Plasmonic enhancement of upconversion emission in Ag@NaYF4:Er3+/Yb3+ phosphor

Recent advances of lanthanide nanomaterials in Tumor NIR fluorescence detection and treatment

Lanthanide nanomaterials have garnered significant attention from researchers among the main near-infrared (NIR) fluorescent nanomaterials due to their excellent chemical and fluorescence stability, narrow emission band, adjustable luminescence color, and long lifetime. In recent years, with the preparation, functional modification, and fluorescence improvement of lanthanide materials, great progress has been made in their application in the biomedical field. This review focuses on the latest progress of lanthanide nanomaterials in tumor diagnosis and treatment, as well as the interaction mechanism between fluorescence and biological tissues. We introduce a set of efficient strategies for improving the fluorescence properties of lanthanide nanomaterials and discuss some representative in-depth research work in detail, showcasing their superiority in early detection of ultra-small tumors, phototherapy, and real-time guidance for surgical resection. However, lanthanide nanomaterials have only realized a portion of their potential in tumor applications so far. Therefore, we discuss promising methods for further improving the performance of lanthanide nanomaterials and their future development directions.

Surface plasmon resonance of Au/Ag metals for the photoluminescence enhancement of lanthanide ion Ln3+ doped upconversion nanoparticles in bioimaging

Deep tissue penetration, chemical inertness and biocompatibility give UCNPs a competitive edge over traditional fluorescent materials like organic dyes or quantum dots. However, the low quantum efficiency of UNCPs becomes an obstacle. Among extensive methods and strategies currently used to prominently solve this concerned issue, surface plasmon resonance (SPR) of noble metals is of great use due to the agreement between the SPR peak of metals and absorption band of UCNPs. A key challenge of this match is that the structures and sizes of noble metals have significant influences on the peak of SPR formants, where achieving an explicit elucidation of relationships between the physical properties of noble metals and their SPR formants is of great importance. This review aims to clarify the mechanism of the SPR effect of noble metals on the optical performance of UCNPs. Furthermore, novel research studies in which Au, Ag or Au/Ag composites in various structures and sizes are combined with UCNPs through different synthetic methods are summarized. We provide an overview of improved photoluminescence for bioimaging exhibited by different composite nanoparticles with respect to UCNPs acting as both cores and shells, taking Au@UCNPs, Ag@UCNPs and Au/Ag@UCNPs into account. Finally, there are remaining shortcomings and latent opportunities which deserve further research. This review will provide directions for the bioimaging applications of UCNPs through the introduction of the SPR effect of noble metals.

Enhanced up-conversion photoluminescence in fluoride-oxyfluoride nanophosphor films by embedding gold nanoparticles

Owing to their unique non-linear optical character, lanthanide-based up-converting materials are potentially interesting for a wide variety of fields ranging from biomedicine to light harvesting. However, their poor luminescent efficiency challenges the development of technological applications. In this context, localized surface plasmon resonances (LSPRs) have been demonstrated as a valuable strategy to improve light conversion. Herein, we utilize LSPR induced by gold nanoparticles (NPs) to enhance up-conversion photoluminescence (UCPL) in transparent, i.e. scattering-free, films made of nanophosphors formed by fluoride-oxyfluoride host matrix that feature high thermal stability. Transparency allows excitation by an external source without extinction losses caused by unwanted diffuse reflection. We provide a simple method to embed gold NPs in films made of YF/YOF:Yb³⁺,Er³⁺ UC nanophosphors, via preparation of a viscous paste composed of both UC nanophosphors and colloidal gold NPs, reducing complexity in sample fabrication. The dimensions of gold NPs are such that their associated LSPR matches spectrally with the green emission band of the Er³⁺ doped nanophosphors. In order to demonstrate the benefits of plasmonic nanoparticles for UCPL in nanophosphor films, we provide a careful analysis of the structural properties of the composite thin films along with precise characterization of the impact of the gold NPs on the photophysical properties of UC nanophosphors.

Structures, plasmon-enhanced luminescence, and applications of heterostructure phosphors

Heterostructure phosphor composites have been used widely in the fields of targeted bio-probes and bio-imaging, hyperthermia treatment, photocatalysis, solar cells, and fingerprint identification. The structures, plasmon-enhanced luminescence and mechanism of metal/fluorophore heterostructure composites, such as core-shell nanocrystals, multilayers, adhesion, islands, arrays, and composite optical glass, are reviewed in detail. Their extended applications were explored widely since the surface plasmon resonance effect increased the up-conversion efficiency of fluorophores significantly. We summarize their synthesis methods, size and shape control, absorption and excitation spectra, plasmon-enhanced up-conversion luminescence, and specific applications. The most important results acquired in each case are summarized, and the main challenges that need to be overcome are discussed.

Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO2 Upconversion Phosphor Composites

Sol–gel synthesized N-doped and carbon–nitrogen–sulfur (CNS)-doped TiO2 solutions were deposited on upconversion phosphor using a dip coating method. Scanning electron microscopy (SEM) imaging showed that there was a change in the morphology of TiO2 coated on NaYF4:Yb,Er from spherical to nanorods caused by additional urea and thiourea doping reagents. Fourier transform infrared (FTIR) spectroscopy further verified the existence of nitrate–hyponitrite, carboxylate, and SO42− because of the doping effect. NaYF4:Yb,Er composites coated with N- and CNS-doped TiO2 exhibited a slight shift of UV-Vis spectra towards the visible light region. Photodecomposition of methylene blue (MB) was evaluated under 254 nm germicidal lamps and a 300 W Xe lamp with UV/Vis cut off filters. The photodegradation of toluene was evaluated on TiO2/NaYF4:Yb,Er and CNS-doped TiO2/NaYF4:Yb,Er samples under UV light illumination. The photocatalytic reactivity with CNS-doped TiO2/NaYF4:Yb,Er surpassed that of the undoped TiO2/NaYF4:Yb,Er for the MB solution and toluene. Photocatalytic activity is increased by CNS doping of TiO2, which improves light sensitization as a result of band gap narrowing due to impurity sites.

Precise Control of Copper-Localized Surface Plasmon Resonance in the Near Infrared Region for Enhancement of Up-Conversion Luminescence

The surface plasmon resonance of copper in the near infrared region provides a novel method for enhancement of up-conversion luminescence compared to using gold and silver, as the former grants significant cost savings. In this study, we made a flat Cu film covered TiO2 to enhance the up-conversion fluorescence intensity. The results show that the deposition of copper/TiO2 dioxide nanocomposite film prepared via spin-coating has no effect on the structure of NaGdF4:Er3+, Yb3+. The absorption wavelength of the copper film moved from the original visible wavelength (~600 nm) range to the infrared wavelength after covering TiO2, and most obviously, the copper film covered two layers of TiO2 by about 16 nm; the peak of the absorption appeared at 835 nm due to the enhanced excitation field. The behavior of the nanocomposite film with NaGdF4:Er3+ and Yb3+ under 980 nm excitation was investigated; it provides a novel way for studying mental-enhanced fluorescence. Besides, the peaks of the fluorescence spectrum show different emissions at 542 nm and 660 nm, respectively. The copper nanoparticles-covered TiO2 layer can obviously enhance the fluorescence intensity, and the maximum enhancement factors of emission of NaGdF4:Er3+ and Yb3+ nanoparticles are 3.1 and 1.9 on the nanocomposite film, respectively.

Scheelite like NaTb(WO4)2 nanoparticles: Green fluorescence and in vitro cell imaging applications

This study highlights the investigation of the green fluorescence in NaTb(WO₄)₂ materials (NaTbW Bulk and NaTbW Nano) synthesized via template free hydrothermal method as a function of particle size and morphology. Herein, we demonstrated the biocompatibility and intracellular green fluorescence of NaTbW Nano samples using HeLa cells for cell imaging applications. Powder X-ray diffraction studies showed that the as synthesized NaTbW Bulk and NaTbW Nano crystallize in the Scheelite like tetragonal crystal system with the I4₁/a space group. The reaction pH and solvent is observed to play a critical role in determining particle size, shape and morphology of these luminescent materials. Furthermore, size dependent optical properties were systematically studied by diffuse reflectance, steady state photoluminescence; time resolved fluorescence lifetime and quantum yield measurements. Both the materials have shown bright green fluorescence upon UV excitation as a function of particle size. Remarkable high quantum yield of NaTbW Bulk indicated its greater luminescence efficiency and the closer CIE coordinates to the commercial green illuminant suggested their potential use in solid state display systems. On the other hand the observed biocompatibility of NaTbW Nano particles towards mammalian cancer HeLa cells, Staphylococcus aureus, Escherichia coli and the intracellular green fluorescence rightly proved its functionality as active bio-probes. Thus, our work summarize the potential use of these Scheelite like NaTb(WO₄)₂ material for solid state display and bio-imaging applications.

Plasmon-enhanced upconversion luminescence in pyrochlore phase Yb x Er2-x Ti2O7 thin film

Pyrochlore phase Yb _x Er_2-x Ti₂O₇ (YETO) thin films have been prepared by employing a facile sol-gel method combining with spin-coating technique and post-annealing treatment at 700 °C. High concentration of Yb³⁺ ions can promote the transformation from Yb³⁺/Er³⁺ co-doped anatase phase TiO₂ to pyrochlore phase YETO at 700 °C temperature. We find that the YETO thin film with 30 mol% Yb³⁺ ions exhibits the brightest upconversion (UC) emission. Moreover, the introduction of Au nanorods (Au NRs) in the YETO thin film can further enhance the UC fluorescence. By adjusting the density of Au NRs, the UC emission intensity is increased by about 2.8-fold due to the excitation field enhancement caused by the localized surface plasmon resonance effect.