A ready-for-use metal reinforcer, for example, of the wire or cord type, made of brass-coated carbon steel, is capable of adhering directly by vulcanization to a matrix of unsaturated rubber such as natural rubber. The surface of the reinforcer is provided with nanoparticles of at least one sulfide of a metal chosen from cobalt, copper, iron, zinc and the alloys comprising at least one of these elements. Such a reinforcer can be used as the reinforcing element of a finished article made of rubber, such as a tire.