Visualization and analysis of nanoparticle transport and ageing in reactive porous media.

We present quasi-3D visualization and analysis of engineered nanoparticle (ENP) transport behavior in an experimental setup that uses a transmitted light imaging technique. A flow cell was packed with specially adapted, water-transparent, spherical polyacrylamide beads, which carry a negative surface charge representative of many natural environments. Ubiquitous, oppositely-charged ENPs - Au and Ag NPs - were synthesized and introduced into a flow cell subjected to a macroscopically uniform flow field via point source pulse injection, at three different flow rates. The negatively-charged ENPs behaved like a conservative tracer, in terms of spatio-temporal plume evolution. The positive AgNPs, however, displayed a decrease in their initially strong tendency to attach to the oppositely-charged porous medium. As a result, immobilization of the positive AgNPs was spatially and temporally limited to the vicinity of the point of injection; beyond this region, the AgNPs were mobile and effluent contained AgNPs with hydrodynamic diameters significantly larger than those of the injected AgNPs. This behavior is understood by dynamic light scattering and ζ potential measurements, which showed aggregation processes and inversion in particle surface charge to occur during transport of the positive ENPs. These findings have broad implications for ENP mobility and reactivity in the environment.

Water organization between oppositely charged surfaces: implications for protein sliding along DNA.

Water molecules are abundant in protein-DNA interfaces, especially in their nonspecific complexes. In this study, we investigated the organization and energetics of the interfacial water by simplifying the geometries of the proteins and the DNA to represent them as two equally and oppositely charged planar surfaces immersed in water. We found that the potential of mean force for bringing the two parallel surfaces into close proximity comprises energetic barriers whose properties strongly depend on the charge density of the surfaces. We demonstrated how the organization of the water molecules into discretized layers and the corresponding energetic barriers to dehydration can be modulated by the charge density on the surfaces, salt, and the structure of the surfaces. The 1-2 layers of ordered water are tightly bound to the charged surfaces representing the nonspecific protein-DNA complex. This suggests that water might mediate one-dimensional diffusion of proteins along DNA (sliding) by screening attractive electrostatic interactions between the positively charged molecular surface on the protein and the negatively charged DNA backbone and, in doing so, reduce intermolecular friction in a manner that smoothens the energetic landscape for sliding, and facilitates the 1D diffusion of the protein.