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)-4H-chromen-4-one (BTC) has been rationally designed and synthesized. BTC can subsequently detect Ga³⁺ and PPi ions through the absorption and emission off-on-off response with high specificity. Importantly, fluorescent probe BTC can well discriminate Ga³⁺ from Al³⁺ and In³⁺. The association constant (K) was calculated as 2.06 × 10⁴M^-1, and the limit of detection (LOD) was calculated as 4.88 × 10^-2μM. Competitive binding studies also illustrated good results of the probe BTC towards Ga³⁺. Job's plot and HRMS results substantiated the 1:1 stoichiometry between BTC and Ga³⁺ ion. The interaction binding mode of BTC with Ga³⁺ was proposed by HRMS, ¹H 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-Ga³⁺ could further monitor pyrophosphate ion (PPi) by demetallization process with quenching fluorescence emission. Additionally, the BTC and BTC-Ga³⁺ showed good cell permeability and could detect Ga³⁺ and PPi ions in onioninner epidermal cells, respectively.