Nanocavity-in-Multiple Nanogap Plasmonic Coupling Effects from Vertical Sandwich-Like Au@Al2O3@Au Arrays for Surface-Enhanced Raman Scattering

Controllable preparation of Ti3C2Tx/Ag composite as SERS substrate for ultrasensitive detection of 4-nitrobenzenethiol

Currently, metal carbonitride (MXene) has been identified as a hot research topic in the research area of surface-enhanced Raman scattering (SERS). In this study, Ti3C2Tx/Ag composite was fabricated as SERS substrate with different Ag contents. The fabricated Ti3C2Tx/Ag composites show good SERS behavior by detecting 4-Nitrobenzenethiol (4-NBT) probe molecules. Through calculation, the SERS enhancement factor (EF) of the Ti3C2Tx/Ag substrate was as high as 4.15 × 106. It is worth noting that the detection limit of 4-NBT probe molecules can be achieved ultralow concentration of 10-11 M. In this system, electromagnetic enhancement mechanism and chemical enhancement mechanism have synergistic effects on SERS phenomenon. Meanwhile, the Ti3C2Tx/Ag composite substrate exhibited good SERS reproducibility. In addition, the SERS detection signal hardly changed after 6 months of natural standing, and the substrate showed good stability. This work suggests that the Ti3C2Tx/Ag substrate could be used as a sensitivity SERS sensor for practical application, and could be applied in the field of environmental monitoring.

Open nanocavity-assisted Ag@PDMS as a soft SERS substrate with ultra-sensitivity and high uniformity

To achieve high sensitivity and uniformity simultaneously in a surface-enhanced Raman scattering (SERS) substrate, this paper presents the preparation of a flexible and transparent three-dimensional (3D) ordered hemispherical array polydimethylsiloxane (PDMS) film. This is achieved by self-assembling a single-layer polystyrene (PS) microsphere array on a silicon substrate. The liquid-liquid interface method is then used to transfer Ag nanoparticles onto the PDMS film, which includes open nanocavity arrays created by etching the PS microsphere array. An open nanocavity assistant soft SERS sample, "Ag@PDMS," is then prepared. For electromagnetic simulation of our sample, we utilized Comsol software. It has been experimentally confirmed that the Ag@PDMS substrate with silver particles of 50 nm in size is capable of achieving the largest localized electromagnetic hot spots in space. The optimal sample, Ag@PDMS, exhibits ultra-high sensitivity towards Rhodamine 6 G (R6G) probe molecules, with a limit of detection (LOD) of 10^-15 mol/L, and an enhancement factor (EF) of ∼10¹². Additionally, the substrate exhibits a highly uniform signal intensity for probe molecules, with a relative standard deviation (RSD) of approximately 6.86%. Moreover, it is capable of detecting multiple molecules and can perform real detection on non-flat surfaces.

Hierarchically Assembled Plasmonic Metal-Dielectric-Metal Hybrid Nano-Architectures for High-Sensitivity SERS Detection

In this work, we designed and prepared a hierarchically assembled 3D plasmonic metal-dielectric-metal (PMDM) hybrid nano-architecture for high-performance surface-enhanced Raman scattering (SERS) sensing. The fabrication of the PMDM hybrid nanostructure was achieved by the thermal evaporation of Au film followed by thermal dewetting and the atomic layer deposition (ALD) of the Al2O3 dielectric layer, which is crucial for creating numerous nanogaps between the core Au and the out-layered Au nanoparticles (NPs). The PMDM hybrid nanostructures exhibited strong SERS signals originating from highly enhanced electromagnetic (EM) hot spots at the 3 nm Al2O3 layer serving as the nanogap spacer, as confirmed by the finite-difference time-domain (FDTD) simulation. The PMDM SERS substrate achieved an outstanding SERS performance, including a high sensitivity (enhancement factor, EF of 1.3 × 108 and low detection limit 10-11 M) and excellent reproducibility (relative standard deviation (RSD) < 7.5%) for rhodamine 6G (R6G). This study opens a promising route for constructing multilayered plasmonic structures with abundant EM hotspots for the highly sensitive, rapid, and reproducible detection of biomolecules.

Biologically-Inspired Water-Swelling-Driven Fabrication of Centimeter-Level Metallic Nanogaps

Metallic nanogaps have great values in plasmonics devices. However, large-area and low-cost fabrication of such nanogaps is still a huge obstacle, hindering their practical use. In this work, inspired by the cracking behavior of the tomato skin, a water-swelling-driven fabrication method is developed. An Au thinfilm is deposited on a super absorbent polymer (SAP) layer. Once the SAP layer absorbs water and swells, gaps will be created on the surface of the Au thinfilm at a centimeter-scale. Further experimentation indicates that such Au gaps can enhance the Raman scattering signal. In principle, the water-swelling-driven fabrication route can also create gaps on other metallic film and even nonmetallic film in a low-cost way.

Tip-to-tip assembly of urchin-like Au nanostar at water-oil interface for surface-enhanced Raman spectroscopy detection

Au Nanostar (NS) monolayer as a surface enhanced Raman scattering (SERS) substrate has been synthesized by self-assembly at a water-oil interface. It is confirmed from the experiment and simulation results that the Au NS monolayer includes lots of "hot spots" at or between the tips of the Au NSs, enhancing the local electromagnetic fields and giving rise to strong SERS signals sequentially. The limit of detection is determined to be down to 4.2 × 10^-12 M for rhodamine 6G. Furthermore, the Au NS monolayer can detect multiple molecules, including thiabendazole, methylene blue, 4-mercaptobenzoic acid, and p-amino thiophenol, indicating that the SERS substrate composed of Au NS monolayer has potential applications in analytical chemistry, food safety, and environmental safety.

Boosting the Raman signal on a semiconductor-nanotube membrane for reporting photocatalytic reactions on site

A smooth TiO₂ nanotube-based membrane (S-TiNM) is demonstrated to be effective in reducing the Raman-scattering loss in 3D porous structures. Using the photocatalytic reduction of 4-nitrothiophenol as a model, the S-TiNMs show promising potential as sensitive SERS substrates to investigate photocatalytic reactions on site.

3D Ultrasensitive Polymers-Plasmonic Hybrid Flexible Platform for In-Situ Detection

This paper introduces a three-dimensional (3D) pyramid to the polymers-plasmonic hybrid structure of polymethyl methacrylate (PMMA) composite silver nanoparticle (AgNPs) as a higher quality flexible surface-enhanced Raman scattering (SERS) substrate. Benefiting from the effective oscillation of light inside the pyramid valley could provide wide distributions of 3D “hot spots” in a large space. The inclined surface design of the pyramid structure could facilitate the aggregation of probe molecules, which achieves highly sensitive detection of rhodamine 6G (R6G) and crystal violet (CV). In addition, the AgNPs and PMMA composite structures provide uniform space distribution for analyte detection in a designated hot spot zone. The incident light can penetrate the external PMMA film to trigger the localized plasmon resonance of the encapsulated AgNPs, achieving enormous enhancement factor (~6.24×108). After undergoes mechanical deformation, the flexible SERS substrate still maintains high mechanical stability, which was proved by experiment and theory. For practical applications, the prepared flexible SERS substrate is adapted to the in-situ Raman detection of adenosine aqueous solution and the methylene-blue (MB) molecule detection of the skin of a fish, providing a direct and nondestructive active-platform for the detecting on the surfaces with any arbitrary morphology and aqueous solution.

In situ detection of trace pollutants: a cost-effective SERS substrate of blackberry-like silver/graphene oxide nanoparticle cluster based on quick self-assembly technology

To realize fast detection of trace hazardous chemicals, a SERS substrate with the structure of a blackberry-like silver/graphene oxide nanoparticle cluster (Ag/GO NPC) has been designed and prepared through a quick capillarity-assistant self-assembly technology in this paper. Benefitting from the abundant "hot spots" and active oxygen sites brought by this Ag/GO NPC, the substrate shows good Raman performance for malachite green (MG), a common abusive germicide in aquaculture, with lowest limit of detection below 0.1 µg/L (3.48 × 10^-10 mol/L). Detailed analyses are taken on both the formation process and enhancement mechanism of this SERS substrate, and the finite-difference time-domain simulations are utilized as well to prove our hypotheses. Further constructing this structure on polyethylene terephthalate (PET) film, a translucent flexible SERS substrate can be obtained, realizing a fast in situ detection of trace MG in the fishpond subsequently. In consideration of the facile preparation process, good SERS enhancement and affordable materials (PET, Cu, Ag and GO, etc.), this substrate presents high cost performance and a promising application prospect.