Microcontact Printing Using Poly(dimethylsiloxane) Stamps

The patterning of a surface using microcontact printing (μCP) generally employs a hydrophobic ..... Journal of Physics: Conference Series 2010 252, 0...
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Langmuir 2003, 19, 8749-8758

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Microcontact Printing Using Poly(dimethylsiloxane) Stamps Hydrophilized by Poly(ethylene oxide) Silanes Emmanuel Delamarche,*,† Christian Donzel,‡ Fadhil S. Kamounah,§ Heiko Wolf,† Matthias Geissler,† Richard Stutz,† Patrick Schmidt-Winkel,† Bruno Michel,† Hans Jo¨rg Mathieu,‡ and Kjeld Schaumburg§ IBM Research, Zurich Research Laboratory, Sa¨ umerstrasse 4, 8803 Ru¨ schlikon, Switzerland, Materials Institute, School of Engineering, EPFL, CH-1015 Lausanne, Switzerland, and Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark Received March 3, 2003. In Final Form: July 11, 2003

The patterning of a surface using microcontact printing (µCP) generally employs a hydrophobic micropatterned stamp made from poly(dimethylsiloxane) (PDMS) to place ink molecules on a surface with spatial control. We present a simple procedure to hydrophilize PDMS stamps based on the O2 plasma oxidation of PDMS (referred to as PDMSox) and the grafting of poly(ethylene oxide) silanes (PEO-Si) to the oxidized surface. The wetting properties of a PDMSox surface derivatized with PEO having none, one, or two silanes and having chains with 7-70 EO units are inspected. All PDMSox surfaces treated with PEO-Si are hydrophilic and have advancing and receding contact angles of ∼40° and ∼30°, respectively. These surfaces remain hydrophilic for periods longer than 7 days, which saves having to hydrophilize stamps freshly prior to their usage. In particular, grafting a PEO having two triethoxysilane end groups and a molecular weight (MW) of 3400 g mol-1 enables inking and microcontact printing a polar Pd/Sn catalyst for electroless deposition (ELD) from a stamp to an amino-functionalized glass surface. The printed pattern of colloids has high accuracy and contrast, as reflected by the selective ELD of NiB in the printed regions of the glass. The same stamp can be reused for many cycles of inking and printing without degradation of the quality of the final NiB patterns. The hydrophilic layer provided by the grafted PEO molecules is, in some cases, not sufficiently thick to incorporate and print enough polar ink to form a complete monolayer of cysteamine, for example, onto printed Au substrates. Oxidizing a planar PDMS surface through a mask permits the patterning of PEO onto PDMSox. It then becomes possible to ink the stamp with proteins either by depositing proteins from solution onto the areas left underivatized with PEO or by printing proteins in the PEO-derivatized areas only. The proteins on the planar PDMS/PDMSox-PEO surface in turn are microcontact printed with high accuracy onto glass. This work may help expand µCP to applications in which it is desirable to use polar inks or proteins.

1. Introduction Microcontact printing is a surface-patterning technique that employs a micropatterned stamp to print molecules from an ink to a surface.1,2 Stamps are made by curing PDMS on a mold, and the mold itself is usually patterned by using electron-beam lithography or photolithography. Microcontact printing is versatile because it does not require expensive or sophisticated instruments except for the fabrication of molds that have features with dimensions below a micrometer. Microcontact printing has the potential of patterning curved layers3,4 as well as surfaces having a large area,1,5,6 and many of the principles for devising and preparing stamps for µCP are recurrent in * To whom correspondence should be addressed. E-mail: emd@ zurich.ibm.com. † IBM Research, Zurich Research Laboratory. ‡ Materials Institute, School of Engineering, EPFL. § Department of Chemistry, University of Copenhagen. (1) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (2) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498-1511. (3) Jackman, R. J.; Wilbur, J. L.; Whitesides, G. M. Science 1995, 269, 664-666. (4) Rogers, J. A.; Jackman, R. J.; Whitesides, G. M. Adv. Mater. 1997, 9, 475-477. (5) Xia, Y.; Qin, D.; Whitesides, G. M. Adv. Mater. 1996, 8, 10151017. (6) Delamarche, E.; Vichiconti, J.; Hall, S. A.; Geissler, M.; Graham, W.; Michel, B.; Nunes, R. Langmuir 2003, 19, 6567-6569.

“soft lithography” techniques.7 Microcontact printing is suited to pattern self-assembled monolayers (SAMs) that can be used as an etch resist or to affect the properties of surfaces.2,8,9 Similarly, µCP can also derivatize surfaces with catalysts,10-13 polymers,14,15 dendrimers,16,17 and proteins18-20 with spatial control. Molecules used to prepare an ink should ideally be soluble in a solvent that (7) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 551575. (8) Xia, Y.; Zhao, X.-M.; Whitesides, G. M. Microelectron. Eng. 1996, 32, 255-268. (9) Geissler, M.; Schmid, H.; Bietsch, A.; Michel, B.; Delamarche, E. Langmuir 2002, 18, 2374-2377. (10) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-1380. (11) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209-5215. (12) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367-6373. (13) Geissler, M.; Kind, H.; Schmidt-Winkel, P.; Michel, B.; Delamarche, E. Langmuir 2003, 19, 6283-6296. (14) Granlund, T.; Nyberg, T.; Stolz Roman, L.; Svensson, M.; Ingana¨s, O. Adv. Mater. 2000, 12, 269-273. (15) Zheng, H.; Rubner, M. F.; Hammond, P. Langmuir 2002, 18, 4505-4510. (16) Li, H.; Kang, D.-J.; Blamire, M. G.; Huck, W. T. S. Nano Lett. 2002, 2, 347-349. (17) Wu, X. C.; Bittner, A. M.; Kern, K. Langmuir 2002, 18, 49844988. (18) Bernard, A.; Delamarche, E.; Schmid, H.; Michel, B.; Bosshard, H. R.; Biebuyck, H. A. Langmuir 1998, 14, 2225-2229. (19) James, C. D.; Davis, R. C.; Kam, L.; Craighead, H. G.; Isaacson, M.; Turner, J. N.; Shain, W. Langmuir 1998, 14, 741-744.

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does not swell PDMS, diffuse inside PDMS or at least have an affinity for the surface of PDMS, have low vapor pressure under ambient conditions, have limited surface diffusion characteristics to keep the prints accurate, and have sufficient affinity for the substrate to transfer from the stamp. 1.1. Hydrophilic Stamps for Microcontact Printing. PDMS is a nonpolar elastomer and has a hydrophobic surface. The versatility and wide range of applications promised by µCP call for having the possibility of engineering the surface of stamps as a function of the ink/ substrate system used. The patterning of a Au substrate with a polar alkanethiol using a PDMS stamp is usually indirect: for example, first a nonpolar alkanethiol is microcontact printed onto the substrate and then the polar one is adsorbed from solution in the nonprinted areas.21,22 The hydrophilization of PDMS is a prerequisite for employing polar inks for µCP that do not have an affinity for native PDMS.12,23,24 The presence of a thick (compared with the size of the active molecule in the ink) hydrophilic layer on the surface of a stamp is beneficial for transferring large amounts of polar substances to a surface. This approach has been successfully pursued to prepare hydrogel-covered stampers to place proteins on surfaces or amines on polymer surfaces.25-27 Forming a few-nanometer-thick hydrophilic film on the stamp can also be sufficient to uptake enough of a polar ink to derivatize a surface with a SAM and may provide stamps that are more accurate than swollen hydrogel stampers. The simplest approach to prepare a hydrophilic stamp for µCP is probably to oxidize its surface using an O2-based plasma.28 This oxidation is self-limiting and produces a layer of silica only a few nanometers thick on the surface of PDMS.29,30 Stamps hydrophilized by this method do not remain hydrophilic for long because mobile, low-molecular-weight (MW) silicone residues migrate from the bulk to the airstamp interface.31,32 This makes the oxidized stamp hydrophobic again in the course of a few hours after the plasma treatment, unless these stamps are carefully stored under water.33 We showed in previous work that crosslinking poly(ethylene oxide) to plasma-oxidized stamps helped retaining the hydrophilic character of the stamps for long periods of time (>7 days) and relieved a user from having to store a stamp under water before using it.34 This approach was based on a four-step chemistry, which started with the grafting of aminopropyltrimethoxysilane from an aqueous solution to a freshly O2 plasma treated (20) Martin, B. D.; Gaber, B. P.; Patterson, C. H.; Turner, D. C. Langmuir 1998, 14, 3971-3975. (21) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 27902793. (22) Kumar, A.; Whitesides, G. M. Science (Washington, D.C.) 1994, 263, 60-62. (23) Yang, Z.; Chilkoti, A. Adv. Mater. 2000, 12, 413-417. (24) Yan, L.; Huck, W. T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208-1214. (25) Markowitz, M. A.; Turner, D. C.; Martin, B. D.; Gaber, B. P. Appl. Biochem. Biotechnol. 1997, 68, 57-68. (26) Martin, B. D.; Gaber, B. P.; Patterson, C. H.; Turner, D. C. Langmuir 1998, 14, 3971-3975. (27) Martin, B. D.; Brandow, S. L.; Dressick, W. J.; Schull, T. L. Langmuir 2000, 16, 9944-9946. (28) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Macromolecules 1993, 26, 5870-5875. (29) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991-1998. (30) Owen, M. J.; Smith, P. J. J. Adhesion Sci. Technol. 1994, 8, 1063-1075. (31) Fritz, J. L.; Owen, M. J. J. Adhesion 1995, 54, 33-45. (32) Kim, J.; Chaudhury, M. K.; Owen, M. J. IEEE Trans. Dielectrics Electron. Insul. 1999, 6, 695-702. (33) Delamarche, E.; Bernard, A.; Schmid, H.; Michel, B.; Biebuyck, H. A. Science (Washington, D.C.) 1997, 276, 779-781. (34) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamarche, E. Adv. Mater. 2001, 13, 1164-1167.

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Figure 1. Two-step hydrophilization of a PDMS stamp starts with oxidizing the surface of the stamp using an O2-plasma followed by the grafting of PEO silanes.

stamp. The amine functions introduced at the surface of the stamp were then reacted with a bis-homofunctional cross-linker, and finally an amino-functionalized polymer of ethylene oxide was grafted to the stamp. The resulting stamps could uptake polar molecules such as Pd complexes from an ethanolic ink and print them homogeneously on a substrate, whereas untreated PDMS stamps lacked this ability. We aim to extend this approach toward making PDMS stamps hydrophilic by simplifying the grafting procedure, Figure 1. We will first evaluate how the type of PEO grafted to PDMSox affects the wetting properties of the stamp and then inspect if and how this method of hydrophilization benefits some examples of µCP that employ polar inks or proteins. 2. Results and Discussion We chose PEO to hydrophilize PDMS because on Au and oxides this polymer, functionalized with thiol or silane anchoring groups, forms monolayers having contact angles of ∼40° or less,35,36 is stable in many chemical environments and solvents,37 prevents the deposition of proteins from solution,38,39 and is commercially available as many different types of functionalized EO polymers. We desired, in particular, to derivatize PDMS with PEO molecules in a simple manner with the goal of performing reproducible microcontact-printing experiments using polar inks. Additionally, we were curious to see how the MW of the grafted PEO and the number of their silane-anchoring groups influence the wetting characteristics of the treated PDMS surface. Therefore, we synthesized most of the PEO molecules used here, even though grafting a commercially available PEO would be desirable. 2.1. Synthesis of PEO Silanes. Scheme 1 depicts a strategy to prepare the intermediate and final EO molecules. The synthesis of heptaethylene glycol triethoxysilane (4) starts with the preparation of heptaethylene glycol monomethyl ether (2) by alkoxylation of commercially available tetraethylene glycol with triethylene (35) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20. (36) Papra, A.; Gadegaard, N.; Larsen, N. B. Langmuir 2001, 17, 1457-1460. (37) Harris, J. M. In Poly(ethylene glycol) chemistry. Biotechnological and biomedical applications; Plenum Press: New York, 1992. (38) Prime, K. L.; Whitesides, G. M. Science (Washington, D.C.) 1991, 252, 1164-1167. (39) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10 714-10 721.

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Scheme 1

glycol monomethyl ether chloride (1) in the presence of NaH/THF in 50% yield. Allylation of 2 to 3 was effected in 72% yield, using allyl chloride and sodium metal in THF. Silylation of 3 was achieved with triethoxysilane in the presence of H2PtCl6 in toluene at 90 °C to give 4 in 88% yield by adopting a procedure reported for esters of oligoethylene glycol silanes.40 Compound 8 was obtained by stepwise synthesis involving first the monoallylation of tetraethylene glycol to give 5 in 65% yield. The monoallyl 5 was monotosylated41 using p-toluenesulfonyl chloride in dichloromethane in the presence of KOH at 0 °C to give 6 in 86% yield. Reaction of 6 with 2 in the presence of NaH in THF led to the formation of 7 in 62% yield. Silylation of the allyl group in 7 by using triethoxysilane and H2PtCl6 in toluene proceeded smoothly, giving the silane 8 in 82% yield. The undecaethylene glycol monomethyl ether (10) was prepared from the heptaethylene glycol monomethyl ether p-toluene sulfonate (9) and tetraethylene glycol in 43% yield, by adopting the methodology of Allcock and co-workers.42 Alkoxylation of tetraethylene glycol monoallyl ether 5 with triethylene glycol di(p-toluene sulfonate) in the presence of t-BuOK in THF gave compound 11 in 38% yield, which reacted with undecaethylene glycol monomethyl ether (10) in the presence of NaH in THF to give octadecaethylene glycol allyl methyl ether (12) in 42% yield. This was silylated with triethoxysilane in the presence of H2PtCl6 in toluene to give the corresponding triethoxysilane 13 in 73% yield. The synthesis of the remaining PEO compounds proceeded similarly as above and is depicted in Scheme 1 and detailed in the Experimental Section. 2.2. Two-Step Grafting of PEO Molecules to PDMS Stamps. Grafting of the different types of PEO silanes (40) Klein, K.-D.; Knott, W.; Koerner, G. US Patent 5430167, 1995. (41) Keegstra, E. M. D.; Zwikker, J. W.; Roest, M. R.; Jenneskens, L. W. J. Org. Chem. 1992, 57, 6678-6680. (42) Allcock, H. R.; O’Conor, S. J. M.; Olmeijer, D. L.; Napierala, M. E.; Cameron, C. G. Macromolecules 1996, 29, 7544-7552.

Figure 2. Temporal evolution of the advancing (top) and receding (bottom) contact angles of water on PDMS stamps hydrophilized using an O2-plasma only (PDMSox) or by grafting, after the plasma treatment, PEO molecules having none (EO18), one (Si-EO18), or two (Si-EO18-Si) trimethoxysilane anchoring groups. 4-Step-EO〈70〉 refers to the grafting of EO〈70〉 in four steps (including the plasma treatment) to the stamp such as was done in our prior work. The lines serve as guide to the eyes.

starts by exposing a planar or micropatterned PDMS stamp, cleaned with ethanol, to a 30-s-long O2 plasma. The advancing and receding contact angles of these stamps H2O 2O with water, ΘH adv and Θrec , respectively, change from ∼115° and ∼95° to 0° after this treatment.34 The plasmatreated PDMS surface recovers most of its hydrophobicity with time, however, and the “plasma” curve in Figure 2 H2O 2O shows that ΘH adv and Θrec increase by nearly 50° during the first 120 min that follow the plasma treatment. This property is termed hydrophobic recovery and attributed principally to the migration of silicone residues from the bulk, contaminating the oxidized PDMS surface.29,31,32 Hydrophobic recovery is desirable to prevent electric shorts on high-voltage PDMS insulators that are subject to corona discharges in a humid environment.43,44 Inversely, the hydrophobic recovery of polymers hydrophilized poses a problem for the fabrication of contact lenses enriched with organosiloxanes.45,46 Cross-linking EO〈70〉 to PDMS in four steps (referred to as “4-step-EO〈70〉”) yields a hydrophilic (43) Kim, J.; Chaudhury, M. K.; Owen, M. J. J. Colloid Interface Sci. 2000, 226, 231-236. (44) Frost, N. E.; Krenceski, M. IEEE Int. Symp. Electron. Insul. 2002, 220-223. (45) Fakes, D. W., Newton, J. M., Watts, J. F., Edgell, M. J. Surf. Interface Anal. 1987, 10, 416-423.

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surface that initially has larger contact angles than those of a plasma-treated PDMS surface but slows the hydrophobic recovery. In each graph reporting the temporal evolution of the wetting properties of the treated PDMS surfaces, we added a vertical line at 100 min to emphasize that much of the decrease in wettability occurs rapidly. This time corresponds to 1% of the total duration of the experiments, and it might be desirable to keep stamps hydrophilic for much longer times. A benefit of this hydrophilization is in particular to keep the receding contact angle low, which prevents uncontrolled dewetting of a polar ink when a stamp is dried at the end of the inking step. The hydrophobic recovery of freshly oxidized PDMS renders the hydrophilization of stamps in a batchwise manner inconvenient unless they are temporarily stored under water. For this reason, stamps removed from the plasma chamber were placed in a water-filled Petri dish and covered with water until it was time to perform the PEO-grafting step.47 It usually took 1 min to remove each stamp (1 cm2 in size) from the water, rinse it with ethanol, dry it, and cover it with a 10 mM solution of PEO. After discarding the solution of PEO, stamps were characterized or used for inking and printing experiments. 2.3. Varying the Number of Anchoring Groups of PEO Molecules and Its Effect on the Hydrophilicity of Stamps. The wetting properties of PDMSox grafted with PEO having 18 EO units and no (EO18), 1 (Si-EO18), or 2 (Si-EO18-Si) triethoxysilane anchoring groups are compared with those of PDMS treated only with the O2 plasma and grafted with 4-step-EO〈70〉, Figure 2. The wetting characteristics of PDMSox and PDMSox grafted with EO18 increasingly approach those of PDMSox treated with EO〈70〉 in four steps when the number of anchoring groups increases from none to two. It is difficult to predict how the MW of the EO polymers and the number of silane anchoring groups influence the composition and hydrophilicity of the resulting layer: the conformation (coiled vs stretched) of the PEO in solution, the number and accessibility of the anchoring groups, the density and lifetime of adsorption sites on PDMSox, and chemical yields when successive steps are employed to graft PEO all convolve to define the final structure of the air-stamp interface.48-51 We found it impractical to vary too many of these parameters and difficult to characterize the resulting interface in a direct manner. Therefore, we performed several series of ellipsometry-based experiments on grafting PEO to the native oxide of a Si wafer that was freshly cleaned using an O2-based plasma. We found, for example, that 10 min were sufficient to graft these PEO-silanes from a 10 mM solution in ethanol to the Si/SiO2 surface. Si-EO18 and Si-EO18-Si had a similar thickness but slightly different contact angles as H2O 2O already suggested in Figure 2: ΘH adv ≈ 41° and Θrec ≈ 30° H2O H2O for Si-EO18-Si and Θadv ≈ 30° and Θrec ≈ 20° for Si-EO18. The apparent grafting of EO18 to PDMSox is, in contrast, surprising because ellipsometry and contact angle microscopy indicated that no PEO adsorbed from solution to a Si/SiO2 surface. Specifically, a layer having a thickness (46) Fakes, D. W.; Davies, M. C.; Brown, A.; Newton, J. M. Surf. Interface Anal. 1988, 13, 233-236. (47) Storing the stamps in water for longer times (up to 12 h) before grafting the PEO-silane did not seem to have an influence on the grafting procedure. (48) Yang, Z.; Galloway, A.; Yu, H. Langmuir 1999, 15, 8405-8411. (49) Zhu, X.-Y.; Jun, Y.; Staarup, D. R.; Major, R. C.; Danielson, S.; Boiadjiev, V.; Gladfelter, W. L.; Bunker, B. C.; Guo, A. Langmuir 2001, 17, 7798-7803. (50) Sofia, S. J.; Premnath, V.; Merrill, W. Macromolecules 1998, 31, 5059-5070. (51) Norrman, K.; Papra, A.; Kamounah, F. S.; Gadegaard, N.; Larsen, N. B. J. Mass. Spectrosc. 2002, 37, 699-708.

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Figure 3. Temporal evolution of the advancing (top) and receding (bottom) contact angles of water on PDMS stamps hydrophilized with an O2-plasma only (PDMSox) or derivatized with PEO molecules having various numbers of EO units. SiEO〈40〉 has 40 EO units on average, and EO〈70〉 was grafted to the stamp in four steps. The lines serve as guide to the eyes.

of 0.1 nm or less was found to deposit onto the Si/SiO2 surface, which retained advancing and receding contact angles with water of 0°. The oxidizing plasma may produce peroxides and metastable radicals on the surface of PDMS52 that may bind to PEO or fragments of PEO molecules and account for this difference. The wetting characteristics of PDMSox derivatized with EO18 are the closest to those of PDMS treated with plasma only and increase by 50° within 7 days. This suggests that for EO18, the oxidized PDMS contributes to the wetting characteristics of the PDMSox-EO18 surface to a larger extend than for Si-EO18 and Si-EO18-Si-treated PDMS. In summary, it seems preferable to employ PEO with silane groups to derivatize PDMSox. 2.4. Varying the Length of PEO Molecules and Its Effect on the Hydrophilicity of Stamps. The next set of contact-angle experiments explores the influence of the number of EO units on the hydrophilicity of the treated layer. The wetting characteristics of PDMSox derivatized with PEO with a number of unit between 7 and 40 (on average) are largely similar with maybe the exception of the shortest polymer being more hydrophilic initially, Figure 3. Grafting these EO polymers onto freshly O2 plasma cleaned Si/SiO2 resulted in having surfaces with (52) Hillborg, H.; Gedde, U. W. IEEE Trans. Dielectrics Electron. Insul. 1999, 6, 703-717.

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Figure 4. Temporal evolution of the advancing (top) and receding (bottom) contact angles of water on PDMS stamps hydrophilized using an O2-plasma only (PDMSox) or derivatized with EO〈70〉 in four steps (4-step-EO〈70〉) or with Si-EO〈70〉-Si in two steps. The lines serve as guide to the eyes.

similar wetting characteristics. Si-EO7, Si-EO11, Si-EO18, 2O and Si-EO〈40〉 had contact angles of ΘH adv ≈ 35 ( 5° and H2O Θrec ≈ 23 ( 3° on Si/ SiO2. Interestingly, the wetting characteristics of derivatized PDMSox seem to be more strongly influenced by the number of silane anchoring groups than by the length of the EO chain. 2.5. Preferable Method of Hydrophilization. The above-described experiments suggested that having PEO with silane anchoring groups and seven or more EO units in the chain is helpful to prepare PDMS surfaces that are hydrophilic for an extended period of time. We therefore attempted to hydrophilize PDMS stamps in a stable manner using the commercial silane Si-EO〈70〉-Si. The wettability of this surface is reported in Figure 4 separately from Figure 3 for better clarity. The hydrophilic character of PDMSox derivatized with Si-EO〈70〉-Si is more stable than when PDMS is only oxidized or oxidized and treated with 4-step-EO〈70〉. However, a strict comparison of the wetting properties of PDMS modified with 4-step-EO〈70〉 with PEOsilanes directly grafted is difficult because the relative yields of the various steps involved play a role in defining the ultimate modified PDMSox surface. PEO grafted onto PDMS may slow the hydrophobic recovery of plasmatreated PDMS by providing a barrier for the migration of silicone residues to the surface.29,31,32,52 Consequently, it can be desirable to graft a PEO layer as dense and thick as possible in order to obtain more stable hydrophilic

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PDMS stamps, but grafting a polymer to a surface quickly finds a practical limitation owing to the coiling of the polymer in solution.53 We do not expect that further increasing the MW of such a PEO would result in obtaining a thicker and more stable hydrophilic layer on PDMS: The thickness of Si-EO〈70〉-Si grafted on Si/SiO2 using similar conditions as for PDMS was 2.7 ( 0.4 nm. The corresponding advancing and receding contact angles of this layer with water were ∼37° and ∼30°, respectively. A thickness of 2.7 nm suggests that the immobilized molecules have a compact conformation but not a stretched conformation such as (short) PEO-derivatized alkanethiols adopt on Au.38,39 Derivatizing PDMSox with Si-EO〈70〉-Si was repeated with independent experiments 20 times and the averaged data in Figure 4 underline the improved stability of the grafted PDMSox surface. We handled stamps with care to minimize mechanical stress because the silica layer produced by plasma oxidation is brittle, and producing cracks in it may accelerate the hydrophobic recovery.29,54,55 We selected Si-EO〈70〉-Si to hydrophilize PDMS stamps in view of the stable hydrophilic character that it provides to PDMSox and because it is commercially available. The availability of many other molecules comprising an EO〈70〉 moiety might also be advantageous. 2.6. Microcontact Printing Pd/Sn Colloids. Next, we evaluate which types of µCP that use polar inks may benefit from PEO-hydrophilized stamps. First, we inspect inking and printing of a colloidal Pd/Sn catalyst for electroless deposition (ELD).56,57 ELD is a technique in which metallic ions are reduced from solution to metallize the surface of a substrate. A catalyst needs to be present on this substrate to initiate ELD before it proceeds autocatalytically. ELD is particularly interesting to combine with µCP because printing the catalyst can be an elegant method to direct the deposition of a metal on an insulating substrate, Figure 5.10-12 The process, which is shown in Figure 5A, starts with inking the Pd/Sn colloids onto a PEO-derivatized stamp. After rinsing and drying the stamp, the inked stamp is printed onto a glass substrate pretreated with 3-(2-aminoethylamino)propyltrimethoxysilane (EDA-Si). Derivatization of the glass is, like for the stamp, necessary to promote the affinity between the catalyst and the glass.58-60 The glass with the printed colloids is immersed into a solution of accelerator (an aqueous solution of HBF4) to enhance the catalytic activity of the printed particles. ELD proceeds by immersing the accelerated substrate into a plating bath (NiB, in this case). In previous work, we found it necessary to coat oxidized PDMS stamps with a polyelectrolyte, to ink stamps with polar Pd/Sn colloids.13 The polyelectrolyte helped inking the stamp homogeneously with enough catalyst to ensure high catalytic activity for ELD of NiB in the printed regions of a glass substrate. Interestingly, we found that the Pd/Sn colloids have a high affinity for (53) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677-710. (54) Hillborg, H.; Sandelin, M.; Gedde, U. W. Polymer 2001, 42, 73497362. (55) Schmid, H.; Wolf, H.; Allenspach, R.; Riel, H.; Karg, S.; Michel, B.; Delamarche, E. Adv. Func. Mater. 2003, 13, 145-153. (56) Electroless Plating: Fundamentals and Applications; Mallory, G., Hajdu, J. B. Eds.; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990. (57) Matijevic, E.; Poskanzer, A. M.; Zuman, P. Plat. Surf. Finish. 1975, 62, 958-965. (58) Brandow, S. L.; Dressick, W. J.; Marrian, C. R. K.; Chow, G.-M.; Calvert, J. M. J. Electrochem. Soc. 1995, 142, 2233-2243. (59) Dressick, W. J.; Dulcey, C. S.; Georger, J. H.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210-220. (60) Delamarche, E.; Geissler, M.; Vichiconti, J.; Graham, W. S.; Andry, P. A.; Flake, J. C.; Fryer, P. M.; Nunes, R. W.; Michel, B.; O’Sullivan, E. J.; Schmid, H.; Wolf, H.; Wisnieff, R. L. Langmuir 2003, 19, 5923-5935.

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Figure 5. PDMS stamp hydrophilized using an O2-plasma treatment and grafting Si-EO〈70〉-Si uptakes polar Pd/Sn colloids from an ink and transfers them to a substrate in an amount that is sufficient to catalyze the electroless deposition of NiB in the printed areas of a glass substrate. (A) The ELD of NiB results from inking the stamp with an acidic solution of Pd/Sn colloids, rinsing it with water, printing the catalytic colloids onto a glass previously derivatized with aminosilanes, “accelerating” the printed catalyst with an acidic solution of HBF4, and immersing the accelerated sample into a NiB ELD bath. (B) This XPS survey reveals the presence of a large amount of Pd/Sn colloids on the surface of the PEO-derivatized stamp. (C) These optical micrographs show the high contrast and accuracy of the 400-nm-thick pattern of NiB electroless deposited in the printed areas of the glass substrate.

the PEO-derivatized stamp as well; the XPS survey in Figure 5B shows the large Pd 3d and Sn 3d signals from the colloids present on a Si-EO〈70〉-Si-derivatized stamp that was inked and dried. The O 1s, C 1s, and Si signals are attributed to atoms from the PDMS and the grafted layer of PEO. The transfer of colloid from the stamp to the pretreated glass substrate proceeded with high yield and the optical microscope images in Figure 5C reveal that the ELD of NiB occurred specifically in the microcontact printed regions of the substrate. The stability of the PEO layer on the PDMS stamp against the low pH of the ink solution (pH < 1) was surprisingly high. We could use, for example, a stamp treated with Si-EO〈70〉-Si 10 times and more than 12 h without noticing, contrary to our previous work, any degradation of the stamp or of the NiB patterns.13 2.7. Microcontact Printing “Polar” Chemisorbing Molecules. We evaluated whether a stamp derivatized with Si-EO〈70〉-Si could print a polar thiol directly onto

Delamarche et al.

Au instead of printing a nonpolar alkanethiol first and then adsorbing the polar thiol in the nonprinted regions of the substrate from solution. We used cysteamine (hydrochloride salt) with the intent of first microcontact printing it onto Au and then adsorbing ECT from solution in the remaining regions of the Au. We had adopted a similar strategy earlier for positive microcontact printing: the printed thiol does not protect the substrate from wet etching, whereas the second thiol, adsorbed from solution, does.61 This strategy allows the Au layers to be structured with the inverse pattern of the one present on the stamp. The hydrochloride salt of cysteamine was too polar to be inked sufficiently onto a PDMS stamp derivatized with Si-EO〈70〉-Si. In contrast, a PDMS stamp freshly oxidized was sufficiently inked with cysteamine salts to form a layer on the Au substrate that could block the adsorption of ECT from solution in the printed regions of the Au surface. Another limitation of the PEO-derivatized stamps appeared with microcontact printing alkanephosphonic acids onto the native oxide of Al substrates. Alkanephosphonic acids, such as octadecanephosphonic acid, can be inked onto the surface of a hydrophobic PDMS stamp by inking the stamp with an ethanolic solution of ink and drying it. Multilayers of alkanephosphonic molecules accumulate on the stamp during the drying step and can be transferred to an Al surface by microcontact printing.62 Using a PDMS stamp derivatized with Si-EO〈70〉-Si resulted in a markedly improved distribution of the alkanephosphonic molecules on the stamp as a result of the more homogeneous drying of the ink on the PDMSox-PEO surface. The amount of ink accumulated on the stamp was insufficient, however, to microcontact print a layer of alkanephosphonic acid that was thick enough to protect the Al substrate. This demonstrates that although the derivatization of PDMSox with PEO improves the wetting properties of the stamp with the ink, the number of active molecules that can be accumulated from an ink to a stamp remains of course an important aspect of the system used for µCP. We encountered similar limitations with the inking and printing of polar alkanethiols but the “amphiphilic” thiol HS(CH2)11(EO)5OCH3, in contrast, could be inked from a 2 mM solution in ethanol on a Si-EO〈70〉-Si-treated stamp and printed onto Au. The printed layer had a thickness of ∼2.0 nm (as measured using ellipsometry), which corresponds to 80% of a complete monolayer of HS(CH2)11(EO)5OCH3 on Au.35 The density of EO on the surface38,39 was large enough to prevent the deposition of bovine serum albumin from a 1% solution in phosphate buffered saline for at least 20 min. 2.8. Microcontact Printing Proteins Using a “Positive” or “Negative” Strategy. PEO molecules grafted onto PDMSox can impart interesting protein-repellency properties to a stamp. It is possible to oxidize PDMS locally by placing a metal mask on a PDMS surface during the O2-plasma treatment, Figure 6. The subsequent grafting of Si-EO〈70〉-Si proceeds in the unmasked regions of the PDMS layer and it is possible to prevent the deposition of proteins from solution in these areas. We employed fluorescently tagged proteins and surface fluorescence microscopy to see where these proteins deposit from solution onto a planar, chemically patterned stamp and if they transferred to a printed glass substrate, Figure 6. This relatively simple patterning strategy seems suited to pattern proteins with high accuracy and contrast on (61) Delamarche, E.; Geissler, M.; Wolf, H.; Michel, B. J. Am. Chem. Soc. 2002, 124, 3834-3835. (62) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182-1191.

Microcontact Printing with PDMS Stamps

Figure 6. Selective oxidation of a layer of PDMS through a metal mask (visible in the optical micrograph) followed by the grafting of Si-EO〈70〉-Si in the oxidized areas permits directing the deposition of proteins from solution to the hydrophobic areas of PDMS and to microcontact print them onto a glass substrate. The surface fluorescence microscope images of IgGs labeled with TRITC on the stamp and on the printed glass surface are equivalent and have high contrast and accuracy.

substrates. It is constrained by the geometry of the mask but not by the mechanical stability of the stamp.63-66 It is even possible to expand this strategy toward the transfer of proteins in cascade from one surface to another.67 We deposited a layer of the fluorescently tagged proteins from solution onto a hydrophobic, planar PDMS layer and printed parts of this layer onto a flat layer of PDMS selectively derivatized as described above with Si-EO〈70〉Si. The proteins transfer from the hydrophobic PDMS layer to the regions of PDMSox derivatized with Si-EO〈70〉-Si only68 and are microcontact printed onto a glass surface in a second step. The fluorescence microscope images in Figure 7 show the patterns of protein as they were expected. They are similar on the PEO-treated stamp and on final glass substrate and complementary to the pattern of protein left on the hydrophobic PDMS layer. The transfer of proteins from a micropatterned elastomeric (63) Geissler, M.; Bernard, A.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. J. Am. Chem. Soc. 2000, 122, 6303-6304. (64) Delamarche, E.; Biebuyck, H. A.; Schmid, H.; Michel, B. Adv. Mater. 1997, 9, 741-746. (65) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310-4318. (66) Hui, C. Y.; Jagota, A.; Lin, Y. Y.; Kramer, E. J. Langmuir 2002, 18, 1394-1407. (67) Renault, J. P.; Bernard, A.; Juncker, D.; Michel, B.; Bosshard, H. R.; Delamarche, E. Angew. Chem., Int. Ed. 2002, 41, 2320-2323. (68) Tan, J. L.; Tien, J.; Chen, C. S. Langmuir 2002, 18, 519-523.

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Figure 7. Using hydrophobic and Si-EO〈70〉-Si-treated PDMS layers allows proteins to be transferred from one surface to another and printed onto a glass substrate. A homogeneous layer of IgGs fluorescently labeled with TRITC is deposited from solution onto a surface of PDMS that is used as an inker pad. Contacting this inker pad with a flat stamp of PDMS patterned with Si-EO〈70〉-Si results in transferring the proteins to the regions of the stamp derivatized with PEO. Finally, the proteins can be printed from this stamp to a glass substrate. The surface fluorescence microscope images reveal the pattern of protein on the Si-EO〈70〉-Si-treated stamp, the complementary pattern left on the inker pad, and the final pattern of protein on the printed glass substrate.

stamp was found to occur when the wettability by water of the substrate is superior to that of the stamp.68 This important finding suggests that stamps could be derivatized with variable amount of PEO silanes to modulate the ability of proteins to transfer sequentially from one stamp to another and to a final substrate. A result of such a method could be to prepare, using µCP, surfaces patterned with multiple types of proteins.67,69,70 3. Conclusion The experiments described here demonstrate that modifying the surface chemistry of a PDMS stamp can change the ability to ink a stamp and, hence, the type and quality of patterns formed using µCP. The grafting of a PEO silane onto PDMSox results in stable hydrophilic stamps, is simple, and is particularly effective when polar (69) Bernard, A.; Renault, J.-P.; Michel, B.; Bosshard, H. R.; Delamarche, E. Adv. Mater. 2000, 12, 1067-1070. (70) Irenowicz, H. D.; Howell, S.; Regnier, F. E.; Reifenberger, R. Langmuir 2002, 18, 5263-5268.

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Pd/Sn colloids or proteins are the active molecules of an ink. The Pd/Sn colloids are catalysts for ELD, and it is not necessary to accumulate a large number of them on the stamp for printing patterns on substrates that have sufficient catalytic activity. Proteins adsorb from solution onto the areas of a stamp left free of PEO. In this case, the thickness and density of the grafted PEO layer do not play a major role for repelling proteins from the stamp. The ability to print proteins from a PDMS surface onto areas of a stamp derivatized with PEO opens the possibility to transfer biomolecules “in cascade”. The attachment of chains of PEO to PDMS can be performed with spatial control by using a mask during the plasma-oxidizing treatment of PDMS. The resulting stamps are planar but have a chemical pattern; a similar strategy to chemically pattern PDMS might be to photograft PEO fragments or other molecules to PDMS. The grafting of PEO molecules onto PDMSox finds a practical limit when the hydrophilic volume of the surface of the stamp is too small to host enough molecules to, for example, microcontact print an etch resist onto a substrate. We suggest that “grafting from” strategies53 could extend our work by increasing and controlling the thickness of a layer grafted to PDMS better. Overall, hydrophilic stamps will probably enhance the range of applications available to µCP. 4. Experimental Section Substrates and Chemicals. All chemicals were of the best grade available and obtained from Fluka or Aldrich unless indicated otherwise. Si-EO〈70〉-Si ((EtO)3Si(CH2CH2O)nSi(OEt)3, MW 3400) was from Shearwater Polymers Inc. (Huntsville, AL). Water was deionized and had a resistivity >18.2 MΩ cm-1. Tetrahydrofuran (THF) was freshly distilled from sodium benzophenone. Toluene was dried on metallic sodium, filtered, and kept on activated molecular sieves. Triethylene glycol, tetraethylene glycol, and triethylene glycol monomethyl ether were dried on activated molecular sieves. All reactions were performed under a nitrogen atmosphere. Triethylene glycol ditosylate was prepared by the method of Keegsra et al.41 in 90% yield, as a white crystalline solid. 1H and 13C NMR spectra were recorded on a Varian Unity 400 spectrometer at 400 and 100 MHz, respectively, in CDCl3, using tetramethylsilane (TMS) as internal reference. Fast atom bombardment (FAB) mass spectra were taken on a JEOL JMS-HX/HX 110A mass spectrometer. Preparative flash column chromatography was performed on MN silica gel 60 (0.015-0.04 mm, Machery-Nagal) with the solvents specified. Triethylene Glycol Monomethyl Ether Chloride (1). A solution of triethylene glycol monomethyl ether (280.0 g; 1.72 mol) and pyridine (5.0 mL) in benzene (200 mL) was cooled using an ice bath and stirred while adding thionyl chloride (301.8 g, 2.8 mol) for 2 h. The mixture was refluxed for another 2 h, cooled and poured into ice (2 L), extracted with diethyl ether (4 × 150 mL), dried over anhydrous MgSO4, filtered, and concentrated in vacuo. The oil residue was distilled at 115 °C under reduced pressure (10 mmHg) to give 1 (225.0 g; 72% yield) as a colorless oil. 1H NMR (CDCl3) δ 3.30 (s, 3 H), 3.50-3.67 (m, 12 H). 13C NMR (CDCl3) 42.6, 59.0, 70.6, 70.7, 72.0. FABMS (Xe; mnitrobenzyl alcohol matrix) 183 (MH+). Heptaethylene Glycol Monomethyl Ether (2). Tetraethylene glycol (388.6 g; 2.0 mol) in dry THF was treated with metallic Na (11.5 g; 0.5 mol) under a nitrogen atmosphere at 80 °C until complete dissolution of the Na. The solution was warmed to 100 °C, and compound 1 (91.2 g; 0.5 mol) was added slowly over 2 h. The mixture was stirred for an additional 3 h, cooled to room temperature and filtered. The solid obtained was washed with methanol. The organic phase was concentrated under vacuum to obtain an oil residue that was distilled at reduced pressure to give a pure, colorless oil of 2 (85.0 g; 50% yield). 1H NMR (CDCl3) δ 2.98 (br s, 1 H), 3.29 (s, 3 H), 3.44 (m, 2 H), 3.54 (m, 2 H), 3.56-3.58 (m, 22 H), 3.62 (m, 2 H); 13C NMR (CDCl3), 58.6, 61.3, 70.0, 70.1, 70.2, 70.3, 71.6, 72.2. FABMS (Xe; m-nitrobenzyl alcohol matrix) 341 (MH+).

Delamarche et al. Heptaethylene Glycol Allyl Methyl Ether (3). A solution of 2 (3.72 g; 14.7 mmol) in dry THF (40 mL) was added to a suspension of NaH (0.57 g; 23.8 mmol, 80% in mineral oil) in dry THF (5.0 mL). The mixture was stirred for 2 h, and the resulting pale yellow solution was cooled to 0 °C before allyl chloride (3.38 g; 44.2 mmol) was slowly added to it. The mixture was stirred at room temperature for 24 h, diluted with ethyl acetate, filtered, and concentrated in a vacuum. An oily residue was obtained that was subjected to flash column chromatography on silica using ethyl acetate as the eluant to give compound 3 (4.2 g; 72% yield) as a colorless oil. 1H NMR (CDCl3) δ 3.38 (s, 3 H), 3.523.74 (m, 28 H), 4.02 (t, 2 H), 5.15-5.31 (m, 2 H), 5.84-5.99 (m, 1 H). 13C NMR (CDCl3) 58.6, 69.1, 70.1, 70.2, 70.3, 71.6, 71.8, 116.6, 134.4. FABMS (Xe; m-nitrobenzyl alcohol matrix) 381 (MH+). Heptaethylene Glycol Methyl (3-Triethoxysilyl)propyl Ether (4). This compound was synthesized from compound 3 (1.14 g; 3 mmol) in dry toluene (5.0 mL) and from triethoxylsilane (0.76 mL; 6.0 mmol) with a catalytic amount of H2PtCl6 (0.15 mL; 78 mM solution in isopropyl alcohol). The mixture was heated at 90 °C for 96 h, and the solvent and excess triethoxysilane were evaporated in a vacuum to afford a light brown oil (1.51 g; 88% yield). 1H NMR (CDCl3) δ 0.69 (m, 2 H), 1.15 (t, 9 H), 1.58 (m, 2 H), 3.27 (s, 3 H), 3.31-3.35 (m, 2 H), 3.58 (q, 6 H), 3.52-3.58 (m, 28 H). 13C NMR (CDCl3) 4.84, 17.8, 22.3, 50.1, 58.6, 58.8, 69.6, 70.1, 70.2, 71.5, 73.0. Tetraethylene Glycol Monoallyl Ether (5). A mixture of tetraethylene glycol (224.0 g; 1.16 mol) and 50% aqueous solution of NaOH (15.8 mL; 0.2 mol) was stirred at 50 °C for 4 h. Allyl chloride (15.31 g; 0.2 mol) was added slowly over 30 min. The mixture was stirred at 60 °C for 2 h and at 80-85 °C for 22 h. The mixture was cooled to room temperature and diluted with ethyl acetate (300 mL), filtered, and concentrated in a vacuum; the residue was subjected to flash column chromatography on silica with up to 5-10% gradient elution of methanol in dichloromethane to give compound 9 (30.5 g; 65% yield) as a colorless oil. 1H NMR (CDCl3) δ 2.99 (br s, 1 H), 3.05 (t, 2 H), 3.43-3.58 (m, 14 H), 3.88 (t, 2 H), 5.18 (m, 2 H), 5.74-5.86 (m, 1 H). 13C NMR (CDCl3) 20.5, 61.2, 63.1, 68.7, 69.0, 69.9, 70.1, 70.2, 71.8, 72.2, 116.6, 134.3, 170.6. FABMS (Xe; m-nitrobenzyl alcohol matrix) 235 (MH+). Tetraethylene Glycol Monoallyl Ether p-Toluene Sulfonate (6). Monohydric glycol 5 (13.7 g; 71.6 mmol) and dry dichloromethane (125 mL) were added to a 500-mL three-neckflask equipped with magnetic stirrer and a nitrogen inlet. The homogeneous mixture was stirred at 0 °C. Freshly powdered KOH (15.8 g; 280.8 mmol) was added in small portions with vigorous stirring under nitrogen atmosphere at 0-5 °C over 3 h. After complete addition, the mixture was further stirred at 0 °C for 3 h, after which cold dichloromethane (150 mL) and water (100 mL) were added at 5 °C. The organic phase was separated, the aqueous portion extracted with dichloromethane (2 × 75 mL), and the combined organics were washed with brine and dried over anhydrous MgSO4. A pale yellowish oil was obtained after filtration and evaporation of the solvent. The oil was subjected to flash chromatography on silica, using ethyl acetate/petroleum ether (2:1) as eluant to give the pure mono(p-toluol sulfonate) 10 (23.52 g; 86% yield) as a colorless oil. 1H NMR (CDCl3) δ 2.41 (s, 3 H), 3.57-3.66 (m, 12 H), 3.98 (t, 2 H), 4.11-4.13 (m, 2 H), 7.30 (d, 2 H), 7.65 (d, 2 H). 13C NMR (CDCl3) 21.4, 61.5, 68.5, 69.0, 69.2, 70.1, 70.3, 70.4, 70.5, 72.0, 72.3, 116.8, 127.8, 129.6, 132.8, 134.6, 144.5. FABMS (Xe; m-nitrobenzyl alcohol matrix) 389 (MH+). Undecaethylene Glycol Allyl Methyl Ether (7). A solution of the monohydric glycol 2 (9.95 g; 29.3 mmol) in anhydrous THF (75 mL) was treated with NaH (1.1 g; 37.0 mmol, 80% in mineral oil) at room temperature, stirred for 24 h, and cooled to 0 °C, and the allyl p-toluene sulfonate 6 (11.37 g; 29.3 mmol) in dry THF (25 mL) was added dropwise over 30 min at 0-5 °C. After complete addition, the mixture was stirred at room temperature for 72 h and filtered. The filtrate was washed with THF and concentrated by using rotary evaporation, redissolved in chloroform (100 mL), washed with brine, and dried over anhydrous MgSO4. Evaporation of the solvent gave a pale yellow oil, which was subjected to flash chromatography on silica with up to 5% gradient elution of methanol in dichloromethane to give 7 (6.9 g; 62% yield). 1H

Microcontact Printing with PDMS Stamps NMR (CDCl3) δ 3.32 (s, 3 H), 3.51 (m, 2 H), 3.58-3.65 (m, 42 H), 3.98 (t, 2 H), 5.18 (m, 2 H), 5.81-5.92 (m, 1 H). 13C NMR (CDCl3) 58.8, 69.2, 70.3, 70.4, 71.7, 72.0, 116.8, 134.5. FABMS (Xe; m-nitrobenzyl alcohol matrix) 557 (MH+). Undecaethylene Glycol Methyl (3-Triethoxysilyl)propyl Ether (8). This compound was synthesized from compound 7 (0.561 g; 1.48 mmol) in dry toluene (5.0 mL), with triethoxysilane (0.5 mL; 3.9 mmol) and a catalytic amount of H2PtCl6 (0.1 mL; 78 mM solution in isopropyl alcohol), in a similar manner as described for compound 4 to give 8 as a pale yellow oil (0.62 g; 82% yield). 1H NMR (CDCl3) δ 0.68 (m, 2 H), 1.22 (t, 9 H), 1.69 (m, 2 H), 3.38 (s, 3 H), 3.45 (m, 2 H), 3.60 (q, 6 H), 3.62-3.71 (m, 44 H). 13C NMR (CDCl3) 5.0, 17.7, 22.4, 50.3, 58.7, 58.9, 62.6, 68.4, 68.6, 69.7, 69.8, 70.3, 71.7, 72.0, 72.8, 73.1, 127.7, 129.5. Heptaethylene Glycol Monomethyl Ether p-Toluene Sulfonate (9). A solution of sodium hydroxide (3.54 g; 88 mmol) in water (45 mL) was added at 0 °C to a solution of heptaethylene glycol monomethyl ether 2 (21.0 g; 61.8 mmol) dissolved in dry THF (60 mL) under nitrogen atmosphere. The mixture was kept at 0 °C with vigorous stirring. Tosyl chloride (11.79 g; 61.8 mmol) in dry THF (120 mL) was added slowly over 1.5 h at 0 °C. After compete addition, the mixture was further stirred at 0 °C for 3 h, then poured into crushed ice (500 mL) and extracted with dichloromethane (2 × 100 mL). The organic extract was washed with 10% aqueous NaHCO3 solution and brine and dried over anhydrous MgSO4 before it was filtered and concentrated in a vacuum to give the pure 9 (22.19 g; 73% yield) as a viscous colorless oil. 1H NMR (CDCl3) δ 2.41 (s, 3 H), 3.54 (s, 3 H), 3.58-3.66 (m, 24 H), 3.99 (m, 2 H), 4.12 (m, 2 H), 7.29-7.31 (d, 2 H), 7.75-7.77 (d, 2 H). 13C NMR (CDCl3) 21.4, 61.5, 68.5, 69.0, 69.2, 70.1, 70.3, 70.4, 70.5, 72.0, 72.3, 144.5. FABMS (Xe; m-nitrobenzyl alcohol matrix) 495 (MH+). Undecaethylene Glycol Monomethyl Ether (10). A solution of tetraethylene glycol (13.60 g; 70 mmol) in dry THF (75 mL) was treated with NaH (0.34 g; 14 mmol, 80% in mineral oil) at room temperature, stirred for 24 h, and cooled to 0 °C before compound 9 (6.92 g; 14 mmol) in dry THF (25 mL) was added dropwise over 30 min at 0-5 °C. After complete addition, the mixture was stirred at room temperature for 96 h. The mixture was filtered, the resulting solid was washed first with THF and then with ethyl acetate, and the combined solvents were evaporated in a vacuum. The oil residue was subjected to flash column chromatography on silica with up to 1-5% gradient elution of methanol in ethyl acetate to give 10 (3.1 g; 43% yield) as a colorless viscous oil. 1H NMR (CDCl3) δ 2.98 (bs, 1 H), 3.43 (m, 2 H), 3.52 (m, 2 H), 3.55-3.58 (m, 38 H), 3.62 (m, 2 H). 13C NMR (CDCl3) 58.7, 61.3, 69.9, 70.1, 70.2, 70.5, 71.6, 72.2. FABMS (Xe; m-nitrobenzyl alcohol matrix) 539 (MNa+), 517 (MH+). Heptaethylene Glycol Monoallyl Ether p-Toluene Sulfonate (11). Under nitrogen atmosphere, tetraethylene glycol monoallyl ether 5 (11.70 g; 50 mmol) in dry THF (100 mL) and t-BuOK (5.63 g; 50 mmol) were added successively to a 500-mL three-neck flask equipped with condenser, nitrogen inlet, and rubber septum. The reaction mixture was stirred at room temperature for 45 min, and the resulting pale yellow solution was added slowly via a cannula to a solution of triethylene glycol di(p-toluene sulfonate) (45.85 g; 100 mmol) in dry THF (100 mL) at 0 °C over 2 h. After the addition was completed, the reaction mixture was stirred at room temperature for 6 h and at 60 °C for 4 h. It was then cooled to room temperature and filtered and the solid was washed with THF and ethyl acetate. Evaporation of solvents in a vacuum gave an oil, which was subjected to flash column chromatography on silica with up to 10-30% gradient elution of n-hexane in ethyl acetate to give compound 11 (9.88 g; 38% yield) as a colorless viscous oil. 1H NMR (CDCl3) δ 2.42 (s, 3 H), 3.54 (s, 3 H), 3.57-3.68 (m, 24 H), 3.98 (m, 2 H), 4.14 (m, 2 H), 7.28-7.31 (d, 2 H), 7.76-7.78 (m, 1 H). 13C NMR (CDCl3) 21.4, 61.6, 68.5, 69.0, 69.2, 70.1, 70.3, 70.4, 70.5, 72.0, 72.3, 116.8, 127.8, 129.6, 132.8, 134.5, 144.5. FABMS (Xe; m-nitrobenzyl alcohol matrix) 521 (MH+). Octadecaethylene Glycol Allyl Methyl Ether (12). A solution of the monohydric glycol 10 (2.06 g; 4.0 mmol) in dry THF (20 mL) was treated with NaH (0.11 g; 4.5 mmol, 80% in mineral oil) at room temperature, stirred for 24 h, and cooled to 0 °C, and compound 11 (2.08 g; 4.0 mmol) in dry THF (20 mL) was added over 45 min at 0-5 °C. The mixture was then stirred

Langmuir, Vol. 19, No. 21, 2003 8757 at room temperature for 96 h. Next it was filtered, and the solid was washed with THF and ethyl acetate; these solvents were removed in a vacuum and yielded a pale yellow oil, which was subjected to flash chromatography on silica with up to 5-10% gradient elution of methanol in dichloromethane to give pure 12 (1.45 g; 42% yield) as a white wax. 1H NMR (CDCl3) δ 3.33 (s, 3 H), 3.51 (m, 2 H), 3.56 (m, 2 H), 3.59-3.62 (m, 68 H), 3.96 (m, 2 H), 5.14-5.24 (m, 2 H), 5.83-5.87 (m, 1 H). 13C NMR (CDCl3) 58.8, 69.2, 70.3, 70.4, 71.7, 72.0, 116.8, 134.5. FABMS (Xe; m-nitrobenzyl alcohol matrix) 887 (MNa+), 865 (MH+). Octadecaethylene Glycol Methyl (3-Triethoxysilyl)propyl Ether (13). This compound was synthesized from compound 12 (0.60 g; 0.7 mmol) in dry toluene (5.0 mL), with triethoxysilane (0.2 mL; 1.56 mmol) and a catalytic amount of H2PtCl6 (0.08 mL; 78 mM in isopropyl alcohol), in a similar manner as for 4, to afford 13 as a white wax (0.52 g; 73% yield). 1H NMR (CDCl3) δ 0.69 (m, 2 H), 1.24 (t, 9 H) 1.71 (m, 2 H), 3,37 (s, 3 H), 3.48 (m, 2 H), 3.62-3.78 (m, 72 H), 3.81 (q, 6 H). 13C NMR (CDCl3) 18.01, 22.4, 50.2, 58.7, 62.8, 68.6, 69.7, 69.8, 70.3, 71.6, 72.0, 72.7, 73.2, 127.8, 129.6. Octadecaethylene Glycol Dimethyl Ether (14). To a suspension of NaH (0.48 g; 20 mmol, 80% in mineral oil) in dry THF (10 mL), a solution of 2 (5.11 g; 15 mmol) in dry THF (50 mL) was added. The mixture was stirred for 2 h, and the resulting pale yellow solution was cooled to 0 °C. Tetraethylene glycol bis(p-toluene sulfonate) (8.54 g; 17 mmol) in dry THF (10 mL) was then added slowly. The mixture was stirred at room temperature for 36 h, diluted with ethyl acetate, filtered, and concentrated in a vacuum. The highly viscous oily residue was subjected to flash column chromatography on silica, eluting with ethyl acetate/methanol (10:1) to afford compound 14 (3.59 g; 57% yield) as a white waxy material.1H NMR (CDCl3) δ 3.36 (s, 6 H), 3.54-3.58 (m, 64 H), 3.68 (t, 4 H), 3.72 (m, 4 H). 13C NMR (CDCl3) 56.4, 68.1, 70.1, 71.6, 71.8, 116.2, 134.8. FABMS (Xe; mnitrobenzyl alcohol matrix) 839 (MH+). Octadecaethylene Glycol Diallyl Ether (16). This compound was synthesized from the reaction of heptaethylene glycol monoallyl ether (15) with tetraethylene glycol di(p-toluene sulfonate), as described for 14, as a white wax obtained with 64% yield. 1H NMR (CDCl3) 3.50-3.72 (m, 72 H), 4.04 (t, 4 H), 5.165.30 (m, 4 H), 5.81-5.98 (m, 2 H). 13C NMR (CDCl3) 58.5, 69.3, 70.1, 70.3, 71.6, 71.9, 116.8, 134.4. FABMS (Xe; m-nitrobenzyl alcohol matrix) 850 (MH+). Octadecaethylene Glycol Bis[(3-triethoxysilyl)propyl] Ether (17). This compound was synthesized from compound 16 (4.26 g; 5 mmol) in dry toluene (10 mL) and triethoxysilane (1.9 mL; 15 mmol) with a catalytic amount of H2PtCl6 (0.38 mL; 78 mM solution in isopropyl alcohol), in a similar manner as described for compound 4, to afford 17 as a white wax (4.4 g; 72% yield). 1H NMR (CDCl3) 0.68 (m, 4 H), 1.26 (t, 18 H), 1.73 (m, 4 H), 3.44 (m, 4 H), 3.6-3.74 (m, 72 H), 3.86 (q, 6 H). 13C NMR (CDCl3) 18.1, 22.5, 49.9, 62.8, 69.9, 71.4, 72.5, 72.8, 73.4, 127.5, 129.8. PEO〈40〉 Allyl Methyl Ether (18). This compound was synthesized from monomethoxy PEO〈40〉 monomethyl ether (MW 2000) and allyl chloride in THF in a similar manner as described for compound 3. The highly viscous colorless oil was solidified on standing to a waxy material, 90% yield. The 1H and 13C NMR spectra of the compound obtained confirmed that it had the expected structure. PEO〈40〉 Methyl (3-Triethoxysilyl)propyl Ether (19). This compound was synthesized from 18 and triethoxysilane in toluene as described for 4. The waxy product was formed in 88% yield. This compound had the expected structure as confirmed by 1H and 13C NMR. Preparation and Hydrophilization of the Stamps. Stamps were made by curing the prepolymer components of PDMS (Sylgard 184, Dow Corning, Midland, MI), for at least 12 h at 60 °C, on a 4-in. Si wafer covered with a patterned resist. Smaller stamps were cut from the stamp replicated on the 4-in. wafer and used directly for printing unless they were hydrophilized. Flat stamps were prepared by curing PDMS poured in a polystyrene Petri dish (Falcon). Before oxidation, stamps were sonicated in ethanol/water (1:1) for 3 min, rinsed with ethanol, and dried under a stream of N2. The stamps were oxidized in an oxygen plasma (Technics Plasma 100-E, Florence, KY) for 30 s

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using a coil load power of 200 W and a P(O2) of 0.36 mbar. Stamps were always placed at the same position in the chamber, using a glassware support. Selective oxidation of stamps was done by covering the stamp with a metal mask (35-µm-thick Ni grid from TECAN having circles filled with 35-µm-wide and 1-mm-long lines with a periodicity of 70 µm). After this oxidation step, the stamps were stored under water until the grafting step. They were removed from water, rinsed with ethanol, dried, and placed in a Petri dish and covered with a 10 mM solution of the desired PEG in ethanol for 10 min. The solution was then discarded, and the stamps were rinsed with 10 mL of ethanol, finally dried with N2, and stored in a polystyrene box. Grafting the various PEO molecules onto Si/SiO2 was done similarly as for PDMS stamps and started by exposing the Si samples to the O2-based plasma. Microcontact Printing the Catalyst for Electroless Deposition. N-(2-Aminoethyl)-3-(trimethoxysilyl)propylamine (from Gelest, Tullytown, PA) was grafted onto glass slides from a 1% (vol) aqueous solution in 3 min. The glass substrate was copiously rinsed with water and dried. The Pd/Sn colloids (Crimson Activator 5300 B, Shipley) used to ink the stamps were received in an acidic solution and diluted to 5% with concentrated HCl. Stamps were inked for 1 min, rinsed with 50 mL of water, and manually applied to a derivatized glass substrate for 30 s. The printed glass substrate was immersed into a solution of “accelerator” (Accelerator 19 H, Shipley, used after dilution to 10% with water) for 30 s and rinsed with 50 mL of water. The printed, accelerated glass substrate was then immersed into a Ni ELD bath (Niposit 468, Shipley, prepared as recommended) operated at 57 °C, having a pH of 7.2 (adjusted using ammonia), with moderate stirring. The rate of the bath at these conditions was ∼50 nm min-1. The sample was removed after having the desired amount of NiB deposited, then rinsed with water, and dried. Microcontact Printing of Proteins. Planar stamps oxidized using the Ni metal mask and selectively hydrophilized with SiEO〈70〉-Si were inked with a 0.1 mg mL-1 solution of anti-mouse IgG (developed in goat, conjugated with TRITC, Sigma) in PBS for 15-30 min. The stamps were rinsed with 20 mL of PBS and water and dried using a stream of N2. Within 30 s, they were then placed by hand in contact with a glass slide (previously cleaned with cleanroom wipes and 2-propanol) for 15 min. Alternatively to this direct inking of the derivatized stamp from solution, “dry” inking of the stamp was performed. In this case, a hydrophobic, PDMS layer was covered with a 0.1 mg mL-1 solution of the IgGs in PBS for 15-30 min to form a nearly

Delamarche et al. complete layer of IgG on its surface. The PDMS layer was rinsed with PBS and water and dried using N2 before using it as an inker pad to selectively transfer the proteins to the PEG regions of the derivatized stamp during a 15-min contact. The proteins were then printed from the PEG-derivatized stamp to a glass substrate, cleaned as described above, for a printing time of 15 min. Instruments. The wettability of the stamps was evaluated by using a Kru¨ss contact-angle goniometer (Hamburg, Germany) equipped with a motorized pipet (Matrix Technology, Nashua, NH). Advancing and receding contact angles were measured using water as the probe liquid on at least three locations of two similarly prepared stamps, and the corresponding values were averaged. Most of the stamp preparations were done in triplicate, and the grafting of Si-EO〈70〉-Si onto oxidized PDMS was verified in more than 20 experiments and using distinct batches of silanes. A Rudolph AutoEL automatic thin-film ellipsometer equipped with a 632-nm He-Ne laser and having an operating angle of incidence of 70° was used to characterize the thickness of the PEO layers deposited on Si/SiO2 samples. Stamps and samples were inspected using a Leica Polyvar optical microscope equipped with a CCD camera (Coolpix 990, Nikon) or a surface fluorescence microscope (Nikon Labophot-2) equipped with a CCD camera cooled at ∼0 °C (ST8, SBIG, Santa Barbara, CA). XPS spectra were acquired on a Sigma Probe VG Scientific spectrophotometer operating at a base pressure of ∼2% 10-9 mbar and equipped with a monochromatized Al KR source (E ) 1486.6 eV). The X-ray spot was focused at 400 µm for these experiments. The analyzer was at an angle of 37° to the sample, and ∼0.5% 0.5 cm2 PDMS stamps were prepared to investigate the amount of Pd/Sn colloids inked onto them. Spectra were referenced to the O 1s peak (532 eV) and acquired with a pass energy of 80 eV (0.1 eV steps for 20-ms acquisition time). The samples were investigated by using a charge compensation gun (∼0.15 µA emission current).

Acknowledgment. We are grateful to our colleagues H. Berney, A. Bietsch, H. Schmid, M. Wolf, and D. Juncker and to Prof. Hilborn (University of Uppsala) for discussions. We thank P. F. Seidler for his continuous support, and C. Donzel acknowledges financial support from the Swiss Commission for Technology and Innovation. LA034370N