The molecular binding between the glutamine binding‐protein (GlnBP) from Escherichia coli and L‐glutamine (Gln) is detected by means of an optical biosensor based on porous silicon technology. The binding event is optically transduced in the wavelength shift of the porous silicon optical microcavity (PSMC) reflectivity spectrum. The hydrophobic interaction links the GlnBP, which acts as a molecular probe for Gln, to the hydrogenated porous silicon surface area. We can thus avoid any preliminary surface functionalization process. The protein infiltrated PSMC results stable to oxidation at least for few cycles of wet measurements. The penetration of the proteins into the pores of the porous silicon matrix has been optimized: a strong base post‐etch process increases the pore size and removes any nanostructure on top and inside the porous silicon multilayer while does not degrade the optical response and the …