An NBD-based fluorescent and colorimetric chemosensor for detecting S²⁻: Practical application to zebrafish and water samples

A novel 7-nitro-1,2,3-benzoxadiazole (NBD)-based chemosensor BOP ((5-bromopyridin-2-yl)(4-(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)piperazin-1-yl)methanone) was synthesized. BOP could detect S²⁻ through fluorescent quenching and colorimetric change. The detection limit was calculated to be 10.9 µM through fluorescence titration. The reaction mechanism of BOP towards S²⁻ was estimated to be thiolysis of NBD amine, producing the cleavage products, NBD-S⁻ and BP ((5-bromopyridin-2-yl)(piperazin-1-yl)methanone). The thiolysis was demonstrated by ¹H NMR titrations, ESI-mass analysis and theoretical calculations. Importantly, BOP was able to successfully monitor S²⁻ in zebrafish and water samples. Additionally, test strips coated with BOP were applied to the in-the-field measurements of S²⁻.