highly crystalline [69, 70]. Proteins were bound to AuNPs
and AgNPs through free amino groups and carboxylic
groups of the amino acid residues, respectively. Oat (Avena
sativa) biomass has also been used for AuNP formation
with tetrahedral, decahedral, hexagonal, icosahedral
multitwinned and irregular shapes in which the pH of the
reaction influenced the size of particles [71]. At pH values
of 3–4, smaller nanoparticles were observed, whereas at pH
2, larger nanoparticles were observed. AuNPs were
synthesised using oat and wheat (Triticum sp.) biomass in
aqueous solutions under acidic conditions (pH 4) [72]. The
resulting nanoparticles were then extracted with sodium
citrate or cetyltrimethylammonium bromide (capping agents
which coat the nanoparticle) allowing the nanoparticles to
be isolated from the biomass and analysed. Using pear fruit
extract (Pryus sp.), triangular and hexagonal nanoplates of
AuNPs were synthesised under alkaline conditions [73].
The edge lengths of these nanoplates ranged from 200 to
500 nm without any substantial impurities. Biomass of the
edible brown alga Fucus vesiculosus (also called bladder
wrack) reduced Au(III) to Au(0) within an optimum pH
range of 4–9 [9]. Hydroxyl groups in algal polysaccharides
are abundant and contribute to gold reduction. Researchers
suggested that the reduction of Au(III) to Au(0) occurs
through the oxidation of hydroxyl groups to carbonyl
groups as shown below [9]
AuCl −
4
+ 3R − OH Au0 + 3R = O + 3H+ + 4Cl−