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.

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.