Abstracts

Certain optically-triggered nanoparticles (NPs) greatly facilitate the light-into-heat conversion in a region of interest, thus serving for photoacoustic imaging (PAI) and photothermal therapy (PTT), which are attractive in vivo biomedical applications both requiring local and controlled delivery of heat. More specifically, plasmonic NPs with their optical extinction engineered to lie within the third near-infrared window (NIR-III, 1500-1800 nm) allow higher penetration depth into tissues and fewer side-effects (e.g. heating of non-targeted regions) than those working in the NIR-II (1000-1350 nm). That is because of the minimal absorption and scattering of the excitation beam at NIR-III wavelengths when propagating through tissues. Moreover, beyond the use of such plasmonic NPs to build an all-optical system achieving local heat release for PAI and PTT, the combination of luminescent and plasmonic NPs can form an integrated system combining detection and therapy (so-called theranostic). Currently, the broadly extended use of luminescent NPs with excitation in the first (NIR-I, 750-950 nm) and emission within NIR-II, respectively, leaves NIR-III available for optical-triggering of plasmonic NPs for the previously mentioned applications (PAI and PTT).This work addressed the lack of NPs working exclusively in such NIR-III range by preparing indium oxide plasmonic NPs doped with tin (ITO), purposedly tuning their optical extinction to the 1600-1800nm range. Furthermore, these ITO NPs show a heat-conversion-efficiency (HCE) value above 80% under irradiation at 1700 nm. Remarkably, a potential theranostic system was straightforwardly mimicked by mixing the ITO NPs with luminescent Nd-doped NaGdF₄ NPs -nowadays a widely used workhorse for NIR-II in vivo imaging and temperature sensing. Spectra obtained after excitation of the nanofluid at 800 nm showed a minimum spectral distortion and very low reduction in Nd-doped NP emission intensity. Hence, it demonstrates the minimal interference from ITO absorption over the information-carrying emission from those Nd-doped NPs acting as imaging agents/sensors and located nearby. The spectral profile of ITO NPs´ optical extinction and their high HCE value make them ideal candidates for PAI and PTT and as a novel tool to be combined with other (luminescent) NPs for an integrated theranostic platform.