Encapsulation and Release of Recognition Probes Based on a Rigid Three-Dimensional DNA "Nanosafe-box" for Construction of a Electrochemical Biosensor.

Herein a rigid three-dimensional (3D) DNA "nanosafe-box" (DNB) for encapsulation and release of a recognition probe (N3) is designed to construct an electrochemical biosensor with the use of electroactive two-dimensional metal-organic framework (2D MOF) nanosheets as signal tags for ultrasensitive detection of mercury ion (Hg2+). Initially, N3 is locked in the 3D cavity of DNB by blocker DNA. After addition of target Hg2+, exonuclease III (Exo-III) digestion is initiated to the liberate DNA "key" (K); thereby, the free K triggers a strand displacement reaction for exposing the prelocked N3 to successfully ligate dibenzocyclooctyne (DBCO)-tagged anchor via metal-catalyst-free click chemistry, in which amounts of 2D MOF nanosheets containing Co(II) as electron mediator are introduced accompanied by significant electrochemical response. Compared with traditional linear or stem-loop DNA nanostructure, the well-designed 3D DNB possesses remarkably enhanced mechanical rigidity and structural stability, resulting in improved accessibility of probes and increased loading amounts of signal tags. More importantly, by this way of encapsulation and release of recognition probes, the background signal is decreased dramatically, leading to increased sensitivity of the proposed biosensor. Consequently, this electrochemical biosensor exhibits outstanding analytical performance for Hg2+ detection with a low detection limit of 33 fM and dynamic linear range of 0.1 pM to 10 nM. This strategy offers an ingenious method for detection of metal ions and biomarkers, possessing potential applications in environmental tests and clinical diagnosis.