Lei Liu

Controllable Nanotribological Properties of Graphene Nanosheets

Graphene as one type of well-known solid lubricants possesses different nanotribological properties, due to the varied surface and structural characteristics caused by different preparation methods or post-processes. Graphene nanosheets with controllable surface wettability and structural defects were achieved by plasma treatment and thermal reduction. The nanotribological properties of graphene nanosheets were investigated using the calibrated atomic force microscopy. The friction force increases faster and faster with plasma treatment time, which results from the increase of surface wettability and the introduction of structural defects. Short-time plasma treatment increasing friction force is due to the enhancement of surface hydrophilicity. Longer-time plasma treatment increasing friction force can attribute to the combined effects of the enhanced surface hydrophilicity and the generated structural defects. The structural defects as a single factor also increase the friction force when the surface properties are unified by thermal reduction. The surface wettability and the nanotribological properties of plasma-treated graphene nanosheets can recover to its initial level over time. An improved spring model was proposed to elaborate the effects of surface wettability and structural defects on nanotribological properties at the atomic-scale.

Voltage-driven translocation behaviors of IgG molecule through nanopore arrays

Nanopore-based biosensing has attracted more and more interests in the past years, which is also regarded as an emerging field with major impact on bio-analysis and fundamental understanding of nanoscale interactions down to single-molecule level. In this work, the voltage-driven translocation properties of goat antibody to human immunoglobulin G (IgG) are investigated using nanopore arrays in polycarbonate membranes. Obviously, the background ionic currents are modulated by IgG molecules for their physical place-holding effect. However, the detected ionic currents do ‘not’ continuously decrease as conceived; the currents first decrease, then increase, and finally stabilize with increasing IgG concentration. To understand this phenomenon, a simplified model is suggested, and the calculated results contribute to the understanding of the abnormal phenomenon in the actual ionic current changing tendency.

Detecting a single molecule using a micropore-nanopore hybrid chip

Nanopore-based DNA sequencing and biomolecule sensing have attracted more and more attention. In this work, novel sensing devices were built on the basis of the chips containing nanopore arrays in polycarbonate (PC) membranes and micropores in Si3N4 films. Using the integrated chips, the transmembrane ionic current induced by biomolecules translocation was recorded and analyzed, which suggested that the detected current did not change linearly as commonly expected with increasing biomolecule concentration. On the other hand, detailed translocation information (such as translocation gesture) was also extracted from the discrete current blockages in basic current curves. These results indicated that the nanofluidic device based on the chips integrated by micropores and nanopores possessed comparative potentials in biomolecule sensing.

Detecting DNA Using a Single Graphene Pore by Molecular Dynamics Simulations

DNA charged negatively could be transported through a solid nanopore by the force of an electrical field. Recently, the nice properties of graphene attract a lot of researchers. In this paper, A single graphene membrane was punched to form a nanopore and a ds-DNA was driven to pass through the pore by all-atom molecular dynamics simulations. The single graphene membrane was demonstrated useful in DNA sequencing. It suggested that the velocity of DNA translocating through a single graphene pore could be controlled by adjusting the appropriate voltage and the diameter of the nanopore.

Thermally modulated biomolecule transport through nanoconfined channels.

In this work, a nanofluidic device containing both a feed cell and a permeation cell linked by nanopore arrays has been fabricated, which is employed to investigate thermally controlled biomolecular transporting properties through confined nanochannels. The ionic currents modulated by the translocations of goat antibody to human immunoglobulin G (IgG) or bovine serum albumin (BSA) are recorded and analyzed. The results suggest that the modulation effect decreases with the electrolyte concentration increasing, while the effects generated by IgG translocation are more significant than that generated by BSA translocation. More importantly, there is a maximum decreasing value in each modulated current curve with biomolecule concentration increasing for thermally induced intermolecular collision. Furthermore, the turning point for the maximum shifts to lower biomolecule concentrations with the system temperature rising (from 4°C to 45°C), and it is mainly determined by the temperature in the feed cell if the temperature difference exists in the two separated cells. These findings are expected to be valuable for the future design of novel sensing device based on nanopore and/or nanopore arrays.

Water and ion distributions in a silicon nanochannel: a molecular dynamics study

By way of molecular dynamics simulation, a physically realistic nanochannel with consideration of the thermal motion of channel walls is proposed, and the distributions of water and ions in the electrical double layer region on a charged silicon surface are observed. It is found that the distribution profile of Na+ ions has double peaks corresponding to the Stern electrical double layer model. Also, the water distribution profile in the 3.0 nm nanochannel agrees very well with that observed in a previous experiment (Cheng et al. Phys. Rev. Lett. 2001; 87: 156103). In addition, the height of nanochannel has negligible effect on the thicknesses of the water layers, but affects the peak numbers of water density distributions dramatically.

Theoretical and experimental studies on ionic currents in nanopore-based biosensors.

Novel generation of analytical technology based on nanopores has provided possibilities to fabricate nanofluidic devices for low-cost DNA sequencing or rapid biosensing. In this paper, a simplified model was suggested to describe DNA molecules translocation through a nanopore, and the internal potential, ion concentration, ionic flowing speed and ionic current in nanopores with different sizes were theoretically calculated and discussed on the basis of Poisson-Boltzmann equation, Navier-Stokes equation and Nernst-Planck equation by considering several important parameters, such as the applied voltage, the thickness and the electric potential distributions in nanopores. In this way, the basic ionic currents, the modulated ionic currents and the current drops induced by translocation were obtained, and the size effects of the nanopores were carefully compared and discussed based on the calculated results and experimental data, which indicated that nanopores with a size of 10 nm or so are more advantageous to achieve high quality ionic current signals in DNA sensing.

Simulations and Experimental Studies on Biomolecules Passing through Polycarbonate Ultrafiltration Membrane

Nanopore and nanopore based biosensing and DNA sequencing have attracted more and more interests in the past ten years. In this paper, a simplified model is addressed to depict biomolecules passing through ultrafiltration membrane (containing nanopores). Based on this model, the passing velocity of biomolecules will not increase continuously but first increase, then decrease and stabilize with the IgG concentration increasing. Due to the physical place-holding effects and the simulation results, it can be predicted that, with biomolecules concentration increasing, the ionic current will first decrease, then increase and finnally stabilize. These predictions based on the simulation match our experimental results well.

Fabrication of Organic-Inorganic Hybrid Nanopore and its Application in Biosensing

Nanopore and nanopore based bio-sensing technology have become into more and more interesting research area in the past ten years. In this work, micro-pore in Si-S3N4 chips was fabricated and characterized by Focused Ion Beam (dual Beam), and then the S3N4 pore was covered by Polycarbonate (PC) membrane containing 50nm nanopores and sealed by using polydimethylsiloxane (PDMS) to get hybrid micro-nanopores. The obtained chip with hybrid nanopores together with two liquid cells was integrated into an ionic current detection device for biosensing. Based on this device, λ-DNA in the electrolytic solution can be detected when it is electrophoretically driven through the hybrid nanopores, and different gestures of λ-DNA in translocation also can be discriminated.

A Spike-Like Ionic Current Behavior via Graphene Nanopore

Ionic current characterization is critical for the application of nanopores with sub 5 nm as bio medical sensors and devices. Here, we demonstrate an eccentric ionic current behavior in graphene nanopore fabricated by high resolution transmission electron microscopy (HR-TEM). A spike-like current enhancement is shown in the absence of any bio molecule or nanoparticle in the LaCl3 and KCl solution. By tuning the hydrophobicity of the graphene surface, the spikes diminish in the current recordings acquired in graphene nanopore after 20 seconds plasma etching. We consider that the hydrophocity-induced nanobubble is present in the nanopore area, leading to the currents change as the bubbles deformation due to the voltage driven electrostatic forces on the transported ions surrounding the bubble surface.