PEI-coated Prussian blue nanocubes as pH-Switchable nanozyme: Broad-pH-responsive immunoassay for illegal additive.

Nanozymes are commonly used in the construction of immunosensors, yet they are generally susceptible to pH condition, which greatly hindered their practical use. To break the limitation of pH conditions, polyethyleneimine-coated Prussian blue nanocubes (PBNCs@PEI) were synthesized as the pH-switchable nanozyme, which can show peroxidase-like and catalase-like activity in acidic and alkaline condition, respectively. Besides, the modification of PEI can largely improve the catalytic activity of PBNCs. Herein, the pH-switchable catalytic property of PBNCs@PEI was used to construct the dual-mode immunosensor for the detection of illegal additive, rosiglitazone. In acidic condition, PBNCs@PEI showed excellent peroxidase-like activity, which can trigger the colorimetric reaction of Au nanostars with TMB/CTAB. In alkaline condition, the catalase-like activity of PBNCs@PEI prevailed, thus the decomposition of HO can generate O to initiate the aerobic oxidation of 4-chloro-1-naphthol (4-CN), which can decrease the fluorescence intensity of 4-CN. Based on the competitive immunoassay, both the localized surface plasmon resonance wavelength shift of Au nanostars and the fluorescence intensity change of 4-CN were quantitatively related with rosiglitazone concentration, thus shedding a new light on the construction of broad-pH-responsive immunosensor. Besides, a smart device was developed to transfer the chroma value of Au nanostars into the RSG concentration, making this sensor a promising method in on-site and point-of-care detection.