Upconversion Properties and Temperature-Sensing Behaviors of Alkaline-Earth-Metal Scandate Nanocrystals Doped with Er3+/Yb3+ Ions in the Presence of Alkali Ions (Li+, Na+, and K+)

Exploring the potential of lanthanide-doped oxyfluoride materials for bright green upconversion and their promising applications towards temperature sensing and drug delivery

The most efficient upconversion (UC) materials reported to date are based on fluoride hosts with low phonon energies, which reduce the amount of nonradiative transitions. In particular, NaYF4 doped with Yb3+ and Er3+ at appropriate ratios is known as one of the most efficient UC phosphors. However, its low thermal stability limits its use for certain applications. On the other hand, oxide hosts exhibit better thermal stability, yet they have higher phonon energies and are thus prone to lower UC efficiencies. As a result, developing host nanomaterials that combine the robustness of oxides with the high upconversion efficiencies of fluorides remains an intriguing prospect. Herein, we demonstrate the formation of ytrrium doped oxyfluoride (YOF:Yb3+,Er3+) particles, which are prepared by growing a NaYF4:Yb3+,Er3+ layer around SiO2 spherical particles and consecutively applying a high-temperature annealing step followed by the removal of SiO2 template. Our interest lies in employing these materials as Boltzmann type physiological range luminescence thermometers, but their weak green emission is a drawback. To overcome this issue, and engineer materials suitable for Boltzmann type thermometry, we have studied the effect of introducing different metal ion co-dopants (Gd3+, Li+ or Mn2+) into the YOF:Yb3+,Er3+ particles, focusing on the overall emission intensity, as well as the green to red ratio, upon 975 nm laser excitation. These materials are explored for their use as ratiometric thermometers, and further also as drug carriers, including their simultaneous use for these two applications. The investigation also includes examining their level of toxicity towards specific human cells - normal human dermal fibroblasts (NHDFs) - to evaluate their potential use for biological applications.

Regulating the upconversion luminescence properties of Tm3+/Yb3+-codoped ZrScW2PO12 microparticles with a negative thermal expansion effect through thermal stimulation for optical thermometry

Tm3+/Yb3+-codoped ZrScW2PO12 microparticles were prepared in order to solve the problems of the severe thermal quenching and unsatisfactory thermometric properties of most luminescent materials. The synthesized materials exhibit a rarely observed negative thermal expansion (NTE) effect, which was verified by in situ X-ray diffraction experiments, performed under high temperature conditions. Upon excitation with a 980 nm laser, bright blue upconversion (UC) emissions originating from Tm3+ were observed. Moreover, owing to the promoted energy transfer, cross-relaxation and non-radiative decay processes at high temperatures triggered by the NTE effect, the observed UC emissions arising from 1G4 and 3F2,3 levels show non-monotonic responses to temperature. By analysing the temperature-dependent luminescence intensity ratio of these UC emissions originating from the non-thermally coupled levels of Tm3+ (1G4 and 3F2,3), the thermometric properties of the prepared microparticles were investigated in detail. Interestingly, the maximum absolute and relative sensitivities of the synthesized compounds are 0.09 and 1.45% K-1, respectively, which are independent of Yb3+ content, but they can be manipulated by employing different sensing modes. Our results manifest that the exploitation of the NTE effect is an efficient way to control the UC luminescence features of rare earth ions and to realize high performance optical thermometry.

Manipulating the thermometric behaviors of Er3+/Yb3+/Ho3+-tridoped La2Mo3O12 polychromatic upconverting microparticles via adjusting spatial mode and a sensing strategy

To meet the needs of contactless optical thermometry, Er3+/Yb3+/Ho3+-tridoped La2Mo3O12 (LMO) microparticles were designed and synthesized. Upon exciting with 980 nm light, the synthesized compounds emit glaring upconversion (UC) emissions and their emission colors can be tuned from green to yellow by altering the Ho3+ content. It is found that the optimal doping contents for Yb3+ and Ho3+ in LMO are 9 and 1 mol%, respectively, and the UC emission mechanism involved is a two-photon harvest process. Using the fluorescence intensity ratio (FIR) technique to analyze the temperature responses of the UC emissions arising from thermally coupled levels (TCLs) and non-thermally coupled levels (non-TCLs), the temperature sensing abilities of the synthesized samples were investigated. When the TCLs of Er3+ (2H11/2, 4S3/2) are used, the synthesized microparticles present the highest absolute and relative sensitivities of 0.0085 and 1.0236% K-1, respectively. Moreover, when the non-TCLs of Er3+ (2H11/2) and Ho3+ (5F5) are used, the maximum absolute and relative sensitivities of the synthesized compounds are 0.0296 and 0.6287% K-1, respectively. Clearly, the thermometric characteristics of the final products can be regulated via using different sensing strategies (i.e., TCLs and non-TCLs) and emission combinations (i.e., spatial mode). However, the change of the Ho3+ content has little impact on the temperature sensing capacity of the synthesized products. These results indicate that Er3+/Yb3+/Ho3+-tridoped LMO microparticles are promising candidates for optical thermometers and our findings also provide possible strategies for regulating the thermometric properties of rare-earth ion doped luminescent materials.

Up-conversion luminescence properties and temperature sensitivity of AgBi(MoO4)2: Yb3+/Er3+/Ho3+/Tm3+ phosphors

This work begins with a study of the synthesis of multi-color luminescence phosphors, and it is followed by a discussion of the temperature sensitivity of AgBi(MoO4)2:Yb3+/Er3+. Uniform and approximately spherical...

An overview of boosting lanthanide upconversion luminescence through chemical methods and physical strategies

Lanthanide-doped upconversion nanoparticles have attracted extensive research interest due to their promising applications in various fields.