Surface Acoustic Wave Sensors for the Detection of Hazardous Compounds in Indoor Air ^†

Abstract

In this work, the authors show the capabilities of Surface Acoustic Wave (SAW) devices coupled with various absorbents to probe the properties of gas sensitive materials for the manufacturing of hazardous gas sensors. The great capabilities of cobalt corroles for the trapping of carbon monoxide (CO) were exploited to produce selective sensors. These corroles were deposited on SAW delay lines surfaces and then exposed to carbon monoxide (CO) in standard conditions. Concentrations of a few hundreds of ppb were measured emphasizing the interest of such sensors for the detection of CO. Another type of sensitive layers exhibiting specific porosity adapted to the trapping of formaldehyde (CH₂O) were deposited on similar delay lines. A detection threshold of 140 ppb was shown. These encouraging results pave the way for the development of a functionalized SAW sensors network for multi-gas detection in indoor air.

1. 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 formaldehyde (CH₂O) [1] and carbon monoxide (CO) [2]. Formaldehyde being a carcinogenic, the World Health Organization (WHO) recommends exposure limit as low as 80 ppb of formaldehyde during 30 min [3]. The French Institute for Health Surveillance (InVS) reports that accidental domestic poisoning by the CO affects about 1000 households in France each year [4], and are responsible for about 100 deaths. The detection of these two compounds has consequently become a need. To address these needs, we here report results on the capability of functionalized Surface Acoustic Wave (SAW) devices for the selective detection of CO and CH₂O.

2. Materials and Methods

SAW delay lines based on Love waves, shown in Figure 1, are used to probe mass, density or elastic properties of a sensitive layer deposited on its surface. In this context, together with the manufacturing of the SAW device, the authors have developed sensitive layers capable of efficiently and selectively trap a target gas.

2.1. Formaldehyde Sensor

For the production of the CH₂O sensor, two different sensitive layers were tested; a first layer referred as L1 displays a specific nano-porosity allowing the trapping of small molecules such as CH₂O. A second sensitive layer referred as L2 displays functional groups aimed at improving the molecular interaction with CH₂O. Both materials were produced via the sol-gel process and were deposited as thin films on the SAW device by spin-coating.

2.2. Carbon Monoxide Sensor

For the functionalization of the CO sensors, we took advantage of the great capabilities of cobalt corroles to trap this molecule [5,6]. Because of the structure of the corroles, small molecules such as N₂, O₂ and CO 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. The use of a variety of cobalt corroles including new structures of corroles complexes represents an innovative aspect of this work.

3. Results

Modifications of the physical properties of the sensitive layer consecutive to gas adsorption lead to a shift of the synchronous frequency of the delay line. Since the phase is linear with the frequency for such a device, the frequency shift can be revealed by a phase shift measurement at constant frequency. The characterization of the adsorption of the targeted pollutant onto the surface is then achieved by calculating the slope of the phase at the beginning of its decrease. This approach allows to measure concentrations of gas within a few tens of seconds.

3.1. Formaldehyde Measurements

As shown on Figure 2, with the first layer L1 displaying a specific porosity adapted to the trapping of CH₂O, a drift of the signal in the presence of N₂ used as a carrier gas is observed indicating that N₂ molecules are also adsorbed in the nano-porous layer. This lack of selectivity induces a weak sensitivity of the sensor to CH₂O. As a matter of fact, it enables the detection of CH₂O only at concentrations superior or equal to 2.5 ppm which do not fit with the required WHO detection threshold.

Based on these results, the second functionalized sensitive layer L2 was used to enhance CH₂O adsorption at the expense of N₂ adsorption. One can see in Figure 3a that with L2, the drift caused by the exposure to N₂ is substantially reduced and the sensitivity is strongly improved, allowing the detection of CH₂O at concentrations as low as 80 ppb with a sensitivity of 2.6 e⁻³ °.s⁻¹.ppm⁻¹.

3.2. Carbon Monoxide Measurements

SAW delay line covered with new structures of corroles was exposed to CO at low concentration. Figure 4a represents the sensor response to CO diluted in N₂ in the 10 ppm to 200 ppb range. This sensor has shown a great sensitivity to CO allowing the detection of concentration as low as 200 ppb. One can see a drift of the signal following the detection. This behavior is attributed to temperature changes when shifting from the carrier gas to the target gas. In order to address a response to that issue, a second delay line used as a temperature reference is currently under development. Figure 4b shows that despite of the temperature-related measurement problems, the signals observed are correlated to the CO concentration with a sensitivity of 1.32 e⁻³ °.s⁻¹.ppm⁻¹.

4. Conclusions

A linear correlation between the phase shifts velocity of the Love wave and the target gases concentrations has been demonstrated. The experiments have been repeated several times to validate the results under various concentrations. It was shown that the use of Love wave devices functionalized with sensitive materials such as corrole complexes or nano-porous layers enable gas detection at concentrations as low as a few tens of ppb for CH₂O and a few hundreds of ppb in the case of CO. These results pave the way to investigating other gas detection with specific functionalized acoustic waves devices.