Thiadiazole Functionalized Salicylaldehyde-Schiff Base as a pH-responsive and chemo-reversible "Turn-Off" fluorescent...

A novel thiadiazole functionalized schiff base chemoreceptor (E)-2,4-dichloro-6-(((5-mercapto-1,3,4-thiadiazol-2-yl)imino)methyl)phenol (SB-1) has been synthesized and characterized spectroscopically by using various techniques. Its photophysical behaviour was scanned towards a variety of metal ions in mixed aqueous media. The chemosensor (SB-1) displayed excellent selectivity towards Cu²⁺ ion through fluorescent diminishment (turn-off phenomenon). Colorimetric analyses showed a rapid colour change from yellow to dark red under visible light upon addition of Cu²⁺ ions. Interestingly, the original yellow colour reappeared back instantly after the addition of EDTA^2- anions, thus confirming the reversible nature of SB-1. Competitive experiments validated no interference from the other co-existing metal ions in the recognition process of SB-1 towards Cu²⁺ ion. Job's plot confirmed 1:1 binding stoichiometry between SB-1 and Cu²⁺ ion with the binding constant value of 3.87 × 10⁴ M^- 1. The limit of detection was determined to be 1.01 × 10^- 7 M suggesting good sensitivity of SB-1 towards Cu²⁺ ions. Furthermore, pH-dependent UV-Vis spectral behaviour of SB-1 confirmed that it could act as an effective optical pH-sensor for highly acidic environment as well. Portable nature of probe SB-1 was explored by fabricating "easy-to-use" paper test strips, which allow robust and rapid detection of Cu²⁺ ions. Based on the multi-responsive properties of SB-1, a 'NOR' logic gate was constructed by applying Cu²⁺ and EDTA^2- as chemical inputs (ln1: Cu²⁺, ln2: EDTA^2-) while emission intensity observed at 560 nm was considered as output signal (O1). DFT optimized geometries confirmed that chemosensor SB-1 exists in Azo form (Enol form) in its ground state. Molecular docking of the SB-1 and its copper complex, into the binding site of TRK protein tyrosine kinase (PDB: 1t46) was also carried out to explore their biological activity and their potential use as TRK inhibitors.