(397r) Comparison of Size Distributions of Hollow Shell Nanoparticles Determined By Particle Tracking and Direct Imaging
Particle tracking methods determine nanoparticle size distributions by measuring the independent diffusive Brownian motions of a large number of individual nanoparticles for fixed time intervals, and converting the diffusive motion into an effective “spherical” particle diameter using the Stokes – Einstein equation. Little information is available about the validity of these methods for non-spherical particles or porous hollow nanoshells. We are interested in hollow gold nanoshells (HGNs) as tunable near infra-red light absorbers; the absorption maximum depends on the ratio of the nanoshell thickness to the nanoshell diameter. Hence, a reliable method of measuring the nanoshell diameter is essential to optimizing HGN synthesis. Currently, the HGN synthesis, which involves the galvanic exchange of gold on a silver nanoparticle template, provides a relatively polydisperse population, which broadens the absorption maximum (1, 2). Direct imaging of the nanoparticles by depositing the particles on electron microscope grids and imaging with transmission electron microscopy (TEM), provides a precise measure of the size distribution, although the sample size is small and sizing and counting individual particles from the images is cumbersome and time consuming.
The alternative to both sizing and estimating the particle concentration in solution is particle tracking using a commercial Nanosight particle analysis system. However, it is unclear how accurate particle tracking and the Stokes-Einstein equation might be for hollow, porous, spheroidal nanoparticles. A direct comparison of the mean sizes and size distributions obtained via TEM and particle tracking will provide the necessary calibration and show if particle tracking is a viable method of obtaining size distribution for hollow, porous nanoparticles. We can then isolate the steps in the synthesis that lead to the greatest polydispersity; likely choices include the synthesis of the silver templates, the variation in gold nanoshells thickness caused during the reduction step, or the choice of capping ligands such as citrate or thiols to promote HGN stability in salt solutions. The effects of decreasing the polydispersity on the near infra-red light absorption will also be discussed.
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