Benefits of Silica Core-Shell Structures on the Temperature Sensing Properties of Er,Yb:GdVO4 Up-Conversion Nanoparticles

Er3+-activated Ba2V2O7 upconversion nanosheets for dual-mode temperature sensing

So far, there has been substantial research on non-contact luminescence thermometry approaches that rely on luminescence intensity ratio (LIR) technology. However, there is limited availability of phosphors doped with Er3+ ions that exhibit on-par luminescence and high sensitivity. In this work, samples of Ba2V2O7:Er3+ were synthesized using a sol-gel method aided by citric acid. The luminescence properties of these samples, including upconversion and down-shifting, were investigated using both ultraviolet and 980 nm laser stimulation. When subjected to ultraviolet (UV) light, the sample exhibits a distinct broadband emission that appears pale green. This emission is a distinguishing property of the sample and is attributed to the presence of V2O72- ions. Upon being stimulated by a 980 nm laser, the sample exhibits standard green up-conversion Er3+ emission bands. Concurrently, an assessment was conducted on the phosphor's ability to measure temperature by analysing the LIR between the thermally coupled 2H11/2, 4S3/2 energy levels (TCELs) and the non-thermally coupled 2H11/2, 4F9/2 energy levels (NTCELs) of the Er3+ ion. The corresponding highest sensitivity of temperature for TCELs and NTCELs can position Ba2V2O7:Er3+ nanosheets as a capable option for materials utilized in temperature-sensing applications.

Multifunctional Optical Sensing with Lanthanide-Doped Upconverting Nanomaterials: Improving Detection Performance of Temperature and Pressure in the Visible and NIR Ranges

Temperature and pressure are fundamental physical parameters in the field of materials science, making their monitoring of utmost significance for scientists and engineers. Here, the NaSrY(MoO4)3:0.02Er3+/0.01Tm3+/0.15Yb3+ nanophosphor is developed as an optical sensor material. Under 975 nm laser excitation, the upconversion characteristics and optical detection performance of the multifunctional sensing platform of temperature and pressure (vacuum) are investigated. We have successfully developed a novel detection platform that enables optical detection of pressure (vacuum) and temperature. This platform utilizes thermally coupled levels (TCLs) and non-TCLs of Er3+ and Tm3+ to achieve ratiometric detection. The multimodal optical temperature and pressure detection based on TCLs and non-TCLs is successfully realized by using different emission bands of double emission centers, which makes it possible for self-referencing optical temperature and pressure measurement modes. These results indicate that the developed nanophosphor is a promising candidate for optical sensors, and our findings suggest potential strategies for modulating the sensor properties of luminescent materials doped with rare-earth ions.

Luminescence Thermometry Beyond the Biological Realm

As the field of luminescence thermometry has matured, practical applications of luminescence thermometry techniques have grown in both frequency and scope. Due to the biocompatibility of most luminescent thermometers, many of these applications fall within the realm of biology. However, luminescence thermometry is increasingly employed beyond the biological realm, with expanding applications in areas such as thermal characterization of microelectronics, catalysis, and plasmonics. Here, we review the motivations, methodologies, and advances linked to nonbiological applications of luminescence thermometry. We begin with a brief overview of luminescence thermometry probes and techniques, focusing on those most commonly used for nonbiological applications. We then address measurement capabilities that are particularly relevant for these applications and provide a detailed survey of results across various application categories. Throughout the review, we highlight measurement challenges and requirements that are distinct from those of biological applications. Finally, we discuss emerging areas and future directions that present opportunities for continued research.

Advanced temperature sensing with Er3+/Yb3+ co-doped Ba2GdV3O11 phosphors through upconversion luminescence

Optical thermometry is a non-contact temperature sensing technique with widespread applications. It offers precise measurements without physical contact, making it ideal for situations where contact-based methods are impractical. However, improving the accuracy of optical thermometry remains an ongoing challenge. Herein, enhancing the thermometric properties of luminescent thermometers through novel materials or strategies is crucial for developing more precise sensors. Hence, the present study focuses on the application of four-mode luminescence thermometric techniques in sol-gel synthesized Er3+/Yb3+ co-doped Ba2GdV3O11 phosphors for optical temperature sensing in the temperature range of 298-573 K. The upconversion (UC) luminescence is achieved under excitations of 980 nm or 1550 nm, resulting in bright yellow-green emission in the visible spectral range. Temperature sensing is realized by exploiting the UC emissions of 4S3/2, 2H11/2 and 4F7/2 bands, which represent intensity ratios of thermally coupled levels (TCELs) and non-thermally coupled levels (NTCELs) of Er3+/Yb3+, along with the emission lifetimes at 4S3/2. The relative sensitivity (Sr) values for TCELs exhibit a gradual decrease with rising temperature, reaching a maximum of 1.1% K-1 for 980 nm excitation and 0.86% K-1 for 1550 nm excitation at 298 K. Conversely, for NTCELs, the highest Sr value observed is 0.9% K-1 at 298 K for 1550 nm excitation. Moreover, the emission lifetimes at 4S3/2 yield notably high Sr values of up to 5.0% μs K-1 (at 425 K). Furthermore, the studied phosphors have a sub-degree thermal resolution, making them excellent materials for accurate temperature sensing. Overall, this study provides a promising new direction for the development of more precise and reliable optical thermometry techniques, which could have important implications for a range of scientific and industrial optical temperature sensing applications.

Swallowed Embedding of Nanopetal-Rich Microflowers in Flexible Photocatalytic and Thermoresponsive Functional Composite Fibers

Developing efficient catalysts to degrade pollutants in water is a very important way to alleviate water pollution. However, it is crucial but challenging to broaden the functions of conventional photocatalysts and improve their environmental adaptability. In this paper, Bi(Er3+/Yb3+)OBr/polyacrylonitrile (BOB-EY/PAN) composite fibers with a swallowed-embedded structure assembled with nanopetal-rich microflowers were designed and fabricated, integrating photocatalytic and temperature-monitoring functions simultaneously. Their unique structure brings a large specific surface area, and the doping of rare earth ions improves the separation efficiency of electron-hole pairs, which enhances the photocatalytic efficiency and endows the fibers with a temperature-monitoring function at the same time. Under simulated sunlight irradiation, the nanofibers show a maximum degradation efficiency of 99.2% for tetracycline hydrochloride (TC) with a degradation constant of K as high as 0.078 min-1. Based on the fluorescence intensity ratio (FIR), the two thermally coupled levels of Er3+ in the nanofibers, 2H11/2 and 4S3/2, provide real-time temperature feedback, displaying a maximum relative sensitivity as high as 0.0215 K-1 at 303 K. Dual-functional BOB-EY/PAN composite nanofibers show great potential for industrial wastewater disposition, providing solutions for wastewater purification in special scenarios.

Upconversion Nanoparticle-Based Fluorescent Film for Distributed Temperature Monitoring of Mobile Phones' Integrated Chips

As one of the most critical parameters to evaluate the quality and performance of mobile phones, real-time temperature monitoring of mobile phones' integrated chips is vital in the electronics industry. Although several different strategies for the measurement of chips' surface temperature have been proposed in recent years, distributed temperature monitoring with high spatial resolution is still a hot issue with an urgent need to be solved. In this work, a fluorescent film material with photothermal properties containing thermosensitive upconversion nanoparticles (UCNPs) and polydimethylsiloxane (PDMS) is fabricated for the monitoring of the chips' surface temperature. The presented fluorescent films have thicknesses ranging from 23 to 90 μm and are both flexible and elastic. Using the fluorescence intensity ratio (FIR) technique, the temperature-sensing properties of these fluorescent films are investigated. The maximum sensitivity of the fluorescent film was measured to be 1.43% K^-1 at 299 K. By testing the temperature at different positions of the optical film, distributed temperature monitoring with a high spatial resolution down to 10 μm on the chip surface was successfully achieved. It is worth mentioning that the film maintained stable performance even under pull stretching up to 100%. The correctness of the method is verified by taking infrared images of the chip surface with an infrared camera. These results demonstrate that the as-prepared optical film is a promising anti-deformation material for monitoring temperature with high spatial resolution on-chip surfaces.

Improved relative temperature sensitivity of over 10% K-1 in fluoride nanocrystals via engineering the interfacial layer

Real-time in situ temperature sensing is of significance in the bio-medical field; however, the low relative temperature sensitivity S_r is one of the major obstacles in the development of nanothermometers. Herein, we provide an effective route that engineers the interfacial layer in a core/shell/shell nanostructure to enlarge the temperature-dependent luminescence intensity ratio (LIR) variations followed by an improved S_r. The CaF₂ interlayer is employed to inhibit the interaction between the core and outer shell, and increase the interfacial phonon energy to enhance the negative thermal quenching effect (TQE) of Nd³⁺ ions in the outer shell and positive TQE of Er³⁺ ions in the core layer. Based on the temperature-dependent LIR variations of Er (650 nm) to Nd (800 nm), the maximum S_r of 10.01% K^-1 and minimum S_r of % 2.56% K^-1 are achieved.

A Dual-Mode Optical Thermometer with High Sensitivity Based on BaAl12O19:Sm2+/SrAl12O19:Sm3+ Solid Solution Phosphors

A series of BaAl₁₂O₁₉:Sm²⁺/SrAl₁₂O₁₉:Sm³⁺ mixed-phase phosphors were produced in one step using the traditional high-temperature solid-phase process. Because Sm is divalent in BaAl₁₂O₁₉ and trivalent in SrAl₁₂O₁₉, the coexistence of Sm²⁺ and Sm³⁺ is realized in the mixed-phase host. Since the temperature sensitivity of Sm²⁺ and Sm³⁺ in the solid solution host is significantly different, this makes it possible for the sample to measure temperature based on the fluorescence intensity ratio (FIR). The crystal model, ion emission spectrum, and temperature sensitivity of these phosphors are studied in detail. Under the co-excitation of a 410 nm excitation source, this sample has excellent temperature measurement performance in the range of 313-513 K. Based on the FIR method, the maximum absolute temperature sensitivity (S_a) is 0.55 K^-1 at 513 K, and the maximum relative temperature sensitivity (S_r) is 2.47%K^-1 at 453 K. Moreover, based on the photoluminescence lifetime temperature measurement mode, the largest value of S_a at 413 K is 0.046 K^-1, and the maximum value of S_r at 473 K is 3.10%K^-1. In short, the BaAl₁₂O₁₉:Sm²⁺/SrAl₁₂O₁₉:Sm³⁺ solid solution is a kind of phosphor with nice temperature measurement ability, and it has very strong potential in the application of noncontact optical thermometers.

A dual-ratiometric optical thermometry based on Sr2LaF7:Er3+ crystal-implanted pliable fibers

Sr2(La1-xErx)F7/polyacrylonitrile composite fibers with special pliability and excellent crystal dispersibility have been fabricated, which provide the smaller size and appropriate temperature sensitivity. Up-conversion emission shows quadratic dependence of the photoluminescence...

Frequency upconversion based thermally stable molybdate phosphors in temperature sensing probe

Er3+-Yb3+ co-doped NaGd(MoO4)2 phosphors with different concentrations of Er3+ and Yb3+ ions have been successfully synthesized via a high-temperature solid-state reaction method. Phase confirmation and morphological studies have been done...

Recent advances in colorimetry/fluorimetry-based dual-modal sensing technologies

Tailored to the increasing demands for sensing technologies, the fabrication of dual-modal sensing technologies through combining two signal transduction channels into one method has been proposed and drawn considerable attention. The integration of two sensing signals not only promotes the analytical efficiency with reduced assumption, but also improves the analytical performances with enlarged detection linear range, enhanced accuracy, and boosted application flexibility. The two top-rated output signals for developing dual-modal sensors are colorimetric and fluorescent signals because of their outstanding merits for point of care applications and real-time sensitive sensing. Given the rapid development of material chemistry and nanotechnology, the recent decade has witnessed great advance in colorimetric/fluorimetric signal based dual-modal sensing technologies. The new sensing strategy leads to a broad avenue for various applications in disease diagnosis, environmental monitoring and food safety because of the complementary and synergistic effects of the two output signals. In this state-of-the-art review, we comprehensively summarize different types of colorimetric/fluorimetric dual-modal sensing methods by highlighting representative research in the last 5 years, digging into their sensing methodologies, particularly the working principles of the signal transduction systems. Then, the challenges and future prospects for boosting further development of this research field are discussed.

Fluorescent polymeric nanoparticle for ratiometric temperature sensing allows real-time monitoring in influenza virus-infected cells

Temperature is a key indicator of infection and disease, however, it is difficult to measure at a cellular level. Nanoparticles are applied to measure the cellular temperature, and enhancement of the stability and reliability of the signal and higher biocompatibility are demanded. We have developed fluorescent polymeric nanoparticles loaded with temperature-sensitive units (as rhodamine B) and internal reference units (as coumarin) for imaging and ratiometric sensing of the cellular temperature in the physiological range. The fluorescence signal of the nanoparticles was stable in the bio-environment and the ratiometric sensing strategy could overcome the concentration effect of nanoparticles. The nanoparticles were endocytosed by cells and partially presented in mitochondria. The fluorescence intensity ratio of rhodamine B and coumarin using nanoparticles showed good linear correlations in buffer solutions, cell suspensions, and imaging of living cells. Using the fluorescent polymeric nanoparticles, the change of temperature of cells during influenza virus infection could be individually monitored.

Photoluminescence properties of Er3+ and Eu3+ ions based on oxide host for optical temperature sensing with high sensitivity

Motivated from increasing demands of non-contact optical temperature sensing, the Yb³⁺-Er³⁺ and Eu³⁺ doped NaY₉Si₆O₂₆ (NYS) oxide phosphors were designed by high-temperature solid-state reaction method. The phase purity of the as-prepared samples was checked from XRD patterns. For the upconversion luminescence in NYS:Yb³⁺,Er³⁺, the optimal Er³⁺ doping content was determined to be 1 mol%, and the characteristic emission peaks of Er³⁺ were observed in the region of 500-700 nm. In Eu³⁺ activated NYS phosphors, it has been found that the emissions originating in ⁵D₁ and ⁵D₀ levels demonstrate different change tendencies in intensity with Eu³⁺ doping concentration. By studying the temperature-dependent luminescent spectra, it was indicated that the emissions intensities from different excited states of Er³⁺ change differently with temperature. Two kinds of fluorescence intensity ratio (FIR) strategies were used in NYS:10%Yb³⁺,1%Er³⁺, containing thermally-coupled levels and non-thermally-coupled levels. In the NYS:3%Eu³⁺ phosphor, it was found that the FIR for 577 and 536 nm emissions follows a linear relation with temperature. The high sensitivities in the present phosphors indicate the potential application in optical temperature sensing.

Lanthanide doped luminescence nanothermometers in the biological windows: strategies and applications

The development of lanthanide-doped non-contact luminescent nanothermometers with accuracy, efficiency and fast diagnostic tools attributed to their versatility, stability and narrow emission band profiles has spurred the replacement of conventional contact thermal probes. The application of lanthanide-doped materials as temperature nanosensors, excited by ultraviolet, visible or near infrared light, and the generation of emissions lying in the biological window regions, I-BW (650 nm-950 nm), II-BW (1000 nm-1350 nm), III-BW (1400 nm-2000 nm) and IV-BW (centered at 2200 nm), are notably growing due to the advantages they present, including reduced phototoxicity and photobleaching, better image contrast and deeper penetration depths into biological tissues. Here, the different mechanisms used in lanthanide ion-doped nanomaterials to sense temperature in these biological windows for biomedical and other applications are summarized, focusing on factors that affect their thermal sensitivity, and consequently their temperature resolution. Comparing the thermometric performance of these nanomaterials in each biological window, we identified the strategies that allow boosting of their sensing properties.

Enhanced up-conversion luminescence and temperature-sensing of GdVO4:Ln3+ with dual-wavelength excitation

There exists a tendency in the research of up-conversion materials to shift the excitation from 980 nm to multiple excitation wavelengths. A series of GdVO4:Ln3+ with similar sizes and irregular prism morphology were successfully prepared by a co-precipitation technique. A wide multi-color emission corresponding to different Ln3+ doping was also obtained under single excitation. It is worth pointing out that among all the studied samples, only the emission intensity of GdVO4:Yb3+/Er3+ excited at two-wavelengths (980 nm + 1550 nm) simultaneously was enhanced by a factor of 1.87, compared to the sum of emission intensities excited at two single-wavelengths separately. Moreover, to further enhance the up-conversion luminescence intensity, cation ions (Lu3+/Y3+) and anion ions (PO43-) were also doped into the host, and the luminous intensity was also improved to a certain extent. A possible mechanism for energy transfer and possible transitions were also suggested and discussed in detail using an energy level diagram. In addition, not only a high record value of Sa (0.0069 K-1) but also a high Sr (1.13% K-1) is achieved for GdVO4:Yb3+/Er3+ in the physiological temperature range (273-453 K). Combining a much intensified dual-wavelength up-conversion signal and good temperature-sensing properties, this work can be extended to the surges of other lanthanide ion doped systems pumped by using multiple-wavelength lasers, and can also open new possibilities for up-conversion color displays and anti-counterfeiting applications.

Upconversion luminescence and temperature sensing properties of NaGd(WO4)2:Yb3+/Er3+@SiO2 core-shell nanoparticles

Optical thermometry based on the fluorescence intensity ratio (FIR) of two thermally coupled levels in lanthanide ions has potential application in non-contact optical temperature sensing techniques. In this work, a shell of SiO2 with tunable thickness was uniformly coated on NaGd(WO4)2:Yb3+/Er3+ core upconversion nanoparticles (UCNPs). The effects of the silica shell on UC luminescence and thermal sensing properties of core-shell NaGd(WO4)2:Yb3+/Er3+@SiO2 UCNPs were investigated. Under 980 nm laser excitation, the temperature-dependent UC emission spectra of obtained samples were measured. The FIR was analyzed based on the thermally coupled 2H11/2 and 4S3/2 levels of Er3+ in the biological temperature range of 300-350 K, in which the Boltzmann distribution is applied. The emission from the upper 2H11/2 state within Er3+ was enhanced as temperature increased due to the thermal effect. Absolute sensitivities (S A) and relative sensitivities (S R) of the core and core-shell UCNPs were calculated. It was found that after SiO2 coating, the maximum S A was enhanced by ∼2-fold (1.03% K-1 at 350 K). Especially, S A was as high as 2.14% K-1 at 350 K by analyzing the FIR of the non-thermally coupled 2H11/2 and 4F9/2 levels.

Luminescent Nanothermometer Operating at Very High Temperature-Sensing up to 1000 K with Upconverting Nanoparticles (Yb3+/Tm3+)

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K^-1) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb³⁺-Tm³⁺ codoped YVO₄ nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm³⁺ emissions associated with the ³F_2,3 and ³H₄ thermally coupled levels, that is, ³F_2,3 → ³H₆/³H₄ → ³H₆ (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb³⁺ (²F_5/2 → ²F_7/2) and Tm³⁺ (³H₄ → ³H₆). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.

Bifunctional Tm3+,Yb3+:GdVO4@SiO2 Core-Shell Nanoparticles in HeLa Cells: Upconversion Luminescence Nanothermometry in the First Biological Window and Biolabelling in the Visible

The bifunctional possibilities of Tm,Yb:GdVO₄@SiO₂ core-shell nanoparticles for temperature sensing by using the near-infrared (NIR)-excited upconversion emissions in the first biological window, and biolabeling through the visible emissions they generate, were investigated. The two emission lines located at 700 and 800 nm, that arise from the thermally coupled ³F_2,3 and ³H₄ energy levels of Tm³⁺, were used to develop a luminescent thermometer, operating through the Fluorescence Intensity Ratio (FIR) technique, with a very high thermal relative sensitivity . Moreover, since the inert shell surrounding the luminescent active core allows for dispersal of the nanoparticles in water and biological compatible fluids, we investigated the penetration depth that can be realized in biological tissues with their emissions in the NIR range, achieving a value of 0.8 mm when excited at powers of 50 mW. After their internalization in HeLa cells, a low toxicity was observed and the potentiality for biolabelling in the visible range was demonstrated, which facilitated the identification of the location of the nanoparticles inside the cells, and the temperature determination.

Encapsulation of Dual Emitting Giant Quantum Dots in Silica Nanoparticles for Optical Ratiometric Temperature Nanosensors

Accurate temperature measurements with a high spatial resolution for application in the biomedical fields demand novel nanosized thermometers with new advanced properties. Here, a water dispersible ratiometric temperature sensor is fabricated by encapsulating in silica nanoparticles, organic capped PbS@CdS@CdS “giant” quantum dots (GQDs), characterized by dual emission in the visible and near infrared spectral range, already assessed as efficient fluorescent nanothermometers. The chemical stability, easy surface functionalization, limited toxicity and transparency of the silica coating represent advantageous features for the realization of a nanoscale heterostructure suitable for temperature sensing. However, the strong dependence of the optical properties on the morphology of the final core–shell nanoparticle requires an accurate control of the encapsulation process. We carried out a systematic investigation of the synthetic conditions to achieve, by the microemulsion method, uniform and single core silica coated GQD (GQD@SiO2) nanoparticles and subsequently recorded temperature-dependent fluorescent spectra in the 281-313 K temperature range, suited for biological systems. The ratiometric response—the ratio between the two integrated PbS and CdS emission bands—is found to monotonically decrease with the temperature, showing a sensitivity comparable to bare GQDs, and thus confirming the effectiveness of the functionalization strategy and the potential of GQD@SiO2 in future biomedical applications.

Origin of the giant thermal enhancement of the Er3+ ion's 4I9/2-4I15/2 photoluminescence

The Er³⁺ ion's ⁴I_9/2-⁴I_15/2 emission spectrum, which was rarely reported before, was successfully observed in the as-prepared scheelite-structured CaWO₄:Yb³⁺,Er³⁺ phosphors, upon excitation at 980 nm. This photoluminescence, peaking at ca. 800 nm, was found to undergo a monotonous and giant enhancement with increasing the temperature from 333 to 813 K. Its dependence on pump power revealed that this emission spectrum was from one-photon pumping mechanism. Together with the analysis on luminescence intensity ratio thermometry, it was confirmed that the giant enhancement of the 800 nm emission spectrum was most likely to come from the adjacent lower ⁴I_11/2 state via a thermally coupled way.

808 nm-Light-Excited Near-Infrared Luminescent Lanthanide Metal-Organic Frameworks for Highly Sensitive Physiological Temperature Sensing

Ongoing demand for accurate self-calibrated noninvasive thermometers for micro-/nano-scale applications, particular biomedical diagnosis, is driving the development of temperature sensors. Here a new type of lanthanide metal-organic framework having near-infrared absorption and near-infrared emission features is presented, and it is based on efficient Nd³⁺ -to-Yb³⁺ energy transfer in 808 nm photoexcitation. The results show that the ratiometric parameter of Nd_0.5 Yb_0.5 TPTC (TPTC= 1,1':4',1''-terphenyl]-3,3'',5,5''-tetracarboxylic acid) can deliver good exponential-type luminescence response to temperature in the physiological regime (293-328 K) with high relative sensitivity and accurate temperature resolution, as well as good biocompatibility and chemical stability. Such lanthanide-based materials are especially useful in biomedical applications.

Temperature-sensing luminescent materials La9.67Si6O26.5:Yb3+–Er3+/Ho3+ based on pump-power-dependent upconversion luminescence

Rare earth ion doped upconversion (UC) luminescent materials could show potential applications in optical temperature sensing.

Er3+/Yb3+ co-doped nanocrystals modified with 6-aminocaproic acid for temperature sensing in biomedicine

We report β-PbF2:Er3+/Yb3+ nanocrystals (NCs) modified with 6-aminocaproic acid (6AA) via wet chemical etching of glass ceramics (GCs). NCs body-doped with trivalent rare-earth (RE3+) ions were released from the GCs by etching of the glass matrix and modified with bifunctional 6AA ligands to enhance their water solubility. They have good stability in water with an average diameter of 56 nm and display efficient green (521, 550 nm) and red (660 nm) emission under the excitation of a 940 nm laser. High absolute sensitivity (S A) and relative sensitivity (S R) (0.0027 K-1 and 1.18% K-1, respectively) and high resolution (0.25 K) were achieved for temperature sensing in the biological temperature range, using the fluorescence intensity ratio (FIR) technique. All of the experimental results indicate that the Er3+/Yb3+ co-doped NCs modified with 6AA may potentially be useful as fluorescent biological temperature sensors.

One-pot synthesis of SiO2-coated Gd2(WO4)3:Yb3+/Ho3+ nanoparticles for simultaneous multi-imaging, temperature sensing and tumor inhibition

Rare earth ion-doped fluoride upconversion nanoparticles (UCNPs), emerging as a novel class of probes and drug carriers, exhibit superior promise for bio-applications in diagnostics and treatment on account of their strong luminescence, fine biocompatibility, and high drug loading. However, the fine control and manipulation of particle size and the distribution of rare earth ion-doped oxides has remained an insurmountable challenge to date. In this work, we construct and synthesize silica-coated Gd2(WO4)3:Yb3+/Ho3+ nanoparticles by one-pot co-precipitation, with uniform distribution (∼130 nm) and enhanced yellow fluorescence. Particularly, the nanoparticles not only possess outstanding temperature sensing performance at biological temperatures in water by utilizing the fluorescence intensity ratio (FIR) method, but also allow a further serviceable contrast effect in vitro and in vivo based on the prominent T1-weighted magnetic resonance (MR) signal of Gd3+. Compared with cisplatin and platinum(iv) (DSP), the Gd2(WO4)3@SiO2 nanoparticles functionalized with DSP (Gd2(WO4)3@SiO2-Pt-PEG) exert higher lethality against CT26 cells and significantly inhibit the growth of tumors at the same concentration of Pt. This effect occurs through the greater level of cell endocytosis. The lethality value of the latter is 10 times higher than the former after the same length of time according to inductively coupled plasma-mass spectrometry (ICP-MS) results. In short, the monodisperse and strongly fluorescent Gd2(WO4)3@SiO2-Pt-PEG nanoparticles are endowed with dual-mode imaging, temperature sensing and anticancer functions, which provide a significant guide for synthesis and bio-application of lanthanide ion-doped oxides.

Classification, Synthesis, and Application of Luminescent Silica Nanoparticles: a Review

Luminescent materials are of worldwide interest because of their unique optical properties. Silica, which is transparent to light, is an ideal matrix for luminescent materials. Luminescent silica nanoparticles (LSNs) have broad applications because of their enhanced chemical and thermal stability. Silica spheres of various sizes could be synthesized by different methods to satisfy specific requirements. Diverse luminescent dyes have potential for different applications. Subject to many factors such as quenchers, their performance was not quite satisfying. This review thus discusses the development of LSNs including their classification, synthesis, and application. It is the highlight that how silica improves the properties of luminescent dye and what role silica plays in the system. Further, their applications in biology, display, and sensors are also described.

Comprehensive Study on the Combined Effect of Laser-Induced Heating and Laser Power Dependence on Luminescence Ratiometric Thermometry

Luminescence ratiometric thermometry, on the basis of nonthermally linked states of lanthanides, became a hot research issue recently because of its several attractive features. Here, the ⁵F₄,⁵S₂/⁵F₅-⁵I₈ transitions of Ho³⁺ embedded in calcium tungstate host are taken as an example to show the influence of laser pump power on this temperature detection technology. The luminescence intensity ratio between the ⁵F₄,⁵S₂/⁵F₅-⁵I₈ upconversion emission lines was found to respond monotonously to the temperature between 303 and 603 K and could be fitted well with the use of an empirical function. It suggested that this ratio might be suitable for temperature measurement. However, at 303 K, the temperature readout derived from this ratio decreased from 303 to 248 K on increasing the laser pump power from 35 to 205 mW (the irradiated spot's area: ca. 2 mm²). This uncommon phenomenon differs from the conventional laser-induced heating effect. With the help of the Boltzmann distribution based on the two Stark components of the ⁵F₅ state of Ho³⁺, the laser-induced heating was calculated to be ca. 20 K when the excitation power was 205 mW. Thus, this suggested that there should be a mechanism responsible for the gradually decreasing temperature readout. It was then confirmed that this mechanism was the different dependences for the ⁵F₄,⁵S₂/⁵F₅-⁵I₈ transitions on laser pump power, which was much stronger than the laser-induced heating effect. A calibration method to eliminate the influence of laser power dependence on luminescence ratiometric thermometry was then proposed.

Sensitivity improvement induced by thermal response behavior for temperature sensing applications

Overcoming the restriction of the energy gap (700-800 cm-1) in Er3+-doped upconversion (UC) materials to achieve high detection accuracy is crucial for practical temperature detection applications. Herein, we design a feasible route based on the different thermal response behaviors of various hosts to enhance the SA value in a double perovskite NaLaMgWO6:Er,Mo system. The maximum SA value is 222.8 × 10-4@423 K in the NLMW:5%Er3+ host, and this can be promoted to 275.4 × 10-4 K-1@323 K in the NaLaMgWO6:Er,Mo system. The SR values decrease monotonously as the temperature rises, and this is due to the dependency of the SR values on the energy gap. A mechanism that is ascribed to the different thermal response behaviors of the various hosts is proposed, and this mechanism is further proved by investigating the temperature sensing properties of barium gadolinium zincate phosphors that possess the same thermal response behaviors. In addition, this study introduces the idea that a host with a high emission intensity for the 2H11/2 level and a lower emission intensity for the 4S3/2 level is highly suitable for temperature measurements. A thorough investigation of this system offers a strategy to acquire a high SA value and reveals the broad prospects of NaLaMgWO6:Er,Mo in the temperature detection field.

A non-invasive luminescent nano-thermometer: approaching the maximum thermal sensitivity of Er3+ ion's green emission

The maximum relative thermal sensitivity for the green luminescence of the Er³⁺ ion is reported.

Apparent self-heating of individual upconverting nanoparticle thermometers

Individual luminescent nanoparticles enable thermometry with sub-diffraction limited spatial resolution, but potential self-heating effects from high single-particle excitation intensities remain largely uninvestigated because thermal models predict negligible self-heating. Here, we report that the common "ratiometric" thermometry signal of individual NaYF4:Yb3+,Er3+ nanoparticles unexpectedly increases with excitation intensity, implying a temperature rise over 50 K if interpreted as thermal. Luminescence lifetime thermometry, which we demonstrate for the first time using individual NaYF4:Yb3+,Er3+ nanoparticles, indicates a similar temperature rise. To resolve this apparent contradiction between model and experiment, we systematically vary the nanoparticle's thermal environment: the substrate thermal conductivity, nanoparticle-substrate contact resistance, and nanoparticle size. The apparent self-heating remains unchanged, demonstrating that this effect is an artifact, not a real temperature rise. Using rate equation modeling, we show that this artifact results from increased radiative and non-radiative relaxation from higher-lying Er3+ energy levels. This study has important implications for single-particle thermometry.

Double NIR laser stimulation and enhancing the thermal sensitivity of Er3+/Tm3+/Nd3+ doped multilayer core-shell nanoparticles

Non-contact thermal sensors are important devices to study cellular processes and monitor temperature in vivo. Herein, a novel highly sensitive nanothermometer based on NaYF₄:Yb,Er@ NaYF₄@NaYF₄:Yb,Tm@ NaYF₄:Nd (denoted as Er@Y@Tm@Nd) was prepared by a facile solvothermal method. When excited by the near-infrared (NIR) light of 808 and 980 nm, the as-prepared Er@Y@Tm@Nd nanoparticles could emit both blue and green light, respectively, since the lanthanide cations responsible for these emissions are gathered inside this nanostructure. The green and blue light intensity ratio exhibits obvious temperature dependence in the range of the physiological temperature. Additionally, the fluorescence intensity of Er³⁺ and Tm³⁺ are also greatly enhanced due to the multilayer structure that implies avoiding the Er³⁺ and Tm³⁺ energy cross-relaxation by introduction of a NaYF₄ wall between them. The as-prepared core-shell-shell-shell structure with Er³⁺ and Tm³⁺ in different layers improves dozens of times of the thermal sensitivity based on the non-thermal coupling levels of the probe: the maximum values for the sensitivity are 2.95% K^-1 (I _Er-521/I _Tm-450) and 6.30% K^-1 (I _Tm-474/I _Er-541) when excited by 980 and 808 nm laser sources, respectively. These values are well above those previously reported (<0.7% K^-1), indicating that the prepared nanostructures are temperature sensors with excellent thermal sensitivity and sensitive to NIR wavelength excitation that makes them highly preferred for thermal detection.

Dual-Emissive Phosphorescent Polymer Probe for Accurate Temperature Sensing in Living Cells and Zebrafish Using Ratiometric and Phosphorescence Lifetime Imaging Microscopy

Temperature plays an important part in many biochemical processes. Accurate diagnosis and proper treatment usually depend on precise measurement of temperature. In this work, a dual-emissive phosphorescent polymer temperature probe, composed of iridium(III) complexes as temperature sensitive unit with phosphorescence lifetime of ∼500 ns and europium(III) complexes as reference unit with lifetime of ∼400 μs, has been rationally designed and synthesized. Upon the increase of the temperature, the luminescence intensity from the iridium(III) complexes is enhanced, while that from the europium(III) complexes remains unchanged, which makes it possible for the ratiometric detection of temperature. Furthermore, the polymer also displays a significant change in emission lifetime accompanied by the temperature variation. By utilizing the laser scanning confocal microscope and time-resolved luminescence imaging systems, ratiometric and time-resolved luminescence imaging in Hela cells and zebrafish have been carried out. Notably, the intensity ratio and long-lifetime-based imaging can offer higher sensitivity, decrease the detection limit, and minimize the background interference from biosamples.

Upconversion thermometry: a new tool to measure the thermal resistance of nanoparticles

The rapid evolution in luminescence thermometry in the last few years gradually shifted the research from the fabrication of more sensitive nanoarchitectures towards the use of the technique as a tool for thermal bioimaging and for the unveiling of properties of the thermometers themselves and of their local surroundings, for example to evaluate heat transport at unprecedented small scales. In this work, we demonstrated that KLu(WO4)2:Ho3+,Tm3+ nanoparticles are able to combine controllable heat release and upconversion thermometry permitting to estimate its thermal resistance (in air), a key parameter to model the heat transfer at the nanoscale.

MgTiO3:Mn4+ a multi-reading temperature nanoprobe

MgTiO₃ nanoparticles doped with Mn⁴⁺, with homogeneous size ranging about 63.1 ± 9.8 nm, were synthesized by a molten salt assisted sol gel method. These nanoparticles have been investigated as optical thermal sensors. The luminescence of tetravalent manganese ion in octahedral environment within the perovskite host presents drastic variations with temperature. Three different thermometry approaches have been proposed and characterized. Two luminescence intensity ratios are studied. Firstly between the two R-lines of Mn⁴⁺ emission at low temperature (−250 °C and −90 °C) with a maximal sensitivity of 0.9% °C⁻¹, but also secondly between ²E → ⁴A₂ (R-line) and the ⁴T₂ → ⁴A₂ transitions. This allows studying the temperature variation within a larger temperature range (−200 °C to 50 °C) with a sensitivity between 0.6% °C⁻¹ and 1.2% °C⁻¹ over this range. The last proposed method is the study of the lifetime variation versus temperature. The effective lifetime value corresponds to a combination of transitions from two excited energy levels of the tetravalent manganese (²E and ⁴T₂) in thermal equilibrium toward the fundamental ⁴A₂ state. Since the more energetic transition (⁴T₂ → ⁴A₂) is spin-allowed, contrary to the ²E → ⁴A₂ one, the lifetime drastically decreases with the increase in temperature leading to an impressive high sensitivity value of 4.1% °C⁻¹ at 4 °C and an exceptional temperature resolution of 0.025 °C. According to their optical features, MgTiO₃:Mn⁴⁺ nanoparticles are indeed suitable candidates for the luminescence temperature probes at the nanoscale over several temperature ranges.

Luminescence properties of MgTiO₃ nanoparticles doped with Mn⁴⁺ ions are investigated for precise temperature determination.

Charge convertibility and near infrared photon co-enhanced cisplatin chemotherapy based on upconversion nanoplatform

Optimal nano-sized drug carrier requires long blood circulation, selective extravasation, and efficient cell uptake. Here we develop a charge-convertible nanoplatform based on Pt(IV) prodrug loaded NaYF₄:Yb,Tm upconversion nanoparticles (UCNs), followed by coating a layer of PEG-PAH-DMMA polymer (UCNs-Pt(IV)@PEG-PAH-DMMA). The polymer endows the platform with high biocompatibility, initial nano-size for prolonged blood circulation and selective extravasation. Especially, the anionic polymer can response to the mild acidic stimulus (pH ∼6.5) of tumor extracellular microenvironment and experience charge-shifting to a cationic polymer, resulting in electrostatic repulsion and releases of positive UCNs-Pt(IV). The positive UCNs-Pt(IV) nanoparticles have high affinity to negative cell membrane, leading to efficacious cell internalization. Simultaneously, the ultraviolet (UV) light emitted from UCNs upon near-infrared (NIR) light irradiation, together with the reductive glutathione (GSH) in cancer cells efficiently activate the Pt(IV) prodrug to highly cytotoxic Pt(II), realizing NIR photon improved chemotherapy. The experimental results reveal the charge convertibility, low adverse effect and markedly enhanced tumor ablation efficacy upon NIR laser irradiation of this smart nanoplatform. Moreover, combining the inherent upconversion luminescence (UCL) and computed tomography (CT) imaging capabilities, an alliance of cancer diagnosis and therapy has been achieved.

Markedly enhanced up-conversion luminescence by combining IR-808 dye sensitization and core–shell–shell structures

The up-conversion emission of core–shell–shell structured nanoparticles has been greatly enhanced by IR-808 dye sensitization of 808 nm photons.

Upconversion based near white light emission, intrinsic optical bistability and temperature sensing in Er3+/Tm3+/Yb3+/Li+:NaZnPO4 phosphors

NaZnPO₄:Er³⁺/Tm³⁺/Yb³⁺/Li⁺ phosphors have been prepared which show UC based near white light emission, intrinsic optical bistability and temperature-dependent population re-distribution ability.

Terbium and holmium codoped yttrium phosphate as non-contact optical temperature sensors

A new sensing mechanism is presented and a high relative sensitivity is achieved in our sample YPO₄:3.2%Tb³⁺,0.8%Ho³⁺.

Sensitivity Modulation of Upconverting Thermometry through Engineering Phonon Energy of a Matrix

Investigation of the unclear influential factors to thermal sensing capability is the only way to achieve highly sensitive thermometry, which is greatly needed to meet the growing demand for potential sensing applications. Here, the effect from the phonon energy of a matrix on the sensitivity of upconversion (UC) microthermometers is elaborately discussed using a controllable method. Uniform truncated octahedral YF₃:Er³⁺/Yb³⁺ microcrystals were prepared by a hydrothermal approach, and phase transformation from YF₃ to YOF and Y₂O₃ with nearly unchanged morphology and size was successfully realized by controlling the annealing temperature. The phonon energies of blank matrixes were determined by FT-IR spectra and Raman scattering. Upon 980 nm excitation, phonon energy-dependent UC emitting color was finely tuned from green to yellow for three samples, and the mechanisms were proposed. Thermal sensing behaviors based on the TCLs (²H_11/2/⁴S_3/2) were evaluated, and the sensitivities gradually grew with the increase in the matrix's phonon energy. According to chemical bond theory and first-principle calculations, the most intrinsic factors associated with thermometric ability were qualitatively demonstrated through analyzing the inner relation between the phonon energy and bond covalency. The exciting results provide guiding insights into employing appropriate host materials with desired thermometric ability while offering the possibility of highly accurate measurement of temperature.

Thermometry and up-conversion luminescence of Yb3+–Er3+ co-doped Na2Ln2Ti3O10 (Ln = Gd, La) phosphors

Yb³⁺/Er³⁺-ion co-doped Na₂Ln₂Ti₃O₁₀ (Ln = Gd, La) up-conversion phosphors were successfully synthesized by a sol–gel method, and their crystal structures were characterized by powder X-ray diffraction.