Routes towards sensor selectivity

Although SWCNT gas sensors exhibit a high sensitivity, these devices have a poor selectivity. Several mature routes to acquire gas selectivity are explored in this section. 

As has been pointed out by Cantalini et al. (2003), the cross-sensitivity of SWCNT gas sensors can lead to false alarms in a complex atmosphere. To exploit CNT-based devices, it is crucial to find ways to give them selectivity. To modify the chemical activity of SWCNTs and enhance their sensitivity to a specific gas, the common approach is to functionalise the nanotube side-wall. There are two types of functionalisation depending on the binding of the functional group onto the nanotube sidewall covalent functionafisation and non-covalent functionalisation. Covalent functionalisation of SWCNTs is often obtained from the esterification of carboxylic add groups formed on 

Gas sensors based on bio-functionalised SWCNTs with DNA strands were explored by the University of Pennsylvania group (Staii et al, 2005). DNA preferentially wraps around SWCNTs via Ti-stacking interaction. Compared with CNTFET based on pristine SWCNTs, DNA functionahsed CNTFETs 

An important issue consists in the development of new strategies to suppress the influence of atmospheric humidity on sensor response. As reported in Section 10.3.2, moisture has a dramatic effect on the CNTFET Ids(Vgs) characteristic, which can lead to erroneous measurements. 

The adsorption of gas molecules near the dopant induces a large charge transfer which shifts the Fermi level of the SWCNT and modifies the doped-SWCNT conductivity to a considerable degree. Increasing the dopant concentration increases the number of active sites and the sensor becomes progressively more sensitive (Hamadanian et /.,2011). 

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