Langmuir 1993,9, 3660-3667
3660
Characterization of Octadecanethiol-CoatedGold Electrodes as Microarray Electrodes by Cyclic Voltammetry and ac Impedance Spectroscopy Harry 0. Finklea,' Daniel A. Snider, and John Fedyk Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506
Eyal Sabatani, Yael Gafni, and Israel Rubinstein' Department of Materials and Interfaces, Weizmann Institute of Science, 76100 Rehovot, Israel Received July 2, 1993. I n Final Form: September 27, 199P Organized monolayers of octadecanethiol on gold are examined by cyclic voltammetry (CV) and ac impedance spectroscopy (ACIS) in electrolytes containing 1-electron redox couples. Pinholes in the monolayer lead to voltammetric behavior resembling that of a microarray electrode. Contour plots are given for linear scan voltammetric parameters (peak current, peak potential and half-peak potential) from the theoretical treatment of microarray electrodes. The theory of ac impedance spectroscopy is developed for a microarray electrode with the area fraction of active sites less than 0.1. The behavior of monolayercoated electrodes conforms qualitatively to the predictions for microarray electrodes with pinhole radii of ca. 0.1-10 pm and pinhole separations of 1-100 pm, but significant deviations from the predictions are observed. The deviations are discussed in terms of the nonuniform distribution of the diameter and separation of the pinholes. ACIS data in particular suggest that the pinholes are grouped in patches. Introduction Self-assembled monolayers of alkanethiols on gold are intriguing structures because the monolayers survive the voltammetry experiment in aqueous electrolyte.142 The 0
Abstract published in Advunce ACS Abstracts, November 1,
1993. (1) Finklea, H. 0.;Avery, S.; Lynch, M.; Furtach, T. Langmuir 1987, 3, 409-13. Snider, D. A.; Fedyk, J. Langmuir 1990,6,371-6. (2) Finklea, H. 0.; (3) Finklea, H. 0.;Hanshew, D. D. J.Am. Chem. SOC. 1992,114,317381. (4) Finklea, H. 0.; Ravenscroft, M. S.; Snider, D. A. Langmuir 1993, 9, 223-7. (5) Finklea, H. 0.;Hanshew, D. D. J. Electroanal. Chem. 1993,347, 327-340. (6) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987,219, 365-71. (7) Sabatani, E.; Rubinstain, I. J. Phys. Chem. 1987,91,6663-9.
(8) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A,; Sagiv, J. Nature 1988,332,426-9. (9) Rubinstein, I.; Steinberg, S.;Tor, Y.; Shanzer, A,; Sagiv, J. Nature 1989,337, 216-7. (10) Steinberg, S.; Tor, Y.; Sabatani, E.; Rubinstein, I. J. Am. Chem. SOC. 1991,113, 5176-82. (11) Steinberg, S.;Rubinstein, I. Langmuir 1992,8, 1183-7. (12) Porter, M. D.; Bright, T. B.; Allara, D.; Chidsey, C. E. D. J.Am. Chem. SOC. 1987,109,3559-68. (13) Popenoe, D. D.; Deinhammer,R. S.;Porter, M. D.Langmuir 1992, 8, 2521-30. (14) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6,682-691. (15) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC. 1990,112,4301-6. (16) Chidsey, C. E. D. Science 1991,251, 919-22. (17) Miller, C.; Cuendet, P.; Gratzel, M. J.Phys. Chem. 1991,95,87786. (18) Miller, C.; Gratzel, M. J.Phys. Chem. 1991,95, 5225-33. (19) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992,96, 2657-68. (20) Creager, S.E.; Collard, D. M.; Fox, M. A. Langmuir 1990,6,161720. (21) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192-7. (22) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307-2312. (23) Creager, S. E.; Rowe, G. K. Anal. Chim. Acta 1991,246, 233-9. (24) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854-61. (25) Diem, T.; Czajka, B.; Weber, B.; Regen, S.L. J. Am. Chem. SOC. 1986,108,6094-5. (26) Fabianowski, W.; Coyle, L. C.; Weber, B. A.; Granata, R. D.; Castner, D. G.; Sadownik, A.; Regen, S. L. Langmuir 1989,5, 35-41. (27) Sun, L.; Crooks, R. M. J. Electrochem. SOC.1991, 138, L23-25. (28) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993,9, 632-6.
monolayers strongly block many electrode processes including gold oxidation in acidic electrolyte and electron transfer with otherwise reversible redox ~ o ~ p l e s ? A ~ ~ ~ J 4 J ~ - 1 9 Only in strong base does the thiol head group become electrochemically oxidizable or reducible.The monolayer-modified electrodes have stimulated considerable (29) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7, 1419-25. (30) Kwan, V. W. S.;Cammarata, V.; Miller, L. L.; Hill, M. G.; Mann, K. R. Langmuir 1992,8, 3003-7. (31) Tarlov,M. J.;Bowden,E.F. J.Am. Chem.Soc. 1991,123,1847-9. (32) Collinson, M.; Bowden, E. F.; Tarlov, M. J. Langmuir 1992, 8, 1247-50. (33) Kunitake, M.; Akiyoshi, K.; Kawatana, K.; Nakashima, N.; Manabe, 0. J.Electroanal. Chem. 1990,292, 277-80. (34) Bilewicz, R.; Majda, M. J. Am. Chem. SOC. 1991,123, 5464-66. (35) De Long, H. C.; Buttry, D. A. Langmuir 1990,6,1319-22. (36) Nordyke, L. L.; Buttry, D. A. Langmuir 1991, 7,380-8. (37) De Long, H. C.; Donohue, J. J.; Buttry, D. A. Langmuir 1991,7, 21962202. (38) De Long, H. C.; Buttry, D. A. Langmuir 1992,8,2491-6. (39) Lee, K. A. B. Langmuir 1990,6, 709-12. (40) Kim, J.-H.;Lee, K. A. B.; Uphaus, R. A.; Cotton,T. M. ThinSolid Films 1992,2101211, 825-7. (41) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510-4. (42) Shimazu,K.;Yagi,I.;Sato,Y.;Uosaki,K.Langmuir 1992,8,13857. (43) Uosaki, K.; Sato, Y.; Kita, H. Electrochim. Acta 1991,36,1799801. (44)Wang, J.; Frostman, L. M.; Ward, M. D. J.Phys. Chem. 1992,96, 5224-8. (45) Obeng, Y. S.;Bard, A. J. Langmuir 1991, 7, 195-201. (46) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688-91. (47) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; ZJU, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992,8, 357-9. (48) Frisbie, C. D.; Fritach-Faules, I.; Wollman, E. W.; Wrighton, M. S. Thin Solid Films 1992,210/211, 341-7. (49) Takehara, K.; Yamada, S.; Ide, Y. J.Electroanal. Chem. 1992, 333,339-44. (50) Sondag-Huethorst,J. A. M.; Fokkink, L. G. J. Langmuir 1992,8, 2560-6. (51) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37-41. (52) Doblhofer, K.; Figura, J.; Fuhrhop, J.-H.Langmuir 1992,8,18116. (53) Widria, C. A.: Chuna, - C.:- Porter, M. D. J. Electroanal. Chem. 1991,310,335-359. (54) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-93. (55) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323-9.
0743-7463/93/2409-3660$04.00/0 0 1993 American Chemical Society
Monolayer-Coated Gold as Microarray Electrodes interest because of their applications to fundamental studies of long range electron transfer1t=J4-19*31$2 and to selectiveelectrodes.7-11 However, the blocking properties of the monolayers are seldom perfect. In most cases, a monolayer-coated electrode exhibits behavior resembling a microarray electrode, i.e. an electrode containing small active sites embedded in an insulating plane. The active sites are identified as pinholes in the monolayer. Both the electrolyte and the redox couple have access to the gold substrate at the pinholes. Pinholes are the result of imperfect adsorption of the alkanethiol to the gold during the self-assemblystep and/or subsequent loss of the thiol during rinsing, storage, or use. Methods for creating monolayers which are essentially free of pinholes have been de~eloped.~-llJ~-l9@ However, this work focuses on the characterization of pinholes in the monolayer as a microarray electrode. Our study is motivated by the potential applicationsof microarray electrodesfor creating selective voltammetric detectorall or for measuring very fast electron transfer kinetics.7eap" This paper is divided into two sections. In the fiist section, we discuss the theory of linear scan voltammetry (LSV) and ac impedance spectroscopy (ACIS) at an ideal microarray electrode. For the LSV experimentwe provide contour plots for three observable quantities (peak potential, half-peak potential, and normalized peak current) and show how the contour plots can be used to analyze experimental linear scan voltammograms. We derive equations for the faradaic impedance of a microarray electrode and demonstrate how limiting behavior of the in-phase faradaic impedance at high and low frequencies leads to a complete characterization of all microarray parameters. In the second section, we apply these analyses to voltammetry of gold electrodes coated with octadecanethiol (ClaSH). The monolayer-coated electrodes are treated to generate pinholes. The pinholes are characterized by oxide stripping in dilute sulfuric acid and by LSV and ACIS in the presence of several well-behaved redox couples. The degree to which the monolayer-coated electrodes deviate from the ideal microarray electrodeand the cause for the deviation are the major subjects of the second section.
Langmuir, Vol. 9, No.12, 1993 3661
I
I
I
Figure 1. Microarray parameters and diffusion profiles. R. is the radius of the microelectrode site and ROis the radius of the inactive area surrounding the microelectrode site. Diffusion layers indicated by semicircles are isolated at short times (high frequencies) and overlapped at long times (low frequencies).
of the microelectrodes is represented as 1- 8, where 8 is the coverage of the blocking layer. When 1- 6 is small (less than O.l), then 1- 8 = RR /::
(1)
where R, is the radius of the microelectrode and Ro is the radius of the inactive domain surrounding the microelectrode; 2 R o is the distance between the centers of adjacent microelectrodes. Matsuda and co-workers described the expected CAF7CV,B2and ACIW behavior of a microarray electrode for the case of 1- 8 greater than 0.1. Amatore et al.63 pointed out that Matsuda's assumption of uncoupled diffusion parallel and perpendicular to the electrode is inappropriate for the case of 1- 8 less than 0.1. They analyzed the problem for the case in which the pinhole radii and separation are small compared to the thickness of the total diffusion layer. The following expressions for calculating LSV's were derived: $ = Ae"Y(1- I$ - $/B) - (I$ + $/A)e'l $ = ~/[~FAC(~FDV/RT)''~] t
= -(nF/RT)(E - E O ' )
(2)
(3) (4) (5)
Theory for an Ideal Microarray Electrode The expected behavior of microarray electrodeshas been examined for several voltammetric methods, including chronoamperometry (CA),S741cyclic voltammetry (CV) and linear scan voltammetry (LSV),S2ts3and ac impedance spectroscopy (ACIS).84 In most cases, the theoretical treatments assume an evenly spaced array of disk-shaped microelectrodesof uniform radii embedded in an insulating plane. Figure 1illustratesthe relevant parameters interma of a blocking monolayer with pinholes. The area fraction (SB)Bindra, P.; Brown, A. P.; Fleischmann, M.; Pletcher, D. J. Electroanal. Chem. 1976,58,31 and 39. (57)Gueahi,T.;Tokuda, K.; Matauda, H. J. Electroanal. Chem. 1978, 89,247-60. (58) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Electroanal. Chem. 1982,138,65-72. (59)Reller, H.;Kirowa-Eiener, E.; Gileadi, E. J. Electroanal. Chem. 1984,161,247-68. (Sa)Shoup, D.; Szabo, A. J. Electroanal. Chem. 1984,160,l-17and 19-26. (61)SchariRer, B. R. J. ElectroanaZ. Chem. 1988,240,61-76. (62)Gueahi, T.;Tokuda, K.; Matauda, H. J. Electroanal. Chem.1979, mi. - -,-24-38. - - -. (63)Amatore, C.; Saveant, J. M.; Tmier, D. J. Electroanal. Chem. 1983,147,39-51. (64)Tokuda, K.; Gueehi, T.;Matauda, H. J. Electroanal. Chem. 1979, 102,41-8.
B = (DRT(1- ~ ) / ~ F V ) ' / ~ / ( O . ~ R & (7) $ is the dimensionless current,
t the dimensionless potential, I$ the convolution integral of the current with t1l2,D the diffusion coefficient, and v the scan rate. The shape of the cyclic voltammogram is a function of two dimensionless parameters A and B. Limiting cases noted by Amatore et al.89 include reversible LSV's, irreversible LSV'swiththeapparentkO given bykO(l-andpl plateau currents which are independent of scan rate. The last case appears when the separation between microelectrodes is relatively large compared to their diameters, i.e. when B is less than ca. 0.2. Since the parameters of a microarray electrode can in principal be uniquely determined by a simple LSV experiment, we have developed contour plots for the parameters obtained in a LSV peak current (Figure 2), peak potential (Figure 3),and half-peak potential (Figure 4). The cyclic voltammetry parameter of peak splitting is omitted because the position of the peak on the return scan is sensitive to the switching potential and because the defect currents we observe (see below) make the determination of the baseline difficult on the return scan.
3662 Langmuir, Vol. 9, No. 12, 1993
Finklea et al.
ID Contours
-2
1
0
-1
2
Log(B)
Figure 2. Contour plot for the normalized peak current (I,? of a LSV. The contours are spaced at intervals of 0.05 increasmg from 0.05 (left) to 0.95 (right). See the text for details.
E, Contours
solution is assumed to contain only the oxidized form of the redox couple. Each contour plot (generated by AXUM software) is based on a grid of log(A) and log@) spaced in increments of 0.2. We find that the range of -2 to 1.8 for log(A) and log(B) allows the analysis of most LSV's generated with monolayer-coated electrodes. In favorable cases, contours for two of the three parameters (e.g. Ipand Ep)intereect and define both log(A)and log@), from which, if ko and D are known, the pinhole parameters (1- 8, R, and Ro) can be calculated. When Ip is less than 0.3 (corresponding to plateau currents), Epbecomes indeterminable on the LSV, and the Ip contours are nearly independent of the value of log(A). In this case log@) (and hence a relation between Ro and 1- 8) can be obtained without requiring knowledge of ko. In this section we derive the ACIS equations for the microarray electrode for 1 - 8 less than 0.1. In the derivation, we assume that the electrolyte contains equal concentrations of the oxidized and reduced forms of the redox couple and that their diffusion coefficientsare equal. According to Amatore et al,,es the nonlinear diffusion problem for a microarray electrode is formally equivalent to an electron transfer step with preceding and succeeding chemical steps: ki xi20
2a00
ka
3
0 + ne-
-1
i2 R
ka
R e Y ki - 2 " " " " ' " " " " ' ' '
Figure 3. Contour plot for the peak potential (E,) of a LSV. The peak potentials are relative to the formal potential for a reduction (negative-going scan). The contours are spaced at intervals of 0.02 increasing from -0.22 V (bottom) to -0.04 V (top).
E,n
Contours
2r
1
t
1 1
'
1
F(1- 8) = 0.3(1- 8)-'12 (for disk-shaped pinholes) (13) Substituting eqs 11-13 into the general expression for faradaic impedance for coupled reaction schemes,M*67 noting that 8 is approximately 1,and rearranging terms, we obtain the following expressions for the in-phase (24) and out-of-phase (Z(') components of the faradaic impedance
2; = RJ(1- 8) + g/l/G; + (u/(i - e)){[(w2+ q2)1/2+ qi/[w2
n
sa00
+ q 2 i p 2 (14)
v --0.10 v .-0.05
I
-2
-1
0
1
2
Log@) Figure 4. Contour plot for the half-peak potential (Ep/*)of a LSV. The half-peakpotentials are relative tothe formalpotential as in Figure 3. The contours are spaced at intervals of 0.01 V increasing from -0.13 V (bottom) to +0.02 V (top).
For convenience the peak current (Ip)is expressed as a ratio of the peak or plateau current for the coated electrode relative to the peak for a clean electrode with the same area in the same solution; Ipis obtained by dividing the peak of the dimensionless current $ by 0.4463.= Both the peak position (E,) and the half-peak position (E,,/Z)are relative to Eo' with the scan going in the negative 'direction; the
R, = (RT/F)/(FAk"C)
(17)
q = D/(0.36R;)
(18) w is the radial frequency (2q9,uis the Warburg impedance slope with C being the concentration of either the 0 or R forms, and R, is the charge transfer resistance. Equations (65) Bard, A. J.; Faulkner, L. R. Electrochemical Methode: Fundomentala and Applicationa, John Wdey 8t Sons: New York, USO,p 218. (66)Smith, D. E.In Electroanalytical Chembtry; Bard, A. J., Ed., Marcel Dekker: New York, 1968;pp 1-166. (67)Sluytara-Rehbnch,M.;S l u m , J. InElectroanalvtical Chemistry: -. Bard, A. J;, Ed.;Marcel Dekkeri New York, 1970; pp1-128.
Monolayer- Coated Gold as Microarray Electrodes
Langmuir, Vol. 9, No. 12, 1993 3663
14and 15 are equivalent to the equationsgiven by Matsuda et aleufor the case of 1- 6 greater than 0.1,with the only difference being the expression for q
-e)
q = [u>]/[R,,%(l
I n ( l + 0.27/(1-e)1/2)~(19) Limiting cases are obtained by setting o much larger or much smaller than q. The high-frequency case (o>> q, which corresponds to nearly isolated diffusion profies for each microelectrode) yields the following expressions for the faradaic impedance components:
z;
= RJ(I - e)
z;
= u/&
+ u/G+ u/[(i- e)&] + - e)&]
(20)
(21) The equivalent expressions for the low frequency case (w
