Direct evidence for Scheme 2 was obtained from the LC–MS spectra of RATU-Hg2+. As shown in Fig. 10, a peak at m/z 1242.7964 corresponded to [RATU + 2Hg2+] verified a 1:2 RATU-Hg2+ complex. The other peak at m/z 1043.3502, 629.3616, 522.1772 and 315.1834 belonged to [RATU + Hg2+ + H+], [RATU′ + H+], 1/2[RATU + Hg2+ + 2H+] and 1/2[RATU′ + 2H+], respectively. These results suggest a new interaction way between Hg2+and rhodamine-bsed chemosensor, in which one RATU molecule binds two Hg2+ ions to two S atoms, followed by the removal of HgS and phenyl fragment to produce some new structures.