In this work, the binding of the recombinant glutamine-binding protein (GlnBP) from Escherichia coli to gliadin peptides, toxic for celiac patients, was investigated by mass spectrometry experiments and optical techniques. Mass spectrometry experiments demonstrated that GlnBP binds the following amino acid sequence: XXQPQPQQQQQQQQQQQQL, present only into the toxic prolamines. The binding of GlnBP to gliadin suggested us to design a new optical biosensor based on nanostructured porous silicon (PSi) for the detection of trace amounts of gliadin in food. The GlnBP, which acts as a molecular probe for the gliadin, was covalently linked to the surface of the PSi wafer by a proper passivation process. The GlnBP-gliadin interaction was revealed as a shift in wavelength of the fringes in the reflectivity spectrum of the PSi layer. The GlnBP, covalently bonded to the PSi chip, selectively recognized the toxic peptide. Finally, the sensor response to the protein concentration was measured in the range 2.0-40.0 microg/L and the sensitivity of the sensor was determined.