Selective SAW-Based Carbon Monoxide Differential Sensor Functionalized with Cobalt Corroles: Influence of Interfering Gases

Introduction During the last two decades, the potential impact of indoor air quality on human health has stimulated an interest in hazardous compounds survey such as carbon monoxide (CO) [1]. The French Institute for Health Surveillance (InVS) reports that accidental domestic poisoning by the CO affects about 1000 households in France each year [2], and is responsible for about 100 deaths. The detection of this compounds has consequently become a need. To address it, we here report results on the capability of functionalized Surface Acoustic Wave (SAW) devices for the selective detection of CO. Here we insist on the necessity to detect CO in presence of interferent such as O 2 that is obviously present in the air and CO 2 present in significant quantity in urban area. Material SAW delay lines based on Love waves, shown in figure 1, are used to probe mass of sensitive materials deposited as a thin layer on its surface. These devices consist in two-port delay lines built on quartz. The Love wave is generated and detected using interdigited transducers (IDTs) and the frequency operation is in the vicinity of 125 MHz. A 1.5 µm thick silica guiding layer is deposited onto the IDTs providing a propagation path which permit the guidance of the acoustic wave. For the functionalization of the sensor, we take advantage of the great capabilities of cobalt corroles to trap CO molecules at the sensor’s surface with selectivity [3]. Because of the structure of the corroles, small molecules such as N 2 , O 2 and CO 2 can be trapped within by mean of weak interactions. In the particular case of CO, stronger chemical interactions are involved reaching high selectivity for this gas. In addition to this intrinsic selectivity, a second corrole has been selected to be used as a reference. This corrole exhibits the same structure as the cobalt corrole, so that it interacts with interferents in a similar way, but has no affinity to the target gaz. Method Two SAWs delay lines (A and B) shown in figure 1 are respectively coated with cobalt- and copper-corroles by mean of a spray coating method. The quantity of corroles deposited on the device is monitored during the process to avoid the deposition of an excessive amount of corroles that would damage the sensor signal compromising its proper functioning. They are then heated at 90°C for an hour under vacuum to remove the ammonia ligands present on the cobalt to prevent the corrole’s degradation in solid state. A gas-mixing bench co