Complexation of Linear DNA and Poly(styrenesulfonate) with Cationic Copolymer Micelles: Effect of Polyanion Flexibility

The complexation of linear double stranded DNA and poly­(styrenesulfonate) (PSS) with cationic poly­(dimethylamino ethyl methacrylate)-<i>block</i>-poly­(<i>n</i>-butyl methacrylate) micelles was compared in aqueous solutions at various pH values and ionic strengths. The complexation process was monitored by turbidimetric titration, as a function of the ratio (<i>N</i>/<i>P</i>) of amine groups in the micelle corona to the number of phosphates (or sulfonates) in the polyanion. The size, structure and stability of the resulting micelleplexes were studied by dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM). In the short chain regime, where the contour lengths of the polyanions are shorter than or comparable to the micelle corona thickness, micelleplexes with DNA oligomers show very similar behavior to complexes with short PSS chains, in terms of titration curves and structural evolution of the complexes as a function of charge ratio. However, in the long chain regime, where the contour length of the polyanion far exceeds the micelle radius, micelleplexes of linear DNA show titration curves shifted toward lower <i>N</i>/<i>P</i> ratios, reduced stability at <i>N</i>/<i>P</i> < 1, and a higher percentage of small complexes at <i>N</i>/<i>P</i> > 1 compared to complexes with long chain PSS. Furthermore, at 1 M ionic strength, the cationic micelles could still complex with long chain PSS, but not with DNA of the same total charge. These differences are attributed to the flexibility difference between the polyanion chains, and possible mechanisms are proposed. This work highlights the importance of chain flexibility in complexation of dissimilar polyelectrolyte pairs, a factor that could therefore help guide the future design of micelleplexes for various applications

Photoexcitation Dynamics of Coupled Semiconducting Carbon Nanotube Thin Films

Carbon nanotubes are a promising means of capturing photons for use in solar cell devices. We time-resolved the photoexcitation dynamics of coupled, bandgap-selected, semiconducting carbon nanotubes in thin films tailored for photovoltaics. Using transient absorption spectroscopy and anisotropy measurements, we found that the photoexcitation evolves by two mechanisms with a fast and long-range component followed by a slow and short-range component. Within 300 fs of optical excitation, 20% of nanotubes transfer their photoexcitation over 5–10 nm into nearby nanotube fibers. After 3 ps, 70% of the photoexcitation resides on the smallest bandgap nanotubes. After this ultrafast process, the photoexcitation continues to transfer on a ∼10 ps time scale but to predominantly aligned tubes. Ultimately the photoexcitation hops twice on average between fibers. These results are important for understanding the flow of energy and charge in coupled nanotube materials and light-harvesting devices

Image-based visual hull of a tennis racket

Researchers are often interested in tracking object movement in sport. This could be useful to identify equipment designs that match the technique of players. Previously, stereo camera systems have been used to track markers attached to striking implements to measure their movement in three dimensions. However, manual selection of markers on the image plane can be time consuming and inaccurate. There is potential however to reduce these drawbacks associated with marker-based analysis by tracking a striking implement using its visual hull. The closest geometric approximation of an object that can be reconstructed using only its silhouette images is its visual hull. Early applications of visual hulls include size and shape estimation of objects such as stones. Recently, subject specific visual hulls constructed from multiple camera views combined with anatomical tracking algorithms have measured human motion through markerless motion capture. However, multiple camera systems are not practical for real play conditions in most sports. The application of visual hulls to measure movements of striking implements used in sport has not yet been explored. A set of calibrated views of a tennis racket were captured and segmented into binary images to obtain silhouettes. The visual hull of a tennis racket was constructed by intersection of the volume of space formed by back-projecting the silhouettes from all input views. This research is the first stage in the development of a system that measures movement of a striking implement in real play conditions by combining its visual hull with footage from a single camera