Sensitive electrochemical DNA-based biosensors for the determination of Ag⁺ and Hg²⁺ ions and their application in analysis of amalgam filling

In the present paper, we used single-stranded poly-T (100% thymine bases) and poly-C (100% cytosine bases) nucleic acids as DNA probes for selective and sensitive individual electrochemical determination of Hg²⁺ and Ag⁺_, respectively, on the multi-walled carbon nanotube paste electrodes (MWCNTPEs) using [Fe(CN)₆]^3−/4− as electroactive labels. In the presence of Hg²⁺ and Ag⁺, the probe–Hg²⁺/Ag⁺ interactions through T–Hg²⁺–T and C–Ag⁺–C complexes formation could cause the formation of a unimolecular hybridized probe. This structure of probe led to its partial depletion from electrode surface and facilitation of electron transfer between [Fe(CN)₆]^3−/4− redox couple and electrode surface, resulting in the enhanced differential pulse voltammetry (DPV) oxidation current of [Fe(CN)₆]^3−/4− at the probe-modified electrode surface. We applied the difference in the oxidation peak currents of [Fe(CN)₆]^3−/4− before and after Hg²⁺/Ag⁺–DNA probe bonding (∆I) for electrochemical determination of these heavy metal ions. Detection limits were 8.0 × 10⁻¹² M and 1.0 × 10⁻¹¹ M for Hg²⁺ and Ag⁺ ions determination, respectively. The biosensors were utilized to determine the weight percent of toxic metals, i.e., silver and mercury in dental amalgam filling composition. The results of their practical applicability in analysis of the amalgam sample were satisfactory.