Self-Assembled Plasmonic Pyramids from Anisotropic Nanoparticles for High-Efficient SERS

Selective recognition of the amyloid marker single thioflavin T using DNA origami-based gold nanobipyramid nanoantennas

The development of effective methods for the detection of protein misfolding is highly beneficial for early stage medical diagnosis and the prevention of many neurodegenerative diseases. Self-assembled plasmonic nanoantennas with precisely tunable nanogaps show extraordinary electromagnetic enhancement, generating extreme signal amplification imperative for the design of ultrasensitive biosensors for point of care applications. Herein, we report the custom arrangement of Au nanobipyramid (Au NBP) monomer and dimer nanoantennas engineered precisely based on the DNA origami technique. Furthermore, we demonstrate the SERS based detection of thioflavin T (ThT), a well-established marker for the detection of amyloid fibril formation, where G-Quadruplexes govern the site-specific attachment of ThT in the plasmonic hotspot. This is the first study for the SERS based detection of the ThT dye attached specifically using a G-Quadruplex complex. The spectroscopic signals of ThT were greatly enhanced due to the designed nanoantennas demonstrating their potential as superior SERS substrates. This study paves the way for boosting the design of next-generation diagnostic tools for the specific and precise detection of various target disease biomarkers using molecular probes.

Enhancement of plasmonic photovoltaics with pyramidal nanoparticles

Light trapping as a result of embedding plasmonic nanoparticles (NPs) into photovoltaics (PVs) has been recently used to achieve better optical performance compared to conventional PVs. This light trapping technique enhances the efficiency of PVs by confining incident light into hot-spot field regions around NPs, which have higher absorption, and thus more enhancement of the photocurrent. This research aims to study the impact of embedding metallic pyramidal-shaped NPs inside the PV's active region to enhance the efficiency of plasmonic silicon PVs. The optical properties of pyramidal-shaped NPs in visible and near-infrared spectra have been investigated. The light absorption into silicon PV is significantly enhanced by embedding periodic arrays of pyramidal NPs in the cell compared to the case of bare silicon PV. Furthermore, the effects of varying the pyramidal-shaped NP dimensions on the absorption enhancement are studied. In addition, a sensitivity analysis has been performed, which helps in identifying the allowed fabrication tolerance for each geometrical dimension. The performance of the proposed pyramidal NP is compared with other frequently used shapes, such as cylinders, cones, and hemispheres. Poisson's and Carrier's continuity equations are formulated and solved for the current density-voltage characteristics associated with embedded pyramidal NPs with different dimensions. The optimized array of pyramidal NPs provides an enhancement of 41% in the generated current density when compared to the bare silicon cell.

Partial Leidenfrost Evaporation-Assisted Ultrasensitive Surface-Enhanced Raman Spectroscopy in a Janus Water Droplet on Hierarchical Plasmonic Micro-/Nanostructures

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro-/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/nanostructures, which are fabricated by integrating nanolaminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10^-9 M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 10⁷. Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.

Self-assembly and characterization of 2D plasmene nanosheets

Freestanding plasmonic nanoparticle (NP) superlattice sheets are novel 2D nanomaterials with tailorable properties that enable their use for broad applications in sensing, anticounterfeit measures, ionic gating, nanophotonics and flat lenses. We recently developed a robust, yet general, two-step drying-mediated approach to produce freestanding monolayer, plasmonic NP superlattice sheets, which are typically held together by holey grids with minimal solid support. Within these superlattices, NP building blocks are closely packed and have strong plasmonic coupling interactions; hence, we termed such freestanding materials 'plasmene nanosheets'. Using the desired NP building blocks as starting material, we describe the detailed fabrication protocol, including NP surface functionalization by thiolated polystyrene and the self-assembly of NPs at the air-water interface. We also discuss various characterization approaches for checking the quality and optical properties of the as-obtained plasmene nanosheets: optical microscopy, spectrophotometry, transmission/scanning electron microscopy (TEM/SEM) and atomic force microscopy (AFM). With regard to different constituent building blocks, the key experimental parameters, including NP concentration and volume, are summarized to guide the successful fabrication of specific types of plasmene nanosheets. This protocol, from initial NP synthesis to the final fabrication and characterization, takes ~33.5 h.

The importance of plasmonic heating for the plasmon-driven photodimerization of 4-nitrothiophenol

Metal nanoparticles form potent nanoreactors, driven by the optical generation of energetic electrons and nanoscale heat. The relative influence of these two factors on nanoscale chemistry is strongly debated. This article discusses the temperature dependence of the dimerization of 4-nitrothiophenol (4-NTP) into 4,4′-dimercaptoazobenzene (DMAB) adsorbed on gold nanoflowers by Surface-Enhanced Raman Scattering (SERS). Raman thermometry shows a significant optical heating of the particles. The ratio of the Stokes and the anti-Stokes Raman signal moreover demonstrates that the molecular temperature during the reaction rises beyond the average crystal lattice temperature of the plasmonic particles. The product bands have an even higher temperature than reactant bands, which suggests that the reaction proceeds preferentially at thermal hot spots. In addition, kinetic measurements of the reaction during external heating of the reaction environment yield a considerable rise of the reaction rate with temperature. Despite this significant heating effects, a comparison of SERS spectra recorded after heating the sample by an external heater to spectra recorded after prolonged illumination shows that the reaction is strictly photo-driven. While in both cases the temperature increase is comparable, the dimerization occurs only in the presence of light. Intensity dependent measurements at fixed temperatures confirm this finding.