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High sensitizer and activator concentrations have been increasingly examined to improve the performance of multi-color emissive upconversion (UC) nanocrystals (UCNC) like NaYF 4 :Yb,Er and first strategies were reported to reduce concentration quenching in highly doped UCNC. UC luminescence (UCL) is, however, controlled not only by dopant concentration, yet by an interplay of different parameters including size, crystal and shell quality, and excitation power density ( P ). Thus, identifying optimum dopant concentrations requires systematic studies of UCNC designed to minimize additional quenching pathways and quantitative spectroscopy. Here, we quantify the dopant concentration dependence of the UCL quantum yield ( Φ UC ) of solid NaYF 4 :Yb,Er/NaYF 4 :Lu upconversion core/shell nanocrystals of varying Yb 3+ and Er 3+ concentrations (Yb 3+ series: 20%–98% Yb 3+ ; 2% Er 3+ ; Er 3+ series: 60% Yb 3+ ; 2%–40% Er 3+ ). To circumvent other luminescence quenching processes, an elaborate synthesis yielding OH-free UCNC with record Φ UC of ∼9% and ∼25 nm core particles with a thick surface shell were used. High Yb 3+ concentrations barely reduce Φ UC from ∼9% (20% Yb 3+ ) to ∼7% (98% Yb 3+ ) for an Er 3+ concentration of 2%, thereby allowing to strongly increase the particle absorption cross section and UCNC brightness. Although an increased Er 3+ concentration reduces Φ UC from ∼7% (2% Er 3+ ) to 1% (40%) for 60% Yb 3+ . Nevertheless, at very high P (> 1 MW/cm 2 ) used for microscopic studies, highly Er 3+ -doped UCNC display a high brightness because of reduced saturation. These findings underline the importance of synthesis control and will pave the road to many fundamental studies of UC materials.

Over the past two decades, lanthanide-based upconversion nanoparticles (UCNPs) have been fascinating scientists due to their ability to offer unprecedented prospects to upconvert tissue-penetrating near-infrared light into color-tailorable optical illumination inside biological matter. In particular, luminescent behavior UCNPs have been widely utilized for background-free biorecognition and biosensing. Currently, a paramount challenge exists on how to maximize NIR light harvesting and upconversion efficiencies for achieving faster response and better sensitivity without damaging the biological tissue upon laser assisted photoactivation. In this review, we offer the reader an overview of the recent updates about exciting achievements and challenges in the development of plasmon-modulated upconversion nanoformulations for biosensing application.

Lanthanide‐doped upconversion nanoparticles (UCNPs) exhibit unique luminescence properties, making them promising for applications in displays, sensors, security labels, and solar cells. Embedding UCNPs in polymer films can enhance their functionality; however, the properties of the polymer matrix significantly influence the dispersion and loading capacity of UCNPs, ultimately affecting optical performance. In this study, we investigate the incorporation of UCNPs into two distinct polymer matrices, poly(3‐hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA), via spin coating at different speeds. Our findings demonstrate that UCNP dispersion and monodispersity are governed by polymer polarity, viscosity, and UCNP concentration in the suspension. To enhance UCNP loading, multiple spin coatings were explored. In UCNP−P3HT films, the volume fraction of UCNPs increased from 26.1% to 51.4% after three consecutive spin coatings, while maintaining a uniform distribution. In contrast, the lower miscibility and higher viscosity of PMMA restricted UCNP loading to 12.0% before significant clustering occurred. Although multiple spin coatings increased the total UCNP content in PMMA films, the volume fraction decreased to 8.0% due to film thickening. This comparative analysis highlights the critical role of polymer matrix properties in UCNP embedding and provides valuable insights for optimizing UCNP−polymer composites for advanced optical applications. We study the fabrication of nanocomposite thin films incorporating upconversion nanoparticles into two types of polymer matrices, demonstrating the key roles of nanoparticle surface–polymer compatibility and processing conditions in determining particle dispersity and loading capacity.

The intracellular metabolism of organelles, like lysosomes and mitochondria, is highly coordinated spatiotemporally and functionally. The activities of lysosomal enzymes significantly rely on the cytoplasmic temperature, and heat is constantly released by mitochondria as the byproduct of adenosine triphosphate (ATP) generation during active metabolism. Here, we developed temperature-sensitive LysoDots and MitoDots to monitor the in situ thermal dynamics of lysosomes and mitochondria. The design is based on upconversion nanoparticles (UCNPs) with high-density surface modifications to achieve the exceptionally high sensitivity of 2.7% K−1 and low uncertainty of 0.8 K for nanothermometry to be used in living cells. We show the measurement is independent of the ion concentrations and pH values. With Ca2+ ion shock, the temperatures of both lysosomes and mitochondria increased by ∼2 to 4°C. Intriguingly, with chloroquine (CQ) treatment, the lysosomal temperature was observed to decrease by up to ∼3 °C, while mitochondria remained relatively stable. Lastly, with oxidative phosphorylation inhibitor treatment, we observed an ∼3 to 7°C temperature increase and a thermal transition from mitochondria to lysosomes. These observations indicate different metabolic pathways and thermal transitions between lysosomes and mitochondria inside HeLa cells. The nanothermometry probes provide a powerful tool for multimodality functional imaging of subcellular organelles and interactions with high spatial, temporal, and thermal dynamics resolutions.

Upconversion (UC) nanostructures, which can upconvert near-infrared (NIR) light with low energy to visible or UV light with higher energy, are investigated for theranostic applications. The surface of lanthanide (Ln)-doped UC nanostructures can be modified with different functional groups and bioconjugated with biomolecules for therapeutic systems. On the other hand, organic molecular-based UC nanostructures, by using the triplet-triplet annihilation (TTA) UC mechanism, have high UC quantum yields and do not require high excitation power. In this review, the major UC mechanisms in different nanostructures have been introduced, including the Ln-doped UC mechanism and the TTA UC mechanism. The design and fabrication of Ln-doped UC nanostructures and TTA UC-based UC nanostructures for theranostic applications have been reviewed and discussed. In addition, the current progress in the application of UC nanostructures for diagnosis and therapy has been summarized, including tumor-targeted bioimaging and chemotherapy, image-guided diagnosis and phototherapy, NIR-triggered controlled drug releasing and bioimaging. We also provide insight into the development of emerging UC nanostructures in the field of theranostics.

An acetamiprid-binding aptamer (ABA), gold nanoparticles (AuNPs) and upconversion nanoparticles (UCNPs) are used in a colorimetric and fluorometric method for the ultrasensitive and selective detection of the pesticide acetamiprid. The ABA is first configured into a duplex with a complementary DNA covalently attached to AuNPs. The resulting dsDNA-functionalized AuNP probe is not stable in 0.15 M NaCl solution and aggregates. This causing the color to change from red to purple. In the presence of acetamiprid, the ABA undergoes a structural switch from a DNA duplex to an aptamer-acetamiprid complex and consequently dissociates from the AuNPs. The partially unhybridized AuNPs are stable against salt-induced aggregation and show red color. The ratio of absorbances at 524 nm (red) and 650 nm (purple blue) varies with the concentration of acetamiprid in the 0.025–10 μM concentration range. The colorimetric signal can be further amplified by introducing DNA-modified carboxylated UCNPs (silica-coated NaYF 4 :Yb,Er) which display red and green fluorescence under 980 nm excitation. An inner filter effect occurs between DNA-modified UCNPs and dsDNA-modified AuNPs. The fluorometric assay is based on the measurement of the ratio of red (654 nm) and green (540 nm) fluorescence and works in the 0.025 to 1 μM acetamiprid concentration range and has a 0.36 nM detection limit (at a signal-to-noise ratio of 3). Because of the specificity of the aptamer, the assay is high selective. It was successfully used to quantify acetamiprid in contaminated real samples. Graphical abstract Schematic presentation of an upconversion fluorescent assay for acetamiprid. It involves the principle of analyte-triggered structural switch of aptamers, salt-induced AuNP aggregation, and signal amplification from UCNP.

An upconversion luminescence (UCL) nanosensor was developed based on the inner filter effect (IFE) mechanism. This involved the interaction between polyacrylic acid (PAA)-coated NaYF 4 :Yb/Er upconversion nanoparticles (UCNPs) and the oxidation product (oxDPD) of N ,  N -diethyl- p -phenylenediamine (DPD). DPD was oxidized to oxDPD through the combined action of H 2 O 2 and horseradish peroxidase (HRP). As a result, there was a large overlap between the UCL spectrum (545 nm) of Er-doped UCNPs and the UV–Vis absorption spectrum (552 nm) of oxDPD, which subsequently caused the quenching of UCL via the IFE. When 6-mercaptopurine (6-MP) reacted with oxDPD, the absorption intensity of the resulting product at 552 nm decreased, thereby restoring the intensity of UCL. Therefore, 6-MP can be sensitively detected according to the intensity change of UCL. The resulting detection range and the limit of detection (LOD) were 0–15 μg·mL −1 and 0.56 μg·mL −1 , respectively. The “off–on” UCL nanosensor designed for the detection of 6-MP exhibits the advantages of easy operation, rapid response time, and high sensitivity, demonstrating promising potential for application in clinical monitoring. Graphical Abstract

Rare earth (RE 3+ )-doped phosphors generally suffer from thermal quenching, in which their photoluminescence (PL) intensities decrease at high temperatures. Herein, we report a class of unique two-dimensional negative-thermal-expansion phosphor of Sc 2 (MoO 4 ) 3 :Yb/Er. By virtue of the reduced distances between sensitizers and emitters as well as confined energy migration with increasing the temperature, a 45-fold enhancement of green upconversion (UC) luminescence and a 450-fold enhancement of near-infrared downshifting (DS) luminescence of Er 3+ are achieved upon raising the temperature from 298 to 773 K. The thermally boosted UC and DS luminescence mechanism is systematically investigated through in situ temperature-dependent Raman spectroscopy, synchrotron X-ray diffraction and PL dynamics. Moreover, the luminescence lifetime of 4 I 13/2 of Er 3+ in Sc 2 (MoO 4 ) 3 :Yb/Er displays a strong temperature dependence, enabling luminescence thermometry with the highest relative sensitivity of 12.3%/K at 298 K and low temperature uncertainty of 0.11 K at 623 K. These findings may gain a vital insight into the design of negative-thermal-expansion RE 3+ -doped phosphors for versatile applications. Rare-earth doped phosphors with negative thermal expansion (NTE) may display thermally-enhanced emission, but their performance is generally limited. Here the authors report thermally-boosted green upconversion luminescence and near-infrared downshifting luminescence in Sc 2 (MoO 4 ) 3 :Yb/Er phosphors with two-dimensional NTE, and their application in temperature sensing.

Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications 1 – 6 , with stereolithography a particularly successful approach 4 , 7 – 9 . However, owing to the linear absorption of light, this technique requires photopolymerization to occur at the surface of the printing volume, imparting fundamental limitations on resin choice and shape gamut. One promising way to circumvent this interfacial paradigm is to move beyond linear processes, with many groups using two-photon absorption to print in a truly volumetric fashion 3 , 7 – 9 . Using two-photon absorption, many groups and companies have been able to create remarkable nanoscale structures 4 , 5 , but the laser power required to drive this process has limited print size and speed, preventing widespread application beyond the nanoscale. Here we use triplet fusion upconversion 10 – 13 to print volumetrically with less than 4 milliwatt continuous-wave excitation. Upconversion is introduced to the resin by means of encapsulation with a silica shell and solubilizing ligands. We further introduce an excitonic strategy to systematically control the upconversion threshold to support either monovoxel or parallelized printing schemes, printing at power densities several orders of magnitude lower than the power densities required for two-photon-based 3D printing.   . Triplet fusion upconversion nanocapsules dispersed in a photopolymerizable resin allow for volumetric 3D printing at low-power continuous-wave excitation without support structures.