Yuki Takeuchi

The adsorption of the nitrate ion by the cylindrical pore of single-walled carbon nanotubes (SWCNT) was found to be aided by an acidic adsorbed layer. Adsorbed water in the vicinity of the pore wall can supply protons through ionization, forming the acidic layer, according to Raman spectra and results of solution pH fluctuations caused by ion species adsorption. Such an acidic adsorbed layer leads to surplus adsorption of anionic species where the adsorbed amount of nitrate ions is much larger than that of cations. Also, we could observe the Raman bands being assignable to the symmetrical stretching mode at an extremely high-frequency region for nano-restricted nitrate ions compared to any other bulk phases. The abnormal band shift of adsorbed nitrate ions indicates that the nitrate ions are confined in the pore under the effects of nanoconfinement by the pore and the strong interaction with the acidic layer in the pore. Our results warn that we have to construct the adsorption model of aqueous electrolytes confined in carbon pores by deliberating the acid layer formed by the adsorbed water.

We evaluated the properties of hot carrier generation on Au and Ag nanoparticle (NP) 2D arrays by monitoring chemical transformation. The arrays were fabricated by assembling the Au and AgNPs, respectively, with a diameter of ∼30 nm. The plasmon resonance peaks of both the arrays were tuned to around 671 nm, where the intraband transition was the dominant pathway for the surface plasmon excitation since the incident photon energy is below the interband transition threshold for both Au and Ag. Time-resolved surface-enhanced Raman scattering (SERS) spectroscopy was utilized to monitor the changes in the molecular structure sensitively. We selected para-aminothiophenol as the molecule, which was well known to transform to 4,4′-dimercaptoazobenzene (DMAB) by hot carriers generated from the plasmonic excited surface. It was observed that the peak intensity of DMAB increased as the total exposure increased due to the increase in the number of DMAB molecules for both metals, similar to our previous reports. The analysis of the SERS spectra showed that larger laser intensity progressed the transformation more rapidly. The analysis also clarified that the chemical transformation occurs more efficiently on the AgNP array compared to the AuNP array. The superiority in the efficiency for Ag may be derived from contribution of more hot carriers generated in the material with the smaller plasmon damping constant.

In this study, we focus on the antimicrobial properties of tempeh, a soybean fermented food, against oral bacteria.Tempeh showed antimicrobial activity against dental caries pathogenic bacterium Streptococcus mutans at a final concentration of 1 mg/mL. An antimicrobial substance contained in tempeh was present in the 100 kDa or greater fraction generated by ultrafiltration, but it was found not to be proteinaceous by native-PAGE, SDS-PAGE and protein degradation tests. Next, when the fraction was purified with an ODS column, the 80% and 100% methanol eluates showed antimicrobial activity against S. mutans. The 100% methanol eluate was further subjected to a 2nd column purification, and isolation of the target was confirmed by HPLC. When the isolated material was analyzed by ESI-MS, the m/z was 279.234. Further analysis by Raman spectroscopy revealed a peak similar to linoleic acid. This substance also possessed antimicrobial properties equivalent to linoleic acid.

We evaluated the hot carrier generation in two-dimensional (2D) silver nanoparticle (AgNP) arrays under light illumination at different wavelengths, 458, 532, 671, and 785 nm. The 2D AgNP arrays were tailored to match the plasmon resonance to each excitation wavelength in order to fulfill the on-resonant condition. We selected para-aminothiophenol (p-ATP) as a probe molecule, which is chemically transformed into 4,4'-dimercaptoazobenzene (DMAB) upon light illumination. The reaction is driven by hot carriers emitted from a plasmonic surface. For evaluation of hot carrier generation, we monitored the chemical transformation from p-ATP into DMAB with surface-enhanced Raman scattering. The normalized Raman intensity of DMAB was plotted against the total exposure, where the peak intensity increased as the total exposure increased because of the increase of the number of DMAB molecules. The saturation of the peak growth was observed, indicating that the chemical transformation was completed, at different exposures for each wavelength. The total exposure required for completing the chemical transformation was smaller at 458 nm by at least similar to 10(5) times than that at 785 nm, although the difference of the photon energy was only 1.7 times. The growth of the Raman peak was related to the laser intensity as well, where the higher laser intensity showed a more rapid growth. These results indicated that more hot carriers with sufficient energy for the chemical transformation were generated at shorter excitation wavelengths as well as at higher laser intensities.

Surface-enhanced Raman scattering (SERS) spectra of organic compounds, para-methylthiophenol (p-MT), decylamine and 1-butanethiol (1-BT), were measured using a two-dimensional silver nanoparticle array at 532 nm excitation. For p-MT, it was observed that Raman peaks grew at 1580 and 1690 cm(-1), which were never observed in the normal Raman spectrum, indicating oxidation from the methyl to carboxyl group. For both decylamine and 1-BT, an intensive SERS peak grew at 1580 cm(-1). We measured the time-resolved SERS spectra of 1-BT at the laser intensity of 185 W mm(-2) and confirmed that the spectral shapes changed as the total exposure increased. Another SERS peak was also observed at 3050 cm(-1) for decylamine and 1-BT. From these results, it was considered that unsaturated bonds were formed in the alkanes, meaning that alkenes were produced from alkanes. Additionally, the SERS spectrum revealed that the chemically transformed alkane possesses a methyl group. The result indicates that dehydrogenation preferentially occurs at the secondary carbons, which is consistent with the stability of radicals on carbon atoms. The laser intensity threshold for plasmon-mediated chemical transformation was experimentally observed to be 2.7 and 40 W mm(-2) for p-MT and 1-BT, respectively. The higher laser intensity is necessary for oxidation of alkanes compared with aromatic compounds, which is consistent with the chemical stability of organic compounds.