Magnetic-Based Double Fano Resonances in Au-SiO2-Si Multilayer Nanoshells

Plasmonics - Compared to metallic nanostructures employed in plasmonics, dielectric materials with high refractive index can directly engineer magnetic responses in addition to the electric...     

Simultaneous Synthesis of Single-walled Carbon Nanotubes and Graphene in a Magnetically-enhanced Arc Plasma

Carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene attract a deluge of interest of scholars nowadays due to their very promising application for molecular sensors, field effect transistor and super thin and flexible electronic devices^1-4. Anodic arc discharge supported by the erosion of the anode material is one of the most practical and efficient methods, which can provide specific non-equilibrium processes and a high influx of carbon material to the developing structures at relatively higher temperature, and consequently the as-synthesized products have few structural defects and better crystallinity.To further improve the controllability and flexibility of the synthesis of carbon nanostructures in arc discharge, magnetic fields can be applied during the synthesis process according to the strong magnetic responses of arc plasmas. It was demonstrated that the magnetically-enhanced arc discharge can increase the average length of SWCNT ⁵, narrow the diameter distribution of metallic catalyst particles and carbon nanotubes ⁶, and change the ratio of metallic and semiconducting carbon nanotubes ⁷, as well as lead to graphene synthesis ⁸. Furthermore, it is worthwhile to remark that when we introduce a non-uniform magnetic field with the component normal to the current in arc, the Lorentz force along the J×B direction can generate the plasmas jet and make effective delivery of carbon ion particles and heat flux to samples. As a result, large-scale graphene flakes and high-purity single-walled carbon nanotubes were simultaneously generated by such new magnetically-enhanced anodic arc method. Arc imaging, scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman spectroscopy were employed to analyze the characterization of carbon nanostructures. These findings indicate a wide spectrum of opportunities to manipulate with the properties of nanostructures produced in plasmas by means of controlling the arc conditions.     

Enhanced Optical Transmission Assisted Near-Infrared Plasmonic Optical Filter via Hybrid Subwavelength Structures

Plasmonics - Enhanced optical transmission (EOT), which results from the incident light interacting with the subwavelength nanostructures patterned in a metallic film, showcases the excitation of...     

Switchable DNA-origami nanostructures that respond to their environment and their applications

Structural DNA nanotechnology, in which Watson-Crick base pairing drives the formation of self-assembling nanostructures, has rapidly expanded in complexity and functionality since its inception in 1981. DNA nanostructures can now be made in arbitrary three-dimensional shapes and used to scaffold many other functional molecules such as proteins, metallic nanoparticles, polymers, fluorescent dyes and small molecules. In parallel, the field of dynamic DNA nanotechnology has built DNA circuits, motors and switches. More recently, these two areas have begun to merge—to produce switchable DNA nanostructures, which change state in response to their environment. In this review, we summarise switchable DNA nanostructures into two major classes based on response type: molecular actuation triggered by local chemical changes such as pH or concentration and external actuation driven by light, electric or magnetic fields. While molecular actuation has been well explored, external actuation of DNA nanostructures is a relatively new area that allows for the remote control of nanoscale devices. We discuss recent applications for DNA nanostructures where switching is used to perform specific functions—such as opening a capsule to deliver a molecular payload to a target cell. We then discuss challenges and future directions towards achieving synthetic nanomachines with complexity on the level of the protein machinery in living cells.     

A Paper-Fiber-Supported 3D SERS Substrate

Plasmonics - Surface-enhanced Raman scattering (SERS) spectroscopy is an effective approach for trace-level detection of molecular substance. Plasmonic metallic nanostructures with high...     

Organizing protein-DNA hybrids as nanostructures with programmed functionalities

The structural and functional information encoded in the base sequence of nucleic acids provides a means to organize hybrid protein-DNA nanostructures with pre-designed, programmed functionality. This review discusses the activation of enzyme cascades in supramolecular DNA-protein hybrid structures, the bioelectrocatalytic activation of redox enzymes on DNA scaffolds, and the programmed positioning of enzymes on 1D, 2D and 3D DNA nanostructures. These systems provide starting points towards the design of interconnected enzyme networks. Substantial progress in the tailoring of functional protein-DNA nanostructures has been accomplished in recent years, and advances in this field warrant a comprehensive discussion. The application of these systems for the control of biocatalytic transformations, for amplified biosensing, and for the synthesis of metallic nanostructures are addressed, and future prospects for these systems are highlighted.     

Annealing Process in the Refurbishment of the Plasmonic Photonic Structures Fabricated Using Colloidal Gold Nanoparticles

Solution-processible fabrication techniques have been demonstrated with promising features for realizing different types of plasmonic devices, which combine interference lithography, spin-coating of the colloidal gold nanoparticles, and subsequent annealing process at a temperature of 200–300 °C. However, the resultant device needs to be improved in the following considerations: (1) The photoresist master grating needs to be removed for the applications in optoelectronic or sensor devices and (2) each lattice site of the photonic crystals is still composed of closely contacted gold nanoparticles. Actually, these metallic photonic structures can be refurbished through a further annealing process. Using an annealing temperature above 450 °C, we have successfully removed the remaining photoresist and make the gold nanoparticles join into a solid homogenous unit on each lattice site after being fully molten. Thus, high-quality gold nanostructures with excellent plasmonic response can be obtained. This accomplished an improved recipe for the solution-processible fabrication of plasmonic nanostructures. The corresponding devices with improved optical properties become more suitable for biosensors and optoelectronic devices.     

Plasmon-Enhanced Fluorescence Biosensors: a Review

Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

     

Biological synthesis of gold nanowires using extract of Rhodopseudomonas capsulata

An environmentally friendly method using a cell-free extract (CFE) of Rhodopseudomonas capsulata is proposed to synthesize gold nanowires with a network structure. This procedure offers control over the shapes of gold nanoparticles with the change of HAuCl4 concentration. The CFE solutions were added with different concentrations of HAuCl4, resulting in the bioreduction of gold ions and biosynthesis of morphologies of gold nanostructures. It is probable that proteins acted as the major biomolecules involved in the bioreduction and synthesis of gold nanoparticles. At a lower concentration of gold ions, exclusively spherical gold nanoparticles with sizes ranging from 10 to 20 nm were produced, whereas gold nanowires with a network structure formed at the higher concentration of gold ions in the aqueous solution. This method is expected to be applicable to the synthesis of other metallic nanowires such as silver and platinum, and even other anisotropic metal nanostructures are expected using the biosynthetic methods.     

The Effect of Negative Curvature on the Plasmonic Coupling of Concentric Core–Shell Metallic Nanostructures

Negative curvature-dependent localized surface plasmon resonance (LSPR) properties of concentric core–shell metallic nanostructure have been studied using quasistatic approach and plasmon hybridization theory. Whether in single-layered gold nanoshell or double gold nanoshells, the oscillating surface charges always concentrate close to the poles of the metal surface with negative curvature, which results in the anisotropic local electric field distribution and affects both the inter-surface plasmonic coupling and inter-shell plasmonic coupling. Therefore, the change of the radius of the gold surface with negative curvature could modulate the plasmon hybridization and lead to the LSPR shifting. The physical mechanism of the negative curvature-dependent LSPR presents a potential for design and fabrication of nanoscale optical device based on core–shell type metallic nanostructures.     

Plasmonic Focusing in Nanostructures

In this review, we show that by designing the metallic nanostructures, the surface plasmon (SP) focusing has been achieved, with the focusing spot at a subwavelength scale. The central idea is based on the principle of optical interference that the constructive superposition of SPs with phase matching can result in a considerable electric-field enhancement of SPs in the near field, exhibiting a pronounced focusing spot. We first reviewed several new designs for surface plasmon focusing by controlling the metallic geometry or incident light polarization: We made an in-plane plasmonic Fresnel zone plates, a counterpart in optics, which produces an obvious SP focusing effect; We also fabricated the symmetry broken nanocorrals which can provide the spatial phase difference for SPs, and then we propose another plasmon focusing approach by using semicircular nanoslits, which gives rise to the phase difference through changing refractive index of the medium in the nanoslits. Further, we showed that the spiral metallic nanostructure can be severed as plasmonic lens to control the plasmon focusing under a linearly polarized light with different angles.

     

Gyroid Nickel Nanostructures from Diblock Copolymer Supramolecules

Nanoporous metal foams possess a unique combination of properties - they are catalytically active, thermally and electrically conductive, and furthermore, have high porosity, high surface-to-volume and strength-to-weight ratio. Unfortunately, common approaches for preparation of metallic nanostructures render materials with highly disordered architecture, which might have an adverse effect on their mechanical properties. Block copolymers have the ability to self-assemble into ordered nanostructures and can be applied as templates for the preparation of well-ordered metal nanofoams. Here we describe the application of a block copolymer-based supramolecular complex - polystyrene-block-poly(4-vinylpyridine)(pentadecylphenol) PS-b-P4VP(PDP) - as a precursor for well-ordered nickel nanofoam. The supramolecular complexes exhibit a phase behavior similar to conventional block copolymers and can self-assemble into the bicontinuous gyroid morphology with two PS networks placed in a P4VP(PDP) matrix. PDP can be dissolved in ethanol leading to the formation of a porous structure that can be backfilled with metal. Using electroless plating technique, nickel can be inserted into the template''s channels. Finally, the remaining polymer can be removed via pyrolysis from the polymer/inorganic nanohybrid resulting in nanoporous nickel foam with inverse gyroid morphology.     

Broadening of Plasmonic Resonance Due to Electron Collisions with Nanoparticle Boundary: а Quantum Mechanical Consideration

We present a quantum mechanical approach to calculate broadening of plasmonic resonances in metallic nanostructures due to collisions of electrons with the surface of the structure. The approach is applicable if the characteristic size of the structure is much larger than the de Broglie electron wavelength in the metal. The approach can be used in studies of plasmonic properties of both single nanoparticles and arrays of nanoparticles. Energy conservation is insured by a self-consistent solution of Maxwell's equations and our model for the photon absorption at the metal boundaries. Consequences of the model are illustrated for the case of spheroid nanoparticles, and results are in good agreement with earlier theories. In particular, we show that the boundary-collision broadening of the plasmonic resonance in spheroid nanoparticles can depend strongly on the polarization of the impinging light.     

Metal-enhanced fluorescence: an emerging tool in biotechnology

Over the past 15 years, fluorescence has become the dominant detection/sensing technology in medical diagnostics and biotechnology. Although fluorescence is a highly sensitive technique, where single molecules can readily be detected, there is still a drive for reduced detection limits. The detection of a fluorophore is usually limited by its quantum yield, autofluorescence of the samples and/or the photostability of the fluorophores; however, there has been a recent explosion in the use of metallic nanostructures to favorably modify the spectral properties of fluorophores and to alleviate some of these fluorophore photophysical constraints. The use of fluorophore-metal interactions has been termed radiative decay engineering, metal-enhanced fluorescence or surface-enhanced fluorescence.     

Nanostructures as tailored biological probes

A new generation of spectroscopic dyes is gradually becoming available to biological researchers, from an unexpected source: materials chemists who study the synthesis and properties of nano-sized inorganic objects. Research into tailoring the optical properties, surface chemistry and biocompatibility of metallic and semiconductor nanoparticles, exemplified in part by a recent report by Mirkin, Schatz and coworkers, is fulfilling the promise of these nanostructures as customizable substitutes for organic molecular probes.     

SERS Properties of Different Sized and Shaped Gold Nanoparticles Biosynthesized under Different Environmental Conditions by Neurospora crassa Extract

Surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces. It is known that metal nanoparticles, especially gold and silver nanoparticles, exhibit great SERS properties, which make them very attractive for the development of biosensors and biocatalysts. On the other hand, the development of ecofriendly methods for the synthesis of metallic nanostructures has become the focus of research in several countries, and many microorganisms and plants have already been used to biosynthesize metallic nanostructures. However, the majority of these are pathogenic to plants or humans. Here, we report gold nanoparticles with good SERS properties, biosynthesized by Neurospora crassa extract under different environmental conditions, increasing Raman signals up to 40 times using methylene blue as a target molecule. Incubation of tetrachloroauric acid solution with the fungal extract at 60°C and a pH value of a) 3, b) 5.5, and c) 10 resulted in the formation of gold nanoparticles of a) different shapes like triangles, hexagons, pentagons etc. in a broad size range of about 10-200 nm, b) mostly quasi-spheres with some different shapes in a main size range of 6-23 nm, and c) only quasi-spheres of 3-12 nm. Analyses included TEM, HRTEM, and EDS in order to corroborate the shape and the elemental character of the gold nanoparticles, respectively. The results presented here show that these ‘green’ synthesized gold nanoparticles might have potential applicability in the field of biological sensing.     

Fluorescence fluctuations analysis in nanoapertures: physical concepts and biological applications

During the past years, nanophotonics has provided new approaches to study the biological processes below the optical diffraction limit. How single molecules diffuse, bind and assemble can be studied now at the nanometric level, not only in solutions but also in complex and crowded environments such as in live cells. In this context fluorescence fluctuations spectroscopy is a unique tool since it has proven to be easy to use in combination with nanostructures, which are able to confine light in nanometric volumes. We review here recent advances in fluorescence fluctuations’ analysis below the optical diffraction limit with a special focus on nanoapertures milled in metallic films. We discuss applications in the field of single-molecule detection, DNA sequencing and membrane organization, and underscore some potential perspectives of this new emerging technology.     

Enhancing LSPR Sensitivity of Au Gratings through Graphene Coupling to Au Film

A particular interesting plasmonic system is that of metallic nanostructures interacting with metal films. As the localized surface plasmon resonance (LSPR) behavior of gold nanostructures (Au NPs) on the top of a gold thin film is exquisitely sensitive to the spacer distance of the film-Au NPs, we investigate in the present work the influence of a few-layered graphene spacer on the LSPR behavior of the NPs. The idea is to evidence the role of few-layered graphene as one of the thinnest possible spacer. We first show that the coupling to the Au film induces a strong lowering at around 507 nm and sharpening of the main LSPR of the Au NPs. Moreover, a blue shift in the main LSP resonance of about 13 nm is observed in the presence of a few-layered graphene spacer when compared to the case where gold nanostructures are directly linked to a gold thin film. Numerical simulations suggest that this LSP mode is dipolar and that the hot spots of the electric field are pushed to the top corners of the NPs, which makes it very sensitive to surrounding medium optical index changes and thus appealing for sensing applications. A figure of merit of such a system (gold/graphene/Au NPs) is 2.8, as compared to 2.1 for gold/Au NPs. This represents a 33 % gain in sensitivity and opens-up new sensing strategies.     

Selecting peptides for use in nanoscale materials using phage-displayed combinatorial peptide libraries

Materials that combine inorganic components and biological molecules provide a new paradigm for synthesizing nanoscale and larger structures with tailored physical properties. These synthesis techniques utilize the molecular recognition properties of many biological molecules to nucleate and control growth of the nanoscale structure. Phage-displayed peptide libraries are a powerful tool to identify peptides that selectively recognize and bind to a variety of inorganic surfaces that are utilized in electronic and photonic devices. These libraries have been used extensively to study the peptide-mediated nucleation and growth of some metallic and semiconducting materials, and the application to designed nanostructures has been demonstrated.