Disclosed are assays, methods, and kits for the screening of test compounds for their capability to induce cardiotoxicity in a subject. In particular, whether a test compound has the effect to prolong the Q-T interval as measured by an electrocardiogram in a human. The assays, methods, and kits disclosed herein make use of the binding interaction between novel fluorescent tracers and the hERG K+ channel, and the propensity of a test compound to influence that binding interaction.