Dynamic Behavior of Noble Metal Nanoparticle Assemblies in Solution
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Dynamic Behavior of Noble Metal Nanoparticle Assemblies in Solution Anthony S. Stender1,2* and Ning Fang1,2 1 Department of Chemistry, Iowa State University, Ames, IA 50011, U.S.A. 2 Ames Laboratory, U.S. Department of Energy, Ames, IA 50011-3020, U.S.A. *Corresponding author: [email protected] ABSTRACT Nanoparticle functionalization and assembly is undergoing a period of challenging yet exciting development. Much of this research effort has been focused on the development of new functionalities that might enable strategically-directed assembly of nanoparticles into structures that can be utilized in other important applications. In order to determine the success of such experiments, researchers often prepare a dried sample and study the assembly patterns with electron microscopy. However, these imaging techniques can be expensive and do not provide a complete illustration of what the three dimensional nanoparticle assemblies truly look like in solution. Moreover, sample preparation presents its own challenges. Most notably, sample preparation may cause alteration to the individual assemblies or unwanted aggregation of the assemblies upon removal of the solution. To address these concerns, assemblies of anisotropic nanoparticles were tracked freefloating in solution with differential interference contrast (DIC) microscopy. DIC microscopy is an optical technique based on interferometry with high lateral resolution and shallow depth of field. After functionalizing gold and silver nanoparticles for self-assembly, aliquots of the nanoparticle solutions were examined in real-time. Nanoparticle assemblies were observed undergoing rotations, internal vibrations, structural modifications, and interactions with other assemblies. Observations of the dynamic behaviors of nanoparticle assemblies serve as a complement to imaging with electron microscopy and provide new insights into the actual assembly process.
INTRODUCTION Directed assembly of anisotropic nanoparticles has garnered much research attention in recent years. Many researchers view directed assembly as a means for building structures or designing sensors that are not made possible through manufacturing techniques. Although some kinetic studies have been completed on nanoparticles during the assembly process, real time imaging of the assembly process and of free-floating aggregates is lacking from the literature. Recently it was shown that differential interference contrast (DIC) microscopy is wellsuited for tracking individual nanoparticles within complex environments.1 DIC microscopy also provides a shallow depth of field and has a high lateral resolution. Due to the presence of two orthogonal polarizers and two Nomarski prisms along the light path, nanoparticles have a black and white “shadow-cast” appearance when viewed at their plasmonic wavelengths. An anisotropic nanoparticle has varying degrees of white and black signal as the particle is rotated within the x, y plane. This variance in the signal follows a sinusoidal relationship and can be used to determ
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