Langmuir 2000, 16, 9687-9689
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Electrochemically Induced Deposition of Thiol-Based Monolayers onto Closely Spaced Microelectrodes Joseph Wang,* Mian Jiang, A. M. Kawde, and Ronen Polsky Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003 Received July 6, 2000. In Final Form: October 6, 2000 We report on an electrochemical protocol for the selective formation of self-assembled alkanethiol monolayers on a given microelectrode in the presence of a nearby second one. The procedure relies on the accelerated self-assembly of alkanethiols in ethanolic solutions under a cathodic polarization. Voltammetric “blocking” and electrochemical quartz crystal microbalance experiments, along with optical surface imaging, are used for demonstrating the selective electrochemisorption process. Using closely spaced electrodes (with 6 µm gaps), we demonstrate that only electrodes kept at negative potentials have been coated. Such attractive behavior is attributed to the simultaneous cathodic removal of an inhibitory surface layer. The high spatial resolution opens the way for localizing different reactive chemistries onto predetermined locations of sensor arrays.
Introduction
Results and Discussion
The formation of self-assembled organosulfur monolayers (SAMs) has attracted considerable attention due to its potential to many scientific and technological applications.1,2 Such applications include chemical sensors and biosensors, gene chips, information storage devices, or lithography, as well as fundamental studies on longrange charge transfer and surface structure or reactivity. While the fine control of the surface chemistry offered by SAMs is extremely attractive for diverse applications,3 various areas (e.g., high-density arrays, gene chips) would greatly benefit from further control of the interface reactivity down to the micrometer domain. In this Letter we describe the electrochemically triggered growth of SAMs on gold that permits selective cathodic deposition on specific electrodes. A recent study by Freund, Ferguson, and co-workers4 demonstrated that the electrochemical oxidation of alkyl thiosulfate on gold electrodes results in selective self-assembly in specific locations. Positive potentials were also used by Ron et al.5 for facilitating the self-assembly of alkanethiols. Selective placement of SAMs on closely spaced electrodes was reported also by Tender et al.6 or Imabayashi et al.7 In the present work we used negative potentials to accelerate dramatically the formation of SAMs during the immersion of gold electrodes in ethanolic solutions containing the alkanethiol precursor. Substantial differences in the adsorption kinetics under open- and close-circuit conditions have thus been exploited for selective self-assembly on a given microelectrode in close (6 µm) proximity to a second one. Such high spatial resolution paves the way for localizing different surface chemistries at individual elements of high-density arrays.
Cyclic voltammetric (CV) probing of the barrier properties at closely spaced disk electrodes was used for demonstrating the selective SAM formation on gold microelectrodes. The electrode block of a dual-electrode thin-layer flow detector, containing two identical closely spaced gold electrodes (Figure 1, bottom), was used in connection to the ferricyanide redox marker. The dualelectrode assembly was immersed into an ethanolic solution containing 1 × 10-4 M 1-dodecanethiol. Holding the right-side electrode (R) at -1.5 V (vs Ag/AgCl) for 1 min resulted in a complete blocking of the heterogeneous electron transfer (Figure 1a′). The second electrode (L), kept at open circuit during the same period, displayed a well-defined cyclic voltammogram (Figure 1a), similar to that observed in the “thiol-free” control experiment (Figure 1c,c′). The experiment was then repeated/reversed, after polishing the electrodes and restoring the CV peak, by applying the negative potential onto the left-side electrode (L), while keeping the right one (R) at open circuit. This resulted in reversal of the surface blockage, with a defined response and no signal for the open and close circuit operations, respectively (Figure 1b′,b). A similar (“on/off”) switching of the barrier properties, with a highly reproducible response, was observed upon repeating/reversing the process over four additional cycles (not shown). The corresponding voltammograms obtained in the thiol-free open-circuit control experiment (Figure 1c,c′) were similar to those observed at the bare electrodes and in the thiolfree close-circuit control experiment (not shown). The similar marker response obtained in the presence and absence of the thiol using open-circuit conditions (a and b′ vs c and c′) indicates a negligible chemisorption during the 1 min period. Optical micrographs of closely spaced interdigitated microband electrodes were also used for demonstrating the localized electrochemical assembly of the alkanethiol monolayer (Figure 2). The interdigitated electrode array contained two opposing branches of 14 µm wide gold band electrodes. The array was placed in an ethanolic solution containing 1 × 10-3 M 11-mercaptoundecanoic acid for 1 min. During that period, one electrode branch was kept at -1.5 V, while the second one was kept at open circuit. Subsequently, the surface was washed and incubated in a Rhodamine 6G solution. The resulting image (Figure
* Corresponding author. E-mail:
[email protected]. (1) Finkela, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335. (2) Wang, J. Analytical Electrochemistry, 2nd ed.; Wiley: New York, 2000. (3) Herne, T. M.; Tarlov, M. J. J. Am. Chem. Soc. 1997, 119, 8916. (4) Hsueh, C. C.; Lee, M. T.; Freund, M.; Ferguson, G. S. Angew. Chem., Int. Ed. 2000, 39, 1228. (5) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116. (6) Tender, L. M.; Worley, R. L.; Fan, H. Y.; Lopez, G. P. Langmuir 1996, 12, 5515. (7) Imabayashi, S.; Hobara, D.; Kakiuchi, T., Knoll, W. Langmuir 1997, 13, 4502.
10.1021/la000956n CCC: $19.00 © 2000 American Chemical Society Published on Web 11/16/2000
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Figure 1. Cyclic voltammograms of 1 × 10-3 M potassium ferricyanide in 1 M KCl using a dual-gold electrode assembly. a, b, and c are voltammograms obtained at the “left” (L) electrode, while a′, b′, and c′ are voltammograms recorded at the “right” (R) electrode. (a, a′) The electrode pair was first immersed in ethanolic solution containing 1 × 10-4 M 1-dodecanethiol and 0.5 M NaClO4 for 1 min with the R electrode at -1.5 V (a′) and the L electrode at open circuit (a) before recording the voltammograms. (b, b′) The electrode pair was immersed in ethanolic solution containing 1 × 10-4 M 1-dodecanethiol/ 0.5 M NaClO4 solution for 1 min, with L at -1.5 V (b) and R at open circuit (b′) before recording the voltammograms. (c, c′) Control: the dual-electrode block was immersed in a “thiolfree” ethanolic solution containing 0.5 M NaClO4 for 1 min (at open circuit) and was dried before exposure to potassium ferricyanide solution. Scan rate was 50 mV/s. Between a,a′, b,b′, and c,c′ the electrodes were polished with alumina slurry and were ultrasonically washed with water. The dual-electrode block is shown in the bottom.
2A) indicates selective adsorption of the dye over the branch held at the negative potential (i.e., onto every second “finger”). The second branch, kept at open circuit, remained uncoated, despite the extremely narrow gap (6 µm) between the close- and open-circuit electrodes. Apparently, the mercaptoundecanoic acid SAM layer, formed only under potential control, attracted electrostatically the cationic dye. Larger spacings (of 70-80 µm) were employed in connection to anodic deposition of SAM films.4 No surface coating on either branch is observed for the control experiment, involving immersion of the electrode array in the dye solution without prior exposure to the thiol (Figure 2B). We also employed the electrochemical quartz crystal microbalance (EQCM) technique, with its unique ability to probe in real time minute mass changes at gold surfaces,8 for supporting the potential-induced electrochemisorption. Figure 3 compares typical time-frequency QCM profiles for the dodecanethiol/ethanol (A) and ethanol (B) solutions upon stepping the potential to -1.5 V (at the points indicated by the arrows). By use of the dodecanethiol/ethanol medium, the crystal electrode displays a rapid decrease of the resonance frequency upon applying the negative potential, reflecting the increased mass (8) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391.
Letters
Figure 2. Optical micrograph (500× magnification, using an Olympus BH2-UMA microscope and an Mitsubishi color video printer) of the gold interdigitated microband electrode (NTT, Japan) after incubation in a rhodamine 6G solution. The widths of the gold lines and the spacing between them are 14 and 6 µm, respectively. (A) A SAM-covered interdigitated electrode. The electrode was immersed into an ethanolic solution containing 1 × 10-3 M of 11-mercaptoundecanoic acid and 0.5 M NaClO4 for 1 min, while applying a potential of -1.5 V onto one branch and keeping the second one at open circuit. Then, it was thoroughly washed with ethanol and water. It was subsequently incubated with 20 µL of 0.05 M phosphate buffer containing 1 × 10-3 M rhodamine 6G for 30 min before it was thoroughly rinsed with water and dried. (B) Control: an untreated interdigitated electrode after a 30 min incubation with 20 µL of 0.05 M phosphate buffer containing 1 × 10-3 M rhodamine 6G, rinsing with water, and drying.
associated with the self-assembly process (A). Such a large frequency change suggests the formation of a multilayer. No frequency change is observed in control experiments in the absence of the alkanethiol (B) or without applying the cathodic potential (C). The rapid SAM formation is indicated also from comparison of the double-layer capacitance of the bare and dodecanethiol “electrochemically coated” electrodes, 23.7 and 8.4 µF/cm2, respectively [based on the slope of the charging current vs scan-rate plot (at +0.0 V) in a phosphate-buffer solution]. A control experiment, involving holding the potential for 1 min at -1.5 V in the alkanethiol-free ethanol/electrolyte solution, yielded a capacitance value of 31.7 µF/cm2. This value is larger than that of the bare electrode, indicating a potential-induced removal of a surface layer. Comparison of reflectance FTIR spectra of SAM films prepared with and without the cathodic potential control revealed a significantly larger CH-stretching band at ca. 2900 cm-1 (characteristic to SAM layers) following the electrochemical deposition.
Letters
Figure 3. EQCM response in a 5 × 10-4 M 1-dodecanethiol/ ethanol (A, C) and ethanol (B) solutions (containing 0.5 M NaClO4) upon stepping the potential to -1.5 V (at the points indicated by the arrows; no potential step in C). Details of the EQCM system were given earlier.11
The selective self-assembly on a given electrode in the presence of neighboring ones relies on differences in the adsorption kinetics under open- and close-circuit conditions. A voltammetric blocking experiment indicated complete suppressions of the ferricyanide peak for periods longer than 50 s and 70 min under close- and open-circuit conditions, respectively (using a 1 × 10-3 M 1-hexadecanethiol solution, not shown). Examining the influence of the applied potential upon the electrochemical-induced adsorption (under the above conditions) indicated that the process starts at potentials more negative than -0.2 V, with 64, 78, and 100% diminutions of the ferricyanide peak using -0.5, -1.0, and -1.5 V, respectively. Such rapid (
