Introduction

Inorganic nanoparticles (NPs) such as metal NPs, metal oxide NPs, and quantum dots are attracting considerable interest as optical materials and for use in electrical devices and biosensors because of their unique optical and electronic properties. Various in-situ preparation methods of metal NPs to control and uniform their size have been demonstrated. However, the methods require multistep processes including purification and fractionation using ligand exchange, centrifugal filtration and/or polymer gel electrophoresis. In addition, it is an onerous task to remove by-products completely. Recently, simple processes for preparing metal NPs that do not produce by-products have been demonstrated. These processes are based on vacuum metal evaporation using a limited number of low-vapor-pressure liquids at room temperature. For example, metal NPs were produced by vapor deposition in silicone oils and by sputtering deposition in ionic liquids. However, nanometer-order size control and high dispersibility of metal NPs in various matrices have not been achieved using the vacuum methods.

In this study, I developed a new vacuum method of inorganic NP preparation, which I term molten matrix sputtering (MMS). Using this method, water-soluble luminescent AuNPs exhibiting a large Stokes shift were synthesized. Furthermore, AuNPs/thiourethane and AuNPs/urethane hybrid optical resins were prepared using the matrix sputtering method. The AuNPs thus prepared were characterized using various methods and their optical properties were analyzed.

MMS synthesis of water-soluble luminescent AuNPs with a large Stokes shift

The MMS method is illustrated in Figure 1. Temperature control of the disperse medium was introduced during sputtering deposition of MMS. Specifically, disperse media that are in solid state at room temperature were heated above their melting points to keep them in liquid state during sputtering. The MMS method can apply to many kinds of disperse media in solid state at room temperature.

A molten (6-mercaptohexyl)trimethylammonium bromide (6-MTAB) (m.p. 83 °C) was heated at 110 °C for 3 min in a vessel, and then sputtering deposition into molten 6-MTAB was carried out. AuNPs could be prepared with sputtering times of 5-20 min. The samples obtained were then dispersed into 3 mL of water (these samples are referred to as s-AuNPs). Figure 2 shows transmission electron microscope (TEM) images and size-distribution histograms of s-AuNPs produced with sputtering times of 5 min and 20 min. The average sizes of the s-AuNPs for both sputtering times are 1.3nm with a very narrow distribution (± 0.3nm). These results suggest that s-AuNPs do not grow in 6-MTAB; rather, additional nucleation occurs so that the number of s-AuNPs increases with increasing sputtering time. Careful observation of molten 6-MTAB during sputtering revealed that the local concentration of s-AuNPs close to the surface became very high. The strong adsorption of the thiol group of 6-MTAB to the sputtered species (i.e., Au atoms and/or clusters) could suppress the growth and aggregation of s-AuNPs.

Figure 3a shows 3D photoluminescence (PL) spectra of s-AuNPs produced with a sputtering time of 20 min. s-AuNPs that formed with different sputtering times have similar spectral profiles and PL peak wavelengths. This result is in good agreement with the similar size distributions of s-AuNPs produced using different sputtering times. Figure 3b shows a UV-Vis absorption spectrum of s-AuNPs in which the absorbance increases continuously as the wavelength decreases from 350nm; in contrast, its PL excitation (PLE) spectrum has a peak at around 300nm. It is a fascinating result that excitation with UV radiation at 300nm gives a low-energy emission at 770nm that extends to the near-IR region. The Stokes shifts of the s-AuNPs exhibit large values (2.4-2.7 eV).

Click Preparation of AuNPs-Dispersed Optical Resins Using the Matrix Sputtering Method

The low vapor pressure of pentaerythritol tetrakis(3-mercaptopropionate) (PEMP) enables matrix sputtering. Sputtering deposition of Au into PEMP was carried out with sputtering times of 5-15 min at room temperature (these samples are referred to as AuNPs/PEMP). The absorption spectra of AuNPs/PEMP are similar to those of s-AuNPs. AuNPs/PEMP also show luminescence at ca. 690nm.

The thiourethane resins embedded with AuNPs (AuNPs/thiourethane) were prepared from curing the mixture solutions of AuNPs/PEMP and m-xylylene diisocyanate (m-XDI). The transmittance spectra of AuNPs/thiourethane hybrid resins exhibit good transparency at wavelength longer than 450nm, and no surface plasmon absorption peaks around 530nm are observed (Figure 4a). No growth and aggregation of AuNPs was observed during polymerization ca. 130 °C. A high angle annular dark field scanning transmission electron microscope (HAADF-STEM) image of AuNPs/thiourethane hybrid resin agrees with the UV-Vis spectrum. The luminescence of AuNPs/thiourethane resins is kept at around 690nm as well as AuNPs/PEMP (Figure 4b). This indicates that the thiol group of PEMP which can strongly bind to sputtered Au surface is responsible to keep the same optical properties of AuNPs/PEMP in the resins.

AuNPs/pentaerythritol ethoxylate (PEEL) was synthesized by the matrix sputtering, and then Au NPs/urethane hybrid resin was obtained immediately by its polymerization with m-XDI (fresh-AuNPs/urethane resin). The resin shows good transparency with brownish tint. Its UV-Vis transmittance spectrum shows little surface plasmon band (Figure 5), which is consistent with the size of fresh-AuNPs/PEEL from the TEM image (2.1 ± 0.7nm). The difference of the sizes of AuNPs/PEMP and fresh-AuNPs/PEEL can be attributed to the difference of the coordination affinities of thiol and hydroxy groups to gold surface. The weak interaction between the hydroxy group and gold surface allowed coalescence of AuNPs in PEEL with the passage of time. The size of AuNPs/PEEL after 65 days (matured-AuNPs/PEEL) from the TEM image is 4.5 ± 3.4nm. The UV-Vis spectra of matured-AuNPs/PEEL and its hybrid resin exhibit surface plasmon absorption peaks at around 520nm (Figure 5a). In addition, once the resin was formed, no further UV-Vis spectral change was detected, indicating no more coalescence and aggregation of AuNPs in the solid matrix.

Conclusion

I developed a simple method, MMS, for synthesizing water-soluble luminescent AuNPs. The obtained AuNPs have a uniform size, a narrow size distribution and a luminescence with a large Stokes shift for all the sputtering times used in this study. In addition, AuNPs/thiourethane and AuNPs/urethane hybrid resins with good transparency were prepared by the matrix sputtering method. AuNPs/thiourethane resins do not show surface plasmon absorption but red luminescence. In contrast, AuNPs/urethane resins exhibit surface plasmon absorption, of which intensity can be tuned by change the size of AuNPs/PEEL with time before polymerization. This study expands the possibilities to obtain various functionalized NPs and their hybrid materials.

本論文は4章と付録からなり、第1章では研究の背景および目的、第2章ではモルテンマトリックススパッタリング(MMS)法による水溶性・発光性金ナノ粒子の合成とその物性評価、第3章ではマトリックススパッタリング法による金ナノ粒子/チオウレタン及び金ナノ粒子/ウレタン光学用ハイブリッド樹脂の作製とその物性評価、第4章では研究成果のまとめと展望について述べられている。以下に各章の概要を記す。

第1章では、研究の背景について述べている。無機ナノ粒子はバルク材料には見られない発光、融点の低下、高い表面積、透過性の維持、などの興味深い性質を示すため、様々な分野で応用されている。無機ナノ粒子の合成法はウエット法とドライ法の2つに大別されるが、前者は副生成物の生成や煩雑な操作が必要、後者は粒子の溶媒への再分散性が低い、といったデメリットがある。そこで本研究において、通常、基板に薄膜を作製する方法の1つであるスパッタリング法を応用した簡便な無機ナノ粒子合成法の開発と、得られたナノ粒子やハイブリッド材料の物性評価を行うことが目的として示されている。

第2章では、スパッタリング時に使用する分散媒へ温度パラメータを導入し、室温で固体のナノ粒子保護剤を分散媒として使用できるMMS法の開発と、そこで得られた金ナノ粒子の物性について述べている。4級アンモニウム塩とチオール基を末端に持つ分子を分散媒として得られた金ナノ粒子の解析を行い、分散媒へのスパッタ量はスパッタ時間から推測できること、金ナノ粒子はスパッタ時間に関わらず1.3nmの粒子サイズを示すこと、770nmの蛍光発光を示し、この発光がこれまで報告されている金ナノ粒子よりもストークスシフトが大きいこと、を明らかにしている。

第3章では、マトリックススパッタリング法を用いて光学樹脂原料中に金ナノ粒子を分散させ、そこからハイブリッド樹脂を作製し、得られた粒子やハイブリッド樹脂の物性について述べられている。分散媒として用いる樹脂原料の官能基を変更することで異なる光学特性を示すことを明らかとしており、チオール基の場合、1nm以下の粒子サイズで690nmの蛍光発光を示した。一方、ヒドロキシ基の場合、2nm以上の粒子サイズで表面プラズモン吸収を示し、スパッタ後から重合開始までの時間を変更することで粒子サイズが変化し、その粒子サイズに応じた表面プラズモン吸収特性を示した。またいずれの樹脂の場合も、樹脂化前のナノ粒子のサイズや光学特性を樹脂化後も維持していることを明らかとしている。

第4章では、以上の結果を総括し、今後の研究展望を述べている。またAppendixとして、第2章で分散媒として使用した分子の合成を記載している。

以上、本論文では、MMS法を開発し、ストークスシフトの大きな水溶性・発光性金ナノ粒子を得られたこと、マトリックススパッタリング法によって透明性を有するハイブリッド樹脂を作製できること、分散媒の種類を変更することでハイブリッド樹脂の光学特性を変えられること、を記述している。本博士論文において示されたナノ粒子合成法は金属と分散媒の自由な組み合わせが可能であり、様々な機能を持ったナノ粒子やハイブリッド材料へ応用できると考えられる。したがってナノ科学の分野を大きく進展させることが期待される。なお、本論文第2章は、米澤 徹、河合 功治、西原 寛との共同研究、第3章は、米澤 徹、宇田川 智史、長谷 要、西原 寛との共同研究であり、一部は既に学術雑誌として出版されたものであるが、論文提出者が主体となって実験および解析を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。