Self-assembled colloidal nanostructures for photochemical applications

Abstract

Colloidal inorganic nanomaterials have been widely utilized in a broad range of practical applications such as catalysts, electronics and photoelectric devices, biosensors, and drug delivery systems because of their excellent physical and chemical properties. However, the changes in the chemical environments around the nanomaterials due to various chemical side reactions in a solution often cause undesirable agglomeration and precipitation of the colloidal nanomaterial, which consequently leads to severe degradation of the intrinsic properties of the nanomaterial. The immobilization of the nanomaterials on the templates can alleviate the structural instability of the colloidal nanomaterials. Colloidal self-assembly allows the incorporation of nanomaterials onto planar or particulate templates and their porous templates, and can also impart unique three-dimensional (3D) structural characteristics through layer-by-layer (LbL) self-assembly of nanomaterials. For the stable immobilization of nanomaterials, it is essential to modify the surface of the templates and optimize the interfacial properties with nanomaterials for providing the stable and robust chemical linkages. This dissertation reports a colloidal self-assembly of functional nanomaterials on 3D complex structures using various self-assembly methodologies including messel-inspired polydopamine chemistry, polyphenol-mediated assembly, and LbL assembly. I demonstrate that the 3D self-assembled multifunctional hybrid materials can be utilized for a wide range of photochemical applications including a reactive oxygen species (ROS)-suppressive ultraviolet (UV) filter and plasmonic solar water splitting. First, I attempt to functionalize colloidal porous polymer microsphere templates with various metal nanoparticles (e.g., Au and Ag) using a mussel-inspired polydopamine (pD) chemistry in an aqueous milieu. The pD coating, inspired from mussel adhesive proteins in marine environments, is conducted on the porous polymer microspheres via oxidative polymerization of dopamine at an alkaline pH, which provides a stable adhesive layer. In particular, the pD-coated surfaces exhibit the mild reducing capability which allows the in-situ immobilization of metal nanoparticles within the porous polymer microspheres via the spontaneous reduction of noble metal ion precursors into solid metal nanoparticles. In addition, the pD layer mediates the silicification to fabricate porous metal-silica hybrid microspheres. The hybrid colloids exhibit an excellent dispersion property and a structural durability of the immobilized metal nanoparticles, allowing them to be utilized as recyclable SERS-active colloidal substrates. The mussel-inspired catechol oxidative chemistry for the synthesis of colloidal metal nanoparticles is further investigated using a catechol-grafted polymer. The catechol-grafted polymer acts as a polymer template, which leads to the formation of template-free mesoporous metal nanoparticles via the spontaneous reduction of metal ions complexed with the catechol-grafted polymer. Second, inorganic UV filters, titanium dioxide nanoparticles ($TiO_2$ NPs), are assembled on colloidal porous microspheres via tannin-mediated assembly for efficient UV filtering while suppressing the ROS generation. Tannin allows the direct deposition of $TiO_2$ NPs onto both of the outer and internal surfaces of the porous structures via ligand-to-metal complexation. Multi-layered tannin-$TiO_2$ hybrid colloids are prepared by the LbL deposition of tannin and $TiO_2$ NPs, which exhibit strong absorption in the UV region and an excellent sun protection factor (SPF) due to the unique porous structures and good dispersity without the aggregation of $TiO_2$ NPs. In addition, tannin changes the redox microenvironment of $TiO_2$ NPs and efficiently scavenges the photochemically generated ROS from the $TiO_2$ NPs under UV exposure. The tannin-$TiO_2$ hybrid colloids significantly suppress the generation of superoxide anions and hydroxyl radicals compared to colloidal $TiO_2$ NPs under UV irradiation. Furthermore, histological examination of a mouse model shows that the hybrid colloids exhibit excellent anti-UV skin protection in terms of epidermal hypertrophy, inflammatory infiltrates, and keratinocyte apoptosis without significant long-term toxicity due to efficient scavenging of ROS by tannin. Lastly, LbL assembled metal nanostructures are fabricated as a plasmonic light-harvesting material for light-driven water splitting. Plasmonic metal nanostructures exhibit the excellent chemical stability and strong light absorption in the visible region due to the localized surface plasmon resonance (LSPR). The 3D porous networks of colloidal gold nanoparticles (AuNPs) are prepared by the LbL self-assembly of negatively charged AuNPs and cationic polyethyleneimine (PEI). The absorption properties of the 3D Au nanostructures are easily controlled by the number of assembled layers, which affect the hot carrier generation in the Au nanostructures. Furthermore, colloidal water oxidation catalysts, IrO2 hydrosols, are introduced to the metal nanostructures, and the photocatalytic activities of the plasmonic photoanodes with the LbL-assembled catalyst-metal hybrid nanostructures for light-driven water oxidation are analyzed depending on the structural and optical properties of the assembled metal nanostructures. Plasmonic photocurrents significantly increase with the increased number of AuNP layers under visible light illumination, indicating that the hot carrier generation in the Au due to the SPR increases with the enhanced light absorption. These results suggest that the optimized plasmonic nanostructures via LbL self-assembly can provide a promising light-harvesting platform for sustainable and efficient solar water splitting driven by the visible light.