Quantitative Comparison of the Light-to-Heat Conversion Efficiency in Nanomaterials Suitable for Photothermal Therapy

Abstract

Functional colloidal nanoparticles capable of converting between various energy types are finding an increasing number of applications. One of the relevant examples concerns light-to-heat-converting colloidal nanoparticles that may be useful for localized photothermal therapy of cancers. Unfortunately, quantitative comparison and ranking of nanoheaters are not straightforward as materials of different compositions and structures have different photophysical and chemical properties and may interact differently with the biological environment. In terms of photophysical properties, the most relevant information to rank these nanoheaters is the light-to-heat conversion efficiency, which, along with information on the absorption capacity of the material, can be used to directly compare materials. In this work, we evaluate the light-to-heat conversion properties of 17 different nanoheaters belonging to different groups (plasmonic, semiconductor, lanthanide-doped nanocrystals, carbon nanocrystals, and metal oxides). We conclude that the light-to-heat conversion efficiency alone is not meaningful enough as many materials have similar conversion efficiencies─in the range of 80-99%─while they significantly differ in their extinction coefficient. We therefore constructed their qualitative ranking based on the external conversion efficiency, which takes into account the conventionally defined light-to-heat conversion efficiency and its absorption capacity. This ranking demonstrated the differences between the samples more meaningfully. Among the studied systems, the top-ranking materials were black porous silicon and CuS nanocrystals. These results allow us to select the most favorable materials for photo-based theranostics and set a new standard in the characterization of nanoheaters.

Keywords: gold nanoparticles; lanthanide-doped nanomaterials; nanoheaters; photothermal conversion efficiency; photothermal treatment; porous silicon; semiconductor nanocrystals.

Figure 1

Schematic setup and methodology for measuring the efficiency of light-to-heat conversion of colloidal nanomaterials. (a) Experimental “droplet” setup (PM, power meter; TGC, thermographic camera). (b) Photography of a typical droplet irradiated by a 668 nm laser beam. (c) Exemplary data analysis. The graph above shows the optical power behind a drop of sample (I_s) or water I_H2O and the reference power (I_ref) measured simultaneously. The temperature rise (graph below) in the sample (T_s) water colloid and the solvent itself (T_H2O) must be known to evaluate the light-to-heat conversion efficiency. (d) Typical image of a droplet during heating; a scale is visible above the droplet—elements with a measured distance of 6.1 mm.

Figure 2

Mechanisms of heat generation and absorption spectra in different classes of NHs. (a) Localized surface plasmon resonance in plasmonic NHs. (b) Absorption spectra of gold nanospheres and gold nanorods. (c) Cross-relaxation in Nd³⁺ ions, which is the heat generation explanation for Nd³⁺ ions. (d) Absorption spectra of NaNdF₄, NaDyF₄, and NaSmF₄ nanoparticles dispersed in chloroform. (e) Mechanism of heat generation in semiconductors. (f) Absorption spectra of semiconductor nanocrystals investigated in this study.

Figure 3

Ranking of the nanoheaters studied in this work. (a) Light-to-heat conversion efficiency as a function of wavelength. (b) External light-to-heat conversion efficiency as a function of wavelength.

Figure 4

External light-to-heat conversion efficiency at 794 nm: ranking of nanoheaters investigated in this study.