Hyperporphyrin Effect-Driven Bandgap Narrowing in Cationic Fe3O4@ICOF for Boosted Photothermal Immunoassay

Molecular orbital engineering tackles the wide bandgaps and limited near-infrared (NIR) response that constrain conventional immunochromatographic assays (ICAs) by precisely tailoring the electronic properties of covalent organic frameworks (COFs), enabling ultrasensitive pathogen detection. Here, we report a methylation-mediated cationization strategy to construct core-shell Fe3O4@ICOF, where a cationic porphyrin COF shell is grown on a magnetic Fe3O4 core. The methylation triggers a hyperporphyrin effect that dramatically narrows the HOMO-LUMO gap from 2.634 eV to 1.997 eV, redirecting energy dissipation toward non-radiative thermal relaxation and yielding an ultrahigh photothermal conversion efficiency of 71.9% under 808 nm irradiation. The magnetic core enables facile enrichment, while the cationic surface promotes electrostatic capture of negatively charged Salmonella, synergistically enhancing sensitivity. Integrating colorimetric and photothermal readouts, the dual-modal ICA achieves detection limits of 100 CFU/mL (colorimetric) and 50 CFU/mL (photothermal), representing 500- to 1000-fold improvement over conventional AuNPs-based ICA, which shows satisfactory recoveries in real food matrices. This work establishes a clear structure-electronics-photothermal performance correlation, providing a design method for high-performance point-of-care pathogen sensors.