Self-assembly-based integration of Ag@Au oligomers and core/shell nanoparticles on polymer chips for efficient sensing devices.

Two distinct Ag/Au nanocomposites, namely, hetero-oligomers and eccentric core/shells were obtained by one-step polymer self-assembly-based fabrication. The Ag concentration-dependent, facet-specific passivation, and presence or absence of anisotropic facets were the main factors responsible for controlling the structures of the final products. Based on an understanding of the role of Ag+ ions in controlling the shape of anisotropic gold nanoparticles (AuNPs), tailored concentrations of Ag+ were applied to design nanoparticles with desired anisotropic surface facets to allow site-specific Ag coatings on AuNPs. The Ag additives acted as shape-directing agents due to an underpotential deposition (UPD) that was responsible for stabilizing the various surface facets that enclose the AuNPs. The morphology differences between the substrate samples resulted in discrete plasmonic and sensing features. In surface-enhanced Raman spectroscopy (SERS) studies, we showed that the site-selective deposition of Ag on anisotropic gold nanohexagons (AuNHs) delivers more advantages as compared to their hetero-oligomer nanostructured counterparts due to synergistic effects.

Precise control of the size and gap between gold nanocubes by surface-based synthesis for high SERS performance.

The optical properties of a monolayer of nanocomposite film (PMMA/gold nanocubes) were provided by fitting a proposed theoretical model to spectroscopic ellipsometry (SE) measurements. For such a thin film, these features cannot be successfully determined by means of experimental and conventional effective medium theory such as Maxwell-Garnett or Bruggeman. To make it possible, we developed a model of two classical Lorentz oscillators; one for a PMMA layer and the other for GNCs, revealing one homogeneous layer and rapid analysis without the need for large computational resources. Additionally, we tailored both the size and number of GNCs in the PMMA layer by tuning the synthesis parameters as seen in scanning electron microscopy (SEM) images. In parallel, SE measurements clearly highlighted the change in the optical properties of GNCs as a function of their density on the substrate and dimensions. Our findings demonstrate that SE is an alternative method to characterize layered GNCs on opaque substrates efficiently, which has potential implications for designing other morphologies in the future.

Femtosecond Direct Laser-Induced Assembly of Monolayer of Gold Nanostructures with Tunable Surface Plasmon Resonance and High Performance Localized Surface Plasmon Resonance and Surface Enhanced Raman Scattering Sensing.

We show femtosecond direct laser-induced assembly of gold nanostructures with plasmon resonance band variable as a function of laser irradiation in a wide range of visible wavelengths. A system of 2-photon lithography is used to achieve site-selectively controlled dewetting of a thin gold film into nanostructures in which size and shape are highly dependent on the laser power. Simultaneous measurements of localized surface plasmon resonance (LSPR) and surface enhanced Raman scattering (SERS) in the presence of various concentrations of trans-1,2-bis(4-pyridyl) ethylene (BPE) as target molecule are performed in order to highlight the relationship between structural dimensions, plasmonic effect, and detection activity. The resulting gold NPs exhibit high sensitivity as both LSPR and SERS sensors and allow the detection of picomolar concentrations of BPE with a SERS enhancement factor (SEF) of 1.33 × 109 and a linear detection range between 10-3 and 10-12 M.

Ellipsometry of Colloidal Solutions: New Experimental Setup and Application to Metallic Colloids.

An ellipsometric cell is developed to simultaneously determine the shape distribution, the volume fraction, and the complex refractive index of gold and silver colloids. Simulation reveals that this cell drastically improves the detection limit of ellipsometry. Indeed, Ag and Au nanoparticles (NPs) are detected at the ppmv level. We demonstrate that the NPs shape distribution can be estimated from ellipsometric measurements by analyzing them with a shape distributed effective medium theory (SDEMT). The obtained distributions from ellipsometry are in agreement with those deduced from transmission electron microcopy (TEM). Contrary to TEM, ellipsometry probes a large number of NPs estimated at about 1011 NPs. Finally, we show that the complex refractive index of colloids as determined from ellipsometry is sensitive to the optical properties of the solvent and the plasmonic properties of NPs.

Enhanced Photocatalytic Activity and Photoluminescence of ZnO Nano-Wires Coupled with Aluminum Nanostructures.

In this study, we fabricated a hybrid plasmonic/semiconductor material by combining the chemical bath deposition of zinc oxide nanowires (ZnONWs) with the physical vapor deposition of aluminum nanostructures (AlNSs) under controlled temperature and atmosphere. The morphological and the optical properties of the ZnONWs/AlNSs hybrid material fabricated at different temperatures (250, 350, and 450 °C) and thicknesses (5, 7, and 9 nm) of Al layers were investigated. By adjusting the deposition and annealing parameters, it was possible to tune the size distribution of the AlNSs. The resonant coupling between the plasmonic AlNSs and ZnONWs leads to an enhanced photoluminescence response. The photocatalytic activity was studied through photodegradation under UV-light irradiation of methylene blue (MB) adsorbed at the surface of ZnO. The MB photodegradation experiment reveals that the ZnONWs covered with 7 nm aluminum film and annealed at 450 °C exhibit the highest degradation efficiency. The comparison between ZnONws and ZnONws/AlNSs shows a photoluminescence enhancement factor of 1.7 and an increase in the kinetics constant of photodegradation with a factor of 4.

Self-Assembled Ag Nanocomposites into Ultra-Sensitive and Reproducible Large-Area SERS-Active Opaque Substrates.

This work describes a novel, one-shot strategy to fabricate ultrasensitive SERS sensors based on silver/poly(methyl methacrylate) (PMMA) nanocomposites. Upon spin coating of a dispersion of PMMA and silver precursor on N-doped silicon substrate, closely separated silver nanoparticles were self-assembled into uniform nanospheres. As a result, a thin hydrophobic PMMA layer embedded with Ag nanoparticles (AgNPs) was obtained on the whole silicon substrate. Consequently, a large-scale, reproducible SERS platform was produced through a rapid, simple, low-cost, and high-throughput technology. In addition, reproducible SERS features and high SERS enhancement factors were determined (SEF ~1015). This finding matches the highest SEF reported in literature to date (1014) for silver aggregates. The potential and novelty of this synthesis is that no reducing agent or copolymer was used, nor was any preliminary functionalization of the surface carried out. In addition, the AgNPs were fabricated directly on the substrate's surface; consequently, there was no need for polymer etching. Then, the synthetic method was successfully applied to prepare opaque SERS platforms. Opaque surfaces are needed in photonic devices because of the absence of secondary back reflection, which makes optical analysis and applications easier.

Advanced Large-Scale Nanofabrication Route for Ultrasensitive SERS Platforms Based on Precisely Shaped Gold Nanostructures.

One of the key issues for SERS-based trace applications is engineering structurally uniform substrates with ultrasensitivity, stability, and good reproducibility. A label-free, cost-effective, and reproducible fabrication strategy of ultrasensitive SERS sensors was reported in this work. Herein, we present recent progress in self-assembly-based synthesis to elaborate precisely shaped and abundant gold nanoparticles in a large area. We demonstrated that shape control is driven by the selective adsorption of a cation (Na+, K+, and H+) on a single facet of gold nanocrystal seeds during the growth process. We studied SERS features as a function of morphology. Importantly, we found a correlation between the shape and experimental SERS enhancement factors. We observed a detection threshold of 10-20 M of bipyridine ethylene (BPE), which matches the lowest value determined in literature for BPE until now. Such novel sensing finding could be very promising for diseases and pathogen detection and opens up an avenue toward predicting which other morphologies could offer improved sensitivity.