We describe an optomagnetic bionanotechnology for rapid and sensitive solution-based affinity assays. Nanoactuators made from bioactive magnetic nanoparticles undergo rotational motion in the volume of a fluid under frequency-controlled magnetic actuation. The nanoactuators show a time-dependent scattering cross-section to an incoming light beam. We demonstrate that the temporal behavior of the scattered light intensity relates to the number, the magnetic properties and the size distribution of the nanoactuators, independently revealing the average value and variation in the magnetic properties of the nanoparticles as well as the concentration of nanoactuators. The method is applied to detect biomolecules in fluid by interparticle binding. In a total assay time of less than 3 min, we demonstrate a limit of detection lower than 400 fM in buffer and 5 pM in human plasma.