Absorption cross section of gold nanoparticles based on NIR laser heating and thermodynamic calculations

We present a method for measuring the optical absorption cross section ( σ_abs ) of gold nanoparticles (GNPs) based on optically heating the solution of GNPs with an 808 nm near-infrared (NIR) laser and measuring the temperature increase of the solution. We rely on the theoretical calculations based on the heat diffusion equations and experimental measurements based on the energy balance equations to measure the σ_abs and the temperature distribution of single GNPs. Several morphologies, including gold nanospheres (GNSs), spherical gold nanoparticle conjugate (AuNPC), which are 20 nm GNSs surface-functionalized with an IR 808 dye, gold nanorods (GNRs), and gold nanourchins (GNUs), were studied. The study found that a single 20 nm GNS has the lowest σ_abs and temperature distribution as compared to 100 nm GNUs. By increasing the size of GNSs from 20 to 30 nm, the magnitude of σ_abs as well as temperature distribution increases by a factor of 5. The σ_abs values of 20 and 30 nm GNSs calculated by Mie theory and the experimentally measured are in a good agreement. GNRs with equivalent radius ( R_eq ) 9.16 nm show the second lowest σ_abs . By increasing the R_eq by a factor of 2 to 19.2 nm, the measured σ_abs and temperature distribution also increased by a factor of 2. We also estimated σ_abs for GNUs with diameters at 80 and 100 nm, which also have higher σ_abs values. This work confirms that we can use temperature to accurately measure the σ_abs of a variety of GNPs in solution.