Electrografted Aryl Diazonium Initiators for Surface ... - ACS Publications

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Electrografted Aryl Diazonium Initiators for Surface-Confined Photopolymerization: A New Approach to Designing Functional Polymer Coatings )

Sarra Gam-Derouich,† Benjamin Carbonnier,‡ Mireille Turmine,§ Philippe Lang,† Mohamed Jouini,† Dalila Ben Hassen-Chehimi, and Mohamed M. Chehimi*,† ITODYS, University Denis Diderot & CNRS (UMR 7086), 15 rue Jean de Baı¨f, 75013 Paris, France, ‡Institut de Chimie et des Mat
eriaux Paris Est, CNRS UMR 7182, Facult
e des Sciences Universit
e Paris Est Cr
eteil, 2 rue Henri Dunant 94320, Thiais, France, §LISE, CNRS (UPR 15) and University Pierre & Marie Curie, Case 133, 4 Place Jussieu, 75005 Paris, France, and Laboratoire d’Application de la Chimie aux Ressources et Substances Naturelles et a l’Environnement, D
epartement de Chimie, Facult
e des Sciences de Bizerte, Zarzouna, Bizerte 7021, Tunisia )

†

Received March 2, 2010. Revised Manuscript Received May 21, 2010 This article reports on the preparation of polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-hydroxyethyl methacrylate) (PHEMA) ultrathin grafts on gold substrates modified by 4-benzoylphenyl (BP) moieties derived from the electroreduction of the parent diazonium salt BF4-, þN2-C6H4-CO-C6H5 (DS). The grafted organic species -C6H4-CO-C6H5 was found to be very effective in the surface-initiating photopolymerization (SIPP) of vinylic monomers in the presence of an aromatic tertiary amine co-initiator acting as a hydrogen donor. This novel tandem diazonium salt electroreduction/SIPP was found to be effective in grafting PS, PMMA, and PHEMA from the surface of gold-coated silicon wafers. The polymer films were characterized in terms of chemical structure and wettability by infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy, and contact angle measurements, respectively. The polymer grafts were further evaluated as adsorbents for bovine serum albumin (BSA) used as a model protein. It was found gold/PHEMA resisted BSA adsorption because of its hydrophilic character, whereas PS and PMMA grafts adsorbed BSA via interfacial hydrophobic interaction. The XPS-determined extent of adsorbed BSA was found to increase linearly with the hydrophobic character of the polymer grafts as measured by water contact angles. This work shows that this novel tandem diazonium salt electroreduction/SIPP is a facile, ultrafast, efficient protocol for grafting polymer chains to surfaces. It broadens the enormous possibilities offered by aryl diazonium salts to generate functional organic coatings.

1. Introduction The recent years have witnessed a quantum jump in the number of publications pertaining to surface chemistry and applications to aryl diazonium salts.1,2 The interest in using aryl diazonium salts lies in their ease of preparation, rapid (electro)reduction, large choice of reactive functional groups, and strong aryl-surface covalent bonding.3,4 These attractive properties were exploited for the design of novel high-performance redox surfaces,5-7 (bio)sensors,8,9 biocatalysts,10 specific and selective metal ion (1) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429, and references therein. (2) Aswal, D. K.; Koiry, S. P.; Jousselme, B.; Gupta, S. K.; Palacin, S.; Yakhmi, J. V. Physica E 2009, 41, 325. (3) (a) Jiang, D. E.; Sumpter, B. G.; Dai, S. J. Am. Chem. Soc. 2006, 128, 6030. (b) Jiang, D. E.; Sumpter, B. G.; Dai, S. J. Phys. Chem. B 2006, 110, 23628. (4) Boukerma, K.; Chehimi, M. M.; Pinson; Blomfield, J. C. Langmuir 2003, 19, 6333. (5) (a) Saby, C.; Ortiz, B.; Champagne, G. Y.; B
elanger, D. Langmuir 1997, 13, 6805. (b) Ghodbane, O.; Chamoulaud, G.; B
elanger, D. Electrochem. Commun. 2004, 6, 254. (6) Agnes, J. C.; Arnault, F.; Omnes, B.; Jousselme, M. Billon.; Bidan, G.; Maille, P. Phys. Chem. Chem. Phys. 2009, 11, 11647. (7) Boland, S.; Foster, K.; Leech, D. Electrochim. Acta 2009, 54, 1986. (8) Wang, J.; Carlisle, J. A. Diamond Relat. Mater. 2006, 15, 279. (9) Radi, A. E.; Munoz-Berbel, X.; Lates, V.; Marty, J. L. Biosens. Bioelectron. 2009, 24, 1888. (10) Pellissier, M.; Barriere, F.; Downard, A. J.; Leech, D. Electrochem. Commun. 2008, 10, 429. (11) (a) Betelu, S.; Vautrin-Ul, C.; Ly, J.; Chauss
e, A. Talanta. 2009, 80, 372. (b) Ust€undaga, Z.; Solak, A. O. Electrochim. Acta 2009, 54, 6426. (12) Liu, G.; Gooding, J. J. Langmuir 2006, 22, 7421. (13) Evrard, D.; Lambert, F.; Policar, C.; Balland, V.; Limoges, B. Chem.;Eur. J. 2008, 14, 9286.

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adsorbents,11 bioactive surfaces,12,13 and hybrid molecule-onsemiconductor nanoelectronic devices,2,14 to name but a few. In macromolecular science, it has been shown recently that aryl diazonium salts are effective in binding biomacromolecules,8,10,13 epoxy resins,15 and polymers16-20 onto surfaces via strong, primary covalent bonding. In particular, Pinson and co-workers16 grafted polystyrene (PS) chains to 4-benzoylphenyl-functionalized iron substrates under UV irradiation at 360 nm, with the 4-benzoylphenyl grafted groups acting as photoactivators. It is (14) Lu, M.; Nolte, W. M.; He, T.; Corley, D. A.; Tour, J. M. Chem. Mater. 2009, 21, 442. (15) Delamar, M.; D
esarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Sav
eant, J. M. Carbon 1997, 35, 801. (16) Adenier, A.; Cabet-Deliry, E.; Lalot, T.; Pinson, J.; Podvorica, F. Chem. Mater. 2002, 14, 4576. (17) Mahouche, S.; Mekni, N.; Abbassi, L.; Lang, P.; Perruchot, C.; Jouini, M.; Mammeri, F.; Turmine, M.; Ben Romdhane, H.; Chehimi, M. M. Surf. Sci. 2009, 603, 3205. (18) (a) Matrab, T.; Chehimi, M. M.; Perruchot, C.; Adenier, A.; Guillez, V.; Save, M.; Charleux, B.; Cabet-Deliry, E.; Pinson, J. Langmuir 2005, 21, 4686. (b) Matrab, T.; Save, M.; Charleux, B.; Pinson, J.; Cabet-Deliry, E.; Adenier, A.; Chehimi, M. M.; Delamar, M. Surf. Sci. 2007, 601, 2357. (19) (a) Matrab, T.; Chehimi, M. M.; Pinson, J.; Basinska, T.; Slomkowski, S. Surf. Interface Anal. 2006, 38, 565. (b) Matrab, T.; Chancolon, J.; Mayne-L'Hermite, M.; Rouzaud, J. N.; Deniau, G.; Boudou, J. P.; Chehimi, M. M.; Delamar, M. Colloids Surf., A 2006, 287, 217. (c) Matrab, T.; Nguyen, M. N.; Mahouche, S.; Lang, P.; Badre, C.; Turmine, M.; Girard, G.; Bai, J.; Chehimi, M. M. J. Adhes. 2008, 84, 684. (d) Mahouche, S.; Abbas, N.; Matrab, T.; Turmine, M.; Bon Nguyen, E.; Losno, R.; Pinson, J.; Chehimi, M. M. Carbon 2010, 48, 2106. (20) (a) Matrab, T.; Chehimi, M. M.; Boudou, J. P.; Benedic, F.; Wang, J.; Naguib, N. N.; Carlisle, J. A. Diamond Relat. Mater. 2006, 15, 639. (b) Dahoumane, S. A.; Nguyen, M. N.; Thorel, A.; Boudou, J. P.; Chehimi, M. M.; Mangeney, C. Langmuir 2009, 25, 9633.

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worth noting that in these publications the substrate-aryl polymer junctions were reported to be strong and to resist prolonged ultrasonic treatment. Because PS chains are photografted to the 4-benzoylphenyl groups which are themselves bound to substrates, it implies that the polymer is also linked to the underlying metal via covalent bonds through the intermediate aryl layer. On the basis of the density functional theory (DFT) calculations of Jiang et al.3a and considering the simple phenyl group to be the grafted species, a carbon-metal σ bond is favored in the upright fashion and a carbon-metal π bond is favored if the phenyl is tilted. For metallic surfaces (including gold), it was shown that in the upright position phenyl groups bind with an energy that is at least 3 times the 5-8 kcal/mol usually reported for van der Waals and hydrogen bonds. Metal-aryl bond energy can even be within the 50-250 kcal/mol range, a requisite for species to be considered as coupling agents.21 On the one hand, aryl groups form primary bonds with surfaces; on the other hand, they bind macromolecular species by covalent bonding (e.g., photoactivation for PS, ring opening of epoxy resin, or NHS/ EDC coupling procedure for proteins). It follows that aryl diazonium salts act as true coupling agents in the same sense as the more traditional silane coupling agents. The concept of diazonium salt coupling agents is new and only recently has it attracted the interest of materials scientists investigating reactive and functional polymer coatings, bioactive surfaces, chelatant supports, and so on. As far as we are concerned, we found that aryl diazonium salts are excellent alternatives for silanes because the latter bind polymers mostly to ceramics. Concerning thiols, although recent theoretical computations indicate comparable aryl-Au(111) and thiol-Au(111) bond strengths (maxima of 31.8 and 28.4 kcal/mol, respectively),22 experimental work by Shewchuk and McDermott23 provided direct evidence for the enhanced stability of diazonium-derived films over thiol monolayer analogues. Given the strong adhesion of polymers to aryl-modified substrates,15,18a the experimental spectroscopic proof for covalent bonding with surfaces,4 and DFT calculations,3 we undertook a series of studies pertaining to polymer grafts on surfaces via aryl layers. In this regard, we designed aryl diazonium salts that are able to surface-initiate atom-transfer radical polymerization (ATRP) on iron,18 sp2 graphene-based carbonaceous solids,19 ultrananocrystalline diamond plates,20a and diamond nanoparticles.20b In this way, it was possible to design high-performance materials with controlled hydrophilic/hydrophobic character,19a bioactive platforms,20b and electrochemical sensors based on carbon fiber/ chelatant polymer for the uptake of metal ions.19d Elsewhere, tandem aryl diazonium salts and surface-initiated ATRP served as the design of drug eluting stents24 whereas Combellas and coworkers have shown in an elegant way how such a combination permits one to prepare patterned polymer brushes.25 As alternatives to surface-initiated ATRP, diazonium salts were employed for surface-initiated reversible addition-fragmentation (21) Walter, P. In Handbook of Adhesive Technology, 2nd ed.; Pizzi, A., Mittal, K. L., Eds.; Marcel Dekker: New York, 2003; Chapter 10, pp 205-221. (22) de la Llave, E.; Ricci, A.; Calvo, E. J.; Scherlis, D. A. J. Phys. Chem. C 2008, 112, 17611–17617. (23) Shewchuk, D. M.; McDermott, M. T. Langmuir 2009, 25, 4556–4563. (24) Shaulov, Y.; Okner, R.; Levi, Y.; Tal, N.; Gutkin, V.; Mandler, D.; Domb, A. J. ACS Appl. Mater. Interfaces 2009, 1, 2519. (25) Hauquier, F.; Matrab, T.; Kanoufi, F.; Combellas, C. Electrochim. Acta 2009, 54, 5127. (26) Wang, G. J.; Huang, S. Z.; Wang, Y.; Liu, L.; Qiu, J.; Li, Y. Polymer. 2007, 48, 728. (27) Deniau, G.; Azoulay, L.; Bougerolles, L.; Palacin, S. Chem. Mater. 2006, 18, 5421. Tessier, L.; Deniau, G.; Charleux, B.; Palacin, S. Chem. Mater. 2009, 21, 4261.

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chain transfer (RAFT),26 surface-electroinitiated emulsion polymerization,27 and the preparation of conjugated polymer films by electrodeposition.28 Instead of these surface-confined polymerization methods, we have recently proposed the binding of oligoethylene glycols to aryl layers via the well-known Huisgen 1,3-cycloaddition reaction.17 Similarly, Evrard et al. explored diazonium salts to click proteins to glassy carbon electrodes.13 In a continuous effort to explore the surface reactivity and applications of aryl diazonium salts as a new class of coupling agents, we aimed at examining the propensity of 4-benzoylphenyl groups grafted on metallic substrates to surface initiate photopolymerization (SIPP) of a range of monomers instead of photografting prefabricated polymers.16 Indeed, this approach could be applied to a large variety of classical or specialty monomers, the corresponding polymers of which are not available commercially. SIPP is very well documented29-35 and has several advantages: it applies to a wide range of monomers without the necessity to remove the inhibitor; it is extremely fast (a few minutes could be enough to prepare ultrathin coatings); it can be performed in a variety of ways with either grafted photoinitiators or initiators in solution; it applies to plates and (nano)particles with different shapes (planar, curved, lamellar, etc.); and it is a low-cost method. In this context, the aim of this article is to surface initiate the photopolymerization of model vinylic monomers, namely, styrene, methyl methacrylate, and hydroxyethyl methacryalte by diazonium salt-derived aryl groups, on gold-coated silicon substrates. These substrates were electrografted with 4-benzoyl phenyl groups from parent diazonium salt BF4-, þN2-C6H4CO-C6H5 (BP). The Au-polymer hybrids were characterized by XPS, PM-IRRAS, and water contact angles. The surface interactions of the Au-polymer hybrids with bovine serum albumin (BSA), a model protein, were investigated by XPS and PMIRRAS in relation to the hydrophilic/hydrophobic character of the polymer grafts.

2. Experimental Section 2.1. Materials. Gold-coated silicon wafers with a thickness of about 1000 A˚ were purchased from Aldrich and cut into 1  2 cm2 slides. Just before use and in order to remove the organic residues on the surface, the slides were ultrasonically rinsed with acetone, water, and ethanol, dried in a stream of argon, cleaned in a UV cleaner (Boekel, Inc., model 135500), and rinsed with acetonitrile (ACN). Styrene (S, 105.15 g/mol, Fluka), methyl methacrylate (MMA, 100.11 g/mol, Acros), and 2-hydroxyethyl methacrylate (HEMA, 130.14 g/mol, Aldrich) were used without purification. 4-Aminobenzophenone (Alfa Aesar), N,N-dimethylaniline (DMA, Fluka), bovine serum albumin (BSA, Sigma), and a solution of Tween 20 were purchased from Aldrich and used as received. (28) Stockhausen, V.; Ghilane, J.; Martin, P.; Tripp
e-Allard, G.; Randriamahazaka, H.; Lacroix, J. C. J. Am. Chem. Soc. 2009, 131, 14920. Santos, L. M.; Ghilane, J.; Fave, C.; Lacaze, P. C.; Randriamahazaka, H.; Abrantes, L. M.; Lacroix, J. C. J. Phys. Chem. C 2008, 112, 16103. (29) Dyer, D. J. Adv. Polym. Sci. 2006, 197, 47, and references therein. (30) Steenackers, M.; K€uller, A.; Stoycheva, S.; Grunze, M.; Jordan, R. Langmuir 2009, 25, 2225. Schmelmer, U.; Paul, A.; K€uller, A.; Steenackers, M.; Ulman, A.; Grunze, M.; G€olzh€auser, A.; Jordan, R. Small 2007, 3, 459. Steenackers, M.; Lud, S. Q.; Niedermeier, M.; Bruno, P.; Gruen, D. M.; Feulner, P.; Stutzmann, M.; Garrido, J. A.; Jordan, R. J. Am. Chem. Soc. 2007, 129, 15655. (31) K
ız
ılela, S.; Sawardeckerb, E.; Teymourb, F.; P
erez-Luna, V. H. Biomaterials 2006, 27, 1209. (32) Janorkar, A. V.; Proulx, S. E.; Metters, A. T.; Hirt, D. E. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6534. (33) Maguis, S.; Laffont, G.; Ferdinand, P.; Carbonnier, B.; Kham, K.; Mekhalif, T.; Claude Millot, M. Opt. Express 2008, 16, 19049. (34) Bernand-Mantel, D.; Chehimi, M. M.; Millot, M. C.; Carbonnier, B. Surf. Interface Anal. In press. DOI: 10.1002/sia.3469. (35) Shah, D.; Fytas, G.; Vlassopoulos, D.; Di, J.; Sogah, D.; Giannelis, E. P. Langmuir 2005, 21, 19.

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Article Chloroform (CHCl3), methanol (MeOH), toluene, and phosphatebuffered saline (PBS, 10 mM, pH 7.4) were all analytical-grade reagents from Acros Organics and used as received. Water was deionized using a Millipore purification system.

2.2. Synthesis of the Diazonium Salt BF4-, þ N2-C6H4CO-C6H5. The synthesis of the starting diazonium salt (DS)

was conducted according to Pinson and co-workers.16,36 1H NMR (200 MHz, DMSO): δ 8.87 and 8.25 (d, 4H), aromatic group to the diazonium function, 7.5 to 7.8 (m, 5H aromatics) from the commercial 4-aminobenzophenone.

2.3. Electrochemical Reduction of the Diazonium Salt on a Gold Surface. Gold substrates were modified by the electro-

chemical reduction of BF4-, þN2-C6H4-CO-C6H5. The grafting reaction was carried out in a degassed solution of acetonitrile medium (ACN) containing 5 mM BP and 0.1 M supporting electrolyte tetrabutylammonium tetrafluoroborate (NBu4BF4, Alfa Aeser) using chronoamperometry for 300 s at a potential of -700 mV. After modification, the gold electrode (Au-BP) was rinsed by sonication in acetonitrile, ethanol, and dichlomethane and subsequently dried in a stream of argon. The Au-BP substrates were used as macrophotoactivators for SIPP. 2.4. Light Source. UV irradiation was carried out in the Spectrolinker XL 1500 UV (Spectronics Corp.) commercial ultraviolet processor. This processor was equipped with six tubes (8 W) having a wavelength range of 365 nm and an intensity of 17.6 mW/ cm2 at 365 nm. 2.5. Surface-Initiated Photopolymerization. The growth of polymer grafts on a BP-modified gold surface was performed as follows. A homogeneous mixture of monomer (4 mmol) and 4 wt % DMA in chloroform (4 mL) was prepared. The glass vessel containing Au-BP plates dipped in a polymerization mixture was degassed by bubbling with argon for 5 min. The slides were then exposed to UV light at 365 nm at room temperature for a period of 100-1000 s to prepare PHEMA grafts and for 800 s for PMMA. We found that for styrene a much longer photopolymerization time was necessary; we set it to 2 h. After irradiation, the slides were taken out, thoroughly sonicated in chloroform for 4 min to remove the unreacted monomer, and then washed with ethanol and dichloromethane to remove organic species. The polymercoated slides were dried and stored under argon.

2.6. Adsorption of Bovine Serum Albumin on Polymer Grafts. The gold-grafted polymers were incubated for 24 h at room temperature in 10 mL of a PBS solution (pH 7.4) of bovine serum albumin (400 μg/mL). The plates were then thoroughly rinsed with a 5% v/v aqueous solution of Tween 20 to remove loosely bound protein and then with distilled water. The Aupolymer-BSA specimens were analyzed by attenuated total reflection infrared spectroscopy and XPS. 2.7. XPS. The spectra were recorded using a Thermo VG Scientific ESCALAB 250 system fitted with a microfocused, monochromatic Al KR X-ray beam (1486.6 eV, 500 μm spot size). The samples were stuck on sample holders using conductive double-sided adhesive tape and pumped overnight in the fast entry lock at ∼5  10-8 mbar prior to transfer to the analysis chamber. Avantage software, version 3.51, was used for digital acquisition and data processing. The analyses were performed without the assistance of the static charge-compensation flood gun. The spectra were calibrated against the Au 4f7/2 peak at 84.0 eV. However, in the case of Au-PHEMA and Au-PMMA, whereas the Au substrate exhibited a negligibly small charging effect, the top polymer layer had a positive static charge of ∼0.4 eV. The surface composition was determined using the integrated peak areas and the corresponding Scofield sensitivity factors corrected for the analyzer transmission function. Additional C 1s peak fittings were performed using peak components of comparable full width at half maxima and Lorentzian shape in the 0-30% range. (36) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Sav
eant, J. M. J. Am. Chem. Soc. 1997, 119, 201.

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Gam-Derouich et al. The lateral uniformity of PHEMA grafts along the main axis of the plate sample was checked using a 120 μm X-ray spot and setting the step size to 200 μm. In this way, it was possible to analyze about 90 small regions that are located over an 18 mm line. 2.8. PM-IRRAS. PM-IRRAS spectra were recorded with a Nicolet 860 FTIR (Thermo-Electron) spectrometer with a resolution of 8 cm-1 by adding 2000 scans with an optical mirror velocity of 0.474 cm-1/s. For full details of the PM-IRRAS measurements, see ref 17. 2.9. Contact Angle Measurements. Water contact angles were determined with a Kr€ uss DSA 100 instrument (Hamburg, Germany) fitted with a drop-shape analyzer. The DSA 100 traces the profiles of a sessile drop, from which the contact angle is computed. The plates were mounted on a sample holder placed between a horizontal light source and a CCD video camera. Live video images of the samples are obtained by adjusting the lighting and focus. After loading water in the syringe, droplets were gently placed on the glass plates under an optical vessel to minimize evaporation. The entire system was located in a thermostatted chamber at 25.0 ( 0.2 °C. The drops were left to interact with the glass plates for 20 min, a period over which the contact angles were automatically measured every 5 s. 2.10. AFM. Untreated, electrochemically modified, and polymer-coated gold surfaces were imaged by a Nanoscope III Digital Instrument in tapping mode using a Si3N4 tip cantilever. The cantilever oscillation frequency was set at 320 kHz. The tips of the cantilever were characterized by their radius of curvature, which was equal to 7 ( 2 nm. No computer filtering procedure was used to treat the images. Tapping-mode imaging was recorded with 256 pixels per line at a scan rate of 1.2 Hz.

3. Results and Discussion 3.1. Strategy of Surface-Confined Photopolymerization Initiated by Grafted Aryl Diazonium Photoinitiators. Aupolymer hybrids were obtained in two steps by the diazonium salt electrografting/photopolymerization protocol. First, 4-benzoylphenyl groups (BP) were electrografted onto the gold surface; second PMMA, PS, and PHEMA were prepared by SIPP using UV irradiation at a wavelength of 365 nm. The overall synthesis process is illustrated in Scheme 1. 3.2. Electrografting of the Photoinitiator onto a Gold Surface. The noncommercial 4-benzoyl benzenediazonium tetrafluoroborate was readily electrochemically reduced on gold surfaces to yield attached 4-benzoylphenyl (-C6H4-CO-C6H5, BP) groups as shown in Scheme 1. Cyclic voltametry was used for the determination of the electrochemical reduction potential. One can observe a broad, irreversible monoelectronic wave at Epc= -350 mV/SCE, which corresponds to the reduction of the diazonium salt (Figure 1A, plot a). The electron transfer is concerted with the cleavage of dinitrogen, giving an aryl radical that strongly binds to the gold surface,37-39 and the height of this wave decreases dramatically on the second scan (Figure 1A, plot b), which is a sign of the grafting of the aryl radical. For the purpose of this work and once the reduction peak potential was determined, all gold surfaces were electrochemically pretreated by chronoamperometry for 300 s at a potential of -700 mV, reference SCE (Figure 1B). The very steep decrease in the current with time is characteristic of the formation of the organic layer, which hampers the electron transfer from the electrode. The electrochemically (37) Bernard, M. C.; Chausse, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15, 3450. (38) Benedetto, A.; Balog, M.; Viel, P.; Le Derf, F.; Sall
e, M.; Palacin, S. Electrochim. Acta 2008, 53, 7117. (39) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. J. Am. Chem. Soc. 2008, 130, 8576.

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Scheme 1. Strategy of Tandem Diazonium Salt Electrografting and Surface-Initiated Photopolymerizationa

a 4-Benzoylphenyl initiators are attached to a gold surface via electrochemical reduction of the parent DS and serve to graft photopolymerized monomers (here S, MMA, and HEMA) using dimethylaniline as a co-initiator (H donor). Note that aryl groups generally do not form monolayers but multilayers. Here we display a single 4-benzoylphenyl group attached to the surface for the sake of clarity.

Figure 1. (A) Cyclic voltammogram of a gold-coated silicon wafer grafted with 5 mM DS in ACN þ 0.1 M NBu4BF4, v = 0.2 V s-1. Reference SCE. (B) Chronoamperometry of a gold plate grafted with 5 mM DS in ACN þ 0.1 M NBu4BF4, 700 mV negative potential, t = 300 s, and v = 0.2 V s-1. Reference SCE.

treated gold plates (Au-C6H4-CO-C6H5, Au-BP in short) served as photoinitiators for SIPP of S, MMA, and HEMA. 3.3. Preparation and Surface Characterization of Polymer Grafts. In SIPP, solvent40 and UV irradiation time41 are two important parameters that can affect the thickness of the grafted (40) Ma, H.; Davis, R. H.; Bowman, C. N. Polymer 2001, 42, 8333. (41) Rohr, T.; Hilder, E. F.; Donovan, J. J.; Svec, F.; Fr
echet, J. M. J. Macromolecules 2003, 36, 1677.

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film. The influence of these principal factors on SIPP was tested with HEMA in the presence of Au-BP and the co-initiator. First, grafting the BP layer to gold has been investigated by XPS. Figure 2a shows a drastic change in the XPS survey region of Au: Au-BP exhibits an attenuation of Au4f and a relative increase in the C 1s peak intensity. The O 1s signal is now visible in comparison to Au. One can also note the presence of an N 1s peak at ∼400 eV that is invariably detected for diazonium-treated surfaces. It is assigned to -NdN- azo linkages within the aryl DOI: 10.1021/la100880j

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Figure 2. XPS analysis of the solvent effect on SIPP of HEMA: (a) survey scans of Au and Au-BP; (b) survey regions of Au-BP-PHEMA hybrids prepared in various solvents; and (c) C 1s/Au 4f intensity ratio for SIPP of HEMA in various solvents. Note that in part a C 1s at the Au surface is due to adventitious hydrocarbon contamination.

Figure 3. Lateral uniformity of PHEMA grafts prepared in chloroform by photopolymerization initiated on a gold surface by 4-benzoylphenyl electrografted groups: (a) survey scans and valence band (inset) and (b) C and O atom % and C/O atomic ratio.

layer.23 This peak vanishes for a short time during chronoamperometric grafting of the aryl groups, which proves that nitrogen atoms are within the aryl layer and not at the gold-aryl interface (Supporting Information SI1). Grafted BP groups resulted in an ∼8-fold increase in the C/Au atomic ratio and an increase in the inelastically scattered photoelectrons from gold (increase in the background intensity). The effect of solvent on the SIPP of HEMA was studied in chloroform, toluene, and ethanol. Figure 2b displays XPS survey scans of Au-BP-PHEMA hybrids in different solvents. The Au core-hole peaks are clearly detected in the case of ethanol, which implies weak attenuation due to a very thin PHEMA layer. For toluene, the Au photopeaks are no longer visible but still yield an increase in the background intensity in the 84-533 eV apparent binding energy range, that is, in the 1402-953 eV kinetic energy range (region corresponding to the inelastic energy loss of Au 4f core-hole electrons). In the case of chloroform, the survey spectrum has a horizontal background with a quasi-total absence of Au peaks and without any sign of inelastically scattered photoelectrons from the underlying gold, an indication of the massive grafting of PHEMA. Au-BP-PHEMA even has a wide

scan that is similar to that of a pure PHEMA surface.42 The decreasing trend in the C 1s/Au 4f intensity ratio is CHCl3 > toluene . EtOH (Figure 2c). These findings indicate more efficient SIPP in aliphatic and aprotic solvents.43,44 In the case of SIPP in ethanol, a hydrogen atom may be abstracted from the protic solvent, leading to a possible termination reaction that limits the growth of the PHEMA grafts.44 The lateral uniformity of PHEMA grafts was investigated via the line-scan procedure. Figure 3 shows the survey scans (Figure 3a) and the carbon and oxygen atom % (Figure 3b) from PHEMA grafted onto a gold surface. All spectra were taken over an 18-mm-long line. The spectra have, from one spot to another, nearly the same structure and intensity and almost identical C/O ratios of around 2, emphasizing the uniform lateral growth of PHEMA on the gold surface via SIPP. The inset in Figure 3a shows the valence band region over 0-40 eV recorded for AuBP-PHEMA. This valence band region is a fingerprint of the polymer and has exactly the same structure as that published for pure PHEMA.42 The second parameter that can affect the growth of polymers is the irradiation time. Figure 4 shows the effect of SIPP time on the

(42) Beamson, G., Briggs, D., Eds. High Resolution XPS of Organic Polymers. The Scienta ESCA300 Database; John Wiley: Chichester, U.K., 1992.

(43) Wang, H.; Brown, H. R. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 263. (44) Deng, J.; Wang, L.; Liu, L.; Yang, W. Prog. Polym. Sci. 2009, 34, 156.

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Figure 4. Effect of irradiation time on the C/O atomic ratio determined by XPS for Au-PHEMA hybrids. At t = 0, the sample is Au-BP.

C/O ratio for the polymerization of PHEMA in chloroform. The C/O atomic ratio decreases to 2.2, which matches the value for pure PHEMA. Therefore, in CHCl3, 200 s is enough to grow PHEMA films that completely screen the underlying gold substrate. Figure 5 displays the surface morphology of Au, Au-BP, and Au-BP-PHEMA slides as imaged by AFM. First, the change from untreated gold to Au-BP results in an increase in the maximum height. However, the grafted 4-benzoyl phenyl layer (in Au-BP) fits the morphology of the bare gold. An important change is obtained after the SIPP of HEMA; indeed Au-BPPHEMA exhibits a thick but porous structure with a height of up to 300 nm. This conforms to the XPS spectrum (in Figure 2b) generated by the PHEMA graft that is identical to the spectrum obtained from a few-micrometers-thick PHEMA film prepared by solvent casting.42 SIPP was tested with two other monomers (S and MMA) under the same optimal conditions found for PHEMA, that is, in chloroform and by setting the polymerization time to 800 s, except for styrene for which a much longer time was necessary (2 h). Figure 6 shows the survey scans (Figure 6a) of Au-BP-PS, Au-BP-PMMA, and Au-BP-PHEMA prepared in CHCl3 and the well-known QUASES method of Tougaard45 for the assessment of overlayer thickness (Figure 6b). The main peaks C 1s, O 1s, and Au 4f7/2 are centered at 285, 533, and 84 eV, respectively. Survey regions, from Au-BP-PMMA and Au-BP-PS, exhibit important inelastic backgrounds because of the attenuation of gold slides by polymer coatings. In the case of Au-BP-PHEMA, gold core-level peaks have vanished and the inelastic background is flat. The background of Au-BP-PHEMA (Figure 6b) was fitted using QUASES software (www.QUASES.com, see Supporting Information SI2). The red box indicates the start and the end depths where the polymer is located. The software is limited to 100 nm, so XPS also indicates an important grafting of PHEMA under optimal conditions. This QUASES result supports the important heights (up to 300 nm) observed in the AFM of AuBP-PHEMA (Figure 5). Table 1 reports the XPS-determined surface chemical compositions of the polymer-modified gold slides and the organic coating (BP-polymer) thickness determined using QUASES. (45) (a) Tougaard, S. J. Vac. Sci. Technol., A 1996, 14, 1415. (b) Tougaard, S. Surf. Interface Anal. 1998, 26, 249.

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Figure 5. AFM images of Au, Au-BP, and Au-BP-PHEMA prepared in chloroform.

All Au/C atomic ratios are much lower than 1.9 determined for gold. O/C decreases for PS grafts because this polymer contains no oxygen atoms. However, because PMMA and PHEMA have higher O/C ratios than the grafted BP (theoretical value = 0.08), Table 1 indicates for the polymethacrylates O/C ratios higher than that found for Au-BP. However, for PMMA grafts the ratio is slightly lower than the expected 0.4. Similarly, for AuBP-PHEMA, the O/C ratio is slightly lower than the expected 0.5. These results are probably due to some unavoidable adventitious hydrocarbon contamination from the atmosphere or from residual gases in the XPS analysis chamber.42,46 The total absence of Cl 2p (200 eV) in the survey spectra rules out any possible contamination by chloroform. As far as the thickness is concerned, the results indicate that the combination of diazonium salts and SIPP permit one to obtain ultrathin polymer grafts. As we have shown above, control over the HEMA polymerization time can yield a thickness well below 100 nm. Figure 7 displays the C 1s high-resolution regions from the polymer grafts and macroinitiator Au-BP. (See Supporting Information SI3 for peak-fitting parameters.) The Au-BP substrate has a C 1s peak fitted with four components centered at 285 (C-C/C-H), 286.7 (C-O/C-N), 288 (CdO), and 291.2 eV (π f π*) for the shakeup satellite transition. The latter is a fingerprint of the attachment of the aromatic aryl groups. The C 1s shakeup satellite and the ketone function (CdO) testify to the successful electrografting of the -C6H4-CO-C6H5 photoinitiator groups. Note that in the case of Au-BP there is a 286.7 eV component assigned to C-O and C-N bonds. C-O could be due to surface contamination and/or oxidation of the aryl layers as suggested elsewhere20a whereas the C-N bonds are due to the azo linkages within the layer: C(aryl)-NdN-C(aryl). Interestingly, and as mentioned above and in SI1, short chronoamperometric grafting resulted in survey spectra of Au-BP without any sign of the N 1s peak. For this reason, the C 1s peak acquired for Au-BP after 1 s of electrochemical treatment shows a better-resolved C 1s (46) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis: By Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; Wiley: Chichester, U.K., 1983; Vol. 1, p 543.

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Figure 6. (a) Survey scans of gold slides after the growth of polymer chains in chloroform and (b) background fitting by QUASES of the Au-BP-PHEMA near the O 1s region. Table 1. Apparent Surface Chemical Compositions of Au-BP-PS, Au-BP-PMMA and Au-BP-PHEMA As Determined by XPS Au/C

O/C

thickness (A˚)a

Au-BP-PS 0.30 0.19 18 Au-BP-PMMA 0.075 0. 31 50 -5 0.44 >1000 Au-BP-PHEMA 4  10 a See Supporting Information SI2 for parameters employed in determining the overlayer thickness using QUASES.

peak because of the absence of the C-N component. This is a second proof that the azo linkages are within the aryl layer and not at the substrate-aryl interface (Supporting Information SI1). This result supports the grafting mechanisms proposed by Shewchuk and McDermott23 and ToF-SIMS studies47 demonstrating the existence of these azo groups in the polyphenylene chains. The question that still arises concerning aryl groups pertains to their effective covalent bonding to gold. In the ideal case of a monolayer, one would obtain Au-C13H9O. It follows that 1 carbon atom among 13 binds to the surface. However, Pinson and co-workers have proven by ToF-SIMS that the aryl groups have a polyphenylene structure,47,48 which implies that the C-Au carbon types have a relative concentration well below 1/13. Elsewhere, we have shown by XPS that ATRP initiators based on diazonium salts form an oligophenylene-like layer with up to six repeat units.18b Therefore, it is extremely difficult to distinguish the C 1s component because of the interfacial C-Au from the numerous other carbon atoms that are near the interface. This contrasts with the thiol situation where sulfur is a unique elemental marker for the molecules. Therefore, any specific interaction with gold would result in a detectable XPS shift. Returning to diazonium salts and even in the case of a high relative concentration of C-Au bonds, we believe it remains difficult to prove by XPS the existence of such a bond, probably because gold and carbon have comparable electronegativity values. Tentatively, we fitted the C 1s spectrum from Au-BP obtained after only 1 s of chronoamperometric grafting with a component centered at ∼284.3 eV assigned to C-Au, (See Supporting Information SI4 for the fitted spectrum and discussion.) In Figure 7, PS graft exhibits the characteristic (π f π*) shakeup satellite transition. However, between the main C-C/C-H and (47) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Chem. Mater. 2007, 19, 4570. (48) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Langmuir 2005, 21, 280.

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the (π f π*) shakeup satellite, there are intermediate components arising from the underlying Au-BP because Figure 6 shows a survey spectrum of Au-BP-PS with Au photopeaks. It follows that BP groups are also detected. The shakeup of the C-C/C-H atomic ratio is ∼0.07 for Au-BP-PS, which is slightly lower than 0.1 reported for pure PS.42 For PMMA and PHEMA grafts, the ketone contribution is zero, meaning that the BP initiators are very well screened or are below the detection limit. The (C-C/C-H)/ C-O/O-CdO contributions are in the ratios of 4:1.5:1 and 3.1:2.1:1, to be compared to 3:1:1 and 3:2:1 for pure PMMA and PHEMA, respectively. For PMMA, there is a sign of possible surface contamination whereas for PHEMA the C 1s peak fitting reflects pure PHEMA. The high-resolution O 1s regions from Au-BP, Au-BP-PMMA, and Au-BP-PHEMA are displayed in Supporting Information SI5. Grafted BP exhibits an O 1s region centered at 532.0 eV, in line with CdO groups, and a shoulder at 533.5 eV. The latter is due to surface contamination and/or oxidation of the aryl layer, as demonstrated for -C6H4-CH(CH3)Br groups on nitrogendoped diamond surfaces.20a For this reason, the Au-BP survey scan in Figure 2 exhibits quite a large O 1s/C 1s intensity ratio. PMMA grafts have a well-resolved doublet with components centered at about 532.3 and 533.8 eV corresponding to CdO and O-CH3, respectively. As far as PHEMA grafts are concerned, the O 1s region is unresolved because of the additional hydroxyl (OH) O 1s core-level peak expected at 533 eV,42 which is in an intermediate position between those of the CdO and the O-CH2 O 1s peaks. At this stage, it is essential to know if the grafted BP and the coinitiator are indeed important for SIPP or if the grafts can be prepared spontaneously under UV light. Two samples were prepared in this manner: Au was dipped into the SIPP medium containing HEMA and co-initiator DMA and Au-BP dipped into the SIPP medium containing HEMA but lacking DMA. The C 1s regions in the latter (Supporting Information SI6) have structures that completely differ from those of the polymer grafts prepared using the Au-BP macrophotoinitiator in the presence of the DMA co-initiator. It is also interesting to know whether 4-benzoyl phenyl groups could be simply physisorbed on gold and could initiate SIPP. To address this question, we deliberately physisorbed pure benzophenone (BPphys) on gold plates that were then washed in ethanol prior to SIPP. Supporting Information SI7 compares the survey spectra obtained with grafted and physisorbed BP. With physically Langmuir 2010, 26(14), 11830–11840

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Figure 7. Peak-fitted C 1s regions from Au-BP, Au-BP-PS, Au-BP-PMMA, and Au-BP-PMEMA.

adsorbed BP, Au-BPphys and Au-BPphys-PHEMA have similar survey spectra comparable to that of clean gold shown in Figure 2a, and the C 1s region from Au-BPphys-PHEMA differs completely from that of PHEMA. These results stress the paramount importance of efficient grafting of the photoinitiator to the surface and have implications on the interaction of PHEMA grafts with biomacromolecules. (See below.) The chemical composition changes at the surfaces of gold plates were further investigated by infrared spectroscopy. The PM-IRRAS spectra of Au-BP and the KBr pellet IR diazonium salts DS are displayed in Supporting Information SI8. In the 2800-1500 cm-1 spectral domain (SI8, part b) the spectrum of Au-BP lacks the band at 2294 cm-1 characteristic of the þN2 group, which indicates that 4-benzoyl benzenediazonium tetrafluoroborate has indeed been grafted from the reduction of the parent diazonium salt onto a gold surface.16 There are similarities between these two spectra in the 6002000 cm-1 spectral region (SI8, part a). The presence of 4-benzoylphenyl groups on the surface is particularly evidenced by the presence of the carbonyl band at 1657 cm-1 and also by the inplane CH bending vibrations between 1315 and 1278 cm-1. However, the differences can be noticed between the two spectra. The aromatic CdC stretching vibrations at 1596 cm-1 become very weak in Au-BP. Also, most interesting is the presence of the 740 cm-1 band in the reference spectrum of 4-benzoyl benzenediazonium tetrafluoroborate that is typical of the out-of-plane CH deformation of monosubstituted benzene and indicates the Langmuir 2010, 26(14), 11830–11840

presence of five adjacent hydrogen atoms. This band becomes very weak in the spectrum of Au-BP, and new structural signals appear at 832 cm-1 that are indicative of a 1,4-disubstituted benzene ring. This confirms that the reduction of the diazonium group has indeed occurred. In addition, it shows that, on the one hand, aryl groups attach to the surface and, on the other hand, form oligomers.16 Figure 8 displays the PM-IRRAS spectra of Au-BP-PMMA, Au-BP-PS, and Au-BP-PHEMA. In the 800-2000 cm-1 spectral domain (Figure 8a), PMMA and PHEMA grafts exhibit important spectral similarities characteristic of polymethacrylates: (i) a sharp CdO stretch at 1735 cm-1 due to the ester carbonyl group of polymethacrylates and polyacrylates and (ii) complex bands centered at 1150 and 1267 cm-1 characteristic of the C-O stretching vibrations. In addition, convoluted peaks centered at 1449 cm-1 are due to the -CH2- and C-CH3 deformations.18a The striking difference between the two polymethacrylate grafts is in the 2000-3800 cm-1 spectral domain (Figure 8b): PHEMA grafts exhibit an OH stretching vibration broad band at 3200-3600 cm-1 (Figure 8b), but PMMA grafts do not. The peaks at 2947, 2935, and 2850 cm-1 are characteristic of symmetric and antisymmetric vibrations of CH2 and CH3 of PHEMA and PMMA. The Au-BP-PS spectrum presents a series of bands which are characteristic of polystyrene. In Figure 8b, two bands at 3022 and 3062 cm-1 are due to aromatic CH stretching vibrations and three bands between 2945 and 2850 cm-1 are due to methylenic stretching. DOI: 10.1021/la100880j

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Figure 8. PM-IRRAS spectra of Au-BP-PHEMA, Au-BP-PS, and Au-BP-PMMA in (a) the 800-2000 cm-1 region and (b) the 2000-3800 cm-1 region.

Figure 9. Plots of water contact angle vs contact time for Au-BP-PS, Au-BP-PMMA, and Au-BP-PHEMA.

In the 800-2000 cm-1 spectral domain (Figure 8a), typical peaks for polystyrene surfaces shown at 1601, 1494, and 1453 cm-1 are due to aromatic ring stretching.49 The hydrophilic/hydrophobic properties of polymer grafts were investigated by water contact angle measurements. Figure 9 shows θw versus time plots for the Au-BP-PS, Au-BP-PMMA, and Au-PHEMA hybrids and the corresponding live video images representative of the various water drop-substrate systems. The initial water contact angles θw were found to be 44.5, 57, and 78° (all values (1°) for Au-BP-PHEMA, Au-BP-PMMA, and Au-BP-PS, respectively. The PS graft is significantly more hydrophobic than PMMA, which is in line with the published water contact for pure PS 80°.50 The Au-BP-PHEMA hybrid is the most hydrophilic because of the OH groups in the HEMA repeat units. PHEMA is well known to be a relatively hydrophilic polymer, a characteristic that imparts a substantial resistance to nonspecific protein adsorption PHEMA when it is grafted on glass slides or silica particles.51,52 (49) Benabderrahmane, S.; Bousalem, S.; Mangeney, C.; Azioune, A.; Vaulay, M. J.; Chehimi, M. M. Polymer. 2005, 46, 1339. (50) Gallardo-Moreno, A. M.; Gonz
alez-Mart
ıı´ n, M. L.; P
erez-Giraldo, C.; Gardu~no, E.; Bruque, J. M.; G
omez-Garcı´ a, A. C. Appl. Environ. Microbiol. 2002, 65, 2610. (51) Mrabet, B.; Nguyen, M. N.; Majbri, A.; Mahouche, S.; Turmine, M.; Bakrouf., A.; Chehimi, M. M. Surf. Sci. 2009, 603, 2422. (52) Tsukagoshi, T.; Kondo, Y.; Yoshino, N. Colloids Surf., B 2007, 54, 101.

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Figure 10. PM-IRRAS

spectra of Au-BP-PMMA, Au-BP-PS, and Au-BP-PHEMA after interaction with BSA and rinsing with Tween 20 and water and a KBr pellet IR of BSA.

It is interesting to note that despite the control of temperature and water liquid/vapor equilibria, the contact angles tended to decrease, probably because of the porous aspect of the polymer graft surface shown by AFM (Figure 5). However, the difference in contact angle (Δθ) between the initial and final contact times decreases in the order PHEMA (Δθ = 19°) > PMMA (Δθ = 17°)>PS (Δθ = 10.5°), which is the exact trend of hydrophilic character as measured by the simple contact angle θw. A faster decrease of θw on PHEMA grafts reflects the hydrophilic character of this polymer. Surface wettability by water drops is in line with the chemical structure of the ultrathin polymer layers grown by aryl-initiated surface photopolymerization. PHEMA has a stronger hydrophilic character with its pendant OH groups than the sole ester groups of PMMA. The latter is in turn more hydrophilic than PS, which could weakly interact specifically with water via its π electrons. These trends in wettability of the organic polymer grafts prepared so far indicate that the latter behave as pure thick films or sheets. 3.4. Interaction of Proteins with Polymer Grafts. The effect of the hydrophilic/hydrophobic character of polymercoated gold substrates was further evaluated by BSA adsorption. Langmuir 2010, 26(14), 11830–11840

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Figure 11. Peak-fitted C 1s regions from BSA and from Au-BP-PS, Au-BP-PMMA, and Au-BP-PHEMA after BSA adsorption.

Figure 10 displays the 1900-1200 cm-1 region of the PM-IRRAS spectra of Au-BP-PMMA, Au-BP-PS, and Au-BP-PHEMA after interaction with BSA. Au-BP-PS and Au-BP-PMMA exhibit two new narrow bands at 1674 and 1545 cm-1 that are assigned to amide groups from BSA. Though less clear, the amide band peaks at 1545 cm-1 are slightly better detected in the case of polystyrene. At this spectral position, no similar signal is observed for PHEMA grafts. This difference confirms literature53 reports indicating that the BSA adsorption was more favorable on the hydrophobic polymer grafts. The XPS spectra of the same surfaces also showed significant modifications after incubation with BSA. Figure 11 displays the C 1s regions of Au-BP-PMMA, Au-BP-PS, and Au-BPPHEMA. Before BSA attachment, PS brushes have a C 1s structure comparable to that of pure PS with the main feature at 285 eV and a shakeup satellite at 291.5 eV. After protein adsorption, two prominent shoulders were observed at 286.5 and 288 eV corresponding to C-N/C-O bonds and amide groups, respectively. For PMMA, the changes are less significant because PMMA already has oxidized carbon atom types (C-O and O-CdO), the components of which are centered at 286.5 and 289 eV, respectively. However, there is evidence for an additional component at 288 eV due to the peptidic linkages (N-CdO) of the protein. For PHEMA, the spectra remain unchanged. These high-resolution C 1s spectra before and after attachment of protein are identical and exhibit a fine structure that is comparable to that reported for pure PHEMA.42 This stresses the total (53) Murthy, R.; Shell, C. E.; Grunlan, M. A. Biomaterials 2009, 30, 2433.

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resistance of the as-prepared PHEMA graft to nonspecific BSA adsorption. To monitor the protein adsorption onto the polymers grafts, one can simply use the surface (O þ N)/C ratio (noted R). The ratio can then be compared before (R°) and after (R) adsorption, and the normalized increase was set as   R - R°  100% RE ¼ R° The elemental descriptor RE is expected to be high in the case of BSA adsorption because all proteins are indeed nitrogen- and oxygen-rich. Another way to monitor BSA adsorption is to plot the increase in the sum of contributions of C-O/C-N and N-CdO components to the C 1s regions. (See the Table in SI3 for substrates before BSA adsorption.) The spectra displayed in Figure 11 were peak fitted with four components centered at 285, 286.5, 288, and 289 eV corresponding to chemical environments C-C/C-H, C-O/C-N, CdO/N-CdO, and O-CdO, respectively. For PS, it was necessary to add a π-π* satellite at 291.5 eV. For BSA, the most important components are the C-O/C-N and the N-CdO components at 286.5 and 288 eV, respectively. These peak components are noted as C286.5 and C288. It is worth noting that there is a massive change in the C 1s structure from PS grafts: prominent C286.5 and C288 are clearly visible for the BSA-coated Au-BP-PS hybrid compared to the pristine PS graft. For PMMA, there is a slight increase in C286.5. More importantly, a feature centered at ∼288 eV, which is not available for pristine DOI: 10.1021/la100880j

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Figure 12. Elemental and chemical descriptors, RE and RC, of BSA adsorption versus water contact angles for Au-BP-PS, Au-BP-PMMA, and Au-BP-PHEMA substrates.

PMMA, can be assigned to only peptidic links from BSA. The C288 component reflects then the immobilization of BSA onto PS and PMMA. Instead of the elemental descriptor RE defined above, one can thus determine a chemical descriptor from the C 1s peak fitting, defined as RC ¼

½C286:5 þ C288  - ½C286:5 þ C288 ° ½C286:5 þ C288 °

 100%

Figure 12 shows plots of RC and RE versus the initial water contact angles. The XPS elemental and chemical descriptors of BSA adsorption increase linearly with the water contact angle on the polymer grafts. This is strong evidence that the polymers grafts prepared by this approach reflect the nature of the polymers and the effect of their hydrophilic/hydrophobic character on protein adsorption, which is obviously driven by interfacial hydrophobic interactions.54 To verify the importance of electrografting BP and the subsequent efficient SIPP on the reactive and functional aspects of PHEMA grafts, BSA was adsorbed onto the sample noted AuBPphys-PHEMA prepared by SIPP of HEMA in the presence of gold that had possibly been modified by physisorbed benzophenone. The XPS results indicated 72 and 71% for elemental and chemical descriptors RE and RC whereas they equal 0 and ∼10% (Figure 12) in the case of electrografting BP groups, followed by SIPP (Au-BP-PHEMA specimen).

4. Conclusions Surface-initiated photopolymerization (SIPP) was conducted on gold surfaces modified by the electrochemical reduction of the aryl diazonium salt BF4-, þN2-C6H4-CO-C6H5. The grafted

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4-benzoyl phenyl (BP) groups were found to be very effective in SIPP of styrene, methyl methacrylate, and hydroxyethyl methacrylate on gold to provide model PS, PMMA, and PHEMA grafts. The covalent attachment of BP species was evidenced by PM-IRRAS whereas XPS confirmed the aromatic nature of the grafted BP and the presence of ketone groups. SIPP was very efficient in chloroform and to a lesser extent in toluene (both nonprotic solvents, confirming the free radical mechanism). Massive grafting of PHEMA was achieved in about 200 s, and the screening of gold decreased in the order of PHEMA > PMMA . PS. The hydrophobic character of the grafts decreased in the order of PS >PMMA >PHEMA as judged from contact angle measurements and was found to govern nonspecific protein adsorption. XPS and PM-IRRAS results have indeed shown that PHEMA grafts resisted bovine serum albumin adsorption. The results obtained so far show that this new protocol combining diazonium salt-based photoinitiators and SIPP is a new, fast, elegant way to prepare ultrathin polymer grafts with antifouling and other potential properties. All in one, the process of surface photopolymerization initiated by grafted 4-benzoyl phenyl moieties takes less than 1 h (e.g., for PHEMA), starting from the as-received gold plate to the end gold-polymer hybrid. This is an important aspect of SIPP employing diazonium saltbased photoinitiators as the latter covalently bind to the surface in less than 5 min, via the UV/ozone cleaning procedure in less than 15 min, and via SIPP in less than 5-10 min. Nevertheless, it would also be important to compare the practical adhesion of polymer grafts prepared by this process to that obtained using thiols and other linkers. Beyond these results, this work broadens the enormous possibilities offered by diazonium salts as true novel coupling agents for polymers and thus for the design of functional organic coatings. Acknowledgment. We are indebted to Professor Jean Pinson (Emeritus Professor at University Paris Diderot and ESPCI) for helpful discussions. Mrs. S. Truong and Mrs. C. Connan are acknowledged for their assistance with AFM and XPS analyses, respectively. Supporting Information Available: Survey scans of gold slides. Contributions (in %) of functional groups to C 1s regions from Au-BP and gold-grafted polymers. Peak-fitted C 1s regions from Au-BP after 1s of electrochemical treatment. O 1s regions from Au-BP, Au-BP-PHEMA, and Au-BP-PMMA. Effect of photoinitiator and co-initiator for PHEMA photografting (SIPP) by XPS. Comparison of survey and C 1s spectra obtained with physisorbed and electrografted 4-benzoylphenyl groups. PM-IRRAS spectra of Au-BP and the KBr pellet IR of diazonium salt. This material is available free of charge via the Internet at http:// pubs.acs.org. (54) Azioune, A.; Chehimi, M. M.; Miksa, B.; Basinska, T.; Slomkowski, S. Langmuir 2002, 18, 1150.

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