A novel triphenylamine based push–pull fluorophore bearing a 2-thiohydantoin unit for toxic Hg2+ ion detection: exploring its potential for live cell imaging

Abstract

A novel fluorescent chemosensor (TPA-2TH) containing triphenylamine (TPA) appended in conjugation with 2-thiohydantoin (2TH) is developed and confirmed by various analytical techniques like FT-IR spectroscopy, NMR spectroscopy, HR-MS and single crystal X-ray diffraction (SC-XRD). The crystal structure analysis revealed a propeller-shaped TPA unit and a co-facial herringbone packing arrangement stabilized by N–H⋯S, N–H⋯O and C–H⋯O in the crystal lattice. TPA-2TH exhibited a strong absorption band at ∼431 nm and emitted a yellow-green fluorescence at λ_max = 522 nm in THF, attributed to an efficient charge-separated state resulting from ICT between TPA and 2TH. A solvatochromic effect was observed with a red shift in the emission wavelength as the polarity of the solvent increased. TPA-2TH showed unprecedented selectivity with turn-off fluorescence upon complexation with diamagnetic Hg²⁺ ions, attributed to an increase in the spin–orbit coupling and a weakened ICT effect caused by Hg²⁺ binding. The addition of Hg²⁺ to TPA-2TH resulted in the formation of a 2 : 1 complex (TPA-2TH–Hg²⁺), confirmed by the coordination of a sulphur atom of TPA-2TH with Hg²⁺ ions, as evidenced by ¹H NMR spectroscopy and Job's plot analysis. A linear relationship between the fluorescence intensity of TPA-2TH and the concentration of Hg²⁺ ions is observed with a LOD of 3.3 pM. Furthermore, the toxicological effect of TPA-2TH is explored in zebrafish larvae and embryos to provide a sensitive bio-indicator for metal ion contamination and fluorescence imaging. This comprehensive approach combining fluorescence quenching and behavioural toxicity assessment offers a practical method for evaluating metal ion contamination and its potential toxic effects on aquatic organisms with acute behavioural toxicity end points using adult zebrafish.