Synthesis and Catalytic/Optical Properties of Dealloying-Based Porous Nanoparticle and Plasmonic Nanogap Nanoparticle

Abstract

Nanoporous metallic structure, which are composed by a three-dimensional open-cell network of interconnected ligaments on the order of 10 to 100 nm, are very promising materials in various applications such as catalysis and sensing owing to their high surface-to-volume ratio and excellent surface reactivity. Especially, nanoporous gold (Au) structures have been attracted much attention for practical applications due to the good chemical/biological stability, unique mechanical rigidity, electrical conductivity, and high corrosion resistance. Previously, the porous materials have been made by casting into the interstices of microphase-separated block copolymers, colloidal crystal, self-assembled surfactants, and biologically formed porous skeletal structures. Recently, with the advent of nanotechnology, a dealloying which is defined as selective dissolving of the less stable metal elements from alloy system has been used as a processing tool to fabricate the well-controllable nanoporous metallic structures. By many efforts in theoretical and experimental studies, the nanoporous metallic structures have been well-defined and precisely controlled, and their structure-dependent properties have also been investigated a lot. For recent decades, various studies and techniques have been reported and developed for fabricating well-controlled nanoporous metallic structures, however, most of nanoporous metallic structures are mainly focused on the three- or two- dimensional structures which have limitations for specific applications such as nanomedicine, drug delivery, and labeling in biosensing. In contrast, the nanoporous metallic nanoparticles are most promising materials for biomedical and sensing applications due to their large surface which is capable for loading of drug and targeting molecule, and their tunable and unique SPR properties for enlarging the sensing signal. Furthermore, those nanoparticles are relatively easy and fast to synthesize and is also highly capable for mass production, resulting in in practical applications. However, the precise and reproducible synthesis of nanoporous metallic nanoparticle (especially in Au) is quite difficult and have rarely been reported because the fabrication of uniform alloy nanoparticles with suitable composition and size is still quite challenging due to the different chemical properties of each metal elements. Although some recent studies cover the fabrication of nanoporous metallic nanoparticle, those synthesis methods need complex route such as annealing process, and resulting products are usually not uniform in size, which cannot provide reproducibility and reliability in sensing application. Therefore, it is still highly challenging to precisely and reproducibly synthesize the uniform and pore/ligament size-controllable nanoporous metallic nanoparticles for utilizing them to a wide range of applications. In this thesis, dealloying-based synthetic strategies to fabricate highly porous nanoparticles and plasmonic interior-nanogap nanoparticles are presented. These facile and straightforward synthetic strategies allow us to manipulate the nanoporosity and interior nanogap which highly affect to catalytic performance and optical signal enhancement. By introducing co-reduction chemistry and by adjusting the Ag to Au ratio, the atomic distribution of Au can be highly controlled in Au-Ag alloy shells, resulting in nanoporous Au nanoparticles with different porosity via dealloying process. Using the Au core/porous shell nanoparticles which contain networked thin ligaments, surface defects with well-controlled nanoporosity, the porosity-dependent catalytic activity is investigated. When Ag to Au ratio is further adjusted during co-reduction chemistry, Au-Ag dealloyed nanogap nanoparticles with interior nanogap can be formed despite of the absence of an interlayer, such as DNA, silica ore polymer layer via dealloying process. With dealloyed nanogap nanoparticles of which size of well-defined interior nanogap is as smaller as ~2 nm, quantitative studies for surface-enhanced Raman scattering (SERS) are investigated. In addition, estimation of enhancement factor and polarization-dependent SERS property are also carried out in single-particle-level. Finally, highly quantitative DNA detection and highly sensitive/selective target cell imaging is presented using the bio-functionalized interior-nanogap nanoparticles. Understanding and utilizing the relationships between dealloying-based structural changes and catalytic/optical properties in highly controlled nanostructures can give advanced insight in designing and synthesizing the desired nanostructures for the targeted applications. The chapter 1 provides an overview and perspective of recent advances in the use of dealloying-based nanoporous Au nanostructures for various applications. Next, the mechanism of evolution of nanoporosity from Au-Ag alloy system during dealloying is introduced. Because the morphologies (e.g., size or distribution of pore/ligament) of nanoporous metals obtained from dealloying process are highly influenced by the dealloying condition, such as alloy composition, dealloying time, temperature, kinds of solutions, and intrinsic property of noble metals, the effects of dealloying conditions in adjusting structure of dealloyed nanoporous metals are also discussed. Finally, various examples of nanoporous Au structures produced by well-controlled dealloying process and their representative applications are introduced and discussed. In chapter 2, a facile and straightforward synthetic strategy for controlling and forming Au core/porous shell nanoparticles (CPS NPs) by a selective dealloying (Ag-etching process) process of Au/Au–Ag core/alloy shell NPs (CAS NPs) is presented. The shell of CPS NPs contained networked thin ligaments, catalytically-active surface defects and ultra-high porosity throughout the shell. Using the robust CPS NPs, the porosity-dependent catalytic activity with a well-known chemistry [the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) in this case] was investigated. The CPS NPs exhibited a very short induction time, high conversion rate constant, low activation energy and high turnover frequency due to their catalytically active porous shells containing networked thin ligaments, surface defects, ultra-high porosity. The CPS NPs could further enhance the catalytic reactivity due to their photothermal activation. In addition, the highly porous CPS NPs showed much better catalytic performance in comparison with galvanic replacement-based Au core/hollow shell nanoparticles (CHS NPs). In chapter 3, a highly controllable dealloying chemistry for synthesizing Au-Ag dealloyed nanogap nanoparticles (DAG NPs) with a dealloyed interior nanogap is presented. The interior nanogap was formed as small as ~2 nm and controlled without the need for an interlayer, such as DNA, silica or polymer layer, in the gap in a high yield (~95 %). Owing to the strong electromagnetic field enhancement in the nanogap, DAG NPs facilitate generation of strong, tunable, and reproducible SERS signals with narrowly distributed enhancement factors. In addition, the SERS signal intensity from DAG NPs linearly increased as particle concentration increased, allowing for highly reliable, quantitative SERS. Importantly, single-particle laser polarization study suggests that the SERS signals from a DAG NP are polarization-independent, which is critical in getting reproducible signals from individual particles. Finally, the bio-functionalized DAG NPs showed that they can be used as ultrasensitive, highly quantitative DNA detection probes and targeted cell imaging probes with high sensitivity and selectivity.