A fluorescence spectroscopic study of phenanthrene sorption on porous silica

Fluorescence spectroscopic characteristics of sorbed phenanthrene in porous silica provide information about its chemical state such as monomer vs dim er or higher aggregates, as well as a basis for high sensitivity detection. In this study, the chemical state and distribution of phenanthrene sorbed in two types of porous silica particles, mesoporous silica (365 mum particl e diameter, 150 Angstrom average pore diameter) and microporous silica (cus tom synthethized, 1 mum particle diameter, 20 Angstrom pore diameter), is d etermined by fluorescence spectroscopy, fluorescence lifetime measurements, and scanning two-photon excitation fluorescence profiling. From the charac teristic fluorescence emission spectra, it is found that at loading levels of less than or equal to4.7 mg/g (phenanthrene/silica) phenanthrene exists as monomers in both meso- and microporous silica particles for phenanthrene loaded from super critical CO2 (SCF). Two-photon excitation fluorescence i ntensity distribution profiles indicate that for the mesoporous silica part icles phenanthrene is adsorbed throughout the entire silica particle. Intro duction of water into phenanthrene-loaded mesoporous silica particles cause s instantaneous conversion of phenanthrene from monomer to crystalline form at phenantherene loading levels greater than or equal to4.7 mug/g due to h ydration of the silica surface. In this process, sorption of water molecule s expels phenanthrene from the surface sorption sites and causes localized phenanthrene concentration beyond its solubility limit, resulting in crysta llization. In comparison this fast conversion is not observed for phenanthr ene-loaded microporous silica particles that show extremely slow conversion even for phenanthrene loading levels as high as 4.7 mg/g. This difference is interpreted as reflecting hindered diffusion of phenanthrene in the near ly monodispersed micropores with pore sizes close to the molecular diameter of phenanthrene.