Ultrasensitive Electrogenerated Chemiluminescence Biosensor for the Determination of Mercury Ion Incorporating G4 PAMAM Dendrimer and Hg(II)-specific Oligonucleotide

A novel electrogenerated chemiluminescence (ECL) biosensor for highly sensitive and selective detection of mercury ion was developed on the basis of mercury-specific oligonucleotide (MSO) served as a molecular recognition element and the ruthenium(II) complex (Ru1) as an ECL emitting species. The biosensor was fabricated on a glassy carbon electrode coated with a thin layer of single wall carbon nanotubes, where the ECL probe, NH(2)-(CH(2))(6)-oligo(ethylene oxide)(6)-MSO↔Dend-Ru1, was covalently attached. The Dend-Ru1 pendant was prepared by covalent coupling Ru1 with the 4th generation polyamidoamine dendrimer (Dend), in which each dendrimer contained 35 Ru1 units so that a large amplification of ECL signal was obtained. Upon binding of Hg(2+) to thymine (T) bases of the MSO, the T-Hg-T structure was formed, and the MSO changed from its linear shape to a "hairpin" configuration. Consequently, the Dend-Ru1 approached the electrode surface resulting in the increase of anodic ECL signal in the presence of the ECL coreactant tri-n-propylamine. The reported biosensor showed a high reproducibility and possessed long-term storage stability (92.3% initial ECL recovery over 30 day's storage). An extremely low detection limit of 2.4 pM and a large dynamic range of 7.0 pM to 50 nM Hg(2+) were obtained. An apparent binding constant of 1.6 × 10(9)M(-1) between Hg(2+) and the MSO was estimated using an ECL based extended Langmuir isotherm approach involving multilayer adsorption. Determination of Hg(2+) contents in real water samples was conducted and the data were consistent with the results from cold vapor atomic fluorescence spectroscopy.