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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
(a) Real-time frequency response of the optimized FBAR sensor exposed to three typical formaldehyde concentrations at the relative humidity of 40%, 60% and 80%. (b) Plots of frequency shift as a function of formaldehyde concentration at the relative humidity of 40%, 60% and 80%.