The present disclosure provides a thin film transistor, a thin film transistor array, and a method for detecting an object to be detected, wherein the thin film transistor is configured to detect a parameter of an object to be detected bound with a metal ion and includes an active layer, wherein: a carrier of the active layer without a metal element contained in the metal ion bound is of a first mobility, and a carrier of the active layer with the metal element bound is of a second mobility different from the first mobility.