Metal-organic frameworks (MOFs) are nanoporous materials with high prospects as recognition elements for enhanced gas sensors owing to their sensitivity and selectivity. MOFs offer large internal surface areas and, often, expose functional groups and open metal sites to the pore space. As a result, gas molecules concentrate inside the pores of the MOF by (selectively) adsorbing onto the pore walls. The amount of gas adsorbed in a MOF at thermodynamic equilibrium depends upon the chemical potential of each component in the gas phase (and the temperature). Therefore, observation of the total mass of gas adsorbed in a MOF -or some other property that depends on the gas adsorbed in the MOF, such as electrical resistance , color , luminescence , or strain -provides information about the composition of the gas. Fig. illustrates a miniaturized gravimetric, MOF-based sensor composed of a quartz crystal microbalance (QCM) coated with a thin film of MOF -a surface mounted MOF (SURMOF) . Nanogram-scale changes in the adsorbed mass of gas in the MOF film can be inferred from changes in the resonance frequency of the piezoelectric quartz crystal, forced by an alternating voltage . Several gravimetric, MOF-based sensors have been experimentally demonstrated using QCMs and surface acoustic wave devices (SAWs) as signal transducers.