Plant extracts have also been used for Ag nanoparticle
synthesis. The reactions have generally been carried out at
room temperature, although in some processes a heating
was applied, mainly below 100 8C. For example, silver
nanoparticles have been synthesized (at room temperature
and 60 8C) using Polyalthia longifolia leaf extract as a
reducing and capping agent along with D-sorbitol used
to increase the stability of the nanoparticles [50]. The
synthesized silver nanoparticles were more highly toxic
to Gram-positive bacteria than Gram-negative bacteria. In
addition, silver nanoparticles have been synthesized using
sulfated polysaccharide isolated from marine red algae
(Porphyra vietnamensis) [51]. The resulting silver nanoparticles
showed surface plasmon resonance centered at
404 nm with average particle size measured to be 13
3 nm. The involvement of the sulfate moiety of the polysaccharide
for reduction of silver nitrate was revealed
spectroscopically.