Photophysical and structural impact of phosphorylated anions associated to lanthanide complexes in water

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.

Design and applications of metal-based molecular receptors and probes for inorganic phosphate

Inorganic phosphate has numerous biomedical functions. Regulated primarily by the kidneys, phosphate reaches abnormally high blood levels in patients with advanced renal diseases. Since phosphate cannot be efficiently removed by dialysis, the resulting hyperphosphatemia leads to increased mortality. Phosphate is also an important component of the environmental chemistry of surface water. Although required to secure our food supply, inorganic phosphate is also linked to eutrophication and the spread of algal blooms with an increasing economic and environmental burden. Key to resolving both of these issues is the development of accurate probes and molecular receptors for inorganic phosphate. Yet, quantifying phosphate in complex aqueous media remains challenging, as is the development of supramolecular receptors that have adequate sensitivity and selectivity for use in either blood or surface waters. Metal-based receptors are particularly well-suited for these applications as they can overcome the high hydration enthalpy of phosphate that limits the effectiveness of many organic receptors in water. Three different strategies are most commonly employed with inorganic receptors for anions: metal extrusion assays, responsive molecular receptors, and indicator displacement assays. In this review, the requirements for molecular receptors and probes for environmental applications are outlined. The different strategies deployed to recognize and sense phosphate with metal ions will be detailed, and their advantages and shortfalls will be delineated with key examples from the literature.

Cationic Europium Complexes for Visualizing Fluctuations in Mitochondrial ATP Levels in Living Cells

The ability to study cellular metabolism and enzymatic processes involving adenosine triphosphate (ATP) is impeded by the lack of imaging probes capable of signalling the concentration and distribution of intracellular ATP rapidly, with high sensitivity. We report here the first example of a luminescent lanthanide complex capable of visualizing changes in the concentration of ATP in the mitochondria of living cells. Four cationic europium(III) complexes [Eu.1-4]+ have been synthesized and their binding capabilities towards nucleoside polyphosphate anions examined in aqueous solution at physiological pH. Complexes [Eu.1]+ and [Eu.3]+ bearing hydrogen bond donor groups in the pendant arms showed excellent discrimination between ATP, ADP and monophosphate species. Complex [Eu.3]+ showed relatively strong binding to ATP (logKa =5.8), providing a rapid, long-lived luminescent signal that enabled its detection in a highly competitive aqueous medium containing biologically relevant concentrations of Mg2+ , ADP, GTP, UTP and human serum albumin. This EuIII complex responds linearly to ATP within the physiological concentration range (1-5 mm), and was used to continuously monitor the apyrase-catalyzed hydrolysis of ATP to ADP in vitro. We demonstrate that [Eu.3]+ can permeate mammalian (NIH-3T3) cells efficiently and localize to the mitochondria selectively, permitting real-time visualization of elevated mitochondrial ATP levels following treatment with a broad spectrum kinase inhibitor, staurosporine, as well as depleted ATP levels upon treatment with potassium cyanide under glucose starvation conditions.

Aqueous Lanthanide Chemistry in Asymmetric Catalysis and Magnetic Resonance Imaging

This account describes lanthanide coordination chemistry with a focus on the similarities between lanthanide complexes used in catalysis and those used as contrast agents in magnetic resonance imaging.

Tb(III)-doped silica nanoparticles for sensing: effect of interfacial interactions on substrate-induced luminescent response

The present work introduces the easy modification of the water-in-oil microemulsion procedure aimed at the doping of the Tb(III) complexes within core or shell zones of the silica nanoparticles (SNs), which are designated as "core-shell", "shell", and "core". The dye molecules, chelating ligands, and copper ions were applied as the quenchers of Tb(III)-centered luminescence through dynamic or/and static mechanisms. The binding of the quenchers at the silica/water interface results in the quenching of the Tb(III) complexes within SNs, which, in turn, is greatly dependent on the synthetic procedure. The luminescence of "core" SNs remains unchanged under the binding of the quenchers at the silica/water interface. The quenching through dynamic mechanism is more significant for "core-shell" and "shell" than for "core" SNs. Thus, both "core-shell" and "shell" SNs have enough percentage of the Tb(III) complexes located close to the interface for efficient quenching through the energy transfer. The quenching through the ion or ligand exchange is most efficient for "core-shell" SNs due to the greatest percentage of the Tb(III) complexes at the silica/water interface, which correlates with the used synthetic procedure. The highlighted regularities introduce the applicability of "core-shell" SNs used as silica beads for phosphatidylcholine bilayers in sensing their permeability toward the quenching ions.

Multidentate europium chelates as luminoionophores for anion recognition: impact of ligand design on sensitivity and selectivity, and applicability to enzymatic assays

The design of photoluminescent molecular probes for the selective recognition of anions is a major challenge for the development of optical chemical sensors. The reversible binding of anions to lanthanide centers is one promising option for the realization of anion sensors, because it leads in some cases to a strong luminescence increase by the replacement of quenching water molecules. Yet, it is an open problem to gain control of the sensitivity and selectivity of the luminescence response. Primarily, the selective detection of (poly)phosphate species such as nucleotides has emerged as a demanding task, because they are involved in many biological processes and enzymatic reactions. We designed a series of pyridyl-based multidentate europium complexes (seven-, six-, and five-dentate) including sensitizing chromophores and studied their luminescence intensity and lifetime responses to different (poly)phosphates (adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), cyclic adenosine monophosphate (cAMP), pyrophosphate, and phosphate anions), and carboxyanions (citrate, malate, oxalacetate, succinate, α-ketoglutarate, pyruvate, oxalate, carbonate). The results reveal that the number of free coordination sites has a significant impact on the sensitivity and selectivity of the response. Because of its reversibility, the lanthanide probes can be applied to monitor the activity of ATP-consuming enzymes such ATPases and apyrases, which is demonstrated by means of the five-dentate complex.

Bright mono-aqua europium complexes based on triazacyclononane that bind anions reversibly and permeate cells efficiently

A series of five europium(III) complexes has been prepared from heptadentate N5O2 ligands that possess a brightness of more than 10 mM(-1) cm(-1) in water, following excitation over the range λ=330-355 nm. Binding of several oxy anions has been assessed by emission spectral titrimetric analysis, with the binding of simple carboxylates, lactate and citrate involving a common ligation mode following displacement of the coordinated water. Selectivity for bicarbonate allows the rapid determination of this anion in human serum, with K(d)=37 mM (295 K). The complexes are internalised quickly into mammalian cells and exhibit a mitochondrial localisation at early time points, migrating after a few hours to reveal a predominant lysosomal distribution. Herein, we report the synthesis and complexation behaviour of strongly emissive europium (III) complexes that bind oxy-anions in aqueous media with an affinity and selectivity profile that is distinctively different from previously studied systems.

A new bis-tetraamine ligand with a chromophoric 4-(9-anthracenyl)-2,6-dimethylpyridinyl linker for glyphosate and ATP sensing

The synthesis of a new linear bis-tetraamine ligand L1, based on two 1,4,8,11-tetraazaundecane units grafted at the 2 and 6 positions of a pyridinyl linker substituted by an anthracenyl fluorophore in the para position, is described and anion complexation studies of L1 with anionic substrates are reported. The protonation pattern and the study of the binding properties of L1 in an aqueous medium with two anionic substrates, the nucleotide adenosine triphosphate (ATP) and the herbicide glyphosate (N-(phosphonomethyl)glycine, PMG), were investigated by means of potentiometry, NMR spectroscopy and absorption and emission spectroscopic techniques. To decipher the impact of the chromophoric linker on the complexation process and to highlight its optical properties, a comparison is established with its previously reported analog L2 devoid of the anthracenyl group. The results unambiguously show that the protonation and complexation properties are preserved despite the presence of the bulky linker, allowing for the use of L1 as a fluorescent sensor for ATP and PMG.

A luminescent lanthanide complex-based anion sensor with electron-donating methoxy groups for monitoring multiple anions in environmental and biological processes

We designed a ternary europium (III) tris(2-thenoyltrifluoroacetonate) with 2-(3,4,5-trimethoxyphenyl)imidazo[4,5-f]-1,10-phenanthroline (1) ligands for the luminescent detection of various anions, such as fluoride, acetate and dihydrogen phosphate. Characterization of the sensor's photophysical properties and via NMR showed that the sensor exhibits striking emission changes to fluoride (purple), acetate (green) and dihydrogen phosphate (blue) anions, respectively. Its fluorescence lifetime was determined to be 1.10 ms for europium ions and the complex showed an overall quantum yield of 10% in DMSO. Furthermore, transparent hybrid thick films composed of the europium complex and poly-methyl methacrylate matrix were successfully prepared via copolymerization. The resulting film overall displayed intense red emissions associated with europium ions. Fluorescence microscopic evaluation showed a homogenous distribution of aggregates with average diameters of 30-50 μm throughout the film. The accordingly produced film could give rise to a luminescence change to purple in response to fluoride anions.

A supramolecular sensing platform for phosphate anions and an anthrax biomarker in a microfluidic device

A supramolecular platform based on self-assembled monolayers (SAMs) has been implemented in a microfluidic device. The system has been applied for the sensing of two different analyte types: biologically relevant phosphate anions and aromatic carboxylic acids, which are important for anthrax detection. A Eu(III)-EDTA complex was bound to β-cyclodextrin monolayers via orthogonal supramolecular host-guest interactions. The self-assembly of the Eu(III)-EDTA conjugate and naphthalene β-diketone as an antenna resulted in the formation of a highly luminescent lanthanide complex on the microchannel surface. Detection of different phosphate anions and aromatic carboxylic acids was demonstrated by monitoring the decrease in red emission following displacement of the antenna by the analyte. Among these analytes, adenosine triphosphate (ATP) and pyrophosphate, as well as dipicolinic acid (DPA) which is a biomarker for anthrax, showed a strong response. Parallel fabrication of five sensing SAMs in a single multichannel chip was performed, as a first demonstration of phosphate and carboxylic acid screening in a multiplexed format that allows a general detection platform for both analyte systems in a single test run with μM and nM detection sensitivity for ATP and DPA, respectively.

Infinite terbium(III) chains formed from a tetra-phosphorylated resorcinarene cavitand

Reaction of [Tb(III)(DMSO)(8)](CF(3)SO(3))(3) with a resorcinarene cavitand substituted by four -CH(2)P(O)Ph(2) pendant arms afforded an infinite 1D inorganic polymer, in which the consecutive resorcinarene units are linked through two terbium strands, this connection involves pairs of phosphoryl groups occupying adjacent positions of the resorcinarene units.

Prototype Protein Assembly as Scaffold for Time-Resolved Fluoroimmuno Assays

Turnip yellow mosaic virus (TYMV) is an icosahedral plant virus with an average diameter of 28 nm and can be isolated in gram quantities from turnip or Chinese cabbage inexpensively. In this study, it was selected as a prototype bionanoparticle for time-resolved fluoroimmuno assay (TRFIA). Two types of reactive amino acid residues were employed to anchor luminescent terbium complexes and biotin groups based on orthogonal chemical reactions. While terbium complexes were used as luminescent signaling groups, biotin motifs acted as a model ligand for protein binding. The bioconjugation results were confirmed by MS and Western blot analysis. Steady-state and time-resolved luminescence study of the dual-modified viruses demonstrated that the spectroscopic properties of the Tb complex are unperturbed by the labeling procedure. The dual-modified particle was probed by fluorescence resonance energy transfer (FRET) experiments using avidin labeled with an Alexa488 fluorophore, which bound to the biotin on the surface of the particle, as an energy acceptor, and terbium complexes as an energy donor. The emission and excitation spectra of the dual-labeled TYMV particle displayed residual virus fluorescence and Tb luminescence upon ligand-centered excitation. The Tb luminescence lifetime was 1.62 ms and could be effectively fitted with a single-exponential behavior. In the TRFIA, an efficient transfer of 66% was observed, and the calculation using the Förster radius of 41 A allowed for an estimation of the average donor-acceptor distance of 36 A. Our studies show that the two reactive sites can communicate with each other on the surface of a nanoscale biological assembly. In particular, the ligand-receptor binding (biotin and avidin in this paper) was not interfered with when anchored to the surface of TYMV. Therefore, as a prototype of polyvalent bionanoparticles, TYMV can be used as scaffold for sensor development with TRFIA.

A disymmetric terpyridine based ligand for the formation of luminescent di-aquo lanthanide complexes

The synthesis of ligand H3 based on a disymmetrically substituted terpyridine core functionalised by a carboxylic acid in the 6-position and a bis(carboxymethyl)aminomethyl function in the 6''-position is described. The coordination behaviour of this heptadentate (4N/3O) ligand with lanthanide cations (Ln=Eu, Gd and Tb) was studied in solution showing the formation of complexes with [Ln] stoichiometry. Complexes with general formula [Ln(H2O)2] were isolated from neutral water solutions containing equimolar amounts of cations and ligands, and the complexes were characterized in the solid state (elemental analysis, IR) and in solution (mass spectrometry). The photo-physical properties of the luminescent complexes of Eu and Tb were studied in water solution by means of absorption, steady state and time-resolved emission spectroscopies. Evolution of the luminescence lifetimes of the Eu and Tb complexes in H2O and D2O reveals the presence of two water molecules coordinated in the first coordination sphere of the cations. Despite this important hydration number, the overall luminescence quantum yields of the complexes remained elevated, especially in the case of Tb (Phi=22.0 and 6.5% respectively for Tb and Eu). Upon crystallisation the Gd complex formed dimeric species in which two gadolinium atoms are each heptacoordinated by one ligand, the coordination sphere being completed by a single water molecule and a bridging carboxylate function, pointing to different behaviours in the solid and liquid states.

A Cl? anion-responsive luminescent Eu3+ complex with a chiral tripod: ligand substituent effects on ternary complex stoichiometry and anion sensing selectivity

A new series of N3,O-mixed donor tripods was prepared for luminescent Eu3+ complexes, in which the soft quinoline nitrogen, tertiary amine nitrogen, and hard amide oxygen donors were cooperatively involved. The mixed donor tripods formed more stable 1 : 1 complexes with Eu(NO3)3, La(NO3)3 and Tb(NO3)3 than the corresponding N4 donor tripods, and their Eu3+ complexes particularly exhibited anion-responsive luminescence properties. NMR, UV, and luminescence spectroscopic characterizations revealed that -CH3 substitution on the tripod skeleton remarkably altered the preferred stoichiometry of the "tripod-Eu3+-anion" ternary complex and gave anion-dependent europium luminescence. Although the disubstituted tripod preferred to form non-luminescent 2 : 1 (tripod : Eu3+) complexes with Eu(NO3)3 and other salts, it formed a luminescent 1 : 1 complex with EuCl3. Thus, this type of tripod offered Cl- anion-selective luminescence enhancement that was easily observed by the naked eye.

Enhancement of near-IR emission by bromine substitution in lanthanide complexes with 2-carboxamide-8-hydroxyquinoline

Three novel 2-carboxamide-8-hydroxyquinoline derivatives wrap helically around trivalent lanthanide ions to form monometallic 3 : 1 complexes possessing strong NIR emission.

Relationship Between the Ligand Structure and the Luminescent Properties of Water-Soluble Lanthanide Complexes Containing Bis(bipyridine) Anionic Arms

A series of six new ligands (L(1)-L(6)) suitable for the formation of luminescent lanthanide complexes in water is described. Ligands L(1)-L(4) are constructed from two 6'-carboxy-6-methylene-2,2'-bipyridine chromophoric arms bonded to the amino function of a 2-aminomethylene-6-carboxy-pyridine (L(1)), an N,N-diacetate-ethylene diamine (L(2)), a serine (L(3)), or an aminomalonic acid (L(4)). For ligands L(5) and L(6), the linking amino function is provided by a glutamic acid, and the anionic functions at the 6'-position of the bipyridyl arms are made of the sodium salts of monoethylphosphonic ester (L(5)) and phosphonic acid (L(6)). The synthesis and characterisation of the ligands are described, together with the study of the formation of lanthanide complexes with europium and terbium. In the case of L(3), the europium complex obtained in acidic conditions was crystallised and the X-ray crystal structure is depicted. Photophysical properties of the complexes were studied by means of UV-visible absorption, and steady-state and time-resolved luminescence spectroscopy. Excited-state luminescence lifetimes of the complexes were determined in water and deuterated water to gain insight into the number of water molecules directly coordinated in the first coordination sphere of the complexes. The coordination behaviour of the series of ligands is questioned in the light of the spectroscopic data and discussed in terms of protection of the cation towards water molecules and their impact on the luminescence efficiency.