Enzyme-Free Colorimetric Immunoassay for Protein Biomarker Enabled by Loading and Disassembly Behaviors of Polydopamine Nanoparticles.

In this work, a mussel-inspired polydopamine (PDA) nanoparticle with biocompatible and reactive surface characteristics was desirably incorporated with high loading capacity and pH-induced disassembly-releasing behavior toward Fe(III) ions to develop an enzyme-free colorimetric immunoassay. The catechol functional network of PDA can act as a scaffold to coordinate with large amounts of Fe(III) ions to form the PDA-Fe(III) NPs whose coordination state can be tailored by changing the pH values. In detail, PDA-Fe(III) NPs can be maintained in a disassembled tri-coordinate state under alkaline conditions while the highly loaded Fe(III) ions can be easily released under acid conditions to react with ferrocyanide for the in situ generation of Prussian blue (PB) NPs. The detection sensitivity of prostate-specific antigen (PSA) was significantly improved, owing to the high peroxidase-like activity of PB NPs that triggered excellent catalytic effect by the colorimetric reaction. In addition, favorable linearity was found in the range of 0.0005-20 ng mL with a low detection limit of 0.84 pg mL (S/N = 3). Furthermore, excellent selectivity and reproducibility were exhibited by the developed enzyme-free colorimetric immunoassay. It is believed that this proposed PDA-Fe(III) NP-based enzyme-free colorimetric system will offer a facile and reliable tool for the sensitive detection of PSA and other cancer biomarkers in human serum.