A　novel　chemosensor　based　on　unsymmetrical　squaraine　dye　(USQ-1)　for　the　selective　detection　of　Hg2+　in　aqueous　media　is　described.　USQ-1　in　combination　with　metal　ions　shows　dual　chromogenic　and　＂turn-on＂　fluorogenic　response　selectivity　toward　Hg2+　as　compared　to　Li+,　Na+,　K+,　Mg2+,　Ca2+,　Ba2+,　Al3+,　Cu2+,　Cd2+,　Mn2+,　Fe3+,　Ag+,　Pb2+,　Zn2+,　Ni2+　and　Co2+　due　to　the　Hg2+-induced　deaggregation　of　the　dye　molecule.　A　recognition　mechanism　based　on　the　binding　mode　is　proposed　based　on　the　absorption　and　fluorescence　changes,　H-1　NMR　titration　experiments,　ESI-MS　study,　and　theoretical　calculations.