Au-Ag nano-garlands as a versatile SERS substrate: Two-step synthesis realizes the growth of petal-shaped branches on hollow Au-Ag nanoshells

In this work, we synthesized Au-Ag alloy nano-garlands with multiple petal-like branches and internal cavities in two steps. First, we etched silver nanospheres with HAuCl₄ to obtain Au-Ag alloy nanoshells with different shell layer thicknesses and hollow core sizes. After that, based on different nanoshells, 4-NTP was used as a shape-directing agent, HAuCl₄ and AgNO₃ as precursor raw materials, and AA as a reducing agent for the growth of petals to form Au-Ag bimetallic nano-garlands with multi-branching structure. The regulation of the branching morphology of the nano-garlands could be achieved by changing the amounts of 4-NTP and HAuCl₄. The SERS activity of the nano-garlands could be further improved by changing the AA concentration, the size of nanoshells, and the Au/Ag ratio of the precursor raw materials. Because the bimetallic nano-garlands have hollow cavities and petal-like branches, there are couplings between the cavities and the branches, between the adjacent branches, and between the metals inside and outside of the cores, which can achieve a good enhancement of SERS. The highest SERS enhancement factor of Au-Ag bimetallic nano-garlands can reach 4.47×10⁸, which can be used as a good candidate for SERS substrate for the trace detection of biomolecules, pesticides, and other substances.