No abstract

Growth of Cu nanoparticles in a pulsed gas aggregation cluster source was studied. The cluster growth is enhanced by an energy transport inside the aggregation chamber fed by argon that is delivered in short pulses repeated with low frequency. The effect of pressure, varied during the pulse, on the cluster growth was estimated from time‐resolved measurements of mass/size cluster distribution. The cluster mass and the cluster production well correlate with pressure changes. Several stages of cluster growth during the gas‐pulse were recognized. Not only larger clusters but also significantly enhanced particle and mass fluxes were observed.

Radical induced modification of DNA plays an important role in many pathological pathways like cancer development, aging, etc. In this work, we quantify radical-induced DNA damage that causes transitions from double to single stranded DNA using atomic force microscopy (AFM). The plasmid pBR322 is attacked by free hydroxyl radicals that are produced by Fenton's reaction; the strength of the radical attack is controlled via the ratio of hydroxyl radical molecules to DNA base pairs. The extent of DNA modification is assessed by AFM tapping mode (TM) imaging of the plasmids (after adsorption onto PAH-functionalized mica) in air. As single stranded DNA chains (height ∼2 Å) are much smaller than intact DNA strands (∼5 Å), their fraction can be quantified based on the height distribution, which allows a simplified data analysis in comparison to similar AFM-based approaches. It is found that the amount of damaged DNA strands increases with increasing strength of radical attack, and decreases if ROS scavengers like sodium acetate are added. Competition curves are calculated for the interaction of hydroxyl radicals with DNA and sodium acetate, which finally allows calculation of relative rate constants for the respective reactions.

Positively charged, branched polyethylenimine (PEI) adsorbed onto silicon wafers and silica surfaces are attacked by free hydroxyl radicals. With AFM colloid probe technique, the surface forces between PEI layers are measured. Force profiles show that an electrostatic repulsion dominates the surface forces between freshly deposited PEI layers. After radical attack, both surface potential and surface charge density are reduced by a factor of about 0.5, while the Debye length remains unchanged. Adsorbed gold nanoparticles and force volume measurements show a homogeneous distribution of the surface charge on length scales between 40 nm and 30 μm. For radical attacked PEI layers, we find a 10% decrease of saturation coverage of gold nanoparticles. This is consistent with the decreased surface charge density, if the electrostatic three-body interaction is taken into account. Nevertheless, the NP adsorption kinetics are slowed down suggesting that the PEI layer is inhomogeneous on the nm-scale after radical attack.

The paper is focused on investigation of enhanced metal (Cu) cluster growth in a source of Haberland's type using pulsed gas aggregation. The aggregation Ar gas was delivered into the cluster source in a pulse regime, which results in the formation of well pronounced aggregation pressure peaks. The pressure peaks were varied by varying the different pulse gas frequency at the same mean pressure kept for all experiments. Hence, we were able to study the effect of enhanced aggregation pressure on cluster formation. Time-resolved measurements of cluster mass distribution were performed to estimate the mass and particle flux. The paper demonstrates that pulse gas aggregation influences growth of Cu nanoparticles, i.e., cluster mass/size, mass flux, and particle flux emitted from the cluster source. It was found that cluster mass related quantities are strongly influenced by pulsed gas frequency; the highest value of mass flux appears at the most pronounced pressure peaks. On the other hand, the particle flux depends only slightly on the gas pulse frequency. The explanation based on cooling and thermalization of sputtered particles is discussed in the paper.