Polymer coated MEMS resonator for room temperature NH3 sensing

This work presents a feasibility study of using a polymer coated MEMS doubly-clamped beam resonator for low power sensing of NH₃ at sub-ppm level in air at room temperature. Upon NH₃ absorption, the polymer swells leading to an increase in the effective mass of the beam and buildup of stress in the polymer layer, that result in a decrease in the resonance frequency. The normalized frequency shift (Δf/f₀) of such beams is larger than that of conventional cantilever type resonators. Poly(acrylic acid) (PAA) and polypyrrole (PPY) were compared using a quartz crystal microbalance and PAA was selected for inkjet printing onto the resonators thanks to its high NH₃ uptake. The sensor with 12 printed PAA layers shows a NH₃ sensitivity (Δf/f₀ per ppm) of 0.7 × 10^-3 ppm^-1 at 40% relative humidity and a detection limit below 100 ppb. The sensor has a response time of 5 minutes, which scales linearly with the number of the printed polymer layers. The sensitivity was found to increase with RH.