Rational design and evaluation of the sensing mechanism of a europium(III)-based luminescent turn-ON chemosensor for citrate

Abstract

Citrate plays a multifaceted and crucial role as a key intermediate in the Krebs cycle (citric acid cycle or TCA cycle), the central metabolic pathway for energy production or a source of ATP in aerobic organisms. Besides a metabolic regulator, dysregulation of citrate levels is intricately linked to the pathology of multiple diseases, including metabolism in cancer cells. Abnormal citrate concentrations have been associated with kidney stones, inflammation, metabolic disorders, cancers, non-alcoholic fatty liver diseases, neurological disorders, etc., highlighting its diagnostic and prognostic value. Therefore, the selective recognition and monitoring of changes in the citrate levels is useful for advancing our understanding of these diseases, enabling early diagnosis, and guiding effective therapeutic strategies. Developing a selective receptor for citrate is challenging due to its close structural resemblance to competing biological anions (PO43-, NO3-, tartrate, HCO3-, etc.) and its lower levels than other common anions in biological media. The uniquely attractive optical properties of luminescent Ln(III) probes, narrow emission bands, photobleaching resistance, photostability, especially long luminescence lifetime, and hard oxophillic nature of tripositive Ln(III) ions, make them ideal for time-gated luminescence measurements for trinegative hard [citrate]3- ions with enhanced S/N ratio and minimized scattering and autofluorescence from the media. We report here the rationale and fundamental design principles for an emissive Eu(III)-probe: [Eu(EDTA3AQ)(H2O)3]Cl (Eu.1), containing EDTA-bisamide-linked to two 3-aminoquinolines as antennae for the selective sensing of citrate3- as an important regulatory metabolite at physiological pH using highly sensitive and selective time-resolved luminescence (TRL) from Eu.1. The facile displacement of three labile inner-sphere H2O molecules and coordinative unsaturation at Eu(III) centre in Eu.1 satisfying the steric demand for preferential 1:1 binding of [citrate]3- at Eu(III) are found to be important design criteria, which were comprehensively studied using various solution-based spectroscopic studies and DFT. The [EuL-citrate] complexation, solution speciation, effects of variation of coordination number of Eu(III), and change in hydration numbers (q) were studied and validated to gain an insightful correlation of the rationale of Eu.1 probe design based on luminescence sensing mechanism of Eu.1 for citrate. The strong electrostatic binding of [citrate]3- via displacing H2Os in the 1st-sphere in Eu.1 suppresses the nonradiative vibrational energy transfer (VET), resulting in enhanced TRL from the f-f transitions, and thus acts as a reversible, selective and sensitive turn-ON sensor at the ppb level of citrate using both the TRL intensity and lifetime-based modalities.