with µ and ρ being the shear modulus (29.47 GPa) and the density (2.65 g/cm 3 ) of quartz, f 0 the resonance frequency of the coated QCMs, and A QCM the sensitive area (0.2043 × 10 -4 m 2 ). After film deposition and DUV exposure, the mass of the hybrid films (four LbL-SC cycles) was calculated using this equation, under consideration of the frequency shift ∆f after film deposition with regard to the resonance frequency of the bare QCM. The masses of the films ranged between ∼1.7 -2.3 µg, with the highest mass determined for the film with the lowest degree of reduction. In accordance with our above discussed observations (cf. Figure )) we attribute the reduced mass after DUV exposure to the decomposition of the ligands on the TNC surface as well as the removal of GO functional groups. All coated QCMs showed a shift of resonance frequency of up to 400 Hz when exposing them to humidified air (cf. the Supporting Information, Section S12). The water sorption and desorption was fast and reversible, as shown by the rectangular transient shapes and the stable baseline frequencies. The relative mass uptake ∆m/m of the films was determined for all tested r.h. levels. Figure ) shows the mass uptake for selected r.h. levels as a function of the UV exposure time of the sensors. Here, no distinct trend of the relative mass change with increasing degree of GO reduction of the films is observed. The r.h. leveldependent sorption isotherms (cf. Supporting Information, Section S12) resemble type II isotherms, corresponding to unrestricted monolayer-multilayer sorption to the material at high r.h. levels. These findings indicate that secondary water sorption occurs at r.h. levels > 40%, leading to multilayered water films. The maximum relative mass uptake for all films, regardless of their degree of reduction, is ∼17%. Hence, the distinct resistive responses of the films to r.h. are not caused by varying amounts of sorbed analyte, but have to be attributed to specific interactions of water molecules with the internal chemical structure of the composites, as well as their different electrical properties. A schematic of the proposed sensing mechanism at different r.h. levels for (i, iii) GO-and (ii, iv) rGO-rich hybrids is illustrated in Figure . (i) GO-rich films provide a high density of polar binding sites, such as epoxy and hydroxy groups, that allow for water sorption via H-bonds (pink dashed lines).