The thermal boundary conductance of Al/SiO2, Al/Si, Au/SiO2, and Au/Si are measured by a femtosecond laser transient thermoreflectance technique. The distinct differences of the interfacial thermal conductance between these samples are observed. For the same metal film, the thermal boundary conductance between metal and substrate decreases with the thermal conductivity of the substrate. The measured results are explained with the phonon diffusion mismatch model by introducing a phonon transmission coefficient across the interface.
The surface and adhesion forces between chitosan- coated mica surfaces in an acetic acid buffer solution were measured using a surface force apparatus （SFA）. The force- distance profiles were obtained under different pressure conditions. It was found that the chitosan was adsorbed on the mica surface and formed a stable nanofilm under acid conditions. The adsorbed chitosan nanofilms induced a short- range monotonically steric force when two such surfaces came close in the acid buffer. The adhesion forces between the two chitosan-coated mica surfaces varied with the loads. Strong adhesion between the two chitosan-coated mica surfaces was observed at high pressure. Such pressure-dependent adhesion properties are most likely related to the molecular configurations and hydrogen bonds reordering under high confinement.
The forces between two molecularly smooth mica surfaces are measured in monovalent and divalent cations electrolyte solutions by a surface force apparatus （SFA）. The properties of K＋, Na＋, and Mg2＋ between molecularly smooth mica surfaces are investigated. The Derjagui-Landau- Verwey-Overbeek （DLVO） force and the hydration force are detected in the experiment. The results show that in lower concentrations of a monovalent electrolyte solution （about 10-4 mol/L）, the force curves are completely in good agreement with those computed by the DLVO theory. However, additional short-range repulsive forces which deviate from the DLVO theory are observed when the concentrations of cations are above the critical bulk concentration, which is different for each electrolyte. The results show the properties of these cations on both the screening effect adsorbed on the mica surface and the hydration in solution. From the results, the interaction energy between two hydrated ions of potassium or sodium can also be estimated.
The nanopore size effect on translocation of poly(dT)30through Si3N4 membrane is investigated.In this paper,we report that the speed of the poly(dT)30 transport through Si3N4 nanopores can be slowed down by half through increasing the nanopore diameter from 4.8 nm to 10.8 nm.The results are consistent with our simulation results.Besides,the current blockage induced by DNA passing through the nanopore is less obvious as pore diameter is larger,which is in good agreement with the theoretical prediction.The conclusion about DNA transport through nanopores is beneficial for the design of DNA sequencing devices.
Ion specificity of Na＋ and C1- ions for NaCI solution confined in silicon nanochannels is investigated with molecular dynamics （MD） simulations. The MD simulation results demonstrate that ion specificity for Na＋ and C1- ions exhibits clearly in na- nochannels with high surface charge density. The two types of ions show different density distributions perpendicular to the channel surface due to the ion specificity when they act as countefions near negatively and positively charged surfaces, respec- tively. Both the two counterion distributions cannot be predicted by Poisson-Boltzmann equation within 0.75 nm near the sur- face. In addition, the ion specificity is also demonstrated through affecting the water density distributions. In the nanochannel with negatively charged surfaces, the presence of the Na＋ ions reduces the number of water peaks in water density distribution profile. In comparison, when the C1- ions act as counterions near positively charged surfaces, they do not affect the number of the water peaks. Besides the influence on the water density distribution, ion specificity also exhibits through affecting the wa- ter molecule orientation in the adsorbed layer. It is found that C1- ions make the water molecules in the adsorbed layer align more orderly than Na~ ions do when the two types of ions act as the counterions near the positively and negatively charged surfaces with the same surface charge density.
Nanopores are emerging sensitive sensors that can detect and analyze single charged molecule.Nanopores present a promising approach for sequencing human genome below US$1,000 because of its superior performance,such as high throughput and low cost.However,a dominant bottleneck,that is,the high translocation speed of DNA molecules,has to be overcome.This property decreases accuracy of nanopore sensors to the single-base level.In this review,we mainly introduce the recent research works of retarding and manipulating of DNA motion through nanopores by actively control of three forces,which are the driving force,interaction force between nanopore and molecule,and exterior drag force.Lastly,conclusion and further outlook are presented on future directions of nanopore-based DNA sequencing technology.
DNA sequencing based on nanopore sensors is a promising tool for third-generation sequencing technology because of its special properties,such as revolutionized speed and low cost.With about two decades of nanopore technology development,the pioneering work has demonstrated the ability of nanopores to perform single-molecule detection and DNA sequencing.However,the microscopic mechanisms of DNA transport dynamics through nanopores remain largely unknown.Currently,DNA microscopic transport in a nanopore is difficult to characterize and several unexpected experimental observations are equivocal.This limitation can be resolved using theoretical calculations and simulations.These computational methods can monitor the entire dynamic process that DNA undergoes in solution at a single-atom resolution that can accurately unveil the mystery of DNA transport dynamics and predict certain unexpected phenomena.This paper mainly reports the recent applications of computational and simulation methods applied to the study of DNA transport through both biological and synthetic nanopores.We hope the theoretical calculations and simulations of DNA transport through nanopores can benefit the design of DNA sequencing devices.
Porous graphene has a high mechanical strength and an atomic-layer thickness that makes it a promising material for material separation and biomolecule sensing. Electrostatic interactions between charges in aqueous solutions are a type of strong long-range interaction that may greatly influence fluid transport through nanopores. In this study, molecular dynamic simulations were conducted to investigate ion and water transport through 1.05-nm diameter monolayer graphene nanopores, with their edges charge-modified. Our results indicated that these nanopores are selective to counterions when they are charged. As the charge amount increases, the total ionic currents show an increase-decrease profile while the coion currents monotonically decrease. The co-ion rejection can reach 76.5% and 90.2% when the nanopores are negatively and positively charged, respectively. The Cl-ion current increases and reaches a plateau, and the Na＋current decreases as the charge amount increases in systems in which Na＋ions act as counterions. In addition, charge modification can enhance water transport through nanopores. This is mainly due to the ion selectivity of the nanopores. Notably, positive charges on the pore edges facilitate water transport much more strongly than negative charges.
