A novel hydrazone-based fluorescent “off-on-off” probe for relay sensing of Ga3+ and PPi ions

A novel hydrazone-based fluorescent probe ( E )-3-((2-(benzo[ d ]thiazol-2-yl)hydrazono)methyl)-4 H -chromen-4-one ( BTC ) has been rationally designed and synthesized. BTC can subsequently detect Ga 3+ and PPi ions through the absorption and emission off–on-off response with high specificity. Importantly, fluorescent probe BTC can well discriminate Ga 3+ from Al 3+ and In 3+ . The association constant (K) was calculated as 2.06 × 10 4   M −1 , and the limit of detection (LOD) was calculated as 4.88 × 10 -2   μM. Competitive binding studies also illustrated good results of the probe BTC towards Ga 3+ . Job's plot and HRMS results substantiated the 1:1 stoichiometry between BTC and Ga 3+ ion. The interaction binding mode of BTC with Ga 3+ was proposed by HRMS, 1 H NMR spectral titration, UV–vis absorption and fluorescence spectral measurements. The combination of the restraint of the photo-induced electron transfer (PET) process and the chelation enhanced fluorescence (CHEF) process is responsible for the fluorescence enhancement of this probe. The in situ chelated BTC -Ga 3+ could further monitor pyrophosphate ion (PPi) by demetallization process with quenching fluorescence emission. Additionally, the BTC and BTC -Ga 3+ showed good cell permeability and could detect Ga 3+ and PPi ions in onion   inner epidermal cells, respectively.