Fig. 1.

Characteristic peak centered

(a) UV–visible spectra of 1 mM HAuCl₄ solution recorded before and after the addition of hordenine, showing maximum intensity peak at 540 nm, (b) Vials containing 1 mM HAuCl₄ solution (right) and HD‐AuNPs (left) showing change in colour of the reaction mixture from pale yellow to ruby red