A Fluorogenic Triphenyl‐Amine‐Naphthyl‐Hydrazide Probe Selective for Cu 2+ and Cysteine Detection via an ON‐OFF‐ON Logic path with Real Applications

1,1′‐((1E,1′E)‐((2E,2′E)(((phenylazanediyl)bis(4,1‐phenylene))bis‐(methanylylidene))bis‐ (hydrazine‐2,1‐diylidene))bis(methanylylidene))bis(naphthalen‐2‐ol), H 2 L shows high fluorescence emission (λ em , 550 nm; λ abs , 420 nm) which is selectively quenched by Cu 2+ in presence of many other biologically important ions. The limit of detection (LOD), 7.3 nM is much lower than WHO recommended maximum permissible limit (20 μM) of Cu 2+ consumption in human body. The binding fashion of the probe to Cu 2+ ion is 1 : 2 mole ratio and has been confirmed by Job's plot and ESI‐MS spectral data. The binding constant for Cu 2+ is 4.93×10 10  M −2 which also indicates sufficient stability and 1 : 2 complexation. On addition of cysteine to L‐Cu 2+ ensemble,the emission intensity (550 nm) enhances which accounts the release of Cu 2+ from the non‐emissive complex and restores the total fluorogenic efficiency of probe, H 2 L (LOD (Cysteine), 36 nM). Thus, ON‐OFF‐ON mechanism of H 2 L has been utilized for selective and specific detection of Cu 2+ and Cysteine over several other amino acids. In addition to this, the probe is treated on WI‐38 cell line to check cytotoxicity. This chemosensor, H 2 L has been successfully applied to examine intracellular trace quantity of Cu 2+ and Cysteine concentration in Hep G2 Cell lines.