(228b) Electrochemically Functionalized Single-Walled Carbon Nanotubes Based Gas Sensors

Semiconducting
single-walled carbon nanotube (SWNT) and conducting polymers nanowires are
promising 1-D nanomaterials for chemical and biological sensor applications.
With its unique electrical, thermal and mechanic properties, SWNT has shown
promise for superior sensitivity compared to conventional bulk sensing
materials in a transistor configuration for detection of NH₃ and NO₂.
Conducting polymers such as polyaniline and polypyrrole, with their tunable
electoral properties, high surface area and unique redox properties, have also
made themselves very attractive in various sensing devices. Limitations,
however, prevent both from replacing conventional sensors in practical
applications: the presence of metallic and semiconducting SWNT mixture display
less than ideal sensitivity due to the insensitivity of metallic SWNT to
analytes, while conducting polymers suffer from low mechanical strength and
thermal instability. Combining the two by functionalizing SWNT with appropriate
conducting polymers creates a synergistic combination with improved mechanical
stability, efficient electrical conduction and large surface/interface area for
better sensitivity and broader selectivity.

In this study, we demonstrate
a facile electrochemical method to functionalize SWNT with polyaniline
(SWNT-PANI). This simple and efficient fabrication method enables targeted
functionalization to allow for creation of high-density individually
addressable nanosensor arrays. Nanosensors made by SWNT-PANI networked across
interdigitated electrodes were tested for on-line monitoring of ammonia gas.
The SWNT-PANI sensors exhibited improved sensitivity, detection limit, recovery
time, and reproducibility over intrinsic SWNT sensors. SWNT-PANI based sensor
displayed a relative electrical resistance change DR/R of 2.24% per ppm_v
NH₃, a detection limit as low as 50 ppb_v, and excellent
reproducibility when repeatedly exposed to 10ppm_v NH₃. 
Response time was on the order of minutes and recovery time from less than 1
hour to around 4 hours corresponding to various NH₃
concentrations. Compared to purified SWNT based sensor, the SWNT-PANI sensor
exhibited several to tens times higher sensitivity upon exposure to NH₃.
Recent results including sensor arrays as well as sensing of other gaseous
compounds will be presented and discussed at the conference.