Fabrication of optical functional thin film for an application as a molecular sensor

• Material Type: 博士論文
• Title: Fabrication of optical functional thin film for an application as a molecular sensor
• Author/Editor: Fujita, Takashi; フジタ, タカシ; 藤田, 隆史
• Author Heading: Fujita, Takashi; フジタ, タカシ; 藤田, 隆史
• Publication Date: 2014-03-25
• Publication Date (W3CDTF): 2014-03-25
• Alternative Title: 光機能性薄膜の作製と分子センサーへ応用
• Pages: 1-153
• Degree grantor/type: 首都大学東京
• Date Granted: 2014-03-25
• Dissertation Number: 甲第409号
• Degree Type: 博士（理学）
• Text Language Code: eng
• NDLC: UT51
• Target Audience: 一般
• Note (General): Optical functional thin films can cause various surface effects by their photo excitation. These surface effects are divided into two types. One is surface reaction with adsorbate. The other is unique career movement such as surface plasmon (SP) excitation, charge separation and so on. These effects lead to various applicatons such as photocatalyst, solar cell, optical sensors and so on. Therefore, fabrication of optical functional thin films gives us not only scientific interest but also various practical uses. In this thesis, the author fabricated two optical functional thin films (titania thin film and two dimensional array (2D-array) of gold nanoparticle) for application of optical molecular sensor with surface assisted laser desorption/ionization (SALDI-MS) and surface enhanced Raman scattering (SERS) measurements. Photocatalytic effect on titania surface causes interesting reaction such as degradation of organic molecules and site-selective redox reaction and so on. However, these reactions are very complex. Therefore, simple observation methods for revealing these reaction products in whole are required. In order to observe these reaction products, the author focused on SALDI-MS because SALDI-MS spectra can reflect surface properties of SALDI-MS ionization substrates according to our previous studies. In this thesis, the author fabricated titania thin film, and then applied the thin film to SALDI-MS substrates for monitoring degradation of organic molecules with oligosaccharides. SERS is an attractive technique for ultra-trace analysis of analyte due to its huge enhancement effect caused by SP resonance on a metal nanostructured surface. However, it is difficult to control the enhancement effect because the efficiency of SP resonance strongly depends on a surface morphology of the SERS active substrate. From this reason, conventional SERS measurement has not applied for practical optical molecular sensor due to its low-reproducibility. Therefore, in order to solve this problem, uniform surface morphology of nanostructure is required as a SERS active substrate. To create a uniform surface roughness of a nanostructured surface, the author focused on a highly-ordered 2D-array of metal nanoparticles (2DMA) with a constant inter-particle distance by using same metal nanoparticle because fabricated SERS active substrates are expected to have same enhancement effect at any position of the substrate. Highly-ordered 2DMAs requires elimination of aggregations because aggregations cause not only a random efficiency of SP excitation but also a decrease of enhancement effect. Generally, in order to prevent nanoparticles from aggregating, surface modifier are used because the modifier plays a role as a spacer between nanoparticles. However, the surface modifier changes its optical property and generates background signals which interfere with the detection of target molecules. From these reasons, it is expected that the 2DMA composed by metal nanoparticles without any surface modifier has a large potential for a molecular sensor, but fabrication methods of the 2DMA still have not been proposed. On the other hand, our group succeeded in fabrication of highly-ordered 2DMA without any surface modifier by using the novel technique, which is called sandwich (SW) method. The SW method is a very simple technique which is only by sandwiching the gold nanoparticle (AuNP) solution between two flat plates and drying naturally. However, the 2DMA at this stage could not work for SERS analysis. The reason was that an area of the 2DMA was too small to obtain many SERS spectra from one substrate and to estimate the quantitative performance. In this thesis, the fabrication condition of the SW-method has been optimized in order to expand the area of the 2DMA and a fabrication mechanism in the SW-method has been studied. Then the 2DMA has been applied to SERS measurement in order to develop a molecular sensor with a high reproducibility and an ultra-high sensitivity. Chapter 1 deals with general introduction. Importance of optical functional thin films was discussed. Proposed mechanisms of photocatalytic effect and SERS enhancement effect and purpose of this thesis are discussed. In chapter 2, SALDI-MS measurements of oligosaccharides on the titania substrate, which were prepared by using oil/water interface trapping method, were discussed. A lot of fragment ions of oligosaccharides were observed. They had periodical molecular weight which is correspond to oligosaccharide units only by our substrate. It is suggested that the unique fragmentation process on the titania SALDI-MS substrate relates to photocatalytic reaction. In chapter 3, fundamentals of localized SP (LSP), calculation of optical properties of AuNPs and inter-particle coupling effect are discussed in order to understand a relationship between LSP excitation and electromagnetic enhancement in SERS. Optimum diameter of AuNP was estimated for the SERS measurement. To be from 60 nm to 100 nm are suitable because they have high efficiencies of SP excitation. In chapter 4, common fabrication methods of two-dimensional metal nanostructure and the reason why attention was paid to the 2DMA as quantitative SERS active substrates are discussed. And, the theory for interactions between colloids in solution is discussed. As a result, a guideline for fabricating the highly-ordered 2DMA composed by AuNPs without any surface modifier was obtained. In chapter 5, SW method and characterization of the 2DMA fabricated by SW method using scanning electron microscope are discussed. And, experimental conditions of the SW method were optimized. As a result, the area of the 2DMA composed by AuNPs without any surface modifier was expanded compared with that at early stage of this method. Moreover, the 2DMA had a higher packing rate compared with those fabricated by conventional methods. In chapter 6, a uniformity of the 2DMA of the SW-method and a mechanism of arrangement in the SW-method are discussed by using SEM analysis with Fourier Transform analysis, Vis absorption spectroscopy and optical microscope. These analyses showed that the 2DMA had a high uniformity and a hexagonal periodicity with inter-particle distance of 4nm in the arrangement. From these results, it is considered that the 2DMA have a high uniformity and a constant inter-particle distance (4.5nm) which has a large potential to generate a giant enhancement effect. Moreover, the assembly mechanism of SW method was investigated by observations of assembly process with an optical microscope. As a result, it was found that the SW method had three important processes; 1. Supplying AuNPs toward the contact line. 2. Maintaining dispersive nature just before formation of the 2DMA. 3. Smooth movement of the contact line toward inside In chapter 7, SW-substrates were applied as SERS active substrates, and their analytical performances such as sensitivity and reproducibility were evaluated. Raman measurements revealed that SW substrates gave us an enhancement in Raman signals of CV by 5 to 6 orders of magnitude. Therefore, SW substrates had high SERS enhancement effect. Next in order to evaluate a reproducibility of SERS signals detected on the SW substrate, spot-to-spot SERS signals from the SW-substrate were measured. The SW substrate enabled a high reproducible detection of SERS signals. It is considered that the SW-substrate had uniform EM fields. In conclusion, the author fabricated two optical functional thin films. One is titania thin film. SALDI-MS measurement of oligosaccharides on the titania thin film showed specific fragment pattern which indicates photocatalytic degradation. Second is highly-ordered 2DMA of AuNPs. The 2DMA had a higher uniformity than those of conventional methods and a hexagonal periodicity with an inter-particle distance of 4 nm in the arrangement. And, in SERS measurement, the 2DMA showed a remarkably high spot-to-spot reproducibility and a high sensitivity. Therefore, the 2DMA had a large potential for molecular sensor with high reproducibility and ultra-high sensitivity.; 首都大学東京, 2014-03-25, 博士（理学）, 甲第409号
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• Original Data Provider (Database): 東京都立大学 : みやこ鳥