Abstract: The system disclosed comprises an apparatus and a method for the detection and quantification of targeted molecules in vivo. The apparatus comprises an implantable biosensor and an AC magnetic detection device. The implantable biosensor includes functionalized nanoparticles functionalized with one or more moieties that bind to a molecular target of interest. The nanoparticles are retained in a biocompatible container which allows the molecular target of interest to enter the biosensor, for example through a semipermeable port. The biosensor can be implanted minimally-invasively into humans or animals. Upon exposure of the nanoparticles to the molecular target, a change in Neel relaxation time can be externally detected and correlated to the target analytes concentration.