Oxidase-mimicking peptide-copper complexes and their applications in sandwich affinity biosensors

This work reported that the peptide-Cu²⁺ complexes reveal oxidase-mimicking activities for oxygen reduction and ascorbate oxidation. The peptide features a histidine (His) residue in the first position with respect to the free N-terminal amine. The complexes can be easily integrated with nanomaterials such as gold nanoparticles (AuNPs) to form nanocatalysts or nanozymes, which allows for the fabrication of sensitive sensing platforms. To explore the analytical performances of the peptide-Cu²⁺ complexes in biosensing, sandwich electrochemical and fluorescent immunoassays of prostate specific antigen (PSA) were performed with AuNP/peptide-Cu²⁺ conjugates as the nanolabels. In the electrochemical immunoassays, the nanolabels coined as nanocatalysts promoted the electrocatalytic reduction of oxygen. For the fluorescent assays of PSA, the nanolabels coined as nanozymes catalyzed the oxidation of ascorbic acid (AA), and the resulting oxidization product dehydroascorbic acid (DHA) was reacted with o-phenylenediamine (OPD) to form fluorescent 3-(dihydroxyethyl)furo [3,4-b]quinoxaline-1-one (DFQ). The electrochemical and fluorescent signals are proportional to the concentration of PSA in the linear range of 0.001-0.50 and 0.001-2.00 ng/mL, respectively. The low detection limits of the methods (0.40 and 1.00 pg/mL) enabled clinical detection of PSA in serum samples. The findings should be valuable for understanding the biological functions of peptide-Cu²⁺ complexes, and provide new insight to develop optical and electrochemical sensing platforms for clinical diagnosis.