Highly efficient recyclable antibacterial magnetite nanoparticles consisting of a magnetic Fe3O4 core with an antibacterial poly(quaternary ammonium) (PQA) coating were prepared in an efficient four-step process. The synthetic pathway included: (1) preparation. of Fe3O4 nanoparticles via coprecipitation of Fe2+/Fe3+ in the presence of an alkaline solution; (2) attachment of an ATRP initiating functionality to the surface of the nanoparticles; (3) surface-initiated atom transfer radical polymerization (ATRP) of 2(dimethylamino)ethyl methacrylate (DMAEMA); and (4) transformation of PDMAEMA brushes to PQA via quaternization with ethyl bromide. The success of the surface functionalization was confirmed by FT-IR, thermal gravimetric analysis (TGA), elemental analysis, and transmission electron microscopy (TEM). The PQA-modified magnetite nanoparticles were dispersed in water and exhibited a response to an external magnetic field, making the nanoparticles easy to remove from water after antibacterial tests. The PQA-modified magnetite nanoparticles retained 100% biocidal efficiency against E. coli (10(5) to 10(6) E. coli/mg nanoparticles) during eight exposure/collect/recycle procedures without washing with any solvents or Water.