Tuning the upconversion light emission by bandgap engineering in bismuth oxide-based upconverting nanoparticles

Effect of Sr2+ ions on the structure, up-conversion emission and thermal sensing of Er3+, Yb3+ co-doped double perovskite Ba(2-x)SrxMgWO6 phosphors

Investigating the effect of different phases on the optical performance is crucial for thermal sensing phosphor materials. Ba_(2-x)Sr_xMgWO₆:Er³⁺, Yb³⁺, K⁺ double perovskite phosphors were successfully prepared using a high-temperature solid-phase method. The dominant up-conversion luminescent (UCL) mechanism was deduced by analyzing the power-dependence spectra and energy level diagrams. By X-ray diffraction tests and tolerance factor calculations, it was demonstrated that the substitution of Sr²⁺ ions for Ba²⁺ ions led to the phase changing from cubic to tetragonal. The phase transition led to a decrease in the crystallographic symmetry of the compounds and changes in the optical thermometric properties. The optical temperature sensing properties were investigated using the fluorescence intensity ratio of thermally coupled energy levels (²H_11/2 and ⁴S_3/2 to the ground state energy level ⁴I_15/2) of Er³⁺ ions in Ba₂MgWO₆, BaSrMgWO₆ and Sr₂MgWO₆. The maximum absolute sensitivities obtained for Ba₂MgWO₆, BaSrMgWO₆ and Sr₂MgWO₆ doped with 7% Er³⁺, 2% Yb³⁺ and 9% K⁺ were 6.77 × 10^-4 K^-1, 10.09 × 10^-4 K^-1 and 23.4 × 10^-4 K^-1, respectively. The comparison revealed that the phase transition caused an increase in the luminescence intensity and absolute sensitivity. This provides a useful pathway for modulating the subsequent thermometric performance.

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.

Rapid aqueous-phase synthesis of highly stable K0.3Bi0.7F2.4 upconversion nanocrystalline particles at low temperature

Lanthanide-doped K_0.3Bi_0.7F_2.4 nanocrystalline particles are synthesized through an ultrafast (only 1 min) and aqueous-phase chemical method at low temperature (room temperature ∼ 90 °C), which can be used as pigments for anti-counterfeiting.

Confined-Melting-Assisted Synthesis of Bismuth Silicate Glass-Ceramic Nanoparticles: Formation and Optical Thermometry Investigation

Bismuth-based (nano)materials have been attracting increasing interest due to appealing properties such as high refractive indexes, intrinsic opacity, and structural distortions due to the stereochemistry of 6s² lone pair electrons of Bi³⁺. However, the control over specific phases and strategies able to stabilize uniform bismuth-based (nano)materials is still a challenge. In this study, we employed the ability of bismuth to lower the melting point of silica to introduce a new synthetic approach able to confine the growth of bismuth-oxide-based materials into nanostructures. Combining in situ temperature-dependent synchrotron radiation X-ray powder diffraction (XRPD) with high-resolution transmission electron microscopy (HR-TEM) analyses, we demonstrate the evolution of a confined Bi₂O₃-SiO₂ nanosystem from Bi₂SiO₅ to Bi₄Si₃O₁₂ through a melting process. The silica shell acts as both a nanoreactor and a silicon source for the stabilization of bismuth silicate glass-ceramic nanocrystals keeping the original spherical shape. The exciton peak of Bi₂SiO₅ is measured for the first time allowing the estimation of its real energy gap. Moreover, based on a detailed spectroscopic investigation, we discuss the potential and the limitations of Nd³⁺-activated bismuth silicate systems as ratiometric thermometers. The synthetic strategy introduced here could be further explored to stabilize other bismuth-oxide-based materials, opening the way toward the growth of well-defined glass-ceramic nanoparticles.

Sodium niobate based hierarchical 3D perovskite nanoparticle clusters

We report a microwave assisted synthesis of NaNbO3 perovskite mesocrystals with a hierarchical morphology formed by the self-assembly of nanoparticles in particle clusters. The synthesis method combines non-aqueous sol-gel synthesis and microwave heating in a single step process that allows us to isolate crystalline single phase NaNbO3 in few minutes. A detailed investigation of the effect of the reaction temperature on the crystallinity and morphology of the product was conducted. The synthesis stabilizes the unusual orthorhombic phase Pmma, a property that can be ascribed to the crystal size (24 nm). TEM and SAED analyses show that the hierarchical polycrystalline particles behave as single crystals, a feature related to a non-classical crystallization mechanism. Moreover, the optical bandgap of this NaNbO3 phase was estimated for the first time. The results suggest the potential of this synthetic procedure for the fast production of high quality tertiary oxide nanocrystals.

Tailoring local coordination structure of the Er3+ ions for tuning the up-conversion multicolor luminescence

The regulation of the local structure around Er³⁺ ions is an important channel for adjusting the characteristic of up-conversion luminescence. In this paper, the cubic-phased Er³⁺:CaF₂ crystals with different Er³⁺ doping concentrations were fabricated with temperature gradient technique (TGT) method and the effect of the local coordination structure of the Er³⁺ ions on its luminescence performance was investigated. The local coordination structure of Er³⁺ ions was simulated by density functional theory. The computational results show that clusters evolve from low order to high order with the increase of Er³⁺ ion doping concentration. In this evolution process, the local structure transforms from cubic structure to the co-existence of cubic and lower symmetric square anti-prism structures. Meanwhile, the distance between Er³⁺ ions in the cluster decreased first and then increased slightly, and in dimers and trimers this distance reached the minimum. Under 980 nm excitation, with the increase of Er³⁺ ion concentration, the intensity ratios of the red and green emissions of Er³⁺:CaF₂ first increased from 0.61 to 42.03 and then decreased to 12.11. The corresponding up-conversion luminescence gamut was adjusted from monochrome green to red to red-yellow. This work provides a new thread for realizing upconversion multicolor luminescence by regulating the clusters of rare earth ions.

Mesoporous Bi-Containing Radiosensitizer Loading with DOX to Repolarize Tumor-Associated Macrophages and Elicit Immunogenic Tumor Cell Death to Inhibit Tumor Progression

Tumor-associated macrophages (TAMs) were a major component of tumor, which comprised up to 50% of tumor mass, and correlated with poor prognosis in more than 80% of cases. TAMs were resistant to radiotherapy and chemotherapy, and radiation could further activate TAMs to promote tumor progression. Herein, we explored a kind of Bi-based mesoporous upconversion nanophosphor (UCNP) loaded with doxorubicin (UCNP-DOX) to elicit immunogenic tumor cell death and repolarize TAMs to an antitumor M1-like type for strengthening the tumor-specific antitumor immune effects of X-ray radiotherapy. The repolarization effect of UCNP-DOX with X-ray was confirmed in THP-1 cell line, in vivo mouse model, and hydrothorax of a non-small-cell lung carcinoma patient. Moreover, the UCNP-DOX and X-ray radiation could elicit immunogenic tumor necrosis, presenting more tumor antigens for tumor-specific immune response. In a cell co-incubation system, activated macrophages could significantly inhibit cancer colony formation, migration, and invasion. After treatment, xenografted tumor in mice was also found to be significantly regressed and presented substantial CD8-positive T cells. This study opens the door to further enhance the abscopal effects and inhibit the metastasis in radiotherapy.

Excitation dependent visible and NIR photoluminescence properties of Er3+, Yb3+ co-doped NaBi(MoO4)2 nanomaterials

Due to the exceptional luminescence properties of lanthanide doped nanomaterials, they have applications in various field such as sensing, photocatalysis, solar cells, bio-imaging, therapy, diagnostics, anti-counterfeiting, latent fingerprint development, optical amplifiers, solid state lighting, etc. Here, we report the excitation dependent photoluminescence properties of Yb³⁺, Er³⁺ co-doped NaBi(MoO₄)₂ nanomaterials in both the visible and NIR regions upon UV, visible and NIR excitation. These photoluminescence properties show that strong energy transfer occurs from the host to the Yb³⁺, Er³⁺ ions. These materials show major emission bands at 530, 552 (green) and 656 nm (red) in the visible region and 1000 and 1534 nm in the NIR region. The intensity ratio between green and red bands is dependent on the excitation wavelength, whereas the intensity ratio of the 1000 and 1534 nm bands relies on the excitation wavelength and Er³⁺ doping concentration. These materials also exhibit host emission and upconversion luminescence properties in the visible region.

Er³⁺, Yb³⁺ co-doped NaBi(MoO₄)₂ nanomaterials show excitation dependent photoluminescence properties in the visible and NIR regions upon excitation with UV, visible and NIR light.

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.

Bismuth titanate-based UV filters embedded mesoporous silica nanoparticles: Role of bismuth concentration in the self-sealing process

The development of new safe inorganic UV filters to effectively protect the skin from ultraviolet (UV) radiation effects is an emerging issue. Bismuth titanate-based UV filters embedded into mesoporous silica nanoparticles (MSN) represent a new class of inorganic sunscreens, with excellent UVA and UVB shielding properties. In addition, the presence of bismuth ions promotes a self-sealing process, allowing (i) the entrapment of the active phases in the deepest core of the system and (ii) the formation of an external glassy silica layer with a consequent suppression of the photocatalytic activity. In this work, aimed at studying in detail the self-sealing mechanism and accessing the role of bismuth ions in the formation of the system, a series of samples impregnated with a different amount of bismuth were investigated. The self-sealing process already occurs at the lowest content of bismuth and the mechanism is demonstrated to be triggered by the ability of Bi to work as a low-melting point agent for silica. Finally, a sunscreen formulation containing the new UV filter was prepared and the Sun Protection Factor (SPF), the pH and the viscosity were measured, demonstrating the potential of the proposed material for large-scale applications.

Bi2SiO5@g-SiO2 upconverting nanoparticles: a bismuth-driven core-shell self-assembly mechanism

Core-shell systems have attracted increasing interest among the research community in recent years due to their unique properties and structural features, and the development of new synthetic strategies is still a challenge. In this work, we have investigated lanthanide-doped Bi2SiO5 nanocrystal formation inside mesoporous silica nanoparticles (MSNs). The role of both synthesis temperature and concentration of the bismuth precursor impregnated into the MSNs is discussed, showing an unprecedented strategy for the simultaneous stabilization of a crystalline core and a glassy shell. Temperature dependent synchrotron radiation X-ray powder diffraction (SR-XRPD) and high resolution transmission electron microscopy (HR-TEM) analyses allow one to follow the crystalline core growth. A mechanism for the formation of a Bi2SiO5@g-SiO2 core-shell nanosystem is proposed. In addition, the easy tunability of the color output of the upconverting system is demonstrated by means of suitable doping lanthanide ions with potential applications in several fields.

Ultrafast synthesis of fluorine-18 doped bismuth based upconversion nanophosphors for tri-modal CT/PET/UCL imaging in vivo

A fluorine-18 doped bismuth upconversion luminescence (UCL) nanoprobe (¹⁸F-UNBOF) was quickly synthesized within 1 min at room temperature, and it could be utilized for computed tomography (CT), positron emission tomography (PET) and UCL imaging in vivo.

Revisiting Cr3+-Doped Bi2Ga4O9 Spectroscopy: Crystal Field Effect and Optical Thermometric Behavior of Near-Infrared-Emitting Singly-Activated Phosphors

The increasing interest in the development of ratiometric optical thermal sensors has led to a wide variety of new systems with promising properties. Among them, singly-doped ratiometric thermometers were recently demonstrated to be particularly reliable. With the aim to discuss the development of an ideal optical thermal sensor, a combined experimental and theoretical insight into the spectroscopy of the Bi₂Ga₄O₉:Cr³⁺ system is reported showing the importance of an insightful analysis in a wide temperature range. Low-temperature photoluminescence analysis (from 10 K) and the temperature dependence of the lifetime investigation, together with the crystal field analysis and the modeling of the thermal quenching process, allow the estimation of key parameters such as the Debye temperature (cutoff frequency), the Huang-Rhys parameter, and the energy barrier between ²E_g and ⁴T_2g. Additionally, by considering the reliable class of singly-doped ratiometric thermometers based on a couple of excited states obeying the Boltzmann law, the important role played by the absolute sensitivity was discussed and the great potential of Cr³⁺ singly-activated systems was demonstrated. The results may provide new guidelines for the design of reliable optical thermometers with outstanding and robust performances.

Lanthanide Doped Near Infrared Active Upconversion Nanophosphors: Fundamental Concepts, Synthesis Strategies, and Technological Applications

Near infrared (NIR) light utilization in a range of current technologies has gained huge significance due to its abundance in nature and nondestructive properties. NIR active lanthanide (Ln) doped upconversion nanomaterials synthesized in controlled shape, size, and surface functionality can be combined with various pertinent materials for extensive applications in diverse fields. Upconversion nanophosphors (UCNP) possess unique abilities, such as deep tissue penetration, enhanced photostability, low toxicity, sharp emission peaks, long anti-Stokes shift, etc., which have bestowed them with prodigious advantages over other conventional luminescent materials. As new generation fluorophores, UCNP have found a wide range of applications in various fields. In this Review, a comprehensive overview of lanthanide doped NIR active UCNP is provided by discussing the fundamental concepts including the different mechanisms proposed for explaining the upconversion processes, followed by the different strategies employed for the synthesis of these materials, and finally the technological applications of UCNP, mainly in the fields of bioimaging, drug delivery, sensing, and photocatalysis by highlighting the recent works in these areas. In addition, a brief note on the applications of UCNP in other fields is also provided along with the summary and future perspectives of these materials.

A Hollow NaGdF4 /AFn Nanosystem Based on "Relay Race" Release for Therapy

To develop a multifunctional nanomaterial for dual-mode imaging and synergetic chemotherapy, curcumin (CUR) was physically entrapped into hollow upconversion NaGdF₄ nanomaterial, then apoferritin (AFn) loaded with doxorubicin (DOX) was attached to the NaGdF₄ surface. Subsequent modification with the targeting reagent folic acid (FA) led to generation of the CUR/NaGdF₄ -DOX/AFn-FA conjugate for cancer treatment. X-ray diffraction, scanning (SEM) and transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy demonstrated the successful preparation of hexagonal-phase NaGdF₄ and NaGdF₄ -AFn-FA. Moreover, no toxicity was observed for NaGdF₄ -AFn-FA. In vitro and in vivo experiments demonstrated that the two drugs are sequentially released from the nanocomposites. This two-drug system showed strong growth inhibitory effects on MCF-7 cells. Upconversion luminescence imaging and magnetic resonance (MR) imaging of NaGdF₄ -AFn-FA were carried out. The results of this study show that NaGdF₄ -AFn-FA can be used for targeted anticancer drug delivery as well as imaging, a novel multi-pronged theranostic system for tumor treatment.