Physical Mechanics Research On The Transport Properties Of Confined Water Molecules At Nanoscale

Water molecules confined to structures at nanoscale has unique properties that are exploited inbiology, which plays important roles in protein folding, transmembrane water diffusion, andtransmission of neural signals, etc. The peculiar thermodynamic properties of confined watermolecules and its interaction with proteins hold promise for a wide range of biomimetic andnanotechnological applications. Due to its hollow structure and hydrophobic properties, carbonnanotube is a popular nanochannel for study the behavior of confined water molecules, which hasattracted much attention in theoretical and experimental investigations. Especially, when confined in aCNT with appropriate radius, water molecules can form a single-file hydrogen-bonded chain. Thealignment, dipole orientation, thermal motion and flow of the single-file water chain can be affectedby thermal fluctuation, external electrostatic field, temperature, pressure gradient or the geometry,chiral, length and the chemical modify of the CNT. The behind energetic analysis and physicalmechanism becomes one of the focuses in nanobionic and nanofluidic research, which has the benefitfor the design of functional nanochannel, nanosensor, and even Brown motor, with perspective in drugdelivery, desalination and other applications. The findings are briefly concluded as below:(1) Transport properties of single-file water chain in biomimicking water channel.In biological water channel, there are three important polar residues inside the selectivity filter, whichgenerates asymmetric electrostatic potential governing the transport properties of single-file waterchain. Then a bimimicking water channel has been proposed, which consists of a (6,6) carbonnanotube with different types of external point charges. It is expected that the asymmetric electrostaticpotential could drive the single-file water molecules flow unidirectionally. However, we find thatthere is no robust unidirectional water flow in CNT. Thermal fluctuation in bulk water competes withcharge affinity to steer the water transport, resulting in nonmontonic flow with intermittent reversal oftransport direction. The energetic analysis suggests that the water-water interaction, determined bydipole orientation configuration, influences the transport rate significantly. Meanwhile, the potencialof mean force (PMF) profiles show slight difference between the two entrances of the CNT, implyingpoor control of unidirectional water flow under the asymmetric electrostatic potential caused by theexternal charges. It is important that, when MD simulations are carried our on such a tricky issue,different results may be yielded in different running events on the exact same modeling system, due tothe statistical nature of MD process at finite temperature. Our results show that the biomimic CNT channel has the same diffusion property as the mimicked biological water channels, rather than apump, which has been strongly supported by the later communication published on NatureNanotechnology. These findings can provide correct biomimic understanding of water transportproperties and will benefit the design of efficient functional nanofluidic devices.(2) Water transport driven by asymmetric structure. In systems possessing spatial ordynamical symmetry breaking, Brownian motion combined with unbiased external input signals,deterministic and random alike, can assist directed motion of particles at nanoscale. According to theworking principle of cellular molecular machinery in biology, people are continually taking efforts todesign Brown motor and try to optimize its charicteristics. Here, we propose an interestingnanochannel based on ‘ratchet mechanism’, which has realized unidirectional water flow driven byasymmetric nanostructure. This design is inspired by the geometric structure of biological waterchannel: there exists a transition for confined water molecules from cone area to single-file. Theasymmetric free energy barriers at two ends of the nanochannel can effectively drive the single-filewater molecules move from the cone end to the other end. It should be mentioned that the angle of thenanocone, the length of the CNT, and the simulation temperature have influences on the directionalwater flow. Under290K, the unidirectional efficiency is up to100%during longer simulation time,which holds a perspective for future pathways and potential new applications, especially in efficientparticle separation and pump. The primary and extremely encouraging results shown by moleculardynamics simulation demonstrate that spontaneous directed water transport can be achieved by aratchet-like mechanism. Meanwhile, we study the mutation of biological water channel according tothe novel mechanism. The mutation of polar ARG197, which is located at the above transition zonewill change the structure of confined water cluster distinctly, while, the desired change of watertransport properties disappears in the complicated water-protein interaction.(3) Self-adjusted&self-sustained oscillation mechanism of water moleculesconfined in carbon nanotube. The growth of living organism is a spontaneous process of localentropy loss, typically the protein folding passes through entropy loss on the molecular level. Here wefind entropy loss also exists in nonbiological confined environment: the isolated water moleculesconfined in CNTs at ambient temperature can self-adjust into regular oscillation with remarkablylower entropy from random thermal motion with higher entropy. The characteristic frequency is up totens of gigahertz. The tuning between the random motion and regular oscillation is dominated by theself-adjusted dipole orientation of the H-bond donor water molecule at one end of the water chain,which has significant lower energetic cost of reorientation than other water molecules. Meanwhile,radii and chiral of CNTs have important effect on the stability of water oscillation. The ingenious intermittent oscillating mechanism is stable enough at room temperature according to Boltzemanndistribution law and robust against variations in simulation parameters and boundary conditions of themodel system, thus, holding prospective realistic significance for wide applications in emergingNEMS driven by environmental energy.