Characterization of Changes in the Surface Properties of Silicon and Porous Silicon after Interaction with Hydroxyl Radicals

Free radicals (FR) can be formed inside cells as a result of physiological processes such as food metabolism, respiration or pollutants generated by effects such as snuff radiation, among others. Its accumulation can be harmful to nearby cells or in contact with a tissue or organ [ [i] ] . It is for this reason that various investigations have focused their interest to the detection and quantification of these FR, in order to find materials that enable in-vivo sensing these FR and in particular the hydroxyl radical (–OH), because of its high oxidation potential +2.32 V [ [ii] ]. In this study we examined the changes on the surface properties of crystalline silicon (n -Si) and porous silicon (n-PS) as a function of exposition time to hydroxyl radicals generated by Fenton reaction and by means of the reaction between hydrogen peroxide electrochemically generated with iron (II) ions (electrofenton). Morphological, optical and wetting degree characterizations were performed for the evaluation of changes on the surface properties of these substrates. For morphological analysis, Atomic Force Microscope (AFM) was employed, evaluating pore size distribution on n-PS and y¡the influence of the exposition to FR. Changes in optical properties were analyzed by means of photoluminescence (PL) of n-PS exposed at –OH radical at different periods and analyzing changes on the intensity of this signal. Finally, the wetting degree was analyzed by contact angles in two stages: (i) sessile drop method ( ex- situ ); (ii) captive bubble method ( in- situ ). The results obtained indicate a change in the silicon hydrophobicity/hydrophilicity as a function of exposition time to –OH radicals. These changes have been associated with the dynamic surface transformation of silicon surface groups from Si - H (after cleaning treatment ) to Si - OH (after exposure to hydroxyl radical). [[i] ] Robert E. Huie and P. Neta. Chemistry of reactive oxygen species Part I. Kluwer academic publishers, (2002) New York, 33 -63pp. [[ii] ] Fritz Scholz, Gabriela López de Lara González, Leandro Machado de Carvalho, Mauricio Hilgemann, Khenia Z. Brainina, Heike Kahlert, Robert Smail Jack, and Dang Truong Minh. Angewandtechemie international edition. 46 , (2007) 8079 –8081.