Fabrication of porous/hollow tin (IV) oxide skeletons from polypeptide mediated self-assembly
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B.Y. Tay Singapore Institute of Manufacturing Technology, Singapore 638075 (Received 9 October 2006; accepted 1 February 2007)
We demonstrate the aqueous self-assembly of ligand-assisted SnO2 sol precursors onto preformed poly-l-lysine templates through interfacial electrostatic forces (COO−/NH3+). On the removal of organics, two unique coral-like and sea worm-like textures consisting of hierarchical pores (macropores and mesopores) and nanocrystalline SnO2 frameworks are synthesized, mainly depending on the chelator/Sn molar ratio. Structural formation is discussed based on acid-base interaction and interfacial charge density matching. For the first time, metal oxide structures mediated by polypeptides are reported. More importantly, the method described here might open a generally attractive route for synthesizing complex nanostructures of other oxides (e.g., ZnO, TiO2, and ZrO2).
I. INTRODUCTION
A great variety of biomaterials have been utilized to control the inorganic textures at different scales (nanometer to micron) because of their attractive shapes and chemically active surfaces. Typical examples include homo-/heteropolypeptides, collagen fibers, diatoms, marine algae, single cells, bacterial superstructures, and deoxyribonucleic acid (DNA)/ribonucleic acid (RNA) strands, to name a few.1–11 The discovery of synthetic block copolypeptides with well-defined architecture and narrowmolecular-weight distribution initiated the use of its secondary and/or tertiary structures as novel templates.12 Using Lys200Cys30, Wong et al.4 and Cha et al.13–15 synthesized microspheres of silver, gold, and CdSe/CdS. These nanoparticle vesicles could be mechanically stabilized by an outer shell of amorphous silica. Later, they further reported that homopolypeptide (e.g., poly-Lys, -Orn, and -His) microspheres could be stabilized by certain small and functionalized multivalent anions (e.g., citric acid), and subsequently used to direct the formation of silicate shell.3 With the use of LysxPhey (x/y ⳱ 1), Jan et al.16 also demonstrated the successful synthesis of AgBr and AgBr/silica core-shell nanospheres with a controllable size in the range of 25–250 nm. Furthermore, block copolypeptides were also applied in controlling the nucleation growth, morphology (e.g., hexagonal platelets), and pore architecture of silica material.1,2,17–20 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0303 2448
J. Mater. Res., Vol. 22, No. 9, Sep 2007
To date, despite the stimulating findings in the field, there has been no report on the successful synthesis of polypeptide-templated metal oxides, to the best of our knowledge. The reasons are probably because of the poor understanding of interfacial chemical behavior as well as the uncontrollable inorganic sol stability. It is wellknown that due to a high electropositive character and the coordination number of metal atoms/ions, the rate of hydrolysis and the condensation of metal oxide precursors (i.e., metal organic compounds or metal salts)
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