Molecular simulations of charged complex fluids: A review

Finely tuned water structure and transport in functionalized carbon nanotube membranes during desalination

Molecular dynamics simulations were performed to tune the transport of water molecules in nanostructured membrane in a desalination process. Four armchair-type (7,7), (8,8), (9,9) and (10,10) carbon nanotubes (CNTs) with pore diameters around 1 nm were chosen, their interior surfaces were modified with -OH, -CH3 and -F groups. Simulation results show that water transport in nanochannel depends on confined water structures which could be regulated by precisely controlled channel diameter and chemical functionalization. Increasing CNT diameter changes water structures from single-file-like to be square and hexagonal-like, then into a disordered pattern, resulting in a concave-shaped trend of water permeance. The -OH functional groups promote structural ordering of water molecules in (7,7) CNT, but disrupt water structures in (8,8) and (9,9) CNTs, and reduce the order degree of water molecules in (10,10) CNT, moreover, exert an attraction to enhance surface friction inside channel. The -CH3 groups induce more strictly single-file movement of water molecules in (7,7) CNT, turning water structures in (8,8) and (9,9) CNTs into two and triangular column arrangements, improving water transport, however, causing again square-like water structure in (10,10) CNT. Fluorinations of CNT make water structure more disordered in (7,7), (9,9) and (10,10) CNTs, while enhance the square water structure in (8,8) CNT with a lower water permeance. Through changing channel diameter and functionalization, the low tetrahedral order corresponds to a more single-file-like water structure, associated with rapid water diffusion and high permeability; an increase in tetrahedrality results in more ice-like water structures, lower water diffusion coefficients, and permeability. The results of this study demonstrate that water transport could be finely regulated via a functionalized CNT membrane.

Modulating the adsorption orientation of methionine-rich laccase by tailoring the surface chemistry of single-walled carbon nanotubes

Achieving fast electron transfer process between oxidoreductase and electrodes is pivotal for the biocathode of enzymatic biofuel cells (EBFCs). However, in-depth understanding of the interplay mechanism between enzymes and electrode materials remains challenging when designing and constructing EBFCs. Herein, atomic-scale insight into the direct electron transfer (DET) behavior of Thermus thermophilus laccase (TtLac) with a special methionine-rich β-hairpin motif adsorbed on the carboxyl-functionalized carbon nanotube (COOH-CNT) and amino-functionalized carbon nanotube (NH₂-CNT) surfaces were disclosed by multi-scale molecular simulations. Simulation results reveal that electrostatic modification is an effective way to tune the DET behavior for TtLac on the modified-CNTs electrode surface. Surprisingly, the positively charged TtLac can be attracted by both negatively charged COOH-CNT and positively charged NH₂-CNT surfaces, yet only the latter is capable to trigger the DET process due to the 'lying-on' adsorption orientation. Specifically, the T1 copper site is near the methionine-rich β-hairpin motif, which is the key binding site for TtLac binding onto the NH₂-CNT surface via electrostatic interaction, π-π stacking and cation-π interaction. Moreover, TtLac on the NH₂-CNT surface undergoes less conformational changes than those on the COOH-CNT surface, which allows the laccase stability and catalytic efficiency to be well preserved. These findings provide a fundamental guidance for future design and fabrication of methionine-rich laccase-based EBFCs with high power output and long lifespan.