The　yolk-shell-structured　SnO2-C　nanospheres　have　been　prepared　by　a　hydrothermal　reaction　of　SnCl2　center　dot　2H(2)O　and　glucose,　followed　by　carbonization　under　500　degrees　C　in　air　conditions.　Then,　an　inorganic/organic　hybrid　film　bearing　SnO2-C　and　polytyrosine　(pTyr)　is　fabricated　by　electro-polymerization　of　the　SnO2-C-modified　electrode　in　tyrosine　solution.　The　modified　electrode　is　utilized　as　a　supporting　platform　for　covalent　immobilization　of　cauliflower　mosaic　virus　35s　(CaMV35s)　promoter　gene　fragments　to　construct　an　electrochemical　DNA　sensor.　Chronocoulometric　experiments　show　that　the　loading　density　of　probe　DNA　(pDNA)　and　hybridization　efficiency　are　determined　to　be　as　high　as　4.54　X　10(13)　strands　cm(-2)　and　83.2%,　respectively.　Upon　hybridization　with　target　DNA　(tDNA),　the　probe　DNA　that　lay　flat　on　the　electrode　surface　through　hydrogen　bonding　with　pTyr　is　erected,　[Fe(CN)(6)](3-/4-)　diffusion.　Therefore,　a　＂signal-off＇　response　strictly　reducing　the　charge　repulsion　and　steric　hindrance　for　dependent　on　the　hybridization　reaction　is　achieved　in　electrochemical　impedance　spectroscopy.　The　response　mechanism　is　predicted　by　theoretical　calculation.　Owing　to　the　high　probe　density　and　hybridization　efficiency　of　the　sensor,　a　wide　kinetic　linear　range　of　1.0　aM　to　100　pM　and　an　ultralow　detection　limit　of　0.53　aM　for　the　target　sequence　are　obtained.　The　biosensor　also　presents　high　recognition　ability　toward　the　DNA　samples　extracted　from　real　transgenic　and　nontransgenic　soybeans,　showing　great　promise　for　facile　monitoring　of　the　transgenic　product.