Single-walled carbon nanotubes-calixarene hybrid for sub-ppm detection of NO₂

Calixarenes are exciting class of organic macromolecules and proved to be an excellent sensing material for optical and mass transaction based sensors. The limited conductivity of calixarenes is a major obstacle for the development of calixarene-only chemiresistive sensors. Here we report on a single-walled carbon nanotube (SWNT)-calixarene based chemiresistive senor for room temperature detection of NO₂ that is based on a hybrid material obtained by non-covalent functionalization of SWNTs with calixarene. This has two beneficial effects: (i) the use of SWNTs eliminates the conductivity issue, and this enables low-power chemiresistive sensing; (ii) the excellent affinity of calixarenes for certain analytes improves sensitivity. The hybrid was fabricated by solvent casting technique and its formation was confirmed through structural (SEM and TEM) and electrical (I_D-V_D and I_D-V_G) characterizations. The hybrid exhibited higher sensitivity to NO₂ gas at concentrations as low as 0.25 ppm when compared to pristine SWNTs. The sensitivity and the limit of detection (LOD) of the hybrid was found to be ~283/ppm of NO₂ and ~25 ppb of NO₂ respectively. An improvement of the response time of the sensor was observed and the average response time of the hybrids was found to be <~1 min for all concentration of NO₂ exposure more than 1 ppm.