3.4. Effect of pH
The color of reaction mixture and the intensity of the SPR peaks
were pH dependent; when the reaction was conducted at pH
2.0 neither color change nor characteristic SPR peak of silver nanoparticles were observed (Fig. 1d). At pH values 2.0e4.0 a
white precipitate was obtained and no color change was
observed also. Varying shades of reddish brown color were
observed at pH values 4.5e6.5. The highest color intensity was
obtained at pH 4.5. In agreement, Roopan et al. (2013) reported
that at pH 2.0 no reaction occurred while at pH 11 highly
monodispersed nanoparticles were obtained with an average
size of 23 ± 2 nm.A variety of biomolecules are postulated to be
involved in biological nanoparticle synthesis, such biomolecules
are likely to be inactivated under the extremely
acidic conditions (pH 2.0). The differences in the colors obtained
over the range of pH could be ascribed to a variation in
the dissociation constants (pKa) of functional groups on the
biomass that are involved (Pimprikar et al., 2009).