Understanding metal-enhanced fluorescence and structural properties in Au@Ag core-shell nanocubes

Surface Plasmonic Core-Shell Nanostructures in Surface Enhanced Raman Scattering and Photocatalysis

Core-shell nanostructures are a typical material design. Usually, it consists of a core wrapped in a shell. It has attracted much attention due to its tunable structure and composition, high surface area, and high programmability. The properties and resonance frequency of their surface plasmons can be adjusted by regulating the shape, size, and composition of metal core-shell nanostructures. This interaction makes core-shell nanostructures an excellent platform for plasmon-enhanced optical effects. This Perspective explores the categories of core-shell nanostructures, their exchanges with excitons in two-dimensional materials, their spectrum-enhanced aspects, and prospects for future applications of core-shell nanostructures.

Preparation of a Three-Dimensional Composite Structure Based on a Periodic Au@Ag Core-Shell Nanocube with Ultrasensitive Surface-Enhanced Raman Scattering for Rapid Detection

The absorption and scattering frequencies of surface plasmon resonance can be selectively adjusted by changing the morphology, size, structure, arrangement, and gap between noble metal nanoparticles so that the local electromagnetic field on the substrate surface can be further enhanced. This change will promote and popularize surface-enhanced Raman spectroscopy. This paper reports the research results and improvement scheme of surface enhanced Raman scattering (SERS) activity of silver-coated gold nanocubed/organism (Au@Ag/CW NCs) prepared by three-phase self-assembly. In the experiment, the uppermost oil phase in the three-phase self-assembly process was optimized as ethanol and n-hexane solution containing a specific concentration of a probe molecule rhodamine 6G or aspartame. The probe molecules were directly self-assembled on the surface of the composite substrate to avoid the possible loss and pollution during immersion and preservation and achieve the purpose of rapid detection. The results show that the Au@Ag/CW NC array substrate is a periodic cubic ring structure. The sensitivity, uniformity, reproducibility, and stability of composite Au@Ag/CW NC array substrates are verified by comparing the Raman activities of various substrates. The feasibility of using the substrate to realize rapid SERS detection, compared with the advantages and disadvantages of the traditional soaking method, proved that the prepared substrate and improvement direction have excellent potential for application and development prospects in the field of rapid food additive detection.

Plasmon-Enhanced Fluorescence Emission of an Electric Dipole Modulated by a Nanoscale Silver Hemisphere

The spontaneous emission of a fluorophore is altered by the surrounding electromagnetic field. Therefore, the radiation of the fluorophore can be engineered by inter-coupling with the nanoscale plasmons. This work proposes a nanoscale hemisphere structure that enhances the electric field and further modulates its effects on fluorophores by adjusting the radius of the hemisphere. A full-wave simulation is carried out using the finite element method, and the radiation characteristics of the nanoscale hemisphere are studied in detail. Compared with free space, the structure has generated significant enhancement exceeding 30. Through curve fitting, the relationship between the enhanced peak wavelength and the radius of the hemisphere is obtained.

Two-Dimensional Self-Assembly of Au@Ag Core-Shell Nanocubes with Different Permutations for Ultrasensitive SERS Measurements

Different self-assembly methods not only directly change the arrangement of noble metal particles on the substrate but also indirectly affect the local electromagnetic field distribution and intensity of the substrate under specific optical excitation conditions, which leads to distinguished different enhancement effects of the structure on molecular Raman signals. In this paper, first, the gold species growth method was used to prepare the silver-coated gold nanocubes (Au@Ag NCs) with regular morphology and uniform size, and then the two-phase and three-phase liquid-liquid self-assembly and evaporation-induced self-assembly methods were used to obtain the substrate structure with different NC arrangement patterns. The optimal arrangement of NCs was found by transverse comparison of Raman signal detection of probe molecules with the same concentration. Subsequently, surface-enhanced Raman scattering (SERS) measurements of Rhodamine (Rh6G) and aspartame (APM) were carried out. Furthermore, the finite element method (FEM) was employed to calculate the local electromagnetic fields of the substrates with different Au@Ag NC arrangements, and the calculated results were in agreement with the experimental results. The experimental results show that the SERS-active substrate was largely associated with the different arrangements of Au@Ag NCs, and the island membrane Au@Ag NCs array substrate obtained by evaporation-induced self-assembly can generate a strong local electromagnetic field due to the edge and corner bonding gap between the tightly arranged NCs; this endows the substrate with benign sensitivity and reproducibility and has great potential in molecular detection, biosensing, and food safety monitoring.

Recent Developments in Plasmonic Nanostructures for Metal Enhanced Fluorescence-Based Biosensing

Metal-enhanced fluorescence (MEF) is a unique phenomenon of surface plasmons, where light interacts with the metallic nanostructures and produces electromagnetic fields to enhance the sensitivity of fluorescence-based detection. In particular, this enhancement in sensing capacity is of importance to many research areas, including medical diagnostics, forensic science, and biotechnology. The article covers the basic mechanism of MEF and recent developments in plasmonic nanostructures fabrication for efficient fluorescence signal enhancement that are critically reviewed. The implications of current fluorescence-based technologies for biosensors are summarized, which are in practice to detect different analytes relevant to food control, medical diagnostics, and forensic science. Furthermore, characteristics of existing fabrication methods have been compared on the basis of their resolution, design flexibility, and throughput. The future projections emphasize exploring the potential of non-conventional materials and hybrid fabrication techniques to further enhance the sensitivity of MEF-based biosensors.