Fabrication of electric-field aligned carbon nanotube network suspended within a microfluidic channel for chemical sensor application

Abstract

Incorporation of sensors into microfluidic channels has been the subject of recent interest in bionanotechnology applications. Of these sensors, carbon nanotube based gas and molecular adsorption sensors have been successfully integrated for the detection of specific gases and DNA fragments. The usefulness of carbon nanotubes comes from the fact that they are chemically inert, have high surface to volume ratio, show surface conduction, and high mechanical strengths. By placing a carbon nanotube network based sensors in suspension perpendicular to the flow of the microfluid, it may be possible to detect minute amounts of analyte normally undetectable with planar sensors. Here, we introduce a method to fabricate carbon nanotube network sensors suspended within the cross-sectional area of a microfluidic channel. The upper and lower layers of the glass channels ~20 μm wide are patterned using optical lithography and chemical etching. Then Cr/Au electrodes are pattered across the lower channel. When placed on top, the upper channel intersects with the lower channel only above the electrode gaps, which forms a continuous microfluidic channel. After thermal bonding of the layers, we introduce a solution which contains suspended single-wall carbon nanotubes. When the channel was filled with the nanotube suspension, 10 V, 5 MHz ac electric fields were applied to the electrodes guiding the nanotubes between the electrodes. This dielectrophoresis process forms the suspended nanotube network sensor within the channel with several kΩ resistance. The initial conductivity increases as the additional nanotubes entangle with the existing network. After network formation, a series of voltage pulses were applied to remove unstable contacts. We will report on the fluid flow dependent conductivity and chemical and ion sensing results of the suspended carbon nanotube network fluidic sensors.