New magnetic hybrid nanoparticles of Fe3O4 coated with organosulfur-squaramide compounds are prepared. The modified-nanoparticles show a good coordination for Ag+ and Hg2+ cations in water, and present a high affinity for Au3+ ions. The behaviour of the squaramide-coated nanoparticles differs significantly from that previously reported for nanoparticles used as Au3+ scavengers. In the presence of organosulfur-squaramide, the Au3+ salt is reduced to gold nanoparticles that are deposited upon Fe3O4 nanoparticles. For the first time, the coordination capacity of the carbonyl squaramide groups with the gold cation, based on purely electrostatic cation-dipole interactions, is proved.