the present invention provides a method for identifying a test compound that binds to a target species . the method includes : incubating at least one test mixture under isothermal denaturing conditions , each test mixture comprising at least one test compound , and at least one target species , wherein the isothermal denaturing conditions are effective to cause at least a portion of the target species to denature to a measurable extent ; detecting a denaturation signal of each target species in the presence of the at least one test compound by a change in the diffusion properties of the target molecule using fluorescence correlation spectroscopy ; and comparing the denaturation signal of each target species in the presence of at least one test compound with a denaturation signal of the same target species in the absence of the at least one test compound under the same isothermal denaturing conditions .