© Copyright 2000 American Chemical Society
DECEMBER 12, 2000 VOLUME 16, NUMBER 25
Letters Fabrication of Microelectrode Arrays Using Microcontact Printing H. X. He,*,† Q. G. Li,‡ Z. Y. Zhou,‡ H. Zhang,‡ S. F. Y. Li,*,§ and Z. F. Liu*,‡ Center for Nanoscience & Technology (CNST), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China, and Institute of Materials Research and Engineering (IMRE), Blk S7, Level 3, Singapore 119260 Received May 3, 2000. In Final Form: October 12, 2000 Geometrically defined gold microelectrode arrays are fabricated, in which a self-assembled monolayer of hexadecylmercaptan acts as passivation layer and is region-selectively formed by microcontact-printing (µCP). By use of differently designed stamps in the µCP procedure, the dimension of individual microelectrode and the pitch between in the microelectrode array can be easily controlled. The prepared microelectrode arrays are characterized by scanning force microscopy and cyclic voltammetry. The results demonstrate that the cross-talk between the individual microelectrodes, which causes current shielding effect, directly correlates the arrangement and the size of the microelectrodes in the arrays. This is in good agreement with the standard microelectrode theory.
Introduction A microelectrode array may increase the current for electrochemical measurements without losing the special features of a singe microelectrode, such as high mass flux, steady-state current, etc. It has shown great impact in electrochemical sensors and biosensors.1-3 To practically attain these characteristics. it is realized that the size, the shape, and the distance between individual electrodes (which contributes to the current shielding effect due to diffusional overlap) in the microelectrode array must be * To whom correspondence should be addressed. He: telephone, 305-348-6264; fax, 305-348-6700; e-mail,
[email protected]. Li: 65874-2681; e-mail,
[email protected]. Liu: 86-10-6275-7157; e-mail,
[email protected]. † Present address: Department of Physics, Florida International University, University Park, Miami, FL 33199. ‡ Peking University. § Institute of Materials Research and Engineering. (1) Wollenberger, U.; Hintsche, R.; Scheller, F. Microsyst. Technol. 1995, 1, 7. (2) Mohr, A.; Finger, W.; Fo¨hr, K. J.; Go¨pel, W.; Ha¨mmerle, H.; Nisch, W. Sens. Actuators, B 1996, 34, 265. (3) Gross, G. W.; Rhoades, B. K. Biosens. Bioelectron. 1995, 10, 553.
controlled.4,5 A number of techniques have been proposed to prepare microelectrodes and microelectrode arrays.6-8 In particlular, fabrication of microelectrodes based on molecular self-assembly was recently reported. It is wellknown that alkanethiols form very stable and well organized monolayers on gold9 and that these monolayers can effectively block electron transfer and mass transport between the electrode and redox couples diffused in the aqueous electrolytes.10-13 In the fabrication process, the gold electrode was first modified with an alkyl monolayer, (4) Scharifker B. R. J. Electroanal. Chem. 1988, 240, 61. (5) Heinze, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 1268. (6) Buss, G.; Scho¨ning, M. J.; Lu¨th, H.; Schultze, J. W. Electrochim. Acta 1999, 44, 3899. (7) Grzybowski, B. A.; Haag, R.; Bowden, N.; Whitesides, G. M. Anal. Chem. 1998, 70, 4645. (8) Sreenivas, G.; Ang, S. S.; Fritsch, I.; Brown, W. D.; Gerhardt, G. A.; Woodward, D. J. Anal. Chem. 1996, 68, 1858. (9) Ulman, A. Chem. Rev. 1996, 96, 1533. (10) Bader, W. S.; Crooks, R. M. J. Phys. Chem. B 1998, 102, 10041. (11) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (12) Abbott, N. L.; Rolison, F. R.; Whitesides, G. M. Langmuir 1994, 10, 2672. (13) Ohtani, M.; Sunagawa, T.; Kuwabata, S.; Yoneyama H. J. Electroanal. Chem. 1995, 396, 97.
10.1021/la000635b CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/2000
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and then the monolayer was partially removed to provide bare gold areas. These bare gold areas serve as microelectrodes. The experimental methods used to remove alkanethiol molecules in the monolayer includes etching by cyanide,10 micromachining by the scanning tunneling microscope tip11 or a surgical scalpel,12 and eliminating by UV irradiation through a mask.13 In this work, we report a new approach using a reverse process to facilely fabricate geometrically defined microelectrode arrays. The idea is to take the best advantage of the microcontact printing technique (µCP)14 to selectively form self-assembled monolayers (SAMs) on designed areas at the Au electrode surface. It is conceptually different from the previously reported methods, which partially destroy an existing SAM to construct geometrically defined microelectrode arrays. The merit of this method is that a microelectrode array containing a large number of microelectrodes with given dimensions and separations can be constructed within a single step. This will impact on both fundamental research and development of electrochemical sensor devices. Experimental Section Hexadecylmercaptan (HS(CH2)15CH3) was purchased from Aldrich; all other reagents were analytical grade and used as obtained. Absolute ethanol was further purified by distillation. Gold substrates were prepared by sputtering high-purity gold onto cleaned Si(100) wafers with a Ti adhesion layer (250 nm of Au and 10 nm of Ti). The obtained gold substrates were cleaned in piranha solution (98% H2SO4:30% H2O2 ) 3:1, v/v) prior to modification. Caution: piranha solution reacts violently with most organic materials and must be handled with extreme care. The µCP technique was used to fabricate microelectrode arrays. Briefly, elastomeric stamps with recess features were prepared by casting poly(dimethylsioxane) (PDMS, Dow-Corning Corp., Sylgard 184) against masters prepared by UV-lithography. The stamp was inked with 10 mM ethanol solution of HS(CH2)15CH3 and brought into contact with a piece of clean Au substrate for 10 s. A patterned methyl-terminated monolayer is transferred from the stamp onto the Au substrate leaving an array of bare Au dots which acted as microelectrodes. Before characterization by atomic force microscopy (AFM), friction force microscopy (FFM) (Nanoscope III A, Digital Instruments, Santa Barbara, CA), and cyclic voltammetry (CV), the microelectrode array was sonicated in distilled ethanol and water successively and then dried under a stream of nitrogen. All electrochemical measurements were performed using a Hokuto Denko HA-150 potentiostat and a HB-111 function generator. The experiments were carried out in a one-compartment cell. An Ag/AgCl/saturated KCl electrode was used as the reference electrode, a Pt wire was used as the counter electrode, and 1 mM K3Fe(CN) 6 and 0.1 M NaClO4 were used as electrolyte solution. The cyclic voltammograms were recorded with a Riken Denshi D-72DG X-Y recorder.
Results and Discussion The key point of SAM-based microelectrode is that a closely packed SAM terminated with hydrophobic functional groups can effectively block electron transfer and mass transport between the Au substrate and redox couples in electrolyte solutions. The blocking effect of SAMs that were prepared by dipping clean Au electrode into dilute alkanethiol solution for 24 h has been very well studied.15 The microcontact printing (µCP) technique now becomes a routine way to fabricate chemical patterns with micrometer dimensions.14 Since it is principally a “dry” process, it only involves transient contact between the Au substrate and the inked stamp. No liquid drops are present during the formation of monolayers, which might influence (14) Kumar, A.; Whitesides G. M. Appl. Phys. Lett. 1993, 63, 2002. (15) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657.
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Figure 1. Cyclic voltammograms in 1 mM K3Fe(CN)6, 0.1 M NaClO4 supporting electrolyte solution at a scan rate of 50 mV/s: (a) the naked Au electrode; (b) the unpatterned SAMcovered Au electrode prepared by µCP method, the inking solution is 10 mM hexadecylmercaptan ethanol solution.
the quality of the film.16 So to form a defect-free and closely packed SAM during the µCP process becomes the key issue in successful fabrication of microelectrode arrays. Our work begins with examining the blocking effect of a SAM formed by µCP. A CH3(CH2)15SH SAM modified Au electrode was prepared using an unpatterned PDMS stamp following the same µCP procedure as described by Larsen et al.17 Figure 1 shows the cyclic voltammograms of a bare Au electrode and a SAM-covered electrode obtained in 1.0 mM Fe(CN)63-/0.1 M NaClO4 electrolyte solution. As shown in Figure 1a, the bare Au electrode exhibits well-defined reversible electrochemical responses, which are attributed to the reduction/oxidation of Fe(CN)63-/4-. The separation between the anodic and cathodic peak potentials is ∼70 mV. A relationship of ip ∼ ν0.5 between cathodic peak current (ip) and scan rate was obtained, which is typical for a macroelectrode with planar diffusion.18 However, for the SAM-modified Au electrode, the electrochemical response of Fe(CN6)3- is barely discerned, indicating that the electrochemical reaction was suppressed by the CH3(CH2)15S SAM on the electrode surface. The current scale in Figure 1b can be compared with that of a CH3(CH2)15S SAM modified Au electrode which was prepared by soaking gold electrodes in 1 mM CH3(CH2)15SH/CH3CH2OH solution for 24 h, suggesting that the CH3(CH2)15S SAM formed by the µCP method can be used as an effective blocking layer for electrochemical reactions on the gold electrode. Note that when we use 1 mM CH3(CH2)15SH solution as ink to prepare the SAM, the electrochemical reaction cannot be blocked completely. In fact, inking the stamp with 10 mM or greater of CH3(CH2)15SH in ethanol produces SAMs on gold having the same blocking effect as those SAMs prepared by equilibration of a gold substrate in solution. When the inking solution is lower than 10 mM, the blocking function of the SAM diminishes. This is completely in accord with the wettability and STM studies, which demonstrated that the SAM formed with lower coverage and more defects.16,17 To fabricate a microelectrode array, in which microelectrodes were embedded in the SAM “sea” by the µCP method, a special PDMS stamp was designed which consists of orderly arranged recessed cylindrical features, 1 µm in diameter, 1.5 µm in depth, and 5 µm in pitch. This PDMS stamp was then inked with a 10 mM CH3(CH2)15(16) Biebuyck, H. A.; Larsen, N. B.; Delamarche, E.; Michel, B. IBM J. Res. Dev. 1997, 41, 159. (17) Larsen, N. B.; Bietsch, H.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017. (18) Bard, A. J.; Fan, F.-R. Electrochemical Methods, Fundamentals and Applications; Wiley: New York, 1980.
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Figure 2. Friction force images of the prepared SAM-based microelectrode arrays: (a) 1 µm in diameter and 5 µm in pitch; (b) 3 µm in diameter and 5 µm in pitch acquired with Si3N4 tip. These two images were collected concurrently with their topographic atomic force images, and the scan direction is from left to right.
SH ethanol solution and placed into contact with Au substrate. In the regions contacted with the stamp, a CH3terminated SAM was formed by transferring CH3(CH2)15SH molecules from the PDMS stamp to the Au substrate. Periodically arranged bare gold dots (the uncontacted regions, i.e., the recessed cylinder regions on the PDMS stamp) were left on the surface, which are expected to act as microelectrode array. Figure 2a shows the FFM image of this patterned SAM. In the AFM topographic image, only a faint contrast was observed (which is not shown here) because of the large difference in friction forces between Si3N4/Au and Si3N4/CH3, which cause cantilever buckling that leads to errors in measuring the sample heights during scanning.19 However, it is because of the large different friction forces between the two regions that the concurrent friction image clearly exhibits the periodic array of bright Au dots of about 1 µm diameter and 5 µm pitch (referred to as a 1 × 5 microelectrode array). Both the shape and the dimension of the pattern are consistent with our expectations except for a few discernible defects due to the deformations of the PDMS stamp in the casting process. Cyclic voltammetry of thus prepared microelectrode array was carried out to further charaterize its electrochemical behavior. As shown in Figure 3a, the voltammogram (scan rate ν ) 10 mV/s) obtained on this microelectrode array has a sigmoidal shape, a characteristic feature that results from the development of radial diffusion at each microelectrode surface, giving a limiting current (ilim) about 1.8 µA. For a defect-free microelectrode array and pure radial diffusion without cross-talk between individual electrodes, the current response from the array should be a simple sum of the current from each electrode, given by20
ilim ) 4nFDCrAgq where Ag is the geometric area of the electrode (0.05 cm2), C is the concentration, D is the diffusion coefficient, r is the microelectrode radius, and q is the electrode number (19) Warmack, R. J.; Zheng, X.-Y.; Thundt, T.; Allison, D. P. Rev. Sci. Instrum. 1994, 65, 394. (20) Cheng, F.; Whiteley, L. D.; Martin, C. R. Anal. Chem. 1989, 61, 762.
Figure 3. Cyclic voltammograms of SAM-based microelectrode arrays in 1 mM K3Fe(CN)6, 0.1 M NaClO4 supporting electrolyte solution at varied scan rates (from top to bottom, 10, 50, and 100 mV/s): (a) 1 µm in diameter and 5 µm in pitch; (b) 3 µm in diameter and 5 µm in pitch.
density (0.0284 electrodes/µm2, obtained from the FFM image assuming no electrode missing in the array). We would expect the limit current should be 2.4 µA, higher than the measured value. We believed that it is due to the missing electrodes and irregular edges of some of the individual electrodes in the array, which causes reduced microelectrode number and reduced electrode area of some individual electrodes. In addition, some overlap of the individual diffusion layers might be another contribution. The ilim in the voltammogram at 100 mV/s (Figure 3a) is obviously increased, and the hysteresis of the curve between forth and back scanning becomes more obvious. The limit currents at varied potential sweep rates between 1 and 100 mV/s were related to the scan rates by an exponent of 0.15 (ilim ∼ ν0.15). For pure radial diffusion without cross-talk between microelectrodes, the limit currents should be independent of the potential sweep rate. For pure linear diffusion an exponent of 0.5 would be expected over this range of scan rates.18 The currently obtained exponent of 0.15 can be understood as the result of radial diffusion and the diffusion layer developed at each microelectrode overlapped to some extent. According to the theory of microelectrode arrays,21 the size (diameter (21) Wightman, R. M.; Wipf, D. O. Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1989; Vol. 15.
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r) and the distance (d) between the centers of two adjacent microelectrodes determine the diffusion behavior of the microelectrode array. The larger the ratio of r/d, the more diffusion cross-talk between the electrodes occurs, and the more planar electrode characteristic will be shown. The voltammogram curves from another microelectrode array of 3.0 µm in diameter and 5.0 µm in pitch (referred to as a 3 × 5 microelectrode array) (Figure 2b), shown in Figure 3b, provided more evidence. Compared to Figure 3a, a non-steady-state behavior develops even at a 10 mV/s scan rate, a peak on top of the steady-state voltammogram starts to develop, and the shape and voltammetric current are dependent upon the scan rate. An exponent of 0.26 (ilim ∼ ν0.26) was obtained from various scan rates, the voltammetric response exhibits characteristic of both radial and linear diffusion, and the linear diffusion behavior and the shielding effect, caused by the overlap of diffusion layers, becomes more obvious than the 1 × 5 microelectrode array, consistent with the microelectrode array theory as discussed above. In terms of the microelectrode array theory, a better design for microelectrode array without shielding effect can be achieved by either enlarging the separation between the individual microelectrode in the array or reducing their size, even down to a nanometer-scale, which is more interesting and useful.22 While microcontact printing is only a routine way to make micro-sized patterns, note that we did not find any electrode missing in the 3 × 5 microelectrode array and the features agree well with the design (in Figure 2b). As the size becomes smaller, making defect-free patterns becomes more difficult because of an increasing convolution between the effects of stamp formation and ink localization. During our experiment, we indeed met some difficulties to make defect-free microelectrodes (smaller than 1 µm) because of the experimental situation in our lab to make the defect-free master and accordingly the PDMS stamp. But this is not the situation found in industry, where 193-nm features can be routinely fabricated nowadays. In addition, there are some efforts reported on how to make nanometersized patterns using the microcontact-printing technique, and the results are encouraging.23,24 The geometrically defined nanoelectrode arrays by µCP are believed to be coming out soon. (22) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 238, 1118. (23) Libioulle, L.; Beitsch, A.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 1999, 15, 300. (24) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324.
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The stability of the microelectrode array is vital for any further applications. In our experiment, no discernible changes in the CV curves were observed even after several tens of cycles. After one year of storing, very similar AFM and FFM images on the patterned SAM still can be obtained, providing strong evidence of its high stability. And it could possibly be stored for a much longer period in darkness, as UV light may cause oxidation of alkanethiolates to alkanesulfonates.25 Since the microelectrode array is a thiol SAM based array, the stability of microelectrode array is the stability of the SAM, which is based on the power of the covalent binding of thiols on a Au surface. There are an abundant number of papers that examined the stability of alkanethiol SAMs on gold in both aqueous and nonaqueous solution over the past decade.26-28 It has a wide electrochemical potential stable window, stable until ∼1.2 V (vs SCE), above where the oxidation of thiols takes place. Decreasing the potential to ∼-0.6 V causes thiol desorption or some structure changes in the SAM to begin to occur.29,30 In summary, the present results demonstrate that the µCP technique can be successfully applied to fabricate microelectrode arrays. Once the stamp is available, any features on it can be transferred to the Au substrate in a single step. We can design any pattern of microelectrode arrays to meet research and application demands. Since the reproduction is easy and cost effective, the microelectrode array can even be used as a disposable electrode. With respect to some considerations, for example, highspeed electrochemical reactions, these arrays offer no fundamental advantage over single microelectrode as the RC and iR effect becomes larger. However, there is a practical gain. The arrays produce much higher currents than a single microelectrode so that routine electroanalytical measurements can be made without using sophisticated instrumentations. LA000635B (25) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342. (26) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (27) Everett, W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292. (28) Beulen, M. W. J.; Kastenberg, M. I.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1998, 14, 7463. (29) Riepl, M. Mirsky, V. M.; Wolfbeis, O. S. Mikrochim. Acta 1999, 131, 29. (30) Tender, L. M.; Opperman, K. A.; Hampton, P. D.; Lopez, G. P. Adv. Mater. 1998, 10, 73.
