Description Application The iron oxide containing conjugated polymer nanoparticles are water-soluble nanoparticle compromising of light emitting polymer and iron oxide. These nanoparticles possess ′core-shell′ structure where light emitting polymer and iron oxide are encapsulated in biocompatible surfactant. The surfactant essentially increases hydrophilicity and allows formation of stable nanoparticles. The ′core-shell′ structure consisting of polymer and iron oxide in the core and the biocompatible surfactant in surrounding shell allows easy covalently bonding of these nanoparticles with biomolecules such streptavidin, antibodies, proteins, nucleic acids etc and make them suitable for applications in flow cytometry, western blotting, immunochemistry etc. The presence of iron oxide in core makes these fluorescent nanoparticles distinctly different than conventional dyes as it allows their manipulation with magnetic field and renders them suitable for applications in magnetic resonance imaging as well.