Determination of magnesium ion in serum samples by a DNAzyme-based electrochemical biosensor

A DNAzyme-based electrochemical biosensor for magnesium ion (Mg2+) has been developed. Firstly, the Mg2+-dependent DNAzyme with thiol at the 5′-terminus is immobilized on a gold electrode through thiol–Au interaction. Then the substrate strand with ferrocene (Fc) at the 5′-terminus is attached to the gold electrode via hybridization with DNAzyme. In the absence of Mg2+, this sensor shows an obvious faradaic current from Fc. In the presence of Mg2+, the Fc-labeled substrate strand is cleaved into two pieces by catalytic action of DNAzyme and moves away from the electrode surface, which reduces the faradaic current. The current change provides quantitative measurement of Mg2+. The linear response range for the proposed biosensor is 0.2–5.0 mmol L−1, and the detection limit is 0.05 mmol L−1. Other divalent metals present do not interfere with Mg2+ detection. The sensor was applied to detect Mg2+ in serum with satisfactory results.