A ratiometric electrochemical strategy based on Fe (iii) and Pt (iv) for immobilization-free detection of Escherichia coli

A new ratiometric electrochemical strategy for immobilization-free detection of Escherichia coli (E. coli) was constructed by using a capture DNA-polyaniline/copper ferrite nanoparticles/graphene oxide (cDNA-PANI/CuFe2O4/GO) composite as capture probes, which has a high specific surface area and good magnetic properties. Then trigger DNA/Au nanoparticles (tDNA/Au NPs) were used as signal amplification labels, and Pt (iv) and Fe (iii) were chosen as the signal probes. In the presence of targets, the sandwich format among cDNA-PANI/CuFe2O4/GO, E. coli and auxiliary DNA (aDNA) was realized by using the aptamer recognition system. Then, the tDNA/Au binding could be anchored on the sandwich format due to the principle of base complementation between unpaired aDNA and tDNA. And the unbounded tDNA of tDNA/Au NPs could bind an amount of Pt (iv). After separation using a magnet, a handful of unbound Pt (iv) which remained in the supernatant reacted with a large number of Fe (iii) ions, leading to a markedly increased I-Fe(iii)/I-Pt(iv) value. Oppositely, the sandwich format could not appear in the absence of targets, and even the tDNA/Au could not be immobilized on it. So, the redox reaction between a large amount of Pt (iv) residue in the supernatant and Fe (iii) was significantly successful, causing a low I-Fe(iii)/I-Pt(iv) value. Under optimal conditions, we found that I-Fe(iii)/I-Pt(iv) was linearly related to the logarithmic E. coli concentration with a low limit of detection (1.862 x 10(3) cfu mL(-1)). This devised ratiometric electrochemical method may develop into a powerful and effective means for the detection of E. coli in real samples, which may also be developed as a universal tool for another microorganism.

Automated summary

A new strategy for immobilization-free detection of E. coli was developed using a composite as capture probes and signal amplification labels, achieving target detection through a specific sandwich format. This method showed linear detection of E. coli concentration with a low limit of detection.