We investigate hollow photonic crystal structures consisting in air-slot photonic crystal nanocavities. The high quality factor similar to 2.6x10(4) of such cavities along with a strong overlap between the resonant mode and the hollow core region allows for a strong interaction with an optically active medium in the air slot. This is illustrated in achieving highly sensitive refractive index sensing of a gas infiltrated in the slot. An experimental sensitivity of 610 nm per refractive index unit (RUI) and a detection limit below 1x10(-5) RUI are demonstrated. The device has a remarkably low sensing volume of 40 attoliters, which at atmospheric pressure and room temperature contains as little as 1x10(6) molecules.