Applications of Europium Tetracycline Complex: A Review

Europium solutions present low absorption coefficients and, due to the energy transfer between europium ions and the OH vibration of coordinated water molecules, there is a significant non-radiative relaxation and no emission is observed. However, when europium complexes with tetracycline (EuTc), it exhibits and intense and narrow emission band centered on 615 nm, characteristic of the 5D0 → 5F2 transition within the lanthanide ion. The tetracycline molecule acts as an “antenna” or sensitizer, absorbing the excitation light and transferring the energy to the lanthanide ion. A major increase in the luminescence of the EuTc complex was observed in the presence of hydrogen peroxide and also urea hydrogen peroxide. The possibility of quantifying the hydrogen peroxide using luminescence measurements is also promising for the indirect quantification of glucose and cholesterol since enzymatic reactions generate hydrogen peroxide. This principle can be applied to determine, indirectly, concentrations of lactose and galactose, alcohol, catalase, etc. There are also other possibilities of EuTc applications: assay for phosphate, sensing and imaging of malate in aqueous solution, spectrofluorimetric determination of lecithin, heparin, human serum albumin, citrate and other intermediates in the citric acid cycle, low density lipoproteins (LDL), the determination of the activity of phosphorylation, immunoassays, etc. The purpose of this review is to describe and discuss the latest advances in analytical methodologies based in the luminescence properties of EuTc probes.