shifts to 1491 cm1. The absorption due to C–H stretching vibration of hydrocarbon around 2926 cm1 is shifted to 3080 cm1.
Early reports show that the essential oil of A. occidentale contains
various terpenoids [35]. Of these spathulenol, Globulol, Muurolol
and Cadinol contain tertiary hydroxyl groups and have less reducing
capacity. But at higher temperatures in presence of acetone, they
may reduce Au3+ to Au NPs and gets oxidized to carboxylic acids.
The carboxylic acids easily release H+ ions and the carboxylate ions
gets attached to Au NPs having positive surface charge which is supported by the shifted bands at 1324 cm1 and 1491 cm1 in Fig. 6b.
These capping agent may cause steric and electrostatic barriers
around the particle surface and thereby stabilizes it [36]. The carboxylate ion stabilized Au NPs have negative charge which prevents
them from aggregation. The long hydrocarbon chain or ring also provides electrosteric stabilization. The suggested mechanism is schematically represented in Fig. 7.