Person:

Kolle, Mathias

We present a reproducible, one-pot colloidal co-assembly approach that results in large-scale, highly-ordered porous silica films with embedded, uniformly-distributed, accessible gold nanoparticles. The unique coloration of these inverse opal films combines iridescence with plasmonic effects. The coupled optical properties are easily tunable either by changing the concentration of added nanoparticles to the solution before assembly or by localized growth of the embedded Au nanoparticles upon exposure to tetrachloroauric acid solution, after colloidal template removal. The presence of the selectively absorbing particles furthermore enhances the hue and saturation of the inverse opals color by suppressing incoherent diffuse scattering. The composition and optical properties of these films are demonstrated to be locally tunable using selective functionalization of the doped opals.

Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.

Colloidal particles can assemble into ordered crystals, creating periodically structured materials at the nanoscale without relying on expensive equipment. The combination of small size and high order leads to strong interaction with visible light, which induces macroscopic, iridescent structural coloration. To increase the complexity and functionality, it is important to control the organization of such materials in hierarchical structures with high degrees of order spanning multiple length scales. Here, a bottom-up assembly of polystyrene particles in the presence of a silica sol-gel precursor material (tetraethylorthosilicate, TEOS), which creates crack-free inverse opal films with high positional order and uniform crystal alignment along the (110) crystal plane, is combined with top-down microfabrication techniques. Micrometer scale hierarchical superstructures having a highly regular internal nanostructure with precisely controlled crystal orientation and wall profiles are produced. The ability to combine structural order at the nano- and microscale enables the fabrication of materials with complex optical properties resulting from light-matter interactions at different length scales. As an example, a hierarchical diffraction grating, which combines Bragg reflection arising from the nanoscale periodicity of the inverse opal crystal with grating diffraction resulting from a micrometer scale periodicity, is demonstrated.

Nature provides a multitude of examples of multifunctional structural materials in which trade-offs are imposed by conflicting functional requirements. One such example is the biomineralized armor of the chiton Acanthopleura granulata, which incorporates an integrated sensory system that includes hundreds of eyes with aragonite-based lenses. We use optical experiments to demonstrate that these microscopic lenses are able to form images. Light scattering by the polycrystalline lenses is minimized by the use of relatively large, crystallographically aligned grains. Multiscale mechanical testing reveals that as the size, complexity, and functionality of the integrated sensory elements increase, the local mechanical performance of the armor decreases. However, A. granulata has evolved several strategies to compensate for its mechanical vulnerabilities to form a multipurpose system with co-optimized optical and structural functions.

Mechanical forces in the cell’s natural environment have a crucial impact on growth, differentiation and behavior. Few areas of biology can be understood without taking into account how both individual cells and cell networks sense and transduce physical stresses. However, the field is currently held back by the limitations of the available methods to apply physiologically relevant stress profiles on cells, particularly with sub-cellular resolution, in controlled in vitro experiments. Here we report a new type of active cell culture material that allows highly localized, directional, and reversible deformation of the cell growth substrate, with control at scales ranging from the entire surface to the subcellular, and response times on the order of seconds. These capabilities are not matched by any other method, and this versatile material has the potential to bridge the performance gap between the existing single cell micro-manipulation and 2D cell sheet mechanical stimulation techniques.