Electromagnetic absorption mechanisms in metal nanospheres: Bulk and surface effects in radiofrequency-terahertz heating of nanoparticles

Abstract

We report on the absorption of electromagnetic radiation by metallic nanoparticles in the radio and far infrared frequency range, and subsequent heating of nanoparticle solutions. A recent series of papers has measured considerable radio frequency (RF) heating of gold nanoparticle solutions. In this work, we show that claims of RF heating by metallic nanoparticles are not supported by theory. We analyze several mechanisms by which nonmagnetic metallic nanoparticles can absorb low frequency radiation, including both classical and quantum effects. We conclude that none of these absorption mechanisms, nor any combination of them, can increase temperatures at the rates recently reported. A recent experiment supports this finding.

Electromagnetic heating effect of aggregated gold nanoparticle colloids

“…For uncoated neutral GNPs, the heating mechanisms have been discussed extensively in Hanson's work. 15,17 For simplicity, the following model is used to estimate the heating effect in a order of magnitude manner.…”

“…However, experimentally, Gannon 7 reported that GNP incubated tissue could be heated under 13.56 MHz, resulting in the temperature increase of the malignant tissue by several centigrade. Clearly, this heating effect could not be due to mono-dispersed naked GNPs, whose effects have been disproved both theoretically 17,18 and experimentally. 13,14 There shall have other mechanisms responsible for the reported temperature rise.…”

“…Later on, Li 13 and Liu 14 experimentally reported that the heating effect of GNP colloids was not due to the presence of GNPs, but the ionic impurities within the colloid. Theoretically, Hanson 17 investigated the heating mechanisms of GNPs from various aspects, including the dielectric loss, phonon transfer, and the surface electron effect, and concluded that naked GNPs could not be heated by 13.56 MHz RF field. Similarly, another theoretical work by Sassaroli 18 also concluded that naked GNPs could hardly be heated by low frequency RF fields.…”

“…One of the mechanisms that has been mentioned in both Refs. 17 Aggregation of GNPs is very likely to happen when ligands coated GNPs are injected to target cancer cells. 7 It has been reported that aggregated particles can increase the absorption cross section in the IR region.…”

Parameter studies on optimal absorption and electrophoretic resonances in lossy media

“…One hypothesis is then that the charged GNPs will facilitate an electrophoretic current that can destroy the cancer under electromagnetic radiation [1,4,5]. However, it is also important to observe the complexity of the clinical application with many possible physical and biophysical phenomena to consider, as well as the fact that several authors have questioned whether metal nanoparticles can be heated in radio frequency at all, see e.g., [1,3]. We have recently performed a study on the physical limitations for radio frequency absorption in a spherical suspension where the main theoretical result is the derivation of an optimal conjugate match with respect to the surrounding medium [4].…”