Microscale Patterning of Organic Films on Carbon Surfaces Using

Jul 18, 2006 - In the build-up patterning approach, the PDMS mold is sealed to the modified PPF surface, and a chemical coupling reaction is effected ...
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Langmuir 2006, 22, 10739-10746

10739

Microscale Patterning of Organic Films on Carbon Surfaces Using Electrochemistry and Soft Lithography† Alison J. Downard,* David J. Garrett, and Emelyn S. Q. Tan MacDiarmid Institute for AdVanced Materials and Nanotechnology, Department of Chemistry, UniVersity of Canterbury, PriVate Bag 4800, Christchurch, New Zealand ReceiVed April 27, 2006. In Final Form: June 5, 2006 We have demonstrated three simple strategies employing poly(dimethylsiloxane) (PDMS) molds for patterning carbon surfaces with two different modifiers in an 18 µm line pattern. The PDMS molds are patterned with microfluidic channels (approximately 22 µm wide and 49 µm deep) and form a reversible, conformal seal to the pyrolyzed photoresist film (PPF) and modified PPF surfaces. Modifiers are electrochemically grafted to the PPF surface by the reduction of aryl diazonium salts and the oxidation of primary amines. For the fill-in patterning approach, the first modifier is electrografted to the PPF surface exposed within the microchannels, and in a second grafting step after removal of the PDMS mold, the second modifier fills in the remaining surface. The selective conversion strategy involves electrografting a continuous film of the modifier to the PPF surface, sealing the PDMS mold to the modified surface and carrying out an irreversible electrochemical reaction of the modifier exposed within the microchannels. In the build-up patterning approach, the PDMS mold is sealed to the modified PPF surface, and a chemical coupling reaction is effected in the microchannels to build up the pattern. The patterns are characterized using SEM, optical microscopy, the formation of condensation figures, and SEM imaging after the assembly of Au nanoparticles.

Introduction Since the first report of electrochemically assisted covalent modification of carbon surfaces in 1990,1 many studies have demonstrated the utility of this class of modification methods.2,3 Electrochemically assisted methods are those in which the addition or subtraction of an electron from a species in solution at the electrode generates a reactive radical that couples to the surface with the formation of a covalent bond. The best-studied reaction is the reduction of an aryl diazonium cation to yield an aryl radical;4,5 fewer studies have been reported of the oxidation of primary amines6-8 and aryl acetates9,10 giving amine- and methylene-based radicals, respectively. The modification method has been extended to other conducting and semiconducting materials,2,11-15 and it has also been demonstrated that grafting †

Part of the Electrochemistry special issue. * Corresponding author. E-mail: [email protected]. Tel: 64-3-3642501. Fax: 64-3-3642110. (1) Barbier, B.; Pinson, J.; Desarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 1757-1764. (2) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429-439. (3) Downard, A. J. Electroanalysis 2000, 12, 1085-1096. (4) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201-207. (5) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883-5884. (6) Liu, J.; Dong, S. Electrochem. Commun. 2000, 2, 707-712. (7) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306-1313. (8) Adenier, A.; Chehimi, M. M.; Gallardo, I.; Pinson, J.; Vila, N. Langmuir 2004, 20, 8243-8253. (9) Brooksby, P. A.; Downard, A. J.; Yu, S. S. C. Langmuir 2005, 21, 1130411311. (10) Andrieux, C. P.; Gonzalez, F.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 4292-4300. (11) deVilleneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415-2420. (12) Adenier, A.; Bernard, M.-C.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 45414549. (13) Bernard, M. C.; Chausse, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15, 3450-3462. (14) Allongue, P.; De Villeneuve, C. H.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791-2798. (15) Chausse, A.; Chehimi, M. M.; Karsi, N.; Pinson, J.; Podvorica, F.; VautrinUl, C. Chem. Mater. 2002, 14, 392-400.

from aryl diazonium salt solutions can be carried out using solution-based redox agents.16 Even in the absence of deliberately added redox agents, a surface film can be grafted from solutions of aryl diazonium salts.17-20 Nevertheless, at present, the electrochemical formation of the reactive modifier appears to offer the greatest degree of control over the surface concentration of the resulting film. An attractive feature of the radical-based grafting methods is that the modifier is attached to the surface via a covalent C-substrate or N-substrate bond. In the case of diazoniumgenerated layers on carbon materials, the attachment has been shown to be stable at high temperatures, in a range of aggressive solvents and over a relatively wide potential range.4,5,21,22 In these aspects, the layers appear to have superior properties to those of alkanethiol self-assembled monolayers (SAMs) on gold surfaces. Patterning of surfaces with thin organic films is a desirable goal for numerous applications ranging from bio- and chemical sensors to molecular electronics. Patterning SAMs onto gold surfaces can be achieved using many different techniques and is a mature research area. In contrast, we very recently reported the first example of a pattern incorporating two different modifiers that were attached to a planar carbon surface through radical reactions.23 In that work, we electrochemically grafted a continuous layer of the first modifier to a sample of pyrolyzed photoresist film (PPF) followed by the removal of selected areas of the layer using mechanical scribing with an atomic force (16) Pandurangappa, M.; Lawrence, N. S.; Compton, R. G. Analyst 2002, 127, 1568-1571. (17) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491-501. (18) Hurley, B. L.; McCreery, R. L. J. Electrochem. Soc. 2004, 151, B252B259. (19) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370-378. (20) Combellas, C.; Delamar, M.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Chem. Mater. 2005, 17, 3968-3975. (21) Yu, S. S. C.; Downard, A. J. e- J. Surf. Sci. Nanotech. 2005, 3, 294-298. (22) D′Amours, M.; Belanger, D. J. Phys. Chem. B 2003, 107, 4811-4817. (23) Brooksby, P. A.; Downard, A. J. Langmuir 2005, 21, 1672-1675.

10.1021/la061148k CCC: $33.50 © 2006 American Chemical Society Published on Web 07/18/2006

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Figure 1. Strategies for patterning the carbon surface. The first step of each approach involves electrografting of the first modifier. (a) Fill-in, (b) selective conversion, and (c) build-up.

microscope (AFM) tip. The second modifier was then electrochemically grafted to the exposed areas, giving a nanoscale pattern that was imaged using AFM. Although high-resolution patterns can be prepared and visualized in this manner, the practical usefulness of the technique is limited because it is slow and tedious to carry out. For further development of methods for patterning two or more modifiers that are compatible with electrochemically grafted thin films, we have looked to the soft lithographic techniques that have become very widely utilized for generating micro- and nanoscale patterns of SAMs.24 Soft lithography is based on the use of a patterned elastomer (most commonly poly(dimethylsiloxane), PDMS) as a stamp, mask, or mold. The methods are generally inexpensive, simple, and fast to use and, apart from master preparation, can be carried out in standard chemistry laboratories. In this work, we have explored the patterning of PPF surfaces using a slab of PDMS patterned with approximately 22-µmwide microfluidic channels. The PDMS mold forms a conformal and reversible seal to the PPF and modified PPF surface. Three approaches are demonstrated, as shown in Figure 1. The first will be referred to as “fill-in” (Figure 1a) and involves sealing the PDMS mold to the carbon surface, filling the microfluidic channels with modifier solution, and electrochemically grafting the first modifier to the carbon surface exposed within the channels. After the removal of the PDMS mold, the second modifier is grafted to the areas of surface previously in contact with the PDMS. In the second strategy (Figure 1b), termed “selective conversion”, the modifier is electrochemically grafted to the surface as a continuous film, and the PDMS mold is sealed to the modified surface. An electrochemical reaction selectively converts the functional group of the modifier layer, exposed in the microchannels, to a different functionality. The third strategy, referred to as “build-up” (Figure 1c), entails electrochemically grafting the first modifier to the carbon surface, sealing the PDMS mold to the modified surface, and filling the microfluidic channels with a solution of the second modifier that couples to the terminal groups in the already attached layer. This latter strategy has recently been demonstrated for patterning monolayers formed by alkenes on hydrogen-terminated Si(111).25 A second important goal of the present study was to investigate methods for the visualization of patterned carbon surfaces. In particular, we are interested in methods that are more convenient and rapid than AFM imaging and are suitable for “proof of concept” patterning experiments. Experimental Section Materials. Tetraethyleneglycol diamine (Molecular Biosciences), AZ4620 photoresist (Clariant), and PDMS elastomer and curing agent (Sylgard 184, Dow Corning) were used as received. Nano (24) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551-575.

Downard et al. SU-8 50 negative photoresist and nanoremover PG (SU-8 cleaner) were obtained from Microlithography Chemicals Corporation, and SU-8 developer (1-methoxy-2-propyl)acetate) was from Merck. The tetrafluoroborate salts of 4-methylbenzenediazonium, 4-nitrobenzenediazonium, and 4-carboxybenzenediazonium were synthesized using standard procedures.26 Pyrolyzed photoresist films (PPFs) were prepared following reported methods.27 The preparation and drying of tetrabutylammonium tetrafluoroborate and acetonitrile for electrochemistry have been described previously.27 Citrate-capped Au nanoparticles were prepared by the method of Natan and co-workers.28 The as-prepared solution of colloidal particles was characterized by an absorption maximum at 520 nm. The average particle diameter, 11 nm, was measured from transmission electron microscopy (TEM) images obtained using a JEOL 1200EX electron microscope operating at 80 keV. Samples for TEM were prepared by depositing 1 drop of nanoparticle solution onto standard Formvar-coated copper grids and drying in air. The nanoparticle concentration was calculated assuming all Au was reduced to form spherical particles of diameter 11 nm, giving 1.3 × 1016 nanoparticles L-1. Preparation of Mask, Master, and Molds. Microfluidic channels were formed by replica molding29 of PDMS on a SU-8 photoresistpatterned silicon master. To prepare the master, a silicon wafer was spin coated with two layers of SU-8 photoresist. The first layer was coated at 1500 rpm for 15 s, and the second, at 3000 rpm for 20 s. The wafer was baked at 110 °C for 10 min, cooled for 10 min, and then exposed to 365 nm light through a mask for 20 s using a Suss MA 6 mask aligner. After 10 min of baking at 110 °C and cooling to room temperature, the wafer was developed, followed by washing and drying. A final 5 min bake at 95 °C completed the master preparation. The master has two parallel (22 ( 2) µm lines of height 49 µm and separation 380 µm. The mask was chrome on glass. PDMS prepolymer and curing agent (10:1) were mixed thoroughly, degassed under vacuum, and poured onto the master. The polymer was cured at 95 °C for 20-30 min and then carefully peeled off of the master. The PDMS was rinsed with ethanol and dried with N2. Small pieces of patterned PDMS were cut with a pen knife. For the preparation of the pattern based on the selective conversion of a nitrophenyl film, some experiments used PDMS that had been solvent extracted to remove non-cross-linked materials.30 The PDMS was successively swelled in pentane for 48 h, deswelled in toluene for 24 h and ethyl acetate for 24 h, and then dried at 120 °C for 48 h. The outcome of the patterning experiments did not appear to be dependent on whether the mold had been extracted. Electrochemistry. All electrochemical measurements were performed using a computer-controlled EG & G PAR model 173 potentiostat coupled to a Powerlab 4SP (ADInstruments). Samples (approximately 15 mm × 15 mm) of PPF were mounted horizontally on an insulated metal stage under a glass cell held down by four springs. A hole in the bottom of the cell was positioned on top of a viton O-ring (diameter ) 5.5 ( 0.2 mm), which sealed the solution above the PPF. For electrochemical experiments utilizing patterned PDMS, a larger O-ring was used (diameter ) 9.4 ( 0.2 mm), and the sample of PDMS (approximately 5 mm × 8 mm) was sealed to the PPF within the area defined by the O-ring prior to assembling the cell. Electrical contact was made using a copper strip placed on the PPF surface not exposed to solution. The auxiliary electrode was a Pt wire, and the reference electrode was a Ag/Ag+ (10-2 M AgNO3 in acetonitrile-0.1 M [Bu4N]BF4) or a Ag wire pseudoreference. All reported potentials are referenced to Ag/Ag+, against which the ferrocenium/ferrocene (Fc+/Fc) couple appeared at E1/2 ) 0.01 V. (25) Perring, M.; Dutta, S.; Arafat, S.; Mitchell, M.; Kenis, P. J. A.; Bowden, N. B. Langmuir 2005, 21, 10537-10544. (26) Saunders: K. H.; Allen, R. L. M. Aromatic Diazo Compounds, 3rd ed.; Edward Arnold: London, 1985. (27) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038-5045. (28) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735-43. (29) Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974-4984. (30) Lee, J. N.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 65446554.

Microscale Patterning of Organic Films All electrochemical measurements were made at room temperature in an N2 atmosphere. Grafting of the First Modifier to PPF. Unless otherwise stated, electrochemical grafting of tetraethylene glycol diamine (TGD), methylphenyl (MP), and nitrophenyl (NP) layers to PPF was carried out in 0.1 M [Bu4N]BF4-acetonitrile solutions containing approximately 5 mM TGD or diazonium salt. To graft carboxyphenyl (CP) layers, the diazonium salt concentration was 0.6 mM. A fresh sample of PPF was used for every modification. The standard modification procedure for grafting TGD, MP, and NP layers entailed an initial cyclic scan from 0 V to Eapp at 100 mV s-1, followed by a potential step from 0 V to Eapp for 10 min (TGD and MP) or 2 min (NP). Eapp was 1.20, -1.12, and -0.72 V for TGD, 4-methylbenzenediazonium, and 4-nitrobenzenediazonium, respectively. For electrografting within the PDMS microchannels, the modifier concentration was 20 mM, and the initial cyclic scan was obtained at 50 mV s-1. Unless stated otherwise, CP was grafted using Eapp ) -0.85 V for 120 s. Pattern Formation. Background films for the fill-in approach were grafted using a modifier concentration of 5 mM and a single cyclic scan at 100 mV s-1. The potential limits were the same as those given above. For selective conversion patterning, a solution of 0.02 M benzoic acid in 0.1 M [Bu4N]BF4-acetonitrile was introduced into the microchannels, and cyclic scans were recorded at 50 mV s-1 between 0 and -2.0 or 0.3 and -2.3 V. After patterning via the fill-in and selective conversion strategies, the PDMS mold was removed from the surface, and the modified PPF samples were rinsed successively with acetonitrile and water and dried with compressed N2(g) prior to further use or storage under vacuum. For the build-up patterning approach, a DMF solution of 0.1 M ethylenediamine (en), 20 mM 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), and 4 mM N-hydroxysuccinimide (NHS) was introduced into the microchannels and allowed to react with the CP film for 2 h. After reaction, the PPF-PDMS assembly was rinsed twice in DMF, and the PDMS was then removed. The patterned PPF surface was rinsed twice with Milli-Q water, dried with N2(g), and stored under vacuum. Characterization of Films and Patterns. AFM (Digital Instruments Dimension 3100) depth profiling measurements were performed on modified PPF samples, as previously described.27,31 The technique involves using the AFM tip to scratch a section of film from the PPF surface. From the cross-sectional profile across the film and scratch, the film thickness can be determined. The uncertainties reported with film thickness measurements include the uncertainty arising from instrumental (AFM) limitations, the variation in the depth of PPF removed when the film is scratched from the surface, and the variation in the measured film thickness for replicate samples. An Olympus BX60 inverted light microscope equipped with an Olympus DP10 camera was used for optical microscopy. A polarizer was used for viewing the carbon surfaces. To obtain static water contact angles, the PPF sample was placed on a horizontal stage, and 1 or 2 µL of Milli-Q water was dispensed onto the surface from a microsyringe. The water droplet image was captured by an Edmund Scientific video camera and video for Windows NT software. Three measurements were taken from each side of the drop, and two drops were applied, sequentially, to each modified surface. The average water contact angle of two separate samples (i.e., of 24 measurements) was calculated, and the stated errors include all of the measurements taken. Condensation figures (CFs) were generated on modified PPF surfaces by two methods. In one method, a video was recorded of steam condensing on the surface, and a still photograph was extracted from the video using Windows Movie Maker software. The simpler approach was to breathe on the surface and obtain an optical micrograph of the water condensation. Gold nanoparticles were assembled on modified PPF surfaces by immersion in an as-prepared nanoparticle solution for the selected (31) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 87918798.

Langmuir, Vol. 22, No. 25, 2006 10741 time at room temperature. The assembly time was 3 h for all nonpatterned films. After removal from the nanoparticle solution, the samples were gently rinsed with water, dried with a gentle stream of N2(g), and stored under vacuum. Scanning electron microscopy (SEM) images were obtained using a Raith 150 e-beam lithography system operating with a 10 kV acceleration voltage. The quantification of nanoparticle assemblies utilized 200 000× SEM images. Nanoparticles were counted manually or using Image Pro Plus software. To ensure an accurate representation of the nanoparticle assembly, the area over which nanoparticles were counted was varied according to the density of the assembly. The reported values are standardized to a surface area of 1 µm2 and are the averages of nanoparticle counts for all samples prepared using the same conditions. The associated uncertainties indicate the full range of values obtained for all samples.

Results and Discussion The modifiers chosen for electrochemical grafting were the 4-methylbenzene-, 4-nitrobenzene-, and 4-carboxybenzenediazonium cations and tetraethyleneglycol diamine (TGD). Equations 1 and 2 show the electrochemically assisted grafting reactions for the two types of modifiers, where only the grafting of the first layer to the carbon surface is represented. Continued generation of radicals that couple to the already-grafted layer has been demonstrated to give multilayer films during the reduction of aryl diazonium salts,27,31-33 and similarly, the oxidation of diamines can give films of thickness greater than a monolayer.34,35

In initial experiments, continuous films of each modifier were grafted to PPF, water contact angles were measured, the thickness of MP and TGD films was measured, and the assembly of citratecapped Au nanoparticles on each surface was examined. The characteristics of aminophenyl (AP) films, formed by the electrochemical conversion of NP films (eqs 3 and 4)4,36,37 were also examined, as were en-CP surfaces that resulted from coupling ethylenediamine (en) to CP films via amide bond formation (eq 5).

PPF-Ph-NO2 + 4H+ + 4e- f PPF-Ph-NHOH + H2O (3) PPF-Ph-NHOH + 2H+ + 2e- f PPF-Ph-NH2 + H2O (4) PPF-Ph-COOH + H2NCH2CH2NH2 f PPF-Ph-CONHCH2CH2NH2 (5) Characterization of MP, NP, AP, TGD, CP, and en-CP Films and Assembly of Au Nanoparticles. MP, NP, CP, and (32) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947-5951. (33) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 38373844. (34) Antoniadou, S.; Jannakoudakis, A. D.; Jannakoudakis, P. D.; Theodoridou, E. J. Appl. Electrochem. 1992, 22, 1060-1064. (35) Downard, A. J.; Jackson, S. L.; Tan, E. S. Q. Aust. J. Chem. 2005, 58, 275-279. (36) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. Carbon 1997, 35, 801-807. (37) Ortiz, B.; Saby, C.; Champagne, G. Y.; Belanger, D. J. Electroanal. Chem. 1998, 455, 75-81.

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Table 1. Water Contact Angles and Density of Au Nanoparticle Assemblies on Films Grafted to PPF Surfaces film water contact angles (deg) nanoparticle assembly (np/µm2)

MP

TGD

NP

AP

CP

en-CP

65 ( 4 63 ( 27 (2)a

46 ( 3 2875 ( 400 (3)

64 ( 4