The Fluoride Host: Nucleation, Growth, and Upconversion of Lanthanide‐Doped Nanoparticles

Abstract

The rapid ascent of nanoscience has garnered significant attention in recent years. Much of the interest generated has dealt with the integration of nanoparticles in various applications ranging from automotive and textiles to bioimaging and nanomedicine. In order for the realization of this potential, their synthesis and chemistry need to be thoroughly understood. One particularly interesting class of nanoparticles comprises a lanthanide‐doped inorganic matrix. Due to their physicochemical and optical properties, these lanthanide‐doped nanoparticles are undergoing widespread investigation in many fields, particularly for in vitro and in vivo imaging, as well as theranostics. They offer significant advantages in biological applications, particularly the extension of the system applicability to deep tissue regions of the body, a reduced scattering of the excitation wavelength, reduction of autofluorescence, and decrease in thermal loading and photodamage to the system under study. Specifically, lanthanide‐doped fluoride hosts are being propelled to the forefront of the current research efforts as they offer several advantages relative to other studied upconverting host materials. This review will take an in‐depth look at lanthanide‐doped upconverting fluoride nanoparticles with a particular emphasis on the synthesis, nucleation, and growth mechanisms and, finally, the potential to tailor particle properties.

Towards near-infrared photosensitization of tungsten trioxide nanostructured films by upconverting nanoparticles

“…These two characteristics explain why UCNPs exhibit sharp optical emission, and relatively long lifetime for their excited states (ranging from µs to ms) [43]. Since 4f n transitions are Laporte-forbidden 2 in order to preserve the dipole moment, there is only a small probability of transition, which results in long excited state lifetime [44]. If n represents the number of electrons in 4f orbitals, there are 14 n possible electronic configurations: it explains the rich energy level diagram of lanthanides.…”

“…Their high efficiency can partly be explained by the choice of the host material. The fluoride host is a material of choice, because it has low lattice phonon energy, which considerably reduces the probability of non-radiative processes [46,44]. Fluoride hosts are also very stable from a chemical viewpoint.…”

Photoluminescent nanoplatforms in biomedical applications

“…An ideal host must possess high chemical stability, low lattice phonon energies that minimize non-radiative process and maximize radiative emissions, and close lattice matches to the dopant ions. These characteristics are exhibited by highly stable fluorides such as NaYF 4 , NaGdF 4 , LiYF 4 , NaLuF 4 with phonon energies ~350 cm −1 [38]. Renero-Lecuna et al concluded that low-symmetry hosts offer a more highly disorder structures to the emitting ions resulting in a large site distribution.…”

Pure red visible emission via three-photon excitation of colloidal Na3ZrF7:Er nanoparticles using a telecom-band laser

“…Similar to other nonlinear optical technologies, the common multiphoton effect has very low efficiency to upconvert the energy of excited photons, as the intermediate levels are virtual [11] . In comparison, lanthanide-doped upconversion nanoparticles (UCNPs) can effectively convert low-energy photons (always infrared light) into ultraviolet, visible, or near-infrared (NIR) photons, owing to their ladderlike system of energy levels [12] . Until now, owing to their high efficiency, the most frequently used UCNPs have been hexagonal β-NaYF 4 nanoparticles (NPs) doped with Yb 3þ ∕RE 3þ (RE ¼ Er, Tm, Ho) [13,14] .…”