Ratiometric dual-emitting thermometers based on rhodamine B dye-incorporated (nano) curcumin periodic mesoporous organosilicas for bioapplications

This study explores the potential of combining periodic mesoporous organosilicas (PMOs) with a fluorescent dye to develop a ratiometric thermometry system with enhanced stability, sensitivity, and biocompatibility. PMOs, ordered porous materials known for their stability and versatility, serve as an ideal platform. Curcumin, a natural polyphenol and fluorescent dye, is incorporated into PMOs to develop curcumin-functionalized PMOs (C-PMO) and curcumin-pyrazole-functionalized PMOs (CP-PMO) via hydrolysis and co-condensation. These PMOs exhibit temperature-dependent fluorescence properties. The next step involves encapsulating rhodamine B (RhB) dye within the PMO pores to create dual-emitting PMO@dye nanocomposites, followed by a lipid bilayer (LB) coating to enhance biocompatibility and dye retention. Remarkably, within the physiological temperature range, C-PMO@RhB@LB and CP-PMO@RhB@LB demonstrate noteworthy maximum relative sensitivity (Sr) values of up to 1.69 and 2.60% K-1, respectively. This approach offers versatile means to create various ratiometric thermometers by incorporating different fluorescent dyes, holding promise for future temperature sensing applications.

Intensity and lifetime ratiometric luminescent thermometer based on a Tb(III) coordination polymer

A three-dimensional terbium(III) coordination polymer of formula [Tb(bttb)0.5(2,5-pzdc)0.5]n (1) [H4bttb = 1,2,4,5-tetrakis(4'-carboxyphenyl)benzene and H2-2,5-pzdc = 2,5-pyrazinedicarboxylic acid] was obtained under hydrothermal conditions. The bttb4- tetraanion in 1 adopts the bridging and chelating-bridging pseudo-oxo coordination modes while the 2,5-pzdc2- dianion exhibits a rather unusual bis-bidentate bridging pseudo-oxo coordination mode, both ligands being responsible for the stiffness of the resulting 3D structure. Solid-state photoluminescent measurements illustrate that 1 exhibits remarkable green luminescence emission, the most intense band occurring in the region of 550 nm (5D4 → 7F5) with lifetimes at the millisecond scale. Thermometric performances of 1 reveal a maximum relative sensitivity (Sm) of 0.76% K-1 at 295 K (δT = 0.05 K), constituting a TbIII ratiometric solid luminescent thermometer over the physiological temperature range. Variable-temperature static (dc) magnetic susceptibility measurements for 1 in the temperature range 2.0-300 K show the expected behavior for the depopulation of the splitted mJ levels of the 7F7 ground state of the magnetically anisotropic terbium(III) ion plus a weak antiferromagnetic interaction through the carboxylate bridges. No significant out-of-phase magnetic susceptibility signals were observed for 1 in the temperature range 2.0-10.0 K, either in the absence or presence of a static dc magnetic field.

Luminescent Behavior of Sb3+-Activated Luminescent Metal Halide

Metal halide perovskites have unparalleled optoelectronic properties and broad application potential and are expected to become the next epoch-making optoelectronic semiconductors. Although remarkable achievements have been achieved with lead halide perovskites, the toxicity of lead inhibits the development of such materials. Recently, Sb3+-activated luminescent metal halide perovskite materials with low toxicity, high efficiency, broadband, large Stokes shift, and emission wavelengths covering the entire visible and near-infrared regions have been considered one of the most likely luminescent materials to replace lead halide perovskites. This review reviews the synthesis, luminescence mechanism, structure, and luminescence properties of the compounds. The basic luminescence properties of Sb3+-activated luminescent metal halide perovskites and their applications in WLED, electroluminescence LED, temperature sensing, optical anti-counterfeiting, and X-ray scintillators are introduced. Finally, the development prospects and challenges of Sb3+-activated luminescent metal halide perovskites are discussed.

Spotlight on Luminescence Thermometry: Basics, Challenges, and Cutting-Edge Applications

Luminescence (nano)thermometry is a remote sensing technique that relies on the temperature dependency of the luminescence features (e.g., bandshape, peak energy or intensity, and excited state lifetimes and risetimes) of a phosphor to measure temperature. This technique provides precise thermal readouts with superior spatial resolution in short acquisition times. Although luminescence thermometry is just starting to become a more mature subject, it exhibits enormous potential in several areas, e.g., optoelectronics, photonics, micro- and nanofluidics, and nanomedicine. This work reviews the latest trends in the field, including the establishment of a comprehensive theoretical background and standardized practices. The reliability, repeatability, and reproducibility of the technique are also discussed, along with the use of multiparametric analysis and artificial-intelligence algorithms to enhance thermal readouts. In addition, examples are provided to underscore the challenges that luminescence thermometry faces, alongside the need for a continuous search and design of new materials, experimental techniques, and analysis procedures to improve the competitiveness, accessibility, and popularity of the technology.

Structure and luminescence properties of Dy3+ doped quaternary tungstate Li3Ba2Gd3(WO4)8 for application in wLEDs

Quaternary tungstates with the composition Li3Ba2Gd3(WO4)8 doped with different concentrations of Dy3+ (from 0.5 to 10 at%) were prepared by the solid-state reaction method at 900 °C. Their structural, spectroscopic and optical properties were studied systematically in this work. X-ray diffraction analysis confirmed the crystallization of Li3Ba2Gd3(WO4)8 to have a monoclinic structure (sp. gr. C2/c); the lattice constants for 1 at% doping concentration of Dy3+ are a = 5.2126(2) Å, b = 12.7382(1) Å, c = 19.1884(3) Å, Vcalc = 1273,40(4) Å3 and β = a × c = 91.890(9)°. The first principles calculations for the undoped crystal revealed a direct bandgap of 2.45 eV, which is very close to the experimental one. The identified broad, and strong excitation peak at 450 nm indicates that Li3Ba2Gd3(WO4)8:Dy3+ phosphors are suitable to be pumped by a blue laser diode (LD). Under excitation at 445 nm, the phosphor showed a stronger luminescence peak at 575 nm which corresponds to the Dy3+:4F9/2 → 6H13/2 transition, and three weaker emissions peaks at 477, 661, and 750 nm. Meanwhile, the effect of different Dy3+ contents on the luminescence properties was investigated. The optimum concentration to minimize the quenching effect was 4 at% and the critical distance is 31.209 Å. The phosphor emitted strong greenish-yellow light situated at (0.425, 0.472) in CIE coordinates with a color temperature of 3652 K. All the measured luminescence lifetime curves exhibited a single-exponential nature. Excellent thermal stability was found for this tungstate phosphor (the activation energy is 0.352 ± 0.01 eV). The measured absolute photoluminescence quantum yield was around 10.5%. The results presented in this work show that Li3Ba2Gd3(WO4)8:Dy3+ phosphors with strong yellow emission are promising candidates for white-light emitting LED (wLED) applications.

Lanthanide molecular cluster-aggregates as the next generation of optical materials

In this perspective, we provide an overview of the recent achievements in luminescent lanthanide-based molecular cluster-aggregates (MCAs) and illustrate why MCAs can be seen as the next generation of highly efficient optical materials. MCAs are high nuclearity compounds composed of rigid multinuclear metal cores encapsulated by organic ligands. The combination of high nuclearity and molecular structure makes MCAs an ideal class of compounds that can unify the properties of traditional nanoparticles and small molecules. By bridging the gap between both domains, MCAs intrinsically retain unique features with tremendous impacts on their optical properties. Although homometallic luminescent MCAs have been extensively studied since the late 1990s, it was only recently that heterometallic luminescent MCAs were pioneered as tunable luminescent materials. These heterometallic systems have shown tremendous impacts in areas such as anti-counterfeiting materials, luminescent thermometry, and molecular upconversion, thus representing a new generation of lanthanide-based optical materials.

Luminescent Properties of Phosphonate Ester-Supported Neodymium(III) Nitrate and Chloride Complexes

This study examines the synthesis of two geminal bisphosphonate ester-supported Ln³⁺ complexes [Ln(L3)₂(NO₃)₃] (Ln = Nd³⁺ (5), La³⁺ (6)) and optical properties of the neodymium(III) complex. These results are compared to known mono-phosphonate ester-based Nd³⁺ complexes [Nd(L1/L2)₃X₃]_n (X = NO₃^-, n = 1; Cl^-, n = 2) (1-4). The optical properties of Nd³⁺ compounds are determined by micro-photoluminescence (µ-PL) spectroscopy which reveals three characteristic metal-centered emission bands in the NIR region related to transitions from ⁴F_3/2 excited state. Additionally, two emission bands from ⁴F_5/2, ²H_9/2 → ⁴I_J (J = 11/2, 13/2) transitions were observed. PL spectroscopy of equimolar complex solutions in dry dichloromethane (DCM) revealed remarkably higher emission intensity of the mono-phosphonate ester-based complexes in comparison to their bisphosphonate ester congener. The temperature-dependent PL measurements enable assignment of the emission lines of the ⁴F_3/2 → ⁴I_9/2 transition. Furthermore, low-temperature polarization-dependent measurements of the transitions from R₁ and R₂ Stark sublevel of ⁴F_3/2 state to the ⁴I_9/2 state for crystals of [Nd(L3)₂(NO₃)₃] (5) are discussed.

Expansion of thermometry in magnetic hyperthermia cancer therapy: antecedence and aftermath

Magnetic hyperthermia cancer therapy (MHCT) is a promising antitumor therapy based on the generation of heat by magnetic nanoparticles under the influence of an alternating-current magnetic field. However, an often-overlooked factor hindering the translation of MHCT to clinics is the inability to accurately monitor temperature, thereby leading to erroneous thermal control. It is significant to address 'thermometry' during magnetic hyperthermia because numerous factors are affected by the magnetic fields employed, rendering traditional thermometry methods unsuitable for temperature estimation. Currently, there is a dearth of literature describing appropriate techniques for thermometry during MHCT. This review offers a general outline of the various modes of conventional thermometry as well as cutting-edge techniques operating at cellular/nanoscale levels (nanothermometry) as prospective thermometers for MHCT in the future.

Investigation of Intermetallic Energy Transfers in Lanthanide Coordination Polymers Molecular Alloys: Case Study of Trimesate-Based Compounds

Reactions in water at ambient temperature and pressure between a lanthanide ion and benzene-1,3,5-tricarboxylate (or trimesate) lead to two series of iso-structural coordination polymers. Their general chemical formula is [Ln(tma)(H₂O)₆]_∞ for the lightest lanthanide ions (Ln = La-Dy except Pm), while it is [Ln(tma)(H₂O)₅·3.5H₂O]_∞ for the heaviest ones (Ho-Lu plus Y). For the heaviest lanthanide ions, reactions at 50 °C lead to a third structural family with the general chemical formula [Ln(tma)(H₂O)₃·1.5H₂O]_∞ with Ln = Ho-Lu plus Y. Homo-lanthanide coordination polymers that belong to the latter two families do not exhibit luminescence in the visible region. Therefore, we used a phase induction strategy to obtain molecular alloys that belong to these structural families and show sizeable emission. The random distribution of the lanthanide ions over the metallic sites has been investigated using ⁸⁹Y and ¹³⁹La solid-state NMR spectroscopy experiments. Luminescent properties of homo- and hetero-nuclear coordination polymers based on Eu³⁺ and Tb³⁺ have been studied in detail and compared. As a result, this study strongly suggests that exchange-based intermetallic energy transfer mechanisms play an important role in these systems. It also suggests the presence of an intermetallic exchange pathway through π-stacking interactions.

Temperature-Sensitive Chameleon Luminescent Films Based on PMMA Doped with Europium(III) and Terbium(III) Anisometric Complexes

The spin-coating technique was used to produce composite films consisting of PMMA polymer doped with anisometric complexes of Eu(III) and Tb(III). It was found that an increase in the content of Tb3+ complexes intensifies emission of both ions due to the intermolecular energy transfer from the Tb(III) complex to the Eu(III) complex, which results in the increase in the relative luminescence quantum yield of Eu(III) ion by 36%. The temperature sensitivity of the film luminescence intensity and lifetime in the range of 296–363 K was investigated. The maximum relative sensitivity of the films reaches 5.44% × K−1 and exceeds that of all known lanthanide-containing thermal sensors designed for measuring physiological temperatures. In combination with changing luminescence color, such a sensitivity makes these films promising colorimetric thermal sensors for in situ temperature measurements.

Photoluminescence of Lanthanide-Titanium-Oxo Clusters and Based on a β-Diketone Ligand

A series of acetylacetone-protected lanthanide-titanium-oxo clusters (LTOCs), formulated as [La₆Ti(μ₃-OH)₈(acac)₁₂(CH₃O)₂(CH₃OH)₆] (La₆Ti; Hacac = acetylacetone) and [Ln₉Ti₂(μ₄-O)(μ₃-OH)₁₄(acac)₁₇(CH₃O)₂(CH₃OH)₃] [Ln = Eu (Eu₉Ti₂) and Tb (Tb₉Ti₂)], were synthesized through the reactions of LnCl₃·6H₂O (Ln = La, Eu, and Tb), Hacac, Ti(OⁱPr)₄, and triethylamine in methanol. Crystal structural analysis shows that La₆Ti exhibits an hourglass-like structure consisting of two La₃Ti cubane subunits by sharing one Ti⁴⁺ ion, while Eu₉Ti₂ can be viewed as a combination of four Eu₃Ti cubane subunits by sharing three corners and one side. The photoluminescence (PL) measurements show that Tb₉Ti₂ exhibits excellent PL properties with a high quantum yield (QY) of 34.8%, while Eu₉Ti₂ only has a QY of 1.4% because of the different photosensitizations of ligands to Eu³⁺ and Tb³⁺ ions in the photophysical process.

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 Highly Stable Yttrium Organic Framework as a Host for Optical Thermometry and D2 O Detection

The yttrium organic framework (Y_0.89Tb_0.10Eu_0.01)₆(BDC)₇(OH)₄(H₂O)₄ (BDC=benzene‐1,4‐dicarboxylate) is hydrothermally stable up to at least 513 K and thermally stable in air in excess of 673 K. The relative intensities of luminescence of Tb³⁺ and Eu³⁺ are governed by Tb³⁺‐to‐Eu³⁺ phonon‐assisted energy transfer and Tb³⁺‐to‐ligand back transfer and are responsible for the differing temperature‐dependent luminescence of the two ions. This provides a ratiometric luminescent thermometer in the 288–573 K temperature range, not previously seen for MOF materials, with a high sensitivity, 1.69±0.04 % K⁻¹ at 523 K. In aqueous conditions, loosely bound H₂O can be replaced by D₂O in the same material, which modifies decay lifetimes to yield a quantitative luminescent D₂O sensor with a useful sensitivity for practical application.

Ratiometric Thermometers Based on Rhodamine B and Fluorescein Dye-Incorporated (Nano) Cyclodextrin Metal-Organic Frameworks

Macro- and nanosized core, as well as core-shell, γ-cyclodextrin metal-organic frameworks (γ-CD-MOFs) have been designed and used as platforms for the encapsulation of dye molecules to develop the first CD-MOF-based ratiometric optical thermometer materials. A novel dye combination was employed for this purpose, i.e., the duo rhodamine B (RhB) and fluorescein (FL). RhB is highly temperature-sensitive, whereas FL is less temperature-sensitive, and its luminescence emission peak is used as a reference. Promising results in terms of thermometric properties were obtained for a series of dye-encapsulated γ-CD-MOF materials based on this dye combination, with high relative sensitivities, even up to 5%K^-1, for the dye-encapsulated 75%RhB-25%FL nanosized γ-CD-MOF, among the highest performance values reported so far for luminescent dual thermometers. In our study, we have additionally developed a simple yet effective preparation method for core-shell γ-CD-MOFs, allowing effective manipulation of the γ-CD-MOF shell growth. The proposed method allows incorporation of the FL and RhB dyes in the γ-CD-MOFs in a more controlled manner, enhancing the efficiency of the developed ratiometric (macro) γ-CD-MOF thermometers.

Enhancing the temperature sensitivity of Cr3+ emissions by modification of the host's composition for fluorescence thermometry applications

The fluorescence intensity ratio (FIR) technique is widely adopted in thermometric phosphor materials, but the improvement of relative sensitivity is normally limited by the fixed energy gap between two thermally-coupled emitting levels of luminescent ions. Herein, LnAl₃(BO₃)₄:Cr³⁺ (LnAB:Cr³⁺, Ln = Gd, Y, Lu) phosphors are found to simultaneously show ⁴T₂ and ²E emissions of Cr³⁺, and their FIR is sensitive to temperature and suitable for fluorescence thermometric applications. Moreover, the energy gap between the ⁴T₂ and ²E levels of Cr³⁺ is tunable and the relative sensitivity can be greatly improved by modifying the host's composition. Structural analysis and spectroscopic data confirm that the enhanced crystal-field of the Cr³⁺/Al³⁺ sites caused by incorporating smaller Ln³⁺ ions into the host contributes to the improvement of relative sensitivity. This work would provide new insights into the development of novel FIR thermometric materials with high-sensitivity.

1D-Zigzag Eu3+/Tb3+ Coordination Chains as Luminescent Ratiometric Thermometers Endowed with Multicolor Emission

Two homometallic Coordination Polymers (CPs) with composition [Ln(hfac)₃bipy]_n (Ln³⁺ = Eu³⁺, 1, and Tb³⁺, 2; hfac = hexafluoroacetylacetonato, bipy = 4,4′-bipyridine) were used to develop a family of ratiometric luminescent thermometers containing Eu³⁺ and Tb³⁺ as red and green emitters, respectively. The thermometric properties of pure CPs and of their mixtures having an Eu³⁺/Tb³⁺ molar ratio of 1:1, 1:3, 1:5, and 1:10 (samples: Eu1Tb1, Eu1Tb3, Eu1Tb5, and Eu1Tb10) were studied in the 83–383 K temperature range. Irrespective of the chemical composition, we observed similar thermometric responses characterized by broad applicative temperature ranges (from 100 to 165 K wide), and high relative thermal sensitivity values (S_r), up to 2.40% K⁻¹, in the physiological temperature range (298–318 K). All samples showed emissions endowed with peculiar and continuous color variation from green (83 K) to red (383 K) that can be exploited to develop a colorimetric temperature indicator. At fixed temperature, the color of the emitted light can be tuned by varying composition and excitation wavelength.

Dual interactions and thermo-optical analysis of YAGG:Ce/Eu nanophosphor

Present article report on structural and optical investigations of doubly doped Y₃(Al_5-xGax)O₁₂:Ce/Eu nanophosphor. Efforts have been made to explore the interaction between the Ce³⁺ and Eu³⁺ ions and subsequently variation in overall color perception. Experimental techniques namely XRD, HR-TEM, and SEM reveal precipitation of tiny particles with diameter ~22 nm. Structural refinement has been carried out by Rietveld refinement. Photo-excitation by 342, 405, 450, and 464 nm wavelengths triggers the emission from Eu³⁺ and Ce³⁺ ions. Further, the laser excitations by 405 nm and 450 nm radiations, induced YAGG:Ce/Eu nanophosphor to yield combinatorial emission of both Ce³⁺ and Eu³⁺ ions; though, the intensity was found to be altered due to the two-way ionic interaction between the doped ions. The resultant emission extended in the red region. Also, the doubly doped sample exhibits strong temperature reliance on the emission intensity in the temperature range extended over 273-388 K. The color perception of the nanophosphor was observed to be significantly modified at different excitation wavelengths, temperatures, and laser powers as reflected by CIE coordinates.

Luminescent PMMA Films and PMMA@SiO2 Nanoparticles with Embedded Ln3+ Complexes for Highly Sensitive Optical Thermometers in the Physiological Temperature Range*

In recent years, luminescent materials doped with Ln³⁺ ions have attracted much attention for their application as optical thermometers based on both downshifting and upconversion processes. This study presents research done on the development of highly sensitive optical thermometers in the physiological temperature range based on poly(methyl methacrylate) (PMMA) films doped with two series of visible Ln³⁺ complexes (Ln³⁺ =Tb³⁺ , Eu³⁺ , and Sm³⁺ ) and SiO₂ nanoparticles (NPs) coated with these PMMA films. The best performing PMMA film doped with Tb³⁺ and Eu³⁺ complexes was the PMMA[TbEuL₁ tppo]1 film (L₁ =4,4,4-trifluoro-1-phenyl-1,3-butadionate; tppo=triphenylphosphine oxide), which showed good temperature sensing of S_r =4.21 % K^-1 at 313 K, whereas for the PMMA films doped with Tb³⁺ and Sm³⁺ complexes the best performing was the PMMA[TbSmL₂ tppo]3 film (L₂ =4,4,4-trifluoro-1-(4-chlorophenyl)-1,3-butadionate), with S_r =3.64 % K^-1 at 313 K. Additionally, SiO₂ NPs coated with the best performing films from each of the series of PMMA films (Tb-Eu and Tb-Sm) and their temperature-sensing properties were studied in water, showing excellent performance in the physiological temperature range (PMMA[TbEuL₁ tppo]1@SiO₂ : S_r =3.84 % °C at 20 °C; PMMA[TbSmL₂ tppo]3@SiO₂ : S_r =3.27 % °C at 20 °C) and the toxicity of these nanoparticles on human cells was studied, showing that they were nontoxic.

1,3-Diketone Calix[4]arene Derivatives-A New Type of Versatile Ligands for Metal Complexes and Nanoparticles

The present review is aimed at highlighting outlooks for cyclophanic 1,3-diketones as a new type of versatile ligands and building blocks of the nanomaterial for sensing and bioimaging. Thus, the main synthetic routes for achieving the structural diversity of cyclophanic 1,3-diketones are discussed. The structural diversity is demonstrated by variation of both cyclophanic backbones (calix[4]arene, calix[4]resorcinarene and thiacalix[4]arene) and embedding of different substituents onto lower or upper macrocyclic rims. The structural features of the cyclophanic 1,3-diketones are correlated with their ability to form lanthanide complexes exhibiting both lanthanide-centered luminescence and magnetic relaxivity parameters convenient for contrast effect in magnetic resonance imaging (MRI). The revealed structure–property relationships and the applicability of facile one-pot transformation of the complexes to hydrophilic nanoparticles demonstrates the advantages of 1,3-diketone calix[4]arene ligands and their complexes in developing of nanomaterials for sensing and bioimaging.

Strontium-Based MOFs Showing Dual Emission: Luminescence Thermometers and Toluene Sensors

This work reports on the preparation and optical characterization of two metal-organic frameworks (MOFs) based on strontium ions and 2-amino-1,4-benzenedicarboxylate (NH₂-bdc) ligand: i.e., [Sr(NH₂-bdc)(DMF)]_n (1) and {[Sr(NH₂-bdc)(Form)]·H₂O}_n (2) (where DMF = dimethylformamide and Form = formamide). Compound 1 has a 3D architecture built up from the linkage established by NH₂-bdc among metal-carboxylate rods, leaving significant microchannels that are largely occupied by DMF molecules coordinated to strontium centers. The solvent molecules play a crucial role in the photoluminescence (PL) properties, which has been deeply characterized by diffuse reflectance and variable-temperature emission. Interestingly, both materials present intriguing photoluminescence (PL) properties involving intense short-lived and long-lasting phosphorescence (LLP), though the latter is especially remarkable for compound 2 with a lifetime of 815 ms at low temperature. Conversely, the strong PL shown by 1 may be successfully exploited due to both its luminescent thermochromism observed in the RT to 10 K range and its solvent-dependent PL sensing capacity, imbuing this material with potential activity as a PL thermometer as well as a toluene detector in water solutions.

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.

Standardizing luminescence nanothermometry for biomedical applications

Luminescence nanothermometry enables accurate, remote, and all-optically-based thermal sensing. Notwithstanding its fast development, there are serious obstacles hindering reproducibility and reliable quantitative assessment of nanothermometers, which impede the intentional design, optimization and use of these sensors. These issues include ambiguities or absence of established universal rules for quantitative evaluation, incorrect assumptions about the mechanisms behind the thermal response of the sensors as well as the dependence of the nanothermometers readout on external conditions and host materials themselves. In this perspective article, we discuss these problems and propose a series of standardization guidelines to be followed. This critical discourse constitutes the first required step towards the ubiquitous acceptance, by the scientific community, of luminescence thermometry as a reliable tool for remote temperature determination in numerous practical biomedical implementations.

Peasecod-Like Hollow Upconversion Nanocrystals with Excellent Optical Thermometric Performance

Trivalent lanthanide (Ln3+)-doped hollow upconversion nanocrystals (UCNCs) usually exhibit unique optical performance that cannot be realized in their solid counterparts, and thus have been receiving tremendous interest from their fundamentals to diverse applications. However, all currently available Ln3+-doped UCNCs are solid in appearance, the preparation of hollow UCNCs remains nearly untouched hitherto. Herein, a class of UCNCs based on Yb3+/Er3+-doped tetralithium zirconium octafluoride (Li4ZrF8:Yb/Er) featuring 2D layered crystal lattice is reported, which makes the fabrication of hollow UCNCs with a peasecod-like shape possible after Ln3+ doping. By employing the first-principle calculations, the unique peasecod-like hollow nanoarchitecture primarily associated with the hetero-valence Yb3+/Er3+ doping into the 2D layered crystal lattice of Li4ZrF8 matrix is revealed. Benefiting from this hollow nanoarchitecture, the resulting Li4ZrF8:Yb/Er UCNCs exhibit an abnormal green upconversion luminescence in terms of the population ratio between two thermally coupled states (2H11/2 and 4S3/2) of Er3+ relative to their solid Li2ZrF6:Yb/Er counterparts, thereby allowing to prepare the first family of hollow Ln3+-doped UCNCs as supersensitive luminescent nanothermometer with almost the widest temperature sensing range (123-800 K). These findings described here unambiguously pave a new way to fabricate hollow Ln3+-doped UCNCs for numerous applications.

A simple synthetic approach to enhance the thermal luminescence sensitivity of Tb3+ complexes with thiacalix[4]arene derivatives through upper-rim bromination

The present work for the first time reports an application of the thiacalix[4]arene scaffold for the preparation of Tb3+ complexes possessing high thermal luminescence sensitivity in the physiological temperature range of 20-50 °C. Non-substituted thiacalix[4]arenes form luminescent complexes with Tb3+ ions, but they do not reveal any meaningful thermal sensitivity. To solve this problem, an upper-rim bromination of thiacalix[4]arenes, as well as distal bromination along with the embedding of two 1,3-diketone substituents are proposed as new simple synthetic approaches to enhance the thermal luminescence sensitivity of the Tb3+ complexes. A combination of mass spectrometry, NMR, UV-Vis and luminescence spectroscopy with quantum chemical calculations reveals a dimeric structure of the complexes formed by thiacalix[4]arenes with Tb3+ ions in DMF solutions. The steady-state luminescence of the Tb3+ complexes has demonstrated more than one order higher thermal sensitivity for the complexes of bromo-substituted ligands in comparison with the non-substituted thiacalix[4]arenes. The reasons for such behavior are discussed. The results highlight new opportunities for the thiacalix[4]arene platform for controlling ligand-to-metal energy transfer in terbium complexes and tuning their thermo-responsive luminescence properties.

Thermo-responsive light-emitting metal complexes and related materials

This review discusses the fundamentals and design strategies for the development of thermo-responsive metal–ligand coordination materials and the applications of these materials in temperature sensing, bioimaging, information security, etc.

A closer look at the defects and luminescence of nanocrystalline fluorides synthesized via ionic liquids: the case of Ce3+-doped BaF2

Phase pure BaF₂ doped with Ce³⁺ (0.1%) nanocrystals are synthesized using an ionic liquid (IL) ([C₄mim][BF₄]) assisted solvothermal method where the IL is not only used as a reaction medium and a capping agent, but also as a reaction partner.

NaYF4 Microstructure, beyond Their Well-Shaped Morphology

Lanthanide-doped nanoparticles are widely investigated for their optical properties. However, the sensitivity of the lanthanide ions’ luminescence to the local symmetry, useful when investigating structural environments, becomes a drawback for optimized properties in the case of poorly controlled crystallinity. In this paper, we focus on β-NaYF₄ nanorods in order to provide a detailed description of their chemical composition and microstructure. The combination of detailed XRD analysis and TEM observations show that strong variation may be observed from particles from a same batch of synthesis, but also when considering small variations of synthesis conditions. Moreover, also the nanorods observed by SEM exhibit a very nice faceted shape, they are far from being monocrystalline and present significant local deviation of crystalline symmetry and orientation. All these structural considerations, sensitively probed by polarized emission analysis, are crucial to analyze for the development of optimal systems toward the targeted applications.

Prediction of Thermal-Coupled Thermometric Performance of Er3

Predicting the thermometric performance of diverse materials will facilitate the selection and design of nanothermometers to suit complex environments and specific signal outputs while saving much time and expense. Herein we explore and unveil the thermal-coupled thermometric performance of Er³⁺/Yb³⁺ codoped in a set of host lattices via the chemical bond theory of complex crystals. The unknown B and ΔE values of the thermometry are accurately estimated by the chemical bond parameters, further deepening our cognition of the correlation between the luminescence properties of Er³⁺ ions and the microscopic crystal structure. This allows us to precisely forecast the thermal-coupled thermometric performance of Er³⁺ for varying host lattices in advance.

Strong photoluminescence and sensing performance of nanosized Ca0.8Ln0.1Na0.1WO4 (Ln = Sm,Eu) compounds obtained by the dry "top-down" grinding method

Two lanthanide doped nanosystems Ca_0.8Ln_0.1Na_0.1WO₄ (Ln = Eu, Sm), denoted as Eu@CWO and Sm@CWO, were prepared by a "top-down" approach in three simple steps: activation, miniaturization by high-energy milling, and further calcination. The solids were thoroughly characterized by X-ray powder diffraction (XRPD) and Scanning-electron microscopy (SEM). Also, analyses of the structure of the compounds and the impact of milling on the crystallite shape and size were carried out through Rietveld refinements. Solid-state photoluminescence was studied in terms of excitation, emission, lifetimes (τ_obs) and europium-quantum yields. Finally, the Eu@CWO sample was employed as a potential water-stable chemical sensor towards toxic cations, showing a quenching effect in the presence of iron ions.

Eu3+, Tb3+- and Er3+, Yb3+-Doped α-MoO3 Nanosheets for Optical Luminescent Thermometry

Here we report a novel synthesis approach for the preparation of α-MoO₃:Ln³⁺ materials employing a two-step synthesis. Additionally, in this work the α-MoO₃:Ln³⁺ materials are reported as potential optical thermometers for the first time. In this synthesis approach, first MoS₂ 2D nanosheets were prepared, which were further heat treated to obtain α-MoO₃. These materials were fully characterized by powder X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), thermogravimetry (TG) and differential thermal analysis (DTA), transmission electron microscopy (TEM), and luminescence spectroscopy. Temperature-dependent luminescence measurements were carried out to determine the optical thermometric properties of two different types of α-MoO₃:Ln³⁺ materials (Eu³⁺/Tb³⁺ downshifting and Er³⁺/Yb³⁺ upconversion luminescence systems). We demonstrate in this study that this class of material could be a potential candidate for temperature-sensing applications.

Photo-isomerization of azobenzene containing surfactants induced by near-infrared light using upconversion nanoparticles as mediator

Here we report on photo-isomerization of azobenzene containing surfactants induced during irradiation with near-infrared (NIR) light in the presence of upconversion nanoparticles (UCNPs) acting as mediator. The surfactant molecule consists of charged head group and hydrophobic tail with azobenzene group incorporated in alkyl chain. The azobenzene group can be reversible photo-isomerized between two states: trans- and cis- by irradiation with light of an appropriate wavelength. The trans-cis photo-isomerization is induced by UV light, while cis-trans isomerization proceeds either thermally in darkness, or can be accelerated by exposure to illumination with a longer wavelength typically in a blue/green range. We present the application of lanthanide doped UCNPs to successfully switch azobenzene containing surfactants from cis to trans conformation in bulk solution using NIR light. Using Tm³⁺ or Er³⁺ as activator ions, the UCNPs provide emissions in the spectral range of 450 nm  <  λ _em  <  480 nm (for Tm³⁺, three and four photon induced emission) or 525 nm  <  λ _em  <  545 nm (for Er³⁺, two photon induced emission), respectively. Especially for UCNPs containing Tm³⁺ a good overlap of the emissions with the absorption bands of the azobenzene is present. Under illumination of the surfactant solution with NIR light (λ _ex  =  976 nm) in the presence of the Tm³⁺-doped UCNPs, the relaxation time of cis-trans photo-isomerization was increased by almost 13 times compared to thermally induced isomerization. The influence of thermal heating due to the irradiation using NIR light was shown to be minor for solvents not absorbing in NIR spectral range (e.g. CHCl₃) in contrast to water, which shows a distinct absorption in the NIR.

Luminescent Lanthanide Metal Organic Frameworks as Chemosensing Platforms towards Agrochemicals and Cations

Since the first studies of luminescent sensors based on metal organic frameworks (MOFs) about ten years ago, there has been an increased interest in the development of specific sensors towards cations, anions, explosives, small molecules, solvents, etc. However, the detection of toxic compounds related to agro-industry and nuclear activity is noticeably scarce or even non-existent. In this work, we report the synthesis and characterization of luminescent lanthanide-based MOFs (Ln-MOFs) with diverse crystalline architectures obtained by solvothermal methods. The luminescent properties of the lanthanides, and the hypersensitive transitions of Eu³⁺ (⁵D₀→⁷F₂) and Tb³⁺ (⁵D₄→⁷F₅) intrinsically found in the obtained MOFs in particular, were evaluated and employed as chemical sensors for agrochemical and cationic species. The limit of detection (LOD) of Tb-PSA MOFs (PSA = 2-phenylsuccinate) was 2.9 ppm for [UO₂²⁺] and 5.6 ppm for [Cu²⁺]. The variations of the 4f–4f spectral lines and the quenching/enhancement effects of the Ln-MOFs in the presence of the analytes were fully analyzed and discussed in terms of a combinatorial “host–guest” vibrational and “in-silico” interaction studies.

Methylene Blue-Loaded Upconverting Hydrogel Nanocomposite: Potential Material for Near-Infrared Light-Triggered Photodynamic Therapy Application

The property of upconverting nanoparticles to convert the low-energy near-infrared (NIR) light into high-energy visible light has made them a potential candidate for various biomedical applications including photodynamic therapy (PDT). In this work, we show how a surface functionalization approach on the nanoparticle can be used to develop a nanocomposite hydrogel which can be of potential use for the PDT application. The upconverting hydrogel nanocomposite was synthesized by reacting 10-undecenoic acid-capped Yb3+/Er3+-doped NaYF4 nanoparticles with the thermosensitive N-isopropylacrylamide monomer. The formation of hydrogel was completed within 15 min and hydrogel nanocomposites showed strong enhancement in the visible light emission compared to the emission obtained from 10-undecenoic acid-capped Yb3+/Er3+-doped NaYF4 nanoparticles via the upconversion process (under 980 nm laser excitation). The upconverting hydrogel nanocomposites displayed high swelling behavior in water because of their porous nature. The porous structure ensured a higher loading of methylene blue dye (∼78% in 1 h) into the upconverting hydrogel, which was achieved via the swelling diffusion phenomenon. Upon excitation with the NIR light, the visible light emitted from the hydrogel activated the photosensitizer methylene blue which generated reactive oxygen species. Our results were able to show that the methylene blue-loaded composite hydrogel can be a potential platform for the future of NIR-triggered PDT in skin cancer treatment.

Intracellular ratiometric temperature sensing using fluorescent carbon dots

Highly sensitive non-invasive temperature sensing is critical for studying fundamental biological processes and applications in medical diagnostics. Nanoscale-based thermometers are promising non-invasive probes for precise temperature sensing with subcellular resolution. However, many of these systems have limitations as they rely on fluorescence intensity changes, deconvolution of peaks, or the use of hybrid systems to measure thermal events. To address this, we developed a fluorescence-based ratiometric temperature sensing approach using carbon dots prepared via microwave synthesis. These dots possess dual fluorescence signatures in the blue and red regions of the spectrum. We observed a linear response as a function of temperature in the range of 5-60 °C with a thermal resolution of 0.048 K-1 and thermal sensitivity of 1.97% C-1. Temperature-dependent fluorescence was also observed in HeLa cancer cells over a range of 32-42 °C by monitoring changes in the red-to-blue fluorescence signatures. We demonstrate that the ratiometric approach is superior to intensity-based thermal sensing because it is independent of the intracellular concentration of the optical probe. These findings suggest that dual-emitting carbon dots can be an effective tool for in vitro and possibly in vivo fluorescence nanothermometry.