Characterization of Hydrophobic Forces for in Liquid Self-Assembly of Micron-Sized Functional Building Blocks
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Characterization of Hydrophobic Forces for in Liquid Self-Assembly of Micron-Sized Functional Building Blocks M. R. Gullo1, L. Jacot-Descombes1, L. Aeschimann2, J. Brugger1 1
Microsystems Laboratory, Institute of Microtechnology, Ecole Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne, Switzerland 2 Nanoworld AG, 2000 Neuchâtel, Switzerland ABSTRACT This paper presents the experimental and numerical study of hydrophobic interaction forces at nanometer scale in the scope of engineering micron-sized building blocks for self-assembly in liquid. The hydrophobic force distance relation of carbon, Teflon and dodeca-thiols immersed in degassed and deionized water has been measured by atomic force microscopy. Carbon and dodeca-thiols showed comparable attractive and binding forces in the rage of pN/nm2. Teflon showed the weakest binding and no attractive force. Molecular dynamic simulations were performed to correlate the molecular arrangement of water molecules and the hydrophobic interactions measured by atomic force microscopy. The simulations showed a depletion zone of 2Å followed by a layered region of 8Å in the axis perpendicular to the hydrophobic surface. INTRODUCTION Capillary forces have often been used to perform templated self-assembly (SA) [1] and folding [2] of micron-sized building blocks. However capillarity has proven to be hardly controllable and thus not fitted to achieve controlled and selective SA. This is even more the case for the SA of a defined number of functional building blocks inside the bulk of the liquid. An alternative and more controlled way to self-assemble micron-sized building blocks would be to use the hydrophobic force. It has been proven that specific and orthogonal hydrophobic patterning can be applied to target areas of the building blocks by local chemical functionalization [3]. Moreover hydrophobic gradients might be engineered to guide the SA and induce the self-repairing of miss aligned assemblies [4]. In order to optimize the design of such functionalized building blocks it is necessary to understand the nature of the hydrophobic force. Especially the force to distance relation of the attraction and the magnitude of binding force is of great interest [5, 6]. The surface force apparatus has extensively been used to study hydrophobic interactions. However it is limited in the materials that can be measured. One way to overcome this limitation is to use atomic force microscopes (AFM) to perform the force distance measurements. AFM is the first choice for many biologists to precisely measure protein unfolding forces and has proven to be a very reliable and sensitive tool [7]. The force distance measurements in this paper are exclusively done by AFM. There are many models that aim to explain the hydrophobic interaction: a) entropic effects due to molecular rearrangement of water, b) electrostatic effects, c) submicroscopic bubbles and d) cavitation [5, 6, 8]. Each one of these models describes only a part of the highly complex nature of the hydrophobic interaction. The final
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