Ratiometric fluorescence detection of Cu(II) with a keto-dipicolylamine ligand: A mechanistic implication

Cu(II) affects our environment and living systems to various aspects, demanding its detection and quantification methods. Fluorescent detection has its own advantages, and many fluorescent sensing systems are known. As Cu (II) quenches fluorescence, a key challenge in developing fluorescent sensors is to induce fluorescence signal enhancement rather than quenching. A further challenge is to develop probes that provide ratiometric fluorescence changes at two different wavelengths. We have developed such a ratiometric sensing system for Cu(II), which is an acedan-derived dipicolylamine ligand. The probe responded to Cu(II) with a large emission wavelength shift (104 nm) from green to blue in aqueous buffer at pH 7.4; the response was specific to Cu(II) among various other metal ions. The probe was also highly sensitive (LOD = 89 nM), and thus used to detect trace amounts of Cu(II) ions in nearby river and sea water samples. Also, the probe enabled us to image intracellular Cu(II) ions through ratiometric two-photon microscopic imaging. We performed fluorescence titrations toward Cu(II) and Cu(I) at pH 7.4 and 9.0, respectively, and computational calculations on plausible metal complex intermediates, which provided insights on the sensing mechanism to propose that it involves Cu(II)-promoted enolization and subsequent reduction of Cu(II) to Cu(I). The mechanistic implication would guide us to develop other types of probes and also to re-consider the sensing mechanism involved in the known probes that involve the intramolecular charge-transfer upon metal binding.