Simultaneous quasi-one-dimensional propagation and tuning of upconversion luminescence through waveguide effect

Upconversion luminescence and temperature sensing properties of Ho3+,Yb3+-codoped Bi2WO6

Ho3+ and Yb3+-codoped Bi2WO6 upconversion luminescent materials at different concentrations were prepared via a high-temperature solid-phase method. The X-ray diffraction patterns showed that Ho3+ and Yb3+ doping basically did not affect the orthorhombic crystal system structure of the Bi2WO6 matrix material. Scanning electron microscopy images showed that 3%Ho3+,10%Yb3+:Bi2WO6 consisted of irregular bulk particles with sizes in the range of 0.5-2 μm and some powder agglomeration. SEM mapping and EDS measurements of the powder showed that the elements were relatively uniformly distributed. Under 980 nm excitation, the emission intensity of Ho3+ was the largest for the 3%Ho3+- and 10%Yb3+-doped sample. With an excitation power ranging from 45 mW to 283 mW for the 3%Ho3+,10%Yb3+:Bi2WO6 sample, the relationship between the luminescence intensity and pump power was determined; the results indicated that the Ho3+ (538 nm, 546 nm, 660 nm, 756 nm) emission peaks originated from two-photon absorption. In the temperature range of 298 K-573 K, under 980 nm laser excitation, the maximum absolute temperature sensitivity Sa was 0.029% K-1 (373 K), the maximum relative temperature sensitivity Sr was 0.034% K-1 (348 K) for the Ho3+ thermally coupled energy levels 5F4/5S2, and the minimum temperature resolution δT was 1.2857 K (298 K). Under the same conditions, the maximum Sa was 51.02% K-1 (573 K), the maximum Sr was 1.85% K-1 (523 K) for the Ho3+ nonthermally coupled energy levels 5F5/5F4, and the minimum δT is 0.2477 K (448 K). The colour coordinates showed that the luminescence of the 3%Ho3+,10%Yb3+:Bi2WO6 sample gradually shifted from the green region to the red region with increasing temperature.

Low-temperature open-air synthesis of PVP-coated NaYF4:Yb,Er,Mn upconversion nanoparticles with strong red emission

Cubic (α-phase) NaYF₄:Yb,Er upconversion nanoparticles (UCNPs) are uniquely suited to biophotonics and biosensing applications due to their near-infrared excitation and visible red emission (λ _ex approx. 660 nm), enabling detection via thick overlying tissue with no bio-autofluorescence. However, UCNP synthesis typically requires high temperatures in combination with either high pressure reaction vessels or an inert atmosphere. Here, we report synthesis of α-phase NaYF₄:Yb,Er,Mn UCNPs via the considerably more convenient PVP40-mediated route; a strategy that requires modest temperatures and relatively short reaction time (160°C, 2 h) in open air, with Mn²⁺ co-doping serving to greatly enhance red emission. The optimal Mn²⁺ co-doping level was found to be 35 mol %, which decreased the average maximum UCNP Feret diameter from 42 ± 11 to 36 ± 15 nm; reduced the crystal lattice parameter, a, from 5.52 to 5.45 Å; and greatly enhanced UCNP red/green emission ratio in EtOH by a factor of 5.6. The PVP40 coating enabled dispersal in water and organic solvents and can be exploited for further surface modification (e.g. silica shell formation). We anticipate that this straightforward UCNP synthesis method for producing strongly red-emitting UCNPs will be particularly beneficial for deep tissue biophotonics and biosensing applications.

The upconversion quantum yield (UCQY): a review to standardize the measurement methodology, improve comparability, and define efficiency standards

Advancing the upconversion materials field relies on accurate and contrastable photoluminescence efficiency measurements, which are characterised by the absolute upconversion quantum yield (UCQY). However, the methodology for such measurements cannot be extrapolated directly from traditional photoluminescence quantum yield techniques, primarily due to issues that arise from the non-linear behaviour of the UC process. Subsequently, no UCQY standards exist, and significant variations in their reported magnitude can occur between laboratories. In this work, our aim is to provide a path for determining and reporting the most reliable UCQYs possible, by addressing all the effects and uncertainties that influence its value. Here the UCQY standard, at a given excitation power density, is defined under a range of stated experimental conditions, environmental conditions, material properties, and influential effects that have been estimated or corrected for. A broad range of UCQYs reported for various UC materials are scrutinized and categorized based on our assertion of the provided information associated with each value. This is crucial for improved comparability with other types of photoluminescent materials, and in addition, the next generation of UC materials can be built on top of these reliable standards.

Enhancing the static green up-conversion luminescence of NaY(MoO4)2:Yb/Er microcrystals via an annealing strategy for anti-counterfeiting applications

The majority of the fabrication procedures of lanthanide-doped materials involve thermal treatment that often results in crystallite regrowth, stabilizing the specific crystal structure and resulting in luminescence enhancement. The efficiency and intensity of up-conversion luminescence are closely related to the structure and synthesis process of the materials. Herein, well-crystallized and pure tetragonal NaY(MoO4)2 microcrystals with a uniform octahedral shape have been successfully synthesized via an environmentally friendly hydrothermal method, followed by annealing treatment. The phases, structures, morphologies, and compositions of the synthesized products annealed at 500-1000 °C remain unchanged, indicating high thermal stability. Furthermore, the NaY(MoO4)2:Yb3+/Er3+ microcrystals exhibit strong green emission when irradiated using infrared (980 nm) or ultraviolet (378 nm) wavelengths. Upon 980 nm excitation, up to 37-fold luminescence enhancement is achieved when the samples are annealed at about 700 °C. Interestingly, the high colour purity of the strong green emission is not only independent of the dopant concentration and heat treatment temperature, but it is also independent of the excitation conditions, including power and wavelength, and this makes it particularly suitable as a green safety signal light and luminescent security ink in paintings. As-prepared safety inks with NaY(MoO4)2:Yb3+/Er3+ microcrystals were used for visual fingerprint recognition printed on A4 paper with three-level fingerprint security features, significantly increasing the difficulty of illegal imitation and enhancing the levels of anti-counterfeiting.

An Integrated System to Remotely Trigger Intracellular Signal Transduction by Upconversion Nanoparticle-mediated Kinase Photoactivation

Upconversion nanoparticle (UCNP)-mediated photoactivation is a new approach to remotely control bioeffectors with much less phototoxicity and with deeper tissue penetration. However, the existing instrumentation on the market is not readily compatible with upconversion application. Therefore, modifying the commercially available instrument is essential for this research. In this paper, we first illustrate the modifications of a conventional fluorimeter and fluorescence microscope to make them compatible for photon upconversion experiments. We then describe the synthesis of a near-infrared (NIR)-triggered caged protein kinase A catalytic subunit (PKA) immobilized on a UCNP complex. Parameters for microinjection and NIR photoactivation procedures are also reported. After the caged PKA-UCNP is microinjected into REF52 fibroblast cells, the NIR irradiation, which is significantly superior to conventional UV irradiation, efficiently triggers the PKA signal transduction pathway in living cells. In addition, positive and negative control experiments confirm that the PKA-induced pathway leading to the disintegration of stress fibers is specifically triggered by NIR irradiation. Thus, the use of protein-modified UCNP provides an innovative approach to remotely control light-modulated cellular experiments, in which direct exposure to UV light must be avoided.

Unique adjustable UC luminescence pattern and directional radiation of peculiar-shaped NaYF4: Yb3+/Er3+ microcrystal particle

A new design of peculiar-shaped β-NaYF₄: 20% Yb³⁺/2% Er³⁺ hexagonal microcrystal (PSβHM) is proposed and its upconversion (UC) luminescence with adjustable color pattern is studied with near infrared excitation. Flower-like green UC luminescence emission pattern from blooming to withering process and intensive directional red emission are achieved by simply adjusting the focal point position of the excitation light. The mechanism that determines the unique UC luminescence phenomena are investigated systematically. The function of the internal light reflection and waveguide effect with annular microcavity is explored based on the structure characteristic of the hexagonal microplate. The current work may have great significant and potential applications in the development of optoelectronic device, color display, directional light and laser emission of microsystem.

Simultaneous spectra and dynamics processes tuning of a single upconversion microtube through Yb3+ doping concentration and excitation power

Doping and varying pump laser parameters are the widely applied technological processes for tuning spectra to yield desirable luminescence properties and functions.

Color-tunable upconversion photoluminescence and highly performed optical temperature sensing in Er3+/Yb3+ co-doped ZnWO4

Er³⁺/Yb³⁺ co-doped ZnWO₄ phosphors were synthesized by a solid state reaction method and their structure, photoluminescence and temperature sensing properties were characterized. The color-tunable upconversion emissions (from green to red) were observed by increasing the doped Er³⁺/Yb³⁺ concentration. The temperature sensing properties were studied by using the fluorescence intensity ratio technique in the temperature range of 83-583 K, and high performance was obtained. The maximum sensitivity is found to be 0.0099 K^-1 at 583 K. The XRD Rietveld refinement revealed that the phosphors crystallized in monoclinic structure with the space group P2/c (13) at room temperature. The results suggest that the phosphors could be an exceptional choice for next generation luminescence-based temperature sensing devices as well as in multiple biolabels.

Facile synthesis of Er3+/Yb3+-codoped NaYF4 nanoparticles: a promising multifunctional upconverting luminescent material for versatile applications

The Er³⁺/Yb³⁺-codoped NaYF₄ upconverting nanoparticles were prepared by a facile hydrothermal method.