Songmei Wu

Facile fabrication of nanofluidic diode membranes using anodic aluminium oxide

Active control of ion transport plays important roles in chemical and biological analytical processes. Nanofluidic systems hold the promise for such control through electrostatic interaction between ions and channel surfaces. Most existing experiments rely on planar geometry where the nanochannels are generally very long and shallow with large aspect ratios. Based on this configuration the concepts of nanofluidic gating and rectification have been successfully demonstrated. However, device minimization and throughput scaling remain significant challenges. We report here an innovative and facile realization of hetero-structured Al(2)O(3)/SiO(2) (Si) nanopore array membranes by using pattern transfer of self-organized nanopore structures of anodic aluminum oxide (AAO). Thanks to the opposite surface charge states of Al(2)O(3) (positive) and SiO(2) (negative), the membrane exhibits clear rectification of ion current in electrolyte solutions with very low aspect ratios compared to previous approaches. Our hetero-structured nanopore arrays provide a valuable platform for high throughput applications such as molecular separation, chemical processors and energy conversion.

Nafion/chitosan-wrapped CNT nanocomposite membrane for high-performance direct methanol fuel cells

Here we show that the transport properties and electrochemical performance of polyelectrolyte membranes are improved through the dispersion of chitosan-wrapped carbon nanotubes, for direct methanol fuel cell applications. Methanol permeability is reduced via improving the interfacial interactions and the solubilization of CNTs in the Nafion matrix, as well as inducing the formation of long-range oriented conduction pathways in the vicinity of the decorated one-dimensional nanostructure. The improved membrane selectivity results in a considerably enhanced fuel cell efficiency (16% vs. 11%) and a power generation capacity more than two times higher (110 mW cm−2vs. 47 mW cm−2) in a concentrated methanol solution (5 M), in comparison with the commercial Nafion®117 membrane.

Field effect modulated nanofluidic diode membrane based on Al2O3/W heterogeneous nanopore arrays

We developed Al2O3/W heterogeneous nanopore arrays for field effect modulated nanofluidic diodes. They are fabricated by transferring self-organized nanopores of anodic aluminium oxide into a W thin film. The nanopores are ∼20 nm in diameter and 400 nm in length. After mild oxidation, approximately 10 nm WO3 grows on the surface of W, forming a conformal and dense dielectric layer. It allows the application of an electrical field through the surrounding W electrode to modulate the ionic transport across the entire membrane. Our experimental findings have potential applications in high throughput controlled delivery and electrostatic sorting of biomolecules.

Ultrathin alumina membranes as scaffold for epithelial cell culture from the intestine of rainbow trout

Permeable membranes are indispensable for in vitro epithelial barrier models. However, currently available polymer-based membranes are low in porosity and relatively thick, resulting in a limited permeability and unrealistic culture conditions. In this study, we developed an ultrathin, nanoporous alumina membrane as novel cell culture interface for vertebrate cells, with focus on the rainbow trout (Onchorynchus mykiss) intestinal cell line RTgutGC. The new type of membrane is framed in a silicon chip for physical support and has a thickness of only 1 μm, with a porosity of 15% and homogeneous nanopores (Ø = 73 ± 21 nm). Permeability rates for small molecules, namely lucifer yellow, dextran 40, and bovine serum albumin, exceeded those of standard polyethylene terephthalate (PET) membranes by up to 27 fold. With the final goal to establish a representative model of the fish intestine for environmental toxicology, we engineered a simple culture setup, capable of testing the cellular response toward chemical exposure. Herein, cells were cultured in a monolayer on the alumina membranes and formed a polarized epithelium with apical expression of the tight junction protein ZO-1 within 14 days. Impedance spectroscopy, a noninvasive and real time electrical measurement, was used to determine cellular resistance during epithelial layer formation and chemical exposure to evaluate barrier functionality. Resistance values during epithelial development revealed different stages of epithelial maturity and were comparable with the in vivo situation. During chemical exposure, cellular resistance changed immediately when barrier tightness or cell viability was affected. Thus, our study demonstrates nanoporous alumina membranes as promising novel interface for alternative in vitro approaches, capable of allowing cell culture in a physiologically realistic manner and enabling high quality microscopy and sensitive measurement of cellular resistance.

Al 2 O 3 /W hetero-structured nanopore membranes: From native to tunable nanofluidic diodes

We present here Al2O3/W hetero-structured nanopore membranes which function as native and electrical field tunable nanofluidic diodes. A typical membrane is 100×100 μm2 in size with pore density of ~20/μm2. The nanopores are 26 nm in diameter and 400 nm in length. Owing to the opposite surface charge states of Al2O3 (positive) and W (negative with native oxide), the membrane exhibits clear rectification of ion current in electrolyte solutions. After thermal heating at 350°C for 2 hrs, approximately 10 nm WOx grows on the surface of W, forming a conformal and dense dielectric layer. The W layer allows the application of an electrical field to further modulate the ionic transport through the nanopores with low gate potentials and ultra low gate leakage current. We have demonstrated the control of rectifying factor from 2 to 11. Our experimental findings have a valuable potential for controllable high throughput molecular separation and chemical processors.

High aspect ratio etching of nanopores in PECVD SiC through AAO mask

We present in this work the fabrication of high aspect ratio nanopores in 500 nm PECVD SiC films through AAO (anodic aluminum oxide) mask. The initial AAO thin film is 180 nm thick and the diameter of nanopores is 33 ± 7 nm. We have used three plasma chemistries: CF₄, Cl₂ /Ar, and SF₆/O₂ to study the pattern transfer process into SiC at sub-50 nm scale by deep reactive ion etching. CF₄ and Cl₂/Ar etchings show highly anisotropic features. Vertical pores with similar diameter as the AAO mask (33 ± 12 nm) and as deep as 400 nm (aspect ratio > 10) can be achieved by CF₄ reactant. As comparison, SF₆ /O₂ chemistry generates very different etching profiles, causing trenches both in vertical and lateral directions. Our PECVD SiC nanopores are promising candidates for robust biosensing and nanofiltration applications.

Solid-state nanopore array membranes patterned by electron beam lithography, nanosphere lithography and aluminum anodization

We present the fabrication of thin membranes with dense arrays of nanometer and submicrometer pore arrays by the integration of standard micromachining with three pore patterning techniques: electron beam lithography (EBL), nanosphere lithography (NSL) and aluminum anodization. Using a serial top-down EBL technique we exploit a fine size, positioning and flexibility of this tool. NSL and aluminum anodization, as self-organized bottom-up processes, guaranties cost efficiency and throughput. In our work, we have fabricated silicon nitride (SiN) and alumina (Al(2)O(0)3) membranes with a thickness down to 100 nm, side length ranging from 200 mu m up to 2.4 mm and pore size ranging from 20 nm to 500 nm.