A stoichiometric terbium-europium dyad molecular thermometer: energy transfer properties

Luminescence of nanocrystalline BaFCl codoped with Eu2+/3+ and Tb3

BaFCl:Eu,Tb nanocrystals were synthesized via hot-injection thermolysis of metal chlorodifluoroacetates and their luminescence response characterized between 80 and 430 K. Though unintended, partial reduction of Eu3+ to Eu2+ occurred during the synthesis stage, leading to luminescent nanocrystals coactivated by Eu2+, Eu3+, and Tb3+. Their emission was dominated by Eu2+, specifically by radiative transitions 4f7 (6P7/2) → 4f7 (8S7/2) and 4f65d1 (lowest level) → 4f7 (8S7/2). Owing to the thermal coupling between 4f7 (6P7/2) and 4f65d1 (lowest) levels, line-like emission from this manifold at 80 K evolved into band-like emission at 430 K. The change in the shape of the emission profile of Eu2+ was used to demonstrate that BaFCl:Eu,Tb may serve to realize bandshape luminescence thermometry as a more straightforward alternative to the usual ratiometric approach. The energy gap between 4f7 (6P7/2) and 4f65d1 (lowest) levels and their radiative transition probabilities were estimated from time-resolved variable-temperature decays.

Cathodoluminescent and Characteristic X-Ray-Emissive Rare-Earth-Doped Core/Shell Protein Labels for Spectromicroscopic Analysis of Cell Surface Receptors

Understanding the localization and the interactions of biomolecules at the nanoscale and in the cellular context remains challenging. Electron microscopy (EM), unlike light-based microscopy, gives access to the cellular ultrastructure yet results in grey-scale images and averts unambiguous (co-)localization of biomolecules. Multimodal nanoparticle-based protein labels for correlative cathodoluminescence electron microscopy (CCLEM) and energy-dispersive X-ray spectromicroscopy (EDX-SM) are presented. The single-particle STEM-cathodoluminescence (CL) and characteristic X-ray emissivity of sub-20 nm lanthanide-doped nanoparticles are exploited as unique spectral fingerprints for precise label localization and identification. To maximize the nanoparticle brightness, lanthanides are incorporated in a low-phonon host lattice and separated from the environment using a passivating shell. The core/shell nanoparticles are then functionalized with either folic (terbium-doped) or caffeic acid (europium-doped). Their potential for (protein-)labeling is successfully demonstrated using HeLa cells expressing different surface receptors that bind to folic or caffeic acid, respectively. Both particle populations show single-particle CL emission along with a distinctive energy-dispersive X-ray signal, with the latter enabling color-based localization of receptors within swift imaging times well below 2 min perμ m $\umu\text{m}$ 2 while offering high resolution with a pixel size of 2.78 nm. Taken together, these results open a route to multi-color labeling based on electron spectromicroscopy.

Insights into NdIII to YbIII Energy Transfer and Its Implications in Luminescence Thermometry

This work challenges the conventional approach of using NdIII 4F3/2 lifetime changes for evaluating the experimental NdIII → YbIII energy transfer rate and efficiency. Using near-infrared (NIR) emitting Nd:Yb mixed-metal coordination polymers (CPs), synthesized via solvent-free thermal grinding, we demonstrate that the NdIII [2H11/2 → 4I15/2] → YbIII [2F7/2 → 2F5/2] pathway, previously overlooked, dominates energy transfer due to superior energy resonance and J-level selection rule compatibility. This finding upends the conventional focus on the NdIII [4F3/2 → 4I11/2] → YbIII [2F7/2 → 2F5/2] transition pathway. We characterized Nd0.890Yb0.110(BTC)(H2O)6 as a promising cryogenic NIR thermometry system and employed our novel energy transfer understanding to perform simulations, yielding theoretical thermometric parameters and sensitivities for diverse Nd:Yb ratios. Strikingly, experimental thermometric data closely matched the theoretical predictions, validating our revised model. This novel perspective on NdIII → YbIII energy transfer holds general applicability for the NdIII/YbIII pair, unveiling an important spectroscopic feature with broad implications for energy transfer-driven materials design.

White-emitting orthosilicate phosphor α-Sr2SiO4:Ce3+/Eu2+/K+: a bimodal temperature sensor with excellent optical thermometric sensitivity

Non-contact temperature sensors with low cost, high reliability and high sensitivity have attracted increasing research interest in recent years. In this study, we synthesized a bimodal optical temperature sensor Sr2SiO4:Ce3+/Eu2+/K+ with excellent thermometric sensitivity through a high-temperature solid-state reaction method. In the matrix of α-Sr2SiO4, Ce3+ luminescence exhibits excellent thermal stability (∼129.1%@250 °C), while Eu2+ shows strong thermal quenching (∼21.7%@250 °C), leading to a significant change in the fluorescence intensity ratio (FIR) of Ce3+ (437 nm) and Eu2+ (550 nm) as a function of temperature. This feature enables the phosphor exhibiting outstanding sensitivity in the temperature range of 298-523 K. To be exact, it demonstrates a maximal relative sensitivity of 0.93% K-1 at 348 K. Its absolute sensitivity linearly increases and reaches 3.46% K-1 at 523 K. Besides, it has a large chromaticity shift (ΔE = 228 × 10-3 in 298-523 K) against temperature, making the temperature change visible to the naked eye. We first demonstrate a CIE chromaticity coordinate technique for temperature sensing with high accuracy and good sensitivity by using the function of x or (x2 + y2)0.5 against T. These unique optical thermometric features allow Sr2SiO4:Ce3+/Eu2+/K+ to serve as an accurate and reliable thermometer probe candidate for temperature sensing applications.

Composition Control in Molecular Cluster-Aggregates: A Toolbox for Optical Output Tunability via Energy Transfer Pathways

Composition control is a powerful tool for obtaining high-performance lanthanide (Ln) luminescent materials with adjustable optical outputs. This strategy is well-established for hierarchically structured nanoparticles, but it is rarely applied to molecular compounds due to the limited number of metal centers within a single unit. In this work, we present a series of molecular cluster-aggregates (MCAs) with an icosanuclear core {Ln2Eu2Tb16} (Ln = Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, and Yb) in which we explore composition control, akin to nanoparticles, to modulate the optical output. More specifically, we target to understand how the presence of a third LnIII doping ion would impact the well-known TbIII → EuIII energy transfer and the ratiometric optical thermometry performance based on the TbIII/EuIII pair. Photophysical properties at room and at varying temperatures were investigated. Based on experimental data and well-established intrinsic features, such as spin-orbit coupling strength and LnIII 4f energy levels' structure, we discuss the possible luminescent processes present in each MCA and provide insight into qualitative trends that can be rationally correlated throughout the series.

A portable test strip fabricated of luminescent lanthanide-functionalized metal-organic frameworks for rapid and visual detection of tetracycline antibiotics

Tetracycline antibiotics (TCs) are commonly used antibiotics in the treatment of infections, but their overuse has a negative impact on human health and ecosystems. Thus, the development of a facile and on-site visualization method for TC detection is necessary. Here, we propose the potential of using lanthanide-functionalized metal-organic framework (MOF) composites (Ag+/Tb3+@UiO-66-(COOH)2, ATUC) as a probe for the rapid detection of tetracycline (TC), chlortetracycline (CTC), oxytetracycline (OTC), and doxycycline (DOX) residues, in which UiO-66-(COOH)2 (UC) could be utilized to provide an interaction microenvironment, Tb3+ as recognition units and Ag+ as a fluorescence enhancer. Upon exposure to TCs, significant luminescence quenching of ATUC excited at 255 nm was observed due to the inner filter effect (IFE) and photo-induced electron transfer (PET), and the established strategy has a detection limit (LOD) of 11.0, 20.1, 9.1, and 22.5 nM for TC, CTC, OTC, and DOX, respectively. More importantly, given its portability and conspicuous luminescence color gradation variation, a portable test strip based on ATUC was manufactured and the results could be distinguished immediately by the naked eye and smartphone analysis, allowing for on-site rapid quantitative assay of TCs, not only in the laboratory but also in a point-of-care setting.

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.

Desolvation-Induced Highly Symmetrical Terbium(III) Single-Molecule Magnet Exhibiting Luminescent Self-Monitoring of Temperature

A conjunction of Single-Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read-out of temperature in future SMM-based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII -based emissive SMMs formed in a cyanido-bridged framework whose properties are governed by the reversible structural transformation from [TbIII (H2 O)2 ][CoIII (CN)6 ] ⋅ 2.7H2 O (1) to its dehydrated phase, TbIII [CoIII (CN)6 ] (2). The 8-coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal-prismatic TbIII complexes in 2, showing the SMM features up to 42 K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18) cm-1 (854(26) K), one of the highest among the TbIII -based molecular nanomagnets. Both systems exhibit emission related to the f-f electronic transitions, with the temperature variations resulting in the optical thermometry below 100 K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6 K to 42 K. These functionalities are further enriched after the magnetic dilution. The role of post-synthetic formation of high-symmetry TbIII complexes in achieving the SMM effect and hot-bands-based optical thermometry is discussed.

The Upconversion Luminescence of Ca3Sc2Si3O12:Yb3+,Er3+ and Its Application in Thermometry

To develop novel luminescent materials for optical temperature measurement, a series of Yb³⁺- and Er³⁺-doped Ca₃Sc₂Si₃O₁₂ (CSS) upconversion (UC) phosphors were synthesized by the sol-gel combustion method. The crystal structure, phase purity, and element distribution of the samples were characterized by powder X-ray diffraction and a transmission electron microscope (TEM). The detailed study of the photoluminescence emission spectra of the samples shows that the addition of Yb³⁺ can greatly enhance the emission of Er³⁺ by effective energy transfer. The prepared Yb³⁺ and Er³⁺ co-doped CSS phosphors exhibit green emission bands near 522 and 555 nm and red emission bands near 658 nm, which correspond to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2 transitions of Er³⁺, respectively. The temperature-dependent behavior of the CSS:0.2Yb³⁺,0.02Er³⁺ sample was carefully studied by the fluorescence intensity ratio (FIR) technique. The results indicate the excellent sensitivity of the sample, with a maximum absolute sensitivity of 0.67% K^-1 at 500 K and a relative sensitivity of 1.34% K^-1 at 300 K. We demonstrate here that the temperature measurement performance of FIR technology using the CSS:Yb³⁺,Er³⁺ phosphor is not inferior to that of infrared thermal imaging thermometers. Therefore, CSS:Yb³⁺,Er³⁺ phosphors have great potential applications in the field of optical thermometry.

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.

Boosting the sensitivity with time-gated luminescence thermometry using a nanosized molecular cluster aggregate

Luminescence thermometry with trivalent lanthanide ions is a promising avenue for contactless temperature probing. The area has been growing exponentially for the last two decades, and its viability has been successfully demonstrated in various research domains. However, moving from laboratory equipment to real-life applications remains a challenging task. One of the reasons is the possibility of a background luminescence from the probing device or probed environment. To tackle this issue, we elegantly incorporate a rarely explored thermometric approach called time-gated luminescence thermometry (TGLT). Furthermore, we demonstrate an enhanced relative sensitivity through this innovative approach and a path to move toward practical application.

Trends in energy and charge transfer in 2D and integrated perovskite heterostructures

Two-dimensional (2D) van der Waals (vdW) heterostructured transition metal dichalcogenides (TMDs) open up new possibilities for a wide range of optoelectronic applications. Interlayer couplings are responsible for several fascinating physics phenomena, which are in addition to the multifunctionalities that have been discovered in the field of optoelectronics. These couplings can influence the overall charge, or the energy transfer processes via stacking, separation, and dielectric angles. This focused review article summarizes the most recent and promising strategies for interlayer exciton emission in 2D or integrated perovskites and TMD heterostructures. These types of devices require a thorough comprehension and effective control of interlayer couplings in order to realize their functionalities and improve performance, which is demonstrated in this article with the energy or charge transfer mechanisms in the individual devices. An ideal platform for examining the interlayer coupling and the related physical processes is provided by a summary of the recent research findings in 2D perovskites and TMDs. Furthermore, it would encourage more investigation into the comprehension and regulation of excitonic effects and the related optoelectronic applications in vdW heterostructures over a broad spectral response range. Finally, the current challenges and prospects are summarized in this paper.

Characteristic stacked structures and luminescent properties of dinuclear lanthanide complexes with pyrene units

A novel design strategy of stacked organic fluorophores using dinuclear lanthanide (Ln(III)) complexes is demonstrated for the formation of excimer. The dinuclear Ln(III) complexes are composed of two Ln(III) (Eu(III) or Gd(III)) ions, six hexafluoroacetylacetonate (hfa), and two pyrene-based phosphine oxide ligands. Single-crystal analysis revealed a rigid pyrene-stacked structure via CH-F (pyrene/hfa) intramolecular interactions. The rigid aggregation structures of the two-typed organic ligands around Ln(III) resulted in high thermal stability (decomposition temperature: 340°C). The aggregated ligands exhibited excimer-type green emission from the stacked pyrene-center. The change in the Ln(III) ion promotes effective shifts of excimer emissions (Gd(III):500 nm, Eu(III):490 nm). The organic aggregation system using red-luminescent Eu(III) also provides temperature-sensitive ratiometric emission composed of π-π* and 4f-4f transitions by energy migration between aggregated ligands and Eu(III).

Layered Gadolinium-Europium-Terbium Hydroxides Sensitised with 4-Sulfobenzoate as All Solid-State Luminescent Thermometers

Ternary layered gadolinium-europium-terbium basic chlorides were synthesised using a facile hydrothermal-microwave technique. A continuous series of solid solutions was obtained in a full range of rare earth concentrations. To sensitise the luminescence of Eu3+ and Tb3+, a 4-sulfobenzoate anion was intercalated in the ternary layered rare earth hydroxides using one of two methods—a high-temperature ion exchange or a single-stage synthesis. The luminescent colour of the materials was governed by the gadolinium content: at low and medium gadolinium concentrations (0–70%), layered Gd-Eu-Tb basic sulfobenzoate exhibited a bright red europium luminescence; at high gadolinium content (70–90%), a bright green terbium luminescence was observed. The colour coordinates of layered Gd-Eu-Tb basic sulfobenzoate luminescence depended on the temperature in the physiological range (20–50 °C). The relative thermal sensitivity of the obtained materials was up to 2.9%·K−1.

Bifunctional Temperature and Oxygen Dual Probe Based on Anthracene and Europium Complex Luminescence

In this work, we synthesized a polydimethylsiloxane membrane containing two emitter groups chemically attached to the membrane structure. For this, we attached the anthracene group and the [Eu(bzac)₃] complex as blue and red emitters, respectively, in the matrix via hydrosilylation reactions. The synthesized membrane can be used as a bifunctional temperature and oxygen ratiometric optical probe by analyzing the effects that temperature changes and oxygen levels produce on the ratio of anthracene and europium(III) emission components. As a temperature probe, the system is operational in the 203-323 K range, with an observed maximum relative sensitivity of 2.06% K^-1 at 290 K and temperature uncertainties below 0.1 K over all the operational range. As an oxygen probe, we evaluated the ratiometric response at 25, 30, 35, and 40 °C. These results show an interesting approach to obtaining bifunctional ratiometric optical probes and also suggest the presence of an anthracene → europium(III) energy transfer, even though there is no chemical bonding between species.

Tunable Optical Molecular Thermometers Based on Metallacrowns

The effect of ligands' energy levels on thermal dependence of lanthanide emission was examined to create new molecular nanothermometers. A series of Ln₂Ga₈L₈^'L₈^″ metallacrowns (shorthand Ln₂L₈^'), where Ln = Gd³⁺, Tb³⁺, or Sm³⁺ (H₃L' = salicylhydroxamic acid (H₃shi), 5-methylsalicylhydroxamic acid (H₃mshi), 5-methoxysalicylhydroxamic acid (H₃moshi), and 3-hydroxy-2-naphthohydroxamic acid (H₃nha)) and H₂L″ = isophthalic acid (H₂iph), was synthesized and characterized. Within the series, ligand-centered singlet state (S₁) energy levels ranged from 23,300 to 27,800 cm^-1, while triplet (T₁) energy levels ranged from 18,150 to 21,980 cm^-1. We demonstrated that the difference between T₁ levels and relevant energies of the excited ⁴G_5/2 level of Sm³⁺ (17,800 cm^-1) and ⁵D₄ level of Tb³⁺ (20,400 cm^-1) is the major parameter controlling thermal dependence of the emission intensity via the back energy transfer mechanism. However, when the energy difference between S₁ and T₁ levels is small (below 3760 cm^-1), the S₁ → T₁ intersystem crossing (and its reverse, S₁ ← T₁) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln³⁺-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb₂L₈^' and from 0.0267(7)% to 2.27(5)% for Sm₂L₈^'. The maximal thermal dependence varies over a wide thermal range (ca. 150-350 K) based on energy gaps between relevant ligand-based and lanthanide-based electronic states. By mixing Tb₂moshi₈^' with Sm₂moshi₈^' in a 1:1 ratio, an optical thermometer with a relative thermal sensitivity larger than 3%/K at 225 K was created. Other temperature ranges are also accessible with this approach.

Construction of novel luminescent thermometers based on dual-emission centers of rare-earth and bismuth ions

Optical thermometry based on fluorescence intensity ratio (FIR) technology has several advantages for industrial and medical applications such as remote signaling, non-invasiveness, and excellent spatial resolution. Here, an approach to the construction of luminescent thermometers is proposed based on high-temperature solid-state reactions through doping of rare earth (RE) elements (e.g., samarium (Sm³⁺) or europium (Eu³⁺)) into Ca₂Y_0.97Bi_0.03SbO₆ (CYBS) phosphors. The tuning of the CYBS:Eu³⁺ and CYBS:Sm³⁺ ratios in the phosphors provided a wide range of color changes from purplish blue to red and from purplish blue to pink, respectively. The superiority of optical thermometer is validated by higher values of absolute sensitivity (S_a) and relative sensitivity (S_r). As such, both phosphors exhibit excellent temperature sensing performance with S_a/S_r values (at 483 K) of 4.945 × 10^-2/0.968 × 10^-2 K^-1 (CYBS:0.05Eu³⁺) and 2.964 × 10^-2/0.864 × 10^-2 K^-1 (CYBS:0.05 Sm³⁺). Thus, RE-doped CYBS materials with color tuning properties and superior temperature sensing performance are recommended for the construction of novel luminescent optical thermometers.

New Carboxylate Anionic Sm-MOF: Synthesis, Structure and Effect of the Isomorphic Substitution of Sm3+ with Gd3+ and Tb3+ Ions on the Luminescent Properties

Two new compounds, namely {(NMe2H2)}[Ln(TDA)(HCOO)] 0.5H2O, Ln = Sm3+ (Sm-TDA) and Gd3+ (Gd-TDA), where TDA3− is the anion of 1H-1,2,3-triazole-4,5-dicarboxylic acid (H3TDA), were synthesized by the solvothermal method in a DMF:H2O mixture. According to single-crystal X-ray diffraction data, the compounds are 3d-MOFs with an anionic lattice and dimethylammonium cations occupying part of the cavities. Based on these compounds, two series of mixed-metal complexes, [NMe2H2][SmxLn1-x(TDA)(HCOO)], (x = 0.9 (Sm0.9Ln0.1-TDA), x = 0.8 (Sm0.8-Ln0.2-TDA)…Sm0.02Ln0.98-TDA, Ln = Tb, Gd), were also obtained and characterized by powder XRD. The luminescent properties of the compounds were studied and it was shown that the resulting compounds are two- or three-component emitters with the possibility of fine color tuning by changing the intensities of fluorescence and phosphorescence of the ligand, as well as the luminescence of Sm3+ and Tb3+ f-ions.

Hollow nanoparticles synthesized via Ostwald ripening and their upconversion luminescence-mediated Boltzmann thermometry over a wide temperature range

Upconversion nanoparticles (UCNPs) with hollow structures exhibit many fascinating optical properties due to their special morphology. However, there are few reports on the exploration of hollow UCNPs and their optical applications, mainly because of the difficulty in constructing hollow structures by conventional methods. Here, we report a one-step template-free method to synthesize NaBiF₄:Yb,Er (NBFYE) hollow UCNPs via Ostwald ripening under solvothermal conditions. Moreover, we also elucidate the possible formation mechanism of hollow nanoparticles (HNPs) by studying the growth process of nanoparticles in detail. By changing the contents of polyacrylic acid and H₂O in the reaction system, the central cavity size of NBFYE nanoparticles can be adjusted. Benefiting from the structural characteristics of large internal surface area and high surface permeability, NBFYE HNPs exhibit excellent luminescence properties under 980 nm near-infrared irradiation. Importantly, NBFYE hollow UCNPs can act as self-referenced ratiometric luminescent thermometers under 980 nm laser irradiation, which are effective over a wide temperature range from 223 K to 548 K and have a maximum sensitivity value of 0.0065 K^-1 at 514 K. Our work clearly demonstrates a novel method for synthesizing HNPs and develops their applications, which provides a new idea for constructing hollow structure UCNPs and will also encourage researchers to further explore the optical applications of hollow UCNPs.

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.

Thermally Activated Photophysical Processes of Organolanthanide Complexes in Solution

The effect of temperature upon the lanthanide luminescence lifetime and intensity has been investigated in toluene solution for the complexes LnPhen(TTA)3 (Ln = Eu, Sm, Nd, Yb; Phen = 1,10-phenanthroline; TTA = thenoyltrifluoroacetonate). Thermally excited back-transfer to a charge transfer state was found to occur for Ln = Eu and can be explained by lifetime and intensity back-transfer models. The emission intensity and lifetime were also quenched with increasing temperature for Ln = Sm, and the activation energy for nonradiative decay is similar to that for the thermal population of Sm3+ excited states. Unusual behavior for lifetime and intensity was found for both Ln = Nd, Yb. The usually assumed equivalence of τ/τ0 = I/I0 (where τ is lifetime and I is intensity) does not hold for these cases. We infer that for these lanthanide systems the intensity decreases with temperature in the stage prior to population of the luminescent state. The lifetime changes are discussed.

Toward Opto-Structural Correlation to Investigate Luminescence Thermometry in an Organometallic Eu(II) Complex

Lanthanide-based luminescent materials have unique properties and are well-studied for many potential applications. In particular, the characteristic 5d → 4f emission of divalent lanthanide ions such as Eu^II allows for tunability of the emissive properties via modulation of the coordination environment. We report the synthesis and photoluminescence investigation of pentamethylcyclopentadienyleuropium(II) tetrahydroborate bis(tetrahydrofuran) dimer (1), the first example of an organometallic, discrete molecular Eu^II band-shift luminescence thermometer. Complex 1 exhibits an absolute sensitivity of 8.2 cm^-1 K^-1 at 320 K, the highest thus far observed for a lanthanide-based band-shift thermometer. Opto-structural correlation via variable-temperature single-crystal X-ray diffraction and fluorescence spectroscopy allows rationalization of the remarkable thermometric luminescence of complex 1 and reveals the significant potential of molecular Eu^II compounds in luminescence thermometry.

Fluorescence Thermometer: Intermediation of the Fontal Temperature and Light

The rapid advance of thermal materials and fluorescence spectroscopy has extensively promoted micro-scale fluorescence thermometry development in recent years. Based on the advantages of fast response, high sensitivity, simple operation,...

Facile synthesis of rare earth-doped CeF3 two-dimensional nanosheets and their application in ratiometric luminescence temperature sensing

Rare earth-doped CeF3 two-dimensional nanosheets have been successfully synthesized and their potential application as a ratiometric luminescent thermometer.

The design of responsive luminescent lanthanide probes and sensors

The principles of the design of responsive luminescent probes and sensors based on lanthanide emission are summarised, based on a mechanistic understanding of their mode of action. Competing kinetic pathways for deactivation of the excited states that occur are described, highlighting the need to consider each of the salient quenching processes. Such an analysis dictates the choice of both the ligand and its integral sensitising moiety for the particular application. The key aspects of quenching involving electron transfer and vibrational and electronic energy transfer are highlighted and exemplified. Responsive systems for pH, pM, pX and pO₂ and selected biochemical analytes are distinguished, according to the nature of the optical signal observed. Signal changes include both simple and ratiometric intensity measurements, emission lifetime variations and the unique features associated with the observation of circularly polarised luminescence (CPL) for chiral systems. A classification of responsive lanthanide probes is introduced. Examples of the operation of probes for reactive oxygen species, citrate, bicarbonate, α₁-AGP and pH are used to illustrate reversible and irreversible transformations of the ligand constitution, as well as the reversible changes to the metal primary and secondary coordination sphere that sensitively perturb the ligand field. Finally, systems that function by modulation of dynamic quenching of the ligand or metal excited states are described, including real time observation of endosomal acidification in living cells, rapid urate analysis in serum, accurate temperature assessment in confined compartments and high throughput screening of drug binding to G-protein coupled receptors.

Overview of N-Rich Antennae Investigated in Lanthanide-Based Temperature Sensing

The market share of noncontact temperature sensors is expending due to fast technological and medical evolutions. In the wide variety of noncontact sensors, lanthanide-based temperature sensors stand out. They benefit from high photostability, relatively long decay times and high quantum yields. To circumvent their low molar light absorption, the incorporation of a light-harvesting antenna is required. This Review provides an overview of the nitrogen-rich antennae in lanthanide-based temperature sensors, emitting in the visible light spectrum, and discusses their temperature sensor ability. The N-rich ligands are incorporated in many different platforms. The investigation of different antennae is required to develop temperature sensors with diverse optical properties and to create a diverse offer for the multiple application fields. Molecular probes, consisting of small molecules, are first discussed. Furthermore, the thermometer properties of ratiometric temperature sensors, based on di- and polynuclear complexes, metal-organic frameworks, periodic mesoporous organosilicas and porous organic polymers, are summarized. The antenna mainly determines the application potential of the ratiometric thermometer. It can be observed that molecular probes are operational in the broad physiological range, metal-organic frameworks are generally very useful in the cryogenic region, periodic mesoporous organosilica show temperature dependency in the physiological range, and porous organic polymers are operative in the cryogenic-to-medium temperature range.

Mono- and Mixed Metal Complexes of Eu3+, Gd3+, and Tb3+ with a Diketone, Bearing Pyrazole Moiety and CHF2-Group: Structure, Color Tuning, and Kinetics of Energy Transfer between Lanthanide Ions

Three novel lanthanide complexes with the ligand 4,4-difluoro-1-(1,5-dimethyl-1H-pyrazol-4-yl)butane-1,3-dione (HL), namely [LnL₃(H₂O)₂], Ln = Eu, Gd and Tb, were synthesized, and, according to single-crystal X-ray diffraction, are isostructural. The photoluminescent properties of these compounds, as well as of three series of mixed metal complexes [Eu_xTb_1-xL₃(H₂O)₂] (Eu_xTb_1-xL₃), [EuxGd_1-xL₃(H₂O)₂] (Eu_xGd_1-xL₃), and [Gd_xTb_1-xL₃(H₂O)₂] (Gd_xTb_1-xL₃), were studied. The Eu_xTb_1-xL₃ complexes exhibit the simultaneous emission of both Eu³⁺ and Tb³⁺ ions, and the luminescence color rapidly changes from green to red upon introducing even a small fraction of Eu³⁺. A detailed analysis of the luminescence decay made it possible to determine the observed radiative lifetimes of Tb³⁺ and Eu³⁺ and estimate the rate of excitation energy transfer between these ions. For this task, a simple approximation function was proposed. The values of the energy transfer rates determined independently from the luminescence decays of terbium(III) and europium(III) ions show a good correlation.

Upconversion Nanoparticle Decorated Spider Silks as Single-Cell Thermometers

Noninvasive and sensitive thermometry of a single living cell is crucial to the analysis of fundamental cellular processes and applications to cancer diagnosis. Optical fibers decorated with temperature-sensitive nanomaterials have become widely used instruments for biosensing temperature. However, current silica fibers exhibit low compatibility and degradability in biosystems. In this work, we employ spider silks as natural optical fibers to construct biocompatible thermometers. The spider silks were drawn directly from Araneus ventricosus and were decorated with core-shell upconversion nanoparticles (UCNPs) via a photophoretic effect. By measuring the fluorescence spectra of the UCNPs on the spider silks, the membrane temperature of a single breast cancer cell was obtained with absolute and relative sensitivities ranging from 3.3 to 4.5 × 10^-3 K^-1 and 0.2 to 0.8% K^-1, respectively. Additionally, the temperature variation during apoptosis was monitored by the thermometer in real time. This work provides a biocompatible tool for precise biosensing and single-cell analysis.

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

By choosing C3 symmetric 2,4,6-tris-(4-carboxyphenyl)-1,3,5-triazine (TCTZ) as the ligand, a series of lanthanide metal-origanic complexes Tb_1-xEu_x-TCTZ(DMF)·2H₂O(x = 0, 0.01, 1) have been successfully synthesized via solvothermal reaction. The complexes present intense emission although with coordinationofwater molecules. The temperature-dependent photoluminescent (PL) properties of Tb-TCTZ is investigated both in terms of emission intensity and lifetime in order to establish their potentials as luminescent themometers. It shows excellent responseto temperature from 303 to 403 K and exhibits the maximum relative sensitivity(S_r) as high as 5.36% K^-1 at 403 K. Tb_0.99Eu_0.01-TCTZ is evaluated for application as ratiometric luminescence thermometers, which exhibits high sensitivity to temperature in range of 303-403 K, with the maximum absolute sensitivity (S_a) and S_r as 5.16% and 3.22% K^-1 respectively. The obtained maximum sensitivities in this study is superior to many materials reported. Moreover, the emission color changes from green at 303 K to red at 403 K, so that it is also suitable to act as colorimetric luminescent probes.

N-Rich Porous Polymer with Isolated Tb3+ -Ions Displays Unique Temperature Dependent Behavior through the Absence of Thermal Quenching

The challenge of measuring fast moving or small scale samples is based on the absence of contact between sample and sensor. Grafting lanthanides onto hybrid materials arises as one of the most promising accurate techniques to obtain noninvasive thermometers. In this work, a novel bipyridine based porous organic polymer (bpyDAT POP) was investigated as temperature sensor after grafting with Eu(acac)₃ and Tb(acac)₃ complexes. The bpyDAT POP successfully showed temperature-dependent behavior in the 10-310 K range, proving the potential of amorphous, porous organic frameworks. We observed unique temperature dependent behavior. More intriguingly, instead of the standard observed change in emission as a result of a change in temperature for both Eu³⁺ and Tb³⁺ , the emission spectrum of Tb³⁺ remained constant. This work provides framework- and energy-based explanations for the observed phenomenon. The conjugation in the bpyDAT POP framework is interrupted, creating energetically isolated Tb³⁺ environments. Energy transfer from Tb³⁺ to Eu³⁺ is therefore absent, nor energy back transfer from Tb³⁺ to bpyDAT POP ligand (i.e. no thermal quenching) is detected.

[Ga3+ 8 Sm3+ 2 , Ga3+ 8 Tb3+ 2 ] Metallacrowns are Highly Promising Ratiometric Luminescent Molecular Nanothermometers Operating at Physiologically Relevant Temperatures

Nanothermometry is the study of temperature at the submicron scale with a broad range of potential applications, such as cellular studies or electronics. Molecular luminescent-based nanothermometers offer a non-contact means to record these temperatures with high spatial resolution and thermal sensitivity. A luminescent-based molecular thermometer comprised of visible-emitting Ga³⁺ /Tb³⁺ and Ga³⁺ /Sm³⁺ metallacrowns (MCs) achieved remarkable relative thermal sensitivity associated with very low temperature uncertainty of S_r =1.9 % K^-1 and δT<0.045 K, respectively, at 328 K, as an aqueous suspension of polystyrene nanobeads loaded with the corresponding MCs. To date, they are the ratiometric molecular nanothermometers offering the highest level of sensitivity in the physiologically relevant temperature range.

Holmium(iii) molecular nanomagnets for optical thermometry exploring the luminescence re-absorption effect

Coordination complexes of lanthanide(3+) ions can combine Single-Molecule Magnetism (SMM) with thermally modulated luminescence applicable in optical thermometry. We report an innovative approach towards high performance SMM-based optical thermometers which explores tunable anisotropy and the luminescence re-absorption effect of HoIII complexes. Our concept is shown in dinuclear cyanido-bridged molecules, {[HoIII(4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, 1; Rh, 2; Ir, 3) and their magnetically diluted analogues, {[HoIII x YIII 1-x (4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, x = 0.11, 1@Y; Rh, x = 0.12, 2@Y; Ir, x = 0.10, 3@Y). They are built of pentagonal bipyramidal HoIII complexes revealing the zero-dc-field SMM effect. Experimental studies and the ab initio calculations indicate an Orbach magnetic relaxation with energy barriers varying from 89.8 to 86.7 and 78.7 cm-1 K for 1, 2, and 3, respectively. 1-3 also differ in the strength of quantum tunnelling of magnetization which is suppressed by hyperfine interactions, and, further, by the magnetic dilution. The YIII-based dilution governs the optical properties as 1-3 exhibit poor emission due to the dominant re-absorption from HoIII while 1@Y-3@Y show room-temperature blue emission of 4-pyridone. Within ligand emission bands, the sharp re-absorption lines of the HoIII electronic transitions were observed. Their strong thermal variation was used in achieving highly sensitive ratiometric optical thermometers whose good performance ranges, lying between 25 and 205 K, are adjustable by using hexacyanidometallates. This work shows that HoIII complexes are great prerequisites for advanced opto-magnetic systems linking slow magnetic relaxation with unique optical thermometry exploiting a luminescence re-absorption phenomenon.

A Dual-Excitation Decoding Strategy Based on NIR Hybrid Nanocomposites for High-Accuracy Thermal Sensing

Optical thermal sensing holds great promise for disease theranostics. However, traditional ratiometric thermometry methods, in which intensity ratio of two nonoverlapping emissions is defined as the thermosensitive parameter, may have a limited accuracy in temperature read-out due to the deleterious interference from wavelength- and temperature-dependent photon attenuation in tissue. To overcome this limitation, a dual-excitation decoding strategy based on NIR hybrid nanocomposites comprising self-assembled quantum dots (QDs) and Nd3+ doped fluoride nanocrystals (NCs) is proposed for thermal sensing. Upon excitation at 808 nm, the intensity ratio of two emissions at identical wavelength (1057 nm) from QDs and NCs, respectively, is defined as the thermometric parameter R. By employing another 830 nm laser beam following the same optical path as 808 nm laser to exclusively excite QDs, the two overlapping emissions can be easily decoded. The acquired R proves to be inert to the detection depth in tissue, with a minimized temperature reading error of ≈2.3 °C at 35 °C (at a depth of ≈1.1 mm), while the traditional thermometry mode based on the nonoverlapping 1025 and 863 nm emissions may exhibit a large error of ≈43.0 °C. The insights provided by this work pave the way toward high-accuracy deep-tissue biosensing.

Thermochromic Luminescent Nanomaterials Based on Mn4+/Tb3+ Codoping for Temperature Imaging with Digital Cameras

A new thermographic nanocrystalline Sr4Al14O25:Mn4+,Tb3+ phosphor was developed, and the concentrations of both dopants and the synthesis conditions were optimized. The combination of the thermally quenched luminescence from the Mn4+ ions to the almost temperature-independent emission from Tb3+ provides a sensitive luminescent thermometer (SR = 2.8%/°C at 150 °C) with strong emission color variability. In addition, a figure of merit for this luminescence thermochromism was proposed, as the relative sensitivities of the x and y CIE coordinates, which for this phosphor reaches at 150 °C SR(x) = 0.6%/°C and SR(y) = 0.4%/°C, respectively. Noncontact thermal imaging was demonstrated with this phosphor using a single consumer digital camera and exploiting the ratio of red (R) and green (G) channels of the RGB images, thereby confirming the high application potential of Sr4Al14O25:Mn4+,Tb3+ nanocrystals for thermal sensing and mapping.

How does the ligand affect the sensitivity of the luminescent thermometers based on Tb-Eu complexes

Theoretical description of a four-level system for luminescent thermometry was proposed. Based on this description, the sensitivity was proved not to exceed Sr =|E23 - E12|/T2, where T is the temperature and |E23 - E12| is the absolute energy difference between the excited state energy gaps. The analysis was verified for the terbium-europium coordination compounds, the most studied class of four-level thermometry systems, which revealed that most of the examples fall within either this theory or represent three-level systems, where the ligand is not involved in the thermally-activated processes. From this description, it follows that the highest Sr can be reached either when the triplet state is not involved or when the ligand triplet state is very high, which in the case of terbium-europium coordination compounds means exceeding 26 800 cm-1.

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

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

Monoanionic Anilidophosphine Ligand in Lanthanide Chemistry: Scope, Reactivity, and Electrochemistry

We present the synthesis of a series of new lanthanide(III) complexes supported by a monoanionic bidentate anilidophosphine ligand (N-(2-(diisopropylphosphanyl)-4-methylphenyl)-2,4,6-trimethylanilide, short PN^-). The work comprises the characterization of a variety of heteroleptic complexes containing either one or two PN ligands as well as a study on further functionalization possibilities. The new heteroleptic complexes cover selected examples over the whole lanthanide(III) series including lanthanum, cerium, neodymium, gadolinium, terbium, dysprosium, and lutetium. In case of the two diamagnetic metal cations lanthanum(III) and lutetium(III), we have furthermore studied the influence of the lanthanide ion (early vs. late) on the reactivity of these complexes. Thereby we found that the radius of the lanthanide ion has a major influence on the reactivity. Using sterically demanding, multidentate ligand systems, e.g., cyclopentadienide (Cp^-), we found that the lanthanum complex La(PN)₂Cl (1-La) reacts well to the corresponding cyclopentadienide complex, while for Lu(PN)₂Cl (1-Lu) no reaction was observed under any conditions tested. On the contrary, employing monodentate ligands such as mesitolate, thiomesitolate, 2,4,6-trimethylanilide or 2,4,6-trimethylphenylphosphide, results in the clean formation of the desired complexes for both lanthanum and lutetium. All complexes have been studied by various techniques, including multi nuclear NMR spectroscopy and X-ray crystallography. ³¹P NMR spectroscopy was furthermore used to evaluate the presence of open coordination sites on the complexes using coordinating and noncoordinating solvents, and as a probe for estimating the Ce-P distance in the corresponding complexes. Additionally, we present cyclic voltammetry (CV) data for Ce(PN)₂Cl (1-Ce), La(PN)₂Cl (1-La), Ce(PN)(HMDS)₂ (8-Ce) and La(PN)(HMDS)₂ (8-La) (with HMDS = hexamethyldisilazide, (Me₃Si)₂N^-) exploring the potential of the anilidophosphane ligand framework to stabilize a potential Ce(IV) ion.

Luminescent lanthanide metal-organic framework test strip for immediate detection of tetracycline antibiotics in water

Tetracycline antibiotics (TCs) are a kind of commonly used antibiotics for treating infections, however, the overuse of TCs has adversely affected human health and the ecosystem. Thus, detection of TCs in water is important but challenging. In this work, a luminescent lanthanide metal-organic framework (LnMOF) sensor (1) for immediate detection of oxytetracycline (OTC) and tetracycline (TC) is developed. The sensor has high acid-base and water stability. Investigation reveals that among the 27 species of antibiotics, anions and cations under investigation, 1 shows highly selective sensing towards OTC and TC, and the detection is not disturbed by the presence of other species. The limit of detection (LOD) for OTC and TC are ultra-sensitive value of 1.95 and 2.77 nM, respectively. Investigation reveals the sensing mechanism is due to the inner filter effect. Further studies reveal that the sensor can be used in real sample monitoring. More importantly, test strips based on 1 are manufactured. They are an easy-to-use, low-cost, highly selective and sensitive sensing device for detecting OTC and TC. The sensing can be distinguished immediately and easily by the naked eyes, making it an excellent candidate to monitor OTC and TC in real use.