Luminescent Eu(III) complexes as probes for citrate sensing in complex matrix.

Rationally designed Eu(III) complexes can be exploited as probes in the detection of important analytes in biological fluids, by means of luminescence.[1] These complexes must be stable in aqueous solution, absorb and efficiently transfer the UV excitation to the metal ion (antenna effect). We succeeded to obtain this goal by including isoquinoline antenna into the ligand backbone. In fact, the luminescence of [Eu(bisoQcd)]+ and Eu(isoQC3A) complexes significantly increases in the presence of the main analytes which constitute the interstitial extracellular fluid (i.e. hydrogen carbonate, serum albumin (SA) and citrate at their typical concentration).[2,3] The optical response of our Eu(III)-based probes is selective towards citrate molecule, when a complex matrix is considered. Citrate molecule is a very important bio-analyte and since it is involved in many metabolic pathway, the monitoring of its concentration is crucial in order to identify the presence of metabolic disease.[4] When a simulated interstitial extracellular fluid is employed, the change of the citrate concentration gives rise to an increase of the Eu(III) luminescence emission intensity of [Eu(bisoQcd)]+. On the other hand, negligible or no change of the luminescence intensity was detected when the concentration of both hydrogen carbonate and SA is changed. Such a finding opens up the possibility to determine quantitatively the citrate anion in complex matrices up to concentrations of 500 μM. Actually, all the main analytes present in this biological fluid show a similar affinity for the Eu(III) complexes and directly interact with the metal ion displacing a variable number of water molecules from the inner coordination sphere. As well known, this phenomenon produces a peculiar increase of the luminescence intensity stemming from Eu(III). Differently than hydrogen carbonate and SA, we demonstrate that citrate is capable to displace a higher number of water molecules giving a distinctive increase of the luminescence intensity. Moreover, the Eu(III) intrinsic quantum yield of the adduct is higher. The obtained results candidate our europium complexes as viable probes for luminescence analysis of biofluids with complex matrix. Acknowledgments: The authors thank the Italian Ministry of education, university and research for the received funds (PRIN (Progetti di Ricerca di Rilevante Interesse Nazionale) project "CHIRALAB", grant n. 20172M3K5N). References: [1] J.-C.G. Bünzli, Chem. Rev. 2010, 110 (5), 2729-2755. [2] C. De Rosa, A. Melchior, M. Sanadar, M. Tolazzi, A. Giorgetti, R. P. Ribeiro, C. Nardon, F. Piccinelli, Inorg Chem. 2020, 59 (17), 12564-12577. [3] C. De Rosa, A. Melchior, M. Sanadar, M. Tolazzi, A. Duerkop, F. Piccinelli, Dalton Trans. 2021, 50 (13), 4700-4712. [4] L. C. Costello, R. B. Franklin, Prostate Cancer Prostatic Dis. 2009, 12 (1), 17-24.