Single- and Multiphoton Turn-On Fluorescent Fe3+ Sensors Based on Bis(rhodamine)

Selective and sensitive turn-on fluorescent Fe3+ sensors based on novel bis(rhodamine) dye molecules are reported. The compounds are synthesized with very high yields and characterized with NMR, ESI mass spectrometry, and elemental analysis. Single- and two-photon fluorescence enhancement is observed for both molecules in the presence of Fe3+. High selectivity and sensitivity is observed over other metal ions and is shown to be due mainly to the spirolactam ring-opening power of Fe3+. All measurements are made in buffer environments simulating biological conditions to facilitate single- and multiphoton fluorescence imaging of Fe3+ in vivo and in vitro. Larger enhancement of fluorescence for both one- and two-photon excitation makes them suitable candidates for fluorescent labeling of biological systems. Two photon cross-section and time-resolved fluorescence measurements are utilized to understand the selectivity of the present sensors for Fe3+-sensing.