We report measurements on planar optical waveguides having an aqueous core and a low-index nanoporous dielectric cladding. Spin-on deposition of the nanoporous dielectric results in a thin (~0.8 µm) low-index cladding layer on a higher-index fused silica substrate, which produces leaky waveguide modes; however, for the aqueous layer thickness needed in most microfluidic applications, a large number of low-loss modes exist. We demonstrate that such a waveguide can be used for efficient collection and transport of fluorescence generated within the aqueous core and show that the use of these nanoporous materials offers advantages over the principal alternative, Teflon® AF.