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Posted on September 10, 2009 by Teresa Henry

Carbon nanotube sensors detect minute levels of oxygen

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A team led by chemistry professors Alexander Star and St├ęphane Petoud at University of Pittsburgh revealed the development of a highly sensitive, fluorescent oxygen sensor that can detect minute amounts of Oxygen.
 
Oxygen (O2) detectors are important safety devices in confined spaces where people work or travel such as in mines, pressure vessels and aircraft. They are also used to detect oxygen in some chemical manufacturing processes.
 
The recently developed sensor that consists of carbon nanotubes coated with a luminescent compound incorporating europium, a reactive metal found in fluorescent bulbs, television/computer screens, and lasers, among other applications is able to gauge minute amounts of oxygen by measuring the intensity of its glow when exposed to ultraviolet light and the tubes' change in electrical conductance.
 
The oxygen sensor combines small-scale carbon nanotubes, which are one-atom thick rolls of graphite 100,000 times smaller than a human hair, with the reactivity of the europium compound coating. This produces a platform for low-cost, room-temperature detectors that are particularly sensitive to oxygen but less complicated than existing sensors.

Oxygen sensors in confined spaces

Data gathered in the late seventies and early eighties indicated that 65% of all those who died in confined spaces were unaware that the space they were entering was a potential hazard. Over 50% of confined space deaths occur to the rescuers and over one third of the fatalities occurred after the space was tested and declared safe and the gas detector was removed. The new oxygen sensor would be small enough to incorporate into a portable or wearable device which would notify workers or rescuers any change in oxygen levels.
 
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Commercial oxygen sensors used today

Currently, commercial O2 gas sensors include high and low temperature units. High temperature units are typically used for automobile engines, and are composed of semiconducting metal-oxide materials where a change in the O2 concentration will produce a change in the sensor current or voltage. The most common low temperature O2 sensors rely on optical methods such as infrared (IR) transmittance of a material or the luminescence of an oxygen sensitive molecule.

Problems with oxygen sensors

The pitfalls of high temperature O2 sensors are obvious, in the sense that they require high operating temperatures. While they are quite robust and have long operation lifetimes, the requirement for high temperatures makes it difficult to incorporate this technology into a portable or wearable sensor. The problems with low-temperature optical-based sensors are that they often require expensive measurement equipment such as light sources and photo-detectors. This technology is also difficult to miniaturize into a wearable sensor platform due to the required instrumentation.

How can commercial sensors used today be improved

Professor Alexander Star explained to Printed Electronics World how this new technology improves upon the commercial sensors used today because it relies on the electrical conductance of carbon nanotubes (CNTs) decorated with a polymeric material to operate at room temperature without the need for complicated optical equipment. He said, "For example, after brief illumination with a UV light source, which can be quite compact and inexpensive (i.e. an LED), the electrical conductance of the CNT-based sensor devices changes in proportion to the concentration of O2 gas. An added benefit of this approach is that the luminescence of the polymeric material is also sensitive to O2, so one could design a measurement technique that simultaneously monitored the electrical and optical signal of the sensor - providing bimodality to the device and increasing the accuracy of the measurement."
 
Initially the researchers were looking at detecting environmentally relevant concentrations of O2 for personal safety, which would correspond to levels commonly found in air (~22%) and lower. However, although the tubes demonstrated sensitivity to oxygen concentrations as low as 5 percent, the scientists calculated that the detector can indicate a level as low as 0.4 percent, which is unaffected by other atmospheric gases, such as carbon dioxide and nitrogen.

Dectecting low levels of oxygen

There are many applications where low temperature measurement of low O2 levels is important, such as in streams of natural gas or during some manufacturing processes where combinations of liquid, vapour and gas are used - if a small amount of oxygen is found the possibility of an explosion increases. Star adds, "The fact that our un-optimized laboratory prototypes could detect such low concentrations of O2 holds promise for this technology."
 
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The ability to operate in a wide range of O2 concentrations, i.e. well below 1% to over 20%, will provide more opportunity for adoption of this technology.
 
According to Star, "There are not many researchers working on solid-state sensor platforms that can operate at room temperature and ambient pressure. While some researchers have observed O2 sensitivity with CNTs, they typically required the aid of vacuum instrumentation."