Distinct mechanisms for the upconversion of NaYF4:Yb3+,Er3+ nanoparticles revealed by stimulated emission depletion

Recent progress of UCNPs-MoS2 nanocomposites as a platform for biological applications

Composite materials can take advantages of the functional benefits of multiple pure nanomaterials to a greater degree than single nanomaterials alone. The UCNPs-MoS2 composite is a nano-application platform that combines upconversion luminescence and photothermal properties. Upconversion nanoparticles (UCNPs) are inorganic nanomaterials with long-wavelength excitation and short-wavelength tunable emission capabilities, and are able to effectively convert near-infrared (NIR) light into visible light for increased photostability. However, UCNPs have a low capacity for absorbing visible light, whereas MoS2 shows better absorption in the ultraviolet and visible regions. By integrating the benefits of UCNPs and MoS2, UCNPs-MoS2 nanocomposites can convert NIR light with a higher depth of detection into visible light for application with MoS2 through fluorescence resonance energy transfer (FRET), which compensates for the issues of MoS2's low tissue penetration light-absorbing wavelengths and expands its potential biological applications. Therefore, starting from the construction of UCNPs-MoS2 nanoplatforms, herein, we review the research progress in biological applications, including biosensing, phototherapy, bioimaging, and targeted drug delivery. Additionally, the current challenges and future development trends of UCNPs-MoS2 nanocomposites for biological applications are also discussed.

Efficient monodisperse upconversion composite prepared using high-density local field and its dual-mode temperature sensing

Enhanced upconversion via plasmonics has considerable potential in biosensors and solar cells; however, conventional plasmonic configurations such as core-shell assemblies or nanoarray platforms still suffer from the compromise between the enhancement factor and monodispersity, which has failed to meet the requirement of the materials for the in vivo all-solution-prepared solar cells and biosensors. We herein report a monodisperse metal-dielectric-metal (MDM) type upconverted hybrid material with high efficiency. The lanthanide-doped upconversion nanoparticles (UCNPs) were sandwiched by two gold nanodisk mirrors, and the highly localized excitation field around the UCNPs together with the efficient coupling enhanced the upconversion. The upconversion intensity can then be effectively regulated and improved by three to four orders of magnitude. As per the measurement of the temperature-dependent fluorescence intensity and spectra shift, a dual-mode nanothermometer based on our proposed hybrid materials was demonstrated. This MDM-type upconverted hybrid material demonstrated the merits of high efficiency and monodispersity, which demonstrated promise in in vivo biosensors and solar cell fabrication techniques such as spin-coating and roll-to-roll.

Synthesis of near-infrared-responsive hexagonal-phase upconversion nanoparticles with controllable shape and luminescence efficiency for theranostic applications

Over the past few decades, photodynamic therapy has been studied as a therapeutic method by generating singlet oxygen through activation of a photosensitizer (PS) to kill cancer cells. However, the light within the activating wavelength range of commercial photosensitizers has a low penetration depth. In this study, we designed multifunctional upconversion nanoparticles (UCNs) that can emit high-energy light by absorbing low-energy near-infrared (NIR) light with excellent tissue permeability through a fluorescence resonance energy transfer procedure. This process can produce reactive oxygen species by activating the PS. We aimed to optimize the thermal decomposition synthesis procedure to produce lanthanide-doped UCNs with a uniform size and improve the photoluminescence efficiency for an NIR-regulated theranostic system. It was confirmed that the morphologies of UCNs can be controlled by varying the reaction time, reaction temperature, and feed molar ratio of the solvent and reactant. The crystalline morphology of the synthesized UCNs showed a thermodynamically stable hexagonal phase. The photoluminescence efficiency of the UCNs also was influenced by size, surface area, crystalline property, and stability in aqueous solution. Furthermore, the surface-modified UCNs with a folic acid-conjugated block copolymer and PS exhibited enhanced singlet oxygen generation and significantly improved aqueous solubility and photoluminescence efficiency.

Orthogonal Emissive Upconversion Nanoparticles: Material Design and Applications

Upconversion nanoparticles (UCNPs) have gone beyond traditional fluorophores in a lot of fields due to the outstanding features such as sharp excitation and emission bands, chemical and photo stability of high quality, low auto fluorescence, and high tissue permeation depth of the near-infrared irradiation light used for excitation. Conventional UCNPs carrying single/multiple emissions under a single excitation wavelength can be only employed in concurrent activation, orthogonal emissive upconversion nanoparticles (OUCNPs) with the emissions, a kind of luminescence reliant on excitation, in which by switching the external excitation different lanthanide activators can adopt independent way to control the emission, is more like an ideal UCNPs nanoplatform which can switch their activated emissions depending upon the different application for which it is used at the right time when necessary. This review summaries what has been achieved on the synthesis optimization of designed OUCNPs in recent years and sums up various applications including bioimaging, photo-switching, and programmable control process. And also, the limitations OUCNPs face, and the efforts that have been made to overcome these limitations are discussed.

Upconversion properties in lanthanide doped layered-perovskite, CsBiNb2O7

Despite advances of lanthanide-doped upconversion (UC) materials, the applications such as light-emitting diode and biological imaging are limited by low quantum efficiency. For this context, the understanding of unique interactions between the doped-lanthanides and the host crystals has attracted a huge amount of the researcher's interest. In particular, it was revealed that doping lanthanide ions in a non-centrosymmetric site of host lattice is the cause of relaxation of the Laporte selection rule in the 4f-4f transition of lanthanide ions. One of the layered perovskites CsBiNb₂O₇ is known to have non-centrosymmetric sites, which would lead to highly bright UC emission. Nevertheless, to our knowledge, there has been no research on the UC comparison between host materials of CsBiNb₂O₇ with other hosts. In this article, we present the UC intensity comparison of Yb³⁺-Er³⁺ ion doped CsBiNb₂O₇, NaYF₄, BaTiO₃, and SrTiO₃ hosts (the UC in CsBiNb₂O₇:Er³⁺,Yb³⁺ was 2.4 times that of NaYF₄:Er³⁺,Yb³⁺ and ∼70 times that of SrTiO₃:Er³⁺,Yb³⁺). After that, we dig into UC, downshifting, and double beam system UC properties. The activator concentration was optimized by varying the doping ratio of Yb³⁺ and Er³⁺, and we found out the main reason for the concentration quenching behavior in Er³⁺ ion doped CsBiNb₂O₇ is dipole-dipole interaction. In addition, the double excitation experiment indicates that the absorption (⁴I_15/2 → ⁴I_13/2) factor is stronger than the stimulated emission (⁴I_13/2 → ⁴I_15/2) factor in CsBiNb₂O₇ under 1540 nm laser irradiation.

Nanorods with multidimensional optical information beyond the diffraction limit

Precise design and fabrication of heterogeneous nanostructures will enable nanoscale devices to integrate multiple desirable functionalities. But due to the diffraction limit (~200 nm), the optical uniformity and diversity within the heterogeneous functional nanostructures are hardly controlled and characterized. Here, we report a set of heterogeneous nanorods; each optically active section has its unique nonlinear response to donut-shaped illumination, so that one can discern each section with super-resolution. To achieve this, we first realize an approach of highly controlled epitaxial growth and produce a range of heterogeneous structures. Each section along the nanorod structure displays tunable upconversion emissions, in four optical dimensions, including color, lifetime, excitation wavelength, and power dependency. Moreover, we demonstrate a 210 nm single nanorod as an extremely small polychromatic light source for the on-demand generation of RGB photonic emissions. This work benchmarks our ability toward the full control of sub-diffraction-limit optical diversities of single heterogeneous nanoparticles.

Development of functional nanostructures can enable a range of applications in imaging and nanoscale science. Here, the authors fabricate and characterize complex heterogeneous nanorods with diverse, tunable sub-wavelength structures.

Origin of strong red emission in Er3+-based upconversion materials: role of intermediate states and cross relaxation

Among the various upconversion (UC) materials, sodium yttrium fluoride doped with ytterbium and erbium (NaYF₄:Yb³⁺,Er³⁺) is the most widely studied owing to its high UC efficiency. Nonetheless, UC mechanisms are not yet fully understood and, in particular, near-infrared-to-red UC mechanisms are still under debate. Herein, we examine UC mechanisms in Er³⁺-based UC materials. Most importantly, the ⁴F_3/2 and ⁴F_5/2 states of Er³⁺ were found to be important intermediate states for strong red emission, for the first time. The cross relaxation between the Er³⁺ ions, back energy transfer from Er³⁺ to Yb³⁺, and relative doping concentrations of Er³⁺ and Yb³⁺ in NaYF₄:Yb³⁺,Er³⁺ were found to play important roles in the relative intensity between red and green emissions. The proposed UC mechanism will provide design principles for various Er³⁺-based UC materials.

Direct Observation of Nanoparticles within Cells at Subcellular Levels by Super-Resolution Fluorescence Imaging

Direct observation of nanoparticles with high spatial resolution at subcellular levels is of great importance to understand the nanotoxicology and promote the biomedical applications of nanoparticles. Super-resolution fluorescence microscopy can break the diffraction resolution limit to achieve spatial resolution of tens of nanometers, making it ideal for highly accurate observation of nanoparticles in the cellular world. In this study, we introduced the employment of super-resolution fluorescence imaging for monitoring nanoparticles within cells. Carbocyanine dyes Alexa Flour 647 labeled mesoporous silica nanoparticles (designated as MSNs-AF647) were constructed as the super-resolution imaging nanoplatform in this work as proof of concept. The MSNs-AF647 were incubated with Hela cells, and the nanoparticles within cells were further monitored by super-resolution fluorescence microscopy. The fluorescence images of MSNs-AF647 within cells captured with the super-resolution fluorescence microscopy showed a much higher spatial resolution than that obtained using conventional fluorescence microscopy, showing that super-resolution fluorescence images can provide more accurate information to locate the nanoparticles at the subcellular levels. Moreover, other functional molecules can be easily loaded into the MSNs-AF647 super-resolution imaging nanoplatform, which suggested that super-resolution fluorescence imaging can further be applied to various bioimaging-related areas, such as imaging-guided therapy, with the aid of the MSNs-AF647 nanoplatform. This study demonstrates that super-resolution fluorescence microscopy offers a highly accurate method to study nanoparticles in the cellular world. We anticipate this strategy may further be applied to research areas such as studying the nanotoxicology and optimization of nanoparticle-based bioprobes or drugs by designing new nanostructured materials with multifunctional properties based on MSNs-AF647.

Two-Dimensional and Three-Dimensional Single Particle Tracking of Upconverting Nanoparticles in Living Cells

Lanthanide-doped upconversion nanoparticles (UCNPs) are inorganic nanomaterials in which the lanthanide cations embedded in the host matrix can convert incident near-infrared light to visible or ultraviolet light. These particles are often used for long-term and real-time imaging because they are extremely stable even when subjected to continuous irradiation for a long time. It is now possible to image their movement at the single particle level with a scale of a few nanometers and track their trajectories as a function of time with a scale of a few microseconds. Such UCNP-based single-particle tracking (SPT) technology provides information about the intracellular structures and dynamics in living cells. Thus far, most imaging techniques have been built on fluorescence microscopic techniques (epifluorescence, total internal reflection, etc.). However, two-dimensional (2D) images obtained using these techniques are limited in only being able to visualize those on the focal planes of the objective lens. On the contrary, if three-dimensional (3D) structures and dynamics are known, deeper insights into the biology of the thick cells and tissues can be obtained. In this review, we introduce the status of the fluorescence imaging techniques, discuss the mathematical description of SPT, and outline the past few studies using UCNPs as imaging probes or biologically functionalized carriers.

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.

Advances in highly doped upconversion nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting near-infra-red excitation into visible and ultraviolet emission. Their unique optical properties have advanced a broad range of applications, such as fluorescent microscopy, deep-tissue bioimaging, nanomedicine, optogenetics, security labelling and volumetric display. However, the constraint of concentration quenching on upconversion luminescence has hampered the nanoscience community to develop bright UCNPs with a large number of dopants. This review surveys recent advances in developing highly doped UCNPs, highlights the strategies that bypass the concentration quenching effect, and discusses new optical properties as well as emerging applications enabled by these nanoparticles.

Three- and Two-Photon NIR-to-Vis (Yb,Er) Upconversion from ALD/MLD-Fabricated Molecular Hybrid Thin Films

We report blue, green, and red upconversion emissions with strongly angular-dependent intensities for a new type of hybrid (Y,Yb,Er)-pyrazine thin films realized using the atomic/molecular layer deposition thin-film fabrication technology. The luminescence emissions in our amorphous (Y,Yb,Er)-pyrazine thin films of a controllable nanothickness originate from three- and two-photon NIR-to-vis excitation processes. In addition to shielding the lanthanide ions from nonradiative de-excitation, the network of interconnected organic molecules serves as an excellent matrix for the Yb³⁺-to-Er³⁺ excitation energy transfer. This suggests a new approach to achieve efficient upconverting molecular materials with the potential to be used for next-generation medical diagnostics, waveguides, and surface-sensitive detectors.

Biocompatible Chitosan-Functionalized Upconverting Nanocomposites

Simultaneous integration of photon emission and biocompatibility into nanoparticles is an interesting strategy to develop applications of advanced optical materials. In this work, we present the synthesis of biocompatible optical nanocomposites from the combination of near-infrared luminescent lanthanide nanoparticles and water-soluble chitosan. NaYF4:Yb,Er upconverting nanocrystal guests and water-soluble chitosan hosts are prepared and integrated together into biofunctional optical composites. The control of aqueous dissolution, gelation, assembly, and drying of NaYF4:Yb,Er nanocolloids and chitosan liquids allowed us to design novel optical structures of spongelike aerogels and beadlike microspheres. Well-defined shape and near-infrared response lead upconverting nanocrystals to serve as photon converters to couple with plasmonic gold (Au) nanoparticles. Biocompatible chitosan-stabilized Au/NaYF4:Yb,Er nanocomposites are prepared to show their potential use in biomedicine as we find them exhibiting a half-maximal effective concentration (EC50) of 0.58 mg mL-1 for chitosan-stabilized Au/NaYF4:Yb,Er nanorods versus 0.24 mg mL-1 for chitosan-stabilized NaYF4:Yb,Er after 24 h. As a result of their low cytotoxicity and upconverting response, these novel materials hold promise to be interesting for biomedicine, analytical sensing, and other applications.

Upconversion Luminescence Sensitized pH-Nanoprobes

Photon upconversion materials, featuring excellent photophysical properties, are promising for bio-medical research due to their low autofluorescence, non-cytotoxicity, low photobleaching and high photostability. Upconversion based pH-nanoprobes are attracting considerable interest due to their superiority over pH-sensitive molecular indicators and metal nanoparticles. Herein, we review the advances in upconversion based pH-nanoprobes, the first time in the seven years since their discovery in 2009. With a brief discussion on the upconversion materials and upconversion processes, the progress in this field has been overviewed, along with the toxicity and biodistribution of upconversion materials for intracellular application. We strongly believe that this survey will encourage the further pursuit of intense research for designing molecular pH-sensors.