Fivefold Symmetry-Retained Formation of Pentagrammic Nanorattles Through Galvanic Replacement With Co-Reduction

We report a fivefold symmetry-retained transformation of pentatwinned Au@Ag nanorods into non-convex pentagrammic prism nanorattles through galvanic replacement coupled with ascorbic acid-mediated co-reduction. The coupled redox process enables facet-selective growth while preserving the underlying pentatwinned symmetry, yielding hollow nanorattles composed of a solid Au core enclosed within a gold-rich alloy shell featuring five ridged arms and concave pentagrammic caps. High-resolution microscopy reveals anisotropic deposition localized between twin boundaries, while controlled kinetic modulation enables precise control over morphology evolution. The resulting nanostructure exhibits red-shifted and broadened plasmonic modes and significantly enhanced SERS activity relative to rod-shaped analogues. The retention of fivefold symmetry preserves the crystallographic equivalence of the pentagrammic architecture, contributing to a uniform plasmonic response and reproducible SERS performance. These findings establish a coupled galvanic replacement/co-reduction strategy for engineering complex symmetry-retained, concave plasmonic nanostructures with tailored optical properties.