Controllable synthesis of gold nanoparticle dimers via site-selective growth

Surface-enhanced Raman scattering based on noble metal nanoassemblies for detecting harmful substances in food

Residues of harmful substances in food can severely damage human health. The content of these substances in food is generally low, making detection difficult. Surface-enhanced Raman scattering (SERS), based on noble metal nanomaterials, mainly gold (Au) and silver (Ag), has exhibited excellent capabilities for trace detection of various substances. Noble metal nanoassemblies, in particular, have extraordinary flexibility and tunable optical properties, which cannot be offered by single nanoparticles (NPs). These nanoassemblies, with their various morphologies synthesized using NPs through artificially induced self-assembly or template-driven preparation, can significantly enhance the local electric field and create "hot spots" due to the gaps between adjacent NPs. Consequently, the SERS properties of NPs become more prominent, leading to improved performance in the trace detection of various substances and detection limits that are considerably lower than the current relevant standards. Noble metal nanoassemblies show promising potential in ensuring food safety. This review discusses the synthesis methods and SERS properties of noble metal nanoassemblies and then concentrates on their application in detecting biotoxins, drug residues, illegal additives, and heavy metals. The study provides valuable references for further research into the application of nanoassemblies in food safety detection.

Plasmonic Cyclic Au Nanosphere Hexamers

Rational design of plasmonic colloidal assemblies via bottom-up synthesis is challenging but would show unprecedented optical properties that strongly relate to the assembly's shape and spatial arrangement. Herein, the synthesis of plasmonic cyclic Au nanosphere hexamers (PCHs) is reported, wherein six Au nanospheres (Au NSs) are connected via thin metal ligaments. By tuning Au reduction, six dangling Au NSs are interconnected with a core hexagon nanoplate (NPL). Then, Pt atoms are selectively deposited on the edges of the spheres. After etching of the core, necklace-like nanostructures of Pt framework are obtained. Deposition of Au is followed, leading to PCHs in high yield (≈90%). Notably, PCHs exhibit the combinatorial plasmonic characteristics of individual Au NSs and the in-plane coupling of the six linked Au NSs. They yield highly uniform, reproducible, and polarization-independent single-particle surface-enhanced Raman scattering signals, which are attributed to the 2-dimensional isotropic alignment of the Au NSs. Those are applied to a SERS-based immunoassay as quantitative and qualitative single particle SERS nanoprobes. This assay shows a low limit-of-detection, down to 100 pm, which is orders of magnitude lower than those based on Au NSs, and one order of magnitude lower than an assay using analogous particles of smooth Au nanorings.