Terahertz spectroscopy provides unique access to molecular fingerprint regions, but suffers from weak light-matter interaction due to the dimension mismatch between the terahertz wavelength and the molecule. Here, an all-dielectric metasurface strategy inspired by bound states in the continuum (BICs) is proposed for nanoscale-level terahertz molecular sensing with tailored functionalities. Through the temporal coupled-mode theory, it reveals that the molecular signal can be enhanced by two efficient approaches: increasing the coupling coefficient $\mu$, or decreasing the radiation rate ${\gamma }_r$. Using the lossless symmetry-broken quasi-BIC dimerized grating coupled with the $\alpha$-lactose molecules as an example, it demonstrates the significant enhancement, yet with different trends for $\mu$-tailoring and ${\gamma }_r$-tailoring sensing approaches. Further combined with the angle-multiplexed mechanism, the symmetry-broken quasi-BICs enable the retrieval of terahertz vibrational fingerprints from molecular layers as thin as 1 nm, with an enhancement factor of three orders of magnitude. This work not only reveals the powerful capability of BIC within an all-dielectric platform for nanoscale terahertz detection, pushing terahertz sensing into unprecedented deep-nano levels, but also opens new avenues for developing advanced terahertz photonic devices.

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• Bound states in the continuum
• Light propagation, transmission & absorption
• Light-matter interaction
• Metasurfaces
• Nanophotonics
• Coupled mode theory