Multi-branched gold nanoflower-embedded iron porphyrin for colorimetric immunosensor

Here, we for the first time used a thiolated amino-ligand modified multi-branched gold nanoflower as skeleton to encapsulate iron porphyrins (AuNF@FeTPPCl) as alternatives to horseradish peroxidase (HRP). After the FeTPPCl encapsulation, the HRP mimicking activity of FeTPPCl was effectively blocked by the steric hindrance of the hydrophobic layer on the AuNF surface. Upon the addition of ethanol, the loaded FeTPPCl was released into the solution in its free format and exposed the catalytic sites, resulting in the recovery of catalytic activity. The proposed encapsulation method effectively avoided the loss of catalytic activity that originated from the blocking of the catalytic active sites during immobilization. Additionally, the resultant AuNF@FeTPPCl nanocomposite exhibited a high loading level with 2.4 × 10⁶ FeTPPCl molecules per AuNF, and showed considerably high catalytic activity for hydrogen peroxide and 3, 3' 5, 5'-tetramethylbenzidine, which is approximately 40- and 172-folds higher than native HRP. Through using the as-prepared AuNF@FeTPPCl as an alternative of HRP for trace labeling, we successfully developed colorimetric immunosensors for fumonisin B₁ (FB₁) and hepatitis B surface antigen (HBsAg) with competitive and sandwich-type formats, respectively. The developed AuNF@FeTPPCl-based colorimetric immunosensor exhibited higher detection sensitivity for FB₁ and HBsAg than the corresponding HRP-based immunosensors. Thus, the proposed AuNF@FeTPPCl can be used as HRP mimicking analogs for developing highly sensitive colorimetric immunosensor and for loading other hydrophobic iron porphyrins or catalysts.