A　new　electrochemical　sensing　platform　was　designed　for　sensitive　detection　of　copper(II)　(Cu2+)　based　on　click　conjugation　of　two　short　oligonucleotides　by　using　methylene　blue-functionalized　hairpin　DNA　as　the　template.　The　analyte　(Cu2+)　was　in　situ　reduced　to　Cu+　by　sodium　ascorbate,　which　catalyzed　the　click　conjugation　between　two　single-stranded　oligonucleotides　one　was　labelled　with　a　5＇-alkyne　and　the　other　with　3＇-azide　group　via　the　Cu+-catalyzed　azide-alkyne　cycloaddition.　The　newly　formed　long-chain　oligonucleotide　induced　the　conformational　change　of　hairpin　DNA　to　open　the　hairpin,　resulting　in　methylene　blue　far　away　from　the　electrode　for　the　decrease　of　redox　current.　Under　optimal　conditions,　the　decrease　in　the　electronic　signal　was　directly　proportional　to　target　Cu2+　concentration,　and　allowed　the　detection　of　Cu2+　at　a　concentration　as　low　as　1.23　nM.　Our　strategy　also　displayed　high　selectivity　for　Cu2+　against　other　metal　ions　owing　to　the　highly　specific　Cu+-catalyzed　click　chemistry　reaction,　and　was　applicable　for　monitoring　of　Cu2+　in　drinking　water　with　satisfactory　results.