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Chemical Force Titrations of Functionalized Si(111) Surfaces Tadesse Z. Mengistu,† Vishya Goel,†,§ J. Hugh Horton,*,‡ and Sylvie Morin*,† Department of Chemistry, York UniVersity, 4700 Keele Street, Toronto, Ontario, Canada, M3J 1P3, and Department of Chemistry, Queen’s UniVersity, Chernoff Hall, Kingston, Ontario, Canada, K7L 3N6 ReceiVed October 14, 2005. In Final Form: February 6, 2006 Chemical force titrationssplots of the adhesive force between an atomic force microscope tip and sample as a function of pHswere acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amineterminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK1/2 of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.
Introduction The ability to probe interfacial forces with nanometer-scale resolution is critical to develop a molecular-level understanding of a variety of phenomena such as adhesion and fracture at interfaces. Two variants of atomic force microscopy (AFM) are fast becoming important tools for the characterization of interfacial forces. One class directly or indirectly measures the compliance of materials. This includes such techniques as interfacial force microscopy,1 nanoindentation,2 and phase imaging methods.3 A second class of methods is typified by chemical force microscopy (CFM),4,5 a variation of traditional AFM in which chemical specificity is added by deliberate derivatization of an AFM probe. When measuring tip-sample interactions at a single point on the surface, as opposed to imaging, this technique is more properly called chemical force spectrometry. By utilizing chemically functionalized tips, this method can be used to probe forces between different molecular groups, measure surface energetics on a nanometer scale, and determine pK1/2 values (the solution pH value at which half the surface sites are ionized) of the surface acid and base groups locally.6-8 This latter approach, on which we will focus here, has been termed chemical force titration. * To whom correspondence should be addressed. E-mail: hortonj@ chem.queensu.ca (J.H.H.);
[email protected] (S.M.). Phone: (613)-5332379/(613)-533-6704 (J.H.H.); (416)-736-2100, ext. 22303 (S.M.). Fax: (613)-533-6669 (J.H.H.); (416)-736-5936 (S.M.). † York University. ‡ Queen’s University. § Present address: Department of Chemistry, Queen’s University, Chernoff Hall, Kingston, Ontario, Canada, K7L 3N6. (1) Burns, A. R.; Houston, J. E.; Carpick, R. W.; Michalske, T. A. Phys. ReV. Lett. 1999, 82, 1181. (2) VanLandingham, M. R.; Villarrubia, J. S.; Guthrie, W. F.; Meyers, G. F. Macromol. Symp. 2001, 167, 15. (3) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H.-J. Langmuir 1997, 13, 3807. (4) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. ReV. Mater. Sci. 1997, 27, 381. (5) Finot, M. O.; McDermott, M. T. J. Am. Chem. Soc. 1997, 119, 8564. (6) Smith, D. A.; Wallwork, M. L.; Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; Wong, M. J. Phys. Chem. B 2000, 104, 8862. (7) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, 1991. (8) Giesbers, M.; Kleijn, J. M.; Fleer, G. J.; Cohen Stuart M. A. Colloids Surf., A 1998, 142, 343.
Previous force-titration studies have focused on systems in which tip and sample have been functionalized using alkanethiol self-assembled monolayers (SAMs) on Au. This has the dual advantage of giving well-characterized surfaces, which are of effectively the same substrate type, on both tip and sample. Results from such relatively simple systems have been used to examine more complex situations (such as “mixed” tip-sample systems, where the functional groups on tip and sample differ from one another) on colloidal particles, and on polymer surfaces.9 Another important class of chemical surfaces is one by which crystalline silicon is used as a substrate. Crystalline-silicon-based chemical surfaces have gained growing interest for biosensor development and the design of bioreactive surfaces.10-13 Alkyl monolayer derivatization of H-terminated Si(111) surfaces with ω-functionalized 1-alkenes via hydrosilylation reactions is one interesting approach that has been used to prepare surfaces with various structural and chemical properties.14-19 The most common methods are thermal and photochemical hydrosilylation strategies. Stable Si-C bond linked alkyl monolayers containing reactive terminal groups (such as NH2 and COOH) can be prepared by employing such methods. Such reactive surfaces can also be used to attach biological molecules to silicon through other mild derivatization reactions.10-12 Application of force titration studies to these systems could possibly provide useful information on the structural makeup and chemical properties of such technologically important surfaces. Here, we report for the first time (9) Wang, B.; Oleschuk, R. D.; Horton, J. H. Langmuir 2005, 21, 1290. (10) Strother, T.; Hamers, R. J.; Smith, L. M. Nucl. Acids Res. 2000, 28, 3535. (11) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205. (12) Wei, F.; Sun, B.; Guo, Y.; Zhao, X. S. Biosens. Bioelectron. 2003, 18, 1157. (13) More´, S. D.; Hudecek, J.; Urisu, T. Surf. Sci. 2003, 532-535, 993. (14) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (15) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (16) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513. (17) Buriak, J. M. Chem. ReV. 2002, 102, 1272. (18) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759. (19) Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2001, 17, 7554.
10.1021/la052776p CCC: $33.50 © 2006 American Chemical Society Published on Web 05/04/2006
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on force titration curves obtained on chemically modified Si(111) surfaces bearing methyl, carboxyl, and amine end groups. These systems have been fully characterized using standard surface science techniques (AFM, FTIR, and XPS). Experimental Section Chemical Modification of Silicon Surfaces. All reagents used in our experiments were highest purity grade chemicals available from the respective suppliers and were used as received. 1-Decene and methyl-10-undecenoate were purchased from Aldrich. A 10amino-dec-1-ene derivative, where the amine functional group is protected with tert-butyloxycarbonyl (t-BOC), was synthesized from the corresponding free amine product (10-amino-dec-1-ene) according to procedures described in the literature.10 The latter was synthesized from 10-undecenoyl chloride (Aldrich), tetrabutylammonium bromide (Sigma-Aldrich), and sodium azide (Sigma). The substrates used in our experiments were Si(111) n-type wafers (singleside polished, doped with phosphorus, 0.02-0.5 Ω cm resistivity) purchased from Virginia Semiconductor, Inc. (Fredericksburg, VA), and Si(111) n-type ATR elements (25 × 4.5 × 1 mm3, 45°) obtained from Harrick Scientific Corporation. The substrates were functionalized through thermal or photochemical hydrosilylation reactions between H-terminated surfaces and ω-functionalized 1-alkene reagents.10,12,19,20 Initially, shards of Si(111) wafer (and similarly ATR elements) were cleaned in a hot (ca. 90 °C) mixture of H2O2 (30%) and H2SO4 (96%) (1:1, v/v) in Teflon vials and rinsed with copious amounts of deionized water. The cleaned samples were then etched in a deoxygenated 40% NH4F solution for 30 min in order to produce H-terminated surfaces.21 Deoxygenation was performed by bubbling argon through the solution (note: the solution was not bubbled during etching, but argon was continually streamed above it in a partially closed Teflon vial). The etched samples were removed from the ammonium fluoride solution and immediately transferred to deoxygenated neat 1-alkene reagents in Schlenk tubes for functionalization. Thermal and photochemical hydrosilylation strategies produce high-quality alkyl monolayer-derivatized silicon surfaces and are relatively simple to perform.17 In our experiments, surfaces modified with alkyl monolayer containing methyl or methylester terminal groups were prepared through thermal hydrosilylation methods. In each case, reactions were carried out for 23 h with continuous purging of argon through the reagents. The reaction temperatures for the preparation of methyl- and methyl-esterterminated surfaces were set at 150 and 115 °C, respectively. Surfaces with amine (t-BOC protected) terminal groups were prepared by employing a photochemical hydrosilylation strategy. This was carried out by irradiating the hydrogenated silicon sample (in a deoxygenated 1-alkene reagent) with ultraviolet (UV) light (from a UV lamp) at room temperature for 3 h. Quartz Schlenk tubes were used in such reactions. After functionalization, the samples were thoroughly rinsed with the appropriate organic solvents. Samples functionalized with alkyl monolayers containing methyl or amine (t-BOC protected) terminal groups were rinsed with ethanol and tetrahydrofuran (THF). THF and water were used for rinsing methyl-ester-terminated surfaces. Carboxyl-terminated surfaces were prepared from methylester-terminated samples through acid-catalyzed hydrolysis of the ester functional group.16 This was performed by treating esterterminated samples with 2.4 M HCl at 70 °C for 2.5 h followed by rinsing with water and THF. Characterization of Modified Silicon Surfaces: AFM, FTIR, and XPS Analysis. The topographic measurements of functionalized silicon wafer samples prepared as described above were performed under atmospheric air using a commercial AFM instrument (Dimension 3100, NanoScope IIIa, Digital Instruments) in contact mode. FTIR spectroscopic analysis of similarly fucntionalized silicon ATR elements were run using a Nexus 870 Spectrometer (ThermoNicolet) and a 4X Beam Condenser (Harrick Scientific Corporation) in dry (20) Liu, Y.-J.; Navasero, N. M.; Yu, H.-Z. Langmuir 2004, 20, 4039. (21) Allongue, P.; de Villeneuve, C. H.; Morin, S.; Boukherroub, R.; Wayner, D. D. M. Electrochim. Acta 2000, 45, 4591.
Figure 1. Contact mode AFM images of alkyl monolayer-modified flat Si(111) surfaces with (a) methyl-ester and (b) carboxyl terminal groups. Both images are 2 µm × 2 µm size. The images also show that the miscut of the Si wafer employed in these experiments is relatively large. air atmosphere. XPS measurements were performed on functionalized samples prepared from silicon wafers using the instrument model Kratos Axis Ultra XPS. XPS analyses were performed using a monochromated Al KR X-ray source and charge neutralizer and with an accelerating voltage of 14 kV and a current of 10 mA. Survey scans were run at a pass energy of 160 eV. Species detected by survey scans were then analyzed at a pass energy of 20 eV and quantified. All spectra were calibrated to C 1s 285 eV. Chemical Force Titrations. Chemical force titrations were used to measure adhesive interactions between a SAM-modified AFM tip and Si(111) samples prepared by the methods described above. The titration procedure has been outlined previously.22 Briefly, the functionalized tips were prepared from contact mode silicon AFM tips (MikroMasch) coated by thermal evaporation with a 5 nm layer of chromium to promote the adhesion of the following layer of gold (10 nm). The tip was then immersed in a solution of 1 × 10-3 mol L-1 16-thiohexadecanoic acid (Aldrich) in ethanol for 24 h at 298 K to create the SAM on the surface. The tip radius as quoted by the manufacturer was
