The thiol group is known to have a high affinity for gold nanoparticles and is capable of displacing other capping agents. We report a fluorescent indicator displacement technique for the detection of biothiols based on this principle. The displacement of BODIPY from the surface of gold nanoparticles served as a sensitive method for the detection of cysteine, homocysteine, and glutathione under a variety of conditions. It is noteworthy that the turn-ON fluorescence mechanism enabled the differentiation of normal cells from cancer cells because of the inherently higher concentrations of biothiols in the latter.