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

Abstract

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 O2 that is obviously present in the air and CO2 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 N2, O2 and CO2 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 composed of mass-flow controllers allow for generating various gas mixtures. The so prepared mixtures are sent to a dedicated test chamber that may feature up to six delay lines. Once the sensors are in the chamber, a primary vacuum and a heating is applied to remove any contaminant. A carrier gas composed of different gas of interest (N2, O2, CO2) then flows at 500 sccm through the chamber and CO is then injected at different concentrations. The shift of the synchronous frequency of the delay lines consecutive to gas adsorption is then monitored by mean of a dedicated electronics that delivers information similar to those from a classical network analyzer. Results and Conclusions As we can see in figure 2 the differential acquisition using a copper corrole allows stabilizing the sensor’s signal. Indeed, the drift of the signal on the delay line functionalized with cobalt corrole can be compensated by subtracting the one from the delay line functionalized with copper corrole. This differential acquisition results in a stable basic level of the phase signal and also an improved repeatability of the measurements (figure 3).As expected from previous work [4], we noticed a linear correlation between CO concentration, in the 100 to 7000 ppm range, and the phase shift velocity undergone by the sensors regardless of the composition of the carrier gas (figure 4). From there, we determined the sensitivity of the sensors in presence of interfering gas. It appeared that the presence of 20 % of oxygen, known as the main interferent in this measurement, in the carrier gas tends to lower by 5.5 % the sensitivity in comparison with a pure nitrogen carrier gas (figure 4). The presence of CO2 at 20 000 ppm, which represents 50 times the mean concentration in Paris, lowers the sensitivity by 19.3 % (figure 4). From these results, the selective interaction between the cobalt corrole and CO has been verified and the possibility to use it as part of a selective CO sensor has been evidenced.