pH-Responsive Magnetic I-Motif Container Coupled with DNA Walker for Construction of Dual-Signal Electrochemical Biosensor

Herein, a dual-signal electrochemical biosensor has been developed by self-assembly of pH-activatable i-motif probes on magnetic microparticles (MMPs) coupled with DNA walker for signal amplification. In this study, the cytosine (C)-rich single-stranded DNAs are hybridized with DNA initiators to obtain the long-nicked duplexes with repeated units, which are further captured on MMPs to form the magnetic i-motif containers. The resulting duplexes contain abundant G–C base pairs, thus providing extensive binding sites for doxorubicin (DOX). At acidic pH, the C-rich sequences are folded into i-motif structure, resulting in the release of DOX and walker initiators. In this case, the liberated DOX is adsorbed on graphene quantum dots-modified glassy carbon electrode via π–π interaction, while the walker initiators as a moving part can catalyze the hybridization between MB-modified fueler DNA and tracker DNA on electrode, contributing to the generation of dual electrochemical signals induced by MB and DOX. Importantly, the magnetic separation can effectively reduce the background, achieving sensitive biosensing of pH ranging from 4.0 to 7.4 with excellent stability. Moreover, the proposed dual-signal electrochemical biosensor has been successfully applied for accurate monitoring of pH in human serum, which holds great potential in pH-dependent bioassays, especially in ultra-micro analysis for clinical applications.