Reactions of Antimicrobial Species to Imidazole-Microwave Plasma

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Reactions of Antimicrobial Species to Imidazole-Microwave Plasma Reacted Poly(dimethylsiloxane) Surfaces 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 April 30, 2004. In Final Form: July 1, 2004 Microwave plasma reactions of imidazole, 2-methylimidazole, and 2-ethylimidazole on poly(dimethylsiloxane) (PDMS) surfaces resulted in the formation of species containing conjugated surface domains which can be utilized for further reactions. When imidazole and its derivatives were used, polymerization of imidazole and the formation of CdC and CtN conjugated species occurred. However, the extent of reactions for each monomer depends on not only the reaction time but also the molecular structure. For methyl- and ethyl-substituted imidazole, more stable radical species are generated and sustain their excited state in the high-energy plasma environments. Specifically, dehydrogenated 2-methyl, 2-ethylimidazole radicals and •NdCRsNH• (R ) -CH3, -CH2CH3) species exhibit higher stability than dehydrogenated imidazole radicals and •NdCH-NH• species under plasma reaction conditions. Such prepared surfaces are capable of attaching antimicrobial drugs via the Pinner synthesis. These studies show that it is possible to react antimicrobial species such as chloramphenicol, and this promising approach offers numerous applications of microwave plasma reactions in biotechnology. Quantitative analysis of the depth of surface reactions was accomplished by using variable angle ATR FT-IR spectroscopy.

Introduction Although microwave energy represents one of the commonly utilized energy sources, its use in synthesis has been limited despite numerous advantages in generating microwave and radio frequency plasmons.1,2 For example, surface plasma reactions not only are fast but also usually do not alter bulk properties of a substrate and require no volatile organic solvents.3-5 Our earlier studies6-11 indicated that microwave plasma energy can be successfully used for reacting monomers to elastomeric surfaces and revealed numerous advantages of microwave plasma reactions. For instance, reaction mechanisms of imidazole onto poly(dimethylsiloxane) (PDMS) substrate under various conditions including reaction times, plasma gas, and the substrate were investigated.6-11 Furthermore, significant effects of substrate stability and cross-linking as well as the effect of surface reactions on substrate surface morphology were also investigated. During the course of these studies it was found that plasma conditions as well as the monomers being utilized in surface reactions also play an important role. Since the electron-withdrawing or -donating groups on monomers utilized in the plasma reactions have a significant effect on reactivity, these studies focus on the effect of methyl and ethyl substituents on imidazole reactions to PDMS. To the best * To whom all correspondence should be sent. (1) Loupy, A. Microwaves in Organic Sythesis; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2002. (2) Nuchter, M.; Ondruschka, B.; Jungnickel, A.; Muller, U. J. Phys. Org. Chem. 2000, 13, 579-586. (3) Stewart, M. D., E.; Urban, M. W. In US Patent 5,364,662. (4) Yasuda, H. Plasma Polymerization; Academic: Orland, FL, 1985. (5) Yasuda, H.; Sharma, A. K.; Yasuda, T. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1285-1291. (6) Gaboury, S. R.; Urban, M. W. Langmuir 1993, 9, 3225-3233. (7) Kim, H.; Urban, M. W. Langmuir 1995, 11, 2071-2076. (8) Kim, H.; Urban, M. W. Langmuir 1996, 12, 1051-1055. (9) Kim, H.; Urban, M. W. Langmuir 1999, 15, 3499-3505. (10) Kim, H.; Urban, M. W.; Lin, F.; Meier, D. J. Langmuir 1996, 12, 3282-3288. (11) Zhao, Y.; Urban, M. W. Langmuir 1999, 15, 3538-3544.

of our knowledge, the only studies that discuss effect of alkyl substituent groups of imidazole derivatives were conducted on copper-imidazole complexes.12 While the focus of these studies is reactions of imidazole, 2-methylimidazole, and 2-ethylimidazole with PDMS surfaces using open plasma reactor conditions,7 an ultimate goal is to compare reaction patterns for reactive monomers and reaction times. Similar to the previous studies, attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy will be used for quantitative analysis of imidazole and 2-alkylimidazole reactions under microwave plasma open reactor condition. To gain further understanding of plasma-induced reactions, we will attempt to quantify the distribution of imidazole derivatives across the PDMS surfaces by surface depth profiling experiments and surface characterization methods. These approaches will allow us to determine the depth of surface plasma reactions and what morphological changes take place. In an effort to determine usefulness of this approach antimicrobial molecules were attached to surface-modified PDMS surfaces for potential medical device applications. Experimental Section Poly(dimethylsiloxane) (PDMS) was prepared from a linear trimethylsiloxyl-terminated vinylmethylsiloxane-dimethylsiloxane copolymer (Mn ) 28000; 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 4 h at 210 °C. Surface contaminants and residual low-molecular-weight species were removed by washing PDMS substrates in methylene chloride for 1 h, followed by slow deswelling and drying in air and vacuum desiccating in 1.3 Pa for 24 h at room temperature. (12) Yoshida, S.; Ishida, H. Appl. Surf. Sci. 1985, 20, 497-511.

10.1021/la048916x CCC: $27.50 © 2004 American Chemical Society Published on Web 08/18/2004

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Figure 1. (a) ATR FT-IR spectra in the 1800-1350 cm-1 region of imidazole plasma modified PDMS reacted for 5, 10, and 15 s (open system). (b) ATR FT-IR spectra in the 2350-2050 cm-1 region of imidazole plasma modified PDMS reacted for 5, 10, and 15 s (open system). Plasma reactions were conducted using open reactor conditions, as described elsewhere.7 Cross-linked PDMS substrate, with approximate dimensions of 16 × 7 × 1.5 mm, and about 50, 60, and 70 mg of imidazole, 2-methylimidazole, and 2-ethylimidazole (Aldrich Chemical Co.) in a form of fine white powder were placed into the reactor chamber. PDMS substrate and the reactant powders were 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, a reactor is evacuated to 10 mTorr followed by purging it with Ar gas to reach the steadystate pressure (200 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 imidazole into the PDMS network is detected. Attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra were collected using a Bio-Rad FTS-6000 FT-IR single-beam spectrometer set at a 4 cm-1 resolution equipped with a deuterated triglycine sulfate (DTGS) detector and a 45° face angle Ge crystal. To obtain surface depth profiling data, a Ge crystal and KRS-5 crystal with 45° angle parallelogram 52.5 × 20 × 3 mm was used. Each spectrum represents 200 co-added scans ratioed against a reference spectrum obtained by recording 200 co-added scans of an empty ATR cell. All spectra were corrected for spectral distortion using Q-ATR software.13 Determination of extinction coefficients of imidazole, 2-methylimidazole, and 2-ethylimidazole was accomplished by preparing of standard solutions with known concentrations of imidazole and 2-ethylimidazole (0.2, 0.4, 0.6, and 0.8 mol/L) and 2-methylimidazole (0.05, 0.1, 0.15, and 0.2 mol/L) followed by transmission FT-IR measurements recorded as a function of concentration. A liquid transmission cell with 0.1 mm path length was utilized. A plot of the band intensity vs concentration was used to determine the extinction coefficients which for imidazole, 2-methylimidazole, and 2-ethylimidazole were 11.83, 15.64, and 14.94 L/(mol cm), respectively. Internal reflection IR image (IRIRI) experiments were conducted on a Digilab FTS 6000 Stingray system with a Ge internal reflection element.14 This system consists of a Digilab FTS 6000 (13) Huang, J. B.; Urban, M. W. Appl. Spectrosc. 1992, 46, 16661672.

spectrometer, an ImagIR focal plane array (FPA) image detector, and a semispherical Ge crystal. IRIR images were collected using the following spectral acquisition parameters: undersampling ratio of 4, step-scan speed of 1 Hz, 1777 spectrometer steps, 160 images per step, and 8 cm-1 spectral resolution. In a typical experiment, a spectral data set acquisition time was approximately 30 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. Environmental scanning electron microscope (ESEM) measurements were conducted on a New Quanta FEG series 200 FEG (FEI Co.). We prepared virgin PDMS, 5 s imidazole plasma reacted PDMS, and 10 s imidazole plasma reacted PDMS, which are loaded on the scanning electron microscopy (SEM) specimen mount with double-coated tapes. Each sample was investigated at a 45° angle of scanning electron beam. We obtained ESEM images using 3000× magnifications under 1 Torr pressure. To modify PDMS-imidazole surfaces with chloramphenicol, we utilized Pinner synthesis.15 In a 250 mL dry flask, 1.08 × 10-3 mol of chloramphenicol was added to 100 mL of 95 wt % acetone. The flask was placed in ice bath to keep temperature around 0 °C and 3 mL of 37 wt % hydrochloric acid in water was decanted in the flask slowly. 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 CtN functional groups on the surface. Such surface-functionalized PDMS was added to the reaction flask under ice bath conditions and allowed to react for 30 min. Such prepared PDMS specimen was washed with the excess of acetone and analyzed again using ATR FT-IR.

Results and Discussion In the previous studies, when microwave plasma reactions were conducted under the open-flow plasma reactor conditions, imidazole molecules were reacted to the PDMS surface by ring opening to form CtN and conjugated CdC double bond surface species.10 If one considers the stability of the radicals generated during these reactions, important features in determining their (14) Otts, D. B.; Zhang, P.; Urban, M. W. Langmuir 2002, 18, 64736477. (15) Roger, R.; Neilson, D. G. Chem. Rev. 1961, 61, 179-211.

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Figure 2. (a) ATR FT-IR spectra in the 1800-1350 cm-1 region of 2-methylimidazole plasma modified PDMS reacted for 5, 10, and 15 s (open system). (b) ATR FT-IR spectra in the 2350-2050 cm-1 region of 2-methylimidazole plasma modified PDMS reacted for 5, 10, and 15 s (open system).

Figure 3. (a) ATR FT-IR spectra in the 1800-1350 cm-1 region of 2-ethylimidazole plasma modified PDMS reacted for 5, 10, and 15 s (open system). (b) ATR FT-IR spectra in the 2350-2050 cm-1 region of 2-ethylimidazole plasma modified PDMS reacted for 5, 10, and 15 s (open system).

stability are (1) steric effect, which isolates radical reactive sites from its environment and allow radical persistence, and (2) delocalization of the odd electron, which can reduce reactivity by reducing the spin density on the radical center.16,17 In view of the above considerations and the results of the previous studies, it is anticipated that the substituted alkyl groups of imidazole will alter reaction mechanisms as well as produced surface species on PDMS. While one objective of this study is to advance current (16) Fleming, I. Frontier Orbitals and Organic Chemical Reactions; Wiley: New York, 1976. (17) Viehe, H. G.; Janousek, Z.; Merenyi, R.; Stella, L. Acc. Chem. Res. 1985, 18, 148-154.

knowledge on the effect of substituted groups in surface reactions between imidazole and PDMS, an ultimate goal is to develop biocompatible surfaces. Thus, the first part of this paper will focus on the effect of imidazole substituents on surface reactions, whereas the second part will describe their biological importance. Figure 1a illustrates ATR FT-IR spectra obtained from the surface of PDMS (trace A) as well as imidazole-reacted PDMS after microwave exposure for 5 s (trace B), 10 s (trace C), and 15 s (trace D). As seen in trace B, the appearance of the -NHsCHdN- band at 1692 cm-1 after 5 s of microwave plasma exposure is detected. Furthermore, the presence of the 1622, 1596, 1550, and 1379 cm-1

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Figure 4. (A) A schematic representation of PDMS surface structure resulting from imidazole reaction in the presence of Ar microwave plasma for each reaction time. (B) Proposed reactive radical species along with continuous reaction times.

bands is detected, which are due to -CHdCHsCHdCHconjugation species (1622 cm-1), -CdC- (1596 cm-1), -CdN- (1550 cm-1), and CsN (1379 cm-1) species, respectively.10 As reaction times are extended to 10 s (trace C), it is apparent that the relative intensities of these bands change, and the 1622 cm-1 band increases remarkably, while the 1550 and 1379 cm-1 bands become weaker. These observations indicate that the degree of conjugation of -CHdCHsCHdCH- increases, whereas -NHsCHd N- entities cleave at extended reaction times due to highly energetic plasmon generated by microwave energy. As previous studies using opening reactor showed,10 the bands due to -CdC- and -CdN- of imidazole decrease, which contrasts the results obtained under closed reactor conditions. Trace D of Figure 1 shows that upon a 15 s exposure, degradation of previously created species occurs, although the possibility of cross-linking should not be ruled out. Analysis of the same spectra in the 2350-2050 cm-1 region (Figure 1b) also indicates the presence of the 2240, 2216, 2178, and 2139 cm-1 bands when reaction time is 10 s, which is due to the -CtN species (trace C). Specifically, these bands are due to RsCtN, conjugated CtN, [NsCtN T NdCdN], and -N+tC-, respectively.18 These data also show that after 5 s of reaction time (trace B) there are no functionalized CtN groups on the PDMS surface, which is in agreement with the data shown in (18) Pretsch, E. Tables of spectral Data for Structure Determination of Organic Compounds; Springer-Verlag: New York, 1989.

Figure 1a, and the presence of -HNsCHdN- and CsN groups at the 1692 and 1379 cm-1. After 10 s, several nitrile groups are detected and, at the same time, degradation of -HNsCHdN- and CsN groups is detected, as demonstrated by the bands at 1692 and 1379 cm-1. Nitrile groups result from cleavage of the CsN bonds of N-substituted imine groups. Furthermore, the content of conjugated CtN and [NsCtN T NdCdN] groups increases when reaction times are extended to 10 s (trace C). Apparently, the plasma surface reactions with imidazole lead to the formation of conjugated CtN groups, which are conjugated with ends of the -CHdCHsCHd CH- species (2216 cm-1). These observations are consistent with the increase of the -CHdCHsCHdCHconjugated species at 1622 cm-1 shown in Figure 1a. With these results in mind let us focus on the effect of imidazole alkyl substituents on PDMS microwave plasma surface reactions. For that reason we conducted a series of parallel plasma reaction experiments in which we utilized 2-methyl and 2-ethylimidazole monomers. Figures 2 and 3 illustrate a series of ATR FT-IR spectra recorded from modified PDMS surfaces in the presence of 2-methyland 2-ethylimidazole, respectively. As seen, the primary spectral features in the 1800-1350 cm-1 region (Figures 2 and 3, part a) are very similar to that of imidazole, and the major difference is the band due to -NsCRdN- (R ) -CH3, -CH2CH3) entities at 1701 cm-1. As seen in Figure 2b, a single band at 2214 cm-1 is detected when

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Figure 6. Plots of volume concentrations of imidazole, 2-methylimidazole, and 2-ethylimidazole at 5 and 10 s reaction.

Figure 5. ATR FT-IR spectra in the 780-600 cm-1 region of imidazole (Im), 2-methylimidazole (Mim), and 2-ethylimidazole (Eim) to compare the entities of cis-CdC out-of-plane bending vibration band intensity for 5 and 10 s treatments.

reaction times are 15 s (trace D). For the same reaction times, however, only traces of -CdN- (1701 cm-1) and CsN (1383 cm-1) species of -NsCRdN- entities are detected in the 1800-1350 cm-1 region (Figure 2a, trace D). As illustrated before, the 2214 cm-1 band represents conjugated CtN species which are not detected after 10 s of microwave exposure, and the band due to [NsCtN T NdCdN] species at 2182 cm-1 is very weak. Similarly to the imidazole plasma reactions, these observations indicate that the concentration level of -Ns CRdN- species is consistent with band intensity changes of -CtN as a result of elimination of R groups and the cleavage of CsN linkages and weaker [NsCtN T Nd CdN] bands at 2182 cm-1 are expected because •NsCRd N• radical entities are more sustainable than •NsCHdN• under plasma reaction conditions due to the stabilizing effect of alkyl groups. When 2-ethylimidazole is utilized, similar spectral features in the 1800-1350 cm-1 region (Figure 3a) are detected after 5 s of microwave exposure (trace B). However, the -CtN stretching region exhibits significant differences. As shown in trace C of Figure 3b, a 10 s exposure results in two bands which are due to RsCtN at 2239 cm-1 and [NsCtN T NdCdN] at 2182 cm-1. When reaction times are extended to 15 s (trace D), two -CtN stretching bands are not detected, and a single band due to conjugated -CtN at 2217 cm-1 is present. Furthermore, the band at 2139 cm-1 due to -N+tC- is not detected for both 2-methylimidazole and 2-ethylimidazole reactions. The above data allow us to determine the effect of substituents on imidazole-PDMS reactions. Examining the 1800-1350 cm-1 regions of each monomer reaction, the -NsCRdN- (R ) -CH3, -CH2CH3) band at 1701 cm-1 for 2-methylimidazole and 2-ethylimidazole is detected, but -NsCHdN- entities for imidazole are observed at 1692 cm-1. At the same time, the bands at 1383 cm-1 due to C-N species of alkylimidazoles shifts as compared to imidazole (1379 cm-1) due to the effect of alkyl substituents. Similar significant band intensity

changes between imidazole and alkylimidazole spectra are detected at the 2350-2050 cm-1 regions of Figures 1, 2, and 3 after 5 and 10 s reactions. At 10 s reactions, band intensities due to nitrile are increased as well as exhibit different vibrational features. The -N+tC- group is only detected at 2139 cm-1 in imidazole reactions, and furthermore, the band intensity due to [NsCtN T NdCd N] group at 2178 cm-1 is stronger than that of conjugated CtN entities after 15 s. These observations suggest that the CsC bond of alkyl imidazole is more stable than the CsH bond of imidazole upon exposure to microwave plasma. In essence, the CtN groups are generated from -NsCRdN- (R ) H, -CH3, -CH2CH3) groups at 1692 and 1701 cm-1, as shown by the decreased intensity of the -NsCRdN- vibrations in the 1800-1350 cm-1 region during plasma reactions. It should be also noted that the degradation of surfaces at 15 s plasma discharge is detected, but the bands due to nitrile species continue to increase. Thus, various reaction times generate different species and their concentration levels also vary, which is shown in Figure 4A. The proposed mechanisms of their formation are depicted in Figure 4B for 5, 10, and 15 s time intervals. Predominant species are dehydrogenated imidazole, hydrogen, CdC, and N-substituted imine radicals, but extended reaction times to 10 and 15 s lead to cleavage of N-substituted imine radical and generate several nitrile moieties. A number of attempts have been made to correlate polymer deposition rates with such operational variables such as flow rates, discharge power, current density, and system pressure.4 For example, Yasuda and Wang examined polymer deposition rates, and these data revealed simplified kinetic aspects of reactions and their relationship to monomer structures.19 In essence, the amount of generated radicals contributes to reaction patterns reacting on the PDMS surface and also affects their selfcondensation. However, free radicals can be stabilized kinetically by steric effects, and thermodynamically by the electron dispersion. Steric effects isolate radical reactive sites from their environment and, thus being more stable, lead to delocalization of odd electrons, which ultimately reduces reactivity by lowering the spin density on the radical center.16,17 Because the stability of NsCRd (19) Yasuda, H.; Wang, C. R. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 87-106.

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Figure 7. ESEM images of PDMS surface after plasma reaction with imidazole (A) PDMS, (B) after 5 s plasma reaction, and (C) after 10 s reaction; (D) IRIR imaging data (Inset, IRIR image at 1710 cm-1) for PDMS surface after 5 s plasma reaction.

N radical species are affected by the presence of alkyl groups due to steric effect and delocalization of electrons, alkyl-substituted •NsCRdN• (R ) -CH3, -CH2CH3) radical species exhibit higher stability than •NsCHdN• radicals. Moreover, dehydrogenated 2-methylimidazole and 2-ethylimidazole radical species exhibit higher stability than dehydrogenated imidazole in the plasma reaction chamber. As seen in Figure 5, the band at 719 cm-1 due to out-of-plane bending vibrations of cis-1,2-double bonds is detected for 5 s plasma discharge reactions and its intensity is significantly reduced after 10 s. This observation indicates that more quantities of dehydrogenated alkylimidazole ring are detected than those of dehydrogenated imidazole rings after 5 s, thus suggesting that the presence of substituent groups such as methyl and ethyl groups stabilizes radical species under microwave plasma environments. Since ATR FT-IR spectroscopy also allows determination of surface contents at various depths,8,11,13,20 let us examine how the content of newly formed species changes as a function of depth from the surface. Figure 6 illustrates the results of quantitative analysis of the CdC due to imidazole ring for the examined reaction conditions. As seen, as the depth of penetration increases from 0.33 to 3.86 µm, volume concentrations of imidazole reacted on (20) Urban, M. W. Attenuated Total Reflectance Spectroscopy of Polymers Theory and Practice; American Chemical Society: Washington, DC, 1996.

the PDMS surface decrease from 1.24 × 10-5 mol/L to 0.066 × 10-5 mol/L after 5 s reaction times, which correspond to surface concentrations levels ranging from (4.09 to 2.55) × 10-13 mol/cm2. However, extended reaction times to 10 s result in lower concentrations. On the basis of these measurements, surface concentration changes can be obtained for respective reaction conditions in terms of times and structure of imidazole molecules at various depths. As seen, while concentrations of alkyl imidazole are higher than those of imidazole up to about 600 nm, above that depth, concentration levels of imidazole are higher than those of alkylimidazoles. The next question is how surface morphology is affected by microwave plasma reactions. For that purpose we collected a series of ESEM images of PDMS surfaces exposed to 5 and 10 s of microwave plasma. Figure 7 illustrates images of unreacted PDMS (A) and 5 s (B) and 10 s (C) imidazole/PDMS microwave plasma exposures. As seen, while there are no significant morphological changes after 5 s, there is apparent morphology development after 10 s. To correlate surface morphology and surface chemical changes resulting from the open flow reactor conditions on PDMS, we utilized IRIRI and analyzed surfaces which appear as heterogeneous domains after plasma reactions. These experiments showed only minute chemical differences between the areas of heterogeneity (Figure 7D). This is consistent with the earlier atomic force microscopy studies10 of the imidazole reacted

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Figure 8. ATR FT-IR spectra of (A) CtN containing PDMS surfaces and (B) the same surface after reacting chloramphenicol.

of the surface. In the 3700-2800 cm-1 region the appearance of hydroxyl groups at 3394 cm-1 and the C-H stretching vibrations at the 2924-2852 cm-1 due to chloramphenicol are detected. Finally, the 1800-1300 cm-1 region illustrates the presence of generated ester groups at 1742 cm-1, the CH2 bending vibrations at 1457 cm-1, and aromatic nitro compounds at 1521 cm-1 (asymmetric stretch) and 1380 cm-1 (symmetric stretch), thus further substantiating surface reactions between the surface CtN and chloramphenicol which are depicted in Figure 9. Figure 9. Chemical attachment of chloramphenicol on nitrilefunctionalized PDMS surfaces using Pinner synthesis.

to the PDMS surface under open flow reactor conditions which showed surface elevations reaching over 3500 nm. One of the objectives of these studies is to attach antimicrobial species to PDMS. For that purpose we utilized Pinner synthesis in an effort to react chloramphenicol which contains reactive primary and secondary hydroxyl groups via nitrile groups of imidazole-modified PDMS surfaces. Figure 8 illustrates a series of ATR FTIR spectra of imidazole-modified PDMS (10 s plasma exposure) surface (trace A) and chloramphenicol reacted to modified PDM (trace B). As shown, the band at 2178 cm-1 which is due to [NsCtN T NdCdN] species and the band of conjugated CtN groups at 2216 cm-1 disappear after reactions with chloramphenicol. Moreover, unstable -N+tC- species represented by the 2139 cm-1 band also react with the hydroxyl group, but the RsCtN groups at 2240 cm-1 remain stable. It should be noted that since there are four different CtN groups in close proximity, their intensities may vary due to either spectral overlap or conformational changes resulting from the dynamics

Conclusions These studies show that the presence of imidazole under Ar microwave plasma conditions generates several reactive radical species which are chemically attached to the surface of PDMS substrate. During these studies, we found the alkyl-substituent effect on microwave plasma surface reaction. When free radicals contain alkyl substituent, kinetic (steric effect) and thermodynamic (electron dispersion) stabilization is obtained. Using this experimental setup, plasma surface reaction generates several nitrile groups created by splitting -NsCRdN- groups. Nitrile groups were further reacted and in this study chloramphenicol was attached thus opening a number of opportunities for creating antimicrobial surfaces. Acknowledgment. Major support for these studies from the National Science Foundation Materials Research Science and Engineering Center (DMR 0213883) is acknowledged. LA048916X