Microwave gas sensor based on graphene aerogel for breath analysis

Abstract

Exhaled breath can be used for early detection and diagnosis of diseases, monitoring metabolic activity, and precision medicine. In this work, we design and simulate a microwave sensor in which thin graphene aerogels are integrated into rectangular microwave waveguides. Graphene aerogels are ideal sensing platforms for gases and volatile compounds as they combine extremely high surface-to-volume ratio and good electrical conductivity at RF and microwave frequencies. The latter is modified by exposure to different gases, and -when integrated into a waveguide-these changes result in significant shifts in transmission and reflection scattering parameters. We model the aerogel as a graphene grid with hexagonal openings of size 22.86x10.16x0.1 mm 3 , characterized by an air volume equal to about 90% of its entire volume. This grid is used as a building block for modeling thicker samples (up to 9 mm). To simulate the variation in the dynamic conductivity of the graphene sheets as a consequence of the absorption of gaseous molecules, a sweep of the chemical potential from 0.0 eV to 0.5 eV with steps of 0.1 eV was used. The results show a significant variation of the waveguide transmission scattering parameters resulting from the gas-induced modification of the graphene conductivity, and hence the potential of the proposed sensor design for breath analysis.

Microwave Gas Sensor Based on Graphene Aerogels

“…In this article, we experimentally demonstrate that the GA microwave conductivity varies upon exposure to air or vacuum, and that, therefore, sensors can be made by using this highly-porous material, as predicted by our simulations [26]. The article is organized as follows.…”

“…where: ω = 2πf is the angular frequency, E F is the Fermi energy (also called chemical potential µ c ), τ is the scattering time assumed to be independent of energy, T is the temperature expressed in Kelvin, e ≈ 1.6 ) shows how changes in Fermi level (chemical potential) induced by molecules or compounds adsorbed on the graphene surface modify its DC and AC conductivity, leading to modification of wave propagation [26]. The modification of graphene conductivity by gas adsorbates takes place via a direct charge transfer mechanism, where graphene Fermi level is shifted depending on its relative position with respect to the HOMO/LUMO of the adsorbed gas molecules [43], [44].…”

“…Motivated by the need of increasing the surface area of graphene microwave gas sensors while keeping a small formfactor, we designed and computationally investigated a novel microwave sensor using three-dimensional macroassemblies of graphene sheets, known as graphene aerogels (GAs), which we integrated within a rectangular waveguide coupled with a gas inlet [26]. In GAs, graphene sheets form an interconnected network [27], leading to a highly porous structure ideal for gas sensing.…”

“…best of our knowledge, the use of graphene aerogel for microwave gas sensing application has not been experimentally demonstrated. In our previous work, we developed a model to simulate GAs at microwave frequencies and showed that, when incorporated in a waveguide, variations of GA's chemical potential due to absorption of gas molecules alter both transmission and reflection parameters of the waveguide, significantly enhancing the variation due to direct interaction of the propagating wave with the gases [26].…”