Synthetic strategies and optical properties of anisotropic plasmonic nanostructures

다국어 초록 (Multilingual Abstract)

Designing and synthesizing plasmonic nanostructures allow us to manipulate and mix the optical, magnetic and electrical characteristics of materials. Optical response of individual nanoparticles mainly dominated by localized surface plasmon resonance (LSPR) strongly depends on their size, shape, composition. This structure- and environment-dependent LSPR has been used to enhance optical signals such as Raman scattering or fluorescence in a wide range of fields from optics and spectroscopy to biomedical applications. Important reason that enhances such an optical signal is the extraordinarily amplified electromagnetic fields (EM fields) at the junction area positioned between a pair of nanoparticles or at internal crevice of the single nanoparticle. Among many structures, dimeric nanostructures with inter-particle spacing have been the most heavily studied because it is simple and they generate a very strong EM field in their junctions. In a homodimer system, redshift of longitudinal bonding plasmon modes is typically observed while decreasing inter-particle distance (hybridization of individual particle plasmons) but antibonding mode is dark. However, recent results show that diverse plasmon modes can be shown and excitable in the heterodimers with different size of nanoparticles or composition. This phenomenon is due to the symmetry breaking of plasmon modes and resulted in strong enhancement of various plasmon coupling. In both cases, it is particularly important to control ~1 nm gap because effective plasmon coupling can be generated in ~1 nm gap or less. Therefore, it is still highly challenging to precisely and reproducibly synthesize and assemble them into well-defined structures at nanometer scale for making them ideal for a range of fundamental studies and applications.
In this thesis, we introduce DNA-based synthetic strategies to study optical properties and design plasmonic nanostructures with high structural controllability. These strategies allow us to manipulate the shape and assembly of nanostructures, such as dimer/trimmers with inter-particle spacing, single particles with intra-nanogap or different compositions. We investigate the relationships between plasmon response, signal amplification and structural changes of those plasmonic nanostructures such as inter-/intra-particle distance and particle size/composition. In the assembled system with inter-particle spacing, we performed single-molecule correlation studies on single-DNA-tethered Au-Ag core-shell dimeric nanostructures for surface-enhanced Raman scattering (SERS). We showed strategies to design the assembly of plasmonic nanoparticles and control the distance between particles for amplifying SERS signal. With DNA-modified AuNPs, We developed a new salt-tuned synthetic strategy for anisotropic growth of secondary nanoparticles on primary nanoparticles to form Au-Ag bimetallic heterodimers. We further focused in controlling the extent and sharpness of overlapped region on merged Au-Ag heterodimers and investigated the plasmon response dependent on structural changes and also size of nanoparticles. Understanding and utilizing the relationships between structural changes and plasmon coupling in nanostructures herein could greatly increase our knowledge in plasmonics, give insights in designing and synthesizing the plasmonic nanostructures.
The chapter 1 provides an overview and perspective of recent advances in the use of DNA-tailored plasmonic nanostructures in biosensing applications. The plasmonic properties of metallic nanoparticles (NPs) such as Au and Ag NPs and the plasmonic coupling between them are of enormous interest for their strong and controllable optical signal enhancement and manipulation capabilities. The strong optical properties of these plasmonic structures are promising for various biosensing applications, but the widespread use of these structures is limited largely due to the absence of high-yield synthetic method for targeted nanoprobes with nanometer precision and the poor understanding of the plasmonics of these structures. DNA is a promising material that can be used as both specific biorecognition and versatile synthetic template in forming and controlling plasmonic nanostructures and their aggregations.
In chapter 2, two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various inter-particle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed inter-particle gap size and varying particle size from a 23-30 nm pair to a 50-60 nm pair. With two probe design, we extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, inter-particle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm inter-particle gap, an EF value of as high as 5.9 × 1013 (average value = 1.8 × 1013) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 1012 and 5.9 × 1013. In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with<1nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed.
In chapter 3, we performed single-molecule correlation studies on DNA-tethered Au-Ag core-shell heterodimers to find out the relationship between SERS and LSPR. Using a multistep AFM tip-matching strategy that enables us to gain the optical spectra with the optimal signal-to-noise ratio as well as high reliability in correlation measurement between LSPR and SERS, the coupled longitudinal dipolar and high-order multipolar LSPs were detected within a dimeric structure, where a single Raman dye is located via a single-DNA hybridization between two differently sized Au-Ag core-shell particles. On the basis of the characterization of each LSP component, the distinct phase differences, attributed to different quantities of the excited quadrupolar LSPs, between the transverse and longitudinal regimes were observed for the first time. By assessing the relative ratio of dipolar and quadrupolar LSPs, we found that these LSPs of the dimer with ∼1 nm gap were simultaneously excited, and large longitudinal bonding dipolar LSP/longitudinal bonding quadrupolar LSP value is required to generate high SERS signal intensity. Interestingly, a minor population of the examined dimers exhibited strong SERS intensities along not only the dimer axis but also the direction that arises from the interaction between the coupled transverse dipolar and longitudinal bonding quadrupolar LSPs. Overall, our high-precision correlation measurement strategy with a plasmonic heterodimer with ∼1 nm gap allows for the observation of the characteristic spectral features with the optimal signal-to-noise ratio and the subpopulation of plasmonic dimers with a distinct SERS behavior, hidden by a majority of dimer population, and the method and results can be useful in understanding the whole distribution of SERS enhancement factor values and designing plasmonic nanoantenna structures.
In chapter 4, we report a salt-tuned synthetic strategy using DNA-modified Au nanoparticles (DNA-AuNPs) to form Au-Ag head-body nanosnowman structures in >95% yield. We propose a mechanism for the formation of asymmetric Au-Ag nanosnowmen from DNA-AuNPs, salts, and Ag-precursor-loaded polymers. Importantly, we show that oriented assemblies of various nanostructures are readily obtained using nanosnowmen with asymmetrically modified DNA as building blocks. Synthesizing and assembling nanoscale building blocks to form anisotropic nanostructures with the desired composition and property are of paramount importance for the understanding and use of nanostructured materials.
In chapter 5, we further developed the DNA-based synthetic strategies for asymmetric Au-Ag bimetallic heterodimers with structural controllability especially within overlapped region. These strategies allow us to manipulate the shape and composition of nanostructures within single particle level. We investigate the relationships between plasmon response and structural changes. We have shown that the extent of overlap and sharpness of anti-wedge region in merged conductive area along with symmetry breaking by different composition and size are responsible for various plasmon modes. Especially, the charge transfer plasmon and capacitive plasmon modes at low frequency showed most sensitive response on those changes. We have further shown that reproducible SERS signal can be generated from this structures that show linear dependence on particle concentration (5 fM). SERS enhancement factor was confirmed to 1.3 × 106 ~ 1.9 × 106. Besides, Tunable and wide range of LSPR from visible to near IR makes this structure attractive for many plasmon-based applications.

목차 (Table of Contents)

• Contents
• Abstract.......................................................................................................................ⅰ
• Contents......................................................................................................................ⅶ
• List of Figures............................................................................................................ⅹ
• List of Tables..........................................................................................................ⅹⅸ
• Chapter 1. Introduction : DNA-tailored Plasmonic Nanoparticles for Plasmonics and Their Nanobio-applications...................................................................................1
• 1.1 Introduction............................................................................................................2
• 1.2 Plasmon Coupling between Nanoparticles...........................................................5
• 1.3 Nanobio-applications............................................................................................7
• 1.4 Conclusion and Perspective................................................................................16
• 1.5 References...........................................................................................................18
• Figures.........................................................................................................................25
• Chapter 2. Tuning and Maximizing the Single-Molecule Surface-Enhanced Raman Scattering from DNA-Tethered Nanodumbbells.......................................................35
• 2.1 Introduction..........................................................................................................36
• 2.2 Experiment Section..............................................................................................40
• 2.3 Result and Discussion..........................................................................................45
• 2.4 Conclusion.............................................................................................................55
• 2.5 References...........................................................................................................57
• Figures and Tables....................................................................................................62
• Chapter 3. Single-Molecule and Single-Particle-Based Correlation Studies between Localized Surface Plasmons of Dimeric Nanostructures with ∼1 nm Gap and Surface-Enhanced Raman Scattering........................................................71
• 3.1 Introduction..........................................................................................................72
• 3.2 Experiment Section..............................................................................................74
• 3.3 Result and Discussion..........................................................................................75
• 3.4 Conclusion.............................................................................................................89
• 3.5 References...........................................................................................................91
• Figures.........................................................................................................................94
• Chapter 4. Directional Synthesis and Assembly of Bimetallic Nanosnowmen with DNA...........................................................................................................................103
• 4.1 Introduction.........................................................................................................104
• 4.2 Experiment Section............................................................................................106
• 4.3 Result and Discussion........................................................................................110
• 4.4 Conclusion...........................................................................................................117
• 4.5 References..........................................................................................................118
• Figures.......................................................................................................................120
• Chapter 5. Plasmonically Tunable Nanosnowmen as Anisotropic Nanoantenna and Surface-Enhanced Raman Scattering Probes.........................................................129
• 5.1 Introduction.........................................................................................................130
• 5.2 Experiment Section............................................................................................134
• 5.3 Result and Discussion........................................................................................140
• 5.4 Conclusion...........................................................................................................151
• 5.5 References..........................................................................................................153
• Figures.......................................................................................................................156
• Abstract (in Korean).................................................................................................168
• Curriculum Vitae.......................................................................................................172