We report on a photonic sensor with an ultralow limit of detection (LoD) based on a phase interrogation readout scheme together with an anisotropic porous silicon (PSi) membrane. First, the fabrication of porous free-standing membranes from medium doped (100) surface oriented silicon, with pore diameters suitable for the infiltration of biomolecules, around 50 nm, is reported. Then, the phase interrogation scheme for characterizing the PSi membranes is presented whose results show that while volumetric sensitivity increases with the membrane thickness, the resolution in the birefringence measurements decrease dramatically due to depolarization effects. The best LoD was found to be equal to $6.25 \times 10^{-6}$ RIU, from the thinnest 10-$μ\hbox{m}$-thick membrane. Finally, the thermooptic coefficient of the 10 $μ\hbox{m}$ membrane was measured in an aqueous environment and shown to be equal to $8 \times 10^{-4} \ \hbox{rad}/^{∘}\hbox{C}$.