A system for measuring analyte concentrations has porous-walled nanocontainers containing multiple magnetic nanoparticles, the magnetic nanoparticles coated with a selective binder that is analyte-responsive and binds a the analyte, an indicator substance releasable from the selective binder by the analyte, or an indicator substance cleavable by the analyte, apparatus for exposing the nanocontainers to a fluid potentially containing the analyte, and magnetic spectroscopy of Brownian motion sensing apparatus for detecting agglutination of the nanoparticles or binding of analyte to the nanoparticles. The system is used in a method comprising coating magnetic nanoparticles with a selective binder, encapsulating the magnetic nanoparticles in porous nanocontainers, exposing the nanocontainers to a fluid potentially containing analyte, using magnetic spectroscopy of Brownian motion sensing apparatus to detect agglutination or binding of the nanoparticles, and translating Brownian motion spectra to analyte concentrations.