Advancing neodymium single-band nanothermometry

Near-infrared (NIR) emitting contrast agents with integrated optical temperature sensing are highly desirable for a variety of biomedical applications, particularly when subcutaneous target visualization and measurement of its thermodynamic properties are required. To that end, the possibility of using Nd³⁺ doped LiLuF₄ rare-earth nanoparticles (RENPs) as NIR photoluminescent nanothermometers is explored. These RENPs are relatively small, have narrow size distribution, and can easily be core/shell engineered - all combined, these features meet the requirements of biologically relevant and multifunctional nanoprobes. The LiLuF₄ host allows to observe the fine Stark structure of the ⁴F_3/2→⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 optical transitions, each of which can then be used for single-band NIR nanothermometry. The thermometric parameter defined for the most intense Nd³⁺ emission around 1050 nm, shows high temperature sensitivity (∼0.49% °C^-1), and low temperature uncertainty (0.3 °C) as compared to the thermometric parameters defined for the 880 and 1320 nm Nd³⁺ emissions. Additionally, transient temperature measurements through tissue show that these RENPs can be used to assess fast temperature changes at a tissue depth of 3 mm, while slower temperature changes can be measured at even greater depths. Nd³⁺ doped LiLuF₄ RENPs represent a significant improvement for Nd³⁺ based single-band photoluminescence nanothermometry, with the possibility of its integration within more sophisticated multifunctional theranostic nanostructures.

Tuning the sensitivity of lanthanide-activated NIR nanothermometers in the biological windows

Lanthanide-activated SrF₂ nanoparticles with a multishell architecture were investigated as optical thermometers in the biological windows. A ratiometric approach based on the relative changes in the intensities of different lanthanide (Nd³⁺ and Yb³⁺) NIR emissions was applied to investigate the thermometric properties of the nanoparticles. It was found that an appropriate doping with Er³⁺ ions can increase the thermometric properties of the Nd³⁺-Yb³⁺ coupled systems. In addition, a core containing Yb³⁺ and Tm³⁺ can generate light in the visible and UV regions upon near-infrared (NIR) laser excitation at 980 nm. The multishell structure combined with the rational choice of dopants proves to be particularly important to control and enhance the performance of nanoparticles as NIR nanothermometers.

Double rare-earth nanothermometer in aqueous media: opening the third optical transparency window to temperature sensing

Owing to the alluring possibility of contactless temperature probing with microscopic spatial resolution, photoluminescence nanothermometry at the nanoscale is rapidly advancing towards its successful application in biomedical sciences. The emergence of near-infrared nanothermometers has paved the way for temperature sensing at the deep tissue level. However, water dispersibility, adequate size at the nanoscale, and the capability to efficiently operate in the second and third biological optical transparency windows are the requirements that still have to be fulfilled in a single nanoprobe. In this work, these requirements are addressed by rare-earth doped nanoparticles with core/shell-architecture, dispersed in water, whose excitation and emission wavelengths conveniently fall within the biological optical transparency windows. Under heating-free 800 nm excitation, double nanothermometry is realized either with Ho³⁺-Nd³⁺ (1.18-1.34 μm) or Er³⁺-Nd³⁺ (1.55-1.34 μm) NIR emission band ratios, both displaying equal thermal sensitivities around 1.1% °C^-1. It is further demonstrated that, along with the interionic energy transfer processes, the thermometric properties of these nanoparticles are also governed by the temperature dependent energy transfer to the surrounding solvent (water) molecules. Overall, this work presents a novel water dispersible double ratiometric nanothermometer operating in the second and third biological optical transparency windows. The temperature dependent particle-solvent interaction is also presented, which is critical for e.g. future in vivo applications.

Covering the optical spectrum through collective rare-earth doping of NaGdF4 nanoparticles: 806 and 980 nm excitation routes

Sensitization of numerous emission bands (from ultraviolet to near-infrared) in rare-earth doped multilayered nanoparticles: 806 versus 980 nm excitation.