Zinc oxide nanoparticles was synthesized by alginate (A) through a rapid and a facile step in the aqueous solution condition at room temperature. Fabrication of zinc oxide nanoparticles was characterized by ATR-FTIR, TEM and XRD. ATR-FTIR analysis confirmed that the A/ZnO NPs were encapsulated by the polymerized alginate. Their shape, structure and composition were assessed by SEM. TEM and XRD analysis indicated that the A/ZnO NPs give evidence of the crystalline nature of ZnO and hybrid NPs structure, which is suitable for ammonia gas sensor development. The controlled size of the A/ZnO NPs obtained using this innovative synthesis strategy minimizes the response time of 2–3s to sense the ammonia gas significantly with a detection limit of 1ppm were found at room temperature. The antibacterial tests revealed that the A/ZnO NPs exhibits a potent activity against gram positive and gram negative bacteria.