pubs.acs.org/Langmuir © 2009 American Chemical Society
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Self-Assembled Monolayers of Aromatic ω-Aminothiols on Gold: Surface Chemistry and Reactivity Paul M. Dietrich,† Nora Graf,† Thomas Gross,† Andreas Lippitz,† Bj€orn Sch€upbach,‡ Asif Bashir, Christof W€oll,§ Andreas Terfort,*,‡ and Wolfgang E. S. Unger*,†
BAM Bundesanstalt f€ ur Materialforschung und -pr€ ufung, VI.43 - Schicht- und Oberfl€ achenanalytik, D-12203 Berlin, Germany, ‡Institut f€ ur Anorganische und Analytische Chemie, Johann Wolfgang Goethe-Universit€ at Frankfurt, D-60438 Frankfurt, Germany, §Lehrstuhl f€ ur Physikalische Chemie I, Ruhr-Universit€ at Bochum, D-44780 Bochum, Germany, and Max-Planck-Institut f€ ur Eisenforschung GmbH, Max-Planck-Strasse 1, D-40237 D€ usseldorf, Germany )
†
Received September 2, 2009. Revised Manuscript Received December 8, 2009 Amino-terminated self-assembled monolayers on gold substrates were studied by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) measurements, and atomic force microscopy (AFM). Two different ω-amino-4,40 -terphenyl substituted alkanethiols of the general structure H2N-(C6H4)3-(CH2)n-SH (ATPn) were used: 2-(400 -amino-1,10 :40 ,100 -terphenyl-4-yl)ethane-1-thiol (n = 2, ATP2) and 3-(400 -amino-1,10 :40 ,100 -terphenyl-4yl)propane-1-thiol (n = 3, ATP3). Moreover, the addressability of amino groups within the films was investigated by chemical derivatization of ATPn SAMs with 3,5-bis(trifluoromethyl)phenyl isothiocyanate (ITC) forming fluorinated thiourea ATPn-F films. Evaluation of high-resolution C 1s and N 1s XPS data revealed successful derivatization of at least 50% of surface amino species. Furthermore, it could be demonstrated by angle-resolved NEXAFS spectroscopy that chemical derivatization with ITC has no noticeable influence on the preferential upright orientation of the molecules in the SAMs.
Introduction A multiplicity of relevant technological applications from heterogeneous catalysis over chemical sensors to medical diagnostics is based on highly functionalized solid supports. In those cases, a precise knowledge of surface functionalities is mandatory. A strategy to achieve ordered surfaces with specified functionalities utilizes self-assembly of suitable molecules on miscellaneous substrates.1-3 Self-assembled monolayers (SAMs) of thiolates on gold were used extensively in the past.4,5 Diagnostic devices as for example microarrays can be made from SAMs of ω-functionalized thiols and subsequent immobilization of the biomolecules of interest, for example, proteins, carbohydrates, and DNA.5-9 Among the vast amount of functional groups capable of binding biological compounds, the amino functionality is one of the most versatile for tethering many different biomolecules onto an underlying surface.10 In the course of the evaluation of aminated substrates suitable for potential microarray applications, we identified a new type of amino-terminated thiolate SAM on gold. Aminoterphenyl *To whom correspondence should be addressed. E-mail: Aterfort@chemie. uni-frankfurt.de (A.T.);
[email protected] (W.E.S.U.). (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 014109. (3) Ozin, G. A.; Hou, K.; Lotsch, B. V.; Cademartiri, L.; Puzzo, D. P.; Scotognella, F.; Ghadimi, A.; Thomson, J. Mater. Today 2009, 12, 12. (4) Kind, M.; W€oll, C. Prog. Surf. Sci. 2009, 84, 230. (5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (6) Pirrung, M. C. Angew. Chem., Int. Ed. 2002, 41, 1276. (7) Zhu, H.; Snyder, M. Curr. Opin. Chem. Biol. 2003, 7, 55. (8) Shumaker-Parry, J. S.; Zareie, M. H.; Aebersold, R.; Campbell, C. T. Anal. Chem. 2004, 76, 918. (9) Dhayal, M.; Ratner, D. M. Langmuir 2009, 25, 2181. (10) Hermanson, G. T. Bioconjugate Techniques, 2nd revised ed.; Academic Press (Elsevier): London, 2008.
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molecules of a general structure H2N-(C6H4)3-(CH2)n-SH (cf. ATPn) with two different alkyl linker (n = 2, 3) units were synthesized and used for monolayer formation. The structure of these molecules is a rationale of two principles. First, araliphatic thiols (aromatic thiols that bear an alkyl chain between the aromatic part and the thiol group) are known to form welldefined and highly oriented SAMs on gold, as they combine the advantages of saturated and aromatic thiols.11-22 Thus, the order results from the strong interaction between the stiff aromatic parts, while the alkyl chains primarily provide a somewhat flexible anchoring to the surface, permitting the aromatic systems to adopt optimal packing. Since the interaction is strong, it is not even broken up by secondary interactions such as hydrogen bonds, that are known to cause serious disorder in aliphatic (11) Rong, H.-T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wuhn, M.; W€oll, C.; Helmchen, G. Langmuir 2001, 17, 1582. (12) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408. (13) Fuxen, C.; Azzam, W.; Arnold, R.; Witte, G.; Terfort, A.; W€oll, C. Langmuir 2001, 17, 3689. (14) Heister, K.; Rong, H. T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 6888. (15) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 14462. (16) Cyganik, P.; Buck, M.; Wilton-Ely, J. D. E. T.; W€oll, C. J. Phys. Chem. B 2005, 109, 10902. (17) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Johansson, L. S. O.; Grunze, M.; Zharnikov, M. Langmuir 2005, 21, 4370. (18) Shaporenko, A.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Electron Spectrosc. Relat. Phenom. 2006, 151, 45. (19) Vericat, C.; Vela, M. E.; Benitez, G. A.; Gago, J. A. M.; Torrelles, X.; Salvarezza, R. C. J. Phys.: Condens. Matter 2006, 18, R867. (20) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Wilton-Ely, J. D. E. T.; Zharnikov, M.; W€oll, C. J. Am. Chem. Soc. 2006, 128, 13868. (21) Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; W€oll, C. Langmuir 2006, 22, 3647. (22) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Witte, G.; Zharnikov, M.; W€oll, C. J. Phys. Chem. C 2007, 111, 16909.
Published on Web 12/30/2009
DOI: 10.1021/la903293b
3949
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Dietrich et al. Scheme 1. Derivatization of ATPn SAMs with 3,5-Bis(trifluoromethyl)phenyl Isothiocyanate (ITC)
thiolate monolayers.23,24 Thus, it could be expected that the presence of the amino group would not alter the monolayer structure significantly. The second principle involves the use of an aromatic amine as terminal group: Primary alkyl amines can become protonated by thiols (both have pKa values of about 10-11), resulting in the significant formation of ammonium ions in solution and on the SAM, too. In contrast to this, aromatic amines are much less basic (pKa of corresponding ammonium salt 3-5), keeping the thiols unaltered. This should result in improved stability in both the deposition solution as well as the monolayer. Accessibility and reactivity of surface amino groups on those SAMs were tested by chemical derivatization with an isothiocyanate reagent. The reaction of ATPn SAMs with 3,5-bis(trifluoromethyl)phenyl isothiocyanate (ITC) leading to fluorine-containing thiourea ATPn-F products is shown in Scheme 1. For establishing labels at amines with ITC, side reactions have to be considered. The reaction is based on the nucleophilic attack at the central carbon atom of the isothiocyanate functionality. Primary amines as strong nucleophiles react readily with an isothiocyanate. Other nucleophiles such as water or hydroxyl groups may react as well. Especially the reaction with water may reduce the labeling capabilities of isothiocyanates due to condensation reactions.25 However, hydrolysis can be minimized by using anhydrous reaction conditions and an excess of ITC agent. Derivatization of surface amines using ITC has been successfully tested in a detailed study recently.26 Native and derivatized terphenyl thiolate SAMs were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) measurements and atomic force microscopy (AFM) in order to gain insight into chemical bonding, orientation, and surface morphology.
Experimental Details Materials. If not noted otherwise, all chemicals were purchased from Sigma-Aldrich and were of highest available purity. The synthesis of 2-(400 -amino-1,10 :40 ,100 -terphenyl-4-yl)ethane-1thiol (n = 2, ATP2) and 3-(400 -amino-1,10 :40 ,100 -terphenyl-4-yl)propane-1-thiol (n = 3, ATP3) will be described elsewhere. The Au substrates (Georg Albert PVD, Germany), which were used for the deposition of self-assembled monolayers, were prepared by thermal evaporation of 30 nm of Au (purity 99.99%) onto polished single-crystal Si(100) wafers that had been precoated with a 9 nm titanium adhesion layer. The gold substrates were cleaned with “piranha solution” (96% H2SO4/30% H2O2 7/3, v/v) by dipping for less than 30 s and then rinsing with water and ethanol. Caution: “Piranha” solution reacts violently with organic materials and must be handled with extreme care. SAMs were prepared in a sealed container under an atmosphere of nitrogen by immersing pieces of Au into an ethanolic solution (1 mM) of the (23) Dannenberger, O.; Weiss, K.; Himmel, H. J.; J€ager, B.; Buck, M.; W€oll, C. Thin Solid Films 1997, 307, 183. (24) Himmel, H.-J.; Terfort, A.; W€oll, C. J. Am. Chem. Soc. 1998, 120, 12069. (25) Blanco, J. L. J.; Barria, C. S.; Benito, J. M.; Mellet, C. O.; Fuentes, J.; Santoyo-Gonzalez, F.; Fernandez, J. M. G. Synthesis 1999, 1907. (26) Graf, N.; Lippitz, A.; Gross, T.; Pippig, F.; Holl€ander, A.; Unger, W. E. S. Anal. Bioanal. Chem. 2010, 396, 725.
3950 DOI: 10.1021/la903293b
respective ATPn for 24 h at room temperature. After immersion, samples were rinsed with ethanol. The aminated ATPn samples were derivatized with a solution of 3,5-bis(trifluoromethyl)phenyl isothiocyanate (ITC) (1 mL) and triethylamine (1 mL) dissolved in dimethyl sulfoxide (DMSO) (100 mL) for 3 h at room temperature. Afterward, the samples were washed in DMSO, methanol, and acetone each once to remove physisorbed materials.26,27 Instrumentation. XPS measurements were carried out with an AXIS Ultra DLD electron spectrometer (Kratos Analytical, U.K.). XP spectra were recorded using monochromated Al KR excitation at pass energies of 80 eV for survey and 20 eV for highresolution core level spectra. The electron emission angle was 0°, and the source-to-analyzer angle was 54°. The binding energy scale of the instrument was calibrated following a Kratos Analytical procedure, which uses ISO 15472 binding energy (BE) data.28 Spectra were taken by setting the instrument to the hybrid lens mode and the slot mode providing approximately a 300 μm 700 μm analysis area. The binding energy scale was calibrated using the C 1s component of aromatic hydrocarbon at BE = 284.7 eV.29,30 C 1s, N 1s, and S 2p high-resolution core level spectra were analyzed using the CasaXPS peak fit program, version 2.3.14. In curve fitting of C 1s spectra, components were allowed to have full widths at half-maximum (FWHMs) which were up to 1.5 times higher than those of the aromatic component at 284.7 eV. Specific components were additionally constrained in their position by the following chemical shifts 0.8 eV (C-S, C-N, Cβ-CF3), 1.5 eV (C-O, CdN), and 2.5 eV (CdO) referring to the aromatic component at 284.7 eV. Shake-up structures were formally fitted with the least number of components adopting the strategy by Beamson and Briggs.30 FWHMs for all component peaks in N 1s and S 2p spectra were constrained to be equal. FWHM values ranging from 0.8 to 1.3 eV were obtained for all high-resolution XP spectra. A Gaussian/ Lorentzian product function peak shape model (G/L = 30) was used in combination with a Shirley (C 1s) or linear (N 1s, S 2p) background. S 2p core level spectra were fitted with a minimum set of doublets of equal FWHM, a doublet separation of 1.2 eV, and an S 2p3/2/S 2p1/2 peak area ratio of 2:1.31 NEXAFS spectroscopy was carried out at the HE-SGM monochromator dipole magnet beamline at the synchrotron radiation source BESSY II (Berlin, Germany). Spectra were acquired at the C, N, and F K-edges in the TEY (total energy electron yield) mode.32 The resolution E/ΔE of grid 1 at the carbonyl π* resonance of CO (hν = 287.4 eV) was found to be in the order of 2500. The slit width used was 150 μm. Raw spectra were divided by the monochromator transmission function which was obtained with a freshly sputtered Au sample.32 C K-edges were recorded at four different angles (20°, 30°, 55°, and 90°) (27) Graf, N.; Yegen, E.; Gross, T.; Lippitz, A.; Weigel, W.; Krakert, S.; Terfort, A.; Unger, W. E. S. Surf. Sci. 2009, 603, 2849. (28) ISO 15472:2001, Surface chemical analysis - X-ray photoelectron spectrometers - Calibration of energy scales. (29) ISO 19318:2004, Surface chemical analysis - X-ray photoelectron spectroscopy - Reporting of methods used for charge control and charge correction. (30) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; Wiley: Chichester, UK, 1992. (31) Willey, T. M.; Vance, A. L.; van Buuren, T.; Bostedt, C.; Terminello, L. J.; Fadley, C. S. Surf. Sci. 2005, 576, 188. (32) St€ohr, J. NEXAFS Spectroscopy; Springer: Heidelberg, Germany, 1992.
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Figure 1. High-resolution C 1s (left), N 1s (middle), and S 2p (right) XP spectra of ATP2 (a-c, top) and ATP2-F (d-f, bottom). measured between the surface plane of the sample and the direction vector of the incident linearly polarized light beam. Energy alignment of the energy scale was achieved by using an I0 feature referenced to a C1s f π* resonance measured with a fresh surface of a HOPG (highly ordered pyrolytic graphite, Advanced Ceramic Corp., Cleveland, OH) at 285.4 eV.33 Spectra are shown with the pre-edge count rate subtracted and after normalization in units of the absorption edge jump.32 The AFM data were recorded using a Multimode Nanoscope IIIa (Digital Instruments) equipped with a J scanner and operated in tapping mode using ArrowTM NC tips (tip radius,
