FRET-Based Ratiometric Detection of Metal Ions

Abstract

Fluorescent sensors for detecting transition- and heavy-metal ions in environmental and biological systems have received considerable attention. In comparison with single emission intensity-based “turn-off” or “turn-on” sensors, ratiometric sensors can afford self-calibration by dual-channel detection and thus perform precise analyses against various environmental and instrumental factors. One of the most frequently applied mechanisms for ratiometric sensing is the binding-induced modulating of intramolecular fluorescence resonance energy transfer (FRET), which is highly dependent on the distance between the donor and acceptor, the overlap integral between the donor emission and acceptor absorption, and the relative orientation of the transition dipoles. Hence the binding-induced changes of the spatial and electronic structure can effectively modulate the FRET efficiency within a donor-acceptor system, consequently leading to change in the fluorescence intensity of the donor and acceptor.In order to develop heavy-metal ion sensors, we have proposed a new strategy for ratiometric fluorescent detection that is based on the rational integration of two sensing mechanisms of a binding-induced modulating of FRET efficiency coupled with a metal-chelating quenching of fluorescence. Synergistic coupling of these two functions leads to exceptionally large change in the emission ratio, thus allowing for ratiometric detecting of metal ions with improved sensitivity. As proof-of-concepts, a BODIPY-porphyrin dyad (1) and a series of phthalocyanine-porphyrin(Zn) hetero-oligomers (2-4) have been designed for ratiometric fluorescent detection, which show excellent selectivity and sensitivity toward Ag+ and Pb2+ respectively. Figure 1. Ratiometric fluorescent detection of Ag+ by FRET-based BODIPY-porphyrin dyad (1).