Steffen Franzka

Nanostructured Surfaces by Deposition of Metal Nanoparticles by Means of Spray Techniques

An electrospray unit was built in order to deposit preformed metal nanoparticles on various substrates. Furthermore, due to treatment of the deposited particles with an oxygen plasma, the particles were stripped of their ligands and strongly fixed to the surface. These treated surfaces can be expected to exhibit properties that differ characteristically from bare surfaces. Depending on the substrate itself, but even more on the size and the density of the deposited nanoparticles, the contact angles and thereby the wettability could be varied drastically. Gold nanoparticles of 17 nm diameter on titanium decreased the contact angle from 82° to 34°, whereas 6–8 nm palladium particles increased the wettabilty to such a degree that a contact angle could not be determined. In contrast to the electrospray technique, a pneumatically operated device was constructed, allowing simultaneous spraying and plasma pyrolysis.

Preparation of two-dimensionally patterned layers of functionalised calcium phosphate nanoparticles by laser direct writing

Calcium phosphate nanoparticles were functionalised using suitable polymeric additives and electrophoretically deposited on titanium and silicon substrates. Subsequently the coated substrates were patterned with a focused beam of an argon ion laser at a wavelength of 514 nm. Depending on the incident laser power and the writing speed, different processes were observed. At high laser power and writing speed, the particle layer was locally removed down to the substrate (“laser cleaning”). In contrast, at low laser power and writing speed, the particles melted and formed larger structures. The width of the laser written lines is of the order of the laser spot size (about 3 µm). This approach opens a way to two-dimensionally structured layers of nanoparticles with a predefined thickness.

Surface-Initiated Polymerization on Laser-Patterned Templates: Morphological Scaling of Nanoconfined Polymer Brushes

Nonlinear laser processing of silane-based monolayers is used to fabricate nanostructured chemical templates for the selective growth of polymer brushes in confined domains via surface-initiated polymerization (SIP). Upon varying the laser parameters, reactive domains with lateral dimensions from several micrometers down to the sub-100-nm range are fabricated. This provides a versatile means for studying the morphological scaling behavior of confined polymer brushes. Here, the surface-initiated growth of a stimuli-responsive polymer, poly(N-isopropylacrylamide) (PNiPAAm), via atom transfer radical polymerization (ATRP) is investigated. Polymer chains at the domain boundaries extend into the surrounding polymer-free areas. For this reason the width of confined polymer brushes is significantly larger than that of the underlying domains. Within experimental error, though, the excess width does not depend on the domain size. In contrast, the brush height decreases more and more when the domain size falls below a certain value. Simple considerations point to a geometrical scaling relation between height and width of the polymer brushes. These results are considered as essential for implementation of SIP routines in laser-assisted fabrication schemes targeting micro- and nanofluidic applications.

Subwavelength patterning of alkylsiloxane monolayers via nonlinear processing with single femtosecond laser pulses

Femtosecond laser patterning of octadecylsiloxane monolayers on quartz glass at λ=800nm, τ<30fs, and ambient conditions has been investigated. Selective decomposition of the coating with single laser pulses at subwavelength resolution can be carried out over a wide range of fluences from 4.2 down to 3.1J∕cm2. In particular, at a 1∕e laser spot diameter of 1.8μm, structures with a width down to 250nm and below were fabricated. This opens up a facile route towards laser fabrication of transparent templates with chemical structures down into the sub-100-nm-regime.

Dose-dependent surface endothelialization and biocompatibility of polyurethane noble metal nanocomposites.

Surface pre-endothelialization is a promising approach to improve the hemocompatibility of implants, medical devices, and artificial organs. To promote the adhesive property of thermoplastic polyurethane (TPU) for endothelial cells (ECs), up to 1 wt % of gold (Au) or platinum (Pt) nanoparticles, fabricated by pulsed laser ablation in polymer solution, were embedded into the polymer matrix. The analysis of these nanocomposites showed a homogenous dispersion of the nanoparticles, with average diameters of 7 nm for Au or 9 nm for Pt. A dose-dependent effect was found when ECs were seeded onto nanocomposites comprising different nanoparticle concentrations, resulting in a fivefold improvement of proliferation at 0.1 wt % nanoparticle load. This effect was associated with a nanoparticle concentration-dependent hydrophilicity and negative charge of the nanocomposite. In dynamic flow tests, nanocomposites containing 0.1 wt % Au or Pt nanoparticles allowed for the generation of a confluent and resistant EC layer. Real-time polymerase chain reaction quantification of specific markers for EC activation indicated that ECs cultivated on nanocomposites remain in an inactivated, nonthrombogenic and noninflammatory state; however, maintain the ability to trigger an inflammatory response upon stimulation. These findings were confirmed by a platelet and leukocyte adhesion assay. The results of this study suggest the possible applicability of TPU nanocomposites, containing 0.1 wt % Au or Pt nanoparticles, for the generation of pre-endothelialized surfaces of medical devices.

Photothermal micro- and nanopatterning of organic/silicon interfaces.

Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at lambda = 514 nm and a 1/e(2) spot diameter of 2.6 microm results in local decomposition of the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for subwavelength patterning, with line widths between 0.4 and 1.4 microm. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 nm are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g., for hybrid microelectronic and sensor applications.

Direct Laser Patterning of Soft Matter: Photothermal Processing of Supported Phospholipid Multilayers with Nanoscale Precision

Direct laser patterning of supported phospholipid multilayers is investigated. Spin coating is used to fabricate stacked bilayers of 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA). Photothermal processing with a focused laser beam at lambda = 514 nm allows removal of the coating at predefined positions without causing any significant change in adjacent areas. Moreover, processing with nanoscale precision is feasible despite the soft and fluid nature of phospholipid films. In particular, holes with diameters from 1.8 microm down to 300 nm and below are fabricated by using a 1/e(2) laser spot size of about 2.5 microm. In addition, patterning is also very flexible and can be carried out over macroscopic length scales and at short processing times. Considering these features photothermal laser processing constitutes a powerful tool for micro- and nanopatterning of phospholipid films.

Poly(N,N-dimethylaminoethyl methacrylate) Brushes: pH-Dependent Switching Kinetics of a Surface-Grafted Thermoresponsive Polyelectrolyte.

The temperature-dependent switching behavior of poly(N,N-dimethylaminoethyl methacrylate) brushes in alkaline, neutral, and acidic solutions is examined. A novel microscopic laser temperature-jump technique is employed in order to study characteristic thermodynamic and kinetic parameters. Static laser micromanipulation experiments allow one to determine the temperature-dependent variation of the swelling ratio. The data reveal a strong shift of the volume phase transition of the polymer brushes to higher temperatures when going from pH = 10 to pH = 4. Dynamic laser micromanipulation experiments offer a temporal resolution on a submillisecond time scale and provide a means to determine the intrinsic rate constants. Both the swelling and the deswelling rates strongly decrease in acidic solutions. Complementary experiments using in situ atomic force microscopy show an increased polymer layer thickness at these conditions. The data are discussed on the basis of pH-dependent structural changes of the polymer brushes including protonation of the amine groups and conformational rearrangements. Generally, repulsive electrostatic interactions and steric effects are assumed to hamper and slow down temperature-induced switching in acidic solutions. This imposes significant restrictions for smart polymer surfaces, sensors, and devices requiring fast response times.

Photothermal laser microsintering of nanoporous gold.

Photothermal processing of nanoporous gold using a microfocused continuous-wave laser at a wavelength of 532 nm and a 1/e(2) spot diameter of 2.9 μm has been studied. In addition, complementary experiments have been carried out via conventional annealing. Scanning electron microscopy has been used for characterization. Local laser irradiation at distinct laser powers and pulse lengths results in coarsening of the porous gold structures. During laser processing the pore size of the native nanoporous gold increases to maximum values in the range of 0.25-3 μm. The affected areas exhibit lateral dimensions in the range of 2-10 μm. Overall two regions are distinguished. An inner region, where large pores and ligaments are formed and an outer region, where the pore size and ligament size gradually change and approach the feature sizes of the native material. A qualitative thermokinetic model allows one to reproduce the experimentally observed dependence of the laser-induced morphologies on the laser parameters. On the basis of this model the underlying processes are attributed to sintering and melting of the gold structures. The presented results demonstrate the prospects of photothermal laser processing in engineering porous gold with spatially varying porosities on micrometer to nanometer length scales.

Photothermally Induced Microchemical Functionalization of Organic Monolayers

Photopatterning of organic coatings represents a key step in many technological applications ranging from microchip fabrication to the design of bioarrays and microfluidic devices. Fundamentally, these applications rely on photochemical proACHTUNGTRENNUNGcesses, in which chemical reactions are initiated via direct or substrate-mediated electronic excitations. In the simplest case decomposition of the coating takes place. A broad range of photochemical routines, though, also allows for local functionalization of organic coatings. The lateral resolution, in turn, usually is limited by optical diffraction, that is, the fabricated structures are not much smaller than the wavelength even when highly focusing optics is used. Scanning near-field photolithography, of course, allows for sub-wavelength patterning. Processing, though, is very slow and restricted to small areas. A means to enhance the lateral resolution of far-field optical techniques takes advantage of nonlinear effects. In photothermal laser processing, for example, a focused laser beam is used to locally heat the substrate surface and to thermally initiate chemical reactions. For this reason, photothermal processing is highly nonlinear in laser power density and facilitates sub-wavelength patterning. In recent years, organic monolayers have gained particular attraction as photothermally patternable platforms. Generally, the lateral resolution depends on the thermal and chemical stability of the coating. Strongly bound coatings, for example silane-based monolayers, can be patterned from the micrometer range down to the sub 100 nm range. Such patterns have been used as chemical templates to build up functional surface architectures from nanoscopic components. These results emphasize the capabilities of photothermal routines in microand nanofabrication of organic interfaces. Commonly, though, photothermal processing of organic monolayers results in local decomposition of the coating. In analogy to photochemical routines, of course, it is tempting to explore photothermal procedures which allow to locally functionalize organic monolayers. Such procedures open up a facile avenue towards more complex chemical surface structures such as multifunctional templates and chemical gradients. As a prototype example, we here address a photothermal procedure for local functionalization of alkylsiloxane monolayers on surface-oxidized silicon substrates in a gaseous bromine ambient. As outlined below, this procedure takes advantage of some characteristic features of common photobromination reactions. Photobromination of hydrocarbons represents a classical reaction in organic synthesis. Previous contributions also investigated large-area photobromination of polymer interfaces and organic monolayers. In conjunction with other chemical transformations this provides an efficient route to a broad variety of functional groups. Reactions (1)–(5) recapitulates the underlying radical reaction mechanism.