Methods　based　on　metal　nanotags　have　been　developed　for　metallobioassay　of　nucleic　acids,　but　most　involve　complicated　labeling　or　stripping　procedures　and　are　unsuitable　for　routine　use.　Herein,　we　report　the　proof-of-concept　of　a　novel　and　label-free　metallobioassay　for　ultrasensitive　electronic　determination　of　human　immunodeficiency　virus　(HIV)-related　gene　fragments　at　an　ultralow　concentration　based　on　target-triggered　long-range　self-assembled　DNA　nanostructures　and　DNA-based　hybridization　chain　reaction　(HCR).　The　signal　is　amplified　by　silver　nanotags　on　the　DNA　duplex.　The　assay　mainly　consists　of　capture　probe,　detection　probe,　and　two　different　DNA　hairpins.　In　the　presence　of　target　DNA,　the　capture　probe　immobilized　on　the　sensor　sandwiches　target　DNA　with　the　3＇　end　of　detection　probe.　Another　exposed　part　of　detection　probe　at　the　5＇　end　opens　two　alternating　DNA　hairpins　in　turn,　and　propagates　a　chain　reaction　of　hybridization　events　to　form　a　nicked　double-helix.　Finally,　numerous　silver　nanotags　are　immobilized　onto　the　long-range　DNA　nanostructures,　each　of　which　produces　a　strong　electronic　signal　within　the　applied　potentials.　Under　optimal　conditions,　the　target-triggered　long-range　DNA　nanostructures　present　good　electrochemical　behaviors　for　the　detection　of　HIV　DNA　at　a　concentration　as　low　as　0.5.　fM.　Importantly,　the　outstanding　sensitivity　can　make　this　approach　a　promising　scheme　for　development　of　next-generation　DNA　sensors　without　the　need　of　enzyme　labeling　or　fluorophore　labeling.　©　2013　Elsevier　B.V.