Bioinspired Photonic Crystals: Self-Assembly, Surface Functionalization, and Sensing in Inverse Opals

Abstract

From moth wings to lotus leaves, the natural world has many examples of combining self-assembled nanostructured surfaces and surface energy control to manipulate the interactions between liquids and surfaces.1,2 These functional nanostructures and their assembly pathways can inspire the creation and functionalization of engineered nanostructured materials with widespread applications. In the initial chapters of this thesis, I explore two case studies, where I examine the formation of natural photonic structures in begonia leaves and investigate the potential for shape manipulation in crystals of guanine, which is a commonly used natural photonic material, before moving to fully engineered photonic crystal systems and their applications. Engineers can utilize a different and often more varied palette of materials than those used in nature to optimize for more complex applications, and we can adopt design principles from biological photonic structures to create bioinspired self-assembled nanostructured surfaces over which we have much more control and can customize as an adaptable platform technology. Inverse opals are a particularly interesting platform as they have a tunable interconnected porous geometry that can produce iridescent structural color and can be functionalized with surface-modifying molecules to customize the interactions between the surface of pores and fluids or particles. Inverse opals have been used as structurally colored optical materials,3 catalytic support materials,4,5 and battery electrodes,6 but one application in which they truly shine is as sensors,7–9 where their structural color, porous geometry, and customizable surface chemistry contribute to their ability to function in differentiating between liquids. Several key design elements of inverse opals – namely their pore geometry, surface chemistry control, and photonic performance – can be adapted to use these sensors in different systems. In this thesis, I have explored the self-assembly and functionalization of photonic crystals in natural and engineered materials with a focus on using inverse opals in sensing applications. I have created self-assembled inverse opals with colloids in several size ranges and explored the self-assembly of colloids with silica sol-gel precursors and titania nanocrystals as well as the post processing of inverse opals using atomic layer deposition in order to control the pore geometry, surface material, and refractive index. I have further used several strategies for customizing the surface functionalization: using gradients of silanes to distinguish between concentrations of bile salts for neonatal jaundice testing, connecting further surface modifying molecules to linker silanes to produce dynamic surface chemistries that react to UV light, and selectively functionalizing the sharp features at pore necks for attachment of particles that could enable occlusion-based sensing. Through this research, I have explored the implications of pore geometry and surface energy on the wetting of ordered porous films, introduced dynamic photoswitchable molecules that can change the wettability of the surface in response to light, and explored the possibility of using inverse opals as label-free viral sensors by occlusion-based wetting modulation.