Indium-Based Fluoride Nanoparticles Doped with Chromium for Near-Infrared Luminescence

Abstract

Transition metal (TM) and rare earth (RE) ion-doped nanoparticles (NPs) are photoluminescent materials of technological relevance in bioimaging, sensing, and light conversion. Fluoride NPs are particularly attractive in this context, since they combine low-energy phonons, high chemical stability, optical transparency, size, and architecture tunability. Yet, nearly all reported colloidal fluoride NPs (e.g., NaYF₄ and LiYF₄) can only be efficiently doped with RE³⁺ and not with luminescent TM ions. Herein, we contribute to filling this gap in materials science by reporting Na₃InF₆ NPs doped with Cr³⁺ as a model luminescent TM ion. We unveil the heat-driven NP formation mechanism, which involves a cubic-to-monoclinic phase conversion, similarly to the cubic-to-hexagonal phase conversion in NaYF₄. Reaction temperatures above 225 °C and reaction time have a limited impact on the NP morphology, while the amount of fluoride precursor and oleylamine grants control over the NP size. After verifying that Na₃InF₆ NPs show negligible cytotoxicity toward U-87 cell line, we study the optical properties of these NPs upon Cr³⁺ doping. Temperature-dependent photoluminescence measurements indicate that Cr³⁺ ions experience a weak crystal field in the Na₃InF₆ host lattice, while their photoluminescence lifetime varies linearly in the 20–50 °C range. These results set the ground for further studies of photoluminescent TM-doped fluoride NPs, toward their applications in bioimaging, sensing, and light-converting devices.