Near-Infrared-to-Near-Infrared Optical Thermometer BaY2O4: Yb3+/Nd3+ Assembled with Photothermal Conversion Performance.

Nowadays, the construction of photothermal therapy (PTT) agents integrated with real-time thermometry for cancer treatment in deep tissues has become a research hotspot. Herein, an excellent photothermal conversion material, BaY2O4: Yb3+/Nd3+, assembled with real-time optical thermometry is developed successfully. Ultrasensitive temperature sensing is implemented through the fluorescence intensity ratio of thermally coupled Nd3+: 4Fj (j = 7/2, 5/2, and 3/2) with a maximal absolute and relative sensitivity of 68.88 and 3.29% K-1, respectively, which surpass the overwhelming majority of the same type of thermometers. Especially, a thermally enhanced Nd3+ luminescence with a factor of 180 is detected with irradiation at 980 nm, resulting from the improvement in phonon-assisted energy transfer efficiency. Meanwhile, the photothermal conversion performance of the sample is excellent enough to destroy the pathological tissues, of which the temperature can be raised to 319.3 K after 180 s of near-infrared (NIR) irradiation with an invariable power density of 13.74 mW/mm2. Besides, the NIR emission of Nd3+ can reach a depth of 7 mm in the biological tissues, as determined by an ex vivo experiment. All the results show the potential application of BaY2O4: Yb3+/Nd3+ as a deep-tissue PTT agent simultaneously equipped with photothermal conversion and temperature sensing function.

Constructing ultra-sensitive dual-mode optical thermometers: Utilizing FIR of Mn4+/Eu3+ and lifetime of Mn4+ based on double perovskite tellurite phosphor.

A strategy of optical temperature sensing was developed by using various thermal quenching of Mn4+ and Eu3+ for double perovskite tellurite phosphor in optical thermometers. Herein, SrGdLiTeO6 (SGLT): Mn4+,Eu3+ phosphors were synthesized by a high-temperature solid-state reaction method. The temperature-dependent emission spectra indicated that two distinguishable emission peaks originated from Eu3+ and Mn4+ exhibited significantly diverse temperature responses. Therefore, optical thermometers with a dual-mode mechanism were designed by employing a fluorescence intensity ratio (FIR) of Mn4+ (2Eg→4A2g) and Eu3+ (5D0→7F1,2) and the decay lifetime of Mn4+ as the temperature readouts. The temperature sensing of the phosphors ranging from 300 to 550 K were studied. The maximum relative sensitivities (Sr) are obtained as 4.9% K-1 at 550 K. Meanwhile, the 695 nm emission of Mn4+ possessed a temperature-dependent decay lifetime with Sr of 0.229% K-1 at 573 K. Relevant results demonstrate the SrGdLiTeO6:Mn4+, Eu3+ phosphor as an optical thermometer candidate and also provide constructive suggestions and guidance for constructing high-sensitivity dual-mode optical thermometers.

Strategy for optical thermometry based on temperature-dependent charge transfer to the Eu3+ 4f-4f excitation intensity ratio in Sr3Lu(VO4)3:Eu3+ and CaWO4:Nd3.

Optical thermometry has been developed as a promising temperature-sensing technique. We propose a new, to the best of our knowledge, strategy of fluorescence intensity ratio (FIR), based on an abnormal thermal quenching effect. In the phosphors of Sr3Lu(VO4)3:Eu3+ and CaWO4:Nd3+, the f-f emission intensity of the doped lanthanide ions increases with raising temperature upon the excitation of the charge transfer band (CTB) of the host. The abnormal thermal quenching is caused by the thermally activated absorption, which is proved by temperature-dependent diffuse reflectance spectra. The opposite change tendency of M-O (M=V5+ or W6+) CTB and Ln3+ (Ln=Eu3+ or Nd3+) f-f transitions has been observed in the temperature-dependent excitation spectra and employed as the thermometric probe in ratiometric luminescent thermometry. The strategy applies to the FIR technique in lanthanide singly doped phosphors and eliminates the limitation of thermal-coupled levels. It opens up new possibilities of ratiometric optical thermometry. In addition, the derived maximum relative sensitivity is larger than the value obtained via thermal-coupled levels in the same sample. This illustrates that optical thermometry based on abnormal thermal quenching might be a feasible and effective method.

Deep-Tissue Temperature Sensing Realized in BaY2O4:Yb3+/Er3+ with Ultrahigh Sensitivity and Extremely Intense Red Upconversion Luminescence.

In this paper, BaY2O4:Yb3+/Er3+, a high efficient red upconversion (UC) material, is first utilized as an optical thermometer in the biological window, accomplished through the fluorescence intensity ratio (FIR) of thermally coupled Stark sublevels of 4F9/2 (FIR(654/663)). The maximum absolute sensitivity of FIR(654/663)) is 0.19% K-1 at 298 K, which is much higher than most previous reports about FIR-based temperature sensors located in the biological windows. More importantly, the groove of FIR(654/663) for thermometry is nicely located in the physiological temperature range, indicating its potential thermometry application value in biomedicine. Furthermore, a simply ex vivo experiment is implemented to evaluate the penetration depth of the red emission in biological tissues, revealing that a detection depth of 6 mm can be achieved without any effect on the FIR values of I654 to I663. Beyond that, the temperature sensing behaviors of the thermally coupled levels 2H11/2 and 4S3/2 (FIR(523/550)) are also investigated in detail. In the studied temperature range, the absolute sensitivity of FIR(523/550) monotonously increases with the rising temperature and reaches its maximum value 0.31% K-1 at 573 K. All the results imply that BaY2O4:Yb3+/Er3+ is a promising candidate for deep-tissue optical thermometry with high sensitivity.

Multifunctional Luminescent Material Eu(III) and Tb(III) Complexes with Pyridine-3,5-Dicarboxylic Acid Linker: Crystal Structures, Tunable Emission, Energy Transfer, and Temperature Sensing.

A series of lanthanide organic complexes, namely, [Ln2PDC3(H2O)3]H2O (Ln = Eu, Tb, Eu xTb2- x, H2PDC: pyridine-3,5-dicarboxylic acid), were synthesized via hydrothermal method. Eu-PDC was characterized by single-crystal X-ray diffraction. The powder X-ray diffraction, Fourier transform infrared spectra, and thermogravimetric analysis illustrate that Eu-PDC, Tb-PDC, and Eu xTb2- x-PDC complexes are isostructural. All the complexes exhibit strong emission even though water molecules coordinated in the structure. Tunable emission color from green, yellow to red is realized by adjusting the Eu/Tb ratio in mixed Eu xTb2- x-PDC complexes. Energy transfer from Tb3+ to Eu3+ is discussed in detail via fluorescence decay and time resolution spectra. The detailed energy transfer process sees that the emitting color of Eu0.15Tb1.85-PDC shifts from green to orange in a time interval of 153-790 µs. In addition, temperature-dependent emission of the Eu0.05Tb1.95-PDC complex indicates that it is a potential solid state material for a luminescence thermometer in the range of 293-333 K. The temperature resolution is less than 0.16 K. The optical properties of the EuTb-PDC complex indicate that it is a multifunctional luminescent material with promising applications in temperature sensing, time resolution imaging, lighting, and displaying.

Dual-Mode Optical Thermometry Based on the Fluorescence Intensity Ratio Excited by a 915 nm Wavelength in LuVO4:Yb3+/Er3+@SiO2 Nanoparticles.

The optical thermometry properties of LuVO4:Yb3+/Er3+@SiO2 nanoparticles (NPs) are studied in detail. In order to avoid the overheating effect for biological tissue caused by 980 nm radiation, 915 nm is employed as the excitation wavelength to investigate the upconversion (UC) and optical thermometry properties of the as-prepared NPs. In the visible region, the fluorescence intensity ratio (FIR) of the 2H11/2 and 4S3/2 levels of Er3+ is utilized to measure the temperature. The relative sensitivity SR in this case can be written as 1077/ T2, which is higher than that of ß-NaYF4:Yb3+/Er3+ NPs, ß-NaLuF4:Yb3+/Er3+ NPs, YVO4:Yb3+/Er3+ NPs, etc. In the near-infrared (NIR) region, an anomalous enhancement of the 4I13/2 → 4I15/2 transition with increasing temperature is observed. What is more, the FIR of peak 2 (located at 1496 nm) to peak 1 (located at 1527 nm) is changed regularly with increasing temperature, which can also be used to measure the temperature. The combination of the visible and NIR regions for optical thermometry can provide a self-referenced temperature determination to make measurement of the temperature more precise. In addition, the UC mechanism is also investigated, especially the population route of the 4F9/2 level of Er3+. Through analysis of the decay curves, we propose that the dominant way for populating the Er3+ 4F9/2 level is energy transfer from the Yb3+ 2F5/2 level to the Er3+ 4I13/2 level. All of the results reveal the potential application of LuVO4:Yb3+/Er3+@SiO2 NPs for dual-mode optical thermometry.

Improvement of Green Upconversion Monochromaticity by Doping Eu3+ in Lu2O3:Yb3+/Ho3+ Powders with Detailed Investigation of the Energy Transfer Mechanism.

The monochromaticity improvement of green upconversion (UC) in Lu2O3:Yb3+/Ho3+ powders has been successfully realized by tridoping Eu3+. The integral area ratio of green emission to red emission of Ho3+ increases 4.3 times with increasing Eu3+ doping concentration from 0 to 20 mol %. The energy transfer (ET) mechanism in the Yb3+/Ho3+/Eu3+ tridoping system has been investigated carefully by visible and near-infrared (NIR) emission spectra along with the decay curves, revealing the existence of ET from the Ho3+5F4/5S2 level tothe Eu3+5D0 level and ET from the Ho3+5I6 level to the Eu3+7F6 level. In addition, the population routes of the red-emitting Ho3+5F5 level in the Yb3+/Ho3+ codoped system under 980 nm wavelength excitation have also been explored. The ET process from the Yb3+2F5/2 level to the Ho3+5I7 level and the cross-relaxation process between two nearby Ho3+ ions in the 5F4/5S2 level and 5I7 level, respectively, have been demonstrated to be the dominant approaches for populating the Ho3+5F5 level. The multiphonon relaxation process originating from the Ho3+5F4/5S2 level is useless to populate the Ho3+5F5 level. As the energy level gap between the Ho3+5I7 level and Ho3+5I8 level matches well with that between Eu3+7F6 level and Eu3+7F0 level, the energy of the Ho3+5I7 level can be easily transferred to the Eu3+7F6 level by an approximate resonant ET process, resulting in a serious decrease in the red UC emission intensity. Since this ET process is more efficient than the ET from the Ho3+5F4/5S2 level to the Eu3+5D0 level as well as the ET from the Ho3+5I6 level to the Eu3+7F6 level, the integral area ratio of green emission to red emission of Ho3+ has been improved significantly.

Investigation of the Energy-Transfer Mechanism in Ho3+- and Yb3+-Codoped Lu2O3 Phosphor with Efficient Near-Infrared Downconversion.

A high-temperature solid-state method was used to synthesize the Ho3+- and Yb3+-codoped cubic Lu2O3 powders. The crystal structures of the as-prepared powders were characterized by X-ray diffraction. The energy-transfer (ET) phenomenon between Ho3+ ions and Yb3+ ions was verified by the steady-state spectra including visible and near-infrared (NIR) regions. Beyond that, the decay curves were also measured to certify the existence of the ET process. The downconversion phenomena appeared when the samples were excited by 446 nm wavelength corresponding to the transition of Ho3+: 5I8→5G6/5F1. On the basis of the analysis of the relationship between the initial transfer rate of Ho3+: 5F3 level and the Yb3+ doping concentration, it indicates that the ET from 5F3 state of Ho3+ ions to 2F5/2 state of Yb3+ ions is mainly through a two-step ET process, not the long-accepted cooperative ET process. In addition, a 62% ET efficiency can be achieved in Lu2O3: 1% Ho3+/30% Yb3+. Unlike the common situations in which the NIR photons are all emitted by the acceptors Yb3+, the sensitizers Ho3+ also make contributions to the NIR emission upon 446 nm wavelength excitation. Meanwhile, the 5I5→5I8 transition and 5F4/5S2→5I6 transition of Ho3+ as well as the 2F5/2→2F7/2 transition of Yb3+ match well with the optimal spectral response of crystalline silicon solar cells. The current research indicates that Lu2O3: Ho3+/Yb3+ is a promising material to improve conversion efficiency of crystalline silicon solar cell.

Enhancement of Eu3+ Red Upconversion in Lu2O3: Yb3+/Eu3+ Powders under the Assistance of Bridging Function Originated from Ho3+ Tridoping.

The red upconversion (UC) emission of Eu3+ ions in Lu2O3: Yb3+/Eu3+ powders was successfully enhanced by tridoping Ho3+ ions in the matrix, which is due to the bridging function of Ho3+ ions. The experiment data manifest that, in Yb3+/Eu3+/Ho3+ tridoped system, the Ho3+ ions are first populated to the green emitting level 5F4/5S2 through the energy transfer (ET) processes from the excited Yb3+ ions. Subsequently, the Ho3+ ions at 5F4/5S2 level can transfer their energy to the Eu3+ ions at the ground state, resulting in the population of Eu3+5D0 level. With the assistance of the bridging function of Ho3+ ion, this ET process is more efficient than the cooperative sensitization process between Yb3+ ion and Eu3+ ion. Compared with Lu2O3: 5 mol % Yb3+/1 mol % Eu3+, the UC intensity of Eu3+5D0→7F2 transition in Lu2O3: 5 mol % Yb3+/1 mol % Eu3+/0.5 mol % Ho3+ is increased by a factor of 8.

Importance of suppression of Yb(3+) de-excitation to upconversion enhancement in ß-NaYF4: Yb(3+)/Er(3+)@ß-NaYF4 sandwiched structure nanocrystals.

Nanosized Yb(3+) and Er(3+) co-doped ß-NaYF4 cores coated with multiple ß-NaYF4 shell layers were synthesized by a solvothermal process. X-ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near-infrared spectra as well as the decay curves were also measured. A 40-fold intensity increase for the green upconversion and a 34-fold intensity increase for the red upconversion were observed as the cores are coated with three shell layers. The origin of the upconversion enhancement was studied on the basis of photoluminescence spectra and decay times. Our results indicate that the upconversion enhancement in the sandwiched structure mainly originates from the suppression of de-excitation of Yb(3+) ions. We also explored the population of the Er(3+4)F9/2 level. The results reveal that energy transfer from the lower intermediate Er(3+4)I13/2 level is dominant for populating the Er(3+4)F9/2 level when the nanocrystal size is relatively small; however, with increasing nanocrystal size, the contribution of the green emitting Er(3+4)S3/2 level for populating the Er(3+4)F9/2 level, which mainly comes from the cross relaxation energy transfer from Er(3+) ions to Yb(3+) ions followed by energy back transfer within the same Er(3+)-Yb(3+) pair, becomes more and more important. Moreover, this mechanism takes place only in the nearest Er(3+)-Yb(3+) pairs. This population route is in good agreement with that in nanomaterials and bulk materials.

The energy transfer mechanism in Pr(3+) and Yb(3+) codoped ß-NaLuF(4) nanocrystals.

The Pr(3+) and Yb(3+) codoped ß-NaLuF4 hexagonal nanoplates with a size of 250 nm × 110 nm were synthesized by a solvothermal process. X-Ray diffraction and scanning electron microscopy were used to characterize the crystal structure and morphology of the materials. The visible and near infrared spectra as well as the decay curves of Pr(3+):(3)P0 level were used to demonstrate the energy transfer from Pr(3+) ions to Yb(3+) ions. The downconversion phenomenon has been observed under the direct excitation of the (3)P2 level of Pr(3+). According to the analysis of the dependence of the initial transfer rate upon Yb(3+) ion concentration, it indicates that the ET from Pr(3+) ions to Yb(3+) ions is only by a two-step ET process when the Yb(3+) concentration is very low; however, with the increase of the Yb(3+) concentration, a cooperative ET process occurs and gradually increases; when the Yb(3+) ion concentration increases to 20 mol%, the ET from Pr(3+) ions to Yb(3+) ions occurs only by the cooperative ET process. When the doping concentration of Yb(3+) ions reaches 20 mol% at a fixed concentration of Pr(3+) ions (1 mol%), the theoretical quantum efficiency is 192.2%, close to the limit of 200%. The current research has great potential in improving the conversion efficiency of silicon solar cells.

CDs-ICG@BSA nanoparticles for excellent phototherapy and in situ bioimaging.

For the diagnosis and treatment of cancer, a great challenge is the fabrication of straightforward, non-toxic, multifunctional green nanomaterials. In this study, carbon quantum dots self-assembled with indocyanine green dye at bovine serum albumin for phototherapy and in situ bioimaging are produced by a flexible hydrothermal method. We find that the synthesized nanoparticles have high tumor photothermal therapeutic activity when exposed to 808 nm light, with a photothermal conversion efficiency up to 61 %. The phototoxicity study revealed the excellent phototherapy of the nanoparticles mainly arises from photothermal therapeutic effect other than photodynamic therapy effect. Simultaneously, it allows biological imaging in the visible and near-infrared ranges because of the significant absorption at 365 nm and 840 nm. The current work offers a simple, environmentally friendly, and reasonable method for developing photothermal drugs with a high photothermal conversion efficiency in the near-infrared region, as well as good biosafety for multifunctional nanomaterials for bioimaging tumor diagnosis and direct phototherapy.

Designing dual-emission phosphors for temperature warning indication and dual-mode luminescence thermometry.

Color tunable phosphors of Mn4+ and Tb3+ co-doped double-perovskite SrGdLiTeO6 (SGLT) were synthesized in this work. The crystal parameters and photoluminescence performances were investigated in detail. By taking advantage of the different thermal quenching strengths between Mn4+ and Tb3+ ions, the emission color of SGLT:0.7%Mn4+/1%Tb3+ changed from red to green, which could be used for high-temperature temperature warning indication. Moreover, according to the luminescence intensity ratio (LIR) technique, wide temperature-range optical thermometry was developed and further, the maximum relative sensitivity (SR1) value of the SGLT:0.7%Mn4+/5%Tb3+ phosphor was determined to be 1.49% K-1 at 560 K. On the other hand, the sensing properties were also analyzed based on the temperature-dependent lifetime method. The most interesting thing is that the maximum SR2 value reached 1.88% K-1 at 573 K. This work proved that the Mn4+ and Tb3+ co-doped double-perovskite SrGdLiTeO6 could be potentially used in temperature warning indication and high sensitivity luminescence thermometry.

A and B sites dual substitution by Na+ and Cu2+ co-doping in CsPbBr3 quantum dots to achieve bright and stable blue light emitting diodes.

Light-emitting perovskite quantum dots (PeQDs) are extensively investigated owing to their evident merits. However, it is still a challenge to adjust their intrinsic emissions and enhance their thermal stability to achieve full-color highly emissive QD-based light-emitting diodes (QLEDs), especially blue QLEDs. Herein, we demonstrate an effective strategy to fundamentally stabilize the crystal structure of CsPbBr3 QDs by codoping Na+ and Cu2+ ions, which are designed to substitute Cs+ (A sites) and Pb2+ (B sites), respectively. It is found out that the codoping metal ions have significantly improved the thermal stability and the optical properties of the QDs. 40% of the emission intensity can be remained after 8 thermal cycles (20-120 °C) for CsPbBr3: Na+/Cu2+ QDs, whilst less than 10% is maintained for undoped CsPbBr3 QDs. Accordingly, stable blue QLEDs are packed by CsPbBr3: Na+/Cu2+ QDs. Strong electroluminescence with the maximum luminance of 7161 cd m-2 and low turn-on voltage of 2.4 V are realized. The CIE coordinates are tuned from green (0.10, 0.74) to blue (0.17, 0.25) via Na+ and Cu2+ codoping. The maximum external quantum efficiency (EQEmax) is obtained as 4.52% for PeLEDs based on codoped QDs. The proposed metal ions A and B sites dual substitution strategy guarantees PeQDs as an extremely promising prospect in potential applications as high-resolution displays and high-quality lightings.

The explanation of abnormal thermal quenching of the charge transfer band based on thermally coupled levels and applications as temperature sensing probes.

Because of thermal quenching, conventional luminescent materials suffer from severe problems when employed at high temperatures. Based on the thermally coupled energy levels (TCLs) of rare-earth ions, we report and explain an abnormal thermal quenching phenomenon in the excited state of the charge transfer band (CTB), which is expected to bring out a solution to the problems of the low sensitivity of temperature-sensing materials and applications at high temperature. Temperature-dependent excitation spectra of Er3+ or Eu3+-doped CaMoO4, CaWO4, and LuVO4 phosphors are recorded. It was found that CTB exhibited two abnormal thermal quenching phenomena. One is that the intensity of the whole CTB increases with the rising temperature, named totally abnormal thermal quenching (TATQ), and the other is the integrated intensity decrease but the edge of the CTB at longer wavelengths enhanced with temperature, named edge abnormal thermal quenching (EATQ). The temperature-dependent excitation and diffuse reflectance spectra of the host and rare earth ions with moderate (Er3+) and large (Eu3+) energy separation between TCLs are investigated. One photodynamic model, considering influential factors, such as the absorption of the phosphor, energy transfer efficiency between CTB and dopants, and thermal coupling effect, is proposed and explains the unusual thermal response of CTB. Luminescence thermometry based on the abnormal thermal quenching is realized with the obtained relative sensitivity Sr of 4.65% K-1 @ 328 K, which is four times the value derived from the classic TCLs in the same phosphor.

Synthsis and characterization of Tb3+/Eu3+ complexes based on 2,4,6-tris-(4-carboxyphenyl)-1,3,5-triazine ligand for ratiometric luminescence temperature sensing.

By choosing C3 symmetric 2,4,6-tris-(4-carboxyphenyl)-1,3,5-triazine (TCTZ) as the ligand, a series of lanthanide metal-origanic complexes Tb1-xEux-TCTZ(DMF)·2H2O(x = 0, 0.01, 1) have been successfully synthesized via solvothermal reaction. The complexes present intense emission although with coordinationofwater molecules. The temperature-dependent photoluminescent (PL) properties of Tb-TCTZ is investigated both in terms of emission intensity and lifetime in order to establish their potentials as luminescent themometers. It shows excellent responseto temperature from 303 to 403 K and exhibits the maximum relative sensitivity(Sr) as high as 5.36% K-1 at 403 K. Tb0.99Eu0.01-TCTZ is evaluated for application as ratiometric luminescence thermometers, which exhibits high sensitivity to temperature in range of 303-403 K, with the maximum absolute sensitivity (Sa) and Sr as 5.16% and 3.22% K-1 respectively. The obtained maximum sensitivities in this study is superior to many materials reported. Moreover, the emission color changes from green at 303 K to red at 403 K, so that it is also suitable to act as colorimetric luminescent probes.

High-sensitivity and wide-temperature-range dual-mode optical thermometry under dual-wavelength excitation in a novel double perovskite tellurate oxide.

Novel double perovskite SrLaLiTeO6 (abbreviated as SLLT):Mn4+,Dy3+ phosphors synthesized using a solid-state reaction strategy exhibit distinct dual-emission of Mn4+ and Dy3+. High-sensitivity and wide-temperature-range dual-mode optical thermometry was exploited taking advantage of the diverse thermal quenching between Mn4+ and Dy3+ and the decay lifetime of Mn4+. The thermometric properties in the range of 298-673 K were investigated by utilizing the fluorescence intensity ratio (FIR) of Dy3+ (4F9/2→6H13/2)/Mn4+ (2Eg→4A2g) and the Mn4+ (2Eg→4A2g) lifetime under 351 nm and 453 nm excitation, respectively. The maximum relative sensitivities (SR) of the resultant SLLT:1.2%Mn4+,7%Dy3+ phosphor under 351 nm and 453 nm excitation employing the FIR technology were determined to be 1.60% K-1 at 673 K and 1.44% K-1 at 673 K, respectively. Additionally, the maximum SR values based on the lifetime-mode were 1.59% K-1 at 673 K and 2.18% K-1 at 673 K, respectively. It is noteworthy that the SR values can be manipulated by different excitation wavelengths and multi-modal optical thermometry. These results suggest that the SLLT:Mn4+,Dy3+ phosphor has prospective potential in optical thermometry and provide conducive guidance for designing high-sensitivity multi-modal optical thermometers.

Upconversion nanoparticles modified by Cu2S for photothermal therapy along with real-time optical thermometry.

Highly effective photothermal conversion performance coupled with high resolution temperature detection in real time is urgently needed for photothermal therapy (PTT). Herein, ultra-small Cu2S nanoparticles (NPs) were designed to absorb on the surface of NaScF4: Yb3+/Er3+/Mn2+@NaScF4@SiO2 NPs to form a central-satellite system, in which the Cu2S NPs play the role of providing significant light-to-heat conversion ability and the Er3+ ions in the NaScF4: Yb3+/Er3+/Mn2+ cores act as a thermometric probe based on the fluorescence intensity ratio (FIR) technology operating in the biological windows. A wavelength of 915 nm is used instead of the conventional 980 nm excitation wavelength to eliminate the laser induced overheating effect for the bio-tissues, by which Yb3+ can also be effectively excited. The temperature resolution of the FIR-based optical thermometer is determined to be better than 0.08 K over the biophysical temperature range with a minimal value of 0.06 K at 298 K, perfectly satisfying the requirements of biomedicine. Under the radiation of 915 nm light, the Cu2S NPs exhibit remarkable light-to-heat conversion capacity, which is proved by photothermal ablation testing of E. coli. The results reveal the enormous potential of the present NPs for PTT integrated with real-time temperature sensing with high resolution.

Design of a bi-functional NaScF4: Yb3+/Er3+ nanoparticles for deep-tissue bioimaging and optical thermometry through Mn2+ doping.

An approximately monochromatic red upconversion (UC) emission is successfully realized in NaScF4: Yb3+/Er3+ nanoparticles (NPs) through Mn2+ ions doping without phase transition. The Mn2+ ions play a role of bridge during the energy transfer process from green emission state 2H11/2/4S3/2 of Er3+ to red emission state 4F9/2 of Er3+, which significantly accelerates the red UC enhancement. The strongest red luminescence is observed in the sample containing 10% Mn2+ ions (Mn-10) with an enhancement factor of 7.5 times. Meanwhile, an ultrasensitive optical thermometry in the physiological temperature region can be realized by utilizing the fluorescence intensity ratio (FIR) between two thermally coupled Stark transitions of Er3+: 4I13/2 â†’ 4I15/2, locating in the near-infrared (NIR) long wavelength region of the second biological window. Its relative sensitivity SR can be expressed by 340/T2, which is much higher than most optical thermometers based on thermally coupled Stark sublevels reported by the previous papers. Beyond that, an ex vivo experiment is designed to evaluate the penetration depth of the red and NIR emission of Mn-10 in the biological tissues, revealing that they can reach depth of at least 3 mm and 5 mm respectively. More importantly, the increasing tissue thickness has almost no effect on the FIR values. All the results show that the present sample is a promising bi-functional nano probe which can be used for bioimaging and temperature sensing in the deep tissues through the strong red UC emission and ultrasensitive NIR optical thermometer, respectively.

Optical thermometry based on the thermally coupled energy levels of Er3+ in upconversion materials.

Contactless thermometry with the requirements of high accuracy and high efficiency is an extremely acute need in many fields. Optical thermometers based on the fluorescence intensity ratio (FIR) of the thermally coupled energy levels of Er3+ have been demonstrated to be excellent candidates to afford that due to their advantages of high spatial resolution, rapid response, anti-jamming capability, etc. In this paper, we summarize the recent developments in optical thermometry based on the FIR of the electronic levels Er3+:2H11/2/4S3/2 and the Stark sublevels of Er3+:4F9/2 and Er3+:4I13/2 manifolds, including physical mechanism, improvement of thermometric sensitivity, biological application and so on. Moreover, the challenges in creating novel Er3+-based optical thermometers and potentially new research directions for future work are discussed in detail. Overall, the Er3+-based optical thermometers have exhibited outstanding advantages for non-contact temperature sensing, but great efforts are still needed to improve their key performance indicators for meeting the demands of practical applications.