Bifunctional chelators (BFCs) are a critical component of metal-based contrast agents used in magnetic resonance imagine (MRI), time-gated luminescence microscopy and positron emission tomography (PET).1"4 Effective BFCs must meet a number of requirements. First, they must be unimolecular constructs that can be easily conjugated to antibodies, peptides or receptor-targeting ligands. Second, they must bind to a given ion with high thermodynamic stability and kinetic inertness. While stability and inertness are paramount, the ability to form metal complexes under mild physiological conditions is desirable so that antibody conjugates or other heat- or pH-sensitive preparations can be used. Third, the synthetic route to preparation must be efficient and adaptable so that ligands' coordination profiles or functional substitutions can be easily varied. For luminescent lanthanide complexes, the synthesis must allow incorporation of sensitizing chromophores.
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