Dual-signal assay based on carbon dots and Fe/Co/Cu layered double hydroxide nanosheets for efficient monoamine oxidase B sensing

Monoamine oxidase B (MAO-B) is a critical enzyme responsible for neurotransmitter metabolism and is implicated in the pathogenesis of neurodegenerative disorders and hepatic diseases. Here, we reported a dual-signal ratiometric fluorescence and colorimetric method for the analysis of MAO-B by integrating carbon dots (CDs) with outstanding fluorescent properties and Fe/Co/Cu layered double hydroxide (Fe/Co/Cu-LDH) nanosheets exhibiting prominent peroxidase-mimicking activity, for the first time. MAO-B could catalyze the oxidative deamination of phenethylamine to generate hydrogen peroxide (H₂O₂). This generated H₂O₂ then enabled Fe/Co/Cu-LDH nanosheets to catalyze the oxidation of o-phenylenediamine, forming the oxidative product 2,3-diaminophenazine (DAP). The resulting DAP displayed characteristic optical signals at 570 nm (fluorescence emission) and 430 nm (UV-Vis absorption). Subsequently, the fluorescence of CDs at 445 nm was markedly quenched due to the absorption of DAP at 430 nm via an inner filter effect. The ratiometric fluorescence signal (F₅₇₀/F₄₄₅) exhibited a linear response to MAO-B concentrations across two ranges: 0.003-0.08 U/L and 0.08-10 U/L. Concurrently, the colorimetric assay demonstrated effective detection within the range of 0.03-10 U/L. The proposed dual-signal detection strategy based on CDs and Fe/Co/Cu-LDH nanosheets substantially enhanced the sensitivity (limits of detection: 0.00067 U/L and 0.0085 U/L for the ratiometric fluorescence and colorimetric methods, respectively) and accuracy (recovery rates: 97.0%-104.9%; relative standard deviations: less than 5.3%) of the sensor. Furthermore, the proposed sensing platform was effectively utilized for measuring MAO-B levels in human serum and for screening inhibitors, highlighting its potential in clinical diagnostics and pharmaceutical research.