In situ monitoring of the photoluminescence (PL) of undecylenic acid functionalized porous silicon (PS) was carried out in bovine serum albumin (BSA) solution. Two factors dictated the PL behavior during the protein incubation process. A PL redshift was observed within a few hours of PS incubation in BSA solution followed by an overall gradual blueshift of the PL energy. Correlating this result with Fourier transform infrared (FTIR) spectroscopy performed before and after BSA incubation as well as with the PL and FTIR studies performed in parallel on planar Si, the redshift was attributed to the adsorption (or incorporation) of protein in the porous matrix while the blueshift was assigned to surface oxidation duel to the erosion of Si nanocrystallites in the PS matrix. Understanding the protein incorporation in the PS matrix has important implications in utilizing the PL as an optical transduction method in a PS-based biosensor device. (c) 2006 American Vacuum Society.