A conjugate consisting of a thrombin aptamer tethered to the thrombin, Th, with a sensing nucleic acid (1) is used for the optical detection of DNA. The thrombin/aptamer complex blocks the biocatalytic functions of Th. Hybridization of the analyte DNA (2) to the sensing nucleic acid 1 yields a rigid duplex that detaches the aptamer from Th, a process that activates the protein toward the hydrolysis of bis(p-tosyl-Gly-Pro-Arg)-R110 (3) to the rhodamine 110 fluorophore (4). The system allows the DNA sensing with a sensitivity limit of 1 x 10-8 M. The aptamer/Th conjugate is also immobilized on glass slides for the optical detection of DNA. The dissociation of the aptamer/Th complex upon hybridization and the subsequent dehybridization of the duplex and the regeneration of the catalytically inactive Th/aptamer complex duplicate machinery functions.