Metal-Polydopamine Framework: An Innovative Signal-Generation Tag for Colorimetric Immunoassay

In this work, an innovative enzyme-free colorimetric immunoassay was proposed for the sensitive detection of alpha-fetoprotein (AFP) by introducing thymolphthalein-modified metal-polydopamine framework (MPDA@TP) for the signal generation and amplification. Using zeolitic imidazolate framework (ZIF-67) as the template, the hollow-structured metal-polydopamine framework (MPDA) with high surface recovery and abundant groups was synthesized and functionalized with thymolphthalein (TP) molecules via typical π-stacking reaction. In the presence of target AFP, an MPDA@TP-linked immunosorbent assay (MLISA) was implemented on the capture antibody-modified microplate by using detection antibody-labeled MPDA@TP as the secondary antibody. Upon alkaline solution introduction, the coated hydrophobic TP on the MPDA was deprotonated into hydrophilic TP^2- ion and dissolved in the solution, thereby resulting in the color change of the solution from nearly colorless to deep blue, and the increasing absorbance of the solution at 595 nm. Importantly, the MPDA@TP-based immunoassay could exhibit high sensitivity for the quantitative detection of target AFP on the basis of the absorbance within a linear range of 10-1000 pg mL^-1 at a low detection limit of 2.3 pg mL^-1. Furthermore, this system was validated preliminarily to screen human serum specimens with well-matched results for the referenced AFP ELISA kit. Taking advantage of simplicity, enzyme-free, convenience, and sensitivity, MPDA@TP-linked immunosorbent assay has the potential for the application in scientific research and clinical diagnosis.