text: Iron is used a great deal in biological systems, a fact that is well known due to its role in Hemoglobin. For it, there are many small molecule sensors including:
  *Pryrene-TEMPO, in which the binding of iron to TEMPO quenches the fluorescence of pyrene when no Fe(II) is bound. Upon binding however, TEMPO is reduced and pyrene regains fluorescence. This probe is limited in that an analogous response can be generated by unwanted free radicals, and that it can only by used in acidic solution.
  *DansSQ, in which Fe(II) binding increases fluorescence at 460 nm. It consists of a Dansyl group bound to styrylquinoline and operates by the disruption of intra-molecular charge transfer upon the binding of Fe(II). It is limited in that it is only soluble in acetonitrile in 10% HO.