Langmuir 2007, 23, 3927-3931
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Surface Reactivity of Pulsed-Plasma Polymerized Pentafluorophenyl Methacrylate (PFM) toward Amines and Proteins in Solution L. Francesch,†,‡ S. Borros,† W. Knoll,‡ and R. Fo¨rch*,‡ Institut Quı´mic de Sarria` -UniVersitat Ramon Llull, Via Augusta 390, 08017 Barcelona, Spain, and Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ReceiVed August 16, 2006. In Final Form: December 28, 2006 Pulsed-plasma polymerization has been used to deposit ultrathin layers of pentafluorophenyl methacrylate by using low duty cycles and low power input. The monomer structure can be retained such that the chemical reactivity of the active ester group could be studied using the reaction with a simple amine. The film properties in aqueous phosphate buffer have been investigated using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and real time surface plasmon resonance spectroscopy. The films react readily with diaminohexane and immunoglobulin (IgG), yet the reactivity shows a dependence on the extent of hydrolysis of the ester group.
Introduction Targeting materials toward specific cells,1 designing materials that can sense biochemical signals,2 and developing surfaces with improved biocompatibility remain challenges in biomaterials development.3 Modified materials can stimulate cell adhesion, growth, and proliferation4 (while maintaining the mechanical attributes of the bulk material5-7) on bioactive surfaces through biological ligands specific for cell-substrate interactions.8,9 One broadly used approach to immobilizing ligand motifs on biomaterial surfaces consists of creating covalent attachments to reactive polymer side chains.10 Plasma polymerization of surface-active coatings is an attractive method to obtain these reactive side chains and allows for the deposition of polymer-like films with tailored functional group densities. The structure of the polymer deposit is controlled by controlling the experimental conditions. The chemical structures obtained can range from highly functional polymer networks of low cross-link densities to polymer networks of low functional groups but high cross-link densities.11 This has been shown for a number of precursors, even for some containing very labile functional groups.12,13 It has also been shown that by using a modulated plasma, the damage to the monomer and the growing polymer layer is minimized, and thus the loss of its reactive groups is also minimized. It enables conventional polymerization * To whom correspondence should be addressed. Telephone: +49-6131379487. Fax: +49-6131-379100. E-mail:
[email protected]. † Institut Quı´mic de Sarria ` -Universitat Ramon Llull. ‡ Max Planck Institute for Polymer Research. (1) Quintana, A.; Raczka, E.; Piehler, L.; Lee, I.; Myc, A.; Majoros, I.; Patri, A. K.; Thomas, T.; Mule, J.; Baker, J. R. Pharm. Res. 2002, 19, 1310. (2) Mark, S. S.; Sandhyarani, N.; Zhu, C. C.; Campagnolo, C.; Batt, C. A. Langmuir 2004, 20, 6808. (3) Langer, R.; Tirrell, D. A. Nature 2004, 428, 487. (4) Long, S. F.; Clarke, S.; Davies, M. C.; Lewis, A. L.; Hanlon, G. W.; Lloyd, A. W. Biomaterials 2003, 24, 4115. (5) Jeong, S. I.; Kim, S. H.; Kim, Y. H.; Jung, Y.; Kwon, J. H.; Kim, B. S.; Lee, Y. M. J. Biomater. Sci., Polym. Ed. 2004, 15, 645. (6) Xu, H. H. K.; Smith, D. T.; Simon, C. G. Biomaterials 2004, 25, 4615. (7) Kokubo, T.; Kim, H. M.; Kawashita, M. Biomaterials 2003, 24, 2161. (8) Liu, Y.; De Groot, K.; Hunziker, E. B. Ann. Biomed. Eng. 2004, 32, 398. (9) Tessmar, J.; Mikos, A.; Gopferich, A. Biomaterials 2003, 24, 4475. (10) Francesch, L.; Garreta, E.; Balcells, M.; Edelman, E. R.; Borros, S. Plasma Processes and Polymers 2005, 2, 60. (11) Friedrich, J.; Kuhn, G.; Mix, R.; Unger, W. Plasma Processes Polym. 2004, 1, 28. (12) Han, L. M.; Timmons, R. B.; Bogdal, D.; Pielichowski, J. Chem. Mater. 1998, 10, 1422. (13) Calderon, J. G.; Timmons, R. B. Macromolecules 1998, 31, 3216.
reaction pathways to take place during the duty cycle off period, achieving the retention of the desired functional groups.14,15 However, the integrity of the functional groups remains a challenge in the plasma polymerization process. There has been an increasing interest in the use of plasma-deposited layers, and a number of different surface chemical structures have been investigated,16 ranging from amines17,18 to acid functionalities19-21 as well as surfaces containing alcohol groups.22 Plasma-polymerized pentafluorophenyl methacrylate (ppPFM) offers a highly reactive ester group that can potentially be used to react with the amino groups on proteins, for example, integrins and other biological ligands.10 The reaction of the active ester with diaminohexane leads to the formation of strong amide linkages, see Figure 1, which can easily be monitored using Fourier transform infrared (FTIR) spectroscopy.23 On the plasma polymer of pentafluorophenyl methacrylate, the diaminohexane will probably bond via both amino groups; however, this cannot be distinguished by any of the analytical methods used in this work. The interaction of biological molecules with reactive surfaces always occurs in an aqueous environment and often in the presence of other, possibly competing fluid components. To achieve a fundamental understanding of the chemical reactivity of the pp-PFM surface toward proteins, the present work investigates the basic reaction of the perfluoroester group with a simple amine using FTIR, X-ray photoelectron spectroscopy (XPS), and surface plasmon resonance (SPR) spectroscopy. This was done by reacting the surface with an amine in an aqueous (14) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S. Chem. Mater. 1996, 8, 37. (15) Coulson, S. R.; Woodward, I. S.; Badyal, J. P. S.; Brewer, S. A.; Willis, C. Chem. Mater. 2000, 12, 2031. (16) Kelly, J. M.; Daw, R.; Short, R. D.; Brook, I. M. J. Dent. Res. 1999, 78, 1064. (17) Harsch, A.; Calderon, J.; Timmons, R. B.; Gross, G. W. J. Neurosci. Methods 2000, 98, 135. (18) Eves, P. C.; Beck, A. J.; Shard, A. G.; Mac Neil, S. Biomaterials 2005, 26, 7068. (19) De Bartolo, L.; Morelli, S.; Lopez, L. C.; Giorno, L.; Campana, C.; Salerno, S.; Rende, M.; Favia, P.; Detomaso, L.; Gristina, R.; d’Agostino, R.; Drioll, E. Biomaterials 2005, 26, 4432. (20) Feng, X.; Zhang, J.; Xie, H.; Hu, Q.; Huang, Q.; Liu, W. Surf. Coat. Technol. 2003, 171, 96. (21) Sardella, E.; Gristina, R.; Ceccone, G.; Gilliland, D.; PapadopoulouBouraoui, A.; Rossi, F.; Senesi, G. S.; Detomaso, L.; Favia, P.; d’Agostino, R. Surf. Coat. Technol. 2005, 200, 51. (22) France, R. M.; Short, R. D.; Duval, E.; Jones, F. R.; Dawson, R. A.; MacNeil, S. Chem. Mater. 1998, 10, 1176. (23) Eberhardt, M.; Mruk, R.; Zentel, R.; Theato, P. Eur. Polym. J. 2005, 41, 1569.
10.1021/la062422d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007
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Figure 1. Schematic of the reaction between the active ester and diaminohexane.
buffer solution (PBS) after different immersion times in pure buffer. Some preliminary work showing the reactivity toward IgG in PBS is also shown. Experimental Section Materials and Substrates. Pentafluorophenyl methacrylate (98%) was purchased from Polysciences Europe GmbH (Germany) and freeze-dried three times to remove excess adsorbed gases. It was not purified further. Immunoglobulin from sheep serum (IgG; 95%), bovine serum albumin (BSA), 1,6-diaminohexane (99%), and phosphate buffered saline (PBS) tablets were purchased from Sigma (Germany). The solutions were freshly prepared before each experiment. High refractive index LaSFN9 glass (n ) 1.9 at l ) 633 nm), required for SPR measurements, was purchased from Hellma Optik, Jena, Germany. Chromium (1.5 nm) and gold (50 nm) were thermally evaporated onto the LaSFN9 glass slides and used as substrates for SPR measurements. The substrates for FTIR in the reflection mode were also glass slides coated with 1.5 nm Cr and 80 nm gold. All gold-coated substrates in this work were functionalized with an adhesion layer consisting of a 1-hexanethiol self-assembled monolayer. The procedure for this has been described elsewhere.24,25 Plasma Polymerization. Plasma polymerization was carried out in a home-built 30 cm long cylindrical Pyrex reactor using an excitation frequency of 13.56 MHz. Gases that are fed through the system pass through a glass liquid nitrogen-cooled trap for collection of excess reactant before reaching the pump (Leybold Trivac, D16BCS/PFPE). A MKS baratron (Type 122) is connected near the inlet to monitor the reaction pressure. A home-built pulse generator controls the pulsing of the rf signal, which is amplified by an ENI 300 W amplifier and passed via an analogue wattmeter (BIRD 4410A) and a matching network to two concentric rings located around the exterior of the reactor. The rings are separated by ∼13 cm. The typical base pressure prior to all experiments was 1 × 10-4 mbar. Pentafluorophenyl methacrylate monomer vapor was introduced at a constant pressure of 0.2 mbar via a needle valve. The plasma polymers were deposited using continuous wave and pulsed-plasma conditions, with duty cycles (DC ) (ton/ton+ toff)) ranging from 2/4 to 1/101 ms at input powers ranging from 10 to 150 W. Optimum functional group retention was obtained using a DC of 2/52 ms at a Ppeak of 50 W (Peq ) 1.9 W). The deposition rate under these conditions was 28 nm‚min-1. The film thickness used throughout this work was 60 ( 4 nm obtained after ∼2 min of deposition time. The samples were kept under monomer flow (without plasma) for another 15 min in an attempt to deactivate any remaining reactive sites. They were then removed from the reaction chamber and stored under Ar until further use. The reaction chamber was cleaned after each deposition using 100 W Ar/O2 plasma exposure for 30 min. Surface reactivity of the plasma-polymerized active ester was studied using (a) 0.01 M PBS, (b) a 10 mM solution of 1,6diaminohexane in 0.01 M PBS, and (c) 1 mg/mL immunoglobulin (sheep IgG) in 0.01 M PBS Because of the high reactivity of the films, great care was taken to minimize the exposure of the surfaces to air. Thus, film deposition, surface analysis, and SPR were generally performed on the same day, keeping storage times as low as possible. Exposure to air was, however, unavoidable in all cases. (24) Jenkins, A. T. A.; Hu, J.; Wang, Y. Z.; Schiller, S.; Foerch, R.; Knoll, W. Langmuir 2000, 16, 6381. (25) Schiller, S.; Hu, J.; Jenkins, A. T. A.; Timmons, R. B.; Sanchez-Estrada, F. S.; Knoll, W.; Forch, R. Chem. Mater. 2002, 14, 235.
Film Analysis. FTIR analysis was carried out using a Nicolet 850 spectrometer in the reflection mode on ∼80 nm thick gold films. X-ray photoelectron spectroscopy was performed within 2 days using a PHI 5500 spectrometer, equipped with a 300 W monochromatized Mg KR X-ray source. The pass energy for the acquisition of C 1s photoelectron narrow scans was 11.75 eV. All XPS spectra were recorded at a takeoff angle of 45° relative to the surface. Surface charging was controlled using a flood gun. Data analysis was carried out using Multipax (PHI) and ORIGIN software. Deconvolution of the C 1s peak was performed using ORIGIN software assuming a 70:30 Lorentzian-Gaussian peak shape. The C 1s peak could be deconvoluted into a minimum of five individual peak components at binding energies of 285, 286.2 , 288.2, 289.4, and 295 eV, which can be associated with C-C, C-CF, C-F, and O-C)O as well as a πfπ* shake up satellite, respectively.15,26-28 Surface Plasmon Resonance Spectroscopy. SPR was carried out on a home-built spectrometer equipped with a Teflon reaction cell. Surface plasmon resonance spectroscopy is a highly sensitive optical method for probing changes in the optical thickness (∆nd) of ultrathin adlayers on a metallic (generally gold or silver) substrate. The fundamentals of surface plasmons and the experimental setup are well-established and have been summarized in a number of reviews and articles.29-31 SPR is a particularly valuable tool for quantitatively studying binding reactions on surfaces if the surface itself does not change its optical properties during the binding event. In previous work, we have generally studied the reaction kinetics of “stable” surfaces (i.e., those surfaces which were fully swollen).32,33 In the present work, the SPR technique was used to monitor a fast adsorption process which is accompanied by a slow swelling and desorption process. Experimentally, this leads to an unstable baseline in the kinetic measurements and we observe a broadening of the plasmon peak as well as a shift in the resonance angle. The combination of these effects makes it extremely difficult to discriminate between the different processes seen in the reflectivity mode at a constant angle. To at least partially overcome this, the baseline was monitored over a long time and kinetic measurements were performed by a dynamic tracking of the minimum, giving direct access to the actual resonance angle. The SPR data in this work is thus only used to measure relative differences and to determine correlations to the FTIR and XPS data.
Results and Discussion Plasma Polymerization and Surface Analysis. Surface analysis of the pp-PFM films deposited at different process conditions has once again shown that the film chemistry obtained is dependent on the duty cycle and the equivalent power used during the deposition.34 Thus, continuous wave plasma processes (26) Hynes, A.; Badyal, J. P. S. Chem. Mater. 1998, 10, 2177. (27) Bodas, D. S.; Mandale, A. B.; Gangal, S. A. Appl. Surf. Sci. 2005, 245, 202. (28) Mackie, N. M.; Castner, D. G.; Fisher, E. R. Langmuir 1998, 14, 1227. (29) Kambhampati, D. K.; Knoll, W. Curr. Opin. Colloid Interface Sci. 1999, 4, 273. (30) Knoll, W. In Handbook of Optical Properties; Hummel, R. E., Wissmann, P., Eds.; 1997. (31) Nakamura, R.; Muguruma, H.; Ikebukuro, K.; Sasaki, S.; Nagata, R.; Karube, I.; Pedersen, H. Anal. Chem. 1997, 69, 4649. (32) Zhang, Z.; Chen, Q.; Knoll, W.; Fo¨rch, R. Surf. Coat. Technol. 2003, 174-175, 588. (33) Zhang, Z.; Menges, B.; Timmons, R. B.; Knoll, W.; Fo¨rch, R. Langmuir 2003, 19, 4765. (34) Fo¨rch, R.; Zhang, Z. H.; Knoll, W. Plasma Processes Polym. 2005, 2, 351.
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Figure 3. XPS C 1s spectrum of plasma-deposited PFM under conditions of 2/52, Peq ) 2 W. Figure 2. FTIR and XPS C 1s spectra of PFM plasma polymerized under different process conditions.
led to films showing much less functional group retention than the low power deposits and the pulsed-plasma deposited layers. Figure 2 shows the FTIR and XPS spectra of pp-PFM films in order of decreasing input power (Pinput), which corresponds to an equivalent power (Peq ) PinputDC) for the pulsed-plasma deposited layers. The FTIR spectra are dominated by three main absorption bands. These can be associated with the carboxyl groups sOsCdO (1730 cm-1), a sharp and high intensity band as a result of the pentafluorophenol group (PFP; 1525 cm-1), and a broad band encompassing different CsF stretching modes (1000-1400 cm-1). Under continuous wave conditions, it can be seen that relatively few PFP groups are present in the polymer, whereas carboxyl groups are fairly abundant. This is also seen in the contact angle of θ ) 45.4°, showing a hydrophilic rather than a hydrophobic surface. Decreasing the input energy from 150 to 10 W for the continuous wave process leads to an increase in the contact angle to 75.4° ( 2° (see Figure 2), with the FTIR spectra showing an increase in the intensity of the PFP group at 1525 cm-1 under these conditions. When introducing a low duty cycle of 2/52 ms/ms (Peq ) 2 W), excellent structural retention was achieved, as seen by the high relative intensity of the sharp symmetric peak at 1525 cm-1 and the contact angle of 98.6° ( 2°, indicating a hydrophobic, fluorine-rich surface. A further decrease in the duty cycle (Peq ) 0.5 W) did not lead to further improvement in the film properties, and the deposition rate was unacceptably low. Thus, for this particular monomer, only a small range of process parameters provide optimum polymerization conditions for functional group retention. Once this window of experimental conditions had been determined, no further work was performed to study the reaction mechanisms for the deposition. Also shown in Figure 2 are the C 1s X-ray photoelectron peaks for the same films. When deposited under continuous wave (CW) conditions, the C 1s spectrum shows very broad unsymmetrical C 1s peaks, suggesting a variety of different C-O and C-F functional groups. When deposited using a duty cycle of 2/52 at 50 W input power, the C 1s peak shows two very distinct peaks with a low intensity, high energy shoulder. C 1s peak deconvolution for the films deposited under optimum conditions allows for at least five peaks to be fitted under the experimental curve as shown in detail in Figure 3. The low binding energy peak component (285 eV) represents C-C from the hydrocarbon backbone. The low intensity peak component at 286.2 eV can be associated with carbon in the C-O and C-C-F groups. The peak centered around 288.2 eV represents the aromatic C-F bonds, while the small shoulder at 289.4 eV can be associated with the ester carbon atom. The πfπ* shake up satellite at 295
Table 1. XPS Elemental Composition Data for pp-PFM before and after Reaction with Diaminohexane in PBS pp-PFM sample theory expt after 1 h 10 mM diaminohexane in PBS
F/C ((0.02) O/C ((0.02) N/C ((0.01) 0.5 0.48 0.29
0.2 0.23 0.16
0.03 0.07
eV suggests a significant aromatic character within the film and thus a high retention of the monomer structure. The O/C and F/C ratios were 0.23 and 0.48, respectively, which are similar to the theoretical values from the monomer structure (Table 1). A detailed study of the aging of these films in air has not yet been performed. However, the results of the XPS analysis 2 days after deposition seem to suggest reasonable stability in air. The FTIR and XPS analyses thus suggest that the low DC conditions lead to the polymerization of the precursor mostly by activation of the double bond and a polymer structure with a high density of active ester groups. Because of the high functional group retention achieved under the conditions of DC ) 2/52 at 50 W, only these films were used in subsequent tests. Surface Reactivity. Before the reactivity of the plasmapolymerized PFM film toward amines or proteins could be tested, it was necessary to understand the reactivity toward the phosphate buffer saline (PBS) solvent. The FTIR spectra before and after immersion (Figure 4a) show that the plasma-polymerized PFM film undergoes some change in chemical structure upon immersion. The reaction of the ester group with the water is observed as decrease of the band at 1730 cm-1 accompanied by an increase in the relative intensity of the band at 1650 cm-1. At the same time, a decrease in the intensity of the band at 1000-1400 cm-1 indicates a loss of the fluorinated group. As a summary of the data, Figure 4b shows that, after ∼100 min, the films appeared to be sufficiently stable. From the changes in the FTIR spectra, it is expected that with extended immersion time in PBS the active ester group dissociates to give the free acid and the PFP group. Since, however, there is still significant relative intensity at the wavenumbers 1730 and 1525 cm-1, we also assume that only the perfluorinated esters within the uppermost layers are reacting and that the functional groups deeper within the film remain intact over the time period studied here. Knowing that the solvent plays a small but measurable part in any subsequent chemical reaction, we were interested in investigating the reactivity of the active ester toward a simple diamine and a protein (IgG) in PBS solution. The relative change in the chemical structure of the polymer film subjected to 10 mM diaminohexane in PBS, without prior immersion/stabilization in PBS, can be seen in the FTIR spectra (Figure 5) and XPS spectra
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Figure 4. (a) FTIR spectra of pp-PFM before and after immersion in PBS for 1 h. (b) Summary of approximate relative intensity changes observed in the FTIR spectra for the reaction with PBS (the data points are joined by a dotted line for clarity).
Figure 5. (a) FTIR spectra of pp-PFM before and after reaction with 10 mM diaminohexane in PBS. (b) Summary of approximate relative intensity changes observed in the FTIR spectra for the reaction (the data points are joined by a dotted line for clarity).
(Figure 6). The change in the relative FTIR peak intensities for the bands at 1650 cm-1 (amide formation, OdCsNHs), 1525 cm-1 (the PFP group), and 1125 cm-1 (CsF) with reaction time suggests a rapid conversion of the ester to the amide (Figure 5a). The reaction between the ester groups in the film (of d ) 60 nm) and the diamine seems to be complete after 10 min (Figure 5b), suggesting that the rate of reaction of the diamine is a factor of
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Figure 6. XPS C 1s and N 1s spectra of pp-PFM before and after reaction with diaminohexane.
10 faster than that of the solvent (PBS). The contribution due to the aromatic ring (at 1525 cm-1) was not seen to change significantly over the time studied, while the C-Fx bands decreased in intensity. We associate this with differences in the sensitivity of the respective group to the infrared analysis. Similar data analysis was not performed for the ester group (1730 cm-1) since this was not a clearly defined band in all the spectra. With increasing immersion time in diaminohexane/PBS, the FTIR spectra also showed an increase in the intensity of the hydrocarbon component (∼3000 cm-1) and a decrease in the band at 2250 cm-1. The latter could suggest that there is some dissolution of unbonded dissociation products containing CtC. The intensity increase of the hydrocarbon band probably originates from the surface-attached amines. XPS elemental analysis of the surfaces (Table 1) shows that the reaction of the amine leads to a loss of fluorine and oxygen, accompanied by a gain in nitrogen. The XPS C 1s spectra (Figure 6a) of the surfaces before and after reaction with diaminohexane support the FTIR observations. The intensity of the high binding energy peak in the C 1s spectrum, which can be associated with C-F, decreased significantly, indicating a loss of C-F bonds upon reaction. Any changes due to the reaction of the ester to the amide are unfortunately hidden under the tail of the high energy peak at 288.9 eV. The N 1s spectrum of the reacted pp-PFM surface (Figure 6b) peaks at a binding energy of ∼400 eV, which is the typical binding energy observed for amide groups.35 Complementary experiments were done using SPR to follow the reaction of the pp-PFM with the diamine in PBS. This was done by first exposing the pp-PFM films to PBS for a few minutes to obtain a baseline. The solution in the SPR reaction cell was then exchanged for 10 mM diaminohexane in PBS, while a kinetic SPR scan was recorded. The initial immersion of the pp-PFM film in PBS solution led to a slow decrease in the resonance (35) Choukourov, A.; Biedermann, H.; Kholodkov, I.; Slavinska, D.; Trchova, M.; Holla¨nder, A. J. Appl. Polym. Sci. 2004, 92, 979-990.
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Figure 7. SPR kinetic measurements of the reaction of diaminohexane in PBS with pp-PFM.
angle (Figure 7), which is due to a number of effects taking place simultaneously that cannot be distinguished in the SPR data. First, as demonstrated above, the active ester group slowly hydrolyzes in water, leading to some loss of the PFP group, which may or may not be able to desorb from the surface. Second, at low duty cycle plasma deposition conditions, unreacted monomers or dimers may become trapped within the structure and may be slowly released from the film when it is immersed in solution. Third, all polymers swell in aqueous solution, which should result in an increase in film thickness. The overall effect observed in the SPR experiment is merely the sum of these three basic phenomena. Care has been taken to include this in the experimental procedure and in the interpretation. A more quantitative evaluation of the data is therefore difficult. The addition of a solution of the amine in PBS led to an increase in the surface plasmon resonance angle, which can be directly related to the reaction of the amine to the pp-PFM surface (Figure 7). After immersion for ∼10 min, no further significant change was observed in the resonance angle, suggesting that the reaction was complete. This is in agreement with the data observed by FTIR. Rinsing the surface with PBS led to a small decrease in the resonance angle, which may be associated with the loss of physisorbed material from the surface. First tests to study the reactivity of the pp-PFM surface toward immunoglobulin (sheep IgG) are shown in Figure 8. The kinetics of the reaction were monitored by measuring the change in reflectivity over time. This was, again, done with the minimum possible immersion time in pure buffer to establish a baseline. Upon exchange of the solution to IgG in PBS buffer, a very fast surface binding reaction could be observed in the SPR spectrum,
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Figure 8. SPR kinetic measurements on the reactivity of the ppPFM surface toward IgG in PBS solution.
reaching equilibrium within ∼5 min. Rinsing the surface with excess buffer did not lead to a removal of the IgG. This suggests that the antibodies are not just unspecifically bound to the surface but are covalently bonded to the surface. Similar unpublished data studying the reaction of laminin with pp-PFM have also shown promising results. Thus, the present study has provided basic insights into the reactivity of these surfaces, which provide a basis for further work using biological molecules.
Conclusions It has been shown that even monomers containing highly reactive groups, such as pentafluorophenyl methacrylate, can be polymerized with high functional group retention using pulsedplasma deposition conditions. The surfaces of the deposits are highly reactive and react readily in aqueous solution and with primary amines. It has been shown that the reaction with aqueous buffer is much slower than that with primary amines. Thus, the reaction between an amine-terminated reagent and the ester groups will always dominate reaction pathways in a solvent environment. These insights are of particular importance when applying these surfaces as supports for the covalent attachment of peptides and proteins containing amines or alcohols. First results showing the binding of immunoglobulin (IgG) have been presented. Acknowledgment. We would like to thank Daniela Mo¨ssner (IMTEC, Albert-Ludwig Universita¨t Freiburg) for XPS analysis. We would also like to acknowledge Prof. Diethelm Johannsmann and Dr. Maximilian Kreiter for their help with the SPR dynamic minimum tracking and the interpretation of the results. This work was partly supported by a DAAD-La Caixa fellowship. LA062422D
