Magnetite and metallic cobalt-based nanoparticles with sizes ranging from 10 to 300 nm and surface-functionalized with poly(hexamethylene biguanide) (PHMBG) are introduced as capable lipopolysaccharide (LPS)-sequestering agents. The nanoparticles efficiently bind to whole E. coil cells and can be used to separate the cells effectively from suspension using a magnet. A fluorescence dye displacement assay shows strong affinities of the nanoparticles for lipid A. the glycolipid component of LPS responsible for septic shock. The particle-lipid A affinity is of the same order of magnitude or higher than that of polymyxin B. The affinity of smaller (< 50 nm) magnetite particles modified with PHMBG to lipid A is several-fold higher than that of their larger counterparts (> 100 nm) due to their higher surface area to volume ratio. The nanoparticles possess high saturation capacity for double-tranded lambda DNA from E. coli, with which particle polyelectrolyte complexes are formed. The PHMBG-modified nanoparticles are potent bactericides, inhibiting E. coli viability and growth at concentrations at <= 10 mu g/mL.