Bright photon upconversion in LiYbF4:Tm3+@LiYF4 nanoparticles and their application for singlet oxygen generation and in immunoassay for SARS-CoV-2 nucleoprotein

Photon upconversion is an intensively investigated phenomenon in the materials sciences due to its unique applications, mainly in biomedicine for disease prevention and treatment. This study reports the synthesis and properties of tetragonal LiYbF4:Tm3+@LiYF4 core@shell nanoparticles (NPs) and their applications. The NPs had sizes ranging from 18.5 to 23.7 nm. As a result of the energy transfer between Yb3+ and Tm3+ ions, the synthesized NPs show intense emission in the ultraviolet (UV) range up to 347 nm under 975 nm excitation. The bright emission in the UV range allows for singlet oxygen generation in the presence of hematoporphyrin on the surface of NPs. Our studies show that irradiation with a 975 nm laser of the functionalized NPs allows for the production of amounts of singlet oxygen easily detectable by Singlet Oxygen Sensor Green. The high emission intensity of NPs at 800 nm allowed the application of the synthesized NPs in an upconversion-linked immunosorbent assay (ULISA) for highly sensitive detection of the nucleoprotein from SARS-CoV-2, the causative agent of Covid-19. This article proves that LiYbF4:Tm3+@LiYF4 core@shell nanoparticles can be perfect alternatives for the most commonly studied upconverting NPs based on the NaYF4 host compound and are good candidates for biomedical applications.

Uncovering Different Responses and Energy Mechanisms of Sensitizer and Activator in Host Manipulation for Upconversion Nanoparticles

Agile and efficient upconversion luminescence (UCL) fine-tuning strategies are the most demanded for in the frontier applications of highly doped upconversion nanoparticles (UCNPs). By doping Zn²⁺ ions into NaHoF₄ and NaGdF₄:Yb³⁺ shells using the oleate method, the separate influences of Zn²⁺ on Ho³⁺ and Yb³⁺ ions in UCL-related processes were analyzed in detail, revealing relevant UCL changes and underlying energy mechanisms from a novel but explicit perspective. Different behaviors of green and red UCL before and after Zn²⁺-ion doping were attributed to the disparities in the energy pathways and features of the sample structures. Herein, the populations of ⁵S₂/⁵F₄ and ⁵F₅ states, not the usually mentioned decay time, decided the UCL intensities of the NaHoF₄@NaYbF₄-structured highly doped UCNPs. The advantageous small sizes and intense single-band red UCL of these UCNPs were further developed by combining our previous strategies with introducing Zn²⁺ ions into the NaHoF₄ matrix. Overcoming energy loss by surface quenchers and Zn²⁺-triggered inner defects is the key factor in maximizing 4f-4f transitions. To the best of our knowledge, the current study is the first attempt to date to experimentally reveal separate impacts of the heteroions on activators and sensitizers in UCL-related processes and can deepen the theoretical investigation of Ho-based UCL for the broadened applications of NaHoF₄ UCNPs.

NaBiF4:Yb3+,Tm3+ submicron particles as luminescent probes for in vitro imaging of cells

Upconversion materials have attracted considerable research interest for their application in bioimaging due to their unique optical properties. NaREF₄ (RE = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) based host lattice, which is widely used for upconversion, requires expensive rare-earth elements and tedious reaction conditions. Hence there is a need to develop environmentally friendly and cost effective materials for upconversion. In this study, we propose NaBiF₄ as a host material for upconversion which is based on environmentally friendly and cost-effective bismuth. NaBiF₄ has not been explored as an imaging probe before. We report efficient Yb³⁺/Tm³⁺ doped NaBiF₄ based upconversion submicron particles which exhibit a photostable, wide upconversion emission range (NIR-to-NIR and Vis) under NIR (980 nm) excitation, and in-vitro non-cytotoxic uptake by mammalian cancer cell lines as well as bacterial cells with a high signal to background ratio. The synthesis of the chosen host material co-doped with Yb³⁺/Tm³⁺ has not been reported earlier through such a non-aqueous quaternary reverse micelle route. Here, we functionally validate these submicron particles as viable alternatives to currently available upconversion nanomaterials and highlight their potential as luminescent probes for bioimaging.

Photobase-catalysed anionic thiol-epoxy click photopolymerization under NIR irradiation: from deep curing to shape memory

A photobase generator absorbing upconversion fluorescence can efficiently catalyze anionic thiol-epoxy click photopolymerization under 980 nm NIR light irradiation.

Upconversion lanthanide nanomaterials: basics introduction, synthesis approaches, mechanism and application in photodetector and photovoltaic devices

Upconversion (UC) of lanthanide-doped nanostructure has the unique ability to convert low energy infrared (IR) light to high energy photons, which has significant potential for energy conversion applications. This review concisely discusses the basic concepts and fundamental theories of lanthanide nanostructures, synthesis techniques, and enhancement methods of upconversion for photovoltaic and for near-infrared (NIR) photodetector (PD) application. In addition, a few examples of lanthanide-doped nanostructures with improved performance were discussed, with particular emphasis on upconversion emission enhancement using coupling plasmon. The use of UC materials has been shown to significantly improve the NIR light-harvesting properties of photovoltaic devices and photocatalytic materials. However, the inefficiency of UC emission also prompted the need for additional modification of the optical properties of UC material. This improvement entailed the proper selection of the host matrix and optimization of the sensitizer and activator concentrations, followed by subjecting the UC material to surface-passivation, plasmonic enhancement, or doping. As expected, improving the optical properties of UC materials can lead to enhanced efficiency of PDs and photovoltaic devices.

808 nm Near-Infrared Light-Triggered Payload Release from Green Light-Responsive Donor-Acceptor Stenhouse Adducts Polymer-Coated Upconversion Nanoparticles

Owing to deep activation in biotissues and enhanced targeting efficiency, developing photoresponsive polymer-upconversion nanoparticles (PP-UCNPs) nanovectors has witnessed rapid growth in the past decade. However, up to date, all developed nanovectors require high-order photon processes to initiate the release of cargos. The photodamage caused by high-power near-infrared laser light may be a critical obstacle to their clinical application. Here, for the first time, by leveraging absorption-emission spectral matching between donor-acceptor Stenhouse adducts (DASA) PP and UCNPs (λ_ex , 808 nm) in the green region (≈530 nm), the designed nanovector is capable of releasing cargos at a low-power 808 nm excitation (0.2 W). Considering the high molar absorptivity, biobenign, and synthetic tunability of DASA, DASA PP can be utilized as an up-and-coming candidate to design and synthesize the next generation of upconversion nanovectors without photodamage.

Low-Power Near-Infrared-Responsive Upconversion Nanovectors

Activating upconversion nanoparticle-based photoresponsive nanovectors (UCPNVs) by upconversion visible light at low-power near-infrared (NIR) excitation can realize deeper biotissue stimulation with a minimized overheating effect and photodamage. Here, we demonstrate a facile strategy to construct new surface-decorated UCPNVs based on Passerini three-component reaction (P-3CR) in highly convenient and effective manners. Such UCPNVs materials have a tailored deprotecting wavelength that overlaps upconversion blue light. By using 3-perylenecarboxaldehyde, Tm³⁺/Yb³⁺ ion-doped UCNP-containing isocyanides, and antitumor agent chlorambucil as the three components, the resulting monodisperse UCPNV exhibits an efficient release of caged chlorambucil under a very low 976 nm power. This approach expands the synthetic toolbox to enable quick development of UCPNVs for UCNP-assisted low-power NIR photochemistry.

Current Advances in 3D Tissue and Organ Reconstruction

Bi-dimensional culture systems have represented the most used method to study cell biology outside the body for over a century. Although they convey useful information, such systems may lose tissue-specific architecture, biomechanical effectors, and biochemical cues deriving from the native extracellular matrix, with significant alterations in several cellular functions and processes. Notably, the introduction of three-dimensional (3D) platforms that are able to re-create in vitro the structures of the native tissue, have overcome some of these issues, since they better mimic the in vivo milieu and reduce the gap between the cell culture ambient and the tissue environment. 3D culture systems are currently used in a broad range of studies, from cancer and stem cell biology, to drug testing and discovery. Here, we describe the mechanisms used by cells to perceive and respond to biomechanical cues and the main signaling pathways involved. We provide an overall perspective of the most recent 3D technologies. Given the breadth of the subject, we concentrate on the use of hydrogels, bioreactors, 3D printing and bioprinting, nanofiber-based scaffolds, and preparation of a decellularized bio-matrix. In addition, we report the possibility to combine the use of 3D cultures with functionalized nanoparticles to obtain highly predictive in vitro models for use in the nanomedicine field.

Upconversion nanoparticle-assisted cationic and radical/cationic hybrid photopolymerization using sulfonium salts

Several sulfonium salts as unimolecular photoinitiators can efficiently initiate UCNP-assisted direct cationic and radial/cationic photopolymerization under 980 nm NIR light irradiation.

Lanthanide-Doped Core@Multishell Nanoarchitectures: Multimodal Excitable Upconverting/Downshifting Luminescence and High-Level Anti-Counterfeiting

The development of luminescent materials with concurrent multimodal emissions is a great challenge to improve security and data storage density. Lanthanide-doped nanocrystals are particularly appropriate for such applications for their abundant intermediate energy states and distinguishable spectroscopic profiles. However, traditional lanthanide luminescent nanoparticles have a limited capacity for information storage or complexity to shield against counterfeiting. Herein, it is demonstrated that the combination of upconverting and downshifting emissions in a particulate designed lanthanide-doped core@multishell nanoarchitecture allows the generation of multicolor dual-modal luminescence over a wide spectral range for complex information storage. Precise control of lanthanide dopants distribution in the core and distinct shells enables simultaneous excitation of 980/808 nm focusing/defocusing laser and 254 nm light and produces complex upconverting emissions from Er, Tm, Eu, and Tb via multiphoton energy transfer processes and downshifting emissions from Eu and Tb via efficient energy transfer from Ce to Eu/Tb in Gd-assisted lattices. It is experimentally proven that multiple visualized anti-counterfeit and information encryption with facile decryption and authentication using screen-printing inks containing the present core@multishell nanocrystals are practically applicable by selecting different excitation modes.

Synthesis and characterization of LiLuF4:Er3+ and LiLuF4:Yb3+,Er3+ exhibiting upconversion fluorescence pumped by a 1560 nm laser

The UC emission at ∼1 μm could be enhanced by doping Yb³⁺ ions in LiLuF₄:Er microcrystals under 1560 nm excitation.

Core-mediated synthesis, growth mechanism and near-infrared luminescence enhancement of α-NaGdF4@β-NaLuF4:Nd3+ core–shell nanocrystals

High-performance α-NaGdF₄@β-NaLuF₄:Nd³⁺ nanocrystals have been constructed based on a core-mediated method. Their near-infrared emission intensity was eventually enhanced by more than 2 times.

Highly efficient dandelion-like near-infrared light photoinitiator for free radical and thiol-ene photopolymerizations

Efficient photopolymerization activated by nonharmful near-infrared (NIR) light is important for various biological applications. Here we propose a NIR light free-radical photoinitiator (PI) fabricated by incorporating oxime-ester coumarin functionality on the surface of upconversion nanoparticles (UCNPs). The coumarin groups of PI absorb the light emitted from the UCNP core, whereas the oxime ester groups undergo cleavage to form radicals. Upon irradiation at 980 nm, the mobile radicals, formed in a manner similar to that of dandelion seed release, initiate both free-radical and thiol-ene photopolymerizations. The superior efficiency of dandelion-like PIs assisted photopolymerizations can be attributed to the reduction of energy loss and increased local PI concentration due to Förster resonance energy transfer process and confinement effect, respectively. Moreover, the proposed PI system can initiate polymerization under low-power NIR laser and reduces the thermal side effects. The possibility of its potential use in deep curing applications was also demonstrated.

Low-Dose X-ray Excited Photodynamic Therapy Based on NaLuF4:Tb3+-Rose Bengal Nanocomposite

X-ray excited photodynamic therapy (X-PDT), which utilizes X-rays as the energy source and X-ray luminescent nanoparticles (XLNPs) as the transducer to excite photosensitizers (PS), resolves the penetration problem of light in traditional PDT to enable the treatment of deep-seated tumors. Nevertheless, the high X-ray dosage used in X-PDT hampers its potential applications in clinics. In this study, to alleviate the dose problem, β-NaLuF₄:Tb³⁺ spherical nanoparticles (NPs) with ultrastrong green X-ray excited optical luminescence (XEOL) due to the less nonradiative relaxation probability and high X-ray absorption mass coefficient, which perfectly matches the absorption spectrum of a photosensitizer named rose bengal (RB), were synthesized and employed as the energy transducer for X-PDT. After covalent conjugation of NPs with RB, high Förster resonant energy transfer (FRET) efficiency up to 94.29% was achieved, leading to high production of singlet oxygen. In vivo X-PDT efficacy was evaluated by nude mice with a HepG2 tumor xenograft. With excellent biocompatibility, the synthesized NPs-RB nanocomposite showed significant antitumor efficiency up to 80 ± 12.3% with a total X-ray dose of only 0.19 Gy, demonstrating the feasibility of low-dose X-PDT in vivo for the first time. The present work provides a promising platform for X-PDT in deep-seated tumors.

Synthesis and Rational design of Europium and Lithium Doped Sodium Zinc Molybdate with Red Emission for Optical Imaging

Highly efficient fluorescent and biocompatible europium doped sodium zinc molybdate (NZMOE) nanoprobes were successfully synthesized via Polyol method. Non-radiative defect centres get reduced with Li+ co-doping in NZMOE nanoprobes. XRD spectra and Rietveld refinement confirmed successful incorporation of lithium ion and crystallinity was also improved with Li+ co-doping. The shape of phosphor is rod shaped, as determined by TEM. Significant enhancement in photoluminescence intensity was observed with 266, 395 and 465 nm excitations. Profound red emission was recorded for 5 at% Li+ co-doped NZMOE nanoprobes with 266 nm excitation. It shows high asymmetry ratio (~15), color purity (94.90%) and good quantum efficiency (~70%). Judd Ofelt parameters have been calculated to measure intensity parameters and radiative transition rates. In order to measure biocompatibility of the nanoprobes, cytotoxicity assays were performed with HePG2 cells. The fluorescence emitted from phosphor material treated HePG2 cells was also measured by Laser Scanning Confocal Microscopy. The bright red fluorescence in HePG2 cells treated with very low concentration (20 μg/ml) of phosphor material indicates that it could be a promising phosphor for biological detection or bio-imaging.

Preparation, growth mechanism, size manipulation and near-infrared luminescence enhancement of β-NaYF4:Nd3+ microcrystals via Ca2+ doping

The near-infrared emission intensity of NaYF₄:3% Nd³⁺ doped with 20 mol% Ca²⁺ is 3 times that of the Ca²⁺-free samples.

Synthesis and photocatalytic activity of δ-doped hexagonal NaYF4:Yb,Tm@TiO2/RGO nanocrystals

Improved photocatalytic activity of δ-doped β-NaYF₄:Yb,Tm@TiO₂/RGO nanocrystals.

Near-infrared light induced cationic polymerization based on upconversion and ferrocenium photochemistry

Ferrocenium salts as unimolecular photoinitiators can efficiently initiate upconversion nanoparticle-assisted cationic photopolymerization under near-infrared light excitation.

Near infrared emission properties of Er doped cubic sesquioxides in the second/third biological windows

In the recent years, there is an extensive effort concentrated towards the development of nanoparticles with near-infrared emission within the so called second or third biological windows induced by excitation outside 800-1000 nm range corresponding to the traditional Nd (800 nm) and Yb (980 nm) sensitizers. Here, we present a first report on the near-infrared (900-1700 nm) emission of significant member of cubic sesquioxides, Er-Lu2O3 nanoparticles, measured under both near-infrared up-conversion and low energy X-ray excitations. The nanoparticle compositions are optimized by varying Er concentration and Li addition. It is found that, under ca. 1500 nm up-conversion excitation, the emission is almost monochromatic (>93%) and centered at 980 nm while over 80% of the X-ray induced emission is concentrated around 1500 nm. The mechanisms responsible for the up-conversion emission of Er - Lu2O3 are identified by help of the up-conversion emission and excitation spectra as well as emission decays considering multiple excitation/emission transitions across visible to near-infrared ranges. Comparison between the emission properties of Er-Lu2O3 and Er-Y2O3 induced by optical and X-ray excitation is also presented. Our results suggest that the further optimized Er-doped cubic sesquioxides represent promising candidates for bioimaging and photovoltaic applications.

Simultaneous enhancement of red upconversion luminescence and CT contrast of NaGdF4:Yb,Er nanoparticles via Lu3+ doping

To date, lanthanide-doped upconversion nanoparticles (UCNPs) have been widely reported as a promising CT contrast agent because they have high atomic numbers and big X-ray attenuation coefficient values. However, it is still a challenge to fabricate a simple multimodal imaging probe with improved image quality for early cancer diagnosis in clinical medicine. Herein, ultra-small, uniform and monodisperse β-NaGdF4:Yb,Er,X% Lu (X = 0, 1, 2.5, 4, 6, 7.5) UCNPs were prepared through a solvothermal method with high-level modulation of both the phase and morphology. Meanwhile, a remarkably enhanced red upconversion luminescence (UCL) in the β-NaGdF4:Yb,Er,X% Lu NPs was successfully realized via Lu3+ doping. It is found that as the content of Lu3+ increases from 0 to 7.5 mol%, the UCL intensity of the red emission first increases and then decreases, with the optimum doping content of Lu3+ ions of 2.5 mol%. The red UCL enhancement is ascribed to the change of the Yb-Er interionic distance controlling the Yb-Er energy transfer rate and the distortion of the local environment of Er3+ ions influencing the 4f-4f transition rates of Er3+ ions, which has been further confirmed by the experimental check of the crystallographic phase and by photoluminescence spectroscopy employing Eu3+ as the structural probe, respectively. More importantly, after being modified with the HS-PEG2000-NH2 ligand, the NH2-PEGylated-NaGdF4:Yb,Er,X% Lu NPs exhibited low cytotoxicity, high biocompatibility, and remarkably enhanced contrast performance in in vitro UCL and in vivo CT imaging. On the basis of our findings, the as-obtained functionalized UCNPs could be considered as a promising versatile dual-mode imaging probe for bioimaging, tumor diagnosis, and cancer therapy.

Nanocomplexes of Photolabile Polyelectrolyte and Upconversion Nanoparticles for Near-Infrared Light-Triggered Payload Release

A new approach to encapsulating charged cargo molecules into a nanovector and subsequently using near-infrared (NIR) light to trigger the release is demonstrated. NIR light-responsive nanovector was prepared through electrostatic interaction-driven complexation between negatively charged silica-coated upconversion nanoparticles (UCNP@silica, 87 nm hydrodynamic diameter, polydispersity index ∼0.05) and a positively charged UV-labile polyelectrolyte bearing pendants of poly(ethylene glycol) and o-nitrobenzyl side groups; whereas charged fluorescein (FLU) was loaded through a co-complexation process. By controlling the amount of polyelectrolyte, UCNP@silica can be covered by the polymer, whereas remaining dispersed in aqueous solution. Under 980 nm laser excitation, UV light emitted by UCNP is absorbed by photolytic side groups within polyelectrolyte, which results in cleavage of o-nitrobenzyl groups and formation of carboxylic acid groups. Such NIR light-induced partial reversal of positive charge to negative charge on the polyelectrolyte layer disrupts the equilibrium among UCNP@silica, polyelectrolyte, and FLU and, consequently, leads to release of FLU molecules.

Sub-10 nm Water-Dispersible β-NaGdF4:X% Eu3+ Nanoparticles with Enhanced Biocompatibility for in Vivo X-ray Luminescence Computed Tomography

As a novel molecular and functional imaging modality, X-ray luminescence computed tomography (XLCT) has shown its potentials in biomedical and preclinic applications. However, there are still some limitations of X-ray-excited luminescent materials, such as low luminescence efficiency, poor biocompatibility, and cytotoxicity, making in vivo XLCT imaging quite challenging. In this study, for the very first time, we present on using sub-10 nm β-NaGdF₄:X% Eu³⁺ nanoparticles with poly(acrylic acid) (PAA) surface modification, which demonstrate outstanding luminescence efficiency, uniform size distribution, water dispersity, and biosafety, as the luminescent probes for in vivo XLCT application. The pure hexagonal phase (β-) NaGdF₄ has been successfully synthesized and characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM), and then the results of X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectrometry  (EDX), and elemental mapping further confirm Eu³⁺ ions doped into NaGdF₄ host. Under X-ray excitation, the β-NaGdF₄ nanoparticles with a doping level of 15% Eu³⁺ exhibited the most efficient luminescence intensity. Notably, the doping level of Eu³⁺ has no effect on the crystal phase and morphology of the NaGdF₄-based host. Afterward, β-NaGdF₄:15% Eu³⁺ nanoparticles were modified with PAA to enhance the water dispersity and biocompatibility. The compatibility of in vivo XLCT imaging using such nanoparticles was systematically studied via in vitro cytotoxicity, physical phantom, and in vivo imaging experiments. The ultralow cytotoxicity of PAA-modified nanoparticles, which is confirmed by over 80% cell viability of SH-SY5Y cells when treated by high nanoparticle concentration of 200 μg/mL, overcome the major obstacle for in vivo application. In addition, the high luminescence intensity of PAA-modified nanoparticles enables the location error of in vivo XLCT imaging less than 2 mm, which is comparable to that using commercially available bulk material Y₂O₃:15% Eu³⁺. The proposed nanoparticles promote XLCT research into an in vivo stage. Further modification of these nanoparticles with biofunctional molecules could enable the potential of targeting XLCT imaging.

Stable High-Performance Flexible Photodetector Based on Upconversion Nanoparticles/Perovskite Microarrays Composite

Methylammonium lead halide perovskite has emerged as a new class of low-temperature-processed high-performance semiconductors for optoelectronics, but with photoresponse limited to the UV-visible region and low environmental stability. Herein, we report a flexible planar photodetector based on MAPbI₃ microarrays integrated with NaYF₄:Yb/Er upconversion nanoparticles (UCns) that offers promise for future high performance and long-term environmental stability. The promise derives from the confluence of several factors, including significantly enhanced photons absorption in the visible spectrum, efficient energy transition in the near-infrared (NIR) region, and inhibition of water attack by the hydrophobic UCns capping layer. The UCns layer aided in remarkably enhanced photodetection capability in the visible spectrum with detectivity (D*) reaching 5.9 × 10¹² Jones, among the highest reported values, due to the increased photocarrier lifetime and decreased reflectivity. Excellent NIR photoresponse with spectral responsivity (R) and D* as high as 0.27 A W^-1 and 0.76 × 10¹² Jones were obtained at 980 nm, respectively, superior to the reported values of state-of-the-art organic-perovskite NIR photodetectors. Moreover, the hydrophobic UCns capping layer serving as a moisture inhibitor allowed significantly enhanced long-term environmental stability, e.g., 70% vs 27% performance retained after 1000 h exposure in 30-40% RH humidity air without encapsulation for the bilayer and the neat MAPbI₃ devices, respectively. These results suggest that the composite based on perovskite and UCns is promising for constructing high-performance broadband optoelectronic devices with long-term stability.

Lanthanide-Doped KLu2F7 Nanoparticles with High Upconversion Luminescence Performance: A Comparative Study by Judd-Ofelt Analysis and Energy Transfer Mechanistic Investigation

The development, design and the performance evaluation of rare-earth doped host materials is important for further optical investigation and industrial applications. Herein, we successfully fabricate KLu₂F₇ upconversion nanoparticles (UCNPs) through hydrothermal synthesis by controlling the fluorine-to-lanthanide-ion molar ratio. The structural and morphological results show that the samples are orthorhombic-phase hexagonal-prisms UCNPs, with average side length of 80 nm and average thickness of 110 nm. The reaction time dependent crystal growth experiment suggests that the phase transformation is a thermo-dynamical process and the increasing F^-/Ln³⁺ ratio favors the formation of the thermo-dynamical stable phase - orthorhombic KLu₂F₇ structure. The upconversion luminescence (UCL) spectra display that the orthorhombic KLu₂F₇:Yb/Er UCNPs present stronger UCL as much as 280-fold than their cubic counterparts. The UCNPS also display better UCL performance compared with the popular hexagonal-phase NaREF₄ (RE = Y, Gd). Our mechanistic investigation, including Judd-Ofelt analysis and time decay behaviors, suggests that the lanthanide tetrad clusters structure at sublattice level accounts for the saturated luminescence and highly efficient UCL in KLu₂F₇:Yb/Er UCNPs. Our research demonstrates that the orthorhombic KLu₂F₇ is a promising host material for UCL and can find potential applications in lasing, photovoltaics and biolabeling techniques.

Multifunctional BaYbF5: Gd/Er upconversion nanoparticles for in vivo tri-modal upconversion optical, X-ray computed tomography and magnetic resonance imaging

Development of high-quality upconversion nanoparticles (UCNPs) with combination of the merits of multiple molecular imaging techniques, such as, upconversion luminescence (UCL) imaging, X-ray computed tomography (CT), and magnetic resonance (MR) imaging, could significantly improve the accuracy of biological diagnosis. In this work, multifunctional BaYbF₅: Gd/Er (50:2mol%) UCNPs were synthesized via a solvothermal method using oleic acid (OA) as surface ligands (denoted as OA-UCNPs). The OA-UCNPs were further treated by diluted HCl to form ligand-free UCNPs (LF-UCNPs) for later bioimaging applications. The cytotoxicity assay in HeLa cells shows low cell toxicity of these LF-UCNPs. Owing to the efficient UCL of BaYbF₅: Gd/Er, the LF-UCNPs were successfully used as luminescent bioprobe in UCL bioimaging. And, X-ray CT imaging reveals that BaYbF₅: Gd/Er UCNPs can act as potential contrast agents for detection of the liver and spleen in the live mice owing to the high-Z elements (e.g., Ba, Yb, and Gd) in host matrix. Moreover, with the addition of Gd, the as-designed UCNPs exhibit additional positive contrast enhancement in T₁-weighted MR imaging. These findings demonstrate that BaYbF₅: Gd/Er UCNPs are potential candidates for tri-modal imaging.

Focal photodynamic intracellular acidification as a cancer therapeutic

Cancer cells utilize an array of proton transporters to regulate intra- and extracellular pH to thrive in hypoxic conditions, and to increase tumor growth and metastasis. Efforts to target many of the transporters involved in cancer cell pH regulation have yielded promising results, however, many productive attempts to disrupt pH regulation appear to be non-specific to cancer cells, and more effective in some cancer cells than others. Following a review of the status of photodynamic cancer therapy, a novel light-activated process is presented which creates very focal, rapid, and significant decreases in only intracellular pH (pHi), leading to cell death. The light-activation of the H⁺ carrier, nitrobenzaldehyde, has been effective at initiating pH-induced apoptosis in non-cancerous and numerous cancerous cell lines in vitro, to include breast, prostate, and pancreatic cancers. Also, this intracellular acidification technique caused significant reductions in tumor growth rate and enhanced survival in mice bearing triple negative breast cancer tumors. The efficacy of an NBA-upconverting nanoparticle to kill breast cancer cells in vitro is described, as well as a discussion of the potential intracellular mechanisms underlying the pH-induced apoptosis.

Synthesis of Multicolor Core/Shell NaLuF₄:Yb3+/Ln3+@CaF₂ Upconversion Nanocrystals

The ability to synthesize high-quality hierarchical core/shell nanocrystals from an efficient host lattice is important to realize efficacious photon upconversion for applications ranging from bioimaging to solar cells. Here, we describe a strategy to fabricate multicolor core @ shell α-NaLuF₄:Yb³⁺/Ln³⁺@CaF₂ (Ln = Er, Ho, Tm) upconversion nanocrystals (UCNCs) based on the newly established host lattice of sodium lutetium fluoride (NaLuF₄). We exploited the liquid-solid-solution method to synthesize the NaLuF₄ core of pure cubic phase and the thermal decomposition approach to expitaxially grow the calcium fluoride (CaF₂) shell onto the core UCNCs, yielding cubic core/shell nanocrystals with a size of 15.6 ± 1.2 nm (the core ~9 ± 0.9 nm, the shell ~3.3 ± 0.3 nm). We showed that those core/shell UCNCs could emit activator-defined multicolor emissions up to about 772 times more efficient than the core nanocrystals due to effective suppression of surface-related quenching effects. Our results provide a new paradigm on heterogeneous core/shell structure for enhanced multicolor upconversion photoluminescence from colloidal nanocrystals.

A dual-functional NaLuF4:Yb3+/Er3+ material for enhancing photon harvesting in dye-sensitized solar cells

A new strategy to improve the efficiency of dye-sensitized solar cells by using the external near-infrared light harvesting and light-scattering dual-functional upconversion (UC) material NaLuF₄:Yb³⁺/Er³⁺ is proposed.

Growth phase diagram and upconversion luminescence properties of NaLuF4:Yb3+/Tm3+/Gd3+ nanocrystals

Relationships between the phase, reaction time, and UCL intensity of NaLuF₄ nanocrystals are investigated.

Deep Photoacoustic/Luminescence/Magnetic Resonance Multimodal Imaging in Living Subjects Using High-Efficiency Upconversion Nanocomposites

A gadolinium-doped multi-shell upconversion nanoparticle under 800 nm excitation is synthesized with a 10-fold fluorescence-intensity enhancement over that under 980 nm. The nanoformulations exhibit excellent photoacoustic/luminescence/magnetic resonance tri-modal imaging capabilities, enabling visualization of tumor morphology and microvessel distribution at a new imaging depth.

Phase and Size Control of Core-Shell Upconversion Nanocrystals Light up Deep Dual Luminescence Imaging and CT In Vivo

Upconversion nanocrystals (UCNCs) have recently been explored as optical imaging nanoprobes. However, conventional β-NaLuF4 (-) based UCNCs often suffer from large particle size and weak upconversion luminescence (UCL) intensity, leading to poor biocompatibility and low detection sensitivity. Here, a novel strategy for controlling the crystalline phase and size of UCNCs has been developed by doping of yttrium ions, resulting in particle size reduction and phase transition. The total UCL intensity of prepared core-shell UCNCs is significantly enhanced up to ≈4.9 and ≈17.4 times after Tm(3+) and Er(3+) doping than that of core UCNCs, offering deeper tissue UCL imaging with a depth of 8 mm in vivo. Moreover, the CT signal of core-shell UCNCs is ≈1.5 and ≈3.5 times brighter than that of core UCNCs and commercial ioversol agent because of increasing contents of Lu(3+) doped in UCNCs. The synthesized core-shell UCNCs hold a great promise in deep UCL and CT dual-modality imaging in vitro and in vivo.