Spacer dependency of the Hg2+-selective fluorescent signalling by desulfurization of thioamide as a turn-on switch

A series of simple thioamides with 7-nitrobenz-2-oxa-1,3-diazole (NBD) signalling moiety were prepared and their Hg2+-selective signalling behaviour was investigated. The signalling was due to the selective Hg2+-assisted desulfurization of thioamides to their corresponding amides analogue which resulted in the switching on of the fluorescence signalling by the NBD moiety. Sarcosine (N-methyl glycine)-based compound with a single methylene group as a spacer between the thioamide signalling switch and the NBD fluorophore exhibited a pronounced Hg2+-selective chemodosimetric behaviour in aqueous 50% DMSO solution. In contrast, ethylene- and propylene-spaced analogues exhibited much poorer selectivity towards Hg2+ ions and lower fluorescence enhancement, and the efficiency of photoelectron transfer between NBD fluorophore and thioamide switch was dramatically decreased as the spacer length increased. The sarcosine-based thioamide could be used as a probe for the selective determination of Hg2+ ions in aqueous solution with a detection limit of 2.7 × 10−6 M.