Dual-mode colorimetric and RGB detection of sulfide based on a magnetic Fe₃O₄/CoFe₂O₄/Ce-BDC hybrid nanozyme

A dual-mode sensing strategy is presented by integrating a magnetic Fe₃O₄/CoFe₂O₄/Ce-BDC (denoted as FCF/Ce-BDC) nanozyme with TMB/H₂O₂ chromogenic chemistry to construct peroxidase-mimetic hybrids. The FCF/Ce-BDC catalyst efficiently oxidizes 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H₂O₂), generating a blue product (oxTMB, λmax = 651 nm). The subsequent reaction of sulfide under acidic conditions scavenges reactive oxygen species (ROS), predominantly hydroxyl radicals (•OH), and reduces oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB), leading to a pronounced decrease in absorbance at 651 nm and a visually discernible color transition from blue to colorless. This dual-signal response enables quantitative determination of sulfide via both conventional spectrophotometric absorbance measurement and smartphone-based RGB colorimetry, achieving detection limits of 0.29 μM (absorbance mode) and 7.43 μM (RGB mode), respectively. We fabricated three portable, field-deployable sensors for on-site sulfide detection: a hydrogel film, a cotton fabric strip and a cotton swab. The hydrogel sensor leverages ion-mediated crosslinking to modulate reaction kinetics and ensure reproducible color development, whereas the cotton-based substrates exploit capillary-driven fluid transport to enrich analyte concentration, thereby enhancing sensitivity and mitigating matrix interference. This study not only advances the rational design of multifunctional nanozymes but also delivers a validated, application ready framework for their safe and selective use in environmental water matrices and human whole blood, enabling real time, cost effective, and operationally simple tracking of target pollutants without compromising analytical reliability.