Lanthanide-doped Upconversion Nanoparticles Research Articles

Significant Enhancement of the Upconversion Emission in Highly Er3+ -Doped Nanoparticles at Cryogenic Temperatures.

Relatively low efficiency is the bottleneck for the application of lanthanide-doped upconversion nanoparticles (UCNPs). The high-level doping strategy realized in recent years has not improved the efficiency as much as expected. It is argued that cross relaxation (CR) is not detrimental to upconversion. Here we combine theoretical simulation and spectroscopy to elucidate the role of CR in upconversion process of Er3+ highly doped (HD) UCNPs. It is found that if CR is purposively suppressed, upconversion efficiency can be significantly improved. Specifically, we demonstrate experimentally that inhibition of CR by introducing cryogenic environment (40 K) enhances upconversion emission by more than two orders of magnitude. This work not only elucidates the nature of CR and its non-negligible adverse effects, but also provides a new perspective for improving upconversion efficiency. The result can be directly applied to cryogenic imaging and wide range temperature sensing.

Lanthanide-Doped Upconversion Nanoparticles: Exploring A Treasure Trove of NIR-Mediated Emerging Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) possess the remarkable ability to convert multiple near-infrared (NIR) photons into higher energy ultraviolet-visible (UV-vis) photons, making them a prime candidate for several advanced applications within the realm of nanotechnology. Compared to traditional organic fluorophores and quantum dots (QDs), UCNPs possess narrower emission bands (fwhm of 10-50 nm), large anti-Stokes shifts, low toxicity, high chemical stability, and resistance to photobleaching and blinking. In addition, unlike UV-vis excitation, NIR excitation is nondestructive at lower power intensities and has high tissue penetration depths (up to 2 mm) with low autofluorescence and scattering. Together, these properties make UCNPs exceedingly favored for advanced bioanalytical and theranostic applications, where these systems have been well-explored. UCNPs are also well-suited for bioimaging, optically modulating chemistries, forensic science, and other state-of-the-art research applications. In this review, an up-to-date account of emerging applications in UCNP research, beyond bioanalytical and theranostics, are presented including optogenetics, super-resolution imaging, encoded barcodes, fingerprinting, NIR vision, UCNP-assisted photochemical manipulations, optical tweezers, 3D printing, lasing, NIR-II imaging, UCNP-molecule nanohybrids, and UCNP-based persistent luminescent nanocrystals.

Tandem fabrication of upconversion nanocomposites enabled by confined protons.

Lanthanide-doped upconversion nanoparticle (UCNP)-based nanocomposites can address the intrinsic limitations associated with UCNPs and bestow new functions on UCNPs, which can facilitate the development and application of UCNPs. However, the fabrication of UCNP-based composites typically suffers from complex operations, long-drawn-out procedures, and even loss or damage of UCNPs. Herein, we report a tandem fabrication strategy for the preparation of UCNP-based nanocomposites, in which protons, confined in the non-aqueous polar solvent, can produce ligand-free UCNPs for the direct fabrication of a composite without further treatment. Our studies show that the confined protons can be generated by diverse materials and can yield different types of ligand-free nanomaterials for desired composites. This versatile strategy enables a simple but scalable fabrication of UCNP-based nanocomposites, and can be extended to other nanomaterial-based composites. These findings should provide a platform for constructing multifunctional UCNP-based materials, and benefit potential applications of UCNPs in varied fields.

Crystalline phase-dependent cations migration in core-shell lanthanide-doped upconversion nanoparticles

Core-shell structures are widely used to modulate upconversion emission or mitigate surface quenching and cross-relaxation in lanthanide-doped upconversion nanoparticles (UCNPs) to meet the requirements of various applications. Interfacial cation migration has been found recently to deteriorate the core-shell structures and thus affect their upconversion luminescence when they are subjected to annealing post-treatment. Herein, we demonstrate that the interfacial cation migration largely depends on the crystalline phase of the host lattice. Significant Er3+/Y3+(Yb3+) diffusion and migration occur across the core-shell interface when cubic NaREF4 (RE = Y, Yb, Er) core-shell or core-shell-shell UCNPs are annealed at 200 °C in the solid state, while the cation migration in hexagonal counterparts is negligible and the core-shell (core-shell-shell) structure can be well maintained in the same condition. The loosely packed atoms and large cubic void surrounded by eight F- ions in cubic NaREF4 lattice may facilitate cations diffusion and migration, enabling interfacial cations migration at relatively lower temperature. This finding may help us better understand temperature-dependent upconversion luminescent properties of core-shell UCNPs and better utilize various core-shell structured UCNPs according to different requirements of applications.

Enhancement of upconversion photoluminescence in phosphor nanoparticle thin films using metallic nanoantennas fabricated by colloidal lithography.

Lanthanide-doped upconversion nanoparticles (UCNPs), as multifunctional light sources, are finding utility in diverse applications ranging from biotechnology to light harvesting. However, the main challenge in realizing their full potential lies in achieving bright and efficient photon upconversion (UC). In this study, we present a novel approach to fabricate an array of gold nanoantennas arranged in a hexagonal lattice using a simple and inexpensive colloidal lithography technique, and demonstrate a significant enhancement of UC photoluminescence (UCPL) by up to 35-fold through plasmon-enhanced photoexcitation and emission. To elucidate the underlying physical mechanisms responsible for the observed UCPL enhancement, we provide a comprehensive theoretical and experimental characterization, including a detailed photophysical description and numerical simulations of the spatial electric field distribution. Our results shed light on the fundamental principles governing the enhanced UCNPs and pave the way for their potential applications in photonic devices.

Engineered rare-earth nanomaterials for fluorescence imaging and therapy.

Early diagnosis and treatment are of great significance for hindering the progression of brain disease. The limited effects of available treatments and poor prognosis are currently the most pressing problems faced by clinicians and their patients. Therefore, developing new diagnosis and treatment programs for brain diseases is urgently needed. Near-infrared (NIR)-light-responsive, lanthanide-doped upconversion nanoparticles (UCNPs) provide great advantages both in diagnosis and therapy. Hence, we synthesised nanoparticles comprised of a UCNPs core with surface functionalization. UCNPs@Au was used for NIR fluorescence imaging in the brain and inhibiting the growth of mouse glioma 261 (GL261) cells depending on photothermal properties. In addition, a UCNPs core and a mesoporous silica layer as the outer shell with a tannic acid-Al3+ ions (TA-Al) complex as a "gatekeeper" were used for pH-triggered doxorubicin/small interfering ribonucleic acid delivery in vitro. Based on our preliminary results, we expect to develop more multifunctional nanoscale diagnostic and therapeutic agents based on UCNPs for the diagnosis and treatment of brain diseases, including Alzheimer's disease, Parkinson's disease, and brain tumours.

A dye-quenched/sensitized switching upconversion nanoprobe for high-contrast mapping of the pH-related tumor microenvironment.

Nanoprobes based on lanthanide-doped upconversion nanoparticles (UCNPs) exhibit promising potential in bioimaging and biosensing due to their unique optical properties. However, conventional UCNP nanoprobes based on the dye quenching effect are still limited in biosensing due to their low upconversion efficiency. The advent of dye-sensitized upconversion has resulted in nanoprobes with significantly enhanced efficiency; however, these still suffer from a high initial emissive background. In view of this, herein, we have constructed a dye-quenched/sensitized switching upconversion nanoprobe for high-contrast imaging of the pH-related tumor microenvironment. Under normal conditions, the luminescence of the nanoprobe at 540 nm (UCL540) was significantly quenched by the employed dye. Upon being triggered by an acid, the dye would switch to its fluorescent form to sensitize the luminescence of UCNPs, affording a significant enhancement of UCL540. The switching from dye-quenched UCL to dye-sensitized UCL jointly enables the detection of a high signal-to-background ratio (high up to 50), allowing for high-contrast mapping of the tumor specific acidic microenvironment in vivo. We believe that this nanoplatform holds considerable promise for acid-related sensing.

An engineered cascade-sensitized red-emitting upconversion nanoplatform with a tandem hydrophobic hydration-shell and metal-phenolic network decoration for single 808 nm triggered simultaneous tumor PDT and PTT enhanced CDT.

Near-infrared light (NIR) driven lanthanide-doped upconversion nanoparticle (UCNP) based photo-dynamic therapy (PDT) holds a great promise for the non-invasive treatment of deep-seated tumors. However, it has also been highly hindered by the low upconversion luminescence (UCL) efficiency, hypoxia nature of solid tumors, and low therapeutic efficiency using single modality. Herein, we present a novel Nd3+ → Yb3+ → Tm3+ → Er3+ cascade-sensitized red-emitting UCNP with tandem hydrophobic hydration-shell (HHS) and metal-phenolic network (Fe-tannic acid, Fe-TA) decoration (UCNP@HHS@Fe-TA, denoted as UCFS@Fe-TA) for single 808 nm triggered simultaneous tumor PDT and photothermal therapy (PTT) enhanced chemo-dynamic therapy (CDT). The UCNP can supply intense red emission under high tissue penetrating/minimized tissue overheating 808 nm excitation, and their HHS coating with perfluorocarbon/photosensitizer Ce6 co-doping can not only realize UCL-based PDT, but also strengthen PDT of as-formed UCFS via O2-carrying/UCL protection capacity of the HHS. Fe-TA coating can supply 808 nm triggered PTT, and the rise in temperature during PTT leads to enhanced Fenton catalytic activity of Fe-TA and faster ˙OH production rate of CDT to match with the real-timely released 1O2 in PDT. The as-designed UCFS@Fe-TA thus can achieve a single 808 nm triggered simultaneous PDT and PTT enhanced CDT, leading to a PTT-assisted reactive oxygen species storm for efficient tumor suppression. Such a design also renders the nanoplatform lower cell dark toxicity. In addition, the single excitation-triggered multimodal therapy mode might address the excitation wavelength mismatch issue in dual laser-triggered PTT/PDT mode. This study has therefore presented an efficient nanotherapeutic platform enabling synergistic multimodal tumor therapies with high biocompatibility.

Coordination polymer nanospheres based on folic acid: Upconversion properties, cytotoxicity study and drug delivery

Rare earth doped upconversion nanoparticles have been widely studied for their applications in bioanalysis and medical imaging. In this work, rare earth doped coordination polymer nanospheres with folic acid as ligand were prepared via a general and simple solvothermal method. A series of characterization results showed that the nanoparticles were an amorphous hollow nanosphere structure with diameter of ca.150 nm. Under NIR irradiation at 980 nm, the upconversion emission around 663 nm originated from 4F9/2→4I15/2 of Er3+ was successfully realized. In addition, the nanospheres was proved to have good biocompatibility and high loading efficiency for doxorubicin.

Fluorescence Sensor Based on Lanthanide Nanostructure for Full Range Detection of Ethylene Glycol

The ethylene glycol (EG) proportion plays critical role in the performance of secondary refrigerant and antifreezing fluid. Here, we reported an accurate and fast optical detection engineering strategy of EG in aqueous solution by a proposed ligand-free active-core @ inert-shell lanthanide-doped up-conversion nanoparticles (CS-UCNPs). The relevant mechanism was proved to be the fluorescence quenching of up-conversion nanoparticles caused by the controllable energy transfer between up-conversion nanoparticles and H2O molecular. And the three successive steps of this process were confirmed and discussed in detail. Profit from the optical detection, these results provided a highly accurate, full range and stable approach in monitoring the EG proportion.

Upconversion Nanoparticles for Cancer Therapy

Cancer Therapies In article number 2200092, Yang Li and Guanying Chen summarize the fundamental roles of Lanthanidedoped upconversion nanoparticles in cancer therapy and highlight types of cell death caused by excess reactive oxygen species and also highlight cutting-edge applications in multimode cancer therapy and imaging-guided cancer therapy.

Lanthanide-Based Nanoprobes for Time-Resolved Luminescence Imaging on Various Ions and Molecules

Lanthanide-doped upconversion nanoparticles (Ln-UCNPs) have been extensively explored in the biological field. In particular, Ln-UCNPs with near-infrared (NIR) fluorescence have tremendous potential for biological imaging because of their outstanding photo-and chemo-stability, extended photoluminescence lifetimes, low long-term toxicities and narrow photoluminescence bandwidths as well as minimal background interferences. Using predesigned energy transfer routes makes it possible to get upconversion luminescence from lanthanides' 4f-4f optical transitions. This article clarifies the key working principles and superiorities of Ln-UCNPs for bioimaging. A crucial overview of recent advances in biological detection adopting lanthanide-based luminescence resonance energy transfer (LRET) mechanisms is presented while emphasizing the importance of modifying Ln-UCNPs to obtain a more efficient energy transfer mechanism.

Excitation Pulse Duration Response of Upconversion Nanoparticles and Its Applications.

Lanthanide-doped upconversion nanoparticles (UCNPs) have rich photophysics exhibiting complex luminescence kinetics. In this work, we thoroughly investigated the luminescence response of UCNPs to excitation pulse durations. Analyzing this response opens new opportunities in optical encoding/decoding and the assignment of transitions to emission peaks and provides advantages in applications of UCNPs, e.g., for better optical sectioning and improved luminescence nanothermometry. Our work shows that monitoring the UCNP luminescence response to excitation pulse durations (while keeping the duty cycle constant) by recording the average luminescence intensity using a low-time resolution detector such as a spectrometer offers a powerful approach for significantly extending the utility of UCNPs.

Off-Angle Amplified Spontaneous Emission of Upconversion Nanoparticles by Propagating Lattice Plasmons.

Lanthanide-doped upconversion nanoparticles (UCNPs) are appealing for light emitting applications because their high internal conversion efficiency facilitates the amplified spontaneous emission (ASE) under low pumping. In addition, the integration of photonic crystals and microcavities with optical quantum emitters provides a unique opportunity to manipulate their light emissions and generate coherent light sources for quantum photonics. Here, this work describes a two-dimensional (2D) plasmonic lattice of Al nanocone array (Al NCA), which can confine the light at the tip. Light confinement by the enhancement effect supports narrow linewidth resonances as optical feedback for the ASE of UCNPs doped with sensitizer Yb3+ ions/emitter Ho3+ ions/relaxator Ce3+ ions. An off-angle ASE with an enhancement of 19-fold from UCNPs is achieved by propagating lattice plasmons from the Al NCA. Moreover, this upconverting ASE can be switched on or off by adjusting the polarization state of the incident pump light, and photonic band engineering can be used to manipulate it intentionally. This composite plasmonic system opens prospective applications for the ASE as directional emission, real-time tunable wavelengths, controlled multimode lasing, and optical switches.

Low-lying excited state energy trap induced by cross-relaxation – The main origin of concentration quenching in lanthanide upconversion nanoparticles

In lanthanide-doped upconversion nanoparticles (UCNPs), the concentration of emitter ions, also known as activator ions, is usually limited to 1 − 5 mol% due to concentration quenching effects. This circumstance limits the luminescent efficiency of UCNPs’ and their use in a variety of application areas. Earlier studies have attributed the activator concentration quenching to migration of energy to the nanoparticle surface, while indicating that cross-relaxation between activator ions had a minor role therein. In this work, we carried out comparative studies on Er3+-doped and Yb3+-Er3+ codoped UCNPs and could, in contrast to this notion, prove a general adverse effect of cross-relaxation between activator ions, here Er3+ ions, on upconversion luminescence (UCL). The direct result of the cross-relaxation is that the energy of the excitation light is accumulated into a low-lying excited state of Er3+ in the infrared region, so forming a "low-lying excited state energy trap". As a result, the excitation energy is used for generating down-conversion luminescence or for indirectly facilitating UCL channels that are directly related to the low-lying excited state energy trap. The identified effect can be used to regulate UCL channels to achieve a concentrated UCL band that is more favorable for certain applications, e.g., biological imaging.

Transient energy trapping as a size-conserving surface passivation strategy for producing bright ultrasmall upconversion nanoprobes

Lanthanide-doped upconversion nanoparticles (UCNPs) have been widely exploited as nanoprobes or energy transducers in traditional as well as emerging biological applications, such as bioimaging, photodynamic therapy, optogenetics, gene editing. However, the breadth and depth of their utility in the biomedical areas are still not comparable to conventional luminescent probes, such as fluorescent dyes and semiconductor quantum dots. Their application is largely limited by their large size, typically > 20 nm, to ensure a sufficient luminescence brightness. In order to enhance the brightness of UCNPs without exceeding the critical size limitations for biomedical applications, we employ here a transient energy trapping effect as a nanoprobe surface passivation strategy to prevent deleterious distant energy migration in the host lattice, which is particularly prevalent in ultrasmall UCNPs and leads to luminescence quenching. We demonstrate this strategy by incorporating Tm3+ ions as energy trapping centers near the surface of sub-10 nm NaYF4: Yb, Er UCNPs and obtain an emission enhancement by almost one order of magnitude without any increment on the nanoparticle size. Our work presents a promising strategy for the preparation of ultrasmall and bright upconversion nanoprobes that are less vulnerable to surface quenching and that potentially minimize the interference with the object. This facilitates their biomedical applications as here demonstrated by unprecedented high-quality cell labeling and imaging, featured with very uniform nanoparticle distribution in the outer nuclear region.

A Lanthanide Upconversion Nanothermometer for Precise Temperature Mapping on Immune Cell Membrane.

Cell temperature monitoring is of great importance to uncover temperature-dependent intracellular events and regulate cellular functions. However, it remains a great challenge to precisely probe the localized temperature status in living cells. Herein, we report a strategy for in situ temperature mapping on an immune cell membrane for the first time, which was achieved by using the lanthanide-doped upconversion nanoparticles. The nanothermometer was designed to label the cell membrane by combining metabolic labeling and click chemistry and can leverage ratiometric upconversion luminescence signals to in situ sensitively monitor temperature variation (1.4% K-1). Moreover, a purpose-built upconversion hyperspectral microscope was utilized to synchronously map temperature changes on T cell membrane and visualize intracellular Ca2+ influx. This strategy was able to identify a suitable temperature status for facilitating thermally stimulated calcium influx in T cells, thus enabling high-efficiency activation of immune cells. Such findings might advance understandings on thermally dependent biological processes and their regulation methodology.

Influence of TiO2 Space layer on plasmon enhanced Luminescence of Ag/TiO2/NaGdF4:Er3+/Yb3+/Al3+ composite Films

ABSTRACT Lanthanide-doped upconversion nanoparticles (UCNPs)have potential applications in optical and optoelectronic devices due to their novel optical properties. The surface plasmon is an effective method in improving the upconversion luminescence (UCL) properties, and it is essential to tunee the distance between the metal NPs and UCNPs for improving UCL. In this work, the Ag/TiO2/NaGdF4:Er3+/Yb3+/Al3+ composite films were fabricated, and the effect of the TiO2 layer thickness on spectral characteristics in the composite films was investigated. It is shown that the thickness of TiO2 film has a significant effect on the Ag NPs plasma resonance and fluorescence intensity. The maximum upconversion emission enhancement factors of the red and green emission were obtained for the sample with 15-nm-thickness TiO2 under the 980 nm excitation. That is mainly originated from the enhancement of the resonate energy transfer (Yb3+→Er3+) rate under the competition between non-radiative transition and local field effects.

Aligned Plasmonic Antenna and Upconversion Nanoparticles toward Polarization-Sensitive Narrowband Photodetection and Imaging at 1550 nm.

Lanthanide-doped upconversion nanoparticles (UCNPs) are rising as prospect nanomaterials for constructing polarization-sensitive narrowband near-infrared (NIR) photodetectors (PDs), which have attracted significant interest in astronomy, object identification, and remote sensing. However, polarized narrowband NIR photodetection and imaging based on UCNPs have yet to be realized. Herein, we demonstrate that NIR photodetection and imaging are capable of sensing polarized light as well as affording wavelength-selective detection at 1550 nm by integrating directional-Au@Ag nanorods (D-Au@Ag NRs) with NaYF4:Er3+@NaYF4 UCNPs. Monolayer and large-area D-Au@Ag NRs polarization-sensitive plasmonic antenna films are obtained, and the center of their localized surface plasmon resonance (LSPR) peak is located at around 1550 nm. Experimental and theoretical results reveal that D-Au@Ag NRs have a sharp localized LSPR peak with a dominant scattering cross section. The UCNPs coupled with D-Au@Ag NRs exhibit significantly enhanced and strongly polarization-dependent luminescence with a high degree of polarization (DOP) of 0.72. The first polarization-resolved UC narrowband PD at 1550 nm is achieved, which delivers a DOP of 0.63, a detectivity of 1.69 × 1010 Jones, and a responsivity of 0.32 A/W. Finally, we develop a polarized imaging system for 1550 nm with visual photoelectric detection based on the aforementioned PDs. Our work opens up possibilities for manipulating UC and developing next-generation polarization-sensitive narrowband infrared photodetection and imaging technology.

Upconversion Luminescence via Anion Exchange in Perovskite Quantum Dots for Anticounterfeiting Inkjet Printing.

Lanthanide-doped upconversion nanoparticles (UCNPs) and cesium lead halide perovskite quantum dots (PeQDs) are highly compatible with each other: UCNPs produce anti-Stokes upconversion luminescence (UCL) under near-infrared (NIR) excitation and the emissive profiles of PeQDs can be conveniently tuned by varying the halide composition ratio. Therefore, in this study, UCNPs and PeQDs are mixed together, producing colorful UCL under 980 nm laser excitation. In addition, ZnI2 is used to vary the halide composition ratio of PeQDs and manipulate UCL in situ, thus adding more flexibility in UCL regulation. Finally, based on the above-mentioned discussion, a double-encrypted anticounterfeiting pattern is generated via sequentially printing ZnI2 solution and UCNP suspension on an A4 paper. Using PeQDs as the decrypting reagent, under the NIR excitation and decryption channel, the hidden information can be fully decrypted. The combination of UCNPs and PeQDs greatly expands the upconversion possibility, offers more feasibility in UCL regulation, and further promotes the practical applications.