Moisture enhances ammonia sensitivity of carbon nanotubes

(Nanowerk Spotlight) In spite of the numerous research efforts regarding the development of miniaturized, low-cost, and highly sensitive sensors based on different organic and semiconducting materials, carbon nanotubes still remain the most promising ones.
A team of researchers from Italy, France and Russia has now developed a simple way for fabrication and operation of carbon nanotube-based chemical sensors.
The sensor consists of carboxylated single-walled carbon nanotubes, which were spin-coated over the polymer substrate between sputtered metal electrodes. Exposure tests were performed in a wide range of ammonia gas concentrations from 20 ppb to 30 ppm in the atmosphere with controlled humidity up to 80%.
The researchers observed that the sensor response to ammonia is highly sensitive to humidity. When humidity increases, the sensor has higher response to ammonia vapors.
This finding is totally different from well known results for organic thin-films as far as humidity decreases the sensitivity to ammonia, and such sensors demands on complicated compensation schemes.
The research team reported their findings in Nanotechnology ("Humidity-enhanced sub-ppm sensitivity to ammonia of covalently functionalized single-wall carbon nanotube bundle layers").
Chemo-resistive sensor based on carbon nanotubes and the electric scheme of gas registration
Chemo-resistive sensor based on carbon nanotubes and the electric scheme of gas registration. (© IOP)
Previously, the increase of the binding energy between carbon nanotubes and ammonia molecules in the presence of water molecules was predicted by calculations. Thus, the fabricated nanotube sensors behave similar to animal nose receptors which is always covered by moister and provides better binding of odors. Also, sensor construction does not require any heating elements and recovery occurs at normal conditions for a relatively fast time (about 10 seconds).
All that makes the suggested method very promising for using chemo-resistive sensors based on carbon nanotubes in medical applications and for analyzing human health by breath analysis. Since the relative humidity of the exhaled air is close to 100% and the content of detectable substances is in the sub-ppm range, the ideal sensor must be able to operate in highly humid conditions.
This study makes a significant progress in the area of a new direction of biomedicine in predicting and diagnosing of a person's health based on the analysis of volatile organic components in the air.
In particular, the analysis of the sub-ppm amount of ammonia molecules makes it possible to use a noninvasive method to detect lung and kidney diseases through human breath. The main criteria in this direction are: simplicity, manufacturability and low fabrication costs.
A distinctive feature is that the functionalized nanotubes exhibit increased sensitivity to ammonia under conditions of high humidity. Therefore, in comparison with analogues of sensors based on organic films, there is no need to use more complicated compensated schemes to separate the contribution of humidity to conductivity.
Source: A Nanowerk exclusive provided by National Research University of Electronic Technology

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