Jan Timper

Controlled Nucleation of DNA Metallization

The outstanding self-recognition properties of DNA have been exploited in the use of this biomolecule as a template for the construction of nanoscale assemblies and hybrid structures. 3] To further expand the utility of DNA for other applications, research is currently aimed at increasing its intrinsically low electric conductivity. The selective coating of DNA with a thin layer of a conductive element, such as Ag, Pd, Pt, Cu, or Co, has emerged as a promising avenue. Most coating procedures involve the reduction of electrostatically bound metal ions on the DNA by an exogeneous reductant to give small metal clusters attached to the DNA. In a second, development step, known from black-andwhite photography, these metal clusters function as nucleation sites for the reductive deposition of metal atoms until a continuous conductive coating is formed. Novel procedures aimed at increasing the selectivity of the metallization process are the decoration of DNA with functional groups to control the spatial distribution of the nucleation sites, the photochemical deposition of silver on DNA strands, and the formation of DNA–Pt adducts as precursors for metal deposition on DNA. The most critical step in the whole metallization process is the initial nucleation step. The uniformity and the distribution of the metal clusters define the homogeneity of the development

Spontaneous Assembly of Miktoarm Stars into Vesicular Interpolyelectrolyte Complexes

Mixing a bis-hydrophilic, cationic miktoarm star polymer with a linear polyanion leads to the formation of unilamellar polymersomes, which consist of an interpolyelectrolyte complex (IPEC) wall sandwiched between poly(ethylene oxide) brushes. The experimental finding of this rare IPEC morphology is rationalized theoretically: the star architecture forces the assembly into a vesicular shape due to the high entropic penalty for stretching of the insoluble arms in non-planar morphologies. The transmission electron microscopy of vitrified samples (cryogenic TEM) is compared with the samples at ambient conditions (in situ TEM), giving one of the first TEM reports on soft matter in its pristine environment.

Size-dependent multispectral photoacoustic response of solid and hollow gold nanoparticles

Photoacoustic (PA) imaging attracts a great deal of attention as an innovative modality for longitudinal, non-invasive, functional and molecular imaging in oncology. Gold nanoparticles (AuNPs) are identified as superior, NIR-absorbing PA contrast agents for biomedical applications. Until now, no systematic comparison of the optical extinction and PA efficiency of water-soluble AuNPs of various geometries and small sizes has been performed. Here spherical AuNPs with core diameters of 1.0, 1.4 and 11.2 nm, nanorods with longitudinal/transversal elongation of 38/9 and 44/12 nm and hollow nanospheres with outer/inner diameters of 33/19, 57/30, 68/45 and 85/56 nm were synthesized. The diode laser set-up with excitations at 650, 808, 850 and 905 nm allowed us to correlate the molar PA signal intensity with the molar extinction of the respective AuNPs. Deviations were explained by differences in heat transfer from the particle to the medium and, for larger particles, by the scattering of light. The molar PA intensity of 1.0 nm AuNPs was comparable to the commonly used organic dye methylene blue, and rapidly increased with the lateral size of AuNPs.

Hierarchical structures of carbon nanotubes and arrays of chromium-capped silicon nanopillars: formation and electrical properties.

The formation of stochastically oriented carbon-nanotube networks on top of an array of free-standing chromium-capped silicon nanopillars is reported. The combination of nanosphere lithography and chemical vapor deposition enables the construction of nanostructures that exhibit a hierarchical sequence of structural sizes. Metallic chromium serves as an etching mask for Si-pillar formation and as a nucleation site for the formation of carbon nanotubes through the chemical vapor deposition of ethene, ethanol, and methane, respectively, thereby bridging individual pillars from top to top. Iron and cobalt were applied onto the chromium caps as catalysts for CNT growth and the influence of different carbon sources and different gas-flow rates were investigated. The carbon nanotubes were structurally characterized and their DC electrical properties were studied by in situ local- and ex situ macroscopic measurements, both of which reveal their semiconductor properties. This process demonstrates how carbon nanotubes can be integrated into Si-based semiconductors and, thus, this process may be used to form high-surface-area sensors or new porous catalyst supports with enhanced gas-permeation properties.

Size dependent photoacoustic signal response of gold nanoparticles using a multispectral laser diode system

Photoacoustic imaging is based on the generation of ultrasound using laser irradiation. Recently, gold nanoparticles received a lot of attention due to their excellent suitability for use as contrast agents. For contrast agent analysis, Nd:YAG laser systems are commonly employed, but pulsed laser diodes, which are much smaller and cheaper, can be an attractive alternative. They emit significantly lower pulse energies, but fast averaging is feasible due to high repetition rates. Analysis of contrast agents using laser diodes suffers from vastly different properties of different laser diodes. Here, we present a calibration procedure to correct for some of these differences and enable comparison of results across multiple laser diodes. The calibration procedure is performed for analysis of nanoparticles ranging from diameters of 1 nm to 85 nm. The method is analyzed using two reference materials, Copper Chloride (CuCl2) and Copper Sulfate (CuSO4).

Highly Efficient Silver Nanoparticle Formation on Dialdehyde-Modified DNA

We investigated the efficient nucleation of silver nanoparticles on aldehyde-modified DNA strands. The aldehyde-bearing DNA was prepared by PCR incorporation of diol-modified synthetic deoxyuridine derivatives and subsequent cleavage of the diol groups to aldehyde functions using sodium periodate. Formerly unavailable dialdehyde-bearing DNA could be prepared by this mild and selective method. Both dialdehyde-DNA and monoaldehyde-DNA are capable of inducing silver nanoparticle formation without the addition of any external reductant. Interestingly, dialdehyde-DNA shows a different kinetic behavior and is about four times as efficient in silver nanoparticle formation as monoaldehyde-DNA. These data suggest a potentially unique mechanism of silver nanocluster formation induced by the dialdehyde groups. SEM measurements of fully metallized DNA strands show homogeneous metal coverage and selective metal deposition exclusively on the DNA template.