DNA-modified Electrodes For Electrochemical Assay Of Heavy Metal Ions

Electrochemical analysis is an important component element of instrument analysis. Owing to its unique advantages of simple equipment, ease to operate and high sensitivity, electrochemical analysis has been widely applied in many fields such as trace element detecting, chemical reactive mechanisim, microdomain structure and surface charactercistics. Electrochemical biosensor is a new analytical technology which combines the virtues of electrochemical analysis and the high specification of biomolecular recognition.Functional DNA find growing interest as recogniton elements for development of novel electrochemical biosensors. For example, aptamers were immobilized on electrode to electrochemically determine biological small molecules. In this paper, we use metal ion-specific DNA as recogniton probes to constuct electrochemical biosensor for high toxicity silvber ions (Ag+) and mercury ions (Hg2+):1. Electrochemical Ag+sensor based on C-Ag+-C interaction-mediated tunneling current effect of single-walled carbon nanotube (SWCNT) adsorped on a dodecanethiol monolayer.In chapter1, we have developed a novel electrochemical sensor for Ag+detection based on the C-Ag+-C interaction chemistry and the tunnaling current effect of SWCNT assembled on a dodecanethiol monolayer.In this experiment, We investigated the electrochemical redox response of ferricyanide at the modified dodecanethiol(H25C12S-Au), the single-walled carbon nanotubes-dodecanethiol modified electrode (SWCNT-H25C12S-Au) and the DNA-SWCNT-H25C12S-Au. The results showed that SWCNT adsorped on the surfaced of the H25C12S-Au resored the restrained redox current of ferricyanide on the H25C12S-Au modified electrode, and this so-called tunneling current effect could be flexibly modiated by C-Ag+-C interaction between Ag+and the Ag+-specified oligoculetide probe adsorped on the sidewall of SWCNT.Based on this unique phenomenon, a sensitive, selective and simple electrochemical platform for Ag+detection was successfully developed.2. Oligonucleotide probes applied for sensitive enzyme-amplified electrochemical assay of mercury(II) ionsIn chapter3, we developed a novel electrochemical biosensor for sensitive and selective detecting of Hg2+through the high specific thymine-Hg2+-thymine (T-Hg2+-T) interaction and the enzyme-amplified technology. The thiolated, Hg2+-specific DNA capture probe was immobilized on a gold electrode. In the presence of Hg2+, the thymine-Hg2+-thymine (T-Hg2+-T) interaction between the mismatched T-T base pairs directed the biotinated signal probe hybridizing to the capture probe and yielded a biotin-functioned electrode surface. HRP was then immobilized on the biotin-modified substrate via biotin-streptavidin interaction. The immobilized HRP catalyzed the oxidation of hydroquinone (H2Q) to benzoquinone (BQ) by hydrogen peroxide (H2O2) and the generated BQ was further electrochemically reduced at the modified gold electrode, producing a readout signal for quantitative detection of Hg2+. The results showed that the enzyme-amplified electrochemical sensor system was highly sensitive to Hg2+in the concentration of0.5nM to1μM with a detection limit of0.3nM, and it also demonstrated excellent selectivity against other interferential metal ions.3. A bifunctional oligonucleotide signal probe for label-free electrocatalytic assay of mercury(II) ionsSome oligonucleotides with guanine (G)-rich segments, can assemble into G-quartets, planar structures of four H-bonded Gs, which stack on top of each other to form G-quadruplexes that are essential for nucleotide fuctions. The hemin can be obtained with the guanine carbonyl in the plane or between the four-plane of G-quartets, not only greatly enhanced the stability of four-helix structure, and this compound also had a similar horseradish oxidase activity. In the field of biochemical analysis,in view of many advantages of the the G-rich nucleic acid sequences,such as a strong force and high affinity target binding specificity, simple synthesis, biological activity and stability, easy to store, etc, its application is in wide use.In chapter4, we use this unique feature of that G4-Hemin complex can show excellent catalytic activity for the reduction of hydrogen peroxide, to design a bifunctional oligonucleotide signal probe for mercury(II) ions detection. The bifunctional oligonucleotide signal probe contain a Hg2+-specific oligonuotide sequence and a G4sequence, forming a label-free electrocatalytic Hg2+biosensor. The sensor recognition elements comprise two Hg2+-specific oligonuotide sequences(one is a thiolated capture probe, one is a single probe which contains a12-mer Hg2+-specific sequence in its3’-end and a17-mer G4sequence in its5’-end, the two probes include ten complementary base pairs and two mismatched T-T base pairs). In the presence of Hg2+, the two sequences can hybridize and form a stable duplex structure. Then the duplex structure combine with Hemin and form G4-Hemin complex on the electrode surface. Because of the amount of the G4-Hemin complex on the electrode surface depending on the concentration of mercury ions in test solution, the resulting electrocatalytic reduction current of H2O2could provide a quantitative measure for Hg2+.