Two-Photon Fluorescent Probes for Biological Mg2+ Detection Based on 7‑Substituted Coumarin

Two novel water-soluble coumarin-based compounds (OC7, NC7) were designed and synthesized as two-photon fluorescent probes for biological Mg2+ detection. The compounds feature a β-keto acid as a high selective binding site for Mg2+ and the coumarin framework as the two-photon fluorophore. OC7 and NC7 show significant “off-on” detecting signals (9.05-fold and 23.8-fold fluorescence enhancement) and lower detection limits compared with previous reported two-photon fluorescent probes for Mg2+. Moreover, OC7–Mg2+ and NC7–Mg2+ exhibit large two-photon absorption cross sections (340 and 615 GM) at the near-infrared wavelengths (740 and 860 nm), which indicates that the probes are very suitable for detection of Mg2+ in vivo. Both OC7 and NC7 are pH-insensitive and of low cytotoxicity and can be applied to image intracellular Mg2+ under two-photon microscopy (TPM). Our results provide a strategy to modify the coumarin fluorophore to get better two-photon fluorescent properties. And the results also suggest that electronic density of β-keto acid plays a very important role in the recognition of Mg2+.