J . Phys. Chem. 1991, 95, 9042-9044
9042
From a practical point of view, the present study suggests that monitoring the change in an image as the force is varied will enhance the ability of AFM to characterize surfaces of molecular crystals. Combined with bulk crystallographic data and numerical simulations AFM has the capacity to become an important surface
crystallographic tool.
Acknowledgment. Support from NSF, NIH, and DARPA is gratefully acknowledged. We thank John Milligan for assistance with the graphics.
Electrochemical Preparatlon of Ultrathin Polypyrrole Film at Mkroarray Electrodes M. Nishizawa, M. Shibuya, T. Sawaguchi, T. Matsue,* and I. Uchida*
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Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tokoku University, Sendai 980, Japan (Received: August 6, 1991; In Final Form: September 25, 1991)
Electrosynthesized polypyrrole grows effectively at a hydrophobic substrate. Hydrophobic pretreatment of interdigitated microarray electrodes with silanization reagents prompts the lateral growth of polypyrrole film,resulting in the interconnection of arrays with a thin, uniform film. On the basis of this phenomenon, the micropatterning with polypyrrole can successfully be carried out by electropolymerization at a substrate with a silanized pattern.
Introduction Electrosynthesized conducting polymers such as polypyrrole and polyaniline have been studied extensively because of the fundamental interests and the potentiality in practical applications.’ The conductivity characteristics of electrosynthesized conducting polymers have recently been investigated by in situ measurements using microarray electrodes coated with the polymers.2-s Such array electrodes also serve as electrochemical devices responsive to chemicals influencing the polymer condu~tivities.~*~ We recently reported the characterization of the copolymer of pyrrole and N-methylpyrrole by in situ conductivity measurements8 and the fabrication of an enzyme-based switching deviceg showing the on off response upon addition of NADH. Through these researches, we found that formation of thin polymer films at the arrays is essential to improve the sensitivities and reproducibilities of the devices. The thin film can be formed by promoting the lateral growth of the polymer along to the substrate upon poon the control lymerization. So far, there are a few of growing direction of electrosynthesized polymers. In this paper, we report the promotion of the lateral growth of polypyrrole by hydrophobic pretreatment of the substrate. The pretreatment of the glass area of the microarray electrodes is effective for the interconnection of the arrays with thin polymers. Since the polymerization selectively proceeds on the hydrophobically pretreated area, the patterned pretreatment results in a micropatterning of a glass substrate with polypyrrole.
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a r Ia y el e c l r o d e GI a s s subst rat e
Figure 1. Schematic drawing of microarray electrode used in the present study. Electrode length: 3 mm. Electrode width: IO pm. Gap width: IO p m .
Thin Polypyrrole Film Connecting the Array Electrodes The microarray electrode used in this study was interdigitated type with two sets of Pt arrays separated by IO pm from the adjacent elements (see Figure I ) . The array electrode was fabricated on a glass substrate by photolithography. The hydrophobic pretreatment of the microelectrode was carried out by silanization” in a 20 mM octadecyltriethoxysilane/benzene solution for 12 h, followed by a through rinse with pure benzene and drying. Judging from the wettability of the treated electrode, the silanization proceeds mainly at the glass area. The array electrode was placed at a bottom hole of the electrochemical cell with an O-ring (4-mm diameter); the total electrode area exposed to the solution was ca. 5 X IO-) cm2. The electropolymerization at the hydrophobically pretreated electrode was conducted in an aqueous solution containing 0.1
*To whom correspondence should be addressed. 0022-3654/9 1 /2095-9042%02.50/0
Time / mln
Time / min
Figure 2. Variation of currents during the controlled potential polym-
erization at the microarray electrodes in 0.1 M pyrrole/O.l M KNOl aqueous solution. (A) Untreated electrode. (e) Hydrophobically pretreated electrode. (-) Current at the array held at 670 mV vs SCE. (- - -) Current at the array held at 650 m V vs SCE. Arrow indicates the moment of interconnection between the arrays.
M KNO) and 0.1 M pyrrole under potentiostatic conditions. Pyrrole was used immediately after being purified by distillation under a reduced pressure. The polymerization potential for one of the two arrays was set at 650 mV and the other at 670 mV vs SCE by means of a bipotentiostat. Since the potential of the two arrays are slightly different, the ohmic current is superposed 0 199 1 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9043
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electrode polypyrrole electrode polypyrrole contact point contact line Figure 3. S E M images of polypyrrole films a t the microarray electrodes. (A) Untreated electrode (total charge, 11.91 mC). (B) Hydrophobically pretreated electrode (total charge, 0.98 mC).
on the polymerization current after achievement of the interconnection of arrays with polypyrrole. Therefore, it is possible to detect the moment of the interconnection. Figure 2 shows typical variations of the currents during polymerization at (A) the untreated (hydrophilic) array and (B) the treated (hydrophobic) electrodes. The bending points (shown by arrows) indicate the moment of the interconnection between the arrays with polypyrrole. The charge required for the interconnection at the hydrophobic electrode is less than one-tenth that observed at the untreated, hydrophilic electrode. This resuit indicates that the hydrophobic treatment is effective for lateral growth of polypyrrole along the glass surface. Hydrophobic interaction concentrates pyrrole monomer in the alkyl chains of silanized layers at the glass surface, as is observed in monolayer assemblies on e1e~trode.l~ Therefore, the polymerization proceeds preferentially in the lateral direction. Figure 3 shows the SEM images of electrosynthesized polypyrrole films at the untreated (A, total charge, 11.91 mC) and the hydrophobically pretreated (B, total charge, 0.98 mC) microarray electrodes. The images indicate the great morphological difference, although the apparent resistances between the two arrays are on the same order (A, 5.1 X lo3 Q;B, 3.5 X lo3 Q) after the preparation. The film at the treated electrode is very thin (the thickness at the electrode element, ca. 0.3 pm) and connects the 10-pm gap of the arrays. The pretreatment obviously promotes the lateral growth of the polymer; the lateral growth rate is at least 16 times faster than the vertical growth rate at (1) Skotheim, T. A. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986; Vol. 1, 2. (2) Schiavon, G.; Sitran, S.; Zotti, G . Synth. Met. 1989, 32, 209. (3) White, H. S.; Kittlesen, G. P.; Wrighton, M. S . J. Am. Chem. SOC.
1984, 106, 5375. (4) Kittlesen, G . P.; White, H. S.; Wrighton, M. S . J. Am. Chem. SOC. 1984, 106,7389. ( 5 ) Paul, E. W.; Ricco, A. J.; Wrighton, M. S . J. Phys. Chem. 1985,89, 1441. (6) Thackeray, J. W.; White, H. S.; Wrighton, M. S . J. Phys. Chem. 1985, 89, 5 133. ( 7 ) Thackeray, J. W.; Wrighton, M. S . J. Phys. Chem. 1986, 90, 6674. (8) Nishizawa, M.; Sawaguchi, T.; Matsue, T.; Uchida, I. Synth. Met., in
Dual band electrode
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