Signal “Off–On Model’’ for Colorimetric Sensing Proanthocyanidin B₂ Achieved by the Organic Solvent-Processed Amorphous BiVO₄ Nanozyme

Here, we report a "off-on model"-based colorimetric sensor without the assistance of H₂O₂, achieved by organic solvent-processed amorphous BiVO₄ nanoparticles favorable for large-scale manufacturing. The amorphous material beyond traditional crystalline nanozymes has both vanadium vacancy (<i>V</i>_v) and oxygen vacancy (<i>O</i>_v), as well as single oxidase-like activity with both hydroxyl radical (^•OH) and superoxide anion (O₂^•-) as electron acceptors. Despite the high enzymatic performance, the dual-defect-rich amorphous BiVO₄ preserves good long-term activity in either buffer solution or the solid state. The specially designed BiVO₄ is capable of accelerating TMB oxidation in the presence of polyphenols, thus leading to an interesting signal-intensified colorimetric response. This is probably due to the coordination of the ortho dihydroxyl group in polyphenols with the catalyst by replacing the surface-adsorbed NO₃^-, which results in faster charge transfer between BiVO₄ and substrate and, thus, more rapid depletion of radicals for TMB oxidation. Based on these findings, a sensor platform for proanthocyanidin (PAC) B₂ is established with a limit of detection (LOD) as low as 65 nM and good specificity. The sensor shows higher sensitivity than the standard Porter method (LOD: 0.29 μM) and comparable accuracy when analyzing PAC B₂ in commercially available grape seed capsules, with quantitative recoveries varying from 104.0 to 108.3% and relative standard deviations ranging from 1.7 to 5.7%. To date, this is the only nanozyme-based chemical sensor that is active for the signal "off-on model" for the detection of reducing polyphenols. Also, this method seems quite general, and it can be used for sensing of a series of specific polyphenols in addition to PAC B₂.