Colloidal nanoparticles (NPs) have been significant research subjects for decades. The study of these subjects can range widely from relatively simple colloidal synthesis to extremely complex assembly. Recently, colloidal NPs as building blocks for fabricating various functional nanomaterials, just like atoms are the bricks of molecules, have attracted much attention. To precisely control the self-assembly of colloidal NPs, the features of both building blocks and assemblies should be studied systematically.

Brief introduction for synthesis and self-assembly of colloidal NPs, and the structure of doctoral thesis are presented in chapter 1. Four types of colloidal NPs, including silica nanospheres (SNSs), mesoporous silica nanospheres (MSNs), hollow silica nanospheres, and metal oxide NPs are reviewed in detail. Furthermore, one-, two-, and three-dimensional (1D, 2D, and 3D) self-assemblies of colloidal NPs are introduced in this chapter.

Yokoi et al. (JACS, 2006) developed a facile method based on hydrolysis and condensation of tetraethoxysilane (TEOS) with amino acid (e. g., lysine) under weakly basic condition (pH ~9.8) in liquid-liquid (TEOS-water) biphasic system for synthesis of uniform colloidal SNSs. It is effective for precisely controlling the SNSs size and polydispersity. Syntheses of various colloidal NPs containing SNSs, MSNs, and hollow silica nanospheres, as building blocks for the assembly are described in chapter 2. A modified approach, in which primary amine is used as a catalyst for the synthesis of uniform SNSs in liquid-liquid (TEOS-water) biphasic system at relatively high pH conditions (10.8-11.4), has been developed. The size of SNSs can be tuned from 12 to 36 nm by changing the initial pH values of the aqueous phase in reaction mixtures. This biphasic reaction method is applied to synthesize MSNs. Discrete MSNs with a narrow size distribution can be prepared by using biphasic reaction system containing TEOS, water, cationic surfactant, and primary amines under basic conditions (pH 11.3-11.5). The MSNs with diameter in the range of 15 to 30 nm have uniform mesopores about 3 nm in size. The size of the mesopores can be finely tuned by changing the initial pH of the solution, or by the addition of pore expanding agent, N, N-dimethylhexadecylamine. Finally, a facile synthesis method for preparing hollow silica nanospheres with size of about 20 nm and cavity diameter of about 7 nm has been developed by using block copolymer F127 as a soft template in the presence of basic amino acids under neutral condition (pH ~7.5).

Monodisperse colloidal SNSs synthesized by primary amine have been chosen as the starting building blocks for investigating 3D self-assembly process. In chapter 3, the influences of chemical additives on the 3D self-assembly of SNSs are demonstrated. Yokoi et al. (JACS, 2006) reported that uniform SNSs synthesized by basic amino acid assembled into well-ordered nanostructure (face-centered-cubic). For example, SNSs with diameter about 20 nm assemble into close packed SNSs arrays possessing about 5.8 nm interparticle mesopores. In this chapter, SNSs arrays with large interparticle mesopores about 13 nm are observed upon solvent evaporation of as-synthesized sol in the presence of primary amine. This indicates that loosely packed nanostructure are formed. In contrast, SNSs arrays having smaller mesopores in the range of 4.7-6.8 nm are achieved with the aid of the organic buffer or a basic amino acid, lysine. It is suggested that the chemical additives with the ability to maintain relatively strong repulsive interaction until the final stage of evaporation play a vital role in the fabrication of well-ordered SNSs arrays.

In chapters 4 and 5, 1D self-assembly of SNSs with basic amino acids in liquid phase is described. The detailed study of amino acid-assisted 1D assembly of SNSs is discussed in chapter 4. A facile solution process for the preparation of anisotropic silica nanoparticles (ASNPs) is presented in chapter 5. ASNPs are prepared via controlled self-assembly of spherical silica seeds (22 nm) in alcohol-water solution, followed by in situ fixation and overgrowth with TEOS. Ethanol and arginine are used to modify the dielectric constant and ionic strength of the reaction media, by which seed assembly is controlled through the adjustment of electrostatic interaction. Ethanol and arginine also serve as a cosolvent and a catalyst for hydrolysis and condensation of TEOS, respectively, which enables us to produce ASNPs in a simple one-pot process. In addition to ASNPs with wormlike structures, different kinds of NPs (bimodal spherical NPs, monodisperse spherical NPs, and spherical aggregates) have also been obtained by changing the concentrations of ethanol and arginine. The length, thickness, or both of ASNPs are controlled systematically by varying the concentrations of arginine, seed NPs, and TEOS. Other alcoholic cosolvents, such as methanol, 1-propanol, 2-propanol, and t-butanol, are also effective to give ASNPs when the dielectric constant of the alcohol-water mixed media is properly adjusted, showing the versatility of the present method.

Previously, Fukao et al. (JACS, 2009) found that SNSs (ca. 15 nm) synthesized by basic amino acid (lysine) assembled into 1D chain-like nanostructure in the liquid phase with the aid of an amphiphilic block copolymer, F127. However, the formation mechanism of 1D chain-like nanostructure and the role of lysine playing in the 1D self-assembly have not been clarified yet. In an effort to confirm the role of lysine in the 1D assembly process, the experiments on 1D self-assembly of lysine-free SNSs are investigated in chapters 6 and 7. It is shown in chapter 6 that 1D chain-like nanostructure is formed at different pH conditions when changing the type of silica sols. The optimal pH of lysine-free system is not consistent with the pH in the case of lysine. It suggests that lysine is not indispensable for 1D self-assembly of SNSs. It seems that ionic strength of the suspension affects the interparticle interaction, leading to a minor shift in the optimal pH. Instead of the addition of HCl for pH adjustment, dialysis of suspension against distilled water is performed to adjust pH. The dialyzed silica sols without containing amino acid can also assemble into 1D chain-like nanostructure. These findings suggest that fine-tuning of the interparticle interaction by using dialysis method is also effective for preparing 1D chain-like nanostructure. This method can be applied to other types of silica NPs with different surface properties, such as MSNs, which is summarized in chapter 7.

The dialysis method is applied for assembly of other metal oxide colloids into 1D as presented in chapter 8, since basic amino acids are not indispensable for 1D self-assembly of NPs with block copolymer. Titanium oxide NPs are used as building blocks for 1D self-assembly, because titanium oxide NPs are widely studied owing to their broad applications and it is extremely difficult to assemble them into 1D nanostructure in aqueous system. Monodisperse titanium oxide sol is prepared by acid peptization. Dialysis method is used to adjust the pH for controlling the charge density of NPs' surface. After dialysis to the proper pH, F127 is introduced into the titanium oxide sol. 1D nanostructure can be formed after heat treatment of this sol for several days. The effects of pH, the amount of F127, and the heating time are investigated systematically. Finally, it is found that oriented attachment proceed between adjacent NPs in aqueous system.

The driving force for 1D self-assembly of SNSs with block copolymer may be related to depletion interaction in the system. The free micelles of block copolymer are formed in the suspension, and this micelle-mediated depletion interaction may be one of the driving forces for 1D self-assembly. To validate this hypothesis, other water soluble polymers (e. g. polyvinyl alcohol, polyvinylpyrrolidone) that can not form micelle in the solution are examined in chapter 9.

Finally, chapter 10 presents general conclusions and future perspectives of this doctoral thesis.

ナノテクノロジーの進展に伴い、コロイドナノ粒子系が注目されている。原子をビルディングブロックとして分子が構成されていることに倣い、ナノ粒子を利用した機能性ナノ材料の開発に期待が集まっている。とりわけ、簡便で精度のよいナノ粒子合成法の開発や自己集合プロセスの解明と制御法の開発が重要な研究課題となっている。

本博士論文は、様々な単分散コロイドナノ粒子の合成法とこれらの粒子の精密な配列法に関する研究成果をまとめたものである。

Chapter 1では、単分散コロイドナノ粒子の合成および配列制御に関する既往の研究成果をまとめ、同分野における現状の問題点を明らかにした上で、本研究の目的と本論文の構成が述べられている。

Chapter 2では、様々なナノ粒子の合成を検討した結果が報告されている。まず、テトラエチルオルソシリケート(TEOS) をシリカ源に用いた二相反応法によるシリカナノ粒子の合成において、従来必須であった塩基性アミノ酸の代わりに有機アミンを溶解させた水溶液を用いることにより、12-36 nmの均一な球状シリカ粒子の合成が可能であることが報告されている。また二相反応法に界面活性剤を添加した系においては、メソポーラスシリカナノ粒子の合成に成功している。更にN,N-ジメチルヘキサデシルアミンを添加し、pHを調整することで細孔径および細孔容積の制御にも成功している。両親媒性のブロックコポリマーF127を用いた系で、中空シリカナノ粒子の合成も可能であることが報告されている。

Chapter 3では、Chapter 2で得られた均一なシリカナノ粒子を用い、添加物が粒子の三次元配列に与える影響について検討が行われている。アミンや緩衝剤、塩基性アミノ酸を添加した場合に、立方最密充填構造が形成されている。これらの添加が、シリカナノ粒子集合体の充填率に及ぼす影響について、検討が行われている。

Chapter 4では、塩基性アミノ酸を用いて合成したシリカナノ粒子の一次元配列について検討した結果が報告されている。シリカナノ粒子のコロイド溶液に,塩基性アミノ酸とエタノールを添加すると,ナノ粒子が一次元状に自己集合することが見出されている。更に、粒子配列に影響を与える因子についての検討が行われている。

Chapter 5では、Chapter 4の結果を踏まえ、溶液中において形成されるシリカナノ粒子一次元配列体上に、シリカシェルをその場で形成することで異方性シリカナノ粒子の調製が可能であることが報告されている。塩基性アミノ酸やシリカシード、TEOSの濃度を変化させることによって、異方性シリカナノ粒子の長さと太さを精密に制御できることが明らかにされている。また、エタノール以外のアルコールの適用についても検討されている。

Chapter 6とChapter 7では、アミノ酸を用いずに合成されたシリカナノ粒子のF127を用いた一次元配列についての検討結果が報告されている。アミノ酸を用いて合成した系と比較し、一次元配列の最適条件のpHが異なること、イオン強度の影響が強くあらわれることが明らかにされている。従来塩酸を加えてpHを調整していたが、これに代わり、透析を用いてpHを調整する方法の有用性が提案されている。加えて、Chapter 2で得られたメソポーラスシリカナノ粒子の一次元配列に対して、透析法の利用が検討されている。

Chapter 8では、代表的な結晶性金属酸化物である酸化チタンおよび酸化スズナノ粒子の一次元配列に関する検討結果が報告されている。様々な因子の配列プロセスに与える影響及び配向付着の可能性が検討されている。透析法を利用し、F127を添加した条件下で、アスペクト比の高い酸化チタンおよび酸化スズの一次元配列体の調製に成功している。

Chapter 9では水溶性ポリマーであるポリビニルアルコールとポリビニルピロリドンを用いたシリカナノ粒子の一次元配列が検討されている。これらの水溶性ポリマーを利用し、シリカナノ粒子の一次元配列が可能であることが示されている。

Chapter 10では本研究で得られた成果が総括されている。

以上、本論文では様々な単分散コロイドナノ粒子の合成およびその配列制御について詳細な検討が行われ、新規な合成法および一次元配列法が開発されており、これらの成果は、化学システム工学ならびにコロイド化学の発展に寄与するところが大きい。

よって本論文は博士(工学)の学位請求論文として合格と認められる。