PubMed · 15487051
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Katie Cottingham. 2004-10-01. Using dispersion to detect binding.. https://doi.org/10.1021/ac041634h
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We report sedimentation velocity and equilibrium measurements performed with an analytical ultracentrifuge to elucidate the effects of limited flexibility on the transport properties of semiflexible, monodisperse, double-stranded, blunt-ended DNA restriction fragments. We study a homologous series of fragments with 400, 800, and 1600 base pairs (3 to 11 persistence lengths), which are specifically designed and synthesized for this purpose (Part I). The molecular weights following from the sedimentation measurements agree well with the values expected on the basis of the number of base pairs. The sedimentation coefficients at infinite dilution are in good agreement with theoretical predictions for wormlike cylinders. The first order in volume fraction (varphi) coefficient K of the varphi-dependent sedimentation coefficient s(varphi)=1-Kvarphi decreases from 1178 for the shortest fragment to 882 for the longest fragment. These values are much larger than predicted for uncharged rigid rods, indicating the presence of associates with an enhanced aspect ratio and excluded volume. The precise match of the molecular weights obtained from exponential sedimentation-diffusion equilibrium distributions with weights calculated from the number of base pairs shows that any association is reversible and disappears at sufficiently low DNA concentration.
Dielectrophoresis (DEP), the motion of polarizable particles in non-uniform electric fields, has become an important tool for the transport, separation, and characterization of microparticles in biomedical and nanoelectronics research. In this article we present, to our knowledge, the first molecular dynamics simulations of DEP of nanometer-sized colloidal particles. We introduce a simplified model for a polarizable nanoparticle, consisting of a large charged macroion and oppositely charged microions, in an explicit solvent. The model is then used to study DEP motion of the particle at different combinations of temperature and electric field strength. In accord with linear response theory, the particle drift velocities are shown to be proportional to the DEP force. Analysis of the colloid DEP mobility shows a clear time dependence, demonstrating the variation of friction under non-equilibrium. The time dependence of the mobility further results in an apparent weak variation of the DEP displacements with temperature.
Effects of side reactions during the formation of high quality colloidal nanocrystals were studied using ZnO as a model system. In this case, an irreversible side reaction, formation of esters, was identified to accompany formation of ZnO nanocrystals through the chemical reaction between zinc stearate and an excess amount of alcohols in hydrocarbon solvents at elevated temperatures. This irreversible side reaction made the resulting nanocrystals stable and with nearly unity yield regardless of their size, shape, and size/shape distribution. Ostwald ripening and intraparticle ripening were stopped due to the extremely low solubility/stability of the possible monomers because all free ligands in the solution were consumed by the side reaction. However, focusing on size distribution and 1D growth that are needed for the growth of high quality nanocrystals could still occur for high yield reactions. Upon the addition of a small amount of stearic acid or phosphonic acid, immediate partial dissolution of ZnO nanocrystals took place. Although the excess alcohol could not react with the resulting zinc phosphonic acid salt, it could force the newly formed zinc stearate gradually but completely back onto the existing nanocrystals. The results in this report indicate that side reactions are extremely important for the formation of high quality nanocrystals by affecting their quality, yield, and stability under growth conditions. Due to their lack of information in the literature and obvious practical advantages, studies of side reactions accompanying formation of nanocrystals are important for both fundamental science related to crystallization and industrial production of high quality nanocrystals.