Showing papers in "Plasmonics in 2009"

Shape-Controlled Synthesis of Silver Nanoparticles for Plasmonic and Sensing Applications

Abstract: The localized surface plasmon resonance of a silver nanoparticle is responsible for its ability to strongly absorb and scatter light at specific wavelengths. The absorption and scattering spectra (i.e., plots of cross sections as a function of wavelength) of a particle can be predicted using Mie theory (for a spherical particle) or the discrete dipole approximation method (for particles in arbitrary shapes). In this review, we briefly discuss the calculated spectra for silver nanoparticles with different shapes and the synthetic methods available to produce these nanoparticles. As validated in recent studies, there is good agreement between the theoretically calculated and the experimentally measured spectra. We conclude with a discussion of new plasmonic and sensing applications enabled by the shape-controlled nanoparticles.

Surface Plasmon Enhancement of Optical Absorption in Thin-Film Silicon Solar Cells

Abstract: Strong electromagnetic field enhancement that occurs under conditions of the surface plasmon excitation in metallic nanoparticles deposited on a semiconductor surface is a very efficient and promising tool for increasing the optical absorption within semiconductor solar cells and, hence, their photocurrent response. The enhancement of the optical absorption in thin-film silicon solar cells via the excitation of localized surface plasmons in spherical silver nanoparticles is investigated. Using the effective medium model, the effect of the nanoparticle size and the surface coverage on that enhancement is analyzed. The optimum configuration and the nanoparticle parameters leading to the maximum enhancement in the optical absorption and the photocurrent response in a single p-n junction silicon cell are obtained. The effect of coupling between the silicon layer and the surface plasmon fields on the efficiency of the above enhancement is quantified as well.

Recent Advances in the Synthesis of Plasmonic Bimetallic Nanoparticles

Abstract: The importance of plasmonic nanoparticles in sensor development, imaging, photodiagnostics, and optoelectronics has resulted in a strong interest toward the development of straightforward synthetic methods for their preparation. In this article, we review some of the most significant advances that have been made in the synthesis of plasmonic bimetallic nanoparticles. Approaches to control morphology, with an emphasis on reactions that produce uniform particles with well-defined sizes and shapes and in high yields, are described. In addition, several characterization techniques that have been employed to elucidate the morphology of the particles are illustrated.

Preparation of Gold Nanoparticles and their Applications in Anisotropic Nanoparticle Synthesis and Bioimaging

Abstract: In this review, we highlight our recent achievements in using colloidal gold nanoparticles as building blocks for fabrication of anisotropic and multicomponent nanoparticles (e.g., nanoshells, semiconductor nanocrystals, and gold nanorods). The tunable optical properties of these nanoparticles are well suited for various biomedical and biophotonic applications.

Localization of near-field resonances in bowtie antennae: influence of adhesion layers

Abstract: A plasmonic nanostructure for enhanced light excitation is disclosed. The plasmonic nanostructure includes a substrate, an adhesion layer disposed on top of the substrate, a surface plasmon resonance layer, and a cavity that extends into the surface plasmon resonance layer. The surface plasmon resonance layer is configured to concentrate an applied plasmon field to a bottom portion of the cavity.

Colloidal Synthesis of Plasmonic Metallic Nanoparticles

Abstract: Solutions of Ag and Au nanoparticles are strongly colored because of localized surface plasmon resonance in the UV/visible spectral region. The optical properties of these nanoparticles may be tuned to suit the needs of the application. This article summarizes our work in recent years on the solution synthesis of nanoparticles with tunable optical properties. The systems of interest include zero-dimensional bimetallic Ag–Au nanoparticles with different structures, one-, two-, and three-dimensional anisotropic monometallic Ag or Au nanoparticles. All of these nanosystems were prepared from colloidal synthesis through simple changes in the synthesis conditions. This is a demonstration of the versatility of colloidal synthesis as a convenient scalable technique for tuning the properties of metallic nanoparticles.

Theoretical and Experimental Investigation of Enhanced Transmission Through Periodic Metal Nanoslits for Sensing in Water Environment

Abstract: Experimental and theoretical study of sensors based on enhanced transmission through periodic metal nanoslits is presented. Our approach consists of the design of one-dimensional nanoslits array and its application in sensing for water quality control. Rigorous coupled waves analysis was used for the design and fit to the experimental data. Two types of surface plasmon resonance excitations are shown to be possible, one at the upper grating–analyte interface and one at the lower grating–substrate interface. This latter resonance is shown to be affected by the multiple interference or cavity-type effects. Those structures were fabricated by deposition of the metal layer and electron beam lithography of the nanostructure. We found that Ag-based periodic array exhibits the highest sensitivity to refractive index variations. Sensitivity enhancement was measured by ethanol concentrations in water. Stability of the Ag-based sensor was improved by covering the grating with less than 15 nm polymethyl methacrylate capping layer without deterioration of the sensitivity.

Surface Plasmon Effects Excited by the Dielectric Hole in a Silver-Shell Nanospherical Pair

Abstract: Using the finite-element method, the surface plasmon effects in a three-dimensional silver-shell nanospherical pair with five different dielectric holes (DHs) that interact with a transverse magnetic mode incident plane wave are investigated. The proposed structure exhibits a red-shifted localized surface plasmon that can be tuned over an extended wavelength range by varying the dielectric constant and the radii in DHs. The increase in the near-field intensity is attributed to a larger effective size of DH that is filled with a higher refractive index medium. The predictive character of these calculations allows one to tailor the shape of the nanoparticle to achieve excitation spectra on demand with a controlled field enhancement.

Superfocusing and Light Confinement by Surface Plasmon Excitation Through Radially Polarized Beam

Abstract: We theoretically demonstrate the possibility of obtaining nanosources through an original schema based on the generation of the radially polarized surface plasmon mode of a cylindrical metallic tip. This mode has no cutoff radius and can propagate along the tip walls until its nanometric-sized apex. Instead of radiating from the tip end, the guided mode will give rise to a nanospotlight via the well-known antenna effect. 3D calculations demonstrate that both surface plasmon-guided mode and antenna effect are directly involved in the light confinement. Near-field optical microscopy can benefit significantly from this kind of probe because the sample does not need to be directly illuminated.

Unidirectional Surface Plasmon Polariton Excitation on Single Slit with Oblique Backside Illumination

Abstract: We have theoretically investigated the unidirectional surface plasmon polariton (SPP) excitation on single slits with oblique backside illumination. An aperture diffraction method is devised, from which the conditions of slit width and beam illumination angle for the unidirectional SPP excitation are formulated analytically. The derived unidirectional conditions are validated with vectorial electromagnetic simulation using the rigorous coupled wave analysis.

Tuning Optical Discs for Plasmonic Applications

Abstract: We present simple physical and chemical procedures that allow tuning and modification of the topography of gratings present in optical storage discs into geometries optimal for grating coupled plasmon resonance excitation. After proper metal coating, the tuned surfaces exhibit sharp plasmon resonances that can be excited at wavelengths ranging from 260 nm to over 2.7 μm with relatively high quality factors. As an immediate exemplary application, use of such optimized gratings in aqueous medium for refractive index measurement is demonstrated.

Preparation of Nanostructured Film Arrays for Transmission Localized Surface Plasmon Sensing

Abstract: This article presents a concise review of preparation methods for transparent nanostructured films, with an emphasis on their current applications in transmission-localized surface plasmon resonance (T-LSPR) sensing. One of the first methods used for the fabrication of transparent nanostructured metal films is a direct vacuum evaporation of thin gold films. Self-induced formations of small gold islands result in transparent nanostructured gold arrays. The most well-established method is a nanosphere lithography developed by Van Duyne. Nanotriangular island arrays with controlled size and optical properties can be fabricated by this protocol. A different nanolithography method known as focused ion beam milling is reported and used for the fabrication of nanohole arrays. Simple assembly of solution-phase synthesized nanoparticles has also been utilized for the preparation of nanoparticle arrays capable of T-LSPR sensing. Lastly, this article also describes a new preparation strategy, in which self-assembly/thermolysis of nanoparticle multilayers is employed to obtain transparent nanoisland architectures on glass substrates.

SERS Study of Tetrodotoxin (TTX) by Using Silver Nanoparticle Arrays

Abstract: The optical properties of tetrodotoxin (TTX), a scarce toxin with anesthetic properties, were studied using nanoparticle arrays-assisted surface-enhanced Raman scattering (SERS). The nanoparticles arrays were fabricated using nanosphere lithography and a metallic lift-off process to control the particle size, shape, and spacing in the arrays. Using density functional methods, the Raman spectrum of TTX was also calculated with Gaussian03 software. The main peaks of the spectrum are originated from the vibration of the NH2 molecule group. In the SERS experiment, we were able to measure the Raman spectrum with a TTX concentration as less as 0.9 ng/mL. This sensitivity is comparable to that from high performance liquid chromatography.

Plasmonic Properties of Silver Nanoparticles on Two Substrates

Abstract: In this paper, we examine the plasmonic properties of silver nanoparticles, with an emphasis on the sensitivity of the extinction spectra on the supporting substrate: silica (SiO2) microsphere and indium tin oxide (ITO) coated glass slide, on which silver particles are deposited electroless and electrochemically, respectively. The microstructures and phases of these nanoparticles are characterized by transmission electron microscopy, field emission electron microscopy and X-ray diffraction analysis. The surface plasmon resonance (SPR) properties which are experimentally measured in the ultraviolet-visible-near infrared spectral region are compared to electrodynamics calculations based on the discrete dipole approximation. A wide SPR band ranging from 400 to 800 nm is observed for the silver nanoparticles on a silica microsphere, which is similar to the plasmon resonance characteristics of metal nanoshells. The SPR of a conducting substrate, however, has an effect on the plasmonic properties of silver nanoparticles at longer wavelength.

Oligonucleotide-Mediated Au–Ag Core–Shell Nanoparticles

Abstract: The bimetallic core–shell nanoparticles show unique plasmonic properties and their preparations and characterizations are currently under investigation. A new type of Au core–Ag shell (Au@Ag) nanoparticles is prepared by sandwiching the chemically attached Raman reporter molecules (RRMs) and a 12-base-long oligonucleotide between the 13 nm average size core-gold nanoparticles (AuNPs) and 9 nm and 21 nm average size of Ag shell. The synthesized Au@Ag nanoparticles are tested for their surface-enhanced Raman scattering (SERS) performance. It is found that the chemical attachment of the oligonucleotides along with the RRM improved the enhancement in Raman scattering more than one order of the magnitude with the Au@Ag nanoparticles with an average 9-nm shell thickness while the Au@Ag nanoparticles with 21 nm average shell thickness have poor SERS activity. A minimum enhancement factor of 1.0 × 107 is estimated for the SERS active oligonucleotide-mediated Au@Ag nanoparticles. The approach may provide new routes for preparation of highly sensitive new generation of bimetallic core–shell nanoparticles.

Functionalization of Gold Nanorods Toward Their Applications

Abstract: The topics focusing on functionalization of gold nanorods have been reviewed with a view toward their advanced uses. In most cases, as-prepared gold nanorods are hydrophilic and protected by surfactants, since anisotropic growth of gold nanorods by chemical, electrochemical, and photo-induced methods is carried out in aqueous media in the presence of surfactants and additives. Since solvophilicity of gold nanorods predominantly affects on their optical properties, the control of dispersity of gold nanorods in matrices has been performed, without loss of their optical characters, by surface modification and hybridization with small molecules or polymers. As a result of the functionalization procedure, the capability of self-assembly of gold nanorods has been improved. Furthermore, the examples of application using gold nanorods demonstrate that gold nanorod is a promising material.

On the Surface Plasmon Resonance Modes of Metal Nanoparticle Chains and Arrays

Abstract: A method for estimating the surface plasmon resonance modes of metal nanoparticle chains and arrays within a multilayered medium is proposed. In this fully retarded point-dipole method, an inhomogeneous background is replaced with a homogeneous one, based on an effective refractive index approximation. The proposed method includes the effects of retardation, radiative damping, and dynamic depolarization due to the finite size of the nanoparticles. The use of diagonal terms of dyadic Green’s functions and different polarizability coefficients along the semi-axes of ellipsoidal nanoparticles provides a complete set of both longitudinal and transverse resonance modes. Numerical results are compared with experimental results found in the literature.

Van der Waals Forces Between Plasmonic Nanoparticles

Abstract: We propose a new approach to calculate van der Waals forces between nanoparticles where the van der Waals energy can be reduced to the energy of localized plasmons in nanoparticles. The general theory is applied to describe the interaction between two metallic nanoparticles and between a nanoparticle and a perfectly conducting plane. Our results could be used to prove experimentally the existence of new, recently predicted type of plasmon oscillation (Klimov and Guzatov, Phys Rev B 75:024303, 2007a; Klimov and Guzatov, Quantum Electron 37:209, 2007b) and to elaborate new control mechanisms for the adherence of nanoparticles between each other or onto surfaces.

Polarization-Dependent Confocal Imaging of Individual Ag Nanorods and Nanoparticles

Abstract: Polarization-dependent inelastic optical scattering (IOS) of individual Ag nanorods and nanoparticles are studied by confocal imaging. Stronger IOS is observed at two ends of the nanorod with laser polarizing parallel to the rod long axis while the IOS images of Ag nanoparticles elongate along laser polarization direction. The correlation between the far-field IOS image and near-field spatial distributions of the nanostructures′ electric field can be obtained. The IOS imaging is demonstrated to be an effective technique to study the optical properties of metal nanostructures, which also provides information for their applications in surface-enhanced Raman scattering.

Near-Infrared Metal-Enhanced Fluorescence Using a Liquid–Liquid Droplet Micromixer in a Disposable Poly(Methyl Methacrylate) Microchip

Abstract: Solution-based metal-enhanced fluorescence of near-infrared fluorophores in a poly(methyl methacrylate) microchip is studied. A liquid–liquid droplet micromixer is used for rapid mixing of fluorophores with silver nanoparticles while maintaining discrete packets of known analytes for reproducible quantitative analysis. Nanoparticle aggregation within the microchip is controlled by individually adjusting salt concentration, colloid concentration, and mixing efficiency. Results identify an optimal salt concentration for aggregate formation and enhanced fluorescence, while the impacts of colloid concentration and mixing efficiency increase linearly, suggesting the possibility of further enhancement. Fluorescence enhancements of 35-fold were achieved on a microfluidics device using metal-enhanced fluorescence in a discrete solution-based system with exposure times of only 50 ms.

Wavelength-Ratiometric Plasmon Light Scattering-Based Immunoassays

Abstract: The application of a wavelength-ratiometric plasmon light scattering technique to immunoassays is demonstrated. A model immunoassay for anti-immunoglobulin G (IgG), constructed in gold colloid-modified high-throughput screening wells, was monitored by the changes in the intensity of scattered light (with transmitted light) from gold colloids as a result of antibody–antibody interactions. The quantitative determination of anti-IgG was undertaken by measuring the ratio of intensity of scattered light at both 590 and 500 nm. A white light-emitting diode (LED) and a fiber optic coupled fluorometer was used as an excitation source and the detection system, respectively. The visual confirmation of the quantitative nature of the measurement technique was done by digital photography. A lower detection limit of 0.05 µg/mL for anti-IgG was determined. The wavelength-ratiometric plasmon light scattering technique offers several advantages: (1) light at >500 nm can be used for reduced biological autofluorescence; (2) due to the ratiometric nature of these measurements, the fluctuations in the excitation or ambient light do not perturb the measured signal; and (3) with the addition of automated detection systems, multiple samples in a high-throughput format can potentially be assessed quickly and more efficiently.

A Simple and Sensitive SPRS Method for Trace H2O2 Based on the TiO2+ Complex and Gold Nanoparticles Aggregation Reactions

Abstract: In the medium of H2SO4 and in the presence of TiO2+, gold nanoparticles in size of 10 nm exhibited a weak surface plasmon resonance scattering (SPRS) peak at 775 nm. Upon addition of trace H2O2, the yellow complex [TiO(H2O2)]2+ formed that cause the gold nanoparticles aggregations to form bigger gold nanoparticle clusters in size of about 900 nm, and the SPRS intensity at 775 nm (I) enhanced greatly. The enhanced intensity ΔI was linear to the H2O2 concentration in the range of 0.025–48.7 μg/mL, with a detection limit of 0.014 μg/mL H2O2. This SPRS method was applied to determining H2O2 in water samples with satisfactory results.

Synthesis, Conforming, Linear, and Non-linear Optical Properties of Gold Nanoparticles-sepiolite Compacts

Abstract: Gold monodispersed nanoparticles (smaller than 10 nm) have been embedded into sepiolite fibres and sintered by spark plasma sintering (SPS) in order to preserve their nanometre size. The optical properties of these nanoparticles have been measured showing the surface plasmon resonance. Plasmon absorption curves have been successfully fitted by using the quasistatic approximation for gold nanoparticles. It has been observed that the sintering process decreases the plasmon width. SPS sintered nanostructured compacts present large values of the non-linear third-order dielectric susceptibility related to the gold concentration.

Surface Plasmon as a Probe of Local Field Enhancement

Abstract: New method of experimental determination of local field enhancement at metal nanoparticles is suggested. It uses surface plasmon as a probe. Alternating-sign shift of surface plasmon resonance in copper nanoparticles incorporated in silica matrix has been observed under irradiation by intense femtosecond laser pulse. The red shift of plasmon observed during the action of pump pulse is interpreted as a result of change of dielectric constant of silica matrix due to optical Kerr effect in electric field of pump pulse enhanced in a vicinity of metal nanoparticles. The field enhancement factor is estimated from the value of the observed red shift of plasmon resonance.

Solution-Deposited Thin Silver Films on Plastic Surfaces for Low-Cost Applications in Plasmon-Coupled Emission Sensors

Abstract: We report the deposition of highly uniform thin silver films on plastic materials using a wet-chemistry method, suitable for surface plasmon-coupled emission (SPCE). This approach is reproducible for diverse low-cost applications and versatile to generate silver surfaces on various plastics substrates. An oxygen plasma pretreatment of the plastic provides for rapid silvering, leading to a 47-nm-thick continuous film for SPCE applications. The surface smoothness and thickness of the films have been estimated using atomic force microscope. The higher refractive index of polycarbonate, resulted in an SPCE angle of θF = 470 for Rhodamine B, compared to glass (θF = 500). The current study presents details on film deposition conditions, appropriate choice of index matching fluids, substrates, and light sources that play a vital role to augment SPCE emission intensity.

Silver Fused Conducting Fiber Formation of Au–Ag Core-Shell Nanoparticles Mediated by Ascorbic Acid

Abstract: In this paper, we report the spontaneous formation of fibrous structures consisting of assemblies of Au–Ag core-shell nanoparticles (NPs) from a solution consisting of Au–Ag core-shell NPs and l-ascorbic acid (AA). AA acted both as the reducing agent for the generation of NPs and also as the mediator for the formation of fibers. The process of fiber formation involved three steps—reduction of HAuCl4 to Au NPs by AA, subsequent formation of Au–Ag core-shell NPs after addition of AgNO3, and spontaneous formation of fibers from the mixtures in water. It took typically about 30 days to form complete fibers that are of lengths of several hundred micrometers to millimeters, although nanofibers started forming from the first day of solution preparation. The width of each of these fibers was typically about 1–4 µm with length of each segment of fiber bundle, on the order of 40 µm. Formation of fibers was also observed in absence of AgNO3. These fibers consisted of Au NPs and polymer of AA degradation products and were not electrically conducting. Also, low concentrations of AgNO3 produced fibers with low electrical conductivity. However, it was observed that increase in the amount of AgNO3 leads to the formation of fibers that were electrically conducting with conductivity values in the range of metallic conductivity. Spectroscopic and electron microscopic investigations were carried out to establish the formation of fibers. The details of fiber formation mechanism under different conditions and electrical conductivities of the fibers are discussed in the article.

Modulation of Main Lobe for Superfocusing Using Subwavelength Metallic Heterostructures

Abstract: A subwavelength metallic heterostructure is put forth for the purpose of suppressing sidelobes and improving superfocusing at a quasi-far field region. Improvement has been made by means of optimization of the heterostructure composed of structured Au and Ag thin films. By tuning thicknesses of both the structured Au and Ag films, we can modulate propagation distance of the plasmonic lens and beam width of main lobe for the superfocusing. A finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation was carried out for analyzing the focusing performance and tuning ability of the metal films. Our computational calculation results show that the sidelobes which play negative role for the focusing can be suppressed significantly in the case of the metal film thicknesses of hAu = 50 nm and hAg = 10 nm. Theoretically, the metallic structure with smaller thicknesses of the structured Au and Ag films is helpful for improving the focusing performance. This heterostructure-based device is possible to be used as a superlens or nanoprobe in data storage, nanometrology/inspection, and biosensing etc.

Features of the Secondary Emission Enhancement Near Plasmonic Gold Film

Abstract: Plasmonic gold films (PGF) prepared by vacuum deposition of gold onto quartz slides possess unique property to enhance electromagnetic signal in the near field. Spectral tuning of PGF’s plasmon band to resonance with the electronic spectra of adsorbed molecules provides selective enhancement of fluorescence or surface-enhanced Raman scattering in the far field. Plasmon-enhanced fluorescence (PEF) of mitoxantrone (mitox) as a function of the distance between gold surface and adsorbed molecules for different polarization and incidence angle of exciting light is analyzed in this work. Spectrophotometric data reveal that probability of localized plasmon excitation in gold grains increases with growth of incidence angle for s-polarized and decrease for p-polarized excitation. This fact correlates well with oblate shape of gold particles detected by Atomic force microscope. However, the fluorescence intensity of dyes deposited at fixed distance from gold surface increase with angle of incidence of p-polarized light more noticeably than for s-polarized one. Nevertheless, the behavior of mitox PEF signal upon p-polarized laser excitation and different angle of incidence are similar in appearance to such phenomenon as selective photoelectric effect. According to this observation, the near-field interactions between plasmons and molecule as possible mechanism of PEF is discussed.

Gold Nanoparticle Ring and Hole Structures for Sensing Proteins and Antigen–Antibody Interactions

Abstract: A new experimentally simple nanosphere lithography method was used to fabricate gold ring and nanohole structures. The method is based on the simultaneous self-assembly of polystyrene microspheres and gold colloids in multilayers, by a vertical deposition method. Dissolution of polystyrene microspheres resulted in the formation of a monolayer, where holes are surrounded by gold nanoparticles. The dependency of the sensitivity of the sensor platform on the size of the holes and their density has been demonstrated. Furthermore, sensing experiments have shown a high sensitivity of the hole structure toward fibrinogen, amyloid-derived diffusible ligands, and a plant protein (AT5G07010.1). It was found that the position and shape of the localized surface plasmon resonance band changed significantly as a result of the antigen–antibody recognition event.

Effect of Gold Coating on Sensitivity of Rhombic Silver Nanostructure Array

Abstract: The sensitivity is the most important parameter in the sensing field. Effort was made to study the effect of gold coating on the sensitivity of rhombic silver nanostructure array through numerical simulation using the discrete dipole approximation method. This study shows that thickness of the gold coating can be varied to tune the sensitivity of the rhombic silver nanostructure array. The Au-Ag nanostructure array is found to possess the maximum refractive index sensitivity of 714 nm/RIU when thickness of gold is 20 nm, thickness of silver is 25 nm, and refractive index of the medium is around 1.35. The condition for achieving the maximum refractive index sensitivity can be used for detecting many species of biomolecules and drugs in the future.