ABSTRACT
Among chemical sensors, metal oxide nanostructures for gas sensing constitute a frontier research area of paramount importance. The sensing mechanism of metal-oxide sensors must be properly understood with reference to vital parameters controlling their sensitivities. The influence of heat treatment on sensor performance and that of additives in imparting selectivity to sensor response are discussed. Carbon nanotube–based gas sensors have played a significant role in this field. After unfolding the gas-sensing properties of CNTs, responses of SWCNTs and MWCNTs towards gases, and modification of CNTs for gas sensing, a CNT-based FET-type sensor, an MWCNTs/SnO2 ammonia sensor and a CNT-based acoustic gas sensor are described. Examples of porous silicon-based gas sensor, thin organic polymer film-based gas sensors and electrospun polymer nanofibers as humidity sensors are followed by consideration of large nanosensor arrays and nanoelectronic nose. Among CNT-, nanowire- and nanobelt-based chemical nanosensors, a CNT-based ISFET works as a nano pH sensor, besides NW nanosensor for pH detection and ZnS/silica nanocable FET pH sensor. Bridging nanowire is used as a vapor sensor while palladium functionalized Si NW acts as a hydrogen gas sensor. A polymer-functionalized piezoelectric-FET humidity nanosensor has been developed. In the optochemical nanosensor category, low-potential quantum dot ECL sensor for metal ion, a BSA-activated CdTe QD nanosensor for Sb3+ ion and a functionalized CdSe/ZnS QD nanosensor for Hg(II) ion are notable achievements. Marine diatom gas sensors also hold promise.
