The fabrication and optical properties of a nanostructured system consisting of free-standing metallic nanorods surrounded by a dielectric nanoshell are reported. The adjustable core-shell geometry allows the resonant optical properties of the system to be tuned throughout the visible spectrum in the 500-900 nm range. The shell presents a tubular geometry of uniform thickness that can be varied from a fraction of a nanometer to a few tens of nanometers. This creates a well-defined subattoliter volume for the integration of a substance to be excited or probed. To exemplify such an application, the sensitivity of the nanostructured system to the variation of the shell's index of refraction is studied as a function of shell thickness and nanorod aspect ratio. The spectral sensitivity of the longitudinal resonance of the structure was found to be about 100 nm per refractive index unit with a figure of merit (sensitivity/line-width) of about 1.