Surface acoustic wave CO2 gas sensor based on electric loading effect

This letter describes a novel Surface Acoustic Wave (SAW) sensor for carbon dioxide (CO
₂
) detection based on electric loading effect with a graphene (G
_r
)/nickel (Ni) - L-alanine- Al sandwich structure (MIM) as a sensing capacitance. This capacitance, which is fully coated on the IDTs and reflectors of SAW device and is sensitive to the pH value of the environment, is paralleled with the static capacitance (C
_S
) of IDTs constructing an equivalent static capacitance (C
_S
'). When the concentration of CO
₂
increased from 0 ppm to 40000 ppm, the equivalent static capacitance decreased from 5.402pF to 4.358pF. Then the center frequency of SAW sensor increases 20 kHz due to the electric loading effect. And the sensitivity of the fabricated SAW sensor for CO
₂
gas was about 0.38 MHz/ (ppm*m
²
).