Heat Generation in Gold Nanorods Solutions due to Absorption of Near-Infrared Radiation

Hyperthermia treatment of tumours surrounded by healthy tissues can be enhanced using radiative heating of embedded gold nanoparticles (GNPs) due to their optical resonance absorption in the so-called optical therapeutic window. In this paper resonance absorption of gold nanorods (GNRs) and correspondent heat generation in GNR solutions was studied both numerically and experimentally. The calculations based on the discrete-dipole approximation (DDA) showed a consistent relationship between the maximum absorption efficiency and the nanorod orientation with respect to the incident radiation. Additionally, the plasmonic wavelength and the maximum extinction efficiency of a single nanorod were shown to increase linearly with its aspect ratio when the nanorod volume was fixed. The wavelength of the surface plasmonic resonance (SPR) was found to change when the gold nanorods were closely spaced. Specifically, a shift and broadening of the resonance peak in the optical spectrum was obtained when the distance between the nanorods was about 50 nm or less. In parallel to numerical simulations, the optical experiment was performed where the transmission and reflection of suspended nanorods at various volume fractions were measured by a spectrophotometer to investigate their capability of absorption and heating. The plasmonic wavelength of the nanorods solution was shown to be around 780 ± 10 nm, which was in good agreement with computational predictions for coupled side-by-side nanorods. The temperature of solution heated by near infrared light was also measured in the laboratory experiments at various volume fractions of suspended nanorods. It was found that the rate of increase for both the temperature of solution and the absorbed light diminished when the volume fraction of suspended nanorods reached about 1.24 × 10−6. This can be explained by partial clustering of nanorods at their high volume fractions in water.