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 nano-technology. The binding event is revealed as a wavelength shift in the reflectivity spectrum of a porous silicon optical microcavity (PSMC). Exploiting the hydrophobic interaction between GlnBP and the porous silicon surface "as etched", we avoid any preliminary surface functionalization process. The unmodified porous silicon optical microcavity, infiltrated by the protein, results stable to oxidation for few cycles of wet measurements.