H₂-Based Electrochemical Biosensor with Pd Nanowires@ZIF-67 Molecular Sieve Bilayered Sensing Interface for Immunoassay

With the introduction of gas-based contactless electrochemical biosensors lies the prospects of separating the sensing interface from the bioassembly platform, enhancing stability, and exploring signal transduction mechanism, all intimately linking to development of immunoassay. Herein, we report on a H₂-based electrochemical biosensor whose signals derived from the chemical signal transduction between a H₂ and Pd nanowires@ZIF-67 (ZIF: Zeolitic Imidazolate Frameworks) bilayered sensing interface for immunoassay. Dendritic Pt nanoparticles (DPNs) conjugated on the detection antibody were introduced on the interface of a magnetic microsphere according to an immune sandwich assembly between the antigen and antibody. H₂ as a bridge originates from DPNs catalyzing NH₃BH₃ and links biological signals to electrical signals by reacting with Pd nanowires. Nevertheless, the response of Pd nanowires being extremely effected by O₂ in air due to the competitive adsorption on the surface of Pd nanostructures as well as the reaction between chemisorbed O (Pd-O) and adsorbed dihydrogen lead to a decrease in H absorption into PdH_x and poor sensing responses under low target concentration. Porous ZIF-67 (window aperture 0.331 nm) as a molecular sieve self-assembling on the surface of the Pd nanowires film could easily permeate H₂ (kinetic diameter of 0.289 nm), while interferential O₂ (kinetic diameter of 0.346 nm) has difficultly passing through the ZIF-67 layer to contact Pd nanowires and achieves a response of a lower concentration target as well as faster response rate. Under optimal conditions, H₂-based electrochemical biosensors exhibit great response toward target alpha-fetoprotein (AFP) within a dynamic working range of 0.1-50 ng mL^-1 at a detection limit of 0.04 ng mL^-1. Our strategy provides a reusable sensing interface, high specificity, and acceptable accuracy for the immunoassay. In addition, it also expands a promising platform for application as a molecular sieve in electrochemical biosensors.