Multi-walled carbon nanotubes (MWCNTs) were grown on a stainless-steel foil by thermal chemical vapor deposition (CVD) process. The MWCNTs were functionalized with carboxylic groups (COOH) on their surfaces by using oxidation and acid (3:1 H2SO4/HNO3) treatments for improving the solubility property of them in the solvent. The functionalized MWCNTs (f-MWCNTs) were conducted to prepare the solution by continuous stir in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), dimethyl sulfoxide (DMSO), ethylene glycol (EG) and Triton X-100. The solution was deposited onto a bendable substrate such as polyethylene terephthalate (PET) with a fabricated silver interdigitated electrode for application in a room-temperature gas sensor. A homemade-doctor blade coater, an UNO R3 Arduino board and a L298N motor driver are presented as a suitable system for screen printing the solution onto the gas-sensing substrates. The different contents of f-MWCNTs embedded in PEDOT:PSS were compared in the gas response to ammonia (NH3), ethanol (C2H5OH), benzene (C6H6), and acetone (C3H6O) vapors. The results demonstrate that the 3.0% v/v of f-MWCNT solution dissolved in 87.8% v/v of PEDOT:PSS, 5.4% v/v of DMSO, 3.6% v/v of EG and 0.2% v/v of Triton X-100 shows the highest response to 80 ppm NH3. Finally, the reduction in the NH3 response under heavy substrate-bending is also discussed.

Growth of helical carbon coils can be achieved by sputtered Inconel® 600 films on silicon (Si) substrates followed by thermal chemical vapor deposition (CVD) using acetylene as a carbon source along with the injection of sulfur hexafluoride (SF6). The coils were used to prepare electronic ink for fabrication of flexible room temperature gas sensors. The ink as a sensing film was deposited onto silver-screen printed plastic substrates using a droplet coating technique. Before dripping the sensing film, the coils were purified using oxidation and acid treatments. The purified coils were then dispersed in different solvents such as deionized water (DI water), ethanol and dimethyl sulfoxide (DMSO) for comparisons. The performance of sensors was investigated for its response to ammonia (NH3) and volatile organic compounds (VOCs) including ethanol, methanol, and dimethylformamide (DMF) in concentration of 1000 ppm at room temperature. Because the baseline resistance of sensor falls within the working range (i.e. kΩ), the coils dispersed in DI water are performed to show the highest selectivity and sensitivity to NH3. The sensing mechanism of helically coiled carbon gas sensors has been also discussed based on the reducing reaction process between NH3 and chemisorbed oxygen on the surface of purified carbon coils.