This characteristic is useful in hazardous applications when continuous monitoring of gases is necessary [10].In order to increase the reactivity of CNTs, metal nanoparticles (NPs) are frequently incorporated along their structures, acting as active sites during the interaction between target analytes and CNTs’ walls [10�C12]. The interest of using metal NPs is due to their high catalytic activity, adsorption capacity and efficient charge transfer [13]. Metal NPs are highly sensitive to changes in their environment, providing a wide range of reactivity towards different gases. The selectivity and sensitivity of sensors can be tuned using several types of NPs because their reactivity depends on the element constituting the NPs.

The key concept in CNTs decorated with nanoparticles (CNTs-NP) is that upon adsorption of a target molecule, a significant amount of charge is donated or accepted by the NPs, and as a result the electron transport in the CNTs is affected and this change is easily measureable [10].Ozone is a principal cause of photochemical smog and atmospheric contamination. It is harmful to human health; breathing ozone-containing air reduces lung function, aggravating asthma and other respiratory conditions [14]. Furthermore, high ozone levels can also harm sensitive vegetation in forested ecosystems [14]. The standard method for ozone detection is based on a UV adsorption method [15]. Accuracy and high sensitivity are reported for this method, however, it presents drawbacks such as complexity of the apparatus with high cost and large detector size.

Due to their compact size, adsorption capacity, wide range of reactivity of several gases, simplicity in fabrication, operation in harsh environmental and at high temperatures sensors base CNT-nanoparticles are an alternative to replace the conventional method for detection of ozone.Few works have reported the detection of ozone gas molecules Brefeldin_A with CNTs used as the sensitive material [15�C18]. Park et al. [15] showed the response of a sensor using single wall CNT (SWCNT) networks; their sensor was sensitive to ozone down to 50 ppb with a rapid response as well as a fast recovery. Wongwiriyapan et al. [16] reported ozone sensitivity when using SWCNT networks directly grown on a conventional sensor substrate. Ozone was detected down to 6 ppb at room temperature while operating with a fast response.

Ghaddab et al. [17] compared the gas sensing properties between three types of materials: SWCNT, SnO2 and SWCNT/SnO2 hybrid materials. Among these, the latter were significantly more responsive to ozone and ammonia than pure SnO2 or SWCNTs. The detection limit at room temperature was evaluated to be lower than 20 ppb.To the best of our knowledge, although Pd decorated CNTs have been used for hydrogen detection [19,20], it has not been used for ozone sensing before.