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A photonic crystal (PhC) biosensor is presented to have improved sensitivity for detecting surrounding media by incorporation of nanophotonic cavities. This device, consisting of a tailored PhC defect cavity placed next to a PhC waveguide (PhC-WG), is fabricated on silicon-on-insulator (SOI) substrate. Unlike conventional PhC defects, this tailored cavity creates two decoupled resonant modes that both have high quality factor (Q-factor). Moreover, this new arrangement of PhC cavity enhances optical coupling between the cavity and waveguide, leading to a strong interaction between light and the surrounding medium in the vicinity of cavity. The sensor was characterized to be with sensitivity of 180 nm/RIU, Q-factor of about 8,000 and limit of detection (LOD) of 5.56 × 10-5 RIU. The analytes under testing can be gases or liquids with a very small amount required: 17.5 pl for bare devices or 72 nl for microfludic devices covered with PDMS microchannels.