We report on the development of a Quartz Crystal Microbalance-based sensor targeting the monitoring of the pertechnetate ion (TcO4-) in groundwater exploiting the anion-specific adsorptive behaviour of a novel Ag-4,4-bipyridine metal-organic framework (MOF) thin film. This MOF was first reported by Zhu et al. [Environ. Sci. Technol. Lett., 4, 316, (2017)] to selectively and irreversibly adsorb ReO4-, a non-radioactive chemical surrogate for TcO4−, in the form of bulk crystals. The structure of our film was characterised by XRD and SEM and its response and selectivity were evaluated by monitoring the changes in the resonant frequency of the QCM as a function of the concentration of ReO4- and other interferents. Finally, the interaction of all the analytes with the film was modelled with adsorption isotherms including Langmuir, Freundlich and Sips. We show that the behaviour of the thin film is substantially different from that of the bulk crystals.