Fluorescein hydrazide‐appended metal–organic framework as a chromogenic and fluorogenic chemosensor for mercury ions

In this work, we prepared a fluorescein hydrazide‐appended Ni(MOF) (Metal–Organic Framework) [Ni3(BTC)2(H2O)3].(DMF)3(H2O)3 composite, FH@Ni(MOF). This composite was well-characterized by PXRD (powder X‐ray diffraction), FT‐IR (Fourier transform infrared spectroscopy), N2 adsorption isotherm, TGA (thermogravimetric analysis), XPS (X‐ray photoelectron spectroscopy), and FESEM (field emission scanning electron microscopy). This composite was then tested with different heavy metals and was found to act as a highly selective and sensitive optical sensor for the Hg²⁺ ion. It was found that the aqueous emulsion of this composite produces a new peak in absorption at 583 nm, with a chromogenic change to a pink color visible to the naked eye upon binding with Hg²⁺ ions. In emission, it enhances fluorescence with a fluorogenic change to green fluorescence upon complexation with the Hg²⁺ ion. The binding constant was found to be 9.4 × 10⁵ M⁻¹, with a detection limit of 0.02 μM or 5 ppb. This sensor was also found to be reversible and could be used for seven consecutive cycles. It was also tested for Hg²⁺ ion detection in practical water samples from ground water, tap water, and drinking water.