Through-Bond Energy Transfer-Based Ratiometric Two-Photon Probe for Fluorescent Imaging of Pd2+ Ions in Living Cells and Tissues

Palladium can cause severe skin and eye irritation once it enters the human body. Ratiometric two-photon fluorescent probes can both eliminate interference from environmental factors and realize deep-tissue imaging with improved spatial localization. To quantitatively track Pd2+ in biosystems, we report here a colorimetric and two-photon ratiometric fluorescent probe, termed Np-Rh-Pd, which consists of a two-photon fluorophore (naphthalene derivative with a D-pi-A structure) and a rhodamine B dye. The two fluorophores are directly linked to form a two-photon ratiometric fluorescent probe for Pd2+ based on a through-bond energy transfer (TBET) strategy. It exhibits highly efficient energy transfer (90%) with two well-resolved emission peaks (wavelength difference of 100 nm), which could efficiently diminish the cross talk between channels and is especially favorable for ratiometric bioimaging applications. A signal-to-background ratio of 31.2 was observed for the probe, which affords a high sensitivity for Pd2+ with a detection limit of 2.3 X 10(-7) M. It was also found that acidity does not affect the fluorescent response of the probe to Pd2+, which is favorable for its applications in practical samples. The probe was further used for fluorescence imaging of Pd2+ ions in live cells and tissue slices under two-photon excitation, which showed significant tissue-imaging depths (90-270 mu m) and a high resolution for ratiometric imaging.