Green synthesis of silver nanoparticles using the seaweed Gracilaria birdiae and their antibacterial activity

This work presents a simple method for the green synthesis of silver nanoparticles (AgNPs) using as reducing and stabilizing agent a polysaccharide extracted from red algae Gracilaria birdiae present in the coast of Piauí. The AgNPS were prepared using three polysaccharide concentrations (0.02, 0.03 and 0.05% v/v) and two pHs (10 and 11) at stirring for 30 min at 90 °C. The formation of silver nanoparticles was monitored by measurements of UV–vis and FTIR and characterized by size and zeta potential measurements using DLS and morphologically by TEM. The UV–vis absorption spectrum showed the surface plasmon peak at 410 nm, which is characteristic peak of silver nanoparticles. The functional biomolecules present in the polysaccharide and the interaction between the nanoparticles were identified by the Fourier transform infrared spectroscopy (FTIR) analysis. The stability of the synthesized silver nanoparticles was analyzed during four months and no significant agglomeration was observed. The hydrodynamic diameter of the AgNPs varied between 20.2 nm and 94.9 nm. The AgNPs were tested for antimicrobial activity using Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) and all samples showed antimicrobial activity against E. coli. Using an environment-friendly, the AgNPs were synthesized in a simple, rapid and one-step process using natural sources as red algae with favorable characteristics such as spherical shape, small size and zeta potential negative. The results suggest that the polysaccharide mediated synthesized silver nanoparticles could be used as a model for future projects of nano-medicines or drug delivery systems.