Persistent Luminescence Intensity Research Articles

Dual co-doping strategy for prolonged luminescence in laser-sintered BaAl2O4:Eu2+, RE3+ ceramics

This work reports on the influence of co-doping on the luminescence and persistent luminescence properties of BaAl2O4 (BaO·Al2O3) laser-sintered ceramics. The samples were doped with Eu and doubly co-doped with different rare earths (Dy, Er, Nd, Pr, and Tm) followed by laser-sintering without an atmospheric control, at a power density of 1.6 W/mm2. All sintered ceramics contained the hexagonal BaAl2O4 phase and had a homogeneous microstructure. A photoluminescence emission band centered at 495 nm was observed, which was due to the 4f65 d1 → 4f7 Eu2+ transition in the BaAl2O4 matrix. The persistent luminescence emission was monitored for 15 min after charging at 345 nm for 3 min. The combination of Nd and Dy co-doping led to substantial improvements in decay time and persistent luminescence intensity compared to individual co-doping. These results suggested that the two co-dopants interacted by modifying the trap configurations in the BaAl2O4 matrix.

High-sensitive detection of H2O2 in biological systems by persistent luminescent nanoprobes

Autofluorescence interference in the conventional fluorescence detection of hydrogen peroxide (H2O2) in biological systems represents a long-lasting problem in the field of biosensing. Here, we report a Cr3+, Y3+ co-doped ZnGaSnO4 (ZGSCY) nanoprobes which show long persistent luminescence (PL) that can be specifically enhanced by H2O2 without background fluorescence interference. The mesoporous organosilica are coated on the surface of ZGSCY to enhance the biocompatibility of the nanoprobes. Importantly, PL intensity of the nanoprobes can be further improved, because of the specific response of tetrasulfide bond in organosilica to H2O2. ZGSCY@MON show high-sensitive detection of H2O2 in living cells and human serum with the advantages of high specificity, low detection limit (91 nM), and wide linear range (0.5–60 μM, 60–400 μM). Furthermore, the extraordinary detection capability of H2O2 enables that the successful differentiate normal breast cells from breast cancer cells and the highly sensitive determination of serum glucose and cholesterol. We also clarified that the endogenous response originates from the UV-induced the interfacial electron transfer on ZGSCY regulated by H2O2 and exogenous response stems from the elimination of the adsorption of PL from the organosilica by H2O2. These findings open up a new effective way for autofluorescence-free detection of H2O2 in biological systems.

Controlling a defect structure of the ZnGa2O4:Cr3+ spinel through synthesis parameters for persistent luminescence optimization

ZnGa2O4:Cr3+ (ZGO:Cr) is one of the most promising red persistent phosphors for in-vivo imaging due to its 650–750 nm emission band in the range of the so-called first biological window. In this paper, we report on how to control the high brightness persistent luminescence of the smallest ZGO:Cr nanoparticles by optimization of hydrothermal synthesis parameters. The influence of pH solution, the temperature of synthesis, and calcination on the degree of inversion, lattice parameters, morphology, and crystallite size were systematically investigated. The results have shown that different synthesis conditions modify the trap depths distribution, by mixing the occupancy of Zn2+ and Ga3+ sites in the spinel structure, which affects the duration and intensity of persistent luminescence. The correlation between structural changes and optical properties is established and it is possible to control them by selecting the appropriate hydrothermal synthesis parameters.

Nonstoichiometric Nanocubes with a Controllable Morphology and Persistent Luminescence for Autofluorescence-Free Biosensing.

Persistent luminescence nanoparticles (PLNPs) have shown special advantages in areas such as bioimaging, cancer therapy, stress sensing, and photo-biocatalysis. However, the lack of methods for controllable synthesis of PLNPs with uniform morphologies and strong persistent luminescence has seriously hindered the applications of PLNPs. Herein, we reported that modifying the electronic structures of zinc gallogermanate (ZGGO) PLNPs by nonstoichiometric reactions can produce highly uniform nanocubes with controllable size and persistent luminescence. By nonstoichiometric increase of the Ge/Ga ratio in ZGGO, the ZGGO PLNPs were transformed from a mixture of nanocubes and small nanospheres into highly symmetrical and uniform large nanocubes, accompanied by the enhancement of persistent luminescence intensity by about 3.7 times. Moreover, we found that ZGGO PLNPs were responsive to reactive oxygen species (ROS), that is, the persistent luminescence of ZGGO can be quenched by ROS. Autofluorescence-free serum ROS detection was achieved with the developed PLNPs. Further, a biosensing assay for glucose oxidase (GOx) was designed based on the responsiveness of ZGGO PLNPs to H2O2. This study may pave a new way for better control of PLNPs' size, morphology, and persistent luminescence, and it can further promote the applications of PLNPs in areas ranging from theranostics to solar energy utilization.

Optimizing crystallinity to enhance X-ray excited persistent luminescence intensity of NaLuF4: Gd/Tb nanoparticles

Lanthanide doped fluoride nanoparticles (NPs) exhibit tunable X-ray excited persistent luminescence (XEPL), which show promising applications in optical imaging, cancer therapy and information storage. Although a few progresses have been achieved in enhancing the XEPL of lanthanide activators, there is no report about the influence of crystallinity on the XEPL intensity. In this work, we verify that the XEPL intensity can be improved by optimizing the NP crystallinity. Our results reveal that better crystallinity benefit the lower non-radiative relaxation possibility but also the restriction of the formation of Frenkel defects. This work provides important information for the development of lanthanide doped fluoride nanoparticles with superior XEPL performances.

Competitive ELISA based on pH-responsive persistent luminescence nanoparticles for autofluorescence-free biosensor determination of ochratoxin A in cereals.

An accurate and sensitive competitive enzyme-linked immunosorbent assay (ELISA) based on persistent luminescence nanoparticles Zn2GeO4:Mn2+, Eu3+ (ZGME) was developed for detecting ochratoxin A (OTA), a powerfully toxic mycotoxin usually found in grains. As a signal output element of autofluorescence-free biosensors, ZGME can be integrated into ELISA with glucose oxidase (GOx)-binding OTA molecules due to its excellent pH-responsive persistent luminescence. In the absence of OTA, the OTA-GOx conjugate was captured by the anti-OTA monoclonal antibody (anti-OTA mAb) pre-coated on the 96-well plate. The results indicate a decrease in the pH value of the solution, which triggered the quenching of ZGME luminescence due to GOx-dependent gluconic acid production. The presence of OTA inhibited the binding of OTA-GOx on the plate, thus decreasing the production of gluconic acid and increasing the persistent luminous intensity of ZGME. Under the optimized concentrations of anti-OTA mAb and OTA-GOx, quantitative determination of OTA was achieved by plotting the increase or decrease in persistent luminescence intensity of ZGME at 535nm. In this study, the linear range was from 0.1μg L-1 to 63μg L-1, and the limit of detection (LOD) was as low as 0.045μg L-1. In five food samples (corn grit, brown rice, soybean, rice, and wheat), the results exhibited good stability and repeatability, with a recovery range from 81.3% to 94.4% and a relative standard deviation (RSD) of less than 4.2%. Hence, the established method provides a sensitive, accurate, and autofluorescence-free approach for the determination of OTA in different grain samples.

Doping and luminescence mechanisms of broadband NIR‐II emitting Zn2(1−x)Ni2xGa3Ge0.75O8 nanoparticles

AbstractIn this study, Zn2(1−x)Ni2xGa3Ge0.75O8 (x = 0.0002, 0.001, 0.002, 0.010, 0.020, and 0.030) nanoparticles with broadband NIR‐II emissions were synthesized by a hydrothermal synthesis combined with a vacuum annealing. For the Ni2+‐doped ZGGO samples (x = 0–0.03), with increasing concentration, the particle shape gradually becomes spherical and the average particle size decreases from 124.4 to 74.2 nm. Meanwhile, for the ZGGO:Ni2+0.01 nanoparticles, the asymmetrically broad emission peak around 1290 nm, which is the superposition of the two peaks locating at 1280 and 1450 nm, can be observed and the afterglow time exceeds 30 min. Based on the spectral data, luminescence decay curves, first‐principles calculations, and Tanabe–Sugano theory, it is found that Ni2+ ions can occupy not only tetrahedral but also octahedral Zn2+ sites (locating in anti‐site defects pair) in the spinel ZGGO host, and they have the contributions to the 1450 and 1280 nm emission peaks, respectively. Furthermore, the surface‐modified ZGGO:Ni2+ nanoparticles exhibited good stability in the H2O and HSA (5% human serum albumin, pH = 7.4) solutions and the occurred agglomeration sinking in the SLS (simulate lysosomal solution, pH = 4.7) solution. Compared to the narrow‐band NIR‐II emitting persistent luminescence nanoparticles (ZGGO:Cr3+,Er3+ and ZGGO:Cr3+,Nd3+), broadband NIR‐II emitting persistent luminescence nanoparticles (ZGGO:Ni2+ NIR‐II) possess stronger persistent luminescence intensity and can effectively avoid the water absorption of biological tissues. Our results suggest that ZGGO:Ni2+ persistent luminescence nanoparticles have a potential to become optical probes for deep‐tissue autofluorescence‐free bioimaging in the biomedical field.

Optimization of persistent luminescence performance of zinc gallogermanates

X-ray diffraction, Fourier transform infrared spectroscopy, differential thermal analysis, and X-ray photoelectron spectroscopy, combined with instrumental analysis, have been employed to identify and characterize the physical differences between Zn3Ga2GeO8 (ZGGO-s, s as in stoichiometric) and Zn3Ga2Ge2O10 (ZGGO-e, e as in excess) solid solutions. The two materials differ in the addition of GeO2 (in the case of the ZGGO-e sample) to the solid solution of ZnGa2O4 and Zn2GeO4. The optimum sintering temperature for these materials is 1000 °C. The photoluminescence spectra comprise a broad feature between 400 and 600 nm which is slightly red-shifted in the case of ZGGO-e. Notably, ZGGO-e exhibits superior performance for both the intensity and duration of persistent luminescence at room temperature, with emission maximum at 515–517 nm. The performance of the chromium-doped sample, with persistent luminescence emission at 697 nm, is also superior for ZGGO-e. The reasons behind such a significant enhancement of persistent luminescence performance for ZGGO-e sample were investigated in this paper. We have found out that long persistent luminescence in both samples is governed by trap depth distributions. In the case of ZGGO-e, the addition of GeO2 promotes the formation of deeper traps changing the shape of the resulting trap distribution and hence improving the room temperature persistent luminescence of this compound.

Persistent visible luminescence of SrF2:Pr3+ for ratiometric thermometry.

Luminescence-based thermometry, especially the ratiometric temperature sensing technology, has attracted considerable attention recently due to its characteristics such as non-contact operating mode and strong capacity of resisting disturbance. Differing from the conventional strategy that usually needs continuous excitation, here an optical thermometry, which we have named the persistent luminescence intensity ratio (PLIR) thermometry, is proposed. The PLIR thermometry relies on the optical material SrF2:Pr3+ that could emit luminescence for several hours and even longer after being charged by X-ray. It has been demonstrated that the PLIR is sensitive to the variation of temperature and complies with the Boltzmann distribution. More importantly, the reliability of the proposed PLIR thermometry is verified. Our work may inspire others to develop more persistent luminescence thermometry.

Boltzmann-distribution-dominated persistent luminescence ratiometric thermometry in NaYF4:Pr3.

A novel, to the best of our knowledge, optical temperature measurement method is proposed, i.e., persistent luminescence intensity ratio (PLIR) thermometry. The PLIR thermometry relies on the micro-sized NaYF4:Pr3+ material that can emit persistent luminescence (PersL) uninterruptedly after being charged by x ray irradiation. The 3P1→3H5 and 3P0→3H5 PersL transitions, locating separately at ∼ 522 and 538 nm, have been confirmed to follow the Boltzmann distribution. The emitting intensity ratio of this pair of PersL lines is thus found to be a good indicator of the variation of temperature. Our work is expected to enrich the optical temperature sensing family.

Persistent luminescence ratiometric thermometry

Over the past twenty years, luminescence intensity ratio (LIR)-based temperature sensing technology has become a research hotspot. Despite the great progress in this field, the main limitation of conventional LIR thermometry is the requirement of continuous exposure to excitation source during measurement that may result in thermal effect and thus disturb the accuracy of temperature readout. In order to solve this problem, a new type of thermometry is proposed here, exploiting NaYF4:Er3+ that exhibits long persistent luminescence (PersL) over almost one day. The LIR between the two green PersL bands associated with the 2H11/2→4I15/2 and 4S3/2→4I15/2 electronic transition of Er3+ ions in NaYF4:Er3+ is demonstrated to be a reliable indicator of temperature, which could effectively avoid the long-term exposure to irradiation source. To clearly emphasize the difference between a regular LIR and this research, the newly proposed strategy here is named as persistent luminescence intensity ratio (PLIR) thermometry. The density functional theory and thermoluminescence measurement indicate the PersL mechanism of NaYF4:Er3+ probably stems from the fluorine vacancy related defects. The proposed PLIR thermometry, different from the traditional LIR counterpart, is believed to open a new road to determine temperature via an interesting and reliable way, which will greatly inspire other researchers in the related fields.

A novel Li+-doped MgLuGaO4 phosphor with stable white-light emission and long persistent luminescence

In recent years, white persistent luminescence materials have received extensive attention due to the potential application of white light in the field of plant growth lighting. The white persistent luminescence materials synthesized by traditional methods either have strict requirements on the concentration of doped ions or cannot produce white light continuously and stably due to different decay rates of various emission centers, which limits their further application. In our work, we successfully synthesized a novel Li+-doped MgLuGaO4 phosphor with stable white-light emission and long persistent luminescence. The broadband emission of MgLuGaO4:Li+ is mainly located at 350–650 nm, which has no change at different decay time, showing stable white-light emission. The persistent luminescence intensity is increased by 3.8 times and the persistent luminescence time is extended from 24 h to more than 36 h by doping Li+ ions. Stable white-light and long persistent luminescence emission make it promising to be used in plant growth lighting.

Designing Next Generation of Persistent Luminescence: Recent Advances in Uniform Persistent Luminescence Nanoparticles.

Persistent luminescence is a unique optical process where long-lasting afterglow persists after the cessation of excitation. Nanoscale persistent luminescent materials are getting increased research interest from various fields due to their unique optical property. In recent years, inspiring achievements have been made to produce uniform persistent luminescence nanoparticles (PLNPs) in a controllable manner, unleashing their fascinating potential, surpassing other types of luminescent materials in a wide variety of application such as high-contrast bioimaging and high-resolution X-ray detection. In this review, the evolution of uniform PLNPs, from their bulk phosphor counterparts, to the "top-down" preparation of nanoscale persistent luminescent materials, to the recent "bottom-up" synthesis of uniform PLNPs is first summarized. The respective milestones of uniform PLNPs prepared by templated synthesis, aqueous synthesis, and colloidal synthesis are highlighted. The key optical properties that can be enhanced in uniform PLNPs, including increasing the persistent luminescence intensity, tuning the excitation irradiance, as well as the emission wavelengths are then analyzed. Detailed strategies to enhance each optical property are also discussed in various sections. Finally, future challenges are highlighted with respect to the perspectives on the development of next-generation PLNPs with novel applications.

Blue Persistent Phosphor of YSiO2N:Ce3+ Developed by Codoping Sm3+ or Tm3+ Ions and Thermoluminescence Analysis of Their Trap Distributions

By estimating the trap depth from the relative position of the zigzag curve of divalent lanthanoid ions and conduction band bottom, the YSiO2N:Ce3+‐based blue persistent phosphors are developed, and their persistent luminescence and thermoluminescence properties are characterized to identify the electron trap distributions. Taking the nephelauxetic effect caused by covalent nitrogen into account for the vacuum referred binding energy diagram, it is predicted that Tm3+ and Sm3+ ions with the trap depth of 0.726 and 1.04 eV are an optimal codopant to obtain Ce3+ persistent luminescence. The prepared YSiO2N:Ce3+‐Ln3+ (Ln = Sm or Tm) samples enhance the Ce3+: 5d → 4f blue persistent luminescence intensity. In the thermoluminescence glow curves, the Ce3+ singly doped sample shows two glow peaks related to intrinsic defects; on the other hand, the Ln3+ codoped samples show an additional glow peak related to the Ln3+ traps. The thermoluminescence glow curve analyses through the initial rise method combined with the thermal cleaning and trap density methods reveal that the electron traps of Ln3+‐related defects have a distribution with ≈0.25 eV width and depths of 0.802 eV (Tm3+) and 1.10 eV (Sm3+).

How Many Electron Traps are formed in Persistent Phosphors?

Persistent luminescence is caused by a charge carrier detrapping process from the carrier traps filled by excitation light. Thus, the carrier storage capacity is an important factor in determining the persistent luminescence intensity. Here, the electron storage capacity was investigated in the YAGG:Ce3+-Yb3+ transparent ceramic persistent phosphor, in which the Ce3+ ion is the luminescence center and the Yb3+ ion acts as the electron trap. The number density of the Yb2+ electron trapping center was estimated to be approximately 1.6 × 1018 ions cm−3 from the absorption coefficient spectrum of Yb2+:4f–5d photochromic absorption center and the Yb-LIII edge XANES spectrum, which means that approximately 12% of the Yb3+ ions in the sample were changed to the divalent state after charging. Although the maximum energy density of 0.61 J cm−3 was calculated as a storage property of persistent phosphors from the Yb2+ number density and the photon energy of Ce3+:5d1−4f luminescence at 520 nm, the actual energy density which was detected as persistent luminescence was 0.011 J cm−3. It is suggested that the recombination efficiency of the detrapped electrons from the Yb2+ ions with the hole-trapped Ce3+ ions is approximately a few percent.

Molten-Salt-Assisted Annealing for Making Colloidal ZnGa2 O4 :Cr Nanocrystals with High Persistent Luminescence.

Persistent luminescent nanocrystals (PLNCs) in the sub-10 nm domain are considered to be the most fascinating inventions in lighting technology owing to their excellent performance in anti-counterfeiting, luminous paints, bioimaging, security applications, etc. Further improvement of persistent luminescence (PersL) intensity and lifetime is needed to achieve the desired success of PLNCs while keeping the uniform sub-10 nm size. In this work, the concept of molten salt confinement to thermally anneal as-synthesized ZnGa2 O4 :Cr3+ (ZGOC) colloidal NCs (CNCs) in a molten salt medium at 650 °C is introduced. This method led to significantly monodispersed and few agglomerated NCs with a much improved photoluminescence (PL) and PersL intensity without much growth in the size of the pristine CNCs. Other strategies such as i) thermal annealing, ii) overcoating, and iii) the core-shell strategy have also been tried to improve PL and PersL but did not improve them simultaneously. Moreover, directly annealing the CNCs in air without the assistance of molten salt could significantly improve both PL and PersL but led to particle heterogeneity and aggregation, which are highly unsuitable for in vivo imaging. We believe this work provides a novel strategy to design PLNCs with high PL intensity and long PersL duration without losing their nanostructural characteristics, water dispersibility and biocompatibility.

Time-dependent photoluminescence patterns based on Cr3+-doped and co-doped Zn3Ga2Ge2O10

In this paper, we pursued developing a kind of novel time-dependent patterns based on persistent luminescence (PersL). We prepared a series of Cr3+-doped PersL: ZGGO: m%Cr3+ (m = 0.5, 1, 2, 5); and co-doped PersL: ZGGO: 1%Cr3+, n%Yb3+, ZGGO: 1%Cr3+, n%Eu3+, (n = 0.5, 1, 2, 5). All the Cr3+-doped and co-doped ZGGOs possessed high-performance persistent luminescence property, which exhibited a 723 nm peaking luminescence and persistent luminescence under UV irradiation. The ZGGOs appeared similar in emission wavelength and color, except persistent luminescence intensity decreased for different degrees. Based on the properties of the materials, we produced the photoluminescence patterns which exhibit a time-dependent factor. Those were a flower-like pattern, a Chinese “Taiji” pattern, and a “solar eclipse” pattern. This strategy could provide a future use for information storage, security signs, and anti-counterfeiting.

White persistent luminescence of La2Hf2O7:Ti,Pr

Abstract White-light persistent luminescence is of practical interest but is not easy to generate, especially over a longer time. In this paper we present the possibility to generate a white-light persistent emission over a few hours by the doubly-doped La2Hf2O7:Ti,Pr phosphor. Its relevant thermoluminescent properties are presented together with adequate photoluminescence characteristics. The singly-activated versions (with Ti or Pr) of the La2Hf2O7 host were also fabricated and characterized for comparison and better understanding the properties of La2Hf2O7:Ti,Pr. Thermoluminescence of all those phosphors showed strong activator-concentration dependence. When the activator contents exceeded 0.1mol% the thermoluminescence intensity lessened quickly and the persistent luminescence intensity and duration got reduced adequately. In La2Hf2O7:0.1%Ti,0.1%Pr both activators contributed to strong thermoluminescence peaking at around 115 °C and resulting from traps showing some distribution of depths. After charging, these traps served as reservoir of energy for white persistent luminescence generated by both Ti and Pr activators and lasting a few hours.

Enhancement of persistent luminescence in Ca2SnO4: Sm3+

Reddish-orange persistent luminescent Ca2SnO4: 0.1% Sm3+ co-doped with 0–10% Gd3+ was prepared using solid state synthesis. The phase formation and morphology was analyzed using X-ray diffraction and scanning electron microscopy. Incorporation of Sm3+ and Gd3+ in Ca2SnO4 lattice was investigated using site-selective spectroscopy and electron paramagnetic resonance. In all samples reddish-orange persistent luminescence and photochromic effect was observed after excitation with UV as well as X-rays. The thermostimulated luminescence curves indicated that the introduction of Gd3+ increases the number of shallow traps of charges which enhance the intensity of persistent luminescence. In samples with high Gd3+ content (5–10%) an additional luminescence band in the near-infrared spectral region was detected. The origin of this band was discussed.

Laser sintering and influence of the Dy concentration on BaAl2O4:Eu2+, Dy3+ persistent luminescence ceramics

This work reports the effect of the Dy concentration on the persistence luminescence properties of Eu doped barium aluminate (BaAl2O4) laser-sintered ceramics. For this study, the ceramics were first sintered using the laser sintering technique, based on a CO2 laser as the heating source, in an ambient atmosphere. The structural and morphology characteristics of the samples were investigated by x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, respectively. The laser-sintered ceramics presented are shown to be phase pure (single phased), and, for highest Dy concentration sample, a spurious Dy-rich phase at the grain boundary was observed. All samples exhibit the characteristic blue-green emission from the Eu2+ ion, due to the 4f65d1–4f7 transition (495 nm), even when they have been sintered in air. Finally, a clear dependence of the persistent luminescence intensity and decay time with the Dy concentration was verified.