A test agent includes a composite probe having at least one nanoparticle having multiple metal atoms, a directing agent, and an enzyme. The directing agent attaches the probe to a target in a test sample. The test sample and bound probe are then treated with an enzyme substrate. A method of detecting a target in a test sample includes exposing the test sample to the probe, then treating the test sample with an enhancement or development solution to deposit at least one of a fluorophore, a chromogen, or a metal.