Surface Functionalization of Ultrananocrystalline Diamond Films by

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Surface Functionalization of Ultrananocrystalline Diamond Films by Electrochemical Reduction of Aryldiazonium Salts Jian Wang,† Millicent A. Firestone,†,‡ Orlando Auciello,†,‡ and John A. Carlisle*,†,‡ Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439 Received May 20, 2004. In Final Form: August 16, 2004 The surface functionalization of ultrananocrystalline diamond (UNCD) thin films via the electrochemical reduction of aryl diazonium cations is described. The one-electron-transfer reaction leads to the formation of solution-based aryl radicals, which in turn react with the UNCD surface forming stable covalent C-C bonds. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), ac impedance spectroscopy, and contact angle measurements have been employed to characterize the organic overlayer and estimate the surface coverage. The grafting of 3,5-dichlorophenyl groups renders the UNCD surface hydrophobic, whereas the attachment of 4-aminophenyl groups makes the surface relatively hydrophilic. The surface coverage, estimated from the electrochemical and XPS measurements, is as high as 70% of a compact monolayer. The aminophenyl terminated surface was obtained by electrochemical reduction of the tethered nitrophenyl groups. This two-step approach yields a UNCD surface with functional moieties available for the potential covalent coupling of a wide variety of biomolecules (e.g., DNA and proteins).

Introduction The coupling of biological molecules with inorganic materials to form hybrid bioinorganic interfaces has attracted considerable attention because of their potential importance in several areas of technological interest, including biosensors, biomedical implants, and organic/ biomolecular electronics.1-5 To date, the development of these interfaces has been primarily based on silicon,3 metal oxides,4 or gold5,6 platforms because of their intrinsic electronic properties and established micromachining and photopatterning technologies. Full implementation of these materials has been limited since the coupling strategies used to immobilize a biomolecule of interest (i.e., the bioinorganic interfaces) often suffer from longterm instability in aqueous environments. For one-timeuse devices, this instability may not be critical, but for biosensors/bioimplants that must work continuously in harsh operational environments, the robustness of the immobilization chemistry is of fundamental importance. Furthermore, the lack of biocompatibility of silicon is problematic. Several studies have revealed fibrosis and over-formation of scar tissue around silicon implants, thus limiting the long-term functioning of the microdevices.7,8 † ‡

Materials Science Division. Center for Nanoscale Materials.

(1) Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; WileyInterscience: New York, 1992; Vol. 22. (2) Electroanalytical Methods for Biological Materials; Brajter-Toth, A., Chambers, J. Q., Eds.; Marcel Dekker: New York, 2002. (3) Buriak, J. M. Chem. Rev. 2002, 102, 1271-1308. (4) Textor, M.; Sittig, C.; Frauchiger, V.; Tosatti, S.; Brunette, D. M. In Titanium in Medicine: Material Science, Surface Science, Engineering, Biological Responses and Medical Applications; Brunette, D. M., Textor, M., Tengvall, P., Eds.; Springer-Verlag: Heidelberg and Berlin, 2001; pp 171-230. (5) Schaeferling, M.; Schiller, S.; Paul, H.; Kruschina, M.; Pavlickova, P.; Meerkamp, M.; Giammasi, C.; Kambhampati, D. Electrophoresis 2002, 23, 3097-3105. (6) Wink, T.; Zuilen, S. J. v.; Bult, A.; Bennekom, W. P. v. Analyst 1997, 122, 43R-50R. (7) Turner, J. N.; Shain, W.; H., S. D.; Anderson, M.; Martins, S.; Isaacson, M.; Craighead, H. Exp. Neurol. 1999, 156, 33-49.

The ideal platform material for biointerfaces should possess several key properties: (1) it should be bioinert; (2) it should be amenable to chemistries that would allow for tailoring of its surface hydrophobicity; (3) it should possess terminal functional groups that allow for the robust covalent attachment of a wide range of biomolecules; and for electrode materials; and (4) as electrode materials it should have overpotentials for the evolution of oxygen and hydrogen as high as possible, to allow for the reversible modification of surface reactivity without the destruction of the biointerfaces. Diamond has revently emerged as a possible candidate material that may satisfy many of the previously mentioned desired properties for a platform material for biointerfaces.9,10 Diamond offers several advantages over traditional materials including extreme hardness and mechanical strength, favorable tribological properties, high thermal conductivity, and inherent bioinertness.11 Furthermore, synthetic diamond thin films can be made electrically conductive by selective incorporation of nitrogen or boron and are well-known to have superior electrochemical properties, including extreme microstructural and morphological stability and high overpotentials for both oxygen and hydrogen evolution.11-17 (8) Edell, D. J.; Toi, V. V.; McNeil, V. M.; Clark, L. D. IEEE Trans. Biomed. Eng. 1992, 39, 635-43. (9) Carlisle, J. A.; Auciello, O. Interface 2003, 12, 28. (10) Yang, W. S.; Auciello, O.; Butler, J. E.; Cai, W.; Carlisle, J. A.; Gerbi, J.; Gruen, D. M.; Knickerbocker, T.; Lasseter, T. L.; Russell, J. N.; Smith, L. M.; Hamers, R. J. Nat. Mater. 2002, 1, 253-257. (11) Plano, L. S. G. In Diamond: Electronic Properties and Applications; Kania, D. R., Ed.; Kluwer Academic Publishers: Norwell, MA, 1995; p 62. (12) Wang, J.; Swain, G. M. Electrochem. Solid State Lett. 2002, 5, E4-E7. (13) Birrell, J.; Gerbi, J. E.; Auciello, O.; Gibson, J. M.; Gruen, D. M.; Carlisle, J. A. J. Appl. Phys. 2003, 93, 5606-5612. (14) Chen, Q.; Gruen, D. M.; Krauss, A. R.; Corrigan, T. D.; Witek, M.; Swain, G. M. J. Electrochem. Soc. 2001, 148, E44-E51. (15) Bhattacharyya, S.; Auciello, O.; Birrell, J.; Carlisle, J. A.; Curtiss, L. A.; Goyette, A. N.; Gruen, D. M.; Krauss, A. R.; Schlueter, J.; Sumant, A.; Zapol, P. Appl. Phys. Lett. 2001, 79, 1441-1443.

10.1021/la048740z CCC: $27.50 © 2004 American Chemical Society Published on Web 11/13/2004

Surface Functionalization of Diamond Films

Ultrananocrystalline diamond (UNCD) thin films are synthesized using microwave plasma-enhanced chemical vapor deposition with argon-rich Ar/CH4 plasmas and consist of 3-5 nm grains of pure sp3-bonded carbons and atomically abrupt grain boundaries consisting of disordered carbons. UNCD differs from boron-doped microcrystalline diamond mainly in its unique electrical properties. UNCD can be doped with nitrogen by adding nitrogen gas to the plasma, yielding films with n-type semiconducting to semimetallic conductivities (up to 250 Ω-1 cm-1 at room temperature). Furthermore, the N-doped UNCD films possess several interesting electrochemical properties, such as wide working potential window, low background current, extreme morphological stability at demanding electrochemical conditions, and high degree of electrochemical activity for simple redox systems.9,14,18-21 As-deposited UNCD hydrophobic and chemically inert characteristics are derived primarily from the hydrogenterminated surface.19,22 The surface hydrophobicity prevents the stabilization of many biomolecules directly at the surface, and the lack of chemically reactive groups precludes the attachment of biomolecules to the surface. Thus, the surface modification of UNCD is required to provide chemical functional groups amenable for the covalent coupling of biomolecules, while preserving its otherwise favorable biomaterials properties (i.e., bioinertness). The surface modification of synthetic diamond materials has been investigated by several groups, and two general approaches have been studied: direct chemical functionalization of the hydrogen-terminated surface10,23-29 or application of a pretreatment method that renders the surface hydrophilic (by generation of oxygen-containing surface functionalities).30-34 In the first approach, the hydrophobic, hydrogen-terminated surfaces were functionalized either photochemically by UV excitation of halogen gases23-27 or alkenes,10,29 thereby introducing chloro-, fluoro-, or aminoalkyl groups, or chemically by thermal decomposition of benzoyl peroxide.28 In the second (16) Xu, J. S.; Granger, M. C.; Chen, Q. Y.; Strojek, J. W.; Lister, T. E.; Swain, G. M. Analyt. Chem. 1997, 69, A591-A597. (17) Granger, M. C.; Xu, J. S.; Strojek, J. W.; Swain, G. M. Anal. Chim. Acta 1999, 397, 145-161. (18) Gruen, D. M.; Pan, X. Z.; Krauss, A. R.; Liu, S. Z.; Luo, J. S.; Foster, C. M. J. Vac. Sci. Technol., A 1994, 12, 1491-1495. (19) Gruen, D. M. Ann. Rev. Mater. Sci. 1999, 29, 211-259. (20) Krauss, A. R.; Auciello, O.; Gruen, D. M.; Jayatissa, A.; Sumant, A.; Tucek, J.; Mancini, D. C.; Moldovan, N.; Erdemir, A.; Ersoy, D.; Gardos, M. N.; Busmann, H. G.; Meyer, E. M.; Ding, M. Q. Diamond Related Mater. 2001, 10, 1952-1961. (21) Gerbi, J. E.; Auciello, O.; Birrell, J.; Gruen, D. M.; Alphenaar, B. W.; Carlisle, J. A. Appl. Phys. Lett. 2003, 83, 2001-2003. (22) Zapol, P.; Sternberg, M.; Curtiss, L. A.; Frauenheim, T.; Gruen, D. M. Phys. Rev. B 2002, 65, 045403. (23) Ando, T.; Yamamoto, K.; Suehara, S.; Kamo, M.; Sate, Y.; Shimosaki, S.; Nishitanigamo, M. J. Chinese Chem. Soc. 1995, 42, 285292. (24) Ando, T.; NishitaniGamo, M.; Rawles, R. E.; Yamamoto, K.; Kamo, M.; Sato, Y. Diamond Related Mater. 1996, 5, 1136-1142. (25) Freedman, A.; Stinespring, C. D. Appl. Phys. Lett. 1990, 57, 1194. (26) Miller, J. B. Surf. Sci. 1999, 439, 21-33. (27) Ohtani, B.; Kim, Y. H.; Yano, T.; Hashimoto, K.; Fujishima, A.; Uosaki, K. Chem. Lett. 1998, 953-954. (28) Tsubota, T.; Tanii, S.; Ida, S.; Nagata, M.; Matsumoto, Y. Phys. Chem. Chem. Phys. 2003, 5, 1474-1480. (29) Strother, T.; Knickerbocker, T.; Russell, J. N.; Butler, J. E.; Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 968-971. (30) Notsu, H.; Yagi, I.; Tatsuma, T.; Tryk, D. A.; Fujishima, A. Electrochem. Solid State Lett. 1999, 2, 522-524. (31) Ando, T.; Yamamoto, K.; Ishii, M.; Kamo, M.; Sato, Y. J. Chem. Soc. Faraday Trans. 1993, 89, 3635-3640. (32) Granger, M. C.; Swain, G. M. J. Electrochem. Soc. 1999, 146, 4551-4558. (33) Notsu, H.; Yagi, I.; Tatsuma, T.; Tryk, D. A.; Fujishima, A. J. Electroanal. Chem. 2000, 492, 31-37.

Langmuir, Vol. 20, No. 26, 2004 11451 Scheme 1

approach, oxygen-containing functional groups were introduced onto polycrystalline diamond surfaces by oxygen plasma treatment, anodic polarization, or the use of strong oxidizing acids. The hydrophilic surface can then be modified using standard silane coupling agents.34 A limitation of this approach is that exposure to oxygen or other reactive gas plasmas has been found to etch the diamond surface and introduces a mixture of oxygen moieties.30 Recently, it has been shown that the trifluoroacetamide-protected amino alkenes can be reacted to yield amine functionalities, which then are used to covalently couple short oligonucleotides.10 Although this work has delineated a path by which DNA can be chemisorbed to diamond surfaces, considerable work remains before a broader range of molecules (e.g., soluble and membrane proteins) can be successfully integrated. On the basis of our interests in the surface functionalization of UNCD as a new platform material for biointerfaces, we have adopted another approach, the electrochemical reduction of aryldiazonium salts, as a facile means for altering the interfacial architectures of conductive UNCD. Electrochemical-assisted surface modification is a approach that has been used to modify sp2bonded carbon materials,35-43 iron,44 and silicon.45 Moreover, it has been demonstrated in work conducted by Kuo et al. that electrochemical reduction of aryldiazonium salts can be used for successful modification of boron-doped microcrystalline diamond.46 As shown in Scheme 1, the electrochemical reduction of aryl diazonium is a oneelectron transfer reaction, leading to the formation of solution-based aryl radicals that covalently attach to the electrode surface.40 The formation of monolayer/submonolayer phenyl-derivatized surfaces has been reported on glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) in low concentration of diazonium salt with short reduction times.42,47,48 Dense and ordered monolayers have been achieved on H-terminated silicon by well controlling the passed electrical charge and by properly choosing the (34) Notsu, H.; Fukazawa, T.; Tatsuma, T.; Tryk, D. A.; Fujishima, A. Electrochem. Solid State Lett. 2001, 4, H1-H3. (35) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883-5884. (36) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303-310. (37) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. Carbon 1997, 35, 801-807. (38) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805-6813. (39) Liu, Y. C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091-2097. (40) Downard, A. J. Electroanalysis 2000, 12, 1085-1096. (41) Downard, A. J.; Prince, M. J. Langmuir 2001, 17, 5581-5586. (42) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 65346540. (43) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 59475951. (44) Adenier, A.; Bernard, M. C.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 4541-4549. (45) Allongue, P.; de Villeneuve, C. H.; Cherouvrier, G.; Cortes, R.; Bernard, M. C. J. Electroanal. Chem. 2003, 550, 161-174. (46) Kuo, T. C.; McCreery, R. L.; Swain, G. M. Electrochem. Solid State Lett. 1999, 2, 288-290. (47) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201-207. (48) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837-3844.

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grafting potential.45 In this work, we report the surface functionalization of UNCD via electrochemical reduction of diazonium salts, in a manner similar to that employed for functionalization of other carbon surfaces. Two diazonium salts were used: 3,5-dichlorophenyl diazonium tetrafluoroborate and 4-nitrophenyl diazonium tetrafluoroborate. Cyclic voltammetry, X-ray photoelectron spectroscopy, ac impedance measurements, and contact angle measurements were used to characterize the organic overlayer and estimate the surface coverage. Experimental Procedures Materials and Methods. All chemicals were reagent grade quality (Aldrich, Milwaukee, WI) and used as received. Ultrapure water (>18 MΩ cm) from a Barnstead E-pure system was used to prepare all aqueous solutions. X-ray photoelectron spectra were collected using an AlKR X-ray (1486.6 eV) beam that was generated with a power of 8 kV/40 mA. The atomic ratios of elements were calculated from the ratio of the areas under the respective peaks, followed by correction with their sensitivity factors. Contact angle measurements were made using a FTA 125 contact angle analyzer (First Ten Angstroms, Portsmouth, VA) with ultrapure water as the liquid phase. Electrochemical (cyclic voltammetry and ac impedance) measurements were performed in a single compartment glass cell using a Solartron 1287A digital potentiostat and a 1260 impedance/gain-phase analyzer (Solartron Analytical, Inc., Hampshire, UK). An Ag/ AgCl electrode and a Pt wire were used as the reference and counter electrode, respectively. All potentials in this study are referred to this reference electrode unless otherwise stated. The diamond film electrodes were pressed against the bottom of the glass cell with the fluid being contained by a Chemraz acetonitrileresistant O-ring. The geometric area exposed was 0.7 cm2, and all currents were normalized to this area. Electrical connection was made to the diamond by scratching the backside of the Si substrate with a diamond scribe and then coating the area with graphite before contacting the aluminum current-collecting backplate. The diamond film working electrodes were pretreated, once mounted in the cell, by a thorough rinsing with ultrapure water, a 20 min soak in distilled 2-propanol, followed by another thorough rinsing with ultrapure water, and finally blown dry using nitrogen gas. Synthesis and Surface Modification of UNCD Thin Films. The microwave plasma-enhanced chemical vapor deposition (MPCVD) of UNCD was previously described.21 The nitrogenincorporated UNCD thin-films, achieved by introduction of 5% (v/v) N2 (g) in the plasma, were deposited on highly conducting n-type Si (111) substrates (Wafernet Inc., San Jose, CA) using CYRANNUS I reactor from IPLAS (Innovative Plasma Systems, Inc., Triosdorf, Germany). Surface modification of the UNCD surface was performed in an inert atmosphere and maintained in a dry glovebox (Coy Labs, Grass Lake, MI) with humidity level