A simple and sensitive method is described for the determination of DNA. It relies on the use of (a) an invasive reaction that is catalyzed by flap endonuclease 1 (FEN 1), and (b) graphene oxide (GO)-based fluorescence signalling. The presence of target DNA mediates the formation of the invasive structure, and this induces FEN 1 to catalyze multiple cycles of cleavage reaction at the junction, thereby liberating numerous fluorophore-labeled flaps. The released flaps are intentionally designed too short to be adsorbed onto GO. Hence, intense green fluorescence whose maximum emission is observed at 520nm after excitation at 480nm is restored even in the presence of GO. The method can be applied to the determination of target DNA from Chlamydia trachomatis, one of the major pathogenic bacteria causing sexually transmitted diseases. The assay is sensitive and specific with the limit of detection of 6.7 pM, and was applied to reliable determination of Chlamydia trachomatis DNA in human serum.