Thiadiazole Functionalized Salicylaldehyde-Schiff Base as a pH-responsive and chemo-reversible "Turn-Off" fluorescent probe for selective cu (II) detection: Logic Gate Behaviour and Molecular Docking Studies.

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 Cu 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 Cu ions. Interestingly, the original yellow colour reappeared back instantly after the addition of EDTA 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 Cu ion. Job's plot confirmed 1:1 binding stoichiometry between SB-1 and Cu ion with the binding constant value of 3.87 × 10 M. The limit of detection was determined to be 1.01 × 10 M suggesting good sensitivity of SB-1 towards Cu 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 Cu ions. Based on the multi-responsive properties of SB-1, a 'NOR' logic gate was constructed by applying Cu and EDTA as chemical inputs (ln1: Cu, ln2: EDTA) 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.