Yasuaki Kumamoto

Plasmon-enhanced UV photocatalysis

We report plasmonic nanoparticle enhanced photocatalysis on titanium dioxide (TiO2) in the deep-UV range. Aluminum (Al) nanoparticles fabricated on TiO2 film increases the reaction rate of photocatalysis by factors as high as 14 under UV irradiation in the range of 260–340 nm. The reaction efficiency has been determined by measuring the decolorization rate of methylene blue applied on the TiO2 substrate. The enhancement of photocatalysis shows particle size and excitation wavelength dependence, which can be explained by the surface plasmon resonance of Al nanoparticles.

A femtosecond laser pacemaker for heart muscle cells

The intracellular effects of focused near-infrared femtosecond laser irradiation are shown to cause contraction in cultured neonatal rat cardiomyocytes. By periodic exposure to femtosecond laser pulse-trains, periodic contraction cycles in cardiomyocytes could be triggered, depleted, and synchronized with the laser periodicity. This was observed in isolated cells, and in small groups of cardiomyocytes with the laser acting as pacemaker for the entire group. A window for this effect was found to occur between 15 and 30 mW average power for an 80 fs, 82 MHz pulse train of 780 nm, using 8 ms exposures applied periodically at 1 to 2 Hz. At power levels below this power window, laser-induced cardiomyocyte contraction was not observed, while above this power window, cells typically responded by a high calcium elevation and contracted without subsequent relaxation. This laser-cell interaction allows the laser irradiation to act as a pacemaker, and can be used to trigger contraction in dormant cells as well as synchronize or destabilize contraction in spontaneously contracting cardiomyocytes. By increasing laser power above the window available for laser-cell synchronization, we also demonstrate the use of cardiomyocytes as optically-triggered actuators. To our knowledge, this is the first demonstration of remote optical control of cardiomyocytes without requiring exogenous photosensitive compounds.

Deep UV resonant Raman spectroscopy for photodamage characterization in cells

We employed deep UV (DUV) Raman spectroscopy for characterization of molecular photodamage in cells. 244 nm light excitation Raman spectra were measured for HeLa cells exposed to the excitation light for different durations. In the spectra obtained with the shortest exposure duration (0.25 sec at 16 µW/µm2 irradiation), characteristic resonant Raman bands of adenine and guanine at 1483 cm−1 and tryptophan and tyrosine at 1618 cm−1 were clearly visible. With increasing exposure duration (up to 12.5 sec), these biomolecular Raman bands diminished, while a photoproduct Raman band at 1611 cm−1 grew. By exponential function fitting analyses, intensities of these characteristic three bands were correlated with sample exposure duration at different intensities of excitation light. We then suggest practical excitation conditions effective for DUV Raman observation of cells without photodamage-related spectral distortion.

Deep ultraviolet resonant Raman imaging of a cell

We report the first demonstration of deep ultraviolet (DUV) Raman imaging of a cell. Nucleotide distributions in a HeLa cell were observed without any labeling at 257 nm excitation with resonant bands attributable to guanine and adenine. Obtained images represent DNA localization at nucleoli in the nucleus and RNA distribution in the cytoplasm. The presented technique extends the potential of Raman microscopy as a tool to selectively probe nucleic acids in a cell with high sensitivity due to resonance.

Efficient UV photocatalysis assisted by densely distributed aluminum nanoparticles

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.

Nano-Raman Scattering Microscopy: Resolution and Enhancement

Raman scattering microscopy is becoming one of the hot topics in analytical microscopy as a tool for analyzing advanced nanomaterials, such as biomolecules in a live cell for the study of cellular dynamics, semiconductor devices for characterizing strain distribution and contamination, and nanocarbons and nano-2D materials. In this paper, we review the recent progress in the development of Raman scattering microscopy from the viewpoint of spatial resolution and scattering efficiency. To overcome the extremely small cross section of Raman scattering, we discuss three approaches for the enhancement of scattering efficiency and show that the scattering enhancement synergistically increases the spatial resolution. We discuss the mechanisms of tip-enhanced Raman scattering, deep-UV resonant Raman scattering, and coherent nonlinear Raman scattering for micro- and nanoscope applications. The combinations of these three approaches are also shown as nanometer-resolution Raman scattering microscopy. The critical issues of the structures, materials, and reproducibility of tips and three-dimensionality for TERS; photodegradation for resonant Raman scattering; and laser availability for coherent nonlinear Raman scattering are also discussed.

Deep-UV biological imaging by lanthanide ion molecular protection.

Deep-UV (DUV) light is a sensitive probe for biological molecules such as nucleobases and aromatic amino acids due to specific absorption. However, the use of DUV light for imaging is limited because DUV can destroy or denature target molecules in a sample. Here we show that trivalent ions in the lanthanide group can suppress molecular photodegradation under DUV exposure, enabling a high signal-to-noise ratio and repetitive DUV imaging of nucleobases in cells. Underlying mechanisms of the photodegradation suppression can be excitation relaxation of the DUV-absorptive molecules due to energy transfer to the lanthanide ions, and/or avoiding ionization and reactions with surrounding molecules, including generation of reactive oxygen species, which can modify molecules that are otherwise transparent to DUV light. This approach, directly removing excited energy at the fundamental origin of cellular photodegradation, indicates an important first step towards the practical use of DUV imaging in a variety of biological applications.

Raman micro-spectroscopy as a viable tool to monitor and estimate the ionic transport in epithelial cells

The typical response to the lowering of plasma Na+ concentration and blood pressure in our body involves the release of aldosterone from the adrenal glands, which triggers the reabsorption of sodium in the kidney. Although the effects of aldosterone on this physiological mechanism were extensively studied in the past decades, there are still some aspects to be fully elucidated. In the present study, we propose for the first time a new approach based on Raman spectroscopy to monitor the ionic activity in aldosterone-treated A6 renal epithelial cells. This spectroscopic technique is capable of probing the cells through their thickness in a non-destructive and nimble way. The spectroscopic variations of the Raman bands associated to the O-H stretching of water were correlated to the variations of ionic concentration in the intracellular and extracellular fluids. The increase of Na+ concentration gradients was clearly visualized in the cytosol of aldosterone-treated cells. The enhancement of the Na+ current density induced by aldosterone was estimated from the variation of the ionic chemical potential across the intracellular space. In addition, the variation of the O-H Raman bands of water was used to quantify the cell thickness, which was not affected by aldosterone.

Rapid and accurate peripheral nerve imaging by multipoint Raman spectroscopy

Raman spectroscopy allows label-free, minimally invasive, and accurate detection of peripheral nerves. However, the conventional Raman imaging technique is time-consuming when measuring a large area of a sample. Establishing a method for rapidly acquiring spatial distribution of a bundle of peripheral nerve fibers is an essential step for Raman spectroscopy towards application in clinical surgery. Here we present a multipoint Raman spectroscopic technique for rapid peripheral nerve imaging. In only 5 seconds, spectra at 32 points situated on ex vivo rat peripheral nerve bundles and adjoining connective tissues were acquired. Principal component regression and discriminant analysis of spectra revealed that the sensitivity, specificity and accuracy for nerve detection were 85.8%, 96.0%, and 90.8%, respectively. Of 158 peripheral nerves, 152 (96.2%) showed ratio of the number of nerve-positive prediction points to the total measurement points being 0.4 or larger, whereas 119 (99.2%) connective tissues among 120 showed ratio smaller than 0.4. Based on the ratio and a bright-field image of the sample, accurate visualization of peripheral nerves was implemented. The results indicated that the multipoint Raman spectroscopic technique is capable of rapid and accurate peripheral nerve imaging.

Label-free detection of myocardial ischaemia in the perfused rat heart by spontaneous Raman spectroscopy

Raman spectroscopy, which identifies intrinsic molecular constituents, has a potential for determining myocardial viability under label-free conditions. However, its suitability for evaluating myocardial ischaemia is undetermined. Focusing on cytochromes, i.e., representative molecules reflecting mitochondrial activity, we tested whether Raman spectroscopy is applicable for evaluating myocardial ischaemia especially during early ischaemic phase. We obtained spontaneous Raman spectra of the subepicardial myocardium in the Langendorff-perfused rat heart upon 532-nm excitation before and during the “stopped-flow,” global ischaemia. Semi-quantitative values of the peak intensities at 750 and 1127 cm−1, which reflect reduced cytochromes c and b, increased immediately and progressively after induction of the stopped flow, indicating progressive reduction of the mitochondrial respiration. Such spectral changes emerged before the loss of 1) mitochondrial membrane potentials measured by the fluorescence intensity of tetramethyl rhodamine ethyl ester or 2) staining of the triphenyl tetrazolium chloride dye in the myocardium. The progressive increases in the Raman peaks by stopped flow were significantly retarded by ischaemic preconditioning. Sequential measurements of the peak intensities at 750 and 1127 cm−1 enabled early detection of the myocardial ischaemia based on the mitochondrial functions. These data suggest that Raman spectroscopy offers the potential to evaluate acute ischaemic heart under label-free conditions.