Numerical Simulation of Light to Heat Conversion by Plasmonic Nanoheaters

Abstract

Plasmonic nanoparticles are widely recognized as photothermal conversion agents, i.e., nanotransducers or nanoheaters. Translation of these materials into practical applications requires quantitative analyses of their photothermal conversion efficiencies (η). However, the value of η obtained for different materials is dramatically influenced by the experimental setup and method of calculation. Here, we evaluate the most common methods for estimating η (Roper's and Wang's) and compare these with numerical estimates using the simulation software ANSYS. Experiments were performed with colloidal gold nanorod solutions suspended in a hanging droplet irradiated by an 808 nm diode laser and monitored by a thermal camera. The ANSYS simulations accounted for both heating and evaporation, providing η values consistent with the Wang method but higher than the Roper approach. This study details methods for estimating the photothermal efficiency and finds ANSYS to be a robust tool where experimental constraints complicate traditional methods.

Keywords: Roper method; Wang method; gold nanorods; hanging droplet; photothermal conversion efficiency; simulation.

Figure 1

General flowchart for the numerical analysis using ANSYS. T is temperature and t is time, and subindexes exp and sim stand for experimental and simulated, respectively. Created in BioRender. Nevárez Martínez, M. (2024) https://BioRender.com/j85p746.

Figure 2

(a) Diagram of the measurement setup and 3D views of the laser area/volume that intersect the droplet. The droplet is in blue and the laser is in red. (b) Planar representation of the laser area that intersects the droplet. (c) Representative droplet-on-needle 3D model with sample temperature distribution.

Figure 3

(a) Normalized absorption spectra of AuNRs and PEG-AuNRs. (b) TEM image of AuNRs.

Figure 4

(a) Heating–cooling curves for the droplets of 0.25 mM AuNRs, 0.50 mM AuNRs, and 0.50 mM PEG-AuNRs. The solid lines correspond to the average temperature of three measurements, and the shadows represent the standard deviation. The insets present example thermograms of a droplet under ambient conditions (left) and irradiation (right). (b) ON-OFF cycle evaluation. The insets include a droplet of 0.50 mM Au NRs before (left) and after a 3-cycle experiment (right).

Figure 5

Light-to-heat conversion efficiency (η, %) calculated by three different methods: Roper’s, Wang’s, and ANSYS. The ANSYS approach has been used to compare the effect of evaporation on the estimation of η. In all cases η is presented as the average of three measurements with error bars indicating the standard deviation.