Target-Controlled Redox Reaction and Ru(II) Release of a Smart Metal-Organic Framework Nanomaterial for Highly Sensitive Ratiometric Homogeneous Electroanalysis of Cadmium(II).

In this work, a highly sensitive ratiometric homogeneous electroanalysis (HEA) strategy of cadmium(II) (Cd) was proposed via a Cd-controlled redox reaction and Ru(bpy) (Ru(II)) release from a smart metal-organic framework (MOF) nanomaterial. For achieving this purpose, Ru(II) was entrapped ingeniously into the pores of an MOF material (UiO-66-NH) and subsequently gated by the double-strand hybrids of a Cd-aptamer (Apt) and its complementary sequences (CP) to form a novel smart nanomaterial (denoted as Ru@UiO-66-NH); meanwhile, Fe(III) was selected as an additional probe present in electrolyte to facilitate the Ru(II) redox reaction: Fe(III) + Ru(II) → Fe(II) + Ru(III). Owing to the strong binding effect of the Cd target to the specific Apt, the Apt-CP hybridization at Ru@UiO-66-NH would be destroyed in the presence of Cd, and the related Apt was further induced away from the smart nanomaterial, leading to the opening of the gate and release of Ru(II). Meanwhile, the released Ru(II) was quickly oxidized chemically by Fe(III) to Ru(III). On the basis of the generated Ru(III) and consumed Fe(III), the ratio of the reduction currents between Ru(III) and Fe(III) exhibits an enhancement and it is dependent on the level of Cd; thus, a novel HEA strategy of Cd was then designed. Under the optimal conditions, the HEA sensor shows a wide linearity ranging from 10.0 pM to 500.0 nM, and the achieved detection limit of Cd is 3.3 pM. The as-designed ratiometric HEA strategy not only offers a unique idea to realize a simple and sensitive assay for Cd but also possesses significant potential as an effective tool to be introduced for other target analysis just via altering the specific Apt.