We experimentally investigate refractive index sensing in silicon slot waveguide ring resonators by the detection of the giant shift of the ring transmission spectrum envelope enabled by the following specific conditions: the slot waveguide cross section as well as the ring couplers have been designed to lead to a V-shaped microring resonator spectrum modulated by the classical frequency comb and exhibiting quality factor peaks of 2000-6000 around λ=1.5 μm. By tracking the spectrum envelope wavelength shift, sensitivity up to S=1,300 nm per refraction index unit (RIU) is reported when the slots are filled by liquids with refraction index values close to 1.33.