3. Results and discussion
3.1. Visual observation and UVevisible spectroscopy
Noble metals are known to exhibit unique optical properties
due to the property of surface plasmon resonance (SPR)
(Bindhu & Umadevi, 2013). The formation of silver nanoparticles
was monitored with color change and UVeVis
spectroscopy. The color of the reaction mixture started
changing to yellowish brown within 10 min and to reddish
brown after 1 h, indicating the generation of silver nanoparticles,
due to the reduction of silver metal ions Agþ into
silver nanoparticles Ag via the active molecules present in
the BPE (Ahmad et al., 2003). This color is attributed to the
excitation of SPR. As shown in Fig. 1a, a characteristic and
well-defined SPR band for silver nanoparticles was obtained
at around l 433 nm (Mulvaney, 1996). Control silver nitrate
solution neither developed the reddish brown color nor did
they display the characteristic band, indicating that abiotic
reduction of silver nitrate did not occur under the used
conditions.