Hyperspectral Imaging and Optical Trapping: Complementary Tools for Assessing Direction‐Dependent Polarized Emission from Single Upconverting LiYF₄:Yb³⁺/Er³⁺ Microparticles

Abstract

Single‐particle fluorescent probes with the capacity to infer specific intracellular conditions, for instance, have great application potential in the realm of biomedicine. Imaging techniques that improve our understanding of the fluorescence processes at a single‐particle level are thus instrumental in actualizing this potential. This study demonstrates the importance of implementing synergistic single‐particle spectroscopic techniques to gain a more comprehensive understanding of the optical anisotropy exhibited by upconverting erbium and ytterbium co‐doped lithium yttrium tetrafluoride (LiYF4:Yb3+/Er3+) microparticles. More specifically, optical trapping and single‐particle polarized emission spectroscopy is herein leveraged to provide a plausible explanation for the spatial emission intensity distribution variation exhibited by LiYF4:Yb3+/Er3+ microparticles during hyperspectral imaging. By probing the polarized emission stemming from a single, optically trapped LiYF4:Yb3+/Er3+ microparticle, it is possible to find evidence that the emission intensity anisotropy exhibited by the respective microparticles during hyperspectral imaging arises as a consequence of the selection rules governing the emission probability in rare‐earth (RE3+) ions doped into a uniaxially birefringent host matrix such as LiYF4.

Tailoring Lanthanide Upconversion Luminescence through Material Designs and Regulation Strategies

“…In this context, some other groups also reported the polarized UCL from NaYF 4 , LiYF 4 , and LiLuF 4 UCNPs. [107][108][109][110][111] To make use of polarized UCL, Paloma et al reported using microsized upconverting particles (UCP) to monitor the suspending medium in real-time via singleparticle spectroscopy. [112] Single UCP was optically trapped and maintained at a certain position, followed by the excitation of linearly polarized 980 nm laser (Figure 10b).…”

“…In this context, some other groups also reported the polarized UCL from NaYF 4 , LiYF 4 , and LiLuF 4 UCNPs. [ 107–111 ] To make use of polarized UCL, Paloma et al. reported using microsized upconverting particles (UCP) to monitor the suspending medium in real‐time via single‐particle spectroscopy.…”

Responsive Sensors of Upconversion Nanoparticles

“…Similarly, the combination of SPPS and optical tweezers allowed gaining insight into the spatial variations in the emission intensities of a thick film of UCNPs obtained by hyperspectral imaging. In the hyperspectral analysis of 36 neutrally (θ = 56°, θ is the angle relative to its optic axis) and 35 non-neutrally oriented (0° ≤ θ ≤ 90°; θ ≠ 56°) LiYF 4 :Yb 3+ /Er 3+ microparticles, the averaged intensities of both the green and red emissions stemming from the latter particles were found to be nearly 2 times higher than the former ones . Using an optical beam, one can further trap single LiYF 4 :Yb 3+ /Er 3+ microparticles in three different spatial orientations, which allowed for its polarized emission to be probed at two different angles, θ = 56° and 90°.…”

“…In the hyperspectral analysis of 36 neutrally (θ = 56°, θ is the angle relative to its optic axis) and 35 non-neutrally oriented (0°≤ θ ≤ 90°; θ ≠ 56°) LiYF 4 :Yb 3+ /Er 3+ microparticles, the averaged intensities of both the green and red emissions stemming from the latter particles were found to be nearly 2 times higher than the former ones. 33 Using an optical beam, one can further trap single LiYF 4 :Yb 3+ /Er 3+ microparticles in three different spatial orientations, which allowed for its polarized emission to be probed at two different angles, θ = 56°and 90°. Recently, the crystal-field (CF) polarization components of a class of bipyramidal LiLuF 4 :Yb 3+ /Er 3+ microcrystals with a mean size of ∼20 μm × 12 μm has been determined by SPPS.…”

Highly Polarized Upconversion Emissions from Lanthanide-Doped LiYF4 Crystals as Spatial Orientation Indicators

“…Lanthanide (Ln 3+ )-doped upconversion (UC) materials, which have exhibited large anti-Stokes shifts, high photochemical stability, and adjustable emission, − have proven to be excellent optical probes. A few reports have revealed the polarized UC from a single particle − or assembly with an orderly alignment. − Compared with the assembly, a single particle is particularly desired for the polarization studies to obtain highly polarized UC emission and explore further applications. It is found that the DOPs of the 3 H 4 → 3 H 6 and 1 G 4 → 3 H 5 transitions of Tm 3+ from a single NaYF 4 :Yb/Tm nanorod were both close to 1 .…”