BODIPY-Hg²⁺ Complex: A Fluorescence "Turn-ON" Sensor for Cysteine Detection

A BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) based pioneering sensing material (HL^Py) having 2-amino pyridine as receptor was synthesized and used for the selective detection of Hg²⁺ ions. The synthesized HL^Py features a high affinity towards Hg²⁺ (k_a = 2.04 × 10⁵ M^-1), accompanied by effective quenching of fluorescence in DMF:H₂O (1:9 v/v, 10 mM HEPES buffer, pH 7.4) with 54 nM limit of detection (LOD). The emission titration experiments (Job's plot) in the presence of varying mole-fraction of Hg²⁺ ions reveals the formation of non-fluorescent 2:1 coordination complex [Hg(L^Py)₂]. The resulting non-fluorescent [Hg(L^Py)₂] was thoroughly characterized using various spectroscopic techniques and analyses. Interestingly, the non-fluorescent complex [Hg(L^Py)₂] is able to specifically respond towards Cys over other biothiols and amino acids through a reversible de-complexation mechanism. As a result, the remarkable recovery of the fluorescence can be observed. The limit of detection (LOD) for Cys detection is estimated to be 29 nM in DMF:H₂O (1:9 v/v, 10 mM HEPES buffer, pH 8.0). The reversibility and reusability of [Hg(L^Py)₂] were achieved by the sequential addition of Cys and Hg²⁺ ions up to five cycles. Moreover, the removal of Hg²⁺ ions up to 89% from aqueous samples using HL^Py was successfully demonstrated.