Showing papers in "Plasmonics in 2006"

Plasmonics in Biology and Plasmon-Controlled Fluorescence

Abstract: Fluorescence technology is fully entrenched in all aspects of biological research. To a significant extent, future advances in biology and medicine depend on the advances in the capabilities of fluorescence measurements. As examples, the sensitivity of many clinical assays is limited by sample autofluorescence, single-molecule detection is limited by the brightness and photostability of the fluorophores, and the spatial resolution of cellular imaging is limited to about one-half of the wavelength of the incident light. We believe a combination of fluorescence, plasmonics, and nanofabrication can fundamentally change and increase the capabilities of fluorescence technology. Surface plasmons are collective oscillations of free electrons in metallic surfaces and particles. Surface plasmons, without fluorescence, are already in use to a limited extent in biological research. These applications include the use of surface plasmon resonance to measure bioaffinity reactions and the use of metal colloids as light-scattering probes. However, the uses of surface plasmons in biology are not limited to their optical absorption or extinction. We now know that fluorophores in the excited state can create plasmons that radiate into the far field and that fluorophores in the ground state can interact with and be excited by surface plasmons. These reciprocal interactions suggest that the novel optical absorption and scattering properties of metallic nanostructures can be used to control the decay rates, location, and direction of fluorophore emission. We refer to these phenomena as plasmon-controlled fluorescence (PCF). We predict that PCF will result in a new generation of probes and devices. These likely possibilities include ultrabright single-particle probes that do not photobleach, probes for selective multiphoton excitation with decreased light intensities, and distance measurements in biomolecular assemblies in the range from 10 to 200 nm. Additionally, PCF is likely to allow design of structures that enhance emission at specific wavelengths and the creation of new devices that control and transport the energy from excited fluorophores in the form of plasmons, and then convert the plasmons back to light. Finally, it appears possible that the use of PCF will allow construction of wide-field optical microscopy with subwavelength spatial resolution down to 25 nm.

Drastic Surface Plasmon Mode Shifts in Gold Nanorods Due to Electron Charging

Abstract: The color of small gold rods changes dramatically when electrons are injected by chemical reductants. The longitudinal and transverse plasmon modes are both found to blue-shift, and the shift is larger for rods with larger aspect ratios. The color changes are visible to the eye for rods with aspect ratios around 2–3. It is found that the surface plasmon band is damped when charging becomes high. The effects are in qualitative agreement with a model in which the gold plasma frequency increases due to an increase in electron density.

Optical Extinction Spectroscopy of Oblate, Prolate and Ellipsoid Shaped Gold Nanoparticles: Experiments and Theory

Abstract: Localized Surface Plasmons (LSP) on metallic nanoparticles of different shapes are investigated by extinction spectroscopy. Experimental results are compared to simulations by a Finite-Difference Time-Domain (FDTD) method. Three different shapes of nanoparticles are compared, oblates, prolates and ellipsoids, in terms of spectral tunability of the LSP resonance (LSPR). It is found that the complete geometry of the nanoparticle must be given to truly define the LSP resonance and that ellipsoids offer the widest spectral tunability.

Photochemical Synthesis and Multiphoton Luminescence of Monodisperse Silver Nanocrystals

Abstract: A rapid, photochemical solution-phase synthesis has been developed for the production of monodisperse, nanometer-sized silver particles. The stabilizer used in the synthesis can be used to control the average diameter of the particles over a range from 1 to 7 nm. The same reaction mixture can also be employed to deposit patterns of nanoparticles with a laser via multiphoton absorption. The particles exhibit strong multiphoton absorption-induced luminescence when irradiated with 800-nm light, allowing emission from single nanoparticles to be observed readily.

Microwave-Accelerated and Metal-Enhanced Fluorescence Myoglobin Detection on Silvered Surfaces: Potential Application to Myocardial Infarction Diagnosis.

Abstract: In this short paper, we describe a novel approach to both significantly accelerate and optically amplify fluorescence-based immunoassays. Our approach utilizes metal-enhanced fluorescence (MEF) to intrinsically optically amplify fluorescence signatures, which, when combined with the use of low-power microwaves to kinetically accelerate assays, provides for both ultrafast and ultrabright immunoassays. Surprisingly, the use of low-power microwaves and silver nanostructures provides for localized heating, concentrating the effect to the particles themselves as compared to the generic heating of the high dielectric assay fluid. We have subsequently applied our microwave-accelerated MEF approach to the detection of myoglobin, where its rapid quantification is paramount for the clinical assessment of an acute myocardial infarction.

Surface Plasmon Polariton Mach–Zehnder Interferometer and Oscillation Fringes

Abstract: We present a quantitative experimental analysis of a surface plasmon polariton (SPP) interferometer relying on elliptical Bragg mirrors. By using a leakage radiation microscope, we observe oscillation fringes with unit visibility at the two interferometer exits. We study the properties of the SPP beam splitter and determine experimentally both the norm and phase of the SPP reflection and transmission coefficients.

Dispersion Study of the Infrared Transmission Resonances of Freestanding Ni Microarrays

Abstract: Nickel films (several-micrometer thickness, with 5.2-μm square holes in a square lattice array with 12.7-μm hole-to-hole spacing) exhibit Ebbesen's extraordinary transmission effect in the infrared; that is, they transmit a higher fraction of incident infrared light than the fractional open area of the holes. The role of surface plasmons (SPs) in this phenomenon is much debated, so we have obtained a data set whereby this idea and others can be tested against empirically determined dispersion curves. Unpolarized, zero-order transmission spectra have been recorded by rotating the mesh (from −2° to 60° in 1° steps) relative to the spectrometer's incident beam about an axis along the mesh's nearest hole-to-hole spacing in order to create the dispersion diagram. The data are numerically analyzed for peak centers that are then projected outside of the light line (by SP momentum matching equations) to two SP dispersion curves that are spaced in frequency by a splitting. With this caveat, all of the observed structure is accounted for by a simple SP model.

Far-Field Raman Imaging of Short-Wavelength Particle Plasmons on Gold Nanorods

Abstract: The surface plasmon fields of gold nanorods with a diameter of 100 nm and lengths of 1–5 $$\mu$$ m are imaged by using far-field Raman scattering of methylene blue adsorbed on the rods. When optically exciting the nanorods under total internal reflection with wave vector and electric field vector orientations along the rod axis, the plasmon field intensity along this axis is observed to be periodically modulated. This modulation is attributable to a beating of the exciting light wave and the nanorod plasmon mode. The plasmon wavelength deduced from the beat frequency is 379 nm, which is considerably smaller than the exciting laser wavelength of 647 nm. In general, Raman imaging is shown to be a powerful technique to probe local plasmon fields using far-field spectroscopy.

Phase-Transfer Identification of Core-Shell Structures in Bimetallic Nanoparticles

Abstract: On the basis of a combination of previously published experimental procedures, ultraviolet–visible spectroscopy, transmission electron microscopy, and energy-dispersive X-ray measurements, a systematic investigation was carried out on the phase-transfer characteristics of different bimetallic nanoparticles (Ag–Au, Ag–Pt, Ag–Ru, Au–Pt, Au–Ru, and Pt–Ru) formed by the seed-mediated growth reactions. The different phase-transfer characteristics of the monometallic nanoparticles of Au, Ag, Pt, and Ru were used to form the basis of differentiation between various possible structures existing in the bimetallic systems (core-shell particles or a physical mixture of nanoparticles). The experimental results indicate clearly the formation of core-shell nanoparticles of Ag–Au, Ag–Pt, Ru–Ag, Pt–Au, Au–Ru, and Pt–Ru when the nanoparticles of the first metal were used as the seeds in the seed-mediated growth reactions. However, when the order of the synthesis was reversed using the nanoparticles of the second metal as the seeds, only a physical mixture of the two metal nanoparticles was obtained instead.

Local Field Spectroscopy of Metal Dimers by TPL Microscopy

Abstract: We report on two-photon photoluminescence (TPL) spectroscopy on metal dimers made of two gold nanoparticles separated by subwavelength distances. A direct comparison with far-field scattering measure- ments shows that TPL provides additional data on the structure modes of major importance for their use in surface-enhanced Raman scattering, enhanced fluores- cence, and sensing.

Near-Infrared Fluorescence Enhancement Using Silver Island Films

Abstract: Near-infrared (near-IR) excitation produces little background signal from biological molecules, making near- IR fluorescence technology highly useful in proteomic and genomic applications. To increase the emissions of near-IR fluorophores, we examined the use of metal-enhanced fluorescence on these longer wavelength dyes. IRDye ® 700- and IRDye ® 800-labeled DNA oligonucleotides and pro- teins were spotted onto silver island film (SIF)-coated glass slides, and analyzed using a LI-COR Odyssey ® IR imaging system. We observed more than 18-fold enhancement of the IRDye ® 700 and 15-fold enhancement of the IRDye ® 800- labeled DNA oligonucleotides when spotted on SIF-coated surfaces compared with uncoated surfaces. We also dem- onstrated that the enhanced emissions produced on the SIF- coated slides remained linear over several orders of magnitude, that the emissions remained reproducible across a slide surface, and that the SIF-coated slide remained effective at enhancing emissions after 9 months of storage. Our results indicate that SIF-coated glass slides are effective at enhancing near-IR fluorescence and could be developed into an effective tool to aid in molecular biological applications.

Plasmonics—A Vision for the Future

Abstract: Plasmonics is quickly becoming a dominant technology for the twenty-first century. The classic text by H. Raether appeared in 1988, but the potential of surface plasmons laid dormant for a decade. It is now recognized that surface plasmons, which can be present on particulate, smooth, or corrugated metallic surfaces, have enormous potential in the fields of optical computing, novel optical devices, and— more recently—biological and medical research. These emerging applications are the result of the unique properties of surface plasmons, which are confined in a two-dimensional surface and can have dimensions considerably smaller than optical wavelengths. Surface plasmons can be, to a reasonable extent, controlled in two dimensions, trapping and transporting optical energy in nanoscale structures. At first glance this possibility seems similar to optical wavelengths. However , typical optical waveguides, in contrast to plas-monics devices, must be three-dimensional and have wavelength-size features. Additionally, the two-dimensional nature of plasmonic structures makes them compatible with modern lithographic methods used for preparation of integrated circuits. The potential of plasmonics in chemical and biological research is just now being realized. One early application is surface-enhanced Raman scattering (SERS), the mechanism of which is still a subject of investigation. Another known application of plas-monics is surface plasmon resonance (SPR), which is used to study biomolecule binding reactions. SERS and SPR depend on the two extremes of metallic structures, random particles and a smooth surface. The applications and understanding of plasmonics are now being facilitated by modern nanofabrication technologies, which allows preparation of numerous metallic nanostructures, in particular regular patterns of particles, holes, or other features. These metallic nanostructures are already known to display unusual and unexpected optical properties, such as anomalously larger optical transmission through subwave-length nanoholes and directional—rather than diffracted—light transmission. Additionally, these are strong optical fields with subwavelength dimensions near such structures. These fields provide opportunities for new experimental capabilities such as sub-wavelength optical imaging. Plasmonics is a highly interdisciplinary science that depends on the efforts of physicists, chemists, and biologists. Many subdisciplines are involved, such as computational electrodynamics, nanofabrication, bottom up chemical self-assembly and biochemical spec-troscopy, to name a few. Consequently, and perhaps appropriately, the many new results using plasmonics are appearing diffusively in many journals. Given the breadth and potential of plasmonics technology, it is now time for a journal dedicated to this important new science. We are thus pleased to introduce Plas-monics, a peer-reviewed journal to serve as the …

Nonradiative Interactions between Biotin-Functionalized Gold Nanoparticles and Fluorophore-Labeled Antibiotin

Abstract: The interaction between antibodies and ligand-functionalized nanoparticles were exploited in this work by taking advantage of the strong influence that metallic surfaces have on emission of fluorescence. The surface of colloidal gold nanoparticles was functionalized with biotin moieties embedded in a nonfouling matrix of di(ethylene glycol) groups to minimize nonspecific interactions. Antibiotin labeled with fluorophore Alexa™ 488 bound to these particles via specific biomolecular recognition interactions. Upon binding of the labeled antibody to the biotinylated nanoparticles, an immediate decrease in emission of fluorescence was observed. Competitive dissociation of the antibody from the nanoparticles with soluble biotin produced a recovery in the intensity of emission of fluorescence. For large concentrations of the antibody, emission of fluorescence (corrected for dilution and absorption/scattering effects) appeared to increase to levels higher than the intensity of emission of the unbound antibody. This apparent increase is ascribed to a decreased extinction coefficient produced during aggregation of the nanoparticles by the bivalent antibodies. This scheme could have applications in detection of small molecules or could be used to study the interactions of ligand functionalized nanoparticles and proteins.

Combination of Nanoholes with Metal Nanoparticles-Fabrication and Characterization of Novel Plasmonic Nanostructures

Abstract: Small metal nanostructures, especially gold and silver nanoparticles, are known for their interesting optical properties caused by plasmonic effects. Molecular plasmonics, a combination of these optically active nanostructures with the molecular world, opens new possibilities for bioanalytics and (bio-) nanophotonics. Isotropic or anisotropic, homogeneous or heterogeneous metal nanoparticles provide a platform for different, highly defined functional units with interesting optical properties such as plasmon waveguides or molecular beacons. Nanohole arrays in metal layers are another promising component for nanophotonics. New photonic materials were realized from combinations of single metal nanoparticles with individual nanoholes in metals. Atomic force microscopic imaging was used to determine the particle location as well as the lateral dimensions and the topography of the resulting structures. Besides ultramicroscopic characterization of the nanoarrangements, such as nanoparticles positioned in nanoholes, far-field optical methods were also applied to investigate their optical properties.

Gold Nanoparticles from Induced Au3+→Au0 Reaction in Polyvinyl Alcohol Molecules in Presence of Sucrose in Hot Water

Abstract: In hot water (50–60°C), polyvinyl alcohol (PVA) molecules have coordination reaction with Au3+ cations, forming an Au3+-PVA polymer complex. In the proposed model reaction in small templates, the complex converts to Au0 capping in PVA molecules. Adding sucrose (5–10 times the PVA in mass) in a typical batch promotes Au3+→Au0 reaction, showing absorption coefficient α in Au0 surface plasmon band to be enhanced as much as 28 times the value in reaction with PVA. The band shifts at 547 nm from 566 nm (α = 21.4 cm−1 mol−1) in the PVA sample. Drying Au0-PVA/sucrose (2–5 wt% Au0) colloid at 60–70°C and then heating at 450°C in air burns off the organic part, leaving behind a light ash colored powder with Au0 nanoprisms or nanofibrils (∼30 nm average width). X-ray diffractogram has six reflections, (111), (200), (220), (311), (222), and (400), of Fm3m fcc Au0 of lattice parameter a = 0.4080 nm. The powder has photoluminescence in transversal and longitudinal Au0 plasmon bands of 535 and 585 nm, respectively.

Wet Chemical Colorimetric Estimation of CO: An Update on the Reduced Silver Sol Spectral and Kinetic Transformations to Silver nanoparticle

Abstract: Measurements of ppm (v/v) level COg concentration is conveniently performed by its preconcentration in alkaline absorber solution of Ag+-(4)- HCO2-C6H4-SO2NH2 complex, followed by a spectral measurement of the reduced silver sol. In this study, the transitory nature of this latter species and its subsequent real-time transformation to silver nanoparticle are presented. These results were based on spectral measurements made under varying concentrations of alkali, (4)-HCO2-C6H4-SO2NH2, and Ag+ in the absorber solution, and in the presence of a wide range of sampled COg concentration. The initially created light yellow colored sol with its broad absorption profile peaking at 380 nm and absorption coefficient 3500 ± 300 cm−1 M−1 (related to the amount of sampled [COg] as standardized by gas chromatographic analysis) changed into the characteristic yellow orange nanoparticle with its plasmon band peak absorption at 425 nm and absorption coefficient 6350 ± 300 cm−1 M−1. Under different sampling conditions, the respective first-order conversion rates varied between 0.03 and 0.15 h−1, whereas simultaneous dynamic light scattering measurements revealed steady growth of the averaged particle size ranging from 60 to 300 nm.

Surface Plasmon Excitation on a Sphere Made of Left-Handed Material

Abstract: We introduce formulae in suitable form for the numerical computation of Mie coefficients and the scattered electrical field in the general case of a sphere with permitivity $$\epsilon$$ and permeability $$\mu$$ . The small particle limit is also investigated. We compute the extinction cross section of a negative index sphere, as well as intensity maps. Excitation of surface plasmons featuring narrow linewidths is observed, which can be attributed to the permitivity or the permeability of the sphere.