Figure 1(a) shows UV–Vis spectra of allamanda flower extract while the inset shows the yellow flower and its extract. The UV–Vis spectra show a strong absorption peak at ~370 nm. Figure 1(b) shows UV–Vis spectra of gold nanoparticles synthesized at different volumes of flower extract i.e. R1 = 5.0 ml, R2 = 7.5 ml and R3 = 10 ml. It can be observed from the spectra that at low volume (R1) of flower extract an absorption band appears at ~ 552 nm which is a characteristic peak of gold nanoparticles due to surface plasmon resonance. When the volume of flower extract was increased to R2, a small absorption band appears at ~ 715 nm along with the characteristic band of Au nanoparticles at ~ 545 nm. These two distinct absorption bands indicate asymmetry in the structure of gold nanoparticles and may be associated with the transverse and longitudinal plasmon absorption. At higher volume of flower extract (R3), a broad absorption band appeared at ~ 590 nm indicating aggregation of gold nanoparticles within the solution. Samples R1 and R2 were used for further characterization and catalytic application.