Fabrication of nanostructured polymer surfaces using colloidal lithography and spin-coating

We present an original method for fabricating nanostructured polymer surfaces exhibiting homogeneous chemistry using colloidal lithography combined with polymer spin-coating. The first step of the strategy consists of creating gold surfaces with hemisphere-like protrusions, by adhesion of negatively charged polystyrene colloidal particles onto smooth positively charged surfaces, followed by coating with a thin layer of gold. In the second step, gold substrata are spin-coated with ultrathin polystyrene films. Using X-ray photoelectron spectroscopy and atomic force microscopy, we show that the nanotopography of the obtained polymer surfaces can be modulated by changing three parameters, i.e., the particle diameter, the particle concentration, and the polystyrene film thickness, while keeping the surface chemistry unmodified

Optical properties of gold nanorings

The optical response of ring-shaped gold nanoparticles prepared by colloidal lithography is investigated. Compared to solid gold particles of similar size, nanorings exhibit a redshifted localized surface plasmon that can be tuned over an extended wavelength range by varying the ratio of the ring thickness to its radius. The measured wavelength variation is well reproduced by numerical calculations and interpreted as originating from coupling of dipolar modes at the inner and outer surfaces of the nanorings. The electric field associated with these plasmons exhibits uniform enhancement and polarization in the ring cavity, suggesting applications in near-infrared surface-enhanced spectroscopy and sensing.Peer reviewe

Protein adsorption on model surfaces with controlled nanotopography and chemistry

To evaluate the influence of substratum surface characteristics on protein adsorption processes, we have investigated the adsorption (adsorbed amount, supramolecular organization) of collagen on model substrata exhibiting controlled topography and surface chemistry. Substrata were prepared in two steps: (i) gold deposition onto silicon wafers (smooth substrata) and onto a support with nanoscale protrusions created by colloidal lithography (rough substrata); (ii) functionalization with CH3 (hydrophobic) and OH (hydrophilic) groups, using alkanethiol self-assembly. Atomic force microscopy (AFM) images were recorded under water, prior to and after collagen adsorption, and the images were analyzed quantitatively using two independent approaches. On smooth substrata, collagen formed a similar to6 nm thick, homogeneous layer with low roughness on hydrophilic surfaces, and a similar to20 nm thick layer exhibiting elongated aggregated structures on hydrophobic surfaces. Film thickness measurements (AFM) together with X-ray photoelectron spectroscopy (XPS) revealed larger adsorbed amounts on hydrophobic surfaces compared to hydrophilic ones. On rough substrata, the adsorbed amounts were similar to those found on smooth substrata; however, the collagen molecules no longer formed aggregated structures on the hydrophobic surfaces. It is concluded that while the adsorbed amount is only affected by the surface chemistry, the supramolecular organization of the adsorbed layer is controlled both by surface chemistry and topography. The approach presented here will have great value in biophysics for investigating bioadsorption and bioadhesion processes on substrata of defined surface properties

Light scattering in gold nanorings

The optical response of gold nanoparticles of disk-like and ring-like shape is studied by analyzing their extinction cross section in the visible and near infrared regions. A strong dependence of the excitation modes on the actual geometry of the nanoparticles is found for particle diameters of ~120nm. The nanorings exhibit pronounced extinction peaks at much larger wavelengths than the nanodisks. We present both experimental results and numerical calculations, which are in excellent agreement when the geometry is correctly described. Nanorings are also shown to enhance the spontaneous emission at the resonance, where the emission is focused primarily along the ring axis.Peer reviewe

Self-organization of nanoparticles - implications for interface biology

Cells bind to their surroundings via proteins displayed on the cell surface. These interactions support the cells and are important for many cellular processes, e.g. cell migration during morphogenesis, wound healing and cancer metastasis. There is a yet unmet need for simple and robust in vitro models mirroring the complex molecular organization found in natural tissue. In this thesis, protein-sized gold nanoparticles were used to introduce morphological and biochemical nanopatterns on material surfaces via nanoparticle self-assembly. These surfaces were used to explore the effect of protein organization and other nanoscopic parameters on cell response. In their simplest form, gold nanoparticles (in solution) are stabilized by negatively charged ions adsorbed onto their surfaces. It was shown that such nanoparticles, 10 nm in diameter, could self-organize on a dithiol modified gold surface under the influence of electrostatic double-layer forces. The distance between the adsorbed particles could be tuned by the ionic composition of the particle solution, which was described using classical DLVO-theory. A novel method to prepare surfaces with nanoparticle gradients, based on this mechanism, was introduced. Prepared surfaces were used as templates for the assembly of nanopatterns of chemical entities and proteins, with a periodicity in the sub 100 nm regime, by site-specific grafting of different molecules to the particle surfaces. Patterns with specific cell-binding proteins and peptides as well as synthetic polymers were realized and characterized with SEM, imaging SPR, QCM-D and TOF-SIMS. Gradient patterns were also assembled with multiple ligands, e.g. RGD-peptides and heparin, allowing the investigation of synergistic cell stimuli. Biochemical nanopatterns were evaluated in studies on human fibroblasts and endothelial cells, e.g. the cellular mobility was explored in response to different gradient stimuli. In a separate study, fimbria mediated adhesion of E. coli bacteria to nanoscopic adhesive domains was investigated. Surfaces decorated with gold nanoparticles were also shown to attenuate the complement protein cascade system via morphological alteration of adsorbed proteins. Altogether, concepts and methods presented in this thesis offer a route to systematically explore the interactions between biology and molecularly organized interfaces