Chemoresistive Response of Carbon Nanotube-Polymer Composites to VOCs
 

Samuel Anin, Catherine Ellen and Anthony Guiseppi-Elie*
Department of Chemical Engineering and Center for Bioelectronics Biosensors
and Biochips, Virginia Commonwealth University, Richmond, Virginia 23284.

Carbon nanotube-polymer composites, at and around the percolation threshold, are shown to be useful chemoresistive artificial sensory receptors in an electronic nose. A toluene suspension of high performance single-walled Carbon Nanotubes (CNT) was formulated by mixing with a host polymer to varying concentrations. The polymer, a sulfonated Kraton® (polystyrene-butadiene), was formulated at 10 different concentrations that ranged from 90 wt. % CNT in 10% polymer to 10 wt. % CNT in 90 wt. % polymer. Formulated composites were spun applied to glass cover slips and onto microlithographically fabricated interdigitated microsensor electrode (IME) devices.  CNT-polymer formulations were characterized by AC–impedance (both liquid and spun film), four-point conductivity, and optical microscopy.  The electrical impedance of each formula was determined over the range of 10 mHz – 100 kHz using a model 1250 Frequency Response Analyzer from Schlumberger.  Four-point DC conductivity was measured using a Keithley 2010 Multimeter. Optical micrographs were obtained on a Licra at x350 magnification.  Finally, CNT- polymer-coated IME chips were tested for their response to water vapor (30% RH), n-butanol (30% VP at RT) and methyl isoamyl ketone (MIAK) (30% VP at RT) in a custom built Natural Olfactory Sensor Emulation System (NOSES).  NOSES simultaneously retrieves multiplexed data from eight Volatile Organic Compound (VOC) sensors, a RH sensor and an RTD. 

Composition dependence of the AC and DC conductivity places the apparent percolation threshold of CNT-polymer at ca. 17 wt. % CNT. This suggests that CNT aggregates behave as discrete particles (and not as dispersed nanotubes) within this particular host polymer. Optical micrographs support this view, as discrete domains of black CNT are visible even at x350 magnification and down to 10 wt. % CNT.  CNT- polymer chips were shown to be responsive to RH, n-BuOH and MIAK and to have a maximum relative resistance response (1-Rt/Ro) at compositions close to percolation threshold.