Upconversion Luminescent Materials: Advances and Applications

Ultraviolet light-mediated drug delivery: Principles, applications, and challenges

UV light has been extensively employed in drug delivery because of its versatility, ease of manipulation, and ability to induce chemical changes on the therapeutic carrier. Here we review the mechanisms by which UV light affects drug delivery systems. We will present the challenges facing UV-induced drug delivery and some of the proposed solutions.

Real-time, non-invasive monitoring of hydrogel degradation using LiYF4:Yb(3+)/Tm(3+) NIR-to-NIR upconverting nanoparticles

To design a biodegradable hydrogel as cell support, one should know its in vivo degradation rate. A technique commonly used to track gel degradation is fluorescence spectroscopy. However, the fluorescence from conventional fluorophores quickly decays, and the fluorophores are often moderately cytotoxic. Most importantly, they require ultraviolet or visible (UV-Vis) light as the excitation source, which cannot penetrate deeply through biological tissues. Lanthanide-doped upconverting nanoparticles (UCNPs) are exciting alternatives to conventional fluorophores because they can convert near-infrared (NIR) to UV-Vis-NIR light via a sequential multiphoton absorption process referred to as upconversion. NIR light can penetrate up to few cm inside tissues, thus making these UCNPs much better probes than conventional fluorophores for in vivo monitoring. Also, UCNPs have narrow emission bands, high photoluminescence (PL) signal-to-noise ratio, low cytotoxicity and good physical and chemical stability. Here, we show a nanocomposite system consisting of a biodegradable, in situ thermogelling injectable hydrogel made of chitosan and hyaluronic acid encapsulating silica-coated LiYF4:Yb(3+)/Tm(3+) UCNPs. We use these UCNPs as photoluminescent tags to monitor the gel degradation inside live, cultured intervertebral discs (IVDs) over a period of 3 weeks. PL spectroscopy and NIR imaging show that NIR-to-NIR upconversion of LiYF4:Yb(3+)/Tm(3+)@SiO2 UCNPs allows for tracking of the gel degradation in living tissues. Both in vitro and ex vivo release of UCNPs follow a similar trend during the first 5 days; after this time, ex vivo release becomes faster than in vitro, indicating a faster gel degradation ex vivo. Also, the amount of released UCNPs in vitro increases continuously up to 3 weeks, while it plateaus after 15 days inside the IVDs showing a homogenous distribution of UCNPs throughout the IVD tissue. This non-invasive optical method for real time, live tissue imaging holds great potential for tissue analysis, biomapping and bioimaging applications.

Interplay between Static and Dynamic Energy Transfer in Biofunctional Upconversion Nanoplatforms

Clarification of the energy-transfer (ET) mechanism is of vital importance for constructing efficient upconversion nanoplatforms for biological/biomedical applications. Yet, most strategies of optimizing these nanoplatforms were casually based on a dynamic ET assumption. In this work, we have modeled quantitatively the shell-thickness-dependent interplay between dynamic and static ET in nanosystems and validated the model in a typical biofunctional upconversion nanoplatform composed of NaYF4:Er, Yb/NaYF4 upconversion nanoparticles (UCNPs), and energy-acceptor photosensitizing molecule Rose Bengal (RB). It was determined that with a proper thickness shell, the energy transferred via dynamic ET as well as static ET in this case could be significantly improved by ∼4 and ∼9 fold, respectively, compared with the total energy transferred from bare core UCNPs. Our results shall form the bedrock in designing highly efficient ET-based biofunctional nanoplatforms.

Photoluminescent ZnO Nanoparticles and Their Biological Applications

During the past decades, numerous achievements concerning luminescent zinc oxide nanoparticles (ZnO NPs) have been reported due to their improved luminescence and good biocompatibility. The photoluminescence of ZnO NPs usually contains two parts, the exciton-related ultraviolet (UV) emission and the defect-related visible emission. With respect to the visible emission, many routes have been developed to synthesize and functionalize ZnO NPs for the applications in detecting metal ions and biomolecules, biological fluorescence imaging, nonlinear multiphoton imaging, and fluorescence lifetime imaging. As the biological applications of ZnO NPs develop rapidly, the toxicity of ZnO NPs has attracted more and more attention because ZnO can produce the reactive oxygen species (ROS) and release Zn²⁺ ions. Just as a coin has two sides, both the drug delivery and the antibacterial effects of ZnO NPs become attractive at the same time. Hence, in this review, we will focus on the progress in the synthetic methods, luminescent properties, and biological applications of ZnO NPs.

In vivo targeted magnetic resonance imaging and visualized photodynamic therapy in deep-tissue cancers using folic acid-functionalized superparamagnetic-upconversion nanocomposites

Multifunctional nanoprobes used in magnetic resonance imaging (MRI) and photodynamic therapy (PDT) also have potential applications in diagnosis and visualized therapy of cancers, and hence it is important to investigate the active-targeting ability and in vivo reliability of these nanoprobes. In this work, folic acid (FA)-targeted, photosensitizer (PS)-loaded Fe3O4@NaYF4:Yb/Er (FA-NPs-PS) nanocomposites were synthesized for in vivo T2-weighted MRI and visualized PDT of cancers by modeling MCF-7 tumor-bearing nude mice. By measuring the upconversion luminescence (UCL) and fluorescence emission spectra, the as-prepared FA-NPs-PS nanocomposites showed near-infrared (NIR)-triggered PDT performance due to the production of a singlet oxygen species. Moreover, by tracing PS fluorescence in MCF-7, HeLa cells and in MCF-7 tumors, the FA-targeted nanocomposites demonstrated good targeting ability both in vitro and in vivo. Under the irradiation of a 980 nm laser, the viabilities of MCF-7 and HeLa cells incubated with FA-NPs-PS nanocomposites could decrease to about 18.4% and 30.7%, respectively, and the inhibition of MCF-7 tumors could reach about 94.9%. The transverse MR relaxivity of 63.79 mM(-1) s(-1) (r2 value) and in vivo MR imaging of MCF-7 tumors indicated an excellent T2-weighted MR performance. This work demonstrated that FA-targeted MRI/PDT nanoprobes are effective for in vivo diagnosis and visualized therapy of breast cancers.

Intracellular Adenosine Triphosphate Deprivation through Lanthanide-Doped Nanoparticles

Growing interest in lanthanide-doped nanoparticles for biological and medical uses has brought particular attention to their safety concerns. However, the intrinsic toxicity of this new class of optical nanomaterials in biological systems has not been fully evaluated. In this work, we systematically evaluate the long-term cytotoxicity of lanthanide-doped nanoparticles (NaGdF4 and NaYF4) to HeLa cells by monitoring cell viability (mitochondrial activity), adenosine triphosphate (ATP) level, and cell membrane integrity (lactate dehydrogenase release), respectively. Importantly, we find that ligand-free lanthanide-doped nanoparticles induce intracellular ATP deprivation of HeLa cells, resulting in a significant decrease in cell viability after exposure for 7 days. We attribute the particle-induced cell death to two distinct cell death pathways, autophagy and apoptosis, which are primarily mediated via the interaction between the nanoparticle and the phosphate group of cellular ATP. The understanding gained from the investigation of cytotoxicity associated with lanthanide-doped nanoparticles provides keen insights into the safe use of these nanoparticles in biological systems.

Power driven tunable white upconversion luminescence from Lu2TeO6 tri-doped with Yb3+, Tm3+ and Ho3+

Tunable power driven white upconversion luminescence was realized in the Yb³⁺, Tm³⁺, and Ho³⁺ tri-doped Lu₂TeO₆.

Phase transition, morphology transformation and highly enhanced luminescence properties of YOF:Eu3+ crystals by Gd3+ doping

We report the crystal phase transition, morphology transformation, and greatly enhanced luminescence properties of YOF crystals by Gd³⁺ doping.

Imaging the lipid bilayer of giant unilamellar vesicles using red-to-blue light upconversion

Red-to-blue triplet–triplet annihilation upconversion was obtained in giant unilamellar vesicles.

The triplet excited state of Bodipy: formation, modulation and application

The accessing of the triplet excited state of one of the most popular fluorophores, boron-dipyrromethene (Bodipy), was summarized.

Photoluminescence properties of BaSiF6:Eu3+,Eu3+/K+and Eu3+/Tb3+co-doped phosphors

An investigation into rare-earth down-converting luminescence properties of hexafluorosilicates as host materials, a case for BaSiF₆.

Intramolecular triplet–triplet energy transfer enhanced triplet–triplet annihilation upconversion with a short-lived triplet state platinum(ii) terpyridyl acetylide photosensitizer

A triplet–triplet annihilation upconversion system with a short-lived triplet state photosensitizer was constructed and enhanced by the design of intramolecular triplet–triplet energy transfer.

Time-resolved confocal microscopy using lanthanide centred near-IR emission

Time-resolved NIR imaging of lanthanide coated silica particles using Photon Arrival Time Imaging allows fast acquisition of high contrast images based on the probe luminescence lifetime.

Silica-coated upconversion lanthanide nanoparticles: The effect of crystal design on morphology, structure and optical properties

NaYF4:Yb(3+)/Er(3+) nanoparticles were synthesized by thermal decomposition of lanthanide trifluoroacetates using oleylamine (OM) as both solvent and surface binding ligand. The effect of reaction temperature and time on the properties of the particles was investigated. The nanoparticles were characterized by transmission electron microscopy (TEM), electron diffraction (ED), energy dispersive spectroscopy (EDX), dynamic light scattering (DLS), thermogravimetric analysis (TGA), elemental analysis and X-ray diffraction (XRD) to determine morphology, size, polydispersity, crystal structure and elemental composition of the nanocrystals. TEM microscopy revealed that the morphology of the nanoparticles could be fine-tuned by modifying of the synthetic conditions. A cubic-to-hexagonal phase transition of the NaYF4:Yb(3+)/Er(3+) nanoparticles at temperatures above 300 °C was confirmed by both ED and XRD. Upconversion luminescence under excitation at 980 nm was observed in the luminescence spectra of OM-NaYF4:Yb(3+)/Er(3+) nanoparticles. Finally, the OM-NaYF4:Yb(3+)/Er(3+) nanoparticles were coated with a silica shell to enable further functionalization and increase biocompatibility and stability in aqueous media, preventing particle aggregation.

Time-resolved luminescent biosensing based on inorganic lanthanide-doped nanoprobes

In this feature article, we review the latest advancements in lanthanide-doped luminescent nanocrystals as time-resolved luminescent nano-bioprobes, from their fundamental optical properties to their potential applications for ultrasensitive biodetection and high-resolution bioimaging.

Interplay between singlet and triplet excited states in a conformationally locked donor–acceptor dyad

The synthesis and photophysical characterization of a palladium(ii) porphyrin – anthracene dyad bridged via short and conformationally rigid bicyclo[2.2.2]octadiene spacer were achieved.