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Silver double nanorings with circular intra-nanogaps between two nanorings of different diameters were synthesized without a linker molecule to confine an incident electromagnetic field in a single entity. We used on-demand, rational, and systematic multi-stepwise reactions consisting of (1) selective etching of gold, (2) rim-on deposition of platinum, (3) eccentric growth of gold, and (4) concentric growth of silver. The resulting silver double nanorings exhibited a high degree of homogeneity in both shape and size, with strongly coupled circular hot zones (or "hot halos", referring to the circular intra-nanogaps capable of focusing the near electromagnetic field) resulting from strong surface plasmon coupling between the inner and outer nanorings. Remarkably, these silver double nanorings exhibited strong, stable, and reproducible single-particle surface-enhanced Raman scattering signals without blinking. The signals appeared independently of polarization directions, which is a unique feature of a circular hot halo. The estimated enhancement factor was between 2 × 108 and 7 × 108. The measured limit of detection was 10-7 M in bulk concentration, and the signal appeared 570 s after sample exposure.

We designed complex Au nanorings with intertwined triple rings (ANITs) in a single entity to amplify the efficacy of near-field focusing. Such a complex and unprecedented morphology at the nanoscale was realized through on-demand multistepwise reactions. Triangular nanoprisms were first sculpted into circular nanorings, followed by a series of chemical etching and deposition reactions eventually leading to ANITs wherein thin metal bridges hold the structure together without any linker molecules. In the multistepwise reaction, the well-faceted growth pattern of Au, which induces the growth of two distinctive flat facets in a lateral direction, is important to evolve the morphology from single to multiple nanorings. Although our synthesis proceeds through multiple steps in one batch without purification steps, it shows a remarkably high yield (>∼90%) at the final stage. The obtained high degree of homogeneity (in both shape and size) of the resulting ANITs allowed us to systematically investigate the corresponding localized surface plasmon resonance (LSPR) coupling with varying nanoring arrangements and observe their single-particle surface enhanced Raman scattering (SERS). Surprisingly, individual ANITs exhibited an enormously large enhancement factor (∼109), which confirms their superior near-field focusing relative to other reported nanoparticles.

Herein, we report the novel strategy for the synthesis of complex 3-dimensional (3D) nanostructures, mimicking the linker molecule-free 3D arrangement of six Au nanospheres at the vertices of octahedrons. We utilized 3D PtAu skeleton for the structural rigidity and deposited Au around the PtAu skeleton in a site-selective manner, allowing us to investigate their surface plasmonic coupling phenomenon and near-field enhancement as a function of sizes of nanospheres, which are directly related to the intrananogap distance and interior volume size. The resulting 3D Au hexamer structures with octahedral arrangement were realized through precise control of the Au growth pattern. The complex 3D Au hexamers were composed of six Au nanospheres connected by thin metal conductive bridges. The standard deviation of the metal conductive bridges and Au nanospheres was within ca. 10%, exhibiting a high degree of homogeneity and precise structural tunability. Interestingly, charge transfer among the six Au nanospheres occurred along the metal conductive bridges leading to surface plasmonic coupling between Au nanospheres. Accordingly, electric near fields were strongly and effectively focused at the vertices, intrananogap regions between Au nanospheres, and interior space, exhibiting well-resolved single-particle surface-enhanced Raman spectroscopy signals of absorbed analytes.