Yuko S. Yamamoto

High-pressure structures of methane hydrate observed up to 8 GPa at room temperature

Three high-pressure structures of methane hydrate, a hexagonal structure (str.A) and two orthorhombic structures (str.B and str.C), were found by in situ x-ray diffractometry and Raman spectroscopy. The well-known structure I (str.I) decomposed into the str.A and fluid at 0.8 GPa. The str.A transformed into the str.B at 1.6 GPa, and the str.B further transformed into the str.C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The structural changes occurring with increasing pressure were observed reversibly with decreasing pressure. The symmetric stretching vibration, ν1, of the methane molecule observed in the Raman spectra changed along with the structural changes. The bulk moduli, K0, for the str.I, str.A, and str.C were calculated to be 7.4, 9.8, and 25.0 GPa, respectively. The difference in the bulk moduli implies the difference in fundamental structure of the high-pressure structures.Three high-pressure structures of methane hydrate, a hexagonal structure (str.A) and two orthorhombic structures (str.B and str.C), were found by in situ x-ray diffractometry and Raman spectroscopy. The well-known structure I (str.I) decomposed into the str.A and fluid at 0.8 GPa. The str.A transformed into the str.B at 1.6 GPa, and the str.B further transformed into the str.C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The structural changes occurring with increasing pressure were observed reversibly with decreasing pressure. The symmetric stretching vibration, ν1, of the methane molecule observed in the Raman spectra changed along with the structural changes. The bulk moduli, K0, for the str.I, str.A, and str.C were calculated to be 7.4, 9.8, and 25.0 GPa, respectively. The difference in the bulk moduli implies the difference in fundamental structure of the high-pressure structures.

High Axial Resolution Raman Probe Made of a Single Hollow Optical Fiber

A ball lens mounted hollow optical fiber Raman probe (BHRP) consisting of a single hollow optical fiber (HOF) and a micro-ball lens was developed for performing a high axial resolution and high-sensitivity remote Raman analysis of biomedical tissues. The total diameter of the probe head is 640 μm. The BHRP is useful in the measurement of thin-layered tissues that are in contact with the probes surface because the probe has a limited depth-of-field optical property. An optical calculation study suggested that it is possible to vary the probes working distance by selecting different materials and diameters for the ball lens. Empirical studies revealed that this probe has a higher axial resolution and a higher sensitivity than an HOF Raman probe without the ball lens. The spectrum of a mouse stomach measured with the BHRP had better quality and considerably lower noise than that measured with a conventional Raman microscope. These results strongly suggest that the BHRP can be used effectively in biomedical applications.

Subsurface sensing of biomedical tissues using a miniaturized Raman probe: Study of thin-layered model samples

A ball lens hollow-fiber Raman probe (BHRP) is a powerful tool for in vivo nondestructive subsurface analysis of biomedical tissues in a living body. It has confocal-like optical properties, but its collection volume is rather large in comparison with that of a conventional confocal Raman system. Therefore, the obtained Raman spectra have contributions from the upper and lower layers at different rates depending on the thickness of the upper layer when the measurement point is close to the boundary surface of the two layers. In the present study, we describe a methodology to extract quantitative information about the thickness of the subsurface layer structure by using a BHRP combined with the partial least-square regression (PLSR) analysis. The simulation study indicates that distribution of the collection efficiency in the collection volume of the BHRP is similar to a Gaussian distribution. The empirical study suggests that the PLSR model built with only a principal component (PC) 1 based on the linearized depth data gives good prediction.

Direct conversion of silver complexes to nanoscale hexagonal columns on a copper alloy for plasmonic applications

We introduced a novel method for the rapid synthesis of silver nanohexagonal thin columns from an aqueous mixture of sodium thiosulfate (Na2S2O3) and silver chloride (AgCl) simply added to a phosphor bronze substrate. The reaction is based on galvanic displacement and the products are potentially useful for plasmonic applications.

One-dimensional plasmonic hotspots located between silver nanowire dimers evaluated by surface-enhanced resonance Raman scattering

Hotspots of surface-enhanced resonance Raman scattering (SERRS) are localized within 1 nm at gaps or crevices of plasmonic nanoparticle (NP) dimers. We demonstrate SERRS hotspots with volumes that are extended in one dimension tens of thousand times compared to standard zero-dimensional hotspots using gaps or crevices of plasmonic nanowire (NW) dimers.

Fluctuating single sp2 carbon clusters at single hotspots of silver nanoparticle dimers investigated by surface-enhanced resonance Raman scattering

We evaluate spectral changes in surface enhanced resonance Raman scattering (SERRS) of near-single dye molecules in hotspots of single Ag nanoparticle (NP) dimers. During the laser excitation, surface enhance florescence (SEF) of dye disappeared and the number of SERRS lines decreased until finally ca. two lines remained around 1600 and 1350 cm−1, those are evidence of G and D lines of single sp2carbon clusters. Analysis of the G and D line intensity ratios reveals the temporal fluctuation in the crystallite size of the clusters within several angstroms; whereas, broadening and splitting in the lines enable us for identifying directly the dynamics of various defects in the clusters. This analysis reveals that the detailed fluctuations of single sp2carbon clusters, which would be impossible to gain with other microscopic methods.

Plasmonic Imaging of Brownian Motion of Single DNA Molecules Spontaneously Binding to Ag Nanoparticles

We find the spontaneous binding of single DNA molecules to uncoated silver nanoparticles (AgNPs) in aqueous solution with Mn(2+) (3 mM). From dark-field optical microscopic imaging of AgNPs bound to DNA molecules, we demonstrate analysis of the Brownian motion of single DNA molecules via plasmon resonance elastic light scattering. Our results provide that the plasmonic imaging technique is free from photobleaching and blinking and thus is useful in long-time observations of single-molecule DNA dynamics.

Darkfield microspectroscopy of nanostructures on silver tip-enhanced Raman scattering probes

We report an evaluation method employing darkfield microspectroscopy for silver probes used in tip-enhanced Raman scattering (TERS). By adjusting the darkfield illumination, the diffracted light from the probe outlines disappears and the diffracted light from the surface nanostructures and tips of the probes appears as colorful spots. Scanning electron microscopy reveals that the spectral variations in these spots reflect the shapes of the surface nanostructures. The tip curvatures correlate to the spectral maxima of their spots. Temporal color changes in the spots indicate the deterioration due to the oxidation of the silver surfaces. These results show that the proposed method is useful for in situ evaluation of plasmonic properties of TERS probes.

Formation mechanism of plasmonic silver nanohexagonal particles made by galvanic displacement reaction

The galvanic displacement reaction (GDR) is a powerful method for the preparation of various plasmonic nanostructures within several minutes. However, the formation mechanism of the nanostructures, which retain plasmonic hotspots, still remains unclear. In this work, X-ray photoelectron spectroscopy (XPS) is applied to silver nanostructures made by newly-discovered GDR between Ag complex solutions and Cu alloy substrates, whose nanostructures form characteristic nanoscale hexagonal columns (NHCs) and generate strong surface-enhanced Raman scattering (SERS) signals of adsorbates. Detailed depth profiles by multi-element XPS analysis revealed that NHCs are made of Ag metallic cores covered with surface layers composed of copper sulfates and copper oxides, which prevent NHCs from fusion, resulting in highly concentrated stable hotspots. These findings explain why NHCs exhibit reproducible SERS signals and the proposed methodology gives new insights for efficient creation of plasmonic nanostructures in a few minutes.

Solid methane behaviours under high pressure at room temperature

High pressure behaviours of solid methane were examined in a pressure range of 0.5 GPa to 86 GPa at room temperature using diamond anvil cell. In-situ X-ray diffractometry and Raman spectroscopy revealed existence of two high pressure phases above 35 GPa. The Raman spectra showed clear peak splitting and appearance of new vibration modes at 35 GPa and 62 GPa. The XRD patterns of the high pressure phases were basically similar to those of the known phases B and HP. These results indicate that the observed high pressure phases have a similar fundamental structure but have different molecular orientations. In addition, an intermediate metastable phase was found between phases A and B. This phase can be thought as that: the configuration of the methane molecules almost attains as that in phase B, and the rotation of molecules still remains as that in phase A.