Surfactant-Layer Engineering Enables Growth Pathway Switching and Reversal of Chiroptical Activity in Helicoid Gold Nanocrystals

Helicoid gold (Au) nanocrystals exhibit pronounced chiroplasmonic responses arising from their three-dimensional chiral morphology and strong light-matter interactions. Achieving precise control over their chirality requires synthetic advances and a deeper understanding of the interplay among nanocrystal growth, structural chirality, and chiroptical activity. Here, we report a surfactant-layer engineering strategy that enables switching of growth pathways and reversal of chiroptical signals in helicoid Au nanocrystals by introducing aromatic additives, particularly 5-bromosalicylic acid (5-BSA), into a synthesis system using cysteine as chiral inducers. The incorporation of 5-BSA alters the growth pathway of helicoid Au nanocrystals, resulting in a transition in chiral morphology from C₄-type pinwheel-like structures to C₃-type propeller-like structures, accompanied by a reversal of the g-factor from -0.14 to +0.09. Mechanistic investigations reveal that 5-BSA modulates the surfactant interface, altering the relative growth rates along the ⟨100⟩ and ⟨111⟩ directions and deepening the concaveness of Au nanocrystals, which contributes to the reversal of chiroptical activity and the large g-factors. The strategy is generalizable to diverse aromatic molecules and chiral inducers, enabling the use of a single chiral inducer in one enantiomeric form to produce nanostructures with opposite signs of the chiroptical response. This work provides a flexible synthesis route for chiroplasmonic materials and deepens the mechanistic understanding linking enantioselective anisotropic growth and chiroptical responses.