There is increasing demand for low-cost gas sensors that can discriminate between low concentrations of analytes. Nanotechnology offers the promise of improved gas sensors with low-power consumption, fast response time which will enable portability for a wide range of applications. It is well documentated that nanostructured materials such as nanotubes and nanowires are suitable for sensing a number of different gases.
In most cases, these sensors were subject to cross interference by other analytes. While arraying of nanostructured gas sensing materials combined with advanced numerical methods such as pattern recognition has the potential to filter out some of these interferences, the development of more analyte specific sensors is highly desirable.
A group of scientists from the states [Zhang et al., Nanotechnology (2009) 20 255501] have successfully manufactured a nanostructured materials sensor for ammonia gas which can be tuned to eliminate the interference of water vapour. By precisely functionalising single walled carbon nanotubes (SWNT) networks with camphorsulphonic acid doped polyaniline (PANI(CSA)), the opposite electrical response toward humid air of CSA doped PANI and SWNTs effectively cancelled the humidity interference.
The morphology of the PANI(CSA) coated SWNT networks was characterized using Atomic force microscope, AFM images and diameter histograms of PANI(CSA) coated SWNTs revealed nodular polymer deposits on the SWNTs.
Temperature dependent I-V curves showed a nonlinear “S” shape, with a nonlinearlity decreasing with increasing temperature. The electrical resistence decreased sharply with the increasing temperature indicating that the PANI(CSA)-SWNT network behaved as a typical semiconductor.
CSA-doped PANI was precisely electro polymerized onto SWNTs with controlled thickness by Zhang and his colleagues. The resulting sensors showed excellent sensitivity toward NH3 at room temperature with minimum interference from H2O vapor. Overall, these results demonstrate that short comings of conventional sensors can be over come by designing novel nano engineered materials. The approach of using nanostructures with opposite electrical responses to interferences should be generally applicable to the development of analyte specific nano sensors.The approach paves the way for the development of more selective gas nano sensors.