A pyrazole derivative-functionalized Fe3O4@SiO2 fluorescent probe was designed and its application for detection of Hg2+ in aqueous solutions was investigated. The sample was characterized by using transmission electron microscopy, Fourier transform infrared spectroscopy, vibrating sample magnetometer, and thermogravimetric-differential. Different cations were investigated against the newly prepared probe and the results showed that Hg2+ was the one with the most fluorescence quenching ability. The sensitivity of the probe was determined and it was shown that the new probe could detect Hg2+ with the detection limit of 7.6 nM. The prepared nanocomposites were easily separated from aqueous solution by an external magnet. Theoretical calculations were used for estimating the most stable bonding mode of the ligand with mercury ion. (C) 2017 Elsevier B.V. All rights reserved.