We show here that L-lysine effectively complexes with phosphomolybdic acid (PMA) and the solution mixture when added to a 10 −3 M aqueous solution of HAuCl 4 after UV-irradiation for 3 h leads to the slow reduction and consequent formation of gold nanotriangles with a high degree of anisotropy. The same reaction carried out in a 12.5 kDa cutoff dialysis bag where the irradiated PMA-lysine solution was kept inside and stirred in a beaker containing aqueous HAuCl 4 , did not lead to the formation of gold nanotriangles. This implies that L-lysine plays the role of a shape-modulating agent and hence this study proves an improvement in the understanding of the role of such organic–inorganic hybrid structures in the synthesis and growth of anisotropic nanoparticles. S Online supplementary data available from stacks.iop.org/MRX/1/015007/ mmedia