Highly plasmon-enhanced upconversion emissions from Au@β-NaYF4:Yb,Tm hybrid nanostructures

Surface plasmon field enhanced upconversion luminescence for the screening and detection of phenolic environmental estrogens

The endocrine system's interference caused by environmental estrogens (EEs) residue in food is a topic of public concern. Here, we construct an aptasensor for the sensitive detection of EEs based on luminescence resonance energy transfer (LRET). With MoS₂ nanosheets acting as the energy acceptor and upconversion luminescence nanoparticles@gold nanoparticles (UCNPs@Au) as the luminescence donor, autofluorescence from food is prevented from interfering. The in-situ deposition of AuNPs not only induces local field enhancement to significantly increase the luminescence intensity of UCNPs, but also conduces to the modification of aptamer through Au-S bond. This aptasensor can respond to multiple estrogens thanks to the choice of a universal aptamer that recognizes phenolic hydroxyl group, and it offers the probability to screen unidentified phenolic estrogens. This method has a high sensitivity and a low limit of detection (LOD), and the satisfactory recovery rates acquired from water and milk samples confirmed its considerable application value.

Hollow nanoparticles synthesized via Ostwald ripening and their upconversion luminescence-mediated Boltzmann thermometry over a wide temperature range

Upconversion nanoparticles (UCNPs) with hollow structures exhibit many fascinating optical properties due to their special morphology. However, there are few reports on the exploration of hollow UCNPs and their optical applications, mainly because of the difficulty in constructing hollow structures by conventional methods. Here, we report a one-step template-free method to synthesize NaBiF₄:Yb,Er (NBFYE) hollow UCNPs via Ostwald ripening under solvothermal conditions. Moreover, we also elucidate the possible formation mechanism of hollow nanoparticles (HNPs) by studying the growth process of nanoparticles in detail. By changing the contents of polyacrylic acid and H₂O in the reaction system, the central cavity size of NBFYE nanoparticles can be adjusted. Benefiting from the structural characteristics of large internal surface area and high surface permeability, NBFYE HNPs exhibit excellent luminescence properties under 980 nm near-infrared irradiation. Importantly, NBFYE hollow UCNPs can act as self-referenced ratiometric luminescent thermometers under 980 nm laser irradiation, which are effective over a wide temperature range from 223 K to 548 K and have a maximum sensitivity value of 0.0065 K^-1 at 514 K. Our work clearly demonstrates a novel method for synthesizing HNPs and develops their applications, which provides a new idea for constructing hollow structure UCNPs and will also encourage researchers to further explore the optical applications of hollow UCNPs.

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.

Near-Infrared-Triggered Nitrogen Fixation over Upconversion Nanoparticles Assembled Carbon Nitride Nanotubes with Nitrogen Vacancies

Photocatalytic artificial fixation of N₂ to NH₃ occurs over NaYF₄:Yb,Tm (NYF) upconversion nanoparticles (NPs) decorated carbon nitride nanotubes with nitrogen vacancies (NYF/NV-CNNTs) in water under near-infrared (NIR) light irradiation. NYF NPs with a particle size of ca. 20 nm were uniformly distributed on the surface of NV-CNNTs. The NYF/NV-CNNTs with 15 wt % NYF exhibited the highest NH₃ production yield of 1.72 mmol L^-1 g_cat^-1, corresponding to an apparent quantum efficiency of 0.50% under NIR light illumination, and about three times higher the activity of the bare CNNTs under UV-filtered solar light. ¹⁵N isotope-labeling NMR results confirm that the N source of ammonia originates from the photochemical N₂ reduction. The spectroelectrochemical measurements reveal that NVs can greatly facilitate the photogenerated electron transfer without energy loss, while the presence of NYF NPs shifts both the deep trap state and the edge of conduction band toward a lower potential. Moreover, NYF NPs endow the photocatalyst with a NIR light absorption via the fluorescence resonance energy transfer process, and NVs have the ability to enhance the active sites for a stronger adsorption of N₂ and decrease the surface quenching effect of NYF NPs, which thus can promote the energy migration within the heterojunctions. This work opens the way toward full solar spectrum photocatalysis for sustainable ammonia synthesis under aqueous system.

Upconversion Nanodevice-Assisted Healthy Molecular Photocorrection

Mushrooms are rich in ergosterol, a precursor of ergocalciferol, which is a type of vitamin D₂. The conversion of ergosterol to ergocalciferol takes place in the presence of UV radiation by the cleavage of the "B-ring" in the ergosterol. As the UV radiation cannot penetrate deep into the tissue, only minimal increase occurs in sunlight. In this study, upconversion nanoparticles with the property to convert deep-penetrating near-infrared radiation to UV radiation have been cast into a disk to use sunlight and emit UV radiation for vitamin D conversion. An engineered upconversion nanoparticle (UCNP) disk with maximum particles and limited clusters demonstrates ∼2.5 times enhanced vitamin D₂ conversion.

Efficient upconverting carbon nitride nanotubes for near-infrared-driven photocatalytic hydrogen production

We report a facile chemical technique for synthesizing nanotube-based hybrid materials for near-infrared-driven photocatalytic hydrogen (H₂) production. Upconversion nanoparticles (UCNPs), NaYF₄:Yb,Tm,Gd (NYFG) and NaYF₄:Yb,Tm (NYF), were engineered on C₃N₄ nanotubes (C₃N₄ NTs) separately to construct heterojunction structures. With a UCNP loading content of 15 wt%, the NYFG/C₃N₄ NT heterojunction exhibits the highest H₂ generation rate of 311.6 μmol g^-1 with an apparent quantum efficiency of 0.80 ‰, about 1.4 times higher than that of the NYF/C₃N₄ NT nanocomposite under 980 nm laser irradiation. Comprehensive characterization reveals that the enhanced photocatalytic performance of the Gd doped nanostructure is attributed to the synergistic effect, stronger interaction, higher emission intensities, and faster charge transfer between the UCNPs and C₃N₄ NTs. Moreover, the steady-state and dynamic fluorescence spectra indicate that the energy from NYFG NPs was transferred to C₃N₄ NTs via a fluorescence-resonance energy-transfer process. Our work demonstrates the potential of developing near-infrared-responsive photocatalysts for energy and environmental applications.

An electrochemiluminescence cytosensor for sensitive detection of HeLa cells based on a signal amplification strategy of Au-NaYF4:Yb,Er nanocomposites

A novel electrochemiluminescence (ECL) cytosensor was proposed for the quantitative detection of HeLa cells (human cervical cancer cells) with the help of a signal amplification strategy. Firstly, the Au-NaYF4:Yb,Er nanocomposites were prepared by a simple in situ hydrothermal method and characterized by transmission electron microscopy (TEM) images, X-ray diffraction (XRD) patterns, UV-vis spectra and Fourier transform infrared (FTIR) spectra. Compared with the bare NaYF4:Yb,Er nanocomposites, the ECL intensity of Au-NaYF4:Yb,Er nanocomposites was greatly enhanced by about 4.2-fold which can be attributed to the good conductivity of gold nanoparticles (Au NPs). The nanocomposites showed high and stable ECL emission, fast response and superior conductivity, all of which were advantageous to the ECL detection. Furthermore, HeLa cells were immobilized on the modified electrode via the interaction between folic acid and a folate receptor present on the cell surface. The ECL cytosensor showed satisfactory sensitive response to HeLa cells in a linear range of 4.25 × 102-4.25 × 105 cells per mL with a low detection limit of 326 cells per mL. The proposed cytosensor had good sensitivity and stability, which can offer a great potential platform for bioassay analysis.

Near-Infrared Manipulation of Membrane Ion Channels via Upconversion Optogenetics

Membrane ion channels are ultimately responsible for the propagation and integration of electrical signals in the nervous, muscular, and other systems. Their activation or malfunctioning plays a significant role in physiological and pathophysiological processes. Using optogenetics to dynamically and spatiotemporally control ion channels has recently attracted considerable attention. However, most of the established optogenetic tools (e.g., channelrhodopsins, ChRs) for optical manipulations, are mainly stimulated by UV or visible light, which raises the concerns of potential photodamage, limited tissue penetration, and high-invasive implantation of optical fiber devices. Near-infrared (NIR) upconversion nanoparticle (UCNP)-mediated optogenetic systems provide great opportunities for overcoming the problems encountered in the manipulation of ion channels in deep tissues. Hence, this review focuses on the recent advances in NIR regulation of membrane ion channels via upconversion optogenetics in biomedical research. The engineering and applications of upconversion optogenetic systems by the incorporation multiple emissive UCNPs into various light-gated ChRs/ligands are first elaborated, followed by a detailed discussion of the technical improvements for more precise and efficient control of membrane channels. Finally, the future perspectives for refining and advancing NIR-mediated upconversion optogenetics into in vivo even in clinical applications are proposed.

Heating-induced abnormal increase in Yb3+ excited state lifetime and its potential application in lifetime luminescence nanothermometry

Heating-induced abnormal increase in Yb³⁺ excited state lifetime is demonstrated with potential application in lifetime luminescence nanothermometry.

Up-conversion luminescence lifetime thermometry based on the 1G4 state of Tm3+ modulated by cross relaxation processes

The first study on the up-conversion luminescence lifetime thermometry based on the ¹G₄ state of Tm³⁺ modulated by cross relaxation processes.

Over 1000-fold enhancement of upconversion luminescence using water-dispersible metal-insulator-metal nanostructures

Rare-earth activated upconversion nanoparticles (UCNPs) are receiving renewed attention for use in bioimaging due to their exceptional photostability and low cytotoxicity. Often, these nanoparticles are attached to plasmonic nanostructures to enhance their photoluminescence (PL) emission. However, current wet-chemistry techniques suffer from large inhomogeneity and thus low enhancement is achieved. In this paper, we report lithographically fabricated metal-insulator-metal (MIM) nanostructures that show over 1000-fold enhancement of their PL. We demonstrate the potential for bioimaging applications by dispersing the MIMs into water and imaging bladder cancer cells with them. To our knowledge, our results represent one and two orders of magnitude improvement, respectively, over the best lithographically fabricated structures and colloidal systems in the literature. The large enhancement will allow for bioimaging and therapeutics using lower particle densities or lower excitation power densities, thus increasing the sensitivity and efficacy of such procedures while decreasing potential side effects.

Energy transfer mechanism dominated by the doping location of activators in rare-earth upconversion nanoparticles

Research on the energy transfer mechanism of rare-earth-doped upconversion nanoparticles (UCNPs) has been an important area due to the increasing demand for tuning multicolor emission and enhancing the upconversion efficiency; however, because of large energy mismatch, many lanthanide activators, such as Eu3+, cannot realize highly efficient near infrared-to-visible upconversion by simple codoping of Yb3+. Therefore, introduction of other ions to assist the energy transfer process is required. Herein, we prepared core-shell nanoparticles with different doping locations to investigate the upconversion energy transfer mechanism. The upconversion luminescence (UCL) of core-shell nanoparticles was investigated by steady-state luminescence and time-resolved luminescence spectra. The UCL behaviors in these different multi-activator core-shell nanoparticles were observed. The results revealed different energy transfer channels influenced by the doping location of activators. This study may open up new avenues of structure design for fine-tuning of multicolor UCL for specific applications.

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, 2D materials, wurtzite-type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.

Enhance the Er3+ Upconversion Luminescence by Constructing NaGdF4:Er3+@NaGdF4:Er3+ Active-Core/Active-Shell Nanocrystals

NaGdF4:12%Er3+@NaGdF4:x%Er3+ (x = 0, 6, 8, 10, and 12) active-core/active-shell nanoparticles (NPs) were peculiarly synthesized via a delayed nucleation pathway with procedures. The phase, shape, and size of the resulting core-shell NPs are confirmed by transmission electron microscopy and X-ray diffraction. Coated with a NaGdF4:10%Er3+ active shell around the NaGdF4:12%Er3+ core NPs, a maximum luminescent enhancement of about 336 times higher than the NaGdF4:12%Er3+ core-only NPs was observed under the 1540 nm excitation. The intensity ratio of green to red was adjusted through the construction of the core-shell structure and the change of Er3+ concentration in the shell. By analyzing the lifetimes of emission bands and exploring the energy transition mechanism, the giant luminescence enhancement is mainly attributed to the significant increase in the near-infrared absorption at 1540 nm and efficient energy migration from the shell to core.

An assembly and interaction of upconversion and plasmonic nanoparticles on organometallic nanofibers: enhanced multicolor upconversion, downshifting emission and the plasmonic effect

We present novel inorganic-organic hybrid nanoparticles (HNPs) constituting inorganic NPs, NaY_0.78Er_0.02Yb_0.2F₄, and organometallic nanofiber, Tb(ASA)₃Phen (TAP). X-ray diffraction, Fourier transform infrared absorption and transmission electron microscopy analyses reveal that prepared ultrafine upconversion NPs (UCNPs (5-8 nm)) are dispersed on the surface of the TAP nanofibers. We observe that the addition of TAP in UCNPs effectively limits the surface quenching to boost the upconversion (UC) intensity and enables tuning of UC emission from the green to the red region by controlling the phonon frequency around the Er³⁺ ion. On the other hand, TAP is an excellent source of green emission under ultraviolet exposure. Therefore prepared HNPs not only give enhanced and tunable UC but also emit a strong green color in the downshifting (DS) process. To further enhance the dual-mode emission of HNPs, silver NPs (AgNPs) are introduced. The emission intensity of UC as well as DS emission is found to be strongly modulated in the presence of AgNPs. It is found that AgNPs enhance red UC emission. The possible mechanism involved in enhanced emission intensity and color output is investigated in detail. The important optical properties of these nano-hybrid materials provide a great opportunity in the fields of biological imaging, drug delivery and energy devices.

Plasmonic enhancement and polarization dependence of nonlinear upconversion emissions from single gold nanorod@SiO2@CaF2:Yb3+,Er3+ hybrid core-shell-satellite nanostructures

Lanthanide-doped upconversion nanocrystals (UCNCs) have recently become an attractive nonlinear fluorescence material for use in bioimaging because of their tunable spectral characteristics and exceptional photostability. Plasmonic materials are often introduced into the vicinity of UCNCs to increase their emission intensity by means of enlarging the absorption cross-section and accelerating the radiative decay rate. Moreover, plasmonic nanostructures (e.g., gold nanorods, GNRs) can also influence the polarization state of the UC fluorescence-an effect that is of fundamental importance for fluorescence polarization-based imaging methods yet has not been discussed previously. To study this effect, we synthesized GNR@SiO₂@CaF₂:Yb³⁺,Er³⁺ hybrid core-shell-satellite nanostructures with precise control over the thickness of the SiO₂ shell. We evaluated the shell thickness-dependent plasmonic enhancement of the emission intensity in ensemble and studied the plasmonic modulation of the emission polarization at the single-particle level. The hybrid plasmonic UC nanostructures with an optimal shell thickness exhibit an improved bioimaging performance compared with bare UCNCs, and we observed a polarized nature of the light at both UC emission bands, which stems from the relationship between the excitation polarization and GNR orientation. We used electrodynamic simulations combined with Förster resonance energy transfer theory to fully explain the observed effect. Our results provide extensive insights into how the coherent interaction between the emission dipoles of UCNCs and the plasmonic dipoles of the GNR determines the emission polarization state in various situations and thus open the way to the accurate control of the UC emission anisotropy for a wide range of bioimaging and biosensing applications.

808-nm-Light-Excited Lanthanide-Doped Nanoparticles: Rational Design, Luminescence Control and Theranostic Applications

808 nm-light-excited lanthanide (Ln³⁺ )-doped nanoparticles (LnNPs) hold great promise for a wide range of applications, including bioimaging diagnosis and anticancer therapy. This is due to their unique properties, including their minimized overheating effect, improved penetration depth, relatively high quantum yields, and other common features of LnNPs. In this review, the progress of 808 nm-excited LnNPs is reported, including their i) luminescence mechanism, ii) luminescence enhancement, iii) color tuning, iv) diagnostic and v) therapeutic applications. Finally, the future outlook and challenges of 808 nm-excited LnNPs are presented.

Near-infrared photochemistry at interfaces based on upconverting nanoparticles

We review near-infrared photochemistry at interfaces based on upconverting nanoparticles, highlight its potential applications, and discuss the challenges.

Current Advances in Lanthanide-Doped Upconversion Nanostructures for Detection and Bioapplication

Along with the development of science and technology, lanthanide-doped upconversion nanostructures as a new type of materials have taken their place in the field of nanomaterials. Upconversion luminescence is a nonlinear optical phenomenon, which absorbs two or more photons and emits one photon. Compared with traditional luminescence materials, upconversion nanostructures have many advantages, such as weak background interference, long lifetime, low excitation energy, and strong tissue penetration. These interesting nanostructures can be applied in anticounterfeit, solar cell, detection, bioimaging, therapy, and so on. This review is focused on the current advances in lanthanide-doped upconversion nanostructures, covering not only basic luminescence mechanism, synthesis, and modification methods but also the design and fabrication of upconversion nanostructures, like core-shell nanoparticles or nanocomposites. At last, this review emphasizes the application of upconversion nanostructure in detection and bioimaging and therapy. Learning more about the advances of upconversion nanostructures can help us better exploit their excellent performance and use them in practice.

Enhanced UV upconversion emission using plasmonic nanocavities

Upconversion of near infrared (NIR) into ultraviolet (UV) radiation could lead to a number of applications in bio-imaging, diagnostics and drug delivery. However, for bare nanoparticles, the conversion efficiency is extremely low. In this work, we experimentally demonstrate strongly enhanced upconversion emission from an ensemble of β-NaYF₄:Gd³⁺/Yb³⁺/Tm³⁺ @NaLuF₄ core-shell nanoparticles trapped in judiciously designed plasmonic nanocavities. In doing so, different metal platforms and nanostructures are systematically investigated. Our results indicate that using a cross-shape silver nanocavity, a record high enhancement of 170-fold can be obtained in the UV band centered at a wavelength of 345 nm. The observed upconversion efficiency improvement may be attributed to the increased absorption at NIR, the tailored photonic local density of states, and the light out-coupling characteristics of the cavity.

Plasmon-Mediated Solar Energy Conversion via Photocatalysis in Noble Metal/Semiconductor Composites

Plasmonics has remained a prominent and growing field over the past several decades. The coupling of various chemical and photo phenomenon has sparked considerable interest in plasmon-mediated photocatalysis. Given plasmonic photocatalysis has only been developed for a relatively short period, considerable progress has been made in improving the absorption across the full solar spectrum and the efficiency of photo-generated charge carrier separation. With recent advances in fundamental (i.e., mechanisms) and experimental studies (i.e., the influence of size, geometry, surrounding dielectric field, etc.) on plasmon-mediated photocatalysis, the rational design and synthesis of metal/semiconductor hybrid nanostructure photocatalysts has been realized. This review seeks to highlight the recent impressive developments in plasmon-mediated photocatalytic mechanisms (i.e., Schottky junction, direct electron transfer, enhanced local electric field, plasmon resonant energy transfer, and scattering and heating effects), summarize a set of factors (i.e., size, geometry, dielectric environment, loading amount and composition of plasmonic metal, and nanostructure and properties of semiconductors) that largely affect plasmonic photocatalysis, and finally conclude with a perspective on future directions within this rich field of research.

On The Latest Three-Stage Development of Nanomedicines based on Upconversion Nanoparticles

Following the "detect-to-treat" strategy, by biological engineering, the emerging upconversion nanoparticles (UCNPs) have become one of the most promising inorganic nanomedicines, and their biomedical applications have gradually shifted from multimodal tumor imaging to highly efficient cancer therapy. The past few years have witnessed a three-stage development of UCNP-based nanomedicines. On one hand, UCNPs can optimize each clinical treatment tool (chemotherapy, photodynamic therapy (PDT), radiotherapy (RT)) by controlled drug delivery/release, near-infrared (NIR)-excited deep PDT, and radiosensitization, respectively, all of which contribute greatly to the optimized treatment efficacy along with minimized side effects. On the other hand, several individual treatments can be "smartly" integrated into a single UCNP-based nanotheranostic system for multimodal synergetic therapy, which can further improve the overall therapeutic effectiveness. Especially, UCNPs provide more-effective strategies for overcoming tumor hypoxia, thus leading to an ideal treatment efficacy for complete eradication of solid tumors. Finally, the critical issues regarding the future development of UCNPs are discussed to promote the clinic-translational applications of UCNP-based nanomedicines, as well as realization of our "one drug fits all" dream.

Local Field Modulation Induced Three-Order Upconversion Enhancement: Combining Surface Plasmon Effect and Photonic Crystal Effect

A 2D surface plasmon photonic crystal (SPPC) is achieved by implanting gold nanorods onto the periodic surface apertures of the poly(methyl methacrylate) (PMMA) opal photonic crystals. On the surface of the SPPC, the overall upconversion luminescence intensity of NaYF4 :Yb(3+) , Er(3+) under 980 nm excitation is improved more than 10(3) fold. The device is easily shifted to a transparent flexible substrate, applied to flexible displays.

Enhanced upconversion quantum yield near spherical gold nanoparticles - a comprehensive simulation based analysis

Photon upconversion is promising for many applications. However, the potential of lanthanide doped upconverter materials is typically limited by low absorption coefficients and low upconversion quantum yields (UCQY) under practical irradiance of the excitation. Modifying the photonic environment can strongly enhance the spontaneous emission and therefore also the upconversion luminescence. Additionally, the non-linear nature of the upconversion processes can be exploited by an increased local optical field introduced by photonic or plasmonic structures. In combination, both processes may lead to a strong enhancement of the UCQY at simultaneously lower incident irradiances. Here, we use a comprehensive 3D computation-based approach to investigate how absorption, upconversion luminescence, and UCQY of an upconverter are altered in the vicinity of spherical gold nanoparticles (GNPs). We use Mie theory and electrodynamic theory to compute the properties of GNPs. The parameters obtained in these calculations were used as input parameters in a rate equation model of the upconverter β-NaYF₄: 20% Er³⁺. We consider different diameters of the GNP and determine the behavior of the system as a function of the incident irradiance. Whether the UCQY is increased or actually decreased depends heavily on the position of the upconverter in respect to the GNP. Whereas the upconversion luminescence enhancement reaches a maximum around a distance of 35 nm to the surface of the GNP, we observe strong quenching of the UCQY for distances <40 nm and a UCQY maximum around 125 to 150 nm, in the case of a 300 nm diameter GNP. Hence, the upconverter material needs to be placed at different positions, depending on whether absorption, upconversion luminescence, or UCQY should be maximized. At the optimum position, we determine a maximum UCQY enhancement of 117% for a 300 nm diameter GNP at a low incident irradiance of 0.01 W/cm². As the irradiance increases, the maximum UCQY enhancement decreases to 20% at 1 W/cm². However, this UCQY enhancement translates into a significant improvement of the UCQY from 12.0% to 14.4% absolute.

Near-Infrared-Sensitive Materials Based on Upconverting Nanoparticles

The near-infrared (NIR) region of the spectrum is called the "therapeutic window" because NIR light can penetrate deeply into tissue. Therefore, NIR-sensitive materials are attractive for biomedical applications. Recently, upconverting nanoparticles (UCNPs) were used to construct NIR-sensitive materials. UCNPs convert NIR light to UV or visible light, which can trigger photoreactions of photosensitive materials. Here, how to use UCNPs to construct NIR-sensitive materials is introduced, applications of NIR-sensitive materials with a focus on biomedical applications are highlighted, and the associated challenges are discussed.

Preparation of a novel nanocomposite NaLuF4:Gd,Yb,Tm@SiO2@Ag@TiO2 with high photocatalytic activity driven by simulated solar light

A novel nanocomposite photocatalyst NaLuF₄:Gd,Yb,Tm@SiO₂@Ag@TiO₂ was developed for the first time.

Core–shell–shell heterostructures of α-NaLuF4:Yb/Er@NaLuF4:Yb@MF2 (M = Ca, Sr, Ba) with remarkably enhanced upconversion luminescence

Core–shell–shell heterostructures of α-NaLuF₄:Yb/Er@NaLuF₄:Yb@MF₂ (M = Ca, Sr, Ba) exhibit remarkably enhanced upconversion luminescence.

Surface modification and characterization of photon-upconverting nanoparticles for bioanalytical applications

Photon-upconverting nanoparticles (UCNPs) can be excited by near-infrared light and emit visible light (anti-Stokes emission) which prevents autofluorescence and light scattering of biological samples. The potential for background-free imaging has attracted wide interest in UCNPs in recent years. Small and homogeneous lanthanide-doped UCNPs that display high upconversion efficiency have typically been synthesized in organic solvents. Bioanalytical applications, however, require a subsequent phase transfer to aqueous solutions. Hence, the surface properties of UCNPs must be well designed and characterized to grant both a stable aqueous colloidal dispersion and the ability to conjugate biomolecules and other ligands on the nanoparticle surface. In this review, we introduce various routes for the surface modification of UCNPs and critically discuss their advantages and disadvantages. The last part covers various analytical methods that enable a thorough examination of the progress and success of the surface functionalization.

A New Cubic Phase for a NaYF4 Host Matrix Offering High Upconversion Luminescence Efficiency

A NaYF4 host matrix with a new cubic phase is fabricated to offer high upconversion luminescence efficiency. The new cubic phase is formed through a hexagonal-to-cubic phase transition by shining intense near-infrared light on lanthanide-doped hexagonal NaYF4 materials.

Simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage

A strategy has been adopted for simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF₄:Yb³⁺/Er³⁺ microcrystals by simply tuning the KF dosage. X-ray power diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectra (PL) were used to characterize the samples. The influence of molar ratio of KF to Y³⁺ on the crystal phase and morphology has been systematically investigated and discussed. It is found that the molar ratio of KF to Y³⁺ can strongly control the morphology of the as-synthesized β-NaYF₄ samples because of the different capping effect of F⁻ ions on the different crystal faces. The possible formation mechanism has been proposed on the basis of a series of time-dependent experiments. More importantly, the upconversion luminescence of β-NaYF₄:Yb³⁺/Er³⁺ was greatly enhanced by increasing the molar ratio of KF to RE³⁺ (RE = Y, Yb, Er), which is attributed to the distortion of local crystal field symmetry around lanthanide ions through K⁺ ions doping. This synthetic methodology is expected to provide a new strategy for simultaneous morphology control and remarkable upconversion luminescence enhancement of yttrium fluorides, which may be applicable for other rare earth fluorides.

Plasmonic enhancement of the upconversion fluorescence in YVO₄:Yb³⁺, Er³⁺ nanocrystals based on the porous Ag film

The upconversion luminescence (UCL) enhancement based on the surface plasmonic resonance (SPR) of noble metals is a promising way to improve UCL efficiency. However, it is still a challenge to achieve stable and effective UCL enhancement. Here, we present the preparation of the porous Ag/YVO4:Yb(3+), Er(3+) composite film via a simple double annealing method. It is exciting to observe that a maximum 36-fold ((2)H11/2-(4)I15/2) and 30-fold ((4)S3/2-(4)I15/2) UCL enhancement in the porous Ag/YVO4:Yb(3+), Er(3+) composite film, attributed to the effective coupling between SPR and the excitation light by adjusting the SPR peak to the excitation wavelength, controlling the effective coupling distance and improving the scattering-absorption ratio. Furthermore, the enhancement factor strongly depended on the excitation power and the Er(3+) concentration.

Shape-controllable synthesis and enhanced upconversion luminescence of Li+ doped β-NaLuF4:Yb3+,Ln3+ (Ln = Tm, Ho) microcrystals

UC luminescence intensities of blue and green emissions are significantly enhanced via Li⁺ doping.

Probing the nature of upconversion nanocrystals: instrumentation matters

Understanding upconversion nanocrystals: this review intends to summarize instrumental matters related to the characterization of upconversion nanocrystals from surface structures to intrinsic properties to ultimate challenges in nanocrystal analysis at single-particle levels.

Highly-reproducible Raman scattering of NaYF4:Yb,Er@SiO2@Ag for methylamphetamine detection under near-infrared laser excitation

NaYF4:Yb,Er@SiO₂@Ag displays highly-reproducible Raman enhancement ability for methylamphetamine detection under near-infrared excitation.

Improving photocatalytic activity by combining upconversion nanocrystals and Mo-doping: a case study on β-NaLuF4:Gd,Yb,Tm@SiO2@TiO2:Mo

A novel double-shell-structured β-NaLuF₄:Gd,Yb,Tm@SiO₂@TiO₂:Mo nanocomposite photocatalyst has been developed for the first time.

Selective enhancement of red emission from upconversion nanoparticles via surface plasmon-coupled emission

Upconversion nanoparticle and gold nanorod heteronanostructures spaced by a polyelectrolyte are prepared by a layer-by-layer assembly process to enhance red emission.

Fabrication of a novel nanocomposite Ag/graphene@SiO2–NaLuF4:Yb,Gd,Er for large enhancement upconversion luminescence

Upconversion nanocrystals have a lot of advantages over other fluorescent materials.

Upconversion luminescence enhancement of Yb3+, Nd3+ sensitized NaYF4 core–shell nanocrystals on Ag grating films

The UCL enhancement of NaYF₄:Yb³⁺,Tm³⁺@NaYF₄:Yb³⁺,Nd³⁺ core–shell NCs resulting from Ag grating structures provides a novel insight for improving UCL.

Plasmon enhancement of luminescence upconversion

This review is aimed at offering a comprehensive framework for plasmon enhanced luminescence upconversion.

Energy transfer in lanthanide upconversion studies for extended optical applications

In this review, the various energy transfer pathways involved in lanthanide-related upconversion emissions are comprehensively discussed.

In situ synthesis and high adsorption performance of MoO2/Mo4O11 and MoO2/MoS2 composite nanorods by reduction of MoO3

MoO₂/Mo₄O₁₁ and MoO₂/MoS₂ composite nanorods with high adsorption performance were successfully synthesized by reducing MoO₃ nanorods.

Stimuli responsive upconversion luminescence nanomaterials and films for various applications

This review highlights recent advances in upconversion luminescence materials in response to various stimuli for a broad spectrum of applications.

Lab on upconversion nanoparticles: optical properties and applications engineering via designed nanostructure

This review aims to summarize recent progress in optical properties and applications engineering of upconversion nanoparticles via the designed nanostructure.