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Peroxynitrite (ONOO-) could deplete ER-associated Ca2+ and cause ER stress, and thereby precise and in situ detection of ONOO- at ER is highly important for the study of its biological roles. Herein, we present a ratiometric two-photon ER-targeting fluorescent probe (NAB) for the detection of ONOO- in living cells and brain tissues. NAB utilized 4-bromomethylphenyl borate as a recognition site for ONOO-, and utilized p -toluenesulfonamide as ER-targeting group. After the response of NAB to ONOO-, the 4-bromomethylphenyl borate converted to hydroxyl, resulting in the change of the fluorescence color from blue to yellow. The probe NAB could show highly sensitive response to ONOO- within 6 s, and exhibited excellent selectivity to ONOO-. The probe NAB mainly located at ER and showed excellent ER-targeting property. Ratiometric two-photon imaging demonstrated that the probe NAB could selectively detect exogenous ONOO- at ER, and also could be applied to image metformin-induced ONOO- in living HepG2 cells. Furthermore, NAB can effectively detect the changes of ONOO- level in brain tissues, and revealed that the endogenous ONOO- level at ER showed increase in hippocampus of the depressive mouse. [Display omitted] • A ratiometric two-photon and ER-targeting probe for ONOO- was developed. • The fluctuation of endogenous ONOO- at ER was imaged in a ratiometric two-photon manner. • ONOO- level at ER showed an increase in hippocampus of the depressive mouse. [ABSTRACT FROM AUTHOR]

Cell death can destroy homeostasis and is a hallmark of many pathological conditions. Discrimination of live and dead cells is a crucial task for the biological, medical and pharmaceutical studies. Herein, we constructed an ESIPT-based fluorescent probe (BTE) on the basis of the different esterase activity in live and dead cells. Under excitation, the probe BTE showed the blue emission peaked at 465 nm, while it mainly displayed green emission peaked at 543 nm after it was hydrolyzed by esterase. Imaging of the cells treated by H 2 O 2 and ultraviolet (UV) radiation demonstrated that the probe BTE is effective in the detection of the health of cells, could help us to better understand cell death and its effects in a range of diseases and treatments. Unlabelled Image • Discrimination of live and dead cells in a ratiometric mode. • Response of the probe to esterase on the basis of ESIPT • Image the unhealthy status the cells treated with H 2 O 2 • Analysis the damage of ultraviolet UV radiation to live cells [ABSTRACT FROM AUTHOR]