Single-chain Janus nanoparticle of the example polymer polystyrene-block-poly(2-vinylpyridine)-block-polyethylene oxide (PS-b-P2VP-b-PEO) was achieved by intramolecular crosslinking via complexation of P2VP with cobalt octacarbonyl, while the metallic complex was incorporated within the crosslinked core. It is crucial to introduce electrostatic interaction along the chain by pretreatment and thus ensure the intramolecular crosslinking in concentrated solutions, for example, 30 mg/mL. After thermolysis in the dispersion, the complex is decomposed achieving cobalt within the P2VP core. A magnetic responsive single-chain Janus composite nanoparticle was derived with the P2VP/Co core conjugated with PS and PEO single chains onto the opposite sides. The Janus composite nanoparticle displays highly catalytic activity and magnetic responsive performance from the metallic cobalt and interfacial activity from the PS and PEO single chains. A highly efficient catalytic reduction of nitroaromatic compounds at emulsion interfaces was demonstrated thereby, which was easily recycled with a magnet.