Bimodal Magneto-Luminescent Response of Lanthanide Metallopolymers for Distinguishing of Phosphates in Aqueous Solutions

Abstract

Linear N-substituted polyacrylamides bearing tricarboxylic moieties pendant to the backbone are synthesized, and coordinative binding approach is developed to convert them into Gd³⁺/Eu³⁺-metallopolymers. Inner-sphere coordination of the lanthanide ions with the pendant chelating moieties, together with their outer-sphere stabilization by the randomly coiled backbone, are the reasons for peculiar water proton relaxation and luminescence properties of the metallopolymers obtained. Indeed, for Gd³⁺-metallopolymer, the values of longitudinal and transverse relaxivity (r₁ and r₂) are 50 and 60 mm⁻¹·s⁻¹, respectively, while for the Eu³⁺-counterpart, Eu³⁺-centered luminescence is significantly raised compared to Eu³⁺ aquaions. Coil-like conformation of the metallopolymer molecules undergoes a swelling at pH values above 5.0 as evidenced by transmission electron microscopy TEM and dynamic light scattering (DLS) data, which is followed by the changes in the Eu³⁺-centered luminescence and r₁₍₂₎-values of Gd³⁺/Eu³⁺-metallopolymers. The phosphates additives (adenosine monophosphoric acid (AMP), adenosine diphosphoric acid (ADP), HPO₄²⁻ and adenosine triphosphoric acid (ATP) induce well detectable changes in both water proton relaxation and luminescence of Gd³⁺/Eu³⁺-metallopolymers. The luminescent changes distinguish AMP from the other phosphates, while the changes in water proton relaxation are greater for HPO₄²⁻ and ATP compared to AMP and ADP. This study explains this by interplay of the effects associated with the formation of ternary phosphate-lanthanide-polymer complexes and stripping of lanthanide ions from the polymer molecules.