Upconverting Near‐Infrared Light through Energy Management in Core–Shell–Shell Nanoparticles

Abstract

Lanthanide-doped upconversion materials, capable of converting low-density (< 1000 W cm À2 ) near-infrared (NIR) excitation to ultraviolet (UV) and visible emissions, have generated a large amount of interests in the areas of information technology, biotechnology, energy, and photonics. [1] Significantly, recent developments in the synthetic and multicolor tuning methods have allowed easy access to upconversion nanoparticles with well-defined phase and size, core-shell structure, optical emission, and surface properties. [2][3][4][5] The technological advances provide promising applications in sensitive biodetection and advanced bioimaging without many of the constraints associated with conventional optical biolabels. [6] Despite the attractions, further progress in using upconversion processes has been largely hindered because upconversion nanoparticles are typically sensitized by Yb 3+ ions that only respond to narrowband NIR excitation centered at 980 nm. The absorption of 980 nm light by the water component in biological samples usually limits deep tissue imaging and induces potential thermal damages to cells and tissues. [7] Excitation of conventional upconversion nanoparticles at other wavelengths has been proposed to minimize the effect of water absorption. [8] But the use of this technique is limited mainly by the largely sacrificed excitation efficiency. Efforts have also been devoted to tuning the NIR response of photon upconversion through integration of various sensitizers such as metal ions (e.g.; Nd 3+ , V 3+ or Cr 5+ ) and organic dyes. [9] The progress has resulted in visible emission by NIR excitation in the 700-900 nm range where the transparency of biological samples is maximal. [9e-h] However, upconversion emission across a broad range of spectra in these systems have not been demonstrated largely owing to the uncontrollable nonradiative processes. Herein, we describe a novel design, based on nanostructural engineering to separate unwanted electronic transitions for constructing a new class of materials displaying tunable upconversion emissions spanning from UV to the visible spectral region by single wavelength excitation at 808 nm. We also show that these nanoparticles can surpass the constraints associated with conventional upconversion nanoparticles for biological studies.The nanostructure design for management of energy transitions is depicted in Figure 1. A core-shell-shell nanoparticle platform is used to host light-harvesting, upconverting, and optical tuning processes at separate layers through doping of appropriate lanthanide ions. Interlayer energy exchange interactions are mediated by arrays of lanthanide migrator ions that can bridge efficient energy transfer across the core-shell interface while filtering unwanted crossrelaxations. As a result, incompatible optical processes can be rationally combined to achieve flexible and efficient photon energy conversions.As a proof-of-concept experiment, we employed a NaYbF 4 @Na(Yb,Gd)F 4 @NaGdF 4 core-shell-shell nanoparticle host....

Excitation energy migration dynamics in upconversion nanomaterials

“…A similar EMU process was also reported by Wen et al in the NaYbF 4 :Nd 3+ @ Na(Yb 3+ ,Gd 3+ )F 4 :Er 3+ @NaGdF 4 core-shell-shell structure. 19 The efficient ''long-range'' EMU process implies that the energy transfer process is actually not a local effect. The energy could be captured by an acceptor far away from the donor (several nanometers) with the assistance of the mediating ions (such as Yb 3+ and Gd 3+ ).…”

Ultra-small fluorescent inorganic nanoparticles for bioimaging

“…178 The authors attributed the high brightness to a combination of high excitation intensity, increased 95 energy migration without long-lived intermediate energy states. 182,183 In up-conversion nanoparticles, minimizing the depletion of excitation energy is the key to tuning their luminescence. The excitation energy can randomly migrate from an atom to its This allows to minimize the concentration quenching of the 5 luminescence, and generates an unusual four-photon-promoted violet up-conversion emission from KYb 2 F 7 :Er (2 mol%) with an intensity more than eight times higher than that previously reported.…”

808 nm driven Nd3+-sensitized upconversion nanostructures for photodynamic therapy and simultaneous fluorescence imaging

“…Several groups spatially separated activators from Nd 3+ ions via core-shell structures to acquire high efficient luminescence under excitation at 800 nm. [16][17][18][19] Although the upconversion luminescence (UCL) caused by a thick Nd 3+ sensitizing shell could be used for bioimaging, this structure is not appropriate for FRET-based applications, such as homogeneous bioassays, biosensing and PDT, which are in general based on an energy transfer mechanism. For example, for UCNP-based PDT, the luminescence activators inside the nanoparticles (as the energy donors) transfer the excitation energy to the acceptors, i.e.…”