A Fluorescein-Based Probe for Selective Detection of ClO− and Resultant Mixture as a Fluorescence Sensor for Br− and I−

This paper presents the design and evaluation of a fluorescent probe based on fluorescein hydrazide for the selective detection of hypochlorite (ClO−), bromide (Br−), and iodide (I−) ions in solution. The starting chemosensor, fluorescein hydrazide, is suitable for detecting hypochlorite anions in solution, as observed for the first time. The Br− and I− ions could be discovered after activating the probe with hypochlorite. Upon interaction with ClO− ions, the proposed probe exhibits a significant increase in fluorescence emission, a sharp rise in absorbance, and a distinct color change, which is attributed to the conversion from the spirolactam closed form to the open form of the fluorescein ring. ClO− and Br− ions added together were found to brominate the probe in an acetonitrile–water mixture, resulting in a pronounced bathochromic shift in both absorption and emission spectra. Notably, the combination of ClO− and I− was more effective in cleaving the spirolactam ring than hypochlorite alone. Quantum chemical calculations were used to understand the detection mechanism of Br and I ions in a probe–hypochlorite mixture. The probe demonstrated exceptional selectivity and rapid response towards the target analytes, with detection limits determined to be 2.61 μM for ClO−, 66 nM for Br−, and 13 nM for I−. Furthermore, it successfully monitored fluctuations in ClO−, Br−, and I− concentrations within complex systems, highlighting its potential application in environmental and biological monitoring.