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Interfacial Nanofiber Composites for Trace Alkane Detection

Written by USTAR professor Ling Zang

Detecting alkane vapors is a major concern for a variety of industries. For example, it would improve the detection of leaks in oil wells and pipelines. Alkane sensors would also enable the detection of the improvised explosive ANFO (ammonium nitrate/fuel oil). Ammonium nitrate is easy to detect, but there are too many similar compounds for it to be a reliable signature of the explosive on its own. Detecting both the ammonium nitrate and fuel oil (alkane) components is critical to detect these improvised explosives without false positives.  However, detecting alkanes is challenging because of their chemical inertness. This inertness makes detection difficult for low cost sensors, such as chemiresistors, because they generally rely on charge transfer between the chemical and the sensor material. Alkanes do not provide charge transfer.

Although they do not interact electronically, alkanes do physically interact with sensor materials through adsorption. We exploited this effect to produce sensitive, selective sensors. Two material systems were investigated. In both cases, an interface was designed to facilitate alkane adsorption and control quantum tunneling. Additionally, both feature one-dimensional nanostructures, which are ideal for sensors because of their large surface area to volume ratio and the porosity of the films they comprise.

First, carbon nanotubes were coated with a designer molecule. In a network structure, the electrically insulating molecule creates a tunnel barrier at the interface between adjacent carbon nanotubes. When an alkane is absorbed, the molecule swells, which increases the thickness of the tunnel barrier. We applied a voltage to the materials to generate a current. Swelling at the interface caused a significant decrease in current. Proof-of-concept of this new sensor mode was published in Advanced Materials and ACS Applied Materials and Interfaces. A press release highlighting these results was AZO Nano’s most read news article of 2014.

Our other approach uses the same swelling effect at a donor-acceptor interface. The interface is comprised of electron donating molecules coated onto an electron accepting nanofiber. Neither material is conductive on its own. However, under illumination, the nanofiber becomes excited and accepts electrons from the molecules. With the extra charges, the nanofiber becomes conductive. Charge transfer is interrupted when an alkane is adsorbed, which decreases conductivity. Swelling at the interface causes this disruption, which increases the distance between the donor and acceptor molecule (effectively, this is tunneling through free space with the thickness of the tunnel barrier increasing as before). Interestingly, by monitoring the rate of recovery once the alkane vapor is removed, we can determine exactly which alkane species was present. Restated, using a single material, we can distinguish between hexane and octane while simultaneously measuring the concentration. This work is currently being prepared for publication.

Both of these sensor materials are compatible with the system being developed by Vaporsens, a start-up company that is a product of the University of Utah and USTAR. Through continued collaboration and development, we aim to take these materials from the lab to the marketplace.

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