With the aid of a dextran matrix, the metal-induced fluorescence loss of bound fluorophores can be greatly reduced, and the distance dependence of the fluorescence yield could be largely convoluted. This is optimized for the limit of detection assessment of surface plasmon fluorescence spectroscopy. The model system was designed as a direct assay with mouse IgG covalently immobilized to the carboxymethyl dextran matrix of a CM5 sensor chip from Biacore. Time-resolved ultratrace detection of fluorophore (Alexa-Fluor 647)-labeled rabbit anti-mouse antibody down to 500 aM (10-18 M) was accomplished, corresponding to a binding rate of approximately 10 molecules mm-2 min-1.