of residual DMF solvent. Insertion of cobalt(I1)into tetrapyridylporphyrin to form CoTPyP was done in an analogous manner. Cobalt(I1) tetrakis(4-N-methy1pyridyl)porphyrin (CoTMF'yP) was prepared by following a literature procedure (21). Exposure of all porphyrin solutions to direct light was minimized
0003-2700/88/0380-1549$01.50/00 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 15, AUGUST 1, 1988
I
10,
Table I. Spectrophotometric Assay of C&oTPyP Adsorbed on Gold Surfaces electrolyte
[C,,CoTPyP]/rM
10%/mol cm-*
0.1 M HC1, 0.1 M KCl
15.OU 0.58 0.12 6.0" 3.0"
8.6 f 0.6 7.6 f 0.6 2.6 +L 0.6 6.8 f 0.8 7.4 f 1.5
0.1 M HzSOj 0.1 M CF&OOH
Concentration of a saturated solution. , I
30
60
90
120
time I rnin
Figure 1. Dependence of the C,&oT!JyP coverage on gold surfaces vs time of exposure to a 15 pM amphiphile solution in pH 1.O HCI, 0.1 M KCI. The data were obtained by the spectrophotometrlcassay (see
Experimental Section). by using a protective aluminum foil cover. Water was purified by passing house-distilled water through a Barnstead Nanopure I1 purification train. All other chemicals were reagent grade and used as received. Gold foils, used as substrates in the C1&oTPyP adsorption studies, were prepared by vapor deposition of an approximately 3 pm thick layer of gold onto glass slides. The gold layer can be easily separated from the glass substrate by peeling it off with a razor blade. The resulting foil was cut into 2.00 cm X 2.25 cm rectangles. The surface roughness of these foils was 20% as determined electrochemically by measuring the charge due to the reduction of gold oxide generated in an anodic oxidation under controlled conditions (22) and comparing it to the gold oxide reduction charge on vapor-deposited gold films on highly polished single crystal silicon wafers (23). The roughness factor of the latter surfaces was assumed to be 1.1 (13). Thus, the surface area of each gold foil was 10.8 cm2. Monolayers of porphyrins were spread at water/air interfaces from ca. 15 pM chloroform solutions. The subphase was 0.5 M NaClO,. Molecular areas for the spread molecules were calculated from pressurearea isotherms resulting from the addition of known aliquots of the stock solution. This procedure minimizes errors due to impurities that are present at the interface prior to addition of the porphyrin stock solution. Spectrophotometric Assays. The ,A values and the corresponding molar absorptivities, c, in DMF for the porphyrin molecules under study are as follows: C16CoTPyP (437 nm, c 85500 M-' cm-'), CoTPyP (433 nm, 62300 M-' cm-'), and CoTMPyP (427 nm, e 77900 M-' cni'). These molar absorptivities were obtained from Beer's law plots which were linear for porphyrin concentrations of 10 pM or less. All the porphyrin solutions used in the analyses were below the 10 pM level. Solutions of porphyrins from which adsorption on gold surfaces was studied (loading solutions) were prepared by dissolving the porphyrin in 25-200 mL of the appropriate solution (see Table I). Larger volumes were used with loading solutions of lower concentration to ensure that the adsorption process did not appreciably altor the solution concentration of porphyrin. Sonication and gentle heating were used to obtain a saturation concentration of porphyrin in the loading solution, when desired. Gold foils or electrodes were submerged in the stirred loading solution for a controlled period of time. The gold substrates were then transferred to a large volume of porphyrin-free loading solvent and gently swirled to remove porphyrin that was not adsorbed to the gold. Gold electrodes were then directly used for electrochemical experiments. Gold foils were transferred to a vial containing 1.0 mL of DMF. The vial and contents were sonicated for ca. 2 min to ensure that all of the adsorbed porphyrin dissolved in the DMF. The DMF solution was syringed into a small volume quartz cell and the UV-vis spectrum was recorded. Knowledge the molar absorptivity of the absorbance of this solution at A,, and the surface area of the gold foil of the porphyrin at A, allowed calculation of the surface coverage of the porphyrin on the gold substrate,.'I The detection limit for porphyrin adsorption with this procedure was 5 X lo-'* mol/cm2.
Instrumentation. UV-vis spectra were obtained by using a Shimadzu UV-160 scanning spectrophotometer. Electrochemical experiments were performed with a PAR Model 173 potentiostat/galvanostat, a PAR Model 175 universal programmer, and a PAR Model 179 current follower. Rotating disk electrode experiments were carried out with an IBM EC/219 electrode rotator. A Veeco Model 7700 vacuum deposition system was used to prepare gold foils and electrodes. Langmuir experiments were done with a KSV 2200 Langmuir trough (KSV-Chemicals, Finland).
RESULTS AND DISCUSSION Adsorption of Cl6CoTPyP on Gold Surfaces. The structure of the hexadecyl derivative of cobalt(I1) tetrapyridylporphyrin (C16CoTPyP) synthesized for the studies described in this paper is shown below. In solutions of pH
1.0 used in this study, the three nonquaternized pyridyl groups are expected to be protonated (18), thus the hydrophilic portion of this amphiphilic molecule is quadruply charged. C&oTPyP and its nonamphiphilic analogues CoTPyP and CoTMPyP are not electrochemically active in aqueous acidic solutions at gold electrodes. In order to study their adsorption, we developed a spectrophotometric assay involving ca. 5-cm2 sections of gold foil as described in the Experimental Section. The adsorption of c&oTPyP on the sections of gold foil (total surface area ca. 10.8 cm2) was carried out from different aqueous pH 1.0 solutions with stirring for a prescribed period of time. The gold substrate was then transferred to an identical electrolyte solution free of the porphyrin amphiphile and gently swirled for several seconds to rinse porphyrin that was not absorbed. The adsorbed C16CoTPyP was then dissolved by sonicating the gold foil in DMF for several minutes. The concentration of porphyrin in this solution was determined spectrophotometrically. This then allowed calculation of the surface coverage, I?, of C16CoTPyP on the gold surface. The surface coverage of C&OTPyP on gold was measured as a function of loading time from a stirred pH 1.0 HC1/0.1 M KCl solution containing a saturation concentration of C16CoTPyP (15 pM). Figure 1 shows that the gold surface coverage increases with time until an approximately constant coverage is achieved for loading times of about 60 min or longer. A series of seven independent determinations gave an average value of r = (8.6 f 0.6) X lo-" mol/cm2 under these conditions. This value does not depend strongly on the C16CoTPyP concentration in solution. As seen in Table I, l7 = 7.6 f 0.6 was obtained when the experiment was repeated in 0.58 pM C&oTPyP, a concentration 26-fold lower than the saturation level above. A lower coverage is obtained when
ANALYTICAL CHEMISTRY, VOL. 60, NO. 15, AUGUST 1, 1988
the solution concentration of the amphiphile is lowered to the 0.1 pM level (see Table I). A significantly longer time was allowed in the last two cases to ensure that the extent of adsorption was not time dependent. On the basis of these data, the maximum coverage reported here can be considered as the limiting coverage for the C&oTPyP adsorption on gold. Experiments were next performed to investigate how porphyrin structure and solution conditions affected the adsorption. Specifically, are the amphiphilic nature of C1&oTPyP and the identity of the counterions in the electrolyte important in facilitating adsorption on the gold surfaces? The adsorption of the nonamphiphilic analogues CoTPyP and CoTMPyP, was studied by using procedures identical with those used in the ClaCoTPyP experimenta. No detectable adsorption of CoTPyP or CoTMPyP from their 1 mM solutions was observed after the transfer to the porphyrin-free electrolyte. This observation is consistent with the lack of previous reports of O2electrocatalysis with cobalt porphyrins adsorbed onto metal surfaces and points out the importance of the hexadecyl chain of ClaCoTPyP in facilitating its adsorption. Since the hydrophilic porphyrin moiety of the adsorbed c 1 6 c o ~ molecule P is quadruply charged in pH 1.0 solutions, there are likely counterions associated with the molecule at the electrode surface. Specifically chloride ions, which are known to be strongly adsorbed a t gold electrodes, could play a role in the porphyrin adsorption via electrostatic interactions (24,W).To test this possibility, the adsorption of c16coTPyP on gold was studied in various p H 1.0 electrolyte solutions. Sulfuric and trifluoroacetic acids were chosen as electrolytes for these studies since adsorption of SO?- or CF3COO- onto gold is expected to be weak (26).The results of the spectrophotometric assay as listed in Table I show that stable adsorption of C1&oTPyP in pH 1.0 H2S04and CF,COOH solutions does occur and that surface coverages are similar to those seen in HCl/KCl solutions. This result indicates that Cl6CoTPyP adsorption does not depend strongly on the identity of the counterion, leaving the amphiphilic nature of C1&oTPyP as the dominant factor in stabilizing adsorption of the porphyrin amphiphile on the gold surfaces. Orientation and Packing Density of ClsCoTPyP on Gold. The limiting surface coverage value obtained by the spectrophotometric assay for the adsorption of C16CoTPyP on gold is roughly three times greater than typical surface coverages for similar mononuclear cobalt macrocycles adsorbed on carbon surfaces. For example, the surface coverages for cobalt(I1) tetrasulfonate phthalocyanine adsorbed on a stress-annealed pyrolytic graphite electrode (27) and for CoTMPyP adsorbed on a glassy carbon electrode (18)have been reported as 3 X lo-" mol/cm2 and (2-3) X 10-l' mol/cm2, respectively. On the basis of a geometric model (neglecting the hexadecyl chain), if the porphyrin ring of C1&oTPyP lies parallel to the gold surface, a molecular area of approximately 236 A2 would be expected. This corresponds to a surface coverage of 7.0 X lo-" mol/cm2. This calculation assumes an area equal to the square of the maximum length of the molecule and assumes no contribution from counterions that may coadsorb. On the other hand, a perpendicular orientation of the porphyrin rings to the surface should give a molecular area of ca. 50 A2resulting in a surface coverage of 3.3 X mol/cm2. In order to better understand the orientation and the packing density of C16CoTPyP molecules adsorbed on gold surfaces, we carried out Langmuir experiments with a nonamphiphilic analogue, CoTPyP (28).In these experiments, stable monolayer films of CoTPyP were spread and compressed under controlled surface pressure at the water/air interface of a Langmuir-Blodgett trough by first carefully
L
I
I00
I
I
I
1551
I
200 300 400 500 Molecular Area (X2/moiecuie)
Surface pressure vs area per molecule isotherm for CoTPyP spread on 0.5 M NaCIO,. T = 21.0 'C. The extrapolation gives the limiting surface area per molecule of 230 A'. Figure 2.
syringing an aliquot of a chloroform solution containing CoTPyP onto the surface of an aqueous 0.5 M NaC104 subphase. A typical surface pressure vs area per molecule isotherm for CoTPyP is shown in Figure 2. Initially, as the CoTPyP monolayer is compressed there is little increase in the surface pressure until the molecules experience a density corresponding to about 250 A2/ molecule. As compression continues, the surface pressure rapidly increases reflecting the increased intermolecular repulsions in the tightly packed monolayer. Pressures as great as 25 dyn/cm can be exerted reversibly on the film without collapse of the monolayer. Extrapolation of the rapidly rising portion of the isotherm to zero pressure gives a value of AIim. The value of AIirnfor a monolayer of CoTPyP calculated as an average of three isotherms is 230 f 7 A2/molecule, giving a packing density of (7.2 f 0.2) X 10-l' mol/cm2. The agreement between this value and that predicted on the basis of molecular model calculations indicates that the orientation of CoTPyP is coplanar with the water/air interface in the Langmuir experiment. Furthermore, the similarity of the I? values for C16COTPyP on the gold surface as determined by the spectrophotometric assay with these geometric and Langmuir surface coverages allows us to postulate that the adsorption of C1&oTPyP on gold surfaces results in a close-packed monolayer in which the porphyrin rings are coplanar with the gold surface. These results indicate that the hexadecyl chains of C16CoTPyP are not directly adsorbed at the gold surface. Instead, they form a hydrophobic layer between the adsorbed porphyrin moieties and the solution. This is consistent with a simple observation that the gold surface, which is initially well wetted by water, becomes hydrophobic when it is withdrawn from a CleCoTPyP solution. We postulate that the hydrophobic layer created this way is a key factor responsible for the irreversible character of the adsorption of the porphyrin amphiphile. We envision that the hydrophobicity in the adsorption layer decreases the extent of hydration of the adsorbed head groups which ultimately hinders the kinetics of the adsorption/desorption processes and renders the process irreversible as observed. Recently, we reported the irreversible adsorption of an octadecylviologen amphiphile, which forms a close-packed monolayers at gold surfaces (8).Other electroactive amphiphiles show similar behavior, which seems to be a general property of amphiphilic monolayers on solid surfaces in aqueous solutions (7). Electrocatalysis of 0, Reduction by C&oTPyP Adsorbed on Gold Electrodes. Two cyclic voltammograms obtained a t a gold electrode in an air-saturated (0,concentration = 0.24 mM) pH 1.0 HCl, 0.1 M KCl solution are shown in Figure 3. The dashed-line voltammogram was obtained with an untreated gold electrode while the solid-line trace corresponds to the same gold electrode with a monolayer of adsorbed C1&oTPyP. At the untreated gold electrode the E3,4value for O2 reduction is -0.31 V, the wave being su-
1552
ANALYTICAL CHEMISTRY, VOL. 60, NO. 15, AUGUST 1, 1988
f , ,
,
0.6
0.4
t
0.2
,
,
I
0
j
,
l
L
-0.2 -0.4
E / V vs, Ag/AgCI (0.1M C I - )
-
Figure 3. Cyclic voltammograms of O2reduction at gold electrodes (A = 0.071 cm2)in air-saturated 0.1 M KCI, 0.1 M HCI, pH 1.0, v = 50 mV/s: dashed line, clean gold electrode;continuous line, the same electrode wRh an adsorbed monolayer of C,,CoTPyP.
40.0-
0
0.12
0.16
I s-"2
Flgure 5. A Koutecky-Levich plot of the limiting current density values obtained in the experiment of Figure 4. The continuous line in the plot represents a calculated current-rotation rate response for the twoelectron, mass transport limited reduction of 02.
20.0-
0.4
02
0
-02
-04
E / V v s Ag/AgCI (0. I M C I - )
Figure 4. A series of the RDE voltammograms of O2 reduction catalyzed by a monolayer of C,&oTPyP adsorbed at the gold disk ( A = 0.071 cm2). The solution conditions are the same as in Figure 3. Y = 10 mV/s. The rotation rate values in revolutions per minute are given in the figure.
perimposed on the rising background caused by the reduction of protons. It can be seen that the presence of a ClaCoTPyP monolayer at the electrode catalyzes the oxygen reduction, shifting the E3,4 value positively by 300 mV to -0.01 V. The potential of the catalytic wave is similar to that reported for such catalysis by cobalt(I1) porphyrins on carbon surfaces. This suggests that the reduction potential of the Co(II)-02 adduct, which is the species being reduced, does not depend strongly on the material of the electrode surface. As mentioned in the previous section, the adsorbed Cl6CoTPyP exhibited no detectable electrochemistry in the accessible potential window of 0.7 to -0.5 V. This is not surprising since the kinetics of the cobalt(III)/cobalt(II) porphyrin couple are sluggish, particularly in solutions of low pH, which often makes the direct electrode reactions of metalloporphyrins difficult to observe (29). While the potential of catalyzed O2reduction is one measure of the effectiveness of a catalyst, the kinetics and overall mechanism of the catalysis are both of practical and fundamental importance. This type of characterization can be obtained by the rotating disk electrode (RDE)methods. Figure 4 shows a family of RDE voltammograms recorded at several rotation rates o for the reduction of O2 in pH 1.0 HC1/0.1 M KC1 solution at a gold rotating disk electrode modified by adsorption of a ClsCoTPyP monolayer. The corresponding Koutecky-Levich plot derived from the limiting current density values at -0.2 V is shown in Figure 5. The equation describing the Koutecky-Levich plot is the following (30): l/& =
0.08
(**w]-"2
3600-
30.0i 3
0.04
l/ik
+ 1/0.62nFD2130'i2v-'i6C*
(1)
The first term on the right-hand side is the inverse of the
kinetic current density discussed below. The second term is the inverse of the Levich current density, which is limited by the mass transport of O2to the electrode surface. In this term n is the number of electrons transferred in the overall electrode reaction. D and C* are the diffusion coefficient and the solution concentration of 02, and v is the kinematic viscosity of the electrolyte solution. An important mechanistic parameter of the O2 electroreduction is the number of electrons transferred in the overall process. Its knowledge allows one to distinguish between the 2e-, 2H+ mechanism leading to H202 and the 4e-, 4H+ mechanism in which oxygen is reduced directly to H20. These two possibilities can be distinguished by examining the slope of the Koutecky-Levich plot which is inversely proportional to n (see Figure 5). The solid line in Figure 5 represents the line calculated for the two-electron reduction of O2 to HzOz. cm2/s was used.) For the calculation, the (Do, = 1.7 X exact value of v was obtained experimentally from an analogous RDE control experiment done with the same gold electrode (untreated) in a K,Fe(CN), solution of known concentration. The agreement of the slope obtained experimentally with ita calculated value in Figure 5 is a strong indication that the electroreduction of dioxygen catalyzed by the adsorbed Cl6CoTPyP proceeds by the 2e-, 2H+pathway to HzOz. The actual n value obtained experimentally was 2.1 f 0.1 for three different electrodes. This mechanistic assignment is consistent with the literature data for a number of cobalt(I1) porphyrin derivatives (18, 29). The intercept of the Koutecky-Levich plot in Figure 5 is dependent solely on the kinetics of the catalyzed O2reduction since its value is a result of an extrapolation to an infinite rotation rate which assures no mass transport limitation. In view of eq 1,one can express the inverse of the intercept value as the kinetic current density, i k , by the following equation:
ik = nFkI'C*
(2)
where k is the second-order rate constant describing the reaction of O2with the immobilized catalyst and r is its surface coverage. According to the scheme proposed by Durant and Anson, the rate-limiting step of the catalytic reaction is the binding of O2to the cobalt porphyrin at the surface (30). The O2-Co(II)porphyrin adduct is rapidly protonated and reduced. The value of r for the gold rotating disk electrodes was assumed to be the same as the one found for the gold foil substrates (r = 8.6 X mol/cm2) since identical procedures
Anal. Chem. 1988, 60, 1553-1562
for the C16CoTPyPadsorption were used in both cases. The average value of the rate constant obtained from three different electrodes based on the Koutecky-Levich analysis and eq 2 was (9.2 f 0.9) X lo4 M-’s-l. This value is in good agreement with those reported by Anson and co-workers for oxygen catalysis by cobalt(I1) porphyrins immobilized at graphite electrodes (typical values of k were (1-2) X lo6 M-l s-l) (15,29). This agreement indicates full catalytic activity of our amphiphilic porphyrin adsorbed at the electrode surface. In order to determine the stability of adsorption of C16CoTPyP at a gold electrode, the treated electrode was rotated at 500 rpm for an extended period of time in an air-saturated HCl/KCl solution with an applied potential of 0.3 V. After 3 h, a voltammetric measurement such as that shown in Figure 3 showed no change in the size or position of the catalyzed O2reduction peak, indicating negligible desorption of the catalyst. Some adverse effect of H2O2generation during the steady-state experiments on the catalyst was indicated by a 10% decrease in the RDE limiting current at 400 rpm which was observed after recording the sequence of the RDE voltammograms shown in Figure 4. CONCLUSIONS The experiments described here demonstrate that the adsorption of the cobalt porphyrin amphiphile is a convenient strategy for producing stable monolayer films of a catalyst at the electrode surface. The kinetic analysis of the system’s performance, which showed full catalytic potency of the immobilized catalyst, substantiate our postulate of the wellorganized character of the adsorption layer. The stable self-assembly of amphiphilic monolayers at solid surfaces in aqueous media appears to be a general phenomenon which could be applied to other systems with additional benefits derived from the ordered nature of the surface assemblies. Registry No. C&oTPyP, 114763-57-4;CoTMPyP, 79346-65-9; CoTPyP, 14244-55-4;Au, 7440-57-5; 02, 7782-44-7; H202, 7722-84-1.
1553
LITERATURE CITED (1) Murray, R. W. Electroanalytcal Chemlshy; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (2) Fromherz, P.; Arden, W. J. Am. Chem. SOC. 1980, 702, 8211. (3) Dalfuku. H.; Aoki, K.; TokNa, K.; Matsuda, H. J. Nectroanal. Chem. Interfacial Electrochem. 1985, 183, 1. (4) Daifuku, H.; Yoshlmura, I.; Hirata, I.; Aoki, K.; Tokuda, K.; Matsuda, H. J . Nectroanal. Chem. Interfacial Electrochem. 1986, 199, 47. (5) Facci, J. S.; Falcigno, P. A.; Gold, J. M. Langmulr 1986, 2, 732. (6) Fujihira, M.; Pooslttlsak. S. J. Nectroanal. Chem. Interfaclal Electrochem. 1986, 199, 481. (7) Faccl, J. S. Langmulr 1987, 3 . 525. (8) Wldrig, C. A.; Majda, M., submitted for publication in J. Phys. Chem. (9) Maoz, R.; Sagiv. J. J. Colloid Interface Sci. 1984, 700, 465. (IO) Gun, J.; Iscovicl, R.; Saglv, J. J. Colloid Interface Sci. 1984, 7 0 7 , 201. (11) Gun, J.; Saglv, J. J. ColloidInterface Sci. 1986, 172, 457. (12) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 7 , 45. (13) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J Am. Chem. Soc. 1982, 709,2358. (14) Forshey, P. A.; Kuwana, T. Inorg. Chem. 1983, 22, 699. (15) Durand, R. R.; Bencosme. C. S.; Collman, J. P.; Anson, F. C. J. Am. Chem. SOC. 1983, 705, 2710. (16) NI, C.-L.; Abdalmuhdl. I.; Chang, C. K.; Anson, F. C. J . fhys. Chem. 1987, 91, 1158. (17) Bettelhelm, A.; White, B. A.; Murray, R. W. J. Electroanal. Chem. 1887, 217, 271. (18) Bettelheim, A.; Chan, R.J.H.; Kuwana, T. J. Nectroanal. Chem. Interfacial Nectrochem 1978, 99, 39 1. (19) Okuno, Y.; Ford, W. E.; Calvin, M. Synthesis 1980, 7 , 537. (20) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem. 1970, 3 2 , 2443. (21) Hambright, P.; Fleischer, E. B. Inorg. Chem. 1970. 9 , 1757. (22) Oesch, M.; Jgnata, J. Electrochem. Acta 1983, 28, 1237. (23) Miller, C. J.; MaJda, M. J . Nectroanal. Chem. 1988, 207, 49. (24) Clavlller, J.; Van Huong, N. J. Electroanal. Chem. 1973, 4 7 , 193. (25) Bellier, J. P. J. Nectroanal. Chem. 1982, 3 0 , 13. (28) Anson, F. C. Acc. Chem. Res. 1975, 8 , 400. (27) Zagal, J.; Sen, R. K.; Yeager, E. J. Electroanal. Chem. 1977, 8 3 , 207. (28) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (29) Durand, R. R.; Anson, F. C. J. Electroanal. Chem. 1982, 734, 273. (30) Bard, A. J.; Faulkner, L. R. Nectrochemical Methods; Wiley: New York. 1980; Chapter 8.
.
RECEIVED for review September 3,1987. Accepted March 16, 1988. The support of this research was provided by the National Science Foundation under Grant CHE-8504368.
Theory of Interfacial Potential Differences: Effects of Adsorption onto Hydrated (Gel) and Nonhydrated Surfaces James R. Sandifer Corporate Research Laboratories, Eastman Kodak Company, Rochester, New York 14650
Potential differences are generated across soiid/soiution interfaces when ions that are initially electrostatically bound to “sites” on the surface (or within the bulk of the solid phase) tend to dissolve In the solution, leaving the oppositely charged sites behind. Based on simple theoretical considerations, rationales are given to explain the effects of concomitant adsorption of both charged and uncharged species from the soiutlon and also to predict the effects of having sites buried in the bulk of the sdld rather than confined only to its surface. The results address questions concerning coated wire electrode responses, ISFET and RefFET (ion-selective and reference field effect transistors, respectively) responses, chemical modification of common ISFET gate materiels to achieve selectivities to different Ions, callbration of conventional pH glass electrodes, and the limitations of surfacemodified fiber optic approaches to pH measurements.
A considerable volume of literature has addressed the question of how interfacial potential differences that are analytically meaningful can be developed, particularly with regard to the mechanism of ion-selective field effect transistor (ISFET) (1-6) and coated wire electrode (CWE) (7) responses. This question has importance beyond ISFETs and CWEs, however, and extends to surface-modified fiber-optic pH sensors as well, as pointed out recently by Janata (8). Such devices rely on color changes at their surfaces induced by bulk solution pH variations. The indicator dye is thus separated from the bulk of the solution by the various planes of ionic charge (Helmholtz layers (9)),which influence the interfacial potential difference. The result is that the pK, depends upon ionic strength and adsorption, as discussed by Janata (8). Two theoretical approaches to treating interfacial potentials have been taken, one by workers in the ISFET field, the other
0003-2700/88/0360-1553$01.50/00 1988 American Chemical Society
