Strong Upconversion Emission Research Articles

Revelation of luminescence-thermometry, light generation in first biological window and IR detection capabilities of sodium yttrium fluoride co-doped with Yb3+ and Er3+ ions synthesized by low-cost approach

Microwave-assisted combustion synthesis approach is adopted to prepare sodium yttrium fluoride co-doped with ytterbium and erbium (NaYF4: Er3+/ Yb3+) nanoparticles. PXRD confirms the formation of mixed phase (cubic and hexagonal) of the NaYF4: Er3+/ Yb3+ crystal lattice. It is determined that the cubic phase dominates over the hexagonal phase. Three characteristic emission peaks appearing at 523, 544 and 672 nm are attributed to the transitions 2H11/2, 4S3/2 - 4I15/2 and 4F9/2 - 4I15/2 respectively which is character of Er3+ ions, confirming the potential in first biological window applications. The optimal dopant concentration is found and the asymmetric ratio is calculated. Strong up-conversion emission has been observed in the sample in the yellowish-green area when excited with 980 nm radiation. The life time of the 4S3/2 (green) and 4F9/2 (red) levels are found to be 1.187 and 0.472 ms respectively. The emission occurring in the synthesized sample follows the two-photon process which is confirmed by double log graph of intensity and pump-power. Thermal sensitivity is recorded in the range of 300–540 K. The thermally and non-thermally coupled levels attributed to the transitions (2H11/2 and 4S3/2) to (2H11/2 and 4F9/2) of Er3+ ions which are employed to study the temperature sensing. Higher relative sensitivities in both thermally and non-thermally coupled levels were observed in the samples with their values being 1.33 % K−1 and 1.81 % K−1 correspondingly. The significant thermal sensitivity of the synthesized phosphor materials improves its possibility in optical thermometric applications. Further to study the light-emitting abilities of the prepared phosphor, a phosphor-in-glass structure was fabricated using a borate-based glass system. The photometric parameters such as luminous efficacy of radiation and luminous efficiency were obtained to be 331 lm/Wopt and 48.46 % respectively. The prepared compound exhibits superior IR sensing which is evident upon exposure of the fabricated P-i-G to 980 nm radiation. The performed studies authenticate the capabilities of the cubic phase NYF in optical thermometry, light generation and IR detection abilities.

Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion.

Triplet-triplet annihilation photon upconversion (TTA-UC) is attracting a great deal of attention as a viable approach to exploit unutilized wavelengths of light in solar-driven devices. Recently, ligand-protected metal nanoclusters have emerged as a compelling platform for serving as triplet sensitizers for TTA-UC. In this study, we developed an atomically precise, triplet-mediator ligand (TL)-protected metal nanocluster, Au2Cu6(S-Adm)6[P(DPA)3]2 (Au2Cu6DPA; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In Au2Cu6DPA, the excitation of the Au2Cu6 core rapidly generates a metal-to-ligand charge transfer state, followed by the formation of the long-lived triplet state (approximately 150 μs) at a DPA site in the TL. By combining Au2Cu6DPA with a DPA annihilator, we achieved a red-to-blue upconversion quantum yield (ΦUCg) of 20.7 ± 0.4% (50% max.) with a low threshold excitation intensity of 36 mW cm-2 at 640 nm. This quantum yield almost reaches the maximum limit achievable using a DPA annihilator and establishes a record-setting value, outperforming previously reported nanocrystal and nanocluster sensitizers. Furthermore, strong upconversion emission based on a pseudo-first-order TTA process was observed under 1 sun illumination, indicating that the Au2Cu6DPA sensitizer holds promise for applications in solar-energy-based systems.

High-performance of optical thermometry based on the non-thermally coupled levels in YVO4: Yb3+/Er3+/Tm3+ nanocrystals

Hydrothermally fabricated nanocrystals YVO4: Yb3+/Er3+/Tm3+ were investigated for up-conversion (UC) luminescence and optical temperature sensing performance. Upon 980 nm excitation, the luminescence intensity firstly increases and then decreases with raising Yb3+ doping content and reaches the maximum at 15 mol%. Considering the strong UC emission, the temperature sensing behavior was investigated based on the fluorescence intensity ratio (FIR) technique in temperature range of 293–573 K. The study was focused on the non-thermally coupled levels (NTCLs) including transitions 3F2,3 → 3H6 and 1G4→3H6 of Tm3+, transitions from the 4S3/2, stark sublevel 4S3/2(2), and 4F9/2 to the ground state 4I15/2 of Er3+ owing to their various temperature-dependent behaviors. Through using the stark sublevel 4S3/2(2) of Er3+, the maximum absolute sensitivity (SA) of 0.0563 K-1, relative sensitivity (SR) of 2.334 %K−1 and the minimum temperature resolution of 0.0058 K was achieved with good repeatability. The experimental results indicate that the as-obtained YVO4: 15%Yb3+/1%Er3+/1%Tm3+ nanocrystals can be potential candidates for nano-dimensional optical thermometry applications.

Preparation of Er3+, Ho3+, Yb3+-doped Bi2WO6 upconversion luminescent materials and their temperature sensing properties

A high-temperature solid-phase method was used to prepare codoped Bi2WO6 upconversion luminescent materials with different concentrations of Er3+, Ho3+, and Yb3+. The microstructure, upconversion luminescence and temperature sensing properties of the synthesized powders were analyzed. X-ray diffraction (XRD) patterns showed that the doping of Er3+, Ho3+, and Yb3+ did not affect the orthorhombic crystal structure of the Bi2WO6 matrix material. As the calcination temperature increased from 800 °C to 1000 °C for 3h, the average particle sizes of the 1% Er3+, 1% Ho3+, 1% Yb3+: Bi2WO6 samples decreased, and the luminescence weakened. Scanning electron microscopy (SEM) images showed that the morphology of the sample for 1% Er3+, 1% Ho3+, 1% Yb3+: Bi2WO6 with calcination at 800 °C was nearly spherical, with particle sizes ranging from 1 to 5 μm and some agglomeration. SEM mapping showed that the elements were more uniformly distributed on the surfaces of the sample particles, and energy-dispersive X-ray spectroscopy (EDS) analysis showed that each element was distributed with in the particles. Er3+, Ho3+, and Yb3+ codoping was used to regulate the Yb3+ doping concentration, and under 980 nm excitation, the strongest upconversion emission was obtained for the prepared 1% Er3+, 1% Ho3+, 1% Yb3+ samples. Compared with that for the Er3+, Yb3+-doped Bi2WO6 sample, the luminescence intensity of the characteristic emission peak of Er3+ at 525 nm for the Er3+, Ho3+, Yb3+-doped sample was weakened, while the luminescence of the characteristic emission peak of Ho3+ was enhanced; these results indicated that energy transfer from Er3+ to Ho3+ occurred. The intensities of the four emission peaks at 525 nm, 546 nm, 660 nm, and 756 nm in the 1% Er3+, 1% Ho3+, 1% Yb3+-doped Bi2WO6 sample increased with increasing excitation pump power from 45 mW to 233 mW. Based on the relationship between the excitation pump power and the emission intensity, all four luminescence peaks originated from two-photon absorption. The fluorescence intensity ratio versus temperature was fitted using three different nonthermally coupled energy level pairs in the range from 298 K to 573 K. The maximum absolute temperature sensitivity of 0.296 K-1 was obtained at 573 K for the Ho3+ nonthermally coupled energy level pair, and the maximum relative temperature sensitivity of 0.279 K-1 was obtained at 298 K. The temperature resolution for characterization using the above nonthermally coupled energy level pairs was calculated, with the temperature resolution δT yielding a minimum value of 0.03 K. The CIE color coordinates at different temperatures were calculated, and the color coordinates gradually moved from the green region to the red region.

Strong green upconversion emission from submicron spindle-shaped SrMoO4:Yb3+,Er3.

Upconversion luminescence (UCL) is a fluorescence process where two or more lower-energy photons convert into a higher-energy photon. Lanthanide (Ln3+)-doped UCL materials often suffer from weak luminescence, especially when directly synthesized by a hydrothermal (HT) process due to the existing hydroxyl group and undesirable arrangement of dopants within host lattices which quench luminescence and limit energy transfer. Therefore, additional heat treatment processes are required to enhance their UCL emission, even though direct hydrothermal synthesis without further heat treatment has the advantages of low energy consumption, fast synthesis, and wide applicability to generate UCL materials. In this study, via a HT process without annealing, we have produced Yb3+ and Er3+ co-doped SrMoO4 submicron spindles with a strong green UCL emission which can be seen with the naked eye, which HT produced oxide-based UCL materials often fail to demonstrate. We have investigated different HT synthesis conditions, such as temperature, time, pH and dopant composition, which control the nucleation, growth, lattice structure arrangement, and ultimately their UCL properties through XRD, SEM, EDS and UCL measurements. The bright green UCL from the SrMoO4:Yb,Er submicron spindles is further enhanced by post-synthesis annealing within a molten NaNO3/KNO3 system to prevent particle size growth. The green UCL intensity from the annealed SrMoO4:Yb,Er submicron spindles surpasses samples produced by the solid-state method and is comparable to that from the commercial NaYF4:Yb,Er sample. We have further studied the temperature-dependent luminescence of both the HT-prepared and molten-salt annealed SrMoO4:Yb,Er submicron spindle samples. The strong UCL from our SrMoO4:Yb,Er submicron spindles could warrant their candidacy for bioimaging and anticounterfeiting applications.

Combustion synthesized La2O3: Er3+/Yb3+ phosphor for simultaneous upconversion emission and photo-thermal signal generation

The Er3+/Yb3+ doped La2O3 phosphor is prepared through combustion synthesis route and then studied using frequency upconversion (UC) and photoacoustic spectroscopic techniques. Prepared phosphor sample was annealed at high temperature ∼2100°C for short time and then structural and optical characterizations were performed. A photoacoustic cell was fabricated to record photoacoustic signal generation. Samples have shown strong upconversion emission at 409 nm, 523 nm, 548 nm, and 672 nm due to the 2H9/2 →4I15/2, 2H11/2 →4I15/2, 4S3/2 → 4I15/2 and 4F9/2→ 4I15/2 transitions, respectively. The photoacoustic measurement has revealed that heat generation in the sample increases in line with the increase in upconversion emission although both properties are opponent to each other. Studies show that prepared phosphor is efficient for generating strong upconversion emission as well as strong photoacoustic signal (also photo-thermal signal) and hence present dual functional sample seems useful for upconversion imaging as well as photo-thermal therapy.

Intense red upconversion luminescence and optical thermometry of a novel Yb3+/Er3+ co-doped Ba3Sc2WO9 phosphor

Yb3+/Er3+ co-doped Ba3Sc2WO9 strong red upconversion luminescent (UCL) phosphors have been achieved by high-temperature solid-phase method. Under 980 nm laser excitation, the phosphor emits strong red light at ∼667 nm and weak green light at ∼560 nm, with the intensity ratio of the red-green integrals up to 23.06. The band gap of the optimally doped Ba3Sc2WO9: 0.1 Yb3+, 0.03 Er3+ sample was calculated to be 3.14 eV. The main excitation process for the red light emission is the transition of Er3+ from the 4I13/2 energy state to the 4F9/2 energy state. In addition, the Yb3+/Er3+ doped Ba3Sc2WO9 red UCL phosphors have great potential for ambient temperature sensing in the temperature range 298–498 K. The Ba3Sc2WO9: 0.1 Yb3+, 0.03 Er3+ phosphor as a temperature sensor has a maximum sensitivity of 0.0096 K−1 at 498 K. This study provides a groundbreaking method for exploring new materials for strong red light upconversion emission.

Multi-shell structured nanomaterials with strong red upconversion emission for trimodal biomedical imaging

Upconversion nanomaterials show promising applications in multimodal imaging due to their advantages such as sharp emission bands and non-autofluorescence in tissue. Obtaining intense red upconversion emission is crucial for fluorescence imaging. Herein, we designed a multi-shell structured NaErF4:0.5%Tm3+@NaYF4:20%Yb3+ @NaLuF4 upconversion nanomaterial exhibiting strong red emission and high red-to-green ratio under 980 nm irradiation. The main mechanism of the strong emission is attributed to the inert NaLuF4 shell which serves to block the energy transfer from Yb3+ to surface defects. In addition, a NaGdF4 shell is further cladded considering the property of Gd3+ of facilitating longitudinal relaxation rate of water. The properties of X-ray attenuation, longitudinal relaxation rate, and the upconversion emission of the measured sample suggest that our designed nanomaterial possesses potential for X-ray computed tomography (CT), magnetic resonance imaging (MRI), and fluorescence trimodal imaging.

Rectangular cross-sectional Nd3+/Yb3+ co-doped AlN nanorods with strong up-conversion emission for high-sensitivity optical thermometry

Due to the characteristics of non-contact, high sensitivity and spatial resolution, nanoscale optical thermometry is necessary in many applications such as micro/nano electronics, integrated optics and biomedicine. Herein, we report the synthesis of Nd3+/Yb3+ co-doped AlN (AlN:Nd3+/Yb3+) nanorods using an arc discharge method and investigate their phase, composition and morphology using XRD, Raman, XPS, EDS, ICP, SEM and TEM. The nanorods with rectangular cross-section are highly single crystalline and have a uniform smooth surface. Upon excitation at 980 nm, AlN:Nd3+/Yb3+ nanorods exhibit up-conversion (UC) visible emission bands (around 544, 595 and 667 nm) and near-infrared emission bands (around 755 and 806 nm), which correspond to intra-4f electron transitions of the Nd3+ ions. Later, the temperature dependence of UC luminescence behavior was systemically studied using luminescence intensity ratio (LIR) technique. It was verified that LIRs with non-thermally coupled levels (4G7/2/4F5/2 or 4G7/2/4F7/2) are suitable for temperature sensing. The maximum relative sensitivity based on 4G7/2/4F7/2 is 4.63% K−1 at 298 K, which is one of the highest values reported for optical thermometers. This work provides an efficient method for synthesizing wide-band gap semiconductors with doped size-mismatched functional atoms that can be used for high-sensitivity optical thermometry.

Reengineering the thermometric behaviors of Er3+/Yb3+-codoped Gd2Mo3O12 microparticles via dual-mode luminescence manipulation

Series of Er3+/Yb3+-codoped Gd2Mo3O12 (GMO:Er3+/2xYb3+) microparticles with dual-mode luminescence were fabricated with the assistance of sol-gel method. The phase structure, morphological information, upconversion (UC) and down-shifting luminescence properties of the final products were systematically investigated. At 980 nm light irradiation, strong UC emissions rise in all the products and their intensities depend on the dopant concentration, where the optimum status occurs at x = 0.25 and the two-photon capture procedure results in the UC emission mechanism. Besides, excited at 378 nm, the down-shifting photoluminescence is also gained in resulting samples. By means of the fluorescence intensity ratio technique, the thermometric properties of the studied samples were investigated through analyzing the diverse thermal quenching features of the green emissions from thermally coupled levels of Er3+. When the UC emission is selected, the maximum absolute and relative sensitivities of the GMO:Er3+/2xYb3+ microparticles are 0.0076 and 1.15% K−1, respectively, whereas they shift to 0.0036 and 0.89% K−1, respectively, when the down-shifting photoluminescence is adopted. Note that, the sensor sensitivities of developed microparticles are related to the luminescence mode, while they do not depend on Yb3+ content. These results suggest that the GMO:Er3+/2xYb3+ microparticles are promising thermosensitive luminescent materials for optical thermometry and their thermometric features can be tuned through changing luminescence mode.

Down-/Up-conversion luminescence behaviors and energy transfer analysis in Tm3+ and Tm3+/Yb3+ co-doped tellurite glasses

Glasses with the chemical composition of (50-x-y) TeO2-30ZnO-10YF3-10NaF-0.5Tm2O3-yYb2O3 (y = 0.5, 1.0, and 3.0 mol%) are developed using a melt-quenching method. Measurements of absorption, excitation, down and up-conversion emission spectra, and decay times were used to investigate the energy transfer processes. Cross-relaxation mechanisms caused the emission intensity to decrease at higher Tm3+ ion concentrations. The Tm3+/Yb3+ co-doped glasses showed strong up-conversion emission under the diode laser excitation at 978 nm. Studies on the dependence of pump energy revealed the mechanism underlying the up-conversion luminescence. According to the findings, the developed Tm3+/Yb3+co-doped glasses are also found to be a promising option for spectral conversion and emission (visible and near infrared) applications.

Upconversion Nanoparticles as Imaging Agents for Dental Caries.

Dental caries (cavities) is the most prevalent disease worldwide; however, current detection methods suffer from issues associated with sensitivity, subjective interpretations, and false positive identification of carious lesions. Therefore, there is a great need for the development of more sensitive, noninvasive imaging methods. The 30 nm core@shell NaYF4; Yb20%, Er2%@NaYF4 upconversion nanoparticles (UCNPs), exhibiting strong upconversion emission from erbium upon excitation at 975 nm, were used in the imaging of locations of demineralized enamel and oral biofilm formation for the detection of dental caries. UCNPs were modified with poly(acrylic acid) (PAA) or poly-d-lysine (PDL), and targeting peptides were conjugated to their surface with affinity for either hydroxyapatite (HA), the material dentin is composed of, or the caries causing bacteria Streptococcus mutans. A statistical difference in the binding of targeted vs nontargeted UCNPs to HA was observed after 15 min, using both upconversion fluorescence of UCNP (p < 0.001) and elemental analysis (p = 0.0091). Additionally, using the HA targeted UCNPs, holes drilled in the enamel of bovine teeth with diameters of 1.0 and 0.5 mm were visible by the green emission after a 20 min incubation with no observable nonspecific binding. A statistical difference was also observed in the binding of targeted versus nontargeted UCNPs to S. mutans biofilms. This difference was observed after 15 min, using the fluorescence measurements (p = 0.0125), and only 10 min (p < 0.001) using elemental analysis via ICP-OES measurements of Y3+ concentration present in the biofilms. These results highlight the potential of these UCNPs for use in noninvasive imaging diagnosis of oral disease.

Effect of Li+/K+/Zn2+ doping on the optical and temperature sensing properties of Tm3+ and Yb3+ doped YVO4 phosphor

The Pechini sol-gel reaction technique has been used to produce Tm3+, Yb3+ co-doped YVO4 and Li+/K+/Zn2+ codoped YVO4: Tm3+,Yb3+ phosphors. Post-heat treatment (at 800 °C) was given to all the samples to strengthen the material's structural and optical characteristics. Prepared phosphors exhibit blue and near-infrared (NIR) light. The crystalline structure and morphology of the prepared phosphors were studied by using powder X-ray diffraction (PXRD), Fourier transform Infrared spectroscopy (FTIR) and scanning electron microscope (SEM). All prepared phosphors crystallise in the tetragonal crystal structure having space group I41/amd. Prepared phosphors, when pumped with the laser at 980 nm, give strong upconversion emission in blue and NIR regions that correspond to Tm3+ transitions 1G4→3H6 and 3H4→3H6, respectively. Li+/K+/Zn2+ co-doping enhances the luminescence intensity significantly. Among Li+/K+/Zn2+ ions, Zn2+ co-doping increases the upconversion emission intensity mostly compared to others, the blue emission intensity get increased by ∼7 times and the NIR by ∼6 times. The main cause of enhancement in the emission intensity may be primarily due to the charge imbalance and asymmetry in the crystal field.Further, optical temperature sensing was also investigated by using the fluorescence intensity ratio (FIR) of the transitions (3F2,3→3H6) and (3H4→3H6). Co-doping of Li+/K+/Zn2+ ions also affects the temperature sensitivity. The relative sensitivity and absolute sensitivity of the samples were also calculated. This study indicates that this phosphor is a suitable candidate as a blue phosphor for display devices, WLEDs and optical temperature-sensing probes.

Control of red upconversion emission in Er3+–Yb3+– Fe3+ tri–doped biphasic calcium phosphate

This paper reports a novel red upconversion (UC) emission in Er/Yb/Fe tri-doped biphasic calcium phosphate (BCP) synthesized using a solid-state reaction method. The crystal structure of BCP was analyzed using Rietveld refinement, which confirmed the hexagonal structure of hydroxyapatite and the rhombohedral structure of beta-tricalcium phosphate with good crystallinity. The results of Rietveld refinement, Fourier-transform infrared spectroscopy, and diffuse reflectance spectroscopy revealed that the presence of Fe3+ in the host lattices strongly influenced the crystal structure of the phosphors. The phosphors showed intense red UC emission with a maximum at 660 nm upon the near-infrared excitation of a diode laser at 975 nm. Significantly, the red emission intensities of the Fe3+-doped sample were 60-times greater than those of the undoped sample. The preferential enhancement of red UC emission was due to the energy transfer from the high-energy excited state |2F7/2, 4T1g > of the Yb3+-Fe3+ dimer to the 4F9/2 state of the Er3+ ions. The UC emission mechanism was primarily estimated by the dependence of the UC spectral intensities on the laser 975 nm excitation power. The BCP: Er/Yb/Fe phosphor generating strong red UC emission could be used to develop a highly effective bioimaging tool in biomedicine.

Synthesis and up-conversion luminescence of Er3+ and Yb3+ co-doped La14W8O45 phosphors

Pure phase orthorhombic Er3+ and Yb3+ co-doped La14W8O45 up-conversion luminescent materials were successfully prepared by the hydrothermal method combined with calcination treatment. The structures, morphology, and optical properties of the samples were analyzed by X-ray powder diffractometer (XRD), scanning electron microscope (SEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and up-conversion photoluminescence spectroscopy (UPL) with a 980 nm laser. XRD results indicated that the obtained samples are single-phase orthorhombic Er3+ and Yb3+ co-doped La14W8O45 crystals. SEM results showed that the prepared sample exhibits an irregular and melted polyhedral-like morphology with agglomeration in submicron size of particles approximately 50–200 nm. UV–vis DRS results revealed that the La14W8O45 crystal is a direct semiconductor with an optical band gap of 3.84 eV. The prepared Er3+ and Yb3+ co-doped La14W8O45 crystals exhibit strong up-conversion emission peaks at about 526 nm, 548 nm, and 660 nm under 980 nm laser excitation. The intensity of up-conversion luminescence is affected by the Er3+ doping concentration, and concentration quenching will be observed due to the cross-relaxation (CR) process when the Er3+ doping content is above 1%. The pumping power dependence of the luminescence indicated that the up-conversion emissions of samples are excited by a two-photon absorption process. The color coordinates of the samples are located in the green light region.

Enhancement of photoacoustic and upconversion emission in Mg2+/Zn2+ codoped Gd2O3: Er3+/Yb3+ phosphor using 980 nm excitation.

In this work, Er3+/Yb3+ doped Gd2O3 phosphor samples were synthesized through a combustion method and then characterized through X-ray diffraction and FE-SEM techniques. The sample was studied for photoacoustic spectroscopy (PAS) and upconversion (UC) emission spectroscopy techniques using a frequency modulated 980 nm excitation source. A correlation between PA signal (produced due to non-radiative transitions) and UC emission intensity (produced due to radiative transition) is made in order to optimize the sample for both these properties. The phosphor was codoped with Mg2+ and Zn2+ ions to see enhancement in upconversion emission intensity and these ions have enhanced the upconversion emission. Studies show that the present phosphor is appropriate for producing strong upconversion emission intensity along with a strong photoacoustic signal which is beneficial for upconversion-imaging as well as photothermal therapy. The present sample has also shown its potential for detection of fingerprints.

Dual excitable upconversion nanoparticle@polydopamine nanocomposite with intense red emission and efficient photothermal generation

Integration of strong upconversion emission with deep tissue penetration and stable photothermal generation is of great significance for imaging-guided photothermal therapy under infrared excitation. Herein, the NaYF4:Yb3+,Ho3+,Ce3+@NaGdF4:Yb3+,Nd3+@NaGdF4 core-shell-shell upconversion nanoparticles (UCNPs) with increasing Ce3+ doping concentration were firstly synthesized, in which the size manipulation, enhanced red to green emission intensity ratio (IR/IG), 980 and 808 nm dual-modal excitations, as well as improved total upconversion emission intensity can be achieved. After the coating of polydopamine (PDA) through dopamine spontaneous polymerization on the UCNPs surface under alkaline condition, the obtained UCNP@PDA nanocomposites show efficient photothermal effect under 808 nm excitation, while the thickness variation of PDA can be indicated by the upconversion spectra under 980 nm excitation. Interestingly, the photothermal effect of the UCNP@PDA nanocomposites with high IR/IG ratio is more stable with PDA thickness variation, which benefits from the structure design that allows PDA to simultaneously absorb the visible emission from inside UCNPs and the external 808 nm infrared light. Further in vitro cytotoxicity assay and photothermal therapy demonstrate that the UCNP@PDA nanocomposite has good biocompatibility and ability to kill tumor cells by photothermal effect under 808 nm excitation. This research may provide a nanoplatform for integrating deep bioimaging with highly stable photothermal therapy without the sacrifice of radiative transitions of rare earth ions.

Hydrothermal Synthesis and Properties of Yb3+/Tm3+ Doped Sr2LaF7 Upconversion Nanoparticles

We report the procedure for hydrothermal synthesis of ultrasmall Yb3+/Tm3+ co-doped Sr2LaF7 (SLF) upconversion phosphors. These phosphors were synthesized by varying the concentrations of Yb3+ (x = 10, 15, 20, and 25 mol%) and Tm3+ (y = 0.75, 1, 2, and 3 mol%) with the aim to analyze their emissions in the near IR spectral range. According to the detailed structural analysis, Yb3+ and Tm3+ occupy the La3+ sites in the SLF host. The addition of Yb3+/Tm3+ ions has a huge impact on the lattice constant, particle size, and PL emission properties of the synthesized SLF nanophosphor. The results show that the optimal dopant concentrations for upconversion luminescence of Yb3+/Tm3+ co-doped SLF are 20 mol% Yb3+ and 1 mol% Tm3+ with EDTA as the chelating agent. Under 980 nm light excitation, a strong upconversion emission of Tm3+ ions around 800 nm was achieved. In addition, the experimental photoluminescence lifetime of Tm3+ emission in the SLF host is reported. This study discovered that efficient near IR emission from ultrasmall Yb3+/Tm3+ co-doped SLF phosphors may have potential applications in the fields of fluorescent labels in bioimaging and security applications.

Ultraviolet and visible upconversion in Yb/Er-CaSiO3 β-wollastonite phosphors

Wollastonite (CaSiO3) is a well-known rock-forming mineral and an important constituent in ceramics and cement industries due to its outstanding mechanical, chemical, and thermal stabilities. Despite technological importance, functional properties such as photon upconversion in CaSiO3 wollastonite ceramics have not been studied. In this contribution, Yb- and Er-doped CaSiO3 (Yb/Er–CaSiO3) wollastonite ceramics were synthesized via microwave hydrothermal technique followed up by heat-treatment in an air environment. X-ray diffraction and transmission electron microscopy studies confirmed the β-wollastonite (2M) phase in the synthesized samples heat-treated at 1050 °C. X-ray photoelectron spectroscopy analysis has shown that the binding energy of Ca 2p orbitals decreases after doping, indicating a change in the crystal environment of Ca in the CaSiO3 and hence a successful incorporation of Yb3+ and Er3+ ions in the lattice. The 980 nm excitation resulted in ultraviolet, violet and strong green and red upconversion emissions as well as downshifting infrared emissions due to the energy transfer between Yb3+ and Er3+ ions. An absolute upconversion quantum yield in the 400–800 nm range is 0.04%. The most intense phonon band was observed at 969 cm−1 in the Yb/Er–CaSiO3 system. This study demonstrates that the β-wollastonite can be developed as a new kind of efficient upconversion phosphor material.

One-step hydrothermal synthesis of Cit-NaYbF4:Er3+ nanocrystals with enhanced red upconversion emission for in vivo fluorescence molecular tomography

Fluorescence imaging techniques represent essential tools in in vitro, preclinical, and clinical studies. In this study, an improved one-step hydrothermal method to synthesize citric acid (CA) modified α-NaYbF4:2%Er3+ nanocrystals was proposed. The introduction of various doping ions into NaYbF4:2%Er3+ and the different valence states of the same ions affect both the crystal size and upconversion luminescence. Therefore, we investigated the upconversion luminescence enhancement of NaYbF4:2%Er3+ by ion doping and find that the upconversion luminescence intensity of the upconversion nanoparticles (UCNPs) co-doped with 5 mol% Fe2+ ions shows the greatest enhancement, especially for red emission at 654 nm. Furthermore, HeLa cells incubated with UCNPs allow for imaging with strong red upconversion emission detection. Confocal laser scanning microscope (CLSM) fluorescent images of HeLa cells indicate that NaYbF4:2%Er/5%Fe2+ leads to a clear outline and improves visualization of the cell morphology. In addition, the CA coated NaYbF4:2%Er3+/5%Fe2+ nanoparticles and NaYbF4:2%Er3+/5%Fe2+ show low cytotoxicity in HeLa cells. Organ imaging reveals the efficiency of these UCNPs to analyze the lungs, liver, and spleen. Together, these results indicate that the Cit-NaYbF4:2%Er3+/5%Fe2+ UCNPs are efficient nanoprobes for fluorescence molecular tomography.