Colloidal substrate-facilitated synthesis of gold nanohelices.

Helical nanostructures have unique optical and mechanical properties, yet their syntheses had always been quite challenging. Various symmetry-breaking mechanisms such as chiral templates, strain-restriction and asymmetric ligand-binding have been developed to induce the helical growth at nanoscale. In this work, with neither chiral ligands nor templates, gold (Au) nanohelices were synthesized via a facile wet-chemical method, through an asymmetric Active Surface Growth facilitated by colloidal silica nanoparticles (NPs). The one-dimensional growth followed the Active Surface Growth, which employs a thiolated ligand to direct continuous deposition of Au at the interface, known as the active surface, between the Au nanostructures and the silica NPs - the colloidal substrates. More importantly, the colloidal substrates are crucial for the helical growth, as the diameter of the obtained nanohelices was found proportional to the size of the colloidal substrates. We propose that the nanoscale size and the curvature of the silica NPs would reduce the size of anchoring point between Au nanowires and the substrates, causing partial blockage of the active surface by the substrate and divergence of the activity on the active surface towards Au deposition. The subsequent inequivalent deposition, and the dynamic shifting of the blockage lead to the asymmetric growth and the formation of nanohelices. Factors that would affect the asymmetric Active Surface Growth were also identified and discussed, including the reduction kinetics, substrate treatment and the type and concentration of the ligand. In particular, variation of the size of the active surfaces would change the degree of the surface inequivalence, and thus affect the yield of the nanohelices.