We investigate an air-slot photonic crystal cavity for high-precision refractive index sensing. The high quality factor ∼2:6 × 10^4 of the cavity, along with a strong overlap between the resonant mode and the hollow core region, allows us to achieve an experimental sensitivity of 510 nm per refractive index unit (RUI) and a detection limit below 1 × 10^−5 RUI. The device has a remarkably low sensing volume of 40 aliters, holding less than 1 × 10^6 molecules.