The present invention provides a process for preparing the nanoparticles. The process comprises first forming a water-in-oil emulsion from chitosan lactate, amoxicillin, dioctyl sodium sulfosuccinate, glutaraldehyde or bis[sulfosuccinimidyl] suberate, and oil, and sonicating the mixture of to form nanoparticles comprising chitosan crosslinked by dioctyl sodium sulfosuccinate and glutaraldehyde or by dioctyl sodium sulfosuccinate and bis[sulfosuccinimidyl] suberate, wherein the nanoparticles have an average diameter of 100-600 nm and have amoxicillin entrapped by the crosslinked chitosan. The present invention is also directed to nanoparticles comprising crosslinked chitosan and amoxicillin, wherein amoxicillin is entrapped by the crosslinked chitosan. The nanoparticles have an average diameter of 100-600 nm, and the entrapped amoxicillin is at least 5% (w/w) of the total weight nanoparticles.