The experimental study of a gold slot waveguide cavity is presented. The resonance of this cavity working in the telecom wavelength range is highly dependent on the refractive index of the medium located in or around the slots array, because of the high confinement of the electromagnetic field in the structure. We will demonstrate the application of this structure to local refractive index sensors at the nanoscale. The measured sensitivity of this device is S = 730 nm/RIU (refractive index unit). The structure has been optimized by adding another array of slots cascaded with the first one. The consequence is an improvement in the time efficiency of the experiments. A discussion about the effect of the volume of liquids used and the filling percentage of the slots by the liquids is also presented as parameters affecting the measurements and the sensitivity of the sensor.