Europium-doped calcium pyrophosphates: Allotropic forms and photoluminescent properties

In a search for new luminescent biological probes, we synthesized calcium pyrophosphates doped with europium up to an atomic Eu/(Eu+Ca) ratio of 2%. They were prepared by coprecipitating a mixture of calcium and europium salts with phosphate. After heating at 900 °C in air, two phases coexisted, identified as the β calcium pyrophosphate form and EuPO₄. Heating near 1250 °C in air, during the β→α transformation, europium ions substitute for calcium ions in the α calcium pyrophosphate structure as demonstrated by the spectroscopic study. Europium ions with both valence states (divalent and trivalent) were observed in the samples. Following the synthesis procedure, partial reduction of Eu³⁺ took place even in an oxidizing atmosphere. The 0.5%-doped compound could serve as a sensitive probe in biological applications. Depending on the excitation wavelength, the luminescence occurs either in the red or in the blue regions, which discriminates it from parasitic signals arising from other dyes or organelles in live cells.