Diaminomaleonitrile (DAMN)-Based Salen-Type Cobalt Complexes as Cyclable Sensors for H₂S/HS^-: Biological Applications

In the current contribution, the synthesis and characterization of new diaminomaleonitrile-based salen cobalt complexes L₁Co and L₂Co (L₁-H = 2,3-bis[[(2,4-dihydroxy-phenyl)(methylene)]amino]-2-butenedinitrile, L₂-H = 2,3-bis[[(2-hydroxy-4-(diethylamino)phenyl)(methylene)]amino]-2-butenedinitrile) are reported. Structural analysis and electronic properties of the complexes were assessed, providing a basis for their application as H₂S sensors. Spectroscopic investigations revealed an enhancement in the fluorescence intensities upon H₂S addition. A reversible fluorescence response was observed when the systems were shifted from H₂S-rich to oxygenated environments, demonstrating their reusability and adaptability. To further establish the mechanism of interaction of the HS^- with the cobalt complexes, electrochemical analysis was conducted, providing evidence of HS^- coordination to the cobalt centers. Moreover, biological assessment of the sensors was performed in HepG2 cells, revealing their low cytotoxicity and efficient cell permeability. Fluorescence imaging experiments provided evidence of intracellular detection of exogenous H₂S. The reversibility of the sensing mechanism was confirmed in living cells: subsequent addition of HS^- and oxygen-saturated buffer to the cells tunes their fluorescence accordingly, in a cyclable manner, as in the experiments performed in vitro. This study provides evidence that cobalt-based DAMN complexes work as robust, reversible, and selective detection systems for H₂S, demonstrating their suitability for applications in both biological and environmental contexts.