Kenneth H. Sandhage

The ability to synthesize large quantities of 3-D microparticles with tunable adhesion is critically important for a variety of mature and emerging technologies, such as for paints, inks, chemical/water purification, drug delivery, cell manipulation, and assembly of hierarchical structures. Nature provides impressive examples of sustainable, complex-shaped microparticles with chemistries and structures tailored for adhesion, among the most common of which are pollen grains. We have recently used a surface sol–gel (SSG) coating process to generate iron oxide replicas of sunflower pollen grains. While these replicas exhibited multimodal adhesion, the tailorability and predictability of such adhesion was not examined. In the present paper, the layer-by-layer SSG process has been used to carefully adjust the amount of iron oxide deposited onto the pollen grains. Controlled-atmosphere thermal treatments then yielded freestanding replicas with tailored hematite (α-Fe₂O₃) or magnetite (Fe₃O₄) contents. The 3-D morphology of the starting pollen was well-preserved in the all-oxide replicas, and the shrinkage upon firing could be controlled by increasing the number of Fe–O-bearing layers deposited on the pollen. While the short-range van der Waals (VDW) adhesion of the oxide replicas to a variety of surfaces was lower than for the larger starting pollen grains, this difference was not due to shrinkage of the replicas. Analyses with a simple Hamaker model indicated that VDW adhesion of the oxide replicas was governed by the contact of oxide nanocrystals located on the spine tips (as opposed to the curvature of the entire spine tip). The longer-range attraction to a magnetic substrate could be tailored independently of the short-range VDW attraction by controlling the magnetite content of the replicas, and a simple and effective model for describing such magnetic attraction was developed. This work demonstrates that sustainable pollen microparticles can be converted into high-fidelity 3-D oxide replicas with predictable and tailorable multimodal adhesion.

Hierarchically‐porous, rigid inorganic structures have attracted appreciable interest in a wide range of applications, including catalysis, filtration, sensing, and energy storage/harvesting. Naturally‐occurring, hierarchically‐porous, rigid assemblies with a wide range of three‐dimensional (3D) structures are generated by diatoms (photosynthetic aquatic microorganisms); that is, each of the tens of thousands of diatom species forms a macro‐to‐mesoporous, silica‐bearing cell wall (frustule) with a particular, highly‐reproducible 3D morphology. We have recently demonstrated that such intricate biosilica structures can be converted, without loss of 3D morphology, into high surface area (>1300 m²/g) macro‐to‐microporous carbon. Here we demonstrate, for the first time, how the chemical tailoring of such hierarchically‐porous, carbon‐converted, 3D biogenic structures can result in a high degree of enzyme loading for rapid flow‐through catalysis. Two approaches have been developed for enriching such structures with carboxylic acid groups: i) dendritic amplification of partially‐oxidized C replicas, and ii) electrochemical Au deposition followed by self‐assembly of a carboxylic acid‐bearing surface layer. The terminal carboxylic acid groups were then used for electrostatic attachment of a protamine (PA) modified derivative of the model enzyme, glucose oxidase (GOx‐PA). In a flow‐through system, the GOx‐PA‐loaded, diatom‐derived microscale structures displayed a glucose consumption rate more than 80% faster than for GOx‐PA‐loaded C black and Au nanoparticles. The rapid flow‐through catalysis of the carbon and gold‐bearing frustule replicas was enabled by the open 3D morphology of the starting diatom silica templates, along with the enhanced surface area and high enzyme loading resulting from the chemical conversion and surface functionalization processes.

A combined conformal coating and gas/solid reaction process has been used for the first time to convert porous anodic alumina templates into robust, freestanding, multi-wall titania nanotube (MWTNT) arrays that resist appreciable nanotube bundling upon drying. A sol–gel infiltration process was first used to apply a thin conformal titania coating to the alumina templates. After selective etching to generate a gap between the template and the titania coating, exposed alumina was converted into nanocrystalline titania via reaction with titanium tetrafluoride gas and then with humid oxygen. After selective dissolution of residual aluminum-bearing phases and drying, freestanding, non-agglomerated, well-aligned MWTNT arrays were generated (i.e., with coating-derived inner titania tubes resting inside reaction-derived outer titania tubes). When incorporated as aligned electrodes in dye-sensitized solar cells, the dye loading and power conversion efficiencies were higher by factors of 2.2 and 1.8, respectively, than for solar cells with single-wall titania nanotube array electrodes. By controlling the conditions used for alumina template formation, sol–gel coating, and gas/solid reaction, this hybrid process may be used to generate robust, uniformly aligned MWTNT arrays with dimensions and functional chemistries tailored for a variety of electrical, optical, chemical, or biochemical applications.

Conformal, layer-by-layer, surface sol–gel coating and microwave hydrothermal reaction processing were combined for the first time to convert 3D bioorganic templates (butterfly scales) into photoluminescent Eu-doped BaTiO₃ replicas. The use of such complex microscale structures as distinct photo-luminescent patterns for the labelling of documents or goods for tracking or anticounterfeiting purposes is demonstrated.