Yanbo Xie

Electric energy generation in single track-etched nanopores

We investigated the efficiency of electrical power generation in single track-etched nanopores by measuring the streaming currents and conductance. Experimental results indicate that both the efficiency and output power depend on the electrolyte concentration and the dimension of the pore. The highest efficiency of 5% was obtained in nanopores with small radii of 31 nm. The surface property of the track-etched nanopores was found very important to the kinetic-electric behaviors in the pores, especially when the electrolyte concentration was low.

Surface charge density of the track-etched nanopores in polyethylene terephthalate foils

Surface charge is one of the most important properties of nanopores, which determines the nanopore performance in many practical applications. We report the surface charge densities of track-etched nanopores, which were obtained by measuring the streaming current and pore conductance, respectively. Experimental results reveal that surface charge densities depend significantly on the salt concentrations. In addition the values obtained with the pore conductance were always several times higher than those calculated with the streaming current, and the gel-like surface layer on the nanopore was considered to be responsible for this discrepancy.

Surface modification of single track-etched nanopores with surfactant CTAB.

In this letter, we report a method to modify the surface charge property of single track-etched nanopores with a cationic surfactant (CTAB). The dependence of surface charge density on the surfactant concentration was investigated by measuring the streaming current when the nanopore was immersed in 0.01 M KCl solution with different CTAB concentrations. The results showed that, when the concentration of CTAB was increased gradually, the surface charge of the nanopore was inverted from negative to positive. Our calculation indicated that the surface charge density changed from -9 to 8 mC/m2. We utilized this method to modify the surface property of single conical track-etched nanopores. Its current rectification properties (both the direction and magnitude) were successfully tuned by adjusting the CTAB concentration in the solutions.

Molecular dynamics simulations on the ionic current through charged nanopores

Molecular dynamics (MD) simulation was performed to investigate the ionic current through charged nanopores, and the results were compared with the calculation of Poisson-Nernst-Planck (PNP) equations based on the continuum theory. Results show that the current obtained by MD simulation is lower than the current calculated by PNP equations, and the discrepancy depends on the surface charge density of the nanopores. Also, MD simulation shows that the contribution of the electro-osmotic flow effect on ionic current could be 10% higher than the results obtained by solving PNP equations. Since the PNP equations do not take the effect of the pore wall into consideration, we suggest that adjusting the diffusion coefficient in the PNP equations can obtain more accurate results when calculating the ionic current through charged nanopores.

Non-linear streaming conductance in a single nanopore by addition of surfactants

Electrokinetic phenomena in nanofluidic system result in many unique properties and applications due to overlap of electrical double layers. As well known, the streaming current is linear proportional to the pressure gradient of a channel. However, in this Letter, we report a non-linear streaming conductance phenomenon in a single nanopore by addition of ionic surfactants in KCl aqueous solution. The streaming conductance decreases at about 0.8 bar applied pressure in high concentrated surfactant solution that was attributed to the slip of surfactant layer, and the slip length was theoretically estimated.

A capacitive-pulse model for nanoparticle sensing by single conical nanochannels

Nanochannel based devices have been widely used for single-molecule detection. The detection usually relies on the resistive-pulse model, where the change of the monitored current depends on the physical volumetric blocking of the nanochannel by the analyte. However, this mechanism requires that the nanochannel diameter should not be much larger than the analyte size, because, otherwise, the resultant current change would be too small to detect, and therefore poses particular challenges for the fabrication of nanochannels. To circumvent this issue, in this report, we propose a different mechanism of capacitive-pulse model, where the transport signals can be significantly magnified by the capacitive effect of the nanochannel. We experimentally demonstrate that current pulses with an averaged peak height of 0.87 nA can be achieved for transporting 60 nm nanoparticles through a conical nanochannel device, whereas the traditional resistive-pulse model only predicts one-order-of-magnitude lowered value. With further comprehensive simulation, the dependence of this effect on the nanochannel geometry as well as the surface charge density for both the nanochannel and the analyte is predicted, which would provide important guidance for better designing of the nanochannel-based sensors.

Nanofluidic diode generated by pH gradient inside track-etched conical nanopore

A nanofluidic diode is produced with a track-etched single conical nanopore embedded in a Polyethylene terephthalate (PET) film. It has a very strong current rectification behavior which depends on the surface charge densities and the solution salt concentration. We tried to control the current rectification properties by using asymmetric pH solutions on the two side of nanopore membrane, because that this arrangement could adjust the surface charge density distribution inside the nanopore since it depends on the local pH value. This nanofluidic diode is analogous to a bipolar semiconductor diode which is sensitive to the solution pH values.