A two-photon fluorescence probe with endoplasmic reticulum targeting ability for turn-on sensing photosensitized singlet oxygen in living cells and brain tissues

Endoplasmic reticulum (ER) is an indispensable organelle responsible for protein synthesis, transportation, and maintenance of Ca2+ homeostasis in eukaryotic cells. Recent studies highlighted that ER-targeted photosensitizers with high yield of singlet oxygen (1O2) are effective in selectively disrupting ER function and are promising candidates for anticancer therapy. Unfortunately, no ER targetable fluorescent probes for determining 1O2 photosensitized in this photodynamic therapy process is available. In this work, we synthesized an ER-targetable, two-photon fluorescence probe, ER-1O2, for fluorescence turn-on sensing of 1O2. ER-1O2 demonstrated high sensitivity to 1O2 sensing with a wide detection range (0-2.75 μM) and a low detection limit (0.11 μM). ER-1O2 also displayed excellent selectivity toward 1O2 out of other ROS and metal ions. Notably, ER-1O2 exhibited low cytotoxicity but with specific ER targetable capability. On account of these advantageous features, fluctuations of 1O2 in living cells and brain tissues were effectively visualized by ER-1O2.