The unique properties of metallic nanoparticles are key to their increasing prevalence in biomedical imaging and diagnostics. The high saturation magnetization (MS) of Magnetic Iron Oxide Nanoparticles (MIONs), which results in their high transverse relaxivity (r2), makes these materials particularly effective as MRI contrast agents. The characteristic plasmonic properties of gold nanoparticles, on the other hand, render them powerful tools in dark field microscopy. The combination of these two materials in a single nanocomposite, a so-called magnetoplasmonic assembly, is particularly appealing given the complementarities in terms of resolution and 3-D imaging capabilities of the dark field and MR imaging techniques. For biological applications, the necessity for cell uptake of the nanocomposites requires that their size be < 150 nm2 which in turn imposes the use of a core-shell design with an iron oxide core and an outermost gold layer. Such nanoparticles display both magnetic and plasmonic behaviors rendering them suitable for use as imaging agents for both MRI and dark field microscopy.
Although a number of magnetoplasmonic nanoparticles have been reported, a systematic study of the relationship between their structure and their physical properties is still lacking. Assemblies of such composites into dumbbells or 3-dimensional arrays of controlled architecture have not yet been reported. These assemblies are expected to display significantly different magnetic and plasmonic behavior than their corresponding monomers, thereby paving the way for the development of dual responsive MRI/dark field imaging agents. Moreover, as is the case for solid gold nanoparticles, the changes observed upon formation of the dumbbells are predicted to be very sensitive to the distance separating the nanoparticles. Such dumbbells would behave as magnetoplasmonic molecular rulers that should prove to be useful tools in studying biological processes such as protein assemblies. Herein we propose a systematic study of the relationship between the structure and composition of core-shell iron oxide-gold nanoparticles on their magnetic and plasmonic properties, and the use of these composites in the development of responsive dual magnetic/plasmonic imaging agents and molecular rulers.
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