Flexibility-tunable polymeric ionic liquids mediated interface modification of ZIF-67/MWCNTs: triple interfacial mechanisms for selective caffeic acid electrochemical sensing.

As a key component of polyphenols in beverages, the precise quantification of caffeic acid (CA) is crucial for evaluating beverage quality and studying its health-related functions. Ionic liquids (ILs)/deep eutectic solvents (DESs) monomers containing epoxy groups, carbon-carbon double bonds, and carbon-carbon triple bonds were converted into functionalized polymeric ionic liquids (PILs)/polymeric deep eutectic solvents (PDESs) via distinct polymerization modes. These bonding-mode engineered polymers were composited with zeolitic imidazolate framework (ZIF-67)/multi-walled carbon nanotubes (MWCNTs) to construct a ternary electrochemical sensor (ZIF-67/MWCNTs@PIL/PDES) for the detection of CA. The PIL polymerized via carbon-carbon double bonds displayed optimal performance, since its flexible main chain optimized charge distribution and strengthened π-π stacking and electrostatic interactions with CA. Combined with density functional theory (DFT) calculations, the interfacial electrostatic-coordination synergistic mechanism and electron transfer pathway were revealed. Meanwhile, it could fill the pores of ZIF-67 and adhere to the tube walls of MWCNTs through flexible conformational adjustments, increasing the contact area to facilitate interfacial charge transfer. The sensor based on double bond-polymerized PIL exhibited excellent linear response over 0.1-20.0 μM and 20.0-150.0 μM, with a limit of detection (LOD) as low as 0.0734 μM, and was successfully employed for CA determination in coffee, red wine, and green tea specimens.