Two　C-rich　DNA　strands　are　employed　to　fabricate　the　intermolecular　i-switch:　strand　1　with　thiol　at　the　5＇-terminus　immobilized　on　the　gold　electrode　through　the　thiol-Au　interaction,　and　ferrocene　(Fc)-labeled　strand　2　that　hybridizes　with　strand　1　to　form　an　i-switch　nanostructure.　In　neutral　or　basic　aqueous　solution,　the　i-switch　is　present　in　the　form　of　an　extended　duplex　that　makes　Fc　far　away　from　the　electrode;　a　weak　faradaic　current　is　obtained,　so-called　＂OFF＂　state.　While　in　the　slight　acidic　condition,　an　obvious　signal　is　detected　from　Fc　which　approaches　the　electrode　due　to　the　formation　of　the　i-motif,　so-called　＂ON＂　state.　The　faradaic　current　has　a　linear　relationship　with　the　pH　value　in　the　range　of　5.8-8.0.　(c)　2012　Elsevier　B.V.　All　rights　reserved.