Showing papers by "Satoshi Kawata published in 2015"

Structured line illumination Raman microscopy

Abstract: In the last couple of decades, the spatial resolution in optical microscopy has increased to unprecedented levels by exploiting the fluorescence properties of the probe. At about the same time, Raman imaging techniques have emerged as a way to image inherent chemical information in a sample without using fluorescent probes. However, in many applications, the achievable resolution is limited to about half the wavelength of excitation light. Here we report the use of structured illumination to increase the spatial resolution of label-free spontaneous Raman microscopy, generating highly detailed spatial contrast from the ensemble of molecular information in the sample. Using structured line illumination in slit-scanning Raman microscopy, we demonstrate a marked improvement in spatial resolution and show the applicability to a range of samples, including both biological and inorganic chemical component mapping. This technique is expected to contribute towards greater understanding of chemical component distributions in organic and inorganic materials.

Optical antennas with multiple plasmonic nanoparticles for tip-enhanced Raman microscopy

Abstract: Tip-enhanced Raman spectroscopy (TERS) has recently become one of the most important tools for analyzing advanced nano-devices and nano-materials, because it allows strong enhancement of weak Raman signal from the nanometric volume of a sample. However, consistent enhancement in TERS is still an issue and scientists have been struggling to fabricate good tips for reliable, strong and reproducible enhancement. There is a strong need to study the morphology and the arrangement of metal nanostructures near the tip apex for efficient plasmonic enhancement in TERS. Here, we present a study on the metal grains attached to the tip surface for producing higher and much consistent enhancement in TERS. Our study shows that the plasmonic enhancement strongly depends on the number of grains and on the their separations. We found through simulations that multiple grains arranged closely but discretely on a dielectric probe act as an efficient plasmonic antenna and that enhancement in TERS is maximum for an optimized number of grains. The number of grains and the nano-gap between them are crucial for reproducible enhancement. This promising result, which we also demonstrate and prove by experiments, will bring TERS to a new level, where it can be utilized with more confidence of large reproducible enhancement for those nano-sized samples that have extremely weak Raman scattering.

Optical antennas for tunable enhancement in tip-enhanced Raman spectroscopy imaging

Abstract: The use of optical antennas in tip-enhanced Raman spectroscopy (TERS) makes it a powerful optical analysis and imaging technique at the nanoscale. Optical antennas can work as nano-light sources in the visible region. The plasmonic resonance of an antenna depends on its length; thus, by varying the length, one can control the enhancement in TERS. In this study, we demonstrated a fabrication method based on focused ion beam milling to realize optical antennas with desired lengths. We then measured the resonances of these fabricated antennas and performed TERS imaging of carbon nanotubes to demonstrate the antenna length dependence on plasmonic resonance.

Time-lapse Raman imaging of osteoblast differentiation

Abstract: Osteoblastic mineralization occurs during the early stages of bone formation. During this mineralization, hydroxyapatite (HA), a major component of bone, is synthesized, generating hard tissue. Many of the mechanisms driving biomineralization remain unclear because the traditional biochemical assays used to investigate them are destructive techniques incompatible with viable cells. To determine the temporal changes in mineralization-related biomolecules at mineralization spots, we performed time-lapse Raman imaging of mouse osteoblasts at a subcellular resolution throughout the mineralization process. Raman imaging enabled us to analyze the dynamics of the related biomolecules at mineralization spots throughout the entire process of mineralization. Here, we stimulated KUSA-A1 cells to differentiate into osteoblasts and conducted time-lapse Raman imaging on them every 4 hours for 24 hours, beginning 5 days after the stimulation. The HA and cytochrome c Raman bands were used as markers for osteoblastic mineralization and apoptosis. From the Raman images successfully acquired throughout the mineralization process, we found that β-carotene acts as a biomarker that indicates the initiation of osteoblastic mineralization. A fluctuation of cytochrome c concentration, which indicates cell apoptosis, was also observed during mineralization. We expect time-lapse Raman imaging to help us to further elucidate osteoblastic mineralization mechanisms that have previously been unobservable.

Anomalous lattice vibrations of monolayer MoS2 probed by ultraviolet Raman scattering.

Abstract: We present a comprehensive Raman scattering study of monolayer MoS2 with increasing laser excitation energies ranging from the near-infrared to the deep-ultraviolet. The Raman scattering intensities from the second-order phonon modes are revealed to be enhanced anomalously by only the ultraviolet excitation wavelength 354 nm. We demonstrate theoretically that such resonant behavior arises from a strong optical absorption that forms near the Γ point and of the band structure and an inter-valley resonant electronic scattering by the M-point phonons. These results advance our understanding of the double resonance Raman scattering process in low-dimensional semiconducting nanomaterials and provide a foundation for the technological development of monolayer MoS2 in the ultraviolet frequency range.

Superhydrophobic SERS Substrates Based on Silver-Coated Reduced Graphene Oxide Gratings Prepared by Two-Beam Laser Interference.

Abstract: Reported here is the fabrication of reduced graphene oxide (RGO) grating structures by two-beam laser interference (TBLI) for the development of highly efficient SERS substrates via simple physical vapor deposition (PVD) coating of silver. TBLI has been utilized to make hierarchical RGO grating structures with microscale gratings and nanoscale folders through a laser treatment induced ablation and photoreduction process. The hierarchical structures contribute to the formation of plasmonic structures after silver coating, giving rise to the formation of plenty of SERS “hot spots”, while the RGO substrate would provide chemical enhancement of Raman signal through interaction with analytes molecules. The significantly increased roughness with respect to the hierarchical structures in combination with the removal of hydrophilic oxygen-containing groups endow the resultant substrates with unique superhydrophobicity, which leads to the enrichment of analytes and further lowers the detection limit. The synergist...

Far- and Deep-Ultraviolet Spectroscopy

Abstract: This book is the first comprehensive work to be published on far-ultraviolet (FUV) and deep-ultraviolet (DUV) spectroscopy,subjects of keen interest because new areas of spectroscopy have been born in the FUV and DUV regions. For example, FUV spectroscopy in condensed matterhas become possible due to the development of attenuated total reflection/FUV spectroscopy. As other examples, DUV surface-enhanced Raman scattering and DUV tip-enhanced Raman scattering have received great attention. Imaging by DUV spectroscopy has also become an area of interest. More recently, FUV and DUV spectroscopy have shown potential for applications in several fields including industry. All these topics are described in this book. Doctoral students and researchers in universities and national research institutes as well as researchers in various industries will find this volume highly useful

Chirality-Selective Optical Scattering Force on Single-Walled Carbon Nanotubes

Abstract: Single-walled carbon nanotubes (SWCNTs) are promising for a staggering array of applications, ranging from thin-film electronics to sporting goods. Unfortunately, bulk manufacture yields a mixture of nanotubes with different properties, and separating them is very difficult. The authors have found that the tiny optical force from a laser beam can be used to sort SWCNTs according to their sizes and electronic properties. By tuning the laser to an appropriate color, any of multiple types of nanotubes can be selected and separated from a mixture---a discovery that could be the key to realizing new and higher quality applications for this wonder material.

Tip-enhanced THz Raman spectroscopy for local temperature determination at the nanoscale

Abstract: Local temperature of a nanoscale volume is precisely determined by tip-enhanced terahertz Raman spectroscopy in the low temperature range of several tens of degrees. Heat generated by the tip-enhanced electric field is directly transferred to single-walled carbon nanotubes by heat conduction and radiation at the nanoscale. This heating modulates the intensity ratio of anti-Stokes/Stokes Raman scattering of the radial breathing mode of the carbon nanotube based on the Boltzmann distribution at elevated temperatures. Owing to the low-energy feature of the radial breathing mode, the local temperature of the probing volume has been successfully extracted with high sensitivity. The dependence of the temperature rise underneath the tip apex on the incident power coincides with the analytical results calculated by finite element method based on the tip enhancement effect and the consequent steady-state temperature via Joule heat generation. The results show that the local temperature at the nanoscale can be controlled in the low temperature range simply by the incident laser power while exhibiting a sufficiently high tip enhancement effect as an analytical tool for thermally sensitive materials (e.g., proteins, DNA).

Efficient UV photocatalysis assisted by densely distributed aluminum nanoparticles

Abstract: Aluminum nanoparticles fabricated by oblique angle deposition (OAD) successfully increased the yield and reaction rate of UV photocatalysis due to the localized surface plasmon resonance (LSPR) effect. Nanoparticles 20–60 nm in size were formed in an area larger than ~1 cm2 when the film was highly tilted during the thermal deposition process. The size and density of these nanoparticles were readily controlled by the deposition thickness and speed. The yield of photocatalytic reactions increased by a factor of ~2, while the reaction rate increased by up to ~10 times. The aluminum nanostructures presented here are of tremendous advantage for future applications in photocatalysis through efficient coupling with UV light.

Super-Spatial- and -Spectral-Resolution in Vibrational Imaging via Saturated Coherent Anti-Stokes Raman Scattering

Abstract: Coherent anti-Stokes Raman scattering (CARS) is a vibrational spectroscopic technique offering high sensitivity, chemical selectivity, and three-dimensional visualization---but with spatial resolution limited by the wave nature of light. The authors exploit the saturation of the CARS signal to break that limit, increasing both spatial and spectral resolution and reducing signal background, and thus extending the utility of CARS microscopy in materials science, biology, and beyond.

Size dependent nanomechanics of coil spring shaped polymer nanowires.

Abstract: Direct laser writing (DLW) via two-photon polymerization (TPP) has been established as a powerful technique for fabrication and integration of nanoscale components, as it enables the production of three dimensional (3D) micro/nano objects. This technique has indeed led to numerous applications, including micro- and nanoelectromechanical systems (MEMS/NEMS), metamaterials, mechanical metamaterials, and photonic crystals. However, as the feature sizes decrease, an urgent demand has emerged to uncover the mechanics of nanosized polymer materials. Here, we fabricate coil spring shaped polymer nanowires using DLW via two-photon polymerization. We find that even the nanocoil springs follow a linear-response against applied forces, following Hooke's law, as revealed by compression tests using an atomic force microscope. Further, the elasticity of the polymer material is found to become significantly greater as the wire radius is decreased from 550 to 350 nm. Polarized Raman spectroscopy measurements show that polymer chains are aligned in nanowires along the axis, which may be responsible for the size dependence. Our findings provide insight into the nanomechanics of polymer materials fabricated by DLW, which leads to further applications based on nanosized polymer materials.

Analysis of dynamic SERS spectra measured with a nanoparticle during intracellular transportation in 3D

Abstract: We observed intracellular transportation by using surface-enhanced Raman scattering (SERS) from gold nanoparticles moving in the cytosol. Simultaneous detection of the nanoparticle three-dimensional position and SERS spectra presents molecular vibrational modes and contains large amounts of information about bimolecular events. In order to determine the meaningful temporal spectral changes associated with specific intracellular events or functions, we analyzed the time series Raman spectra, considering both the motion of the gold nanoparticle and also the time-varying spectra, revealing that some molecular vibrational modes showed strong correlation with the gold nanoparticle motion. This type of time-space-spectrum variable analysis will enable us to visualize molecular dynamics such as dissociation of proteins in biological reactions. In particular, our approach enables us to study molecules which should be associated with the transportation process such as vesicle transport, nuclear entry and protein diffusion.

Visualizing the appearance and disappearance of the attractor of differentiation using Raman spectral imaging.

Abstract: Using Raman spectral imaging, we visualized the cell state transition during differentiation and constructed hypothetical potential landscapes for attractors of cellular states on a state space composed of parameters related to the shape of the Raman spectra. As models of differentiation, we used the myogenic C2C12 cell line and mouse embryonic stem cells. Raman spectral imaging can validate the amounts and locations of multiple cellular components that describe the cell state such as proteins, nucleic acids, and lipids; thus, it can report the state of a single cell. Herein, we visualized the cell state transition during differentiation using Raman spectral imaging of cell nuclei in combination with principal component analysis. During differentiation, cell populations with a seemingly homogeneous cell state before differentiation showed heterogeneity at the early stage of differentiation. At later differentiation stages, the cells returned to a homogeneous cell state that was different from the undifferentiated state. Thus, Raman spectral imaging enables us to illustrate the disappearance and reappearance of an attractor in a differentiation landscape, where cells stochastically fluctuate between states at the early stage of differentiation.

Visible-wavelength two-photon excitation microscopy for fluorescent protein imaging.

Abstract: The simultaneous observation of multiple fluorescent proteins (FPs) by optical microscopy is revealing mechanisms by which proteins and organelles control a variety of cellular functions. Here we show the use of visible-light based two-photon excitation for simultaneously imaging multiple FPs. We demonstrated that multiple fluorescent targets can be concurrently excited by the absorption of two photons from the visible wavelength range and can be applied in multicolor fluorescence imaging. The technique also allows simultaneous single-photon excitation to offer simultaneous excitation of FPs across the entire range of visible wavelengths from a single excitation source. The calculation of point spread functions shows that the visible-wavelength two-photon excitation provides the fundamental improvement of spatial resolution compared to conventional confocal microscopy.

Dual-polarization Raman spectral imaging to extract overlapping molecular fingerprints of living cells.

Abstract: Raman spectral imaging is gaining more and more attention in biological studies because of its label-free characteristic. However, the discrimination of overlapping chemical contrasts has been a major challenge. In this study, we introduce an optical method to simultaneously obtain two orthogonally polarized Raman images from a single scan of the sample. We demonstrate how this technique can improve the quality and quantity of the hyperspectral Raman dataset and how the technique is expected to further extend the horizons of Raman spectral imaging in biological studies by providing more detailed chemical information. The dual-polarization Raman images of a HeLa cell.

Nonlinear fluorescence imaging by photoinduced charge separation

Abstract: Manipulation of the optical property of fluorescent probes has been a powerful strategy to establish super-resolution microscopy. We describe a new strategy to realize a probe with a nonlinear fluorescence response by using photoinduced charge separation. In this scheme, the first photon is used for the generation of the charge-separated state and the second photon is for fluorescence excitation. This stepwise two-photon absorption was confirmed by detection of a second-order nonlinear fluorescence response. Transient absorption spectra studies and simulation indicate that fluorescence is emitted through the photophysical pathways we proposed. Fluorescence imaging of biological cells showed marked improvements in image contrast and resolution, demonstrating the usefulness of the fluorescent probe in laser scanning confocal microscopy.

Silver hierarchical structures grown on microstructured silicon in chip for microfluidic integrated catalyst and SERS detector

Abstract: In this Letter, silver (Ag) hierarchical nanostructures grown on black silicon (BS) are used as the catalyst and a surface-enhanced Raman scattering (SERS) detector integrated in a microfluid. The BS is fabricated via femtosecond laser ablation in an atmosphere of sulfur hexafluoride, and then hydrogenated with hydrofluoric acid. As formed, the BS substrate directly acts as a reducing template to grow silver hierarchical nano-structures. Particularly, Ag-BS composite micro/nano-structures can be in-situ constructed in silicon-based microchannels. These structures simultaneously serve as integrated catalytic reactors and a SERS substrate for sensing. The sensitivity is tested to be as low as 10-8 mol/L using Rhodamine 6G.

Temperature-dependent Photodegradation in UV-resonance Raman Spectroscopy

Abstract: Temperature-dependent photodegradation during UV-resonance Raman spectroscopy was investigated. Photodegradation was quantitatively probed by monitoring the temporal evolution of UV-resonance Raman spectra obtained from bacteriochlorophyll (BChl) showing, resonance effect at a 355-nm excitation wavelength. At 80 K, the molecular photodecomposition rate was 5-times lower than that at room temperature. The decomposition rates of BChl were analyzed by the Arrhenius formula, indicating that the mechanism of photodegradation includes a thermal process having an activation energy of 1.4 kJ/mol.

Anticrossing behavior of surface plasmon polariton dispersions in metal-insulator-metal structure

Abstract: During the past two decades, strong coupling of SPPs with other excitations, such as localized surface plasmons in nanostructured metals and excitons in semiconductor dots has been the subject of intensive studies [1]. Controlling the coupling of SPPs in plasmonic systems is one of the most interesting and important issues in plasmonics to find potential applications such as biosensors and waveguides. In this work, we study the mutual coupling of SPPs in the metal-insulator-metal (MIM) structure and demonstrate the anti-crossing behavior experimentally and theoretically.

Introduction to FUV and DUV Spectroscopy

Abstract: Recently, both far ultraviolet (FUV) and deep UV (DUV) spectroscopies have received keen interest as new spectroscopies because they offer novel possibilities for studying electronic structure and transition, selective molecular imaging, high resolution microscopy, as well as applications for photoelectric devices. In this chapter after brief history of UV light, UV, FUV, and DUV regions are defined. Then, principles and brief history of FUV spectroscopy are described, followed by the discussion of its characteristics and advantages. Recent progress in FUV spectroscopy is also introduced. In the second part of this chapter, principles, methods, and recent progress of DUV spectroscopy are mentioned. Finally, the characteristics of FUV and DUV spectroscopies are compared.

Alkyne-tag Raman imaging of bio-active small molecules in live cells

Abstract: Raman microscopy is useful for molecular imaging and analysis of biological specimens. Here, we used alkyne containing a carbon-carbon triple bond as a Raman tag for observing small molecules in live cells. Alkyne tags can maintain original properties of target molecules with providing high chemical specificity owing to its distinct peak in a Raman-silent window of biomolecules. For demonstrations, alkyne-tagged thymidine and coenzyme Q analogue in live cells were visualized with high-spatial resolution. We extended the application of alkyne-tag imaging to visualize cell organelles and specific lipid components in artificial monolayer membranes.

Raman spectroscopic detection of bio-active small molecules using alkyne tag

Abstract: We use alkyne as a Raman tag for bio-active small molecules and which shows a distinct peak at the Raman-silent window of biomolecules. Raman scattering microscopy is used to observe the distribution of target small molecules in cells and lipid membranes.

Superresolution imaging based on nonlinearities of plasmonic scattering

Abstract: We demonstrated novel non-bleaching contrast for super-resolution imaging based on saturation and on/off switching of scattering from plasmonic particles, for the first time. Our study opens up new paradigms for both plasmonics and super-resolution microscopy.