Comparative studies of upconversion luminescence and optical temperature sensing in Tm3+/Yb3+ codoped LaVO4 and GdVO4 phosphors

Tm³⁺/Yb³⁺ codoped LaVO₄ and GdVO₄ phosphors are successfully synthesized using solid state reaction methods and then upconversion emission studies are performed. X-ray diffraction has confirmed a pure monoclinic phase of LaVO₄ and a tetragonal phase of GdVO₄. Upconversion emission through 980 nm laser diode excitation has shown a strong blue band at 475 nm and two weak red bands at 647 and 700 nm originating from ¹G₄ → ³H₆, ¹G₄ → ³F₄ and ³F₃ → ³H₆ transitions of Tm³⁺ ions, respectively. Non-thermally coupled levels viz.³F₃ (700 nm) and ¹G₄ (475 nm) in both the phosphors are used for fluorescence intensity ratio based optical thermometric studies and a comparison is made. The FIR data against temperature were fitted with polynomial and exponential fittings. The results show that polynomial fitting has a higher absolute sensitivity of 21.2 × 10^-3 K^-1 at 653 K for the LaVO₄: Tm³⁺/Yb³⁺ phosphor than the exponential fitting sensitivity of 19.0 × 10^-3 K^-1 at 653 K, while in the case of the GdVO₄: Tm³⁺/Yb³⁺ phosphor both fitting functions provided the same value of absolute sensitivity, that is 13.0 × 10^-3 K^-1 at 653 K. A comparison of the sensitivity values shows that the LaVO₄: Tm³⁺/Yb³⁺ phosphor provides higher sensitivity than the GdVO₄: Tm³⁺/Yb³⁺phosphor but the latter one is too high in upconversion emission.

Synthesis and upconversion emission studies of CaYF5:Ho3+/Yb3+ phosphor and its applications in optical thermometry, fingerprint detection, and security ink†

In this work, a CaYF₅:Ho³⁺/Yb³⁺ upconversion phosphor was synthesized and its structural, morphological, and optical properties were studied. The upconversion emission study was performed at an excitation pump power density of 5 W cm⁻² and emissions at 544 nm, 650 nm and 747 nm due to the ⁵F₄(⁵S₂) → ⁵I₈, ⁵F₅ → ⁵I₈ and ⁵F₄(⁵S₂) → ⁵I₇ transitions of the Ho³⁺ ion, respectively were observed. From temperature-dependent upconversion spectra temperature sensitivity was calculated and sensitivity is found to be 14 × 10⁻³ K⁻¹ for the synthesized upconversion phosphor. The photothermal conversion efficiency of the prepared sample in ethanol medium was tested. Moreover, the sample was used to develop latent fingerprints on various surfaces and good contrast in recorded images is observed. The invisible ink was prepared using the upconversion phosphor and then written words were recorded in upconversion emission mode.

In this work, a CaYF₅:Ho³⁺/Yb³⁺ upconversion phosphor was synthesized and its structural, morphological, and optical properties were studied. Apart from these studies, latent fingerprint detection and security ink applications were also demonstrated using this phosphor.

Efficient dual mode emission in Ce3+/Yb3+/Er3+ doped yttrium aluminium gallium garnet for led device and optical thermometry

By utilizing the effective energy transfer from Ce³⁺ to Yb³⁺, Er³⁺ and from Yb³⁺ to Er³⁺ authors have achieved dual mode emission in Y₃Al₄GaO₁₂ activated with Ce³⁺/Yb³⁺/Er³⁺ ions. Surface morphological and elemental compositions of the prepared samples have been examined using field emission scanning electron microscope (FE-SEM) and energy dispersive spectroscopy (EDS) analysis, respectively. The chemical state of the dopant ions is confirmed by the X-ray photoelectron spectroscopy (XPS). On excitation with 438 nm the prepared material has shown broad visible emission band around 511 nm due to the electronic transition of Ce³⁺ ion. In addition to this sample has also shown NIR emission bands centered around 1024 nm and 1480 nm from Yb³⁺ and Er³⁺ ions, respectively. The emission band at 1024 nm could be due to the quantum cutting (QC) process. Furthermore, up-conversion (UC) emission against temperature and laser power variations is studied on 980 nm excitation wavelength. The prepared sample is used to fabricate visible and NIR LED devices by coating of sample on blue LED chip. Along with this demonstration, optical thermometry ability of the material is studied using emission intensity ratio of two emitting levels. Studies suggest that the prepared material could be useful as dual mode emitting phosphor and LED.

Observation of Stark splitting in micro upconversion photoluminescence spectra of polycrystalline Ln3+ doped Y2O3microspheres

In this study, Yb³⁺-Er³⁺-Tm³⁺doped Y₂O₃microspheres were synthesized using the solvothermal method. Structural, morphological, and elemental analysis was done using XRD, FESEM, and EDX characterization techniques. The optical properties of the samples were determined using UV-Vis-NIR spectroscopy and upconversion photoluminescence measurements. The anti-Stokes emission peaks from these polycrystalline phosphor microspheres were obtained using a commercial micro-photoluminescence setup equipped with a 976 nm laser as an excitation source at room temperature. It was compared with the emission spectra taken from a 2-3 mm spot size of 976 nm laser irradiation on the same sample. The micro-emission spectra were analyzed based on possible Stark splitting energy level transitions between the^2S+1L_Jmanifolds of Er³⁺and Tm³⁺ions. A detailed mechanism is outlined for emission in the entire visible region under 976 nm laser excitation.

Near-Infrared-Triggered Upconverting Nanoparticles for Biomedicine Applications

Due to the unique properties of lanthanide-doped upconverting nanoparticles (UCNP) under near-infrared (NIR) light, the last decade has shown a sharp progress in their biomedicine applications. Advances in the techniques for polymer, dye, and bio-molecule conjugation on the surface of the nanoparticles has further expanded their dynamic opportunities for optogenetics, oncotherapy and bioimaging. In this account, considering the primary benefits such as the absence of photobleaching, photoblinking, and autofluorescence of UCNPs not only facilitate the construction of accurate, sensitive and multifunctional nanoprobes, but also improve therapeutic and diagnostic results. We introduce, with the basic knowledge of upconversion, unique properties of UCNPs and the mechanisms involved in photon upconversion and discuss how UCNPs can be implemented in biological practices. In this focused review, we categorize the applications of UCNP-based various strategies into the following domains: neuromodulation, immunotherapy, drug delivery, photodynamic and photothermal therapy, bioimaging and biosensing. Herein, we also discuss the current emerging bioapplications with cutting edge nano-/biointerfacing of UCNPs. Finally, this review provides concluding remarks on future opportunities and challenges on clinical translation of UCNPs-based nanotechnology research.

Synthesis and characterization of Gd2O3: Er3+, Yb3+doped with Mg2+, Li+ions-effect on the photoluminescence and biological applications

Gd₂O₃:1% Er³⁺, 18% Yb³⁺,x% Mg²⁺(x = 0; 2.5; 4; 5; 6; 8;10; 20; 25; 50) and Gd₂O₃:1% Er³⁺, 18% Yb³⁺, 2,5% Mg²⁺,y% Li⁺(y = 0.5-2.5) nanoparticles were synthesized by homogenous precipitation method and calcined at 900 °C for 3 h in air atmosphere. Powder x-ray diffraction, scanning electron microscopy, cathodoluminescence, transmission electron microscopy, energy dispersive x-ray spectroscopy and photoluminescence techniques were employed to characterize the obtained nanoparticles. We observed a 8-fold increase in red luminescence for samples suspended in DMSO solution for 2.5% of Mg²⁺doping. The x-ray analysis shows that for the concentration of 2.5% Mg, the size of the crystallites in the NPs is the largest, which is mainly responsible for the increase in the intensity of the upconversion luminescence. But the addition of Li⁺ions did not improve the luminescence of the upconversion due to decreasing of crystallites size of the NPs. Synthesized nanomaterials with very effective upconverting luminescence, can act as luminescent markers inin vivoimaging. The cytotoxicity of the nanoparticles was evaluated on the 4T1 cell line for the first time.

A flexible and portable all-fiber temperature sensor based on the upconversion luminescence of octahedral NaBi(WO4)2:Er3+/Yb3+ phosphors

Octahedral NaBi(WO4)2:Er3+/Yb3+ phosphors were synthesized by a hydrothermal method. Intense green upconversion emissions from both 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions were observed and the appropriate energy gap between them is very suitable for temperature sensing requirement based on the fluorescence intensity ratio (FIR) technique. A flexible and portable all-fiber temperature sensing device was established and used to assess the temperature sensing characteristics of NaBi(WO4)2:Er3+Yb3+ phosphors. A maximum absolute sensitivity of 0.014 K-1 is achieved at 423 K and the temperature absolute error is -0.5 K to +0.6 K. The stepwise heating and cooling processes confirm the good stability and recyclability of the all-fiber temperature sensor, which lays the foundation for actual temperature measurement. Based on the high flexibility and accuracy of the temperature sensor, the monitoring of body temperature was realized in real time and continuously, which may provide new development prospects for effective health tracking and improvement of medical care quality.

Optical thermometry of Er3+ in electrospun yttrium titanate nanobelts

Er³⁺/Yb³⁺ co-doped Y₂Ti₂O₇ (YTOEY) nanobelts with a thickness of ∼100 nm exhibiting a high sensitivity and rapid responsiveness in temperature sensing are obtained.

Upconversion photoluminescence of Ho3+-Yb3+ doped barium titanate nanocrystallites: Optical tools for structural phase detection and temperature probing

Authors have explored the photo-physical properties of Ho³⁺-Yb³⁺ doped BaTiO₃ nanocrystals and proposed an intuitive method to probe temperature and crystal phase structure of the matrix. Structural phase change of doped crystals was analyzed in terms of their X-ray diffraction, and it was confirmed through second harmonic generation. We give insights on upconversion of energy of light-emission in Ho³⁺-Yb³⁺: BaTiO₃ nanocrystals upon a 980 nm laser-light excitation and subsequently, the excited state dynamics were studied with the help of dependence of upconversion luminescence on excitation power and measuring-temperature. To understand the nature of occupancies of the Ho³⁺ ions at the Ti- and Ba-sites, we performed site-selective, time-resolved spectroscopic measurements at various crystal phases. Based on the lifetime analysis, it is inferred that the Ho³⁺ ions are present at two types of sites in barium titanate lattice. One of those is the 6-coordinated Ti-site of low symmetry, while the other one is the 12-coordinated Ba-site of higher symmetry. The upconversion emission of the nanocrystals are found to be temperature-sensitive (12 to 300 K), indicating possible use as a self-referenced temperature probe. An analysis of the temperature dependent emissions from ⁵F₄ and ⁵S₂ levels of Ho³⁺ ions, gives a maximum value of temperature sensitivity ~ 0.0095 K⁻¹ at 12 K. Furthermore, we observe a sharp change in the luminescence intensity at ~180 K due to a ferroelectric phase change of the sample. The correlation of upconversion luminescence with the results of X-ray diffraction and second harmonic generation at different crystal phases implies that the frequency upconversion may be used as a probe of structural change of the lattice.

Near-infrared excited luminescence and in vitro imaging of HeLa cells by using Mn2+ enhanced Tb3+ and Yb3+ cooperative upconversion in NaYF4 nanocrystals

Advanced biodetection and bioimaging require fluorescent labels which exhibit many, easily distinguishable colors to identify or study numerous biotargets in a single sample. Although numerous different colors have been demonstrated with lanthanide doped nanoparticles, these colors usually originate from various ratios of overlapping multiple emission bands from activators, which severely limits the number of available labels. As a consequence, different lanthanide doped labels cannot be easily distinguished from each other (e.g. Er3+ from Ho3+) in a quantitative way, when such labels are co-localized during microscopy wide-field imaging. It is therefore reasonable to expand the available choice of spectral signatures and not rely on just different colors. Other ions, such as Tb3+ or Eu3+, can offer new possibilities and unique spectral features in upconversion mode in this respect. For example, despite partial overlap with Er3+ or Ho3+ emission spectra, Tb3+ ions display also unique and easily distinguishable spectral features at 580 nm. Unfortunately, in terms of brightness, Tb3+ emission in upconversion mode is typically too weak to be useful. To improve the Tb3+ upconversion emission intensity, a new approach, i.e. Mn2+ co-doping, has been proposed and verified in this work. A versatile optimization of Tb3+, Yb3+ and Mn2+ ion concentrations has been performed based on luminescence spectra and lifetime studies. The most intense emission was achieved for nanoparticles doped with 10% Mn2+ ions, with over 30 times brighter intensity of Tb3+ ions compared to the emission of nanocrystals without the addition of Mn2+ ions. Additionally, as a proof of the concept, the surface of nanoparticles was coated with proteins and conjugated with folic acid, and such biofunctionalized nanoparticles were subsequently used for bioimaging of HeLa cells.

Extending the temperature sensing range using Eu3+ luminescence up to 865 K in a single crystal of EuPO4

Extending the temperature sensing range up to 865 K using an appropriate choice of excitation wavelength and coupling scheme in a single crystal sample of EuPO₄.