Neodymium‐Based Stoichiometric Ultrasmall Nanoparticles for Multifunctional Deep‐Tissue Photothermal Therapy

Nanoparticle‐mediated photothermal therapy (NP‐PTT) constitutes a flexible, highly selective, cost effective, and accurate tool for cancer treatment alone or in combination with other therapies such as radiotherapy or chemotherapy. The future application of NP‐PTT in real life mainly depends on the design and synthesis of novel multifunctional nanoparticles that could overcome the current limitations of NP‐PTT such as limited penetration depth and absence of therapy control. In this work, ultrasmall (≈2.4 nm) NdVO4 stoichiometric (100% constituent Nd3+ ions) nanoparticles are reported, which are capable of in vivo sub‐tissue localized heating under 808 nm optical excitation while providing, simultaneously, the possibility of high penetration near‐infrared fluorescence imaging. Ultrasmall stoichiometric NdVO4 nanoparticles have evidenced a superior light‐to‐heat conversion efficiency. This is explained in terms of their large absorption cross‐section at 808 nm (consequence of the particular spectroscopic properties of neodymium ions in NdVO4 and of the high neodymium content) as well as to their ultrasmall size that leads to large nonradiative decay rates. Results included in this work introduce ultrasmall, NdVO4 stoichiometric nanoparticles to the scientific community as multifunctional photothermal agents that could be considered as an alternative to traditional systems such as metallic, organic, or carbon‐based nanoparticles.