Compared with micrometer chitosan (CS) particles, the adsorption performance could be greatly improved with nanometer chitosan particles. In this work, we present a new W/O nanoemulsion system with Polyethylene glycol octylphenyl ether (Triton X-100) as a surfactant, cyclohexane as the oil phase, n-hexanol as a cosurfactant and dilute acetic acid solution containing chitosan as the aqueous phase. The droplet size of the W/O nanoemulsion was determined using a dynamic light-scattering method. By adding glutaraldehyde or sodium hydroxide (NaOH) solution into the nanoemulsion system, the chitosan polymer was solidified and the nanoparticles could form from the nanoemulsion system. Then, the adsorption capacity of the particles to acidic diuretics furosemide was explored. The nanoemulsion could be formed when the molecular weight of chitosan changed from 105 to 1.5 x 106, and the droplet diameter was in the range from 170 to 550 nm. The size of the nanoparticles prepared with NaOH as the precipitator was well dispersed and ranged from 30 to 150 nm in diameter, while 22 nm on average was prepared with glutaraldehyde as the cross-linking agent. Compared with that of the chitosan particles in micrometer size, the adsorption equilibrium of the nanoparticles was achieved much faster, and the adsorption loading highly increased, the equilibrium could be achieved in 10 min, and the maximum adsorption loading of the particles prepared by NaOH could reach 270 mg/g, while only 87 mg/g using about 60 min to reach equilibrium for the particles with micrometer size. (c) 2005 Elsevier B.V. All rights reserved.