Bimetallic urchin-like Au@Pt/SiO2 hybrid interface realizing a reversed positive signal strategy for electrochemical immunosensing of tetracycline.

A reversed positive-signal electrochemical immunosensor was developed for ultra-sensitive tetracyclines (TC) detection, leveraging a synergistically engineered interface of conductive urchin-like Au@Pt nanoparticles (NPs) and insulating SiO2 nanoprobes. The Au@Pt NPs provide a high-performance electroactive foundation that significantly expands the effective surface area and accelerates interfacial electron transfer kinetics. A ‘signal-on’ mechanism was innovatively constructed by utilizing the mass-transport resistance and steric hindrance of bulky, insulating SiO2-antibody conjugates. In the absence of TC, the anchoring of SiO2 nanoprobes creates a dense insulating barrier that truncates electron tunneling paths. Conversely, the competitive recognition of free TC prevents probe anchoring, removing the barrier and restoring charge transfer, which results in a significantly enhanced signal. This strategy effectively transforms passive dielectric blocking into a sensitive electrochemical response, allowing effective signal amplification of subtle recognition events. Under optimized conditions, the sensor exhibited a linear differential pulse voltammetry (DPV) response from 20 fg/mL to 20 ng/mL, with detection limits of 4.55 fg/mL and 11.58 fg/mL for the glassy carbon electrode and the screen-printed electrode, respectively. The sensor demonstrated excellent specificity against structural analogs, high reproducibility, and reliable recovery in spiked milk samples. This work validates the efficacy of using cost-effective insulating nanomaterials for high-sensitivity biosensing, offering a robust tool for food safety surveillance.