pubs.acs.org/Langmuir © 2009 American Chemical Society
Single Molecules as Sensors for Local Molecular Adhesion Studies Markus Valtiner† and Guido Grundmeier*,†,‡ †
Christian Doppler Laboratory for Polymer/Metal Interfaces, Max-Planck-Insitut f€ ur Eisenforschung GmbH, Max-Planck-Strasse 1, D-40237 D€ usseldorf, Germany, and ‡Technical and Macromolecular Chemistry, University of Paderborn, Warburgerstrasse 100, D-33098 Paderborn, Germany Received June 21, 2009. Revised Manuscript Received July 11, 2009
An experimental approach is presented that allows the measurement of interactions of single macromolecules at the electrolyte/single-crystal interfaces under the continuous variation of electrolyte composition. Single polyelectrolyte desorption experiments with poly(acrylic acid) were performed on atomically defined single-crystal ZnO(0001)-Zn surfaces in aqueous electrolytes of varying pH and constant ionic strength. The corresponding characterized singlecrystalline surface structures were proven to be stable in the pH range of 4-11, enabling the analysis of desorption forces on both surface terraces and step edges as a function of pH. Thereby, contributions of electrostatic and van der Waals forces as well as contributions of coordinative interfacial bonds could be distinguished. The results showed that carboxylic acid functionalities adsorb weakly to hydroxide-stabilized polar ZnO(0001)-Zn surfaces with forces in the range of 60-80 pN, whereas they strongly bind to the separating step-edges between the polar terraces, most probably via coordinative bonds exhibiting forces of up to 700 pN. Thus, by means of single-molecule desorption spectroscopy individual binding sites could be readily identified by distinct features in the force-distance profiles. Moreover, the measurement of desorption forces on the large atomically flat terraces at varying pH proved that a maximum molecular desorption force occurs at pH 7 as a result of increasing repulsive interactions at pH values above the surface point of zero charge and decreasing electrostatic interactions when shifting the pH in the direction of the pKA of the poly(acrylic acid).
Introduction For the understanding and prediction of interfacial processes at polymer/oxide/metal interfaces such as polymer de-adhesion in environments of high water activity or even electrochemical corrosive de-adhesion, a molecular understanding of the desorption of single macromolecules from oxide surfaces is a key topic. Under conditions of corrosive de-adhesion of polymer films from oxide-covered metals, not only the formation of ultrathin water layers at the interface but also the change in the local pH values in these ultrathin water films is an important aspect of a delamination process.1-3 In the last few decades, the development of the surface science approach has allowed the defined understanding of atomic-scale processes and molecular dynamics under welldefined ultrahigh vacuum conditions.4-7 At electrolyte/metal interfaces, an atomic-level understanding of metal deposition and dissolution has been achieved by applying STM in a few outstanding research groups.8-13 However, performing similarly *Corresponding author. E-mail:
[email protected]. (1) Grundmeier, G.; Schmidt, W.; Stratmann, M. Electrochim. Acta 2000, 45, 2515–2533. (2) Leng, A.; Streckel, H.; Stratmann, M. Corros. Sci. 1999, 41, 547–578. (3) Leng, A.; Streckel, H.; Stratmann, M. Corros. Sci. 1999, 41, 579–597. (4) Lin, W. F.; Zei, M. S.; Eiswirth, M.; Ertl, G.; Iwasita, T.; Vielstich, W. J. Phys. Chem. B 1999, 103, 6968–6977. (5) Lin, W. F.; Zei, M. S.; Kim, Y. D.; Over, H.; Ertl, G. J. Phys. Chem. B 2000, 104, 6040–6048. (6) Reuter, K.; Scheffler, M. Phys. Rev. B 2006, 73, 045433. (7) Christmann, K.; Ertl, G. Thin Solid Films 1975, 28, 3–18. (8) Baldauf, M.; Kolb, D. M. Electrochim. Acta 1993, 38, 2145–2153. (9) Magnussen, O. M.; Hotlos, J.; Bettel, G.; Kolb, D. M.; Behm, R. J. J. Vacuum Sci. Technol., B 1991, 9, 969–975. (10) Kolb, D. M.; Ullmann, R.; Will, T. Science 1997, 275, 1097–1099. (11) Zuili, D.; Maurice, V.; Marcus, P. J. Electrochem. Soc. 2000, 147, 1393– 1400. (12) Kitakatsu, N.; Maurice, V.; Hinnen, C.; Marcus, P. Surf. Sci. 1998, 407, 36– 58. (13) Suzuki, T.; Yamada, T.; Itaya, K. J. Phys. Chem. 1996, 100, 8954–8961.
Langmuir 2010, 26(2), 815–820
well defined experiments at solid/electrolyte/polymer interfaces and bridging the gap between the structure of the interface and deadhesion as an interfacial process is still one of the most challenging aspects of adhesion science. With the development of extremely versatile AFM-based single-molecule force spectroscopy techniques in the last few years,14-18 it has become possible to perform mechanical experiments with single molecules on generic surfaces under ambient conditions and within liquid electrolytes. It could be demonstrated that single-molecule experiments allow the precise measurement of inter- and intramolecular forces. These experimental techniques provide the experimental platform for a molecularlevel understanding of adhesion and de-adhesion phenomena on material surfaces. Quite recently, AFM was applied to investigate the desorption of single polyelectrolyte molecules from solid substrates by covalently attaching single polyelectrolyte molecules onto a cantilever tip by employing thiol chemistry19,20 on gold-coated tips. The increase in free energy during the desorption of individual polyelectrolytes was shown to be linearly dependent on the extension length, resulting in terraces of constant force within a force-extension measurement.21 Long-term measurements with poly(acrylic acid)-functionalized tips14 demonstrated the stable (14) Seitz, M.; Friedsam, C.; Jostl, W.; Hugel, T.; Gaub, H. E. ChemPhysChem 2003, 4, 986–990. (15) Friedsam, C.; Seitz, M.; Gaub, H. E. J. Phys.: Condens. Matter 2004, 16, S2369–S2382. (16) Hugel, T.; Seitz, M. Macromol. Rapid Commun. 2001, 22, 989–1016. (17) Jerome, C.; Willet, N.; Jerome, R.; Duwez, A. ChemPhysChem 2004, 5, 147–149. (18) Duwez, A.-S.; Cuenot, S.; Jerome, C.; Gabriel, S.; Jerome, R.; Rapino, S.; Zerbetto, F. Nat. Nanotechnol. 2006, 1, 122–125. (19) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335. (20) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62–63. (21) Hanke, F.; Livadaru, L.; Kreuzer, H. J. Europhys. Lett. 2005, 69, 242–248.
Published on Web 08/17/2009
DOI: 10.1021/la9022322
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Article
Valtiner and Grundmeier
terraces and the step edges, respectively. Consequently, different desorption forces of PAAc can be expected at these two surface sites. By means of single-molecule desorption spectroscopy, individual binding sites could be readily identified by distinct features in the force-distance profiles, as schematically depicted in Figure 1 (right). Moreover, the measurement of desorption forces on the large, atomically flat terraces at varying pH allowed the correlation of desorption forces with material parameters as the point of zero charge (PZC) and the pKA value of the PAAc.
Experimental Section
Figure 1. (Left) The two AFM images represent the distinctly different surface structures that can be prepared on the Zn(0001)Zn surface. Both of these structures are single-crystalline. The step edges are separated either by micrometer-wide atomically flat terraces or by 40-100-nm-wide terraces.22 (Right) Schematics of the setup that was used. The desorption of single poly(acrylic acid) (PAAc) molecules by means of AFM was used as a nanoscopic sensor for measuring local bonding characteristics on single-crystalline ZnO(0001) surfaces.
binding of individual polyelectrolyte molecules to the AFM cantilever, which makes this technique an ideal tool for reproducibly studying local adhesion forces on generic surfaces on a molecular level. Within this work, we demonstrate the successful utilization of this approach in the study of molecular adhesion of macromolecules on metal oxide surfaces. Single-crystalline polar ZnO surfaces were studied because of the importance of such oxide surfaces in the stability of adhesively bonded galvanized steel substrates. In recent studies, we demonstrated that it is possible to prepare single-crystalline polar ZnO(0001)-Zn surfaces with two distinctly different topographies, which are stable over a wide pH range.22-24 On the one hand, it is possible to obtain a singlecrystalline surface with terraces that are between 40 and 100 nm in width and separated by steps with a height of one unit cell (5.2 A˚). On the other hand, it is possible to anneal these surface thermally in order to obtain atomically flat terraces that are in the micrometer range with a step height of 2-10 nm, as can be seen in the AFM images in Figure 1 (bottom and top, respectively). This provided us with the unique possibility to perform singlemolecular adhesion studies on single -crystalline oxide surfaces with distinct differently sized terraces and step edges but similar respective surface chemistry, allowing for a correlation of the measured single-molecule desorption spectra with the relative distribution of two different crystal surface structures in one experiment. As shown in refs 22 and 23, polar ZnO(0001)-Zn, prepared by alkaline etching and alkaline etching combined with thermal annealing, is saturated with strongly bound surface hydroxyls. However, the step-edge surfaces on the ZnO(0001)-Zn surface have dangling bonds at the surface. Thus, it could be assumed that adsorbed poly(acrylic acid) (PAAc) molecules show different bonding characteristics of their carboxylic acid groups to the (22) Valtiner, M.; Borodin, S.; Grundmeier, G. Phys. Chem. Chem. Phys. 2007, 9, 2406–2412. (23) Valtiner, M.; Borodin, S.; Grundmeier, G. Langmuir 2008, 24, 5350–5358. (24) (a) Valtiner, M.; Grundmeier, G. Mater. Res. Soc. Symp. Proc. 2007, 1035E, 1035-L13-02. (b) Valtiner, M.; Todorova, M.; Grundmeier, G.; Neugebauer, J. Phys. Rev. Lett. 2009, 103(6), 065502-1–065502-4.
816 DOI: 10.1021/la9022322
Chemicals and Materials. All chemicals were of p.a. grade (analytical reagent grade) and were used as supplied without any further purification. The following chemical were used: VWR-International. Concentrated H2SO4, ethylenediamine z.S., N-hydroxy-succinimid z.S., n-hexane, and ethanol. Sigma-Aldrich. 11-Mercapto-undecane-1-ol (99%), 99% 15mercapto-penta-decanoic acid, PAAc with Mw = 450.000 g/mol, 99% N-(3-dimethylamino-propyl)-N0 -ethyl carbodiimin, 10 concentrated phosphate-buffered saline solution, ethylendiamine-tetraacetate z.S., 0.2 N NaOH volumetric solution, and 0.01 N HClO4 standard solution for GC. The water used for the experiments was deionized (18.2 MΩ cm -1 resistivity) with a Purelab Plus UV (USF) filtration system. Hydrothermally grown ZnO single crystals with ZnO(0001)-Zn polished faces (
