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We propose two different designs of slotted photonic crystal nanofiber cavities (SPCNCs), dielectric-mode (DM) SPCNC and air-mode (AM) SPCNC, and numerically investigate their refractive index sensing performances when they are immersed in an aqueous environment. By using the three-dimensional finite-difference time-domain method, we demonstrate that the DM-SPCNC (AM-SPCNC) simultaneously possesses a high quality factor of 1.49 × 107 (7.96 × 106) and a high sensitivity of 598 nm/refractive index unit (RIU) (423 nm/RIU) at the resonant wavelength of 1525.94 nm (1585.06 nm), yielding a figure of merit as high as 5.8 × 106 RIU−1 (2.1 × 106 RIU−1). Moreover, the mode volume of the DM-SPCNC's (AM-SPCNC's) fundamental resonant mode is found to be as small as 0.12 (λ/n)3 (0.24 (λ/n)3), resulting in a very compact effective sensing area. With these distinct features, we believe our proposed cavities are potentially ideal platforms for applications in multiplexed biochemical sensing, multielement mixture detection, and optical trapping of single biomolecules or nanoparticles. [ABSTRACT FROM AUTHOR]