Highlights
► Alginate biopolymer is used as controlled environment for preparation of ZnO nanoparticles. ► A novel synthetic procedure is introduced for preparation of ZnO–alginate nanocomposites. ► The obtained ZnO–alginate nanocomposites exhibit strong antimicrobial activity. ► The antimicrobial activity was found to depend slightly on the preparation conditions. ► XPS spectroscopy was used to study the defects on the surface of ZnO nanoparticles.