Immobilization-free and label-free electrochemical DNA biosensing based on target-stimulated release of redox reporter and its catalytic redox recycling

Herein, we demonstrate a simple, homogenous and label-free electrochemical biosensing system for sensitive nucleic acid detection based on target-responsive porous materials and nuclease-triggered target recycling amplification. The Fe(CN)63- reporter was firstly sealed into the pores of Fe3O4 nanoparticles by probe DNA. Target DNA recognition triggered the controllable release of Fe(CN)63- for the redox reaction with the electron mediator of methylene blue enriched in the dodecanethiol assembled electrode and thereby generating electrochemical signal. The exonuclease III (Exo III)-assisted target recycling and the catalytic redox recycling between Fe(CN)63- and methylene blue contributed for the enhanced signal response toward target recognition. The low detection limit toward target was obtained as 478 fM and 1.6 pM, respectively, by square wave voltammetry and cyclic voltammetry methods. It also possessed a well-discrimination ability toward mismatched strands and high tolerance to complex sample matrix. The coupling of bio-gated porous nanoparticles, nuclease-assisted target amplification and catalytic redox recycling afforded the sensing system with well-controllable signal responses, sensitive and selective DNA detection, and good stability, reusability and reproducibility. It thus opens a new avenue toward the development of simple but sensitive electrochemical biosensing platform.

A ratiometric electrochemical biosensor via alkaline phosphatase mediated dissolution of nano-MnO2 and Ru(III) redox recycling for the determination of dimethoate

A sensitive and ratiometric electrochemical biosensor was developed for the determination of dimethoate via alkaline phosphatase (ALP) mediated dissolution of nano-MnO₂ and [Ru(NH₃)₆]³⁺(Ru(III)) redox recycling. The electroactive probe Ru(III) was adsorbed on the nano-MnO₂ with the high specific surface area through electrostatic interaction to form the MnO₂-Ru(III) nanocomposite, which was then fixed on the surface of the glassy carbon electrode. When the dimethoate inhibited the catalytic activity of ALP in a homogeneous system, the hydrolysate L-ascorbic acid (AA) produced by ALP hydrolysis of L-ascorbic acid-trisodium 2-phosphate (AAP) decreased. The solution was then incubated with a glassy carbon electrode modified by MnO₂-Ru(III). At this time, only a small amount of MnO₂-Ru(III) was decomposed and Ru(III) was rapidly electroreduced to Ru(II) on the surface of the electrode. The in-situ produced Ru(II) was chemically oxidized back to Ru(III) by Fe(III). The redox recycling of Ru(III) was completed and the Ru(III) reduction current signal was amplified. The process consumed part of Fe(III) to reduce the reduction current signal of Fe(III), and the ratio of the two reduction currents (I_Ru(III)/I_Fe(III)) increased significantly. The I_Ru(III)/I_Fe(III) value increased with the increase of dimethoate concentration in the linear range of 0.01-300 ng mL^-1, and the detection limit was 6.3 pg mL^-1. It has been successfully applied to the determination of dimethoate in oilseed rape and lettuce with a satisfactory result.