An improved assay for detecting an analyte in a fluid sample includes a step of conducting a photochemical reaction, in which a substrate conversion catalyzed by a photosensitizer into a product of the photochemical reaction is temporary inhibited when the reaction mixture is irradiated with a light at a wavelength within a light absorption spectrum of the photosensitizer. The photosensitizer (or an enzyme to catalyze producing thereof) is attached to an entity having an affinity to the analyte, such entity is bound to the analyte prior to irradiation. To achieve temporary inhibition, certain additives are used such as ascorbic acid or its derivatives. The assay may increase the sensitivity of ELISA 20- to 100-fold.