We investigated the dependence of the surface-enhanced Raman scattering (SERS) activity of densely arranged two-dimensional assemblies of spherical Au(core)-Ag(shell) nanoparticles (Au/AgNSs) on the nanoparticle diameter. The size-controlled Au/AgNSs were synthesized using the Au nanosphere seed-mediated growth method without any bulky stabilizers. The diameters of the Au/AgNSs were 38, 53, and 90 nm and the ratio of the total diameter to the Au core diameter was adjusted to ca. 2.0. Extinction spectra of the colloidal solutions of these nanoparticles exhibited the prominent peak of the localized surface plasmon resonance (LSPR) of Ag and therefore the Au/AgNSs exhibited LSPR properties almost the same as Ag nanospheres. It was confirmed from SEM observation that the organic solvent-mediated liquid-liquid interface assembly technique easily generated densely arranged two-dimensional assemblies of the nanospheres. The extinction spectra of all the assemblies exhibited a prominent broad peak ranging from 500 nm to the near-infrared region, which is assigned to the longitudinal LSPR mode of the coupling nanospheres. The extinction intensity increased with increasing nanosphere diameter. The SERS activities of these assemblies were investigated using p-aminothiophenol as a probe molecule. The result revealed that the enhancement factor (EF) of the Raman signal dramatically increased upon increasing the particle diameter. The maximum EF obtained with a laser excitation wavelength of 785 nm was 1.90 × 10(6) for a nanosphere diameter of 90 nm. This renders the two-dimensional assemblies of the plasmonic Au/AgNSs promising for the development of highly sensitive SERS sensor platforms due to their strong electromagnetic effect.

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Despite many studies on seafloor hydrothermal systems conducted to date, the generation mechanism of seafloor massive sulfide (SMS) deposits is not yet fully understood. To elucidate this mechanism, this study clarifies the three-dimensional regional temperature distribution and fluid flow of a seafloor hydrothermal system of the Iheya North Knoll, middle Okinawa Trough. Lateral flow and boiling of hydrothermal fluids below a caprock were the main features found by the simulation. A caprock formation generated by anhydrite precipitation and hydrothermal alteration is the most plausible cause of these features, because caprocks can increase the temperature and induce boiling of fluids by preventing seawater inflow. Such a formation also gradually makes the top of the conduit less permeable; thus, lateral flow occurs. Consequently, vapor-rich hydrothermal fluids poor in metals are discharged from vents as white smokers, whereas liquid-dominated hydrothermal fluids rich in metals flow laterally below the caprocks, forming subseafloor SMS deposits. Plain Language Summary In seafloor hydrothermal systems, the seawater heated by magma circulates under the seafloor and forms seafloor massive sulfide (SMS) deposits. Recently, SMS deposits have attracted interest as a new metal resource, the generation mechanism and model of which must be established for accurate resource exploration. However, the mechanism is not yet fully understood. To address this problem, we applied a hydrothermal flow simulation and clarified the temperature distribution and fluid flow in the Iheya North Knoll, southwestern Japan. The result revealed that lateral flow and boiling of hydrothermal fluids occur below the seafloor. A low permeability caprock formation generated by anhydrite and clay mineral development is the most plausible cause of these occurrences, because a caprock can increase the temperature and induce boiling of fluids by suppressing the seawater inflow. This formation also makes fluid outlets less permeable, thus induces lateral flow. Consequently, vapor-rich hydrothermal fluids poor in metals are discharged from vents, while liquid-dominated hydrothermal fluids rich in metals flow laterally below the caprocks, forming subseafloor SMS deposits.

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