<p>A dual-mode immunosensor based on polydopamine@Prussian blue nanoparticles (PDA@PB NPs) was developed for rapid and sensitive detection of benzocaine (BZC) in aquatic food. PDA@PB NPs possess excellent peroxidase-like activity, photothermal conversion efficiency, and antibody-binding capacity, enabling simple, crosslinker-free antibody immobilization. Upon target recognition, the sensor produces a distinct blue color via TMB–H₂O₂ oxidation and a robust near-infrared photothermal signal, allowing dual-mode quantitative determination. Under optimized conditions, the sensor showed a linear range of 0.01–1000 ng/mL, with detection limits of 0.33 ng/mL (colorimetric) and 0.82 ng/mL (photothermal). The integration of colorimetric and photothermal outputs improves detection accuracy, reduces matrix interference, and eliminates the need for complex instrumentation. Validation with spiked real samples confirmed its reliability and applicability. This portable, cost-effective immunosensor offers a promising tool for on-site monitoring of BZC residues in aquatic products, contributing to enhanced food safety surveillance.</p> Graphical abstract <p></p>