Gravimetric sensors based on thin-film bulk acoustic wave (BAW) resonators operating between 1-5 GHz have tremendous potential as biosensors because they are inexpensive, label-free, fast and highly sensitive. The two main challenges in this objective are: the conventional longitudinal mode resonance in $c$-axis oriented piezoelectric films suffers from more than 90% damping in liquid; the alternative is the shear mode resonance, with lower damping in liquid but which requires an inclined $c$-axis piezoelectric film, a process that is still not fully scalable. In this thesis, seed layers such as AlN with mainly (103) orientations are used to promote the growth of homogeneously inclined $c$-axis ZnO (inclination of up to $\sim$45$^{\circ}$)filmswithoutsignificantequipmentmodifications.SputteredAlelectrodeswithcontrolledroughnessarethensubstitutedfortheparasiticAlNseedlayerstoimprovetheelectromechanicalperformance.Atasubstratetemperature,T${s}$ = 100 $\mathrm{<mrow\; data-mjx-texclass="ORD">\circ </mrow>}$C, an optimum surface roughness of 9.2 nm yields homogeneously inclined $c$-axis ZnO films with angles $\sim$25$^{\circ}$. Solidly mounted resonators (SMRs) operating in a shear mode at $\sim$1.1GHzwiththeAlelectrodeshaveresonantqualityfactors(Q${r}$)higherthan150andeffectiveelectromechanicalcouplingcoefficients,k$^{2}$${eff}$,of2.9-3.4${m}$ of (4.9 $\pm$ 0:1) kHz$\cdot$cm$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$/ng and temperature coefficients of frequency (TCF) of -(66$\pm$2) ppm/K. Viscosity sensing is carried out with different ethanol-water compositions; the SMRs are functionalised and successfully used in the detection of Rabbit Immunoglobin G. To mitigate the longitudinal mode damping in water, multi-wall carbon nanotube (CNT) forests are grown by chemical vapour deposition (CVD) at 600 $\mathrm{<mrow\; data-mjx-texclass="ORD">\circ </mrow>}$C using Fe/Al layers on the active area of inclined $c$-axis AlN SMRs designed for improved thermal and chemical stability. The dense CNT forest (with 0.5/8 nm Fe/Al) of $\sim$15\mu mheightprovidesanacousticisolationtoDIwaterwithonly50-70$_{r}$ compared to 99% in SMRs without the CNTs. Mass loading is still detected and demonstrated by detecting bovine serum albumin (BSA) in water whereas with forest heights of $\sim$30\mu mandnosignificantfrequencyshiftsduetomassattachmentareobserved.WiththeCNTsthelongitudinalmodeisshownforthefirsttimetobemoresensitivetomass($\sim$7x) than the shear mode in liquid, highlighting the potential of CNTs for the large scale use of the longitudinal mode for in-liquid sensing.