Device for detecting at least one substance of a fluid, comprising a piezoacoustic thin film resonator with - at least one piezoelectric layer, - an electrode layer arranged on the piezoelectric layer, - at least one further electrode layer arranged on the piezoelectric layer and - at least one adsorption surface for adsorbing the substance of the fluid, wherein - the piezoelectric layer, the electrode layers (12, 13) and the adsorption surface are designed and arranged on one another in such a way that - an excitation alternating field can be coupled into the piezoelectric layer by electrically activating the electrode layers (12, 13), - the thin film resonator can be excited to a resonance oscillation at a resonance frequency f R because of an excitation alternating field coupled into the piezoelectric layer, and - the resonance frequency f R depends on an amount of the substance adsorbed on the adsorption surface, characterised in that - at least one electrical insulating layer for electrically insulating the electrode layer and for encapsulating the thin film resonator is arranged directly on a side of at least one of the electrode layers facing away from the piezoelectric layer, - the insulating layer extends over the entire thin film resonator, including laterally, and as a protective layer effects the passivisation of the thin film resonator in respect of the surrounding area, and wherein the device comprises a read-out circuit integrated in the substrate using CMOS technology, an electrical insulating layer consisting of silicon dioxide, an acoustic mirror and contacts, wherein the thin film resonator is arranged above the read-out circuit, the insulating layer and the acoustic mirror, wherein the read-out circuit is connected via the electrical contacts to the electrode layers (12, 13) of the thin film resonator for electrical activation. - at least one piezoelectric layer, - an electrode layer arranged on the piezoelectric layer, - at least one further electrode layer arranged on the piezoelectric layer and - at least one adsorption surface for adsorbing the substance of the fluid, wherein - the piezoelectric layer, the electrode layers (12, 13) and the adsorption surface are designed and arranged on one another in such a way that - an excitation alternating field can be coupled into the piezoelectric layer by electrically activating the electrode layers (12, 13), - the thin film resonator can be excited to a resonance oscillation at a resonance frequency f R because of an excitation alternating field coupled into the piezoelectric layer, and - the resonance frequency f R depends on an amount of the substance adsorbed on the adsorption surface, characterised in that - at least one electrical insulating layer for electrically insulating the electrode layer and for encapsulating the thin film resonator is arranged directly on a side of at least one of the electrode layers facing away from the piezoelectric layer, - the insulating layer extends over the entire thin film resonator, including laterally, and as a protective layer effects the passivisation of the thin film resonator in respect of the surrounding area, and wherein the device comprises a read-out circuit integrated in the substrate using CMOS technology, an electrical insulating layer consisting of silicon dioxide, an acoustic mirror and contacts, wherein the thin film resonator is arranged above the read-out circuit, the insulating layer and the acoustic mirror, wherein the read-out circuit is connected via the electrical contacts to the electrode layers (12, 13) of the thin film resonator for electrical activation.