Showing papers on "Surface plasmon resonance published in 2008"

Biosensing with plasmonic nanosensors

Abstract: Recent developments have greatly improved the sensitivity of optical sensors based on metal nanoparticle arrays and single nanoparticles. We introduce the localized surface plasmon resonance (LSPR) sensor and describe how its exquisite sensitivity to size, shape and environment can be harnessed to detect molecular binding events and changes in molecular conformation. We then describe recent progress in three areas representing the most significant challenges: pushing sensitivity towards the single-molecule detection limit, combining LSPR with complementary molecular identification techniques such as surface-enhanced Raman spectroscopy, and practical development of sensors and instrumentation for routine use and high-throughput detection. This review highlights several exceptionally promising research directions and discusses how diverse applications of plasmonic nanoparticles can be integrated in the near future.

Surface Plasmon Resonance Sensors for Detection of Chemical and Biological Species

Abstract: 5.1. Detection Formats 475 5.2. Food Quality and Safety Analysis 477 5.2.1. Pathogens 477 5.2.2. Toxins 479 5.2.3. Veterinary Drugs 479 5.2.4. Vitamins 480 5.2.5. Hormones 480 5.2.6. Diagnostic Antibodies 480 5.2.7. Allergens 481 5.2.8. Proteins 481 5.2.9. Chemical Contaminants 481 5.3. Medical Diagnostics 481 5.3.1. Cancer Markers 481 5.3.2. Antibodies against Viral Pathogens 482 5.3.3. Drugs and Drug-Induced Antibodies 483 5.3.4. Hormones 483 5.3.5. Allergy Markers 483 5.3.6. Heart Attack Markers 484 5.3.7. Other Molecular Biomarkers 484 5.4. Environmental Monitoring 484 5.4.1. Pesticides 484 5.4.2. 2,4,6-Trinitrotoluene (TNT) 485 5.4.3. Aromatic Hydrocarbons 485 5.4.4. Heavy Metals 485 5.4.5. Phenols 485 5.4.6. Polychlorinated Biphenyls 487 5.4.7. Dioxins 487 5.5. Summary 488 6. Conclusions 489 7. Abbreviations 489 8. Acknowledgment 489 9. References 489

Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine

Abstract: Noble metal nanostructures attract much interest because of their unique properties, including large optical field enhancements resulting in the strong scattering and absorption of light. The enhancement in the optical and photothermal properties of noble metal nanoparticles arises from resonant oscillation of their free electrons in the presence of light, also known as localized surface plasmon resonance (LSPR). The plasmon resonance can either radiate light (Mie scattering), a process that finds great utility in optical and imaging fields, or be rapidly converted to heat (absorption); the latter mechanism of dissipation has opened up applications in several new areas. The ability to integrate metal nanoparticles into biological systems has had greatest impact in biology and biomedicine. In this Account, we discuss the plasmonic properties of gold and silver nanostructures and present examples of how they are being utilized for biodiagnostics, biophysical studies, and medical therapy. For instance, takin...

Surface-enhanced Raman spectroscopy.

Abstract: The ability to control the size, shape, and material of a surface has reinvigorated the field of surface-enhanced Raman spectroscopy (SERS). Because excitation of the localized surface plasmon resonance of a nanostructured surface or nanoparticle lies at the heart of SERS, the ability to reliably control the surface characteristics has taken SERS from an interesting surface phenomenon to a rapidly developing analytical tool. This article first explains many fundamental features of SERS and then describes the use of nanosphere lithography for the fabrication of highly reproducible and robust SERS substrates. In particular, we review metal film over nanosphere surfaces as excellent candidates for several experiments that were once impossible with more primitive SERS substrates (e.g., metal island films). The article also describes progress in applying SERS to the detection of chemical warfare agents and several biological molecules.

Whispering-gallery-mode biosensing: label-free detection down to single molecules

Abstract: Optical label-free detectors, such as the venerable surface plasmon resonance (SPR) sensor, are generally favored for their ability to obtain quantitative data on intermolecular binding. However, before the recent introduction of resonant microcavities that use whispering gallery mode (WGM) recirculation, sensitivity to single binding events had not materialized. Here we describe the enhancement mechanisms responsible for the extreme sensitivity of the WGM biosensor, review its current implementations and applications, and discuss its future possibilities.

Ag@AgCl: A Highly Efficient and Stable Photocatalyst Active under Visible Light

Abstract: Nanoparticles (NPs) of noble metals can strongly absorb visible light because of their plasmon resonance, which is greatly influenced by their morphology and size. The phenomenon of plasmon resonance gives rise to important applications such as colorimetric sensors, photovoltaic devices, photochromic devices, and photocatalysts. Noble metal NPs exhibit characteristic optical and physical properties that are substantially different from those of the corresponding bulk materials. In particular, silver NPs show efficient plasmon resonance in the visible region, which Awazu et al. recently utilized to develop a plasmonic photocatalyst. In their study, TiO2 was deposited on NPs consisting of a silver core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2. Under UV illumination, this plasmonic photocatalyst exhibits enhanced catalytic activity, which increases with decreasing thickness of the SiO2 shell. To enhance the activity of a plasmonic photocatalyst, it is desirable to deposit silver NPs directly onto the surface of an active dielectric substrate without a protective shell, because the near-field effect of the NPs will be more strongly felt by the substrate. Herein we show that such a photocatalyst can be obtained from silver chloride by exploiting its photosensitivity, and the resulting plasmonic photocatalyst is highly efficient and stable under visible-light illumination. Silver halides are photosensitive materials extensively used as source materials in photographic films. On absorbing a photon, a silver halide particle generates an electron and a hole, and subsequently the photogenerated electron combines with an Ag ion to form an Ag atom. Ultimately, a cluster of silver atoms is formed within a silver halide particle upon repeated absorption of photons. Due to this instability under sunlight, which provides the very basis for chemical photography, silver halides are seldom used as photocatalysts. Nevertheless, there have been reports that under UV/Vis illumination AgCl deposited on a conducting support photocatalyzes O2 production from water in the presence of a small excess of silver ions in solution, and that under UV illumination AgBr dispersed on a silica support photocatalyzes H2 production from CH3OH/H2O solution. [21] In their study on the AgBr/SiO2 photocatalyst, Kakuta et al. [21] observed that Ag species are formed on AgBr in the early stage of the reaction, and AgBr is not destroyed under successive UV illumination. As suggested by Kakuta et al., electron–hole separation may occur smoothly in the presence of Ag species, and the latter may catalyze H2 production from alcohol radicals formed by photo-induced holes. If so, silver NPs formed on silver halide particles might be expected to be a stable photocatalyst under visible-light illumination due to their plasmon resonance. This expectation led us to prepare a new photocatalyst active and stable under visible light, namely, AgCl particles with silver NPs formed on their surface, by first treating Ag2MoO4 with HCl to form AgCl powder and then reducing some Ag ions in the surface region of the AgCl particles to Ag species (for details, see the Experimental Section). For convenience, these are referred to as Ag@AgCl particles. The X-ray diffraction (XRD) pattern of the Ag@AgCl product clearly shows that the cubic phase of Ag with lattice constant a= 4.0861 A (JCPDS file: 65-2871) coexists with the cubic phase of AgCl with lattice constant a= 5.5491 A (JCPDS file: 31-1238; see Figure 1). Scanning electron microscopy (SEM) images of the Ag@AgCl product (Figure 2) reveal that silver NPs with diameters in the range of 20–150 nm are deposited on the surface of AgCl particles with diameters in the range of 0.2–1.3 mm. The UV/Vis diffuse-reflectance spectra of Ag@AgCl, AgCl, and N-doped TiO2 (used as reference photocatalyst) are compared in Figure 3. In contrast to AgCl and N-doped TiO2, Ag@AgCl has a strong adsorption in the visible region which is almost as strong as that in the UV region. This is attributed to the plasmon resonance of silver NPs deposited on AgCl particles. To evaluate the photooxidation capability of Ag@AgCl, we examined the decomposition of methylic orange (MO) dye in solution over the Ag@AgCl sample under visible-light irradiation as a function of time (Figure 4). For comparison, we also carried out decomposition of the MO dye in solution over the N-doped TiO2 reference photocatalyst under visible[*] P. Wang, Prof. Dr. B. Huang, X. Qin, Prof. X. Zhang, Dr. J. Wei State Key Lab of Crystal Materials Shandong University, Jinan 250100 (China) E-mail: bbhuang@sdu.edu.cn Homepage: http://www.icm.sdu.edu.cn/index.php

Handbook of surface plasmon resonance

Abstract: Chapter 1: Introduction to Surface Plasmon Resonance Chapter 2: Physics of Surface Plasmon Resonance Chapter 3: SPR Instrumentation Chapter 4: Kinetic Models Describing Biomolecular Interactions at Surfaces Chapter 5: Kinetic and Thermodynamic Analysis of Ligand-Receptor Interactions: SPR Applications in Drug Development Chapter 6: Surface Chemistry in SPR Technology Chapter 7: Measurement of the Analysis Cycle: Scanning SPR Microarray Imaging of Autoimmune Diseases Chapter 8: Advanced Methods for SPR Imaging Biosensing Chapter 9: Surface Plasmon Fluorescence Techniques for Bioaffinity Studies Chapter 10: SPR Imaging for Clinical Diagnostics Chapter 11: The Benefits and Scope of Surface Plasmon Resonance-based Biosensors in Food Analysis Chapter 12: Future Trends in SPR Technology

A Unified Approach to Surface-Enhanced Raman Spectroscopy

Abstract: We present a unified expression for surface-enhanced Raman spectroscopy (SERS). The expression contains a product of three resonance denominators, representing the surface plasmon resonance, the metal-molecule charge-transfer resonance at the Fermi energy, and an allowed molecular resonance. This latter resonance is that from which intensity is borrowed for charge transfer, and when the molecular resonance is active it is responsible for surface-enhanced resonance Raman spectroscopy. We examine this expression in various limits, to explore the relative contribution or each resonance. First, we look at the situation in which only the surface plasmon resonance is active and examine the various contributions to the Raman signal, including the surface selection rules. Then we examine additional contributions from charge-transfer or molecular resonances. We show that the three resonances are not totally independent, since they are linked by a product of four matrix elements in the numerator. These linked matrix elements provide comprehensive selection rules for SERS. One involves a harmonic oscillator in the observed normal mode. This is the same mode which appears in the vibronic coupling operator linking one of the states of the allowed molecular resonance to the charge-transfer state. The charge-transfer transition moment is linked to the surface plasmon resonance by the requirement that the transition dipole moment be polarized along the direction of maximum amplitude of the field produced by the plasmon (i.e., perpendicular to the metal surface). We show that these selection rules govern the observed SERS spectral intensities and apply these to the observed spectra of several molecules. We also suggest a quantitative measure of the degree to which charge transfer contributes to the overall SERS enhancement.

Localized Surface Plasmon Resonances in Aluminum Nanodisks

Abstract: The plasmonic properties of arrays of supported Al nanodisks, fabricated by hole-mask colloidal lithography (HCL), are analyzed for the disk diameter range 61-492 nm at a constant disk height of 20 nm. Strong and well-defined (UV-vis-NIR) localized surface plasmon resonances are found and experimentally characterized with respect to spectral peak positions, peak widths, total cross sections, and radiative and nonradiative decay channels. Theoretically, the plasmon excitations are described by electrostatic spheroid theory. Very good qualitative and quantitative agreement between model and experiment is found for all these observables by assuming a nanoparticle embedded in a few nanometer thick homogeneous (native) aluminum oxide shell. Other addressed aspects are: (i) the role of the strong interband transition in Al metal, located at 1.5 eV, for the plasmonic excitations of Al nanoparticles, (ii) the role of the native oxide layer, and (iii) the possibility of using the plasmon excitation as an ultrasensitive, remote, real-time probe for studies of oxidation/corrosion kinetics in metal nanoparticle systems.

Plexcitonic nanoparticles: plasmon-exciton coupling in nanoshell-J-aggregate complexes.

Abstract: Stable Au nanoshell-J-aggregate complexes are formed that exhibit coherent coupling between the localized plasmons of a nanoshell and the excitons of molecular J-aggregates adsorbed on its surface. By tuning the nanoshell plasmon energies across the exciton line of the J-aggregate, plasmon-exciton coupling energies for these complexes are obtained. The strength of this interaction is dependent on the specific plasmon mode of the nanoparticle coupled to the J-aggregate exciton. From a model based on Gans theory, we obtain an expression for the plasmon-exciton hybridized states of the complex.

Tailoring plasmonic substrates for surface enhanced spectroscopies

Abstract: Our understanding of how the geometry of metallic nanostructures controls the properties of their surface plasmons, based on plasmon hybridization, is useful for developing high-performance substrates for surface enhanced spectroscopies. In this tutorial review, we outline the design of metallic nanostructures tailored specifically for providing electromagnetic enhancements for surface enhanced Raman scattering (SERS). The concepts developed for nanoshell-based substrates can be generalized to other nanoparticle geometries and scaled to other spectroscopies, such as surface enhanced infrared absorption spectroscopy (SEIRA).

Graphene Terahertz Plasmon Oscillators

Abstract: In this paper we propose and discuss coherent terahertz sources based on charge density wave (plasmon) amplification in two-dimensional graphene. The coupling of the plasmons to interband electron-hole transitions in population inverted graphene layers can lead to plasmon amplification through stimulated emission. Plasmon gain values in graphene can be very large due to the small group velocity of the plasmons and the strong confinement of the plasmon field in the vicinity of the graphene layer. We present a transmission line model for plasmon propagation in graphene that includes plasmon dissipation and plasmon interband gain due to stimulated emission. Using this model, we discuss design for terahertz plasmon oscillators and derive the threshold condition for oscillation taking into account internal losses and also losses due to external coupling. The threshold condition is shown to depend on the ratio of the external impedance and the characteristic impedance of the plasmon transmission line. The large gain values available at terahertz frequencies in graphene can lead to integrated oscillators that have dimensions in the 1-10 mum range.

Distance-Dependent Plasmon Resonant Coupling between a Gold Nanoparticle and Gold Film

Abstract: We present an experimental analysis of the plasmonic scattering properties of gold nanoparticles controllably placed nanometers away from a gold metal film. We show that the spectral response of this system results from the interplay between the localized plasmon resonance of the nanoparticle and the surface plasmon polaritons of the gold film, as previously predicted by theoretical studies. In addition, we report that the metal film induces a polarization to the single nanoparticle light scattering, resulting in a doughnut-shaped point spread function when imaged in the far-field. Both the spectral response and the polarization effects are highly sensitive to the nanoparticle−film separation distance. Such a system shows promise in potential biometrology and diagnostic devices.

Semiconductor quantum dots and metal nanoparticles: syntheses, optical properties, and biological applications

Abstract: We review the syntheses, optical properties, and biological applications of cadmium selenide (CdSe) and cadmium selenide-zinc sulfide (CdSe-ZnS) quantum dots (QDs) and gold (Au) and silver (Ag) nanoparticles (NPs). Specifically, we selected the syntheses of QDs and Au and Ag NPs in aqueous and organic phases, size- and shape-dependent photoluminescence (PL) of QDs and plasmon of metal NPs, and their bioimaging applications. The PL properties of QDs are discussed with reference to their band gap structure and various electronic transitions, relations of PL and photoactivated PL with surface defects, and blinking of single QDs. Optical properties of Ag and Au NPs are discussed with reference to their size- and shape-dependent surface plasmon bands, electron dynamics and relaxation, and surface-enhanced Raman scattering (SERS). The bioimaging applications are discussed with reference to in vitro and in vivo imaging of live cells, and in vivo imaging of cancers, tumor vasculature, and lymph nodes. Other aspects of the review are in vivo deep tissue imaging, multiphoton excitation, NIR fluorescence and SERS imaging, and toxic effects of NPs and their clearance from the body.

Optical Properties and Growth Aspects of Silver Nanoprisms Produced by a Highly Reproducible and Rapid Synthesis at Room Temperature

Abstract: A rapid and readily reproducible seed-based method for the production of high quality silver nanoprisms in high yield is presented. The edge-length and the position of the main plasmon resonance of the nanoprisms can be readily controlled through adjustment of reaction conditions. From UV-vis spectra of solutions of the nanoprisms, the inhomogeneously broadened line width of the in-plane dipole plasmon resonance is measured and trends in the extent of plasmon damping as a function of plasmon resonance energy and nanoprism size have been elucidated. In addition, an in-depth analysis of the lamellar defect structure of silver nanoprisms is provided that confirms that the defects can lead to a transformation of the crystal structure in the vicinity of the defects. These defects can combine give rise to lamellar regions, thicker than 1 nm, that extend across the crystal, where the silver atoms are arranged in a continuous hexagonal-close-packed (hcp) structure. This hcp structure has a periodicity of 2.50 A, thus explaining the 2.50 A lattice fringes that are commonly observed in 〈111〉 oriented flat-lying nanoprisms. A new understanding of the mechanisms behind anisotropic growth in silver nanoprisms is presented.

Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays

Abstract: We demonstrate that coupling between grating diffraction and localized surface plasmons in two-dimensional gold nanoparticle arrays in water leads to narrow near-infrared resonance peaks in measured far field extinction spectra. Good agreement is obtained between finite difference time domain (FDTD) calculations and experimental extinction spectra. The FDTD calculations predict that the gold nanoparticle arrays exhibit near-field electric field intensity (E2) enhancements approximately one order of magnitude greater than those of single isolated gold nanoparticles.

Dark-field microscopy studies of single metal nanoparticles: understanding the factors that influence the linewidth of the localized surface plasmon resonance

Abstract: This article provides a review of our recent Rayleigh scattering measurements on single metal nanoparticles. Two different systems will be discussed in detail: gold nanorods with lengths between 30 and 80 nm, and widths between 8 and 30 nm; and hollow gold-silver nanocubes (termed nanoboxes or nanocages depending on their exact morphology) with edge lengths between 100 and 160 nm, and wall thicknesses of the order of 10 nm. The goal of this work is to understand how the linewidth of the localized surface plasmon resonance depends on the size, shape, and environment of the nanoparticles. Specifically, the relative contributions from bulk dephasing, electron-surface scattering, and radiation damping (energy loss via coupling to the radiation field) have been determined by examining particles with different dimensions. This separation is possible because the magnitude of the radiation damping effect is proportional to the particle volume, whereas, the electron-surface scattering contribution is inversely proportional to the dimensions. For the nanorods, radiation damping is the dominant effect for thick rods (widths greater than 20 nm), while electron-surface scattering is dominant for thin rods (widths less than 10 nm). Rods with widths in between these limits have narrow resonances-approaching the value determined by the bulk contribution. For nanoboxes and nanocages, both radiation damping and electron-surface scattering are significant at all sizes. This is because these materials have thin walls, but large edge lengths and, therefore, relatively large volumes. The effect of the environment on the localized surface plasmon resonance has also been studied for nanoboxes. Increasing the dielectric constant of the surroundings causes a red-shift and an increase in the linewidth of the plasmon band. The increase in linewidth is attributed to enhanced radiation damping.

A Label-Free Immunoassay Based Upon Localized Surface Plasmon Resonance of Gold Nanorods

Abstract: Robust gold nanorod substrates were fabricated for refractive index sensing based on localized surface plasmon resonance (LSPR). The substrate sensitivity was 170 nm/RIU with a figure of merit of 1.3. To monitor biomolecular interactions, the nanorod surfaces were covered with a self-assembled monolayer and conjugated to antibodies by carbodiimide cross-linking. Interactions with a specific secondary antibody were monitored through shifts in the LSPR spectral extinction peak. The resulting binding rates and equilibrium constant were in good agreement with literature values for an antibody–antigen system. The nanorod LSPR sensors were also shown to be sensitive and specific. These results demonstrate that given a sufficiently stable nanoparticle substrate with a well defined chemical interface, LSPR sensing yields similar results to the surface plasmon resonance technique, yet with much simpler instrumentation.

Direct observation of chemical reactions on single gold nanocrystals using surface plasmon spectroscopy.

Abstract: Heterogeneous catalysts have been pivotal to the development of the modern chemical industry and are essential for catalysing many industrial reactions. However, reaction rates are different for every individual catalyst particle and depend upon each particle's morphology and size, crystal structure and composition. Measuring the rates of reaction on single nanocrystals will enable the role of catalyst structure to be quantified. Here, using surface plasmon spectroscopy, we have directly observed the kinetics of atomic deposition onto a single gold nanocrystal and also monitored electron injection and extraction during a redox reaction involving the oxidation of ascorbic acid on a gold nanocrystal surface. These results constitute the first direct measurement of the rates of redox catalysis on single nanocrystals.

Shape-dependent plasmon resonances of gold nanoparticles

Abstract: Localized surface plasmon resonances in noble metal nanoparticles cause enhanced optical absorption and scattering that is tunable through the visible and near-infrared. Furthermore, these resonances create large local electric field enhancements at the nanoparticle surfaces, essentially focussing light at the nanometer scale. These properties suggest a range of applications, including biomedical imaging, therapeutics, and molecular sensing. Here we review some recent advances regarding shape-dependent optical properties of two specific nanoparticle geometries: gold nanorods and branched gold nanoparticles.

Ultralow Fouling and Functionalizable Surface Chemistry Based on a Zwitterionic Polymer Enabling Sensitive and Specific Protein Detection in Undiluted Blood Plasma

Abstract: A crucial step in the development of implanted medical devices, in vivo diagnostics, and microarrays is the effective prevention of nonspecific protein adsorption from real-world complex media such as blood plasma or serum. In this work, a zwitterionic poly(carboxybetaine acrylamide) (polyCBAA) biomimetic material was employed to create a unique biorecognition coating with an ultralow fouling background, enabling the sensitive and specific detection of proteins in blood plasma. Conditions for surface activation, protein immobilization, and surface deactivation of the carboxylate groups in the polyCBAA coating were determined. An antibody-functionalized polyCBAA surface platform was used to detect a target protein in blood plasma using a sensitive surface plasmon resonance (SPR) sensor. A selective protein was directly detected from 100% human blood plasma with extraordinary specificity and sensitivity. The total nonspecific protein adsorption on the functionalized polyCBAA surface was very low (<3 ng/cm2 ...

Mapping the Plasmon Resonances of Metallic Nanoantennas

Abstract: We study the light scattering and surface plasmon resonances of Au nanorods that are commonly used as optical nanoantennas in analogy to dipole radio antennas for chemical and biodetection field-enhanced spectroscopies and scanned-probe microscopies. With the use of the boundary element method, we calculate the nanorod near-field and far-field response to show how the nanorod shape and dimensions determine its optical response. A full mapping of the size (length and radius) dependence for Au nanorods is obtained. The dipolar plasmon resonance wavelength I shows a nearly linear dependence on total rod length L out to the largest lengths that we study. However, L is always substantially less than I/2, indicating the difference between optical nanoantennas and long-wavelength traditional I/2 antennas. Although it is often assumed that the plasmon wavelength scales with the nanorod aspect ratio, we find that this scaling does not apply except in the extreme limit of very small, spherical nanoparticles. The plasmon response depends critically on both the rod length and radius. Large (500 nm) differences in resonance wavelength are found for structures with different sizes but with the same aspect ratio. In addition, the plasmon resonance deduced from the near-field enhancement can be significantly red-shifted due to retardation from the resonance in far-field scattering. Large differences in near-field and far-field response, together with the breakdown of the simple scaling law must be accounted for in the choice and design of metallic I/2 nanoantennas. We provide a general, practical map of the resonances for use in locating the desired response for gold nanoantennas.

Surface Plasmon Resonance for Biosensing: A Mini-Review

Abstract: A variety of configurations and formats have been devised to exploit the phenomenon of surface plasmon on metal dielectric interfaces for sensing a variety of significant analytes, such as pesticides and explosives, pathogens and toxins, and diseased tissue. Researchers continue to aim at detecting lower concentrations in smaller volumes of samples in real time. A new research field, called nanoplasmnonics, has emerged in this regard.

Plasmonic Optical Properties and Applications of Metal Nanostructures

Abstract: In this review article, we provide an overview of recent research activities in the study of plasmonic optical properties of metal nanostructures with emphasis on understanding the relation between surface plasmon absorption and structure. Both experimental results and theoretical calculations have indicated that the plasmonic absorption strongly depends on the detailed structure of the nanomaterials. Examples discussed include spherical nanoparticles, nanorods, nanowires, hollow nanospheres, aggregates, and nanocages. Plasmon–phonon coupling measured from dynamic studies as a function of particle size, shape, and aggregation state is also reviewed. The fascinating optical properties of metal nanostructures find important applications in a number of technological areas including surface plasmon resonance, surface-enhanced Raman scattering, and photothermal imaging and therapy. Their novel optical properties and emerging applications are illustrated using specific examples from recent literature. The case of hollow nanosphere structures is highlighted to illustrate their unique features and advantages for some of these applications.

Localized Surface Plasmon Resonance Spectroscopy of Triangular Aluminum Nanoparticles

Abstract: The localized surface plasmon resonance (LSPR) of Al nanoparticles fabricated by nanosphere lithography (NSL) was examined by UV−vis extinction spectroscopy and electrodynamics theory Al triangular nanoparticle arrays can support LSP resonances that are tunable throughout the visible and into the UV portion of the spectrum Scanning electron microscope and atomic force microscope studies point to the presence of a thin native Al2O3 layer on the surface of the Al triangular nanoparticles The presence of the oxide layer, especially on the tips of the nanotriangles, results in a significant red shift in the LSPR λmax The refractive index (RI) sensitivity of the Al triangular nanoparticle arrays in bulk solvents was determined to be 0405 eV/RIU Theoretical results show that the oxide layer leads to a significant decrease in this RI sensitivity compared to unoxidized triangular nanoparticles of similar size and geometry A comparison of Al, Ag, Cu, and Au triangular nanoparticles for a similar shape and g

Visible-Light-Driven Oxidation of Organic Contaminants in Air with Gold Nanoparticle Catalysts on Oxide Supports

Abstract: One of the great challenges in catalysis is to devise new catalysts that have high activity when illuminated by visible light. Solving this challenge will allow us to use sunlight, an abundant and clean low-cost energy source, to drive chemical reactions. Visible light (wavelength l> 400 nm) constitutes around 43% of solar energy, and the energy of sunlight to the Earth is about 10000 times more than the current energy consumption of the world. Many approaches have been proposed to develop visible light photocatalysts, including doping TiO2 with metal ions or metal atom clusters, [4,5] incorporating nitrogen and carbon into TiO2, and employing other metal oxides or polymetallates as catalyst materials. 5,8] Research has been mainly concentrated on semiconductor oxides. Sulfides have also been studied, but they are not suitable catalysts because of their poor chemical stability. However, searching for catalysts that can work under visible light should not be limited to semiconductor materials with band-gap structure, but can be extended to other materials, such as gold nanoparticles. It can be said that glaziers in medieval forges were the first nanotechnologists who produced colors with gold nanoparticles of different sizes, although they had little understanding of the modern day principles which have become a hot topic in the last two decades. In recent years there have been numerous studies on the optical properties of gold nanoparticles. Gold nanoparticles absorb visible light intensely because of the surface plasmon resonance (SPR) effect. The electromagnetic field of incident light couples with the oscillations of conduction electrons in gold particles, resulting in strong-field enhancement of the local electromagnetic fields near the rough surface of gold nanoparticles. The enhanced local field strength can be over 500 times larger than the applied field for structures with sharp apices, edges, or concave curvature (e.g. nanowires, cubes, triangular plates, and nanoparticle junctions). The SPR absorption may cause rapid heating of the nanoparticles. Gold nanoparticles supported on metal oxides are efficient catalysts for important oxidation process, including selective oxidation of hydrocarbons and oxidation of various volatile organic compounds (VOCs), such as CO, CH3OH, and HCHO at moderately elevated temperatures. Therefore, the combination of the SPR absorption and the catalytic activity of gold nanoparticles presents an important opportunity: if the heated gold nanoparticles could activate the organic molecules on them to induce oxidation of the organic compounds, then oxidation on gold catalysts can be driven by visible light at ambient temperature. Moreover, the SPR is a local effect, limited to the noble metal particles, so that the light only heats gold nanoparticles, which generally account for a few percent of the overall catalyst mass. This leads to significant saving in energy consumption for catalyzing organic compound oxidation. To verify the possibility of driving the VOC oxidation with visible light at room temperature, we prepared gold particles supported on various oxide powders. ZrO2 and SiO2 powders were first chosen as supports, because their band gaps are circa 5.0 eV and circa 9.0 eV, respectively, which are much larger than the energies of the photons of visible light (less than 3.0 eV). Thus, the light cannot excite electrons from the valence band to the conduction band. It is also impossible for the gold nanoparticles on ZrO2 to reduce the band gaps of ZrO2 enough for visible light photons to be absorbed and excite electrons in ZrO2. Thus, the catalytic activity is not caused by the same mechanism as occurs in semiconductor photocatalysts, but is due to the SPR effect of gold nanoparticles. The changes in the concentrations of the reactant (HCHO, 100 ppm) and product (CO2), when gold supported on ZrO2 was used as the catalyst, are depicted in Figure 1a. The initial concentration of HCHO in the glass vessel was 100 ppm. HCHO content decreased by 64% in two hours under the irradiation of six light tubes of blue light (with wavelength between 400 and 500 nm and the irradiation energy determined to be 0.17 Wcm 2 at the position of glass slides coated with the gold catalysts), and the CO2 content in the vessel increased accordingly. These results confirm that the oxidation of formaldehyde to carbon dioxide proceeds to a large extent at ambient temperature. The turnover frequency was calculated as being about 1.2 = 10 3 molecules of [*] Dr. X. Chen, Prof. H.-Y. Zhu, Z.-F. Zheng School of Physical and Chemical Sciences Queensland University of Technology Brisbane, Qld 4001 (Australia) Fax: (+61)7-3864-1804 E-mail: hy.zhu@qut.edu.au

Inverted Size-Dependence of Surface-Enhanced Raman Scattering on Gold Nanohole and Nanodisk Arrays

Abstract: Surface-enhanced Raman scattering (SERS) on gold nanohole and nanodisk arrays with precisely controlled size and spacing fabricated via electron beam lithography was investigated. These nanostructures exhibit strong SERS signals at 785 nm excitation but not at 514 nm. When the edge-to-edge distance is maintained, enhancement increases for nanoholes but decreases for nanodisks as diameter is increased. It is shown that the observed enhancement results from the local surface plasmon resonance wavelength shifts to the near-infrared regime as nanohole diameter increases. The large tolerance on dimensions and the empty space confined by nanoholes suggest promise for their use as a functional component in sensing, spectroscopy, and photonic devices.