Surfactant-free synthesis of gold nanoparticle-decorated poly( <i>o</i> -phenylenediamine) sub-microspheres as surface-confined signaling probes for label-free electrochemical immunosensing of <i>E. coli O157:H7</i>

Rapid, sensitive, and efficient detection of pathogenic bacteria is crucial for public health, particularly in food safety, environmental monitoring, and clinical diagnostics. In this paper, we report a simple sol-gel-based synthesis of poly(<i>o</i>-phenylenediamine) (PoPD) and surfactant-free gold nanoparticles (AuNPs) decorated on the PoPD polymer. The amine/imine groups in the polymer reduce gold ions to AuNPs and stabilize them on its surfaces. Reproducible, structure-size-controlled, and electrochemically redox-active oligomeric polymers offer significant advantages for electrocatalysis and sensor applications. Physicochemical characterization revealed the presence of phenazine and quinoid units in the polymer structure, and their protonated forms were electrochemically accessible in a pH 7 buffer at negative potentials. Thus, the synthesized PoPD-AuNPs can serve as a stable electrochemical transducer and biorecognition immobilization matrix for immunosensor applications. By leveraging the electrochemical properties and biocompatibility of the spherical PoPD-AuNP composite, we developed a label-free, surface-confined signaling probe as an electrochemical immunosensor for pathogenic <i>Escherichia coli (E. coli) O157:H7 via</i> specific antigen-antibody interactions. The sensor showed a wide linear detection range between 5 and 10⁸ CFU mL^-1, with a detection limit of 1.1 CFU mL^-1.