## 1 Introduction  
The detection of trace amounts of sequence specific DNA has numerous important applications in clinical diagnosis [1], food safety [2] and environmental monitoring [3]. Many sensitive and selective PCR-based methods have been established in past decade, including real-time fluorescence quantitative polymerase chain reaction (rt-PCR) [4; 5; 6], gene chip [7], and gene sequencing [8]. These techniques exhibit the advantages of excellent sensitivity and rapidity, but the sophisticated, expensive equipment and professional skills needed limit their practical application. ECL is the process whereby species generated at electrodes undergo high-energy electron-transfer reactions to form excited states which will generate light. At present, ECL has become a widely used analytical technique in the areas of immunoassay, food testing, and biomarker agent detection. Thus, many ECL DNA sensors have been proposed as alternatives to PCR-based methods for their simplicity, precise, selectivity, relative low cost and wide dynamic range except sensitivity. ECL DNA sensors typically employ various amplification techniques such as enzymes [9], DNA biobarcodes [10] and nanomaterials [11] to increase sensitivity; however, the sensitivity of such sensors not satisfactory. The recently developed rolling circle amplification(RCA) [12; 13; 14] and target DNA recycling amplification (TRA) [15; 16; 17] techniques demonstrate some improvement in sensitivity; thus, they are widely used in ECL DNA sensor fabrication [18; 19].  
RCA is an isothermal amplification process mediated by certain DNA polymerases (e.g., Phi29 DNA polymerase), which can produce thousands of repeated sequences and lead to significant signal amplification with speed, high efficiency, and specificity [20]. Therefore, RCA has been widely applied for the highly sensitive detection of DNA [21], RNA [22], and protein [23]. Recently, [PERSON] _et al._ developed a strategy that combined RCA with fluorescent _in situ_ hybridization (FISH) and successfully detected microRNAs in tumor cells [24]. The TRA method amplifies signals via cyclical hybridization and digestion with the aid of nucleases such as exonuclease [25], polymerase [26] and nicking endonuclease [27]. DNA nicking endonucleases belong to a special restriction endonuclease family, which can recognize double-stranded DNA and hydrolyze only one objective strand [28]. Nicking endonuclease-assisted TRA has received particular interest due to the virtue its manipulability and isothermal reaction condition. [PERSON] _et al._ have demonstrated colorimetric DNA detection through the use of nicking endonuclease (Nt.A1 wl)-assisted TRA, and this system provides a colorimetric detection limit of 0.5 fM within several hours for single-stranded oligonucleotides [29].  
Herein, we report a novel highly sensitive strategy for ECL detection of DNA sequences using the _S. aureus_ nuc gene (position 894755-894776, GI:47208328), which is highly conserved in all _S. aureus_ strains, as a model target [30; 31; 32]. Our approach relies on the coupling of Nt.BbvCI-assisted TRA with the RCA reaction for signal amplification and then the formation of hemin/G-quadruplex, which is formed by the intercalation of hemin into a guanine-rich nucleic acid sequence [33]. Hemin demonstrate a significant ECL quenching effect toward the O2/S2 Os2- system because the hemin will absorb the oxygen dissolved in the detection buffer like the haemoglobin will absorb the oxygen in blood. Due to Nt.BbvCI-assisted TRA, each target DNA can be used multiple times, leading to the production of numerous intermediate DNA fragments, which can bind specifically designed padlock probes, thus facilitating the RCA reaction and generation of massive hemin/G-quadruplex. In this manner, a considerable quenching effect toward the O2/S2 Os2- system is achieved. The integration ofthese two signal amplification routes and hemin/G-quadruplex in one assay protocol makes it possible to detect an ultralow level of sequence-specific DNA targets.