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This paper presents a study on the physical limitations for radio frequencyabsorption in gold nanoparticle suspensions. A canonical spherical geometry isconsidered consisting of a spherical suspension of colloidal gold nanoparticlescharacterized as an arbitrary passive dielectric material which is immersed inan arbitrary lossy medium. A relative heating coefficient and a correspondingoptimal near field excitation are defined taking the skin effect of thesurrounding medium into account. For small particle suspensions the optimalexcitation is an electric dipole field for which explicit asymptoticexpressions are readily obtained. It is then proven that the optimalpermittivity function yielding a maximal absorption inside the sphericalsuspension is a conjugate match with respect to the surrounding lossy material.For a surrounding medium consisting of a weak electrolyte solution the optimalconjugate match can then readily be realized at a single frequency, e.g., bytuning the parameters of a Drude model corresponding to the electrophoreticparticle acceleration mechanism. As such, the conjugate match can also beregarded to yield an optimal plasmonic resonance. Finally, a convexoptimization approach is used to investigate the realizability of a passivematerial to approximate the desired conjugate match over a finite bandwidth.The relation of the proposed approach to general Mie theory as well as to theapproximation of metamaterials are discussed. Numerical examples are includedto illustrate the ultimate potential of heating in a realistic scenario in themicrowave regime.

On the physical limitations for radio frequency absorption in gold nanoparticle suspensions