Oxygen sensors based on mesoporous silica particles on layer-by-layer self-assembled films

We describe photoluminescent (PL) oxygen sensors based upon phosphorescent dyes adsorbed into the pores of mesoporous silica particles at submonolayer coverage on a layer-by-layer self-assembled film. Eight transition metal dyes (four Pt and Pd porphyrin complexes and four ruthenium complexes) were investigated through monitoring the changes in PL intensity and lifetime upon varying oxygen pressure. The intensity Stern-Volmer (SV) plots were curved. In most systems, the intensity SV plots match the lifetime SV plots, with deviations seen only at high oxygen pressures where static quenching may play a role. The curved intensity Stern-Volmer plots could be fitted with two phenomenological models, a two-site model, and a model based on a Freundlich binding isotherm for oxygen. The Gaussian distribution model was less successful in fitting the data. The most important result to emerge from these data is that the unquenched lifetime of the dye itself is not a sufficient scaling parameter to reduce all of the SV plots to a common line. A second scaling parameter was necessary. This parameter, which measures the capture radius for quenching (R-eff), times the efficiency of quenching per encounter alpha ranged from 0.38 nm for PdOEP and 0.42 for PtTPP to 1.12 for Ru(bpy)(3)]Cl-2 and 1.25 for Ru(phen)(3)]Cl-2 relative to an assumed value of alpha R-eff = 1.0 nm for PtOEP.