The fluorescence emission of the solution formed by reaction of ATP-LW with ONOO -decreased at lower pH, which could be ascribed to the decomposition of ONOO -in acidic solutions (Fig. ). While, the fluorescence intensity at 587 nm decreased significantly at higher pH (pH>8), which is most likely due to hydrolysis of ATP (Fig. ). The presence of ONOO -resulted in an instantaneous and significant fluorescence increase (Fig. ), whilst the reaction of ATP-LW and ATP was complete within 100 min (Fig. ). These promising results in aqueous solution prompted us to explore the use of ATP-LW for imaging live cells. Firstly, we confirmed using an MTT assay that ATP-LW was non-toxic to HL-7702 cells over concentrations ranging from 0 to 1 mM (Fig. ). The ability of ATP-LW to image ONOO -and ATP in living cells was then evaluated using excitation wavelengths of 488 nm and 514 nm, respectively. One-photon imaging was then performed to evaluate the effect of ATP and ONOO -changes stimulated by SIN-1. ATP-LW demonstrated a clear "turn on" response with the addition of SIN-1 in the green channel (Fig. and). Meanwhile, a 0.55-fold decrease in red channel was observed, when compared to the control group (Fig. ). No interference for both green channel excited by 514 nm and red channel under excitation at 488 nm were observed for ATP-LW when HL-7702 cells were exposed to ATP and SIN-1 (a donor of ONOO -) 39 simultaneously (Fig. ). As shown in Fig. , there was almost no fluorescence in Channel 3