A　novel　electrochemiluminescence　(ECL)　＂sandwich＂　biosensor　has　been　developed　to　detect　cocaine.　The　sandwich　biosensor　was　fabricated　on　the　basis　of　the　fact　that　a　single　aptamer　could　be　split　into　two　fragments　and　the　two　dissociated　parts　could　form　a　folded,　associated　complex　in　the　presence　of　targets.　One　of　these　(capture　probe),　which　had　hexane-thiol　at　its　5＇-terminus,　was　immobilized　on　a　gold　electrode　via　thiol-gold　binding.　The　other　one　(detection　probe)　was　labeled　with　the　ECL　reagent　tris(2,2＇-bipyridyl)ruthenium(II)-doped　silica　nanoparticles　(RuSiNPs)　at　its　3＇-terminus.　Owing　to　the　weak　interaction　between　the　two　fragments,　the　sensor　exhibited　a　low　ECL　signal　in　the　absence　of　cocaine.　After　the　target　cocaine　had　been　added　to　the　solution,　it　induced　association　of　the　two　fragments　and　stabilized　the　associated　complexes,　leading　to　immobilization　of　RuSiNPs　on　the　electrode　surface,　and　the　ECL　detected　on　the　electrode　surface　was　enhanced.　The　enhanced　ECL　intensity　was　directly　proportional　to　the　logarithm　of　the　cocaine　concentration　in　the　range　from　1.0　x　10(-9)　to1.0　x　10(-11)　mol/L,　with　a　detection　limit　of　3.7　x　10(-12)　mol/L.　The　biosensor　was　applied　to　detect　trace　amounts　of　cocaine　on　banknotes　with　satisfactory　results.