Upconversion Luminescence Emission Research Articles

Upconversion Luminescence of Tb3+ and Yb3+ Co-doped Hexagonal and Monoclinic GdPO4 Nanophosphors

Tb3+ and Yb3+ codoped hexagonal GdPO4 nanorods have been prepared by a hydrothermal method at 220°C for 12 h and monoclinic GdPO4 nanoparticles have been obtained by sintering the hydrothermal products at 900°C for 5 h. Green upconversion luminescence of Tb3+ has been observed when pumped by 980 nm diodes due to efficiently cooperative energy transfer from codopant Yb3+. The luminescence of Tb3+ in both hexagonal and monoclinic GdPO4 nanophosphors has been comparatively studied. The dependence of the upconversion luminescence intensity on pump laser power shows that two-phonon processes involved in upconversion luminescence of the emission level 5D4 of Tb3+ is populated via two-photon process. Upconversion luminescence emission intensity and the energy transfer efficiency ηT from Yb3+ to Tb3+ ions upon the concentration of Tb3+ have been investigated.

Folic Acid-Conjugated LaF3:Yb,Tm@SiO2 Nanoprobes for Targeting Dual-Modality Imaging of Upconversion Luminescence and X-ray Computed Tomography

Development of multimodal contrast agents for in vivo simultaneous multimodality imaging is an emerging interdiscipline that is paving the avenue toward the goal of personalized medicine. Herein, folic acid-conjugated silica-modified LaF(3):Yb,Tm upconversion nanoparticles (UCNPs@SiO(2)-FA) with high La content in a single particle were strategically designed and prepared for simultaneously targeting dual-modality imaging of upconversion luminescence (UCL) and X-ray computed tomography (CT). LaF(3) UCNPs were synthesized by a novel oleic acid (OA)/ionic liquid (IL) two-phase system. Afterward, a folic acid molecule was covalently anchored on the surface of UCNPs with a silane coupling agent. The UCNPs@SiO(2)-FA exhibits good stability, water dispersibility and solubility, low cytotoxicity, good biocompatibility, highly selective targeting, excellent X-ray attenuation, and UCL emission under excitation at 980 nm. In vivo UCL and CT images of mice show the UCNPs@SiO(2)-FA can be used in targeting dual-modality imaging. These results suggest that the as-prepared nanoprobe is a good candidate with excellent imaging and targeting ability for targeting dual-modality imaging of UCL and CT.

Hydrothermal synthesis of NaLuF4:153Sm,Yb,Tm nanoparticles and their application in dual-modality upconversion luminescence and SPECT bioimaging

Upconversion luminescence (UCL) properties and radioactivity have been integrated into NaLuF4:153Sm,Yb,Tm nanoparticles by a facile one-step hydrothermal method, making these nanoparticles potential candidates for UCL and single-photon emission computed tomography (SPECT) dual-modal bioimaging in vivo. The introduction of small amount of radioactive 153Sm3+ can hardly vary the upconversion luminescence properties of the nanoparticles. The as-designed nanoparticles showed very low cytotoxicity, no obvious tissue damage in 7 days, and excellent in vitro and in vivo performances in dual-modal bioimaging. By means of a combination of UCL and SPECT imaging in vivo, the distribution of the nanoparticles in living animals has been studied, and the results indicated that these particles were mainly accumulated in the liver and spleen. Therefore, the concept of 153Sm3+/Yb3+/Tm3+ co-doped NaLuF4 nanoparticles for UCL and SPECT dual-modality imaging in vivo of whole-body animals may serve as a platform for next-generation probes for ultra-sensitive molecular imaging from the cellular scale to whole-body evaluation. It also introduces an easy methodology to quantify in vivo biodistribution of nanomaterials which still needs further understanding as a community.

Enhanced Red Emission from GdF3:Yb3+,Er3+ Upconversion Nanocrystals by Li+ Doping and Their Application for Bioimaging

Under 980 nm near-infrared (NIR) excitation, upconversion luminescent (UCL) emission of GdF(3):Yb,Er upconversion nanoparticles (UCNPs) synthesized by a simple and green hydrothermal process can be tuned from yellow to red by varying the concentration of dopant Li(+) ions. A possible mechanism for enhanced red upconverted radiation is proposed. A layer of silica was coated onto the surface of GdF(3):Yb,Er,Li UCNPs to improve their biocompatibility. The silica-coated GdF(3):Yb,Er,Li UCNPs show great advantages in cell labeling and in vivo optical imaging. Moreover, GdF(3) UCNPs also exhibited a positive contrast effect in T(1)-weighted magnetic resonance imaging (MRI). These results suggest that the GdF(3) UCNPs could act as dual-modality biolabels for optical imaging and MRI.

Water-stable NaLuF4-based upconversion nanophosphors with long-term validity for multimodal lymphatic imaging

Multimodal imaging is rapidly becoming an important tool for biomedical applications because it can compensate for the deficiencies of individual imaging modalities. Herein, multifunctional NaLuF4-based upconversion nanoparticles (Lu-UCNPs) were synthesized though a facile one-step microemulsion method under ambient condition. The doping of lanthanide ions (Gd3+, Yb3+ and Er3+/Tm3+) endows the Lu-UCNPs with high T1-enhancement, bright upconversion luminescence (UCL) emissions, and excellent X-ray absorption coefficient. Moreover, the as-prepared Lu-UCNPs are stable in water for more than six months, due to the protection of sodium glutamate and diethylene triamine pentacetate acid (DTPA) coordinating ligands on the surface. Lu-UCNPs have been successfully applied to the trimodal CT/MR/UCL lymphatic imaging on the modal of small animals. It is worth noting that Lu-UCNPs could be used for imaging even after preserving for over six months. In vitro transmission electron microscope (TEM), methyl thiazolyl tetrazolium (MTT) assay and histological analysis demonstrated that Lu-UCNPs exhibited low toxicity on living systems. Therefore, Lu-UCNPs could be multimodal agents for CT/MR/UCL imaging, and the concept can be served as a platform technology for the next-generation of probes for multimodal imaging.

Iridium‐Complex‐Modified Upconversion Nanophosphors for Effective LRET Detection of Cyanide Anions in Pure Water

AbstractBecause of the extreme toxicity of cyanide anions, a great deal of effort has been invested in developing probes that use colorimetric and fluorometric methods to detect them. To date, most of the current optical probes require the use of organic solvents or a mixture of organic solvents and water. Upconversion nanophosphors (UCNPs) can serve as excellent emitters as a result of their sharp emission peaks and simple control of the emission wavelength. Here, the fabrication of hybrid material composed of iridium complex and UCNPs to detect cyanide anions selectively in pure water is described. In addition to the colorimetric detection, the upconversion luminescence emission can serve as a detection signal based on an effective luminescent resonance energy transfer (LRET) process within the hybrid nanoparticles.

Core–shell Fe3O4@NaLuF4:Yb,Er/Tm nanostructure for MRI, CT and upconversion luminescence tri-modality imaging

Core–shell Fe3O4@NaLuF4:Yb,Er/Tm nanostructure (MUCNP) with multifunctional properties has been developed using a step-wise synthetic method. The successful fabrication of MUCNP has been confirmed by transmission electron microscopy, powder X-ray diffraction, energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. The MUCNP exhibits superparamagnetic property with saturation magnetization of 15 emu g−1, and T2-enhanced magnetic resonance (MR) effect with an r2 value of 21.63 s−1 mM−1 at 0.5 T, resulting from the Fe3O4 cores. Moreover, the NaLuF4-based MUCNP provides excellent X-ray attenuation and upconversion luminescence (UCL) emission under excitation at 980 nm. In vivo MR, computed tomography (CT) and UCL images of tumor-bearing mice show that the MUCNP can be successfully used in multimodal imaging. In vitro tests reveal that the MUCNP is non-cytotoxic. These results suggest that the developed MUCNP could be served as an MR, CT and UCL probe for tri-modality imaging.

Cubic sub-20 nm NaLuF4-based upconversion nanophosphors for high-contrast bioimaging in different animal species

A new upconversion luminescence (UCL) nanophosphors based on host matrix of cubic NaLuF4 with bright luminescence have been synthesized by a solvothermal method, facilitate the nanocrystals potential candidates for imaging in vivo, especially large-animals. The sub-20 nm NaLuF4 co-doped Yb3+ and Er3+ (Tm3+) showed about 10-fold stronger UCL emission than that of corresponding hexagonal NaYF4-based nanocrystals with a 20 nm diameter. Near-infrared to near-infrared (NIR-to-NIR) UCL emission of PAA-coated NaLuF4:20%Yb,1%Tm (PAA-LuTm) can penetrate >1.5 cm tissue of pork with high contrast. Based on super-strong UCL emission and deep penetration, PAA-LuTm as optical bioprobe has been demonstrated by in vivo UCL imaging of a normal black mouse, even rabbit with excellent signal-to-noise ratio. Furthermore, such cubic NaLuF4-based nanophosphor was applied in lymph node imaging of live Kunming mouse with rich white fur.

Multifunctional rare-earth self-assembled nanosystem for tri-modal upconversion luminescence /fluorescence /positron emission tomography imaging

Rare-earth upconversion nanoparticles (UCNP) which can absorb low-energy photons and emit high energy photons have attracted great interest not only because of their unique application in upconversion luminescence imaging, but also because they can be used as ideal building blocks for multimodal bioimaging probes. Improving the water-solubility of UCNP and functionalizing them are as yet unresolved problems. In this present study, a general strategy was developed to achieve these two aims by converting hydrophobic upconversion nanoparticles into hydrophilic ones. This was based on the self-assembly between oleic acid, which is a capping ligand, as the guest molecule, and alpha-cyclodextrin, as the host molecule, no matter what the particle size was (10–400 nm) or what synthesis method (thermal decomposition, hydrothermal, solvothermal) was used. The synthesized hydrophilic nanoparticles can further load hydrophobic molecule, e.g. Os(II) complex. The process of self-assembly and loading was confirmed by transmission electron microscopy, X-ray powder diffraction, 1H-nuclear magnetic resonance, Fourier transform-infrared and thermogravimetric analysis, upconversion luminescence and fluorescence spectra. Further bioapplication has also been investigated, including cell-labeling, in vivo lymphatic imaging, upconversion luminescence and positron emission tomography imaging of whole-body Kunming mice. The results indicate that this method is a potential candidate for the preparation of hydrophilic UCNP as a multimodal nanoprobe.

Core –shell NaYF 4:Yb 3+,Tm 3+@Fe xO y nanocrystals for dual-modality T 2-enhanced magnetic resonance and NIR-to-NIR upconversion luminescent imaging of small-animal lymphatic node

New core–shell structured NaYF 4:Yb 3+,Tm 3+@Fe xO y nanocrystals, with 20 nm Yb 3+,Tm 3+-co-doped NaYF 4 nanocrystals as a core and 5 nm Fe xO y nanocrystals as a shell, have been synthesized and characterized by TEM and XRD analysis. These core–shell nanocrystals exhibit excellent near-infrared upconversion luminescence (UCL) emission at 800 nm under excitation by a continuous-wave 980 nm laser and superparamagnetic properties with a saturation magnetization (Ms) of ∼12 emu/g. Water-soluble nanocrystals were obtained by surface ligand exchange of oleic acid-coated precursor nanocrystals, and their internalization within living cells has been investigated by laser scanning UCL microscopy. Furthermore, the obtained core–shell nanocrystals have been applied in dual-modality T 2-enhanced magnetic resonance (MR) and UCL imaging in vivo of the lymphatic system. Moreover, the toxicity of NaYF 4:Yb 3+,Tm 3+@Fe xO y nanocrystals has also been evaluated by MTT assay, IC50 values, and histological analysis of lymphatic node sections.

Intense red upconversion fluorescence emission in NIR-excited erbium–ytterbium doped laponite-derived phosphor

In this report the optical properties and energy-transfer frequency upconversion luminescence of Er 3+/Yb 3+-codoped laponite-derived powders under 975 nm infrared excitation is investigated. The 75%(laponite):25%(PbF 2) samples doped with erbium and ytterbium ions, generated high intensity red emission around 660 nm and lower intensity green emission around 525, and 545 nm. The observed emission signals were examined as a function of the excitation power and annealing temperature. The results indicate that energy-transfer, and excited-state absorption are the major upconversion excitation mechanism for the erbium excited-state red emitting level. The precursor glass samples were also heat treated at annealing temperatures of 300 °C, 400 °C, 500 °C, and 600 °C, for a 2 h period. The dependence of the visible upconversion luminescence emission upon the annealing temperature indicated the existence of an optimum temperature which leads to the generation of the most intense and spectrally pure red emission signal.

18F-Labeled Magnetic-Upconversion Nanophosphors via Rare-Earth Cation-Assisted Ligand Assembly

A novel method of rare-earth cation-assisted ligand assembly has been developed to provide upconversion nanophosphors with T(1)-enhanced magnetic resonance (MR), radioactivity, and targeted recognition properties, making these nanoparticles potential candidates for multimodal bioimaging. The process of modifying the surface of the nanophosphors has been confirmed by transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, proton nuclear magnetic resonance, Fourier-transform infrared spectroscopy, energy-dispersive X-ray analysis, and so on. The versatility of this surface modification approach for incorporating functional molecules and fabricating fluorine-18-labeled magnetic-upconversion nanophosphors as multimodal bioprobes has been demonstrated by targeted cell imaging, in vivo upconversion luminescence, MR imaging, and positron emission tomography imaging of whole-body small animals.

High-quality water-soluble and surface-functionalized upconversion nanocrystals as luminescent probes for bioimaging

Rare-earth upconversion nanophosphors (UCNPs) have great potential to become excellent biological luminescent labels for fluorescence bioimaging. However, it is still difficult to directly synthesize high-quality water-soluble UCNPs bearing appropriate functional groups using a one-step synthetic strategy. Herein, we report a one-step synthetic strategy for high-quality water-soluble and surface-functionalized UCNPs using a hydrothermal reaction assisted by binary cooperative ligands (HR-BCL). In this system, 6-aminohexanoic acid and oleate were introduced to control nuclear generation and crystal growth of small nanoparticles. The UCNPs synthesized here showed high crystalline and intense upconversion luminescence emission. Fourier-transform infrared and nuclear magnetic resonance spectroscopy indicated that 6-aminohexanoic acid and oleate cooperate as surface ligands to co-control the surface of UCNPs. Thus, the water-solubility of the as-prepared UCNPs can be tuned by changing the molar ratio of 6-aminohexanoic acid to oleate. The AA-modified UCNPs provided a free amine content of (6.0 ± 0.2) × 10−5 mol/g, which renders them dispersible in aqueous solution and allows further conjugation with folic acid (FA) for targeted bioimaging. Furthermore, the amine-functionalized UCNPs show intense near-infrared upconversion luminescence and were successfully applied in the lymphatic capillary bioimaging of small animals with a high signal-to-noise ratio, suggesting that these surface-functionalized UCNPs are promising candidates for luminescent biolabels.

Hydrothermal synthesis and luminescence properties of octahedral LiYbF4: Er3+ microcrystals

Abstract LiYbF 4 : Er 3+ octahedral microcrystals have been successfully prepared through a facile hydrothermal method assisted with EDTA (ethylenediaminetetraacetic acid). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric and differential scanning calorimeters (TG-DSC), photoluminescence (PL) spectra are used to characterize the samples. Under 976 nm excitation, the upconversion (UC) luminescence emission spectra of LiYbF 4 : Er 3+ microcrystals show the characteristic Er 3+ emissions. The results show that the infrared light emissions at 792 nm of 4 I 9/2 → 4 I 15/2 are dominantly strong unusually, while the green emissions at 526 and 545 nm assigned to 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 , respectively, and the red emission at 667 nm of 4 F 9/2 → 4 I 15/2 are relatively weaker. Most importantly, the samples show more efficient luminescence with further heat treatment.

Fluorine-18-labeled Gd 3+/Yb 3+/Er 3+ co-doped NaYF 4 nanophosphors for multimodality PET/MR/UCL imaging

Molecular imaging modalities provide a wealth of information that is highly complementary and rarely redundant. To combine the advantages of molecular imaging techniques, 18F-labeled Gd 3+/Yb 3+/Er 3+ co-doped NaYF 4 nanophosphors (NPs) simultaneously possessing with radioactivity, magnetic, and upconversion luminescent properties have been fabricated for multimodality positron emission tomography (PET), magnetic resonance imaging (MRI), and laser scanning upconversion luminescence (UCL) imaging. Hydrophilic citrate-capped NaY 0.2Gd 0.6Yb 0.18Er 0.02F 4 nanophosphors (cit-NPs) were obtained from hydrophobic oleic acid (OA)-coated nanoparticles (OA-NPs) through a process of ligand exchange of OA with citrate, and were found to be monodisperse with an average size of 22 × 19 nm. The obtained hexagonal cit-NPs show intense UCL emission in the visible region and paramagnetic longitudinal relaxivity ( r 1 = 0.405 s −1·(mM) −1). Through a facile inorganic reaction based on the strong binding between Y 3+ and F −, 18F-labeled NPs have been fabricated in high yield. The use of cit-NPs as a multimodal probe has been further explored for T 1-weighted MR and PET imaging in vivo and UCL imaging of living cells and tissue slides. The results indicate that 18F-labeled NaY 0.2Gd 0.6Yb 0.18Er 0.02 is a potential candidate as a multimodal nanoprobe for ultra-sensitive molecular imaging from the cellular scale to whole-body evaluation.

Investigation of energy transfer and frequency upconversion in Er 3+/Ho 3+ co-doped tellurite glasses

The energy-transfer mechanisms and frequency upconversion emissions in 0.5Er 3+/ xHo 3+ co-doped tellurite glasses by exciting at 980 nm have been investigated. Three intense upconversion luminescence emissions are observed at around 525, 548, and 660 nm, which correspond to Er 3+: 2H 11/2 → 4I 15/2, Er 3+: 4S 3/2 → 4I 15/2 + Ho 3+: 5S 2( 5F 4) → 5I 8, and Er 3+: 4F 9/2 → 4I 15/2 + Ho 3+: 5F 5 → 5I 8 transitions, respectively. The upconversion emissions reach the maximum values when Ho 2O 3 is 0.5 mol%, and the intensities of the green and red light emissions were about 4.5 and 6 times stronger than those un-doped Ho 2O 3, respectively. The possible upconversion mechanisms and energy transfer between Er 3+ and Ho 3+ were also estimated and evaluated. All the three emissions are based on two photon absorption processes.

Enhancement of luminescence properties in Er 3+ doped TeO 2–Na 2O–PbX (X = O and F) ternary glasses

An enhancement of luminescence properties in Er 3+ doped ternary glasses is observed on the addition of PbO/PbF 2. The infrared to visible upconversion emission bands are observed at 410, 525, 550 and 658 nm, due to the 2H 9/2 → 4I 15/2, 2H 11/2 → 4I 15/2, 4S 3/2 → 4I 15/2, 4F 9/2 → 4I 15/2 transitions respectively, on excitation with 797 nm laser line. A detailed study reveals that the 2H 9/2 → 4I 15/2 transition arises due to three step upconversion process while other transitions arise due to two step absorption. On excitation with 532 nm radiation, ultraviolet and violet upconversion bands centered at 380, 404, 410 and 475 nm wavelengths are observed along with one photon luminescence bands at 525, 550, 658 and 843 nm wavelengths. These bands are found due to the 4G 11/2 → 4I 15/2, 2P 3/2 → 4I 13/2, 2H 9/2 → 4I 15/2, 2P 3/2 → 4I 11/2, 2H 11/2 → 4I 15/2, 4S 3/2 → 4I 15/2, 4F 9/2 → 4I 15/2 and 4S 3/2 → 4I 13/2 transitions, respectively. Though incorporation of PbO and PbF 2 both enhances fluorescence intensities however, PbF 2 content has an important influence on upconversion luminescence emission. The incorporation of PbF 2 enhances the red emission (658 nm) intensity by 1.5 times and the violet emission (410 nm) intensity by 2.0 times. A concentration dependence study of fluorescence reveals the rapid increase in the red ( 4F 9/2 → 4I 15/2) emission intensity relative to the green ( 4S 3/2 → 4I 15/2) emission with increase in the Er 3+ ion concentration. This behaviour has been explained in terms of an energy transfer by relaxation between excited ions.

Host dependent thermal stability and frequency upconversion of Er 3+ -doped heavy metal oxyfluoride germanate glasses

The upconversion properties of Er3+-doped heavy metal oxyfluoride germanate glasses under 975 nm excitation have been investigated. The intense green (551 and 529 nm) and relatively weak red (657 nm) emissions corresponding to the transitions 4S3/2→4I15/2, 2H11/2→4I15/2 and 4F9/2→4I15/2, respectively, were simultaneously observed at room temperature. The content of PbF2 has an important influence on the upconversion luminescence emission. With increasing content of PbF2, the intensities of green (529 nm) and red (657 nm) emissions increase slightly, while the green emission (551 nm) increases markedly. These results suggest that PbF2 has an influence on the green (551 nm) emission more than on the green (529 nm) and red (657 nm) emissions.

Comparative investigation of up-conversion luminescence in Tm 3+-doped oxide–chloride germanate and tellurite glasses

Tm 3+-doped oxide–chloride germanate and tellurite glasses have been synthesized by conventional melting method. Intense up-conversion luminescence emissions were simultaneously observed at room temperature in these glasses. The possible up-conversion mechanisms are discussed and estimated. However, in these Tm 3+-doped glasses, tellurite glass showed weaker up-conversion emissions than germanate glass, which is inconsistent with the prediction from the difference of maximum phonon energy between tellurite and germanate glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Our results confirm that, besides the maximum phonon energy, the phonon density of host glasses is also an important factor in determining the up-conversion efficiency.

Composition dependent upconversion of Er 3+-doped PbF 2–TeO 2 glasses

The upconversion properties of Er 3+ ions were studied for heavy metal oxyfluoride tellurite glass hosts xPbF 2–(100− x)TeO 2 under 975 nm excitation. The intense green (529 and 545 nm) and relative weak red (657 nm) emissions corresponding to the transitions 4S 3/2 → 4I 15/2, 2H 11/2 → 4I 15/2 and 4F 9/2 → 4I 15/2, respectively, were simultaneously observed at room temperature. The PbF 2 content has an important influence on upconversion luminescence emission. With increasing PbF 2 content, the intensities of green (529 nm) and red (657 nm) emissions increase slightly, while the green (545 nm) emission increases significantly. These results indicate that PbF 2 has more influence on the green (545 nm) emission than the green (529 nm) and red (657 nm) emissions. The intense green emission observed suggest that Er 3+-doped heavy metal oxyfluoride tellurite glasses can become candidates for developing upconversion optical devices.