Fingerprint detection. Fingerprints revealed on paper after the application of a solution of ninhydrin. Ninhydrin is a chemical that binds to the proteins in the sweat in fingerprints. It changes colour to purple during this reaction, allowing previously invisible prints to be detected. Ninhydrin is typically used to detect prints on porous surfaces such as paper. Fingerprints are unique to every individual and can thus be used to determine who has handled the letter.