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May 13, 2009 - Present address: Laboratory for Micro and Nanotechnology, Paul Scherrer ... While the general trend in the immobilization of avidin ont...
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Fabrication of Mixed Self-Assembled Monolayers Designed for Avidin Immobilization by Irradiation Promoted Exchange Reaction Nirmalya Ballav,†,§ Andreas Terfort,‡ and Michael Zharnikov†,* †

Angewandte Physikalische Chemie, Universit€ at Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, and ‡Institut f€ ur Anorganische und Analytische Chemie, Goethe-Universit€ at Frankfurt, Max-vonLaue-Strasse 7, 60438 Frankfurt, Germany. §Present address: Laboratory for Micro and Nanotechnology, Paul Scherrer Institut, 5332 Villigen PSI, Switzerland. Received March 2, 2009. Revised Manuscript Received April 9, 2009 An applicability of irradiation-promoted exchange reaction (IPER) to the fabrication of mixed self-assembled monolayers (SAMs) composed of the protein-repelling matrix and the moieties bearing binding sites for specific attachment of a target protein is demonstrated. As test systems, we took mixed films of oligoethylene glycol (OEG)substituted alkanethiols (OEG-ATs) and biotin-substituted alkanethiols (BATs) on Au{111}. Such SAMs are suitable for the specific immobilization of avidin protein and its variants. The composition of the mixed OEG-AT/BAT SAMs could be precisely controlled by varying the irradiation dose, which is important prerequisite for the fabrication of the respective patterns by electron-beam lithography. While the general trend in the immobilization of avidin onto the mixed OEG-AT/BAT SAMs prepared by IPER was found to be consistent with the earlier reports regarding the analogous films fabricated by the coassembly method, the concentration of the BAT component in the mixed SAMs needed for the maximum surface coverage of the specific protein was found to be somewhat lower, and the maximum avidin coverage somewhat higher in the case of IPER as compared to the coassembly method. We ascribe these differences to the lack of phase segregation and better separation of the individual BAT species in the OEG-AT matrix in the case of IPER.

1. Introduction The control over protein adsorption at surfaces and interfaces is one of the current challenges of modern science and technology. In particular, it is important for mimicking biological interfaces, cell research, proteomics, fabrication of different assays, and pharmaceutical and medicine screening. In most cases, this control relies upon a “key-lock-system” for the specific attachment of a particular protein to the engineered surface. A wellestablished system in this regard is the biotin-avidin combination. Avidin is a homotetrameric protein with the individual subunits, each of which can bind to biotin (vitamin B7/H) with a high degree of specificity and affinity (KD ≈ 10-15 M), thereby making the avidin-biotin system one of the strongest known noncovalent binding entities in nature.1-5 This property makes the avidin-biotin system quite useful for a wide range of biological applications, e.g., in diagnostics and therapeutics.6-10 Suitable molecular templates for the immobilization of avidin and its variants (e.g., streptavidin) are self-assembled monolayers (SAMs) of thiolates11 on gold with a biotin tail group as the *To whom correspondence should be addressed: E-mail: Michael. [email protected]. (1) Green, N. M. Biochem. J. 1963, 89, 585–591. (2) Green, N. M.; Toms, E. J. Biochem. J. 1973, 133, 687–700. (3) Green, N. M. Adv. Protein Chem. 1975, 29, 85–133. (4) Green, N. M. Methods Enzymol. 1990, 184, 51–67. (5) Laitinen, O. H.; Marttila, A. T.; Airenne, K. J.; Kulik, T.; Livnah, O.; Bayer, E. A.; Wilchek, M.; Kulomaa, M. S. J. Biol. Chem. 2001, 276, 8219–8224. (6) Wilchek, M.; Bayer, E. A. Methods Enzymol. 1990, 184, 5–45. (7) Diamandis, E. P.; Christopoulos, T. K. Clin. Chem. 1991, 37, 625–636. (8) Hnatowich, D. J.; Virzi, F.; Rusckowski, M. J. Nucl. Med. 1987, 28, 1294–1302. (9) Oehr, P.; Westermann, J.; Biersack, H. J. J. Nucl. Med. 1988, 29, 728–729. (10) Kalofonos, H. P.; Rusckowski, M.; Siebecker, D. A.; Sivolapenko, G. B.; Snook, D.; Lavender, J. P.; Epenetos, A. A.; Hnatowich, D. J. J. Nucl. Med. 1990, 31, 1791–1796. (11) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481–4483.

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functionality exposed at the SAM-ambient interface. In pioneering experiments, Knoll and co-workers12 showed that immobilization of streptavidin could be performed on a specifically designed SAM. Keeping in mind the possibility of both specific and nonspecific adsorption of streptavidin, mixed SAMs of biotin-substituted (BAT) and hydroxyl-substituted alkanethiols on gold were successfully explored, which subsequently provided a platform for the fabrication of novel biosensors in the early 1990s.13-19 Later on, Lopez and co-workers20 reported a more detailed study on these mixed SAMs to optimize the immobilization with respect to surface coverage, specificity, and activity of various types of streptavidin (both wild-type and mutants). Parallel, some other combinations of molecular constituents of mixed SAMs were explored. In particular, oligoethylene glycol (OEG)-substituted alkanethiols (OEG-ATs) were introduced as a promising building block of these films, following the report by Whitesides and Prime,21,22 who demonstrated that suitable (12) H€aussling, L.; Ringsdorf, H.; Schmitt, F. J.; Knoll, W. Langmuir 1991, 7, 1837–1840. (13) H€aussling, L.; Knoll, W.; Ringsdorf, H.; Schmitt, F. J.; Yang, J. L. Makromol. Chem. Macromol. Symp. 1991, 46, 145–155. (14) H€aussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. 1991, 30, 569–572. (15) Schmitt, F. J.; H€aussling, L.; Ringsdorf, H.; Knoll, W. Thin Solid Films 1992, 210, 815–817. (16) Herron, J. N.; M€uller, W.; Paudler, M.; Riegler, H.; Ringsdorf, H.; Suci, P. A. Langmuir 1992, 8, 1413–1416. (17) Spinke, J.; Liley, M.; Schmitt, F. J.; Guder, H. J.; Angermaier, L.; Knoll, W. J. Chem. Phys. 1993, 99, 7012–7019. (18) M€uller, W.; Ringsdorf, H.; Rump, E.; Wildburg, G.; Zhang, X.; Angermaier, L.; Knoll, W.; Liley, M.; Spinke, J. Science 1993, 262, 1706–1708. (19) Spinke, J.; Liley, M.; Guder, H.-J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821–1825. (20) Perez-Luna, V. H.; O’Brien, M. J.; Opperman, K. A.; Hampton, P. D.; Stayton, P. S.; Klumb, L.; Lopez, G. P. J. Am. Chem. Soc. 1999, 121, 6469–6478. (21) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164–1167. (22) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714–10721.

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OEG-AT SAMs on gold are completely resistant to protein adsorption. Thus, a binary mixture of BAT and OEG-AT species was thought to be an ideal molecular template to control the specific immobilization of avidin. Respectively, significant research efforts were made regarding the fabrication of mixed SAMs comprising various types of BATs, OEG-ATs, and other alkanethiols (ATs) to study different aspects of immobilization of avidin and its variants on the engineered, molecular templates for biosensor applications.23-49 The respective templates in all these studies were prepared by the coassembly method (either from solution or gel). Recently, we have developed an alternative approach to prepare mixed SAMs: irradiation-promoted exchange reaction (IPER).50-57 Within this approach, the mixing of both components of a binary film occurs by the exchange reaction between the primary SAM (one component) and a potential molecular substituent (second component). Electron or UV irradiation creates subtle structural and chemical defects in the primary SAM, which (23) Knoll, W.; Liley, M.; Piscevic, D.; Spinke, J.; Tarlov, M. J. Adv. Biophys. 1997, 34, 231–251. (24) Gupta, V. K.; Skaife, J. J.; Dubrovsky, T. M.; Abbott, N. L. Science 1998, 279, 2077–2080. (25) Jung, L. S.; Nelson, K. E.; Campbell, C. T.; Stayton, P. S.; Yee, S. S.; Perez-Luna, V.; Lopez, G. P. Sens. Actuators, B 1999, B54, 137–144. (26) Yousaf, M. N.; Mrksich, M. J. Am. Chem. Soc. 1999, 121, 4286–4287. (27) Yang, Z.; Frey, W.; Oliver, T.; Chilkoti, A. Langmuir 2000, 16, 1751–1758. (28) Jung, L. S.; Nelson, K. E.; Stayton, P. S.; Campbell, C. T. Langmuir 2000, 16, 9421–9432. (29) Hodneland, C. D.; Mrksich, M. J. Am. Chem. Soc. 2000, 122, 4235–4236. (30) Nelson, K. E.; Gamble, L.; Jung, L. S.; Boeckl, M. S.; Naeemi, E.; Gollegde, S. L.; Sasaki, T.; Castner, D. G.; Campbell, C. T.; Stayton, P. S. Langmuir 2001, 17, 2807–2816. (31) Yoon, H. C.; Hong, M.-Y.; Kim, H.-S. Langmuir 2001, 17, 1234–1239. (32) Riepl, M.; Enander, K.; Liedberg, B.; Schaeferling, M.; Kruschina, M.; Ortigao, F. Langmuir 2002, 18, 7016–7023. (33) Ladd, J.; Boozer, C.; Yu, Q.; Chen, S.; Homola, J.; Jiang, S. Langmuir 2004, 20, 8090–8095. (34) Ekgasit, S.; Stengel, G.; Knoll, W. Anal. Chem. 2004, 76, 4747–4755. (35) Hong, M.-Y.; Lee, D.; Kim, H.-S. Anal. Chem. 2005, 77, 7326–7334. (36) Riepl, M.; Stblom, M.; Lundstrm, I.; Svensson, S. C. T.; Denier van der Gon, A. W.; Schferling, M.; Liedberg, B. Langmuir 2005, 21, 1042–1050. (37) Clare, T. L.; Clare, B. H.; Nichols, B. M.; Abbott, N. L.; Hamers, R. J. Langmuir 2005, 21, 6344–6355. (38) Senaratne, W.; Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6, 2427–2448. (39) Christman, K. L.; Requa, M. V.; Enriquez-Rios, V. D.; Ward, S. C.; Bradley, K. A.; Turner, K. L.; Maynard, H. D. Langmuir 2006, 22, 7444–7450. (40) Mali, P.; Bhattacharjee, N.; Searson, P. C. Nano Lett. 2006, 6, 1250–1253. (41) Ducker, R. E.; Janusz, S.; Sun, S.; Leggett, G. J. J. Am. Chem. Soc. 2007, 129, 14842–14843. (42) Chelmowski, R.; Prekelt, A.; Grunwald, C.; Woll, C. J. Phys. Chem. A 2007, 111, 12295–12303. (43) Azzaroni, O.; Mir, M.; Knoll, W. J. Phys. Chem. B 2007, 111, 13499–13503. (44) Liu, J.; Lauterbach, R.; Paulsen, H.; Knoll, W. Langmuir 2008, 24, 9661–9667. (45) Kima, H.; Chob, I.-H.; Park, J. H.; Kima, S.; Paek, Se-H.; Noh, J.; Lee, H. Colloids Surf. A: Physicochem. Eng. Aspects 2008, 313, 541-544. (46) Zareie, H. M.; Boyer, C.; Bulmus, V.; Nateghi, E.; Davis, T. P. ACS Nano 2008, 2, 757–765. (47) Kim, H.; Noh, J.; Hara, M.; Lee, H. Ultramicroscopy 2008, 108, 1140–1143. (48) Zhang, J.; Lao, R.; Song, S.; Yan, Z.; Fan, C. Anal. Chem. 2008, 80, 9029–9033. (49) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1170. (50) Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. Adv. Mater. 2007, 19, 998–1000. (51) Ballav, N.; Shaporenko, A.; Krakert, S.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 7772–7782. (52) Ballav, N.; Weidner, T.; Rossler, K.; Lang, H.; Zharnikov, M. ChemPhysChem 2007, 8, 819–822. (53) Ballav, N.; Weidner, T.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 12002–12010. (54) Ballav, N.; Schlip, S.; Zharnikov, M. Angew. Chem., Int. Ed. 2008, 47, 1421–1424. (55) Ballav, N.; Zharnikov, M. J. Phys. Chem. C 2008, 112, 15037–15044. (56) Winkler, T.; Ballav, N.; Thomas, H.; Zharnikov, M.; Terfort, A. Angew. Chem., Int. Ed. 2008, 47, 7238–7241. (57) Ballav, N.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2009, 113, 3697–3706.

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promote the molecular exchange. As a result, the kinetics and extent of the exchange reaction can be precisely tuned by selection of a proper irradiation dose, so that a mixed SAM of desired composition can be prepared. The tunability of the SAM composition is one of the major advantages of IPER as compared to the previous (lithographic) techniques exploiting UV-promoted molecular exchange.58-60 Using several test systems, with the primary templates composed of nonsubstituted ATs, we have demonstrated that IPER is a promising approach for the fabrication of mixed SAMs, overcoming, in many cases, specific constraints of the coassembly method.50-57 In particular, a stochastic character of IPER leads to a molecular mixture of both SAM constitients, whereas, in the case of the coassembly, phase segregation can occur. In this study, we used protein-repelling OEG-AT SAMs as the primary templates and demonstrated the applicability of IPER to the fabrication of mixed SAMs composed of the protein-repelling matrix and the moieties bearing binding sites for specific attachment of a target protein. As the test system, we used the BAT species suitable for specific immobilization of avidin; as mentioned above, the biotin-avidin combination is a well-established system to study the specific adsorption of proteins on surfaces and interfaces. The major challenge was to test to what extent the specific limitations of IPER51 regarding its low efficiency for the primary templates composed of long molecules can be overcome in this particular case of biological significance. For this purpose, we varied the length of the primary OEG-AT species. The further goal was to compare the performance of the mixed OEG-AT/ BAT SAMs fabricated by IPER with respect to the specific and nonspecific protein adsorption with the analogous films prepared by the coadsorption method (literature data). Finally, we wanted to obtain starting information for the fabrication of complex arrays for immobilization of specific proteins by IPER-based lithography. So far, only inverse patterns (protein repelling features imbedded in a protein-adsorbing matrix), relying on nonspecific adsorption of proteins, could be fabricated by this technique.56

2. Experimental Section 2.1. Substrates and Compounds. The gold substrates were prepared by thermal evaporation of 100 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 5 nm titanium adhesion layer. Such evaporated films are polycrystalline in nature with a grain size of 20-50 nm as observed by atomic force microscopy. The grains predominantly exhibit a {111} orientation, which is particularly supported by the observation of the corresponding forward scattering maxima in the angular distributions of the Au 4f photoelectrons and by the characteristic binding energy (BE) shift of the Au 4f surface component.61,62 They are considered to be standard substrates for thiolate SAMs. As mentioned in 1Section 1, two OEG-AT molecules, differing by the length of the OEG part, were used for the preparation of the mixed OEG-AT/BAT SAMs. A shorter OEG-AT compound, HO(CH2CH2O)3(CH2)11SH (EG3) was purchased from Asemblon, Inc. (USA). The longer OEG-AT compound, HO(CH2CH2O)7(CH2)11SH (EG7) was synthesized in analogy to a previously described method.56 As a BAT compound, we used (58) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 2740–2748. (59) Cooper, E.; Wiggs, R.; Hutt, D. A.; Parker, L.; Leggett, G. J.; Parker, T. L. J. Mater. Chem. 1997, 7, 435–441. (60) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024–1032. (61) Kohn, F. Diploma Thesis, Universit€at Heidelberg, Heidelberg, Germany, 1998. (62) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058–4061.

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Figure 1. Chemical structures of the molecular constituents (EG3Bio, EG3, and EG7) used for the fabrication of mixed SAMs by the IPER method. The EG3-Bio structure is scaled down as compared to the EG3 and EG7 ones. The thicknesses of the single-component EG3-Bio, EG3, and EG7 SAMs on Au{111} are 4.5 nm,30 1.94 nm,63 and 2.82 nm,63 so that the biotin tail group of the EG3-Bio moiety protrudes over the EG3 or EG7 matrix in the mixed EG3/ EG3-Bio and EG7/EG3-Bio films. biotin-terminated tri(ethylene glycol) hexdecanethiol (EG3-Bio) (C36H68N4O6S2), which was purchased from Asemblon, Inc.; this compound is frequently used for the respective studies.28,30 Chemical structures of EG3, EG7, and EG3-Bio are presented in Figure 1. Dodecanethiol (DDT), avidin (Avi), bovine serum albumin (BSA), globulin (Glo), phosphate-buffered saline (PBS) buffer tablets, and all solvents were obtained from Sigma-Aldrich Chemie GmbH (Germany) and used without further purification. 2.2. Fabrication of Mixed SAMs by IPER. Primary, single-component EG3 and EG7 SAMs were formed by immersion of freshly prepared gold substrates into 0.1 mM solutions of the target compounds in ethanol for 24 h at room temperature. After immersion, the samples were thoroughly rinsed with pure ethanol, blown dry with argon, and kept in argon-filled glass containers until the characterization or subsequent experiments. DDT SAMs were prepared using standard protocol;51 these films served as reference samples for the SAM thickness evaluation and calculation of the proteins coverage (it is generally accepted that one-monolayer (ML) protein film is formed on the surface of Langmuir 2009, 25(16), 9189–9196

Article nonsubstituted AT SAMs (NSATs), such as DDT films). The primary EG3 and EG7 SAMs were irradiated with 10 eV electrons from a custom-made flood gun, which are especially effective for a gentle modification of SAMs and AT SAMs in particular.64 The irradiation dose did not exceed 1 mC/cm2, to stay in the dose range where the exchange reaction is progressively promoted by irradiation while the extent of irradiation-induced cross-linking, hindering the exchange, is quite low.50,51 The doses were estimated by multiplication of the exposure time with the current density (∼2.5 μA/cm2). The flood gun was mounted at a distance of ∼11.5 cm from the sample to ensure uniform illumination. The base pressure in the chamber during the irradiation was 1  10-8 mbar. The exchange reactions were performed by immersion of either pristine or irradiated primary EG3 and EG7 SAMs into a 0.1 mM solution of EG3-Bio compound (in ethanol) for 2 h at room temperature. According to our previous results, this immersion time corresponds to the saturation of IPER process.51 After immersion, the samples were thoroughly rinsed with ethanol, blown dry with argon, and kept in argon-filled containers until the characterization or subsequent experiments. 2.3. Protein Adsorption. Pristine, irradiated, and IPERprocessed SAMs were immersed in a solution of the respective protein (c = 1 mg/mL) in PBS-buffer (pH = 7.4 at 25 °C) for 15 min.65 After immersion, the samples were flashed with Millipore water (∼100 mL for each sample), thoroughly rinsed with Millipore water, and dried in a stream of nitrogen. Such a flashing and rinsing procedure was found to be effective in order to avoid formation of protein multilayers.66 2.4. XPS Measurements. The samples were characterized by X-ray photoelectron spectroscopy (XPS). The XPS measurements were performed in a custom-made UHV chamber, using Mg KR X-ray source (1253.6 eV) and a LHS 11 analyzer. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ∼0.9 eV. The X-ray source was operated at a power of 260 W and positioned ∼1.5 cm away from the samples. The energy scale was referenced to the Au 4f7/2 peak of AT-coated gold at a BE of 84.0 eV.67 XPS data were used to determine the effective thickness of the pristine and irradiated EG3 and EG7 SAMs. The thickness was determined on the basis of the IC1s/IAu4f intensity ratios, assuming a standard exponential attenuation of the photoelectron signal and using the attenuation lengths reported in ref 68 (the procedure has been verified for several reference samples); DDT SAMs of known thickness were used as the reference. XPS data were also used to determine the protein coverage on the pristine and irradiated EG3 and EG7 SAMs as well as mixed EG3/EG3-Bio and EG7/EG3-Bio films prepared by IPER. As direct references for the above evaluation, we used the single-component DDT and EG3-Bio SAMs, serving as the references for nonspecific and specific proteins adsorption, respectively. The characteristic N1s emission in the respective XPS spectra was used as a characteristic marker of the EG3-Bio molecule and proteins; this approach is quite common and frequently used for such purposes.

3. Results 3.1. Mixed SAMs. Since both EG3 and EG7 contain no nitrogen while EG3-Bio has nitrogen in its elemental composition, N1s XPS signal was used as a spectroscopic marker to monitor the extent of IPER between the primary EG3 and EG7 SAMs and EG3-Bio substituents. Figure 2a,b presents N1s XPS (63) Ballav, N.; Thomas, H.; Winkler, T.; Terfort, A.; Zharnikov, M. Angew. Chem., Int. Ed., in press. (64) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750–3764. (65) Gu, J.; Yam, C. M.; Li, S.; Cai, C. J. Am. Chem. Soc. 2004, 126, 8098–8099. (66) Herrwerth, S.; Eck, W.; Reinhart, S.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 9359–9366. (67) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1992. (68) Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15, 2037.

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Figure 2. N1s XPS spectra of the mixed EG3/EG3-Bio (a) and EG7/EG3-Bio (b) SAMs prepared by standard exchange reaction (0 mC/cm2) and IPER. Without irradiation, no exchange reactions occurred for the primary EG7 template, so that the bottom curve in panel b corresponds to the single-component EG7 SAM. The irradiation doses (mC/cm2) are given on the left of the respective spectra. The portion of the EG3-Bio moieties referenced to the single-component EG3-Bio films is given on the right of the respective spectra.

spectra of the IPER-processed samples. The pristine EG3 SAM revealed a clear N1s signal after immersion in the avidin solution, thereby indicating exchange reaction and incorporation of the EG3-Bio moieties in the EG3/Au SAM even without the preliminary irradiation. Using the N1s signal of the single-component EG3-Bio SAM (not shown) as the reference (100%), we estimated the portion of EG3-Bio in the mixed EG3/EG3-Bio SAM as 6%. Performing IPER instead of the standard exchange reaction, a stronger N1s signal could be detected already for a small irradiation dose (0.5 mC/cm2). Further on, the intensity of N1s gradually increased with increasing irradiation dose from 0.5 to 0.75mC/cm2, and the portions of EG3-Bio component in the mixed EG3/EG3-Bio SAMs were evaluated to be 21% and 31%, respectively. Considering the previous reports about BAT/OEGAT SAMs,28,30 we were satisfied with this composition and refrained from further increase of the irradiation dose in the present case in view of the onset of extensive cross-linking, hindering the exchange reaction.50,51,57 The primary EG7 SAMs exhibited a somewhat different behavior than the EG3 films. The exchange reaction did not occur without the preliminary irradiation: it was not possible to incorporate the EG3-Bio moieties into the EG7/Au SAM as revealed by the absence of the characteristic emission in the N1s XPS spectrum (bottom curve in Figure 2b). However, promoting the exchange by irradiation, the EG3-Bio component could be easily introduced into the EG7/Au SAM as seen in Figure 2. 3.5% of EG3-Bio was observed at a dose of 0.5 mC/cm2 (the value is normalized to the single-component EG3-Bio film). Similar to the EG3 case, the portion of the EG3-Bio component in the mixed EG7/EG3-Bio SAMs could be increased to 10.5% at higher irradiation dose (0.75 mC/cm2). At the same time, a further increase of the irradiation dose beyond the latter value did not result in a respective increase of the EG3-Bio portion: there was only 9% of the EG3-Bio component in the mixed EG7/EG3-Bio SAM fabricated with the irradiation dose of 1.0 mC/cm2. This behavior was probably related to the onset of the extensive crosslinking, progressing with increasing dose and hindering the exchange reaction.56 9192 DOI: 10.1021/la9007476

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Both the general trend of IPER between the primary EG3 and EG7 SAMs and EG3-Bio substituents (i.e., progressive promotion of the exchange by irradiation with saturation at higher doses) and the observed differences between these two systems are quite consistent with the results for several AT systems reported by us earlier.50-57 Whereas the pristine EG3 films underwent the exchange reaction with EG3-Bio, no exchange occurred in the case of the pristine EG7 SAMs. Further, the extent of IPER for the EG3 films was noticeably higher than that for the EG7 SAMs at the same irradiation dose. This behavior can be explained by a stronger intermolecular interaction in the SAMs composed of the longer molecules, which makes it more difficult to exchange these species. It is well-known that the length of the SAM constituents affects the efficiency of standard exchange reaction, with faster kinetics and larger extent of the reaction for shorter chain films.49 Also, in the case of the irradiation-induced promotion (IPER), the efficiency of the exchange reaction is higher for shorter chain primary SAMs, which has been verified previously by us for several AT SAMs as primary templates and ω-substituted ATs as the substituents,51 and which is exactly what we observed for the EG3 and EG7 systems. On the average, the extent of the exchange reaction with the EG3-Bio substituents for the EG3 SAMs was found to be larger by a factor of 2-3 as compared to the EG7/Au films, which makes the former system more suitable as far as specific immobilization of avidin is aimed. 3.2. Specific Protein Adsorption. In good agreement with the known behavior of OEG SAMs, no adsorption of avidin could be observed for the pristine, single-component EG3 and EG7 films (data not shown). For all other films, the amount of adsorbed avidin correlated with the portion of the EG3-Bio moieties. In particular, after the immersion into EG3-Bio solution (standard exchange reaction), only the EG3-stemming films, which underwent the molecular exchange and contained some amount of the EG3-Bio species (see the bottom curve in Figure 2a), exhibited a gain of the characteristic N1s signal associated with adsorbed avidin, whereas the analogous EG7 films, having no exchange with EG3-Bio under conditions of the standard exchange reaction (see the bottom curve in Figure 2b), did not show any N1s intensity. The respective spectra are presented in Figure 3a,b (bottom curves), along with the corresponding spectra of the mixed EG3/EG3-Bio and EG7/EG3-Bio SAMs prepared by IPER after their incubation in avidin solution. For the latter SAMs, pronounced gain of the N1s signal as compared to the respective spectra in Figure 2 was observed, suggesting the expected adsorption of avidin. The surface coverage of avidin versus surface concentration of EG-Bio component in the mixed EG3/EG3-Bio and EG7/EG3-Bio SAMs is presented in Figure 4a,b, respectively. For the mixed EG3/EG3-Bio SAMs (Figure 4a), the observed trend of the specific immobilization of avidin is very consistent with the observations on the immobilization of various types of streptavidin on similar (EG4/EG3-Bio) mixed SAMs prepared by the coassembly method.28 The amount of avidin increases initially with increasing portion of EG3-Bio, achieves a maximum at 20-30% EG3-Bio coverage, and decreases at higher EG3-Bio portion. A detailed analysis of this behavior will be provided below, in the 4Discussion section . As for the EG7/EG3-Bio (Figure 4b), the general trend in avidin immobilization was found to be similar to that of the EG3/ EG3-Bio films. However, only a starting part of the curve, with a steep increase of the avidin coverage following the EG3-Bio portion in the mixed films could be monitored. This increase was, however, somewhat less steep as compared to the EG3/EG3Bio system, which is presumably related to the differences in the Langmuir 2009, 25(16), 9189–9196

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Figure 3. N1s XPS spectra of the mixed EG3/EG3-Bio (a) and EG7/EG3-Bio (b) SAMs after immersion into the avidin solution. Without irradiation, no exchange reactions occurred for the primary EG7 template, so that the bottom curve in panel b corresponds to the single-component EG7 SAM. The irradiation doses (mC/cm2) and surface coverage of avidin are given, respectively, on the left and right of the corresponding spectra. The same scaling of the intensity axis as in Figure 2 is used, so that the spectra can be directly compared.

Figure 4. Surface coverage of avidin versus the portion of the EG3-Bio component in the mixed EG3/EG3-Bio (a) and EG7/ EG3-Bio (b) SAMs. The values are referenced to the avidin coverage and EG3-Bio density for the single-component EG3-Bio film, which both are set as 100%. A part of the solid curve above 15% of EG3-Bio in panel b is a speculation only. The avidin coverage was calculated on the basis of the spectra presented in Figure 3. The intensities of the characteristic N1s emission were corrected for the signal related to the EG3-Bio species (Figure 2).

exact molecular organization at the SAM-ambient interface of the EG7/EG3-Bio and EG3/EG3-Bio films. As for the residual part of the avidin coverage curve for EG7/EG3-Bio in Figure 4b, beyond the steep increase region, we can only speculate about it, since it was not possible to achieve sufficiently high (higher than 11%) EG3-Bio concentrations for this particular system by IPER. Similar to the case of EG3/EG3-Bio (Figure 4a) and EG4/EG3Bio (coassembly),28 we can assume the presence of a maximum and a slow decrease of the avidin coverage at the intermediate EG3-Bio concentrations (as tentatively shown in Figure 4b). 3.3. Nonspecific Proteins Adsorption. In addition to the specific adsorption of avidin and its derivatives onto specifically designed mixed SAMs bearing BAT as a mandatory component, one should also test the nonspecific adsorption of other proteins, which is quite important as far as biosensor applications of such mixed SAMs are concerned. We found that electron irradiation of Langmuir 2009, 25(16), 9189–9196

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Figure 5. N1s XPS spectra of the pristine and irradiated singlecomponent EG3 and EG7 SAMs after nonspecific proteins adsorption. Avidin adsorption in a nonspecific way (a) and nonspecific adsorption of BSA (b) were carried out on the EG3 SAMs. Nonspecific adsorption of globulin (c) was performed on the EG7 SAMs. The irradiation doses (mC/cm2) are given on the left of the respective spectra. The protein coverage referenced to the singlecomponent DDT film is given on the right of the respective spectra.

the initially completely protein-resistant OEG SAMs, such as, e.g., EG3 and EG7 ones, not only promoted the subsequent exchange reaction with the EG3-Bio component, but also makes these films protein-adsorbing, which is related to significant damage of the OEG backbones occurring already at relatively small irradiation doses.63 In particular, the irradiated EG3 and EG7 SAMs were found to adsorb avidin in a nonspecific way and a wide variety of further, nonspecific proteins, such as, e.g., BSA, globulin, and fibrinogen. Significantly, adsorption of such nonspecific proteins could be finely tuned by selecting a proper irradiation dose. Figure 5 presents N1s XPS signals of various proteins immobilized onto the irradiated EG7 and EG3 SAMs. While no N1s XPS signal (coming exclusively from the proteins) could be detected on the pristine SAMs, certain amounts of avidin, BSA, and globulin could be immobilized on the irradiated SAMs, as evident from the N1s XPS signals. In particular, taking the BSA and globulin coverage on the DDT SAM as a reference (this is a standard approach),66 we estimated the nonspecific adsorption of BSA on the irradiated (0.75 mC/cm2) EG3 SAMs and globulin on the irradiated (1.0 mC/cm2) EG7 films as 38 and 77%, respectively. Figure 5b,c shows a clear dependence of the extent of nonspecific proteins adsorption on the irradiation dose. With increasing irradiation dose, the surface coverage of nonspecific adsorbed proteins increased and gradually approached a saturation at about 1 mC/cm2. The above results cause some doubt regarding the use of EG3/ EG3-Bio and EG7/EG3-Bio templates fabricated by IPER for specific protein adsorption. However, fortunately, the exchange reaction with the EG3-Bio moieties (IPER) significantly suppressed the extent of nonspecific protein adsorption with respect to the irradiated EG3 and EG7 SAMs. In particular, the mixed EG3/EG3-Bio SAMs prepared by IPER with irradiation dose of 0.5 and 0.75 mC/cm2 revealed no increment in the N1s XPS signal (not shown) after immersion into the BSA solution. Thus, nonspecific protein adsorption was fully suppressed after the IPER and the respective film exhibited only specific protein adsorption (see previous section). A probable explanation for this behavior could be a complete removal of the damaged EG3 species by the EG3-Bio molecules, which agrees quite well with the general idea of IPER.50,51 In contrast to the EG3/EG3-Bio SAMs, mixed EG7/EG3-Bio films were found to exhibit nonspecific adsorption to some extent, which was observed in experiments with globulin. However, the surface coverage of globulin on the mixed EG7/EG3-Bio SAMs DOI: 10.1021/la9007476

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Figure 6. Schematic presentation of the individual steps and processes occurring during the fabrication of mixed SAMs designed for avidin immobilization by IPER. The starting point is highlighted by the gray background. Step 1: general effect of electron irradiation on the OEG (EG3 and EG7) SAMs. Step 2: depending on the identity of the primary OEG SAM, the outcome of IPER with a BAT substituent can be different. In the present study, steps 2a and 2b are characteristic of the EG3/EG3-Bio and EG7/EG3-Bio systems, respectively. Step 3: adsorption of a specifically binding protein (avidin) on the mixed OEG/BAT SAMs prepared by IPER. In the present study, steps 3a and 3b are characteristic of the EG3/EG3-Bio and EG7+EG3-Bio systems, respectively. Step 4: verification of nonspecific proteins adsorption onto the mixed OEG/BAT SAMs designed for the specific protein immobilization. In the present study, steps 4a and 4b are characteristic of the EG3/EG3-Bio (no adsorption of BSA) and EG7/EG3-Bio (some adsorption of globulin) systems, respectively. Step 5: nonspecific proteins adsorption (including avidin) onto the irradiated OEG SAMs.

was significantly less than that on the EG7 SAMs, which were only irradiated. In particular, the mixed EG7/EG3-Bio SAM fabricated by IPER with 0.75 mC/cm2 irradiation dose showed 1.6 times less globulin coverage than the EG7 film, which was only irradiated with the same dose. One reason for this behavior could be noncomplete removal of all damaged EG7 species upon the exchange with the EG3-Bio molecules, which can be expected for the comparably long EG7 molecules. Another reason could be that globulin is a much “stickier” protein than BSA, giving thus a worst-case scenario of nonspecific protein adsorption.

4. Discussion 4.1. Irradiation Promoted Exchange Reaction. Figure 6 shows a schematic presentation of the individual steps and processes occurring during the fabrication of mixed SAMs designed for specific immobilization of proteins by IPER. We have depicted five important steps relevant to the present work. These are electron irradiation (step 1); formation of mixed SAMs by IPER (steps 2a and 2b); immobilization of a specifically binding protein onto the mixed SAMs prepared by IPER (steps 3a and 3b); adsorption of nonspecific proteins onto the mixed 9194 DOI: 10.1021/la9007476

SAMs prepared by IPER (steps 4a and 4b); and nonspecific proteins adsorption on the irradiated SAMs (step 5). Generally, electron irradiation of SAMs induces a variety of different processes, which occur simultaneously and are interrelated.69,70 The most prominent processes are (i) cleavage of the individual bonds in molecular backbones with subsequent desorption of the released fragments, (ii) damage of the substrate-headgroup interface, (iii) structural and conformational disordering, and (iv) cross-linking between the residual molecular moieties. The defects associated with these processes affect the exchange reaction differently, promoting (i-iii) or hindering (iv) it.50-57 In the present case, it would be useful to analyze the specific course of these events in the case of OEG-AT SAMs and correlate them with the subsequent immobilization of both specific (avidin) and nonspecific (BSA and globulin) proteins on the irradiated and IPER-fabricated films. The most prominent events that could be clearly detected during electron irradiation of the EG3 and EG7 SAMs in the relevant dose range were (i) significant and (69) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697–2705. (70) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol. B 2002, 20, 1793–1807.

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preferential damage of the OEG units resulting in dramatic reduction of the film thickness63 and (ii) cleavage of the primary S-Au bonds, resulting in a weaker coupling of the affected molecules to the substrate. While the former process was mainly responsible for the nonspecific protein adsorption onto the irradiated OEG-AT SAMs, both processes promoted the exchange reaction. A shorter (after the damage of the OEG part) OEG-AT moiety could be exchanged for EG3-Bio more easily than the nondamaged, long moieties. A weaker bonding to the substrate and molecular disorder associated with the cleavage of the S-Au bond promoted the exchange reaction with the EG3Bio molecules as well. Apart from this general behavior, it was much more difficult to exchange the longer EG7 molecules as compared to the shorter EG3 ones (the thickness of EG3 and EG7 SAMs was found to be ∼18 and 28 A˚, respectively),63 even in the case of their damage. Similar difference between the EG3 and EG7 SAMs was also observed for the pristine films, with the pronounced exchange reaction in the former case and complete lack of this reaction in the latter case. Note that we can not exclude that a significant part of the exchange events for these films occurred at the “grain boundaries” and “domain borders” that were observed before for some other AT films.49,71-81 Note also that an additional reason for the suppression of the exchange reaction with the EG3-Bio molecules in the case of the pristine EG7 SAMs could be a different conformation of the OEG backbones in these films compared to the EG3 layers. According to literature reports, OEG-AT SAMs with a small length of the OEG part showed zigzag conformation of the OEG-backbones in contrast to films with longer OEG parts, exhibiting helix conformation of the EGbackbones.82 In the present case, the most probable helix conformation of the EG-backbones within the EG7 SAMs could prevent the penetration of the EG3-Bio molecules through the dense molecular matrix to reach the suitable sites for a possible exchange reaction. 4.2. Specificity, Nonspecificity, and Comparison with Previous Reports. An important requirement for the specific immobilization of avidin is a proper engineering of the surface with suitable biotin-functionalized molecular constituents. In this respect, mixed SAMs are better molecular templates than the single-component BAT SAMs since the latter show lower avidin coverage as compared to the former ones. The reason behind this phenomenon is the structure-activity relationship.20 One aspect is a structural disorder and intermixing of the densely packed BAT molecules, especially in the region of the terminal biotin tailgroups, which occurs in the single-component BAT SAMs and prevents an extensive adsorption of avidin. Another aspect is the (71) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528–12536. (72) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301–4306. (73) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167–3173. (74) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192–1197. (75) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys., Part 1 1997, 36, 2379–2385. (76) Kajikawa, K.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys., Part 2 1997, 36, 1116–1119. (77) Chung, C.; Lee, M. J. Electroanal. Chem. 1999, 468, 91–97. (78) Felgenhauer, T.; Rong, H. T.; Buck, M. J. Electroanal. Chem. 2003, 550551, 309–319. (79) Yang, G.; Amro, N. A.; Starkewolfe, Z. B.; Liu, G.-Y. Langmuir 2004, 20, 3995–4003. (80) Baralia, G. G.; Duwez, A.-S.; Nysten, B.; Jonas, A. M. Langmuir 2005, 21, 6825–6829. (81) Wrochem,von, F.; Scholz, F.; Schreiber, A.; Nothofer, H.-G.; Ford, W. E.; Morf, P.; Jung, T.; Yashuda, A.; Wessels, J. M. Langmuir 2008, 24, 6910–6917. (82) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. J. Phys. Chem. B 1998, 102, 426–436.

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necessity of definite spatial orientation of the biotin part to bind specifically with avidin, which is also difficult to achieve in the single-component BAT films. In contrast, in the mixed BATOEG SAMs, individual biotin tail groups protruding over the OEG “background” are well separated, do not hinder each other, and have enough flexibility in its lateral movement, allowing for the optimal special orientation. As mentioned above, most of the previous works used coassembly methods to fabricate mixed SAMs composed of nonsubstituted (-CH3) and ω-substituted (e.g., -OH) ATs and OEGAT as diluents in the BAT layers for the immobilization of streptavidin.30 In these systems, the initial mixing ratios of the molecular constituents in the coassembly solution determined the surface coverage of the specific protein receptors (and consequently of the specific proteins), while in the present case it was achieved by selecting a proper irradiation dose. The general trend observed for the immobilization of various types of streptavidin on the mixed SAMs prepared by the coassembly method;“a steep increment with small fraction of surface BAT component until a maximum is reached followed by gradual decay”28;is similar to that observed in our study, for the mixed EG3/EG3-Bio and EG7/EG3-Bio SAMs prepared by IPER (Figure 4a,b). For the mixed EG4/EG3-Bio SAMs fabricated by the coassembly, the surface concentration of the BAT component required for the maximum immobilization of various types of streptavidin was found to be ∼30%.28 In our case, for the EG3/EG3-Bio SAMs prepared by IPER, the maximum surface coverage of avidin was achieved with only ∼20% surface portion of the EG3-Bio component. In addition, this coverage was significantly higher (by a factor of 1.5) as compared to the maximal coverage of streptavidin on EG4/EG3-Bio SAMs fabricated by the coassembly28 as far the single components EG3/EG3-Bio or EG4/EG3Bio films were considered as the reference. The above differences allow us to speculate about a better structural arrangement of the EG3-Bio component within the mixed ML fabricated by IPER than in the case of the coassembly method (in agreement with our previous results for other systems).57 It is well-known that “phase segregation” of different molecular constituents leading to the formation of different molecular domains can occur during coassembly, at least to some extent.83,84 In contrast, fully stochastic distribution of irradiation-induced defects and, subsequently, a homogeneous mixture of both molecular components in the mixed SAMs can be expected in the case of IPER,50-56 which is especially beneficial in the case of OEG-BAT mixtures in view of the above-mentioned steric hindrance and orientation problems of the individual biotin tail groups. Further, apart from revealing a good performance in the specific immobilization of avidin, mixed EG3/EG3-Bio SAMs prepared by IPER exhibited a complete resistance to the adsorption of nonspecific proteins. This suggests that all of the irradiation-affected molecules in the irradiated, primary EG3 films could be removed by EG3-Bio molecules during the exchange reaction (IPER). Unfortunately no data on nonspecific proteins adsorption on the mixed EG4/ EG3-Bio SAMs prepared by the coassembly method is available for direct comparison. However, it looks like IPER has some advantages as compared to the coassembly method in terms of fabricating suitable functional surface templates based on mixed SAMs for the specific immobilization of avidin and its variants with zero nonspecific proteins adsorption. (83) Schonherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H. -J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898–3904. (84) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261–3265.

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Let us consider the EG7/EG3-Bio system. As a result of a lower efficiency of IPER in the case of long-chain molecules, the maximum concentration of the only EG3-Bio species in the mixed SAMs did not exceed 10.5%. This concentration is within “a steep increment with a small fraction of surface BAT component” part of the avidine coverage versus BAT component curve (see Figure 4) and is below the expected optimal concentration characteristic of the maximum of the curve (20-30%). The similarity of the initial parts of the curves for the EG3/EG3-Bio (Figure 4a) and EG7/EG3-Bio (Figure 4b) films allow us to speculate that their general courses, including the presence of the characteristic maximum as shown in Figure 4b, are similar as well. Another consequence of a lower efficiency of IPER in the case of the EG7/EG3-Bio system is a noncomplete exchange of the damaged (by electrons) EG7 species by the EG3-Bio substituents, resulting in the occurrence of nonspecific protein adsorption. Even though the extent of the latter process was significantly (by a factor of 1.5-2) reduced as compared to the irradiated EG7 films (see Figures 5a and 5c), the nonspecific protein could not be completely suppressed, as was shown by the example of globulin. Note, however, that the experiments with globulin represented probably the worst case of nonspecific protein adsorption, since this particular protein is believed to be especially sticky. Since, to our knowledge, no data is available on nonspecific protein adsorption on similar mixed SAMs prepared by coassembly method, we are unable to present a comparative assessment in this direction. However, it has been found recently that a layer of a nonspecific protein (BSA) on such special mixed SAMs designed for streptavidin immobilization can be very useful in regard to the biosensor properties of the immobilized streptavidin.85 4.3. Possible Lithographic Applications. The strength of the IPER approach is not only its applicability to the fabrication of mixed SAMs but also the possibility to combine this approach with lithography,54,56 representing a promising alternative to existing lithographic approaches, exploiting, e.g., the biotinavidin “key-lock system” (see, e.g., refs 27, 41, 65, and 86-93). In the ideal case, for biosensor applications, selected spots or spot array for immobilization of specific protein surrounded by protein-repelling background should be fabricated.63 Considering the EG3/EG3-Bio and EG7/EG3-Bio systems in this regard, we can say they are not completely suitable for the fabrication of the above patterns. The immersion of the primary EG3 template into (85) Esseghaier, C.; Bergaoui, Y.; Fredj, H.; Tlili, A.; Helali, S.; Ameur, S.; Abdelghani, A. Sens. Actuators B: Chem. 2008, 134, 112–116. (86) Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Langmuir 2001, 17, 178–182. (87) Kim, K.; Yang, H.; Jon, S.; Kim, E.; Kwak, J. J. Am. Chem. Soc. 2004, 126, 15368–15369. (88) Jiang, X.; Xu, Q.; Dertinger, S. K. W.; Stroock, A. D.; Fu, T.-M.; Whitesides, G. M. Anal. Chem. 2005, 77, 2338–2347. (89) Rong, D.; Krishnan, S.; Baird, B. A.; Lindau, M.; Ober, C. K. Biomacromolecules 2007, 8, 3082–3092. (90) Jhaveri, S. B.; Beinhoff, M.; Hawker, C. J.; Carter, K. R.; Sogah, D. Y. ACS Nano 2008, 2, 719–727. (91) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem. Rev. 2008, 108, 494–521. (92) Christman, K. L.; Schopf, E.; Broyer, R. M.; Li, R. C.; Chen, Y.; Maynard, H. D. J. Am. Chem. Soc. 2009, 131, 521–527. (93) Gu, Z.; Huang, S.; Chen, Y. Angew. Chem., Int. Ed. 2009, 48, 952–955.

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the EG3-Bio solution will results in the appearance of the EG3Bio component, not only within the selected spots, but also within the background, since the exchange reaction occurs to some extent for the nonirradiated EG3 films as well. As the result, specific adsorption of avidin will occur onto both preselected spots and background, even though to a lesser extent. The nonspecific adsorption will be, however, completely suppressed, on both spots and background. In contrast to the EG3/EG3-Bio case, the background of the EG7/EG3-Bio pattern will be protein-repelling with respect to both specific and nonspecific protein adsorption, since no exchange of the EG7 species for the EG3-Bio moieties occurs without the irradiation. However, the spots fabricated by IPER lithography for specific adsorption of avidin, will exhibit some extent of nonspecific protein adsorption, at least for such sticky proteins as globulin. Presumably, further work on the OEG-AT/ BAT system is necessary, to achieve the optimal performance of the OEG-AT/BAT patterns for specific protein adhesion.

5. Conclusions In summary, by the example of the OEG-AT/BAT system, we have demonstrated that mixed SAMs composed of the proteinrepelling matrix and the moieties bearing binding sites for specific attachment of a target protein can be conveniently fabricated by IPER, which represents an alternative to the commonly used coassembly method. The resulting EG3/EG3-Bio and EG7/EG3Bio SAMs exhibited the characteristic dependence of the specific protein coverage on the portion of the EG3-Bio component in the molecular templates, similar to the behavior of the analogous films prepared by the coassembly method, viz., a steep increment of the protein coverage with increasing EG3-Bio fraction in the SAM until a maximum coverage was reached, followed by a gradual decrease. The optimal EG3-Bio fraction (∼20%) and maximal avidin coverage (∼220%) for the EG3/EG3-Bio SAMs prepared by IPER differ from the analogous values (∼30% and 150%, respectively) observed for the adsorption of wild type streptavidin onto the EG4/EG3-Bio films fabricated by coassembly.28 These differences are explained by the lack of phase segregation and better separation of the individual EG3-Bio species in the EG3 matrix in the case of IPER. For EG7/EG3-Bio, the maximum portion of the BAT component in the mixed SAMs prepared by IPER did not exceed 10.5%, which was related to a lower efficiency of IPER for long-chain primary SAMs. In addition to specific adsorption of avidin, nonspecific adsorption of different proteins onto the EG3/EG3-Bio and EG7/EG3-Bio templates was monitored. This process was completely suppressed in the former case but occurred to some extent in the latter case, parallel to the specific events. The implication of this phenomenon for IPER-based lithography was considered, along with some other relevant findings of the present study. Acknowledgment. We thank M. Grunze for the support. This work has been financially supported by the DFG (ZH 63/9-3 and TE 247/6-2).

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