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Creating Patterned Poly(dimethylsiloxane) Surfaces with Amoxicillin and Poly(ethylene glycol) Woo-Sung Bae and Marek W. Urban* School of Polymers and High Performance Materials, Shelby F. Thames Polymer Science Research Center, The UniVersity of Southern Mississippi, Hattiesburg, Mississippi 39406 ReceiVed June 1, 2006. In Final Form: September 19, 2006 This paper reports a simple microwave plasma patterning of poly(dimethylsiloxane) (PDMS) surfaces, which is accomplished by allowing selective surface areas to microwave plasma exposure in the presence of gaseous monomer. When maleic anhydride is used for microwave plasma reaction in the presence of physical barrier on the PDMS substrate, the resulting patterned surfaces with chemically bonded maleic anhydride and carboxylic acid groups are generated. In this particular study we attached amoxicillin via ammonolysis under weak base conditions in the presence of a catalyst as well as poly(ethyleneglycol) (PEG). A combination of internal reflection IR imaging (IRIRI) and atomic force microscopy (AFM) revealed that amoxicillin and PEG can be readily reacted on the microwave plasma patterned PDMS surfaces. Surface areas directly exposed to microwave plasmons exhibit the highest reactivity due to higher content of functional groups. These studies also show that molecular weight of PEG has also significant effect on kinetics of surface reactions.
Introduction Although microwave-generated plasma reactions have been successfully utilized to create stable surface modifications on polymeric surfaces,1-8 obtaining chemical patterns with desired shapes and spatial morphologies remain to be a challenge. If such reactions can be conducted to obtain desirable patterns on surfaces, such properties as localized adhesion and wettability as well as reactivity may be controlled selectively using further reactions.9-11 However, precise patterned modifications in the presence of microwave plasma have not been reported. Recent efforts dealing with the precise fabrication of patterned surfaces containing controlled functional groups10-16 are particularly of interest in optical devices,17 lithography,18 and bio applications.14 These processes, however, typically involve imprinting specific patterns using controlled UV exposure, microcontact printing, * To whom all correspondence should be addressed. E-mail:
[email protected]. (1) Bae, W.-S.; Urban, M. W. Langmuir 2004, 20 (19), 8372-8. (2) Gaboury, S. R.; Urban, M. W. Langmuir 1993, 9 (11), 3225-33. (3) Kim, H.; Urban, M. W. Langmuir 1995, 11 (6), 2071-6. (4) Kim, H.; Urban, M. W. Langmuir 1996, 12 (4), 1051-5. (5) Kim, H.; Urban, M. W. Langmuir 1999, 15 (10), 3499-505. (6) Kim, H.; Urban, M. W.; Lin, F.; Meier, D. J. Langmuir 1996, 12 (13), 3282-8. (7) Stewart, M. D., E.; Urban, M. W. U.S. Patent 5,364,662, 1994. (8) Zhao, Y.; Urban, M. W. Langmuir 1999, 15 (10), 3538-44. (9) Pallandre, A.; De Meersman, B.; Blondeau, F.; Nysten, B.; Jonas, A. M. J. Am. Chem. Soc. 2005, 127 (12), 4320-5. (10) Pan, F.; Wang, P.; Lee, K.; Wu, A.; Turro, N. J.; Koberstein, J. T. Langmuir 2005, 21 (8), 3605-12. (11) Corcoran, N.; Ho, P. K. H.; Arias, A. C.; Mackenzie, J. D.; Friend, R. H.; Fichet, G.; Huck, W. T. S. Appl. Phys. Lett. 2004, 85 (14), 2965-7. (12) Chen, L.; Zhuang, L.; Deshpande, P.; Chou, S. Langmuir 2005, 21 (3), 818-21. (13) Brooksby, P. A.; Downard, A. J. Langmuir 2005, 21 (5), 1672-5. (14) McCarley, R. L.; Vaidya, B.; Wei, S.; Smith, A. F.; Patel, A. B.; Feng, J.; Murphy, M. C.; Soper, S. A. J. Am. Chem. Soc. 2005, 127 (3), 842-3. (15) Zhou, W.; Kuebler Stephen, M.; Braun Kevin, L.; Yu, T.; Cammack, J. K.; Ober Christopher, K.; Perry Joseph, W.; Marder Seth, R. Science 2002, 296 (5570), 1106-9. (16) Senaratne, W.; Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6 (5), 2427-48. (17) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. W. Nature 1998, 393 (6681), 146-9. (18) Boltau, M.; Walheim, S.; Mlynek, J.; Krausch, G.; Steiner, U. Nature 1998, 391 (6670), 877-9.
and sputtering deposition techniques.14,19,20 Simplicity and speed of microwave plasmons may open another avenue for surface patterning, and this study reports the utilization of microwave plasmons to fabricate patterned poly(dimethylsiloxane) (PDMS) surfaces in which a metal grid serves as screen for microwave plasmon, thus facilitating energetically favored conditions in selected surface reactions. Such prepared surfaces may serve further reactions, and in this particular study, amoxicillin and O-(2-aminoethyl)-O′-methylpoly(ethylene glycol) (APEG) will be reacted in order to create patterned surfaces with antimicrobial and biocompatible properties. While poly(ethyleneglycol) (PEG) and PEG-based polymeric materials are often used in biological applications due to resistance to protein and cell adhesion, nontoxicity, nonimmunogenicity,21-23 blood compatibility, and optimal selectivity, efficiency of surface modification of polymeric materials containing PEG remains a major key in the development of many biomedical diagnostic and sensing applications.24 Along the same lines, amoxicillin is a known antibiotic, and reacting these species to polymeric surfaces may open a number of opportunities for creating antimicrobial surfaces. Experimental Section PDMS substrate was prepared from a linear trimethylsiloxyl terminated vinylmethylsiloxane-dimethylsiloxane copolymer (Mn ) 28 000; VDT-731, Gelest, Inc.). Reactions between vinyl groups forming cross-linked PDMS networks were initiated by the addition of 0.5 wt % of tert-butyl perbenzoate (Aldrich Chemical Co.). The copolymer and the initiator were premixed for 24 h to ensure complete dissolution of initiator in PDMS. Cross-linked PDMS substrates were prepared by pressure molding the oligomer-initiator mixture for 15 min at 149 °C and post-cross-linking for an additional 5 h at 182 °C. Surface contaminants and residual low molecular-weight (19) Xia, Y.; Tien, J.; Qin, D.; Whitesides, G. M. Langmuir 1996, 12 (16), 4033-8. (20) Hu, Z.; Chen, Y.; Wang, C.; Zheng, Y.; Li, Y. Nature 1998, 393 (6681), 149-52. (21) Lee, J. H.; Lee, H. B.; Andrade, J. D. Prog. Polym. Sci. 1995, 20 (6), 1043-79. (22) Lee, K. Y.; Mooney, D. J. Chem. reV. 2001, 101 (7), 1869-79. (23) Andruzzi, L.; Senaratne, W.; Hexemer, A.; Sheets, E. D.; Ilic, B.; Kramer, E. J.; Baird, B.; Ober, C. K. Langmuir 2005, 21 (6), 2495-504. (24) Li, Y.; Neoh, K. G.; Kang, E. T. Polymer 2004, 45 (26), 8779-89.
10.1021/la061571t CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006
10278 Langmuir, Vol. 22, No. 24, 2006 species were removed by washing PDMS substrates in methylene chloride for 5 min, followed by slow deswelling and drying in air and vacuum-desiccating in 1.3 Pa for 24 h at room temperature. Plasma reactions were conducted using open reactor conditions, as described elsewhere.3 Cross-linked PDMS substrate, with approximate dimensions of 10 × 7 × 2 mm, and 100 mg of solid maleic anhydride (Aldrich Chemical Co.) were placed into the reactor chamber and spaced by 8.5 cm. It should be noted that the amount of monomer and the distance between monomer and substrate are significant as they affect the efficiency of surface reactions. In a typical experiment, the reactor is evacuated to 10 mTorr followed by purging it with Ar gas to reach the steady-state pressure (250 mTorr) with a flow rate of 2.96 mL/min. At this point, a microwave radiation of 600 W of power with an output frequency of 2.45 GHz is turned on to induce plasmons. Under these conditions, the reaction chamber pressure increases continuously during microwave plasma discharge. Under the same pressure conditions, but without microwave plasma discharge, the pressure in the reaction chamber remains constant, and no sorption of maleic anhydride into the PDMS network is detected. Ted Pella Inc. grids were attached onto PDMS surface during short microwave plasma discharge. The grids are made of copper and sized by 2000 mesh (hole ) 7.5 µm, bar width ) 5 µm). To modify maleic anhydride patterned PDMS surfaces with amoxicillin, we utilized ammonolysis under weak base condition with catalyst such as 4-(dimethylamino)pyridine (DMAP).25 In a 250 mL dry flask, 10 mg (8.19 × 10-5 mol) of DMAP and 100 mg (2.74 × 10-4 mol) of amoxicillin was added to 100 mL of pyridine. The flask was placed in ice bath to keep temperature around 0 °C. Meantime, to achieve high yields, we utilized freshly modified PDMS using microwave plasma, which was analyzed prior to further reactions to confirm the presence of anhydride groups on the surface. Such surface functionalized PDMS was added to the reaction flask under ice bath conditions and allowed to react for 20 min. Such prepared PDMS specimen was washed with the excess of water for 20 min and dried for 24 h and then analyzed again by using ATR FT-IR. Different molecular weights (750, and 3000 g/mol) of APEG were utilized for surface grafting reactions on the patterned PDMS surfaces using N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDAC) coupling reagent (Aldrich Chemical Co.). Microwave plasma reacted PDMS was placed in the EDAC aqueous solution (0.2 mol/L), which was prepared just before the reaction, and the reactor was shaken for 20 min on the orbit shaker (Lab-Line Instruments Inc.) followed by adding APEG aqueous solution (1.87 × 10-2 mol/L) and allowed to react for 2 and 5 h. After reactions, each sample was rinsed with DI water and dried for 24 h before analyses. Surface morphological features of microwave plasma surface patterned PDMS were characterized by atomic force microscopy (AFM). The images were collected using a Nanoscope IIIa Dimension 3000 scanning probe microscope (Digital Instruments) set at the resonance frequency of 300 kHz and tapping mode equipped with a Si prove (force constant ) 40 N/m). Attenuated total reflectance Fourier transform infrared (ATR FTIR) spectra were collected using a Bio-Rad FTS-6000 FT-IR singlebeam spectrometer set at a 4 cm-1 resolution equipped with a deuterated triglycine sulfate (DTGS) detector and a 45° face angle Ge crystal. Each specimen was pressed against a Ge crystal with a constant force in an effort to maintain the same sample-crystal contact in all measurements. Each spectrum represents 200 co-added scans ratioed against a reference spectrum obtained by recording 200 coadded scans of an empty ATR cell. All spectra were collected for spectral distortion using Q-ATR software.26 Internal reflection IR imaging (IRIRI) experiments were conducted on a Digilab FTS 6000 Stingray system with a Ge internal reflection element allowing spatial resolution of about 1 µm or better.27 This (25) Hoefle, G.; Steglich, W.; Vorbrueggen, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 569-83. (26) Huang, J. B.; Urban, M. W. Appl. Spectrosc. 1992, 46 (11), 1666-72.
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Figure 1. Schematic diagram of IRIRI experimental setup. IR radiation coming from the source is focused on the surface and reflected through the optics to the focal plane analyzer (FPA) detector and magnified, thus allowing to overcome diffraction limits (adapted from ref 27). system consists of a Digilab FTS 6000 spectrometer, an ImagIR focal plane array (FPA) image detector, and a semi-spherical Ge crystal. IRIR images were collected using the following spectral acquisition parameters: under-sampling ratio of 4, step-scan speed of 1 Hz, 1021 spectrometer steps, 160 images per step, and 8 cm-1 spectral resolution. In a typical experiment with an experimental setup shown in Figure 1, a spectral data set acquisition time was approximately 19 min. Image processing was performed using the Environment for Visualizing Images (ENVI) software (Research Systems, Inc., version 3.5). When appropriate, baseline correction algorithms were applied to compensate for baseline deviations which were accomplished by built-in application software supplied by GRAMS/AI v7.02 (Galactic Ind.). All bands were normalized to the C-H stretching region due to the abundance of these groups.
Results and Discussion As was shown previously,2,8 when maleic anhydride is reacted to the surface of PDMS and the reaction time is 5 s, two carbonyl bands at 1781 and 1725 cm-1 are detected, which are due to anhydride and carboxylic acid groups resulting from the anhydride ring opening, respectively. When the same reaction is conducted for 10 s, the presence of the 1725 cm-1 due to acid functionalization dominates the spectrum, and significantly weaker band at 1781 cm-1 due to maleic anhydride is detected. The diminished intensity of the latter was attributed to the maleic anhydride ring opening reactions resulted from extended microwave plasma exposure. Using the same conditions we attached a Cu grid to the PDMS surface, and conducted the same microwave plasma reactions. Figure 2a represent a schematic diagram of individual steps involved in microwave plasma reactions and patterned surface reactions. As shown this process involves attachment of the grid (a1), exposure to microwave plasmon in the presence of maleic anhydride (a2), and the removal of the grid (a3). Figure 2b illustrates an optical image of the surface after microwave plasma reactions in the presence of maleic anhydride. The primary advantage of this approach is that this is one step process, which is fast and clean, and only requires a metal mask that determines the reaction patterns. One drawback is that a poor contact between (27) Otts, D. B.; Zhang, P.; Urban, M. W. Langmuir 2002, 18 (17), 6473-7.
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Figure 2. (a) Schematic diagram of microwave plasma reaction to fabricate grid-like patterned surfaces and further surface reaction. (b) Optical image of patterned PDMS surfaces.
Figure 3. (a) IRIRI collected from microwave plasma patterned PDMS surfaces obtained by tuning into the 1781 cm-1. (b) IR spectra recorded from the lines A-F.
metal and polymer surfaces may result in nonuniformity of reactions. Using this process, selectively functionalized PDMS surface with anhydride and carboxylic acid was generated. In an effort
to chemically characterize patterned PDMS surfaces, we utilized IRIRI, which allowed us to tune into a given IR band associated with a specific functional group, thus providing chemical surface mapping. As shown in Figure 3a, the image tuned to 1781 cm-1
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Figure 4. (a) Height images of contact mode AFM. (b) Friction images of contact mode AFM, (c) Phase images of tapping mode AFM collected from microwave plasma patterned PDMS surfaces. (a′) Magnified height image of contact mode (3 µm × 3 µm). (c′) Magnified 3D phase images of tapping mode AFM of inset square region of panel c.
Figure 5. (a) Schematic diagram of ammonolysis reaction of amoxicillin with anhydride modified PDMS surfaces. (b) IRIR image collected from amoxicillin attached PDMS with grid-like pattern, (c) IR spectra recorded from the region A and B of panel b. (d) Phase image of tapping mode AFM image obtained from amoxicillin attached PDMS with grid-like pattern.
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Figure 6. (a) ATR FT-IR spectra collected from PDMS surface (trace A), maleic anhydride reacted surface (trace B), APEG-700 grafted PDMS surface for 2 h (trace C), APEG-3000 grafted PDMS surface for 2 h (trace D) and 5 h (trace E). (b) IRIRI image collected from APEG-700 grafted on patterned PDMS surface. (c) IR spectra recorded from the region A and B of panel b.
exhibits grid-patterned distribution of maleic anhydride which correspond to the regions exposed or unexposed to microwave plasma reactions. While dark reddish areas represent higher concentration levels of maleic anhydride attached to the surface, greenish regions represent lower concentration content. Figure 3b represents the spectra collected from lines A-F spaced 1 µm apart in Figure 3a and illustrates that line A contain the highest concentration levels of carbonyl group of anhydride and carboxylic acid, which is reflected in highest intensities of bands at 1781 and 1726 cm-1. It should be pointed out that the spectra
shown in Figure 3b were normalized to the methyl C-H bending bands due to Si-CH3 vibrations at 1408 cm-1. Analysis of these spectra also show that the area covered by the grid also contain carbonyl groups, as demonstrated by trace F of Figure 3b, thus indicating substantial penetration of plasmons. This phenomenon appears to be important when reacting other species to polymeric surfaces because the density of surface species will determine the nature of further reactions. To correlate surface reaction products with morphology changes, the surface height, fiction, and phase images were
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Figure 7. (a) Height image and (a′) phase image of tapping mode AFM of APEG-700 tethered (for 2 h reaction) patterned PDMS surface. (b) Height image and (b′) phase image of tapping mode AFM of APEG-3000 tethered (for 5 h reaction) patterned PDMS surface.
collected using contact and tapping mode AFM. Figure 4a shows height image of plasma meshed PDMS surfaces and illustrates that indeed morphological features match spectroscopic analysis. The height of the microwave plasma exposed areas is about 110 nm above the PDMS surface, indicating that it is composed of polymerized maleic anhydride, which agrees with the spectroscopic data shown in Figure 3b. The areas directly exposed to microwave plasmons exhibit 200 nm sphere, as shown in Figure 3a. The friction image of the same area is depicted in Figure 4b and clearly illustrate that two chemically different components exist: area labeled A exhibits lower friction and greater surface hardness, and area labeled B has softer surface, thus indicating that the polymerized maleic anhydride is harder than the PDMS surface. Finally, the phase image shown in Figure 3c illustrates similar features, as shown by the blowup of the areas A and B of Figure 4c′. As we recall, IRIR images shown in Figure 3a demonstrated that anhydride and carboxylic acid functional groups are still present in the covered areas, and AFM images shown in Figure 4 indicate the presence of spikes in area B. In contrast, area A exposed to microwave plasma exhibits high-density anhydride and carboxylic acid functionalization. While one objective of this study was to create patterned reactive surfaces, another one was to obtain patterned biocompatible and/or antimicrobial substances. Figure 5a illustrates the reaction scheme to fabricate amoxicillin-modified PDMS surfaces under base conditions on the maleic anhydride microwave plasma reacted PDMS surfaces, such as shown in Figure 2, a4. The results of the surface analysis after reactions are shown in Figure 5b. As seen, IRIR images of the amoxicillin patterned PDMS surfaces exhibit concentration differences of amide groups, which depend on the concentration of anhydride moieties. The IR spectra collected from regions A and B of Figure 5b are shown in Figure 5c, traces A and B, respectively, and indicate the presence of the 1654 cm-1 band due to amide groups. To identify surface morphology the distribution of these species, the AFM tapping mode image is shown in Figure 5d and corresponds to IRIRI counterpart.
As shown in Figure 2, a5, APEG was also reacted to modified microwave plasma functionalized PDMS surfaces and two different molecular weights for APEG were incorporated using EDAC coupling reactions. Figure 6a illustrates ATR FT-IR spectra for each step of surface modifications. Trace A represents unreacted PDMS surface, which is sequentially modified (Figure 2a), and trace B exhibits anhydride and carboxylic acid functional groups represented by the bands at 1781 and 1726 cm-1, respectively. After EDAC coupling with APEG, the CdO stretching vibrations of carboxylic acid shift from 1726 to 1713 cm-1 due to increased concentration of carboxylic acid groups resulting from hydrolysis of anhydride groups, thus increasing dimeric form of carboxylic acid. The occurrence of amide coupling reaction is confirmed by the presence of amide I band at the 1658 cm-1, the amide II at the 1601 cm-1 and the 1564 cm-1 stretching bands. This is illustrated in traces C, D, and E of Figure 6. The C-O stretching vibrations of ethylene glycol are detected at 1382 cm-1. Comparison of traces C and D of Figure 6 also shows that mol wt significantly affects kinetics of these reactions. After 2 h of APEG-700 reactions, the band intensities are intense as shown in trace C. However, as shown in trace D, reactions of APEG3000 lead to lower concentration levels of the amide I, II, and C-O stretching vibrations. By extending reaction times to 5 h with APEG-3000, the concentration levels of APEG on the PDMS surface increase, which is manifested by the increased band intensities at 1658, 1601, 1564, and 1382 cm-1. This is illustrated in trace E, and this behavior is attributed to diffusion-controlled collision of polymeric chains.28-30 Specifically, for higher molecular weights, the polymerization medium become solventdeficient due to the higher concentration levels of polymeric chains, which affect the polymer morphology in the solution due to hydrodynamic radius and transitional mobility differences. Therefore, the probability of amino groups of ethyleneglycol to (28) Dionisio, J. M.; O’Driscoll, K. F. J. Polym. Sci., Polym. Chem. Ed. 1980, 18 (1), 241-9. (29) Mahabadi, H. K.; O’Driscoll, K. F. J. Polym. Sci., Polym. Chem. Ed. 1977, 15 (2), 283-300. (30) Mahabadi, H. K.; O’Driscoll, K. F. Macromolecules 1977, 10 (1), 55-8.
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encounter activated carboxylic acid on the PDMS decreases with the increasing molecular weight of APEG. As shown in Figure 6b, IRIR imaging tuned to the 1382 cm-1 band revealed that selectively grafted ethyleneglycol on the PDMS surface is detected, which is a function of the location of the surface functional groups. On the basis of the IR spectra collected from areas A and B, higher concentration levels of the C-O groups represented by the 1382 cm-1 band due to ethyleneglycol moieties are detected in the area A. Similarly, surface morphology was investigated using a tapping mode AFM, and the results are shown in Figure 7. The height images of APEG-700 and APEG-3000 grafted PDMS (Figure 7, panels a and b, respectively) represent similar surface topologies to the height images of microwave plasma patterned surface shown in Figure 3a. However, the phase images shown in Figures 7, panels a′ and b′, represent mechanical property variations on the square area. These data indicate that the “grafting to” reaction occur on the carboxylic acid functionalized area, but the reactions are heterogeneous. Compared to the phase image of Figure 4c, lower degree area (dark zone) is attributed to the APEG grafted area, indicating that APEG-700 grafting reactions are more efficient on the carboxylic acid functionalized PDMS surface. This observation is consistent with the results of the ATR FT-IR analysis shown in Figure 6a.
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Conclusions These studies show that using microwave plasma reactions in the presence of maleic anhydride PDMS surfaces can be selectively functionalized patterned metal shield is used. As a result, carboxylic acid patterned surfaces are obtained which are available for further reactions. IRIRI and AFM measurements also showed that, in addition to exposed to microwave radiation areas, microwave plasma surface reactions occur at the areas in contact with a metal grid. However, the density of surface reactions is significantly smaller. Further reactions employed the attachment of amoxicillin, and “grafting to” reactions of amino functionalized ethyleneglycol. These studies showed that the formation of patterned PDMS surfaces that exhibit biocompatible properties is possible. The primary advantage of this approach is that this is one step, very fast and clean process that does not require additional steps, and the shape of a metal mask determines the reaction patterns. One of the drawbacks is that poor contact between a metal and a polymer surface may result in additional reactions, thus affecting uniformity of surface reactions. Acknowledgment. Major support for these studies from the National Science Foundation Materials Research Science and Engineering Center (DMR 0213883) is acknowledged. Supporting Information Available: Two figures. This material is available free of charge via the Internet at http://pubs.acs.org. LA061571T
