0428同步年報-2021-全

Soft Matter 037 Fig. 1 : (a) Diatom―Thalassiosira pseudonana. (b) Possible scheme of bio-silica synthesis in a diatom cell. (i) A cell surrounding silicic acid media. (ii) STV enter the cell by endocytosis. (iii) STV is uptaken into SDV and aquaporines pump water from SDV. (iv,v) Desiccation concentration induces the silica polymerization, followed by bio-silica formation after removal of organic composites. [Reproduced from Ref. 1] U nderstanding molecular engineering holds the key to future nanomaterial technologies. Nature has all answers: build biominerals from a bottom-up approach (piecing atoms or molecules together to produce a more complicated construction). Biomineralization involves forming a hierarchical and well organized structure from the self-assembly of organic template-inorganic minerals, followed by mineralizing for organic removal. These minerals could be silica in diatoms and phosphate in vertebrates. Silica and silicates are generally used in medicine (safe as ingredients for oral delivery), transport ( e.g. , sensors and catalysts), telecommunication ( e.g. , optical sensors), manufacturing ( e.g. , micro-robots), etc . Tailoring the morphology and pore structure of silica has been well investigated for more than 30 years, but nobody has been able to use chemical synthesis to build the complicated patterned diatom cell wall (frustule). Diatoms are unicellular photosynthetic algae with hard and porous frustules and some with extended setae for buoyancy in sizes 2−200 μm, in Fig. 1(a) . The open nano- scale pore and channel structure of micrometre-sized frustule is made almost purely of amorphous silica from silicic acid. In the general paradigm as shown in Fig 1(b) , bio-silica is synthesized within silica deposition vesicles (SDV) inside the cell membrane during frustule formation. This bio-silicification occurs also outside the cell membrane, such as in setae (silica extension of a valve) formation. The bio-silicification process inside or outside a cell involves the Si-transport system (like STV) and silicification mediated by confined compartments (like SDV). Yi-Qi Yeh (NSRRC), U-Ser Jeng (NSRRC) and Chung-Yuan Mou (National Taiwan University) recently proposed the concept of a mechanism of formation of bio-silica like diatoms, so as to understand the mechanism of formation of bio-silica in the confined space of a soft template in the concept of biomineralization. Using small-angle X-ray/neutron scattering (SAXS/SANS) at TLS 23A1 in the NSRRC and freeze-fracture-replication transmission electron microscopy (FFR-TEM) methods, it is found the answer to the formation of a thin silica sheet templated by a ternary surfactant system (SDS-CTAB-P123), nearly that of biomineralization in diatoms. The FFR-TEM ( Fig. 2(a) ) reveals the critical importance of the coexistence of surfactant-P123 self-assembly charged micelles of two kinds named as silica transport micelles (STM) and the silica deposition nanoplates (SDNP). The SANS data ( Fig. 2(b) ) were fitted on the basis of a core-shell ellipsoidal model. The trend indicates that significant adsorption of catanionic surfactant to P123 micelles could lead to a transition of micellar shape from spherical to prolate. The SAXS data ( Fig. 2(c) ) elucidate an intriguing role of the catanionic surfactants in modulating the self-assembly of the surfactant-P123 micelles. With appropriately high adsorption affinity to P123 micelles, SDS can facilitate the adsorption of CTAB to the P123 micelles via co-condensation of the charged ion pairs, loading to optimized prolate micelles for a preferred monolayer self- assembly. Chemical Magicians─Mimic Biosilica from Nature Understanding the mechanism of formation of bio-silica in the confined space of a soft template is the concept of biomineralization.