Luminescent bimetallic lanthanide bioprobes for cellular imaging with excitation in the visible-light range

Thermosensitive visible-light-excited visible-/NIR-luminescent complexes with lanthanide sensitized by the π-electronic system through intramolecular H-bonding

Visible-luminescent lanthanide (LnL) complexes with a highly planar tetradentate ligand were successfully developed for a visible-light solid-state excitation system. L was designed by using two 2-hydroxy-3-(2-pyridinyl)-benzaldehyde molecules bridged by ethylenediamine, which was then coordinated to a series of Ln ions (Ln = Nd, Sm, Eu, Gd, Tb, Dy, and Yb). From the measurement of single-crystal X-ray analysis of EuL, two phenolic O atoms and two imine N atoms in L were coordinated to the Eu ion, and each π-electronic system took coplanar with the edged-pyridine moiety through an intramolecular hydrogen bond. The enol group on the phenolic skeleton changed to the keto form, and the pyridine was protonated. Thus, intramolecular proton transfer occurred in L after the complexation. Other complexes take isostructure. The space group is P-1, and the c-axis shrinks with decreasing temperature without a phase transition in EuL. The yellow color caused by the planar structure of L can sensitize ff emission by visible light, and the luminescence color of each complex depends on central Ln ions. Furthermore, a phosphorescence band also appeared at rt with ff emission in LnL. Drastic temperature dependence of luminescence was clarified quantitatively.

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.

Red-emitting heteroleptic iridium(III) complexes: photophysical and cell labeling study

Two red-emitting heteroleptic iridium(III) complexes (Ir-p and Ir-q) were synthesized and their photophysical and biological properties were analyzed. After their structures have been confirmed by several techniques, such as ¹H NMR, ¹³C NMR, FT-IR, UV-Vis, and MALDI TOF analyses, their luminescence behavior was investigated in ethanol and PBS (physiological medium, pH ~ 7.4) solutions. Emission spectra of both complexes are dominated by ³MLCT states at room temperature in ethanolic solution, but at 77 K the Ir-q exhibits the ³LC as the dominant emission state. The Ir-q complex shows one of the highest emission quantum yields, 11.5%, for a red emitter based on iridium(III) complexes in aerated PBS solution, with color coordinates (x;y) of (0.712;0.286). Moreover, both complexes display high potential to be used as a biological marker with excitation wavelengths above 400 nm, high water solubility (Ir-p 1838 μmol L^-1, Ir-q 7601 μmol L^-1), and distinct emission wavelengths from the biological autofluorescence. Their cytotoxicity was analyzed in CHO-k1 cells by MTT assays, and the IC₅₀ was estimated as being higher than 131 μmol L^-1 for Ir-p, and higher than 116 μmol L^-1 for Ir-q. Concentrations above 70% of viability were used to perform cell imaging by confocal and fluorescence microscopies and the results suggest that the complexes were internalized by the cell membrane and they are staining the cytoplasm region.

Dual-sensitized Eu(III)/Tb(III) complexes exhibiting tunable luminescence emission and their application in cellular-imaging

Two novel dual-photosensitized stable complexes, namely [Eu(dpq)(BTFA)₃] (1) and [Tb(dpq)(BTFA)₃] (2), have been successfully assembled via a mixed ligand approach using dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) and 3-benzoyl-1,1,1-trifluoroacetone (BTFA). The crystallographic data reveal mononuclear lanthanide cores in both 1 and 2, in which each eight-coordinated Ln(III) ion is located in a slightly distorted dodecahedron (D_2d). The room-temperature photoluminescence spectra of complexes 1 and 2 indicate that both BTFA and dpq can effectively sensitize Eu(III) and Tb(III) characteristic luminescence. Moreover, heterometallic Ln-complexes can be synthesized, leading to a new series of differently doped EuxTb1-x complexes. Luminescence experiments on them reveal dual-emission peaks of Eu³⁺ and Tb³⁺, which lead to a gradual change in the luminous colour between yellow-green, yellow, orange, orange-red and red upon increasing the Eu³⁺ content. On the basis of the intrinsic strong emission properties and nontoxic nature of complexes 1 and 2, we explore their potential application as cellular imaging agents. Fluorescence microscopy data suggest the cytosolic and nuclear localization of 1 and 2 in HeLa and MCF-7 cells.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Lysosome-targeting luminescent lanthanide complexes: From molecular design to bioimaging

Lysosomes are essential acidic cytoplasmic membrane-bound organelles in human cells that play a critical role in many cellular events. A comprehensive understanding of lysosome-specific imaging can ultimately help us to...

Bright and Efficient Sensitized Near-Infrared Photoluminescence from an Organic Neodymium-Containing Composite Material System

Intense organic neodymium (Nd³⁺) emission is obtained with near-infrared (NIR) emission equivalent in intensity to that of an organic semiconductor emitting material. The advantage of Nd³⁺ emission is its narrow line width and NIR emission, which is enhanced by ∼3000 times at low excitation power through an efficient sensitization effect from a composite organic sensitizer. This performance is optimized at high concentrations of Nd³⁺ ions, and the organic perfluorinated system provides the ion excitations with a quantum efficiency of ∼40%. The material system is applicable to thin films that are compatible with integrated optics applications.

Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand

EuIII , TbIII , GdIII and YbIII complexes of the nonadentate bispidine derivative L2 (bispidine=3,7-diazabicyclo[3.3.1]nonane) were successfully synthesized and their emission properties studied. The X-ray crystallography reveals full encapsulation by the nonadentate ligand L2 that enforces to all LnIII cations a common highly symmetrical capped square antiprismatic (CSAPR) coordination geometry (pseudo C4v symmetry). The well-resolved identical emission spectra in solid state and in solution confirm equal structures in both media. As therefore expected, this results in long-lived excited states and high emission quantum yields ([EuIII L2 ]+ , H2 O, 298 K, τ=1.51 ms, ϕ=0.35; [TbIII L2 ]+ , H2 O, 298 K, τ=1.95 ms, ϕ=0.68). Together with the very high kinetic and thermodynamic stabilities, these complexes are a possible basis for interesting biological probes.

Luminescent lanthanide(III) complexes of DTPA-bis(amido-phenyl-terpyridine) for bioimaging and phototherapeutic applications

We report here a series of coordinatively-saturated and thermodynamically stable luminescent [Ln(dtntp)(H₂O)] [Ln(III) = Eu (1), Tb (2), Gd (3), Sm (4) and Dy (5)] complexes using an aminophenyl-terpyridine appended-DTPA (dtntp) chelating ligand as cell imaging and photocytotoxic agents. The N,N″-bisamide derivative of H₅DTPA named as dtntp is based on 4'-(4-aminophenyl)-2,2':6',2″-terpyridine conjugated to diethylenetriamine-N,N',N″-pentaacetic acid. The structure, physicochemical properties, detailed photophysical aspects, interaction with DNA and serum proteins, and photocytotoxicity were studied. The intrinsic luminescence of Eu(III) and Tb(III) complexes due to f → f transitions used to evaluate their cellular uptake and distribution in cancer cells. The solid-state structure of [Eu(dtntp)(DMF)] (1·DMF) shows a discrete mononuclear molecule with nine-coordinated {EuN₃O₆} distorted tricapped-trigonal prism (TTP) coordination geometry around the Eu(III). The {EuN₃O₆} core results from three nitrogen atoms and three carboxylate oxygen atoms, and two carbonyl oxygen atoms of the amide groups of dtntp ligand. The ninth coordination site is occupied by an oxygen atom of DMF as a solvent from crystallization. The designed probes have two aromatic pendant phenyl-terpyridine (Ph-tpy) moieties as photo-sensitizing antennae to impart the desirable optical properties for cellular imaging and photocytotoxicity. The photostability, coordinative saturation, and energetically rightly poised triplet states of dtntp ligand allow the efficient energy transfer (ET) from Ph-tpy to the emissive excited states of the Eu(III)/Tb(III), makes them luminescent cellular imaging probes. The Ln(III) complexes show significant binding tendency to DNA (K ~ 10⁴ M^-1), and serum proteins (BSA and HSA) (K ~ 10⁵ M^-1). The luminescent Eu(III) (1) and Tb(III) (2) complexes were utilized for cellular internalization and cytotoxicity studies due to their optimal photophysical properties. The cellular uptake studies using fluorescence imaging displayed intracellular (cytosolic and nuclear) localization in cancer cells. The complexes 1 and 2 displayed significant photocytotoxicity in HeLa cells. These results offer a modular design strategy with further scope to utilize appended N,N,N-donor tpy moiety for developing light-responsive luminescent Ln(III) bioprobes for theranostic applications.

Solution structure of a europium–nicotianamine complex supports that phytosiderophores bind lanthanides

The structures of europium–EDTA (known lanthanide chelator) and europium–nicotianamine (biochemical precursor of phytosiderophores) complexes are resolved, in solution, with ab initio molecular dynamics as well as excitation and emission spectroscopy.

Enhanced Near-Infrared Emission from Eight-Coordinate vs Nine-Coordinate YbIII Complexes Using 2-(5-Methylpyridin-2-yl)-8-hydroxyquinoline

Enhanced near-infrared (NIR) luminescence from two structurally related heterobinuclear Na^IYb^III eight-cooridnate and heterobinuclear Yb^IIINa^I eight-coordinate (CN = 8) complexes is reported and compared to a nine-coordinate (CN = 9) homoleptic complex. For the heteroleptic complex, [Yb(MPQ₂)(acac)], the Yb^III cation is coordinated to two tridentate 2-(5-methylpyridin-2-yl)-8-quinolinate (MPQ) anions, with a bidentate acetylacetonate (acac) anion completing the coordination sphere. Instead, the heterobinuclear [NaYb(MPQ)₄] complex comprises a total of four anionic MPQ ligands, two of which exhibit κ³-coordination to the Yb^III cation. The remaining two MPQ anions are unidentate toward the lanthanide and form μ₂-bridges via the deprotonated quinolinate oxygens to a bound Na^I cation which is also coordinated to the remaining nitrogen donor atoms. The structural properties of these complexes were evaluated by single-crystal X-ray diffraction (SXRD), continuous shape measure (CShM) analysis, and ¹H NMR spectroscopy using a diamagnetic Lu^III analogue. The corresponding photophysical properties were examined in CH₂Cl₂ solution by using absorption and emission spectroscopy. For both the complexes, characteristic Yb^III emission is observed at ca. 980 nm, with recorded photoluminescence quantum yields (Φ_obs) and NIR luminescence lifetimes (τ_obs) of 2.0% and 14.0 μs vs 1.5% and 11.6 μs for the [NaYb(MPQ)₄] and [Yb(MPQ)₂(acac)] complexes, respectively. Interestingly, the eight-coordinate Yb^III complexes both have higher photoluminescence quantum yields when compared to the homoleptic [Yb(MPQ)₃] complex, which has a reported quantum yield of 1.0% and a NIR lifetime determined herein of 13.3 μs under identical conditions. These results have been rationalized by considering the overall efficiency of the ligand-centered sensitization process (η_sens = Φ_isc × Φ_eet), together with subsequent radiative (k_r) and nonradiative (k_nr) deactivation of the Yb^III cation. Moreover, the efficiency of the intersystem crossing (Φ_isc) and electronic energy transfer (Φ_eet) processes involved in the antennae effect have been quantified for the new complexes using a combination of nanosecond and femtosecond transient absorption techniques and have been compared to our previous results using [Ln(MPQ)₃] complexes with Ln = Yb and Lu.

Pyclen-Based Ln(III) Complexes as Highly Luminescent Bioprobes for In Vitro and In Vivo One- and Two-Photon Bioimaging Applications

In addition to the already described ligand L^4a, two pyclen-based lanthanide chelators, L^4b and L^4c, bearing two specific picolinate two-photon antennas (tailor-made for each targeted metal) and one acetate arm arranged in a dissymmetrical manner, have been synthesized, to form a complete family of lanthanide luminescent bioprobes: [EuL^4a], [SmL^4a], [YbL^4b], [TbL^4c], and [DyL^4c]. Additionally, the symmetrically arranged regioisomer L^4a' was also synthesized as well as its [EuL^4a'] complex to highlight the astonishing positive impact of the dissymmetrical N-distribution of the functional chelating arms. The investigation clearly shows the high performance of each bioprobe, which, depending on the complexed lanthanide, could be used in various applications. Each presents high brightness, quantum yields, and lifetimes. Staining of the complexes into living human breast cancer cells was observed. In addition, in vivo two-photon microscopy was performed for the first time on a living zebrafish model with [EuL^4a]. No apparent toxicity was detected on the growth of the zebrafish, and images of high quality were obtained.

Recent Advances in Luminescence Imaging of Biological Systems Using Lanthanide(III) Luminescent Complexes

The use of luminescence in biological systems allows one to diagnose diseases and understand cellular processes. Molecular systems, particularly lanthanide(III) complexes, have emerged as an attractive system for application in cellular luminescence imaging due to their long emission lifetimes, high brightness, possibility of controlling the spectroscopic properties at the molecular level, and tailoring of the ligand structure that adds sensing and therapeutic capabilities. This review aims to provide a background in luminescence imaging and lanthanide spectroscopy and discuss selected examples from the recent literature on lanthanide(III) luminescent complexes in cellular luminescence imaging, published in the period 2016-2020. Finally, the challenges and future directions that are pointing for the development of compounds that are capable of executing multiple functions and the use of light in regions where tissues and cells have low absorption will be discussed.

Luminescent EuIIIand TbIIIbimetallic complexes of N,N′-heterocyclic bases and tolfenamic acid: structures, photophysical aspects and biological activity

The isostructural bimetallic luminescent Eu^IIIand Tb^IIIdimers containing N,N′-heterocyclic bases and tolfenamic acid as a bridging ligands were evaluated for their structures, cellular imaging capability and photocytotoxicity.

Metallogels: Availability, Applicability, and Advanceability

Introducing metal components into gel matrices provides an effective strategy to develop soft materials with advantageous properties such as: optical activity, conductivity, magnetic response activity, self-healing activity, catalytic activity, etc. In this context, a thorough overview of application-oriented metallogels is provided. Considering that many well-established metallogels start from serendipitous discoveries, insights into the structure-gelation relationship will offer a profound impact on the development of metallogels. Initially, design strategies for discovering new metallogels are discussed, then the advanced applications of metallogels are summarized. Finally, perspectives regarding the design of metallogels, the potential applications of metallogels and their derivative materials are briefly proposed.

Polyanionic Polydentate Europium Complexes as Ultrabright One- or Two-photon Bioprobes

A family of europium (III) complexes based on a polydentate ligand functionalized by charge-transfer antennae presents remarkable one- and two-photon photophysical proper-ties in water or buffer. A detailed analysis of their emission properties suggests that the wrapping of the ligand around the central rare-earth ion results in an overall Cs symmetry in agreement with the theoretical simulation and that about 65-70 % of the emission intensity is concentrated in the hypersensitive 5 D0 →7 F2 transition at 615 nm. Their brightness is excellent, in the range of the best lanthanide bioprobes making them very attractive for bio-imaging experiments.

Lanthanide-based tools for the investigation of cellular environments

Biological probes constructed from lanthanides can provide a variety of readout signals, such as the luminescence of Eu(iii), Tb(iii), Yb(iii), Sm(iii) and Dy(iii), and the proton relaxation enhancement of Gd(iii) and Eu(ii). For numerous applications the intracellular delivery of the lanthanide probe is essential. Here, we review the methods for the intracellular delivery of non-targeted complexes (i.e. where the overall complex structure enhances cellular uptake), as well as complexes attached to a targeting unit (i.e. to a peptide or a small molecule) that facilitates delivery. The cellular applications of lanthanide-based supramolecules (dendrimers, metal organic frameworks) are covered briefly. Throughout, we emphasize the techniques that can confirm the intracellular localization of the lanthanides and those that enable the determination of the fate of the probes once inside the cell. Finally, we highlight methods that have not yet been applied in the context of lanthanide-based probes, but have been successful in the intracellular delivery of other metal-based probes.

A water-soluble TbIII complex as a temperature-sensitive luminescent probe

The water soluble [Tb(dipicCbz)3]3− (dipicCbz = 4-(9H-carbazol-9-yl-)pyridine-2,6-dicarboxylato) complex was isolated and evaluated as a temperature sensor in water. The 1:3 (TbIII:dipicCbz2−) stoichiometry in solution was confirmed by luminescence titration and high-resolution mass spectrometry. The quantum yield of sensitized emission is 3.8% ± 0.4% at 25.0 ± 0.1 °C, and the emission intensity depends on the temperature in the range of 5–70 °C with a relative thermal sensitivity of 3.4% °C−1 at 35 °C and temperature resolution < 0.01 °C in the range of 30–40 °C. The reversibility of this behavior was demonstrated for three heating–cooling cycles. Calculations of the energy gap between donor and acceptor show that the temperature dependence of the emission intensity is due to back-energy transfer from the Tb 5D4 excited state to the triplet and twisted intramolecular charge transfer (TICT) states of the dipicCbz. The assignment of one of the energy levels as a TICT state was confirmed by the temperature-dependent behavior of the phosphorescence band.

Copper-Catalyzed Benign and Efficient Oxidation of Tetrahydroisoquinolines and Dihydroisoquinolines Using Air as a Clean Oxidant

A green chemical method for mild oxidation of 1,2,3,4-tetrahydroisoquinolines (THIQs) and 3,4-dihydroisoquinolines (DHIQs) has been developed using air (O₂) as a clean oxidant. DHIQs and THIQs could be efficiently oxidized to isoquinolines in dimethyl sulfoxide at 25 °C under an open air atmosphere with CuBr₂ (20 mol %) as the catalyst; different bases [NaOEt and/or 1,8-diazabicyclo[5,4,0]undec-7-ene] were used for the reaction according to the patterns of substituents (R¹, R²).

Near infrared two photon imaging using a bright cationic Yb(iii) bioprobe spontaneously internalized into live cells

An Yb(iii) complex based on a dimethyl cyclen macrocyclic ligand functionalized by charge transfer antennae was prepared. This cationic [YbL3]⁺ complex is stable and soluble in water and presents interesting photophysical nonlinear properties. It is spontaneously internalized and accumulates in live cells. High quality images have been obtained both in a classical NIR-to-vis configuration and in the more challenging NIR-to-NIR one.

Heterobimetallic Complexes for Theranostic Applications

The design of more efficient anticancer drugs requires a deeper understanding of their biodistribution and mechanism of action. Cell imaging agents could help to gain insight into biological processes and, consequently, the best strategy for attaining suitable scaffolds in which both biological and imaging properties are maximized. A new concept arises in this field that is the combination of two metal fragments as collaborative partners to provide the precise emissive properties to visualize the cell as well as the optimum cytotoxic activity to build more potent and selective chemotherapeutic agents.

A Bimetallic Silver-Neodymium Theranostic Nanoparticle with Multimodal NIR/MRI/CT Imaging and Combined Chemo-photothermal Therapy

An engineered metallic nanostructure is an excellent candidate for "theranosis" of cancer, having intrinsic properties of multimodal imaging and therapy. Toward this target, the development of silver-neodymium bimetallic nanoparticles (Ag-Nd BNPs) via microwave-assisted polyol synthesis is presented. The resultant Ag-Nd BNPs exhibit good monodispersity with average size of 10 nm, fluorescence in the near-infrared (NIR) region, and magnetic properties. The Ag-Nd BNPs also validate MRI, CT, and NIR trimodal imaging ability and enunciate valuable temperature response upon irradiation under a NIR laser. Aided by chitosan functionalization on the surface, the Ag-Nd BNPs deliver good biocompatibility and also promote the loading of paclitaxel, an anticancer drug. Isothermal titration calorimetry affirms the combination of strong binding affinity of drug and high loading efficiency of 7 drug molecules per nanoparticle. Moreover, Ag-Nd BNPs also illustrate a highly efficient photothermal effect in PBS. Therefore, the synergistic effects of paclitaxel and the photothermal effect make BNPs excellent "combined therapeutic agents", and also give them the important ability to destroy cancer cells in vitro at very low dose in comparison to single therapy. Thus, the Ag-Nd BNPs unveil a combination of MRI/CT/NIR imaging and chemo-photothermal therapy that ensures accurate diagnosis at an early stage and comprehensive eradication of tumor cells without affecting healthy cells.

Microwave Synthesis of Chitosan Capped Silver-Dysprosium Bimetallic Nanoparticles: A Potential Nanotheranosis Device

Accurate imaging of the structural and functional state of biological targets is a critical task. To amend paucities associated with individual imaging, there is high interest to develop a multifunctional theranostic devices for cancer diagnosis and therapy. Herein, chitosan coated silver/dysprosium bimetallic nanoparticles (BNPs) were synthesized through a green chemistry route and characterization results inferred that the BNPs are crystalline, spherical, and of size ∼10 nm. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray photoelectron spectroscopy (XPS) confirm the reduced metallic states of Ag and Dy in nanoparticles. These BNPs demonstrate high emission in a second near-infrared (NIR-II, 1000-1400 nm) biological window on excitation at 808 nm. Moreover, magnetization and magnetic resonance imaging (MRI) studies perceive the inherent paramagnetic features of Dy component that displays dark T₂ contrast and high relaxivity. Due to high X-ray attenuation effect, BNPs exhibit better Hounsfield unit (HU) value than the reported contrast agents. BNPs unveil good biocompatibility and also express sturdy therapeutic effect in HeLa cells when tethered with doxorubicin.

Brightly Luminescent and Kinetically Inert Lanthanide Bioprobes Based on Linear and Preorganized Chelators

The synthesis, photophysical properties, and kinetic stability of a series of water-soluble, highly emissive Tb(III) and Eu(III) complexes featuring triethylenetetraamine hexaacetic acid (TTHA) and cyclohexyl triethylenetetraamine hexaacetic acid (cyTTHA) chelator scaffolds and carbostyril sensitizers are reported. The unique and modular design of the chelators gives rise to striking quantum yields of emission in aqueous solutions (up to 54%) as well as the characteristic lanthanides' photophysical properties (long excited-state lifetimes, large effective Stokes shifts, and narrow emission peaks). Furthermore, the preorganized chelators (L3, L4, and L6) bind metal within minutes at ambient temperature yet exhibit substantial resistance to transchelation in the presence of a challenge solution (EDTA, 1 mM). Moreover, the Eu(III) complex of L4 remains stably luminescent in HeLa cells over hours, demonstrating the suitability of these compounds for live-cell imaging applications. Representative chelators suitable for derivatization and protein bioconjugation were also prepared that were functionalized with clickable azide and alkyne moieties, biotin, and trimethoprim (TMP). With exceptional long-wavelength brightness, enhanced kinetic inertness, and an adaptable synthetic route, the reported lanthanide complexes are promising probes and labels for time-gated bioanalysis, biosensing, and optical microscopy.

Lanthanides: Applications in Cancer Diagnosis and Therapy

Lanthanide complexes are of increasing importance in cancer diagnosis and therapy, owing to the versatile chemical and magnetic properties of the lanthanide-ion 4f electronic configuration. Following the first implementation of gadolinium(III)-based contrast agents in magnetic resonance imaging in the 1980s, lanthanide-based small molecules and nanomaterials have been investigated as cytotoxic agents and inhibitors, in photodynamic therapy, radiation therapy, drug/gene delivery, biosensing, and bioimaging. As the potential utility of lanthanides in these areas continues to increase, this timely review of current applications will be useful to medicinal chemists and other investigators interested in the latest developments and trends in this emerging field.

Lanthanide-directed synthesis of luminescent self-assembly supramolecular structures and mechanically bonded systems from acyclic coordinating organic ligands

Herein some examples of the use of lanthanide ions (f-metal ions) to direct the synthesis of luminescent self-assembly systems (architectures) will be discussed. This area of lanthanide supramolecular chemistry is fast growing, thanks to the unique physical (magnetic and luminescent) and coordination properties of the lanthanides, which are often transferred to the resulting supermolecule. The emphasis herein will be on systems that are luminescent, and hence, generated by using either visibly emitting ions (such as Eu(III), Tb(III) and Sm(III)) or near infrared emitting ions (like Nd(III), Yb(III) and Er(III)), formed through the use of templating chemistry, by employing structurally simple ligands, possessing oxygen and nitrogen coordinating moieties. As the lanthanides have high coordination requirements, their use often allows for the formation of coordination compounds and supramolecular systems such as bundles, grids, helicates and interlocked molecules that are not synthetically accessible through the use of other commonly used templating ions such as transition metal ions. Hence, the use of the rare-earth metal ions can lead to the formation of unique and stable species in both solution and in the solid state, as well as functional and responsive structures.

Characterization of the Photophysical, Thermodynamic, and Structural Properties of the Terbium(III)-DREAM Complex

DREAM (also known as K(+) channel interacting protein 3 and calsenilin) is a calcium binding protein and an active modulator of KV4 channels in neuronal cells as well as a novel Ca(2+)-regulated transcriptional modulator. DREAM has also been associated with the regulation of Alzheimer's disease through the prevention of presenilin-2 fragmentation. Many interactions of DREAM with its binding partners (Kv4, calmodulin, DNA, and drugs) have been shown to be dependent on calcium. Therefore, understanding the structural changes induced by binding of metals to DREAM is essential for elucidating the mechanism of signal transduction and biological activity of this protein. Here, we show that the fluorescence emission and excitation spectra of the calcium luminescent analogue, Tb(3+), are enhanced upon binding to the EF-hands of DREAM due to a mechanism of energy transfer between Trp and Tb(3+). We also observe that unlike Tb(3+)-bound calmodulin, the luminescence lifetime of terbium bound to DREAM decays as a complex multiexponential (τaverage ∼ 1.8 ms) that is sensitive to perturbation of the protein structure and drug (NS5806) binding. Using isothermal calorimetry, we have determined that Tb(3+) binds to at least three sites with high affinity (Kd = 1.8 μM in the presence of Ca(2+)) and displaces bound Ca(2+) through an entropically driven mechanism (ΔH ∼ 12 kcal mol(-1), and TΔS ∼ 22 kcal mol(-1)). Furthermore, the hydrophobic probe 1,8-ANS shows that Tb(3+), like Ca(2+), triggers the exposure of a hydrophobic surface on DREAM, which modulates ligand binding. Analogous to Ca(2+) binding, Tb(3+) binding also induces the dimerization of DREAM. Secondary structural analyses using far-UV circular dichroism and trapped ion mobility spectrometry-mass spectrometry reveal that replacement of Ca(2+) with Tb(3+) preserves the folding state with minimal changes to the overall structure of DREAM. These findings pave the way for further investigation of the metal binding properties of DREAM using lanthanides as well as the study of DREAM-protein complexes by lanthanide resonance energy transfer or nuclear magnetic resonance.

Enhanced Resonance Energy Transfer and White-Light Emission from Organic Fluorophores and Lanthanides in Dendron-based Hybrid Hydrogel

In this paper, we have investigated the use of poly(aryl ether) dendron-based gel as a medium for resonance energy transfer (RET) from organic donors (phenanthrene, naphthalene, and pyrene) to lanthanide [Eu(III) and Tb(III)] ions. The gel has been prepared through self-assembly of glucose-cored poly(aryl ether) dendrons in a dimethyl sulfoxide/water mixture (1:9 v/v). The efficiency of RET was calculated by metal-centered emission quantum yield measurements in the gel medium. While there was no resonance energy transfer observed between the donor-acceptor pairs in solution, efficient RET has been observed in the gel medium. The metal-centered quantum yield values were 11.9% for phenanthrene-Eu(III), 3.9% for naphthalene-Eu(III), and 3.6% for pyrene-Eu(III) systems. Partial RET in the system has been utilized to generate white-light emission from the gel by incorporating an additional lanthanide ion, Tb(III), along with the organic donors and Eu(III). The CIE (Commission Internationale d'Eclairage) coordinates obtained for gels formed by phenanthrene-Tb(III)-Eu(III) (PTE), naphthalene-Tb(III)-Eu(III) (NTE), and pyrene-Tb(III)-Eu(III) (PyTE) were (0.33, 0.32) for PTE, (0.35, 0.37) for NTE, and (0.35, 0.33) for PyTE. The correlated color temperatures (CCT) for white-light-emitting gels were calculated, and the values (5520 K for PTE, 4886 K for NTE, and 4722 K for PyTE) suggest that the system generates cool white light.

A lysosome targetable luminescent bioprobe based on a europium β-diketonate complex for cellular imaging applications

A unique bright luminescent europium coordination compound with excellent biocompatibility has been developed that serves as a selective bioprobe for particular organelles within the cells.

Kinetically Stable Lanthanide Complexes Displaying Exceptionally High Quantum Yields upon Long-Wavelength Excitation: Synthesis, Photophysical Properties, and Solution Speciation

We demonstrate how highly emissive, kinetically stable complexes can be prepared using the macrocyclic scaffold of DO3A bearing coordinating aryl ketones as highly effective sensitizing chromophores. In the europium complexes, high quantum yields (up to 18% in water) can be combined with long-wavelength excitation (370 nm). The behavior in solution upon variation of pH, studied by means of UV-vis absorption, emission, and NMR spectroscopies, reveals that the nature of the chromophore can give rise to pH-dependent behavior as a consequence of deprotonation adjacent to the carbonyl group. Knowledge of the molecular speciation in solution is therefore critical when assessing the luminescence properties of such complexes.

Triply phenoxo bridged Eu(iii) and Sm(iii) complexes with 2,6-diformyl-4-methylphenol-di(benzoylhydrazone): structure, spectra and biological study in human cell lines

Two dinuclear lanthanide(iii) complexes, [M₂(HL)₃] (M = Sm(iii) (1), Eu(iii) (2); H₃L, 2,6-diformyl-4-methylphenol-di(benzoylhydrazone)) were generated with good yield and characterised systematically.

Using lanthanide ions in molecular bioimaging

This review presents an accessible discussion of the application of trivalent lanthanide ions in both optical and magnetic resonance imaging.