Size-dependent theoretical and experimental photothermal conversion efficiency of spherical gold nanoparticles

• Irradiating cancer cells with a laser with a wavelength of 650 nm causes a greater death rate than the 808 nm laser. • Value of photothermal efficiency decrease with increase of Au NPs size. • Addition of Au NPs to cells and irradiation resulted in higher mortality of cancer cells than irradiation without Au NPs addition. • Experimental value of Au NPs photothermal efficiency can be correlated with the absorption/extinction ratios calculated by the Mie theory. Due to their biocompatible and plasmonic properties, gold nanoparticles (Au NPs) are good candidates to be photosensitizers in photothermal cancer therapy (PTT). In this paper, the dependence of the NIR-light-to-heat energy on Au NPs size was investigated. Moreover, to determine the photosensitizing properties of gold nanoparticles, PTT was conducted on two colon cell lines: SW480 and SW620 by irradiating them with two lasers having different wavelengths. Transmission electron microscopy showed that the respective sizes of Au NPs were 10 nm, 12 nm and 16 nm. Moreover, local as well as global structural measurements showed that all synthesized Au NPs were crystalline and UV-Vis spectroscopy revealed that with increasing nanoparticles size the position of the surface Plasmon resonance (SPR) peaks is shifted to higher wavelengths. Decrease of cells viability was observed, when they were cultured with Au NPs and irradiated by 650 nm and 808 nm lasers. Moreover, FTIR and Raman spectra of cells, showed structural changes in DNA, phospholipids, proteins and cholesterol caused by the addition of nanoparticles and laser irradiation. The chemical changes were more pronounced in the cells cultured with Au NPs and irradiated by 650 nm lasers and these changes were dependent on the nanoparticle size. Moreover, the viability of cells investigated by the MTS assay showed, that the percentage of dead cells (∼40%) is the highest for cells cultured with 8 nm Au NPs and irradiated by the 650 nm laser. The photothermal conversion efficiency calculated from the experimental results showed a decrease of this parameter from 70% to 55% and from 61% to 48% with increasing particle size, for 650 nm and 808 nm lasers, respectively. The obtained results showed that the photothermal conversion efficiency of Au NPs is size-tunable, and can be correlated with the absorption/extinction ratios calculated by the Mie theory.