Structural Characterization of the Body Frame and Spicules of a Glass Sponge
The nanostructure (atomic-scale structure) and water species in the body frame and spicules of the marine glass sponge, Euplectella aspergillum, collected from the sea floor around Cebu Island was characterized in detail by thermogravimetric differential thermal analysis, nuclear magnetic resonance spectroscopy, Raman and infrared spectroscopies, and X-ray diffraction method. The structural features of the nanostructure in the body frame and spicules were essentially similar to each other,
... to each other, although these were different from those of inorganic amorphous silica materials, such as silica gel and silica glass. In addition, the averaged short and medium range structures of the sponge may be similar to those of tridymite. The water content and water species included in the body frame and spicules were almost the same. More than half of the contained water was physisorbed water molecules, and the rest was attributed to Q 3 and Q 2 silanol groups. Most of the water species may be present at the surface and involved in hydrogen bonding. However, details about the silica nanostructure and the water species included in the skeleton of glass sponges remained poorly understood. Cha et al.  reported that the proteinaceous axial filaments isolated from spicules in a Demospongiae was shown to induce the polymerization of silica from the TEOS (Si-tetraethoxide, Si[C 2 H 5 O] 4 ) substrate when combined with TEOS and axial filament. The silica synthesis was promoted by a protein called silicatein solubilized from the axial filament  . Silicatein-like proteins were also identified in the hexactinellid sponges . The protein "glassin" rapidly accelerates silica polycondensation over a pH range of 6-8, when combined with silicic acid solutions  . In addition, the spicule of Demospongiae can crystallize into cristobalite at lower temperature (850 • C), which is possibly due to the presence of silicatein  . These studies showed that precise structural information on the sponge spicule and body frame designed by some proteins may be essential in the synthesis of amorphous and crystalline silica materials with less environmental burden and the development of new materials. On the other hand, the distribution of the ring structure made of SiO 4 tetrahedra, which is constituted of the amorphous silica, varies greatly according to different silica materials. For example, the average structure of silica gel may be four-membered ring  , although the silica glass may be mainly composed of a ring with more than six-membered ring     . Therefore, it is interesting to elucidate the nanostructure from the perspective of material science, especially the ring structure of biogenic silica, e.g., the skeletons of glass sponge and radiolarian and the frustule (shell) of diatom, compared with the inorganic silica materials. Namely, in order to develop and accelerate the synthesis of biological silica, it is necessary to know the specific features of the formed structure. In addition, we conceive that crystallization of spicules of Demospogiae  at low temperature is derived from an amorphous structural feature. The results may also give new important insight to the protein, which controls the formation of silica structure. In this study, the body frame and spicules of the glass sponge, Euplectella aspergillum, were examined by thermogravimetric differential thermal analysis (TG-DTA), 1 H static NMR and 1 H-29 Si CP-MAS NMR spectroscopies, Raman and infrared spectroscopies, and X-ray diffraction (XRD) analysis. Our aim was to determine precisely the nanostructure and water species in the sponge and to reveal the structural differences between the body frame and spicules. Materials and Methods Sample The hexactinellid sponges, Euplectella aspergillum, used in this study were collected from the sea floor near Cebu Island (Philippines; Figure 1 ). Silica gel synthesized by a typical sol-gel method (described by  ) and commercially available fused silica glass  were used as reference materials. The chemical composition of the skeletal body frame, the spicules, and silica glass were determined by X-ray fluorescence analysis (Rigaku ZXS Primus II, Tokyo, Japan) with an acceleration voltage of 50 kV and current of 20 mA (Table 1 ). The body frame and spicules were composed of >99% SiO 2 . Only spicules were found to contain PdO, but the determination of its content needs more accurate analysis.