Yasushi Inouye

Near-Field scanning optical microscope with a metallic probe tip

A near-field scanning optical microscope with a metallic probe tip was developed for detecting localized photons near the surface of the fine structure of a sample. In this microscope a metallic probe is used for converting the evanescent photons localized near the sample surface to the propagating scattering light wave; the scattered light is detected in the far field with external condenser optics. During the measurement the probe tip vibrates normal to the surface with an amplitude of ~5 nm at 2.5 kHz, and the light intensity modulated with this frequency is lock-in detected. This operation permits the removal of stray-light noise contribution. Experimental results of the measurements of the exponential decay of the evanescent field produced by total internal reflection are given with and without the probe vibration. Image data of the surface profile of an optical compact disk are also shown.

Metallized tip amplification of near-field Raman scattering

We have observed the amplification of near-field Raman scattering by using an apertureless near-field scanning optical microscope the tip of which is a 40 nm silver-layer-coated cantilever of an atomic force microscope. Localized surface plasmon polaritons are excited at the metallized tip apex of the cantilever thereby producing a field enhancement effect. We have unambiguously observed the amplification of the near-field signal due to the field enhancement by measuring the Raman spectra of Rhodamine 6G molecules adsorbed on 10 nm silver islands film when the metallized tip approached the surface of the sample. In addition, due to the nature of near-field optics, spectral mapping of Raman Scattering was attained with a 50 nm super-resolving power.

Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy

A tip-enhanced near-field Raman microscope has been applied to the detection of an individual single-wall carbon nanotube (SWNT). The detected information is a color (Raman-shift) image of molecular distribution without having to resort to staining non-fluorescent molecules of interest. In addition to nanometric-sensing and -imaging capability, local field-enhancement of the metallic tip has been utilized to detect a weak Raman scattering from nanometer region, which cannot be observed by conventional micro-Raman configuration (far-field Raman). The experimental results are shown with analysis of distinct vibration modes of both radial breathing mode and G-band.

Fluorescent Platinum Nanoclusters: Synthesis, Purification, Characterization, and Application to Bioimaging†

Noble-metal nanoclusters consisting of several atoms have been gaining much attention as novel fluorescent markers owing to their optical properties, which include size-dependent emission wavelength and discrete electronic state, features that are similar to semiconductor quantum dots. However, their smaller size and lower cytotoxicity in some ways make metal nanoclusters superior. Dickson and coworkers reported the synthesis and characterization of gold and silver 7] nanoclusters and successful bioimaging of actin filament labeled with these nanoclusters, while Lin et al. used peptide-conjugated gold nanoclusters as nuclear targeting and intracellular imaging probes. However, among the noble-metal clusters, platinum clusters have not yet been used as bioimaging probes except as a reducing catalyst. Herein, we describe water-soluble platinum nanoclusters that are less cytotoxic and emit a brighter fluorescence at 470 nm than other fluorescent nanomaterials, such as gold clusters. The synthesis of such platinum nanoclusters is achieved by a simple chemical reduction; they are purified by size-exclusion high performance liquid chromatography (HPLC). The size of the noble-metal nanoclusters is crucial because it affects the photoluminescence wavelength. 2, 3a, 4a,5a] This parameter can be regulated by a macromolecule-based template such as a dendrimer. One example is the fourth-generation polyamidoamine dendrimer (PAMAM (G4-OH)), which has been used as a molecular template in the synthesis of fluorescent gold nanoclusters. 8a,b] This dendrimer has nanometer-scale size and a uniform structure comprising an internal core and an external shell. Since the internal core contains tertiary amines that can form coordination bonds with the metallic ions, PAMAM dendrimers can trap these metal ions to form metal nanoclusters. Herein, we apply PAMAM (G4-OH) to synthesize Pt nanoclusters. Electronspray ionization (ESI) mass spectrometry confirmed that the synthesized nanoclusters were atomically monodispersed Pt5. Platinum nanoclusters were prepared by reducing H2PtCl6 (6.0 mL, 0.5m) with NaBH4 (3.0 mmol) in the presence of PAMAM (G4-OH) (0.5 mmol, 71.4 mg; Scheme 1). NaBH4

Raman microscopy for dynamic molecular imaging of living cells.

We demonstrate dynamic imaging of molecular distribution in unstained living cells using Raman scattering. By combining slit-scanning detection and optimizing the excitation wavelength, we imaged the dynamic molecular distributions of cytochrome c, protein beta sheets, and lipids in unstained HeLa cells with a temporal resolution of 3 minutes. We found that 532-nm excitation can be used to generate strong Raman scattering signals and to suppress autofluorescence that typically obscures Raman signals. With this technique, we reveal time-resolved distributions of cytochrome c and other biomolecules in living cells in the process of cytokinesis without the need for fluorescent labels or markers.

Single molecule detection from a large-scale SERS-active Au79Ag21 substrate

Detecting and identifying single molecules are the ultimate goal of analytic sensitivity. Single molecule detection by surface-enhanced Raman scattering (SM-SERS) depends predominantly on SERS-active metal substrates that are usually colloidal silver fractal clusters. However, the high chemical reactivity of silver and the low reproducibility of its complicated synthesis with fractal clusters have been serious obstacles to practical applications of SERS, particularly for probing single biomolecules in extensive physiological environments. Here we report a large-scale, free standing and chemically stable SERS substrate for both resonant and nonresonant single molecule detection. Our robust substrate is made from wrinkled nanoporous Au79Ag21 films that contain a high number of electromagnetic “hot spots” with a local SERS enhancement larger than 109. This biocompatible gold-based SERS substrate with superior reproducibility, excellent chemical stability and facile synthesis promises to be an ideal candidate for a wide range of applications in life science and environment protection.

Near-field Raman imaging of organic molecules by an apertureless metallic probe scanning optical microscope

Near-field Raman imaging of organic molecules is demonstrated by an apertureless near-field scanning optical microscope, the tip of which is a silver-layer-coated cantilever of an atomic force microscope (AFM). The virtue of the enhanced electric field at the tip apex due to the surface plasmon polariton excitations enhances the Raman scattering cross sections. This phenomenon allows us to reveal from near-field Raman images the molecular vibrational distributions of Rhodamine6G and Crystal Violet molecules beyond the diffraction limit of a light. These molecular vibrations cannot be distinguished by AFM topographic images.

Gold-bead scanning near-field optical microscope with laser-force position control

We have developed a scanning near-field optical microscope with an optically trapped metallic particle that has a small diameter compared to the wavelength of visible light. In this microscope we employed spot illumination to enhance the intensity of light scattered from a probe particle so we could reduce the diameter of the probe particle to 40 nm. We detected slight irregularities of the surface of the cover glass near 10-nm depth. Also, we observed gold colloidal particles on the surface of the cover glass.

Scanning probe optical microscopy using a metallic probe tip

Abstract We propose a scanning probe optical microscope which features a metallic probe tip for detecting evanescent photons localized near the surface of fine structures of the sample. A metallic probe is used for converting the evanescent photons localized near the sample surface to the propagating photons by scattering, but not for transmitting the converted photons through it as a waveguide. The light scattering is detected in far field with external condenser optics. During the measurement, the probe tip vibrates vertically very near the surface, so that the light intensity related to the conversion from evanescent photons to propagating photons is lock-in detected. Experimental results show the exponential decay due to the evanescent field as a function of distance. The experimental results of surface profiling for a smoothly curved surface and a heavily scattering surface are also presented.

Near-field scanning optical microscope with a laser trapped probe

We made an experiment of near-field microscopic imaging using a laser-beam trapped probe. Differently from a conventional near-field (and/or photon-tunneling) scanning optical microscope, the probe is physically isolated from the scanning microscope system; it is trapped and scanned on the sample surface by the radiation force of near-infrared laser beam. The distance between the probe and the sample surface is maintained to be constant (zero) during scanning. Another laser beam for microscopic imaging is incident on the sample surface in the condition of total internal reflection; the probe on the sample couples with the photons localized near the sample surface as the evanescent filed and scatters out. The scattered photons are collected through an microscope objective lens, which is the same lens as the one used for focusing the infrared laser beam on the probe. A near-field image of the sample surface is formed, as the probe is laterally scanned on the sample. The experimental setup of the proposed microscope is described and the image data obtained with the developed microscope are shown for refractive samples and fluorescent samples with sub micrometer structure.