Surface Modification of Luminescent Porous Silicon by Aqueous Solutions of Amino Acids

Effect of amino acid solutions on the photoluminescence (PL) and chemical stability of porous silicon (PS) was studied. It was found that PS PL increases and blueshifts under the treatment by solutions of amino acids with non-polar radicals (glycine, alanine, valine and proline) and aromatic ones (phenylalanine). The most essential modifications were revealed for glycine. Similar changes in PL, though to a much lesser extent, were observed under the treatment of PS with deionized water. Using infrared spectroscopy, it was shown that the changes in the PS PL in the presence of amino acids, unlike distilled water, were accompanied by significant oxidation of the PS surface. The modifications of PL were explained within the quantum-size model by a decrease in the size of the emitting nanocrystallites. The decrease was caused by the degradation and oxidation of PS in aqueous medium, where amino acids affect both the processes. On the one hand, amino groups of the amino acids increase the PS degradation, since the former can act as nucleophiles and attack PS. On the other hand, amino acids, especially glycine, can perhaps stimulate formation of reactive oxygen species in aqueous solutions in the presence of PS and thus initiate its oxidation.