The photoinduced electron transfer between immobilized thionine and the dinucleotide enzyme cofactors NADH and NADPH in a SiO2 sol-gel matrix is reported. The electron- transfer quenching of thionine luminescence is used to monitor the rate of NADPH oxidation. Using Stern-Volmer quenching curves, the quenching rates in the silica matrix are 1 to 2 orders of magnitude smaller than those in solution. The rate constants for oxidation of ...