Broad-scope dual-mode modulation thermometry based on up-conversion phosphor GaNbO4: Yb3+/Er3+

Fluorescence temperature measurement technology has set off another upsurge in non-contact temperature measurement, but still suffers from the large error for single-mode thermometry. Herein, in a broad temperature range of 93–633 K, a dual-mode modulation thermometry based on up-conversion phosphor of GaNbO₄:Yb³⁺/Er³⁺ is realized with the maximum relative sensitivity (S_r) of 11.7% K⁻¹ (93 K) and 13.1% K⁻¹ (123 K), respectively. GaNbO₄:Yb³⁺/Er³⁺ phosphors were synthesized by high temperature solid-state method. The structure, surface morphology and the optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL). The fluorescence intensity ratio (FIR) readout method based on Er³⁺ thermal-coupled energy level (TCL) and non-thermal-coupled energy level (NTCL) was used to achieve the dual-mode temperature measurement with high temperature resolution and good repeatability in GaNbO₄:5 mol% Yb³⁺ and 5 mol% Er³⁺ phosphors. All the results show that GaNbO₄:Yb³⁺/Er³⁺ phosphors have great application potential in high sensitivity broadband thermometry.     

Multifunctional optical thermometry based on the stark sublevels of Er3+ in CaO-Y2O3: Yb3+/Er3+

A conventional high temperature solid state method was utilized to prepare CaO-Y₂O₃, which is a potential candidate for manufacturing crucible material to melt titanium and titanium alloys with low cost. Meanwhile, Yb³⁺ ions and Er³⁺ ions were selected as the sensitizers and activators respectively to dope into CaO-Y₂O₃, aimed at providing real-time optical thermometry during the preparation process of titanium alloys realized using fluorescence intensity ratio (FIR) technology. The results reveal that a high measurement precision can be acquired by using the Stark sublevels of Er^{3+ 4}F_9/2 to measure the temperature with a maximum absolute error of only about 3 K. In addition, by analyzing the dependence of ⁴I_13/2 → ⁴I_15/2 transition on pump power of 980 nm excitation wavelength, it was found that the laser-induced thermal effect has almost no influence on the temperature measurement conducted by using the FIR of the Stark sublevels of Er^{3+ 4}I_13/2, which means that a high excitation pump power can be used to obtain strong NIR emission and good signal-to-noise ratio for optical thermometry without the influence of the laser-induced thermal effect. All the results reveal that CaO-Y₂O₃: Yb³⁺/Er³⁺ is an excellent temperature sensing material with high measurement precision.     

Enhanced up-conversion luminescence and optical thermometry characteristics of Er3+/Yb3+ co-doped Sr10(PO4)6O transparent glass-ceramics

In this paper, the Yb³⁺/Er³⁺ co-doped parent glass (PG) with composition (in mol%) of 30P₂O₅-10B₂O₃-38SrO-22K₂O and transparent glass-ceramics (GCs) containing hexagonal Sr₁₀(PO₄)₆O nanocrystals (NCs) were synthesized for the first time by melt-quenching method and subsequent heating treatment in air. Under 980 nm laser prompting, the GCs samples showed intense red and green up-conversion emissions compared to those characteristics for the PG sample. The emission intensities varied with Er³⁺ concentration and heat treatment conditions. Furthermore, in Yb³⁺/Er³⁺ co-doped GCs specimens, the optical thermometry was researched by means of fluorescence intensity ratio (FIR) of ⁴S_3/2 and ²H_11/2 levels. The GC sample heated at 620°C for 5 hours possessed a high relative temperature sensitivity (S_r) of 0.769% K⁻¹ at 303 K and the maximal absolute temperature sensitivity (S_a) of 5.951 × 10⁻³ K⁻¹ at 663 K, respectively. It is expected that the as-fabricated GC materials with Sr₁₀(PO₄)₆O NCs are promising efficient up-conversion materials for optical temperature sensor.     

Highly Sensitive Optical Thermometry of Yb3+‐Er3+ Codoped AgLa(MoO4)2 Green Upconversion Phosphor

Er³⁺–Yb³⁺ codoped AgLa(MoO₄)₂ phosphors with intense green emission from ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions and negligible red emission from ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ were synthesized by sol–gel process. Its temperature sensing performance was evaluated based on the temperature dependence of fluorescence intensity ratio (FIR) of two green emission bands in the range 300–510 K. The maximum sensitivity of AgLa(MoO₄)₂: 0.02Er³⁺/0.4Yb³⁺ is approximately 0.018 K^?1 at 480 K, which is much higher than those of reported samples based on green emissions of Er³⁺. Result suggests that AgLa(MoO₄)₂: Er³⁺/Yb³⁺ has a great potential application in optical temperature sensors.     

Yb3+/Er3+ co-doped Gd2Te4O11 nanosheets with intrinsic polarity: One-step hydrothermal synthesis and upconverted optical temperature measuring ability

The digadolinium tellurite phosphors of Gd₂Te₄O₁₁(GTO):Yb³⁺/Er³⁺ have been successfully synthesized as upconversion luminescence (UCL) materials via one-step hydrothermal method. The crystal structure, morphology, and upconversion luminescence property were systematically characterize by XRD, SEM, and spectroscopy techniques. The Rietveld refinements of crystal structure were carried out on the XRD patterns and the feature of crystal structure was analyzed. Under the 980 nm NIR excitation, these materials showed very bright upconverted emissions. The concentrations of Yb³⁺ and Er³⁺ were optimized and the strongest upconverted emissions were achieved in the GTO:15%Yb³⁺/1%Er³⁺. The possible energy transfer mechanism of UCL was proposed based upon the analysis of power-dependent UCL and fluorescence kinetics. Furthermore, the fluorescence intensity ratio (FIR) derive from the two thermally coupled energy levels (²H_11/2 and ⁴S_3/2) of Er³⁺ was employed as indicator for temperature measurement. The maximum absolute sensitivity can be achieved to be 7.34 × 10^?3 K^?1 at 501 K. This material exhibited good reliability and repeatability in optical temperature measurement, which could be a novel promising candidate for noncontact temperature sensors.     

Synthesis and up-conversion of Er3+ and Yb3+ Co-doped LiY(MoO4)2@SiO2 for optical thermometry applications

Non-contact temperature sensors based on the fluorescence intensity ratio (FIR) have been widely investigated owing to their high sensitivity and reliable real-time monitoring. Herein, the SiO₂-coated LiY(MoO₄)₂@SiO₂:Er³⁺,Yb³⁺ phosphor was investigated as an optical thermometry material, which was synthesized using the conventional solid state reaction and coated by a facile wet chemical route. The effect of surface modification on FIR was systematically characterized by structural analyses and spectral measurements and the temperature-dependent up-conversion FIR was investigated from 303 to 603 K under a 980 nm laser excitation. The results showed that the FIR value was thermally stable and the SiO₂ coating led to a higher FIR sensitivity as well as a higher saturation threshold. This work would pave a way to design interesting optical thermometry materials in up-conversion phosphors with better properties.     

Linear response fluorescent temperature-sensing properties based on Stark sublevels of Er3+-doped Pb(Mg1/3Nb2/3)O3-PbTiO3-Pb(Yb1/2Nb1/2)O3 ceramics

The Er³⁺ doped 0.84(PMN-PT)?0.14PYN ceramics were employed as a temperature-sensing material. The obtained compounds exhibit strong visible upconversion (UC) fluorescence under a 980-nm diode laser excitation. On account of the Stark split effect, the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels of Er³⁺ split into two Stark sublevels, respectively. The fluorescence intensity ratios (FIR) between these sublevels were calculated in a temperature range of 133–573?K, and a linear FIR vs. temperature relation has been found. FIR of A/B shows a wide temperature sensing range with a relatively low sensitivity of 0.003?K^?1. Meanwhile E/C illustrates the highest sensitivity of 0.0134?K^?1 but in the smallest temperature range 453–573?K. FIR technique provides us a optical thermometric method with constant sensitivity in a wide temperature range.     

Optical Thermometry Based on Up‐Conversion Luminescence Behavior of Er3+ ‐Doped Transparent Sr2YbF7 Glass‐Ceramics

Transparent novel glass‐ceramics containing Sr₂YbF₇:Er³⁺ nanocrystals were successfully fabricated by melt‐quenching technique. Their structural and up‐conversion luminescent properties were systemically investigated by XRD, HRTEM, and a series of spectroscopy methods. The temperature‐dependent up‐conversion spectra prove that ²H_11/2 and ⁴S_3/2 levels of Er³⁺ are thermally coupled energy levels (TCEL). Consequently, the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 emissions of Er³⁺ in Sr₂YbF₇:Er³⁺ glass‐ceramics can be used as optical thermometry based on fluorescence intensity ratio (FIR) technique. Combined with low phonon energy and high thermal stability, Er³⁺ ions in Sr₂YbF₇ glass‐ceramics present broad operating temperature range (300–500 K), large energy gap of TCEL (786 cm^?1) and high theoretical maximum value of relative sensitivity (62.14 × 10^?4 K^?1 at 560 K), which suggests that Sr₂YbF₇:Er³⁺ glass‐ceramics may be excellent candidates for optical temperature sensors.     

BiLaWO6: Er3+/Tm3+/Yb3+ phosphor: Study of multiple fluorescence intensity ratiometric thermometry at cryogenic temperatures

Highly thermally stable Er³⁺/Tm³⁺/Yb³⁺ tri-doped bismuth lanthanum tungstate phosphors were prepared by high temperature solid-state reaction method. The structural and morphological properties of the prepared phosphors were analysed by X-ray diffraction (XRD), Raman spectroscopy and Scanning electron microscopy (SEM) coupled with energy dispersion spectrum (EDS). Visible upconversion (UC) luminescence was measured by exciting the phosphors with 980 nm laser radiation. The dependence of the UC intensity of each emission band of Er³⁺ and Tm³⁺ ions as a function of temperature in the range from 30 to 300 K was monitored. Fluorescence intensity ratios (FIR) of thermally coupled levels (TCL) and non-thermally coupled levels (NTCL) were analysed and verified with appropriate theoretical validation. The absolute (S_A) and relative sensitivities (S_R) were estimated and compared with the reported systems. In the present case of BiLaWO₆: Er³⁺/Tm³⁺/Yb³⁺, S_R (0.43 % K^?1) related to TCL of Er³⁺ UC is found to have maximum sensitivity compared to any of the NTCL combinations at 300 K. From this study we inferred that the S_R values estimated from NTCL are smaller than that of TCL involved in BLW: Er³⁺/Tm³⁺/Yb³⁺ phosphor. The temperature dependent CIE color coordinates were also evaluated in the cryogenic temperature region.     

Fabrication and optical thermometry of transparent glass-ceramics containing Ag@NaGdF4: Er3+ core-shell nanocrystals

Novel transparent glass-ceramics containing Ag@NaGdF₄:Er³⁺ core-shell nanocrystals were fabricated successfully by a melt-quenching method and subsequent heating. X-ray diffraction and transmission electron microscope images show that precious metal Ag is successfully encapsulated by the NaGdF₄:Er³⁺ nanoparticles to form an Ag@NaGdF₄:Er³⁺ core-shell structure in glass matrix. Compared with the NaGdF₄:Er³⁺ glass-ceramics, Ag@NaGdF₄:Er³⁺ core-shell glass-ceramics shows the great enhancement of emission intensity. The thermometric parameters such as fluorescence emission intensity, fluorescence intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2), and temperature sensitivity can be effectively controlled by changing the Ag concentration. When 0.15 mol% Ag is co-doped, the sensitivity of S_R in Ag@NaGdF₄:Er³⁺ core-shell glass-ceramics reaches a maximum value. This work presents a new method to enhance emission intensity and optical thermometry ability of NaGdF₄:Er³⁺ through constructing Ag@NaGdF₄:Er³⁺ core-shell structure.     

Upconversion emission,cathodoluminescence and temperature sensing behaviors of Yb3+ ions sensitized NaY(WO4)2:Er3+ phosphors

A series of Yb³⁺ ions sensitized NaY(WO₄)₂:Er³⁺ phosphors were synthesized through a solid-sate reaction method. The X-ray diffraction (XRD), upconversion (UC) emission and cathodoluminescence (CL) measurments were applied to characterize the as-prepared samples. Under the excitation of 980 nm light, bright green UC emissions corresponding to (²H_11/2,⁴S_3/2)→⁴I_15/2 transitions of Er³⁺ ions were observed and the UC emission intensities showed an upward trend with increasing the Yb³⁺ ion concentration, achieving its optimum value at 25 mol%. Furthermore, the temperature sensing behavior based on the thermally coupled levels (²H_11/2,⁴S_3/2) of Er³⁺ ions was analyzed by a fluorescence intensity ratio technique. It was found that the obtained samples can be operated in a wide temperature range of 133–773 K with a maximum sensitivity of approximately 0.0112 K⁻¹ at 515 K. Ultimately, strong CL properties were observed in NaY(WO₄)₂:0.01Er³⁺/0.25Yb³⁺ phosphors and the CL emission intensity increased gradually with the increment of accelerating voltage and filament current.     

A Novel Multifunctional Upconversion Phosphor: Yb3+/Er3+ Codoped La2S3

Yb³⁺/Er³⁺ codoped La₂S₃ upconversion (UC) phosphors have been synthesized using high‐temperature solid‐state method. Under 971‐nm excitation, the maximum luminescence power can reach 0.64 mW at the excitation power density of 16 W/cm² and an absolute power yield of 0.36% was determined by an absolute method at the excitation power density of 3 W/cm², and the quantum yield of La₂S₃:Yb³⁺, Er³⁺ (green ~0.18%, red ~0.03%, integration ~0.21) was comparable to that of NaYF₄:Yb³⁺, Er³⁺ nanocrystals (integration ~0.005–0.30). Frequency upconverted emissions from two thermally coupled excited states of Er³⁺ were recorded in the temperature range 100–900 K. The maximum sensitivity of temperature sensing is 0.0075 K^?1. As the excitation power density increases, the temperature of host materials rapidly rises and the top temperature can reach to 600 K. Given the intense UC emission, high sensitivity, as well as good photothermal stability, La₂S₃:Yb³⁺/Er³⁺ phosphor can become a promising composite material for photothermal ablation of cancer cells possessing the functions of temperature sensing and in vivo imaging.     

Multipath optical thermometry realized in CaSc2O4: Yb3+/Er3+ with high sensitivity and superior resolution

Design and fabrication of contactless optical thermometer with rapid and accurate performance has become a research hotspot in recent years. Herein, CaSc₂O₄: Yb³⁺/Er³⁺ is employed as the intermediary for temperature sensing under the excitation of 980 nm, which is proven to afford an ultra-sensitive and high-resolution optical thermometry in multiple ways based on the fluorescence intensity ratio (FIR) technology. The optimal thermal sensing behaviors are realized by the FIR of Er³⁺:²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2 transition, which has a relative sensitivity of 1184/T² and a minimal resolution of 0.03 K along with a maximal absolute error of 0.96 K. Besides that, the FIR between the thermally coupled Stark sublevels of Er³⁺:⁴F_9/2 manifold (FIR_R) as well as that of Er³⁺: ⁴I_13/2 manifold (FIR_N) can also provide excellent optical thermometry. The relative sensitivity of FIR_R-based and FIR_N-based optical thermometers are calculated to be 402/T² and 366/T², respectively, with a same minimal resolution of 0.09 K, which possess the potential to be used for biomedicine due to the inherent advantage of their operating wavelengths located in the biological window. The results demonstrate that CaSc₂O₄: Yb³⁺/Er³⁺ is a promising candidate for temperature sensing with multipath, high sensitivity, and superior resolution.     

High sensitive Ln3+/Tm3+/Yb3+ (Ln3+ = Ho3+, Er3+) tri-doped Ba3Y4O9 upconverting optical thermometric materials based on diverse thermal response from non-thermally coupled energy levels

Upconversion (UC) optical thermometers using the fluorescence intensity ratio (FIR) technique arising from the thermally coupled energy levels (TCLs) are still suffering from low sensitivity owing to the restriction of small energy gap. In the present study, a strategy to strive for superior temperature sensitivity and signal discriminability is employed with the help of non-thermally coupled energy levels (NTCLs). A novel tri-doped Ba₃Y₄O₉: Ho³⁺/Tm³⁺/Yb³⁺ phosphor with rhombohedral symmetry was successfully prepared via a solid-state reaction method, and the temperature sensing performance was evaluated by analyzing temperature-dependent upconversion emission spectra. The emission intensities of both Ho³⁺ and Tm³⁺ activators can be almost completely restored to their original values when the temperature of the sample is cooled to room temperature. The temperature-dependent FIR between NTCLs can be fitted well by a derived three-term equation with the correlation coefficient above 99.6%, and the FIR of NTCLs exhibits high temperature sensitivity over a wide temperature range owing to the different temperature responses of the NTCLs. The maximum absolute sensitivity S_A and relative sensitivity S_R values reaches as high as 0.0552?K^?1 and 1.49% K^?1, respectively, which are much higher than those of the previously reported bulk UC optical temperature sensing materials. Moreover, the emission bands of NTCLs are well separated, which endows the material a good signal discriminability for temperature detection. Excellent temperature sensing performance is also demonstrated in Er³⁺/Tm³⁺/Yb³⁺ tri-doped Ba₃Y₄O₉, evidencing the validity of this strategy. These results indicate that the present UC materials can be potential candidates for optical temperature sensors, and the present strategy will provide a thought for developing other innovative UC temperature sensing materials.     

Ferroelectric,upconversion emission and optical thermometric properties of color-controllable Er3+-doped Pb(Mg1/3Nb2/3)O3-PbTiO3-Pb(Yb1/2Nb1/2)O3 ferroelectrics

The (0.98-x)(0.6Pb(Mg_1/3Nb_1/3)O₃-0.4PbTiO₃)-xPb(Yb_1/3Nb_1/3)O₃-0.02Pb(Er_1/2Nb_1/2)O₃ ((0.98-x)(PMN-PT)-xPYN:Er³⁺) ceramics were prepared through a solid-state reaction method. The phase structure, piezoelectric response, ferroelectric performance and upconversion emission of the ceramics were systematically investigated. The phase structure, the electrical and optical properties are strongly related to the content of PYN. The optimized piezoelectric response and upconversion emissions of the ceramics were achieved near x = 0.12, which locates in the morphotropic phase boundary (MPB) composition. Furthermore, the temperature sensing behaviors of the resultant compounds based on the thermally coupled levels of ²H_11/2 and ⁴S_3/2 of Er³⁺ ions in the temperature range of 133–573 K were studied by utilizing the fluorescence intensity ratio technique. Additionally, the thermal effect, which is induced by the laser pump power, of the studied ceramics is also investigated and the produced temperature is enhanced from 268 to 348 K with the pump power rising from 109 to 607 mW.     

Up-conversion luminescence and optical thermometry properties of transparent glass ceramics containing CaF2:Yb3+/Er3+ nanocrystals

A series of Yb³⁺/Er³⁺ codoped transparent oxyfluoride glass ceramics with various amounts of Yb³⁺ have been successfully fabricated and characterized. Under 980 nm laser prompting, the samples produce intense red, green and blue up-conversion emissions, and the emission intensities increase with Yb³⁺ concentration and heat treatment temperature. Before losing good transparency in the visible region, optimum emission intensities are obtained for the sample with 25 mol% of Yb³⁺ at a heat treatment temperature of 680 °C. A possible up-conversion mechanism is proposed from the dependence of emission intensities on pumping power. The fluorescence intensity ratio between the two thermally coupled levels ²H_11/2 versus ⁴S_3/2 was measured with the laser output power of 57 mW to avoid the possible laser induced heating effect. The fluorescence intensity ratio values in the temperature range from 295 K to 723 K can be well fitted with the equation: A exp (−∆E/k_BT), where A = 6.79 and ∆E=876 cm⁻¹. The relative temperature sensitivity at 300 K was evaluated to be 1.4% K⁻¹. All the results suggest that the Yb³⁺/Er³⁺ codoped CaF₂ glass ceramics is an efficient up-conversion material with potential in optical fiber temperature sensing.     

Thermal enhancement of up-conversion luminescence in Lu2W2.5Mo0.5O12: Er3+, Yb3+ phosphors

Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors were synthesized through high temperature solid state method. Under 980 nm laser excitation, the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ compounds show thermal enhancement of up-conversion luminescence (UCL), which is attributed to the lattice contraction and distortion from negative thermal expansion (NTE) of Lu₂W_2.5Mo_0.5O₁₂ host enhancing the energy transfer of Yb³⁺ to Er³⁺, eliminating the energy transfer of Er³⁺ to Er³⁺ through Er³⁺ single-doped Lu₂W_2.5Mo_0.5O₁₂ phosphors without thermal enhancement of UCL. The green luminescence intensities at 693 K of the Lu_1.98-xW_2.5Mo_0.5O₁₂: 0.02Er³⁺, xYb³⁺ (x = 0.2, 0.3, 0.4) samples are 4.6, 4.3 and 7.0 times as that of 302 K, respectively. And through fluorescence intensity ratio (FIR) technique, the corresponding maximum absolute sensitivities are 0.00741, 0.00744 and 0.00723, respectively. The green monochromaticity of UCL spectra in Er³⁺/Yb³⁺ co-doped samples increase with the increasing of temperature, and the possible UCL mechanism with temperature was discussed. The results indicate that the Lu₂W_2.5Mo_0.5O₁₂: Er³⁺/Yb³⁺ phosphors can be applied at a high temperature as optical thermometer with a good green monochromaticity.     

Multi-phase glass-ceramics containing CaF2: Er3+ and ZnAl2O4:Cr3+ nanocrystals for optical temperature sensing

Oxyfluoride transparent glass-ceramics (GC) containing CaF₂ and ZnAl₂O₄ nanocrystals have been fabricated with melt-quenching method. By carrying out the heat treatment of the precursor glass (PG), Er³⁺ and Cr³⁺ were selectively partitioned into CaF₂ and ZnAl₂O₄ nanocrystals, respectively. The obtained multi-phase GC exhibited strong upconversion (UC) fluorescence of Er³⁺ as well as intense down-conversion (DC) fluorescence of Cr³⁺. Under 980 nm excitation, the green UC fluorescence of Er³⁺ due to ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition and the red DC fluorescence lifetime of Cr³⁺ due to ²E, ⁴T₂ → ⁴A₂ transition were found to be highly dependent on the temperature and makes them possibly suitable for Optical Thermometry. With least-square fitting methods, the FIR of Er³⁺ from thermally coupled energy states (²H_11/2 and ⁴S_3/2) produced maximum temperature sensing sensitivity values of 0.33% K⁻¹ at 437 K and 0.36% K⁻¹ at 267 K, respectively. Similarly, fluorescence lifetime of Cr³⁺ attributed to the parity forbidden (²E → ⁴A₂) and spin allowed (⁴T₂ → ⁴A₂) produced the maximum temperature sensor sensitivity value equal to 0.67% K⁻¹ at 535 K.     

Ultrasensitive optical thermometer based on abnormal thermal quenching Stark transitions operating beyond 1500 nm

Light with wavelength longer than 1500 nm has great potential to afford deep bio-tissue penetration due to its extremely weak photon scattering and undetectable autofluorescence in vivo. Here, in order to satisfy the requirements for thermometry during the tumor hyperthermia process, an ultrasensitive optical thermometer operating beyond 1500 nm is developed by employing the thermally coupled Stark sublevels of Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition based on fluorescence intensity ratio (FIR) technology in Yb³⁺ and Er³⁺ codoped BaY₂O₄. Compared with the typical upconversion (UC) material β-NaYF₄: Yb³⁺/Er³⁺ and Y₂O₃: Yb³⁺/Er³⁺, BaY₂O₄: Yb³⁺/Er³⁺ shows more intense red Er³⁺: ⁴F_9/2 → ⁴I_15/2 transition and 1.5 μm near-infrared (NIR) Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition induced by its larger phonon energy and higher quenching concentration of Er³⁺. An equivalent four-level model is proposed to investigate the temperature characteristics of the NIR emission, from which four Stark transitions are separated from the raw spectra, named α, β, γ, and δ respectively. Then, the NIR thermal sensing performance have been developed by utilizing the FIR of I_β to I_α and I_γ to I_α. More importantly, an ultra-high sensitivity for optical thermometry has been obtained through the combination of transition β and γ, especially in the physiological temperature region. Furthermore, the detection depth of NIR light in bio-tissues is assessed by an ex vivo test, demonstrating that the maximal detection depth of NIR emission can reach to 8 mm without any influence on optical thermometry. These findings indicate that Yb³⁺ and Er³⁺ codoped BaY₂O₄ is a remarkable contender for optical thermometry in deep tissue with ultra-high sensitivity.     

High-sensitive temperature sensing use NIR upconversion luminescence of Er3+-core@Tm3+-shell with good robustness

In recent years, lanthanide doped materials have been extensively studied in the field of fluorescence temperature sensing due to their abundant emission levels and sensitive thermal response. Temperature sensing based on fluorescence intensity ratio (FIR) of upconversion nanoparticles has the advantages of fast temperature response, non-aggressiveness, and high spatial resolution. However, the most reported FIR sensing has limited sensitivity, probably due to the use of thermal coupling levels. Herein, we report a novel FIR temperature measurement based on non-thermal coupling levels of NaGdF₄:Yb³⁺/Er³⁺@NaGdF₄@NaGdF₄:Yb³⁺/Tm³⁺ core-shell-shell nanostructure, which has high sensitivity and robustness simultaneously. The relative sensitivity based on I₈₀₁/I₆₅₄ and I₈₀₁/I₈₄₁ of Tm³⁺ to Er³⁺ can reach up to 4.56 (303 K) and 3.82% K⁻¹ (313 K), respectively. Between them, FIR of I₈₀₁/I₈₄₁ is independent of excitation power and time. These results show the great potential of FIR based on non-thermal coupling levels in high-sensitive and robust temperature sensors.