The localization of the electromagnetic field at the nanoscale can play a key role in many applications, such as sensing, spectroscopy and energy conversion. In the last years, great efforts have been performed to study and realize all-dielectric loss-free nanostructures to confine the radiation without the limits imposed by plasmonic systems. It has been demonstrated that an all-dielectric photonic crystal (PhC) metasurface can support bound states in the continuum (BICs) - resonant states of infinite lifetime - due to the interaction between trapped electromagnetic modes. Experimentally, this involves very narrow coupled resonances, with a high Q-factor and a possible extremely large field intensity enhancement, up to 6 orders of magnitude. Here, we demonstrate that the field enhancement in proximity of the surface can be explored to boost lightmatter interaction in spectroscopic sensing. We design and realize an innovative sensing scheme for bulk and surface measurement with ultra-high figure of merit for the recognition of protein-protein interaction and the detection of low molecular weight molecules. In addition, we design a dielectric dual scheme based on metasurfaces supporting BICs to amplify fluorescence emission and Raman scattering of probe molecules dispersed on the surface of the PhC. Our results provide new solutions for light manipulation at the nanoscale, especially for sensing and nonlinear optics applications.