Abstract: Wearable devices configured to detect the presence, concentration, number, or other properties of magnetic nanoparticles disposed in subsurface vasculature of a person are provided. The wearable devices are configured to detect, using one or more magnetometers, magnetic fields produced by the magnetic nanoparticles. In some embodiments, the magnetometer(s) are atomic magnetometers. In some embodiments, the wearable devices include magnets or other means to magnetize the magnetic nanoparticles. In some embodiments, the wearable devices produce a time-varying magnetic field, and the magnetometer(s) are configured to detect a time-varying magnetic field responsively produced by the magnetic nanoparticles. In some embodiments, the magnetic nanoparticles are configured to bind to an analyte of interest and detected properties of the magnetic nanoparticles can be used to determine the presence, concentration, or other properties of the analyte.