pH-Driven Dynamic Heterojunction Based on 2D ZIF-L@Ti–Fe–O Nanocages for Immobilization-Free Photoelectrochemical Sensing: Subpicomolar Monitoring of Typical Forever Chemical Perfluorooctanoic Acid

Abstract

The ultratrace determination of the typical “forever chemical” perfluorooctanoic acid (PFOA) is an urgent imperative due to its severe bioaccumulation and toxicity even at minute concentrations. However, its lack of redox and optical activity poses a significant challenge for conventional photoelectrochemical (PEC) sensing. Traditional PEC configurations rely on immobilizing recognition probes directly onto the solid electrode, an architecture that inherently suffers from restricted binding sites and poor reusability. To address this interfacial barrier, an electrode-immobilization-free (EIF) PEC aptasensor featuring a pH-driven dynamic heterojunction is designed for the subpicomolar detection of PFOA. In this architecture, hollow Fe2O3–TiO2 nanocages (Ti–Fe–O NCs) serve as the photoanode. Engineered as a bifunctional component, a 2D ZIF-L nanosheet acts as a dispersed carrier for the aptamer in the solution and functions as a mobile signal regulator. During the homogeneous recognition process, the specific binding between PFOA and the aptamer triggers the release of ZIF-L nanosheets. Driven by pH-regulated electrostatic affinity, the positively charged ZIF-L spontaneously migrates to and assembles onto the Ti–Fe–O surface. This target-responsive assembly facilitates the in situ construction of a Type-I heterojunction, generating a significantly enhanced photocurrent response. Consequently, the sensing platform achieves a broad linear range from 0.1 to 500 pM with an ultralow detection limit of 0.03 pM. Ultimately, by pioneering this homogeneous-to-heterogeneous signal transduction strategy, this highly sensitive platform provides a highly effective pathway for the environmental surveillance of inert forever chemicals at the subpicomolar level.