Template-assisted Cu2O@Fe(OH)3 yolk-shell nanocages as biomimetic peroxidase: A multi-colorimetry and ratiometric fluorescence separated-type immunosensor for the detection of ochratoxin A

Highlights: • A template-assisted strategy was developed to fabricate Cu₂O@Fe(OH)₃ yolk-shell nanocages. • Cu₂O@Fe(OH)₃ yolk-shell nanocages were used as biomimetic nanozymes. • The catalytic mechanism of Cu₂O@Fe(OH)₃ nanocages was investigated. • Dual-mode immunosensor from multicolorimetry and ratiometric fluorescence was developed to detect OTA. The convenient and effective detection of toxins is urgently demanded for food security and human health. Herein, based on the catalytic activity of mimetic peroxidase from the Cu₂O@Fe(OH)₃ yolk-shell nanocages, a dual-modal multi-colorimetric and ratiometric fluorescence immunosensor for the sensitive detection of ochratoxin A (OTA) was successfully developed. For the multi-colorimetric detection, H₂O₂ can be effectively decomposed by Cu₂O@Fe(OH)₃ to form ·OH groups, thus Au nanorods (Au NRs) can be etched to exhibit vivid color variations and localized surface plasmon resonance (LSPR) shifts. For the ratiometric fluorescence detection, o-phenylenediamine was oxidized by Cu₂O@Fe(OH)₃ to form 2,3-diaminophenazine (DAP) in the presence of H₂O₂. Interestingly, the exogenous fluorescence signal source of carbon dots can be quenched by DAP via inner filter effect, while a new emission peak at 563 nm can be discovered, forming a ratiometric fluorescence signal. Due to the independent signals and mutual confirmation, the performance of the dual-modal immunosensor for the detection of OTA was significantly improved, where a broad linear range from 1 ng/L to 10 μg/L with a detection limit of 0.56 ng/L (S/N = 3) was achieved. The sensing strategy was also used to monitor OTA in millet and lake water samples with a satisfied performance.