CuBi2O4@NH2-UiO-66 loaded with multi-walled carbon nanotubes self-reference ratiometric electrochemical sensor for highly sensitive detection of caffeic acid in beverages.

Although caffeic acid (CA) has important physiological effects, such as antioxidant and antibacterial properties, high concentrations of CA can increase the risk of cancer. Therefore, it is very necessary to establish a simple and sensitive detection method for it. Herein, a novel CA electrochemical sensing interface was constructed by in-situ growing NH2-UiO-66, a zirconium-based metal-organic framework (MOF), on semiconductor cupric bismuthate CuBi2O4 (CBO) and further loading acidified multi-walled carbon nanotubes (MWCNTs). The obtained nanocomposite fully leverages the respective advantage of each component, including the excellent electrocatalytic activity of CBO, the selective adsorption ability of MOF, and the high conductivity and signal amplification effect of MWCNTs. Particularly, the acid-functionalized MWCNTs possess sufficient surface electroactive groups (sp2-C-OH), which can generate a stable electrochemical response through the reversible transformation between hydroxyl and quinone groups, thereby providing an important guarantee for constructing a self-referenced ratiometric electrochemical sensor. The electrochemical sensor was prepared by modifying the composite CBO@NH2-UiO-66/MWCNTs on glassy carbon electrode (GCE) to realize CA detection in wide range of 0.2-150 μM with a low detection limit (LOD = 0.052 μM) and excellent reproducibility, stability and selectivity. The analysis of actual beverages including red wine, green tea and coffee also achieved satisfactory recovery results and precision.