Bioinspired self-healing luminescent lanthanide bipyridinedicarboxiamide complexes

Polymers with photoluminescent centers in their structure are of great interest in the field of bioimaging and could be artificial analogs to green fluorescent protein (GFP) from jellyfish [1]. Along with intrinsic photoluminescence, jellyfish also demonstrate self-healing of their organs, which allow the creatures to survive in aquatic environments [2]. Polymer metal complexes (PMCs) of europium(III) and terbium(III) could be artificial analogs to GFP due to their excellent luminescent properties (Tb³⁺ and Eu³⁺) and to the bioinertness of PDMS.

Europium(III) and terbium(III)-containing 2,2'-bipyridine-6,6'-dicarboxamide-co-polydimethylsiloxanes (Eu-Bipy-PDMS and Tb-Bipy-PDMS) [3] and their low-molecular complexes [Tb(BDCA)₂(H₂O)]Cl₃ and [Eu(BDCA)₂(H₂O)]Cl₃ [4] were synthetized by polycondensation and complexation reactions. The structure of the obtained complexes was confirmed by NMR, IR spectroscopy, and XRD analysis. A tensile property study was carried out on a Shimadzu EZ-L-5kN testing machine (RT, constant stretching rate of 10 mm∙min^–1, sample shape ISO 37 type 3). Photoluminescence spectra and quantum yields (QYs) were studied using a HORIBA Fluorolog-3 spectrofluorometer with an integrating sphere (101 mm in diameter) at RT.

Eu-Bipy-PDMS and Tb-Bipy-PDMS show QYs of 10.5% and 18.5%. The PMCs' structure enables the formation of coordinatively saturated complexes of lanthanide ions and provides good tensile properties to Eu-Bipy-PDMS (1.55 MPa, 185%) and Tb-Bipy-PDMS (1.48 MPa, 190%). The self-healing efficiency of PMCs exceed 90%. [Tb(BDCA)₂(H₂O)]Cl₃ and [Eu(BDCA)₂(H₂O)]Cl₃ show high QYs of 36.5% and 12.6%, respectively, and can retain them after encapsulation in a semitransparent biocompatible polyethyleneglycol matrix (11.2% and 25.3%, respectively) .

Both obtained self-healing luminescent lanthanide-containing PMCs and their low-molecular analogs could be used in bioimaging and theranostics [5].

Financial support for this study was provided by St. Petersburg State University (project 94124215).

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