Design and synthesis of fluorescence-based siderophore sensor molecules for FeIII ion determination

The design principles of fluorescence-based siderophore sensor molecules for detection of heavy transition-metal (HTM) ions are first reviewed. As an example, fluoresceindesferrioxamine (FlDFO), a highly efficient fluorophore molecule combining a specific Fe ion receptor and a fluorescence-sensitive signalling site has been designed, synthesized, and used for dosing with Fe ions. Its response test shows its high selectivity and sensitivity to FeIII ions and its potential for nanobiosensor design. This work clearly identified that among two FlDFO positional isomers differing by the attachment of DFO at the 5- or 6-position of the bottom benzene ring of Fl, the fluorescence of 6-FlDFO is insensitive to the complexation with Fe ions. This is independent of the linkage used between Fl and DFO. Only 5-FlDFO could be a highly potential sensor molecule since it has been revealed that in a free state without complexation with Fe ions, this fluoroionophore sensor molecule gave a maximum fluorescent signal. With successive Fe ion complexing, the fluorescence of 5-FlDFO decreased very sensitively and proportionally with ion concentration. The response speed has been evaluated as a function of Fe ion concentration. Responses to other metal ions present in the solution, such as Cu²⁺, Ca²⁺, Ni²⁺, and Al³⁺, and the effect of pH value on the efficiency of the sensor molecules have also been investigated.