Low refractive index materials such as SiO2 nanorods or nanoporous SiO2 (np-SiO2) and MgF2 have been attractive for use in various types of optical coatings. Due to the high porosity of the films, however, the material properties of these films are unstable. This phenomenon is confirmed in dry versus humid ambient where both the coefficient of thermal expansion (CTE) and refractive index change dramatically. We report here that the properties of np-SiO2 can be stabilized by depositing a cap layer of Al2O3 using Atomic Layer Deposition (ALD) on top. This stabilizing ability is demonstrated successfully for single layer and distributed Bragg reflectors (DBR). With this method, one can pattern nanoporous sensors so that some regions are stable for packaging while others are exposed to show enhanced interactions with environment for sensing.