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Surface Modification of Elastomeric Stamps for Microcontact Printing of Polar Inks Veera Bhadraiah Sadhu,† Andra´s Perl,† Ma´ria Pe´ter,† Dorota I. Rozkiewicz,† Gerard Engbers,‡ Bart Jan Ravoo,† David N. Reinhoudt,† and Jurriaan Huskens*,† Laboratories of Supramolecular Chemistry and Technology & Molecular Nanofabrication, MESA+ Institute for Nanotechnology, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, and Ssens bV, Boortorenweg 10, 7554 RS Hengelo, The Netherlands ReceiVed December 18, 2006. In Final Form: March 9, 2007 Chemical modification of the surface of a stamp used for microcontact printing (µCP) is interesting for controling the surface properties, such as the hydrophilicity. To print polar inks, plasma polymerization of allylamine (PPAA) was employed to render the surface of poly(dimethylsiloxane) (PDMS), polyolefin plastomers (POP), and Kraton elatomeric stamps hydrophilic for long periods of time. A thin PPAA film of about 5 nm was deposited on the stamps, which increased the hydrophilicity, and which remained stable for at least several months. These surface-modified stamps were used to transfer polar inks by µCP. The employed µCP schemes are as follows: (a) a second generation of dendritic ink having eight dialkyl sulfide end groups to fabricate patterns on gold substrates by positive µCP, (b) fluorescent guest molecules on β-cyclodextrin (β-CD) printboards on glass employing host-guest recognition, and (c) Lucifer Yellow ethylenediamine resulting in covalent patterning on an aldehyde-terminated glass surface. All experiments resulted in an excellent performance of all three PPAA-coated stamp materials to transfer the polar inks from the stamp surface to gold and glass substrates by µCP, even from aqueous solutions.
Introduction Elastomeric stamps constitute the key element in microcontact printing (µCP) because they carry the pattern to be transferred and must form a conformal contact upon contacting a substrate.1,2 In soft lithography, an elastomeric poly(dimethylsiloxane) (PDMS) stamp with patterned relief structures is employed to generate motifs and structures with feature sizes ranging from tens/hundreds of nanometers to centimeters using several techniques, such as µCP, replica molding, microtransfer molding, and micromolding in capillaries.1 The most important limiting factors of µCP are the low mechanical stability of the stamp and ink diffusion or over-inking, especially when printing highresolution or extreme aspect-ratio patterns with high accuracy. Several solutions have been proposed in the literature to overcome these problems such as utilizing chemically patterned flat stamps to avoid stamp collapse during printing,3 inking with ink pads,4 and utilizing heavy-weight inks or catalytic microcontact printing to overcome ink diffusion.5-7 Recently, various stamp architectures were introduced for µCP such as thin PDMS stamps with a rigid back support,8 hard PDMS * To whom correspondence should be addressed. E-mail: j.huskens@ utwente.nl. † University of Twente. ‡ Ssens bv. (1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550-575. (2) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. DeV. 2001, 45, 697-719. (3) Sharpe, R. B. A.; Burdinski, D.; Huskens, J.; Zandvliet, H. J. W.; Reinhoudt, D. N.; Poelsema, B. J. Am. Chem. Soc. 2005, 127, 10344-10349. (4) Geissler, M.; Bernard, A.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. J. Am. Chem. Soc. 2000, 122, 6303-6304. (5) Liebau, M.; Huskens, J.; Reinhoudt, D. N. AdV. Funct. Mater. 2001, 11, 147-150. (6) Li, X.-M.; Peter, M.; Huskens, J.; Reinhoudt, D. N. Nano Lett. 2003, 3, 1449-1453. (7) Perl, A.; Pe´ter, M.; Ravoo, B. J.; Reinhoudt, D. N.; Huskens, J. Langmuir 2006, 22, 7568-7573. (8) James, C. D.; Davis, R. C.; Kam, L.; Craighead, H. G.; Isaacson, M.; Turner, J. N.; Shain, W. Langmuir 1998, 14, 741-744.
(h-PDMS),9 and composite PDMS stamps with different layers of hard backplane, elastomeric cushion, and hard polymer.10 Some alternative stamp materials were introduced to achieve the desired mechanical properties of the stamp, including thermoplastic elastomers, e.g., triblock copolymers such as poly(styreneb-butadiene-b-styrene) (SBS) and poly(styrene-b-(ethylene-cobutylene)-b-styrene) (SEBS) with a high stiffness (Kraton polymers),11 and polyolefin plastomers12 (POPs, the copolymers of ethylene and an R-olefin such as 1-butene or 1-octene). The most widely used elastomer for µCP in soft lithography is PDMS (Sylgard 184; Dow Corning), which is a commercially available two-component kit with a cross-linking agent. While in most applications PDMS is used without any further modification, in other cases the PDMS surface had to be altered physically or chemically to achieve the desired properties, for example, the hydrophilic character of the PDMS surface. Several reports describe the use of oxygen plasma,13,14 UV ozone,15 and subsequent chemical attachment of chlorosilanes and/or grafting of hydrophilic polymers on the PDMS surface to tune the surface energy of the PDMS stamp.16,17 These surface treatments enabled printing of hydrophilic inks that would otherwise not adhere to normal PDMS due to its hydrophobic surface. When the PDMS surface is exposed to oxygen plasma, the surface becomes hydrophilic due to the formation of a thin and brittle silica-like (9) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042-3049. (10) Menard, E.; Bilhaut, L.; Zaumseil, J.; Rogers, J. A. Langmuir 2004, 20, 6871-6878. (11) Trimbach, D.; Feldman, K.; Spencer, N. D.; Broer, D. J.; Bastiaansen, C. W. M. Langmuir 2003, 19, 10957-10961. (12) Csucs, G.; Ku¨nzler, T.; Feldmann, K.; Robin, F.; Spencer, N. D. Langmuir 2003, 19, 6104-6109. (13) Hollahan, J. R.; Carlson, G. L. J. Appl. Polym. Sci. 1970, 14, 2499-2508. (14) Olander, B.; Wirsen, A.; Albertsson, A.-C. J. Appl. Polym. Sci. 2004, 91, 4098-4104. (15) Efimenko, K.; Wallace, W. E.; Genzer, J. J. Colloid Interface Sci. 2002, 254, 306-315. (16) Hu, S.; Ren, X.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. Anal. Chem. 2002, 74, 4117-4123. (17) Delamarche, E.; Donzel, C.; Kamounah, S. S.; Wolf, H.; Geissler, M.; Stutz, R.; Schmidt-Winkel, P.; Michel, B.; Mathieu, H. J.; Schaumburg, K. Langmuir 2003, 19, 8749-8758.
10.1021/la063657s CCC: $37.00 © 2007 American Chemical Society Published on Web 05/05/2007
Modification of Elastomeric Stamps for µCP of Polar Inks
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Table 1. XPS Data for Plasma Polymerization of Allylamine Thin Films on Various Stamp Surfaces atomic concentration (%) substrate
C(1s) a
PDMS-blank PDMS-PPAAb POP-blanka POP-PPAAb Kraton-blanka Kraton-PPAAb a
44.38 50.90 82.98 59.60 76.26 61.96
N(1s) 20.93 26.24 23.66
O(1s)
Si(2p)
29.28 17.18 10.93 10.33 13.92 11.47
26.25 10.05 5.87 2.74 9.46 1.68
N/C 0.411 0.440 0.382 b
F(1s, 0.11-0.36%) and Si(2p) were found as impurities. Cl(2p, 0.45-1.07%) and Zn(2p, 0.01-0.17%) were found as impurities.
layer. The creation of such a silica-like layer causes changes in the mechanical properties of PDMS. Owen and Smith18 reported the formation of cracks in this silica-like layer and these cracks may allow the migration of low molecular weight un-crosslinked PDMS fragments to the surface and thus lead to the recovery of the hydrophobic character of the PDMS surface. Hydrophobic recovery always occurs with time (in a few hours) after exposure to O2 plasma or UV ozone19 and partial discharges.20,21 This instability of the hydrophilic character of oxidized stamps and the lack of ink reservoir of polar inks poses serious limitations to the use of µCP for the transfer of polar ink materials. Plasma-induced surface grafting, for instance, with acrylonitrile and imidazole, have been explored to tune the PDMS surface properties.22,23 These methods were developed using microwave plasma pretreatment to activate PDMS and subsequent monomer grafting onto the PDMS surface. The plasma-assisted deposition of an amino-containing organic monomer is a relatively simple, solvent-free, single-step process to obtain amine-rich surfaces which are of particular interest because they are known to influence protein adsorption and cell adhesion, and provide sites for the covalent immobilization of biomolecules and graft copolymers.24-28 The density of functional groups can be easily controlled by changing various parameters such as plasma power, pulsed plasma duty cycles, process pressure, and deposition time. Many different organic monomers can be used to produce thin films with a great variety of chemical structures, physical properties, and applications.24-28 Several studies have placed emphasis on depositing various thin film structures of allylamine,25-28 allyl alcohol,29 and maleic anhydride30-32 on different substrates like PDMS,33 poly(ethylene terephthalate), and poly(vinylidene fluoride), etc.34 Recently, Barbier et al.35 (18) Owen, M. J.; Smith, P. J. J. Adhes. Sci. Technol. 1994, 8, 1063-1075. (19) Hillborg, H.; Tomczak, N.; Olah, A.; Schonherr, H.; Vancso, G. J. Langmuir 2004, 20, 785-794. (20) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991-1998. (21) Hillborg, H.; Sandelin, M.; Gedde, U. W. Polymer 2001, 42, 7349-7362. (22) He, Q.; liu, Z.; Xiao, P.; Liang, R.; He, N.; Lu, Z. Langmuir 2003, 19, 6982-6986. (23) Bae, W.-S.; Urban, M. W. Langmuir 2004, 20, 8372-8378. (24) Arefi, F.; Andre, V.; Montazer-Rahmati, P.; Amouroux, J. Pure Appl. Chem. 1992, 64, 715-723. (25) Chu, L.-Q.; Knoll, W.; Foerch, R. Langmuir 2006, 22, 5548-5551. (26) Chen, Q.; Foerch, R.; Knoll, W. Chem. Mater. 2004, 16, 614-620. (27) Van Os, M. T.; Menges, B.; Foerch, R.; Vancso, G. J.; Knoll, W. Chem. Mater. 1999, 11, 3252-3257. (28) Scho¨nherr, H.; van Os, M. T.; Foerch, R.; Timmons, R. B.; Knoll, W.; Vancso, G. J. Chem. Mater. 2000, 12, 3689-3694. (29) Rinsch, C. L.; Chen, X.; Panchalingam, V.; Eberthart, R. C.; Wang, J. H.; Timmons, R. B. Langmuir 1996, 12, 2995-3002. (30) Liu, S.; Vareiro, M. M. L. M.; Fraser, S.; Jenkins, A. T. A. Langmuir 2005, 21, 8572-8575. (31) Schiller, S.; Hu, J.; Jenkins, A. T. A.; Timmons, R. B.; Sanchez-Estrada, F. S.; Knoll, W.; Fo¨rch, R. Chem. Mater. 2002, 14, 235-242. (32) Jenkins, A. T. A.; Hu, J.; Wang, Y. Z.; Schiller, S.; Fo¨rch, R.; Knoll, W. Langmuir 2000, 16, 6381-6384. (33) Harsch, A.; Calderon, J.; Timmons, R. B.; Gross, G. W. J. Neurosci. Methods 2000, 98, 135-144.
Figure 1. Contact mode AFM height images of etched gold substrates after µCP of ODT using (a) PDMS, (b) POP, and (c) Kraton stamps. The curved line trace in Figure 1a is due to a baseline artifact.
reported the plasma deposition of acrylic acid on PDMS to control the surface properties. This approach was used to generate double emulsions in PDMS microchannels. In this paper we report that plasma polymerization of allylamine is a suitable method for the functionalization of stamp surfaces with amine groups. In our approach, a uniform thin film of plasmapolymerized allylamine was deposited on different elastomeric substrates, such as PDMS (Sylgard 184; Dow Corning), a polyolefin plastomer (Affinity EG-8200G from Dow), and Kraton polymers (Kraton G-1652 from KRATON GmbH, Germany). Their surface properties were analyzed by water contact angle, X-ray photoelectron spectroscopy (XPS), and ellipsometry. We have demonstrated several µCP schemes using these stamps. These include the following: (a) heavy inks for (+)µCP, a second generation of dendritic ink having eight end groups of dialkyl sulfide (G2-S) to fabricate patterns on gold substrates by (+)µCP;7 (b) polar inks for molecular printboards, fluorescent guest molecules on β-cyclodextrin (β-CD) printboards on glass fixing the fluorescent pattern by host-guest chemistry;36 (c) polar inks for covalent attachment, Lucifer Yellow ethylenediamine resulting in covalent patterning on an aldehyde-terminated glass surface.37
Results and Discussion Surface Modification of the Stamps. We have chosen two materials, which were introduced in the literature as stamp materials, polyolefin plastomers (POPs)12 and Kraton11 polymers (34) Walker, A. K.; Wu, Y.; Timmons, R. B.; Kinsel, G. R. Anal. Chem. 1999, 71, 268-272. (35) Barbier, V.; Tatoulian, M.; Li, H.; Arefi-Khonsari, F.; Ajdari, A.; Tabeling, P. Langmuir 2006, 22, 5230-5232. (36) Onclin, S.; Mulder, A.; Huskens, J.; Ravoo, B. J.; Reinhoudt, D. N. Langmuir 2004, 20, 5460-5466. (37) Rozkiewicz, D. I.; Ravoo, B. J.; Reinhoudt, D. N. Langmuir 2005, 21, 6337-6343.
6852 Langmuir, Vol. 23, No. 12, 2007
Sadhu et al. Chart 1. Structures of Polar Inksa,b
a (A) A second-generation poly(propylene imine) dendrimer with dialkyl sulfide end groups as a positive ink on gold. (B) Fluorescent guest molecule with two adamantyl units and a rhodamine dye to bind on a β-CD printboard. (C) Lucifer Yellow to create covalent patterns on an aldehyde-terminated glass substrate.
along with the conventional PDMS stamps (Sylgard 184). Under optimized conditions (at a power of 300 W for 1 min), uniform plasma-polymerized allylamine (PPAA) thin films were deposited on PDMS, POP, and Kraton stamp surfaces. The deposited films were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and contact angle measurements (CA). For ellipsometry measurements, a small piece of silicon wafer (Si) was put into the plasma chamber along with all three different stamps. The polymer film thickness on Si was analyzed by ellipsometry before and after plasma deposition. The difference of 5.4 nm gave the thickness of the PPAA polymer film. XPS data (Table 1) revealed the presence of Si on the PPAAcoated PDMS surface. This is due to the very thin PPAA film (5.4 nm) on the PDMS surface (PDMS-PPAA) because XPS measures the surface composition up to a depth of about 5-10 nm. It cannot be excluded that the Si is partially due to diffusion of low molecular weight siloxane chains from the bulk to the surface. Other PPAA-coated stamp surfaces also contain some Si as an impurity on their surfaces. Blank substrates are served as reference. The atomic concentrations shown in Table 1 are averaged from the measurements on three different spots on the surface of each stamp with a standard deviation of about 3%. A considerable amount of oxygen was present on all stamps before and after PPAA coating. PDMS stamps showed a higher oxygen content than POP and Kraton because of the oxygen in the siloxane polymer chain. Variation in the carbon and nitrogen contents of PPAA-coated stamps (Table 1) may be attributed to different positions of the stamps placed in the chamber during plasma deposition and possibly also to rapid post-plasma reactions.38 The XPS data show high nitrogen-to-carbon (N/C) ratios (above 0.3), indicating the presence of a high coverage of amine groups on the stamps’ surfaces. Water contact angle (CA) measurements were performed to determine the hydrophilicity of the plasma-coated surfaces. Flat PDMS-PPAA surfaces showed a very low advancing contact angle (θADV) of about 10°, indicating a hydrophilic surface, while POP-PPAA and Kraton-PPAA both showed CAs of about 30°. These values are much lower than the CA values of O2 plasma-oxidized stamps: 1 min of O2 plasma treatment leads to CA (θADV) changes from 110° to 590 nm) was filtered using a U-MWG Olympus filter cube. Laser Scanning Confocal Microscopy. Laser scanning confocal microscopy was carried out on a Carl Zeiss LSM 510 microscope with an excitation Ar laser beam of 458 nm wavelength, and a 40× objective was used. The emitted fluorescence was collected on a PMT R6357 spectrophotometer.
Acknowledgment. The authors gratefully acknowledge support from the European FP6 Integrated project NaPa (Contract No. NMP4-CT-2003-500120). The contents of this work are the sole responsibility of the authors. We are grateful to Gerard Kip for XPS measurements and to our colleagues Christiaan M. Bruinink, Pascale A. Maury, Manon J. W. Ludden, and Xuexin Duan for their help. LA063657S
