Standardization of methodology of light-to-heat conversion efficiency determination for colloidal nanoheaters

Localized Photothermal Therapy (PTT) has been demonstrated to be a promising method of combating cancer, that additionally synergistically enhances other treatment modalities such as photodynamic therapy or chemotherapy. PTT exploits nanoparticles (called nanoheaters), that upon proper biofunctionalization may target cancerous tissues, and under light stimulation may convert the energy of photons to heat, leading to local overheating and treatment of cancerous cells. Despite extensive work, there is, however, no agreement on how to accurately and quantitatively compare light-to-heat conversion efficiency (η_Q) and rank the nanoheating performances of various groups of nanomaterials. This disagreement is highly problematic because the obtained η_Q values, measured with various methods, differ significantly for similar nanomaterials. In this work, we experimentally review existing optical setups, methods and physical models used to evaluate η_Q. In order to draw a binding conclusion, we cross-check and critically evaluate the same Au@SiO₂ sample in various experimental conditions. This critical study let us additionally compare and understand the influence of the other experimental factors, such as stirring, data recording and analysis, and assumptions on the effective mass of the system, in order to determine η_Q in a most straightforward and reproducible way. Our goal is therefore to contribute to the understanding, standardization and reliable evaluation of η_Q measurements, aiming to accurately rank various nanoheater platforms.