Hexagonal-phase NaREF4 upconversion nanocrystals: the matter of crystal structure

Optimization of the structure, morphology and luminescent properties of NaYF4 upconversion nanoparticles

We designed and constructed rare earth doped upconversion nanoparticles β-Na(Y0.78Yb0.18Er0.04)F4, sensitizing layer encapsulated β-Na(Y0.9Er0.1)F4@β-NaYbF4 and inert layer encapsulated β-Na(Y0.9Er0.1)F4@β-NaYbF4@β-NaYF4. Compared with the mononuclear material, the luminescence intensity of the particles encapsulated with double shells in the three main bands of blue, green and red emissions increased by 346, 22, and 54 times respectively. While improving the upconversion luminescence performance, the underlying reasons for this improvement were analyzed in detail. The effects of shell coating on the fluorescence lifetime, thermal stability and energy level transition are discussed. On this basis, the composite film material was constructed by combining the shell coating strategy and the plasma resonance interaction strategy, which further improved the upconversion efficiency. In addition, by combining performance optimized upconversion particles with information coding, we explored its potential as an anti-counterfeiting material.

Manipulation of Luminescence via Surface Site Occupation in Ln3+-Doped Nanocrystals

Ln3+-doped (Ln = lanthanide) nanocrystals are garnering strong interest for their potential as optical materials in various applications. For that reason, a thorough understanding of photophysical processes and ways to tune them in these materials is of great importance. This study, using Eu3+-doped Sr2YF7 as a well-suited model system, underscores the (not unexpected) significance of surface site occupation of Ln3+ and also challenges the prevailing views about their contribution to the luminescence of the system. High-temperature cation exchange and epitaxial shell growth allow nanocrystals to exclusively feature Eu3+ residing at the surface or in the interior, thereby separating their spectral responses. Meticulous experiments reveal that nanocrystals with high doping concentrations exhibit luminescence primarily from surface Eu3+, in contrast to the popular belief that luminescence from surface Ln3+ is largely negligible. The present study shows, on the one hand, the necessity to revise common ideas and also reveals the potential for manipulating the luminescence of such materials through an, until now, unperceived way of surface engineering.

The Coming of Age of Neodymium: Redefining Its Role in Rare Earth Doped Nanoparticles

Among luminescent nanostructures actively investigated in the last couple of decades, rare earth (RE³⁺) doped nanoparticles (RENPs) are some of the most reported family of materials. The development of RENPs in the biomedical framework is quickly making its transition to the ∼800 nm excitation pathway, beneficial for both in vitro and in vivo applications to eliminate heating and facilitate higher penetration in tissues. Therefore, reports and investigations on RENPs containing the neodymium ion (Nd³⁺) greatly increased in number as the focus on ∼800 nm radiation absorbing Nd³⁺ ion gained traction. In this review, we cover the basics behind the RE³⁺ luminescence, the most successful Nd³⁺-RENP architectures, and highlight application areas. Nd³⁺-RENPs, particularly Nd³⁺-sensitized RENPs, have been scrutinized by considering the division between their upconversion and downshifting emissions. Aside from their distinctive optical properties, significant attention is paid to the diverse applications of Nd³⁺-RENPs, notwithstanding the pitfalls that are still to be addressed. Overall, we aim to provide a comprehensive overview on Nd³⁺-RENPs, discussing their developmental and applicative successes as well as challenges. We also assess future research pathways and foreseeable obstacles ahead, in a field, which we believe will continue witnessing an effervescent progress in the years to come.

Extending the dynamic temperature range of Boltzmann thermometers

Lanthanide-doped (nano)crystals are an important class of materials in luminescence thermometry. The working mechanism of these thermometers is diverse but most often relies on variation of the ratio of emission intensities from two thermally coupled excited states with temperature. At low temperatures, nonradiative coupling between the states can be slow compared to radiative decay, but, at higher temperatures, the two states reach thermal equilibrium due to faster nonradiative coupling. In thermal equilibrium, the intensity ratio follows Boltzmann statistics, which gives a convenient model to calibrate the thermometer. Here, we investigate multiple strategies to shift the onset of thermal equilibrium to lower temperatures, which enables Boltzmann thermometry in a wider dynamic range. We use Eu³⁺-doped microcrystals as a model system and find that the nonradiative coupling rates increase for host lattices with higher vibrational energies and shorter lanthanide-ligand distances, which reduces the onset temperature of thermal equilibrium by more than 400 K. We additionally reveal that thermometers with excited states coupled by electric-dipole transitions have lower onset temperatures than those with magnetic-dipole-coupled states due to selection rules. These insights provide essential guidelines for the optimization of Boltzmann thermometers to operate in an extended temperature range.

The importance, status, and perspectives of hybrid lanthanide-doped upconversion nanothermometers for theranostics

Theranostics combines diagnostics and therapy in a single multifunctional system. Multifunctional upconversion luminescent lanthanide-doped nanothermometers for theranostic purposes offer non-invasive and sensitive multimodal performance in the biomedical field over traditional temperature measurement methods. Despite existing challenges, various studies on hybrid upconversion nanothermometers show substantial progress for (bio)imaging, temperature sensing, photodynamic and photothermal therapy, as well as drug delivery applications. The beauty of such an approach is that it unfolds possibilities to combine diagnostics and therapy in a single particle, which can modify the way certain diseases are treated, hence change the entire healthcare scene.