Reivindicaciones

1. A method of sensing a target analyte in a liquid sample including the steps: providing a bulk acoustic wave resonator device; placing a liquid sample on the bulk acoustic wave resonator device; operating the bulk acoustic wave resonator device to generate bulk acoustic waves; and measuring a shift in the fundamental resonant frequency of the bulk acoustic wave resonator device, wherein the bulk acoustic wave resonator device comprises a resonator structure and an acoustic decoupling layer, the resonator structure comprising: a piezoelectric material layer; electrodes arranged to apply a driving signal to the piezoelectric material layer to generate bulk acoustic waves; and a resonator structure surface, wherein: the resonator structure has a resonator structure acoustic impedance and the liquid sample has a liquid sample acoustic impedance, wherein the resonator structure acoustic impedance is the acoustic impedance of the uppermost layer of the resonator structure; the acoustic decoupling layer is formed over the resonator structure surface, there being an interface between the acoustic decoupling layer and the uppermost layer of the resonator structure; and the acoustic decoupling layer has an acoustic decoupling layer acoustic impedance, the acoustic decoupling layer acoustic impedance being: up to 1/5 times or not less than 5 times the resonator structure acoustic impedance, and up to 1/5 times or not less than 5 times the liquid sample acoustic impedance, and wherein the acoustic decoupling layer comprises carbon nanotubes. 2. A method according to claim 1 wherein the thickness of the acoustic decoupling layer is between 1/8λ and 50λ where λ is the wavelength in the decoupling layer at the fundamental resonant frequency of the resonator. 3. A method according to claim 1 or claim 2 wherein the density of carbon nanotubes in the acoustic decoupling layer is at least 0.3 × 10<10> cm<-2>. 4. A method according to claim 1 or claim 2 wherein the density of carbon nanotubes in the acoustic decoupling layer is at least 1 × 10<10> cm<-2>. 5. A method according to any one of claims 1 to 4 wherein the liquid is an aqueous liquid. 6. A method according to any one of claims 1 to 5 wherein the liquid sample acoustic impedance is in the range 1.48 MRayl to 2 MRayl. 7. A method according to any one of claims 1 to 6 wherein the fundamental resonant frequency of the bulk acoustic wave resonator device is in the range 1-5 GHz. 8. A method according to any one of claims 1 to 7 wherein the bulk acoustic wave resonator device is an SMR type device. 9. A method according to any one of claims 1 to 8 wherein the acoustic decoupling layer is formed on an electrode of the resonator structure. 10. A method according to any one of claims 1 to 9 wherein the acoustic decoupling layer includes binding sites specific for the analyte in the liquid sample. 11. A method according to any one of claims 1 to 10 wherein the liquid sample partially penetrates the acoustic decoupling layer. 12. A method according to any one of claims 1 to 11 wherein the bulk acoustic wave resonator device is operated in longitudinal mode.