Small and Bright Lithium-Based Upconverting Nanoparticles

Lanthanide-Based Luminescent Materials for Waveguide and Lasing

Microlasers and waveguides have wide applications in the fields of photonics and optoelectronics. Lanthanide-doped luminescent materials featuring large Stokes/anti-Stokes shift, long excited-state lifetime as well as sharp emission bandwidth are excellent optical components for photonic applications. In the past few years, great progress has been made in the design and fabrication of lanthanide-based waveguides and lasers at the micrometer length scale. Waveguide structures and microcavities can be fabricated from lanthanide-doped amorphous materials through top-down process. Alternatively, lanthanide-doped organic compounds featuring large absorption cross-section can self-assemble into low-dimensional structures of well-defined size and morphology. In recent years, lanthanide-doped crystalline structures displaying highly tunable excitation and emission properties have emerged as promising waveguide and lasing materials, which substantially extends the range of lasing wavelength. In this minireview, we discuss recent advances in lanthanide-based luminescent materials that are designed for waveguide and lasing applications. We also attempt to highlight challenging problems of these materials that obstacle further development of this field.

Dual-Mode Long-Lived Luminescence of Mn2+-Doped Nanoparticles for Multilevel Anticounterfeiting

Luminescent nanoparticles with dual-mode long-lived luminescence are of great importance for their attractive applications in biosensing, bioimaging, and data encoding. Herein, we report the realization of up- and downconversion emission of Mn²⁺ dopants in multilayer nanoparticles of NaGdF₄:Yb/Tm@NaGdF₄:Ce/Mn@NaYF₄ upon excitation at 980 and 254 nm, respectively. The dual-mode emission of the Mn²⁺ dopants at 531 nm have a long-lived lifetime up to ∼30 ms as a result of the spin-forbidden optical transition of Mn²⁺ within the 3d⁵ configuration. After ceasing steady excitation at the two wavelengths, the long-lived feature of Mn²⁺ luminescence allows a longer persistent time than lanthanide emissions, thereby enabling the ease of data decoding by a cell phone camera under a burst mode. The long-lived green upconversion emission also permits the generation of a long green tail emission upon dynamic excitation at 980 nm. These attributes make the as-prepared Mn²⁺-doped multilayer nanoparticles particularly attractive for multilevel anticounterfeiting.

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.

Small Alkaline-Earth-based Core/Shell Nanoparticles for Efficient Upconversion

The optical efficiency of lanthanide-based upconversion is intricately related to the crystalline host lattice. Different crystal fields interacting with the electron clouds of the lanthanides can significantly affect transition probabilities between the energy levels. Here, we investigate six distinct alkaline-earth rare-earth fluoride host materials (M_{1- x}Ln _xF_2+x, MLnF) for infrared-to-visible upconversion, focusing on nanoparticles of CaYF, CaLuF, SrYF, SrLuF, BaYF, and BaLuF doped with Yb³⁺ and Er³⁺. We first synthesize ∼5 nm upconverting cores of each material via a thermal decomposition method. Then we introduce a dropwise hot-injection method to grow optically inert MYF shell layers around the active cores. Five distinct shell thicknesses are considered for each host material, resulting in 36 unique, monodisperse upconverting nanomaterials each with size below ∼15 nm. The upconversion quantum yield (UCQY) is measured for all core/shell nanoparticles as a function of shell thickness and compared with hexagonal (β-phase) NaGdF₄, a traditional upconverting host lattice. While the UCQY of core nanoparticles is below the detection limit (<10^-5%), it increases by 4 to 5 orders of magnitude as the shell thickness approaches 4-6 nm. The UCQY values of our cubic MLnF nanoparticles meet or exceed the β-NaGdF₄ reference sample. Across all core/shell samples, SrLuF nanoparticles are the most efficient, with UCQY values of 0.53% at 80 W/cm² for cubic nanoparticles with ∼11 nm edge length. This efficiency is 5 times higher than our β-NaGdF₄ reference material with comparable core size and shell thickness. Our work demonstrates efficient and bright upconversion in ultrasmall alkaline-earth-based nanoparticles, with applications spanning biological imaging and optical sensing.

Rare Earth Hydroxide as a Precursor for Controlled Fabrication of Uniform β-NaYF₄ Nanoparticles: A Novel, Low Cost, and Facile Method

In recent years, rare earth doped upconversion nanocrystals have been widely used in different fields owing to their unique merits. Although rare earth chlorides and trifluoroacetates are commonly used precursors for the synthesis of nanocrystals, they have certain disadvantages. For example, rare earth chlorides are expensive and rare earth trifluoroacetates produce toxic gases during the reaction. To overcome these drawbacks, we use the less expensive rare earth hydroxide as a precursor to synthesize β-NaYF₄ nanoparticles with multiform shapes and sizes. Small-sized nanocrystals (15 nm) can be obtained by precisely controlling the synthesis conditions. Compared with the previous methods, the current method is more facile and has lower cost. In addition, the defects of the nanocrystal surface are reduced through constructing core–shell structures, resulting in enhanced upconversion luminescence intensity.