Highly Selective and Sensitive Detection of Hg(II) from HgCl2 by a Simple Rhodamine-Based Fluorescent Sensor

N-acryloyl rhodamine B hydrazide, a non-responsive control of a colorimetric Cu2+ sensor, was used as a turn-on fluorescent sensor for Hg(II) from HgCl2 in the presence of AgNO3. The detection was highly selective and sensitive, and a large number of environmentally and biologically relevant metal ions, such as Na+, K+, Ca2+, Mg2+, Fe3+, Cu2+, Zn2+, Cr3+, Pb2+, Ni2+, Fe2+, Mn2+, Co2+, Cd2+, including Hg(II) from the easy dissociated salts, did not show significant interference. The fluorescence of the sensor (10 μM) was enhanced 74 folds by 10 equiv. of Hg(II) from HgCl2 in THF/HAc-NaAc (1/1, v/v, pH = 6) aqueous buffer solution containing 20 equiv. of AgNO3. The maximal fluorescence intensity increased linearly with the concentration of Hg(II) in the range of 0–70 μM. The detection limit of Hg(II) was 0.59 μM. The sensing mechanism was explored by Job’s plot experiment, reversible experiment, mass spectrum analysis, spectroscopic analysis, and thin-layer chromatography.