Enhancing the upconversion of Er3+ incorporated BaTiO3 by introducing oxygen vacancies.

Lanthanide-incorporated crystals display the phenomenon of upconversion (UC), wherein near-infrared (NIR) light is converted into ultraviolet-visible (UV-Vis) emission with a narrow bandwidth. This unique photophysical property renders lanthanide UC materials highly promising for diverse applications. However, the limited quantum efficiency (∼3%) hinders the broader utilization of UC materials. Consequently, numerous studies have focused on overcoming this low efficiency. Notably, it has been observed that manipulation of the site symmetry in UC materials significantly enhances their UC efficiency. In this study, we investigate the UC enhancement of Er3+ incorporated BaTiO3 (E-BT) crystals through the introduction of oxygen vacancies (OV). The OV were created using a post-heat treatment method, and the annealing time was varied to control the quantity of OV. An optimal annealing time of 6 hours was determined for efficient OV generation, beyond which the OV content decreased. Remarkably, E-BT crystals with OV exhibited up to three-fold greater UC compared to those without OV. This outcome suggests that OV induce symmetry changes in the E-BT crystal structure. Furthermore, the degree of UC enhancement in E-BT was found to be proportional to the amount of OV present.

Power-dependent photophysical pathways of upconversion in BaTiO3:Er3.

Lanthanide incorporated perovskite is one of the most promising systems for efficient energy conversion or light-emitting materials in terms of upconversion (UC). Investigation of the photophysical mechanism of UC in the lanthanide-doped system is here continued. However, research on the 4I13/2 energy state in Er3+ is rare and more is still needed. In our work, BaTiO3:Er3+ (E-BT) was irradiated by a 1532 nm laser which is a resonance to the energy between 4I13/2 and the ground 4I15/2 state in Er3+. Bright 1532 nm-pumped UC was generated, and the UC color changed from red to yellow under increasing laser power. In addition, pump-power-dependent UC contained interesting clues about the photophysical pathway of UC. By analyzing photon numbers for each UC (green: 2H11/2/4S3/2 → 4I15/2, red: 4F9/2 → 4I15/2, infrared: 4I9/2 → 4I15/2), we found that changes in photon number with pump-power increase contain three different phases (P). P1 is a red UC phase with a small cross-relaxation between Er3+ ions. However, in P2, there is a rapid decrease in the photon number with green UC generation, which is due to the enhancement of 4I13/2-populating cross-relaxation. Finally, in P3, a saturated 4I13/2 state causes little increase of photon number (compared with P2), with different mechanistic cross-relaxation enhancement. With these three different phases under 1532 nm pumping, photophysical mechanisms in E-BT are interpreted.

Upconversion properties in lanthanide doped layered-perovskite, CsBiNb2O7.

Despite advances of lanthanide-doped upconversion (UC) materials, the applications such as light-emitting diode and biological imaging are limited by low quantum efficiency. For this context, the understanding of unique interactions between the doped-lanthanides and the host crystals has attracted a huge amount of the researcher's interest. In particular, it was revealed that doping lanthanide ions in a non-centrosymmetric site of host lattice is the cause of relaxation of the Laporte selection rule in the 4f-4f transition of lanthanide ions. One of the layered perovskites CsBiNb2O7 is known to have non-centrosymmetric sites, which would lead to highly bright UC emission. Nevertheless, to our knowledge, there has been no research on the UC comparison between host materials of CsBiNb2O7 with other hosts. In this article, we present the UC intensity comparison of Yb3+-Er3+ ion doped CsBiNb2O7, NaYF4, BaTiO3, and SrTiO3 hosts (the UC in CsBiNb2O7:Er3+,Yb3+ was 2.4 times that of NaYF4:Er3+,Yb3+ and ∼70 times that of SrTiO3:Er3+,Yb3+). After that, we dig into UC, downshifting, and double beam system UC properties. The activator concentration was optimized by varying the doping ratio of Yb3+ and Er3+, and we found out the main reason for the concentration quenching behavior in Er3+ ion doped CsBiNb2O7 is dipole-dipole interaction. In addition, the double excitation experiment indicates that the absorption (4I15/2 → 4I13/2) factor is stronger than the stimulated emission (4I13/2 → 4I15/2) factor in CsBiNb2O7 under 1540 nm laser irradiation.

Effect of tetragonal to cubic phase transition on the upconversion luminescence properties of A/B site erbium-doped perovskite BaTiO3.

With the increasing number of applications for upconversion materials, a more detailed understanding of the intrinsic mechanisms of their optical processes is required. Thus far, various lanthanide-doped host materials or nanoparticle systems have been investigated as representative upconversion systems owing to their major advantage as optical probes. As for the energetics of upconversion and the associated upconversion pathways, the role of the host material is very important because it provides a unique microscopic environment; for example, a unique local lattice structure in the case of crystalline samples. In general, the upconversion luminescence intensity decreases as a function of temperature owing to thermally accelerated multiphonon relaxation. Here, we report that the temperature dependence of the upconversion luminescence efficiency is affected differently in an Er3+-doped perovskite material, barium titanate (BaTiO3, BT), than in a general system. We show that Er3+ doped at the A (Ba2+) and B (Ti4+) sites of tetragonal phase BT, referred to as A-BT and B-BT, respectively, show different upconversion behaviors. The slope of the plot of the upconversion emission intensity as a function of temperature changed significantly in case of B-BT, but not for A-BT. This anomalous behavior of Er3+-doped BT is attributed to the phase transition (at ∼120 °C) of BT from tetragonal to cubic phase. Essentially, the temperature-dependent upconversion luminescence trend depends on the doping sites of Er3+, i.e., at A or B sites in BT, which is explained by the difference in the symmetry of the crystalline structure with different crystal phase surrounding the Er3+ ions.

Cellular uptake efficiency of nanoparticles investigated by three-dimensional imaging.

Understanding the interaction of nanoparticles with living cells on the basis of cellular uptake efficiency is a fundamental requisite in biomedical research. Cellular internalization of nanoparticles takes place by mechanisms like ATP hydrolysis-driven endocytosis that deliver nanoparticles to the cytoplasm, organelles and nuclei. Despite its importance in nanomedicine, this uptake procedure is not understood in-depth because of the complexity of the biochemical mechanisms and the lack of available experimental methods for quantitative analysis. The only breakthrough is likely to be the development of imaging techniques that can visualize, monitor and even count the number of nanoparticles inside the cell. To this end, we report here a new, fast and background-free three-dimensional (3-D) imaging technique with quantitative evaluation of the uptake efficiency for NaYF4:Yb3+,Er3+/NaYF4 core/shell upconversion nanoparticles (UCNPs) functionalized with different chemical and biological groups. Furthermore, the multiple 3-D trajectories of the UCNPs have been analyzed to investigate the cellular dynamics. This study reveals the nuclear uptake of UCNPs to be dependent on the specific chemical groups conjugated to the UCNPs. The developed 3-D imaging technique is of great significance for exploring complex biological systems.

Upconversion Luminescence Sensitized pH-Nanoprobes.

Photon upconversion materials, featuring excellent photophysical properties, are promising for bio-medical research due to their low autofluorescence, non-cytotoxicity, low photobleaching and high photostability. Upconversion based pH-nanoprobes are attracting considerable interest due to their superiority over pH-sensitive molecular indicators and metal nanoparticles. Herein, we review the advances in upconversion based pH-nanoprobes, the first time in the seven years since their discovery in 2009. With a brief discussion on the upconversion materials and upconversion processes, the progress in this field has been overviewed, along with the toxicity and biodistribution of upconversion materials for intracellular application. We strongly believe that this survey will encourage the further pursuit of intense research for designing molecular pH-sensors.