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Electrochemically Programmed, Spatially Selective Biofunctionalization of Silicon Wires Yuri L. Bunimovich,† Guanglu Ge,† Kristen C. Beverly,‡ Ryan S. Ries,† Leroy Hood,‡ and James R. Heath*,† Caltech Division of Chemistry and Chemical Engineering, MC 127-72, Pasadena, California 91125, and The Institute for Systems Biology, 1441 North 34th Street Seattle, Washington 98103-8904 Received August 21, 2004 A method for the spatially selective biofunctionalization of silicon micro- and nanostructures is reported, and results are presented for both single-crystal silicon (111) or (100) surfaces. An electroactive monolayer of hydroquinone was formed on the surface of H-terminated silicon working electrodes via an olefin reaction with UV-generated surface radicals. Molecules presenting either cyclopentadiene or a thiol group can be immobilized onto the regions where the hydroquinone has been oxidized. Molecular size and crystal orientation are evaluated as important factors that dictate the electrode stability in aqueous solution under anodic potentials. Monolayers composed of smaller molecules on (111) surfaces exhibit the highest packing density and are more effective in preventing anodic oxidation of the underlying substrate. Voltammetry, X-ray photoelectron spectroscopy, and atomic force and fluorescence microscopy are utilized to interrogate the kinetic rates of biofunctionalization, the extent of surface coverage, monolayer quality, and the spatial selectivity of the process.
Introduction The spatially selective biofunctionalization of surfaces has proven to be an enabling capability, beginning with the early work of Fodor and co-workers1 on utilizing photolabile surface groups to construct DNA libraries, to the use of inkjet technologies for the construction of protein chips.2 The dip-pen lithography methods from Mirkin’s group represent the current limit of patterning density for protein chips.3 Chip-based array methods are largely predicated upon the optical detection of the target/probe binding events, which imposes the optical diffraction limit on pixel density. However, electronically transduced detectors, such as chemically gated silicon nanowires (NW),4,5 may circumvent this limitation. Methods for fabricating ultra-high-density circuits of silicon nanowires6 with the excellent conductivity properties required for biosensors7 have been established. To construct an array of sensors, NWs must be functionalized with different receptor probes, such as antibodies or aptamers, against their designated molecular targets. Here, we describe an electrochemical approach that, while possibly having multiple applications, should be applicable toward the selective biopassivation of silicon nanowire (NW) sensor arrays and is spatially limited only by the ability to electronically address the individual sensor elements. Recent advances in alkylation of H-terminated Si surfaces have made it possible to bypass the necessity of * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Caltech Division of Chemistry and Chemical Engineering. ‡ The Institute for Systems Biology. (1) Fodor, S. P. A.; Read, J. L.; Pirrung, M. C.; Stryer, L.; Lu, A. L.; Solas, D. Science 1991, 251, 767. (2) MacBeath, G.; Schreiber, S. L. Science 2000, 289, 1760. (3) Lee, K. B.; Park, S. J.; Mirkin, C. A.; Smith, J. C.; Mrksich, M. Science 2002, 295, 1702. (4) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (5) Hahm, J.; Lieber, C. M. Nano. Lett. 2004, 4, 51. (6) Melosh, N. A.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Science 2003, 300, 112. (7) Johnston-Halperin, E.; Beckman, R.; Luo, Y.; Melosh, N.; Green, J.; Heath, J. R. J. Appl. Phys. (to appear Nov. 1, 2004).
chemical modification of the native oxide of silicon.8-10 Hydrosylilation produces cleaner, more stable, and more reproducible monolayers than silane-based SAMs.11,12 The benefits of utilizing organic monolayers formed on Hterminated Si for biosensing are multi-fold. First, the removal of the SiOx tunneling barrier brings the target/ probe pair 1-2 nm closer to the conducting surface,13-16 which should presumably translate into an increase in the sensitivity of the device. Second, the electrical properties of sufficiently small-diameter silicon NWs are dominated by the surface characteristics. Removal of what is often an electrically imperfect Si-SiO2 interface17 and a disordered oxide film with a high density of trap sites18 is desirable. Finally, the difficulty in controlling the smoothness of the SiO2 layer results in rough and grainy surfaces upon the growth of the siloxane-anchored monolayers.16,19,20 (8) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (9) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudholter, E. J. R. Adv. Mater. 2000, 12, 1457. (10) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 1 2002, 2, 23. (11) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 6164. (12) Ashurst, W. R.; Yan, C.; Carraro, C.; Howe, R. T.; Maboudian, R. In Proceedings of solid-state sensor and actuator workshop, Hilton Head: Hilton Head Island, SC, 2000, 320. (13) Itsumi, M. SiO2 in Si Microdevices; Springer-Verlag: New York, 2002. (14) Li, Q.; Mathur, G.; Homsi, M.; Surthi, S.; Misra, V.; Malinovskii, V.; Schweikart, K. H.; Yu, L.; Lindsey, J. S.; Liu, Z.; Dabke, R. B.; Yasseri, A.; Bocian, D. F.; Kuhr, W. G. Appl. Phys. Lett. 2002, 81, 1494. (15) Li, Q.; Surthi, S.; Mathur, G.; Gowda, S.; Misra, V.; Sorenson, T. A.; Tenent, R. C.; Kuhr, W. G.; Tamaru, S.; Lindsey, J. S.; Liu, Z.; Bocian, D. F. Appl. Phys. Lett. 2003, 83, 198. (16) Grisaru, H.; Cohen, Y.; Aurback, D.; Sukenik, C. N. Langmuir 2001, 17, 1608. (17) Sze, S. M. The Physics of Semiconductor Devices, 2nd. ed.; Wiley: New York, 1981. (18) Royea, W. J.; Juang, A.; Lewis, N. S. Appl. Phys. Lett. 2000, 77, 1988. (19) Hu, M.; Noda, S.; Okubo, T.; Yamaguchi, Y.; Komiyama, H. Appl. Surf. Sci. 2001, 181, 307. (20) Lenigk, R.; Carles, M.; Ip, N. Y.; Sucher, N. J. Langmuir 2001, 17, 2497.
10.1021/la047913h CCC: $27.50 © 2004 American Chemical Society Published on Web 10/15/2004
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Alkyl monolayers grown on appropriately prepared H-terminated Si(111) can reflect the atomic flatness of the underlying substrate.21,22 Multiple studies have focused on the methods of chemical passivation of silicon via the formation of a Si-C bond. Hydrosylilation has been accomplished with radical initiators,23,24 through thermally induced11,24-28 or photochemical methods,29-33 or utilizing Lewis acid catalysts.34,35 Alkylmagnesium reagents have been successfully employed for the alkylation of H-terminated Si surfaces,36,37 and halogenated surfaces have been alkylated with alkylmagnesium and alkyllithium reagents.30,38-42 Furthermore, electrochemical methods of H-terminated silicon functionalization have been explored.43-46 Several research groups have demonstrated an ability to utilize these functionalization methods to nonspecifically attach DNA to silicon surfaces.47-49 Electroactive monolayers have attracted attention due to the growing interest in selective molecular and cellular immobilization on surfaces. Electrochemical activation of hydroquione50,51 and hydroquinone esters52 on gold, as (21) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (22) Zhang, L.; Li, L.; Chen, S.; Jiang, S. Langmuir 2002, 18, 5448. (23) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (24) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 5. (25) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 1998, 14, 1759. (26) Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G.; Auilhof, H.; Sudholter, E. J. R. Langmuir 2001, 17, 7554. (27) Boukherroub, R.; Wayner, D. D. M.; Lockwood, D. J.; Canham, L. T. J. Electrochem. Soc. 2001, 148, H91. (28) Boukherroub, R.; Wayner, D. D. M.; Lockwood, D. J. J. Electrochem. Soc. 2002, 149, H59. (29) Terry, J.; Mo, R.; Wigren, C.; Cao, R. Y.; Mount, G.; Pianetta, P.; Linford, M. R.; Chidsey, C. E. D. Instrum. Methods. Phys. Res., Sect. B 1997, 133, 94. (30) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R. Y.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056. (31) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. Engl. 1998, 37, 2462. (32) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513. (33) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (34) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 1339. (35) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491. (36) Mitchell, S. A.; Boukherroub, R.; Anderson, S. J. J. Phys. Chem. B 2000, 104, 7668. (37) Yu, H. Z.; Boukherroub, R.; Morin, S.; Wayner, D. D. M. Electrochem. Commun. 2000, 2, 562. (38) Bansal, A.; Li, X. L.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. J. Am. Chem. Soc. 1996, 118, 7225. (39) Bansal, A.; Li, X.; Yi, S. I.; Weinberg, W. H.; Lewis, N. S. J. Phys. Chem. B 2001, 105, 10266. (40) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R. Y.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213. (41) Okubo, T.; Tsuchiya, H.; Sadakata, M.; Yasuda, T.; Tanaka, K. Appl. Surf. Sci. 2001, 171, 252. (42) He, J.; Patitsas, S. N.; Preston, K. F.; Wolkow, R. A.; Wayner D. D. M. Chem. Phys. Lett. 1998, 286, 508. (43) Dubois, T.; Ozanam, F.; Chazalviel, J. N. Electrochem. Soc. Proc. 1997, 97-7, 296. (44) Allongue, P.; de Villeneuve, C. H.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791. (45) Allongue, P.; de Villeneuve, C. H.; Pinson, J. J. Electrochim. Acta 2000, 45, 3241. (46) Fidelis, A.; Ozanam, F.; Chazalviel, J. N. Surf. Sci. 2000, 444, L7. (47) Strother, T.; Cai W.; Zhao, X. S.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205. (48) Lin, Z.; Strother, T.; Cai, W. Cao, X. P.; Smith, L. M., Hamers, R. J. Langmuir 2002, 18, 788. (49) Pike, A. R.; Lie, L. H.; Eagling, R. A.; Ryder, L. C.; Patole, S. N.; Connolley, B. A.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. 2002, 41, 615.
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well as oxidation of thiols on Si-SiO2 surfaces,53 have been demonstrated for such applications. Notably, Mrksich’s group has accomplished a selective attachment of proteins and cells to monolayers on gold via a DielsAlder reaction between 1,4-benzoquinone and cyclopentadiene.50,51 Here, we extend this chemistry of hydroquinone-terminated monolayers54 to H-terminated silicon surfaces. We also demonstrate an alternative method of molecular attachment via Michael addition of thiolterminated molecules to p-benzoquinone.55,56 Results and Discussion Scheme 1 shows the overall strategy of spatially selective immobilization via a Diels-Alder reaction (Step IV) or Michael addition (Step V). The initial step in organic monolayer formation on H-terminated Si involves a wet etch of the native oxide. We employed an established wet etch (40% NH4F) for preparing atomically flat Si(111).57 However, the flattest Si(100) surface that can be generated by a wet etch is substantially rougher, with (111) facets,58 and is thus more prone to oxidation.59-61 Our SNAP method for NW fabrication can be carried out most easily using silicon-on-insulator (SOI) substrates,6 which are commercially available only in the (100) orientation. Bonded Si(111) wafers are also available, although they require substantial additional processing steps before being suitable for NW fabrication. Thus, the Si(111) surface is more ideal, while the Si(100) surface is more practical. The quality of the formed monolayer on silicon is critical in determining the interfacial electrical properties and the susceptibility of silicon to oxidation in air and in aqueous solution under oxidative potentials.62,63 In particular, higher packing density leads to a more-stable silicon electrode. The size of the molecule in the chemisorbed monolayer dictates the susceptibility of the surface to oxidation by limiting the packing density. We explored two hydroquinone hydroxyl protecting groups, THP (Molecule A) and CH3 (Molecule B) (Scheme 2). THP is removed under milder conditions and is compatible with using tri(ethylene glycol) (TEG) in conjunction with the electro-active molecule (Step II in Scheme 1). As a cocomponent of a monolayer, TEG helps prevent the nonselective binding of cells and proteins.64 Investigations of mixed TEG-hydroquinone monolayers are currently underway in our laboratory; this paper, however, reports only on films composed of pure hydroquione (Molecules A and B). (50) Yousaf, M.; Mrksich, M. J. Am. Chem. Soc. 1999, 121, 4286. (51) Yeo, W. S.; Yousaf, M. N.; Mrksich, M. J. Am. Chem. Soc. 2003, 125, 14994. (52) Kim, K.; Jang, M.; Yang, H.; Kim, E.; Kim, Y. T.; Kwak, J. Langmuir 2004, 20, 3821. (53) Pavlovic, E.; Quist, A. P.; Gelius, U.; Nyholm, L.; Oscarsson, S. Langmuir 2003, 19, 4217. (54) Hong, H. G.; Park, W. Langmuir 2001, 17, 2485. (55) Giovanelli, D.; Lawrence, N. S.; Jiang, L.; Jones, T. G. J.; Compton, R. G. Anal. Lett. 2003, 36, 2941. (56) Rousell, C.; Rohner, T. C.; Jensen, H.; Girault, H. H. Chem. Phys. Chem. 2003, 4, 200. (57) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656. (58) Madou, M. J. Fundamentals of Microfabrication, 2nd ed.;; CRC Press LLC: Boca Raton, 2002. (59) Hirose, M.; Yasaka, T.; Takakura, M.; Miyazaki, S. Solid State Technol. 1991, 34, 43. (60) Yasaka, T.; Takakura, M.; Sawara, K.; Uenaga, S.; Yasutake, H.; Miyazaki, S.; Hirose, M. IEICE Trans. Electron. 1992, E75-C, 764. (61) Miura, T.; Niwano, M.; Shoji, D.; Miyamato, N. J. Appl. Phys. 1996, 79, 4373. (62) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 4058. (63) Barrelet, C. J.; Robinson, D. B.; Cheng, J.; Hunt, T. P.; Quate, C. F.; Chidsey, C. E. D. Langmuir 2001, 17, 3460. (64) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714.
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Bunimovich et al. Scheme 1
Scheme 2: Molecules Used in This Study
In Figure 1, we present representative X-ray photoelectron spectroscopy (XPS) survey scans for the photochemical functionalization of Si(111) and Si(100) with Molecules A and B. All of the native oxide has been successfully removed via the wet etch, as evidenced by the absence of the O 1s peak in parts a and b of Figure 1 for (111) and (100), respectively. Furthermore, no Si 2p peaks were observed on high-resolution XPS scans between 100 and 104 BeV, which would be expected if traces of SiO2 remained. No adventitious C 1s peaks at
285 BeV were observed for either (100) or (111) immediately after the wet etch. As has been shown previously, atomic force microscopy (AFM) is a useful tool for an assessment of surface stability.21 Figure 2 presents AFM images of Si(111) and Si(100) surfaces functionalized with Molecule B. Atomically flat terraces with monatomic steps, which resemble those of a well-etched, H-terminated surface,22 are evident on the (111) substrate. The topography of these surfaces does not change after storage in air or application of small
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Figure 1. Survey scan XP spectra of Si(111) (left) and Si(100) (right) surfaces (a-b) cleaned and etched as described, (c-d) functionalized with neat Molecule B and (e-f) neat Molecule A at 10-5 Torr under UV for 2 h.
positive potentials (
