A Convenient and Portable Turn-On Fluorescent Probe for Reliable Detection of Hypochlorite: Smartphone-Based Application

A convenient and portable turn-on fluorescent probe, MITC ((E)-N′-((1-methyl-1H-indol-3-yl)methylene)thiophene-2-carbohydrazide), was synthesized for selective and practical detection of ClO⁻. MITC selectively detected ClO⁻ in water-containing environments through a response that exhibited distinct blue fluorescence. The limit of detection of MITC to ClO⁻ was 7.94 μM, which was significantly below the World Health Organization (WHO) standard (5 mg/L; 70.5 μM) for free chlorine in water. MITC exhibited effective detection ability over a wide pH range of 6–11. Test strip and smartphone-based detection tools using MITC offered the advantage of easy and convenient detection. Notably, MITC showed reliable recoveries for the quantification of ClO⁻ in real water samples (tap, drinking, river, and sea water) using the smartphone-based detection method. The mechanism for ClO⁻ detection by MITC was presumed to be a cleavage process through ESI-MS, NMR titrations, and density functional theory (DFT) calculations.