## 1 Introduction  
Pathogenic _Escherichia coli_ (_E. coli_) is one of the most harmful bacteria related to food-borne diseases and can cause inflammation, abdominal pain, diarrhea and even severe cases like hemorrhagic enteritis and hemolysis, especially in infants and youngsters [1]. A systematic analysis indicated that _E. coli_ infection has already become the second leading cause of death in children younger than 5 years old [2]. Therefore, a fast, sensitive, and easy-to-use method for the detection of _E. coli_ is an urgent demand in clinical diagnosis, environment monitoring, pharmacy and food safety fields. At present, _E. coli_ detection methods mainly include the traditional culture counting method [3], molecular biological detection technology [4], immunological detection method [5], surface plasmon resonance (SPR) [6], Chemiluminescence immunoassay immunoassay [7], etc. The traditional counting method for _E. coli_ detection is based on culturing and isolation of bacteria in a specific medium, followed by observing bacterial colony morphology, color change, and biochemical reactions for quantitative detection. This detection method is economical and relatively simple to operate, but the detection time is generally 48-72 h or even longer, which cannot meet the needs for rapid detection. As an effective molecular biological detection technology, polymerase chain reaction (PCR) and the derived technology of PCR has been widely used in detecting _E. coli_[4; 7]. PCR is based on the principle that DNA templates can denature at high temperatures, reactive and extend (one cycle) at low temperatures by using appropriate DNA polymerase and primers. As the template DNA amplification product increases exponentially, increasing the number of cycles can magnify the specified DNA sequence by several million-fold in a short time, which greatly improves detection sensitivity [8]. However, PCR technology is costly and prone to produce false negatives [9], which limits the extensive application of PCR. In addition, enzyme-linked immunosorbent assay (ELISA), as a common immunological detection technology, has also been used in bacterial test [5; 10]. ELISA technology undergoes a color change by the catalysis of the enzyme in the specific recognition process. Owing to the immune activity between antigen and antibody, ELISA method has superior specificity, high sensitivity and short detection time, and has been used in detection of different subtypes of _E. coli_[11; 12; 13]. However, due to the costly and cumbersome preparation process of antibodies, the detection cost of ELISA has increased greatly [14; 15]. Another disadvantage is their limited shelf lives, which limits the viability of antibody-based biosensors in ELISA technology [16]. Moreover, the SPR method is labor intensive and requires expensive instrumentations, which limits its usage in _E. coli_ detection. Chemiluminescence immunoassay immunoassay has the advantages of simple operation, short detection time, and has been used in rapid detection of microbial [17]. However, few stable and sensitive chemiluminescence systems hinder the development of chemiluminescence immunology in microbial detection. Compared with the above-mentioned methods, electrochemical methods have received considerable interest with their high sensitivity, miniaturized and low-cost devices, and simple operation, especially in medical, pharmaceutical, food and environmental rapid detection of _E. coli_[18; 19; 20; 21]. Recently, great efforts have been devoted to the development of novel modified electrodes for _E. coli_ rapid detection, because the direct oxidation current on the bare electrode was not attractive owing to the high overpotential and sluggish electron transfer processes [22; 23]. To date, three kinds of materials, including enzymes [14; 24], aptamers [25] and nano-composites [26; 27], have been used to modify the electrode surface for better analytical properties. Among them, aptamers are expected to be a promising candidate for the design of electrochemical biosensors for _E.