An efficient coumarin-based thiosemicarbazone probe for ratiometric fluorescent sensing of Zn²⁺ and application in...

Selective recognition of Zn²⁺ in biological and environmental samples is a challenge. An N, N-diethyl substituted coumarin thiosemicarbazone was designed as a fluorogenic sensor, HADICIT, [(E)-2-(1-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)ethylidene)-N-isopropylhy drazine-1-carbothioamide] for the detection of Zn²⁺ to address the issue. The compound was characterized using FTIR, ¹H NMR, ¹³C NMR, and mass spectrometry methods. The fluorogenic probe produces a selectively enhanced red-shift emission response in solution upon the addition of Zn²⁺ compared to other competing metal ions. The optical and fluorescence titration experiments suggest a 2:1 binding ratio of HADICIT to Zn²⁺, with an association constant of 3.33 × 10⁶ M^-1. The limit of detection (LoD) of HADICIT is found to be 86 nM, which is much below the WHO recommended standards in environment samples. Frontier molecular orbital calculations suggest intra-ligand charge transfer (ILCT) transitions. Cell imaging on HEK 293 cells using HADICIT indicates that the probe can detect Zn²⁺ ions in live cells.