Biosensors　based　on　nanomaterials　have　been　used　for　detection　of　various　biological　molecules　with　high　sensitivity　and　selectivity.　Herein,　we　developed　a　simple　and　ultrasensitive　electrochemical　DNA　biosensor　using　long-range　self-assembled　DNA　nanostructures　as　carriers　for　signal　amplification,　which　can　achieve　an　impressive　detection　limit　of　5　aM　human　immunodeficiency　virus　(HIV)　DNA　even　in　complex　biological　samples.　In　this　study,　we　designed　two　auxiliary　probes.　A　cascade　of　hybridization　events　between　the　two　auxiliary　probes　can　lead　to　long-range　self-assembly　and　form　micrometer-long　one-dimensional　DNA　nanostructures.　In　the　presence　of　target　DNA,　each　copy　of　the　target　can　act　as　a　trigger　to　connect　a　DNA　nanostructure　to　a　capture　probe　on　the　electrode　surface.　Then,　a　great　amount　of　redox　indicator　[Ru(NH3)6]3+　can　be　electrostatically　bound　to　the　DNA　nanostructures　and　eventually　result　in　significantly　amplified　electrochemical　signals.　©　2012　American　Chemical　Society.