Nanoscale "fluorescent stone": Luminescent Calcium Fluoride Nanoparticles as Theranostic Platforms

Advances in nanotherapeutics for tumor treatment by targeting calcium overload

Traditional antitumor strategies are facing challenges such as low therapeutic efficacy and high side effects, highlighting the significance of developing non-toxic or low-toxic alternative therapies. As a second messenger, calcium ion (Ca2+) plays an important role in cellular metabolism and communication. However, persistent Ca2+ overload leads to mitochondrial structural and functional dysfunction and ultimately induced apoptosis. Therefore, an antitumor strategy based on calcium overload is a promising alternative. Here, we first reviewed the classification of calcium-based nanoparticles (NPs) for exogenous Ca2+ overload, including calcium carbonate (CaCO3), calcium phosphate (CaP), calcium peroxide (CaO2), and hydroxyapatite (HA), calcium hydroxide, etc. Next, the current endogenous Ca2+ overload strategies were summarized, including regulation of Ca2+ channels, destruction of membrane integrity, induction of abnormal intracellular acidity and oxidative stress. Due to the specificity of the tumor microenvironment, it is difficult to completely suppress tumor development with monotherapy. Therefore, we reviewed the progress based on mitochondrial Ca2+ overload, which improved the treatment efficiency by combining photothermal therapy (PTT), photodynamic therapy (PDT), chemodynamic therapy (CDT), sonodynamic therapy (SDT), immunogenic cell death (ICD) and gas therapy. We further explored in detail the advantages and promising new targets of this combination antitumor strategies to better address future opportunities and challenges.

From ferroptosis to cuproptosis, and calcicoptosis, to find more novel metals-mediated distinct form of regulated cell death

Regulated cell death (RCD), also known as programmed cell death (PCD), plays a critical role in various biological processes, such as tissue injury/repair, development, and homeostasis. Dysregulation of RCD pathways can lead to the development of many human diseases, such as cancer, neurodegenerative disorders, and cardiovascular diseases. Maintaining proper metal ion homeostasis is critical for human health. However, imbalances in metal levels within cells can result in cytotoxicity and cell death, leading to a variety of diseases and health problems. In recent years, new types of metal overload-induced cell death have been identified, including ferroptosis, cuproptosis, and calcicoptosis. This has prompted us to examine the three defined metal-dependent cell death types, and discuss other metals-induced ferroptosis, cuproptosis, and disrupted Ca2+ homeostasis, as well as the roles of Zn2+ in metals' homeostasis and related RCD. We have reviewed the connection between metals-induced RCD and various diseases, as well as the underlying mechanisms. We believe that further research in this area will lead to the discovery of novel types of metal-dependent RCD, a better understanding of the underlying mechanisms, and the development of new therapeutic strategies for human diseases.

Sulfate Doping Promotes Agglomeration of Calcium Fluoride Crystals

Calcium salt precipitation is an effective solution to wastewater fluoride pollution. The purity and precipitation efficiency of calcium fluoride is critical for its removal and recovery. This study aimed to reveal the role of coexisting sulfates in the precipitation of calcium fluoride. A low sulfate concentration promoted calcium fluoride precipitation. The size of calcium fluoride-aggregated particle clusters increased from 750 to 2000 nm when the molar ratio of sulfate to fluoride was increased from 0 to 3:100. Sulfate doped in the calcium fluoride crystals neutralized the positive charge of the calcium fluoride. Online atomic force microscopy measurements showed that sulfate reduced the repulsive force between calcium fluoride crystals and increased the adhesion force from 1.62 to 2.46 nN, promoting the agglomeration of calcium fluoride crystals. Sulfate improved the precipitation efficiency of calcium fluoride by promoting agglomeration; however, the purity of calcium fluoride was reduced by doping. Sulfate reduced the induction time of calcium fluoride crystallization and improved the nucleation rate of calcium fluoride. Sulfate should be retained to improve the precipitation of calcium fluoride and to avoid its loss from the effluents. However, it is necessary to separate sulfate from fluoride to obtain high-purity calcium fluoride. Therefore, sulfate concentration regulation in high-fluoride wastewater is key to achieving the efficient removal and recovery of fluoride ions.

Miniaturized Biosensors Based on Lanthanide-Doped Upconversion Polymeric Nanofibers

Electrospun nanofibers possess a large surface area and a three-dimensional porous network that makes them a perfect material for embedding functional nanoparticles for diverse applications. Herein, we report the trends in embedding upconversion nanoparticles (UCNPs) in polymeric nanofibers for making an advanced miniaturized (bio)analytical device. UCNPs have the benefits of several optical properties, like near-infrared excitation, anti-Stokes emission over a wide range from UV to NIR, narrow emission bands, an extended lifespan, and photostability. The luminescence of UCNPs can be regulated using different lanthanide elements and can be used for sensing and tracking physical processes in biological systems. We foresee that a UCNP-based nanofiber sensing platform will open opportunities in developing cost-effective, miniaturized, portable and user-friendly point-of-care sensing device for monitoring (bio)analytical processes. Major challenges in developing microfluidic (bio)analytical systems based on UCNPs@nanofibers have been reviewed and presented.

Luminescence and stability of Tb doped CaF2 nanoparticles

Luminescence properties of CaF₂:Tb³⁺ nanoparticles were studied in order to investigate the effect of CaF₂ native defects on the photoluminescence dynamics of Tb³⁺ ions. Incorporation of Tb ions into the CaF₂ host was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. Cross-relaxation energy transfer was observed from the photoluminescence spectra and decay curves upon excitation at 257 nm. However, the unusual long lifetime of the Tb³⁺ ion as well as the decreasing trend of emission lifetime of the ⁵D₃ level suggested the involvement of traps, which were further investigated by using temperature-dependent photoluminescence measurements, thermoluminescence and lifetime measurements at different wavelengths. This work highlights the critical role that the CaF₂ native defects play in the photoluminescence dynamics of Tb³⁺ ions incorporated in a CaF₂ matrix. The sample doped with 10 mol% of Tb³⁺ ions was found to be stable under prolonged 254 nm ultraviolet irradiation.

Diffusion 19F-NMR of Nanofluorides: In Situ Quantification of Colloidal Diameters and Protein Corona Formation in Solution

The NMR-detectability of elements of organic ligands that stabilize colloidal inorganic nanocrystals (NCs) allow the study of their diffusion characteristics in solutions. Nevertheless, these measurements are sensitive to dynamic ligand exchange and often lead to overestimation of diffusion coefficients of dispersed colloids. Here, we present an approach for the quantitative assessment of the diffusion properties of colloidal NCs based on the NMR signals of the elements of their inorganic cores. Benefiting from the robust ¹⁹F-NMR signals of the fluorides in the core of colloidal CaF₂ and SrF₂, we show the immunity of ¹⁹F-diffusion NMR to dynamic ligand exchange and, thus, the ability to quantify, with high accuracy, the colloidal diameters of different types of nanofluorides in situ. With the demonstrated ability to characterize the formation of protein corona at the surface of nanofluorides, we envision that this study can be extended to additional formulations and applications.

Connecting Calcium-Based Nanomaterials and Cancer: From Diagnosis to Therapy

As the indispensable second cellular messenger, calcium signaling is involved in the regulation of almost all physiological processes by activating specific target proteins. The importance of calcium ions (Ca2+) makes its "Janus nature" strictly regulated by its concentration. Abnormal regulation of calcium signals may cause some diseases; however, artificial regulation of calcium homeostasis in local lesions may also play a therapeutic role. "Calcium overload," for example, is characterized by excessive enrichment of intracellular Ca2+, which irreversibly switches calcium signaling from "positive regulation" to "reverse destruction," leading to cell death. However, this undesirable death could be defined as "calcicoptosis" to offer a novel approach for cancer treatment. Indeed, Ca2+ is involved in various cancer diagnostic and therapeutic events, including calcium overload-induced calcium homeostasis disorder, calcium channels dysregulation, mitochondrial dysfunction, calcium-associated immunoregulation, cell/vascular/tumor calcification, and calcification-mediated CT imaging. In parallel, the development of multifunctional calcium-based nanomaterials (e.g., calcium phosphate, calcium carbonate, calcium peroxide, and hydroxyapatite) is becoming abundantly available. This review will highlight the latest insights of the calcium-based nanomaterials, explain their application, and provide novel perspective. Identifying and characterizing new patterns of calcium-dependent signaling and exploiting the disease element linkage offer additional translational opportunities for cancer theranostics.

Up and down the spectrum: upconversion nanocrystal and semiconductor material fused into a single nanocomposite

A nanocomposite consisting of a cubic EuSe semiconductor material grown on a hexagonal upconversion nanoparticle has overcome the crystal lattice mismatch that typically prevents the epitaxial growth of such heterogeneous nanocrystals. Eu³⁺ at the interface layer shows its characteristic red emission band both under UV excitation light due to energy transfer from the semiconductor and under NIR excitation light due to energy transfer after photon-upconversion. Data storage and security applications are suggested for this new nanocomposite.

Three-Dimensional Colloidal Controlled Growth of Core-Shell Heterostructured Persistent Luminescence Nanocrystals

Persistent luminescence nanoparticles (PLNPs) are an emerging photonic nanomaterial that possesses uniquely persistent luminescence properties after excitation ceases. They can be repeatedly recharged in vitro and in vivo and hold great promise for numerous areas and applications. Unfortunately, none of the existing synthesis methods can control their composition to grow core-shell structured PLNPs with desirable shapes and enhanced functionalities. Here, we report on straightforward thermolysis-mediated colloidal synthesis of CaF₂:Dy@NaYF₄ core-shell PLNPs that can enhance persistent luminescence under both light and X-ray excitations. Benefitting from the well-matched crystal lattices between CaF₂ and NaYF₄, this colloidal synthesis makes it possible to prepare core-shell PLNPs with exquisite control of the compositions, shapes, and enhanced luminescence. This demonstration of the developing colloidal core-shell PLNPs overcomes the current key bottleneck regarding the synthesis of heterostructured PLNPs and sets the stage for fully exploiting the potential of these fascinating luminous materials.

Fabrication of Nd3+ and Yb3+ doped NIR emitting nano fluorescent probe: A candidate for bioimaging applications

The intentional design of rare earth doped luminescent architecture exhibits unique optical properties and it can be considered as a promising and potential probe for optical imaging applications. Calcium fluoride (CaF₂) nanoparticles doped with optimum concentration of Nd³⁺ and Yb³⁺ as sensitizer and activator, respectively, were synthesized by wet precipitation method and characterized by x-ray diffraction (XRD) and photoluminescence. In spite of the fact that the energy transfer takes place from Nd³⁺ to Yb³⁺, the luminescence intensity was found to be weak due to the lattice defects generated from the doping of trivalent cations (Nd³⁺ and Yb³⁺) for divalent host cations (Ca²⁺). These defect centres were tailored via charge compensation approach by co-doping Na⁺ ion and by optimizing its concentration and heat treatment duration. CaF₂ doped with 5 mol% Nd³⁺, 3 mol% Yb³⁺ and 4 mol% Na⁺ after heat treatment for 2 h exhibited significantly enhanced emission intensity and life time. The ex vivo fluorescence imaging experiment was done at various thickness of chicken breast tissue. The maximum theoretical depth penetration of the NIR light was calculated and the value is 14 mm. The fabricated phosphor can serve as contrast agent for deep tissue near infrared (NIR) light imaging.

Bioactive Core-Shell CaF2 Upconversion Nanostructure for Promotion and Visualization of Engineered Bone Reconstruction

Inorganic ion metabolism plays significant roles in various life processes including signal transduction, substance exchange, and cellular constructions. Regulation and monitoring of ion metabolism offer great promise to modulate biological activities and provide insights into related mechanisms. Here, a synergistic nanodepot based on a bioactive core-shell CaF₂ upconversion nanostructure that integrates multiple mineral ions for metabolic regulation was built for the acceleration and monitoring of the biomineralization process. Multiple mineral ions released from the nanodepots can accelerate the growth of inorganic crystals and enhance the production of organic matrixes, synergistically facilitating the regeneration of bone defects in vivo. During the process, such a nanodepot can be constructed to specifically recognize osteoblasts for the monitoring of biomineralization. This nanoprobe represents an efficient strategy to promote and monitor biomineralization-related metabolic activities with applications in fundamental research, disease diagnosis, and regenerative medicine.

Multifunctional Dendrimer-Entrapped Gold Nanoparticles for Labeling and Tracking T Cells Via Dual-Modal Computed Tomography and Fluorescence Imaging

Nanosystems for monitoring and tracking T cells provide an important basis for evaluating the functionality and efficacy of T cell-based immunotherapy. To this end, we designed herein an efficient nanoprobe for T cell monitoring and tracking using poly(amidoamine) (PAMAM) dendrimer-entrapped gold nanoparticles (Au DENPs) conjugated with Fluo-4 for dual-mode computed tomography (CT) and fluorescence imaging. In this study, PAMAM dendrimers of generation 5 (G5) were modified with hydroxyl-terminated polyethylene glycol (PEG) and then used to entrap 2.0 nm Au NPs followed by acetylation of the excess amine groups on the dendrimer surface. Subsequently, the calcium ion probe was covalently attached to the dendrimer nanohybrids through the PEG hydroxyl end groups to gain the functional {(Au⁰)₂₅-G5.NHAc-(PEG)₁₄-(Fluo-4)₂} nanoprobe. This nanoprobe had excellent water solubility, high X-ray attenuation coefficient, and good cytocompatibility in the given concentration range, as well as a high T cell labeling efficiency. Confocal microscopy and flow cytometry results demonstrated that the nanoprobe was able to fluorescently sense activated T cells. Moreover, the nanoprobe was able to realize both CT and fluorescence imaging of subcutaneously injected T cells in vivo. Thus, the developed novel dendrimer-based nanosystem may hold great promise for advancing and improving the clinical application of T cell-based immunotherapy.

Surfactant-Stripped Pheophytin Micelles for Multimodal Tumor Imaging and Photodynamic Therapy

Porphyrin-based nanomaterials can inherently integrate multiple contrast imaging functionalities with phototherapeutic capabilities. We dispersed pheophytin (Pheo) into Pluronic F127 and carried out low-temperature surfactant-stripping to remove the bulk surfactant. Surfactant-stripped Pheo (ss-Pheo) micelles exhibited a similar size, but higher near-infrared fluorescence, compared to two other nanomaterials also with high porphyrin density (surfactant-stripped chlorophyll micelles and porphysomes). Singlet oxygen generation, which was higher for ss-Pheo, enabled photodynamic therapy (PDT). ss-Pheo provided contrast for photoacoustic and fluorescence imaging, and following seamless labeling with ⁶⁴Cu, was used for positron emission tomography. ss-Pheo had a long blood circulation and favorable accumulation in an orthotopic murine mammary tumor model. Trimodal tumor imaging was demonstrated, and PDT resulted in delayed tumor growth.

Acetylated Polyethylenimine-Entrapped Gold Nanoparticles Enable Negative Computed Tomography Imaging of Orthotopic Hepatic Carcinoma

Developing an effective computed tomography (CT) contrast agent is still a challenging task for precise diagnosis of hepatic carcinoma (HCC). Here, we present the use of acetylated polyethylenimine (PEI)-entrapped gold nanoparticles (Ac-PE-AuNPs) without antifouling modification for negative CT imaging of HCC. PEI was first linked to fluorescein isothiocyanate (FI) and then utilized as a vehicle for the entrapment of AuNPs. The particles were then acetylated to reduce its positive surface potential. The designed Ac-PE-AuNPs were characterized by various techniques. We find that the Ac-PE-AuNPs with a uniform size distribution (mean diameter = 2.3 nm) are colloidally stable and possess low toxicity in the studied range of concentration. Owing to the fact that the particles without additional antifouling modification were mainly gathered in liver, the Ac-PE-AuNPs could greatly improve the CT contrast enhancement of normal liver, whereas poor CT contrast enhancement appeared in liver necrosis region caused by HCC. As a result, HCC could be easily and precisely diagnosed. The designed Ac-PE-AuNPs were demonstrated to have biocompatibility through in vivo biodistribution and histological studies, hence holding an enormous potential to be adopted as an effective negative CT contrast agent for diagnosis of hepatoma carcinoma.

Alleviating luminescence concentration quenching in lanthanide doped CaF2 based nanoparticles through Na+ ion doping

Luminescence concentration quenching, mainly due to a cross relaxation (CR) process between lanthanide ions (Ln3+), widely occurs in Ln3+ doped luminescent materials, setting a limit in the dopant content of Ln3+ emitters to withhold the brightness. Here, we introduced Na+ ions into the CaF2 host lattice codoped with Nd3+ emitters that alleviates concentration quenching greatly. And we show that the optimal dopant concentration of Nd3+ in colloidal CaF2:Nd nanoparticles increased from 10 to 30 mol%, resulting in an ∼32 times near-infrared (NIR) (1052 nm) brightness under 800 nm laser irradiation. Our mechanistic investigation suggests that the enhancement of NIR photo-luminescence (PL) could be attributed to not only the increasing crystallinity of nanoparticles but also the reducing concentration quenching of Nd3+ by improving the dopant distribution of Nd3+ ions in the CaF2 lattice, as evidenced by the high angle annular dark field images. These result in the optimal concentration increase to produce brightness enhancement greatly. This strategy can be utilized for other Ln3+ doped CaF2 based nanomaterials for bio-imaging.

CaF2 nanoparticles as surface carriers of GCAP1, a calcium sensor protein involved in retinal dystrophies

CaF₂-based nanoparticles (NP) are promising biocompatible tools for nanomedicine applications. The structure of the NP crystal lattice allows for specific interactions with Ca²⁺-binding proteins through their EF-hand cation binding motifs. Here we investigated the interaction of 23 nm citrate-coated CaF₂ NP with a calcium sensor protein GCAP1 that is normally expressed in photoreceptor cells and involved in the regulation of the early steps of vision. Protein-NP interactions were thoroughly investigated for the wild type (WT) GCAP1 as well as for a variant carrying the Asp 100 to Glu mutation (D100E), which prevents the binding of Ca²⁺ to the highest affinity site and is linked to cone dystrophy. Circular dichroism and fluorescence spectroscopy showed that protein structure and Ca²⁺-sensing capability are conserved for both variants upon interaction with the NP surface, although the interaction mode depends on the specific occupation of Ca²⁺-binding sites. NP binding stabilizes the structure of the bound GCAP1 and occurs with nanomolar affinity, as probed by isothermal titration calorimetry. Surface plasmon resonance revealed a fully reversible binding compatible with physiologically relevant kinetics of protein release whereas biochemical assays indicated a residual capability for NP-dissociated GCAP1 to regulate the target retinal guanylate cyclase. Our study constitutes a proof of concept that CaF₂ NP could be optimized to serve as biologically compatible carriers of high amounts of functional GCAP1 in photoreceptors affected by retinal dystrophies.

Sequential growth of CaF2:Yb,Er@CaF2:Gd nanoparticles for efficient magnetic resonance angiography and tumor diagnosis

It is a significant challenge to develop nanoscale magnetic resonance imaging (MRI) contrast agents with high performance of relaxation.

GSH-triggered size increase of porphyrin-containing nanosystems for enhanced retention and photodynamic activity

We demonstrate the rational design of a size changeable nanosystem triggered by intracellular GSH for enhanced retention and photodynamic activity.

Site-specific sonocatalytic tumor suppression by chemically engineered single-crystalline mesoporous titanium dioxide sonosensitizers

Mesoporous silica nanocrystals have been developed as sonosensitizers for efficient dynamic therapy of tumors.

Two-photon photodynamic ablation of tumor cells by mitochondria-targeted iridium(iii) complexes in aggregate states

Two iridium(iii) complexes with a rotary phenyl substituent ligand were used as mitochondria-targeted photosensitizers for two-photon photodynamic therapy in aggregate states.

Light-Mediated Deep-Tissue Theranostics

This theme issue provides an overview on recent developments of light-mediated imaging and therapy approaches, with an emphasis on those that transcend the shallow tissue penetration dogma.