Article pubs.acs.org/Langmuir
Adhesion of Colloidal Particles on Modified Electrodes Volodymyr Kuznetsov and Georg Papastavrou* Physical Chemistry II, University of Bayreuth, Universitätsstrasse, 95440 Bayreuth, Germany S Supporting Information *
ABSTRACT: The adhesion between colloidal silica particles and modified electrodes has been studied by direct force measurements with the colloidal probe technique based on the atomic force microscope (AFM). The combination of potentiostatic control of gold electrodes and chemical modification of their surface with self-assembled monolayers (SAMs) allows for the decoupling of forces due to the electrical double layers and functional groups at the solid/liquid interface. Adhesion on such electrodes can be tuned over a large range using the externally applied potential and the aqueous solution’s ionic strength. By utilizing cantilevers with a high force constant, it is possible to separate the various contributions to adhesion in an unambiguous manner. These contributions comprise diffuse-layer overlap, van der Waals forces, solvent exclusion, and electrocapillarity. A quantitative description of the observed adhesion forces is obtained by taking into account the surface roughness of the silica particle. The main component of the adhesion forces originates from the overlap of the electrical double layers, which is tuned by the external potential. By contrast, effects due to electrocapillarity are of only minor importance. Based on our quantitative analysis, a new approach is proposed that allows tuning of the adhesion force as a function of the externally applied potential. We expect this approach to have important applications for the design of microelectromechanical systems (MEMS), the development of electrochemical sensors, and the application of micro- and nanomanipulation.
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INTRODUCTION Adhesion between surfaces has a strong influence on various processes such as colloidal transport in soils, wafer cleaning, colloidal aggregation, and friction between solids.1−3 Adhesion is mediated by surface forces and represents a ubiquitous phenomenon in the colloidal domain. Typically, long-ranged interaction forces determine whether a colloidal particle can approach another surface closely enough to reach contact.1 After this point, additional short-ranged interaction mechanisms have a strong influence on the sticking probability of a colloid and determine whether a given shear force is sufficient to remove an adhering colloidal particle. To tune the adhesion behavior for industrial and technical applications, various approaches have been proposed. Most of these are based on permanently altering the surface properties, for example, using polyelectrolyte coatings or monolayers of silanes or thiols.4,5 Interaction forces of various origins can mediate adhesion. The long-ranged force contributions are summarized in the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) and result from the interaction of the diffuse layers and van der Waals (vdW) forces.2 Short-ranged forces act only in the © 2012 American Chemical Society
contact area. Such forces result from solvent exclusion as well as the formation of chemical bonds and have been studied by chemical force microscopy.6−11 In the case of polymeric interfaces, additional force contributions can arise as a result of steric or bridging forces.12 With increasing miniaturization of mechanical devices down to the micro- and nanometer scales, the control of adhesive properties has become increasingly important for device performance and fabrication.13 In particular, the possibility of direct and instantaneous control of the adhesion would be important for many applications. Suitable external stimuli to trigger changes in adhesion include electrical potential, illumination by light, or variation of the temperature. The latter two approaches have been studied extensively in recent years, and typical examples are thin organic films based on poly(N-isopropylacrylamide) (PNIPAM) and azobenzene moieties, respectively.14,15 However, for many applications, an Received: July 23, 2012 Revised: October 12, 2012 Published: October 16, 2012 16567
dx.doi.org/10.1021/la3029726 | Langmuir 2012, 28, 16567−16579
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modification as received. Ethanolic solutions were prepared from ethanol of analytical grade. All aqueous solutions were prepared using deionized water of Milli-Q grade with a resistivity greater than 18 MΩ. After the pH of the solution had been regulated to pH 4.7 by addition of a traceable volume of 1 M HCl (Sigma Aldrich), the total ionic strength was adjusted to a nominal value of 0.12, 0.34, 0.56, or 5 mM by addition of 1 M KCl (Sigma Aldrich). Preparation of Flat Gold Electrodes. Smooth gold substrates were prepared by a modified “template stripping” method.33 As substrates, we used p-doped (
