The ability to detect nanoscale objects is particular crucial for a wide range of applications, such as environmental protection, early-stage disease diagnosis and drug discovery. Photonic crystal nanobeam cavity (PCNC) sensors have attracted great attention due to high-quality factors and small-mode volumes (Q/V) and good on-chip integrability with optical waveguides/circuits. In this review, we focus on nanoscale optical sensing based on PCNC sensors, including ultrahigh figure of merit (FOM) sensing, single nanoparticle trapping, label-free molecule detection and an integrated sensor array for multiplexed sensing. We believe that the PCNC sensors featuring ultracompact footprint, high monolithic integration capability, fast response and ultrahigh sensitivity sensing ability, etc., will provide a promising platform for further developing lab-on-a-chip devices for biosensing and other functionalities.

Microbubble whispering-gallery resonator with the intrinsic microfluidic channel is a promising platform for biochemical sensing due to their ultrahigh quality (Q) factor and ultralow detection limit. In this work, we realize a packaged microbubble resonator (PMBR) of ultrahigh-Q factor (∼ 3.52×10 $^6$ ) in a moisture curable polymer for practical applications. The long-term stability and thermal response of a PMBR are measured. Further, we demonstrate the dynamic hydrogel gelation transition from hydrophilic to hydrophobic state by detecting the resonant wavelength shift and mode broadening in a filled PMBR simultaneously. The refractive index changes of hydrogel during the transition is explicitly modelled. Our work shows the versatility of PMBR for practical optical sensing and may open a new route to microfluidic dynamic reaction.