Bimetallic Cu-BHQT receptor for selective cysteine detection in extracellular environments via fluorescence quenching-recovery

Abstract

Amino thiols play critical roles in biological redox regulations, though, the abnormal cysteine (Cys) levels are associated with the onset of several pathological conditions. In particular, elevated blood cysteine concentrations (>300 µM) demand sensitive and selective detection methods for early clinical monitoring. Herein, we report a novel fluorescent probe, BHQT, featuring an isoquinoline fluorophore linked to a hydrazine-based metal-chelating unit through a benzyl spacer. The fluorescence of BHQT is selectively quenched upon coordination with Cu²⁺ ions, forming a 1:2 BHQT–Cu²⁺ complex as confirmed by Job’s plot and ESI-MS analysis. It exhibits a binding constant of 1.19 × 10⁻⁶ M and a Cu²⁺ detection limit of 4.5 × 10⁻⁴ M. Notably, this probe, BHQT–Cu²⁺ displays a distinct “off–on” fluorescence response toward cysteine, arising from Cu²⁺ displacement and subsequent Cu–Cys complex formation. This mechanism enables selective cysteine detection with an approximate detection limit of 419 µM, while showing minimal interference from other amino acids and biologically relevant species. The reversible sensing behavior and high selectivity of BHQT highlights its potential as an effective fluorescent probe for extracellular cysteine monitoring and clinically relevant analytical applications.