The synthesis of highly complex two-dimensional (2D) metal nanoframes remains a great challenge. Synthetic strategies for preparing 2D metal nanoframes are few, and rational and systematic synthetic pathways to more complicated architectures have not yet been reported. Herein, we demonstrate a stepwise synthetic strategy for complex 2D metal nanoframes with a high degree of intricacy; the strategy leads to a variety of shapes, including rings, triangles, hexagons, and tripods with tailorable single or double frames in a single entity. These nanoframes of high homogeneity could be obtained through selective combination of four different chemical toolkits consisting of selective etching and deposition on certain facets, and concentric and/or eccentric regrowth by controlling the mismatches of lattice constants of metals. The resulting nanoframes were highly homogeneous in size and shape and had van der Waals interactions that maximized rim-to-rim contact, allowing them to uniquely self-assemble into large-area superstructures.

Herein, plasmonic metal tripod nanoframes with three-fold symmetry were synthesized in a high yield (∼83%), and their electric field distribution and single-particle surface-enhanced Raman scattering (SERS) were studied. We realized such complex frame morphology by synthesizing analogous tripod nanoframes through multiple transformations. The precise control of the Au growth pattern led to uniform tripod nanoframes embedded with circle or line-shaped hot spots. The linear-shaped nanogaps (“Y”-shaped hot-zone) of the frame structures can strongly and efficiently confine the electric field, allowing for strong SERS signals. Coupled with a high synthetic yield of the targeted frame structure, strong and uniform SERS signals were obtained inside the nanoframe gaps. Remarkably, quite reproducible SERS signals were obtained with these structuresthe SERS enhancement factors with an average value of 7.9 × 107 with a distribution of enhancement factors from 2.2 × 107 to 2.2 × 108 for 45 measured individual particles.

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.

In this Letter, a rational and stepwise method for the solution-phase synthesis of asymmetric Au split nanorings by adopting Au nanoprisms as a template has been demonstrated. The selective chemical etching of Au nanoprism tips activated the surface reactivity of edges and led to the selective deposition of Pt at the periphery of Au nanoplates. By controlling the total amount of Pt on the edges, different degrees of split Au@Pt nanorings were obtained; the subsequent Au coating around the Au@Pt scaffold eventually resulted in asymmetric Au hexagonal split nanorings. Their surface plasmonic features as a function of split degrees were investigated, including straight nanorods, bent nanorods, split nanorings, and full nanorings. The electrical field focusing using single-particle surface-enhanced Raman spectroscopy was evaluated under different polarization angles of the incident light for two different structures with the point gap and line gap between two arms.

This paper reports a methodology for synthesizing and ordering gold nanoframes into three-dimensional (3D) arrays with a controlled thickness, leading to homogeneous plasmonic superstructures, with which quantitative analysis via surface-enhanced Raman spectroscopy (SERS) has been successfully demonstrated. Because this preparation method allows for systematic control of nanoframe film thickness and the resulting 3D plasmonic superstructure, which exhibits a unique nanoporous network of hot-spots, detection limits down to 10 −18 M, corresponding to ≈6000 molecules, have been measured. Compared to analogous solid nanoparticle superstructures, the nanoframe superstructures with their unique nanoporous architecture effectively dissipate the heat inevitably generated by laser excitation during measurement, effectively suppressing the formation of carbonaceous materials and therefore their accompanying fluorescence interference, especially important for low concentration detection.