Magnetic nanoparticles (MNP) assume a highlighted position among nanostructures due to their outstanding properties and consequently promising applications in research fields like analytical chemistry, magnetic resonance imaging, catalysis and electronics. The current limitation is the difficulty in the reproduction of coated MNPs with constant surface properties and sizes. The coating steps (hydrophilic shells, linkage of biocompatible polymers and conjugation of selective receptors) are essential for the applications and still a challenge for the industrial use. In this context, this project aims to overcome the current obstacles of MNP application by developing multistep microreactors, which are combined in-line with nanoparticle characterization methods. The microfluidic reactors exploit fluid mechanics to create coated nanoparticles with a narrow size range and offer a finely controllable route to tune the shape and composition of the nanomaterial surface. The integrated characterization methods provide the particle size by using dynamic light scattering (DLS), the electrostatic stability by zeta potential analysis and the number of particles through the use of nuclear magnetic resonance (NMR) particle counter. For the first time, a device will provide controlled coated MNP synthesis combined with quality control check and completely new counter system based on NMR. The characterized MNPs will be applied with international partners to MNP-based chemiluminescence microarray immunoassays, electrochemical immunosensors and pathogen assays.