Fluorescence and 1O2 generation properties of porphyrin molecules immobilized in oxidized nano-porous silicon matrix

Recently it has been shown that nano-porous silicon (pSi), synthetic material produced by electrochemical etching of monocrystalline silicon, is able to provide an efficient electronic energy transfer to molecular oxygen, thus producing singlet (i.e. excited) oxygen in gaseous and liquid environment that opens the possibilities for application of pSi in Photodynamic Therapy (PDT). Here, we study the photophysical properties, including an ability of singlet oxygen (1O2) generation, of a modified pSi matrix impregnated with cationic (TMPyP(4)) or anionic (TPPS) porphyrins, which are known as effective photosensitizers of 1O2 production in solution.It is shown that a use of oxidized pSi matrix instead of as-prepared one results in an increase of an ability of the porphyrins to be immobilized in the matrix by orders of magnitude. A matrix of pSi formed in (1 0 0)-oriented Si wafers ensures, after pSi oxidation, better uptake of porphyrins into the pores as compared with oxidized pSi(1 1 1). The fluorescence decay kinetics of the immobilized porphyrins are found to be non-exponential, and the effective fluorescence lifetime shortens with an increase of the porphyrin concentration. It evidences in favour of a moderate annihilation-type concentration quenching of the fluorescence rather than in favour of any quenching effect of oxidized pSi matrix. It opens perspectives for application of the hybrid material for 1O2 generation. An ability of immobilized TMPyP(4) to generate 1O2 is shown experimentally by direct 1O2 luminescence measurements.