Electrochemical Immunosensors for Sensitive Detection of Neuron-Specific Enolase Based on Small-Size Trimetallic Au@Pd^Pt Nanocubes Functionalized on Ultrathin MnO₂ Nanosheets as Signal Labels

Abstract

Medically, neuron-specific enolase (NSE) as a specific tumor marker has become an important indicator to diagnose small-cell lung carcinoma. In this study, a sandwich-type electrochemical immunosensor was designed to determine NSE sensitively. Au nanoparticle (Au NP)-embedded zinc-based metal-organic frameworks (Au@MOFs) were prepared as the substrate materials to modify the electrode and immobilize the primary antibody (Ab₁). The Au@MOFs with the free amino groups on the MOF surface could effectively increase the immobilization amount of Ab₁ through covalent linkage. Simultaneously, the embedding of Au NPs improved the conductivity of MOFs and accelerated interface electron transfer. Sub-30 nm trimetallic Au@Pd^Pt nanocubes (Au@Pd^Pt NCs) loaded onto ultrathin MnO₂ nanosheets (MnO₂ UNs/Au@Pd^Pt NCs) acted as the labels of secondary antibodies. The small-size Au@Pd^Pt NCs enhanced atomic utilization efficiency and offered more catalytic active sites. The MnO₂ UNs with high external surface areas could improve the dispersion of Au@Pd^Pt NCs. The MnO₂ UNs/Au@Pd^Pt NCs could catalyze the H₂O₂ reduction and promote the oxidation of hydroquinone to quinone effectively because of their synergistic effect; thus, the generated quinone achieved amplification of the highly reductive peak current. Furthermore, under the optimal conditions, the immunosensor exhibited a low detection limit (4.17 fg/mL) and broad linear range (10 fg/mL to 100 ng/mL). The results were satisfactory for NSE detection in human serum samples, implying that the presented method had great application potential in clinical bioanalysis.