Principles of responsive lanthanide-based luminescent probes for cellular imaging

Luminescent lanthanide probes for cations and anions: Promises, compromises, and caveats

The long luminescence lifetimes and sharp emission bands of luminescent lanthanide complexes have long been recognized as invaluable strengths for sensing and imaging in complex aqueous biological or environmental media. Herein we discuss the recent developments of these probes for sensing metal ions and, increasingly, anions. Underappreciated in the field, buffers and metal hydrolysis influence the response of many responsive lanthanide probes. The inherent complexities arising from these interactions are further discussed.

Eu3+ functionalized metal-organic framework for selective monitoring of emerging environmental pollutants non-steroidal anti-inflammatory drugs

Non-steroidal anti-inflammatory drugs (NSAIDs), as a new water pollutant emerging in recent years, has potential hazards to the environment. The difficult degradation characteristics of NSAIDs lead to long-term accumulation in the natural environment, which will inevitably cause incalculable damage to human health. In this work, for practical application considerations, MIL-53(Al) type MOF [Al(OH)(TDC)]‧1.5H₂O‧0.7DMF (MIL-53-TDC, TDC = 2,5-thiophene dicarboxylic acid) with good water stability is selected as the sensing main body. The ligand TDC was chosen for two reasons: one is as an antenna ligand, which can sensitize Eu³⁺ ions to emit characteristic fluorescence; the other is as binding site that the sulfur atoms on the thiophene ring can introduce Eu³⁺ ions through coordination. Thus, Eu³⁺ functionalized MIL-53-TDC hybrid materials (Eu@MIL-53-TDC) were developed as a fluorescence sensor for the detection of two kinds of NSAIDs, S-ibuprofen (S-IBP) and diclofenac (DCF). The concentration range of S-IBP and DCF detected by the prepared sensors is 0.001-0.07 mM (LOD = 0.5 μM) and 0.0005-0.1 mM (LOD = 0.2 μM), respectively. Moreover, this sensor not only can achieve rapid (3 min) and sensitive analysis of these two NSAIDs but also has a satisfactory recovery for the detection of S-IBP and DCF in serum and tap water.

Kinetically Inert Macrocyclic Europium(III) Receptors for Phosphate

The significant role that phosphate plays in environmental water pollution and biomedical conditions such as hyperphosphatemia highlights the need to develop robust receptors that can sequester the anion effectively and selectively from complex aqueous media. Toward that goal, four macrocyclic tris-bidentate 1,2-hydroxypyridonate (HOPO) europium(III) complexes containing either a cyclen, cyclam, TACN, or TACD ligand cap were synthesized and evaluated as phosphate receptors. The solubility of EuIII-TACD-HOPO in water was insufficient for luminescent studies. Whereas EuIII-cyclen-HOPO is eight coordinate with two inner-sphere water molecules, both EuIII-cyclam-HOPO and EuIII-TACN-HOPO are nine coordinate with three inner-sphere water molecules, suggesting that the two coordination states are very close in energy. As observed previously with linear analogues of tripodal HOPO complexes, there is no relationship between the number of inner-sphere water molecules and the affinity of the complex for phosphate. Whereas all three complexes do bind phosphate, EuIII-cyclen-HOPO has the highest affinity for phosphate with the anion displacing both of its inner-sphere water molecules. On the other hand, only one or two of the three inner-sphere water molecules of EuIII-TACN-HOPO and EuIII-cyclam-HOPO are displaced by phosphate, respectively. All three complexes are highly selective for phosphate over other anions, including arsenate. All three complexes are highly stable. EuIII-cyclen-HOPO and, to a lesser extent, EuIII-TACN-HOPO are more kinetically inert than the linear EuIII-Ser-HOPO. EuIII-cyclam-HOPO, on the other hand, is not. This study highlights the significant effect that minor changes in the ligand cap can have on both the ligand exchange rate and affinity for phosphate of tripodal 1,2-dihydroxypyridinonate complexes.

Hydration effects on the vibrational properties of carboxylates: From continuum models to QM/MM simulations

The presence of carboxyl groups in a molecule delivers an affinity to metal cations and a sensitivity to the chemical environment, especially for an environment that can give rise to intermolecular hydrogen bonds. Carboxylate groups can also induce intramolecular interactions, such as the formation of hydrogen bonds with donor groups, leading to an impact on the conformational space of biomolecules. In the latter case, the protonation state of the amino groups plays an important role. In order to provide an accurate description of the modifications induced in a carboxylated molecule by the formation of hydrogen bonds, one needs a compromise between a quantum chemical description of the system and the necessity to take into account explicit solvent molecules. In this work, we propose a bottom-up approach to study the conformational space and the carboxylate stretching band of (bio)organic anions. Starting from the anions in a continuum solvent, we then move to calculations using a microsolvation approach including one explicit water molecule per polar group, immersed in a continuum. Finally, we run QM/MM molecular dynamics simulations to analyze the solvation properties and to explore the anions conformational space. The results thus obtained are in good agreement with the description given by the microsolvation approach and they bring a more detailed description of the solvation shell and of the intermolecular hydrogen bonds.

Luminescent Metal Complexes as Emerging Tools for Lipid Imaging

Fluorescence microscopy is a key tool in the biological sciences, which finds use as a routine laboratory technique (e.g., epifluorescence microscope) or more advanced confocal, two-photon, and super-resolution applications. Through continued developments in microscopy, and other analytical methods, the importance of lipids as constituents of subcellular organelles, signalling or regulating molecules continues to emerge. The increasing recognition of the importance of lipids to fundamental cell biology (in health and disease) has prompted the development of protocols and techniques to image the distribution of lipids in cells and tissues. A diverse suite of spectroscopic and microscopy tools are continuously being developed and explored to add to the "toolbox" to study lipid biology. A relatively recent breakthrough in this field has been the development and subsequent application of metal-based luminescent complexes for imaging lipids in biological systems. These metal-based compounds appear to offer advantages with respect to their tunability of the photophysical properties, in addition to capabilities centred around selectively targeting specific lipid structures or classes of lipids. The presence of the metal centre also opens the path to alternative imaging modalities that might not be applicable to traditional organic fluorophores. This review examines the current progress and developments in metal-based luminescent complexes to study lipids, in addition to exploring potential new avenues and challenges for the field to take.

Luminescent Lanthanide Probes for Inorganic and Organic Phosphates

Inorganic and organic phosphates-including orthophosphate, nucleotides, and DNA-are some of the most fundamental anions in cellular biology, regulating numerous processes of both medical and environmental significance. The characteristic long lifetimes of emitting lanthanides, including the brighter europium(III) and terbium(III), make them ideally suited for the development of molecular probes for the detection of phosphates directly in complex aqueous media. Moreover, given their high oxophilicity and the exquisite sensitivity of their quantum yields to their hydration number, those luminescent lanthanides are perfect for the detection of phosphates. Herein we discuss the principles that have guided the recent developments of molecular probes selective for inorganic or organic phosphates and how these lanthanide complexes facilitate the study of numerous biological processes.

Dual-Labelling Strategies for Nuclear and Fluorescence Molecular Imaging: Current Status and Future Perspectives

Molecular imaging offers the possibility to investigate biological and biochemical processes non-invasively and to obtain information on both anatomy and dysfunctions. Based on the data obtained, a fundamental understanding of various disease processes can be derived and treatment strategies can be planned. In this context, methods that combine several modalities in one probe are increasingly being used. Due to the comparably high sensitivity and provided complementary information, the combination of nuclear and optical probes has taken on a special significance. In this review article, dual-labelled systems for bimodal nuclear and optical imaging based on both modular ligands and nanomaterials are discussed. Particular attention is paid to radiometal-labelled molecules for single-photon emission computed tomography (SPECT) and positron emission tomography (PET) and metal complexes combined with fluorescent dyes for optical imaging. The clinical potential of such probes, especially for fluorescence-guided surgery, is assessed.

Metallointercalators-DNA Tetrahedron Supramolecular Self-Assemblies with Increased Serum Stability

Self-assembly of metallointercalators into DNA nanocages is a rapid and facile approach to synthesize discrete bioinorganic host/guest structures with a high load of metal complexes. Turberfield's DNA tetrahedron can accommodate one intercalator for every two base pairs, which corresponds to 48 metallointercalators per DNA tetrahedron. The affinity of the metallointercalator for the DNA tetrahedron is a function of both the structure of the intercalating ligand and the overall charge of the complex, with a trend in affinity [Ru(bpy)₂(dppz)]²⁺ > [Tb-DOTAm-Phen]³⁺ ≫ Tb-DOTA-Phen. Intercalation of the metal complex stabilizes the DNA tetrahedron, resulting in an increase of its melting temperature and, importantly, a significant increase in its stability in the presence of serum. [Ru(bpy)₂(dppz)]²⁺, which has a greater affinity for DNA than [Tb-DOTAm-Phen]³⁺, increases the melting point and decreases degradation in serum to a greater extent than the Tb^III complex. In the presence of Lipofectamine, the metallointercalator@DNA nanocage assemblies substantially increase the cell uptake of their respective metal complex. Altogether, the facile incorporation of a large number of metal complexes per assembly, the higher stability in serum, and the increased cell penetration of metallointercalator@DNA make these self-assemblies well-suited as metallodrugs.

Self-Assembled Lanthanide Antenna Glutathione Sensor for the Study of Immune Cells

The small molecule 8-methoxy-2-oxo-1,2,4,5-tetrahydrocyclopenta[de]quinoline-3-carboxylic acid (2b) behaves as a reactive non-fluorescent Michael acceptor, which after reaction with thiols becomes fluorescent, and an efficient Eu³⁺ antenna, after self-assembling with this cation in water. This behavior makes 2b a highly selective GSH biosensor, which has demonstrated high potential for studies in murine and human cells of the immune system (CD4⁺ T, CD8⁺ T, and B cells) using flow cytometry. GSH can be monitored by the fluorescence of the product of addition to 2b (445 nm) or by the luminescence of Eu³⁺ (592 nm). 2b was able to capture baseline differences in GSH intracellular levels among murine and human CD4⁺ T, CD8⁺ T, and B cells. We also successfully used 2b to monitor intracellular changes in GSH associated with the metabolic variations governing the induction of CD4⁺ naïve T cells into regulatory T cells (T_REG).

Tb(III)-DO3A and BODIPY dyad as multimode responsive hypochlorite probe

A multimode responsive hypochlorite probe 1, based on Terbium (III)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (Tb(III)-DO3A) and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), is described. We have shown that probe 1 can detect ClO^- by absorption, fluorescence, and phosphorescence simultaneously. The multimode response makes probe 1 a versatile ClO^- probe for practical applications. We have found that probe 1 can be used in naked-eye colorimetric and fluorogenic detection of NaOCl in solution. Also, we have constructed a colorimetric test paper for visual sensing of NaOCl. Furthermore, fluorescence imaging studies indicated that probe 1 was a versatile tool for in vitro imaging of NaOCl in living cells. Thus, to the best of our knowledge, probe 1 represents one of the rare examples of multimode responsive ClO^- probes.

Novel versatile europium and terbium complexes as bioprobes and anticancer agents

Focusing on the pertinent research on dual activity (as probes and cancer inhibitors), two novel Eu and Tb lanthanide complexes were prepared in this work alongside their binuclear counterparts holding Cu as the secondary metal.

A Walk Across the Lanthanide Series: Trend in Affinity for Phosphate and Stability of Lanthanide Receptors from La(III) to Lu(III)

The trend in affinity of two 1,2-hydroxypyridinonate lanthanide(III) receptors-Ln^III-2,2-Li-HOPO and Ln^III-3,3-Gly-HOPO (Ln^III = La^III, Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, Tb^III, Dy^III, Ho^III, Er^III, Tm^III, Yb^III, and Lu^III)-for phosphate across the series was investigated by luminescence spectroscopy via competition against the central europium(III) analog. Regardless of the ligand, the rare earth receptors display a steep and continuous increase in affinity for their phosphate guest across the series, with the later lanthanides displaying the highest affinity for the oxyanion. This trend mirrors that of the stability of the lanthanide receptors, which also increases significantly and continuously from La^III to Lu^III. For these two ligands, the ionic radius of a rare earth, a parameter directly linked to its Lewis acidity, correlates strongly with its affinity for anions, regardless of whether that anion is the one coordinating it (in this case the 1,2-hydroxypyridinonate ligand) or the guest targeted by the lanthanide receptor (in this case phosphate). These observations are indicative of a lack of steric hindrance for coordination of phosphate. Advantageously, increased efficacy of the lanthanide receptor comes with increased stability. The remarkably high stability of Lu^III-2,2-Li-HOPO, combined with its high affinity for phosphate, makes it a particularly promising candidate for translational application to medical or environmental sequestration of phosphate since the higher stability will further reduce the risk of the rare earth leaching during anion separation. The unusually large difference in stability between lanthanide complexes (the Lu^III complex of 2,2-Li-HOPO is at least 7 orders of magnitude more stable than the La^III one) bodes well for potential applications in rare earth separation.

Eu(III)-DO3A and BODIPY dyad as a chemosensor for anthrax biomarker

The sensitive and selective determination of Bacillus anthracis spores before the infection is vital for human health and safety. Dipicolinic acid (DPA) is an excellent biomarker due to its presence in the nucleus of bacterial spores at high concentrations (up to 1 M, about 15% dry weight). In the present work, a new molecular chemosensor 1, based on europium(III)-DO3A and BODIPY dyad, is developed to detect DPA in phosphate-buffered saline (PBS) buffered solution and tap water samples. Also, 1 can be used as a ratiometric optical chemosensor to track DPA.

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.

The emission quenching of upconversion nanoparticles coated with amorphous silica by fluorescence resonance energy transfer: A mercury-sensing nanosensor excited by near-infrared radiation

In this paper, a rhodamine derivative was synthesized as a probe for Hg(II) detection. Its spectral response and sensing mechanism towards Hg(II) were discussed carefully. It was found that its absorption and emission were increased by Hg(II), via a direct bonding stoichiometry of 1:1. Its association constant was determined with absorption titration as 2.59 × 10⁵ M^-1, which suggested a coordination procedure between Hg(II) and this rhodamine probe. It showed good selectivity towards Hg(II) over competing metal cations, no increased emission or absorption was observed in the presence of interfering metal cations. It was then covalently grafted onto silica (SiO₂)-encapsulated upconversion nanoparticles (UCNPs). Upon near-infrared (NIR) excitation (980 nm), RHO accepted energy from these UCNPs through a FRET (fluorescence resonance energy transfer) procedure, quenching their upconversion emission. A sensing response towards Hg(II) was thus constructed. Good linearity and selectivity were still preserved in this composite sample. On the other hand, this work found a different phenomenon from literature cases, which was the emission absence of rhodamine emission in this composite structure. Detailed analysis suggested that rhodamine emission absence was caused by its self-quenching effect.

Lanthanide-Aromatic Iminodiacetate Frameworks with Helical Tubes: Structure, Properties, and Low-Temperature Heat Capacity

A series of lanthanide coordination polymers [LnL(H₂O)₂] _n [Ln = Pr (1), Nd (2), Sm (3), Eu (4), and Gd (5), H₃L = N-(4-carboxy-benzyl)iminodiacetic acid] was hydrothermally prepared and structurally characterized. All the five compounds have been confirmed as 3D Ln-CPs with one-dimensional helical tunnels composed of four helical chains, although there are different coordination geometries around Ln³⁺. Enantiomeric helixes in 1-3, and absolute left-handed and right-handed helical chains in 4 and 5, respectively, lead to different tunnel spaces. Their conformations can also be featured by different space groups and unit cell dimensions. Photoluminescence measurement on 3 and 4 show characteristic emission peaks of Sm³⁺ and Eu³⁺ ions, respectively. The low-temperature heat capacity of 1-4 has been investigated in the temperature range of 1.9-300 K. Their heat capacity values are nearly equal below 10 K and display a crossover with the value order C _p,m(2) > C _p,m(1) ≈ C _p,m(4) > C _p,m(3) above 10 K. The measured heat capacities have been fitted, and the corresponding thermodynamic functions were consequently calculated based on the fitting parameters. The standard molar entropies at 298.15 K have been determined to be (415.71 ± 4.16), (451.32 ± 4.51), (308.53 ± 3.09), and (407.62 ± 4.08) J·mol^-1·K^-1 for 1, 2, 3, and 4, respectively.

Upconversion Nanoparticles Encapsulated with Amorphous Silica and Their Emission Quenching by FRET: A Nanosensor Excited by NIR for Mercury Detection

Near-infrared (NIR) region has been considered as a diagnostic window since it avoids sample autofluorescence and light scattering. Upconversion nanoparticles (UCNPs) convert NIR light into high energy excitation light, making them a suitable excitation source for nanoprobes with deep penetration depth and high signal-to-noise ratio. The current work reported a rhodamine-derived probe for the detection of Hg(II). Corresponding absorption and emission responses for Hg(II) and detailed recognizing mechanism were discussed. An absorption titration experiment was performed. It was found that Hg(II) directly bonded with probe with chemical stoichiometry of 1:1, its association constant was calculated as 2.59 × 105 M−1. Such a high value indicated a direct coordination affinity between Hg(II) and this rhodamine-derived probe. Most metal cations exerted no increasing effect on the probe emission or absorption, exhibiting good sensing selectivity of probe towards Hg(II). Upconversion nanoparticles (UCNPs) were firstly encapsulated with silica (SiO2) and then bonded with the probe via a covalent bond. Given a near-infrared (NIR) laser excitation with wavelength of 980 nm, this probe, (E)-2-((3′,6′-bis(diethylamino)-2′,7′-dimethyl-3-oxospiro[isoindoline-1,9′-xanthen]-2-yl)imino)acetaldehyde (denoted as RHO), captured the energy of UCNPs via a FRET (fluorescence resonance energy transfer) path, resulting in the emission quenching of UCNPs. This composite system showed linear sensing behavior towards Hg(II) with high selectivity, which was similar to the case of pure probe. No probe emission, however, was observed from the composite system, which was different from the case of most literature reports. The self-quenching between probe molecules was claimed responsible for the probe emission, which was confirmed by experiment result and analysis. To the best of our knowledge, this is the first demonstration of covalently integrating SiO2-coated UCNPs with a rhodamine-derived probe for Hg(II) sensing.

Pyridinedicarboxylate-Tb(III) Complex-Based Luminescent Probes for ATP Monitoring

The pyridinedicarboxylate-Tb(III) complexes, TbPDC and Tb(PDC)₃, as luminescent probes for ATP monitoring have been conveniently prepared and characterized by FT-IR, ¹H-NMR, ESI-MS, UV-Vis, excitation, and emission spectroscopy. Interestingly, these two Tb(III) complexes were quenched by ATP by a similar mechanism via π-π stacking interaction between the chelating ligand and adenine moiety. The ability of luminescent probes applied for the determination of ATP in aqueous solution has been investigated. The dynamic ranges for the quantification of ATP are within 10-90 μM and 10-100 μM with detection limits of 7.62 and 11.20 μM for TbPDC and Tb(PDC)₃, respectively. The results demonstrated that these luminescent probes would be a potential candidate assay for ATP monitoring in hygiene assessment.

Structural variation of self-assembled lanthanide supramolecular complexes induced by reaction conditions

The structural variation of self-assembled lanthanide supramolecular complexes which can be induced by different factors such as concentration, anion and solvent, cationic radii, stoichiometric ratio and light.

Luminescent Sensor Based on Ln(III) Ternary Complexes for NAD(P)H Detection

Ln(III) complexes of macrocyclic ligands are used in medicinal chemistry, for example as contrast agents in MRI or radiopharmaceutical compounds, and in diagnostics using fluorescence imaging. This paper is devoted to a spectroscopic study of Ln(III) ternary complexes consisting of macrocyclic heptadentate DO3A and bidentate 3-isoquinolinate (IQCA) ligands. IQCA serves as an efficient antenna ligand, leading to a higher quantum yield and Stokes shift (250-350 nm for Eu, Tb, Sm, Dy in VIS region, 550-650 nm for Yb, Nd in NIR region). The shielding-quenching effect of NAD(P)H on the luminescence of the Ln(III) ternary complexes was investigated in detail and this phenomenon was utilized for the analytical determination of this compound. This general approach was verified through an enzymatic reaction during which the course of ethanol transformation catalyzed by alcohol-dehydrogenase (ADH) was followed by luminescence spectroscopy. This method can be utilized for selective and sensitive determination of ethanol concentration and/or ADH enzyme activity. This new analytical method can also be used for other enzyme systems coupled with NAD(P)H/NAD(P)+ redox pairs.

Two Beta-Phosphorylamide Compounds as Ligands for Sm3+, Eu3+, and Tb3+: X-ray Crystallography and Luminescence Properties

This paper describes the synthesis of two beta-phosphorylamide ligands and their coordination chemistry with the Ln ions Tb³⁺, Eu³⁺, and Sm³⁺. Both the ligands and Ln complexes were characterized by IR, NMR, MS, and X-ray crystallography. The luminescence properties of the Tb³⁺ and Eu³⁺ complexes were also characterized, including the acquisition of lifetime decay curves. In the solid state, the Tb³⁺ and Sm³⁺ ligand complexes were found to have a 2:2 stoichiometry when analyzed by X-ray diffraction. In these structures, the Ln ion was bound by both oxygen atoms of each beta-phosphorylamide moiety of the ligands. The Tb³⁺ and Eu³⁺ complexes were modestly emissive as solutions in acetonitrile, with lifetime values that fell within typical ranges.

A dual-sensitized luminescent europium(iii) complex as a photoluminescent probe for selectively detecting Fe3

A new luminescent Eu^III complex, namely [Eu₂(BTFA)₄(OMe)₂(dpq)₂] (1), in which BTFA = 3-benzoyl-1,1,1-trifluoroacetone and dpq = dipyrido [3,2-d:2',3'-f] quinoxaline, has been designed and synthesized by employing two different ligands as sensitizers. Crystal structure analysis reveals that complex 1 is composed of dinuclear Eu^III units crystallized in the monoclinic P1̄ space group. Notably, 1 exhibits high thermal stability up to 270 °C and excellent water stability. The photoluminescence property of the complex is investigated. Further studies show 1 can recognize Fe³⁺ ions with high selectivity from mixed metal ions in aqueous solution through the luminescence quenching phenomenon. Furthermore, the recyclability and stability of 1 after sensing experiments are observed to be adequate. By virtue of the superior stability, detection efficiency, applicability and reusability, the as-prepared Eu^III complex can be a promising fluorescent material for practical sensing.

An Activatable Lanthanide Luminescent Probe for Time-Gated Detection of Nitroreductase in Live Bacteria

Herein we report the development of a turn-on lanthanide luminescent probe for time-gated detection of nitroreductases (NTRs) in live bacteria. The probe is activated through NTR-induced formation of the sensitizing carbostyril antenna and resulting energy transfer to the lanthanide center. This novel NTR-responsive trigger is virtually non-fluorescent in its inactivated form and features a large signal increase upon activation. We show that the probe is capable of selectively sensing NTR in lysates as well as in live bacteria of the ESKAPE family which are clinically highly relevant multiresistant pathogens responsible for the majority of hospital infections. The results suggest that our probe could be used to develop diagnostic tools for bacterial infections.

Smart lanthanide antennas for sensing water

Two new families of lanthanide antennas are described. 8-Methoxy-4,5-dihydrocyclopenta[de]quinolin-2(1H)-one phosphonates or carboxylates behave as selective antennas exhibiting Eu3+ luminescence in organic solvents, while quinolin-2(1H)-one analogues selectively sensitize the Tb3+ emission. These emissions are quenched by H2O addition. Based on this behaviour, the new lanthanide antennas can be used as highly sensitive water sensors.

Europium-Pyridinedicarboxylate-Adenine Light-Up Fluorescence Nanoprobes for Selective Detection of Phosphate in Biological Fluids

Phosphate (Pi) plays important roles in various physiological processes. Its quantification in biological fluids is highly crucial for timely warning of Pi accumulation. Herein, an europium (Eu)-based coordination polymer nanoprobe (Eu/DPA/Ade) is prepared by coordinating 2,6-pyridinedicarboxylic acid (2,6-DPA) and adenine (Ade) with Eu³⁺. Eu/DPA/Ade exhibits light-up fluorescence response to Pi. The strong coordinating interaction between Eu³⁺ and O atoms in the Pi group not only shortens the Eu³⁺-ligand distance to improve the energy transfer from 2,6-DPA to Eu³⁺ but also attenuates the fluorescence quenching from water molecules in the coordinating sphere of Eu³⁺. Eu/DPA/Ade produces red emission at λ_em 618 nm via the "antenna effect". The coligand Ade further promotes the fluorescent emission. The selective recognition of Pi within 10-60 μM is achieved with a detection limit of 4.65 μM. In addition, a certain level of Pi (100-170 μM) causes an exponential increment on the fluorescence of Eu/DPA/Ade and makes it feasible for visual estimation of Pi under irradiation by an ultraviolet lamp at 254 nm. The quantitative detection and visual estimation of Pi in human urine and saliva have been demonstrated.

Oxyaapa: A Picolinate-Based Ligand with Five Oxygen Donors that Strongly Chelates Lanthanides

Coordination compounds of the lanthanide ions (Ln³⁺) have important applications in medicine due to their photophysical, magnetic, and nuclear properties. To effectively use the Ln³⁺ ions for these applications, chelators that stably bind them in vivo are required to prevent toxic side effects that arise from localization of these ions in off-target tissue. In this study, two new picolinate-containing chelators, a heptadentate ligand OxyMepa and a nonadentate ligand Oxyaapa, were prepared, and their coordination chemistries with Ln³⁺ ions were thoroughly investigated to evaluate their suitability for use in medicine. Protonation constants of these chelators and stability constants for their Ln³⁺ complexes were evaluated. Both ligands exhibit a thermodynamic preference for small Ln³⁺ ions. The log K_LuL = 12.21 and 21.49 for OxyMepa and Oxyaapa, respectively, indicating that the nonadentate Oxyaapa forms complexes of significantly higher stability than the heptadentate OxyMepa. X-ray crystal structures of the Lu³⁺ complexes were obtained, revealing that Oxyaapa saturates the coordination sphere of Lu³⁺, whereas OxyMepa leaves an additional open coordination site for a bound water ligand. Solution structural studies carried out with NMR spectroscopy revealed the presence of two possible conformations for these ligands upon Ln³⁺ binding. Density functional theory (DFT) calculations were applied to probe the geometries and energies of these conformations. Energy differences obtained by DFT are small but consistent with experimental data. The photophysical properties of the Eu³⁺ and Tb³⁺ complexes were characterized, revealing modest photoluminescent quantum yields of <2%. Luminescence lifetime measurements were carried out in H₂O and D₂O, showing that the Eu³⁺ and Tb³⁺ complexes of OxyMepa have two inner-sphere water ligands, whereas the Eu³⁺ and Tb³⁺ complexes of Oxyaapa have zero. Lastly, variable-temperature ¹⁷O NMR spectroscopy was performed for the Gd-OxyMepa complex to determine its water exchange rate constant of k_ex²⁹⁸ = (2.8 ± 0.1) × 10⁶ s^-1. Collectively, this comprehensive characterization of these Ln³⁺ chelators provides valuable insight for their potential use in medicine and garners additional understanding of ligand design strategies.

The Ligand Cap Affects the Coordination Number but Not Necessarily the Affinity for Anions of Tris-Bidentate Europium Complexes

To evaluate the effect of ligand geometry on the coordination number, number of inner-sphere water molecules, and affinity for anions of the corresponding lanthanide complex, six tris-bidentate 1,2-hydroxypyridonate (HOPO) europium(III) complexes with different cap sizes were synthesized and characterized. Wider or more flexible ligand caps, such as in Eu^III-TREN-Gly-HOPO and Eu^III-3,3-Gly-HOPO, enable the formation of nine-coordinate europium(III) complexes bearing three inner-sphere water molecules. In contrast, smaller or more rigid caps, such as in Eu^III-TREN-HOPO, Eu^III-2,2-Li-HOPO, Eu^III-3,3-Li-HOPO, and Eu^III-2,2-Gly-HOPO, favor eight-coordinate europium(III) complexes that have only two inner-sphere water molecules. Notably, there is no correlation between the number of inner-sphere water molecules and the affinity of the Eu(III) complexes for phosphate. Some q = 2 (Eu^III-TREN-HOPO, Eu^III-3,3-Li-HOPO, and Eu^III-2,2-Gly-HOPO) and some q = 3 (Eu^III-TREN-Gly-HOPO) complexes have no affinity for anions, whereas one q = 2 complex (Eu^III-2,2-Li-HOPO) and one q = 3 complex (Eu^III-3,3-Gly-HOPO) have a high affinity for phosphate. For the latter two systems, each inner-sphere water molecule is replaced with a phosphate anion, resulting in the formation of EuLPi₂ and EuLPi₃ adducts, respectively.

Tripyridinophane Platform Containing Three Acetate Pendant Arms: An Attractive Structural Entry for the Development of Neutral Eu(III) and Tb(III) Complexes in Aqueous Solution

We report a detailed characterization of Eu³⁺ and Tb³⁺ complexes derived from a tripyridinophane macrocycle bearing three acetate side arms (H₃tpptac). Tpptac^3- displays an overall basicity (∑ log K_i^H) of 24.5, provides the formation of mononuclear ML species, and shows a good binding affinity for Ln³⁺ (log K_LnL = 17.5-18.7). These complexes are also thermodynamically stable at physiological pH (pEu = 18.6, pTb = 18.0). It should be noted that the pGd value of Gd-tpptac (18.4) is only slightly lower than that of commercially available MRI contrast agents such as Gd-dota (pGd = 19.2). Moreover, a very good selectivity for these ions over the endogenous cations (log K_CuL = 14.4, log K_ZnL = 12.9, and log K_CaL = 9.3) is observed. The X-ray structure of the terbium complex shows the metal coordinated by the nine N₆O₃ donor set of the ligand and one inner-sphere water molecule. DFT calculations result in two Eu-tpptac structures with similar bond energies (ΔE = 0.145 eV): one structure in which the water is coordinated to the metal ion and one structure in which the water molecule is farther away from the ion, bound to the ligand with an OH-π bond. By detailed luminescence experiments, we demonstrate that the europium complex in aqueous solution presents a hydration equilibrium between nine-coordinate, dehydrated [Eu-tpptac]⁰ and ten-coordinate, monohydrated [Eu-tpptac(H₂O)]⁰ species. A similar trend is observed for the terbium complex. Despite the presence of this hydration equilibrium, the H₃tpptac ligand sensitizes Eu³⁺ and Tb³⁺ luminescence efficiently in buffered water at physiological pH. Particularly, the terbium complex displays a long excited-state lifetime of 2.24 ms and an overall quantum yield of 33% with a brightness of 3600 M^-1 cm^-1. Such features of Ln³⁺ complexes of H₃tpptac indicate that this platform appears to be particularly appealing for the further development of luminescent lanthanide labels.

Time-Resolved Terbium-Based Probe for the Detection of Zinc(II) Ions: Investigation of the Formation of a Luminescent Ternary Complex

Because of their unique photochemical and photophysical properties, luminescent lanthanide-based complexes have long captivated chemists. In recent years, the number of reports of luminescent lanthanide complex-based probes for monitoring of biological and environmental processes has dramatically increased, namely, because of their selectivity for particular analytes, lower limits of detection, and the fact that they allow monitoring of analytes in real time. Lanthanide-based probes need to be paired with an appropriate antenna/sensitizer to allow maximum energy transfer, with the antenna typically covalently attached to the stable lanthanide chelate. We have recently investigated "dark" lanthanide-based probes where the sensitizer is not covalently linked to the lanthanide chelate. Herein we report the use of a luminescent lanthanide-based probe system for the detection of Zn²⁺ ions based on the formation of a ternary complex between a "dark" terbium complex, lumazine, and Zn²⁺. The terbium(III)-based probe incorporates a 1,4,7,10-tetraazacyclododecane-1,4,7,10-triacetic acid macrocyclic chelator covalently attached to a cyclen moiety, which is the Zn²⁺ ion binding group. In the presence of Zn²⁺ ions and lumazine (a strongly UV-absorbing sensitizer), a 1:1:1 ternary complex forms. The resulting complex is highly luminescent and selective for Zn²⁺ ions over other cations of environmental significance. Furthermore, with a limit of detection of 1.2 μM, this probe can detect the level of chronic zinc(II) concentrations denoted by the U.S. Environmental Protection Agency.

A Combination of Factors: Tuning the Affinity of Europium Receptors for Phosphate in Water

Although recognition of hard anions by hard metal ions is primarily achieved via direct coordination, electrostatic and hydrogen-bonding interactions also play essential roles in tuning the affinity of such supramolecular receptors for their target. In the case of Eu^III hydroxypyridinone-based complexes, the addition of a single charged group (-NH₃⁺, -CO₂^-, or -SO₃^-) or neutral hydrogen-bonding moiety (-OH) peripheral to the open coordination site substantially affects the affinity of the metal receptor for phosphate in water at neutral pH. A single primary ammonium increases the first association constant for phosphate in neutral water by 2 orders of magnitude over its neutral analogue. The addition of a peripheral alcohol group also increases the affinity of the receptor but to a lesser degree (21-fold). On the other hand, negatively charged complexes bearing either a carboxylate or sulfate moiety have negligible affinity for phosphate. Interestingly, the peripheral group also influences the stoichiometry of the lanthanide receptor for phosphate. While the complex bearing a -NH₃⁺ group binds phosphate in a 1:2 ratio, those with -OH and H (control) both form 1:3 complexes. Although the positively charged Eu^III-Lys-HOPO has the highest K_a1 for phosphate, a greater increase in luminescence intensity (36-fold) is observed with the neutral Eu^III-Ser-HOPO complex. Notably, whereas the affinity of the Eu^III complexes for phosphate is substantially influenced by the presence of a single charged group or hydrogen-bond donor, their selectivity for phosphate over competing anions remains unaffected by the addition of the peripheral groups.

Guanine-guided time-resolved luminescence recognition of DNA modification and i-motif formation by a terbium(III)-platinum(II) complex

Site-specific recognition of DNA modification or the formation of noncanonical structures has important applications in molecular biology, disease diagnosis, and gene expression analysis. In this study, we introduce a guanine-guided sensing tool using a terbium(III)-platinum(II) complex (TPC) as a time-resolved luminescence probe to site-specifically recognize DNA modification and i-motif formation in aqueous solution. The probe is composed of a Tb^III center as the luminescent reporter and two Pt^II units as the receptor for guanine (G) nucleobase. TPC exhibits remarkable reaction selectivity for guanine nucleotides over other nucleotides, giving rise to a significant increase in luminescence. The luminescence enhancement of TPC is mainly attributed to an energy transfer from G base to the Tb^III center after the specific coordination of Pt^II with N7 of guanine (N7-G), which would be facilitated by the phosphates through promoting the departure of coordinated water and bringing G closer to Tb^IIIvia noncovalent interactions. Based on such sensing feature, the enhanced luminescence of TPC sensitized by G nucleotides can correspondingly decrease upon N7-G modifications of DNA or i-motif formation through constructing simple guanine-guided sensing tools. This probe would provide a useful strategy for site-specific recognition of DNA for extensive purposes.

Poly(3-hydroxi-butyrate-co-3-hydroxy-valerate) (PHB-HV) microparticles loaded with holmium acetylacetonate as potential contrast agents for magnetic resonance images

INTRODUCTION

Biodegradable polymers that contain radioactive isotopes such as Holmium 166 have potential applications as beta particle emitters in tumor tissues. Also, Ho(III) is paramagnetic, which makes it suitable as a contrast agent for magnetic resonance (MR) images.

METHODS

Holmium acetylacetonate (Ho(acac)3) loaded poly(3-hydroxy-butyrate-co-3-hydroxy-valerate) microspheres, with 5% or 8% of 3-hydroxy-valerate (HV), were prepared by emulsification/evaporation process within 20-53 μm size. Microspheres characterization was done using scanning electron microscopy, energy-dispersive X-ray, and infrared spectroscopies. The release of holmium(III) in sodium phosphate buffer (pH 7.4) was followed for 9 days with inductively coupled plasma. Finally, T2 and T2* magnetic resonance images (MRI) were acquired and compared with the MRI of the inclusion complex of holmium acetylacetonate in some β-cyclodextrins.

RESULTS

Holmium acetylacetonate loading, evaluated by thermogravimetry, was up to 20 times higher for copolymer with 5% of HV. It was shown that microspheres loaded with Ho(acac)3 exhibited an accumulation of Ho(III) on their surfaces but were stable over time, as no expressive release of holmium(III) was detected in 9-day exposition to sodium phosphate buffer. Holmium acetylacetonate in both microspheres or inclusion complexes was very efficient in obtaining T2 and T2* weighted images in magnetic resonance, thus, might be used as contrast agents.

CONCLUSION

This is the first description of the use of inclusion complexes of holmium acetylacetonate in biodegradable polymers as contrast agents. New investigations are underway to evaluate the resistance of PHB-HV polymer microparticles to nuclear activation to assess their potential for use as radiopharmaceuticals for the treatment of liver cancer.

Terbium(III) Luminescence-Based Assay for Esterase Activity

Esterases catalyze the hydrolysis of esters to form a carboxylic acid and alcohol. These enzymes play a key role in both the detoxification of xenobiotic compounds and the metabolism of drugs and prodrugs. Numerous fluorogenic probes have been developed to monitor esterase activity. Most are based on an aromatic alcohol, and the others are based on an aromatic acid. These restrictions leave unexplored the specificity of esterases for aliphatic esters. Here, we report on the use of esters of thiopheneacetic acid coupled with the luminescence of terbium(III) as the basis for a continuous assay of esterase activity. This probe allows for a wide variation of the alcohol moiety and the detection of its hydrolysis at submicromolar concentrations. The assay verifies steady-state kinetic parameters for catalysis by pig liver esterase from either initial rates or the integration of progress curves, and its utility is evident with unpurified esterases in bacterial and human cell lysates.

Phosphorylation-dependent protein design: design of a minimal protein kinase-inducible domain

Protein kinases and phosphatases modulate protein structure and function, which in turn regulate cellular activities. The development of novel proteins and protein motifs that are responsive to protein phosphorylation provides new ways to probe the functions of individual protein kinases and the intracellular effects of their activation and downregulation. Herein we develop a minimal motif that is responsive to protein phosphorylation, termed a minimal protein kinase-inducible domain. The encodable protein motif comprises a 7- or 8-residue sequence (DKDADXW or DKDADXXW), derived from EF-Hand calcium-binding domains, that is necessary but not sufficient for binding terbium, combined with a protein phosphorylation site (Ser or Thr at residue 9) that, upon phosphorylation, completes the metal-binding motif. Thus, the motif binds metal poorly and exhibits weak terbium luminescence when not phosphorylated. Upon phosphorylation, the peptide binds metal with significantly higher affinity and exhibits robust terbium luminescence. Phosphorylation results in up to a 23× increase in terbium luminescence. Minimal phosphorylation-dependent motifs as small as 9 residues (DKDADGWIS) were developed. NMR spectroscopy on this lanthanum(iii)·phosphopeptide complex confirmed that binding occurs in a manner similar to that in an EF-Hand, despite the absence of the conserved Glu12 typically present in an EF-Hand. By combining molecular design with known protein kinase recognition sequences, minimal protein kinase-inducible domains were developed that were responsive to phosphorylation by Protein Kinase A (PKA: DKDADRRW(S/pS)IIAK), Protein Kinase C (PKC: DKDADGWI(T/pT)FRRKA), and Casein Kinase 1 (CK1: DKDADDWA(S/pS)I). Phosphorylation by PKA was quantified in HeLa cell extracts, with a 4.4× increase in fluorescence (terbium luminescence) observed at 544 nm. The optimized minimal motif includes alternating aspartate residues at positions 1, 3, and 5, plus binding through the main-chain carbonyl at position 7; a lysine at position 2 to provide electrostatic balance and reduce binding in the absence of phosphorylation; an alanine at residue 4 to promote the αL conformation observed at that position of the EF Hand; a tryptophan at residue 7 or 8 to sensitize terbium luminescence; and a phosphorylation site with serine or threonine at residue 9. Residues at positions 6; 7 or 8; and 10 or later may be changed to provide kinase specificity. In the CK1-responsive peptide, the acidic residues in the proto-terbium-binding motif are employed as part of the kinase recognition sequence. This work thus presents fundamental rules for the design of compact phosphorylation-responsive terbium-binding motifs, with potential further application to motifs responsive to other protein post-translational modifications.

Coordination of mycotoxins with lanthanides in luminescent complexes

The ability of several chelating mycotoxins to form coordination complexes with the lanthanide metals europium and terbium was explored. The mycotoxins examined included ochratoxin A, citrinin, cyclopiazonic acid (CPA), kojic acid, and tenuazonic acid (TeA). Of these compounds, TeA and CPA resulted in the greatest luminescence. Parameters influencing luminescence of TeA were investigated further. These included the type of lanthanide and its concentration, certain environmental factors, and the effect of competing metal cations. Of the two lanthanide metals, the terbium coordination complex (TeA-Tb³⁺) showed greater luminescence relative to the europium complex (TeA-Eu³⁺). The effects of solvent type, water content, and pH on the TeA-Tb³⁺ system suggested that optimal conditions for luminescence were in 90% methanol with 10% aqueous buffer at pH 3. In competitive assays, the luminescence of the TeA-Tb³⁺ complex decreased as the concentration of competing metal cations increased. Among the cations tested, Cu²⁺ was the best inhibitor followed by Al³⁺, Au³⁺, Fe³⁺, Co²⁺, Mn²⁺, Mg²⁺, and Ca²⁺. Two cations, Na⁺ and K⁺, showed no significant inhibition. This is the first report to describe the coordination of the metal-chelating mycotoxin TeA with lanthanides and the ability of TeA to serve as an "antenna" for the efficient transfer of energy to the lanthanide with resulting luminescence. Understanding the ability of mycotoxins such as TeA to chelate metals provides insight into how they exert their toxic effects.

Host sensitized lanthanide photoluminescence from post-synthetically modified semiconductor nanoparticles depends on reactant identity

This work investigates the photoluminescence characteristics where cadmium selenide (CdSe) and zinc sulfide (ZnS) nanoparticles are treated post-synthetically by the trivalent lanthanide cations (Ln³⁺) [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] separately to form either CdSe/Ln or ZnS/Ln nanoparticles. Host sensitized Ln³⁺ emission was found to be present only in CdSe/Eu, CdSe/Tb, ZnS/Eu, ZnS/Tb and ZnS/Yb nanoparticles. In all the cases tuning of emission of the nanoparticles has been observed, irrespective of the presence or absence of host sensitization. The elemental compositions of CdSe and ZnS nanoparticles upon post-synthetic treatment show a remarkable difference. Incorporation of lanthanides in the nanoparticles is evident with significant alteration in the anionic content, and complete cation exchange of either Cd²⁺ or Zn²⁺ by Ln³⁺ has not been detected; as evaluated from energy dispersive X-ray spectroscopy. Further evaluation on this comes from considering thermodynamic parameters of inter cation interaction. In cases where the host sensitized Ln³⁺ emission have been observed, luminescence lifetime measurements reveal significant protection of Ln³⁺ in the nanoparticles. Noticeable difference in photophysical properties for a given Ln³⁺ has been realized in the two hosts. The photophysical observations have been rationalized using (i) charge trapping mediated host sensitized dopant emission, (ii) autoionization of excited electrons, and (iii) environment induced photoluminescence quenching. The post-synthetic modification discussed in the present work provides an easy and less synthetically demanding room temperature based protocol to avail lanthanide incorporated (doped) semiconductor nanoparticles that can potentially use the unique emission properties of the lanthanide cations.

Luminescent europium(iii) and terbium(iii) complexes of β-diketonate and substituted terpyridine ligands: synthesis, crystal structures and elucidation of energy transfer pathways

A series of coordinatively saturated Ln^III complexes: [Ln(R-TPY)(TTA)₃] (1–6) were designed and structurally characterized and plausible energy transfer (ET) pathways determined using a theoretical method.

Smartphone-based apparatus for measuring upconversion luminescence lifetimes

Luminescence lifetime detection plays an important role in time-resolved detection and research. However, the traditional instruments always require expensive detectors such as time-correlated single photon counter or streak camera. Herein, a low-cost and miniaturized apparatus for measuring upconversion luminescence lifetimes was developed by using a smartphone equipped with a 980 nm CW laser and a motor. When the motor was driving the sample circling at a high linear velocity, the excited sample would emit a luminescence arc, which could be photographed by the phone camera. The rotating rate could be measured by a tuner APP and then used for transferring arc length to delay times. By analyzing the grayscale distribution of the luminescence arc, the luminescence decay curve was obtained, which was then used for exponential fit and calculating lifetimes. The images captured by different smartphones revealed similar lifetime values, suggesting a wide universality of this method. The whole system was not only remarkably cheaper but also more miniaturized than traditional instruments for measuring luminescence lifetimes, indicating the promising applications in point of care testing for time-resolved luminescence detection for bioanalysis and disease diagnosis.