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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) detection based on electric loading effect with a graphene (G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> )/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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> ) of IDTs constructing an equivalent static capacitance (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> '). When the concentration of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas was about 0.38 MHz/ (ppm*m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ).