754
Anal. Chem. 1987, 59, 754-760
Mediated, Thin-Layer Cell, Coulometric Determination of Monomolecular Films of Trichlorosilane Viologen Derivatives at Gold and Nonconducting Surfaces Cindra A. Widrig and Marcin Majda*
Department of Chemistry, University of California, Berkeley, California 94720
A thln-layer, mediated, coukmetrlc titration method Is developed for the determlnatlon of electroactlve reagents ImmoMlized at nonconductkrg surfaces. The method was used to verify the monomolecular quantltles of N-methyl-N-( 2(trimethoxy~l)et)))bensyd4,4'-Mpyrkllnl~ dlchkwlde and of other vlologen sllane derlvatives lmmoblllzed at gold electrodes. The direct electroreduction gave surface coverages on average 50% smaller than those Owalned by the mediated coulometrlc method. The lack of complete electroacthrlty of the surface-bound materlal In customary voltammetrlc experlments was conflrmed spectrophotometrlcally. This unexpected characteristic Is related to the lntetfaclal structure of the sllane layers of whlch some fragments appear to be passivating to the cUrect heterogeneous electron transfer. The hlgh density of the lateral siloxane brldglng groups In such areas was postulated to be responslble for thls effect. The mediated double layer charglng of an electrically unconnected, Conducting Interface Is described and Illustrated by two examples.
Chemical modification of surfaces and studies of chemically bonded interfaces const3ute a broad area of interdisciplinary research (1-7). The application of silane reagents in these studies is ubiquitous because of their pronounced reactivity with surface hydroxyl groups present on many different solid surfaces and the availability of silane reagents with a wide range of different functional groups (4,7,8). The structure of silanized interfaces is of crucial importance as it determines the behavior of the system. For example, the structure of an alkylsilane layer attached to chromatographic packing material such as porous silica gel determines solute retention in reverse-phase liquid chromatography (5, 9-11). In thfs field, the importance of a detailed understanding of the bonded phases including such elements as the surface density of the alkyl chains and their distribution, proximity, and dynamics prompted extensive investigations of these systems by thermogravimetric analysis (12) and fluorescence (13-16), photoacoustic ( l a ,infrared (18-ZO), and NMR (21,22) spectrometries. Electrochemistry is another area where chemical modification of surfaces via silanization has been used extensively and where attachment of electroactive species to electrode surfaces and related electrochemical studies developed into a separate, rapidly growing subdiscipline (23). Applications of chemically modified electrodes in electrocatalysis (24), in electroanalysis (25), as photodisplay devices (26), in chiral electrosynthesis (27), and in photoelectrochemistry (Z8,29) have been demonstrated and provided major impetus for the development of the field. As in the case of chemically derivatized surfaces in chromatography, the structure and composition of thin electrode films are the key factor in determining the electrochemical properties of the modified electrodes (23). Electrochemical methods, surface electron spectroscopies and spectrophotom0003-2700/87/0359-0754$01.50/0
etry have been used primarily for the characterization of derivatized electrode surfaces (23). The main advantages of electrochemical techniques lie in their excellent sensitivity and their ability to provide direct quantitative and qualitative information concerning the electroactive components of the modifying layers created on a conducting surface of an electrode. In many cases, however, electrode modification involves films of nonconducting supports, e.g., polymers (30) and solid-state materials such as oxides (31),zeolites (32),and clays (33) into which electroactive reagents are introduced by means of chemical derivatization. In those cases not all of the introduced electroactive centers may be electrochemically addressable by the underlying electrode. Yet the knowledge of the quantity of the electroactive material immobilized in a nonconducting support film is necessary for the correct interpretation of the electrochemical experiments. In this report we present a method for the electrochemical determination of the quantity of a surface attached electroactive reagent. Surfaces used for the immobilization need not be conducting. The method consists of coulometric titration of a surface bound reagent with an electroactive mediator generated a t a working electrode of a thin-layer cell. The surface carrying the immobilized reagent to be determined constituted the opposing wall of the thin-layer cell. The immobilization procedure was based on the direct silanization of surfaces with trichloro- and trimethoxysilane derivatives of methyl viologen (34) as well as on a two-step process involving an amide coupling of a carboxyl derivative of methyl viologen to an amine moiety surface bound via silanization (35). The quantities of the bound reagents ranged approximately from IO-*oto mol cm-2. The calibration experiments involved silanization of gold surfaces followed by both the direct and the mediative determination of the quantity of attached reagent. In the direct determination the silanized gold surface was used as an electrode and the determination was done by the usual integration of the voltammograms due to the attached viologen. In the mediative determination the silanized gold surface was not under potential control; it served as the opposing wall of a thin-layer cell in which reactive mediators were generated at a parallel gold surface acting as an electrode. These experiments led to a surprising finding that, on average, only half of the electroactive centers assayed by the mediative coulometry are electroactive in a direct electrochemical experiment in which the silanized gold surface was used as an electrode. This conclusion was supported by the spectroelectrochemical measurements in which the absorption band a t 275 nm characteristic for the oxidized form of methyl viologen exhibited only partial decrease upon the direct electroreduction of the immobilized viologen centers. These measurements are discussed in view of a model which assumes partial passivation of the interface by the high density of siloxane bridges formed laterally within the modifying layer.
EXPERIMENTAL SECTION Reagents and Syntheses. l-(Trichlorosilyl)-2-((chloroethyl)phenyl)ethane,its trimethoxy derivative and N-(2-amino0 1987 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59,
ethyl)(3-aminopropyl)trimethoxysilane (later referred to as amine-silane) were obtained from Petrarch Systems, Inc., and distilled under vacuum immediately prior to use. Ferrocene was sublimed at room temperature under reduced pressure. Anthraquinone was recrystallized from glacial acetic acid. N,N’DirnethyL4,4’-bipyridinium dichloride (methyl viologen) was obtained from Aldrich Chemical Co., Inc., and recrystallized from methanol. Its perchlorate salt was formed by precipitation with NaC104from aqueous solution followed by recrystallizationfrom water. High-purity acetonitrile was obtained from Burdick and Johnson Laboratories, Inc., and was stored over 3A zeolite particles (Davison Chemical Co.). In critical cases the dryness of the electrolyte solutions in the electrochemical cell was assured by an addition of alumina powder (Alumina N-Super I from ICN Biochemicals, Inc.). Water was purified by passing house-distilled water through Barnstead Nanopure I1 purification train. Tetrabutylammonium perchlorate was used as received from G. F. Smith, Inc. All other chemicals were reagent grade. The synthesis of N-methyl-N-(2-(trimethoxysilyl)ethyl)benzyL4,4’-bipyridinium dichloride (referred to later as trimethoxysilylviologen) was carried out according to a literature procedure (34). Its iodide, chloride, trichlorosilyl derivative was synthesized according to the same procedure. The product was washed with diethyl ether and hot acetonitrile. N-Methyl-N-pcarboxyphenyL4,4’-bipyridiniumiodide (later referred to as carboxysilylviologen) was synthesized by refluxing N-methyl4,4’-bipyridinium iodide with 4-(chloromethyl)benzoic acid in acetonitrile for 24 h. The precipitated product was collected and recrystallized from methanol. The synthesis of N-methyl-4,4’bipyridyl iodide was carried out by prolonged refluxing of 4,4’bipyridine with methyl iodide in toluene followed by the usual workup of the precipitate. Surface Reactions. Surface silanization reactions were carried out on gold surfaces prepared by vapor deposition of approximately 800 A thick gold films onto 2.54 X 2.54 cm2 float glass slides. The same gold substrates were also used as electrodes as described in the following subsection. An 80-A chromium layer was deposited first during the same pump down cycle to improve the adhesion of the gold. Prior to all surface derivatization reactions, the gold surface was pretreated by immersion for several minutes in hot chromic acid, rinsing with water and methanol, and drying with a heat gun and/or in a vacuum oven at 70 “C. This procedure produces a hydrophilic gold surface which, due to either the presence of the gold oxide film or an adsorbed hydroxyl layer, is reactive in the subsequent silanization reactions. The gold substrates were transferred to a glovebox in an evacuated glass jar in order to prevent contamination by trace quantities of pump oil vapors in the glovebox antechamber. The gold substrates were reacted with ca. 5 mM solutions of trimethoxysilylviologen in a 75/25 acetonitrile/methanol or with a similar solution of trichlorosilylviologen in 0.5 M NaC10, in acetonitrile. The surface attachment of the trimethoxyaminesilane compound was done in toluene. Subsequently,the gold substrates were rinsed thoroughly with methanol. All surface reactions were continued from 2 to 24 h without noticeable influence on the resulting surface coverage. The silanization of the aluminum oxide surfaces (thermally oxidized, vapor deposited A1 films) followed the same procedure. All solvents used in these reactions were freshly distilled and deaerated by several freeze-pump-thaw cycles. After the transfer to the glovebox, the solvents were stored over molecular sieves. The substrates silanized with the amine-silane derivative were subsequently reacted overnight in 10 mM solution of the carboxylviologen in water in the presence of 20 mM 1(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride at pH 6.5. Afterward, the electrodes were rinsed and dried in a vacuum oven at 50 OC. In order to produce electrode films containing larger quantities of the electroactive silane, a 5% solution of trimethoxysilylviologen in 75/25 CH3CN/CH30Hwas spin-coated at 900 rpm onto a gold substrate while it was washed with a stream of water-saturated nitrogen for 2 to 5 min (34). The substrate was then dried in a vacuum oven at 50 OC for 2-4 h. Thin-Layer Cell and Spectroelectrochemical Experiments. The thin-layer cell consisted of two closely spaced glass slides. One of them was a 2.54 X 2.54 cm2 float glass slide with the disk working electrode (0.37 cm2 surface area) and a ring
NO. 5, MARCH 1, 1987 755
position of the AI spacers
2.54 cm
-1
I-
-
Figure 1. Vapordeposition mask pattern used in the fabrication of the gold electrodes for the thin-layer cell experiments (see the Experimental Section).
counter electrode (0.58 cm2 surface area) produced by the gold vapor deposition as described above. The electrode pattern mentioned here is shown in Figure 1. The radial distance between the working and the counter electrode is 0.17cm. The electrical contacts were made with an electronic edge connector through the contact pads included in the pattern. The other glass slide of the thin-layer cell covered only the upper half of the glass slide with the evaporated electrode pattern and was spaced with two pieces of aluminum foil (25 Mm in thickness) or with two sections of vapor deposited aluminum film (8-15 pm in thickness). The position of the spacers is indicated in Figure 1. Both glass slides were held together with stainless steel clips. The thin-layer assembly was operated in a conventional single-compartment Pyrex cell with a silver wire quasi-reference electrode. After the deaeration of the mediators and the supporting electrolyte solution, the thin-layer cell assembly was lowered down so that the bottom edge of the assembly touched the surface of the electrolyte solution bringiig the solution into the thin-layer space by capillary action. For the mediative, thin-layer cell determinations mentioned in the introductory section, two closely spaced gold surfaces are required, in principle: one which remains at open circuit and which carries an attached electroactive reagent, and the other acting as a working electrode. In practice, some of these experiments were done with only one gold surface in a thin-layer cell which acted as a working electrode and to which an electroactive reagent was attached. This is possible since the attached silane layer is easily permeable to the mediators in solution. This modification was introduced in order to avoid an interference due to the mediation of the double layer charging of the electrically unconnected gold surface. The detailed account of this phenomenon is given in the last subsection of the paper. The spectroelectrochemical measurements were done in the diffuse reflectance mode. The same electrode substrates as those shown in Figure 1 were placed in a conventional spectroelectrochemical cell equipped with a quartz window parallel and closely spaced to the electrode surface. The quartz window was masked so that only the disk of the working electrode was visible. The window of the cell was then pressed against the sample port of the integrating sphere of the diffuse reflectance attachment of the spectrometer. The background spectrum of the gold electrode in the electrolyte solution was recorded each time in reference to barium sulfate and subtracted digitally from the subsequent scans. A spectral bandwidth of 2 nm was used in all experiments. Instrumentation. All electrochemical measurements were done with a PAR Model 173 potentiostat/galvanostat, a PAR Model 175 universal programmer, and a PAR Model 179 current follower. The integration of the thin-layer voltammetric current was done digitally on an Apple IIe computer interfaced to the potentiostat by a commercial interface card (Adalab A113 Analog Input System with a fast analog-to-digital (A/D) converter by Interactive Microware, Inc.). The integration software provided
756
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987 lo
r
8
r
30
6
t
A 4
* .> a
2
0
*
a \
.-
-IO
-20 -30 -40
\I
1 t
\I
t
\WI
I
-0.4
"
I
I
I
I
'
-0.6 -0.8 -1.0
I
-1.2
0.4
0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0
E / V vs Aq/Aq+(O.IM)
E / V vs. A q / A g + ( O . I M )
Flgure 2. Cyclic voltammogram of a silylviologen film immobilized at a gold electrode: 0.1 M tetrabutylammonlum perchlorate in CH,CN; v = 100 mV s-'.
for an adjustable background subtraction of the voltammetric curves by a quartic fit to an arbitrary number of points on both sides of the voltammetric peak. Each time, a number of background fits were examined until a satisfactory result wm produced. The accuracy of the obtained background current was verified visually. Diffuse reflectance spectra were obtained using a Perkin-Elmer Lambda 9 UV-Vis-near-IR spectrometer with a 60-mm integrating sphere attachment. A Veeco Model 7700 vacuum deposition system and tungsten sources were used to prepare electrode substrates. The surface silanization reactions were done in a conventional glovebox (Vacuum Atmospheres, Inc.).
RESULTS AND DISCUSSION The general scheme used in the determination of the quantity of electroactive species immobilized at a surface which may be nonconducting is based on the indirect coulometric titration similar to those used, for example, in the studies of redox proteins (36). This approach was also used recently to assay charge stored in thick polypyrrole films (37). The surface silanized with a silylviologen derivative (MV,2+) is used as an opposing wall of a thin-layer cell where it is positioned parallel to the working electrode across a solution layer 10-25 Km in thickness. The electrolyte solution contains a low level of methyl viologen (MV2+),a precursor of a mediator titrant. The coulometric titration involves then the following reactions:
MV2++ e- = MV'+ MV'+
(1)
+ MVS2+= MV2+ + MV;+
The effectiveness of the mediator titrant in reducing the surface bound viologen centers depends primarily on its redox potential relative to that of the immobilized reactant and also on the kinetics of the mediation reaction. The latter is known to be very fast in homogeneous solutions (38)and was assumed to be fast in the present case. The redox potential of methyl viologen was found to be 110 mV more negative than the redox potential of the surface bound viologen silane. The later was measured directly by cyclic voltammetry with a silanized gold electrode (see Figure 2). This difference is sufficient to assure complete one-electron reduction of the surface methyl viologen. At the same time is not large enough to allow for the reduction of the surface species to a neutral diradical. The second reduction wave of the surface methyl viologen is ob-
Flgure 3. Thin-layer cell cyclic voltammograms of the mediated, coulometric determination of a silylviologen film immobilized at an aluminum oxMe surface. Ferrocene (-0.5 mM) and methyl viologen (-0.5 mM) are used as mediitors. MV2+'+ and MV$+/+ are the redox
potentials of the methyl viologen in solution and that attached to the surface respectively: 0.5 M tetrabutyhmmonium perchlorate In CH,CN; v = 20 mV s-'.
served at a potential 710 mV more negative than the first one. On the basis of this thermodynamic argument, it is also clear that methyl viologen can act only as a reducing mediator and that the full reoxidation of the surface reactant requires another mediator with a sufficiently more positive redox potential. Ferrocene (Fc) was used as the oxidizing mediator. The following reactions complete then the full cycle of the reduction and the oxidation of the surface immobilized species: Fc - e- = Fc+ Fc+
+ MV,"
= Fc
+ MV,"
(3) (4)
This mediated reduction oxidation sequence of reactions is illustrated in Figure 3. All of the charge due to the reduction of surface viologen is passed at the reduction potential of the reducing mediator. The reoxidation of the surface species is mediated to some extent by both mediators. The use of two mediators in this experiment also provides a means of internal calibration of the measurement and removes, conveniently, the requirement of an exact knowledge of the thickness of the thin-layer cell. Thus the quantity of surface-immobilized viologen is obtained from the charge under the second reduction wave (Figure 3) by subtracting from it the charge due to the reduction of the reducing mediator alone. The latter is obtained, in turn, from the charge under the ferrocenium reduction wave in the same experiment and the prior knowledge of the concentration ratio of the standard mediators' solution. The concentration ratio of the reducing and oxidizing mediators was calculated from a blank thin-layer experiment. As always in the case of coulometric measurements, the accuracy of this coulometric assay is high and depends only on the knowledge of the concentration ratio of the two mediators. The precision of the determination is related to the precision of current integration and the signal to noise ratio of the thin-layer voltammograms. The latter depends on the ratio of the charge due to the reduction of the surface species to that of the reducing mediator as well as the level of the background current. The overall absolute precision of a single determination such as shown below is approximately 2 X lo-" mol cm-2.
757
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH I , 1987
Table I. Direct and Mediated Coulometric Determination of Methyl Viologen Coverage at Gold Electrodes (Monomolecular Silane Films) electrode no. 1 2
3 4 5 6 7 8 9 10 11 12
13 14 15 16
17 18 19 20 21 22
23 24 25 26 27 28
29 30 31 32
lolordb&:
mol cm-2
I/
t
0.08-
1O1'rmediated,LI
mol cm-2
C13SiCH2CH2PhCH2MV2+ 2.0 4.0 5.0 18.0 1.0 2.3 4.0 6.2 2.3 5.7 4.3 7.1 2.0 3.2 1.8 4.9 4.1 8.8 3.9 7.8 3.3 8.5 8.2 14.2 6.9 15.0 7.1 17.0 1.2
2.2
5.1
22.0
(CH30)3SiCH2CH2PhCH2MV2t 3.6 6.5 3.6 7.7 4.2 9.1 4.1 9.7 1.4 9.4 1.6 2.4 3.1 3.8 1.5 2.9 3.0 3.0 3.0 3.6 2.1 4.6 2.7 3.8 3.1 4.0 3.1 5.6 3.5 5.6 3.3 4.6
0.06-
rmediated/ rdirsct
-
t
R
b
2.0 3.6b 2.3 1.6
2.5 1.7 1.6 2.7 2.1 2.0 2.6 1.7 2.2
0.007-
2.4 1.8
4.3b 2.1 k 0.4c 1.8 2.1 2.2
2.4 6.gb 1.5 1.2
1.9 1.0 1.2 2.2
1.4 1.3 1.8 1.6
1.4 1.7 f 0.4c
R
0.004-
0.00I 0 300
400 k (nm)
500
Flgure 4. UV-VIS spectra of (A) methyl viologen (5 X lo-' M) in acetonitrile solution of tetrabutylammonium perchlorate (lo-' M) and (6) silylviologen fllm (5.5 X lo-'' mol/cm-*) Immobilized at a gold electrode in the acetonitrile solution of the supporting electrolyte. Both sets of spectra were recorded in a thin-layer spectroelectrochemicai cell in a diffuse reflection mode with a working electrode held at (a) -0.1 V, (b) -0.9 V vs. Ag/Ag+ (0.1 M), and (c) background spectrum.
taining the utmost dryness of the reagents and the reaction As is well-known in the literature, however, the environment. C13SiCH2CH2CH2NHCH2CH2NHCOPhCH~MV2+ strong tendency of the trichloro- and trimethoxysilanes to 1 4.1 9.5 2.3 cross-link by siloxane bridges results frequently in surface 2.1 2 5.8 12.0 attachment of the quantities in excess of one monolayer (23). 3 7.0 17.0 2.4 The value of the monolayer coverage is difficult to define 2.3 f 0.2c rigorously. It could be taken as approximately 2 X mol "All of the experiments were carried out in the acetonitrile cm-2. For the silane compounds typically used in the electrode electrolyte. The absolute precision of a single measurement is 2 X modifications (231,this value represents the surface density lo-" mol cmb2. bThese measurements were considered to be of molecules lower than that predicted by a geometrical close anomalous and were not included in the calculation of the average pack modeling. At the same time, the actual distribution of ratio. The average values of the ratio l'm~iated/l'dbect. the molecules at the surface may already involve surface Direct and Mediated Coulometric Determination of domains containing more than monolayer quantities because Viologen Silane Immobilized at Gold Electrodes. The of the lateral siloxane cross-linking. Considering this unindirect coulometric method of quantifying the surface-bound certainty and the uncertainty of the measurement of the electroactive reactant was tested by binding viologen silanes surface roughness factor, which we estimate to be ca. 1.2 for directly to gold electrode surfaces and then carrying out both our electrodes (31), the majority of the electrodes listed in the direct (based on the integration of the voltammetric peak Table I have coverage of one to two monomolecular layers of the silane viologen based on the direct coulometric meacurrents such as those in Figure 2) and the indirect, mediated surements. coulometric determination as described above. All the exIn more than 30 independent measurements, the coverage periments were done in acetonitrile solution. The data of the three series of such experiments are of the same electrodes obtained by the mediated coulometric presented in Table I. The first two series involved the method gave the value on average 1.9 times higher (Table I). one-step surface binding of the trichloro- or trimethoxysiThese data provide strong evidence that not all of the surlylviologen (see the Experimental Section). The last three face-bound viologen centers are electroactive in the direct entries in Table I represent a two-step attachment strategy electrochemical experiments. The confirmation of this conwhere, in the first step, an amine-silane is immobilized at the clusion was obtained from a spectroelectrochemical experielectrode surface and then a carboxyl derivative of methyl ment where the completeness of the reduction of the viologen viologen is coupled via an amide bond. The surface coupling sites was monitored by observing a decrease of absorption a t reactions were carried out with the goal of limiting the 275 nm, a band characteristic for the oxidized form of the quantity of attached reagent to a single monolayer by mainbound viologen. Figure 4A shows absorption spectra of methyl
758
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
viologen in an acetonitrile solution in ita oxidized and reduced state obtained in a thin-layer spectroelectrochemical experiment done in a diffuse reflection mode. After equilibration of the system following the application of a sufficiently negative potential to reduce all of the viologen ions to the cation radical form, the absorption band at 260 nm disappears and the bands at 385 and 600 dominate the spectrum (39). Figure 4B shows a spectrum in a crucial range around 300 nm of a mol cm-2 layer of viologen silane coupled to a gold 5.5 x electrode before and after its electroreduction. The fact that the spectrum, which was recorded while the electrode was polarized to a reducing potential, still exhibits substantial absorbance a t 275 nm correlates well with our finding that approximately half of the viologen centers cannot be reduced directly by the underlying electrode. There have been only a few literature reports that raised the question of whether the voltammetrically obtained coverages reliably represent the total quantity of all the electroactive centers in silane layers. In one such report, Lenhard and Murray combined electrochemical measurements with the X-ray photoelectron spectrometry (XPS) data in the investigation of a ferrocene derivative coupled to a monolayer of a trimethoxy(alky1amine)silane on platinum electrodes (40). The conclusion that all ferrocene centers are electroactive was deduced based on the observation that the Fe 2pSjzXPS signal disappeared after the electrode was potentiostated at +0.7 V until complete decay of the voltammetric signal due to ferrocene. A good agreement of the electrochemical coverage and that obtained from the quantitative analysis of the XPS spectra was also obtained by Fischer et al. in the study of multimolecular coatings of a silylferrocene derivative (41). Finally, in an investigation of multilayers of an amine-silane with coupled porphyrin centers at Pt electrodes, fluorescence spectrometry of a porphyrin solution obtained after its cleavage from the silane surface was used to compare the electrochemically and spectrometrically obtained coverages (42). Again, a good agreement of both sets of data was obtained. In the last two cases, dichlorosilanes were used and the immobilization procedure resulted, deliberately, in multilayer coatings with substantial internal cross-linking. The stability of these films is not necessarily due to the siloxane bonding to the electrode surface but rather is a result of the internal cross-linking. In view of these literature data, our results are unprecedented in demonstrating lower coverage by the direct electrochemical measurement compared to the mediated coulometric method. The partial electroactivity of the surface viologen centers reported here is independent of the absolute magnitude of the surface coverage (Table I). Also, it appears to be related to the structure of the silane layer a t the very interface of the gold electrodes as the length and the polarity of the spacer arm separating the viologen center from the silane group do not seem to influence the ratio of the mediated to the direct coverage (see the last four entries in Table I). We have also examined a series of electrodes that were coated with larger quantities of silylviologen by a technique involving spin-coating of a silane solution onto the electrode surface followed by exposure to water vapor (see the Experimental Section). As mentioned above, this procedure is known to result in f i i i n g the electrode surface due to multiple internal cross-linking of the silane layer (34, 41, 42). The stability of the silane films in these cases derives from the internal cross-linking and does not necessarily involve siloxane bridges to the gold surface to a comparable extent as that seen in the direct surface silanizations. The data of the coulometric measurements are shown in Table 11. In this case there exists a close agreement between the results of both types of coulometric determinations even if significantly higher coverages
Table 11. Direct and Mediated Coulometric Determination of Methyl Viologen Coverage at Gold Electrodes (Polymeric Silane Films)
101'rdirect, mol cm-2
electrode no.
101'rmediated, mol cm-2
16 13 14
1
2
3
13 13 15 18
20 26 33
4
5 6 7
95
9
47
0.8 1.0 1.1 0.9 1.0
25
33 8.4 81 36
7.8
8
rmediated/rdirect
1.0 1.1 0.9
0.8 1.0 f 0.1"
'Average value. A
0
f .
e
I
I 0
I
0
0
///////////////////////// Electrode
0 I
'
/"'\
0
0
I
0 I
0
0
.................... Elect rode
Figure 5. Schematic structure of a siiylviilogen film at a gold electrode:
(A) an insulating domain containing high density of the lateral siloxane bridges at the interface; (6)an area fully electroactive in direct volt-
ammetric experiments. were produced and, therefore, more highly cross-linked films were generated than those discussed in Table I. Thus, these data provide an additional indication that the structure of the silane layer immediately adjacent to the gold surface, or more specifically the way the silane layer is bonded to the gold surface, is responsible for the partial loss of the direct electroactivity of the viologen centers observed in the case of the silanization of the gold surface hydroxyl groups. In order to explain these observations, we postulate that the silanized surface of gold is partially covered by randomly distributed, insulating patches of the silane layer equivalent, perhaps, to a few monolayers and containing at the interface a high density of the lateral siloxane bridges. A schematic drawing of such an area on the gold surface is shown in Figure 5A. Similar structures have been postulated in the literature (23). The insulating character of these areas may be due to a steric hindrance of the segmental motion of the spacer arms with the viologen centers in their bending motion which brings the electroactive centers close to the electrode surface and enables the electron transfer. The "floppy model" of the mechanism leading to heterogeneous electron transfer invoked here was originally proposed by Murray (43). The insulating
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
759
Table 111. Mediation of the Double Layer Charging
electrode no. mediator (1): ferrocene mediator (2): methyl viologen A E O ' = 0.83 V
Q(l)," pC cm-2 cd1(1)," pF cm-2
1
2 3
25.3 29.7 28.2
Q(2)," pC ern+
Cd](2))(lp F cm-2
24.7 29.3 24.9
29.8 35.3 30.0
30.5 35.8 34.0
Average Value of the Double Layer Capacitance, c d l = 33 f 2.8 fiF cm-' mediator (1): ferrocene mediator (2): anthraquinone AE"' = 1.32 V
1 2
3
39.7 47.0 37.0
30.1 35.6 28.0
41.5 43.0 35.1
31.4 32.6 26.6
Average Value of the Double Layer Capacitance, c d l = 31 f 3.2 pF cm-' "Q(l), Q(2), Cdl(l), and cdl(2) are the charge densities and the double layer capacitance values obtained from the integration of the charging current peaks of the voltammograms due to mediator (1)and mediator (2), respectively. character may also be due to an increased distance between the viologen centers and the electrode surface beyond the effective tunneling range. The rest of the surface, we postulate, is covered by the silane layer free of the high density lateral siloxane bridges (see Figure 5B) and therefore fully electroactive. Only these areas are seen as electroactive in the direct coulometric experiments according to this model. The viologen centers belonging to the insulating areas are addressable only chemically by the mediator titrants from the solution side. The additional charge used up in their reduction accounts for the observed ratio of the mediated to the direct coverage in Table I. The range of this ratio (1.2-2.7)indicates that the size, population density, and the thickness of individual insulating patches are varied. It is difficult to assess these parameters quantitatively. The electroactivity of the insulating domains in the direct voltammetric experiments can be, in principle, promoted by the lateral electron transport from the adjacent electroactive areas. This mechanism is apparently not very effective presumably due to a low diffusion coefficient of such lateral electron propagation and/or due to the size of the insulating surface domains since we did not observe any significant dependence of the charge under voltammetric curves in the direct experiments on the scan rate in the range from 5 to 100 mV s-l.
Mediation of the Double Layer Charging of an Interface at Open Circuit. The thin-layer cell experiments described in the previous section require, in principle, two closely spaced gold surfaces, one remaining at open circuit and carrying an immobilized electroactive layer and the other acting as a working electrode. The data presented above were actually collected in a cell where a separate working electrode was eliminated and where in the mediated determinations the silanized gold surface served as a working electrode. The course of the measurements and the data analysis were the same as described above. The elimination of the separate working electrode in these experiments is possible because the mediators are readily electroactive at electrodes covered with the silane monolayers. The shape of the recorded voltammograms is not different from that showed in Figure 3 because of the proximity of the redox potential of the reducing mediator and the surface viologen species. The main reason why these experiments were done as just described was to avoid an interference due to the mediation of the double layer charging of the electrically unconnected gold surface. The process of the mediated double layer charging is illustrated in Figure 6. Consider a thin layer voltammetric experiment done with two reversible electroactive species in solution (ferrocene and methyl viologen in the experiment of Figure 6). The thin-layer cell consists of two parallel gold surfaces, one of which acts as a working electrode and the other is electrically disconnected. The experiments are done on a time scale long enough to affect the entire quantity of the electroactive species in the cell. The
0.9
0.6
0.3 E
VS.
0
-0.3
-0.6
Ag (quasi-ref.)
Flgure 6. Thin-layer cell voltammograms of ferrocene (-0.5 mM) and methyl viologen (-0.5 mM) in a 0.5 M tetrabutylammonium perchlorate acetonitrile solution; v = 20 mV s-'. The cell consists of two goidcoated glass slides spaced at ca. 15 Mm. One of the gold surfaces serves as a working electrode and the other is at open circuit. The sharp current peaks at -0.30 and +0.45 V are due to the mediated double-layer charging of the gold surface remaining at open circuit.
first scan in the negative direction from 0.0 V, where Fc and MV2+ remain in their original oxidation states, begins the experiment. The first reduction and the oxidation of MV2+ produce the expected bell-shaped voltammetric waves (44). When after the scan reversal the potential reaches the vicinity of the Fc Eo' value, a distinct, additional current peak is observed on the raising portion of the anodic voltammogram. A similar current peak is reproducibly seen on the raising portion of the cathodic voltammogram of M V + beginning with the second potential cycle. These additional quantities of charge are due to the charging of the double layer of the unconnected gold surface over the potential difference equal approximately to the AEo' of MV2+/MV'+and Fc+/Fc. Each time the potential of the working electrode is swept past the Eo'value of one of the redox couples, the gold surface across the thin-layer gap is polarized to that potential due to the presence in the entire solution of both redox forms of an electroactive couple. In the potential range between the redox activity of MV2+and Fc, only one redox form of each couple exists in the solution and, temporarily, the working electrode cannot induce its potential onto the opposing gold surface. Its ability to do so is restored when its potential reaches a value sufficient to generate a missing half of a redox couple. In the case of the oxidation of ferrocene, for example, the newly generated ferrocenium ions become rereduced at the opposing gold surface after they diffuse across the width of the cell since that surface is still charged to a potential of ca. -0.3 V. This process accounts for the adjustment of the potential of the gold surface to a new value close to the Eo' of the ferrocene couple. As could be expected, the current due to the mediated
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
double layer charging is only observed when the immediately preceding electrode reaction was related to the oxidation or reduction of the species belonging to the other mediator. For example (see Figure 6), the mediation of the double layer charging is seen when the oxidation of Fc follows the oxidation of M V ; no effect is observed when the oxidation of Fc follows its reduction. In more general terms, the effect of the mediated double layer charging observed here is a pronouncement of kinetic disequilibrium of the system caused by the relative sluggishness of the diffusion processes compared to the rate of the potential sweep at the working electrode. Integration of the charging current peaks allowed us to estimate the integral capacitance of the double layer of the gold surface over the potential difference between the two Eo‘ values. The results are listed in Table 111. This estimate is not very accurate because of the “diffuse” character of the potential where the mediation of the double layer charging takes place and the resulting uncertainty in the value of the potential difference between the two mediation events. (The of the two couples was used, arbitrarily, in the calculations.) To our best knowledge, this effect, although entirely predictable, has never been reported before. We present these results as an illustration of the process and in order to demonstrate that the integral capacitance values obtained in this way are the same for two different pairs of redox mediators operating in a similar range of potentials. This substantiates our intepretation of the observed effect. In view of the main subject of this paper, the primary importance of dealing with this effect is to eliminate a significant interfering factor in the mediative, indirect measurements of the monolayer coverages at the conducting surfaces. The average value of the double layer charge over the potential range equal to the AE”’of the mediators is comparable to the equivalent charge of a single monolayer of an electroactive reagent.
a”’
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RECEIVED for review August 25,1986. Accepted November 11,1986. The support for this research was provided by the National Science Foundation under Grant CHE-8504368.
