Multiphoton Upconversion Luminescence Research Articles

Multi-shell structured nanocarriers with enhanced multiphoton upconversion luminescence efficiency for NIR-mediated targeted photodynamic therapy

Recently, upconversion nanoparticle (UCNP)-based photodynamic therapy (PDT) has attracted attention as an anticancer treatment that can address the limitations of light penetration depth to activate commercial photosensitizers (PSs) using the upconversion luminescence (UCL) process. However, the low UCL intensity of UCNPs and the overheating effect by the 980 nm NIR laser significantly reduced the therapeutic efficacy. To overcome these problems, it is essential to form UCNP with a multi-shell structure by varying the feed composition of Yb3+, which can improve the UCL intensity. To generate highly efficient theranostic systems, we prepared a multi-shell structured UCNP (UCNC60S3) system with enhanced UCL efficiency in this study that can activate two types of PSs with one laser irradiation and have tumor targeting properties. UCNC60S3 capable of emitting multiple wavelengths with enhanced photoluminescence efficiency (PL) were synthesized by a thermal decomposition method with two kinds of activators in a single nanoparticle. The UCNC60S3 with a multi-shell structure showed 267.6 and 387.3 fold enhancements in UCL efficiency than the core UCNP at wavelengths of 545 nm and 660 nm, respectively. In addition, multi-shell structured UCNPs combined with dual PS and tumor-targeted ligands (F-UCNC60S3-RC) exhibited a significant improvement in the therapeutic effect on cancer cells by near-infrared (NIR) irradiation. When an 808 nm laser at 2 W/cm2 intensity was irradiated for 5 min, F-UCNC60S3-RC showed 10 times higher phototoxicity to cancer cells than free PS. Therefore, the NIR-induced F-UCNC60S3-RC nanocarriers with a multi-shell structure and dual PS binding will be able to provide multiple emissions for bio-imaging as well as improved targeted cancer therapy.

Manipulating the Injected Energy Flux via Host-Sensitized Nanostructure for Improving Multiphoton Upconversion Luminescence of Tm3.

Combating the concentration quenching effect by increasing the concentration of sensitized rare-earth ions in rational design upconversion nanostructure will make it easier to utilize injection energy flux and transfer it to emitters, resulting in improved upconversion luminescence (UCL). We proposed a host-sensitized nanostructure (active core@luminescent shell@inert shell) to improve multiphoton UCL of Tm3+ based on the LiLnF4 host. Yb3+ ions were isolated in the core as energy absorbents, and Tm3+ was doped in the interior LiYbF4 host shell. Compared with sandwich structured nanocrystals (Y@Y:Yb/Tm@Y), reverse structure (YbTm@Yb@Y), and fully doped structure (YbTm@YbTm@Y), the proposed structure achieved the highest efficiency of multiphoton UCL and favored a better FRET-based application performance as the Tm3+ located at an optimized spatial distribution. Furthermore, steady-state and dynamic analysis results demonstrate that by manipulating the spatial distribution of the active ions, excited energy can be tuned to enable multiphoton upconversion enhancement, overcoming the conventional limitations.

Enhancing multiphoton upconversion through interfacial energy transfer in multilayered nanoparticles

Photon upconversion in lanthanide-doped upconversion nanoparticles offers a wide variety of applications including deep-tissue biophotonics. However, the upconversion luminescence and efficiency, especially involving multiple photons, is still limited by the concentration quenching effect. Here, we demonstrate a multilayered core-shell-shell structure for lanthanide doped NaYF4, where Er3+ activators and Yb3+ sensitizers are spatially separated, which can enhance the multiphoton emission from Er3+ by 100-fold compared with the multiphoton emission from canonical core-shell nanocrystals. This difference is due to the excitation energy transfer at the interface between activator core and sensitizer shell being unexpectedly efficient, as revealed by the structural and temperature dependence of the multiphoton upconversion luminescence. Therefore, the concentration quenching is suppressed via alleviation of cross-relaxation between the activator and the sensitizer, resulting in a high quantum yield of up to 6.34% for this layered structure. These findings will enable versatile design of multiphoton upconverting nanoparticles overcoming the conventional limitation.

Tuning epitaxial growth on NaYbF4 upconversion nanoparticles by strain management.

Core-shell structural engineering is a common strategy for tuning upconversion luminescence in lanthanide-doped nanoparticles. However, epitaxial growth on hexagonal phase NaYbF4 nanoparticles typically suffers from incomplete shell coverage due to the large and anisotropic interfacial strain. Herein, we explore the effects of core particle size and morphology as well as reaction temperature on controlling the epitaxial growth of NaGdF4 shells on NaYbF4 nanoparticles with misfit parameters of fa = 1.58% and fl = 2.24% for axial and lateral growth, respectively. Rod-like core particles with a long length and a large diameter are found to promote shell growth with high surface coverage by facilitating the relaxation of lattice strains. Furthermore, the primary NaGdF4 shell can serve as a transition layer to mediate the growth of additional NaNdF4 coating layers that display an even larger lattice misfit with the core (fa = 2.98%; fl = 4.32%). The resultant NaYbF4@Na(Gd/Nd)F4 core-shell nanostructures simultaneously show strong multiphoton upconversion luminescence and superior magnetic resonance T1 ionic relaxivity. Our findings are important for the rational design of core-shell upconversion nanoparticles with optimized properties and functionality for technological applications.

Spectral evidence for multi-pathway contribution to the upconversion pathway in NaYF4:Yb3+,Er3+ phosphors.

Although upconversion phosphors have been widely used in nanomedicine, laser engineering, bioimaging, and solar cell technology, the upconversion luminescence mechanism of the phosphors has been fiercely debated. A comprehensive understanding of upconversion photophysics has been significantly impeded because the number of photons incorporated in the process in different competitive pathways could not be resolved. Few convincing results to estimate the contribution of each of the two-, three-, and four-photon channels of near-infrared (NIR) energy have been reported in yielding upconverted visible luminescence. In this study, we present the energy upconversion process occurring in NaYF4:Yb3+,Er3+ phosphors as a function of excitation frequency and power density. We investigated the upconversion mechanism of lanthanide phosphors by comparing UV/VIS one-photon excitation spectra and NIR multi-photon spectra. A detailed analysis of minor transitions in one-photon spectra and luminescence decay enables us to assign electronic origins of individual bands in multi-photon upconversion luminescence and provides characteristic transitions representing the corresponding upconversion channel. Furthermore, we estimated the quantitative contribution of multiple channels with respect to irradiation power and excitation energy.

Multi-photon upconversion luminescence from a CaxYF3+2x host by doping with Yb3+/Er3+ or Yb3+/Tm3+

Enhanced upconversion (UC) luminescence was obtained from a CaxYF3+2x host by doping with Yb3+/Er3+ or Yb3+/Tm3+. Lattice perturbation of YF3 by doping Ca2+ ions increased the energy transfer process between the absorbed Yb3+ ions and the activated Er3+ or Tm3+ ions, resulting in the emission of a bright multicolored UC luminescence.

Multi-photon Up-conversion Luminescence of ErP5O14 Noncrystalline

In this paper we report the observation of multi-photon up-conversion luminescence phenomenon of ErP5O14 noncrystalline induced by pulse DCM dye laser and clarify that the mechanism of up-conversion is the step by step absorption of single Er3+ ion.