Lanthanide probes
Lanthanides are metal ions which have their 4f energy level filled and generally refer to elements cerium to lutetium in the periodic table.
[3] The term chelate derives from the Greek word for “claw,” and is applied to name ligands, which attach to a metal ion with two or more donor atoms through dative bonds.
[3] In 1942 complexes of europium, terbium, and samarium were discovered to exhibit unusual luminescence properties when excited by UV light.
[3] However, the first staining of biological cells with lanthanides occurred twenty years later when bacterial smears of E. coli were treated with aqueous solutions of a europium complex, which under mercury lamp illumination appeared as bright red spots.
[4] The first analyte is linked to a specific binding agent on a solid support such as a polymer and then another reaction couples the first poorly luminescent lanthanide complex with a new better one.
[5] Lanthanides can be used because their small size (ionic radius) gives them the ability to replace metal ions inside protein complex such as calcium or nickel.
[1] The energies of these levels are well defined due to the shielding of the 4f orbitals by the filled 5s and 5p sub-shells,[4] and are not very sensitive to the chemical environments in which the lanthanide ions are inserted.
Since these transitions are parity forbidden, the lifetimes of the excited states are long, which allows the use of time resolved spectroscopy,[4] a definitive asset for bioassays and microscopy.
[1] The energy absorbed by the organic receptor (ligand) is transferred onto Ln(III) excited states, and sharp emission bands originating from the metal ion are detected after rapid internal conversion to the emitting level.
[1][4] Screening tools for the development of new cancer therapies are in high demand worldwide and often require the determination of enzyme kinetics.
Luminescence of the Tb(III) complex with norfloxacin is sensitive to determine the concentration of phosphate released by the GTP to GDP transformation.
[1] Protonation of basic sites in systems comprising a chromophore and a luminescent metal center leads the way for pH sensors.
[1] More robust sensors have been proposed in which the core is a substituted macrocycle usually bearing phosphinate, carboxylate or four amide coordinating groups.
[7] Some scientist also have used lanthanide based luminescence resonance energy transfer (LRET) which is very similar to FRET to study conformational changes in RNA polymerase upon binding to DNA and transcription initiation in prokaryotes.
The time resolved fluorescence based technique is generally applicable and its performance has also been tested in the assay of viral antigens in clinical specimens.
These qualities are: water solubility, large thermodynamic stability at physiological pHs, kinetic inertness and absorption above 330 nm to minimize destruction of live biological materials.
[1] The chelates which have been studied and utilized to date can be classified into the following groups:[3] The efficiency of the energy transfer from the ligand to the ion is determined ligand-metal bond.
The energy transfer to the metal ion increases as the electronegativity of the fluorinated group makes the europium-oxygen bond of a more covalent nature.
