The 4-hydroxy-1,8-naphthalimide product on reaction of ATP-LW with ONOO -was confirmed by LC-MS analysis. (Scheme S3, Fig. and). Subsequently we evaluated the ability of ATP-LW to detect ATP, a large increase in fluorescence intensity at 587 nm (> 80-fold, see Fig. and Fig. ) was observed following the addition of ATP (0-15 mM) when excited at 520 nm. However, the addition of ATP (0-15 mM) to ATP-LW resulted in only a small increase in fluorescence intensity with excitation at 450 (Fig. ) or 488 nm (Fig. ). Thus, no ATP-mediated increase using excitation at 450 and 488 nm was observed. This confirms the ability of ATP-LW to separately detect ONOO -and ATP in the respective excitation profiles. We then explored the fluorescence response of ATP-LW in the presence of both ONOO -and ATP.