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Lanthanide‐doped upconversion nanoparticles (UCNPs) have attractive emission properties but suffer from weak light‐absorbing capacities and thereby relatively low brightnesses. This motivates using strongly absorbing dye molecules as antennas and sensitizers. However, despite much effort, understanding of this dye‐UCNP interplay is still limited. Major sensitization mechanisms are still under discussion, largely because there is a lack of effective means to observe key factors such as dark state transitions within the dyes. Here, a combined spectroscopic procedure is established to systematically investigate the photophysics behind the dye‐UCNP interaction, embracing fluorescence‐based transient‐state excitation‐modulation, lifetime and correlation spectroscopy, and spectrofluorometry/spectrophotometry. With this procedure the heptamethine cyanine dye IR806, a typical UCNP sensitizer is studied, its photophysical model is established, its photophysics in UCL‐sensitization‐related environments is deciphered, and the energy transfer from the IR806 singlet excited state to Yb3+ (UCNP sensitizer ion) can be identified as the dominant sensitization mechanism. These studies suggest that IR806 can form non‐emissive H‐aggregates at the nanoparticle surfaces, which can be dissociated after certain light excitation duration (typically>100 µs). Moreover, buildup of a non‐fluorescent, photo‐redox state of IR806 after longer irradiation times (10–100 ms) can deleteriously affect its UCL sensitization effect, inferring an optimal excitation duration for dye‐sensitized UCNPs, relevant for, e.g., optical imaging applications. [ABSTRACT FROM AUTHOR]

Triplet‐triplet annihilation upconversion (TTA‐UC) is a photophysical process in which two low‐energy photons are converted into one higher‐energy photon. This type of upconversion requires two species: a sensitizer that absorbs low‐energy light and transfers its energy to an annihilator, which emits higher‐energy light after TTA. In spite of the multitude of applications of TTA‐UC, few families of annihilators have been explored. In this work, we show dipyrrolonaphthyridinediones (DPNDs) can act as annihilators in TTA‐UC. We found that structural changes to DPND dramatically increase its upconversion quantum yield (UCQY). Our optimized DPND annihilator demonstrates a high maximum internal UCQY of 9.4 %, outperforming the UCQY of commonly used near‐infrared‐to‐visible annihilator rubrene by almost double. [ABSTRACT FROM AUTHOR]

(Ti 1-x Si x)O 2 :Er/Yb (0 ≤ x ≤ 0.3) phosphor was synthesized using spray pyrolysis, and the upconversion (UC) properties were optimized by varying the calcination temperature and Si content (x). By introducing Si into the TiO 2 matrix, the thermal stability of titania was effectively improved, allowing the anatase phase to be maintained even when the (Ti,Si)O 2 :Er/Yb phosphor was calcined at temperatures above 800 °C. From investigating the effect of crystal size on the UC emission of (Ti,Si)O 2 :Er/Yb, it was found the optimal crystal size of Antase to achieve the highest UC emission intensity is approximately 5.5 nm, beyond which the UC intensity decreases rapidly. With increasing Si content, the crystalltie size decreased linearly and more amorphous SiO 2 domains were formed in the anatase TiO 2 matrix, which could consume Er/Yb ions to form SiO 2 :Er/Yb with low UC emission. Resultently, the UC emission intensity was significantly affected by Si content. The optimal calcination temperature and Si content to obtain the highest UC luminescence were determined as 700 °C and 10 %, respectively. The optimized (Ti,Si)O 2 :Er/Yb showed approximately 2.7 times improved UC luminescence than TiO 2 :Er/Yb. [ABSTRACT FROM AUTHOR]

Triplet–triplet annihilation (TTA), or triplet fusion, is a biexcitonic process in which two triplet‐excited molecules can combine their energy to promote one into an excited singlet state. To alleviate the dependence of the TTA rate and yield on triplet diffusion in both solid and solution environments, intramolecular TTA (intra‐TTA) has been recently proposed in conjugated molecular systems able to hold multiple triplet excitons simultaneously. Developing from the previous demonstration of TTA performance enhancement in sensitized upconversion solutions, here similar improvements in triplet harvesting in solid‐state films are reported under electrical excitation in organic light emitting diodes (OLEDs). At low dye concentration and low current densities, the intra‐TTA active OLED shows a +40% improved external quantum efficiency with respect to the reference device, and a TTA spin‐statistical factor f 4DPA of 0.4, close to that determined in fluid solution for the individual chromophore (0.45). These results therefore indicate the utility of this molecular design strategy across a wider range of TTA applications, and with particular utility in the further development of low‐power TTA‐enhanced OLEDs. [ABSTRACT FROM AUTHOR]

The exploration of photon avalanching (PA) luminescence in different lanthanide emitters has profound implications in plentiful frontier applications. However, studies for universal mechanisms at the nanoscale to exhibit giant nonlinear responses in various avalanching emitters are limited. Here, record‐breaking nonlinear responses up to 60th‐order in high‐lying emitting levels of various emitters at room temperature are generated by proposing a universal mechanism named energy relay‐mediated photon avalanche (enrePA). By harnessing energy from the avalanching nano‐engine through the energy relay of Yb3+ ions, the brightness of blue emissions from Tm3+ and Ho3+ can be enhanced with giant optical nonlinearity up to 60th and 38th order, respectively. Further incorporating gadolinium‐based systems, more emitters (Tb3+, Eu3+, Dy3+, Nd3+) can be activated with extreme optical nonlinearities up to 48th‐order. By expanding PA into the full‐spectrum range, the enrePA mechanism opens exciting avenues for flexible and high‐efficiency PA modulation in multilayer nanostructures, enabling the applicability of PA in more technologies such as super‐resolution imaging, lithography, and optical detection. [ABSTRACT FROM AUTHOR]

The demand for highly sensitive methods of pathogen detection drives the development of new diagnostic assays. While nucleic acid amplification methods such as PCR are very sensitive and remain widely used, they may be limited in complex sample matrices due to the presence of polymerase inhibitors. On the other hand, the direct detection of nucleic acids by DNA hybridization assays is simple but typically less sensitive. This work combines a branched DNA (bDNA) hybridization assay with upconversion nanoparticle (UCNP) labels to enhance the sensitivity of DNA detection. The anti‐Stokes emission of UCNP labels enables measurements without optical background interference. The bDNA assay relies on a series of oligonucleotide probes creating a branched structure with several binding sites for biotinylated amplification probes and streptavidin‐conjugated UCNPs. Several configurations of the bDNA assay are investigated to achieve the highest signal amplification and the lowest background signal. The optimal configuration of bDNA assay yields a limit of detection (LOD) of 5.9 × 104 cfu mL−1 for the target DNA of the bacteriophage M13. The upconversion‐linked bDNA assay is easily adaptable to other target DNAs by adjusting the oligonucleotide probes. [ABSTRACT FROM AUTHOR]

Herein, we report a large-scale solid-state synthesis method for water-soluble graphitic carbon nitride nanoparticles (g-CNNPs) using urea and trisodium citrate, adhering to the principle of atom economy. The as-synthesized g-CNNPs, approximately 3.0 nm in size with a quasi-spherical structure and having a high water-soluble property, exhibit strong blue fluorescence emission. The surface of g-CNNPs contains six surface active sites and serves as an effective fluorescence sensor for the detection of the insecticide fipronil (FPN) at pH 7.0. The size variation of g-CNNPs (2–4.5 nm) induces size-dependent surface effects, contributing to their upconversion properties. The quasi-spherical g-CNNPs (ΦF 21.08%) demonstrate high selectivity and sensitivity for FPN detection, with a limit of detection (LoD) of 2.79 nM (R2 = 0.99511) and a limit of quantification (LoQ) of 9.30 nM within a linear concentration range of 1 × 104 to 9 × 104 nM (ΦF 17.40%), and an association constant (Ka) of 1.356 × 102 M–1. The observed fluorescence quenching during the sensitivity study is likely due to intermolecular hydrogen bonding between g-CNNPs and FPN. Time-correlated single photon counting (TCSPC) analysis confirms the static quenching of g-CNNPs (lifetime 5.53–5.60 ns) upon FPN detection. To confirm the pH-dependent behavior of g-CNNPs and FPN interaction, the TCSPC technique was carried out in particular at pH 3.0 and 11. Density functional theory studies were conducted to confirm the interaction between g-CNNPs and FPN molecules. Additionally, the g-CNNPs demonstrated practical applicability by detecting FPN in soil samples with recovery rates ranging from 102.5 to 129.7%. Due to their low cytotoxicity and good cell permeability, g-CNNPs were successfully applied for selective cytotoxic effects and bioimaging in zebrafish. [ABSTRACT FROM AUTHOR]

Based on the previous optimal concentration of Er3+/Yb3+/Mo4+ co‐doped BiTa7O19 (BTO) samples, Sc3+ or Sb is planned to be doped into BTO:Er3+/Yb3+/Mo4+ for further improving the upconversion luminescence (UCL) intensity under 980‐nm laser excitation. A series of BTO:Er3+/Yb3+/Mo4+/Sc3+ and BTO:Er3+/Yb3+/Mo4+/Sb samples are prepared by high‐temperature solid‐phase sintering method. The microstructure and UCL properties are investigated by X‐ray diffraction, Raman, X‐ray photoelectron spectroscopy (XPS), diffuse reflection, and luminescence spectrum. The green UCL intensity of BTO:Er3+/Yb3+/Mo4+/Sb is little better than that of BTO:Er3+/Yb3+/Mo4+/Sc3+, which reaches 2.40 times than that of β‐NaYF4:Er3+/Yb3+. XPS results show that Sc is 3+, Mo is 4+, and Sb is 3+ and 5+. Raman spectra show that BTO:Er3+/Yb3+/Mo4+/Sb has lower phonon energy and more disorder than BTO:Er3+/Yb3+/Mo4+/Sc3+. The maximum relative temperature sensitivity is got as 0.01012 K−1 at 303 K based on luminescence intensity ratio technology. BTO:Er3+/Yb3+/Mo4+/Sb has outstanding pure green UCL intensity, which can be applied to luminescence display and temperature sensing. [ABSTRACT FROM AUTHOR]

Upconversion detectors monolithically combining a detection unit and a light emitting unit, enables light detection and visualization in a compact structure, promising great advances in miniaturized multifunctional optoelectronics. The detection range of upconversion detectors usually covers a broadband spectrum, limiting their use in spectroscopic fields. This work investigates two wavelength‐selective upconversion detectors made with organic semiconductors to realize narrowband near‐infrared (NIR) light detection and visualization dual function. Two non‐fullerene‐based NIR‐sensitive bulk‐heterojunctions (BHJs) are exploited to make wavelength‐selective upconversion detectors, achieving peak sensitivity at 860 and 890 nm, with full width at half maximum of 125 and 170 nm, respectively. Each NIR‐sensitive BHJ comprises a donor polymer and a non‐fullerene acceptor, both of which are selectively sensitive to NIR light. The cumulative analysis of the optical properties of the absorber and current–voltage characteristics of the device indicates that the wavelength selectivity stems mainly from the wavelength‐dependent absorption. In particular, the upconversion detectors exhibit wavelength‐selective electronic and optical dual‐readouts, which are appealing for miniaturized spectroscopic applications, including health monitoring, optical communication, and microbead imaging, paving the way for practical applications. [ABSTRACT FROM AUTHOR]

Upconversion luminescence presents obvious advantages in optical temperature sensing, nevertheless the application in complex scenarios is limited by luminescence thermal quenching. Herein, NaYS 2 :Yb3+,Nd3+ phosphors are synthesized by the solid-gas reaction method and used for temperature sensing. A series of emissions for Nd3+ from visible to near-infrared region are achieved based on energy transfer from Yb3+ to Nd3+ under 980 nm excitation. With elevating the temperature, significant thermal enhancement effect of nearly three orders of magnitude is detected in near-infrared emission of 4F 7/2 → 4I 9/2 , which is attributed to phonon-assisted energy transfer of Yb3+→Nd3+ and the corresponding thermal population effect. The thermal behaviors of thermally coupled energy levels for 4G 7/2 /2G 9/2 and 4F 7/2 /4F 5/2 in NaYS 2 :Yb3+,Nd3+ are evaluated by luminescence intensity ratio technique. Remarkably, the optical thermometer based on the enhanced emission of 4F 7/2 /4F 5/2 shows excellent temperature measurement performance, which is expected to be applied in wide-temperature-range and highly-sensitive temperature sensing. These results not only provide a pathway to realize high performance temperature sensing, but also heighten the understanding of UC emission thermal enhancement. [ABSTRACT FROM AUTHOR]

Extremely intense blue upconversion luminescence (UCL) is got in Tm3+/Yb3+ co-doped BiTa 7 O 19 (BTO) phosphors, which are synthesized by solid phase sintering. The doping concentration of Tm3+ is fixed at 7 mol%, and the concentration of Yb3+ is adjusted. When the concentration of Yb3+ is 80 mol%, the strongest blue UCL can be obtained under 980 nm laser excitation. The blue UCL integral intensity under 808 + 980 nm laser co-excitation is 1.54 times of the sum of intensity under 808 and 980 nm single excitation, respectively. Blue UCL thermal enhancement behavior was found for all the samples under 808 or 785 nm laser excitation, and blue UCL thermal quenching happened under 660, 660 + 808, and 359 nm excitations. It is demonstrated that UCL thermal enhancement behavior is caused by oxygen defects. The possible electron transition pathways are proposed under excitation with different laser wavelengths. Using the luminescence intensity branching ratio (LIR) technique, the maximum relative temperature sensitivity range is calculated to be from 0.00610 to 0.00732 K−1 at 303 K. The experimental results show that BTO:Tm3+/Yb3+ has excellent blue UCL with thermal enhancement, which can have some applications in the field of UCL display and temperature sensing in high temperature environment. [ABSTRACT FROM AUTHOR]

The perovskite based phosphor materials are widely used to increase the efficiency of solar cells. In this work, Er3+ doped and Er3+/Yb3+ co-doped CaTiO 3 perovskite phosphor samples have been synthesized by solid state reaction technique at 1473 K. The phosphor samples show orthorhombic phase with Pnma (62) space group. The average crystallites and particles size of CaTiO 3 are increased in presence of Er3+ and Yb3+ ions. Er3+ doped CaTiO 3 phosphor samples give downshifting emission under 379 nm excitation. Though upconversion emission is seen in Er3+ under 980 nm excitation without Yb3+ ions in this host. The emission intensity of Er3+ ion is enhanced by 46 and 16 times for green and red emissions, respectively in presence of Yb3+. An intense quantum cutting (QC) emission is observed at 980 nm in presence of Yb3+ in CaTiO 3 :0.5Er3+ phosphor under 379 nm excitation. The QC efficiency has been found to be 119 % for CaTiO 3 :0.5Er3+/5 Yb3+ phosphor. Further an interesting phenomenon of back energy transfer (BEnT) from Yb3+ to Er3+ giving an intense NIR emission from Er3+ at 1002 and 1550 nm have been observed. The phosphor sample also shows an intrinsic optical bistablity (IOB) by upconversion. Thus, the prepared phosphor samples may be useful to increase the efficiency of c-Si solar cell, NIR emission for communication, bistable material and green light emitting source. [Display omitted] [ABSTRACT FROM AUTHOR]

Upconversion nanoparticles (UCNPs) are well-reported for bioimaging. However, their applications are limited by low luminescence intensity. To enhance the intensity, often the UCNPs are coated with macromolecules or excited with high laser power, which is detrimental to their long-term biological applications. Herein, we report a novel approach to prepare co-doped CaF2:Yb3+ (20%), Er3+ with varying concentrations of Er (2%, 2.5%, 3%, and 5%) at ambient temperature with minimal surfactant and high-pressure homogenization. Strong luminescence and effective red emission of the UCNPs were seen even at low power and without functionalization. X-ray diffraction (XRD) of UCNPs revealed the formation of highly crystalline, single-phase cubic fluorite-type nanostructures, and transmission electron microscopy (TEM) showed co-doped UCNPs are of ~ 12 nm. The successful doping of Yb and Er was evident from TEM–energy dispersive X-ray analysis (TEM-EDAX) and X-ray photoelectron spectroscopy (XPS) studies. Photoluminescence studies of UCNPs revealed the effect of phonon coupling between host lattice (CaF2), sensitizer (Yb3+), and activator (Er3+). They exhibited tunable upconversion luminescence (UCL) under irradiation of near-infrared (NIR) light (980 nm) at low laser powers (0.28–0.7 W). The UCL properties increased until 3% doping of Er3+ ions, after which quenching of UCL was observed with higher Er3+ ion concentration, probably due to non-radiative energy transfer and cross-relaxation between Yb3+-Er3+ and Er3+-Er3+ ions. The decay studies aligned with the above observation and showed the dependence of UCL on Er3+ concentration. Further, the UCNPs exhibited strong red emission under irradiation of 980 nm light and retained their red luminescence upon internalization into cancer cell lines, as evident from confocal microscopic imaging. The present study demonstrated an effective approach to designing UCNPs with tunable luminescence properties and their capability for cellular imaging under low laser power. [ABSTRACT FROM AUTHOR]

Alkali‐metal‐based yttrium fluoride host (AY1‒xF4:xEu3+; A: Li/Na/K) nanophosphors were developed using co‐precipitation method. Phase analysis and emission spectral behavior were established for the said nanophosphors. The highest emission observed for NaYF4 doped with Eu3+ was then optimized to achieve maximum photoluminescence (PL) emission for varied dopant amount. The downconversion and upconversion PL studies revealed pure red emission under n‐ultraviolet and infrared excitations. The CIE coordinates are observed to be lie in the red hue of the CIE diagram. The PL decay curves reported to study lifetime of the emitting state and to identify quantum efficiency using Auzel's model for the said nanophosphor. Thermal stability was also evaluated from temperature‐sensitive luminescent spectra. The studies conducted on AYF4:.05Eu3+ (A: Li/Na/K) nanophosphors contemplates that NaYF4 nanophosphors doped with Eu3+ ions are best suited for biophotonic applications as the later one show relatively high emission. [ABSTRACT FROM AUTHOR]

This review delves into the forefront of upconversion luminescence (UCL) research, focusing on KY3F10-based compounds, particularly their cubic α-phase. These materials are renowned for their exceptional luminescent properties and structural stability, making them prime candidates for advanced photonic applications. The synthesis methods and structural characteristics of the existing works in the literature are meticulously analyzed alongside the transformative effects of various doping strategies on UCL efficiency. Incorporating rare earth (RE) sensitizer ions such as Yb3+, along with activator ions like Er3+, Ho3+, Nd3+, or Tm3+, researchers have achieved remarkable enhancements in emission intensity and spectral control. Recent and past breakthroughs in understanding the local structure and phase transitions of single-, double-, and triple-RE3+-doped KY3F10 nanocrystals are highlighted, revealing their pivotal role in fine-tuning luminescent properties. Furthermore, the review underscores the untapped potential of lesser-known crystal structures, such as the metastable δ-phase of KY3F10, which offers promising avenues for future exploration. By presenting a comprehensive analysis and proposing innovative research directions, this review aims to inspire continued advancements in the field of upconversion materials, unlocking new potentials in photonic technologies. [ABSTRACT FROM AUTHOR]

Despite the remarkable luminescent properties and temperature sensing capabilities of lanthanide materials, their photothermal effect in practical temperature measurement systems presents a major challenge to sensing reliability. Herein, taking Pr3+ as a typical exemplar, the influence of excitation wavelength and excitation approach on temperature sensing is systematically examined. The pulsed laser excitation method is identified as an efficient tactic to restrain the self‐heating of luminescence materials. In accordance with this principle, it is discovered that diverse excitation wavelengths give rise to alterations in the internal energy transport channels and further impact the temperature sensing performance. The 980 nm pulsed laser can result in the non‐thermal coupling occupying states of 3P0/3P1, which are recognized as thermal coupling energy levels in the conventional concept, and yield a relative sensitivity (Sr) performance more than three times superior to that of down‐shifting‐based sensing. The work provides a comprehensive analytical solution for achieving highly sensitive and stable temperature sensing based on excitation wavelength and working mode strategies. [ABSTRACT FROM AUTHOR]

The spatially resolvable multicolored microrods have potential applications in many fields. However, achieving spatially resolved multicolor luminescence tuning on the microrod with a fixed composition remains a daunting challenge. Herein, a strategy is proposed that allows for the tuning of spatially resolved, multicolored upconversion (UC) luminescence (UCL) along a 1D heterogeneous microrod by modifying the pulse width of an external laser. NaYbF4:1 % Ho is identified as an UCL color‐adjustable material, exhibiting pulse width‐dependent multicolored UCL, resulting in a significant regulation of the red/green (R/G) ratio from 0.1 to 10.3 as the pulse width is varied from 0.1 to 10 ms. Such variability can be ascribed to differences in the number of photons incident upon the microrod throughout the period necessary for the UC process to occur. Additionally, NaYbF4:1 %Tm and NaYF4:20 %Yb,1 %Ho are employed as materials that emit blue and green light, respectively, with their UCL colors largely unaffected by changes in the pulse width. Subsequently, a tip‐modified epitaxial growth method is utilized to integrate both UCL color‐adjustable and non‐adjustable segments within the same microrod. Comparing with single‐color or fixed multicolor microrods, our developed multisegmented emissive color adjustable 1D heterogeneous microrods have unique optical characteristics and can carry more optical information. [ABSTRACT FROM AUTHOR]

The upconversion response of Er3+ sensitized by Yb3+ in various crystalline hosts and illuminated with a laser light at around 980 nm revealed certain spectral shapes that are typical for each of the crystalline matrices containing the dopants. The purpose of this work was to measure the upconversion response of Er3+ as a dopant in Y2TiO5, sensitized by Yb3+, at different concentrations relative to the substituted Y3+ ion, and to reveal the subtleties of the mechanisms of the energy transfers between them and the lattice. Therefore, we synthesized Y2TiO5 ceramic samples doped with different concentrations of Er3+ and Yb3+, below 10% (mol), in order to minimize the distortion of the lattice. The oxide powders, obtained using the sol–gel method, as well as the ceramics were structurally and morphologically characterized using an X-ray diffraction analysis and scanning electron microscopy. When the ceramic samples were irradiated with an NIR laser light, it was found that, at a wavelength variation of only 2 nm of the incident radiation, from 973.5 nm to 975.5 nm, the upconversion spectra differed significantly. This nonlinearity is notable because it is not present in the case of other crystalline host matrices studied by us since the literature lacks information on this subject. We also correlated this effect with the simulated distribution of the average distances between Er3+ and Yb3+ ions in the host matrix. [ABSTRACT FROM AUTHOR]

The commonly used automotive head‐up display (HUD) technology is confronted with the problems of poor stability in organic resin counterpart and the unsatisfactory display effect of projection lighting source. To solve these issues, developing state‐of‐the‐art materials and designing new HUD schemes with a wide color gamut is of necessity. Herein, a series of pure red/green/blue (R/G/B) upconverting (UC) nano‐glass‐ceramics (GCs) containing lanthanide‐doped NaLuF4 nanocrystals (NCs) are synthesized in a cost‐effective, environment‐friendly, and scalable manner, simultaneously achieving high crystallinity (≈45.44%) and high transparency (>80%). Yb3+ is chosen as the sensitizer to Tm3+ (blue UC)/Ho3+ (green UC), due to its large absorption cross section from ground state 2F7/2 to excited state 2F5/2; meanwhile Er3+, Tm3+ co‐doped into NCs can obtain pure red UC emission via promoting the population of Er3+ red‐emitting 4F9/2 state. Consequently, the as‐prepared materials exhibit high R/G/B color purity of > 80% owing to the energy transfer between different lanthanide ions (Yb3+, Tm3+, Ho3+, and Er3+) upon 980 nm near‐infrared (NIR) laser excitation, and possess superior thermal reversibility/water resistivity. Remarkably, two unprecedented HUD prototypes are constructed to demonstrate the application feasibility of developed pure R/G/B UC GCs. [ABSTRACT FROM AUTHOR]

Visible‐to‐ultraviolet (UV) upconversion provides a fascinating strategy to achieve deep UV emission through readily accessible visible light. However, the intensity of deep UV emission obtained through visible‐to‐UV upconversion progress is still far from satisfactory, severely constraining its practical applications. Herein, a novel class of praseodymium ion (Pr3+)‐doped rare‐earth oxyhalides (YOCl, YOBr, and LuOBr) to achieve efficient upconverted deep UV emission in the spectral range of 250–350 nm is developed. The upconverted UV emission intensity of LuOBr:Pr3+ is determined to be 56.7 times stronger than that of the well‐established Lu7O6F9:Pr3+. When employed as a photon‐converter to activate photocatalytic water splitting reactions, upconverted deep UV emission enables H2 generation under visible light (λ > 420 nm) excitation from a xenon lamp. The efficient deep UV upconversion stems from tuning 4f15d1 state of Pr3+ by oxyhalide constituent which both facilitates the absorption of excitation photons in long‐lived intermediate 4f2 states and suppress the probability of nonradiative relaxation from 4f15d1 state. These findings not only provide new insights into a mechanistic understanding of the host effect on upconversion process but also make a breakthrough in developing efficient deep upconversion materials that will expand their further applications. [ABSTRACT FROM AUTHOR]

The structural, surface, and upconversion (UC) luminescence properties of Y2O3:Ho3+,Yb3+ films grown by pulsed laser deposition, for different numbers of laser pulses, were studied. The crystallinity, surface, and UC luminescence properties of the thin films were found to be highly dependent on the number of laser pulses. The X-ray powder diffraction analysis revealed that Y2O3:Ho3+,Yb3+ films were formed in a cubic structure phase with an Ia 3 ¯ space group. The thicknesses of the films were estimated by using cross-sectional scanning electron microscopy, depth profiles using X-ray photoelectron spectroscopy (XPS), and the Swanepoel method. The high-resolution XPS was used to determine the chemical composition and oxidation states of the prepared films. The UC emissions were observed at 538, 550, 666, and 756 nm, assigned to the 5F4 → 5I8, 5S2 → 5I8, 5F5 → 5I8, and 5S2 → 5I7 transitions of the Ho3+ ions. The power dependence measurements confirmed the involvement of a two-photon process in the UC process. The color purity estimated from the Commission International de I'Eclairage coordinates confirmed strong green UC emission. The results suggested that the Y2O3:Ho3+,Yb3+ UC transparent films are good candidates for various applications, including solar cell applications. [ABSTRACT FROM AUTHOR]

To investigate a near-infrared-to-blue luminescence upconversion curing method for polymerizing resin cements under zirconia discs. Lava zirconia discs of different thicknesses (0.5–2.0 mm) were manufactured. First, the transmittances of the NIR and two blue lights (BLs) (LED and halogen lights) through these discs were measured. Second, NaYF 4 :Yb3+/Tm3+ upconversion phosphor (UP) powder was milled into 0.5-μm particle sizes. A light-curable resin cement VariolinkII base was chosen as the control (UP0), and an experimental cement (UP5) was prepared by adding 5 % UPs. These two cements were examined using multiphoton excitation microscopy for particle distribution. UP5 and UP0 were polymerized with or without zirconia shielding then subjected to a microhardness test. A multifold analysis was performed to examine the effects of zirconia thickness, curing protocols (pure BL or combined BL and NIR curing), and cement type. The transmittance of NIR was superior to that of BL through zirconia discs of all thicknesses. UP particles were homogeneously distributed in UP5 and emitted blue luminescence under 980-nm NIR excitation. UP5 showed higher microhardness values than UP0 under any curing protocol or zirconia shielding condition. The combination of 20-s BL and 40-s NIR curing yielded the highest microhardness in uncovered UP5. However, combining 40-s BL and 20-s NIR curing surpassed the other groups when the zirconia discs were thicker than 0.5 mm. NIR exhibits higher transmission through zirconia than BL. UP particles work as strengthen fillers and photosensitizers in cements. NIR upconversion curing could be a new strategy for polymerizing resin cements under thick zirconia restorations. • An NIR activated upconversion reaction was used to induce blue light (BL) emission. • The transmittance of NIR through zirconia discs is higher than conventional BL. • Phosphor particles work as photosensitizers and strengthen fillers in cements. • Combining BL and NIR effectively enhanced polymerization of cements under zirconia. [ABSTRACT FROM AUTHOR]

Recent reports of upconversion and white light emission from graphitic particles warrant an explanation of the physics behind the process. A model is offered, wherein the upconversion is facilitated by photoinduced electronic structure modification allowing for multiphoton processes. As per the prediction of the model, it is experimentally shown that graphite upconverts infrared light centered around 1.31 μm (0.95 eV) to broadband white light centered around 0.85 μm (1.46 eV). The results suggest that upconversion from shortwave infrared (≈3 μm, 0.45 eV) to visible region may be possible. The experiments show that the population dynamics of the electronic states involved in this upconversion process occur in the timescale of milliseconds. [ABSTRACT FROM AUTHOR]

This work presents a novel paradigm for upconversion‐assisted ferroelectric photovoltaic devices. The system comprises a ferroelectric active layer (BiFeO3), an upconverter layer (Yb; Er‐doped ZnO), a conductive ITO‐coated glass substrate, and a reflective coating (Al) at the rear end of the glass substrate. The photovoltaic efficiency of the single‐layer BFO was found to be 0.71%. With the prescribed device model, the total solar efficiency of BiFeO3 improved significantly and touched solar conversion efficiency of 2.21%. This model's projection widens the future perspectives of device performance in emerging photovoltaic technology, mainly perovskite‐based solar cells. [ABSTRACT FROM AUTHOR]

In this study, TiO2-ZnO:Yb3+/Ho3+ upconversion phosphors are synthesized by a solid-phase reaction method and their Yb3+ concentration dependence is analyzed. The crystal structure of the phosphors is analyzed by X-ray diffractometry, where each sample is composed of Zn2TiO4 and Yb2Ti2O7. Photoluminescence characteristics are analyzed and peaks are observed at wavelengths of 539 nm (5F4, 5S2 → 5I8) and 669 nm (5F5 → 5I8). High intensity luminescence is obtained for samples with Yb = 7–8 mol%. Analysis of temperature characteristics shows absolute and relative sensitivities of 0.0052 K−1 and 0.40% K−1, respectively. The samples are found to be chemically stable, indicating that TiO2-ZnO:Yb3+/Ho3+ could be used as a temperature sensor. [ABSTRACT FROM AUTHOR]

Abstract The structural, surface, and upconversion (UC) luminescence properties of Y2O3:Ho3+,Yb3+ films grown by pulsed laser deposition, for different numbers of laser pulses, were studied. The crystallinity, surface, and UC luminescence properties of the thin films were found to be highly dependent on the number of laser pulses. The X-ray powder diffraction analysis revealed that Y2O3:Ho3+,Yb3+ films were formed in a cubic structure phase with an Ia $$\overline{3 }$$ 3 ¯ space group. The thicknesses of the films were estimated by using cross-sectional scanning electron microscopy, depth profiles using X-ray photoelectron spectroscopy (XPS), and the Swanepoel method. The high-resolution XPS was used to determine the chemical composition and oxidation states of the prepared films. The UC emissions were observed at 538, 550, 666, and 756 nm, assigned to the 5F4 → 5I8, 5S2 → 5I8, 5F5 → 5I8, and 5S2 → 5I7 transitions of the Ho3+ ions. The power dependence measurements confirmed the involvement of a two-photon process in the UC process. The color purity estimated from the Commission International de I’Eclairage coordinates confirmed strong green UC emission. The results suggested that the Y2O3:Ho3+,Yb3+ UC transparent films are good candidates for various applications, including solar cell applications.

Photon avalanching nanoparticles (ANPs) exhibit extremely nonlinear upconverted emission valuable for subdiffraction imaging, nanoscale sensing, and optical computing. Avalanching has been demonstrated with Tm3+-, Pr3+-, or Nd3+-doped nanocrystals, but their emission is limited to a few wavelengths and materials. Here, we utilize Gd3+-assisted energy migration to tune the emission wavelengths of Tm3+-sensitized ANPs and generate highly nonlinear emission from Eu3+, Tb3+, Ho3+, and Er3+ ions. The upconversion intensities of these spectrally discrete ANPs scale with nonlinearity factor s = 10-17 under 1064 nm excitation at power densities as low as 7 kW cm-2. This strategy for imprinting avalanche behavior on remote emitters can be extended to fluorophores adjacent to ANPs, as we demonstrate with CdS/CdSe/CdS core/shell/shell quantum dots. ANPs with rationally designed energy transfer networks provide the means to transform conventional linear emitters into a highly nonlinear ones, expanding the use of photon avalanching in biological, chemical, and photonic applications.

[Display omitted] • A novel ZnO/Bi 3 Ti 2 O 8 F:Yb3+, Er3+ Z-scheme heterojunction was synthesized. • The IEF of ZnO/BTOFYE heterojunction improves the carrier separation efficiency. • The IEF of ZnO/BTOFYE heterojunction improves the UC luminescence intensity. • ZnO/BTOFYE showed better photocatalytic degradation efficiency than BTOFYE. Exploring strategies to improve the near-infrared response of photocatalysts is an urgent challenge that can be overcome by utilizing upconversion (UC) luminescence to enhance photocatalysis. This paper reports the fabrication of a ZnO/Bi 3 Ti 2 O 8 F:Yb3+, Er3+ (ZnO/BTOFYE) Z-scheme heterojunction based on a Bi 3 Ti 2 O 8 F:Yb3+, Er3+ (BTOFYE) UC photocatalyst via electrostatic self-assembly. Fermi energy difference at the interface of BTOFYE and ZnO generates a strong internal electric field (IEF) in the Z-scheme heterojunction, offering a novel charge transfer mode that promotes carrier transfer and separation while retaining the strong redox capability. These results are confirmed through in situ X-ray photoelectron spectroscopy, in situ Kelvin probe force microscopy, electron spin resonance, and density functional theory calculations. In addition, the effect of the IEF on the UC luminescence process of Er3+ enhances the luminescence intensity, considerably improving the UC utilization efficiency. The optimal ZnO/BTOFYE degrades 64 % of ciprofloxacin in 120 min, which is 2.3 times more than that degraded by BTOFYE. Overall, the results of this study offer a reference for the rational development of high efficiency UC photocatalysts by generating IEF in Z-scheme heterojunctions. [ABSTRACT FROM AUTHOR]

Fluoride-upconversion transparent ceramic materials doped with Er ions have attracted extensive interest due to their higher stability, lower phonon energy, and higher upconversion efficiency. xEr:Sr 5 (PO 4) 3 F (S-FAP) (x=0.5–5 at%) transparent ceramics have been synthesized by hot pressing sintering in this work, and the maximum optical transmittance of the sample reached 74.52 and 82.45% at 500 and 1000 nm, respectively. The XRD results of the powder and ceramic samples were consistent with the diffraction characteristics of the hexagonal S-FAP crystal structure. The SEM and TEM images of powder revealed that the morphology of the sample was composed of mostly rice grains with a diameter of around 20 nm. The XPS and EDS results confirmed the presence of Er3+ and the uniform distribution of Er elements. The thermally etched surface and cross section of ceramics revealed residual pores as the most important scattering source, and the SEM images of the ceramics show that Er doping can obviously promote the growth of the average grain size of S-FAP transparent ceramics. Furthermore, the upconversion luminescence characteristics and fluorescence lifespan of S-FAP transparent ceramics with varied Er doping concentrations were examined, as well as the upconversion luminescence process. The findings in this work indicate that Er:S-FAP transparent ceramics can be employed as a potential upconversion fluorescent material in display technology, biomedicine, temperature sensing, and other applications. • The xEr:S-FAP (x=0–0.05) transparent nanostructured ceramics with highly optical quality were prepared by hot-pressed sintering for the first time. • The microstructural characteristics, upconversion luminescence property, and mechanism of as-synthesized xEr:S-FAP transparent ceramics were investigated for the first time. • The wide visible light spectrum from red light to green light was achieved by changing the doping amount of Er3+ from 0.5 to 5 at%. [ABSTRACT FROM AUTHOR]

Photocatalytic technology exhibits promising prospects in environmental remediation owing to its sustainable and environmentally friendly advantages. Among the bismuth-rich halide oxides, Bi4O5I2 has demonstrated remarkable efficacy in dye degradation due to its favorable valence band and conduction band positions. In this study, we successfully synthesized Bi4O5I2/NaYF4:Yb,Tm composites through a solvothermal method. When exposed to visible light, the Bi4O5I2/NaYF4:Yb,Tm composite achieved an impressive degradation rate of 86.7% for RhB solution after 60min of light-induced reaction. Moreover, the incorporation of NaYF4:Yb,Tm into Bi4O5I2 extended the utilization of near-infrared (NIR) spectroscopy. Under 980nm NIR light irradiation, the degradation rate of Rhodamine B (RhB) solution by the Bi4O5I2/NaYF4:Yb,Tm composite reached 43.0% after 240min of light reaction. Free radical capture experiments confirmed that h+ and • O2− played a significant role as the primary active species in the degradation process of RhB by the Bi4O5I2/NaYF4:Yb,Tm composites. Furthermore, we explored the mechanism behind the photocatalytic degradation of RhB solution using the Bi4O5I2/NaYF4:Yb,Tm composites. Bi4O5I2/NaYF4:Yb,Tm holds great potential as a promising candidate for utilization of NIR light for photocatalytic reactions. [ABSTRACT FROM AUTHOR]

Circularly polarized luminescence (CPL)‐active materials are increasingly recognized for their potential applications such as 3D imaging, data storage, and optoelectronic devices. Typically, CPL materials have required high‐energy (HE) photons for excitation to emit low‐energy (LE) circularly polarized light, a process known as downshifting CPL (DSCPL). However, the emergence of upconverted CPL (UCCPL), where the absorption of multi LE photons results in the emission of a single HE photon with circular polarization, has recently attracted considerable attention. This minireview highlights the intricate relationship between upconversion and CPL phenomena. During upconversion, the dissymmetry factor (glum) value can be improved in certain systems. Additionally, the integration of both LE and HE photons in upconversion‐downshifting‐synergistic systems offers avenues for dual‐excitation or dual‐emission CPL functionalities. More in detail, the emerging UCCPL based on various photon upconversion mechanisms and their synergy with DSCPL are introduced. Additionally, several examples that demonstrate the applications of UCCPL are presented to highlight the future opportunities. [ABSTRACT FROM AUTHOR]

MicroRNAs (miRNAs) are promising biomarkers especially for early‐stage cancer diagnostics, but the implementation of miRNA‐based diagnostic tests is still hindered by the limitations of current analytical methods. The small size, low concentrations in biofluids and high sequence homology of miRNAs are challenges for assay development. Currently, most of the sensitive detection methods rely on enzymatic amplification steps, which complicate the analysis and can lead to biases in quantitation. Therefore, there is an increasing need to develop enzyme‐free detection methods that are sensitive, specific and user‐friendly. In this study, a simple direct hybridization assay for the DNA analogue of miR‐20a was developed. The assay is based on upconverting nanoparticle labels, which enable ultrasensitive detection, and hairpin structured probes, which provide additional hybridization stability due to base stacking. The limit of detection was 0.73 fM with plasma recoveries between 76 % and 111 %, demonstrating that the assay could be used for direct detection of miRNAs from complex sample matrices without isolation of RNA. Due to the simplicity and the excellent sensitivity for an amplification‐free method, the assay has a great potential for miRNA‐based clinical applications. [ABSTRACT FROM AUTHOR]

In this work, Yb3+ and Er3+-co-doped Sr 9 In(PO 4) 7 upconversion fluorescent materials with dual functionality were synthesized though simple high-temperature solid state method. Based on International Commission on Illumination (CIE) color coordinates, the calculation of color purity at different temperatures demonstrates the attainment of highly pure green luminescence (color purity >90 %) for the synthesized materials. The analysis of activation energy proves that synthesized materials are stable. Utilizing the fluorescence intensity ratio (FIR) of the thermally coupled energy levels (2H 11/2 , 4S 3/2) as a metric for optical sensing performance, absolute sensitivity and relative sensitivity are determined, and their maximum values are 0.44 % and 1.13 %, respectively. Inspired by the variation in CIE coordinates with temperature, thermally responsive two-dimensional barcode image was developed, demonstrating the practical application of Sr 9 In(PO 4) 7 :Yb/Er materials in the field of information security and anti-counterfeiting. [ABSTRACT FROM AUTHOR]

Pressure‐induced performance enhancement of a given material is an emerging phenomenon, however, among the various known examples very few enhanced properties can retain to ambient conditions. Both new structural mechanism and new material systems are the key challenge to achieve recoverable properties via pressure treatment. Herein, an "Er‐rich" erbium oxyfluoride ErOF is reported with retainable enhanced upconversion (UC) emission by high pressure treatment. The UC emission increases to more than twice the intensity of its pristine state, and followed by additional improvement to 5.1‐fold increase by heat treatment at 100 °C. The site symmetry breaking of Er3+ ions along with the crystal phase transition from R3¯$\bar{3}$m to Pnma, revealed by X‐ray diffraction, is believed to be the underlying reasons for the enhanced UC emission after release. Further, high‐resolution transmission electron microscopy (HRTEM) study reveals that the defects relieved with heat treatment might account for the UC emission enhancement in the released sample. Time‐resolved spectra and I‐P curves are measured comparatively to further illustrate the intrinsic mechanism of UC processes. The findings open up a window to advance UC performance through structure optimization by pressure engineering, thus facilitating its potential applications under ambient conditions. [ABSTRACT FROM AUTHOR]

Alkaline zirconium fluorides (AxZryFx+y, A = Li, Na, and K), featuring unique crystallographic structures, have recently emerged as a class of attractive hosts for fabricating lanthanide (Ln3+)‐doped upconversion nanocrystals (UCNCs) that exhibited distinct morphology, upconversion luminescence (UCL) performance, and physicochemical property. In this paper, for the first time the controlled preparation of Yb3+/Er3+‐doped UCNCs is reported based on the trigonal Cs2ZrF6 host, leading to tunable morphology and size of the resulting UCNCs by varying the reaction temperature and time. By further incorporating Na+ ions into the Cs2ZrF6 crystal lattice, sub‐10 nm Yb3+/Er3+/Na+ tridoped UCNCs with highly improved crystallinity and thus greatly enhanced UCL intensity are obtained. Moreover, these resulting UCNCs display abnormal thermal enhancement of UCL over a temperature range from 333 to 493 K, enabling the fabrication of supersensitive luminescent nanothermometers for temperature sensing. Based on the luminescence intensity ratio of two nonthermally coupled levels (i.e., 4F9/2 and 2H11/2) of Er3+, the as‐prepared Cs2ZrF6:Yb/Er/Na UCNCs exhibit an extremely large absolute sensitivity of 177.3% K−1 and a considerably high relative sensitivity of 1.52% K−1 at 333 K. These results unambiguously demonstrate that Cs2ZrF6 is a suitable host material for preparing small‐sized Ln3+‐doped UCNCs as nanothermometer for high‐performance ratiometric temperature sensing. [ABSTRACT FROM AUTHOR]

Upconversion (UC) process involves diverse energy levels and photoluminescence modes, however, it faces the challenge of concentration quenching of the doped lanthanide ions with multiple luminescence centers. Herein, Yb3+/Ho3+ and Yb3+/Tm3+ are heavily codoped into the lead‐free halide double perovskite single crystal Cs2NaErCl6, enabling the controlled energy transfer UC process, and accordingly, the UC materials exhibit anti‐concentration‐quenching owing to the defect tolerance characters. Depending on different doping couples, the UC materials show green and red emissions under the 980 nm laser excitations. Moreover, the green‐to‐red emission ratio in Yb3+/Tm3+ codoped Cs2NaErCl6 changes dynamically with the increasing pumping intensity due to the different sensitivities of the excited energy levels. This study provides a renewed direction of the lanthanide ions heavily‐doped halide double perovskites toward UC materials and facilitates the development of new smart luminescence materials for optical information storage applications. [ABSTRACT FROM AUTHOR]

Photodynamic therapy (PDT) is a minimally invasive treatment for several diseases. It combines light energy with a photosensitizer (PS) to destroy the targeted cells or tissues. A PS itself is a non-toxic substance, but it becomes toxic to the target cells through the activation of light at a specific wavelength. There are some limitations of PDT, although it has been used in clinical studies for a long time. Two-photon excitation (TPE) and upconversion (UC) for PDT have been recently developed. A TPE nanoparticle-based PS combines the advantages of TPE and nanotechnology that has emerged as an attractive therapeutic agent for near-infrared red (NIR) light-excited PDT, whilst UC is also used for the NIR light-triggered drug release, activation of 'caged' imaging, or therapeutic molecules during PDT process for the diagnosis, imaging, and treatment of cancers. Methods: Nine electronic databases were searched, including WanFang Data, PubMed, Science Direct, Scopus, Web of Science, Springer Link, SciFinder, and China National Knowledge Infrastructure (CNKI), without any language constraints. TPE and UCNP were evaluated to determine if they had different effects from PDT on cancers. All eligible studies were analyzed and summarized in this review. Results: TPE-PDT and UCNP-PDT have a high cell or tissue penetration ability through the excitation of NIR light to activate PS molecules. This is much better than the conventional PDT induced by visible or ultraviolet (UV) light. These studies showed a greater PDT efficacy, which was determined by enhanced generation of reactive oxygen species (ROS) and reduced cell viability, as well as inhibited abnormal cell growth for the treatment of cancers. Conclusions: Conventional PDT involves Type I and Type II reactions for the generation of ROS in the treatment of cancer cells, but there are some limitations. Recently, TPE-PDT and UCNP-PDT have been developed to overcome these problems with the help of nanotechnology in in vitro and in vivo studies. [ABSTRACT FROM AUTHOR]

Highly transparent inorganic photochromic materials have several promising applications, especially for anti‐counterfeiting application. Enriching and enhancing the luminescent properties of perovskite by doping transition metals and rare earth elements into the host is a feasible and attractive route. Codoping of Mn2+ and Er3+ can realize novel anti‐counterfeiting properties with upconversion emission and photochromic ability within a double perovskite structure of Cs2AgInCl6.The Cs2AgInCl6: Mn2+, Er3+ single crystal can be observed in a reversible color variation from yellow to dark violet by implementing under 365, 520 nm illumination or thermal treatment. In addition, the crystal emits red and green light under the excitation of 365 nm ultraviolet and 980 nm laser, respectively. The introduction of Mn2+ endowed Cs2AgInCl6 with photochromic ability, and Mn2+ acted as a bridge for energy transfer from host excitons to Er3+, facilitating emission in the visible region. Er3+ achieve upconversion luminescence in the material through its abundant energy levels and acting synergistically with Mn2+. Herein, the Cs2AgInCl6:Mn2+, Er3+ combines various optical properties and has applications for specific anti‐counterfeiting demands. [ABSTRACT FROM AUTHOR]

The ZnS: Ho3+, Yb3+, Mn2+ nanomaterials fabricated by the co-precipitation method have the zinc-blende structure with the crystallite size about 90 ∼ 100 nm. The X-ray diffraction (XRD), the scanning electron microscope (SEM) and the transmission electron microscope (TEM) characterizations confirm that the doping of the Mn2+ has less effect on the structure of the ZnS: Ho3+, Yb3+, Mn2+ nanomaterials. With the excitation of a 978 nm laser, the upconversion (UC) emissions of the ZnS: Ho3+, Yb3+, Mn2+ nanomaterials include two emission bands located at 551 and 664 nm, which correspond to the transitions 5F4 (5S2)→5I8 and 5F5→5I8 of the Ho3+ ions. The UC emission characterization confirms that the UC emission of the ZnS: Ho3+, Yb3+ nanomaterials can be modulated by the doping of the Mn2+ ions, the increase of the incident laser power and the change of the laser pulse-width. The red emission of the ZnS: Ho3+, Yb3+ nanomaterials particle will increase significantly with the incorporation of the Mn2+ ions. The incident laser power and the laser pulse-width also can modulate the intensity ratio G/R of the green to the red. With the increase of the incident laser power, the emission intensity ratio G/R will also increase. With the increase of laser pulse-width from 0.2 to 0.8 ms, the above intensity ratio G/R will decrease significantly. The modulation between the green and red emission is due to the energy transfer from the 5F4 (5S2) state to the 5F5 state of the Ho3+ ions through the state 4T1 of the Mn2+ ions. [ABSTRACT FROM AUTHOR]

A series of BiTa7O19(BTO):Ho3+/Yb3+ phosphors were prepared by high-temperature solid state sintering, which exhibited outstanding pure green upconversion luminescence (UCL) under 980 nm laser excitation. When Ho3+ doping 10 mol%, Yb3+ has almost no concentration quenching point. By optimizing the preparation process, the phase purity of the sample can be improved. Under 980 nm laser excitation power density of 124.99 W/cm2, the UCL intensity of the optimized sample is 1.79 times that of the non-optimized sample. The maximum relative temperature sensitivity SR of 0.384% K−1 at 557 K and 0.40% K−1 at 723 K can be obtained by using luminescence intensity ratio (LIR) technique. All experiments show that BTO is an excellent UCL host, and BTO:Ho3+/Yb3+ can be used in UCL display and temperature sensing. [ABSTRACT FROM AUTHOR]

[Display omitted] Antibacterial photodynamic therapy (aPDT) is highly effective in killing bacteria, while the problem of hypoxia and limited light penetration in deep tissue has not been properly solved. In addition, few aPDT works take into account the regulation of inflammation, which is an important regulatory process after antimicrobial therapy and the final purpose of treatment. In this work, to address the above isssues, we have designed a multi-functional composite UCNPs-Ce6-Mn(CO) 5 Br@Silane (referred to as UCM@Si), which consists of several key components: Up-conversion nanoparticles (UCNPs: NaErF 4 :Tm3+@NaYF 4 :Yb3+), Chlorin e6 (Ce6) and Manganese pentacarbonyl bromide (Mn(CO) 5 Br). When exposed to near-infrared (NIR) light (980 nm), the UCNPs can emit strong red light at 655 nm which further trigger the aPDT of Ce6. The generated reactive oxygen (ROS) subsequently break the metal carbonyl bond of Mn(CO) 5 Br, leading to the production of carbon monoxide (CO) molecules as well as manganese ions (Mn2+), which further decomposes hydrogen peroxide (H 2 O 2) in the microenvironment to oxygen (O 2). Therefore, this simple nanocomposite not only provides substantial self-oxygen replenishment for enhanced aPDT, but also facilitates effective inflammation regulation via CO across a wide range of deep infections. This approach leverages the unique properties of these materials to combat bacterial infections by simultaneously killing bacteria, regulating inflammation, and enhancing the oxygen levels in the affected microenvironment. This O 2 and CO gas based aPDT treatment system offers a promising approach to comprehensively address microbial-induced infectious diseases, particularly deep infections, holding the potential clinical applications. [ABSTRACT FROM AUTHOR]

[Display omitted] Phototherapy has garnered increasing attention as it allows for precise treatment of tumor sites with its accurate spatiotemporal control. In this study, we have successfully synthesized degradable bifunctional phototherapy agents (UCNPs@mSiO 2 @MPN-MC540/DOX) based on upconversion nanoparticle (UCNPs) and metal phenolic network (MPN), serving as a novel nanoplatform for multimodal tumor treatment in the near-infrared (NIR) biological window. To address the issue of low light penetration depth, the UCNPs we synthesized exhibited efficient light conversion ability under 808 nm laser irradiation to activate the photosensitizer Merocyanine 540 (MC540) for photodynamic therapy. Simultaneously, the 808 nm NIR light can also excite the MPN layer to achieve photothermal therapy for tumors. Additionally, the MPN layer possesses the capability of self-degradation under weakly acidic conditions. Within the tumor microenvironment, the MPN layer gradually degrades, facilitating the controlled release of the chemotherapy drug doxorubicin (DOX), thus achieving pH-responsive drug release and reducing the side effects of chemotherapy. This study provides an example of NIR-excited multimodal tumor treatment and pH-responsive drug release, offering a therapy model for precise tumor therapy. [ABSTRACT FROM AUTHOR]

We report upconversion emission multicolor tuning based on mixing two kinds of lanthanide-doped NaYF4 nanoparticles. The upconversion emission can be continuously and precisely modulated by adjusting the ratio of the nanoparticles with Tm3+ and Er3+. The chromaticity coordinates of the mixed solution are calculated by upconversion fluorescence spectra, which presents linear variation under different mixing ratio. Moreover, the corresponding relational equations of chromaticity coordinate and concentration ratio are established. In this way, it is easier to achieve the desired chromaticity and provides a quantificational method to realize chrominance adjustment. [ABSTRACT FROM AUTHOR]

Cyclometalated iridium (Ir) complexes demonstrate impressive capabilities across a range of fields, including biology and photocatalysis, due to their tunable optical characteristics and structure flexibility. However, generating upconversion luminescence of Ir complexes under near‐infrared light excitation is challenging. Herein, by employing lanthanide‐doped upconversion nanoparticles (UCNPs) as the sensitizer, a new strategy is demonstrated to gain upconversion luminescence of Ir complexes via triplet energy transfer. This design relies on a rationally designed hybrid of core‐shell structured NaYbF4:Tb@NaTbF4 UCNPs and new Ir phosphonate complexes, in which UCNPs can migrate upconverted energy to the surface of nanoparticles through Tb3+‐mediated energy migration and then sensitize the upconversion luminescence of Ir complexes upon 980 nm excitation. Both experimental and theoretical investigations highlight the significance of triplet energy transfer from excited Tb3+ ions to the triplet state of Ir complexes in the sensitization of upconversion luminescence of Ir complexes. These findings may open exciting avenues for fabricating hybrid Ir materials with new functions and driving the development of UCNP‐based nanomaterials. [ABSTRACT FROM AUTHOR]

High-phase purity yttrium aluminum garnet (YAG) nanoparticles (NPs), which account for more than half of the market share in solid-state lasers, are crucial luminescent ceramic matrixes used in upconversion luminescence (UCL) applications. As is well known, the optical performance of YAG NPs is greatly influenced by their phase purity, necessitating a long-duration, high-temperature annealing process to eliminate intermediate impurity phases and subsequent inspection of the crystal phases using traditional methods such as XRD. Here, we report a convenient direct method to evaluate the phase state of YAG NPs by correlating the macroscopic UCL properties with the phase evolution process of YAG NPs, given the strong dependence of UCL on the host phase structure. Yb3+/Er3+-codoped YAG NPs were fabricated using flame spray pyrolysis (FSP), an efficient industrial mass production method. The UCL intensity ratios of 556 and 546 nm (I556/I546) for Er3+ ions are used as a key parameter to assess the YAG phase states. When the I556/I546 ratio is larger than 1.80, FSP-prepared YAG:Yb3+/Er3+ NPs undergo a complete phase transformation from amorphous to hexagonal YAlO3 (YAH) and ultimately to cubic YAG, resulting in pure cubic YAG NPs with orange-to-green UCL color modulation. Our results offer an efficient optical crystal-phase probe applied for YAG and can enable the exploration of industrial phase monitoring for other optical materials. [ABSTRACT FROM AUTHOR]

Reversible optical regulation has potential applications in optical anti-counterfeiting, storage, and catalysis. Compared to common power materials, the reverse opal structure has a larger specific surface area and an increased contact area for optical regulation, which is expected to achieve higher regulation rates. However, it is difficult to achieve reversible and repeatable regulation of the luminescent properties of photonic crystals, especially with the current research on the structural collapse of photonic crystals. In this work, WO3: Yb3+, Er3+ inverse photonic crystals were prepared by the template approach, and reversible multi-mode optical modification was investigated. Upon heat treatment in a reducing atmosphere or air, the color of the photonic crystals can reversibly change from light yellow to dark green, accompanied by changes in absorption and upconversion of luminescence intensity. The stability and fatigue resistance of this reversible optical modification ability were explored through cyclic experiments, providing potential practical applications for photocatalysis, optical information storage, and electrochromism. [ABSTRACT FROM AUTHOR]

The triplet annihilator is a critical component for triplet–triplet annihilation upconversion (TTA-UC); both the photophysical properties of the annihilator and the intermolecular orientation have pivotal effects on the overall efficiency of TTA-UC. Herein, we synthesized two supramolecular annihilators A-1 and A-2 by grafting 9,10-diphenylanthracene (DPA) fragments, which have been widely used as triplet annihilators for TTA-UC, on a macrocyclic host—pillar[5]arenes. In A-1, the orientation of the two DPA units was random, while, in A-2, the two DPA units were pushed to a parallel arrangement by intramolecular hydrogen-bonding interactions. The two compounds showed very similar photophysical properties and host–guest binding affinities toward electron-deficient guests, but showed totally different TTA-UC emissions. The UC quantum yield of A-2 could be optimized to 13.7% when an alkyl ammonia chain-attaching sensitizer S-2 was used, while, for A-1, only 5.1% was achieved. Destroying the hydrogen-bonding interactions by adding MeOH to A-2 significantly decreased the UC emissions, demonstrating that the parallel orientations of the two DPA units contributed greatly to the TTA-UC emissions. These results should be beneficial for annihilator designs and provide a new promising strategy for enhancing TTA-UC emissions. [ABSTRACT FROM AUTHOR]

Integrated photonics (IP) is an emerging technology in photonics in which optical waveguides (WGs) and devices are fabricated as an integrated structure onto the surface of a flat substrate. IP technology has the great potential for reshaping the high-speed optical communication, sensing, and imaging exploiting the benefits of miniaturization, low cost, and high efficiency. Erbium-doped waveguide technology has become a popular area of research as it is compatible with CMOS technology. This is largely due to the success of erbium-doped fiber amplifiers (EDFAs) in achieving high gain, high saturation power, and low noise figure (NF) in high-speed long-haul optical fiber links. In this paper, we report the performance enhancement of Er–Yb: co-doped waveguide amplifier (EYCDWA) for C-band (1530–1565 nm) employing backward pumping with a standard 980-nm pump in the presence of energy transfer upconversion (UC). The performance of EYCDWA is evaluated in terms of signal enhancement (relative gain), net internal gain (NIG), and NF using numerical simulations. The length of WG, ion densities of Er 3 + and Yb 3 , and excess losses are adjusted to achieve record high NIG of 11 dB/cm and signal enhancement of 15.5 dB/cm at signal wavelength of 1530 nm. Also, NF values in the range of 4.9–5.1 dB are observed for C-band wavelengths for input signal power of − 15 dBm. [ABSTRACT FROM AUTHOR]

Visible and near-infrared (NIR) upconversion luminescence (UCL) emissions originating from the BaY2F8: Yb3+, Tm3+ systems were investigated under a laser excitation at 980 nm. The BaY2F8:20 mol% Yb3+, x mol% Tm3+ and BaY2F8: y mol% Yb3+, 0.5 mol% Tm3+ phosphors showed prominent UCL at 800 and 810 nm. The optimized doping concentrations of Yb3+ and Tm3+ in the BaY2F8 host matrix were evaluated, their spectroscopic properties were determined, and studies on their temperature-dependent behaviour were carried out. The temperature-sensing properties were studied by generating the fluorescence intensity ratio (FIR) of the UCL peaks originating from the thermally-coupled energy levels of the Tm3+ ions. The Stark sublevels of 1G4 level of Tm3+ ions were utilized to estimate the temperature-sensing abilities of the phosphor. [ABSTRACT FROM AUTHOR]

The upconversion fluorescence thermometry has been recognized as one of the most promising techniques for the temperature measurement and multi-mode fluorescence temperature sensing can improve the accuracy and widen the temperature range. Herein, a fluorescence thermometry was developed based on the MgAlON: Er3+ transparent ceramic. It was demonstrated that the enrichment of Er3+ in grain boundaries increased susceptibility to temperature variations of the cross-relaxation rate, which endowed the non-thermally coupled energy levels with temperature sensing properties and led to the multi-mode thermometric capability. A novel four-mode temperature sensor based on fluorescence intensity ratios (2H 11/2 /4S 3/2 and 4F 9/2 /4S 3/2) and fluorescent lifetimes (4F 9/2 and 4S 3/2) was realized in the temperature range of 180–500 K, which brought about the maximum absolute and relative sensitivities reached 0.0258 K−1 and 2.95% K−1, respectively. This work reveals that the MgAlON: Er3+ and the grain boundary functionalization of transparent ceramics could have potential applications for fluorescence thermometry. • A novel four-mode MgAlON: Er3+ fluorescent temperature sensor was prepared. • The maximum absolute and relative sensitivities of MgAlON: Er3+ reached 0.0258 K−1 and 2.95% K−1, respectively. • The grain boundary enrichment of Er3+ makes the cross-relaxation rate temperature-sensitive, forming a multi-mode fluorescence thermometry. [ABSTRACT FROM AUTHOR]