Covalent organic frameworks surface-anchored with silver nanoparticles for colorimetric detection of dopamine based on its oxidase-mimicking activity

Sensitive detection of dopamine (DA) is essential for the diagnosis of nervous system diseases. Herein, a nanozyme-based colorimetric platform for DA determination was proposed. Using an imine-type covalent organic framework (COF-TD) as a host matrix, a nanozyme (AgNPs@COF-TD) was fabricated by anchoring silver nanoparticles (AgNPs) on the surface of COF-TD via in situ reduction of silver ions. Various characterization results confirmed the successful synthesis of flower-like AgNPs@COF-TD with surface-anchored AgNPs, which could expose more active sites. The as-prepared AgNPs@COF-TD exhibited satisfactory oxidase-mimicking activity with a low Km, which could catalyze the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxidized TMB (oxTMB) with significant absorbance at 657 nm. The introduction of DA not only reduced blue oxTMB to colorless TMB but also competed with TMB for the active sites of AgNPs@COF-TD, making a significant decrease in absorbance. Therefore, a label-free sensing strategy based on the oxidase-mimicking activity of AgNPs@COF-TD was successfully established for the colorimetric detection of DA, with a detection limit down to 0.2 μM. The practical applicability of this assay was further validated by analyzing DA-spiked human urine samples. This work provided a facile approach to prepare COF-based nanozymes, and demonstrated their great potential in colorimetric detection and bioanalysis.