“…Fluorescein hydrazones can be obtained with high yields and without generating by-products, making them a valuable asset for optical chemosensors. Recently, numerous fluorescent chemosensors for ions such as Cu 2+ [ 35 , 36 , 37 ], Ni 2+ and Al 3+ [ 38 ], Zn 2+ [ 39 ], Cd 2+ [ 40 ], and Hg 2+ [ 27 , 28 , 29 ] were developed based on fluorescein. As for Hg 2+ chemosensors, 7-hydroxy-4-methylcoumarin-8-carbaldehyde [ 27 ], (pyridin-2-ylmethoxy)-naphthalene-1-carbaldehyde [ 28 ], and thiophene-2-aldehyde [ 29 ] were used as receptors.…”