Abstract: A resonant nanosensor apparatus associated with a functionalized monolayer for detecting carbon dioxide and a method of forming the same. A wafer including a sensing vibrating beam and a reference vibrating beam may be functionalized with a functional group in order to form a sensing self monolayer. The sensing self assembled monolayer may be configured by bridging oxygen or carbon atoms covalently bonded with respect to the vibrating beams. A liquid solution of hydrochloric acid may then be applied to the sensing self assembled monolayer at the surface of the reference beam by a direct printing process to obtain a reference monolayer. The liquid solution of HCl transforms the functional groups responsible for the carbon dioxide detection into protonated groups, which do not react with carbon dioxide, but possess visco-elastic properties similar to that of the sensing monolayer.