Enhanced upconversion luminescence in NaGdF4:Yb,Er nanocrystals by Fe3+ doping and their application in bioimaging

Low Temperature Plasma-Assisted Double Anodic Dissolution: A New Approach for the Synthesis of GdFeO3 Perovskite Nanoparticles

Orthorhombic perovskite GdFeO3 nanostructures are promising materials with multiferroic properties. In this study, a new low-temperature plasma-assisted approach is developed via dual anodic dissolution of solid metallic precursors for the preparation of perovskite GdFeO3 nanoparticles (NPs) that can be collected both as colloids as well as deposited as a thin film on a substrate. Two solid metallic foils of Gd and Fe are used as precursors, adding to the simplicity and sustainability of the method. The formation of the orthorhombic perovskite GdFeO3 phase is supported by high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman measurements, while a uniform elemental distribution of Gd, Fe, and O is confirmed by energy dispersive X-ray spectroscopy, proving the successful preparation of ternary compound NPs. The magnetic properties of the NPs show zero remnant magnetization typical of antiferromagnetic materials, and saturation at high fields that can be caused by spin interaction between Gd and Fe magnetic sublattices. The formation mechanism of ternary compound NPs in this novel plasma-assisted method is also discussed. This method is also modified to demonstrate the direct one-step deposition of thin films, opening up opportunities for their future applications in the fabrication of magnetic memory devices and gas sensors.

The luminescence modulation of rare earth-doped/containing lead-free double perovskites toward multifunctional applications: a review

Lead-free double perovskites (DPs) with superior environmental stability and high defect tolerance have attracted considerable attention and exhibit great promise in photodetectors, solar cells, lighting devices, etc. However, achieving optical modulation and high photoluminescence quantum yield using this kind of material remains a challenge. Rare earth ions feature abundant energy levels and outstanding photophysical properties. Incorporating rare earth ions into lead-free DPs is an effective strategy to improve their optical performances, which have great effects on night-vision and light emitting diodes. Consequently, in this mini-review, we summarize the synthesis methods, optical properties, issues, and multifunctional applications of lead-free DPs described in recent years. The performances of DPs can be modulated via rare earth doping, which involves the extension of luminescence range, the improvement of PLQY, the realization of multi-mode excitation, and the regulation of luminescence color. We hope that this review will provide some insights into luminescence modulation and applications of lead-free DPs.

Morphology controlled synthesis of Fe3+-doped upconversion nanomaterials

This work details the synthesis of paramagnetic upconversion nanoparticles doped with Fe3+ in various morphologies via the thermal decomposition method, followed by comprehensive characterization of their structures, optical properties and magnetism using diverse analytical techniques. Our findings demonstrate that by precisely modulating the ratio of oleic acid to octadecene in the solvent, one can successfully obtain hexagonal nanodiscs with a consistent and well-defined morphology. Further adjustments in the oleic acid to octadecene ratio, coupled with fine-tuning of the Na+/F- ratio, led to the production of small-sized nanorods with uniform morphology. Significantly, all Fe3+-doped nanoparticles displayed pronounced paramagnetism, with magnetic susceptibility measurements at 1 T and room temperature of 0.15 emu g-1 and 0.14 emu g-1 for the nanodiscs and nanorods, respectively. To further enhance their magnetic properties, we replaced the Y-matrix with a Gd-matrix, and by fine-tuning the oleic acid/octadecene and Na+/F- ratios, we achieved nanoparticles with uniform morphology. The magnetic susceptibility was 0.82 emu g-1 at 1 T and room temperature. Simultaneously, we could control the nanoparticle size by altering the synthesis temperature. These upconversion nanostructures, characterized by both paramagnetic properties and regular morphology, represent promising dual-mode nanoprobe candidates for optical biological imaging and magnetic resonance imaging.

Achieving Orthogonal Upconversion Luminescence of a Single Lanthanide Ion in Crystals for Optical Encryption

Optical encryption shows great potential in meeting the growing demand for advanced anti-counterfeiting in the information age. The development of upconversion luminescence (UCL) materials capable of emitting different colors of light in response to different external stimuli holds great promise in this field. However, the effective realization of multicolor UCL materials usually requires complex structural designs. In this work, orthogonal UCL is achieved in crystals with a simple structure simply by introducing modulator Tm3+ ions to control the photon transition processes between different energy levels of activator Er3+ ions. The obtained crystals emit red and green UCL when excited by 980 nm and 808 nm lasers, respectively. The orthogonal excitation-emission properties of crystals are shown to be very suitable for high-level optical encryption, which is important for information security and anti-counterfeiting. This work provides an effective strategy for obtaining orthogonal UCL in simple structural materials, which will encourage researchers to further explore novel orthogonal UCL materials and their applications, and has important implications for the development of the frontier photonic upconversion fields.

Temoporfin-Conjugated Upconversion Nanoparticles for NIR-Induced Photodynamic Therapy: Studies with Pancreatic Adenocarcinoma Cells In Vitro and In Vivo

Upconverting nanoparticles are interesting materials that have the potential for use in many applications ranging from solar energy harvesting to biosensing, light-triggered drug delivery, and photodynamic therapy (PDT). One of the main requirements for the particles is their surface modification, in our case using poly(methyl vinyl ether-alt-maleic acid) (PMVEMA) and temoporfin (THPC) photosensitizer to ensure the colloidal and chemical stability of the particles in aqueous media and the formation of singlet oxygen after NIR irradiation, respectively. Codoping of Fe2+, Yb3+, and Er3+ ions in the NaYF4 host induced upconversion emission of particles in the red region, which is dominant for achieving direct excitation of THPC. Novel monodisperse PMVEMA-coated upconversion NaYF4:Yb3+,Er3+,Fe2+ nanoparticles (UCNPs) with chemically bonded THPC were found to efficiently transfer energy and generate singlet oxygen. The cytotoxicity of the UCNPs was determined in the human pancreatic adenocarcinoma cell lines Capan-2, PANC-01, and PA-TU-8902. In vitro data demonstrated enhanced uptake of UCNP@PMVEMA-THPC particles by rat INS-1E insulinoma cells, followed by significant cell destruction after excitation with a 980 nm laser. Intratumoral administration of these nanoconjugates into a mouse model of human pancreatic adenocarcinoma caused extensive necrosis at the tumor site, followed by tumor suppression after NIR-induced PDT. In vitro and in vivo results thus suggest that this nanoconjugate is a promising candidate for NIR-induced PDT of cancer.

Enhancement of photoacoustic and upconversion emission in Mg2+/Zn2+ codoped Gd2O3: Er3+/Yb3+ phosphor using 980 nm excitation

In this work, Er³⁺/Yb³⁺ doped Gd₂O₃ phosphor samples were synthesized through a combustion method and then characterized through X-ray diffraction and FE-SEM techniques. The sample was studied for photoacoustic spectroscopy (PAS) and upconversion (UC) emission spectroscopy techniques using a frequency modulated 980 nm excitation source. A correlation between PA signal (produced due to non-radiative transitions) and UC emission intensity (produced due to radiative transition) is made in order to optimize the sample for both these properties. The phosphor was codoped with Mg²⁺ and Zn²⁺ ions to see enhancement in upconversion emission intensity and these ions have enhanced the upconversion emission. Studies show that the present phosphor is appropriate for producing strong upconversion emission intensity along with a strong photoacoustic signal which is beneficial for upconversion-imaging as well as photothermal therapy. The present sample has also shown its potential for detection of fingerprints.

Nitrogen-doped graphene quantum dot-based portable fluorescent sensors for the sensitive detection of Fe3+ and ATP with logic gate operation

Adenosine triphosphate (ATP) and Fe³⁺ are important "signaling molecules" in living organisms, and their abnormal concentrations can be used for the early diagnosis of degenerative diseases. Therefore, the development of a sensitive and accurate fluorescent sensor is essential for detecting these signaling molecules in biological matrices. Herein, nitrogen-doped graphene quantum dots (N-GQDs) with cyan fluorescence emission were prepared by thermal cleavage of graphene oxide (GO) with N,N-dimethylformamide (DMF) as a solvent. The synergistic effect of static quenching and internal filtration enabled the selective quenching of N-GQD fluorescence by Fe³⁺. With the introduction of ATP, Fe³⁺ in the N-GQDs-Fe³⁺ system formed a more stable complex with ATP via the Fe-O-P bond, thus restoring the fluorescence of the N-GQDs. Fe³⁺ and ATP were detected in the linear ranges of 0-34 μM and 0-10 μM with the limits of detection (LOD) of 2.38 nM and 1.16 nM, respectively. In addition to monitoring Fe³⁺ and ATP in mouse serum and urine, the proposed method was also successfully applied for cytoplasmic imaging of 4T1 cells and in vivo imaging of freshwater shrimps. Moreover, the fluorescence and solution color change-based "AND" logic gate was successfully demonstrated in the biological matrix. Importantly, a complete sensing system was constructed by combining the N-GQDs with hydrogel kits and fluorescent flexible films. Thus, the prepared N-GQDs can be expected to serve as a valuable analytical tool for monitoring Fe³⁺ and ATP concentrations in biological matrices.

Novel Sol-Gel Route to Prepare Eu3+-Doped 80SiO2-20NaGdF4 Oxyfluoride Glass-Ceramic for Photonic Device Applications

Oxyfluoride glass-ceramics (OxGCs) with the molar composition 80SiO₂-20(1.5Eu³⁺: NaGdF₄) were prepared with sol-gel following the "pre-crystallised nanoparticles route" with promising optical results. The preparation of 1.5 mol % Eu³⁺-doped NaGdF₄ nanoparticles, named 1.5Eu³⁺: NaGdF₄, was optimised and characterised using XRD, FTIR and HRTEM. The structural characterisation of 80SiO₂-20(1.5Eu³⁺: NaGdF₄) OxGCs prepared from these nanoparticles' suspension was performed by XRD and FTIR revealing the presence of hexagonal and orthorhombic NaGdF₄ crystalline phases. The optical properties of both nanoparticles' phases and the related OxGCs were studied by measuring the emission and excitation spectra together with the lifetimes of the ⁵D₀ state. The emission spectra obtained by exciting the Eu³⁺-O^2- charge transfer band showed similar features in both cases corresponding the higher emission intensity to the ⁵D₀→⁷F₂ transition that indicates a non-centrosymmetric site for Eu³⁺ ions. Moreover, time-resolved fluorescence line-narrowed emission spectra were performed at a low temperature in OxGCs to obtain information about the site symmetry of Eu³⁺ in this matrix. The results show that this processing method is promising for preparing transparent OxGCs coatings for photonic applications.

Light Conversion upon Photoexcitation of NaBiF4:Yb3+/Ho3+/Ce3+ Nanocrystalline Particles

NaBiF₄ nanocrystalline particles were synthesized by means of a facile precipitation synthesis route to explore upconversion emission properties when doped with lanthanide ions. In particular, the incorporation of the Yb³⁺-Ho³⁺-Ce³⁺ triad with controlled ion concentration facilitates near-IR pumping conversion into visible light, with the possibility of color emission tuning depending on Ce³⁺ doping amount. We observed that introducing a Ce³⁺ content up to 20 at.% in NaBiF₄:Yb³⁺/Ho³⁺, the chromaticity progressively turns from green for the Ce³⁺ undoped system to red. This is due to cross-relaxation mechanisms between Ho³⁺ and Ce³⁺ ions that influence the relative efficiency of the overall upconversion pathways, as discussed on the basis of a theoretical rate equation model. Furthermore, experimental results suggest that the photoexcitation of intra-4f Ho³⁺ transitions with light near the UV-visible edge can promote downconverted Yb³⁺ near-IR emission through quantum cutting triggered by Ho³⁺-Yb³⁺ energy transfer mechanisms. The present study evidences the potentiality of the developed NaBiF₄ particles for applications that exploit lanthanide-based light frequency conversion and multicolor emission tuning.

Enhanced luminescence of erbium silicate: interstitial lithium directly regulates the lattice structure of erbium compound crystals

We prepared high-intensity luminescent films with a high LDP (Er³⁺ luminescence lifetime-concentration product) of 1.54 × 10¹⁹ s × cm^-3, where lithium-doped erbium silicate grains are embedded in cristobalite. The near-infrared and up-conversion luminescence intensities of erbium silicate show ∼55 and 40 times enhancement by lithium-doping, respectively. Lithium-doping directly regulates the lattice structure of erbium silicate to enhance luminescence by reducing the crystal field symmetry around erbium, meanwhile, interstitial lithium does not dilute the concentration of erbium ions. Furthermore, lithium-containing dopants promote silica to crystallize, enhancing the luminescence of erbium silicate by reducing the interface defects. These films are expected to achieve high-gain film waveguide amplifiers in chip-scale optoelectronic integration. And this method opens up possibilities to be universally applicable to erbium compounds for enhancing luminescence by directly regulating the lattice structure.

Time-Gated Imaging of Latent Fingerprints with Level 3 Details Achieved by Persistent Luminescent Fluoride Nanoparticles

The discovery of X-ray-charged persistent luminescence (PersL) in fluoride nanoparticles enables these materials to emit photons without real-time excitation, which provides a great possibility for the development of new luminescent nanotechnologies. In this work, we developed NaLuF₄:Mn nanoparticles with intense green PersL and functionalized surfaces and accordingly achieved time-gated imaging of latent fingerprints (LFPs) with Level 3 details. These surface-modified NaLuF₄:Mn nanoparticles exhibited near-spherical morphology, long-lasting emission for several hours, appropriate trap depth distribution, and tight chemical bonding with amino acids from fingerprints, thus greatly improving the accuracy of LFP imaging in a variety of environments. The developed NaLuF₄:Mn PersL nanoparticles are expected to find broad applications in the fields of LFP imaging and in vivo biological imaging.

Microsphere Photonic Superlens for a Highly Emissive Flexible Upconversion-Nanoparticle-Embedded Film

Increasing upconversion luminescence (UCL) to overcome the intrinsically low conversion efficiency of upconversion nanoparticles (UCNPs) poses a fundamental challenge. Photonic nanostructures are the efficient approaches for UCL enhancement by tailoring the local electromagnetic fields. Unfortunately, such nanostructures are sensitive to environmental conditions, and the regulation strength is varied in flexible applications. Here, we report giant UCL enhancement from a flexible UCNP-embedded film coupled with a microsphere photonic superlens (MPS), by which the enhancement ratio of UCL is over 10⁴-fold under 808 nm excitation down to 0.72 mW. The enhancement pathways of MPS-enhanced UCL are attributed to Mie-resonant nanofocusing for high excitation-photon density, optical whispering-gallery modes (WGMs) for fast radiative decay, and the directional antenna effect for far-field emission confinement. The contribution of optical resonance in the MPS to suppressing the phonon-induced nonradiative transition and thermal quenching is experimentally validated. The UCL quantum yield is therefore improved by 3-fold to 4.20% under 120 mW/cm² near-infrared excitation, consistent with the enhancement ratio via the Purcell effect of WGMs. Furthermore, the MPS demonstrates the robust optical regulation capability toward flexible applications, opening up new opportunities for facilitating multiphoton upconversion in wearable optoelectrical devices for nanoimaging, biosensing, and energy conversion in the future.

Unravelling the benefits of transition-metal-co-doping in lanthanide upconversion nanoparticles

In this review we provide an overview of the current knowledge on lanthanide upconversion materials co-doped with transition metals. We focus on how the co-dopants affect the host lattice and the energy transfer processes.

Subcellular imaging and diagnosis of cancer using engineered nanoparticles

The advances in the synthesis of nanoparticles with engineered properties are reported to have profound applications in oncological disease detection via optical and multimodal imaging and therapy. Among various nanoparticle-assisted imaging techniques, engineered fluorescent nanoparticles show great promise from high contrast images and localized therapeutic applications. Of all the fluorescent nanoparticles available, the gold nanoparticles, carbon dots, and upconversion nanoparticles are emerging recently as the most promising candidates for diagnosis, treatment, and cancer monitoring. This review addresses the recent progress in engineering the properties of these emerging nanoparticles and their application for cancer diagnosis and therapy. In addition, the potential of these particles for subcellular imaging is also reviewed here.

Transparent and flexible resins functionalized by lanthanide-based upconversion nanocrystals

Functional resins with optical adjustment capability own great potential in multiple application scenarios. To this end, we functionalize resins with upconversion nanocrystals (UCNCs), namely an UCNC-Au composite structure, to endow them with the unique ability of converting near-infrared (NIR) radiation into visible-light emission. Such UCNC-functionalized resins with high transparency and flexibility are expected to accelerate the development in the comprehensive utilization of NIR during practical applications.

Nanoscale optical writing through upconversion resonance energy transfer

Nanoscale optical writing using far-field super-resolution methods provides an unprecedented approach for high-capacity data storage. However, current nanoscale optical writing methods typically rely on photoinitiation and photoinhibition with high beam intensity, high energy consumption, and short device life span. We demonstrate a simple and broadly applicable method based on resonance energy transfer from lanthanide-doped upconversion nanoparticles to graphene oxide for nanoscale optical writing. The transfer of high-energy quanta from upconversion nanoparticles induces a localized chemical reduction in graphene oxide flakes for optical writing, with a lateral feature size of ~50 nm (1/20th of the wavelength) under an inhibition intensity of 11.25 MW cm-2 Upconversion resonance energy transfer may enable next-generation optical data storage with high capacity and low energy consumption, while offering a powerful tool for energy-efficient nanofabrication of flexible electronic devices.

Refine the crystallinity of upconversion nanoparticles for NIR-enhanced photocatalysis

A new photocatalyst was synthesized by a combination of the upconversion nanoparticle NaYF4:Yb, Tm, Gd (NYTG) and NH2-MIL-101(Cr) (NMC) to form NYTG/NMC.

Metal Ions Doping for Boosting Luminescence of Lanthanide-Doped Nanocrystals

With the developing need for luminous materials with better performance, lanthanide-doped nanocrystals have been widely studied for their unique luminescence properties such as their narrow bandwidth emission, excellent chemical stability, and photostability, adjustable emission color, high signal-to-background ratio, deeper tissue penetration with less photo-damage, and low toxicity, etc., which triggered enthusiasm for research on the broad applications of lanthanide-doped nanocrystals in bioimaging, anti-counterfeiting, biosensing, and cancer diagnosis and treatment. Considerable progress has been made in the past few decades, but low upconversion luminescence efficiency has been a hindrance in achieving further progress. It is necessary to summarize the recently relevant literature and find solutions to improve the efficiency. The latest experimental and theoretical studies related to the deliberate design of rare earth luminescent nanocrystals have, however, shown the development of metal ion-doped approaches to enhance the luminescent intensity. Host lattice manipulation can enhance the luminescence through increasing the asymmetry, which improves the probability of electric dipole transition; and the energy transfer modulation offers a reduced cross-relaxation pathway to improve the efficiency of the energy transfer. Based on the mechanisms of host lattice manipulation and energy transfer modulation, a wide range of enhancements at all wavelengths or even within a particular wavelength have been accomplished with an enhancement of up to a hundred times. In this mini review, we present the strategy of metal ion-doped lanthanide nanocrystals to cope with the issue of enhancing luminescence, overview the advantages and tricky challenges in boosting the luminescence, and provide a potential trend of future study in this field.

Study on cytotoxicity, cellular uptake and elimination of rare-earth-doped upconversion nanoparticles in human hepatocellular carcinoma cells

The growing use of rare-earth doped upconversion nanoparticles (UCNPs) has caused increasing concern about their biosafety. Here, to understand the toxicity of UCNPs and their mechanism in HepG2 cells, we systematically study the cytotoxicity, uptake and elimination behaviors of three types of UCNPs combined multiple cytotoxicity evaluation means with inductively coupled plasma mass spectrometry (ICP-MS) detection. Sodium yttrium fluoride, doped with 18% (molar ratio) ytterbium and 2% erbium (NaYF₄: Yb³⁺, Er³⁺) was selected as the model UCNPs with two sizes (35 and 55 nm), and the poly(acrylic acid) and polyethylenimine were selected as the representatives of negative and positive surface coating of UCNPs, respectively. UCNPs were found to induce cytotoxicity in time- and dose-dependent manners, which might be mediated by reactive oxygen species generation and oxidative stress. Apoptosis, inflammation, and metabolic process were enhanced after cells exposed to 200 mg/L UCNPs for 48 h. Increase in the protein levels of cleaved caspased-9, cleaved caspase-3 and Bax and decrease in the anti-apoptotic protein, Bcl-2 suggested that the mitochondria mediated pathway was involved in UCNP-induced apoptosis. With the aid of ICP-MS, it demonstrated that the cytotoxicity was associated with internalized amount of UCNPs, which largely relied on their surface properties rather than size in the tested range. By comparing UCNPs with Y³⁺ ions, it demonstrated that NPs properties played a nonnegligible role in the cytotoxicity of UCNPs. These findings provide new insights for fundamental understanding of cytotoxicity of UCNPs and may contribute to more rational use of these materials in the future.

Enhanced photoluminescence and ferro/piezoelectric performance in piezo-luminescent materials with outstanding water resistance and thermal stability

Sm³⁺-Modified (1 -x)Na_0.5Bi_0.5TiO₃-xCaTiO₃ (NBT-xCT:Sm³⁺) piezo-luminescent ceramic materials were synthesized to investigate the influence of CaTiO₃ (CT) concentration on their photoluminescence and electrical performance. Under 480 nm irradiation, the NBT-xCT:Sm³⁺ samples exhibit prominent orange-red emission, with a prime emission peak centered at 597 nm. A moderate amount of CT doping in the Sm³⁺-modified NBT ceramics boosts both their photoluminescence and piezoelectric properties significantly, with NBT-0.04CT:Sm³⁺ showing the optimal performance. Furthermore, outstanding luminescence thermal stability over a temperature range of 293-473 K and superior water resistance behavior are achieved in the NBT-0.04CT:Sm³⁺ ceramic. These results indicate that NBT-xCT:Sm³⁺ piezo-luminescent ceramics show great potential applications in white LEDs, as well as novel optical-electronic multifunctional devices.

Recent advances of lanthanide-doped upconversion nanoparticles for biological applications

Near infrared (NIR) excited lanthanide-doped upconversion nanoparticles (UCNPs) are emerging as a new type of fluorescent tag for biological applications, which can emit multi-photon ultraviolet, visible or NIR luminescence for imaging or activation of photosensitive molecules. Here, we present a comprehensive review on recent advances of UCNPs for a manifold of biological applications, including upconversion mechanisms, building bright multicolor upconversion nanocrystals, single nanoparticle and super resolution imaging, in vivo optical and multimodal imaging, photodynamic therapy, light-controlled drug release, biosensing, and toxicities. Our perspectives on the future development of UCNPs are also described.

Upconversion luminescence enhancement by Fe3+ doping in CeO2:Yb/Er nanomaterials and their application in dye-sensitized solar cells

To make use of broad spectrum solar energy remains a main target in the photoelectrochemical area. Novel promising photoelectrode CeO₂:Fe/Yb/Er nanomaterials supported on upconversion nanomaterials doped with transition-metal ions are reported to improve broad spectrum absorption and scattering properties in dye-sensitized solar cells (DSSCs) for the first time. The results demonstrate that the materials have stronger upconversion luminescence than CeO₂:Yb/Er samples when the Fe³⁺ ion doping concentration is 2 mol% and 33.5% higher photoelectric conversion efficiency than a pure P25 electrode, which are attributed to the special light scattering properties and excellent dye adsorption capacity of the CeO₂:Fe/Yb/Er nanomaterials. Accordingly, doping Fe³⁺ transition metal ions in the upconversion material CeO₂:Yb/Er provides a new research idea for improving the photoelectric conversion efficiency of DSSCs.

The Fe³⁺-doping in CeO₂:Fe/Yb/Er enhances upconversion luminescence by destroying the symmetry of the crystal field around Er³⁺, which improves DSSC performance.

The feasibility of NaGdF4 nanoparticles as an x-ray fluorescence computed tomography imaging probe for the liver and lungs

PURPOSE

As a novel imaging modality, x-ray fluorescence computed tomography (XFCT) can provide distribution and concentration information of contrast agents containing high atomic number elements, such as iodine, gadolinium, barium, gold, and platinum. Since XFCT has a better sensitivity and detection limit of high-Z elements compared with traditional and spectral CT, it becomes a powerful quantitative imaging tool for biological studies. The main problem of current XFCT imaging is its low emission and detection efficiency of x-ray fluorescence (XRF) photons. Increasing XRF photons generation by choosing a high atomic element as a contrast agent is essential to improve the imaging quality of XFCT. Gadolinium emits at least a few times more of XRF photons than gold under the same x-ray excitation condition, leading to a detection limit at a level of sub-mg/mL as the next generation of clinical imaging modality. However, most current XFCT studies have utilized gadolinium salt as the contrast agent, which could not accumulate in organs or tumors efficiently, making in vivo XFCT imaging quite difficult. In this study, we present NaGdF₄ nanoparticles with ultra-small size as nanoprobes to test the feasibility for in vivo XFCT application for the first time.

METHODS

NaGdF₄ nanoparticles with different sizes (3-10 nm) were successfully synthesized via a coprecipitation process by controlling the reaction time at temperature of 290 °C. The morphology, crystal phase, chemical composition, and size of such NPs were further characterized with HR-TEM, XRD, and EDX. The abilities of XRF photons from different sizes of NPs were quantified by our customized XFCT imaging system. To access the in vivo application of as-synthesized NPs, such hydrophobic NPs capped OA molecules were further modified with AEP via a ligand-exchange process and characterized with FT-IR. For in vivo XFCT imaging, 0.1 mL of 30 mg/mL NPs were injected into nude mice via the tail vein. The Varian G-297 x-ray tube was set to 150 kV and 0.5 mA. The XRF photons were captured by a Kromek eV-3500 photon counting detector at each 8° for 10 s.

RESULT

The successfully synthesized NaGdF₄ nanoparticles (3-10 nm) were monodisperse, highly uniform spherical morphology and hexagonal crystalline phases. No significant influence on XRF photons yields or XFCT imaging quality were found by varying the nanoparticle size. The XRF photons were 2.5 times more emitted from NaGdF₄ nanoparticles (NPs) compared to gold nanoparticles, thereby leading to a better image quality. With the AEP surface modification, such NPs were readily adapted for use in in vivo XFCT applications with monodispersity in aqueous solution and negligible cytotoxicity. With the tail-vein injection, the liver, spleen, and lungs could be clearly imaged with XFCT at a sub-mg/mL level.

CONCLUSIONS

Such NaGdF₄ NPs, which were synthesized with coprecipitation process, were modified with AEP for in vivo XFCT applications. With both the phantom and in vivo experiments, such NPs were proved to be appropriate probes for XFCT application with the detection limit at a sub-mg/mL level. In the future research, such NPs could be further functionalized with targeting molecules for early-phase cancer detection.

Breakthroughs in medicine and bioimaging with up-conversion nanoparticles

Nanomedicine is a medical application of biochemistry incorporated with materials chemistry at the scale of nanometer for the purpose of diagnosis, prevention, and treatment. New models and approaches are typically associated with nanomedicine for precise multifunctional diagnostic systems at molecular level. Hence, employing nanoparticles (NPs) has unveiled new opportunities for efficient therapies and remedy of difficult-to-cure diseases. Among all types of inorganic NPs, lanthanide-doped up-conversion nanoparticles (UCNPs) have shown excellent potential for biomedical applications, especially for multimodal bioimaging including fluorescence and electron microscopy. Association of these visualization techniques plus the capability for transporting biomaterials and drugs make them superior agents in the field of nanomedicine. Accordingly, in this review, we firstly presented a fundamental understanding of physical and optical properties of UCNPs and secondly, we illustrated some of the prominent associations with bioimaging, theranostics, cancer therapy, and optogenetics.

Polymer Microfibers Incorporated with Silver Nanoparticles: a New Platform for Optical Sensing

The enhanced sensitivity of up-conversion luminescence is imperative for the application of up-conversion nanoparticles (UCNPs). In this study, microfibers were fabricated after co-doping UCNPs with polymethylmethacrylate (PMMA) and silver (Ag) solutions. Transmission losses and sensitivities of UCNPs (tetrogonal-LiYF4:Yb3+/Er3+) in the presence and absence of Ag were investigated. Sensitivity of up-conversion luminescence with Ag (LiYF4:Yb3+/Er3+/Ag) is 0.0095 K-1 and reduced to (LiYF4:Yb3+/Er3+) 0.0065 K-1 without Ag at 303 K under laser source (980 nm). The UCNP microfibers with Ag showed lower transmission losses and higher sensitivity than without Ag and could serve as promising candidate for optical applications. This is the first observation of Ag-doped microfiber via facile method.

Effect of Mn2+ on Upconversion Emission, Thermal Sensing and Optical Heater Behavior of Yb3+ - Er3+ Codoped NaGdF4 Nanophosphors

In thiswork, we investigate the influence of Mn2+ on the emission color, thermal sensing and optical heater behavior of NaGdF4: Yb/Er nanophosphors, which the nanoparticles were synthesized by a hydrothermal method using oleic acid as both a stabilizing and a chelating agent. The morphology and crystal size of upconversion nano particles (UCNPs) can be effectively controlled through the addition of Mn2+ dopant contents in NaGdF4: Yb/Er system. Moreover, an enhancement in overall UCL spectra of Mn2+ doped UCNPs for NaGdF4 host compared to the UCNPs is observed, which results from a closed back-energy transfer between Er3+ and Mn2+ ions (4S3/2 (Er3+) → 4T1 (Mn2+) → 4F9/2 (Er3+)). The temperature sensitivity of NaGdF4:Yb3+/Er3+ doping with Mn2+ based on thermally coupled levels (2H11/2 and 4S3/2) of Er3+ is similar to that particles without Mn2+ in the 303-548 K range. And the maximum sensitivity is 0.0043 K-1 at 523 K for NaGdF4:Yb3+/Er3+/Mn2+. Interestingly, the NaGdF4:Yb3+/Er3+/Mn2+ shows preferable optical heating behavior, which is reaching a large value of 50 K. These results indicate that inducing of Mn2+ ions in NaGdF4:Yb3+/Er3+ nanophosphors has potential in colorful display, temperature sensor.

Fe3+-codoped ultra-small NaGdF4:Nd3+ nanophosphors: enhanced near-infrared luminescence, reduced particle size and bioimaging applications

Small-sized lanthanide-doped nanoparticles (NPs) exhibiting superior near-infrared (NIR) luminescence properties are highly desired for bioimaging applications. Herein, Fe³⁺ ions are codoped in NaGdF₄:Nd³⁺ nanocrystals via a simple coprecipitation method, which can simultaneously reduce the particle size and enhance the downconverting NIR luminescence of the NPs. The NIR luminescence intensity reaches the maximum for the obtained sub-5 nm NPs when the doping concentration of Fe³⁺ is tuned to 20 mol%, which is ∼1.7 times higher than that of the pristine 8.7 nm NPs without Fe³⁺ doping. After being modified with targeting molecules, the ultra-small NaGdF₄:Nd³⁺,Fe³⁺ NPs were successfully applied as luminescent probes for targeted NIR imaging of tumors in biological tissues. Moreover, they also show great potential as a high contrast agent for T2-weighted MRI imaging.

Small-sized lanthanide-doped nanoparticles (NPs) exhibiting superior near-infrared (NIR) luminescence properties are highly desired for bioimaging applications.

Near-Infrared-Triggered Photodynamic, Photothermal, and on Demand Chemotherapy by Multifunctional Upconversion Nanocomposite

In an attempt to integrate photodynamic therapy (PDT) with photothermal therapy and chemotherapy for enhanced anticancer activity, we have rationally synthesized a multifunctional upconversion nanoplatform using NaYF₄:Yb/Tm/Er/Fe nanoparticles (NPs) as the core and NaYbF₄:1% Tm as a shell. The as-synthesized core-shell upconversion (CSU) NPs exhibited diverse and enhanced photoluminescence emissions in a wide range (UV to NIR) consequent upon Fe³⁺ doping in the core and fabrication of an active shell. Subsequently, CSU was first decorated with titania NPs as photosensitizers. Next, the mesoporous silica (MS) shell loaded with doxorubicin (DOX) via a photocleavable Ru complex as the gating molecule was developed around titania-containing CSU. Finally, gold nanorods (GNRs) with localized surface plasmon resonance (LSPR) at 800 nm were incorporated around the MS layer to obtain the multifunctional nanoplatform. We demonstrated that the UV, blue, and NIR emissions from the CSU produced ROS-mediated PDT through titania activation, induced DOX release through photocleavage of the Ru complex, and generated hyperthermia by LSPR activity of GNRs, respectively, upon a single NIR excitation through FRET. The therapeutic efficacy was validated on HeLa cell lines in vitro by various microscopic and biochemical studies under a significantly milder NIR irradiation and lower dosage of the nanoplatforms, which have been further demonstrated as diagnostic nanoprobes for cell imaging.

NIR Biosensing of Neurotransmitters in Stem Cell-Derived Neural Interface Using Advanced Core-Shell Upconversion Nanoparticles

Nondestructive neurotransmitter detection and real-time monitoring of stem cell differentiation are both of great significance in the field of neurodegenerative disease and regenerative medicine. Although luminescent biosensing nanoprobes have been developed to address this need, they have intrinsic limitations such as autofluorescence, scattering, and phototoxicity. Upconversion nanoparticles (UCNPs) have gained increasing attention for various biomedical applications due to their high photostability, low auto-fluorescent background, and deep tissue penetration; however, UCNPs also suffer from low emission intensities due to undesirable energy migration pathways. To address the aforementioned issue, a single-crystal core-shell-shell "sandwich" structured UCNP is developed that is designed to minimize deleterious energy back-transfer to yield bright visible emissions using low power density excitations. These UCNPs show a remarkable enhancement of luminescent output relative to conventional β-NaYF4:Yb,Er codoped UCNPs and β-NaYF4:Yb,Er@NaYF4:Yb "active shell" alike. Moreover, this advanced core-shell-shell UCNP is subsequently used to develop a highly sensitive biosensor for the ultrasensitive detection of dopamine released from stem cell-derived dopaminergic-neurons. Given the challenges of in situ detection of neurotransmitters, the developed NIR-based biosensing of neurotransmitters in stem cell-derived neural interfaces present a unique tool for investigating single-cell mechanisms associated with dopamine, or other neurotransmitters, and their roles in neurological processes.

Enhanced UV-Vis-NIR activated photocatalytic activity from Fe3+-doped BiOBr:Yb3+/Er3+ upconversion nanoplates: synergistic effect and mechanism insight

A simple strategy for simultaneously enhancing the UC luminescence and UV-Vis-NIR activated photocatalytic activity of BiOBr:Yb³⁺/Er³⁺ nanoplates through Fe³⁺ ion doping.

Multifunctional BiF3:Ln3+ (Ln = Ho, Er, Tm)/Yb3+ nanoparticles: an investigation on the emission color tuning, thermosensitivity, and bioimaging

Pure cubic phase and uniform BiF₃:Ln³⁺ (Ln = Ho, Er, Tm)/Yb³⁺ nanoparticles (NPs) were prepared by coprecipitation. The growth mechanism of BiF₃:2%Er³⁺/20%Yb³⁺ NPs was proposed based on evolution analysis of the time-dependent morphology, in which BiF₃:2%Er³⁺/20%Yb³⁺ was formed through the growth process of “nucleation to crystallization and Ostwald ripening”. The upconversion luminescence (UCL) properties and mechanism of BiF₃:Ln³⁺ (Ln = Ho, Er, Tm)/Yb³⁺ under dual-wavelength excitation were also systematically investigated. The emission intensity of BiF₃:2%Er³⁺/20%Yb³⁺ by dual-wavelength excitation (λ = 980 nm + 1550 nm) was 1.49 times more than that excited by 1550 nm or 980 nm individually. Furthermore, the properties of the bright white and multicolor UCL showed that yellow, purple, green, or pinkish light could be observed by controlling the doping concentration of Ln³⁺ (Ln = Yb, Er, Tm, and Ho), indicating that they had potential applications in backlight sources of color displays and security labeling. The temperature sensitivity of BiF₃:2%Er³⁺/20%Yb³⁺ exhibited a downward tendency and its max value was about 0.0036 K⁻¹ at 273 K. Cell toxicity tests showed that the UCNPs in phospholipid aqueous solution presented low cytotoxicity. Also, in vivo imaging and X-ray imaging revealed that the BiF₃:2%Er³⁺/20%Yb³⁺ NPs had deep penetration and high contrast, which meant it could be used as a potential probe and contrast agent in in vivo optical bioimaging.

Multifunctional BiF₃:Ln³⁺(Ln = Ho, Er, Tm)/Yb³⁺ UCLNPs presented better performances in dual-wavelength synergy, thermosensitivity, emission color tuning, and bioimaging.

Enhanced optical temperature sensing and upconversion emissions based on the Mn2+ codoped NaGdF4:Yb3+,Ho3+ nanophosphor

In this study, to explore new phosphors for temperature sensing with high detection sensitivity, Yb³⁺/Ho³⁺/Mn²⁺ doped hexagonal NaGdF₄ nanoparticles were designed.

Modification of Bi6Fe1.9Co0.1Ti3O18/SiO2/NaGdF4:Yb3+,Er3+ nanocomposites with different SiO2 thicknesses for tunable upconversion luminescent and ferromagnetic properties

Modification of Bi₆Fe_1.9Co_0.1Ti₃O₁₈/SiO₂/NaGdF₄:Yb³⁺,Er³⁺ nanocomposites with different SiO₂ thickness for tunable upconversion emission and ferromagnetic properties.

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.