Surface plasmon-quantum dot coupling from arrays of nanoholes

The coupling of semiconductor quantum dots (QDs) to the surface plasmon (SP) modes of nanohole arrays in a metal film was demonstrated for the first time, showing enhancement in the spontaneous emission by 2 orders of magnitude. The SP-enhanced transmission resonances of the nanohole arrays were tuned around the photoluminescence (PL) peak of polystyrene-b-poly(acrylic acid) (PS-b-PAA)-stabilized cadmium sulfide (CdS) quantum dots (QDs) in contact with the arrays. As a result the overall PL from the SP-QD system was enhanced by 2 orders of magnitude, even after excluding the enhanced transmission of the nanohole array without the QDs. The maximum enhancement occurred when the resonance from the nanohole array matched the QD PL spectrum. Time-resolved PL measurements were used to estimate the relative contribution of different physical mechanisms to the enhanced spontaneous emission. The increased spontaneous emission in the SP-QD system is promising for prospective plasmonic light-emitting devices incorporating QDs.     

Surface plasmon sensor based on the enhanced light transmission through arrays of nanoholes in gold films

Arrays of nanoholes in a gold film were used to monitor the binding of organic and biological molecules to the metallic surface. This technique is particularly sensitive to surface binding events because it is based upon the resonant surface plasmon enhanced transmission through the array of nanoholes. The sensitivity was found to be 400 nm per refractive index unit, which is comparable to other grating-based surface plasmon resonance (SPR) devices. The array of nanoholes is well suited for dense integration in a sensor chip. Furthermore, the optical geometry is collinear, which simplifies the alignment with respect to the traditional Kretschmann (reflection) arrangement for SPR sensing.     

Enhanced molecular fluorescence near thick Ag island film of large pseudotabular nanoparticles

A highly reflective thick Ag island film (TAIF) sputter-grown on mica, consisting of unique large pseudotabular nanoislands, 60-200 nm across and 30-60 nm thick, produced an unusually strong surface enhanced fluorescence (SEF) for rhodamine dyes situated very close to (only approximately 10 A away from) the metal surface. A significantly greater part of the enhanced fluorescence was emitted into the back half space through TAIF and the mica substrate. The detailed fluorescence angular distribution was very similar to that of the light scattering by TAIF, suggesting that the enhanced emission originated from some large induced dipoles in TAIF. For reference, we also present a quantitative analysis of the fluorescent behavior of the same dye but directly coated on a reference glass surface. TAIF showed no distinct dipolar surface plasmon-like bands for excitation at normal incidence, and the light absorption by dye-coated TAIF could be described by simple superposition of the contribution of TAIF and that of the surface-bound dyes. However, the net dye absorptance was increased by 4-5 times due to the strong interactions of the dye transition dipoles with the TAIF-scattered fields. The estimated SEF quantum yield in the low dye coverage limit suggests a markedly high radiative yield of the induced dipole in TAIF around approximately 0.5.     

Uniform gold nanorod arrays from polyethylenimine-coated alumina templates

Monolithic Au nanorod arrays can be grown by electrodeposition in Au-backed nanoporous alumina templates using polyethylenimine (PEI) as an adhesion layer, with excellent height control between 300 nm and 1.4 microm. The local height distribution can be extremely narrow with relative standard deviations well below 2%. The uniform growth rate appears to be determined by the adsorbed PEI matrix, which controls the growth kinetics of the grains comprising the nanorods. The nanorods can be retained as free-standing 2D arrays after careful removal of the AAO template. Reflectance spectroscopy reveals a collective plasmon mode with a maximum near 1.2 microm, in accord with recent calculations for 2D arrays of closely spaced cylindrical nanoparticles.     

Enhanced photoelectrochemical activities of a nanocomposite film with a bamboo leaf-like structured TiO2 layer on TiO2 nanotube arrays

A novel nanocomposite TiO(2) film consisting of a bamboo leaf-like nano TiO(2) layer on a nanotubular TiO(2) arrays surface is synthesized by electrochemical anodization with wet chemical pretreatment; it shows almost three times higher activity as compared to that of nanotubular TiO(2) arrays alone.     

Ordered arrays of gold nanostructures from interfacially assembled Au@PNIPAM hybrid nanoparticles

In this Article, we report on the assembly of hybrid Au@PNIPAM core-shell particles at the air/water interface, their transfer onto solid substrates, and the controlled combustion of the organic material to produce arrays of gold nanoparticles. A detailed investigation on the assembly behavior of such soft hybrid colloids at the air/water interface was performed by correlating the surface pressure-area isotherms with SEM and AFM images from samples transferred at different surface pressures. The hybrid particles display a complex behavior at the interface, and we could distinguish three distinct phases with varying interparticle spacings at different compression. The transfer process presented enables the decoration of topologically structured substrates with gold nanoparticle arrays, and the order of the initial monolayers is retained in the arrays of inorganic gold nanoparticles. The change in monolayer morphology upon compression can therefore be used to tailor the interparticle distance between approximately 650 and 300 nm without exchanging the colloids. More sophisticated gold nanostructures can be patterned into symmetric arrays using a similar protocol, which we demonstrate for nanostars and nanorods.     

Grating-induced plasmon mode in gold nanoparticle arrays

We study the dipolar coupling of gold nanoparticles arranged in regular two-dimensional arrays by extinction micro-spectroscopy. When the interparticle spacing approaches the plasmon resonance wavelength of the individual particles, an additional band of very narrow width emerges in the extinction spectrum. By systematically changing the particles dielectric environment, the particles shape, the grating constant and angle of incidence, we show how this band associated to a grating induced-resonance can be influenced in strength and spectral position. The spectral position can be qualitatively understood by considering the conditions for grazing grating orders whereas the strength can be related to the strength of dipolar scattering from the individual particles.     

Enhanced ALA-induced fluorescence in hyperparathyroidism

INTRODUCTION: Intraoperative localization of parathyroid glands can be challenging especially in minimally invasive surgery. Fluorescence diagnosis using the photosensitizer aminolevulinic acid (ALA) has been described to identify normal parathyroid glands during experimental bilateral neck exploration. The present study evaluated fluorescence differences between hyperplastic and normal parathyroid glands as a precondition for a clinical application of the technique. MATERIALS AND METHODS: Polycystic kidney disease (PKD) rats with hyperparathyroidism due to hyperplastic parathyroid glands and Wistar rats with normal parathyroid glands were photosensitized by peritoneal lavage with ALA solution. After surgical exposure of thyroid and parathyroid glands the operative site was observed under blue light conditions using the d-light system to assess fluorescence characteristics of each tissue. Fluorescence intensities of parathyroid glands and surrounding thyroid tissue were measured by spectrometry. Parathyroid hormone in serum of the rats was determined by enzyme-linked immunosorbent assay (ELISA). RESULTS: Observation of the exposed thyroid site showed a subjectively stronger red fluorescence of the parathyroid glands in the PKD rats in comparison to the Wistar rats, whereas thyroid tissue appeared equally fluorescent. In the PKD animals, spectrometric fluorescence intensity was 10 times higher in the parathyroid glands than in the thyroid gland, whereas in the Wistar rats the ratio was 3.2:1. Fluorescence intensity in the parathyroid glands was more than twice in the PKD rats than in the Wistar rats, however slightly lower in the thyroid tissue. ELISA confirmed the pathophysiological change of a hyperparathyroidism with significantly increased serum levels of parathyroid hormone in the PKD rats. DISCUSSION: Hyperparathyroidism enhances ALA-induced fluorescence of the parathyroid glands. A combined surgical fluorescence strategy may justify a unilateral, minimally invasive approach in selected patients and serve to improve the capability of the surgeon to safely and efficiently manage parathyroid diseases.     

Electrochemical nucleation of gold nanoparticles in a polymer film at a liquid-liquid interface

Simultaneous nucleation of gold nanoparticles and polymerization of tyramine has been carried out at an immiscible electrolyte interface. By transferring the gold ion of tetraoctylammoniumtetracloroaurate (TOAAuCl(4)) from the organic to the aqueous phase, a fast homogeneous electron transfer from the tyramine monomer reduces the gold ion. Electropolymerization then proceeds, and gold nanoparticles form. The newly formed nanoparticles act as nucleation sites for the deposition of the oligomers/polymer (and possibly vice versa). This results in gold nanoparticles stabilized in a polytyramine matrix. The size of the nanoparticles is controlled by the concentration of oligomers/polymer in solution. The polymer nanoparticle composite film was analyzed with TEM, XPS, and AFM.     

Analysis of N-dimensional data arrays from fluorescence spectroscopy of an intermediary sugar product

Unwanted formation of colour takes place during the production of crystalline sugar. The degree of colouration depends partly on the necessary processing conditions, e.g. heating and pH, and partly on the initial composition and condition of the sugar beets used as raw material. Reducing sugars are formed during the process. These are reactive compounds forming a variety of coloured complexes and strong precursors to further formation of colour and many of these compounds contain fluorophores. In the present work it is discussed if spectrofluorometric screening of intermediary sugar products prior to the final heating stages combined with a multi-way chemometric approach can provide information that significantly reflects the condition of the process and the beets. The model used is the N-way PCA (Principal Component Analysis) which is an exploratory model, not necessitating explicit modelling of single parameters nor any assumptions towards parameter interaction. By use of a 4-way PCA of order (3,2,3,3) satisfactory classification of 47 thick juice samples belonging to 5 factories has been obtained from a spectrofluorometric screening method. Also, a temporal trend has been found to evolve during the time of production. The investigation substantiates the use of modern models from data analysis for extracting significant information from large and complex data sets.     

Analysis of N-dimensional data arrays from fluorescence spectroscopy of an intermediary sugar product

Unwanted formation of colour takes place during the production of crystalline sugar. The degree of colouration depends partly on the necessary processing conditions, e.g. heating and pH, and partly on the initial composition and condition of the sugar beets used as raw material. Reducing sugars are formed during the process. These are reactive compounds forming a variety of coloured complexes and strong precursors to further formation of colour and many of these compounds contain fluorophores. In the present work it is discussed if spectrofluorometric screening of intermediary sugar products prior to the final heating stages combined with a multi-way chemometric approach can provide information that significantly reflects the condition of the process and the beets. The model used is the N-way PCA (Principal Component Analysis) which is an exploratory model, not necessitating explicit modelling of single parameters nor any assumptions towards parameter interaction. By use of a 4-way PCA of order (3,2,3,3) satisfactory classification of 47 thick juice samples belonging to 5 factories has been obtained from a spectrofluorometric screening method. Also, a temporal trend has been found to evolve during the time of production. The investigation substantiates the use of modern models from data analysis for extracting significant information from large and complex data sets. Received: 13 January 1997 / Revised: 18 March 1997 / Accepted: 25 March 1997     

The wetting of gold and silicon nanoscale arrays

The relative hydrophobicity of surfaces containing highly regular, nanoscale (<100 nm) topological features (50 nm diameter, 5 nm height, 100 nm spacing) was measured and compared with their flat counterparts. The results are described in the context of both the Wenzel and Cassie models for wetting rough surfaces. Wenzel-type wetting is observed for these high areal density nanostructures (approximately 10(10)/cm2) whose aspect ratios range from 0.2 to 0.7.     

Thin film processing using S-layer proteins: biotemplated assembly of colloidal gold etch masks for fabrication of silicon nanopillar arrays

We explored the bionanofabrication of silicon nanopillar structures using ordered gold nanoparticle arrays generated from microbial surface layer (S-layer) protein templates. The S-layer template used for these thin film processing experiments was isolated from the Gram-positive bacterium Deinococcus radiodurans. In this preliminary work, S-layers preimmobilized onto chemically modified silicon substrates were initially used to template the fabrication of a nanolithographic hard mask pattern comprised of a hexagonally ordered array of 5-nm gold nanoparticles (lattice constant = 18 nm). Significantly, the use of the biotemplated gold nanoparticle mask patterns in an inductively coupled plasma (ICP) etching process successfully yielded silicon nanopillar structures. However, it was found that the resultant nanopillars (8–13 nm wide at the tip, 15–20 nm wide at half-height, 20–30 nm wide at the base, and 60–90 nm tall) appeared to lack any significant degree of translational ordering. The results suggest that further studies are needed in order to elucidate the optimal plasma processing parameters that will lead to the generation of long-range ordered arrays of silicon-based nanostructures using S-layer protein templates.     

Resonant photoconductance of molecular junctions formed in gold nanoparticle arrays

We investigate the photoconductance properties of oligo(phenylene vinylene) (OPV) molecules in metal-molecule-metal junctions. The molecules are electrically contacted in a two-dimensional array of gold nanoparticles. The nanoparticles in such an array are separated by only few nanometers. This allows to bridge the distance between the nanoparticles with molecules considered as molecular wires such as OPV. We report on the photoconductance of electrically contacted OPV upon resonant optical excitation of the molecules. This resonant photoconductance is sublinear in laser intensity, which suggests that trap state dynamics of the optically excited charge carriers dominate the optoelectronic response.     

Growth of gold nanoparticle arrays in TiO2 mesoporous matrixes

An interconnected Au nanoparticle arrangement is obtained by electrodeposition from Au(III) soluble complexes within the pore system of block-copolymer templated mesoporous titania films. The resulting Au@TiO2 nanocomposites (5 nm Au particles, 5.5 nm amorphous titania walls) have the electrochemical behavior of a gold electrode of high surface area. The attenuation of Au surface plasmon due to -OH electroadsorption and the existence of mixed localized states in these Au@TiO2 nanocomposites are observed by in situ spectroelectrochemistry.