Hendrik Reinhardt

Self-organization of multifunctional surfaces--the fingerprints of light on a complex system.

Nanocomposite patterns and nanotemplates are generated by a single-step bottom-up concept that introduces laser-induced periodic surface structures (LIPSS) as a tool for site-specific reaction control in multicomponent systems. Periodic intensity fluctuations of this photothermal stimulus inflict spatial-selective reorganizations, dewetting scenarios and phase segregations, thus creating regular patterns of anisotropic physicochemical properties that feature attractive optical, electrical, magnetic, and catalytic properties.

Photochemical preparation of sub-wavelength heterogeneous laser-induced periodic surface structures.

Laser-induced periodic surface structures (LIPSS) are a phenomenon caused by interaction of light with solid surfaces. We present a photochemical concept which uses LIPSS-related light intensity patterns for the generation of heterogeneous nanostructures. The process facilitates arbitrary combinations of substrate and LIPSS-pattern materials. An efficient method for the generation of organometallic hybrid-nanowire arrays on porous anodic aluminum oxide is demonstrated.

Directed assembly of gold nanowires on silicon via reorganization and simultaneous fusion of randomly distributed gold nanoparticles

Laser-induced reorganization and simultaneous fusion of nanoparticles is introduced as a versatile concept for pattern formation on surfaces. The process takes advantage of a phenomenon called laser-induced periodic surface structures (LIPSS) which originates from periodically alternating photonic fringe patterns in the near-field of solids. Associated photonic fringe patterns are shown to reorganize randomly distributed gold nanoparticles on a silicon wafer into periodic gold nanostructures. Concomitant melting due to optical heating facilitates the formation of continuous structures such as periodic gold nanowire arrays. Generated patterns can be converted into secondary structures using directed assembly or self-organization. This includes for example the rotation of gold nanowire arrays by arbitrary angles or their fragmentation into arrays of aligned gold nanoparticles.

Influence of substrate microcrystallinity on the orientation of laser-induced periodic surface structures

The research in this paper deals with the angular dependence of the formation of laser-induced periodic surface structures (LIPSS) by linearly polarized nanosecond laser pulses on polycrystalline austenitic stainless steel. Incident angles ranging from 45° to 70° lead to the generation of superimposed merely perpendicular oriented LIPSS on steel as well as on monocrystalline (100) silicon which was used as a reference material. Additional extraordinary orientations of superimposing LIPSS along with significantly different periodicities are found on polycrystalline steel but not on (100) silicon. Electron backscatter diffraction measurements indicate that the expansion of these LIPSS is limited to the grain size and affected by the crystal orientation of the individual grains. Atomic force microscopy imaging shows that LIPSS fringe heights are in good agreement with the theoretically predicted penetration depths of surface plasmon polaritons into stainless steel. These results indicate that optical anisotropies must be taken into account to fully describe the theory of light-matter interaction leading to LIPSS formation.

High refractive index TiO2-PHEMA hydrogel for ophthalmological applications

A facile route for the synthesis of PHEMA hydrogel incorporating anatase nanoparticles is presented. The hydrogel features a refractive index of 1.527 and high transparency throughout the visual spectral range. Saturn rings have been machined in order to investigate the workability of the material and its potential applicability as intraocular lens implant. The resulting lenses feature high surface quality, foldability, and shape memory which makes them suitable for implantation via extracapsular cataract extraction.

Site-selective biomineralization of native biological membranes

Biomineralization of silica precursors, mediated by self-assembled proteins, is performed by many organisms. The silica cell walls of diatoms are perhaps the most stunning biomineral structures. Although the mechanisms of biomineralization are still not fully understood, template-assisted formation of silica nanostructures has gained much attention in the materials science community. Precise control of the location and the shape of structures obtained by biomineralization remains a challenge. This paper introduces a versatile biotechnological process that enables site-selective biomineralization of native biological membranes using genetically modified purple membrane (PM) from Halobacterium salinarum as a template. PM is a two-dimensional crystal consisting of bacteriorhodopsin (BR) and lipids. In this work we study PM-E234R7, a genetically modified PM containing mutated BR, where seven amino acids, starting from E234, were replaced by arginine in the C-terminus. The arginine sequence catalyzes silica formation from a tetraethylorthosilicate (TEOS) precursor. Silicification of the mutated PM variant starts with initial formation of membrane-attached spherical silica nanoparticles, which then fuse to form 2D silica nanoflakes, selectively, on the cytoplasmic side of the PM. Genetical modification of membrane proteins with poly-arginine sequences may be a general route for site-selective biomineralization of native biological membranes.

Mechanically metastable structures generated by single pulse laser-induced periodic surface structures (LIPSS) in the photoresist SU8

Laser-induced periodic surface structures (LIPSS) with a periodicity of 351 nm are generated in the negative photoresist SU8 by single nanosecond laser pulse impact. Friction scans indicate the periodic pattern to comprise alternating regions of crosslinked and non-crosslinked SU8. Intriguingly, even minor mechanical stimuli in the order of nanonewtons cause the unfolding or rather the deletion of the characteristic periodic pattern similarly to the release of a pre-loaded spring. This feature combined with high resilience to heat and photon irradiation makes SU8-LIPSS attractive for applications such as mechanical stress monitors, self-destructing memory and passive micro actuators.

TiO2 nanoparticles for enhancing the refractive index of hydrogels for ophthalmological applications

Intraocular lenses (IOL) are currently the only treatment for cataract dependent vision impairment and blindness [1]. A polymer suitable for IOL manufacture needs to meet a plurality of properties, biocompatibility, excellent transmission in the visible range, a high flexibility for micro invasive surgery, a high refractive index as well as a good ABBE-number, just to mention the most important ones [2]. We present the use of in situ generated TiO2-nanoparticles to enhance the refractive index of poly-HEMA hydrogels - with are suitable polymers for IOL manufacture[3] – from 1.44 to 1.527 at 589.3 nm combined with an excellent ABBE-number of 54. The nanoparticles were prepared using titaniumdiisopropoxide- bis(acetylacetonate) as a precursor. First the titanium salt was diffused into the poly-HEMA matrix and then it was transformed into TiO2 in boiling water. The resulting pHEMA [TiO2] hydrogel was dried for 10 days under ambient conditions. By lathing these polymers were machined into lens precursors, the so-called Saturn-rings. After reswelling in physiological saline solutions flexible polymer lenses with high surface quality, shape memory and superior optical properties were obtained. The crystal structure of the formed TiO2 nanoparticles was identified as anatase via Xray. No release of titanium ions or TiO2 nanoparticles was observe under physiological conditions. Such hybrid materials of TiO2 nanoparticles and poly-HEMA like hydrogels are promising materials for IOL.