Ultra-sensitive detection of Pb2+ based on DNAzymes coupling with multi-cycle strand displacement amplification (M-SDA) and nano-graphene oxide

•A multi-cycle strand displacement amplification (M-SDA) was innovatively designed.•A novel fluorescent sensor based on DNAzymes coupling with M-SDA and nano-graphene oxide was established.•The proposed fluorescent sensor can achieve the ultra-sensitive detection of Pb2+ as low as 6.7 pM.•The proposed fluorescent sensor possesses strong selectivity and stability.