ZnO-reduced graphene oxide composite based photoelectrochemical aptasensor for sensitive Cd(II) detection with methylene blue as sensitizer

Abstract

In this paper a photoelectrochemical (PEC) aptasensor based on specific recognition with conformational changed after the target Cd(II) identification was fabricated. A ZnO and reduced graphene oxide (ZnO-rGO) nanocomposite with enhanced PEC activity was designed as photoactive material. After the further incorporation of gold nanoparticles (AuNPs) with ZnO-rGO nanocomposite, the enhanced photocurrent signal could be detected owing to the localized surface plasmon resonance and good conductivity of AuNPs. In addition, AuNPs were used as anchors for immobilization of –SH modified aptamer S1. After that aptamer S2 was paired with S1 sequence to form complementary double stranded DNA (dsDNA) on the electrode surface. Methylene blue (MB) was acted as sensitizer and assembled in dsDNA structure to amplify photocurrent response. When Cd(II) was bound to the aptamer presented on the sensing interface, S2 specifically recognized and captured Cd(II), which resulted in the unwinding of dsDNA structure and the separation of MB molecules from the electrode surface with photocurrent response decreased. The photocurrent was detected by a double-working-electrode system, which used the modified electrode as the first working electrode and glassy carbon electrode (GCE) as the second working electrode. Dopamine (DA) was added to the electrolyte and acted as the electron donor, which could be oxidized on the modified electrode and reduced on the GCE to form a cyclic reaction, leading to the enhanced photocurrent response with improved photocurrent stability. This MB sensitized PEC aptasensor exhibited a high sensitivity with a detection limit of 1.8 × 10−12 mol/L (3σ). Thus, a highly sensitive aptasensor with double-working-electrode detection method for Cd(II) determination were established and further applied to the water samples analysis.