Size-Dependent Photothermal Conversion Efficiencies of Plasmonically Heated Gold Nanoparticles

We report the light-to-heat energy transfer efficiencies of gold nanoparticles with variable sizes by assessing the temperature profiles of laser-activated particle suspensions in water. Gold nanoparticles with sizes ranging from 5 to 50 nm were synthesized by chemical reduction methods using sodium borohydride, sodium citrate, or hydroquinone as reducing agents. As-synthesized gold nanoparticle solution (1 mL) was loaded into a quartz 10 Ire cuvette and exposed to a CW green laser (532 nm). Heat input into the system by energy transfer from nanoparticles equals heat dissipation at thermal equilibrium. The transducing efficiency was then determined by plotting temperature increase as a function of laser power extinction. The efficiency increases from 0.650 +/- 0.012 to 0.803 +/- 0.008 as the particle size decreases from 50.09 +/- 2.34 to 4.98 +/- 0.59 nm, respectively. The results indicate that the photothermal properties of gold nanoparticles are size-tunable, and the variation of efficiency can be correlated to the absorption/extinction ratios calculated by Mie theory for different particle sizes. We further expanded our Mie theory calculations of absorption/extinction ratios to a broader range of diameters and wavelengths. These studies are crucial for practical applications of gold nanoparticles in nanotechnology and bioengineering, such as enhancing the treatment efficiency of laser surgery.