We have developed a selective, sensitive, and re-usable electrochemical sensor for Hg²⁺ ion detection. This sensor is based on the Hg²⁺-induced conformational change of a single-stranded DNA (ssDNA) which involves an electroactive, ferrocene-labeled DNA hairpin structure and provides strategically the selective binding of a thymine–thymine mismatch for the Hg²⁺ ion. The ferrocene-labeled DNA is self-assembled through S–Au bonding on a polycrystalline gold electrode surface and the surface blocked with 3-mercapto-1-propanol to form a mixed monolayer. The modified electrode showed a voltammetric signal due to a one-step redox reaction of the surface-confined ferrocenyl moiety. The ‘signal-on’ upon mercury binding could be attributed to a change in the conformation of ferrocene-labeled DNA from an open structure to a restricted hairpin structure. The differential pulse voltammetry (DPV) of the modified electrode showed a linear response of the ferrocene oxidation signal with increase of Hg²⁺ concentration in the range between 0.1 and 2 µM with a detection limit of 0.1 µM. The molecular beacon mercury(II) ion sensor was amenable to regeneration by simply unfolding the ferrocene-labeled DNA in 10 µM cysteine, and could be regenerated with no loss in signal gain upon subsequent mercury(II) ion binding.

The enantioselective recognition of 3,4-dihydroxyphenylalanine using penicillamine-modified gold nanoparticles has been investigated. Smaller gold nanoparticles with one enantiomeric ligand facilitate the redox reaction of only one enantiomer of 3,4-dihydroxyphenylalanine, with cross inversion for the gold nanoparticles with the other enantiomeric ligand.