Hyper Rayleigh Scattering (HRS) is used to determine the absolute first hyperpolarizability of gold nanorods with an
aspect ratio of 2.2 and 2.7. Two different long axis lengths are used, namely 25.5 nm and 64 nm. This allows for a
discussion of the size effect for these centrosymmetric nanoparticles. A comparison of the first hyperpolarizabilities
obtained with that of spherical nanoparticles, also centrosymmetric particles, is then made to discuss the role of the shape
of these particles on the origin of the response. For the smallest nanorods, a strong hyperpolarizability normalized per
atom is determined underlining the role of the interface and the shape in determining such a large absolute value. For the
larger nanorods, the first hyperpolarizability per atom is smaller than that of the smaller nanorods but remains larger than
the one obtained for gold nanospheres with a similar volume indicating that the shape and the surface response still
continue to play a major role. These results are in agreement with the multipolar theory for the first hyperpolarizability of
centrosymmetric nanoparticles but show that for nanorods, the surface regime pertains over a longer size range.
The unique optical properties associated with nanostructured materials that support the excitation of surface plasmons
offer many new opportunities for the enhanced optical investigation of biological materials that pose a security threat. In
particular, ricin is considered a significant bioterrorism risk due to its high toxicity combined with its ready availability
as a byproduct in castor oil production. Therefore, the development of optical techniques capable of rapid on-site toxin
detection with high molecular specificity and sensitivity continues to be of significant importance. Furthermore,
understanding of the ricin cell entry and intracellular pathways remains poor due to a lack of suitable bioanalytical
techniques. Initial work aimed at simultaneously tackling both these issues is described where different approaches for
the nanoparticle labeling of ricin are investigated along with changes in ricin toxicity associated with the labeling
process.
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