Low Luminescence Research Articles

Improved Quantum Yield and Excellent Luminescence Stability of Europium‐Incorporated Polymeric Hydrogen‐Bonded Heptazine Frameworks Due to an Efficient Hydrogen‐Bonding Effect

AbstractTwo of the most persistent challenges for the high‐end application of luminescent lanthanide (Ln) compounds are a low quantum yield and luminescence quenching caused by a liquid medium. In this work, a type of polymeric hydrogen‐bonded heptazine framework is developed incorporating trivalent europium ions (P‐HHF‐Eu) via a low‐cost and facile low‐temperature thermal condensation reaction. Structural characterization clearly reveals that the solid‐phase pyrolyzation reaction results in the formation of P‐HHF‐Eu. Using time‐resolved and steady state photoluminescence (PL) spectroscopies, the photophysics and photochemistry of P‐HHF‐Eu at different hydration degrees are investigated and the role of hydrogen bonding in the significant enhancement of the emission properties is demonstrated. Furthermore, the P‐HHF‐Eu particles suspended in polyvinyl alcohol hydrogel exhibit excellent luminescence stability with a high PL quantum yield of up to ≈46% and wavelength responsive color‐tunable emission, which holds potential for security applications.

Spatiotemporal variations and implications of luminescence sensitivity of quartz grains on the Chinese Loess Plateau since the last interglaciation

AbstractAlthough luminescence sensitivity of quartz grains of desert sands has been used in discriminating provenance, it still remains unclear about its spatiotemporal variations and climatic implications. In this paper, the luminescence sensitivity of quartz grains from the northern margin of the Chinese Loess Plateau (CLP) was studied using single-aliquot optically stimulated luminescence (OSL) and “pseudo” single-grain OSL measurements. Our results indicate that the OSL sensitivities have lower values in sand/loess beds and higher values in paleosols. We suggest that the variations in OSL sensitivity of quartz grains with depth on the CLP are mainly influenced by the origin of the quartz grains as they are related to the loess-sized material production processes and the migration of desert regions. More quartz grains of glacial origin with lower luminescence sensitivity, together with the reduced durations of irradiation and exposure cycles induced by shorter transport distance due to desert expansion, account for the lower luminescence sensitivity of glacial periods. Moreover, both the mountain processes and the retreat–advance of deserts are ultimately related to climatic changes, therefore, the orbital scale variations of luminescence sensitivity are controlled by glacial–interglacial oscillations on the CLP.

Lanthanide-Doped Near-Infrared Nanoparticles for Biophotonics.

Light in the near-infrared (NIR) spectral region is increasingly utilized in bioapplications, providing deeper penetration in biological tissues owing to the lower absorption and scattering in comparison with light in the visible range. Lanthanide-doped luminescent nanoparticles with excitation and/or emission in the NIR range have recently attracted tremendous attention as one of the prime candidates for noninvasive biological applications due to their unique optical properties, such as large Stokes shift, spectrally sharp luminescence emissions, long luminescence lifetimes, and excellent photostability. Herein, recent advances of lanthanide-doped nanoparticles with NIR upconversion or downshifting luminescence and their uses in cutting-edge biophotonic applications are presented. A set of efficient strategies for overcoming the fundamental limit of low luminescence brightness of lanthanide-doped nanoparticles is introduced. An in-depth literature review of their state-of-art biophotonics applications is also included, showing their superiority for high-resolution imaging, single-nanoparticle-level detection, and efficacy for tissue-penetrating diagnostics and therapeutics.

Mechanism of magnetic field-modulated luminescence from lanthanide ions in inorganic crystal: a review

The luminescence from lanthanide ions has potential applications in light emitting diodes, biomedical, solar cells, sensors, display, etc. However, the luminescence is suffered from the various problems, such as low luminescence efficiency and inharmonious wavelength for energy transfer. Magnetic field is an efficient method to modulate the wavelength and intensity of luminescence from lanthanide ions. Magnetic field redistributes the populated electrons in the excited states to tune the wavelength of lanthanide ions by Zeeman effect, mixing effect, and quantum confinement effect. Magnetic field enhances or suppresses the luminescence intensity by the administration of cross-relaxation, energy transfer, and Boltzmann population. In this review, we first introduce the various phenomena and mechanisms of magnetic field modulated downshift luminescence from lanthanide ions, including Zeeman effect, cross-relaxation, crystal structure, absorption, quantum confinement effect, and magnetic–optical hysteresis. Then, we explain the regulation of upconversion luminescence by magnetic field, containing energy transfer and mixing effect. Finally, different options regarding how to understand the mechanism of magnetic field-modulated luminescence from lanthanide ions in the future are outlined.

Ultrabright bimetallic AuAg complex: From luminescence mechanism to biological application

Metal clusters have attracted wide interests due to their unique electronic and optical properties, but the low luminescence quantum yield (QY) prevents them from potential biomedical applications. In this work, silver-doped Au nanoclusters (NCs) are shown to be able to improve the QY of metal clusters. We succeeded in synthesizing ultrabright glutathione (GSH) protected AuAg clusters with 10.8% QY by a one-pot route. Their florescence is about 7.5 times brighter than pure Au NCs, with super photostability and good biocompatibility in physiological environment. Based on density functional theory (DFT) calculations, we investigated the electronic structures and optical properties of the AuAg NCs. The results show that the increase of the density of states of the lowest unoccupied molecular orbital (LUMO) leads to the fluorescence enhancement. In addition, two-photon excitation fluorescence imaging has been performed to show their great potential for biomedicine.

Luminescence Activity Decreases When v-coelenterazine Replaces Coelenterazine in Calcium-Regulated Photoprotein-A Theoretical and Experimental Study.

Calcium-regulated photoproteins are found in at least five phyla of organisms. The light emitted by those photoproteins can be tuned by mutating the photoprotein and/or by modifying the substrate coelenterazine (CTZ). Thirty years ago, Shimomura observed that the luminescence activity of aequorin was dramatically reduced when the substrate CTZ was replaced by its analog v-CTZ. The latter is formed by adding a phenyl ring to the π-conjugated moiety of CTZ. The decrease in luminescence activity has not been understood until now. In this paper, through combined quantum mechanics and molecular mechanics calculations as well as molecular dynamics simulations, we discovered the reason for this observation. Modification of the substrate changes the conformation of nearby aromatic residues and enhances the π-π stacking interactions between the conjugated moiety of v-CTZ and the residues, which weakens the charge transfer to form light emitter and leads to a lower luminescence activity. The microenvironments of CTZ in obelin and in aequorin are very similar, so we predicted that the luminescence activity of obelin will also dramatically decrease when CTZ is replaced by v-CTZ. This prediction has received strong evidence from currently theoretical calculations and has been verified by experiments.

Development of a highly sensitive microplate luminometer using ATP bioluminescence.

Analytical techniques using ATP bioluminescence, which has a high quantum yield and substrate specificity, are widely used in various assays, such as luciferase reporter assays, in the biological sciences. Although most microplate luminometers can be used to measure ATP luminescence with 96-well or 384-well microplates, their ATP detection limits are typically several tens of amol, which is not sufficient for evaluating cell activities and variability within small samples, such as those containing only a few cells. To analyze cell activities at low ATP concentrations, a more sensitive microplate luminometer is required. Therefore, in this study, we developed an automated highly sensitive microplate luminometer that could perform reagent dispensing and bioluminescence measurement with a 96-well microplate within 10 min. ATP bioluminescence was detected by pressing a photomultiplier tube (PMT) against a microplate surface to seal the measured well with the light-receiving surface of the PMT. This enabled a high light collection efficiency and low luminescence crosstalk, defined as the intensity of stray light from an adjacent well. As a result, the ATP detection limit was 0.97 amol, and the luminescence crosstalk was 4.4 × 10-6 . Both values were one order of magnitude better than that of a typical microplate luminometer. In addition, the same gradient linearity of luminescence intensity against the ATP concentration was confirmed for both high and low ATP concentrations, and the dynamic range of our microplate luminometer was 106 . Overall, our findings demonstrated that our novel microplate luminometer may have wide application in biological sciences research.

Polarization-Sensitive Surface-Enhanced In Situ Photoluminescence Spectroscopy of S. aureus Bacteria on Gold Nanospikes.

We report the possibility of a time-resolved bacterial live/dead dynamics observation with the use of plasmonic nanospikes. Sharp nanospikes, fabricated on a 500-nm thick gold film by laser ablation with the use of 1030-nm femtosecond pulses, were tested as potential elements for antibacterial surfaces and plasmonic luminescence sensors. Staphylococcus aureus bacteria were stained by a live/dead viability kit, with the dead microorganisms acquiring the red colour, caused by the penetration of the luminescent dye propidium iodide through the damaged cell membrane. Photoluminescence was pumped by 515-nm femtosecond laser pulses with linear (Gaussian beam), circular, azimuthal and radial (Laguerre–Gaussian beam) polarizations, exciting the transverse plasmon resonance of the nanospikes and their apex lightning-rod near-field. According to the numerical electrodynamic modeling, the observed strong increase in the photoluminescence yield for radial polarization, while slightly lower for circular and azimuthal polarizations, compared with the low luminescence intensities for the linear laser polarization, was related to their different laser–nanospike coupling efficiencies.

Luminescent properties of biohybrid (kaolinite-proline) materials synthesized by a new boric acid catalyzed route and complexed to Eu3+

The properties of Eu3+-biohybrid (kaolinite-proline) materials were investigated by powder X-ray diffraction, vibrational spectroscopy, scanning electron microscopy, nitrogen adsorption and photoluminescence to understand how Eu3+ interacts with the biohybrid matrices. Biohybrids were obtained through the conventional route and by two new catalytic routes for functionalization of clay minerals, and were then complexed to Eu3+ ions at a constant proline:Eu3+ molar ratio of 3:1, while the water molecules originally existing in the first coordination sphere of Eu3+ ions in the kaolinite-grafted complexes were replaced by 2-thenoyltrifluoroacetone (tta). The typical Eu3+ emission spectra for solids containing tta revealed the characteristic Eu3+ transitions from fundamental 5D0 state to excited 7FJ (J = 0, 1, 2, 3 and 4) states. The lifetime measurements also confirmed that water molecules were exchanged, increasing emission efficiency. The time-resolved spectra allowed to remove the matrix and ligand emissions and thus to evaluate their influence on the emission of the lanthanide ion. After incorporation of Eu3+, the thermal stability of the solids improved. The intercalated europium complexes did not show the same ligand/Eu3+ molar ratio as the free complexes, due to the restricted mobility of the ligands grafted within the interlayer spaces of kaolinite. On the contrary, these complexes showed high internal quantum yields, low luminescence suppression (after tta coordination), preserved the kaolinite layered structure. They also showed a low water molecules coordination number, which made them very attractive for luminescent applications, in addition to bringing high added value, due to a low cost, non-toxicity, non-polluting and high efficiency.

Optimized Multimetal Sensitized Phosphor for Enhanced Red Up-Conversion Luminescence by Machine Learning.

In this research, machine learning including the genetic algorithm (GA) and support vector machine (SVM) algorithm is used to solve the "low up-conversion luminescence (UCL) intensity" problem in order to find the optimal phosphor with enhanced red UCL emission using multielement K/Li/Mn metal modulation. Compared with the first generation of phosphors, the best phosphors' fluorescence intensity occurs in the third generation optimized by the GA, with a stronger brightness (4.91-fold), a higher relative quantum yield (6.40-fold), and an enhanced tissue penetration depth (by 5 mm). The single and multiple dopants effect on the upconversion intensity of K+Li+Mn sensitizers is also studied: the intensity increases first and then decreases with the increase of Yb/Er/K+Li+Mn content, and the optimized K+Li+Mn concentration is 6.03%. In order to confirm the stability of the brightness optimization by the GA, a batch of phosphors was synthesized with the same element proportion, and the similarity of fluorescence intensity of two batches of phosphors was evaluated by the SVM algorithm with the classification accuracy index. Finally, the optimized phosphor was used for bioimaging and phosphor-LED.

Enhancement of luminescence properties of SrAl2Si2O8: Eu3+ red phosphor

Aluminosilicate phosphors have been widely used in the fields of fluorescence and laser technology due to its excellent optical properties. However, red phosphors have low luminescence efficiency, and poor physicochemical stability, which cannot achieve efficient white light emission. Therefore, improving the luminescence performance of aluminosilicate red phosphors is a research hotspot in white LED. In this work, SrAl2Si2O8: Eu3+ red phosphor was synthesized by high temperature solid-phase method. By introducing charge compensator and flux, the luminescence intensity of the phosphor is increased. The best charge compensator is determined as Li+ with the same Eu3+ doping content. The best flux is H3BO3, and the optimal doping concentration of flux is 3 wt%. For SrAl2Si2O8: Eu3+, Li+, H3BO3 red phosphor, the CIE coordinates are (0.619, 0.380) and CCT is 3417 K, which is very close to the standard red light. Through LED packaging mixing phosphor with YAG: Ce3+, the method can significantly improve the color rendering index of WLED, up to 84.7. When the temperature reaches 150 °C, the luminescence intensity decreases to 88.02% of the initial value, which indicates that the SrAl2Si2O8 phosphor is an excellent stable red luminescent material.

Facile synthesis and enhanced luminescence properties of MoO3−x/YF3:Eu3+ composites

At present, the main bottleneck that restricts the application of rare earth luminescent materials is the lower luminescence intensity of materials. Based on the unique local surface plasmon resonance (LSPR) of MoO3 − x, we developed new MoO3 − x / YF3 : Eu3 + composites with strong luminescence. The composites possessed stronger luminescent intensity than individually dispersed YF3 : Eu3 + , exhibiting the MoO3 − x / YF3 : Eu3 + composites, which could be used as highly efficient imaging materials. It is proposed that the ultraviolet-excited LSPR of MoO3 − x is the primary reason for the enhanced luminescence of YF3 : Eu3 + . Meanwhile, the plasmon-enhanced luminescence can be partly absorbed by the MoO3 − x particles to re-excite its higher energy electrons, thus leading to the selective boost luminescence intensity of MoO3 − x / YF3 : Eu3 + composites. We give insight on the development of plasmon-sensitized luminescence materials.

An Enantiopure Propeller-Like Trityl-Brominated Radical: Bringing Together a High Racemization Barrier and an Efficient Circularly Polarized Luminescent Magnetic Emitter.

A new persistent organic free radical has been synthetized with Br atoms occupying the ortho- and para-positions of a trityl core. After the isolation of its two propeller-like atropisomers, Plus (P) and minus (M), their absolute configurations were assigned by a combination of theoretical and experimental data. Remarkably, no hints of racemization were observed up to 60 °C for more than two hours, due to the higher steric hindrance imposed by the bulky Br atoms. Therefore, when compared to its chlorinated homologue (t1/2 =18 s at 60 °C), an outstanding stability against racemization was achieved. A circularly polarized luminescence (CPL) response of both enantiomers was detected. This free radical shows a satisfactory luminescent dissymmetry factor (|glum (592 nm)|≈0.7×10-3 ) despite its pure organic nature and low luminescence quantum yield (LQY). Improved organic magnetic CPL emitters derived from the reported structure can be envisaged thanks to the wide possibilities that Br atoms at para-positions offer for further functionalization.

Synthesis and Inkjet Printing of NaYF₄:Ln3+@NaYF₄ Core-Shell Nanoparticles with Enhanced Upconversion Fluorescence for Anti-Counterfeiting Applications.

Recently, lanthanide-doped upconversion luminescent materials have showed great potential in optical data storage, information encryption and anti-counterfeiting. However, the low upconversion luminescence still limited their applications. In this work, we fabricated RGB NaYF₄:Ln3+@NaYF₄ core-shell nanoparticles (CSNPs) with enhanced upconversion luminescence by coating an inert NaYF₄ shell onto NaYF₄:Ln3+ core nanoparticles via thermal decomposition method. The effect of increased shell temperature and addition of shell precursors on crystal phase, morphology and luminescent property of the synthesized CSNPs were systematically investigated. It was demonstrated that high shell growing temperature facilitated the formation of pure β-NaYF₄ CSNPs. Upon increasing amount of shell precursors, the morphologies of hexagonal phase NaYF₄ CSNPs changed from nanorod to nanocube and showed different luminescent properties. Pure hexagonal phase NaYF₄ CSNPs with highest upconversion luminescence of about 15 times higher than NaYF₄:Ln3+ core nanoparticles can be prepared at 310 °C with the addition of shell precursors at 3 mmol. Moreover, three-primary-color (RGB) CSNPs with enhanced upconversion luminescence were successfully prepared by changing the doping pair of lanthanide ions in core. The synthesized RGB CSNPs were fabricated into environment friendly luminescent ink by sequential surface modification by PAA ligand and dispersing in mixture solvent of ethanol, water and glycerol. Comparative results showed that the fluorescence enhanced RGB CSNPs inks were more suitable for inkjet printing of multicolored, complex and high resolution luminescent anti-counterfeiting patterns on paper substrates.

Removing the Obstacle of Dye‐Sensitized Upconversion Luminescence in Aqueous Phase to Achieve High‐Contrast Deep Imaging In Vivo

AbstractAlthough upconversion nanoparticles (UCNPs) have drawn increasing attention for their unique photophysical characteristics, they suffer from a bottleneck of low luminescence efficiency due to the poor absorption coefficient of Ln3+. Dye sensitization has provided thousands‐fold enhancement of upconversion luminescence (UCL) in organic solvents because of the remarkably improved light absorption ability, but the sensitization of UCL in aqueous phase is only less than 20 folds by far, with unknown restrictive factors. Herein, the aggregation‐caused quenching (ACQ) of dyes is revealed as the most important reason limiting dye sensitization in aqueous phase, and the problem is circumvented through delicately modulating the physical properties of dyes and their assembly manner with UCNPs. By further alleviating energy back transfer (EBT) from Ln3+ to dyes, more than 600‐fold enhancement of UCL is achieved in aqueous phase. The as‐obtained dyes modified UCNPs show good biocompatibility and high signal contrast when applied for deep in vivo imaging.

Phonon coupling and shallow defects in CuInS2

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_{2}$ is an emerging material for solar cells, reaching already efficiencies above 15%. For any solar cell material, it is essential to understand the recombination processes and doping behavior of its defects. Therefore, in this work, we present a detailed photoluminescence and admittance investigation of polycrystalline $\mathrm{CuIn}{\mathrm{S}}_{2}$ thin films to study the shallow intrinsic defects. We find evidence for two acceptors, 105 and 145 meV from the valence band, their predominance depending on the Cu excess, plus one donor, 30 meV from the conduction band. The high crystalline and electronic quality of our samples allows the detection of low intensity luminescence peaks. Based on these emissions we can analyze the phonon coupling of the observed defects and show that an emission previously attributed to a second donor is in fact a phonon replica of the first donor-acceptor transition.

ZnCl2 Enabled Synthesis of Highly Crystalline and Emissive Carbon Dots with Exceptional Capability to Generate O2⋅–

Summary Highly crystalline and emissive carbon dots (CDs) are firstly synthesized via an ionothermal method in organic solvents. In contrast to well-established approaches, ZnCl2 is utilized as the pyrolysis-promoting agent to prepare emissive CDs with tunable wavelengths, directly by carbonization from different O and N precursors under a mild reaction condition at 210°C. More than 50% synthetic yield and 39% photoluminescent quantum yield (blue CD) are obtained. XAFS measurements, in conjunction with TEM, HAADF-STEM, UPS, and ESR analyses, allow us to obtain energy diagram configuration and confirm the luminescent nature of these hard-core structured CDs. Importantly, the resulting CDs exhibit excellent capability to photogenerate superoxide radical anion (O2⋅–), which can be exemplified by both photodynamic therapy and photooxidative cyclization synthesis of tetrahydroquinolines. Representative pharmaceutical mifepristone can be derivatized in 90% yield within 10 min of irradiation of the CDs in an elegant flow reactor.

Sensitized Yb3+ Luminescence in CsPbCl3 Film for Highly Efficient Near-Infrared Light-Emitting Diodes.

Near‐infrared (NIR) light emitting diodes (LEDs) with the emission wavelength over 900 nm are useful in a wide range of optical applications. Narrow bandgap NIR emitters have been widely investigated using organic compounds and colloidal quantum dots. However, intrinsically low charge mobility and luminescence efficiency of these materials limit improvement of the external quantum efficiency (EQE) of NIR LEDs, which is far from practical applications. Herein, a highly efficient NIR LED is demonstrated, which is based on an energy transfer from wide bandgap all inorganic perovskite (CsPbCl3) to ytterbium ions (Yb3+) as an NIR emitter doped in the perovskite crystalline film. High mobility of electrically excited carriers in the perovskite crystalline film provides a long carrier diffusion and enhances radiative recombination of an emission center due to minimized charge trapping losses, resulting in high EQE value in LEDs. The NIR emission of Yb3+ at around 1000 nm is found to be sensitized by CsPbCl3 thin film with a photoluminescence quantum yield over 60%. The LED based on Yb3+‐doped CsPbCl3 film exhibits a high EQE of 5.9% with a peak wavelength of 984 nm, achieved by high carrier transporting ability and effective sensitized emission property in the solid‐film structure.

Morphological and compositional study of quartz and cassiterite of the Mocambo Granite, South Pará Tin Province, Amazonian Craton

Quartz and cassiterite crystals of granite, greisens, and quartz veins from Mocambo Granite, Carajás Province, were studied by scanning electron microscopy — cathodoluminescence and electron microprobe. Five types of quartz were identified: Qz1, magmatic and Qz2, Qz3, Qz4, and Qz5 hydrothermal. Qz1, anhedric to subhedric, luminescent and fractured at different intensities, predominating in less evolved rocks. Qz2, with low luminescence, is younger than Qz1; it occurs as irregular patches, filling fractures or forming veins that cut Qz1. Qz3, non-luminescent, resulting from continuous alteration, dissolution, and recrystallization processes; fills fractures that cut Qz1 and Qz2. Qz4 occurs in the most evolved and hydrothermalized rocks and in greisens bodies mineralized in cassiterite; is euhedral to subhedral, exhibits well-defined zoning, and has been interpreted as later than the previous one. Qz5 has poor luminescence; occurs in mineralized veins with wolframite and, secondarily, cassiterite. Qz1 shows Ti enrichment, while Qz4 associated with cassiterite is enriched with Al. Well-formed cassiterite crystals exhibit concentric zoning and low Fe, Ti, W, Nb and Mn contents. Qz5-associated cassiterite crystals form inclusions in the wolframite, suggesting that cassiterite and wolframite precipitated from hydrothermal processes at different times.

Electronic structure engineering and biomedical applications of low energy-excited persistent luminescence nanoparticles.

Persistent luminescence nanoparticles (PLNPs) are new luminescent materials that can store the excitation energy quickly and persistently emit it after ceasing excitation sources. Due to the advantages of long-lasting luminescence without constant excitation, PLNPs have been widely used in biomedical applications. Visible light excitable PLNPs (VPLNPs) and near-infrared excitable PLNPs (NPLNPs) are two kinds of novel and promising PLNPs. Compared to conventional PLNPs, VPLNPs and NPLNPs have the characteristics of low tissue damage, deep tissue penetration, and high signal-to-noise ratio. With these special features, they have great potential in applications such as long-term tracing, deep-tissue bioimaging, and precise treatment. In this review, we introduce the common strategy of constructing VPLNPs and NPLNPs based on electronic structure engineering and the applications of VPLNPs and NPLNPs in biomedicine. This review article aims to offer valuable information about the progress and development direction of VPLNPs and NPLNPs, promoting more applications in biomedicine, materials science, energy engineering, and environmental technologies in the future.