Lanthanide-doped Upconversion Nanocrystals Research Articles

Topological Control of Lanthanide-Doped Upconversion Nanocrystals for Improved FRET Coupling Efficiency in Second Near-Infrared-Triggered Photodynamic Therapy

Topological Control of Lanthanide-Doped Upconversion Nanocrystals for Improved FRET Coupling Efficiency in Second Near-Infrared-Triggered Photodynamic Therapy

Distinct Luminescent Thermal Behaviors of Yb3+- and Nd3+-Sensitized Core/Shell Upconversion Nanocrystals

Luminescent materials generally exhibit the thermal quenching due to accelerated nonradiative transitions at elevated temperatures. Recently, the discovery of anomalous thermally induced luminescence enhancement in lanthanide-doped upconversion nanocrystals (UCNCs) has aroused wide attention. The research on this topic not only deepens the understanding on the upconversion luminescence (UCL) mechanism but also promotes frontier applications based on UCNCs. Herein, comparative studies on temperature-dependent UCL properties were performed between Yb3+- and Nd3+-sensitized active-core/active-shell UCNCs. Opposite luminescent thermal behaviors were revealed for these two types of core/shell NCs, and the underlying mechanisms were elucidated with the assistance of temperature-dependent steady/transient spectral analysis under various atmospheres. Yb3+-sensitized active-core/active-shell NCs exhibited an anomalous luminescence enhancement with increasing temperature, which was attributed to the thermally alleviated quenching of surface-adsorbed water molecules. By contrast, Nd3+-sensitized active-core/active-shell NCs showed a normal thermal quenching due to rapidly increased deactivation rates of Nd3+ ions at elevated temperatures, although their UCL intensity was also affected by the quenching effect of surface-adsorbed water molecules. Highly secure anti-counterfeiting patterns were also demonstrated using the nanohybrids of Yb3+- and Nd3+-sensitized core/shell UCNCs as luminescent inks on the basis of their opposite luminescent thermal behaviors. This work provides new insights into luminescent thermal behaviors and energy loss pathways of lanthanide-doped UCNCs.

Suppression of inner energy dissipation in Yb-doped NaErF4 upconversion nanocrystals through an energy cycling strategy

Suppression of the inner energy dissipation, related to the lattice phonons and inner defects, in lanthanide doped upconversion luminescent materials remains a formidable challenge. Herein, we reveal an energy cycling strategy capable of suppressing the inner energy dissipation in lanthanide doped upconversion nanocrystals. Yb3+ ions were introduced in Er3+ heavily doped nanocrystals as an energy reservoir to compete with the inner energy dissipation. The detailed energy cycling processes between Er3+ activator and Yb3+ reservoir, responsible for the enhancement of Er3+ upconversion intensity, are proposed and further verified on the basis of spectral observations. The energy cycling strategy, with unique merits like facile and cost-effective preparation as well as broad scope of application, is highly valuable in lanthanide luminescent nanomaterials.

Solid solution Na(Gd/La)(MoO4)2:Yb3+/Er3+ upconversion nanocrystals with simultaneously enhanced photothermal conversion efficiency and luminescence intensity

Lanthanide doped upconversion nanocrystals with remarkable optical properties have been utilized for many emerging applications. Here, solid solution Na(GdxLa1-x)(MoO4)2 nanocrystals are synthesized hydrothermally, and their upconversion photoluminescent and photothermal properties as luminescent host matrix for Yb3+/Er3+ activators are systematically studied. The effect of La3+ doping content on the phase, crystal structure, upconversion luminescence and photothermal conversion are investigated. Rietveld refinements based on XRD data indicate that La3+ occupy Gd3+ sites substitutionally, and pure phase solid solution Na(GdxLa1-x)(MoO4)2 is formed. Due to lattice distortion, lower lanthanide site symmetry and augmented odd-parity crystal field interactions after La3+ doping, the 4f-4f transition probabilities of the lanthanide ions are effectively increased. Na(Gd/La)(MoO4)2 solid solution hosts exhibit enhanced upconversion luminescence from Er3+/Yb3+, compared with single-component NaGd(MoO4)2 counterparts. What's more, based on the calculated temperatures of the samples derived from the luminescence intensity ratio of states 2H11/2 and 4S3/2 for Er3+, enhanced photothermal conversion efficiency is speculated in solid solution Na(GdxLa1-x)(MoO4)2:Yb3+/Er3+ nanocrystals, which is further confirmed by infrared thermographic images. The underling mechanisms of simultaneously enhanced photothermal effects and luminescence intensity in Na(GdxLa1-x)(MoO4)2 solid solution nanocrystals are elucidated. Modulation of composition in solid solution hosts may be an effective method for combating luminescent thermal quenching and/or regulating photothermal conversion and photoluminescence properties simultaneously. Multifunctional solid solution upconversion nanocrystals could be potential for photoluminescent and photothermal applications.

Lanthanides-doped near-infrared active upconversion nanocrystals: Upconversion mechanisms and synthesis

• Well-controlled synthesis lays the foundation for the future success of lanthanide-doped upconversion nanocrystals. • Novel fabrication approaches towards core and core–shell upconversion nanocrystals were developed in recent years. • Exploiting different roles played by dopants in regulating upconversion processes and mechanisms. • Prospective and challenges on upconversion nanocrystals are discussed. In the past few decades, although tremendous advancements have been witnessed by the progress in the fabrication of various kinds of nanomaterials, only those types of nanomaterials with unique features in terms of advantageous morphological attributes and physicochemical properties have become the area of focus towards their applicability in diverse fields. Among them, the lanthanide-doped upconversion nanocrystals (UCNCs) have garnered enormous interest, owing to their unique ability to transform near-infrared (NIR) light to short-wavelength light in the visible and ultraviolet (UV) range. Such UCNCs offer enormous additional advantages which are of particular interest in various applications. Although enormous efforts have been dedicated, in recent years, it is realized that the knowledge and understanding the chemistry of UCNCs is still insufficient, as the more people studied, the more unsolved puzzles are discovered about UCNCs. Therefore, chemistry, which plays a crucial role in regulating the luminescent behavior of UCNCs, needs in-depth elucidations to better understand the UCNCs and their upconversion processes. To facilitate future works and promote the development of UCNCs, in this review, various synthesis approaches and upconversion mechanisms are discussed, with a special focus given on the chemistry behind UCNCs and up-to-date studies.

Coordination mechanism of cyanine dyes on the surface of core@active shell β-NaGdF4:Yb3+,Er3+ nanocrystals and its role in enhancing upconversion luminescence

The sensitization of lanthanide-doped upconversion nanocrystals (UCNCs) using organic dyes with a broad and intense optical absorption is an interesting approach for efficient excitation-energy harvesting and enhancing the upconversion luminescence.

Biodegradable Inorganic Upconversion Nanocrystals for In Vivo Applications.

Lanthanide-doped inorganic upconversion nanocrystals (UCNCs) are promising as fluorescent diagnostic and therapeutic agents for in vivo applications ranging from biological imaging to disease theranostics. However, all currently available lanthanide-doped inorganic UCNCs are not biodegradable and thus cannot be harmlessly eliminated from the body of living organism during a reasonable period of time, making their clinical translations nearly impossible. Here, we report a class of red-emitting biodegradable UCNCs based on Yb3+/Er3+-doped inorganic potassium heptafluozirconate (K3ZrF7:Yb/Er) that features a dynamically "soft" crystal lattice containing water-soluble [ZrF7]3- cluster and a K+ cation. The red-emitting K3ZrF7:Yb/Er UCNCs exhibit a pH-dependent biodegradation capability upon exposure to water both in vitro and in vivo, and the rapid biodegradation rate, monitored using the intrinsic red upconversion luminescence, can be tuned particularly in a mild acidic tumor microenvironment (pH ∼5-6). More importantly, the final biodegradation products of K3ZrF7:Yb/Er UCNCs can be excreted from the body of mice in a short period of time with no evidence of toxicity, in stark contrast to the nondegradable β-NaYF4:Yb/Er UCNCs that primarily accumulate in the main organs of mice. These findings described here unambiguously would benefit the future biomedical applications and clinical translations of lanthanide-doped inorganic UCNCs.

Deep UV random lasing from NaGdF4:Yb3+,Tm3+ upconversion nanocrystals in amorphous borosilicate glass.

The realization of lanthanide-doped upconversion nanocrystals embedded in a robust and transparent solid medium is highly desired to achieve deep UV (<300nm) lasing. Here, we fabricate NaGdF4:Yb3+,Tm3+ upconversion nanocrystals inside amorphous borosilicate glass to support ∼290nm random lasing emission under 980 nm excitation. We found that with careful control of the growth process, which is the key to achieve high-crystallinity nanocrystals, the nanocrystals can suppress defect-related quenching and enhance 1I6→3H6(6IJ→8S7/2) transition of Tm3+ (Gd3+) ions under five-photon absorption excitation so that high optical gain (>45cm-1) at ∼290nm can be obtained.

Sharpening upconversion nanoparticles to reduce surface quenching.

Lanthanide-doped upconversion nanocrystals with different surface sharpness properties were synthesized to study the effects of surface sharpness on optical performance. Nanocrystals with hexagonal sharp surfaces showed a significant reduction in the number of surface defects and increased the luminescence intensity by 2.5 times compared to nanocrystals with spherical surfaces.

Yttrium-dopants-induced phase-controllable and luminescence-tunable lanthanide-doped α/β-NaYbF4 nanocrystals in glass for laser-driven upconverted lighting

In-situ glass crystallization is an effective way to integrate lanthanide-doped upconversion nanocrystals into amorphous glass, producing bulky nanocomposites for specific applications. However, the precipitation of hexagonal NaYbF4 nanocrystals from glass is rarely reported owing to the preferred crystallization of cubic phase. Herein, phase-controllable in-situ crystallization of cubic/hexagonal NaYbF4 nanocrystals in glass is achieved via Y3+ doping. A series of structural and spectroscopic characterizations evidence the formation of Na(Yb/Y)F4 solid-solution nanocrystals in glass and the incorporation of lanthanide emitting centers (Er3+, Tm3+, Eu3+) into the crystalline lattice. Introducing high-content Y3+ dopants is beneficial to induce the crystallization of hexagonal NaYbF4 phase in glass and enables elaborated tunability of Er3+ upconversion emissive colors. Finally, a prototype upconverted solid-state-lighting device is constructed by coupling 980 nm laser with the as-prepared Er/Tm doped monolithic glasses containing Na(Yb/Y)F4 nanocrystals, producing bright upconversion white-light with luminous efficiency of 1 lm/W and energy efficiency of 0.5%.

Simultaneous Tailoring of Dual-Phase Fluoride Precipitation and Dopant Distribution in Glass to Control Upconverting Luminescence.

In situ glass crystallization is an effective strategy to integrate lanthanide-doped upconversion nanocrystals into amorphous glass, leading to new hybrid materials and offering an unexploited way to study light-particle interactions. However, the precipitation of Sc3+-based nanocrystals from glass is rarely reported and the incorporation of lanthanide activators into the Sc3+-based crystalline lattice is formidably difficult owing to their large radius mismatch. Herein, it is demonstrated that lanthanide dopants with smaller ionic radii can act as nucleating agents to promote the nucleation/growth of KSc2F7 nanocrystals in oxyfluoride aluminosilicate glass. A series of structural and spectroscopic characterizations indicate that Ln-dopant-induced K/Sc/Ln/F amorphous phase separation from glass is an essential prerequisite for the precipitation of KSc2F7 and the partition of Ln dopants into the KSc2F7 lattice by substituting Sc3+ ions. Importantly, modifying the Ln-to-Sc ratio in glass enables to control competitive crystallization of KSc2F7 and Ln-based (KYb2F7, KLu2F7, and KYF4) nanocrystals and produce dual-phase fluoride-embedded nanocomposites with distinct crystal fields. Consequently, tunable multicolor upconversion luminescence can be achieved through diversified regulatory approaches, such as adjustment of the dual-phase ratio, selective separation of Ln3+ dopants, and alteration of incident pumping laser. As a proof-of-concept experiment, the application of dual-phase glass as a color converter in 980 nm laser-driven upconverting lighting is demonstrated.

Simultaneous phase and morphology control of Ba2YbF7: Er3+ upconversion nanocrystals through La3+ doping

Lanthanide doped upconversion nanocrystals are widely applied in biological imaging, photonics, photovoltaics and therapeutics. Highly monodispersed and uniform products with controllable optical modulation of upconversion fluorescence really matters their promising applications. Here, Er3+ doped Ba2YbF7 nanocrystals with phase transformation, controllable morphology and tunable upconversion emission color is investigated via varying the concentration of La3+. Accompanied with tetragonal Ba2YbF7 transformed to be cubic phase, nanocubes appear instead of nanorods, and a distinct upconversion color changing from yellow to green is obtained. Crystal structure management of Ba2YbF7 contributing to a suppressive back energy transfer process, which facilitates a controllable upconversion color in monodispersed nanocrystals with a uniform morphology, is demonstrated.

Energy-Transfer Editing in Lanthanide-Activated Upconversion Nanocrystals: A Toolbox for Emerging Applications.

Advanced nanoscale synthetic techniques provide a versatile platform for programmable control over the size, morphology, and composition of nanocrystals doped with lanthanide ions. Characteristic upconversion luminescence features originating from the 4f–4f optical transitions of lanthanides can be achieved through predesigned energy transfer pathways, enabling wide applications ranging from ultrasensitive biological detection to advanced spectroscopic instrumentation with high spatiotemporal resolution. Here, we review recent scientific and technological discoveries that have prompted the realization of these peculiar functions of lanthanide-doped upconversion nanocrystals and discuss the mechanistic studies of energy transfer involved in upconversion processes. These advanced schemes include cross relaxation-mediated depletion, multipulse sequential pumping, and nanostructural configuration design. Our emphasis is placed on disruptive technologies such as super-resolution microscopy, optogenetics, nanolasing, and optical anticounterfeiting, which take full advantage of the upconversion nanophenomena in relation to lanthanide-doped nanocrystals.

Resonant Plasmon-Enhanced Upconversion in Monolayers of Core-Shell Nanocrystals: Role of Shell Thickness.

The upconversion luminescence (UCL) of colloidal lanthanide-doped upconversion nanocrystals (UCNCs) can be improved either by precise encapsulation of the surface by optically inert shells around the core, by an alteration of the nearby environment via metal nanoparticles, or by a combination of both. Considering their potential importance in crystalline silicon photovoltaics, the present study investigates both effects for two-dimensional arrangements of UCNCs. Using excitation light of 1500 nm wavelength, we study the variation in the upconversion luminescence from an Er3+-doped NaYF4 core as a function of the thickness of a NaLuF4 shell in colloidal solutions as well as in spin-cast-assisted self-assembled monolayers of UCNCs. The observed UCL yields and decay times of Er3+ ions of the UCNCs increase with increasing shell thickness in both cases, and nearly no variation in decay times is observed in the transition of the UCNCs from solution to film configurations. The luminescence efficiency of the UCNC monolayers is further enhanced by electron-beam-lithographic-designed Au nanodiscs deposited either on top of or buried within the monolayer. It is observed that the improvement by the nanocrystal shells is greater than that of the Au nanodiscs.

Fast synthesis of sub-10 nm β-NaYF4:Yb3+, Er3+@NaYF4 core-shell upconversion nanocrystals mediated by oleate ligands

We developed a facile approach for the fast synthesis of ultrasmall and monodisperse β-NaYF4:Yb3+, Er3+ nanocrystals down to 5.3 nm with the assistant of oleate ligands, based on which the highly bright sub-10 nm β-NaYF4:Yb3+, Er3+@NaYF4 core-shell nanocrystals with three order of magnitude increased upconversion emission were successfully obtained by a seed-mediated growth method. The reaction time and temperature dependent experimental results implied that the introduciton of extra oleate ligands can both promote the α-to-β phase transition and impeded the crystal growth of NaYF4, which is due to the oleate ligand induced orderly arrangement of Y3+ and capping effect, respectively. It is expected that the oleate ligands mediated phase transition and size evaluation, demonstrated here in NaYF4:Yb3+, Er3+ NCs, is also applicable to other lanthanide-doped upconversion nanocrystals.

Enhanced Cellular Ablation by Attenuating Hypoxia Status and Reprogramming Tumor-Associated Macrophages via NIR Light-Responsive Upconversion Nanocrystals.

Near-infrared (NIR) light-mediated photodynamic therapy (PDT), especially based on lanthanide-doped upconversion nanocrystals (UCNs), have been extensively investigated as a promising strategy for effective cellular ablation owing to their unique optical properties to convert NIR light excitation into multiple short-wavelength emissions. Despite the deep tissue penetration of NIR light in living systems, the therapeutic efficiency is greatly restricted by insufficient oxygen supply in hypoxic tumor microenvironment. Moreover, the coexistent tumor-associated macrophages (TAMs) play critical roles in tumor recurrence during the post-PDT period. Herein, we developed a unique photosensitizer-loaded UCNs nanoconjugate (PUN) by integrating manganese dioxide (MnO2) nanosheets and hyaluronic acid (HA) biopolymer to improve NIR light-mediated PDT efficacy through attenuating hypoxia status and synergistically reprogramming TAMs populations. After the reaction with overproduced H2O2 in acidic tumor microenvironment, the MnO2 nanosheets were degraded for the production of massive oxygen to greatly enhance the oxygen-dependent PDT efficiency upon 808 nm NIR light irradiation. More importantly, the bioinspired polymer HA could effectively reprogram the polarization of pro-tumor M2-type TAMs to anti-tumor M1-type macrophages to prevent tumor relapse after PDT treatment. Such promising results provided the great opportunities to achieve enhanced cellular ablation upon NIR light-mediated PDT treatment by attenuating hypoxic tumor microenvironment, and thus facilitated the rational design of new generations of nanoplatforms toward immunotherapy to inhibit tumor recurrence during post-PDT period.

Inverse thermal quenching effect in lanthanide-doped upconversion nanocrystals for anti-counterfeiting

An inverse thermal quenching effect is actualized in uniform lanthanide-doped Na3ZrF7 by artificially introducing defect state with an appropriate energy level. These kinds of systems are very suitable for anti-counterfeiting with high security.

Synthesis of Core-shell Lanthanide-doped Upconversion Nanocrystals for Cellular Applications

Lanthanide-doped upconversion nanocrystals (UCNs) have attracted much attention in recent years based on their promising and controllable optical properties, which allow for the absorption of near-infrared (NIR) light and can subsequently convert it into multiplexed emissions that span over a broad range of regions from the UV to the visible to the NIR. This article presents detailed experimental procedures for high-temperature co-precipitation synthesis of core-shell UCNs that incorporate different lanthanide ions into nanocrystals for efficiently converting deep-tissue penetrable NIR excitation (808 nm) into a strong blue emission at 480 nm. By controlling the surface modification with biocompatible polymer (polyacrylic acid, PAA), the as-prepared UCNs acquires great solubility in buffer solutions. The hydrophilic nanocrystals are further functionalized with specific ligands (dibenzyl cyclooctyne, DBCO) for localization on the cell membrane. Upon NIR light (808 nm) irradiation, the upconverted blue emission can effectively activate the light-gated channel protein on the cell membrane and specifically regulate the cation (e.g., Ca2+) influx in the cytoplasm. This protocol provides a feasible methodology for the synthesis of core-shell lanthanide-doped UCNs and subsequent biocompatible surface modification for further cellular applications.

Synthesis of Core-shell Lanthanide-doped Upconversion Nanocrystals for Cellular Applications.

Lanthanide-doped upconversion nanocrystals (UCNs) have attracted much attention in recent years based on their promising and controllable optical properties, which allow for the absorption of near-infrared (NIR) light and can subsequently convert it into multiplexed emissions that span over a broad range of regions from the UV to the visible to the NIR. This article presents detailed experimental procedures for high-temperature co-precipitation synthesis of core-shell UCNs that incorporate different lanthanide ions into nanocrystals for efficiently converting deep-tissue penetrable NIR excitation (808 nm) into a strong blue emission at 480 nm. By controlling the surface modification with biocompatible polymer (polyacrylic acid, PAA), the as-prepared UCNs acquires great solubility in buffer solutions. The hydrophilic nanocrystals are further functionalized with specific ligands (dibenzyl cyclooctyne, DBCO) for localization on the cell membrane. Upon NIR light (808 nm) irradiation, the upconverted blue emission can effectively activate the light-gated channel protein on the cell membrane and specifically regulate the cation (e.g., Ca2+) influx in the cytoplasm. This protocol provides a feasible methodology for the synthesis of core-shell lanthanide-doped UCNs and subsequent biocompatible surface modification for further cellular applications.

Luminescent NaYF4:Yb,Er upconversion nanocrystal colloids: Towards controlled synthesis and near‐infrared optical response

AbstractLanthanide‐doped upconversion nanocrystals are an emerging class of near‐IR‐sensitive luminescent nanomaterials that have aroused widespread interest in the investigation of their unique optical properties for sensing, biomedical, and energy applications. In this work, we have hydrothermally prepared NaYF4:Yb,Er upconversion nanocrystals that can respond with brilliant photoluminescence to near‐infrared laser light. The structural phase of the NaYF4:Yb,Er upconversion nanocrystals can be controlled by extending the hydrothermal reaction to visibly tune their emission intensity. The highly photoluminescent NaYF4:Yb,Er colloidal nanocrystals are subsequently assembled in host matrices of hard substrates and polymethyl methacrylate plastics by colloidal solution solidification to fabricate optical display components that may be useful as a potential alternative for fingerprint identification and encryption.