A method to integrate semipermeable membranes with thickness down to 50 nm in soft lithographic structures, is discussed. These membranes act as diffusion barriers for macromolecules, while they are permeable for low moleculer-weight species, which makes them a candidate for separation process. These membranes can be used for creating cavities loaded with high-molecular weight species. Such structures can be potentially used as sensor arrays, it the entrapped molecules have stimuli-responsive properties to probe solution properties such as a pH-dependent fluorescence intensity.