An　ultrasensitive　conformation-dependent　colorimetric　assay　has　been　developed　for　the　detection　of　mercury(II)　ions.　It　is　based　on　the　use　of　exonuclease　III　(Exo　III)-assisted　target　recycling　and　gold　nanoparticles　(AuNPs).　In　the　absence　of　Hg(II),　the　hairpin-shaped　DNA　probe　(H-DNA)　binds　to　AuNPs　and　stabilizes　them　in　solutions　of　high　ionic　strength.　In　the　presence　of　Hg(II),　on　the　other　hand,　the　sticky　termini　of　the　H-DNA　form　a　rigid　DNA　duplex　stem　with　a　blunt　3＇-terminus.　Thus,　Exo　III　is　activated　as　a　biocatalyst　for　selective　and　stepwise　removal　of　mononucleotides　from　the　3＇-terminus　of　the　H-DNA.　As　a　result,　Hg(II)　is　released　from　the　T-Hg(II)-T　complexes.　The　guanine-rich　sequences　released　from　the　H-DNA　are　then　self-assembled　with　potassium　ion　to　form　a　stable　G-quadruplex　conformation.　In　solutions　of　high　ionic　strength,　this　results　in　aggregation　of　AuNPs　and　a　color　change　from　red　to　blue　which　can　be　seen　with　bare　eyes.　The　method　is　highly　sensitive　and　selective.　It　has　a　linear　response　in　the　10　pM　to　100　nM　Hg(II)　concentration　range,　and　the　detection　limit　is　as　low　as　3.2　pM　(at　an　S/N　ratio　of　3).　The　relative　standard　deviation　at　a　level　of　0.5　nM　of　Hg(II)　is　4.9%　(for　n　=　10).　The　method　was　applied　to　the　detection　of　Hg(II)　in　spiked　environment　water　samples,　with　recoveries　ranging　from　92%　to　106%.