This work reports the recent results achieved at the SENSOR Lab, Brescia (Italy) to address the selectivity of metal oxide based gas sensors. In particular, two main strategies are being developed for this purpose: (i) investigating different sensing mechanisms featuring different response spectra that may be potentially integrated in a single device; (ii) exploiting the electronic nose (EN) approach. The former has been addressed only recently and activities are mainly focused on determining the most suitable configuration and measurements to exploit the novel mechanism. Devices suitable to exploit optical (photoluminescence), magnetic (magneto-optical Kerr effect) and surface ionization in addition to the traditional chemiresistor device are here discussed together with the sensing performance measured so far. The electronic nose is a much more consolidated technology, and results are shown concerning its suitability to respond to industrial and societal needs in the fields of food quality control and detection of microbial activity in human sweat.

The possibility of using novel architectures based on carbon nanotubes (CNTs) for a realistic monitoring of the air quality in an urban environment requires the capability to monitor concentrations of polluting gases in the low-ppb range. This limit has been so far virtually neglected, as most of the testing of new ammonia gas sensor devices based on CNTs is carried out above the ppm limit. In this paper, we present single-wall carbon nanotube (SWCNT) chemiresistor gas sensors operating at room temperature, displaying an enhanced sensitivity to NH₃. Ammonia concentrations in air as low as 20 ppb have been measured, and a detection limit of 3 ppb is demonstrated, which is in the full range of the average NH₃ concentration in an urban environment and well below the sensitivities so far reported for pristine, non-functionalized SWCNTs operating at room temperature. In addition to careful preparation of the SWCNT layers, through sonication and dielectrophoresis that improved the quality of the CNT bundle layers, the low-ppb limit is also attained by revealing and properly tracking a fast dynamics channel in the desorption process of the polluting gas molecules.

A low-cost method for carbon nanotubes (CNTs) network production from solutions on flexible polyethylene naphthalate substrates has been adopted to prepare high quality and well characterized SWCNT bundle layers to be used as the active layer in chemiresistor gas sensors. Two types of SWCNTs have been tested: pristine SWCNTs, deposited from a surfactant solution, and covalently functionalized SWCNTs, deposited from a dimethyl-acetamide solution. The humidity effects on the sensitivity of the SWCNTs network to NH ₃ have been investigated. The results show that relative humidity favors the response to NH ₃ , confirming recent theoretical predictions. The COOH-functionalized sample displays the largest response owing to both its hydrophilic nature, favoring the interaction with H ₂ O molecules, and its largest surface area. Compared to data available in the literature, the present sensors display a remarkable sensitivity well below the ppm range, which makes them quite promising for environmental and medical applications, where NH ₃ concentrations (mostly of the order of tens of ppb) have to be detected.