Anal. Chem. 1995, 67,2767-2775
This Research Contribution is in Commemoration of the Life and Science of 1. M. Kolthoff (1894- 1993).
Permselectivity and High Sensitivity at Ultrathin Monolayers. Effect of Film Hydrophobicity Quan Cheng and Anna BrajtelcToth*
Department of Chemistv, Univetsify of Florida, Gainesville, Florida 32611-7200
The effect of monolayer structure and solution composition on electrochemical response at ultrathinmonolayers formed by self-assembly has been investigated. 'he monolayers studied consisted of thioctic acid (1,2-dithiolane-3-pentanoicacid) and 1-hexanethiol,where different degrees of surface hydrophobicity were obtained through the coassembly of these two molecules. On the more hydrophilic thioctic acid monolayers, fast kinetics and high electrochemical sensitivity were obtained for hydrophilic probes Fe(CN)e3-and R u ( N H ~ ) ~ ~Permse+. lectivity at the hydrophilic monolayerelectrodes could be achieved by controlling the extent of dissociation of the monolayer COOH head groups. As the hydrophobicityof the film increased with the coadsorption of hexanethiol, the kinetics of the hydrophilicprobes became slower. For more hydrophobic catecholaminesand quinone probes, slower kinetics (lower sensitivity) were consistently observed at the monolayer electrodes, independent of the monolayer composition. A simple modified Stem model is proposed to represent the relationship between the probe response, the monolayer structure, and the electrolyte composition. The role of the Donnan potential in controlling film permselectivityis addressed,and the roles of monolayer stability and substrate quality on the monolayer electrode response are discussed. Self-assembled monolayers (SAMs) of long alkyl chain thiols have recently attracted tremendous attention.' Applications related to interfacial phenomena such as heterogeneous electron transfer? surface ~ettability,~ metal ion preconcentration,4 protein imm~bilization,~ and molecular recognition6have been reported. SAMs offer a unique strategy for constructing stable, well-defined (1) (a) Dubois, L. H.; Nuzzo, R G. Annu. Rev. Phys. Chem. 1992,43,437. (b) Folkers, J. P.; Zerkowski. J. A.; Laibinis, P. F.; Seto, C. T.; Whitesides, G. M. Supramolecular Architecture: Synthetic Control in Thin Films and Solids: ACS Symposium Series 499; American Chemical Society: Washington, DC, 1993; pp 10-23 and references therein. (2) (a) Chidsey, C. E. D. Science 1991,251,919. (b) Chidsey, C. E. D.; Bertozzi, C. R;Putvinski, T. M.; Mujsce, A. M. J. A m . Chem. SOC.1990, 112,4301. (c) Miller, C. M.; Cuendet. P.; Gratzel, M.]. Phys. Chem. 1991, 95, 877. (d) Acevedo, D.; Abrufia, H. D. J. Phys. Chem. 1991,95,9590. (3) (a) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R G.; Allara, D. L.; Porter, M. D. Lungmuir 1988,4,365. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC. 1988,110, 3665. (c) Bain, C. D.; Evall, J.; Whitesides, G. M.J. Am. Chem. SOC.1989,111, 7155. (d) Laibinis, P. E.; Nuzzo, R G.; Whitesides, G. M. J. Phys. Chem. 1992,96,5097. (4) Steinberg, S.; Tor, Y.; Sabatani, E.; Rubinstein, 1.1. A m . Chem. SOC.1991, 113,5179. (5) Prime, K. I.; Whitesides, G. M. Science 1991,252,1164. 0003-2700/95/0367-2767$9.00/0 0 1995 American Chemical Society
structures on electrodes with controlled chemical For instance, adding hydrophilic head groups to the alkanethiol SAMs has been proposed not only to improve the quality of packing of the monolayerzc but also to provide a platform for chemical modfication of the surface to achieve selectivity and sensitivity. Several applications based on the properties of the terminal groups of such monolayers have been r e p ~ r t e d . ~An ~ ~ion , ~gate effect at a bis(wmercaptoundecy1) (C11) phosphate monolayer functioning in a pH-dependent manner was observed by Nakashima et a1.b Rubinstein et al. reported a monolayer assembly of 22'thiobis(ethy1 acetoacetate) (C4) on a gold electrode which selectively binds and reduces Cu2+in the presence of ferric ions.4 Another advantage of SAMs is the flexibility they provide in the control of film thickness. We have demonstrated that on short chain length self-assembled monolayers of thioctic acid (C5) (see Chart l), fast electron transfer as well as selectivity and high sensitivity can be achieved.9 Our group is particularly interested in the fabrication and manipulation of thin films which may have potential applications in b i o s e n ~ o r s . ~Unlike - ~ ~ many other probes, biological molecules can irreversibly interact with the electrode surface and are typically involved in more complex electron transfer p r o c e ~ s e s . ~At ~-~~ traditional polymer-coated electrodes, complicated morphology makes it difficult to identify the different effects that control the electrode response. A practical strategy for optimizing electrochemical detection is to develop a well-defined electrode surface at which surface features, such as charge and hydrophobicity, can be easily controlled. We have found thioctic acid (1,Zdithiolane3-pentanoic acid, abbreviated as TA) monolayers (of -8 A thickness) promi~ing;~ molecular orientation in the film can be maintained at the monolayer thickness of -8 As7 In the case of TA, this means that the disulfide group of the TA interacts with (6) (a) Nakashima, N.; Taguchi, T.;Takada, Y.; Fujio, IC;Kunitake, M.; Manabe, 0.J. Chem. SOC.,Chem. Commun. 1991,232. (b) Haussling, L.; Michel, B.; Ringsdorf, H.;Rohrer, H. Angetu. Chem., Int. Ed. Engl. 1992,30, 569. (c) Chailapakul, 0.; Crooks, R. M. Lungmuir 1993,9,884. (7) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559. (8) (a) Allara, D. L.; Nuzzo, R. G. Lungmuir 1985,1, 52. 00) Nuzzo, R G.; Fusco, F.; Allara, D. L. J. Am. Chem. SOC.1987,109,2358. (c) Nuzzo, R G.; Dubois, L. H.; Allara, D. L.]. A m . Chem. SOC.1990,112,558. (9) Cheng, Q.; Brajter-Toth, A Anal. Chem. 1992,64,1998. (10) Hsueh, C. C.; Brajter-Toth, A Anal. Chem. 1994,66, 2458. (11) (a) Marino, A; Brajter-Toth, A Anal. Chem. 1993,65,370. (b) Jaramillo, A; Marino, A; Brajter-Toth, A Anal. Chem. 1993,65,3441. (12) Amstrong, F. A; Hill, H. A. 0.; Walton, N. J. Acc. Chem. Res. 1988,21, 407. (13) Brash, J. L.,Horbett, T. A, Eds. Proteins at Inte&ces; ACS Advances in Chemistry Series 347; American Chemical Society: Washington, DC, 1987.
Analytical Chemistry, Vol. 67, No. 17,September 1, 1995 2767
Chart I. Model of the Thioctic Acid Monolayer on Au
the Au surface, the hydrophobic alkyl chains interact with each other, and the hydrophilic COOH head groups favorably interact with the aqueous solution they face. The firm attachment of TA to Au has been proven to be due to the strong chemisorption of TA as a result of the formation of two thiolate bonds with gold after S-S bond cleavage.I6 The control of the electrochemical response on the thin films shifts from electron tunneling,2aor spherical pinhole diffusion on the free sited4 of the compact films, to semiinfinite diffusion. Under these conditions, electron transfer at the electrode no longer limits the electrochemical r e s p ~ n s e .This ~ is significant in sensor design, since most biological molecules have sluggish kinetics on bare electrode surfaces.15 Electrode passivation by thick monolayers slows down electron transfer, resulting in small currents and poor sensitivity. On a thin monolayer, where the current is controlled by mass transport, these problems can be diminished? We report here an expanded study of thin monolayers of thioctic acid and mixed monolayers of TA and 1-hexanethiol (CS SH). Different degrees of surface hydrophobicity were obtained through the coassembly of these two compounds. Monolayers of alkanethiols with more than one functional group at the filmsolution interface have been studied by Whitesides and cow o r k e r ~ . ~Contact angle measurements have shown that the mixed monolayers of carboxylic acid-terminated thiols and alkanethiols display varying hydrophobicity based on the fraction of the carboxylic alkanethiol in the monolayer^;^^ COOH and CH3 belong to different categories of head groups since they exhibit quite different wettabilities. However, based on the wettability measurements, their mixed monolayers were reported to form homogene~usly,~~ and there was no observed segregation. The structural order of the TA monolayer on Au as a result of the TA/Au interactions described above, and the precise orientation of the adsorbed TA molecules on Au, are not known exactly. Surface reflectance IT-IR, which might be able to shed light on the surface structure, becomes less sensitive as the monolayer length decreases.' However, based on the molecular structure of TA (Chart l), the film is expected to be less closely packed than the long alkyl chain thiols and is expected to allow some degree of penetration by solvent and the electrolyte, particularly in the head group region, because of the absence of a second alkyl chain in the assymetric TA disulfide. To form films with (14) (a) Finklea, H. 0.;Avery, S.; Lynch, M.; Furtsch, T. Langmuir. 1987,3,409. (b) Finklea, H. 0.;Snider, D. A.; Fedyk. J. Langmuir 1990,6. 371. (15) Heller. A. Acc. Chem. Res. 1990,23,128. (16) (a) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6,87. (b) Biebuyck. H. A.; Whitesides, G. M. Langmuir 1993,9, 1766. (c) Hagenhoff, B.: Benninghoven, A; Spinke,J.; Liley. M.; Knoll, W. Langmuir 1993,9, 1622.
2768 Analytical Chemistry, Vol. 67, No. 77, September 7, 1995
different degrees of hydrophobicity, the 1-hexanethiol was coassembled with TA because the similarity of alkyl chain lengths of 1-hexanethiol and TA was expected to lead to the formation of mixed films with minimal surface roughness. Cyclic voltammetry was used to characterize the films in this study, where the focus was primarily on the determination of the effect of film structure on the electrochemical response of structurally different probes. Film capacitance was measured to obtain insight into the inner structure of the film, such as the degree of solvent and ion penetration, and the related dielectric constant of the monolayer. While wettability of long alkanethiol SAMs is the extensively studied parameter,16a partitioning of solvent (and ions therein) within the film is in fact as important; the electrochemical response of the monolayers consisting of short molecules is strongly influenced by electrolyte properties such as ion charge and size. Hydrophilic probes Ru(NH3),j3' and Fe(CN)63- were employed to investigate the electrochemistry at the monolayers, especially the effect of film composition on the electrochemical selectivity and sensitivity, under different solution conditions. The responses of probes with hydrophobic features, such as the catecholamines hydroxytyramine (dopamine, or DA) and 3,4dihydroxyphenylacetic acid (DOPAC), and benzoquinone (Q), were also tested. The structure of these probes made it possible to look at the effect of factors other than electrostatic effects on the electrochemical response of the monolayers. Finally, we report the results related to the stability and the memory effects of the films. Such effects may determine the value of the monolayers for practical use in biosensors. The requirement of smoothness of the substrate surface was found to be critical to the selectivity and is discussed as well. EXPERIMENTALSECTION Reagents. Thioctic acid was purchased from Aldrich. Hexaammineruthenium(II1) chloride [Ru(NH3)6C131 was purchased from Alfa Products. Potassium ferricyanide &Fe(NC)G) and benzoquinone were obtained from Fisher Scientific. Tris [tris(hydroxymethyl) aminomethane hydrochloride], dopamine, and DOPAC were purchased from Sigma. Potassium phosphate was used to make phosphate buffer solutions. All chemicals were used as received. Aqueous solutions were freshly prepared from doubly distilled, deionized water. Prior to use, solutions were purged with nitrogen for at least 5 min. Electrode Preparation. The electrode preparation procedure was described in a previous paper.g The electrode area was determined by chronocoulometry in 1 mM Ru(NHJ$+ in 0.1 M phosphate at pH 7.4, with &(Ru(NH~)~~+) = 5.5 x cm2/s.lia The monolayers studied in this work were made of thioctic acid VA), 1-hexanethiol (CBH), 1:l TA/C&H, and 1:lOO TA/CcSH. The molar ratios reflect the solution composition from which the monolayers were assembled and do not necessarily reflect the film composition at the electrode surface. The self-assembly was initiated by immersing the electrode into 0.1%solutions in absolute ethanol for >24 h. The results on bare Au were obtained on a polycrystalline electrode, which was cleaned according to the method described by Oesch and Janata.'* Capacitance measurements were conducted by scanning the potential between 200 and (17) (a) Baur, J. E.: Wightman, R. M.J. Electroanal. Chem. 1991,305, 73. (b) Bard, A. J.; Crayston, J. A,; Kittleson, G. P.; Shea, T. V.; Wrighton. M. S. Anal. Chem. 1986,58, 2321. (18) Oesch, U.; Janata, J. Electrochim. Acta 1983,28, 1237.
Chart 2. Models of (A) the Double Layer Structure for the Self-Assembled TA Monolayer on Au, (6) Equivalent Circuit for the Monolayer Electrode, and (C) Potential Distribution on the Monolayer Surface.
a
qk is the bulk solution potential.
-200 mV vs SCE. The summed cathodic and anodic charging current (a,) at 0 mV was then divided by twice the scan rate and normalized by the electrode area.2c Instrumentation. A Bioanalytical Systems electrochemical analyzer (BASlOO) was used in electrochemical measurements. Data were transferred to a Northgate 386 personal computer for analysis. A conventional threeelectrode setup was employed, with a SAM electrode as the working electrode, FY wire as the auxiliary, and SCE in saturated KC1 as the reference. All potentials are reported versus SCE at room temperature unless specified otherwise. THEORETICAL CONSIDERATIONS When a monolayer is assembled on an electrode, the surface double layer structure is altered. Here we adopt a modified Stern modelIg to describe the double layer on the electrode (see Chart 2A), where the fundamental part of the model is the use of the SAM to replace the concept of the compact part of the double layer.zc The double layer structure may be approximated by a simple two-capacitor system, consisting of an ideal compact monolayer capacitor and a diffuse double layer capacitor connected in series (see Chart 2B). The overall capacitance, CtOtal, is
1 - 1 ctotal
cdl
+-cml1
where cd]is the double layer capacitance and IC, is the monolayer capacitance in pF/cm2. Each term in eq 1 can be calculated individually. Cmlmay be obtained using the Helmholtz capacitor model+
where E is the dielectric constant of the monolayer, EO is the permittivity of free space (8.85 x F/cm), A is the electrode area (cm2),and d is the monolayer thickness (cm). cdl, on the other hand, can be calculated from the GouyChapman For dilute aqueous solutions of 1:l electrolytes (19) Bard, A J.; Faulkner, L. R Electroanalytical Methods; John Wiley & Sons: New York, 1980; pp 500-511.
at 25 "C, cdl is
C,, = 2 2 8 ~ c * "cosh(19.5~6,) ~
(3)
where z is the electrolyte charge, c* is the electrolyte concentration OM), and & is the monolayer surface potential in mV (Chart 2C). The value of & is usually very small for a surface coated with an uncharged long-chain alkanethiol layerFc However, when the surface coating thickness becomes small and/or charged, 42, the potential difference between the film on the surface and the solution bulk (Chart 2C), cannot be assumed to be small. The value of Cpz has an impact on the equilibrium between the externally applied potential (EeJ and the electrochemical rate constant of the probe, Le., the probe kinetics.20 The value of 42 determines the fraction of the applied potential which is available to drive the electron transfer; the rate constants must be adjusted accordingly. The value of & is a function of the experimental parameters such as the external applied potential Using the equivalent circuit shown in Chart 2B, & is described by
(Ea.
(4)
The rate constants for the electrochemical reactions on the monolayer electrode, k , ~(cm/s), are thus influenced by the nonzero values of 42 through2'
k,, = k, exp(-z$,/kT)
(5)
where z is the probe charge, ko (cm/s) is the so-called true standard heterogeneous rate constant, k is Boltzmann's constant, and T (K)is the absolute temperature. Equation 5 has been used by Miller and co-workers to correct the heterogeneous electron transfer rate at different SAMs for the double layer effects at different electrolyte concentrations.2' At potential of zero change (PZC) on Au (ca. -0.07 f 0.10 mV vs SCE,2l where 0.00 V vs SCE was used in this work), an (20) Creager, S. E.; Weber, IC Langmuir 1993,9, 844. (21) (a) Becka, A M.; Miller, C. J. I. Phys. Chem. 1993,97, 6233. (b) Hill, D. T.;Sutton, B. M. Cyst. Struct. Commun. 1980,9, 679.
Analytical Chemistry, Vol. 67,No. 17, September 1, 7995
2769
Table I. Capacitance Measurement Results
electrode
TAb 1:l TA/C6SHb 1:100 TA/C6SHb CeSH’
electrolyte 0.1 M 0.1 M 0.1 M 0.1 M 0.1 M
KCI
NaFC KC1 KCI KC1
C (uF/cm2)
€0
9.2 f 2.2 8.5 5.2 f 0.9 3.0 f 0.1 2.7 f 0.2
8.3 7.7 4.7 2.7 2.4
a Dielectric constant was calculated from eq 2 using d = 8 A (see text). Scan rate, 5.12 V/s. * The monolayer composition (see text). From ref 9.
ionic strength of -0.1 M results in Cdl = 72 pF/cm2 (eq 3). Compared to C,,,,I, which is usually in the range of 1-10 pF/cm2, Cdl is about 7-70 times larger and thus becomes negligible. Therefore,
C,,,,I can be measured from cyclic voltammetry (&,I = i,/ 2vA), where i, is the charging current @A), v is the scan rate (mV/s), and A is the electrode area (cm2). Extensive studies have revealed that the n-alkanethiol monolayers on gold tilt -30” from the surface normal, and the optical ellipsometry and IR spectroscopy data indicate a correlation of 1.3A per CH? group.’J The thickness of the C6SH monolayer is thus estimated to be -8 A,including the Au-S bond (2.3A).2ib The estimation of the film thickness for the TA monolayer is difficult since, as we pointed out earlier, the orientation and the molecular packing of the monolayer are not known exactly. However, Nuzzo and co-workers have suggested that varying the chain-terminating head groups, such as CH3 and COOH, has little effect on the structure of the film in the region of the hydrocarbon chainsxc Our previous results show excellent selectivity and sensitivity of the TA monolayer? which is taken as an indicator that the film order is maintained in spite of the short alkyl chain. The electrochemical results shown here indicate that the order is maintained with the C&H coassembly. Therefore, it is reasonable to propose that the TA monolayers have a molecular orientation similar to that of the n-alkanethiol monolayers, although a difference in tilt would not affect the conclusions drawn here. Assumption of a 30” tilt in the calculations of TA film thickness, where the TA has four CH? and one CH group, results in the same thickness (-8 A) as determined for the C6SH. The dielectric constant, e , for the monolayers can thus be calculated from eq 2. RESULTS AND DISCUSSION Characterization of Monolayer Capacitance. Capacitance measurements have been used previously to evaluate the properties of ordered monolayers such as packing quality, pinhole density, and film Table 1 lists the results of the capacitance measurements for the different thin monolayers investigated in this study and the values of the dielectric constant calculated using eq 2. The results in Table 1 show that capacitance is largest for the TA film and decreases for the 1:land the 1:lOO TA/C&H mixed ~~~
monolayers. The capacitance for the pure CsSH monolayer is the smallest, with a calculated (eq 2) dielectric constant of 2.4.This value agrees well with the dielectric constant of polyethylene (E = 2.3)23 and with the dielectric constants of the alkanethiol monolayers (E = 2.6),‘confirming the CsSH film order. As shown in Table 1, as the surface hydrophilicity increases with the increase in the TA fraction in the film, the dielectric constant of the film increases, with the TA monolayer having the highest value of E . The dielectric constant is a measure of the resistive force the medium exerts against an external electrical field. The extraordinarily high dielectric constant, calculated from the measured capacitance for the TA monolayer, must be a result of film penetration by solvent (and possibly the electrolyte), since water has a dielectric constant of 78, while E values for nonconductive organic films are low ( 6 = 2.3for p~lyethylene).~~ This result is not surprising in view of the TA molecular structure (Chart 1). The lack of a second alkyl chain in TA can lead to a looser packing of the molecules on the Au surface, particularly in the head group region. The implications of the higher measured capacitance and, therefore, the high dielectric constant determined for the TA monolayers compared to that of the pure C6SH monolayer are significant since they mean lower film resistance, or lower potential drop across the film (eq 4). This means that the monolayer functions more like a “conductive” membrane, with monolayer selectivity resulting from the electrostatic interactions at the film solution interface. Porter et al.’ reported that the capacitance of the methylterminated alkanethiol monolayers is much smaller in F- than in C1- solutions. It was argued that the large volume of the hydrated F- can hinder its transport in the monolayer. No major decrease in the capacitance is observed in F- at the TA film (Table l),as expected for a film that can be penetrated by the solvent and the electrolyte. A large capacitance of the TA film, compared to the lower capacitance values for the mixed monolayers (Table 1), indicates that the TA film allows the solvent and the electrolyte to partition into the layer more easily than do the mixed monolayers. It also indicates that the TA film is the least and the C6SH film is the most hydrophobic. The mixed monolayers show an intermediate hydrophobicity, determined by the ratio of the TA to the CsSH in the film. Electrochemical Response of Fe(CN)s3- and R u ( N H ~ ) ~ ~ + on the TA Monolayers. Two hydrophilic probes, Fe(CN)63- and Ru RJH3)63+, were used to investigate the electrochemical properties of the monolayers as a function of solution pH, electrolyte composition, and concentration. Table 2 summarizes the voltammetric results for different film compositions. On the TA electrode, Fe(CN)& has a fast response at pH 1.5 (AEp = 65 mV), while no response is observedg at pH 7.4 when the COOH groups (pKa= 5)gare dissociated. Ru(NHJ& displays a well-defined response at pH 7.4 (AI?,= 75 mV) when the COOH groups dissociate, while at pH 1.5, a relatively small current is observed.9 The i, and the AE, values for Fe(CN)63- (at pH 1.5) and for R u ( N H ~ ) ~(at~ pH + 7.4) are comparable to those observed on the bare Au electrodes, indicating that fast kinetics can be obtained for these probes on the TA monolayer electrodes.9 The cyclic voltammetric results show that the selectivity at the TA electrodes is directly related to both the solution pH and the
(22) (a) Sabatani E , Rubinstein, I J Phys Chem 1987,91, 663 (b) Chidsey,
C E D Loiacono, D N Langmurr 1990,6,682 2770
Analytical Chemistry, Vol. 67, No. 77, September 7, 7995
.. .
(23) Lanza. \’, L.: Herman, D. B. J. Poiym. Sci. 1958.28,622.
Table 2. Cycllc Voltammetry Results for Ru(NHa)r3+and Fe(CN)s3- on Different Monolayer Electrodes
probe RU(NH3)tj3+
electrode bare Au TA 1:l TA/CtjSH
Fe (CN) ~ j ~ -
bare Au
TA 1:l TA/C&H bare Au TA
electrolyte
pH
AEp (mv)
E”’(VvsSCE)
0.1 M KCl, 10 mM Tris 0.1 M KCl, 10 mM Tris 0.05 M phosphate 0.5 M phosphate 0.1 M KCl, 10 mM Tris 0.05 M phosphate 0.5 M phosphate 0.1 M HC10.j 0.1 M HClO4 0.1 M HC10.j 0.5 M NaAc/CF3COOH 0.5 M NaAc/CF3COOH
7.4 7.4 7.4 7.4 7.4 7.4 7.4 1.5 1.5 1.5 3.5 3.5
74 f 4 75 f 2 101 f 0 70 f 0 80 f 10 104 f 1 77i 1 65 65 f 1 153 f 7 69 f 8 90 f 2
-0.18 -0.19 -0.21 -0.24 -0.18 -0.21 -0.24 0.33 0.33 0.33 0.20 0.20
probe charge. We ascribe the observed selectivity to the changes in the electrostatic environment created at the film-solution interface at different solution P H . ~At higher pH, the selectivity is due to the dissociation of the COOH (PK, = 5) group of the TA at the electrode surface, even though the dissociation may only be partial. A recent QCM studyz4 has shown that the dissociation of the COOH group of HS(CH2)15COOH @Ka= 5) on Au occurs at pH zz 6, over a range as wide as 4 pH units, with the extent of the dissociation at pH 7.4 around 40-50% of the total surface population of the COOH groups. Assuming the same dissociation behavior for the TA film, almost half of the surface COOH groups of TA will be negatively charged at pH 7.4. The anticipation of such behavior was the initial motivation for the development of the “chemically modified” z5,z6 electrodes. For Fe(cN)~j~-, when 42 < 0, the exponential term in eq 5 becomes negative, and the apparent rate constant will decrease. It can be generalized on the basis of eq 5 that negatively charged probes will be selectively excluded from the monolayers when b < 0 at the negatively charged monolayer. Because of the low potential drop across the film, the external potential applied for the Fe(cN)63-/4- reaction will produce a positive potential (vs EEC) at the monolayer/solution interface. The permselectivity (i.e., the absence of the response of Fe(CN)e3-, while the response of Ru(NH3),j3+is fully developed) observed on the COO-terminated TA electrodes must result from the negative Donnan potential which must be established at the monolayer surface due to the high negative charge density of the COO- groups.z7 On the other hand, the positively charged probes will be able to access the negatively charged surface, as is observed for Ru(NH3)c3+. However, when b < 0, a resulting increase in the heterogeneous rate constant is predicted by eq 5; it is not observed experimentally for R u ( N H ~ ) ~(which ~ + reacts at potential negative of the PZC) at the COO--terminated monolayer, possibly because of the mass transport (diffusion) limitations of the response at the time scale of the experiment for this already fast probe. A fast response of F ~ ( C N ) Gon ~ - a positively charged alkylammonium-terminated monolayer has been observed.28 At pH 1.5, the surface COOH groups of the TA are neutral. Under these conditions, the sign of b will depend only on the external applied potential, with respect to PZC. When E,, =- PZC, b > 0 and vice versa. At pH 1.5, both Fe(CN)$ and R u ( N H ~ ) ~ ~ + (24) Wmg, J.; Frost”, L. M.; Ward, M. D. J. Phys. Chem. 1992,96, 5224. (25) Lane, R F.; Hubbard, A T. J. Phys. Chem. 1973,77, 1401. (26)Lane, R F.; Hubbard, A T. J. Phys. Chem. 1973,77, 1411. (27) Redepenning, J.; Tunison, H. M.; Finklea, H. 0. Langmuir 1993,9, 1404. (28) Doblhofer, IC; Figura, J.; Fuhrhop, J. Langmuir 1992,8,1811.
ip,c &A cm-2 mM-’) 230 f 8 225 f 5 195 f 2 187 f 1 210 f 5 176 f 4 169 f 1 237 220 f 4 158 f 6 210 f 4 126 f 1
can be expected to display fast responses, since their reactions proceed at the optimum 42 vs PZC 142 > 0 for Fe(cN)63- and & < 0 for R u ( N H ~ ) ~ ~However, +]. the expected behavior is not observed for R u ( N H ~ ) ~ ~The + . response is, in fact, suppressed compared to the response at the bare surface. Originally, the observed decrease in response at low pH with HC104 as the electrolyte at the COOH-terminated surface was attributed to a high concentration of protons at the s ~ r f a c e .Additional ~ experiments with C104-, however, have shown that the response of Ru(NH3)g3+at low pH in HC104 is also affected by the formation of an insoluble salt with the C104-. The precipitate can block the monolayer surface and can also reduce the bulk concentration of the Ru(NH3),j3+,thus reducing the magnitude of the measured current. Experiments at low pH were also conducted in 0.1 M HCl while the solution pH was maintained at the same low value (PH 1.5) as in the HC104 solutions. Here, although a small decrease in the R u ( N H ~ ) ~current ~+ was observed on the TA electrode (-15% decrease compared to the response on the bare Au), the decrease in current was not as sigdcant as was originally observed in the HC104 solutions. Therefore, Clod- must have an effect on the Ru(NH3)c3+current, presumably because of the salt formation. The decrease in the R u ( N H ~ ) ~current ~ + observed in HCl solutions at the TA electrode at the low pH may, in fact, be due to the strong near-neighbor interactions in the hydrophilic head group region of the film at high H30+ concentrations in solution, as suggested earlier? This effect may increase the distance of closest approach for the R u ( N H ~ ) ~to~ the + COOHterminated TA surface and may make ion transport in the R u ( N H ~ ) ~ ~reaction + / ~ + less effective through the film. Since the response of Fe(cN)63- at the COOH-terminated electrode is fast at low pH, the film structure and ion transport remain favorable for the Fe ((“)-I4- reaction at the film, making it sensitive and cation permselective, Le., maintaining good anion response. Response of Fe(CN)e3- and Ru(NH3),j3+ on the Mixed Monolayers. As CsSH is coassembled with the TA, the cyclic voltammetric responses of both probes change. Figure 1 illustrates the response of Ru(NH3),j3+at pH 7.4 on the different monolayers. On the 1:1 monolayr, Ru(NH3)tj3+displays relatively fast kinetics, comparable to those on the TA film. On the 1:lOO TA/C&H, and on the CsSH films, the response of Ru(NH3),j3+ becomes indistinguishable from the background current. Figure 2 shows that the response of Fe(CNh3- at pH 1.5, the pH where at the TA monolayer the response of Fe(CN)63- is maximized, deteriorates when C&H is coassembled with TA. On the 1:1 monolayer, Fe(cN)~j~kinetics are slower than those on Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
2771
300
1507
1
3
. .
i
/--. 2 loo{ E
3
*-
- TA
- TA
-‘0°/ -
3 200
0
0
-200
0 -400
;
monoio er/Au
. - _ _1:1_ TA/C& monolayer Au ..___. 1 : l o 0 CsSH monolayer/Au .......... CBSH monoloyer/Ali 3 -600
1 -800
E/mV Figure 1. Cyclic voltammograms of 1 mM Ru(NH3)$+ in 0.1 M KCI and 10 mM Tris (pH 7.4) on different monolayers. Scan rate, 100 mV/s. 300 3
monola er/Au
_.._ I TA/C&H
monoIayer/ku
...._ 100 TA/CbSH monoloyer/A“ .......... C6SH monolayer/Au -loo/
,
,, ,I
, , -- 0 1 5 800
600
400
200
0
, I I I
,
, , “-r -20C -400
E/mV Figure 3. Cyclic voltammograms of 1 mM Fe(CN)e3- in 0.1 M KCI and 10 mM Tris (pH 7.4) on different monolayers. Scan rate, 100 mVis.
of the COO- head groups at the 1:lmonolayer at pH 7.4 used for the Ru(NH3)2+reaction can be expected to contribute to facilitated reduction of the Ru(NH3)s3+, as predicted by eq 5. More importantly, the negative surface charge density can make up for some of the decrease in 42 caused by the lower capacitance of the mixed monolayer (eq 4). At pH 1.5, where the Fe(CN)63reacts and where the TA/C&H electrode surface is neutral because of the suppressed dissociation of TA, the value of 42 will be controlled primarily by the low capacitance of the mixed monolayer. Therefore, the kinetics of Fe(CN)c3- on the 1:lfilms 3-100 at pH 1.5 are slower than the kinetics of Ru(NH&~+at pH 7.4. 5 TA monoloyer/Au At the more hydrophobic monolayers of the 1:lOO TA/C6SH 1 1 TA/C6SH monolayer/Au and of the C&H, at the solution pH used to obtain the maximum -__.. 1 ’00 TA/C6SH monoloyer/Au ..._._ .... CBSH monolayer/Ab response for each probe on the 1:l monolayers, [i.e., pH 1.5 for -zoo! Fe(CN)s3- and 7.4 for R u ( N H ~ ) ~ ~the + ]kinetics , of the two probes differ considerably, although the trend observed on the 1:lmixed monolayer of a decrease in the electrochemical kinetics of the Figure 2. Cyclic voltammograms of 1 mM Fe(CN)s3- in 0.1 M HC104 hydrophilic probes with the increase in film hydrophobicity (and (pH 1.5) on different monolayers. Scan rate, 100 mV/s. a corresponding decrease in the film capacitance) is still observed. But, in addition, ion partitioning into the monolayer may, in fact, the TA monolayer, as indicated by the larger A,!?,, and the also affect the observed electrochemical responses of the two probes. For example, the A,!?, of Fe(CN)& in 0.1 M KCl is Fe(CN)63- peak current (i,) is smaller than that on the TA film. smaller (380 mv) on the C&H film than on the same film in 0.1 On the 1:lW TA/C6SH monolayer and on the CsSH monolayers, the MPof Fe(CN)& increases further, while the ip decreases. M HC104 (Up = 700 mv). The possible reason is that preferred The slower kinetics of both hydrophilic probes observed on interactions of a hydrophobic C104- with the film may limit the mixed monolayers are consistent with the observed decrease Fe(CN)63- access to the surface. In KCl, neither hydrophilic ion in the capacitance of the mixed monolayers. Table 1 shows that is likely to interact preferably with the film, permitting closer the capacitance of the mixed films decreases as the fraction of access of the negative Fe(CN)63- to the C&H film surface. A hexanethiol in the monolayer increases, which, according to eq related model of ion partitioning into the film at positive potentials 4,predicts that a larger fraction of the applied potential is dropped (vs PZC) at the film-solution interface will be discussed later. across the membrane, since the membrane is now more “resisWhat puzzled us were the significantly different apparent tive”. Consequently, the energy available to conduct the electrokinetics of R U ( N H ~and ) ~ Fe(CN)& ~~ on the C&H monolayer at chemical process decreases, resulting in slower kinetics (eq 5) pH 7.4 in 0.1 M KC1 (Figures 1 and 3). Similar results have been and the experimentally observed lower currents. previously reported,’ but no satisfying explanation has been given On the 1:lTA/C&H monolayer, at the solution pH where the in the literature. In order to obtain a better explanation for the response of each probe is at a maximum on the TA monolayer observed differences in the rates of electron transfer of the two [pH 7.4 for RU(NH&~+and pH 1.5 for Fe(CN)&], R u ( N H ~ ) ~ ~hydrophilic ~ probes, the self-exchange rates, the surface charges, displays much faster kinetics than Fe(CN)& (Figures 1 and 2). the potential windows, and the PZCs of the monolayers were The charge density at the monolayer/solution interface can play analyzed. However, we did not find any correlation between these an important role in the observed response. The negative charge parameters and the electrochemical results. The simplest expla-
1
n..
1
2772 Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
nation is that the high charge density of the R u ( N H ~ ) ~higher ~+, than that of Fe(CN)63-,prevents its access to the hydrophobic 'OI surface. Creager et al. observed that on monolayers containing @c)c&H (Fc = ferrocene) and n-alkanethiol, the formal potentid of Fc shifted to a more positive value and the peaks became broader as the alkanethiol coadsorbate chain length increasedaZ9 -lo] This was used as evidence that the energy difference, caused by the changes in the hydrophobic vs hydrophilic microenvironment, influenced the response. Similar experiments may be able to differentiate the microenvironmental contribution to the kinetics of the Ru(NH)c3+and the Fe(CN),j3- on the CcSH monolayers. Effect of Electrolyte Composition and Concentration on on TA monola er/4u -40 Response. The effect of electrolyte composition and concentra- DA DA on 1 : l TA/C6!& monoloyer/Au _ _ _ - -DOPAC on TA monolo er/Au tion on the electrochemical response of redox couples irreversibly ~.~~~~~~~~ DOPAC on 1 : l TA/C& monoiayer/Au attached to self-assembled monolayers has received recent and is clearly important to monolayer electrode response in view of the results just discussed. The effect of electrolyte composition and concentration on the electrochemical response of the hydrophilic probes present in solution was investigated here at the ultrathin alkyl chain monolayers. The results are summarized in Table 2. Table 3. Cyclic Voltammetry Results for DA on On the TA monolayer, the AE,values for R u ( N H ~ ) decrease ~~+ Different Monolayer Electrodes at pH 7.4 with an increase in the electrolyte concentration,indicating closer EP.& ZP a Ru(NH&~+access to the surface. Changes in the Ru(NH3)s3+ electrode electrolyte (V vs SCE) +A cm-* mM-') peak current, ip,are small, but these currents are higher in C1403 i 3 bare Au 0.1 M KCI, 10 mM Tris 0.18 than in phosphate electrolytes. The effect of electrolyte concen213 i9 TA 0.1 M KC1,lO mM Tris 0.40 i 0.01 251 i 21 0.05 M phosphate 0.37 i 0.02 tration on the cyclic voltammetric results of Ru(NH3)s3+on the 0.48 i 0.01 183 i 15 0.5 M phosphate 1:l TA/CcSH films is similar, with an additional small decrease 232 f 14 1:l TA/C&H 0.1 M KCI, 10 mM Tris 0.48 i 0.01 in the kinetics and the peak currents. 0.05 M phosphate 0.44 i 0.01 278 & 22 208 f 15 0.5 M phosphate 0.52 i 0.03 The simple Gouy-Chapman-Stem model described earlier (Chart 2) of an impermeable compact part of the double layer cannot be used alone to explain these electrolyte effects. The an electrolyte. Table 2 summarizes the results for Fe(CN)63-. On high capacitance of the TA monolayer indicates that the monolayer the bare electrode, the AE, = 69 mV and ip= 210 p A cm-2 mM-I must contain water. At high electrolyte concentrations, a more values are similar to the values obtained in C1-. However, on the compact double layer will have a smaller effect on the kinetics of COOH-terminatedTA monolayer, the Fe (CNh3- peak separation R u ( N H ~ ) ~and ~ +will contribute to a lower potential drop across increases and the peak current decreases, indicating that in this the double layer. In addition, at high electrolyte concentrations, electrolyte the currents are lower than those in C1-. swelling of the hydrophilic head groups and the associated EBect of Monolayers on the Response of Hydrophobic interactions of electrolyte with this region can also Probes. Figure 4 shows cyclic voltammetry results for DA+ @Ka facilitating partitioning of ions into the head group region of the = 8.92) and DOPAC- @Ka= 4.22) at pH 7.4 on the TA and the film. Such effects have been observed in the studies of ionic 1:l TA/C&H monolayer electrodes. Table 3 summarizes the strength effects on the Langmuir-Blogett monolayers, where at cyclic voltammetric results for DA. a constant pressure, surface area/molecule increased with elecAs shown in Table 3, the DA+ oxidation peak potential, Ep,a, trolyte c0ncentration,3~with more electrolyte penetration into the becomes significantly more positive on the TA monolayer elecfilms. The experimentally observed faster kinetics of R u ( N H ~ ) ~ ~ + trode compared to the Ep,a for DA on the bare Au surface, and at high electrolyte concentrations (Table 2) are predicted by eq the DA oxidation peak potential becomes even more positive with 4. Higher diffusion coefficients of Ru(NH3)c3' 32 in C1- than in coassembly of the C6SI-i with TA (Table 3 and Figure 4). On the phosphate electrolytes (5.5 x loT6cm2/s in 0.1 M phosphate17a 1:lOO TA/C&H and the C&H monolayers, DA response disap and 7.5 x cm2/s in 0.1 M K C P ) will contribute to the higher pears. DOPAC shows no response at this pH at any of the peak currents observed in C1-. monolayers. For these more hydrophobic, singly charged probes, Fe(CN),j3- response was tested at low pH, the pH where the the kinetics at the monolayers are significantly slower than those response of Fe(CN)c3- is good, using CF3COONa/CF&OOH as on the bare Au electrode. Since DOPAC- response is not observed on any of the (29) Creager, S. E.; Fox, M. A. J. Electrochem. Suc. 1990, 137, 2151. (30) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. monolayers, this indicates that the access of this probe to the (31) Creager, S. E.; Rowe, G. K. Anal. Chim. Acta 1991,246, 233. monolayer surface is suppressed in spite of the low (-1) probe (32) Shimazu, K.; Yagi, 1.; Sato, Y.; Uosaki, K. Langmuir 1 9 9 2 , 8, 1385. charge. The response of DA disappears on the most hydrophobic (33) De Long, H. C.; Donohue, J. J.; Buttry, D. A. Langmuir 1991, 7, 2196. (34) Smith, C. P.; White, H. S. Anal. Chem. 1992,64, 2398. monolayers, as does the response of the hydrophilic R u ( N H ~ ) ~ ~ + (35) (a) Gh~icha,L.; Leblanc, R. M.; Chattopadhyay, A. K. Longmuir 1993,9, (Figure 1). 288. (b) Chattopadhyay, A. K.; Ghakha, L.; Oh, S. G.; Shah, D. 0 . j .Phys. The slow kinetics and the low currents of DA on the most Chem. 1992,96,6509. (c) Ghai'cha, L.; Chattopadhyay, A IC;Tajmir-Riahi, H. A. Langmuir 1991, 7, 2007. hydrophilic monolayers (TA and 1:l TA/cssH) must result from Analytical Chemistry, Vol. 67,No. 17, September 1, 1995
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Table 4. Cyclic Voltammetry Results for Quinone in 0.1 M KCI/lO mM Tris (pH 7.4) at Different Electrodes
electrode
iP.?(uA cm-2 mM-I)
AU GC TA 1:100 TA/C&H C&H
475 470 240 f 5 237 i 1 230 5 4
(mV vs SCE) - 50 - 108 -274 f 4 -368 i 4 -483 & 5
a larger distance of close approach to the monolayer surface compared to the distance to the bare Au which may result from unfavorable interactions of the probe with the film and from unfavorable molecular orientation of the probe at the monolayer, as well as from possible complications in the electron transfer mechanism of the probe. The 2e, 2H' oxidation of DA may, for example, change the local pH, affecting surface charge density. The results obtained here for DA and DOPAC differ from the results reported recently for DA detection on SAMs with the COOH head groups. The authors reported improved kinetics of DA on the HS(CH&COOH monolayers compared to the kinetics which were measured on the bare Au electr0de.3~In our work, DA oxidation is always more reversible and occurs at less positive potential on the bare Au than on the monolayer electrodes (Table 3), including the TA monolayer. In addition, the authors observed DA response on the C&H monolayer which was not observed in our study. This may be result of a less compact film on the rough glass substrate used in their study. The impact of surface roughness on electrochemical response of the monolayers formed on different substrates is discussed later in this work. According to the results in Table 3, DA shows faster kinetics at lower ionic strength, in contrast to the improved kinetics of R u ( N H ~ ) ~at~higher + ionic strength (Table 2). High electrolyte concentration may, in this case, make the surface more hydrophilic, and screening of the surface charge may make surface access more difficult for the more hydrophobic, singly charge DA. In order to probe the effect of the hydrophobic contribution to the electrochemical response of the more hydrophobic catecholamine probes, the response of a structural analog of the charged catechols, benzoquinone (Q) which is neutral at pH 7, was investigated. Table 4 summarizes the results for Q. On a Au electrode at neutral pH, AE,at -30-60 mV has been reported37for Q, consistent with the results obtained here. On the TA electrode at pH 7.4, Q kinetics are much slower, with a significant negative shift in the cathodic reduction peak, E,,? (Table 4). With incorporation of the C&H into the TA monolayer, the E,,?of Q becomes more negative. The magnitude of the peak current, i,,,, is reduced by half on the monolayers compared to the i,,, on the bare Au electrode. The slow response of Q on the TA electrode at pH 7.4 is similar to the slow response of DA (Tables 3 and 4) at the TA monolayers. The similar slow response of Q and DA at the hydrophilic COO-terminated TA film shows that DA charge does not significantly help its access to the COO--terminated, hydrophilic surface. However, on the more hydrophobic 1:lOO TA/C&H and the c6SH monolayers, uncharged Q continues to show response (Table 4), while the charged DA does not. This reflects closer access of Q to the more hydrophobic surfaces, which may reflect a (36) Malem, F.; Mandler, D.Anal. Chem. 1993,65,37. (37) Bailey, S, I.; Ritchie, I. M.Electrochim. Acta 1985,30,3.
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Analytical Chemistry, Vol. 67, No. 17, September 1, 7995
combination of factors, including probe neutrality and its more compact structure, as well as the reduction pathway of Q. The poor responses of the more hydrophobic DA and Q on the TA monolayers compared to the good kinetics of the hydrophilic Ru (NH3)63+highlight the differences in responses of hydrophobic vs hydrophilic probes on the hydrophilic TA monolayers, although factors other than hydrophobicity may also be important to the response (such as the reaction pathways). In previous work, Miller and co-workers foundZlathat reduction of Fe(CN)& was 20 times faster on -OH- than on CH3-terminated C15 monolayers. Our results with the hydrophilic probes identdy similar slowing of kinetics with an increase in monolayer hydrophobicity. However, we also observed that on short TA/C&H monolayers, the increase in surface hydrophobicity of the mixed monolayers does not improve the response of the hydrophobic probes tested here. Miller and co-workers foundziathat on the films where the carbon chain length was > 10, the hydrophobic Fe(bpy) (CN)4- responded much faster on the CHr than on the -OH-terminated film. Probe Retention, Film Stability, and Substrate Roughness. Retention of the probes on the monolayers was also examined. When the TA monolayer originally used at pH 7.4 in Ru(NH3)$+ was transferred to 0.1 M KCl, no probe retention was detected. Cyclic voltammetric experiments were performed at different scan rates (v) to test probe adsorption on the monolayers. The slopes of the log i, vs log v plots obtained for the R u ( N H ~ ) ~on~ + the TA and the 1:l TA/C&H monolayers were all close to 0.5 and were independent of electrolyte used in the measurements, indicating that diffusion controlled the electrochemical response. The values of the slopes of the log i, vs log v plots for DA were smaller than 0.5, indicating some degree of surface passivation in the electrochemical reaction of DA at the monolayer consistent with the observed slow voltammetric behavior of this probe on the monolayers. Monolayer film stability was tested by testing the effect of the potential window on the electrochemical response of the monol a y e r ~ . The ~ potential window for a stable electrochemical response of the monolayers depends on electrolyte composition, solution pH, and monolayer structure. We have found that TA electrodes are stable between -800 and +lo00 mV vs SCE at neutral pH. However, at very low pH, evolution of hydrogen at the negative potential limit may destroy the monolayers. In 0.1 M HC104, the evolution of hydrogen occurs at ca. -550 mV on the TA film compared to ca. -400 mV vs SCE on the bare Au. On the CRSHmonolayer, even at ca. -800 mV, there is no sign that hydrogen is being generated. An important issue is the thermodynamic stability of the mixed monolayers. Mixed monolayers with long alkyl chains (n 2 10) are stable and behave reproducibly based on the contact angle measurements result^.^ However, stability of the short alkyl chain monolayers such as those used in this study has not been reported. In this study, Au electrodes stored in 1:l TA/C&H solution for 3 months had monolayers with significantly slower kinetics for Ru (NH3)63- than monolayers prepared through much shorter contact with the alkanethiol solution, but the Ru(NH3)c"' current, although much smaller, could still be measured. Theoretically, the alkanethiols may eventually replace the carboxylic thiols from the monolayer since the pure alkanethiol monolayers are thermodynamically more stable in the ethanol
solutions.38 If this were to happen during the 3 months of contact with the 1:lsolution, TA should be largely replaced by the C&H, and the monolayer should display an electrochemical behavior similar to that of the 1:lOOTA/CsSH or even the pure C&H films. However, this was not observed. The slower electrochemical response of the Ru(NH3)s3+on the films prepared by the long exposure to the 1:l solution does not reflect, in our view, fast structural changes in the 1:l films. As our results show, Au substrate preparation procedure is an important factor in fabricating quality monolayers. We examined the response of monolayers prepared on Au vacuumdeposited on silicon wafers and on conventional glass microscope slides. Monolayers of TA formed on a glass slide did not show permselectivity; F ~ ( C N ) Gwas ~ - not completely excluded at pH 7.4, while on the 1:lOO TA/C&H monolayer, the hE, was as small as 72 mV. Clearly, the inhomogeneity and roughness of the substrate can result in a poorly defined film. The capacitance measurements support this observation. The capacitance measured on the TA monolayers assembled on a glass substrate was indistinguishable from that of the bare electrode. Our conclusions which relate the method of substrate preparation to film quality are based on the electrochemicallymeasured selectivity [i.e., Fe(CN)63- response is suppressed, while Ru(NH3)63+ response is fully developed at pH 7.41 and on film capacitance of the monolayers formed on the different substrates. These electrochemical results differ from the previous observations about substrate effect on monolayer quality, which were based on the STM results.38 Since STM is most effective in determining the surface roughness at a nanometer scale, the structural information from the relatively small area STM scans may not always be characteristic of the whole sample. On the other hand, the electrochemical behavior is a sensitive measure of the average substrate quality caused by surface defects, such as cavities, phase boundaries, and phase edges. In our study, the glass slides and the silicon wafers were treated in exactly the same way before the monolayers were assembled on these substrates. Therefore, the observed electrochemically determined selectivity differences of the test Fe(CN)s3- probe on the monolayers formed on the two substrates must result from structural differences in the two substrates, which would cause the films to form differently on these substrates. The observed differences in the electrochemically measured selectivity of Fe(cN)63- on the films assembled on the different substrates are most likely due to the differences in substrate roughness and may explain the various and somewhat controversial reports of the electrochemical responses of SAMs found in the literature. CONCLUSIONS Monolayers prepared by self-assembly and coassembly of two short alkyl chain thiols, thioctic acid and hexanethiol, display properties previously not detected at long alkyl chain monolayers. Through the attachment of a hydrophilic COOH head group to (38) (a) Vancea, J.; Reiss, G.: Schneider, F.; Bauer, K.; Hoffmann, H. Su$ Sci. 1989,218, 108. @) Widrig, C. A; Chung, C.; Porter, M. D. j.Electroanal. Chem. 1991,310, 335.
the short (C5) alkyl chain, such as in TA, a route to surface selectivity is provided which can be modulated by changing the solution pH and electrolyte composition. The hydrophilic TA surface displays higher electrochemical sensitivity than do the long alkanethiol monolayers as a result of the short distance of closest approach of the probes to the electrode surface. A lower potential drop across the short alkyl chain monolayers that terminate in a hydrophilic COOH head group than across structurally related long alkyl chain thiols is caused by higher dielectric constant of the film, in part due to penetration of the film by solvent (water) and electrolytes. The resulting less resistive monolayers are characterized by higher values of the monolayer capacitance and contribute to faster electrochemical kinetics of electroactive probes present in solution. Electrolyte composition and concentration have an important impact on the electrochemical response at the short alkyl chain COOHterminated monolayers, primarily in that they control the compactness of the double layer at the film-solution interface and appear also to influence the response of the more hydrophobic monolayers. At high electrolyte concentrations, hydrophilic films may additionally swell, which can lower the potential drop across the film, contributing to faster probe kinetics. We have demonstrated that hydrophobic mixed TA/hexanethiol monolayers will slow down the response of hydrophilic probes. It was also found that the response of catechol and quinone probes is slow on the hydrophilic and the mixed TA monolayers, even when those probes carry a charge opposite to that of the head groups at the monolayer surface and on the pure C6SH monolayers. Consequently, the short hydrophilic monolayers have an additional degree of selectivity toward hydrophilic probes. Since the kinetics of hydrophilic probes decrease with increasing monolayer hydrophobicity and at the same time, the increased monolayer hydrophobicity does not improve the response of the hydrophobic catechol and quinone probes, hydrophobic monolayers are not the best for direct analysis. For a fast response of hydrophobic probes such as catechols, an ideal surface may need to be hydrophobic but with a low potential drop, a surface which has not yet been successfully built. ACKNOWLEDGMENT We gratefully acknowledge the US. Army and the Division of Sponsored Research at the University of Florida for supporting this work. We thank Prof. Paul Holloway in the Department of Materials Science at the University of Florida for the assistance in the fabrication of the vacuum-deposited Au on silicon wafers.
ScientiFc Parentage ofthe Author: A Brajter-Toth, Ph.D. under J. A Cox, Ph.D. under A M. Hartley, Ph.D. under J. J. Lingane, Ph.D. under I. M. Kolthoff. Received for review October 20, 1994. Accepted May 31, 1995.a AC941031 D @Abstractpublished in Advance ACS Abstracts, July 1, 1995.
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