Rare earth based nanostructures constitute a type of functional materials widely used and studied in the recent literature. The purpose of this review is to provide a general and comprehensive overview of the current state of the art, with special focus on the commonly employed synthesis methods and functionalization strategies of rare earth based nanoparticles and on their different bioimaging and biosensing applications. The luminescent (including downconversion, upconversion and permanent luminescence) and magnetic properties of rare earth based nanoparticles, as well as their ability to absorb X‐rays, will also be explained and connected with their luminescent, magnetic resonance and X‐ray computed tomography bioimaging applications, respectively. This review is not only restricted to nanoparticles, and recent advances reported for in other nanostructures containing rare earths, such as metal organic frameworks and lanthanide complexes conjugated with biological structures, will also be commented on.

Why is the Eu ³⁺ :CaMoO ₄ unit cell bigger than that of CaMoO ₄ ? Does Na ⁺ co-doping improve the quantum yield of Eu ³⁺ :CaMoO ₄ phosphors?

Bi³⁺ codoping has been proposed in this work with a twofold objective, namely, enhancing the luminescence emission of Eu³⁺:LaF₃ nanoparticles (NPs) and increasing their X-ray attenuation capacity, with the purpose of obtaining a bimodal bioprobe for luminescence bioimaging and X-ray computed tomography. The synthesis method, reported here for the first time for LaF₃ particles, allowed obtaining uniform, nonaggregated NPs using a homogeneous precipitation in polyol medium at room temperature in just 2 h. The simplicity of the synthesis method allows the large-scale production of NPs. LaF₃ NPs with different Eu³⁺ contents were first synthesized to find the critical Eu³⁺ concentration, producing the highest emission intensity. This concentration was subsequently used to fabricate Bi³⁺–Eu³⁺-codoped LaF₃ NPs using the same method. The emission intensity of the codoped NPs increased in more than one order of magnitude, thanks to the possibility of excitation through the Bi³⁺ → Eu³⁺ energy-transfer band. The luminescence properties of the codoped NPs were analyzed in detail to find the mechanism responsible for the emission enhancement. Finally, it was demonstrated that the high atomic number of Bi³⁺, higher than that of lanthanides, was an added value of the material because it increased its X-ray attenuation capacity. In summary, the LaF₃ NPs codoped with Eu³⁺ and Bi³⁺ presented in this work are promising candidates as a bimodal bioprobe for luminescence bioimaging and X-ray computed tomography.

The safety assessment of nanoparticles (NPs) is crucial during their design and development for biomedicine. One of the prerequisite steps during this evaluation is in vitro testing that employs cell-based assays not always validated and well-adapted for NPs. Interferences with in vitro assays may arise due to the nano-related optical, oxidative, fluorescent, surface and catalytic properties of NPs. Thus, proper validation of each assay system has to be performed for each NP type. This study aimed to evaluate the applicability of the most common in vitro cytotoxicity assays for the safety assessment of up- and down-converting lanthanide-doped NPs. Conventional cell viability tests and fluorescence-based assays for oxidative stress response were selected to determine the biological effects of up- and down-converting NPs to human brain cells. Comparison with known silver and iron oxide NPs was made for verification purposes. Both the plate reader and flow cytometric measurements were examined. The obtained results indicated that both types of Ln-doped NPs interfered to a much lesser extent than metallic NPs. In addition, the great potential of both up- and down-converting NPs for biomedicine was manifested due to their biocompatibility and low toxicity.