Lanthanide-based luminescence probes and time-resolved luminescence bioassays

Bimodal magnetic-fluorescent probes for bioimaging

Fluorescent optical probes have been intensively used in the area of bio-imaging. In this review article, we describe the recent advancements in the synthesis and application of bimodal magnetic-fluorescent probes for bioimaging. The bimodal probes consist of fluorescent [semiconducting quantum dots (e.g., CdSe/ZnS) or rare-earth doped (e.g., NaYF(4) :Yb,Er)] nanoparticles (NPs) and magnetic (iron oxide or gadolinium based) NPs for optical and magnetic resonance (MR) imaging.

Facile synthesis of lanthanide nanoparticles with paramagnetic, down- and up-conversion properties

Lanthanide materials play an important role in biomedical researches, among which paramagnetic, down- and up-conversion lanthanide nanoparticles are of particular interest. However, little effort has been made to develop versatile lanthanide nanoparticles with these properties combined in a single particle. Herein, we present a facile strategy to make multifunctional lanthanide nanoparticles by doping sensitized lanthanide complexes into porous silica coated on up-conversion NaYF(4) nanocrystals. The nanoparticles not only show strong down- and up-conversion fluorescence but also exhibit high magnetic resonance properties.

Synthesis and characterization of coumarin-based europium complexes and luminescence measurements in aqueous media

A series of new ligands suitable for the formation of luminescent lanthanide complexes in water is described. The chelates are designed for analyte labeling and play the role of fluorescent donor in homogeneous time-resolved fluorescence assays using LEDs as a light source for excitation at 370 nm. Ligands are constructed from a coumarin nucleus, for lanthanide sensitization, and different aminomethylenecarboxy moieties are introduced in positions 7 and 5, 6, or 8 of the sensitizer. A reactive spacer arm under biocompatible conditions (maleimide, azide) is introduced at position 3 for ultimate bioconjugation purposes. The synthesis and characterization of the ligands are described, together with the preparation of their corresponding europium complexes. Photophysical properties of the complexes are investigated in water by means of UV-vis and luminescence spectroscopy.

Cell-penetrating metal complex optical probes: targeted and responsive systems based on lanthanide luminescence

To understand better the structure and function of biological systems, cell biologists and biochemists would like to have methods that minimally perturb living systems. The development of emissive optical probes is essential for improving our observation of intracellular signaling and recognition processes. Following excitation of the probe, photons emitted from the probe may be observed by spectroscopy or microscopy and encode information about their environments in their energy, lifetime, and polarization. Such optical probes may be based on organic fluorophores, quantum dots, recombinant proteins, or emissive metal complexes. In this Account, we trace the emergence of lanthanide coordination complexes as emissive optical probes. These probes benefit from sharp emission bands and long lifetimes. We can design these complexes to report on the concentration of key biochemical variables by modulation of spectral form, lifetime, or circular polarization. These properties allow us to apply ratiometric methods of analysis in spectroscopy or microscopy to report on local pH, pM (M = Ca, Zn), or the concentration of certain anionic metabolites, such as citrate, lactate, bicarbonate, or urate. For optical microscopy studies in living cells, these probes must be cell-permeable and, ideally, should localize in a given cell organelle. We undertook systematic studies of more than 60 emissive complexes, examining the time dependence of cellular uptake and compartmentalization, cellular toxicity, protein affinity, and quenching sensitivity. These results and their relationship to probe structure have allowed us to identify certain structure-activity relationships. The nature and linkage mode of the integral sensitizing group-introduced to harvest incident light efficiently-is of primary importance in determining protein affinity and cellular uptake and trafficking. In many cases, uptake may occur via macropinocytosis. We have defined three main classes of behavior: complexes exhibit predominant localization profiles in protein-rich regions (nucleoli/ribosomes), in cellular mitochondria, or in endosomes/lysosomes. Therefore, these systems offer considerable promise as intracellular optical probes, amenable to single- or two-photon excitation, that may report on the local ionic composition of living cells subjected to differing environmental stresses.

Novel highly charged silica-coated Tb(III) nanoparticles with fluorescent properties sensitive to ion exchange and energy transfer processes in aqueous dispersions

Novel silica-coated Tb(III) nanoparticles with high luminecsence were synthesized using the reverse microemulsion procedure. The quenching of luminescent properties of these nanoparticles can be achieved by ion exchange and energy transfer mechanisms. The quenching through the ion exchange of Tb(III) by H+ or La(III) is time dependent, indicating that the ion exchange is probably diffusion controlled. The quenching by Co(III) complex cations is achieved by the energy transfer mechanism and thus is not time dependent. The analysis of quenching data in Stern-Volmer cooordinates reveal the negative charge of the silica-coated Tb(III)-TCAS nanoparticles and several types of luminophoric species, located within the core and close to the surface of silica nanoparticles.

Spectroscopic analysis of ligand binding to lanthanide-macrocycle platforms

A high-affinity, binary Eu(3+) receptor site consisting of 1,4,7,10-tetraazacyclododecane-1,7-diacetate (DO2A) was constructed with the goal of improving the detection of dipicolinic acid (DPA), a major component of bacterial spores. Ternary Eu(DO2A)(DPA)(-) complex solutions (1.0 microM crystallographically characterized TBA x Eu(DO2A)(DPA)) were titrated with EuCl3 (1.0 nM-1.0 mM); increased Eu(3+) concentration resulted in a shift in equilibrium population from Eu(DO2A)(DPA)(-) to Eu(DO2A)(+) and Eu(DPA)(+), which was monitored via the ligand field sensitive (5)D0 --> (7)F3 transition (lambda(em) = 670-700 nm) using luminescence spectroscopy. A best fit of luminescence intensity titration data to a two-state thermodynamic model yielded the competition equilibrium constant (Kc), which in conjunction with independent measurement of the Eu(DPA)(+) formation constant (Ka) allowed calculation of the ternary complex formation constant (Ka'). With this binding affinity by competition (BAC) assay, we determined that Ka' = 10(8.21) M(-1), which is approximately 1 order of magnitude greater than the formation of Eu(DPA)(+). In general, the BAC assay can be employed to determine ligand binding constants of systems where the lanthanide platform (usually a binary complex) is stable and the ligand bound versus unbound states can be spectroscopically distinguished.

Poly(N-vinylcaprolactam-co-glycidyl methacrylate) aqueous microgels labeled with fluorescent LaF3:Eu nanoparticles

We describe the synthesis and properties of functional microgel particles based on poly(N-vinylcaprolactam-co-glycidyl methacrylate) (PVCL/PGMA) copolymer. A series of colloidally stable microgel particles with a range of glycidyl methacrylate content were prepared by surfactant-free heterophase polymerization in water. The microgel particles obtained had hydrodynamic radii between 250 and 350 nm and were fairly monodisperse in size; however, a broadening of the particle size distribution was observed for samples with a low GMA content. The PVCL/PGMA microgel particles exhibit thermally responsive reversible changes in diameter in water, and the swelling degree increased with the PVCL fraction in the copolymer structure. These microgels were then modified with photoluminescent europium-doped lanthanum fluoride nanoparticles (LaF3:Eu-AEP) through reaction of the 2-aminoethyl phosphate surface ligands with epoxy groups present in the microgel. These hybrid microgels were colloidally stable and thermally responsive in aqueous solution.

Highly Efficient Blue Photoexcitation of Europium in a Bimetallic Pt–Eu Complex

We report the preparation and characterization of dinuclear Pt-Ln complexes constructed from a square-planar Pt(II) core bearing an ethynyl-terpyridine residue connected to platinum by the ethynyl bond. Complexation of the neutral Eu(hfac)3 (hfac = hexafluoroacetylacetonate) fragment to free terpyridine (terpy) gives a stable bimetallic complex (log beta = 6.7). In the crystal structure, the flat Pt[triple bond]terpy core coordinates to Eu(III), which is nonacoordinated with the three nitrogen atoms of the terpy subunit and six oxygen atoms of the three hfac ligands. These atoms form a distorted monocapped square antiprism with a pseudo-C2 symmetry axis passing through the nitrogen atom of the central pyridine ring and the Eu atom. Spectroscopic measurements showed that irradiation with visible light of wavelength up to 460 nm in the 1MLCT state of the Pt subunit resulted in a quantitative energy transfer to the Eu center, which strongly luminesces in the red with an overall luminescence quantum yield of 38%. The energy-transfer process is quantitative and not sensitive to oxygen, and the complexation of Eu to the Pt metallosynthon allows the recovery of the energy lost due to triplet-oxygen quenching of the 3MLCT state observed in the uncomplexed Pt precursor.

An octadentate luminescent Eu(III) 1,2-HOPO chelate with potent aqueous stability

The synthesis, characterization, and photophysical properties of two novel ligands, 5LINMe-1,2-HOPO (1) and H(2,2)-1,2-HOPO (2), which utilize the 1,2-HOPO chelate as a sensitizer for Eu(III) are reported. In addition, the former ligand was structurally characterized as the Eu(III) complex by X-ray crystallography. The [Eu(1)2]- complex of the tetradentate ligand (1) is stable in aqueous solution, to a limiting concentration of ca. 7x10(-9) M, and retains the superior photophysical performance noted for the 1,2-HOPO sensitizer. By contrast, the octadentate ligand (2) has vastly improved stability as the [Eu(2)]- complex upon further dilution, to a limiting concentration of ca. 5x10(-17) M, which is beyond the minimum detectable concentration of most fluorimeters. The presence of a single coordinated water molecule for the latter complex reduces the overall metal-centered luminescence.

Kinetic CE: foundation for homogeneous kinetic affinity methods

Kinetic capillary electrophoresis (KCE) is defined as capillary electrophoresis of species that interact during electrophoresis. KCE can serve as a conceptual platform for development of homogeneous kinetic affinity methods for affinity measurements (measurements of binding parameters and quantitative measurements) and affinity purification (purification of known molecules and search of unknown molecules). A number of different KCE methods can be designed by varying initial and boundary conditions - the way interacting species enter and exit the capillary. KCE methods will find multiple practical applications in the designing of biomedical diagnostics and the development of drug candidates. Here, the concept of KCE, its up-to-date applications, and future prospective are reviewed.

Synthesis of a coumarin-based europium complex for bioanalyte labeling

A coumarin-based europium chelate ready-to-use for analyte labeling and homogeneous time-resolved fluorescence measurements has been designed. Compound 1 displays three functional elements: an azide reactive spacer arm, a coumarin sensitizer, and a seven-coordinate europium complex. That complex can be excited at 370 nm by inexpensive UV-LEDs as a light excitation source.

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.

Practical time-gated luminescence flow cytometry. II: Experimental evaluation using UV LED excitation

In the previous article [Part 1 (8)], we have modelled alternative approaches to design of practical time-gated luminescence (TGL) flow cytometry and examined the feasibility of employing a UV LED as the excitation source for the gated detection of europium dye labelled target in rapid flow stream. The continuous flow-section approach is well suited for rare-event cell counting in applications with a large number of nontarget autofluorescent particles. This article presents details of construction, operation and evaluation of a TGL flow cytometer using a UV LED excitation and a gated high-gain channel photomultiplier tube (CPMT) for detection. The compact prototype TGL flow cytometer was constructed and optimised to operate at a TGL cycle rate of 6 kHz, with each cycle consisting of 100 micros LED pulsed excitation and approximately 60 micros delay-gated detection. The performance of the TGL flow cytometer was evaluated by enumerating 5.7 microm Eu(3+) luminescence beads (having comparable intensity to europium-chelate-labeled Giardia cysts) in both autofluorescence-rich environmental water concentrates and Sulforhodamine 101 (S101) solutions (broadband red fluorescence covering the spectral band of target signals), respectively. The prototype TGL flow cytometer was able to distinguish the target beads, and a maximum signal to background ratio of 38:1 was observed. Neither the environmental water concentrates nor S101 solution contributed to the background in the TGL detection phase. The counting efficiency of the TGL flow cytometer was typically >93% of values determined using conventional counting methods.

Aminopropargyl derivative of terpyridine-bis(methyl-enamine) tetraacetic acid chelate of europium (Eu (TMT)-AP3): a new reagent for fluorescent labelling of proteins and peptides

The synthesis and photophysical properties of a new terpyridine-based europium(III) chelate (Eu (TMT)-AP3) designed for peptide and protein labelling in aqueous solution phase is described. In order to obtain a stable, easy to handle, versatile and efficient labelling agent, a reactive aminopropargyl arm has been introduced onto the terpyridine moiety. As preliminary biochemical applications the chelate has been 1) efficiently covalently attached onto a representative biomolecule-monoclonal antibody-and 2) converted into iodoacetamido and aldehyde derivatives, and the photoluminescent Eu (TMT)-AP3 was grafted onto cysteine and lysine amino acid residues respectively. These two different solution phase labelling methods yielded original fluorogenic FRET based probes suitable for "in vitro" detection of caspase-3 protease, a key mediator of apoptosis of mammalian cells.