Metallic nanoparticles as effective sensors of bio-molecules

This work describes biologically important nanostructures of metals (AgNPs, AuNPs, and PtNPs) and metal oxides (Cu₂ONPs, CuONSs, γ-Fe₂O₃NPs, ZnONPs, ZnONPs-GS, anatase-TiO₂NPs, and rutile-TiO₂NPs) synthesized by different methods (wet-chemical, electrochemical, and green-chemistry methods). The nanostructures were characterized by molecular spectroscopic methods, including scanning/transmission electron microscopy (SEM/TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy (UV-vis), dynamic light scattering (DLS), Raman scattering spectroscopy (RS), and infrared light spectroscopy (IR). Then, a peptide (bombesin, BN) was adsorbed onto the surface of these nanostructures from an aqueous solution with pH of 7 that did not contain surfactants. Adsorption was monitored using surface-enhanced Raman scattering spectroscopy (SERS) to determine the influence of the nature of the metal surface and surface evolution on peptide geometry. Information from the SERS studies was compared with information on the biological activity of the peptide. The SERS enhancement factor was determined for each of the metallic surfaces.

SERS detection of 4-Aminobenzenethiol based on triangular Au-AuAg hierarchical-multishell nanostructure

The surface-enhanced Raman signals of 4-Aminobenzenethiol (4-ABT) adsorbed on the surface of triangular Au-AuAg hierarchical-multishell nanostructure have been investigated. Here, the approach to produce core-cavity-shell sandwich nanostructures presented as Au-AuAg is the same as preparing metal nanoparticles with hollow morphology, in which the galvanic replacement reaction takes place between silver and chloroauric acid. In this paper, we directly mix 4-ABT with gold nanoparticles and drop it on glass slides to study the effect of nanoparticles on signal enhancement of Raman spectrum, avoiding the cumbersome process of preparing metal-molecular-metal three-layer structure as reported. A significant increase in the SERS intensity of b₂ mode around 1140 cm^-1 was observed, which could quantify the concentration of 4-ABT indirectly. In a certain range, the Raman intensity gradually increases with the increasing intermediate gap, which has a strong relationship with dipole plasmon hybridization of core-dielectric-shell sandwich nanostructure. Moreover, Raman spectrum results show that the Au-AuAg substrate can produce signal intensity about 3.8 × 10² times stronger than that of 4-ABT alone and the detection limit was as low as 0.1 μM in solution.

Large-Area, Highly Sensitive SERS Substrates with Silver Nanowire Thin Films Coated by Microliter-Scale Solution Process

A microliter-scale solution process was used to fabricate large-area, uniform films of silver nanowires (AgNWs). These thin films with cross-AgNWs were deposited onto Au substrates by dragging the meniscus of a microliter drop of a coating solution trapped between two plates. The hot spot density was tuned by controlling simple experimental parameters, which changed the optical properties of the resulting films. The cross-AgNW films on the Au surface served as excellent substrates for surface-enhanced Raman spectroscopy, with substantial electromagnetic field enhancement and good reproducibility.

Particle-on-Film Gap Plasmons on Antireflective ZnO Nanocone Arrays for Molecular-Level Surface-Enhanced Raman Scattering Sensors

When semiconducting nanostructures are combined with noble metals, the surface plasmons of the noble metals, in addition to the charge transfer interactions between the semiconductors and noble metals, can be utilized to provide strong surface plasmon effects. Here, we suggest a particle-film plasmonic system in conjunction with tapered ZnO nanowire arrays for ultrasensitive SERS chemical sensors. In this design, the gap plasmons between the metal nanoparticles and the metal films provide significantly improved surface-enhanced Raman spectroscopy (SERS) effects compared to those of interparticle surface plasmons. Furthermore, 3D tapered metal nanostructures with particle-film plasmonic systems enable efficient light trapping and waveguiding effects. To study the effects of various morphologies of ZnO nanostructures on the light trapping and thus the SERS enhancements, we compare the performance of three different ZnO morphologies: ZnO nanocones (NCs), nanonails (NNs), and nanorods (NRs). Finally, we demonstrate that our SERS chemical sensors enable a molecular level of detection capability of benzenethiol (100 zeptomole), rhodamine 6G (10 attomole), and adenine (10 attomole) molecules. This work presents a new design platform based on the 3D antireflective metal/semiconductor heterojunction nanostructures, which will play a critical role in the study of plasmonics and SERS chemical sensors.