Bifunctional heater-thermometer Nd3+-doped nanoparticles with multiple temperature sensing parameters

Smart photopharmacological agents: LaVO4:Eu3+@vinyl phosphonate combining luminescence imaging and photoswitchable butyrylcholinesterase inhibition

The combination of photoswitchability and bioactivity in one compound provides interesting opportunities for photopharmacology. Here, we report a hybrid compound that in addition allows for its visual localization. It is the first demonstration of its kind and it even shows high photoswitchability. The multifunctional nanomaterial hybrid, which we present, is composed of luminescent LaVO4:Eu3+ nanoparticles and vinyl phosphonate, the latter of which inhibits butyrylcholinesterase (BChE). This inhibition increases 7 times when irradiated with a 266 nm laser. We found that it is increased even further when vinyl phosphonate molecules are conjugated with LaVO4:Eu3+ nanoparticles, leading in total to a 20-fold increase in BChE inhibition upon laser irradiation. The specific luminescence spectrum of LaVO4:Eu3+ allows its spatial localization in various biological samples (chicken breast, Daphnia and Paramecium). Furthermore, laser irradiation of the LaVO4:Eu3+@vinyl phosphonate hybrid leads to a drop in luminescence intensity and in lifetime of the Eu3+ ion that can implicitly indicate photoswitching of vinyl phosphonate in the bioactive state. Thus, combining enhanced photoswitchability, bioactivity and luminescence induced localizability in a unique way, hybrid LaVO4:Eu3+@vinyl phosphonate can be considered as a promising tool for photopharmacology.

Mapping Temperature Heterogeneities during Catalytic CO2 Methanation with Operando Luminescence Thermometry

Controlling and understanding reaction temperature variations in catalytic processes are crucial for assessing the performance of a catalyst material. Local temperature measurements are challenging, however. Luminescence thermometry is a promising remote-sensing tool, but it is cross-sensitive to the optical properties of a sample and other external parameters. In this work, we measure spatial variations in the local temperature on the micrometer length scale during carbon dioxide (CO2) methanation over a TiO2-supported Ni catalyst and link them to variations in catalytic performance. We extract local temperatures from the temperature-dependent emission of Y2O3:Nd3+ particles, which are mixed with the CO2 methanation catalyst. Scanning, where a near-infrared laser locally excites the emitting Nd3+ ions, produces a temperature map with a micrometer pixel size. We first designed the Y2O3:Nd3+ particles for optimal temperature precision and characterized cross-sensitivity of the measured signal to parameters other than temperature, such as light absorption by the blackened sample due to coke deposition at elevated temperatures. Introducing reaction gases causes a local temperature increase of the catalyst of on average 6-25 K, increasing with the reactor set temperature in the range of 550-640 K. Pixel-to-pixel variations in the temperature increase show a standard deviation of up to 1.5 K, which are attributed to local variations in the catalytic reaction rate. Mapping and understanding such temperature variations are crucial for the optimization of overall catalyst performance on the nano- and macroscopic scale.

Ratiometric thermometry using single Er3+-doped CaWO4phosphors

Single doped CaWO₄:Er³⁺phosphors were synthesized and studied for application of optical thermal sensing within a wide range of 98-773 K. Ratiometric strategy utilizing two luminescence intensity ratios, one between host and Er³⁺band (LIR₁) and second between different Er³⁺transitions (LIR₂), results in self-referencing temperature readouts. The presence of two temperature-dependent parameters could improve thermometric characteristics and broaden the working temperature range compared to a usual single-parameter thermometer. Thermometric performances of prepared samples were evaluated in terms of thermal sensitivities, temperature resolution and repeatability. The highest sensitivity of 2.09% K^-1@300 K was found for LIR₁, whereas LIR₂provided more accurate thermal sensing with a temperature resolution of 0.06-0.1 K. Effect of Er³⁺doping concentration on sensing properties were studied. The presented findings indicate that CaWO₄:Er³⁺phosphors are perspective in dual-mode thermal sensing with high sensitivity and sub-degree resolution.

A high sensitivity dual-mode optical thermometry based on charge compensation in ZnTiO3:M (M = Eu3+, Mn4+) hexagonal prisms

Optical thermometer based on dual-mode fluorescence intensity ratiometric thermometry has been attracted more attention due to its higher sensitivity. In order to obtain optical thermal probe with high sensitivity, ZnTiO₃ hexagonal prisms with hexagonal perovskite structure were fabricated by using self-assembly method, and Al³⁺ ions were introduced into the crystal lattices of ZnTiO₃ doped with Eu³⁺/Mn⁴⁺ to improve the optical properties. The emission intensity assigned to Eu³⁺ was enhanced about twice with the charge compensation of Al³⁺ between Eu³⁺ and Ti³⁺. While the luminescence ratio between the thermal coupled level of Eu³⁺ revealed poor temperature dependence property. The emission assigned to ²E_g→⁴A_2g (713 nm) transition of Mn⁴⁺ revealed an huge thermal quenching. Using the luminescence ratio between ⁵D₀→⁷F₂ (⁵D₀→⁷F₁) transition of Eu³⁺ to ²E_g→⁴A_2g of Mn⁴⁺, the higher relative sensitivity of 2.7 %K^-1was obtained. The charge compensation of Al³⁺ improved the coordination and reduced the relative sensitivity, Sr =1.85 %K^-1. The results suggested the potential application in optical temperature probes for ZnTiO₃: Mn⁴⁺,Eu³⁺ phosphor.

Dual-center co-doped and mixed ratiometric LuVO4:Nd3+/Yb3+nanothermometers

During last decade luminescence thermometry has become a widely studied research field due to its potential applications for real time contactless temperature sensing where usual thermometers cannot be used. Special attention is paid to the development of accurate and reliable thermal sensors with simple reading. To address existing problems of ratiometric thermometers based on thermally-coupled levels, LuVO₄:Nd³⁺/Yb³⁺thermal sensors were studied as a proof-of-concept of dual-center thermometer obtained by co-doping or mixture. Both approaches to create a dual-center sensor were compared in terms of energy transfer efficiency, relative sensitivity, and temperature resolution. Effect of excitation mechanism and Yb³⁺doping concentration on thermometric performances was also investigated. The best characteristics ofS_r = 0.34% K^-1@298 K and ΔT = 0.2 K were obtained for mixed phosphors upon host excitation.

Single Er3+, Yb3+: KGd3F10 Nanoparticles for Nanothermometry

Among several optical non-contact thermometry methods, luminescence thermometry is the most versatile approach. Lanthanide-based luminescence nanothermometers may exploit not only downshifting, but also upconversion (UC) mechanisms. UC-based nanothermometers are interesting for biological applications: they efficiently convert near-infrared radiation to visible light, allowing local temperatures to be determined through spectroscopic investigation. Here, we have synthesized highly crystalline Er³⁺, Yb³⁺ co-doped upconverting KGd₃F₁₀ nanoparticles (NPs) by the EDTA-assisted hydrothermal method. We characterized the structure and morphology of the obtained NPs by transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and dynamic light scattering. Nonlinear spectroscopic studies with the Er³⁺, Yb³⁺: KGd₃F₁₀ powder showed intense green and red emissions under excitation at 980 and 1,550 nm. Two- and three-photon processes were attributed to the UC mechanisms under excitation at 980 and 1,550 nm. Strong NIR emission centered at 1,530 nm occurred under low 980-nm power densities. Single NPs presented strong green and red emissions under continuous wave excitation at 975.5 nm, so we evaluated their use as primary nanothermometers by employing the Luminescence Intensity Ratio technique. We determined the temperature felt by the dried NPs by integrating the intensity ratio between the thermally coupled ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 levels of Er³⁺ ions in the colloidal phase and at the single NP level. The best thermal sensitivity of a single Er³⁺, Yb³⁺: KGd₃F₁₀ NP was 1.17% at the single NP level for the dry state at 300 K, indicating potential application of this material as accurate nanothermometer in the thermal range of biological interest. To the best of our knowledge, this is the first promising thermometry based on single KGd₃F₁₀ particles, with potential use as biomarkers in the NIR-II region.

Three-dimensional nanothermometry below the diffraction limit

Lanthanide-doped nanothermometers are used to measure temperature through changes in their emission characteristic with sensitivities of up to a few %/K. In contrast to their sensitivity, their spatial resolution, which is of critical importance for various applications, has not been thoroughly studied and optimized. We numerically investigated the improvement in spatial resolution of nanothermometers with a stimulated emission depletion microscopy approach. Fundamental relationships between spatial and temperature resolution were identified by using different beam parameters for the excitation and depletion beams. Our simulations predict contactless temperature measurement below the diffraction limit with temperature resolution of ±1.25K. We further studied the influence of sample thickness and position on both temperature and spatial resolution and showed the potential of three-dimensional measurements.

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.

Multimode high-sensitivity optical YVO4:Ln3+ nanothermometers (Ln3+ = Eu3+, Dy3+, Sm3+) using charge transfer band features

Accurate thermal sensing with good spatial resolution is currently required in a variety of scientific and technological areas. Luminescence nanothermometry has shown competitive superiority in contactless temperature sensing, especially at the nanoscale. To broaden the use of such thermometers, development of a novel sensor type with high sensitivity and resolution is highly demanded. Herein, we report single-phase Ln3+-doped YVO4 nanophosphors synthesized using a modified Pechini method as multimode optical thermometers for wide-range temperature probing (299-466 K). The observed temperature-induced red shift of the charge transfer band was utilized to provide thermal sensing. Temperature sensing was based on the luminescence intensity ratio using emission intensities obtained upon charge transfer and direct lanthanide excitation, the spectral position of the charge transfer band and its bandwidth. The suggested probing strategies provided a high relative thermal sensitivity (up to 3.09% K-1) and a precise temperature resolution (up to 0.1 K). The obtained results can be useful for the design of novel contactless luminescence thermometers.

Terbium(III)-thiacalix[4]arene nanosensor for highly sensitive intracellular monitoring of temperature changes within the 303-313 K range

The work introduces hydrophilic PSS-[Tb₂(TCAn)₂] nanoparticles to be applied as highly sensitive intracellular temperature nanosensors. The nanoparticles are synthesized by solvent-induced nanoprecipitation of [Tb₂(TCAn)₂] complexes (TCAn - thiacalix[4]arenes bearing different upper-rim substituents: unsubstituted TCA1, tert-buthyl-substituted TCA2, di- and tetra-brominated TCA3 and TCA4) with the use of polystyrenesulfonate (PSS) as stabilizer. The temperature responsive luminescence behavior of PSS-[Tb₂(TCAn)₂] within 293–333 K range in water is modulated by reversible changes derived from the back energy transfer from metal to ligand (M* → T₁) correlating with the energy gap between the triplet levels of ligands and resonant ⁵D₄ level of Tb³⁺ ion. The lowering of the triplet level (T₁) energies going from TCA1 and TCA2 to their brominated counterparts TCA3 and TCA4 facilitates the back energy transfer. The highest ever reported temperature sensitivity for intracellular temperature nanosensors is obtained for PSS-[Tb₂(TCA4)₂] (S_I = 5.25% K⁻¹), while PSS-[Tb₂(TCA3)₂] is characterized by a moderate one (S_I = 2.96% K⁻¹). The insignificant release of toxic Tb³⁺ ions from PSS-[Tb₂(TCAn)₂] within heating/cooling cycle and the low cytotoxicity of the colloids point to their applicability in intracellular temperature monitoring. The cell internalization of PSS-[Tb₂(TCAn)₂] (n = 3, 4) marks the cell cytoplasm by green Tb³⁺-luminescence, which exhibits detectable quenching when the cell samples are heated from 303 to 313 K. The colloids hold unprecedented potential for in vivo intracellular monitoring of temperature changes induced by hyperthermia or pathological processes in narrow range of physiological temperatures.

Autofluorescence-Free In Vivo Imaging Using Polymer-Stabilized Nd3+-Doped YAG Nanocrystals

Neodymium-doped yttrium aluminum garnet (YAG:Nd³⁺) has been widely developed during roughly the past 60 years and has been an outstanding fluorescent material. It has been considered as the gold standard among multipurpose solid-state lasers. Yet, the successful downsizing of this system into the nanoregimen has been elusive, so far. Indeed, the synthesis of a garnet structure at the nanoscale, with enough crystalline quality for optical applications, was found to be quite challenging. Here, we present an improved solvothermal synthesis method producing YAG:Nd³⁺ nanocrystals of remarkably good structural quality. Adequate surface functionalization using asymmetric double-hydrophilic block copolymers, constituted of a metal-binding block and a neutral water-soluble block, provides stabilized YAG:Nd³⁺ nanocrystals with long-term colloidal stability in aqueous suspensions. These newly stabilized nanoprobes offer spectroscopic quality (long lifetimes, narrow emission lines, and large Stokes shifts) close to that of bulk YAG:Nd³⁺. The narrow emission lines of YAG:Nd³⁺ nanocrystals are exploited by differential infrared fluorescence imaging, thus achieving an autofluorescence-free in vivo readout. In addition, nanothermometry measurements, based on the ratiometric fluorescence of the stabilized YAG:Nd³⁺ nanocrystals, are demonstrated. The progress here reported paves the way for the implementation of this new stabilized YAG:Nd³⁺ system in the preclinical arena.

Upconversion Nanocrystal Doped Polymer Fiber Thermometer

In recent years, lanthanide-doped nanothermometers have been mainly used in thin films or dispersed in organic solvents. However, both approaches have disadvantages such as the short interaction lengths of the active material with the pump beam or complicated handling, which can directly affect the achievable temperature resolution. We investigated the usability of a polymer fiber doped with upconversion nanocrystals as a thermometer. The fiber was excited with a wavelength stabilized diode laser at a wavelength of 976 nm. Emission spectra were recorded in a temperature range from 10 to 35 ∘C and the thermal emission changes were measured. Additionally, the pump power was varied to study the effect of self-induced heating on the thermometer specifications. Our fiber sensor shows a maximal thermal sensitivity of 1.45%/K and the minimal thermal resolution is below 20 mK. These results demonstrate that polymer fibers doped with nanocrystals constitute an attractive alternative to conventional fluorescence thermometers, as they add a long pump interaction length while also being insensitive to strong electrical fields or inert to bio-chemical environments.

Making Nd3+ a Sensitive Luminescent Thermometer for Physiological Temperatures-An Account of Pitfalls in Boltzmann Thermometry

Ratiometric luminescence thermometry employing luminescence within the biological transparency windows provides high potential for biothermal imaging. Nd3+ is a promising candidate for that purpose due to its intense radiative transitions within biological windows (BWs) I and II and the simultaneous efficient excitability within BW I. This makes Nd3+ almost unique among all lanthanides. Typically, emission from the two 4F3/2 crystal field levels is used for thermometry but the small ~100 cm-1 energy separation limits the sensitivity. A higher sensitivity for physiological temperatures is possible using the luminescence intensity ratio (LIR) of the emissive transitions from the 4F5/2 and 4F3/2 excited spin-orbit levels. Herein, we demonstrate and discuss various pitfalls that can occur in Boltzmann thermometry if this particular LIR is used for physiological temperature sensing. Both microcrystalline, dilute (0.1%) Nd3+-doped LaPO4 and LaPO4: x% Nd3+ (x = 2, 5, 10, 25, 100) nanocrystals serve as an illustrative example. Besides structural and optical characterization of those luminescent thermometers, the impact and consequences of the Nd3+ concentration on their luminescence and performance as Boltzmann-based thermometers are analyzed. For low Nd3+ concentrations, Boltzmann equilibrium starts just around 300 K. At higher Nd3+ concentrations, cross-relaxation processes enhance the decay rates of the 4F3/2 and 4F5/2 levels making the decay faster than the equilibration rates between the levels. It is shown that the onset of the useful temperature sensing range shifts to higher temperatures, even above ~ 450 K for Nd concentrations over 5%. A microscopic explanation for pitfalls in Boltzmann thermometry with Nd3+ is finally given and guidelines for the usability of this lanthanide ion in the field of physiological temperature sensing are elaborated. Insight in competition between thermal coupling through non-radiative transitions and population decay through cross-relaxation of the 4F5/2 and 4F3/2 spin-orbit levels of Nd3+ makes it possible to tailor the thermometric performance of Nd3+ to enable physiological temperature sensing.

Construction of efficient dual activating ratiometric YVO4:Nd3+/Eu3+ nanothermometers using co-doped and mixed phosphors

The development of new contactless thermal nanosensors based on a ratiometric approach is of significant interest. To overcome the intrinsic limitations of thermally coupled levels, a dual activation strategy was applied. Dual activation was performed using co-doped single nanoparticles and a binary mixture of single-doped nanoparticles. Co-doped and mixed YVO₄:Nd³⁺/Eu³⁺ nanoparticles were successfully demonstrated as luminescent nanothermometers and their thermometric performance, in terms of thermal sensitivity, temperature resolution and repeatability, was studied and compared.

Advancing neodymium single-band nanothermometry

Near-infrared (NIR) emitting contrast agents with integrated optical temperature sensing are highly desirable for a variety of biomedical applications, particularly when subcutaneous target visualization and measurement of its thermodynamic properties are required. To that end, the possibility of using Nd³⁺ doped LiLuF₄ rare-earth nanoparticles (RENPs) as NIR photoluminescent nanothermometers is explored. These RENPs are relatively small, have narrow size distribution, and can easily be core/shell engineered - all combined, these features meet the requirements of biologically relevant and multifunctional nanoprobes. The LiLuF₄ host allows to observe the fine Stark structure of the ⁴F_3/2→⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 optical transitions, each of which can then be used for single-band NIR nanothermometry. The thermometric parameter defined for the most intense Nd³⁺ emission around 1050 nm, shows high temperature sensitivity (∼0.49% °C^-1), and low temperature uncertainty (0.3 °C) as compared to the thermometric parameters defined for the 880 and 1320 nm Nd³⁺ emissions. Additionally, transient temperature measurements through tissue show that these RENPs can be used to assess fast temperature changes at a tissue depth of 3 mm, while slower temperature changes can be measured at even greater depths. Nd³⁺ doped LiLuF₄ RENPs represent a significant improvement for Nd³⁺ based single-band photoluminescence nanothermometry, with the possibility of its integration within more sophisticated multifunctional theranostic nanostructures.