pubs.acs.org/Langmuir © 2009 American Chemical Society
Characterization of the Reversible Interaction of Pairs of Nanoparticles Dispersed in Nematic Liquid Crystals Gary M. Koenig Jr., Juan J. de Pablo, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, University of Wisconsin;Madison, 1415 Engineering Drive, Madison, Wisconsin 53706 Received September 14, 2009. Revised Manuscript Received October 20, 2009 Observations of reversible interactions between pairs of chemically functionalized nanoparticles dispersed in nematic liquid crystals (LCs) are reported. In contrast to the irreversible association of microparticles in nematic LCs, by using gold nanoparticles and darkfield microscopy, particle tracking reveals the pairwise interactions of the nanoparticles in nematic LCs to be long-ranged and reversible. The measured range and strength of the pairwise interaction of the nanoparticles in the LCs was found to differ substantially from past theoretical predictions of nanoparticle interactions in LCs. The observation of reversible interactions between nanoparticles in LCs suggests that nematic LCs may provide new routes to spontaneous formation of ordered nanoparticle arrays.
Introduction Nanoparticles have been the subject of many recent investigations, in part because they exhibit unusual optical and electronic properties.1 The collective properties of ordered assemblies of nanoparticles have also been shown to be potentially useful in applications connected with the fields of photonics2 and electronics.3 In this latter context, methods demonstrated to permit the directed assembly of nanoparticles into organized arrays include polymer phase transitions,4 hybridization of sequence-specific DNA oligomers,5 and use of metal-ligand coordination interactions.6 The capability to design and direct the assembly of nanoparticles into a variety of ordered structures requires the ability to rationally manipulate interactions between nanoparticles. Whereas past studies have identified various combinations of van der Waals, depletion, hydrodynamic, electrostatic, hydrogen bonding, and steric interactions to be useful in directing the assembly of nanoparticles,7 herein we describe the first experimental observation and characterization of the reversible interaction between two nanoparticles under the influence of forces generated by a nematic liquid crystalline solvent. A number of past studies have reported on interactions between micrometer-sized particles dispersed in nematic liquid crystals (LCs).8,9 These studies have shown that micrometer-sized particles embedded within LCs generate a range of topological defects in the surrounding LC, and that the symmetry of the resulting interparticle interaction depends strongly on the nature *To whom correspondence should be addressed. E-mail: abbott@engr. wisc.edu.
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13318 DOI: 10.1021/la903464t
of these defects (e.g., defects with dipolar and quadrupolar symmetries/interactions have been observed).8,9 The magnitude of LC-mediated interactions between microparticles has been measured to be ∼103kBT, which is too large to permit reversible associations and thus spontaneous formation of organized assemblies.10 However, with the assistance of external fields (e.g., optical tweezers), microparticles have been directed into stable colloidal crystals that are held together by cohesive forces mediated by nematic LCs. In contrast to these prior studies of micrometer-sized particles, experimental observations of the interactions of discrete numbers of nanoparticles in LCs have not been reported. Numerical simulations, however, have been performed, and these simulations predict that topological defects formed around nanoparticles dispersed in LCs will mediate interactions that are short-ranged (
