Detection of VOCs and Nitrogen Containing Gaseous Molecules By Utilizing Carbon Nanotubes (CNTs) As Sensing Materials
Authors: Zhao Dong , Zhao Bingyuan , Koltsov Dmitri , Chen Shutang , Chen Gugang 
Published: November 2022

Carbon nanomaterials are increasingly attractive as potential candidates to make new generation of sensors due to their unique nanostructures that grant their promising electrical, chemical, and physical properties. Among the group of carbon materials, carbon nanotube (CNT) is one of the most encouraging materials because of its features of high surface-to-volume ratio and unique electronic structure. These features enable CNTs the potential to become a highly sensitive sensing material. Since our project aims on detecting bio-marks from human breath, of which the concentrations are extremely low, the nature of our sensor R&D becomes extremely challenging. Consequently, using CNTs as the sensing materials ought to be our obvious choice at this stage. This choice of carbonous materials as sensing media is also on the hope to simplify the sensor’s instability problems in our R&D effort because carbon itself is very chemically inert toward many chemicals.
This presentation will serve as a report of the preliminary results from a lab experiment setting to detect several human breath related bio-marks. Sensors were chemiresistive typed and constructed through drop casting on interdigitated sensor circuit. Each sensor chip contains 8 sensor pixels, and the test bed can host maximum of 16 sensor chips simultaneously. In other words, our sensor testing chamber can embed up to 128 sensors at the same time. We then performed the sensor testing against several gases. As expected, the application of nanotechnology in using CNTs enabled us to approach high sensitivity towards to several gaseous analytes, ranging from sub-ppb to sub-ppm. Noticed that, the previous study from our lab revealed that the sensitivity of sensors could be promoted by illuminations of UV light.1 It was approved that the detection limit of nitric oxides is about 27 ppm, providing reliable and stable sensitivity. To simplify the sensor fabrication and miniaturization as well as to reduce power consumption of final sensor units, in this work, we employed an external thermal power to improve the reversibility of the sensors. Two external 375W IR bulbs were used as the heating source in this setting. A dimmer and temperature control circuitry were integrated to maintain the intended operating temperatures. Moreover, we employed our test protocols and methods by functionalizing the CNTs with carboxylic group, besides utilizing the pristine CNTs. It was resulted that this sensor array was able to detect various gaseous species, including NH3, Isoprene, acetone, etc., with relatively high sensitivity.
The existence of surfactants in the CNT sensing layer lowered the conductivity of sensor pixels by a great magnitude and resulted in much reduced sensors’ sensitivities. Therefore, removing surfactants in the CNT solution was made, which dramatically improved the sensors’ electric conductivities and boosted sensors’ sensitivities. However, CNT solutions with diluted surfactants destabilize the CNTs’ aqueous suspension, and lead to the non-uniform CNTs layers. The sensor pixels fabricated by using this surfactant deficient CNTs resulted in the formation of CNT bundles or clusters. The gathering of CNTs in a non-uniform fashion could dramatically reduce the sensor’s sensitivity because the bundles would short the interdigitated circuit and disable the CNTs’ sensing capability in most other area of the sensor film. We, therefore, increased the amount of surfactant in the CNT solutions. The sensors fabricated with excess amount of surfactant exhibited highly electric resistance or even non-conductance with very low or no sensitivity. A simple washing process was then developed to wash out the surfactant, which partially resolved the non-uniformity problem. The method to completely prevent the CNT bundles from formation in sensor film is in progress.
In conclusion, we developed a gas sensor array that can detect various VOCs and certain nitrogen containing gas molecules with an extremely high or reasonably high sensitivities. Through applying pristine, modified CNTs or mixtures of both into sensor fabrication, the sensing properties were enhanced under an external heating source in comparison with illumination of UV light. The best sensitivity of the sensors is achieved by removing the surfactants in the sensing films. The application of external thermal energy to help on sensors’ performance gets approved. The benefit of using thermal energy vs. UV light is also discussed.

G. Chen, T. M. Paronyan, E. M. Pigos, and A. R. Harutyunyan, Scientific Reports 2, 343 (2012).