Lanthanide probes' ligands must meet several chemical requirements for the probes to work properly. 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.
The chelates which have been studied and utilized to date can be classified into the following groups:
# Tris chelates (three ligands)
# Tetrakis chelates (four ligands)
# Mixed ligand complexes
# Complexes with neutral donors
# Others such as: phthalate, picrate, and salicylate complexes.
The efficiency of the energy transfer from the ligand to the ion is determined ligand-metal bond. The energy transfer is more efficient when bonded covalently than through ionic bonding. Substituents in the ligand which are of electron-donating such as hydroxy, methoxy and methyl groups increase the fluorescence. The opposite effect is seen when an electron-withdrawing group (such as nitro) is attached.  Furthermore, the fluorescence intensity is increased by fluorine substitution to the ligand. 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. Increased conjugation by aromatic substituents by replacing phenyl by naphtyl groups is shown to enhance fluorescence.