Bismuth-based metal-organic framework peroxidase-mimic nanozyme: Preparation and mechanism for colorimetric-converted ultra-trace electrochemical sensing of chromium ion

A colorimetric-electrochemical dual-mode analytical method based on bismuth metal-organic framework nanozyme was developed for label-free and trace/ultra-trace Cr6+ detection. 3D ball-flower shaped bismuth oxide formate (BiOCOOH) was used as the precursor and template to facilely construct the metal-organic framework nanozyme BiO-BDC-NH2, which possesses intrinsic peroxidase-mimic activity to effectively catalyze the colorless 3,3',5,5'-tetramethylbenzidine into blue oxidation products in the presence of hydrogen peroxide. Based on Cr6+ to promote the peroxide-mimic activity of BiO-BDC-NH2 nanozyme, a colorimetric method for Cr6+ detection was developed with the detection limit of 0.44 ng mL-1. Cr6+ can be electrochemically reduced to Cr3+ that would specifically inhibit the peroxidase-mimic activity of BiO-BDC-NH2 nanozyme. Thus, the colorimetric system for Cr6+ detection was converted into a low-toxic and signal-off electrochemical sensor. The electrochemical model showed upgraded sensitivity and a lower detection limit of 9.00 pg mL-1. The dual-model method was developed for selective appropriate sensing instruments in different detection scenarios, which can provide built-in correction for environmental effects, as well as the development and utilization of dual-signal sensing platforms for trace to ultra-trace Cr6+ rapid assay.