Langmuir 2006, 22, 3453-3455
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Photolithographically Patterned Surface Modification of Poly(dimethylsiloxane) via UV-Initiated Graft Polymerization of Acrylates Natasha Patrito,† Claire McCague,† Swanda Chiang,† Peter R. Norton,*,† and Nils O. Petersen‡ Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, Canada, and National Institute for Nanotechnology, Edmonton, Alberta, Canada ReceiVed NoVember 23, 2005. In Final Form: January 11, 2006 Patterned surface modification of poly(dimethylsiloxane) (PDMS) is achieved by combining ultraviolet-initiated graft polymerization (UV-GP) and photolithography. Poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) patterns were grafted onto PDMS with micrometer-scale feature edge resolution. The morphology and chemical composition of the grafted layers were assessed by optical and atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and XPS imaging. AFM section analyses demonstrated the deposition of 33 ( 1 and 62 ( 8 nm thick patterned films of PAA and PMAA, respectively. Spatially resolved C 1s XPS provided images of carboxylic acid functionalities, verifying the patterned deposition of acrylate films on PDMS. These observations demonstrate the general usefulness of UV-GP and photolithography for micropatterning.
Several characteristics make PDMS a favorable material for microfluidic device fabrication. It is a flexible, durable, and inexpensive polymer that can be readily molded into intricate shapes by soft lithography.1,2 However, closely packed methyl groups on the surface of PDMS render the material hydrophobic and chemically inert, hindering the application of PDMS microfluidic devices to bioanalytical problems.1,3,4 Consequently, the development of methods to tailor the surface chemistry and improve the wettability of PDMS is an active area of research. Grafting is one of the most widespread of numerous surface modification techniques that have been applied to PDMS.5,6,18 * Corresponding author. E-mail:
[email protected]. Tel: (519) 661-4180. Fax: (519) 661-3022. † The University of Western Ontario. ‡ National Institute for Nanotechnology. (1) McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. Electrophoresis 2000, 21, 27-40. (2) Whitesides, G. M.; Ostuni, E.; Takayama, S.; Jiang, X.; Ingber, D. E. Annu. ReV. Biomed. Eng. 2001, 3, 335-373. (3) ) Duffy, D. C.; McDonald, J. C.; Schueller, J. A.; Whitesides, G. M. Anal. Chem. 1998, 70, 4974-4984. (4) Makamba, H.; Kim, J. H.; Lim, K.; Park, N.; Hahn, J. H. Electrophoresis 2003, 24, 3607-3619. (5) Uyama, Y.; Kato, K.; Ikada, Y. AdV. Polym. Sci. 1998, 137, 1-39. (6) Rohr, T.; Ogletree, D. F.; Frantisek, S.; Fre´chet, J. M. J. AdV. Funct. Mater. 2003, 13, 264-270. (7) Hu, S.; Ren, X.; Bachman, M.; Sims, C.; Li, G. P.; Allbritton, N. Anal. Chem. 2002, 74, 4117-4123. (8) Hu, S.; Ren, X.; Bachman, M.; Sims, C.; Li, G. P.; Allbritton, N. Anal. Chem. 2004, 76, 1865-1870. (9) Diaz-Quijada, G. A.; Wayner, D. D. M. Langmuir 2004, 20, 9607-9611. (10) Bruening, M. L.; Zhou, Y. F.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 770-778. (11) Wang, X.; Haasch, R. T.; Bohn, P. W. Langmuir 2005, 21, 8452-8459. (12) Ulbricht, M. React. Funct. Polym. 1996, 31, 165-177. (13) Barbani, N.; Lazzeri, L.; Cristallini, C.; Cascone, M. G.; Polacco, G.; Pizzirani, G. J. Appl. Polym. Sci. 1999, 72, 971-976. (14) Kishi, R.; Miura, T.; Kihara, H.; Asano, T.; Shibata, M.; Yosomiya, R. J. Appl. Polym. Sci. 2003, 89, 75-84. (15) Yamashita, K.; Hashimoto, O.; Nishimura, T.; Nango, M. React. Funct. Polym. 2002, 51, 61-68. (16) Chang, B.; Prucker, O.; Groh, E.; Wallrath, A.; Dahm, M.; Ruhe, J. Colloids Surf., A 2002, 198, 519-526. (17) Lahann, J.; Balcells, M.; Rodon, T.; Lee, J.; Choi, I. S.; Jensen, K. F.; Langer, R. Langmuir 2002, 18, 3632-3638. (18) Since the submission of this letter, Wang and co-workers have published a paper (Wang, Y.; Lai, H.-H.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. L. Anal. Chem. 2005, 77, 7539-7546) describing a complementary technique to achieve PDMS patterning by direct photografting through a mask.
Allbritton and co-workers demonstrated that ultraviolet radiation can initiate the attachment and self-propagating polymerization of acrylic acid, acrylamide, and 2-hydroxyethyl acrylate on PDMS, generating stable coatings in a one-step reaction.7 Recently, this ultraviolet-initiated graft polymerization (UVGP) technique was employed successfully to modify inner surfaces of enclosed PDMS microchannels, confirming the applicability of UV-GP to microfluidic device fabrication.8 However, to be considered a fully viable tool for the spatially controlled modification of microfluidic devices, the deposition of the UV-grafted layers must be localized with near-micrometer precision. Wayner and co-workers achieved the patterned derivatization of PDMS by combining an ozone modification technique with photolithography.9 In the present work, we report that UV-GP can be combined with photolithography to covalently immobilize poly(acrylic acid) and poly(methacrylic acid) films on PDMS with micrometer-scale feature edge resolution. Carboxylic acidfunctionalized films are of particular interest because they are versatile platforms for chemical modification via the formation of amide and ester linkages.10,11 In addition, the physical properties and biocompatibility of such films can be reversibly modulated in response to environmental factors including pH, ionic concentration, and temperature.12-15 We generated PDMS substrates from precursors, Sylgard 184A and B (Dow Corning), combined in a 10:1 mass ratio and subsequently dissolved in heptane (Aldrich Chemicals) to produce a 5% (w/w) solution. The siloxane solution underwent spin casting onto a clean silicon wafer at 3000 rpm for 30 s with a Solitec 850 spin coater (Solitec Wafer Processing). The PDMS was cured at 70 °C for 90 min, and the thicknesses of the resulting films were measured to be 220 ( 3 nm using a Gaertner Scientific L2W16D 1.3 ellipsometer (Supporting Information). Photolithographic processing of PDMS is complicated by insufficient adhesion between the photoresist and the siloxane surface. Wayner and co-workers demonstrated that this problem can be overcome by the deposition of an intermediate aluminum layer.9 The PDMS films were coated with 100 nm of Al using an Edwards Auto 500 Magnetron Sputtering System. Microposit S1813 photoresist (Rohm Haas Electronic Materials) was
10.1021/la0531751 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/18/2006
3454 Langmuir, Vol. 22, No. 8, 2006 Scheme 1. Photolithographic Sequence for Patterned Graft Polymerization on PDMS
deposited onto the samples via spin casting and was exposed through a photomask with a Karl Suss MA-6 optical lithography system. The photoresist was developed with Microposit 319, and the exposed aluminum was subsequently etched with 1.8 M orthophosphoric acid. Acrylic acid (AA), methacrylic acid (MAA), sodium metaperiodate (NaIO4), benzyl alcohol, and all other chemicals used in the UV-graft polymerization process were purchased from Aldrich Chemicals and were used as received. After stripping of the remaining photoresist in acetone, samples of the Al-masked PDMS films were mounted onto a sample holder and inserted into 10-cm-long, 1.5 cm i.d. Kimax borosilicate tubes (KimbleKontes) filled with monomer solutions. The aqueous solutions contained 0.5 mM NaIO4, 0.5% (w/w) benzyl alcohol, and 10% (w/w) AA or MAA. The samples were irradiated for 4 h at 35 °C with a short arc Hg-Xe lamp (XB10003, Microlite Scientifics) delivering an exposure intensity of 7.8 mW/cm2 at a wavelength of 250 nm. To ensure uniform UV exposure, the samples were rotated under the source, and at the conclusion of irradiation, they were rinsed with a direct stream of deionized water for 5 min. To remove adsorbed acrylates, samples were immersed in 250 mL of deionized water (22 °C), with stirring, for 18-24 h. The samples were rinsed with water for an additional 5 min and were dried under a stream of forced air.Optical microscope and AFM images of both the poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) grafts demonstrate that the UV-GP of acrylate polymers on PDMS can be achieved with both high
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patterning fidelity and edge resolution, as shown in Figure 1. AFM images were obtained in contact mode using NP-S20 cantilevers (k ) 0.06 N/m) from Veeco Instruments Inc. and demonstrate that the edges of the pattern have widths at 1090% of full height of approximately 1.5 µm (Figure 1c). All measured edges exhibited this sharpness and indicate that the patterning is of comparable fidelity. In addition, the section analyses of the graft samples indicate that flat, uniform acrylate films have been deposited. The increased roughness at the edges is related to the polymerization and Al removal processes. Under the same experimental conditions, PMAA is deposited more readily than PAA, generating 62 ( 8 and 33 ( 1 nm thick films, respectively. AFM analyses of 5 × 5 µm2 images show that the acrylatemodified regions have increased roughness when compared to pristine PDMS. The calculated roughness for the spin-cast PDMS films (Rrms ) 1.6 nm) compares well with previously reported data.9 Rrms values for the PAA and PMAA graft regions are larger by factors of 1.5 and 1.8, respectively. This increased roughness is consistent with the formation of polydisperse acrylate polymer chains by free-radical polymerization. Water contact angle measurements were made using a model 100 manual goniometer (Rame-Hart) to assess changes in the hydrophobic character of the graft-modified PDMS. A droplet of deionized water was placed on both the unmodified and acrylate-grafted regions of the patterned substrates at room temperature. and after 10 s, a contact angle was recorded. Contact angle data collected from five PAA and five PMAA samples were averaged. Water beaded on the unmodified PDMS surface, and the measured contact angle was 104°. This value compared well with reported contact angles for siloxane thin films.4,7 All UV-initiated graft regions demonstrated a significant decrease in the measured water contact angle after grafting when compared to that of the unmodified PDMS, reflecting an improvement in the hydrophilic character of the treated surfaces. The PAA graft contact angle of 58° is significantly higher than that of bulk PAA but comparable to contact angles measured for similarly grafted acrylic acid films.7 For PMAA-grafted regions, the experimentally observed contact angle of 46° compared well with literature contact angles of the bulk polymeric material.16 To further confirm the presence of grafted acrylates, toluidine blue staining was employed. The PAA and PMAA patterned substrates were immersed in a 1% toluidine blue solution (pH 8) for 5 min and then rinsed under a direct stream of water. Once dry, the samples were imaged with an optical microscope (Figure 2b). Toluidine blue possesses a net positive charge and interacts strongly with the negatively charged carboxyl groups of PAA and PMAA, preferentially staining these regions. The hydrophobic PDMS substrate did not bind the dye. Spectroscopic evidence for the presence of the acrylate films on PDMS was obtained by XPS of the modified substrates. All
Figure 1. (a) Optical microscope image of a patterned PMAA graft sample (dark gray) and PDMS (light gray). (b) AFM height image of the inset region. (c) AFM section analysis of the PDMS/PMAA step edge.
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Figure 2. Optical microscope images of a patterned PAA graft sample (a) before and (b) after staining with toluidine blue dye. Overlaid XPS images of a patterned PAA graft sample (c) before and (d) after reaction with pentafluorophenol (red ) 101.3 eV; green ) 288.75 eV; blue ) 686 eV).
samples were analyzed using a Kratos Axis Ultra spectrometer (Kratos Analytical), employing an Al KR X-ray source (1486.6 eV) operated at a source energy of 210 W. The Kratos chargeneutralizer system was used on all specimens with a filament current of 1.6 A and a charge balance of 2.4 V. Elemental and chemical-state XPS images were collected using low-magnification mode (800 × 800 µm2) and imaging aperture 2. An XPS survey and high-resolution spectra of the C 1s and Si 2p binding energy regions were collected at pass energies of 160 and 20 eV, respectively. XPS spectra were fit and signal areas calculated using CasaXPS software. The binding energy scale was calibrated using the C 1s peak at 284.8 eV. PAA and PMAA graft regions both displayed a C 1s binding energy signal at approximately 289 eV, corresponding to the presence of carboxylic acid functionalities. No similar bands appeared in the pristine PDMS spectrum (Supporting Information, Figure S1). Additionally, carbon, oxygen, and silicon compositions were assessed by XPS before and after grafting. The experimentally quantified atomic compositions for pristine PDMS closely approximated predicted values for the monomer unit. After grafting, the surface carbon content increased while the Si signal was significantly attenuated because of the coverage of PDMS by the acrylate films. C 1s XPS images of a 200-µm-wide PAA patterned region were obtained at 0.25 eV intervals spanning a binding energy range of 275-295 eV. In the image collected at 288.75 eV, which corresponds to the approximate maxima of a carboxylate
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carbon peak, intense signals are displayed across a well-defined strip through the center of the analysis region (Figure 2c). This corresponds to the localization of the PAA graft in the patterned area. The region outside the PAA graft corresponds to the PDMS substrate that does not possess a carboxylic acid functionality and therefore displays a much reduced background signal at 288.75 eV. Further confirmation of the localization of PMAA was provided by the overlaid Si 2p XPS image collected for the same sample at a binding energy of 101.3 eV. In this case, an intense signal is observed from the Si-rich PDMS region whereas the PAA graft region has a small signal because the XPS image probes only the surface of the patterned substrate (Figure 2c). Acrylate films can be derivatized with a wide range of moieties including fluorescent groups, electroactive species, and biocompatible ligands.10,11 Pentafluorophenol esters, generated via the reaction of acrylates with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and pentafluorophenol (PFP), have commonly been used as intermediates to achieve the covalent attachment of amine-terminated molecules.10,11,17 To demonstrate the ability of our substrates to direct surface chemistry, a PAA patterned sample was immersed in an ethanol solution of EDC (0.1 M; Aldrich Chemicals) and PFP (0.2 M, Aldrich Chemicals) for 30 min. After being rinsed with ethanol and dried under a stream of forced air, the sample was imaged by XPS. Intense signals at 686 eV, corresponding to the approximate maxima of fluorocarbon species, are localized to the acrylate graft region (Figure 2d), confirming the patterned formation of pentafluorophenol esters. These reactive esters can then be further derivatized with an array of amine-functionalized molecules including integrin ligand peptides, ethylenediamine, and poly(L-lysine).10,11 In conclusion, we have shown that UV-GP and photolithography can be combined effectively to create patterns of PAA and PMAA on PDMS with micrometer-scale edge resolution. The morphology and thickness of the deposited films were measured by optical and atomic force microscopy, respectively. XPS analyses examined the chemical composition of the patterned substrates and confirmed the localization of the acrylate grafts. The patterning technique reported here is widely applicable and can be used to tailor PDMS surfaces to a variety of applications by varying the grafted polymer. Future research will focus on the coupling of biologically relevant molecules to the patterned acrylate films for the modulation of cell adhesion. Acknowledgment. This research was supported by the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Photonics Innovation. We thank Dr. M. Biesinger and Mr. B. Kobe for XPS technical assistance. Supporting Information Available: Thicknesses of films measured via ellipsometry. PDMS spectrum. This material is available free of charge via the Internet at http://pubs.acs.org. LA0531751
